Mechanical Separations 271
All WILDED CONSTIUCTION. Inlet ond 01,o1let
duct connution1 are weld.cl ond Ron�d.
AUTOMATIC CON-
J 10 LS re;ulote
wo1h-down of moi,-
lure elirriinotor bed
ond opero!lon of
1ludge c.onveyor.
PROTECTIVE COATING
prolec.h all funclionol
C 1teel I componenh.
U------===-- Red'Jce, wear, ero1ion
or corro,ion from wotet
or dull.
STAINLESS sna 1crHn
�:o;t�:!'!�:ton, '---------+-,;· ,111.-!lll'ill!la,'!��
SELF CLEANING detililn prennt1 c.orro1iwe build-
up. There ore no baffles or bladH to wear within
eolleclion area. Entire oreo ii oC"tiwe - no dead
area, for accumulation of dutt,
Overflow-----------
Figure 4-63. Tray-type scrubber
with continuous sludge removal.
Courtesy of National Dust Collec-
tor Corp.
Filters of this type or class may be of the large bag filter Specifications
type for large volumes of low pressure dust laden gases or
vapor, or of the generally smaller cartridge or pack types The details of specifications for bag filter dust collec-
for gas/vapors or liquids containing suspended solid tors are important to a proper and operable design selec-
materials. tion. There are many variables which must be furnished
Figures 4-65, 4-66, and 4-67 show several units of the by the manufacturer so that the user can understand how
bag. The bags may be of cotton, wool, synthetic fiber, and the unit operates mechanically and the unit's dust load-
glass or asbestos with temperature limits on such use as ing capabilities. The larger the air/cloth ratio for the
I80°F, 200°F, 275°F, 650°F respectively, except for unusu- unit, the smaller will be its physical dimensions and gen-
al materials. (See Table 4-12A and B.) These units are erally, cost; however, the higher will be the frequency of
used exclusively on dry solid particles in a gas stream, not cleaning. This can be quite troublesome, therefore low
being suitable for wet or moist applications. The gases values of this ratio are preferable, consistent with the
pass through the woven filter cloth, depositing the dust analysis of overall performance.
on the surface. At intervals the unit is subject to a de-dust- The removal or filtration of the entrained dust from
ing action such as mechanical scraping, shaking or back- the gas stream is accomplished by passing the mixture
flow of clean air or gas to remove the dust from the cloth. through a sufficiently porous fal,ric filter bag(s) (Table 4-
The dust settles to the lower section of the unit and is 14). These bags allow some air to flow through and are
removed. The separation efficiency may be 99%+, but is either cylindrical tubes or oblong tubes/bags. The dust is
dependent upon the system and nature of the particles. retained on the outside or inside (depending on unit
For extremely fine particles a precoat of dry dust similar design) of the bag surface and the small spaces between
to that used in some wet filtrations may be required the fibers of the cloth ( or felt). This dry cake builds up
before re-establishing the process gas-dust flow. and acts as a pre-coat and then as the actual filtering
medium as the dust particles build up. After a period of
For heavy dust loads these units are often preceded by time, unique to the filter system of dust laden air plus bag
a dry cyclone or other separator to reduce the total load type, the pressure drop will build up. (These are low pres-
on the bags. sure and low pressure drop systems.) Therefore, the dust
Suggested air-to-cloth ratios are given in Table 4-13. or "cake" is removed (cleaned) from the outside of the
272 Applied Process Design for Chemical and Petrochemical Plants
LIQUID LI.V£L
D'- Clean gas out
LDW PIU!SSURE
l)flOP V£NTURI ___..#// FLEXITRAV
�
li.NTllY ,J � q9
1
Figure 4-64A. Adjustable "floating" caps for vapor flow. By permis-
sion, Koch Engineering Co., Inc.
Gas plus
suspended
solids In
+A>
�
DUST
STORAGE
HOPPER
FILTERING CLEAN I NG
The scrubber is comprised of one or more trays. Each tray contains
numerous venturi openings. Each of the MultiVenturi openings is Figure 4-65. Bag filtration with mechanical shaking for bag cleaning.
surmounted by a spider cage holding a floating Flexicap (see insert). Courtesy of Dracco Div. Fuller Co.
In addition, each tray Is equipped with one or more "downcomers"
and weir flow baffles that control the scrubbing liquid as it flows
across the tray and then to the tray below.
dust loading [ 47]. The air-to-cloth ratio so often used is
Figure 4-64. Variable orifice MultiVenturi Flexitray® scrubber at only useful when comparing a particular manufacturer's
essentially constant pressure drop maintains good efficiency over equipment for handling different materials, and not for
wide flow rates. By permission, Koch Engineering Co., Inc. comparing manufacturers. Reference [ 49] is an excellent
summary of many details associated with specifying and
selecting bag filters.
bag by internal arrangements in the "bag house" or hous- The following are suggested filter specifications:
ing by such techniques as (1) shaking or vibrating the bag
or bag assembly to drop the dust into an integral hopper l. Performance: define air /gas and dust rates, particle
while there is no-flow of air-dust feed into the unit or com- size distribution, and percent of particle sizes.
partment, or (2) back pulse with jets of air in to each bag
(Figure 4-67). The criterion should be a constant pressure a. Temperature at inlet to baghouse
drop across the fabric for a fixed air flow and a specified b. Moisture concentration, dewpoint
Mechanical Separations 273
c. Chemical compos1t10n of vapor and of dust,
including any abrasive, hydroscopic or other char-
acteristics.
2. Define dust recovery, as percentage below a certain
particle size.
3. Indicate, if known, preferred bag material that
will withstand environment, e.g., fibers of glass,
polyester, TeOon®, Nomex®, polypropylene, poly-
ethylene, cotton, wool, nylon, Orlon®, Dacron®,
and Dyne I®. The type of weave of fiber should be
recommended by the manufacturer. The fabrics
may be felted or woven [47,48] in 'Weaves of
plain, satin, or twill, and should be resistant to
.. � any corrosive material in the solid particles or
Cleaning Air from the gas stream.
Atmosphere
Figure 4-66. Bag filtration with continvous reverse air cleaning. 4. Manufacturer should recommend
Courtesy of W. W. Sly Mfg. Co.
a. Bag size, ( diameter, length).
b. Bag holding hardware: anti-collapse spreader
rings, snap rings, etc.
Primary Manifold c. Number of baghouse compartments, number of
nozz!e
bags per compartment.
�.,__,-secondary d. Air/gas flow cycle to compartments.
nozzle e. Complete description with mechanical details of
Bag
bag cleaning system (shaking, air-jet, etc.)
f. Dust removal arrangement.
The cleaning system set up for a particular bag
house will determine whether the filtering system oper-
ates continuously or batch/intermittently. Some sys-
tems operate as a continuous batch, with sections of the
entering chambers being isolated by valving to auto-
FILTERING CLEANING
POSITION POSITION matically switch from one section of one bag house to
another. Thereby, one or more bag groups/sections fil-
Pulse-jet cleaning (above) uses a ter while another one or more are not operating, but
controlled blast of compressed air from
the primary into the secondary nozzle, are in the dust removal cycle. The permeability of the
which is magnified by induced air being fabric is generally stated as the clean airflow in (cu
drawn into the bag. The sudden release of
air causes the bag to expand fully, ft/min)/square foot of fabric at a pressure differential
throwir.g the dust from the outer surface. of 0.5 inches water as determined by the ASTM stan-
Dislodged dust falls into the collection
hopper. At right. types of duty cycles dard D-737 (Frazier Test) [47]. Whereas this test is use-
ful, several fabrics may have the same permeability yet
Continuous eutometic
have different fiber surfaces, and thereby do not per-
E
u ----- form the same for a specific application.
Operating cycle
i The felted fabrics are generally used for maximum
;;:: 0 - Intermittent recovery of product and are used at high face velocity for
airflow-Lo-cloth-area ratio. The felt promotes the greatest
Cleaning cycle
dust collection surface.
Time-----
Figure 4-67. Pulse-jet air cleaning of fabric bags. By permission, Monofilament fibers require special attention to
Power, November 1975, McGraw-Hill Co., Inc., New York, p. 41. ensure a uniform open space between the filaments.
274 Applied Process Design for Chemical and Petrochemical Plants
Table 4-12A
Partial List of Commercial Crossflow Microfilter
Media-materials and Geometries
Tubulw
1. Po/ymtll"8
Polyvlnylldene fluortm
Ac:ryllc
Alumlnll
. ' ,
,
�
Zlrconle,'carbon , • ,
j
Silica ' . ' ,
Slllcon C8rt)lde ' . "
3. Slntem:I ,,,.,.,
1Typo·11--1 •11C=Multktlannel monolllhlc ••••ta
Other alloys
By permission, Michaels, S. L. [38].
The woven fabrics have various yarn patterns for dif- varies, but one promise seems to be that higher tem-
ferent spacings between the yarn fabrics (Table 4-14). peratures will be handled.
There is a wide variety of choices for not only the
materials of construction but the tightness of weave New cartridge designs for bag houses will allow
and the size of the yarn. All of these factors along with improved servicing and cleaning techniques.
the others noted earlier, make the selection of bag fab-
ric an art that requires manufacturer's and plant's It is important to keep bolts, nuts and other poten-
actual field tests. Woven fabrics have a low ratio of tially loose items to a minimum inside the unit, as
weave openings for yarn area and generally have a lim- vibration from air/ gas flow and bag cleaning can
ited face velocity for air flow of about 1.5 to 3.0 cu loosen nuts, break small welds, and ultimately tear
ft/min/sq ft [47]. holes or rip bags. The bag construction is likewise
extremely important, since loose edges and
Newer fabrics, not in common use but in develop- "unlocked" seams will fray and tear, allowing fibers
ment, test, and field trials, are described for higher into the product dust. The bag construction must have
temperature applications by Reference [50]. Applica- straight seams in order for them to bend and flex
tion to 400°F-2100°F are potentially available using properly on cleaning and/or loading.
ceramic fibers Nextel 312®, laminated membrane of
expanded PTFE on a substrate, polyimid fiber P-84, Cartridge filters may be single units or clusters in a sin-
Ryton® polyphenylene sulfide, and woven fiberglass. gle container or canister. Figures 4-68 to 4-74 illustrate
The heat and acid resistance of these new materials typical units. These may be designed to filter suspended
Mechanical Separations 275
Table 4-12B
Partial List of Crossflow Microfilter Media in Chemical Service Applications
TABLE II I I .! 11 t ihq h
I ti J 11 II i II ii ii II
Chlorlnatecl orpnlc:a,
below 100'C
Eat .... below 100'C
Orpnlce 81 100 • 200'C
Orpnlcll 81 200 • IOO'C
nk:11 11 800 • 900'C
pH = 3 • 7, no chlorides
JIH = 7 • 10, no chlorldN
= O • 3 (except HF)
pH = 3 • 10, chlorldH preunt
HF,wlthpH < 3
pH=10·13
pH> 13
Concentrated acid•
&'teem(> 100'C)
Oxldanta (e.g., bleach)
By permission, Michaels, S. L. [38].
Table 4-13
I
Suggested Air-to-Cloth Ratios for Dust Removal from Air* ,------Ao. o. --,-SERVICE
H SPACE
Type of Dust Ratio I
-------- REQUIRED
Abrasives . 2-2.5
Asbestos . 2.5-3
Blast cleaning . 3-3.5
Carbon . 2-2.5
Cement (mills) . 1.5-2
Cement (conveying and packing) . 2-2.5
Clay . 2-2.5
Coal . 2-2.5
Feed - - ·, · · .. · · · · · · · · · · · 2.5-3
Graphite . 1.5-2 G
Grinders . 3-3.5
Gypsum . 2-2.5
Lampblack · .. 1.5-2
Limestone . 2-2.5
Rubber . 2-2.5
Salt _ 2.5-3
Sand . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3-3.5
Silica Flour . 2-2 5
Soap _ . 2-2.5
Soapstone . 2-2.5
Talc · · ·, .. · · · · · · · · · · · · · · · · · 2-2.5
Wood Flour . 2-2.5
Ratio is the volume in cubic feet per minute of dust-laden air
to each square foot of active cloth area. If grain loading is
above normal, ratios must be reduced.
Figure 4-68. Typical blower intake filter-silencer. Air to blower leaves
*By permission, Bulletin 104 The W. W. Sly Mfg. Co .. Cleve- through pipe connection, which may be screwed or flanged. Cour-
land, Ohio tesy of Dollinger Corp.
276 Applied Process Design for Chemical and Petrochemical Plants
Figure 4-69. Pleated radial-fan filter cartridge. Fiitration is from out-
side to inside. Courtesy of Dollinger Corp.
Figure 4-71. Cluster of filter cartridges in a single chamber. Courtesy
of Filterite Corp.
Figure 4- 70. Wound filter tube on stainless steel core. Courtesy of
Filterite Corp.
solids from gases or liquids. Table 4-14 presents represen-
tative physical property and application data for the more
commonly used filter media. These media may be in fila-
ment, fiber, or "felt" form and arranged by weaving tech-
niques lo control the pore or free spaces to specific size
for removal of various sizes of particles. The particle size
retention listed in the table ranges from 0.006 micron to
over 100 micron. A micron is often termed "micrometer"
or a millionth of a meter, using symbol µm.
Filter cartridges as illustrated are considered "throw-
away" and are removed from service when the pressure
drop builds up to a predetermined value, or when the Figure 4-72. Cartridge-type filter-pleated membrane. Courtesy of
effiuent changes color or becomes opaque with suspend- Gelman Instrument Co.
Mechanical Separations 277
some applications, actual testing in the plant using plant
fluid streams can be the most conclusive. This plant test-
ing is not necessary for every situation because the manu-
facturer has large data files to often aid in a good selec-
tion. Generally the ability to collect solids at low flow rates
is greater for the wound filter.
Because the suspended particles are "captured" by dif-
ferent physical mechanisms depending on the particle
size, shape, density, and concentration, all cartridges do
not perform the same. The "capture" may be by ( 1) direct
interception, (2) sieving, and/or (3) bridging [39]. (See
Figure 4-75.) The cartridges from one manufacturer are
generally consistent in performance; however, all car-
tridges from just any manufacturer may not be inter-
changeable in performance.
The micron ratings of a cartridge are intended to indi-
cate the smallest particle that will be retained by the pores
of the filter element. Often a "rough-cut" pre-filter is
installed ahead of a final or "polishing" filter in order to
increase the life of the final unit. Unfortunately, the
method for determining the micron rating is not a uni-
versal standard between manufacturers. Thus, one manu-
facturer's "50 micron" filter may not perform the same as
This three-dimensional cutaway drawing Illustrates the another manufacturer's with the same rating number.
filtering operation of the GAF� filter-bag pressure filter The only reliable approach is to send the manufacturer
system, showing the flow patterns of unfiltered llquld
through a preselected mlcronrated felt filter bag which an actual sample of the fluid and let him test it to select
renders the desired quality of filtered product. the filter to do your job, or actually test the unit in your
plant's field application [37].
Figure 4-73. Flow scheme for GAF filter-bag pressure filter system
for liquids. Courtesy of GAF Corporation, Chemical Group, Green- An important feature of these cartridge units is the
wich, Conn. mechanism for assembling one or more in the housing.
The top/bottom sealing mechanisms determine what
style of cartridge is required ( open both ends, open one
ed material breaking through. The flow in most applica- end) and the method of pressure loading/sealing each
tions is from outside cartridge to inside and into the hol-
low metal or plastic collection tubes. It then flows into the cartridge into its bracket in the housing. The housing may
outlet pipe to the process. Materials for these cartridges hold one or 40 cartridges, and the assembly inside to pre-
are most commonly selected from cellulose, glass fibers, vent leakage and cross-contamination is essential to good
performance as a filtering device. The housings can be
polypropylene (woven and non-woven) fibers, or made of various metals (carbon steel, stainless, alloy) or
monofilaments, molded resins, ceramics, or resin-impreg- plastic-lined steel using corrosion resistant polymers, or
nated fiberglass. The last three are termed "depth" filters,
as they can hold a large amount of solids before the pres- elastomers, or solid plastic.
sure drop builds up excessively. "Surface" filters are usu- The cartridges can be selected to be useful over the
ally made of paper, non-woven fabrics, or cast membranes range of low to high viscosities, that is, 100,000 cp with
and are usually pleated to provide more working surface temperature ranges to 750°F at higher pressure of up to
area. This type is fabricated from sheets of porous non- 3,000 psi [38]. Usually for the average application, the
woven fabric often used for the absolute capture of sub-- concentration of the suspended solids is not over I 00
micron particles and has a sharp cutoff in particle size ppm, but can be higher. These units do not perforr well
retention [37]. Yarn wound filters often have a graded- with pressure pulsations or surges in the system. Note the
density or decreasing pore size structure. differences in expected performance of Figure 4-76
between a pleated cartridge. This does not necessarily
To aid in selection of the most probable successful fil- mean that all cartridges perform in this manner, but these
ter media for the service, the summaries of Table 4-l 2A are typical of expected performance curves. When exam-
and Table 4-128 can be a useful guide [ 38]; however, for ining particle retention ratings, examine Reference [39].
278 Applied Process Design for Chemical and Petrochemical Plants
Head Assembly
Fluid Outlet
Optional
By-Pass Valve
Rotatable
Permanent
All Metal Stationary
Filter ,-----Cleaning Blade
Cartridge
Assembly
Disc
Spindle
'Spacer
Sump------
Figure 4-74. Edge-type filter with the exter-
nal on-line cleaning. Courtesy of AMF Corp.,
Cuno Div.
Table 4-14
Physical Properties of Filter Media
Cl ,s
!
..
!!
..
.. z '3 .s a
..
0
ci.
Cl
.. -;. e � -= � !!
s
..
..
i
s
Q,
,..c
0
Cl
..
s 8 : ! � .s ..... Cl 0 .. .. .. i .. ::, •
..
Cl
Cl
e:a
Cl
!
0
8.
0
;,,
8 --- --- --- --- --- --- --- --- --- --- --- --- ---
Cl
.!I
ci:I
0
Cl
;,,
;,,
.;i
£
z
0
e
0"'
z
0
if
iii
i
i
Q
f-,
Cl,,
Cl,,
2i,. to 2i,. to 3i,. to 2i,. to 2i,. to 2i,. to 2i,. to 2i,. to 2i,. to 2i,. to 3i,. to li,. to 2i,. Lo li,. Lo
40i,.
> lOOi,.
>40i,.
ioo;
> ioo, > ioo, > ioo, > l(i()11,
> ioo, > ioo, >IOOi,.
>6001L
Particle Retention .......... > lOtii,. --- --- --- --- --- --- --- ---- --- --- --- --- ---
> 10011,
Contaminant Holding
Ability ................. G to E E F to G G to E F to E F to E F to E F to E F to E G Lo E G F to G F toG Fto G
--- --- ---· --- --- --- --- --- --- --- --- ---- ---
Permeability ...•.......... G -- F G G G G G G G G F G F
G
--- --- --- --- -- --- --- --- --- --- --- ---
Chemical Compatibility ..... l<' F G G E G G F E E p E E E
--- --- --- --- --- --- --- --- --- --- --- --- ---
Temperature Limits (°F) .... 200 200 700 300 200 250 450 200 200 450 200 1200 1200 2000
--- --- ---- --- --- --- --- --- --- --- --- --- --- ---
p
G
G
F
Strength .................. G --- --- --- --- --- ----- --- -·-· --- -- --- --- ---
G
G
F
E
G
G
E
G
G
p
Abrasion Resistance ........ --- -- --- --- --- -- ---- -- G --- --- --- --- ---
E
F
G
G
E
E
E
G
E
E
E
G
---
Machineability
(Workability) ........... E E F E E E E G E E F F F F
--- --- --- --- --- --- --- --
--- --- --- --- --- ---
Cleanability ...... ········· G G p G E G G G E E p p G p
--- --- --- --- --- --- --- --- --- --- --- --- ---
Cost ...................... L L M M L M M L L H L H M H
Legend
P-poor F-fair G-good E-excellent L-low M-medium H-high
Mechanical Separations 279
-------Q
A. Direct Interception B. Sieving C. Bridging
Figure 4-75. "Capture" mechanism for cartridge filters. Adapted by permission after Shucosky, A. C., Chemical Engineering, V. 95, No. 1, 1988,
p. 72.
100 pressure drop or lower the holding capacity. In normal
90 operation, the pressure drop initially is quite low, perhaps
1 to 3 psig depending on flow rate, but as the solids build
80
up, the pressure drop will rise to 10 LO 35 psig, in which
,!?.
0 70 range most companies recommend replacement.
� These replaceable cartridges or packs are the most
c:
·u 60 commonly used; however, there are cartridges of wire
Q)
:E 50
(!) mesh, sintered or porous metal which can be removed,
'lij
> 40 cleaned, and replaced. Usually, the fine pores of the metal
0
E become progressively plugged and the cartridges lose
Q)
0: 30
capacity. They are often used for filtering hot fluids, or
20 polymers with suspended particles, pharmaceuticals, and
10 foods (liquids). In the case of polymers and other appli-
cations a special solvent and blow-back cleaning system
may be employed.
Particle size, micrometers The small cartridge units can be conveniently placed
ahead of instruments, close-clearanced pumps, or a
Figure 4-76. Pleated and wound cartridges differ in removal-effi- process to remove last indications of impurities in sus-
ciency profile. By permission, Shucosky, A. C., Chemical Engineer- pension.
ing, V. 95, No. 1, 1988, p. 72.
Other useful cartridges are:
Note: (a) Designations for both nominal and absolute 1. woven stainless steel ( or other wire) wire screen
ratings are based on the measure of a particle mesh, Figure 4-77A and Figure 4-78
size, not a pore size. (b) Ratings are based on 2. wire wound, Figure 4-77 A
arbitrary laboratory tests by the filter manufac- 3. sintered metal, Figure 4-77B
turer and can vary in actual plant conditions as
previously discussed.
The woven wire mesh type are formed to control the
For some critical applications (such as polymer melt, open space between the wires, thereby limiting the maxi-
beverage, or pharmaceutical filtration), it may be impor- mum size particle that can pass through. The cartridge is
tant to avoid cartridges that have a "nap" or "fuzz" on the installed in cases or small vessels Lo facilitate quick
fiber used, because these extremely fine fibers tend to replacement, or they can be arranged for backwash by use
break off and drift through the cartridge and go out with of proper piping connections. The wire wound uni ts have
the finished product, thereby creating a visual acceptance consistent spaces for uniform particle size filtering.
problem, if not outright contamination. The sintered metal units have uniform permeability
with void spaces approximately 50% by volume for some
In actual practice some companies have cartridges that metals and manufacturing techniques. The pore sizes can
will remove to 0.25 micrometer. Of course, the smaller the be graded to remove particles from 1 micron to 20
particle size that is specified to be removed from the microns for liquids and smaller sizes when used in
vapor or liquid, the higher will normally be the ultimate gaseous systems. (See Figure 4-778.)
280 Applied Process Design for Chemical and Petrochemical Plants
---
�-
l'lA .. 011 IQUAH
DUTCH TWIU
Equivalent DUTCH WIAVE
Standard
Nominal Rating Absolute Rating
Microns Microns
2 20
5 25
10 40
20 55
40 90
Figure 4-778. Porous sintered metal filter elements. By permission,
Figure 4-77A. Woven wire mesh filter cartridges. By permission,
AMF Corp., Cuno Div., Catalog MP-20.1. Pall Process Filtration Co.
Metals usually used are stainless steel, nickel, monel, lie unit; therefore, the economics involving the life
inconel, high nickel alloys, and special designs for unique span of each unit should be examined.
services.
Electrical Precipitators
The pressure drops for these units are typically low,
ranging from 0.2 to 10-15 psi. The woven wire mesh The electrical precipitator is a dry dust or liquid mist
runs even lower in pressure drops for the same or larg- removal unit which utilizes the ionization of the process
er flow rates. Consult the manufacturers for specific gas (usually air) to impart electrical charges on the sus-
application data. pended entrained particles and effect particle collection
by attraction to an oppositely charged plate or pipe. This
With some types of particles the porous metal tends type of unit is in use in services which are difficult for
to plug, but they can usually be backwashed or washed other types of entrainment removal equipment. Figures 4-
with a solvent or acid/alkali to remove the particles 79, 4-80, and 4-81 illustrate the usual fundamental action
from within the metal pores. This is one reason why of these units.
manufacturer's testing or plant testing can be impor- For these units the usual particle size for removal is
tant to the proper selection. Once the internal plug- greater than 2 microns with a loading rate of greater
gage has reached a point of reduction in flow-through than 0.1 grains/ cu ft, with a collection efficiency of
capacity, it must be discarded. The actual cost of this 99%±. The pressure drop is very low for a range of gas
type of cartridge is several times that of the non-metal- velocity through the unit of 100-600 ft/min [ 40].
Mechanical Separations 281
D
E
c
Figure 4-81. Electrical precipitator principle of operation. Courtesy
Duo Stzlndard
of Sturtevant Div. Westinghouse Electric Corp.
Figure 4-78. Tubular in-line pressure filter with reusable elements.
The flow: unfiltered liquid enters the inlet port, flows upward,
around, and through the media, which is a stainless steel or fabric Operating temperatures can be as high as I 000°F and
screen reinforced by a perforated stainless steel backing. Filtered above [ 41].
liquid discharges through the outlet (top) port. Because of outside-
to-Inside flow path, solids collect on the outside of the element so To improve the efficiency of collection, several units
screens are easy to clean. By permission, Ronningen-Petter® Engi- can be installed in series. The plate type unit is the
neered Fiiter Systems, Bulletin RP-2. most common design for dry dust removal, while pipe
design is mainly for removal of liquid or sludge parti-
cles and volatilized fumes. The plates/pipes are the col-
lecting electrodes, with the discharge electrodes sus-
pended between the plates or suspended in the pipes
Call1ct1d
Parlicl11 [41,53,57].
In operation, the voltage difference between the dis-
charge and collecting electrodes sets up a strong electri-
(+I C�arg1d
Partlcl11 cal field between them [63]. The "dirty" gas with particles
passes through this field, and the gas ions from the dis-
Figure 4-79. Charging particles in electrostatic precipitator. By per-
mission, adapted after A. Nutting, American Air Filter Co. charge electrode attach to the suspended "dirty" particles,
giving them a negative charge. The charged particles are
_.....-Grounded Eltclro4e(-I then attracted to the positively charged collecting elec-
,#, � l+ICMrgld Parlicl11 trode, discharging their charge on contact, becoming
(+) Ian Path�\ I - -
i: 1'1 • ·� electrically inert.
I l II •
- - \1 ,I./ "" Gas Flaw Through Collected liquids flow down the pipes and drain to a
Positivt Electrode � , • • • CMrg1d Field
( ( \ I ! Su1ptnd1d Parlicln collection sump. Collected solids are washed off the
1 H , .. Charged (+I. plates with water or other liquid. Sometimes the
1
\ht_ .. ·- dust/solids can be removed by mechanically vibrating
Grounded E ltctrodtH
or knocking on the plates while the particles are dry.
Figure 4-80. Particle collection. By permission, Nutting, A., Ameri- The electrical power of the precipitator is applied only
can Air Fiiter Co. (text continued 011 paKP 281)
Table 4-15
Precipitator Operating Data for Common CPI Applications
Ventilating Grinding Drying
... �
�
"Q_
J !. ... iD'
a.
4' b $ � § 'tJ
""
J; t Q G ,� t s a
l: ... b $ s $ e � c 0
CD
�
o
s § .; I ! .... .{' s I (/)
�
·t'
(/)
�
Precipitator operating " qj � .Q; � ... � " � " e 0
e
s
'§
CD
data for common CPI '$ $ ... ;f s .Q,. s f ! J; J; :, co·
�
(/)
applications £ I 9 � tr, (:) � £, s t:$' :::,
� G � (J 0 � 0 (j o'
....
5.0 7.0 94 15.5 10.0 10.0 40 100 11 39 20 62 o
Gas llow, 1,000 cfm. lo lo to Lo to to to to to to to to :::r
CD
24.8 10.0 120 27.0 13.5 13.5 80 150 43 45 187 305 3
�---
-
>----·-- -···--
---- - --- -- o'
100 525 190 53 75 75 100 150 130 350 105 600 a
Gas temperature, F to to to Lo Lo to lo to to to Lo Lo Ill
:::,
150 550 220 80 200 200 280 250 200 400 400 750 a.
'tJ
-4 -6 -10 -10 -10 -10 -10 -4 !a
Gas pressure, in. water to to Nega- Nega- Lo to to to Ncga- - to to a
0
tive
tive
tive
-2 +6 +10 +10 +IO +10 +10 +4 zr
--·------------- ---T-- - CD
3
2 1 1 5 30 5 10 15 5.7 o'
Am bi- Ambi-
Gas moisture, volume % 5 to to to to LO to LO Lo to !!!..
ent ent
3 IO 15 12 35 l!'i 20 100 15.0 'tJ
·- ---·- t----· ii>
0.1 0.66 0.07 7.0 29 29 20 20 6.5 50 0.05 3.7 a
Inlet dust concentration, (/)
grs/cu ft to 10 to lo lo to to to to to to to
0.5 0.80 0.16 17.2 70 70 40 40 150 52 40 43.0
-
0.02 0.13 0.003 0.03 0.021 0.021 0.05 0.02 0.10 0.025 0.02 0.001
Outlet dust concentration, to to to to to to to to to lo to
to
grs/cu ft 0.007 0.13 0.044 0.04 0.4 0.938
0.05
0.03
--------------- - -- 0.044 0.5 0.07 0.30
f----�-1--·
3 7 10 3 4 4 4 15 7 7 2.4
Power input, kw to to to to Lo to to to to 21 to to
7 12 14 7 6 6 12 20 12 --- 24 51.0
90 98 86 98.5 95 95 90 97 99.0 90 80
Collection
efficiency, % to to to to to to to 99+ to lo to Lo
98 98.5 97.6 99.8 99.96 99.96 99 99.7 99.5 99 99.97
- .. --- - --- - --
Table 4-15
Precipitator Operating Data for Common CPI Applications
Ventilating Grinding Drying
... �
�
"Q_
J !. ... iD'
a.
4' b $ � § 'tJ
""
J; t Q G ,� t s a
l: ... b $ s $ e � c 0
CD
�
o
s § .; I ! .... .{' s I (/)
�
·t'
(/)
�
Precipitator operating " qj � .Q; � ... � " � " e 0
e
s
'§
CD
data for common CPI '$ $ ... ;f s .Q,. s f ! J; J; :, co·
�
(/)
applications £ I 9 � tr, (:) � £, s t:$' :::,
� G � (J 0 � 0 (j o'
....
5.0 7.0 94 15.5 10.0 10.0 40 100 11 39 20 62 o
Gas llow, 1,000 cfm. lo lo to Lo to to to to to to to to :::r
CD
24.8 10.0 120 27.0 13.5 13.5 80 150 43 45 187 305 3
�---
-
>----·-- -···--
---- - --- -- o'
100 525 190 53 75 75 100 150 130 350 105 600 a
Gas temperature, F to to to Lo Lo to lo to to to Lo Lo Ill
:::,
150 550 220 80 200 200 280 250 200 400 400 750 a.
'tJ
-4 -6 -10 -10 -10 -10 -10 -4 !a
Gas pressure, in. water to to Nega- Nega- Lo to to to Ncga- - to to a
0
tive
tive
tive
-2 +6 +10 +10 +IO +10 +10 +4 zr
--·------------- ---T-- - CD
3
2 1 1 5 30 5 10 15 5.7 o'
Am bi- Ambi-
Gas moisture, volume % 5 to to to to LO to LO Lo to !!!..
ent ent
3 IO 15 12 35 l!'i 20 100 15.0 'tJ
·- ---·- t----· ii>
0.1 0.66 0.07 7.0 29 29 20 20 6.5 50 0.05 3.7 a
Inlet dust concentration, (/)
grs/cu ft to 10 to lo lo to to to to to to to
0.5 0.80 0.16 17.2 70 70 40 40 150 52 40 43.0
-
0.02 0.13 0.003 0.03 0.021 0.021 0.05 0.02 0.10 0.025 0.02 0.001
Outlet dust concentration, to to to to to to to to to lo to
to
grs/cu ft 0.007 0.13 0.044 0.04 0.4 0.938
0.05
0.03
--------------- - -- 0.044 0.5 0.07 0.30
f----�-1--·
3 7 10 3 4 4 4 15 7 7 2.4
Power input, kw to to to to Lo to to to to 21 to to
7 12 14 7 6 6 12 20 12 --- 24 51.0
90 98 86 98.5 95 95 90 97 99.0 90 80
Collection
efficiency, % to to to to to to to 99+ to lo to Lo
98 98.5 97.6 99.8 99.96 99.96 99 99.7 99.5 99 99.97
- .. --- - --- - --
Calcining /Metals J\cid recovery Miscellaneous
::;<
,� 'l5 ;§
;§
§
e
::;<
e � 'ti a- .,
""
§
�
�
�
� s � ·$ � .;si, e .., r
8 J7 ·S ·S 'if � j � G ·!:/ a-
'if
�
'if
.;
� ...: .; .; � � J '! I � § J -!. ::,
&
'17
e s ! I � e § £ � £ s ·$ .! £ � �
"
0
�
:J
�
... � i 'C s ,J t '$ t � ,Si I..! ?;; -� I
&
�
£ 0 0 � � Qc < � � � � ,l cJ -'l,, ")':;
1S7 3.0 7.6 25 'z.7 3.6 1.5 13 12 10 12 3'2 37 83 32 15 s::
(!)
LO to to to to to to to to to to Lo to to to to 0
346 __ __ 82 13:i 115 600 102 43 rn 37 16 270 45 90 36 20 '::J'
4'1..9
Ill
:i
, -�--- --·-- --·- --- -- -- ---- ·-- ff
35'2 'z.00 250 'z.50 400 120 95 68 520 500 !!!..
Lo to to to to lo to to 140 LO 100 to 650 220 750 129'2 en
(!)
630 300 375 720 7.50 800 170 L70 750 700 -0
...
-- - --- --- --·-- -- � -- Ill
-4 -6 -G -10 -10 -% -5 -0.3 -10 a
Nega- Nega- Nega- 2 s
LO Lo 10 to Live to to to 10 LO 50 Lo tive tive psig :i
+4 -2 -2 +JO +10 0 +5 +0.3 +JO U>
----- --- --- --
25 20 'z.O 40 25 0.5 5 3
to to to to to to to to None 6 Various 'z.7 26 21
15
'z.5
35 40 40 50 50 .----- --- 25 -----· --
9.9 s.o 5.0 100 2 0.6 20 40 1.5 1.0 0.9
LO to to to Lo Lo to to 40 to 1.0 Lo to 2.5 13
53.0 48.0 48.0 iso 17 30 200 100 1.5.0 4.0 l.2
0.006 0.011 0.011 0.04 0.02 0.015 O.:i 0.8 0.02 0.02 0.009
to to to to to Lo to to o.z lo 0.001 to to 0.12 0.08
0.08 0.92 0.92 0.10 0.3 0.8 1.0 5.0 0.04 0.08 0.012
--- -- -- ----
20 5 5 14 13 7 8 8 14
LO Lo to l.O to lO Lo LO 14 15 14 Lo 14 20 12 l4
94 10 10 36 30 30 40 16 :iO
---- -- ---
98.8 95 95 99 98 90 95 90 90
Lo to Lo to Lo to Lo to 99.5 99 99.9 lo 99 9:'i 99 99
99.93 99.94 99.94 99.96 99.5 99.li 99 98 99
-------
II)
By permission, Sickels, R. W. [ 41]. �
Calcining /Metals J\cid recovery Miscellaneous
::;<
,� 'l5 ;§
;§
§
e
::;<
e � 'ti a- .,
""
§
�
�
�
� s � ·$ � .;si, e .., r
8 J7 ·S ·S 'if � j � G ·!:/ a-
'if
�
'if
.;
� ...: .; .; � � J '! I � § J -!. ::,
&
'17
e s ! I � e § £ � £ s ·$ .! £ � �
"
0
�
:J
�
... � i 'C s ,J t '$ t � ,Si I..! ?;; -� I
&
�
£ 0 0 � � Qc < � � � � ,l cJ -'l,, ")':;
1S7 3.0 7.6 25 'z.7 3.6 1.5 13 12 10 12 3'2 37 83 32 15 s::
(!)
LO to to to to to to to to to to Lo to to to to 0
346 __ __ 82 13:i 115 600 102 43 rn 37 16 270 45 90 36 20 '::J'
4'1..9
Ill
:i
, -�--- --·-- --·- --- -- -- ---- ·-- ff
35'2 'z.00 250 'z.50 400 120 95 68 520 500 !!!..
Lo to to to to lo to to 140 LO 100 to 650 220 750 129'2 en
(!)
630 300 375 720 7.50 800 170 L70 750 700 -0
...
-- - --- --- --·-- -- � -- Ill
-4 -6 -G -10 -10 -% -5 -0.3 -10 a
Nega- Nega- Nega- 2 s
LO Lo 10 to Live to to to 10 LO 50 Lo tive tive psig :i
+4 -2 -2 +JO +10 0 +5 +0.3 +JO U>
----- --- --- --
25 20 'z.O 40 25 0.5 5 3
to to to to to to to to None 6 Various 'z.7 26 21
15
'z.5
35 40 40 50 50 .----- --- 25 -----· --
9.9 s.o 5.0 100 2 0.6 20 40 1.5 1.0 0.9
LO to to to Lo Lo to to 40 to 1.0 Lo to 2.5 13
53.0 48.0 48.0 iso 17 30 200 100 1.5.0 4.0 l.2
0.006 0.011 0.011 0.04 0.02 0.015 O.:i 0.8 0.02 0.02 0.009
to to to to to Lo to to o.z lo 0.001 to to 0.12 0.08
0.08 0.92 0.92 0.10 0.3 0.8 1.0 5.0 0.04 0.08 0.012
--- -- -- ----
20 5 5 14 13 7 8 8 14
LO Lo to l.O to lO Lo LO 14 15 14 Lo 14 20 12 l4
94 10 10 36 30 30 40 16 :iO
---- -- ---
98.8 95 95 99 98 90 95 90 90
Lo to Lo to Lo to Lo to 99.5 99 99.9 lo 99 9:'i 99 99
99.93 99.94 99.94 99.96 99.5 99.li 99 98 99
-------
II)
By permission, Sickels, R. W. [ 41]. �
284 Applied Process Design for Chemical and Petrochemical Plants
(lex/ continued from page 281) F, = Total flow rate of both phases, GPM
to the particles collected, thereby allowing for large vol- g = gc = gL = Acceleration due to gravity, 32.2 ft/ (sec) (sec)
umes of gas to be handled with very low pressure drop. h = Distance from center to given chord of aves-
For corrosive gases/liquid particles, corrosion resistant sel, ft
metals can be used for construction. hb = Height of continuous aqueous phase in the
bottom of the vessel, in.
The performance of the unit involves the gas charac- he = Height of a segment of a circle, in.
teristics, analysis, velocity, flow rate, dust or liquid particle h, = Height of continuous hydrocarbon phase in
size and analysis, resistivity and required final particle effi- the top of the vessel, in.
ciency of removal. Some particle materials of high electri- hvi = Cyclone inlet velocity head, in. water
H = Height of a segment of a circle, ft
cal resistivity prevent proper electrical operation. He = Height of rectangular cyclone inlet duct, ft
Table 4-15 illustrates some industrial application of H 0 = Height of dispersion band, ft
electrostatic precipitators; however, it is not intended to I = Width of interface, ft
be all inclusive. k = K = Empirical proportionally constant for cyclone
pressure drop or friction loss, dimensionless
Nomenclature K' = Constant for stationary vane separators, based
on design
Km = Stokes-Cunningham correction factor, dimen-
a = Specific surface area, sq ft/cu ft sionless
a, = Acceleration due to gravity, 32.2 ft/s or 9.8 m/s 2 Kme = Proportionality factor in Stokes-Cunningham
2
A = Area of segment of a circle, sq ft correction factor, dimensionless
or, A = Cross-sectional flow area, sq ft
Ab = Cross-sectional area at bottom of vessel occu- k = Constant for wire mesh separators
1 = Wire mesh thickness, ft
pied by continuous aqueous phase, sq ft
A,, = Cyclone inlet area = WiHc for cyclone with L = Length of vessel from hydrocarbon inlet to
hydrocarbon outlet, or length of decanter, ft
rectangular inlet, sq ft
A 1 = Area of interface, assumes flat horizontal, sq ft L 1 = Liquid entering Webre separator, lbs per minute
A1-1 = Cross-sectional area allocated to heavy phase, per square foot of inlet pipe cross-section
sq ft L,. = Entrainment from Webre unit, lb liquid per
AL = Cross-sectional area allocated to light phase, sq ft minute per square foot of inlet pipe cross
AP = Area of particle projected on plane normal to section
direction of flow or motion, sq ft m = Exponent given by equations
A, = Cross-sectional area at top of vessel occupied mp = Mass of particle, lb mass
by continuous hydrocarbon phase, sq ft n = Constant given in table
ACFS = Actual flow at conditions, cu ft/sec NRc = Reynolds number, dimensionless (use
h, = Constant given in table or (Re) consistent units)
c = Volume fraction solids N, = Number of turns made by gas stream in a
C = Overall drag coefficient, dimensionless cyclone separator
D = Diameter of vessel, ft 6P = Pressure drop, lbs/sq in.
Db = See Dp, min 6p = Pressure drop, in. water
De = Cyclone diameter, ft 6p 0 = Pressure drop, no entrainment, in. water
De = Cyclone gas exit duct diameter, ft 6pL = Pressure drop due to liquid load, in. water
DH = Hydraulic diameter, ft = 4 (flow area for phase 6pT = Pressure drop, total across wet pad, in. water
in question/wetted perimeter); also, D1-1 in Qo = Dispensed phase volumetric flow rate, cu
decanter design represents diameter for heavy ft/sec
phase, ft Qi, = Volumetric flow rate, heavy phase, cu ft/sec
DL = Diameter for light phase, ft Qi. = Volumetric flow rate, light phase, cu ft/sec
DP = Diameter of particle, ft or equivalent diameter r = Vessel radius, ft
of spherical particle, ft SpGr = Specific gravity of continuous phase at flow
Dp-min = Minimum diameter of particle that is com- conditions
pletely collected, ft SpGr P = Specific gravity of settling particle at flow con-
D' P = Diameter of particle, in. or mm ditions
d = Droplet diameter, ft 6SpGr = Difference in specific gravity of the particle
f = Factor relating average velocity to maximum and the surrounding fluid
velocity lavg = Average residence time based on liquid flow
fc = Friction factor, dimensionless race and vessel volume, min
F = Flow rate of one phase, GPM l:n,in = Minimum residence time to allow particles to
Faq = Aqueous phase flow rate, GPM settle based on Stokes Law, min
Fcv = Cyclone friction loss, expressed as number of u = Relative velocity between particle and main
cyclone inlet velocity beads, based on 1\: body of fluid, ft/sec
Fd = Drag or resistance to motion of body in fluid, u. = Terminal settling velocity determined by
poundals Stokes Law, of particle under action of gravity,
Fhc = Hydrocarbon phase flow rate, GPM ft/sec
Mechanical Separations 285
u., = Terminal settling velocity as calculated from Greek Symbols
Stokes Law, ft/sec
v = \'i = Terminal settling velocity, in./min £ = Void fraction of wire mesh, dimensionless
Va = Average velocity of gas, ft/sec T) = Fraction of dispersoid in swept volume collect-
vag = Terminal settling velocity of hydrocarbon ed on target
droplets in aqueous phase in bottom of vessel, 8 = Factor for establishing type of flow for
in./min decanters, Reference [32]
vc = Velocity down flow channel for continuous µ = Viscosity of surrounding fluid, cp, except
where it is lb/ (ft-sec)
phase, ft/sec µc = Viscosity of continuous phase, lb/ (ft) (sec)
"ct = Terminal settling velocity of a droplet, ft/ sec
µH = Viscosity of heavy phase, lb/ (ft) (sec)
vhc = Terminal settling velocity of aqueous droplets u, = Viscosity of fluid, cp
in hydrocarbon phase in top of vessel, in./min µL = Viscosity of light phase, lb/ft sec
v, = Terminal settling velocity of particle under µ = Fluid viscosity, (lb mass) I (ft) (sec) = cen-
action of gravity, ft/sec tipoise/1488
v,., = Terminal settling velocity of particle as calcu- urn = Milli-micron = 0.001 millimeter
lated from Stokes Law, ft/sec 1t = 3.1416
V = Velocity of gas or vapor entering, ft/min p = Pd = Fluid density, or density of fluid in droplet, lb
V (separator)= Separator vapor velocity evaluated for the gas mass/cu ft
or vapor at flowing conditions, ft/sec Pc = Density of fluid continuous phase, lb/en ft
3
V' = Vapor velocity entering unit, lbs, per minute Pr= Density of fluid, lb/ft or kg/m 3
per square foot of inlet pipe cross section PL = Liquid density, lb/ cu ft
Pct= Density of fluid continuous phase, lb/cu ft
Va = Maximum allowable vapor velocity across inlet PL = Density of light phase fluid, lb/ cu ft
face of mesh calculated by relation, fl/sec Pp = Density of particle, lb/cu ft
Vaci= Actual operating superficial gas velocity, ft/sec Ps = p, = True density of particle, lb mass/cu fl
or ft/min, for wire mesh pad o, = Vapor density, lb/ cu ft
Vn = Design vapor velocity (or selected design
value), ft/sec References
Ve = Cyclone inlet velocity, average, based on area
1-\' ft/sec 1. Alden, J. L., Design of Industrial Exhaust Systems, 2nd Ed.
V max = Calculated maximum allowable superficial gas Industrial Press, 1940, New York, N. Y.
velocity, ft/sec, or ft/min wire mesh pad 2. Bulletin, Sales Book Sheet, DC-271, American Air Filter Co.,
V, = Superficial gas velocity, ft/sec 1953, Louisville, Ky.
V,a = Separator vapor velocity evaluated for air-water 3. Bulletin MF.r9-58, Metex Mist Eliminators, Metal Textile
system, ft/sec Corp., 1958, Roselle, N.J.
V,e, = Active volume of settler occupied by one of the 4. Carpenter, C. L., D. Ch. E. Dissertation, Polytechnic Institute
phases, cu ft of Brooklyn, 1951.
\'i = Settling velocity for single spherical particle, 5. Carpenter, C. L. and D. F. Othmer, "Entrainment Removal By
ft/s or m/s a Wire Mesh Separator," A .. I.Ch.E.Joumal, Vol. 1, 1955, p. 549.
Vts = Settling velocity for hindered uniform spheri- 6. Chilton, T. H. and A. P. Colburn, "Heat Transfer and Pres-
cal particle, ft/s or m/s sure Drop in Empty Baffled and Packed Tubes," Part II,
Wi= Width of rectangular cone inlet duct, ft "Pressure Drop in Packed Tubes," Trans. Am. Inst. Chem.
Engrs. 26, 178, 1931.
zh = Heavy phase outlet dimensions of decanter 7. "Cyclone Dust Collectors," Engineering Report, American
measured from horizontal bottom, shown on Petroleum Institute, Division of Refining, 50 \.Vest 50th St.,
Figure 4.-12 New York, N.Y.
z, = Interface of decanter liquids measured from 8. Engineering Manual, Centrifix Corporation, Cleveland, Ohio.
bottom, Figure 4-12 9. Friedlander, S. K., L. Silverman, P. Drinker, and M. ,v. First,
zi = Light phase cutlet measured from bottom of Handbook on Air Cleaning Particulate Renuroal, United States
decanter, Figure 4.-12 Atomic Energy Commission, 1952, Washington, D. C.
10. Kane, John M., Operation, Application and Effectiveness of Dust
Collection Equipment, Heating and Ventilating, August 1952.
Subscripts 11. Kane, John M., "Guideposts Tell How To Select Dust Col-
lecting Equipment," Plant Engineering, November 1954.
L, or I = Light phase 12. Montrose, C. F., "Entrainment Separation," Chem. Eng., Oct.
1953.
H, or h = Heavy phase 13. Perry, John H., Ed. Chemical Engineer's Handbook. 3rd Ed.,
C, or c = Continuous phase "Dust and Mist Collection" by C. E. Lapple, 1950, McGraw-
D, or cl = Dispersed phase Hill Book Co., lnc.
1 = Liquid 14. Pollak, A. and L. T. Work, "The Separation of Liquid from
v = vapor or gas Vapor, Using Cyclones," Amer. Soc. Mech. Engrs. 64, 1942, p. 31.
286 Applied Process Design for Chemical and Petrochemical Plants
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of Industrial Dust, U. S. Public Service, 1935. trolling Fine Particles," Chem. Eng./Deskbook, V. 80, No. 14,
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972, 1939. Cyclone Efficiency," Chern. Eng., Nov. 7, 1977, p. 80.
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34. Sarma, H., "How to Size Gas Scrubbers," Hydrocarbon Process- 63. Van Wassen, R. H., Electrostatic Precipitators for Air Pollution
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35. Roberts, E. J., P. Stavenger, J. P. Bowersox, A. K. Walton and 64. Fair, J. R., "A Half-Century of Progress in Separations Tech-
M. Mehta, "Solids Concentration," Chem. Eng., V. 77, No. 14, nology," Chemical Processing, Mid-March, 1988.
June 29, 1970. 65. Sclker and Slcichcr, Canadianjournal of Chemical Engineering,
36. Toy, D. A. and F. M. Bonady, "Guide to Wet Scrubbers," V.43, 1965, p. 298.
Chemical Processing, Oct. 1983, p. 47.
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Chem. Eng., V. 96, No. i, 1989, p. 84. Anonymous, "Particle Charging Aids Wet Scrubber's Sub-
39. Shucosky, A. C., "Select The Right Cartridge Filter," Chem. Micron Efficiency," Chem. Engineering,July 21, 1975, p. 74.
Eng .. V. 95, No. I, 1988, p. 72. Anonymous, "Wet Electrostatic: Precipitator Tames Oils, Gases
40. Sargent, G. D., "Gas/Solids Separations," Chem. Eng./Desk- and Fines," Chem. Engineering, July 23, 1973, p. 74.
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book, \1. 75. No. 22, 1968. p. 106.
Mechanical Separations 287
Brink.j. A.Jr., et al., "Removal of Mists from Compressed Gases," Gleason, T. G., "How to Avoid Scrubber Corrosion," Chem. Eng.
A.1.Ch.E. Sym. 58th Meeting, A.l.Ch.E., Dallas, Texas, Feb. 1966. Prag., V. 17, No. 3, 1975, p. 43.
Buonicore, A.J., "Monte Carlo Simulation to Predict Collection Lammers, G. C., "Venturi Scrubber Scaleup," Chern. Eng., V. 89,
Efficiencies of Centrifugal Separators," 74th A.I.Ch.E. Meet- No. 15, 1982, p. 80.
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Coopey, W., "Boosting Separator Efficiency," Chem. Engineering, V. 67, No. 11, 1971, p. 54.
Feb. 5, 1962,p. 118. Zanker, A., "Shortcut Technique Gives Size Distribution on
Cross, Frank L. Jr., "Baghouse Filtration of Air Pollutants," Pol- Cornminuted Solids," Chem. Eng., V. 85, No. 10, 1978, p. 101.
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Davis,J. C., "Scrubber-Design Spinoffs from Power-Plant Units?," Chen, N. H., "Liquid-Solid Filtration: Generalized Design and
Optimization Equations," Chem. Eng.,July 31, 1978, p. 97.
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Pollution Engineering, Aug. 1973, p. 28. Gases," Chem. Eng., Oct. 6, 1975, p. 113.
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ing, March 31, 1975, p. 97. No. 2, 1987, p. 179.
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Gases," Chern. Engineering, Oct. 6, 1975, p. 113. 94, No. 3, 1987, p. 73.
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Jan. 27, 1969, p. J 30.
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Drainage, Withdrawal and Removal," Ind. and Engineering
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Chapter
5
Mixing of Liquids
Mixing of fluids is necessary in many chemical process- Mixing applications often include one or more of the
es. It may include mixing of liquid with liquid, gas with liq- following [26]:
uid, or solids with liquid. Agitation of these fluid masses
does not necessarily imply any significant amount of actu- • bulk mixing
al intimate and homogeneous distribution of the fluids or • chemical reaction
particles, and for this reason mixing requires a definition • heat transfer
of degree and/or purpose to properly define the desired • mass transfer
state of the system. • phase interaction (suspending/dispersing)
In order for the mixing operation tu accomplish the
overall process requirement of this step in the system, it is Mixing is accomplished by the rotating action of an
necessary to establish which factors are significant for a impeller in the continuous fluid. This action shears the
mixing device that provides the required end result for fluid, setting up eddies which move through the body of
the industrial application. Because the "art" of mixing is the system. In general the fluid motion involves (a) the
still not an exact science, it is really not practical for the mass of the fluid over large distances and (b) the small
design engineer to expect to totally design a mixer, that is, scale eddy motion or turbulence which moves the fluid
over short distances [21, 15].
define its type, diameter, operating speed, and shape/
type of impeller. Rather it is reasonable for the engineer The size and shape of the vessel to be used for the mix-
to understand the mechanical and processing essentials ing operation is important in achieving the desired mix-
and anticipated performance when dealing technically ing results; therefore, this aspect of the design must
with a mixing equipment representative. For standard accompany the actual mechanical mixer design/size
nomenclature See references [ 4 7, 48]. The technical per- selection.
formance and economics of various designs often need to The performance of mixers involves high volume or
be examined in order to make a good, cost-effective selec- flow operations, or high head or shear operations. Many
tion of the device that will be the "heart" of this step in a mixing processes utilize a combination of these two,
process. In some situations, particularly chemical reaction although, surprisingly enough there are many which can
and/or mass transfer, it may be necessary to conduct test have only high volume or only high head. Some opera-
work to develop a sound basis for a larger scale industrial tions listed in decreasing order of high volume require-
unit. In other cases, the needed data mav be drawn from ments include: blending, heat transfer, solids suspension,
the public technical literature or a man�facturer's appli- solids dissolving, gas dispersion, liquid-liquid dispersion
cation files (see References [l, 4, 10, 11, 19, 20, 24, 25, 26, (immiscible), solid dispersion (high viscosity).
27, 28, 29, 31, 33, 42,43, 44, 45, 46, 47, 48]). Impeller types usually used with mixing and listed in
decreasing order of high volume ability (hence in increas-
Mixer performance is often related in terms of the ing order of high head ability or requirement) are: pad-
fluid velocity during agitation, total pumping capacity dle, turbine, propeller, sawtooth impeller or propeller,
(flow of the fluid in the system) generated by one cut-out impeller disc (no blades), colloid mill.
impeller, and the total flow in the tank ( or sometimes as Figures 5-1 and 5-2 are useful as guides in the general
blending time or a solids-suspension criterion) [25]. selection of mixing impellers and associated vessels. Note
288
Mixing of Liquids 289
i I .,.._
Shope Relationships for
ToM� ......
r+: Viscosity ,Cenlipoises---J Selection Chart ,. Turbine Desl9ns
,.
Solid
ond
Mixing Device 102 103 104 105 Plastic State Service l�!X.!09 Ranoe Criteria Dlo Rollo Dlo.:,1� Pooition
bnptlltr
Stale
Air Aaitotlon --- -- Paddle ... I.Volume 3:1 Unlinited Sintlt or
_
Turbine
Liouid Jets - - Blendin9 Propeller Clrculalion to 6:1 Multlpto
i Paddles i- - • --
Tonk Vol.
Pronellers ...... � Di spm ion PropeJer ... 1,000,000 Gals .
Turbines -- --· ---- Turbine I.Drop Size 3.0:1 1:1 At/orBtlow
Control
Center Lint
Cones ...... _ 2.Re·Circu· to 3.5:! 1:2 in of Liq..14
Stog1d
D!sks --· --- - -- (Immiscible Paddle lallor. Mixers Chori•
Systems)
Screws - Flow � t,000 Gals./Mln .
Barrels i--- Reactions Turbine I. Intensity 2.5:1 1:1 510911 ot
Multlplo
2.Volume
to3.5:I to 3:1
Ball Mills -· --- In Solution Propeller Circulollon
Ribbons (Miscible Paddle
Systems!
-
Kneaders -- -- - ChargeVd. 20,000 Gals. Al/o,BtlDW
I.Sheer
--- -
Colloid Mills i--- Turbine - 2.Volume 1.6 :1 I :2 CutarUnt
103.2:1 to 2:1
Soeciol Mills Dissolution Propeller Circulation ol lkauld
Cho,91
Mu lie rs Paddle 10000 Gals.
ChoroeV
=::
Pua Mills --- -- - -·-· Turbine I.Circulation 2.0:1 1:1
Internal Mixers Solids - to3.5:I to 1:2 t.tm�o;o ... ,.,
2.Velocily
Roll Mills - Suspension Propeller 2.0n Botto,o
on Bottom
Paddle
Conical Mills %Soids - 100% 0
Pan Mills ---- Propelltr .. I.Controlled 2.5:t 4:1 l � -lowolf
Turbine
lmoact Wheels Gos Shear to4.0:I to 1:1 o,.. lff
D�-Ofl
Botto111
Batch Process
- Applications Paddle 2.Clrculotion z.5,11-11111,ct,
Key = - ·- Continuous Process Gos Vol. - 3.Hi � h tust Btlow
Ve ocily
5 000 cu. ff./mln.
'-"•Id lmf
Turbine I.Volume 1.5 :, ,:2 Sln9l1 o,
Hi9h
Clrculotion to 2.s:1 to 2:1 Mu!Mplo
Figure 5-1. Range of operation of mixers. By permission, Quillen, Viscosity Propeller • -
2.Low
�- S., Chem. Engr., June 1954, p. 177 [15]. Applications Poddle Velocity I
Vis. 1,000,000 c.ps ....
Poddle ..... I.Volume Reloted Depends
Turbine
Circulation lo Other onOlher Multlpl1 1
Heot Propeller lmPtlW:r I
that the shape relationships of Figure 5-2 are applicable to Transfer 2.HIQh Veloc. Stlvlces Ser � 0p,..u,1- .....
Across Trans·
ll!in9
hrSu<foco
turbine type impellers only. ChoroeV .. 20,000 Gals. fer Surface formed """''"'"c.ib
Crysta Rizo· Turbine 1.Clrculalion 2.0:1 2:1 5�911,
At/or Btlo•
2.Low
An example of the use of the chart occurs in the lion Propeller 3.Shear ·� 3.2:1 to 1:1 Center Lint I
. Velocity
or
ol li4uld
Alddle
leaching of a 50% water slurry of a 20-mesh, 3.8- Precipitation Charge� 20,000 Gols. Control °'"'it
gravity ore by a dilute acid of equal volume, the heat
or solution to be removed by cooling coils. The con- Figure 5-2. General selection chart for mixing. By permission, Lyons,
trolling factor is suspension of the solids to promote E. J. and Parker, N. H., Chem. Engr. Prog., V. 50, 1954, p. 629 [12].
the reaction in which heat is developed. The criteria
for solid suspension are circulation and liquid veloc- inside the cooling coil [12]. With this size informa-
ity sufficient to overcome the settling rate of the tion and reference to the horsepower charts, the
solids. The same criteria are also pertinent to good preliminary design is complete.
heat transfer and reaction. The large particle size
and gravity difference between solids and solution All styles and designs of mixing impellers produce either
suggest fast settlement. Best impeller position is an axial-flow or a radial-flow of the fluid during the impeller
therefore on the vessel bottom so that its radial dis- rotation. There are, of course, degrees of variation of each
charge will sweep all solids up into the tank. In of these patterns, which then become a pan of the selection
order to maintain rnaxirnurn distribution of solids and specifying process to achieve the mixing objective.
yet allow sufficient depth of liquid for the cooling Axial flow impellers in an unbaffled tank will produce
coils, the maximum tank-height ratio of 1:1 from the vortex swirling about the vertical shaft. This will be dis-
chart would be used. Impeller ratio is regulated by cussed later in more detail.
reaction and suspension, with the latter controlling
because of particle size. Tank depth and particle size
in this case suggest a large impeller diameter, or a Mechanical Components
ratio of about 2.5:�. As the circulation pattern now
established is radially across the bottom and up the Figure 5-3 highlights the most commonly used radial
sides, the slurry will flow up across and through a and axial flow impeller styles for process applications.
helical coil for good transfer rate. This pattern will Other styles/ designs are used for special specific applica-
be assured by four Iull vertical baffles mounted tions to accomplish the mixing objectives (Figures 5-4 and
290 Applied Process Design for Chemical and Petrochemical Plants
-··- MtiAIM .._,.....,I
' pl ratlO
Lft.t ......
L • 1/40
W• 1110
Ohle. • 2/30
b ... lllrllinl
t• 1/40
If - ll'20l'
O dla.•2130
'°'°""
Sl•blada-bol
boltom of a,ppon d
i.....------0-�--
c.AndQ Double
1'1 • 11100 D; 113Do
• 1 00
Two bledes ·m or without u.- Mn 1/20
Figure 5-3. Impeller styles and general sizes commonly in use in process industry plant. By permission, Oldshue, J. Y., "Fluid Mixing Tech-
nology and Practice," Chem. Engr., June 13, 1983, p. 84 [25).
Mixing of Liquids 291
r, B. Open Turbine: Radial
1. Circulates by radially directed centrifugal force
using turbine blades. Circulation good for tank
extremes; less danger of fluid short circuiting in
t 2. Generally limited to a maximum speed, range
tank.
may be narrow for some services.
3. Used for fairly high shear and turbulence.
4. Better than axial unit for tanks with cone bottom
of greater than 15° angle, to iift material from
bottom of cone and mix with bulk of liquid.
Figure 5-4A. Axial-flow pattern produced by a pitched-blade tur-
bine. By permission, Oldshue, J. Y. [25]. 5. Effective in high viscosity systems.
6. Generally requires slower speeds and hence
greater gear reduction than propeller, higher
power per unit volume.
7. Cost: moderate.
C. Open Turbine: Axial Four Blades [25), Most Com-
mon Applications
l. Four-bladed 45° pitched blade, blade width is
function of diameter.
2. Made in wide range of sizes for top entering mix-
ers from l to 500 motor hp, and in diameters of
18 in. to 120 in.
Figure 5-48. Radiai-flow pattern produced by a flat-blade turbine. 3. Primarily for flow controlled requirements, such
By permission, Oldshue, J. Y. [25]. as solids suspensions, heat transfer, and other
high pumping efficiency applications.
5-5), but Figure 5-3 has been found to cover most of the 4. Preferred for shallow tanks of low Z/T with low
common applications. Fluid. flow patterns for the axial liquid cover over turbine of 1.20% of turbine
and radial flows are shown in Figures 5-4A and B. diameter.
5. Cost: moderate.
Impellers D. Open Turbine: Bar Turbine, Radial Flow
1. Produces highest shear rate of any basic impeller.
General Types, Figure 5-5. Runs at high speed, uses lower torque.
A. Propeller, Marine type 2. Blade width and height normally 1 /20 of impeller
1. Circulates by axial flow parallel to the shaft and diameter.
its flow pattern is modified by baffles, normally a
downward flow.
2. Operates over wide speed range. E. Open Turbine: Axial, Three Blades [25]
3. Can be pitched at various angles, most common 1. Provides more flow and less shear than the four
is three blades on square pitch (pitch equal to bladed design.
diameter). 2. Produces nearly constant, uniform velocity across
4. Shearing action very good at high speed, but not entire discharge area; has nearly constant pitch
generally used for this purpose. ratio.
5. At low speed it is not easily destroyed. 3. Close to hub the blade angle is steeper, and blade
6. Economical on power. is wider than at lip.
7. Generally self deaning. 4. Size ranges 20 to 120 in. diameter for motors of I
8. Relatively difficult to locate in vessels to obtain to 500 hp. Impeller speeds range from 56 to 125
optimum performance. rpm.
9. Not effective in viscous liquids, unless special 5. Large flow-directing stabilizer fins improve pump-
design. ing capacity for viscosity ranges 500 to 1500 cen-
10. Cost: moderate (101. continued 011 page 294)
292 Applied Process Design for Chemical and Petrochemical Plants
Figure 5-5A. A flat blade turbine can han- Figure 5-5C. Curved-blade turbine creates
dle the majority of all fluid mixing applica- Figure 5-58. Small openings are no prob- a dual suction flow pattern the same as the
tions when correctly applied. Its high lem with turbines. With blades removed, a flat blade. This design is used when rela-
pumping capacity makes it preferable for turbine can pass through openings about tively low shear is a requirement, when
general mixing operations. It is well adapt- � as large as the assembled impeller, or abrasion must be considered, and when
ed to the application of protective cover- impeller can be split, as above, to pass many other variables are of prime impor-
ings, such as lead, rubber and plastics. through openings 1 /3 of turbine diameter. tance. Courtesy of Lightnin (formerly Mixing
Courtesy of Lightnin (formerly Mixing Courtesy of Lightnin (formerly Mixing Equipment Co.), a unit of General Signal.
Equipment Co.), a unit of General Signal. Equipment Co.), a unit of General Signal.
Figure 5-5E. Marine propeller is designed
with extra section thickness to give longest
life in corrosive or abrasive materials. It is
polished to a high finish and accurately
Figure 5-50. Gate paddle impeller is balanced. Many special propeller types Figure 5-5F. Lifter turbine is efficient for
designed for materials of high viscosity and alloys available. Satisfactory in 95% of pumping large volumes against static
and operates at low shaft speeds. It is applications. It drives liquid ahead in a heli- heads of less than 36 inches. As shown, it
most desirable for shallow, wide tanks and cal cone while doing considerable "work" is used below a draft tube. Inverted, it is
wherever low shear is a requirement. on material passing through it. Courtesy of used above an orifice plane in tank bot-
Courtesy of Lightnin (formerly Mixing Lightnin (formerly Mixing Equipment Co.), tom. Courtesy of Lightnin (formerly Mixing
Equipment Co.), a unit of General Signal. a unit of General Signal. Equipment Co.), a unit of General Signal.
Figure 5-5G. Curved-blade turbine, devel-
oped especially for agitating fibrous mate-
rials such as paper stock. Also used on oil Figure 5-51. Flat blade pitched paddle. A
well drilling muds. This impeller gives fast, simple, low cost design that handles a
thorough turnover without need for the Figure 5-5H. A typical radial impeller agi- wide variety of jobs. Operating at low
usual tank baffling or mid-feather con- tator. Operates as agitating turbine or a speeds, it gives maximum pumping
struction. Courtesy of Lightnin (formerly conventional propeller, wide range of capacity with a minimum of turbulence.
Mixing Equipment Co.), a unit of General applications. Courtesy of Struthers-Wells Courtesy of Lightnin (formerly Mixing
Signal. Corp., Warren, Pa. Equipment Co.), a unit of General Signal.
Mixing of Liquids 293
Figure 5-5J. Bottom of flat-blade turbine Figure 5-5K. Plain cage beater. Imparts a Figure 5-5L. Studded cage beater. Enor-
with stabilizing ring to prevent shaft whip. cutting and beating action. It is usually mous contact area gives extremely violent
Courtesy of Lightnin (formerly Mixing combined with a standard propeller, which cutting and shredding action to certain
Equipment Co.), a unit of General Signal. supplies movement in the mix. Courtesy of emulsions, pulps, etc. Courtesy of Lightnin
Lightnin (fonnerly Mixing Equipment Co.), (fonnerly Mixing Equipment Co.), a unit of
a unit of General Signal. General Signal.
Figure 5-5M. Saw-toothed propeller. Dis- Figure 5-5N. Perforated propeller. Occa- Figure 5-50. Folding propeller. May be
places a large amount of liquid and com- sionally recommended for wetting dry passed through a very small opening.
bines a cutting and tearing action. Suit- powders, especially those that tend to Blades assume working position through
able for fibrous materials. Courtesy of fonn into lumps. Courtesy of Lightnin (for- centrifugal force only while rotating. Cour-
Lightnln (fonner1y Mixing Equipment Co.), merly Mixing Equipment Co.), a unit of tesy of Lightnin (fonnerly Mixing Equip-
a unit of General Signal. General Signal. ment Co.), a unit of General Signal.
Figure 5-5R. Cut-out propeller. Displaces
Figure 5-5Q. Weedless propeller. Handles a small amount of liquid combined with a
Figure 5-5P. Propeller with ring guard. For long fibrous materials that would become high rate of shear for shredding, breaking
extra safety where sounding rods are used entangled in the ordinary propeller. Cour- up pulps, etc. Courtesy of Lightnin (for-
or where samples are taken by hand dip- tesy of Lightnin (fonnerly Mixing Equip- merly Mixing Equipment Co.), a unit of
ping. Courtesy of Lightnin (formerly Mixing ment Co.), a unit of General Signal. General Signal.
Equipment Co.), a unit of General Signal. (Figure .5-.5 continued on next page)
294 Applied Process Design for Chemical and Petrochemical Plants
(Figure 5-5 continued from previous page)
Figure 5-55. Four-blade, vertical Figure 5-ST. Standard six-blade
flat blade turbine Impeller. Very vertical curved blade turbine
versatile, one of the most used impeller. Gives good efficiency
in wide application range. Cour- per unit of horsepower for sus- Figure 5-SX. Type R-500. Very Figure 5-SY. A-31 O Impeller.
tesy of Philadelphia Gear Corp. pensions, mixing fibrous materi- high shear radial flow impeller Develops 50% more action than
als. Gives high pumping capaci- for particle size reduction and ordinary propellers and is geo-
ty. Courtesy of Philadelphia Gear uniform dispersion in liquids. By metrically similar for accurate
Corp.
permission, Lightnin, (Formerly scale-up. By permission, Light-
Mixing Equipment Co.) a unit of nin, a unit of General Signal.
General Signal. (Formerly Mixing Equipment
Co.)
Figure 5-SZ. A-41 O Composite Impeller. Strong axial flow at very
high flow efficiency. Operates through a wide range of viscosities.
By permission, Lightnin, (Formerly Mixing Equipment Co.) a unit of
Figure 5-SU. Shrouded turbine Figure 5-SV. Turbine with three,
for high pumping capacity. Usu- four, or six radial blades. Handles General Signal.
ally used with low static heads, wide range of applications. Cour-
creates minimum of direct shear. tesy of International Process (text continued from page 291)
Courtesy of International Process Equipment Co., Div. of Patterson tipoise. Performs much like a marine propeller,
Equipment Co., Div. of Patterson Foundry and Machine Co. but does not have some of its disadvantages.
Foundry and Machine Co.
6. Cost: moderate, less than marine propeller.
F. Double-Spiral Impeller [25]
1. Used for viscous materials; has inner and outer
;: ::;- - Support.I flights.
2. The inner flights pump down, and the outer
flights pump upward.
3. Diameter of inner flight is one-third the impeller
-----C,gei>1DUl'ld
diameter.
fl\JTllfi''lUC'l.\on
- - -- Ctn1rlh,g,,I
pump 4. Width of outer ribbon is one-sixth diameter of
impeller.
5. Impeller height is equal to its diameter.
Figure 5-SW. Jet-flow mixer. Twin flow jets from submerged cen- 6. Available in 20 in. to 120 in. diameter for one to
trifugal pump allow for a maximum hydraulic shear per unit of power 250 motor hp, and speeds of 5.5 to 45 mph.
input, high velocltles useful for thick slurries. By permission, Penny,
W.R., Chem. Eng., Mar. 22, 1971, p. 97 (31). 7. Cost: moderate
Mixing of Liquids 295
G. Shrouded Turbine O IMPELLER
1. Circulates by radially directed centrifugal force DIAMETER
using enclosed impeller stators. Circulation very
good.
2. Speed range may be limited.
3. At reasonable speeds not easily destroyed.
4. Not self cleaning, fouls and plugs relatively easily.
5. Flow capacity limited, relatively low.
6. Effective in high viscosity systems.
7. Cost: relatively high.
H. Paddle
l. Circulates radially, but has no vertical circulation ....._______...
unless baffles used. ROTIHION
2. Covers wide viscosity range, blending.
IMPELLER CENT ERL I NE I LOCATED
3. Not easily destroyed in operation. AT 1/2 THE BLADE HEIGHT)
4. Not easily fouled.
5. Flow capacity can be high for multiple blades.
6. Cost: relatively low. i::::==;::===:::::::;::::::::::::::::;==��
.20
T
I. Anchor, Two blades, Contoured [25]
l. For higher viscosity applications: 40,000 to 50,000
centipoise. Figure 5-6A. Drawing of typical lifter turbine. By permission, Old-
2. Nominal blade width is impeller/IO, with little shue, J. Y., Fluid Mixing Technology, 1983, Chemical Engineering
McGraw-Hill Publications Co. (29].
power change from D/8 to D/12 (D = impeller
diameter).
28.0
3. Power requirements vary directly with the impeller
height-to-diameter ratio. 24.0
4. Used for blending and heat transfer for viscosities
between 5,000 to 50,000 cp. Pumping capacity f:l 20.0
falls off above 50,000 cp, as it "bores a hole" in the
fluid. Speed range 5.5 to 45 mph, for motor of 1 � ... 16.0
to 150 hp and impeller diameter 24 lo 120 in. N
1- 12,0
J:
J. Lifter Turbine, Figure 5-6A and Figure 5-6B (.!)
t;j
J: 8.0
This type unit [29] is used for a combination of pump- 4.0 4 BAFFLES
so-
ing and mixing purposes. The unit has a closed disk on
the top side. The feed flow into the unit comes from o.o
4.o
12.0 16.0 20.0 24,0
a.o
directly below the rotating impeller. The performance is 4,0 o.o RADIUS R !INCHESl
dependent on the size of the unit and the physical loca-
tion with respect to the distance up from the bottom of Figure 5-68. Velocity vectors in R-Z plane (Lifter Turbine). By per-
the vessel. As this clearance increases, the head decreases mission, Oldshue, J. Y., Fluid Mixing Technology, 1983, Chemical
for constant flow and increases the power requirement. Engineering McGraw-Hill Publications Co. [29].
Figures 5-3 and 5-5 illustrate a few of the types of
impellers used for mixing. They may be basically classified Figure 5-7 is an analysis flow chart for examining types
as axial, radial and mixed. In general the most generally of turbine impeller performance requirements.
applicable are the 3-bladed propeller, the flat-blade tur- For some services there may be more than one
bine, the curved blade turbine, and the paddle. The many impeller on the shaft, attached part-way up the shaft from
other designs are either modifications of these or special- the lower one (Figures 5-SA and 5-8B).
ly designed for a very special purpose with respect to a The use of dual impellers on a shaft should be deter-
fluid system and/or its performance. mined by the physical properties and characteristics of
296 Applied Process Design for Chemical and Petrochemical Plants
.i
.ti
i
G
Figure 5-7. Analysis flow chart for examining types of turbine impeller applications. By permission, Gates, L. E., et al., Chem. Eng., Dec. 8,
1975, p. 110 [26].
Mixing of Liquids 297
the system, in general being a function of viscosity,
impeller diameter and liquid depth in the tank. In gener-
al, dual impellers may be indicated for fluids of 45 cen-
tipoise and greater and where the fluid travels more than
four feet before being deflected.
The circulating capacity of 3-blade square pitch pro-
pellers is theoretically of the magnitude given in Figure 5-
9. The speed ranges indicated may be grouped as [2]:
high speed, 1750 rpm: for low viscosity fluids,
such as water
medium speed, 1150 rpm: for medium viscosity fluids,
such as light syrups and
varnishes
low speed, 420 rpm: for high viscosity fluids
such as oils, paints, or for
tender crystals or fibers: or
if foaming is a problem.
The mixing efficiency is generally higher ( 40%-60%)
for the slow 400 rpm speed and lower (25%-45%) for the
Figure 5-SA. Dual Impeller mixer and drive. Courtesy of Lightnin (for- 1750 and 1150 rpm speeds. This is given in Figure 5-10 for
mer1y Mixing Equipment Co.), a unit of General Signal.
general estimating use. Note that the turnover of tank
capacity is involved through the selected impeller diame-
ter and speed.
Mixing Concepts, Theory, Fundamentals
Slondord Make Righi Angle A mixer unit or impeller, regardless of its physical design
Motoreducer
features, is a pump of varying efficiency for pumping of flu-
ids. Generally speaking, all designs are low heads compared
Double Mechon,col Seals to a conventional centrifugal pump, because there is no
With All Ports Accessible
Without Removing the Drive defined confining casing for the mixing element.
The action of the impeller design produces flow of the
Manhole W,th Hinged Cover fluid, head on the fluid, or shear in the fluid, all to varying
degrees depending on the specific design. A general identi-
fication of these characteristics for several types of impellers
Semi-flexible Coupling
Protects Drive Equipment is given by [27]. (Note: Use consistent dimensions).
ond Pressure Seals
Type Impeller Flow Decreases Head (Shear)
Cooling and Heating Coils from Top Increases from Top
�����--�����
Rakes, Gates
Spirals, Anchors,
Removable Arm Radial
Propeller Agitator ,Giving Paddles
Excellent Shearing Action
ond Rapid Overturning Propellers
of fhe Liquor,Self-
cleaning in Operation Axial Flow Turbines
Flat Blade Turbine
Bar Turbine
Removable Bottom Guide Bladeless Impeller
Bearing
Close Clearance
Impeller and
Stator
Bottom Flush Valve Colloid Mill,
Homogenizer
Figure 5-88. Multiple impellers. Courtesy of Struthers-Wells Corp.
298 Applied Process Design for Chemical and Petrochemical Plants
100
-""'--
....... ..... ....... ...
.. Speed .... ... ... - � ;::::::; I:;::
..
.f 400 Rpm,:....,- �
-
e -r- ... ...- -
-
" 10
ci 1150 - �
�
- - ... -
� i.-- � ... 1750
....... ......-::::: ;......--- Three Blade Square Pitch
�--
� ...... - .....-
� � ....
r> .....
�
100 1,000 10,000 100,000
Circulatina Capacity, Gpm.
Figure 5-9. Theoretical circulating capacity of single propeller mixers. By permission, Fluid Agitation Handbook, Chemineer, Inc.
The horsepower required for any impeller is partly Figure 5-9 indicates the theoretical circulation from a
used for pumping flow and partly for shear requirements. propeller, and Figure 5-10 gives its efficiency for estimat-
To accomplish a given mixing performance for a process ing purposes. Efficiency must be used in converting theo-
operation, the objective usually becomes a matching of retical to actual horsepower, or in converting theoretical
the quantity of flow from an impeller with the shear char- to actual circulation of the propeller.
acteristics at a specific power input. The flow/shear input
ratio to a fluid system can be shifted or changed by chang- Flow Number
ing the type/physical characteristics of the impeller, not
the dimensions of a specific impeller design. For particu- This is probably the most important dimensionless
lar dimensional features (angles of blades, height/ depth group used to represent the actual flow during mixing in
of blades, number of blades, etc.), the performance will a vessel. Flow Number, NQ (or pumping number):
remain the same as long as the dimensions are in the
same relative relationship as the impeller, that is, in the (5-2)
same performance family.
where Nm = impeller speed of rotation, rev per min
Q' = flow rate or pumping capacity, cu ft/min
Flow D = impeller diameter, ft
The quantity of flow is defined as the amount of fluid
that moves axially or radially away from the impeller at the NQ is strongly dependent on the flow regime, Reynolds
Number, NRe, and installation geometry of the impeller.
surface or periphery of rotation. This flow quantity is The flow from an impeller is only that produced by the
never actually measured, but its relative relation to head impeller and does not include the entrained flow, which
characterizes the particular system. The flow rate, Q, is can be a major part of the total "motion" flow from the
usually available from the manufacturer for a given impeller. The entrained flow refers to fluid set in motion
impeller [21].
by the turbulence of the impeller output stream [27]. To
compare different impellers, it is important to define the
(5-1)
type of flows being considered.
It is important to recognize that in the system:
where Q = flow rate from impeller, cu ft/sec
N = speed of rotation, revolutions per sec "Process Result" p Flow
D = impeller diameter, ft
K 1 = proportionality constant, a function of the Figure 5-11 [28] presents an analysis of pumping num-
impeller shape, = 0.40 for 3-blade propeller in ber versus Reynolds Number for various vessel dimen-
water sional relationships, for turbine mixers.
Mixing ot Liquids 299
T ...... / 420 Viscous flow, NRe less than 10 to 300 is expressed:
@....-
60 -;;,"
\ 20 .,., / K
10 / p = _ 2 µ(N.)2 (D)3 (5-4)
/
\ ...... g
\ ,,., ......
\ 50 K 2 = from Table 5-1
100 ,� 15 P = power, not power number, P 0
/
200
300
\ \+ Fully developed turbulent flow, NRe over 10,000, in a tank
r�o 40 10% of the tank diameter:
400 containing four equally spaced baffles having a width of
,.,ooi p = _ 3 p (N )3 (D )5 (5-5)
K
g s
l K3 = from Table 5-1
t 30
..
2,000! "" ... 5 1150 where g = conversion factor, 32.2 lb mass-ft/lb force/
..
(sec) (sec)
e
j.,oo . a: - H = total potential head during flow, ft
c
c.
..
..
...
P = power, ft-lb/sec
• ...
.. � '#, l <I) W = impeller blade width, ft
µ = viscosity, lb/ft-sec
j,.oo,f ;:. • ... p = density, pounds/ cu ft
�
c
.!!
i
e
� ilJ 20 0.. 1750 N, = revolutions/sec
o,
Example: Horsepower, (HP) = P /550 (5-6)
Fiuld Viscosity : IC C P Construct Lines I and 2
Propeller Diameter = 10" Reod Propeller Mixing
Propeller Speed : 420 Rpm Efficiency of 54 %
NN 3 D5Sg·
Figure 5-10. Propeller circulation efficiency. This is used with theo- 1 HP p m I T b' (5-6A)
retical propeller capacity to determine actual capacity. By permis- mp. = 1.523 31013 ur me
sion, Fluid Agitation Handbook, Chemineer, Inc. (Symbols below)
[29]
Power, P; Power Number, P 0; and Reynolds Number,
NR,, Table 5-1 shows that in a cylinder tank, four baffles,
each ).{2 tank diameter above flat bottom, liquid depth is
Power equal to tank diameter, impeller shaft is vertical and at
centerline of tank.
Power is the external measure of the mixer perfor- The Reynolds number NRe for mixing is: (dimension-
mance. The power put into the system must be absorbed less)
through friction in viscous and turbulent shear stresses
and dissipated as heat. The power requirement of a sys-
tem is a function of the impeller shape, size, speed of rota- (5-7)
tion, fluid density and viscosity, vessel dimensions and
internal attachments, and position of the impeller in this or, [29],
enclosed system.
The power requirements cannot always be calculated NRc = (10.754· NmD;2 Sg)/(µ') (5- 8)
for any system with a great degree of reliability. However,
for those systems and/or configurations with known data, where for these units, above equation
good correlation is the result. The relations are [21]:
D = impeller diameter, ft
P = QpH (5-3) Nm = impeller speed, rpm
300 Applied Process Design for Chemical and Petrochemical Plants
Figure 5-11. Pumping number is the basis for design procedures involving blending and motion. By pennission, Hicks, R. W., et. al., Chem.
Eng., Apr. 26, 1976, p. 104 [28).
D; = impeller diameter, in. Representative Cr values from [28) are:
µ' = fluid viscosity, centipoise
µ=fluid viscosity, lb/sec ft
p = fluid density, lb/cu ft NRe c, in.
Sg = fluid specific gravity, (not density) 700 1.0
NP = power no. 400 0.98
200 0.95
The Froude number is [33): 100 0.91
70 0.89
60 0.88
DN 2 (5- 9) 50 0.87
g
g = gravitational constant, 32-ft/sec-sec In general, below a Reynolds number of 50, all
impellers give viscous flow mixing; between 50 and 1,000
the pattern is in the transition range; and from NRe above
Estimated turbine impeller diameter [28): 1000 the action is turbulent.
(5-10) For NRc � 10, the liquid motion moves with the
impeller, and off from the impeller, the fluid is stagnant
[34). The Froude number accounts for the force of gravi-
Calculate Reynolds number, NRc, from Equation 5-8,
then correct for viscosity effects. ty when it has a part in determining the motion of the
fluid. The Froude numbers must be equal in scale-up sit-
uations for the new design to have similar flow when grav-
(5-11) ity controls the motion [16).
Mixing of Liquids 301
Table 5-1
Baffled Cylindrical Tanks
x, Ks
Viscous Turbulent
Propeller, 3.blade, pitch = diameter .... 41.0 0.32
Propeller, 3-blade, pitch = 2 diameters .. 43.5 1.00
Turbine, flat blade, 4 blades . 70.0 4.50
Turbine, flat blade, 6 blades . 71.0 6.30 0=4"
Turbine, flat blade, 8 blades . 72.0 7.80
Fan turbine, blades at 45\ 6 blades . 70.0 1.65
Shrouded turbine, stator ring . 172.5 1.12 0.025
-
Fl � )w1dle 4 2 � I ��� _( ���� l � - ��� -e}'. . 43.0 2.25 0.02
Flat paddles, 2 blades, D/W = 6 . 36.5 1.60
Flat paddles, 2 blades, D/W = 8 . 33.0 1.15
Flat paddles, 4 blades, D/W = 6 . 49.0 2.75 0.015
Flat paddles, 6 blades, D/W = 6 . 71.0 3.82
*By permission, R. H. Rushton and J. Y. Oldshue, Chem. Eng. 0.010 '.J=3"
Prag. 49, 161 (1953)
p
Oldshue [29] points out that to identify the turbulent
range as beginning at a specific NRe may not be exactly
correct, as it actually varies with different impeller 0.004
3
designs. This range may vary from N Re= 10 to N Re = 10 5,
so for common use 1\'Re = 10 is taken as the turbulent 0.003 P VS. N
5
range for all impellers. 0/T=l/3
C/0=1.0
Power Relationship �=1.0
U=l .O
For same family design/styles of impellers [29], see Fig- i!=T
ure 5-12:
0.001'--�...__,_�L.-L.-.L....L....L..JL...LI-L...L.LI
100 1 so 200 250 300 400 500
P ex N 3 (5-12)
N
(5-13)
Figure 5-12. Power vs. RPM with impeller diameter parameters. Illus-
tration of impeller input power versus speed for a family of impeller
p (X p (5-14) designs, but only of various diameters, showing unifonnity of perfor-
mance. By permission, Oldshue, J. Y., Fluid Mixing Technology, 1983,
(5-15) Chemical Engineering, McGraw-Hill Publications Co. [29].
p (X 05 (5-16)
is used in most correlations to represent the relationship
P cx:QHp (5-17) to system performance for turbulent flow in a baffled
tank. For tanks containing no baffles, the fluid motion
Note: (Horsepower) (33,000) = ft lb/min remains swirling and a vortex develops. These conditions
(Horsepower) (550) = ft lb/sec are characterized by the lower curves in Figures 5-13, 3-14,
and 5-15, which include the Froude effect. This effect is
not prominent in baffled tanks.
The power number, P (dimensionless)
O
For unbaffled tanks:
(5-18)
a-log NRc
p = <l> (pNJ 05 ) ( N2 D) h
(5-19) (5- 21)
g g
(5-20) (5-22)
302 Applied Process Design for Chemical and Petrochemical Plants
where <I> is read from the charts and the constants a and ever, in the extremities of a vessel, the motion would be
b are given in Figure 5-16. laminar. In this case, as in all others, the tank baffling is a
Figure 5-17 is useful for determination of horsepower major factor for performance of the system and the power
during turbulent flow for various types of impellers, and and flow results.
Figure 5-18 is useful for laminar flow. Also see Figure 5-19. For NRc > 1000, the properly baffled tank is turbulent
Flow and power numbers each decrease as the throughout. NQ and P O are independent of NRe· If the
Reynolds number increases. In unbaffled tanks, a vortex tank is not baffled, a "forced vortex" dominates the flow
forms that takes over the flow regime and does not allow in the vessel.
the usual relationship to describe the performance of the For NRe > 1000, in fully baffled tank is turbulent.
mixing operation. It is proper and good practice to pro-
vide baffles in all vessels (see later description for the NP= P/(N3 Df)(p) (5-25)
physical configurations).
At high NRe, the power number, P 0, stays reasonably Pumping effectiveness or pumping per power is impor-
constant, thus, viscosity has little effect on the power tant for flow controlled processes [29).
requirements. When moving to lower NRe through the The shape, size, and baffling of a specific mixing vessel
laminar region into the viscous region, the viscosity efTect significantly influences the Reynolds number, flow, and
power numbers.
increases. In the laminar range [29)
Di= 394 (HP/n Sg NJ,)1/5 (5-25A)
(5-23)
Other relationships [29) for one Lype of impeller (not
or, P p µ (5-24) different types)
0
for all other parameters constant.
For 50 < NRe < 1000 [29) is the transition range. In the Q -- ( 1 ) , ratio of flow to power (5- 26)
NQ
2
immediate impeller area, the flow is fully turbulent; how- p po p N 02
- CurYt I Proptlltr. re" Equal to lamtltr , FOllr Its Each 0.1 T
100
�ual to � alltltr I No B3!fl"
J2
•
Propeller , itcb
•
�
4 P � tr , P � tcb Twiet lht Dlamtltr {off tffl11 E,th 0.1 T
111 ' . i Proj>t!ltr , itch Twiet the Diamtltr, No Baffltt
50 '5,&;i;e '- . � lta, ra3 rop/iller, No B � lts bu O - 1ftrtd sllion
.
fflts
lot Si'l • lode urblrit , No
91 ,'\.. ' . i Flot Sia-Bladt Turbint, Fo � r 8afflts Eafh 0.1 T
3,4 -" -, ' 7 Curved Sia-Bladt Turbirie, Hr Bcifflll och 0.1 T
8 Arrowlltod Sla-Blodt Tur Int, Four Bafflu Eoch 0.1 T
20 _1,2 • 9 Fein Turbine, Elght·Blad � our Bcifflts Ecich Of T
10 ..... �;, • 10 Flat Poddlt, Twci-Bladt ur Baffles Each 0.1
� � � � 14 " II � hroudtd Sia-Blade Turblne,FourBofflH Each 0.1 T
�
10 - - • 12 hroudtd Six-Blade Turbine,Stotor Rin9r.w1tb 20 Blodts
13 Paddle,=: l:fflts , Doto of Miiier and ann
"
off In Dcita of White and Sumerfonl
• 14 Paddlt
..... �- I I '::6 - 6
..... �
" -
""'
... �'""" ......... r-s, � r--::,.. ......_ 14" r-s, 7 ._8 ...... __ - 7
8
-
.....
"'-
13
9
2 ' r-, � t-- ..._ -11,12- ·- � i!,O ..... --14 9
10
--- f= pt 3 -13 4
y
1.0 5=Np ""'� ' 14 4 --- -- 12
--- Eactl!I When No Baff111
0.5 ,___ in fN 2 3
- wblch Call for HRf OYtr 300
= t: (� (�l ci·log NRe) -..-.� 2
0.2 ,'ND N' b
·-1 I 1111111 I I
0.1
I 2 5 10 2 2 5 10 1 2 5 2 2 5 io•
D l111p1ller Dia11eter 9 GravltJ Conatont
N lraptlltr Rcitatloncil Speed R17nold1 Nu111b1r T Tonk Dlcimtter
f Liquid Dtnti!J o,b Cautcinta
f Liquid Vlacoslty Np Power Number
P Power fl. lb./Mc. NR, Rt,nolds Number
Figure 5-13. Power consumption of impellers. By permission, Rushton, J. H., Costich, E. w. and Everett, H. L., Chem. Engr. Prog., v. 46, No.
8 and No. 9, 1950 [18].
Mixing of Liquids 303
11111 I I I I I I T
10� Curve No. I O 20" Propeller 21" Pitch in 54" Tonk T/D = 2.7
•• = 3.o
u r a"
"
"
.. 2 CD
6..
..
6..
u
II 3 e 411 II 4 n II II 1311 II II : 3.3
II 4 6) 12" II 11.8" M U 54" " U :4.5
II 11 II u 11 11 u U II
5 0 4 8 13
� fl N 6 • ·411 II 811 II II '' '' : 3,3
-. 13 Baffles 0.1 T l--+---f-4-+-++-1-1-l
,..._ ..-�.---,-..,.......,-,--,-..,..,.,.�---,�.--,......,.....,...,....,�������__J
1
�
2i---t--t--t-t-ir-+t-tt---j � iib,;j-l-i-tti-t------t---lf-4-+++f+1f---+--1--l-+-J'-+l-1+-__j--J-.J.-.jf-l...W-1+ --l---+--+-.l+.U.
I �-..�J. I
f-tt����ii!lf§-�·gif�i·���t�imt-�i��itt11S
�...
_,l" " '�"'---.... - I I
- Propellers ,No Boffles
I.O :: Volues of i P
=
0.5 � Belc,w NRe of 300, j � :Np (a-log NRe) ���lHm�-�j:ee!i��iiiiWi�����il
5
,_
r» D \
- Above Nae of 300, i ,
- (-.!L {_2._\ ---r- - 4:
0.2 - pN3oo/\N2o/
With Boffles, For All NRe f: Np I
0.1-.LLLIIIIII I I I 111111
2 5 2 5 2 5 2 5 2 5 2 5
10
Reynolds Number
Figure 5-14. Reynolds number correlation for propellers. By permission, Rushton, J. H. et ai. [18].
,oo��������������������������:=��:!:!:������tlt!l�����±±liH
I;;
6" Diameter 6 Flot Blade Turbine
"'-
50 � A Tonk Diameter 18"
� Turbine 6" Above Bottom
Liquid Depth 18"
tj., "I.., Curve I 2 Baffles Each 4 % Tonk Diameter
�
a,
10 °lo
�
� -- 20 1---+-11.,io:+ � +-H-1H+--l--+-+-l-+I 11 II 3 II ,, U u 17 Ofo II 11 II 11
� !llii • 4 No Balllta f : Np.;. Nfr r-lo: NR,)
10 5 ; C
f������ff�l���-���-�,-!,�i�-�B���������������������D��
:
2 5 2 5 2 5
D 2 Np
Reynolds Number
f
Figure 5-15. Reynolds number correlation for a flat blade turbine. By permission, Rushton, J. IL et al. [i 8].
This is dependent on the impeller type, speed, diame- (5-30)
ter, and the geometry of the installation.
Flow per power input:
Torquet rv t > NP (pN2D5)/21t (5-27)
Lateral fluid forces on mixer; F = NrP N 0- 1 (5-28) [ N 5/3 l
2
QI p = � r I (N1;3Q213p) (5- 31)
For two different impellers, comparing performance of
power, flow and flow per power at constant flow and speed
[29] in terms of flow, Q, at speed, N: Power, input, net to impeller shaft:
(5-29) (5-32)
304 Applied Process Design for Chemical and Petrochemical Plants
(Ql/3J
1
.
l/3
N 113
i (Flot 6 Blade Turbine) N Q
4 b Diameter, D = --- -- (5 - 33)
2 0 ' I I
b Where using consistent units:
B
!Proptllerl
6
P = impeller power draw, FL/tor ML2/t 3
... I 4 t = time
�· 2 n=O when no Vort11 Pruenl F = fluid force on turbine, perpendicular to shaft, ML/t 2
.... Not,: n = O for Baflltd Tank L = length
&,
..
.
a
n=O Below NRe = 300
8 .
� 0 D = impeller diameter, L
0 Q = volumetric flow, L 3 /t
u
6 T = tank diameter, L
p = fluid density, M/L3
4
.is, µ=fluid viscosity, M/(Lt)
2 (Propeller) - __
I - �--� r---...... 't = torque, FL, or ML2/t2
0 (Flot 6 Blocle Turbine) Z = liquid depth
0 2 4
Ratio of Tank Diameter/Impeller Diameter NP = power number, dimensionless,
Figure 5-16. Factors in Froude number exponent, n. By permission,
Rushton, J. H., et al. [18]. (5-25)
150
140
130
120
110
500 Turbine, Flat Blade(6 or 8)
450 100
400 120 Turbine, Curved Blade (6)
110 1.5 90 Paddle 3/Shaft ( D/W: 6)
350 100
80 Paddle 2/Shaft (D/W=6)
300 80 Paddle I/Shaft (D/W =4
70
4 250 70 Turbine Pitched Blade(6) Figure 5-17. Power con-
Poddlt I/Shaft (D/W=6)
1,000 60 sumption by impeller
Paddle 1/Shaft(D/W: 8) type/dimensions for tur-
200 60 Shrouded Turbine + Stator bulent flow conditions.
-------------- Propeller Pitch = 20 Knowing impeller type,
3 .9 diameter, speed and
10 batch density; connect
150 ... �o RPM with diameter. The
.. 30 ......... ..... ...... intersection with "A,"
2 .. - .. ... .... connected to the density
..
scale, makes an inter-
:II
0
a.
100 .. .I Propeller Pitch = ID section on "B." A line
:! ,..,. 40": from this point to the
..
::,
1.5 90 0 20 .. ....,. impeller scale intersects
u
:c .01 �.6 ...
80 C) .... the horsepower scale at
the correct value. By per-
.. 70 15 -� mission, Quillen, C. S.,
a.
Q. E ts Chem. Engr., June 1954,
a:: 1.0 a:: ct Ill 0 12 en p. 177 [15].
Mixing of Liquids 305
300 5 100 100,000
4.5 90
4 80 1,000
200 3.5
3 70 100
Turbine• Flat Bladt (6)
2.5 60 ------------------- Turbine,Pilchtd Bladt(6}
10 _ .. -- --
2 .. -
100 ---- t-l --
90 1.5_ --. �� -i
80 .. , .. .. a.
70 --- ! 0.1
60 1.0 :z: 1,000 2 Flat Paddles (O/W=6)
0
.9 30
50
I .8 Proptller Pitch =20
40.}, · 7 Flat Paddlt (0/W =41
.6 Prapeller Pitch= 0
30 0.5 20 Flat Paddlt (O/W =6)
:100
.4 .. ·o Flat Paddle (O/W=Bl
a.
20 .. ..
.. 3 .I:: c: Flat Paddle(O/W=IO)
<>
-= �
�
l ..
:,,,.
...
0
s .2 • l., .!! �
a.
a:: IO 0.167; � > 10
Figure 5-18. Laminar flow mixing. For known impeller type, diameter, speed, and viscosity, this nomograph will give power consumption. Con-
nect RPM and diameter, also viscosity and impeller scale. The intersection of these two separate lines with alpha and beta respectively is
then connected to give horsepower on the HP scale. By permission, Quillen, C. S., Chem. Engr., June 1954, p. 177 (15].
NQ = flow number, dimensionless, (5-37)
(5-2) When comparing flow (or pumping) per power, we
determine that it is dependent on the impeller type,
'.\/Re = Reynolds number, dimensionless speed, diameter, and geometry of the installation. The
mixer is not fully specified until torque, 'C, and lateral
(5-7) loads (fluid force, F) are included in the analysis [29].
Nr = force number, '.'ir = F/ (PN D' 1) (5-34) (5-38)
2
when M = mass (5-39)
L = length
T = temperature Table 5-2 presents the effects of expected performance
t = time on various parameters or relationships for mixing. To
F = force, ML/t 2 actually calculate a numerical result of comparing
impeller performances, the dimensionless numbers for
flow power and force are needed. Note that in Table 5-2
Oldshue l29] expresses pumping effectiveness as
pumping per power and recognizes it as a key function for the constant basis is across the horizontal top of the chart
processes that are flow controlled or need more flow than and the function to be examined or compared is along
head or shear. the vertical left side. The functions in the body of the
table are used as ratios for condition (1) and condition
(2), holding the basis constant,
(5-35)
For example, referring to Table 5-2, if power input (P)
(5-36) and impeller diameter, D, are kept constant, then speed,
306 Applied Process Design for Chemical and Petrochemical Plants
�-�� Performance Relationships for Mixing Variables: More
Table 5-2
Than One Variable Changing or Held Constant
(A,) �� : ··· sta . · ;: � siti:Reg ( Basis
,··=
;,i..
gn. (BJ Lam. with stagn. (CJ (D • l Baffled Function N, D *Q,N P,N, t P,D Q,P Q,D
{
State D Stagn. Zone N s;,1
of D Lam. Zone Q
":= Flow N-
i,! 10' D Turb. Zone ,-, N 1;3 N 3/4 NQ
p
p
Iii (Az) lam.nostagn. : I
i � ' -�'-=:.._�
\ (D2) Non- Nt4
\11l
o!( 1Q3 (A)-! ,,·R, baffled D- p
�11:
1 �.,) L .: ---------------7---- N 1/3 Nl/5 N3;,1
p
Q
Q
(BJ I
k
1'\
• 102 ��,:.
NQ
t� 7 -i-,-� i M R {Non-baffled Q- NQ N3;s N 1/3
�� ..).. � ,{1')1__!1,, � ,_S_Ei �� :
w' Baffled
IQ '\. � [CJ-----j_(D)-----1 p I'
� 10 � ' �� .. �,.�---------:--7-----i
[Baffled NP Np
N,-R, \_ Non-baffled N s/s N3
q Q
NP N 3/1 NP
p
t- NP N � 3
0. ,,'----L....,..L--- , '=-02�--,- , 0.,..3�--,c-!o'""•---,,""o,...-----,,c:!os N5;3 N5!4 N2
Q
Q
Q
nd 2
R,=-v-
*Q NQ *N5;3 NQ NQ N3
Q
Q
2
Np = power number R 0 = Reynolds number = No = d np/µ
0 = tank diameter Nqo = discharge flow number= q,lnd 3 p NP Nr N3;s Nl/3 NP
d = impeller diameter p = pressure p p
Z = height of liquid in tank q 0 = discharge flow rate
n = revolutions per minute 0m = mixing time NF NF NF NF N,
T1,1 =time F- NF
N4;3 N4/5 N!"3 N 112N 112 N2
Figure 5-19. Characteristic curves: flow, power, and mixing time. Q Q Q p Q
(H/D = 1, 8-blade paddle H/D = �. d/D = 1/10, Hc,ID = �) By per-
mission, Nagata, S., Mixing Principles and Applications, Halsted
3
Press, John Wiley & Sons, Kodasnsha Scientific. p. 125 [34). *Example: Q/P is proportional to N({ /Np on a comparison basis of
keeping flow Q and speed N constant.
By permission, Oldshue, "Fluid Mixing Technology," Chemical Engineer-
N, is proportional to l/N/13; or holding flow (Q) and ing, McGraw-Hill Publications Co. Inc., 1983 [29].
speed, N, constant then Q/P is proportional to
to the vessel. The manufacturers usually take a conserva-
tive view of this problem, nevertheless it is well to under-
stand the expected operating conditions for any installa-
tion. Normally an impeller-shaft system should operate at
(Q/P)1 (N 5/3'/N 3/4) I about 40% of the critical speed. However, the turbine with
p
l Q
thus --- (5-40)
bottom side stabilizer can go as high as 80% of critical.
The manufacturer should provide this information for
This is a valuable relationship as expressed in Table 5-2, the specific system.
because it expresses the working relationship between all
the important variables. Note that as one variable changes Drive and Gears
all others are changed. One variable cannot be changed
alone without affecting the others. Most mixers are driven by electric motors, or in some
cases by mechanical turbines, with gears ratioed to give
Shaft the proper performance speed of the impeller. A variable
or 2-speed driver or gear system often proves worth the
The proper size of shafi: is very important to avoid whip extra cost, since it is difficult to predict the exact speed
and vibration, destruction of bearings, gears, and damage requirements for some new installations. This is particu-
Mixing of Liquids 307
larly true in continuous chemical processes where the
general nature of the fluid remains constant but the vis-
cosity, density or solid particle content may change as the
plant progress from 'just erected' to steady production
and even on to new and different products. This transi-
tion may take from a few months to several years and
should be economically evaluated. The gear mechanism is
not a place to reduce costs for this equipment, since
improper application can create costly maintenance.
Usual practice, particularly for good estimating is to
assume that the gear drive requires 5% of the impeller
horsepower and that system "surging or variations"
require a minimum of 10% of this impeller horsepower.
Thus
Actual motor hp = impeller required hp/0.85
(minimum) (5-41)
When the actual maximum gear box horsepower is known
from the manufacturer, it should be used as long as it is
equal to or greater than the 5% allowance noted above.
The impeller/fluid horsepower allowable variation
should still be 10% or greater. For example, if the calcu-
lated required motor drive (or turbine drive) hp = 23,
(i.e., 19.55/0.85), the next standard motor is 25 hp, so use
this, never less than the 23 indicated above because 23 hp
is non-standard, and no such motor hp exists.
Figure 5-20 illustrates a vertical propeller mixer assem-
bly, with vertical mounting with gear box and motor. Fig-
ure 5-21 is a typical right angle, vertical impeller shaft with
horizontal gear and motor drive.
The mixer manufacturer should always be consulted
for proper mechanical features design and strength char-
acteristics, such as horsepower, gear rating AGA, shaft Figure 5-20. Portable VektorCTM) vertical propeller mixer assembly.
diameter, shaft deflection, critical speeds, bottom steady By permission, Lightnin (formerly Mixing Equipment Co.), a unit of
bearing, and side shaft bearings. General Signal.
Materials of Construction
Steady Bearings
In general,just about any material that can be worked
into the impeller design is available, including steel,
The installation of mixers on long shafts in tall tanks stainless alloys, copper alloys, nickel and alloys, hard rub-
may become a problem if "whip" of the shaft develops. To ber, and lead, rubber and plastic coatings on impellers
reduce this possibility, a bearing support in the bottom of and shafts.
the tank will hold the shaft steady. Lubrication is by the
tank fluid. Therefore this has limited application if abra- Design
sive particles are present. Normally the manufacturers'
designs avoid this extra bearing. Sometimes a guide bear- Normally the proper impeller selections and horse-
ing is installed about midway in the tanks to steady the power requirements are handled in a cooperative manner
shaft at this point. Again it is preferable to avoid this, if with the manufacturer of this equipment in order to
possible, and the manufacturer should make recommen- obtain the best analysis of a given application. There is no
dations for the installation. substitute for performing the proper test runs to evaluate
308 Applied Process Design for Chemical and Petrochemical Plants
1 . Motor bracket 6. Change pinion 11. Timken tapered roller bearings
2. Standard foot-mounted motor 7. Change gear 12. Removable low-speed coupling half
3. Fabricated housing 8. Change gear cover 13. Dry well oil seal
4. Lifting eyes 9. Drain plug 14. Spiral bevel gear
5. High- speed shaft 1 O. Spiral bevel pinion cartridge 15. Low-speed shaft
Figure 5-21. Right angle drive for vertical impeller shaft. By permission, Chemineer, Inc. Bulletin 711.
one or more types of impellers in a particular application. used in setting forth the known as the desired information
Even is this is carried out on a small scale, properly evalu- with a manufacturer. In general, the specification sheet
ated it can lead to the correct selection of impeller and should not be expected to establish the whole story or
horsepower. The horsepower seems to be the factor that information on a mixing problem, unless the problem is
is missed in some evaluations. Any foreseeable process known to be fairly straight forward or data is known which
changes, or ranges of operation, must be considered in
order to have sufficient power for start-up as well as nor- can be given to the manufacturer (for example, blending,
mal running. dispersing or dissolving crystals, etc.). For the unique
problems, one-of-a-kind, laboratory data should be taken
under the guidance of technical advice from the manufac-
Specifications
turer, or other qualified authority in order that adequate
The suggested specification sheet of Figure 5-22 is help- scale-up data will be taken and evaluated. It is important to
ful as a general checklist for the mixing inquiry and can be either describe and give dimensions for the vessel to be
Mixing of Liquids 309
used, or request the mixer manufacturer to recommend B. Decrease Design Problems
the type best suited to the service. 1. Reduce required shaft diameter and length, while
maintaining complete mixing effectiveness.
Flow Patterns 2. Limit or eliminate the need for submerged or
internal guide bearings.
The pattern of the fluid motion is a function of the
fluid system, impeller, vessel configuration, and location Entrainment
of the impeller in the fluid system relative to the vessel
walls and/ or bottom. The patterns illustrated in Figures 5- Entrainment is an important element in the m1x111g
23A-5-23K indicate that almost any pattern can be estab- operation and involves incorporation of low velocity fluid
lished. provided the particular impeller type is located in into the mass of the fluid stream or jet issuing from a
the proper position. This is easier to accomplish in some source such as a mixing impeller. The axial flow from a
systems than others. propeller under proper physical conditions serves as a cir-
The use of vertical side wall baffles usually destroys the cular cross-section jet to produce mixing by turbulence
rotary and swirling motion in vertical tanks. This also can and entrainment. The flat-blade turbine issues a jet for
be accomplished to a degree by setting the mixer off cen- entrainment at the top and bottom areas of the ring [2].
ter. These baffles should be Yio to Yi2 of the width or diam- It is significant to estimate the relative amount of liquid
eter of the tank. Six baffles generally give slightly better involved due to entrainment, as this helps to describe the
performance than four; although four is the usual num- effectiveness of the operation.
ber, with three not being as good for most situations. From a propeller, the entrainment by circular jet is [9]:
Draft Tubes
(5- 42)
The application of draft tubes as related to various mix-
ing operations is shown in Figures 5-231 and 5-24A-5-241.
The draft tubes are basically a tube or shell around the where Q., = volume entrained, cu ft/sec
shaft of the mixer including the usual axial impeller, X = distance from impeller source, not to exceed 100
which allows a special or top-to-bottom fixed flow pattern jet diameters, ft
0
to be set up in the fluid system. The size and location of D = diameter of jet at origin, ft
the tube are related to both the mechanical and mixing
performance characteristics as well as peculiar problems This relation is sufficiently accurate for large scale
of the system. Usually they are used to ensure a mixing design.
1
flow pattern that cannot or will not develop in the system. The maximum Q/P i 3 for a circular jet is at X = 17.1
Weber gives the following points for draft tubes [23]: D 0 (Refs. [9, 21]) or in other words the optimum jet ori-
With a draft tube inserted in a tank, no sidewall baffles gin diameter is 1/ 17.1 of the distance desired for effective
are required, and, the flow into the axial impeller mount- entrainment. Since the entrainment efficiency does not
ed inside the tube is flooded to give a uniform and high fall off too rapidly, it is not necessary to use only the ratio
flow pattern into the inlet to the impeller. The upflow in given, but rather to stay in close proximity, say ± 25-35
the annulus around the tube has sufficient velocity to percent. Large diameter jet streams are more effective for
keep particles in suspension, if necessary. the same power than small streams [ 17]. Data on flat
blade turbines has not been fully evaluated.
A. Increase mixing efficiency
Batch or Continuous Mixing
l. Prevent short circuiting of fluid, define a specific
path. Often pilot plant or research data for developing a
2. Improve heat transfer coefficient by forcing flow process are obtained on a batch operation. Later, a con-
past coil surfaces. tinuous process will usually prove that smaller equipment
3. Provide more complete reaction in a gas-liquid can be used and that the operation will be more econom-
system by recirculation of unreacted gases. ical. Normally batch mixing requires 10%-25% more
4. Minimize areas of inadequate turbulence in vessel. power than continuous [29] for stable conditions; howev-
5. Accentuate the direct mechanical shearing action er, the reaction time for continuous flow is always longer
of the mixing impeller upon the fluid. than the react.ion time for batch flow, but the practical
6. Amplify mixing action by effectively increasing result may show batch time cycle is increased by filling,
the ratio of mixer to container diameter. (lal continued 011 page 312)
310 Applied Process Design for Chemical and Petrochemical Plants
SPEC. DWG. NO.
A-
-·
Job No.
Page of Pages
Unit Price
B/M No.
No .Units
MIXING EQUIPMENT SPECIFICATIONS
Item No.
PERFORMANCE
Component Wt.% Sp, Gr. Viscosity, Cp Temp.° F
Sp. Gr .. of Mixture Viscosity of Mixture
Solids: CJ Soluble CJ Insoluble D Abrasive CJ Sticky [J Crystal I ine CJ Fluffy
Lbs/Gol. Mixture Sp. Gr. Particle Size
Suspension Sp. Gr. Sett) ing Velocity FPM
Class: [J Blending CJ Di ssol'w'ing D Suspending Selids. CJ Cooking D Emulsifying D Heat Transfer D Gos Dispersion
1
CJ Time Req d
Mixing Type: CJ Violent [J Medium CJ Mild Foam: CJ Slight CJ A v eroge CJ Bod
Cycle: CJ Botch: Smallest Gal. Normal Gal. Large,t Gol.
D Continuou5: Rate GPM
Mixer [JWIII [JWill Not be operated during filling. Sequence of Addition
Vessell Spe�s: Dwg. No,
... MATERIALS OF CONSTRUCTION
Impeller
Vessel Shaft
Mtg. Fig. Stuffing Bax Steady Bearings
Packing Other Wetted Parts
-
SELECTION
Monufocturer Model
Roq'd Vessel Opening Size Pressure Class Facing
Ml,cer Location on Vessel
Mixer Angle 0 with
DESIGN
Impeller: Diameter Type No. Speed RPM
Normal BHP (Excluding Geor)
Type Bearings Sfeady Bearing Req'd? [J Yes CJ No. Guide Bearings Req'd? DY•• DNo
Shalt Seal: CJ Packing D Mechanical. Make Type
Seal Coolant Stuffing Box Lubrication
Shalt Coupling: Type Make
Geor: Manufacturer Type
She Reel. Ratio Rated H.P. Mox. BHP
Mech. Ell. %. No Reductions Output RPM Spec'd Chongoble CJ Yes CJ No --
Driver: Manufacturer Type Speed RPM
Elect. Power: Volts P hose Cycle. BHP
Service Factor Fromc
REMARKS
By lchk'd I App. I Rev. I Rev. I Rev.
Date I I I I I
P.O. To:
Figure 5-22. Mixing equipment specifications.
Mixing of Liquids 311
Jet from Open Pipe
l=f��ai
Vortex
-r- ::J =:;=:
Side Viu, Bottom View Jet from Contracted Jel from Morine Type
Pipt or Nozzle
Propel!er
Figure 5-23A. Fluid flow pattern for propeller mounted center with
no baffles. Note vortex formation. By permission, Lightnin (formerly
Mixing Equipment Co.), a unit of General Signal. l
p "d-
-i==!�=:,=l==
Figure 5-23E. Entraining mixing jets. By permission, Rushton, J. H.,
Petroleum Refiner, V. 33, 1954, p. 101 [17].
Method A Preferred
U nless Liquid Density
j Cousing Covitotion)
Greotly Different er
opor Pressure is Hioh
:::::::::: :::::_
Side View Bottom View - -
_ Turbulent
-
- -.Core
Figure 5-238. Fluid flow pattern for propeller mounted center with B ; ._ -
baffles, axial flow pattern. By permission, Lightnin (formerly Mixing
Equipment Co.), a unit of General Signal.
Figure 5-23F. Introducing liquid during mixer operation. By permis-
sion, Rushton, J. H., Petroleum Refiner, V. 33, 1954, p. 101 [17].
Proper Propeller Position: No Vortex Will Result
Side View Bottom View
Mixer
Clockwi
Figure 5-23C. Fluid flow pattern for turbine mounted on-center with Rotolil
baffles, radial flow. By permission, Lightnin (formerly, Mixing Equip-
ment Co.), a unit of General Signal.
Mixer
Clockwise
Rotation
Side View Side View Propeller Turning Counter· Wrono Positions: A Swirl ond Vortex Will Result
Clockwise Looking Down
on Sha:t
Figure 5-23G. Side-entering propeller mixer position, large tanks. By
Figure 5-230. Fluid flow pattern for propeller mounted in angular- permission, Rushton, J. H., Petroleum Refiner, V. 33, 1954, p. 101
off-center position. By permission, Lightnin (formerly, Mixing Equip- [17].
ment Co.), a unit of General Signal. (Figure 5-2) continued Oil next page)
312 Applied Process Design for Chemical and Petrochemical Plants
(Figure 5-23 continued [rom previous page) (text continued [rom page 309)
Vertical Tu be cleaning, and emptying the reactor (see Figures 5-25A
Baffles
and B).
In batch operations, mixing takes place until a desired
composition or concentration of chemical products or
solids/crystals is achieved. For continuous operation, the
feed, intermediate, and exit streams will not necessarily
be of the same composition, but the objective is for the
end/exit stream to be of constant composition as a result
Helical Coil
of the blending, mixing, chemical reaction, solids suspen-
sion, gas dispension, or other operations of the process.
"Perfect" mixing is rarely totally achieved, but represents
the instantaneous conversion of the feed lo the final bulk
and exit composition (see Figure 5-26).
When conducting pilot plant testing to develop a
0
process involving mixing, which later may be used in the
design of a large scale plant, it is wise to discuss the test-
Figure 5-23H. Liquid motion patterns. A.) Vertical-tube baffles; B.) ing with a mixing specialist and outline the needed pilot
Helical coil, no other baffles. By permission, Dunlap, J. R., Jr. and data required to later scale-up the process, generally from
Rushton, J. H., Al-ChE Symposium Series, No. 5, V. 49, 1953, p. batch pilot plant to continuous commercial process.
137. American Institute of Chemical Engineers [6].
Scale-Up and Interpretation
Scale-up techniques for using the results of pilot plant
or bench scale test work lo establish the equivalent
process results for a commercial or large scale plant mix-
ing system design require careful specialized considera-
tions and usually are best handled by the mixer manufac-
Figure 5-231. Coil used as draft tube. By permission, Dunlap, J. R., turer's specialist. The methods to accomplish scale-up will
Jr. and Rushton, J. H., A./.Ch.E. Symposium Series, No. 5, V. 49, vary considerably, depending on whether the actual oper-
1953, p. 137. American Institute of Chemical Engineers [6].
ation is one of blending, chemical reaction with product
concentrations, gas dispersions, heal transfer, solids sus-
pensions, or others.
These scale-up methods will necessarily at times
include fundamental concepts, dimensional analysis,
feed Pipe for Liquids Heavier rnan
Tank Contents empirical correlations, test data, and experience [32].
Similarity concepts use physical and mathematical rela-
tions between variables to compare the expected perfor-
mance of mixing/agitation in different sized systems
[33]. This is usually only a part answer to the scale-up
problem.
Geometric similarity is often considered the most
Feed Pipe for Gas or Liquids
Lighter than Tank Contents feed Pipe for Gas or Li�uids important feature to establishing similarity in mixing, bas-
Figure 5-23J. Feed of liquids and Figure 5-23K. Feed of liquids ing the scaled-up larger unit on the smaller initial model
gases to turbine. By permission, and gases to dual propellers. By or test unit.
Dunlap, J. R., Jr. and Rushton, J. permission, Dunlap, J. R., Jr. and The scale-up of mixing data has been treated with a
H., A.I.Ch.E. Symposium Series, Rushton, J. H., A.I.Ch.E. Sympo- variety of approaches, some to rather disastrous results.
No. 5, V. 49, 1953, p. 137. Ameri- sium Series, No. 5, V. 49, 1953, p.
can Institute of Chemical Engi- 137. American Institute of The principles are now well established, and it is a matter
neers [6]. Chemical Engineers [6]. of truly understanding the particular systems that poses
Mixing of Liquids 313
.-·Droll Tubt
Gos·fud
Manifold
··coil Bonk
Figure 5-24A. Draft tubes prevent short- Figure 5-248. Forced convection past Figure 5-24C. Gas-liquid mixing is more
circuit!ng of liquid from inlet to outlet in a heat transfer surfaces improves the overall complete when concentric draft tubes are
continuous mixing vessel. By permission, coefficient of heat transfer. By permission, used to recirculate gases. By permission,
Weber, A. P., Chem. Engr., Oct. 1953, p. Weber, A. P., Chem. Engr., Oct. 1953, p. Weber, A. P., Chem. Engr., Oct. 1953, p.
183 (23]. 183 [23]. 183 [23].
Settled Solid
Y j -, \ - :::, ,Concenlric or Layered
Draft Tubu Liquid
�----D-----l
Figure 5-240. Capacity of a draft tube Figure 5-24E. Baffles positioned in the Figure 5-24F. Settled solids or layered liq-
assembly to suck in gases ;s a function of draft tube accentuate the direct mechani- uids are quickly dispersed by the direc-
the liquid height above the rotor hub. By cal action of low speed mixing elements. tionalized flow from the draft tube. By per-
permission, Weber, A. P., Chem. Engr., By permission, Weber, A. P., Chem. Engr., mission. Weber, A. P., Chem. Engr., Oct.
Oct. 1953, p. 183 [23]. Oct. 1953, p. 'i 83 [23]. 1953, p. 183 (23].
--
---... \ - Draft Tu be
1
j I I
-- .:»: -Helix
Continuous
\\
Figure 5-24G. Direct mechanical action Figure 5-24H. Helix-in-draft-tube assem- Figure 5-241. Mechanical design problems
can be increased by the addition of a grat- blies are effective for crutching pastry or may be solved by using a draft tube to
ing piate to the draft tube. By permission, fibrous materials. By permission, Weber, amplify the action of the mixer. By permis-
Weber; A. P., Chem. Engr., Oct. 1953, p. A. P., Chem. Engr., Oct. 1953, p. 183 (23]. sion, Weber, A. P., Chem. Engr., Oct. 1953,
183 (23]. p. 183 [23].
the real problem. The important similitude concept 2. Kinematic similarity requires geometric similarity
involves the following: and requires corresponding points in the system to
have the same velocity ratios and move in the same
l. Geometric similarity requires all corresponding direction between the new system and the model.
dimensions of a new system to have the same ratio
with a test model which has proven acceptable. 3. Dynamic similarity requires geometric and kinemat-
These dimensions should include vessel diameter ic similarity in addition to force ratios at corre-
and liquid level, baffle width and number in vessel, sponding points being equal, involving properties of
impeller diameter, number of blades and width gravitation, surface tension, viscosity and inertia [8,
ratio. For example, a tank four times the diameter of 21]. With proper and careful application of this
the original model also requires a turbine ten times principle scale-up from test model to large scale sys-
the diameter of the original turbine. tems is often feasible and quite successful. Tables 5-
314 Applied Process Design for Chemical and Petrochemical Plants
s.o \ ..___..___..,__ REACTION --+----t----+---+
FIRST-ORDER
8 v (I)
4.0
5 :z: 4
BATCH
2 i:
� 3.0 ... �
!z al
i:c 8 ls
0:
� o,: LL
z
ti 2.0 � �� 21-----1--+---1---1---+----+---''/
z
...
0 � ;:
u
co,n...,usl �
1.0 �
1--- 1L._--==::::::;:;;;;.===::�===::::;:;;�======��
0 0 20 40 60 80 100
TIME PERCENT REACTED
Figure 5-25A. Illustration of chemical reaction in a batch system in Figure 5-258. For a first order reaction, the ratio of time in a contin-
which concentration decreases with time, with a three-stage contin- uous tank to the time in a batch tank for various percentages of
uous mixing system superimposed. By permission, Oldshue, J. Y., reaction completion. By permission, Oldshue, J. Y. [29].
Fluid Mixing Technology, 1983, Chemical Engineering McGraw-Hill
Publications Co., Inc., p. 340, 347, 348 [29].
CONTINUOUS MIXING
VOLUME • Ve,
REQUIRED
,.._..,..._��---....------, BLEND TIHE0<}
CONCENTRATION= C
Figure 5-26. Concept of perfect mixing, in which the feed is �
dispersed instantly into the tank and the exit concentration is CONCENTRATION.-�
equal to the tank concentration. By permission, Oldshue, J. Y.
[29]. TANK VOLUME V
Table 5-3 Table 5-4
Impeller and Flow Characteristics For Turbulent, Impeller and Flow Characteristics-Viscous, Baffled or
Baffled Systems Simple Ratio Relationships Unbaffied Systems Simple Ratio Relationships
At Constant f P, D, N, Q, H, (Q/H), At Constant P, D, N, Q, H,
--- --- ---
Im ener Im eller
6 iameter, D N,a ..... p,11a N, N,2 N;I 6 iarneter, D N2 . ... p,112 N, N,
--- --- ---
r
Speed, N 0,6 p,115 ..... o,a 0,2 D, Speed, N 03 r,1,a . ... 03 . ...
r
r
--- --- ---
Power, P ... N;3t6 D -s;s N;•t6 N,416 N;:sto Power, P .... N;21a 0-312 N;t o-a12
r
'
or
or
or
or
or
D •ta o-•13 D s13 D a12 N,
r
r
r
t
By permission: Chemineer, Inc., Dayton, Ohio 8• By permission: Chemineer, Inc., Dayton, Ohio 8.
Mixing of Liquids 315
3 and 5-4 present the relationships of the major variables the two variables can be established. The third variable is
for the two most important cases of mixing. tied through the power curves (plot of power number v.
NRe• see Figures 5-13, 5-14, and 5-15). Figure 5-28 shows
Often, exact or true kinematic and dynamic similarity that geometric and dynamic similarity can develop useful
cannot be achieved in a system requiring small scale test- relationships for some situations [29], but not all, and it is
ing to determine the effect of the design, or flexibility in not truly possible to prepare one dimensionless group
design to allow for final design "trimming." Consideration expressing a process relationship. This suggests that care
should definitely be given to such flexibility as (a) mixing must be used in resorting only to a dimensionless number
impeller designs that can be modified without excessive for process correlations. Also see Figures 5-29 and 5-30.
cost, or the need to build a completely new/larger/small- Because the most common impeller type is the turbine,
er unit, (b) multiple gear ratios for the gear drive, with most scale-up published studies have been devoted to that
spare ratio gears to adjust speeds, and (c) either variable unit. Almost all scale-up situations require duplication of
speed driver or oversized driver to allow for horsepower process results from the initial scale to the second scaled
adjustments. unit. Therefore, this is the objective of the outline to fol-
The dynamic response used to describe fluid motion in low, from Reference [32]. The dynamic response is used
the system is bulk velocity. Kinematic similarity exists with as a reference for agitation/mixer behavior for a defined
geometric similarity in turbulent agitation [32]. To dupli- set of process results. For turbulent mixing, kinematic
cate a velocity in the kinernatically similar system, the similarity occurs with geometric similarity, meaning fixed
known velocity must be held constant, for example, the ratios exist between corresponding velocities.
velocity of the tip speed of the impeller must be constant. For scale-up procedure, refer to Figure 5-31, which out-
Ultimately, the process result should be duplicated in the lines the steps involved in selecting commercial or industri-
scaled-up design. Therefore, the geometric similarity goes al mechanical agitation equipment when based on test data.
a long way in achieving this for some processes, and the
achievement of dynamic and/or kinematic similarity is • Test data should be planned by knowledgeable special-
sometimes not that essential. ists in order to obtain the range, accuracy, and scope of
For scale-up the "shear-rate" of the fluid, which is a needed data to achieve a pre-established mixing
velocity gradient that. can be calculated from velocity pro- process result.
files at any point in the mixing tank [29], is an important • While obtaining test data, scale-up calculations
concept. The shear rate is the slope of the velocity versus should be made regularly to determine if the end
distance curve. Using the time average velocity yields results will be practical, particuiarly from the avail-
shear rate vaiues between the adjacent layers of fluid that able mixing hardware, motor power, etc.
operate on large particles of about 200 micron or greater.
In Figure 5-27, usually a maximum shear rate will exist at
the impeller jet boundary. The average shear rate is pri- SHEAR RATE = b.. V
b..Y
marily a function of the time average velocity and
impeller speed, and is not a function of any geometric
type of impeller or the impeller diameter [29]. The max-
imum shear rate exists at the jet boundary and is a direct
function of impeller diameter and speed, which is related to the SR (0) = 10 sec: 1
peripheral speed of the impeller. Thus, on scale-up, the SR(Va)=9.5
maximum impeller zone shear rate tends to increase SR(1/4)= 7.0
while the average impeller zone shear rate tends to SR(S/a)=S.O
decrease [29].
The fluid shear stress actually brings about the mixing SR(1°Ui)=O
process, and is the multipiication of fluid shear rate and
viscosity of the fluid [29].
The pumping capacity of the impeller is important in
establishing the shear rate due to the flow of the fluid
from the impeller. SHEAR RATE IS A FUNCTION OF VELOCITY GRADI�NT
There is no constant scale-up factor for each specific
mixing system/process [29J. The two independent Figure 5-27. Shear rate is a function of velocity gradient. By permis-
sion, Lightnin Technology; Lightnin Technology Seminar, 3rd ed.,
impeller variables come from speed, diameter, or power, 1982, Lightnin (formerly Mixing Equipment Co.), a unit of General
because once the impeller type/style has been selected, Signal, p. 1, Section 2A [27].
316 Applied Process Design for Chemical and Petrochemical Plants
100
Equal heat transfer 0 • IMPEUER DIAMETER
per unit volume - L,..,,- I/ T•TANK DIAMETER (Co.lSTANT)
I/ v v 20 IMPELLER TYPE CONSTANT----.11�---+-�
1/
v 1,/ time-T 101--��i---�t---+.,..,....-i�- �- �-+--+-1
T• Turbulent flow
- v �qualblend
L• Laminar flow
......
......
10 - - !,, ..... 8t--��+-��-+--7''+---+--+--. ....... �+-.,,,-.�+-�+--+--t
- 61--����.....,C.-+-�+-'1""+-�-.,.c+-��-+-�+--+--t
� ./ ./ (/)
� v (/) 4t--��'4-��-4-. ....... -+---+ ........ ;.._��+-��-+-�-+--+--t
v ... Equal N r, . er equal heat 1ransfer coetficient uJ
(.)
� on i"ti•ivl 'i"['•:.::.+--+-t-tr i
� v v"" - a.
i g ...... ..... - ... - ..--- ........--r II I I I I I I I I
,,/'
.ii ,...-- / - Equal bubble and drop diameter - T
,..-Equal heat transfer coefficient - L
£ 1.0 «'. ... .u
l ' - - ...... � .. Equal mass and heat transfer coefficient: 1...._��"--��.1--......1.�1......J'-----'��--'� ....... ��
Equal blend time
L
'\. ....
l '\. "\. ....... "'- .... - � particles bubbles, drops T QI Q2 0.4 0.6 I 2 4 6 10
I
I I I
I
I
HORSEPOWER
0 I\. -- ..... ......... -,.... "1-N
.2 \ '� Solids suspension - T Figure 5-30. Effect of power on process result with constant D/T
l \ <, . "'±-. ratio. By permission, Fluid Mixing, Lightnin (formerly Mixing Equip-
[\ <, Equal tip speed·- T r--. ........ ment Co.), a unit of General Signal.
0.1
.....
Equal 11p speed - L
\. <, Oitiign test equipment to mo<al proceu
� <,
Equal NR• (land T) ....
11 I\ ......... .... � Operate equrpmanT for r&q�irod prrn:..u ruult
0.01
1 10 100 1,000
Ratio of mixed volumes, V 2 I V 1 or of impeller diameters cubed, (0 1 I 0 1) 1
Figure 5-28. Mixing scale-up factors referenced to experienced Geomeiric �imilerity for larfl(! scale equipment
ratios of power per unit volume. By permission, Penny, W. A., Chem.
Engr., Mar. 22, 1971, McGraw-Hill, Inc., p. 88 [31]. S�le ratio
Determine imp�lier drameler
Scwle-up upononr
Calcula1e agitator spee�
Compu!e ho""po,..,,
Standard sstect.nns forhor�power/speed
lmpellerl lor standard ,election
Shalt desrgn
Sear desrgn
Cost estimation
0 Q2 0.4 0.8 0.8 1.0
D/T RATIO
Design procedure for agitator scale-up Fig. 1
Figure 5-29. Effect of Off on power requirement for a given process
result. By permission, Fluid Mixing, Lightnin (formerly Mixing Equip- Figure 5-31. Design procedure for agitator scale-up. By permission,
ment Co.), a unit of General Signal. Rautzen, A. A., et al., Chem. Engr., Oct. 25, 1976, p. 119 [32].
Mixing of Liquids 317
• Determine geometric similarity to develop a single ments by back-calculating from the nearest standard
scale ratio R, for the relative magnitudes for all linear mixer diameter or gear speed to be able to use the indus-
dimensions [32). try or manufacturer's standard.
The scale-up exponent, n, is given for typical mixing
(5-43) conditions in Figure 5-32.
"Rules of thumb" regarding scale-up and good design
(5-44)
practice are not suitable for determining cost of perfor-
mance design or physical capital cost.
(5-45)
For geometric similarity of liquid motion (n = 1.0) the
when (1) = small size data unit linear scale ratio of volume is
(2) = proposed larger scaled-up unit
(5-48)
then R = scale ratio= D2/D1 = T2/T1 = W2/W1
= 'Li,/Z1 (5-46) The volume ratio can be related to a speed ratio for a
given scale-up exponent, see Figure 5-32. The usual range
To select a turbine, there must also be geometric simi- for the scale-up exponent is between 0.67 and 1.0. To
larities for the type of turbine, blade width, number of select a scale-up exponent, the following provides a guide:
blades, impeller diameter, etc. From the geometric simi-
larity determination of the turbine diameter, the mixer A. n = I; equal liquid motion
speed can be established to duplicate. The "Scale Ratio R,"
Liquid blending, equivalent liquid motion, corre-
sponding velocities are about equal. Similar results
(5-47)
obtained with equal tip speed or torque per unit
where n is based on theometrical and empirial considera- volume.
tions and varies with the specific type of mixing problem B. n = 0.75; equal solids suspension
[32). See Figure 5-32. Equal suspension of particles referenced to visual
Often the scaled-up design provides equipment or appearances and physical sample testing. Empirical
speeds that are non-standard, which then require adjust- correlations generalized to apply to most problems.
1.0
0.5
�
;;:."
.g� 0.1
�
"'O
8l
a.
(/) 0.05
10 102 103 10 4 10s
Volume ratio, V 2!V,
Figure 5-32. Scale-up exponent characterizes the desired type of agitation in order to determine speed-volume ratios. By permission,
Rautzen, A. A., et al., Chem. Engr., Oct. 25, 1976, p. 119 [32].
318 Applied Process Design for Chemical and Petrochemical Plants
C. n = 0.667; equal mass transfer rate input horsepower of motor and speed of agitator. Select
More directly related to turbulence and motion at the calculated point in the grid, then move to the nearest
the interface. Includes scale-up for rate of dissolving speed and input horsepower (not motor hp). Usually
of solids or mass transfer between liquid phases. adjustments from actual design will be required, but often
Using geometric similarity and equal power per vol- the incremental adjustments will not be great. To aid in
ume results in same n value. these adjustments, equal torque will give equal liquid
D. n = 0.5; equal surface motion motion or solids suspension over a narrow range. To use
Related to vortexes formation, depth of vortex relat- equal torque, set up diagonals from lower left to upper
ed to geometric similarity and equal Froude number. right for the available equipment. Adjust or back-calculate
E. N = O; equal blend time (if necessary) the expected performance and dimensions
Rarely used due to large equipment required to based on the adjustment. Higher speeds require higher
hold speed constant for larger units. horsepower. For equivalent mass transfer, equal power
changes are used.
For turbulent power, (constant power number, P 0): To recheck final design, the diameter of a single,
pitched blade turbine, for turbulent conditions:
p P Ns os (5-49)
Vp 0 3 (5-50) (5-54)
P/V p N 3 D 2 (power per volume) (5-51) OT= impeller diameter, turbulent operation, in.
For P /V constant in two different systems, then Php = horsepower used by system (not motor horsepower
of driver)
(5-52) Sg = specific gravity of fluid
N = agitator impeller speed, rpm
(5-53)
Figure 5-33 provides a selection grid for establishing the Of course economics enters into the solution, so some
industrial standard for the driving equipment, that is, alternate designs may be helpful.
230,-�"T"""��-,.---.---r-"""T"""TTT""T-r-�-r---,.....-..-,-...;.,..---,,---,,---r---r-,-,-�--.-�-,--�--r-�.....-�
____
200�====�==�1-·:-;-,.�.-=t:==� -�==��=:J�:=:ttt�:t:�==::t::==::t==i==1l=:=t==:l:::t=:r,:�;:j:::::;;=::i:=:::j::::::::::r�t--t----l
H-�
1aot====f""="f-='1.'----==i===-l==:l=:::J==�'F1io:l=====.�=4cc=4"'"4-__c-t-,=f:�-=tt:l=°*='t=::t:,=l.-��'==�*-" ·*"=r-.+c---,...J---I
.,.,_ , __
1
l11r--1- � "'--t---t--t-Ht-1-++--,1--i-+-+-+-1 � +1-H-+-l � --+--i-+ � ......1--J
l : ��� 14:= � l=F=l*R*= ����������� · -:!:d
-·
I 80 - i- "- ,-1--,-"1--t--H-t-H---i--ll---+--t--+--t--�-+11-+-+-+----1----1 --J.-1- I....J I i-
-
'f a>t-�f�+--:-:-'r-+-+-+-+tt-t+-�t--+--+-+----+----+-+-H+++---+---+--l--+----1--t-i--l--i---l
&-
C.lcullted horlepowers, hp
Figure 5-33. Scaled-up horsepower/speed requirement for an agitator system is readily related to industrial equipment. By permission,
Rautzen, R. R., et al., Chem. Engr., Oct. 25, 1976, p. 119 (32].
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APPLIED PROCESS DESIGN FOR CHEMICAL AND PETROCHEMICAL PLANTS, Volume 1, 3rd Edition
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