I
DALE C. BERGSTEDT and DONALD A. DAHLSTROM, Eimco Corp., Palatine, 111. VAN
D.
HARMS, Corn Products Refining Co., Argo, 111.
Continuous Pressure Filtration Pilot Plant Application to Vegetable Oils
The growing trend toward continuous processing equipment, so extensively adopted by the chemical and petroleum industries in recent-years, is being further expanded into the food processing field. Progressive firms are making a detailed study of their batch operaticns to determine if the many advantages of continuous processing apply in their specific situations. Filtration of vegetable oils is a particular example of this.
THIS
FILTRATION has been handled conventionally by batch filter presses of the plate and frame, recessed plate, or other applicable designs. The operating costs of such filters include the labor for cleaning after each cycle and the periodic replacement of filter media. These filters have an added disadvantage-that is, a difficult housekeeping step in the refining process. Furthermore, the oil is exposed to air, causing oxidation and discoloration, unless special precautions are taken. Finally, reasonably complete removal of cake oil content requires an air purge after the cake has been prepared fully. This results in a fire hazard when the cake is hot. After air purge, a steaming operation is used which causes undesirable working conditions. For these many reasons, Corn Products Refining Co. studied the possibilities of a closed, continuous filtration operation in two of their corn oil refining steps. One filtration removes the
bleaching clay from the oil following the bleaching step. The second removes the wax from the chilled oil in the “winterizing” step. This pilot plant work is confined to the first type of filtration, although the method with certain modifications could be applied to the second type. Because oil oxidizes easily, has excessive foaming tendency, and has a high viscosity even when hot, continuous vacuum filtration is both uneconomical and undesirable. However, by increasing the filtration pressure drop and maintaining the downstream pressure above atmospheric, the process can be made continuous and simultaneously overcome these difficulties. That is, instead of using the limited pressure differential obtained by a vacuum pump pulling down from atmospheric pressure, a codedesigned, pressure-tight shell is built around the drum filter, and an aboveatmospheric pressure created on the exterior of the drum. Then, if the underside of the filter media is vented to the atmosphere, or to the suction side-of a compressor, any desired pressure drop is available for filtration. Optimum pressure drop appears to be from 40 to 50 pounds per square inch in most cases. The choice of pressure filtration would not preclude the possibility of vacuum bleaching, as is sometimes practiced to eliminate air and moisture during the bleaching step.
Description of Pilot Plant The continuous pressure filter station was constructed by The Eimco Corp. Corn Products Refining Co. installed the filter and completed the pilot plant in their Argo corn oil refinery building. The pressure filter must be a very versa-
tile unit, as it is employed on many other problems. I t can be operated at any degree of submergence u p to 50%, and can function either as a precoat drum filter or with a filter media cover only. The filter is fitted with a two-solution master valve, with adjustable bridge blocks so that the first filtrate can be separated from the second a t any point in the cycle. The master valve also provides for a flow of blow-back gas to aid in cake discharge after the cake has been dried when operating with filter media only. The filter is equipped with a flanged filter drum for precoat operation, and with a variable speed-drive precoat knife advance mechanism. Thus, the precoat can be shaved continuously a t any desired depth of cut as the drum rotates. All wetted metal parts within the filter are Type 316 stainless steel to prevent any possible product contamination. The pressure shell is stainless steel which is fabricated to meet ASME code requirements for unfired pressure vessels, and can be operated a t a maximum working pressure of 55 pounds per square inch gage and temperatures of -20’ to +400’F. Figures 1 and 2 show three different views of the pressure filter unit and the general arrangement of equipment in the pilot plant area. In Figure 3, the flow sheet can be followed by the liquid streams. The oil and clay are mixed in the bleach tank into a slurry which, in turn, is pumped to the filter. The solenoid valve in the feed line controls the rate of flow according to liquid-level controller in the filter tank. An air cushion reduces “hammer” when the valve closes. Valves in the feed line are operated manually so that the feed can be recyVOL. 49, NO. 1 1
NOVEMBER 1957
1863
Figure 1 ( A ) (upper left). 1. 2.
3. 4. 5.
6. 7. 8.
Feed inlet Feed liquid-level controller Agitator drive shaft Filtrate liquid-level controller Gas outlet connections
B 10. 11. 12. 13.
(upper right).
Filtrate pipes from filter to receiver Filtrate receivers Form section filtrate pump 9. Dry section filtrate pump
Exterior view, 180” around
1 864
Drum with precoat and cake formed
The line between the dry port of the master valve and the dry receiver is equipped with a manual valve which can be used to create a back pressure on the dry section. This valve can be used as a throttling valve to reduce the flow of gas through the drying section, and in that way help to maintain the pressure differential across the pickup section under conditions where the drying action is rapid. The filtrate is collected in the two receivers and is pumped to measuring tanks. The pumping cycle is controlled by liquid-level controllers which are mounted within the receivers. The pump control circuit is discussed in the following section. For precoat operation, two receivers are not absolutely necessary as all liquid and gas can be discharged into the same receiver. However, this pilot plant unit was designed to use many methods of the filter operation, and for that reason two receivers were installed. The presence of both receivers has proved bene-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Cake conveyor Cake receiver Cake cleanout door
Interior view of filter
17. Washing and sluicing piping
cled into the main bleach tank until the temperature is stabilized in the filter. Also, part of the slurry can be bled off in the filter tank by “cracking” the drain of the filter tank, and in this way the operating temperature can be maintained by a continuous through-flow of feed. Filtration results from the pressure differential are determined by shell pressure minus the low pressure in the filtrate receivers. This low pressure is maintained by the compressor suction. Gages on the exterior of the pressure shell are connected with the form, or pick-up, and dry sections in the master valve, and these indicate the actual downstream pressure which is used for calculating pressure drop. A mixture of filtrate and gas flows through the master valve and discharges into the receivers. The flow in the pickup receiver is largely liquid, whereas the flow in the dry receiver is mostly gas. The receivers separate these two phases and maintain a flooded suction on the filtrate pumps.
14. 15. 16.
Precoat knife drive shaft Master control panel Precoat knife advance mechanism Gas inlet
C (left). 18.
Exterior view of filter
19. 20.
Cake being shaved off precoat Precoat knife drive shaft
ficial in that it is now possible to observe the effect of drum speed on the rate of deoiling o f the precoat layer. The gas which is separated from the liquid in the receivers passes into the mist trap for removal of the finer particles of liquid. This mist trap is packed with steel wool which provides a large number of “targets” for the liquid drops to coalesce into larger drops and to run to the bottom. Periodically, the trap is drained. The compressor is a single-stage duplex machine driven by a 15-hp. motor. Owing to the compression of 40 to 45 pounds per square inch gage, the temperature of the gas is raised to over 300” F. Some of this heat must be removed so that the gas passing into the shell is approximately the same temperature as that of the slurry. This is accomplished with a water-jacketed cooler, using a very small and continuous flow of water. The cake peeled off by the precoat knife is dropped into a screw conveyor mounted integrally with the filter shell.
C O N T I N U O U S PRESSURE F I L T R A T I O N The screw conveyor discharges into a vertical pipe which permits the cake to drop into a lock-hopper type of cake receiver. Periodically, the hopper is isolated from the shell pressure, vented, and opened for cleaning. This filter station flowsheet is set u p for semiautomatic control of operation, so that a minimum amount of attention is required unless data are being recorded. Those controls, such as gas rate, dry back pressure, and cooling water flow, are operated manually on the pilot plant, but would be automatically controlled on the full-scale unit. Cake discharge can be made automatic in either of two ways-by adding a screw conveyor in the cake receiver and providing for automatic isolation and venting of the cake receiver a t time intervals; or hy replacing a screw conveyor with a repulper and converting the cake to a slurry by adding liquid. This resultant slurry can then automatically be discharged from a receiver through a throttling valve from a slurry-level controlJer on the receiver. Several modifications of this system can be employed to fit the particular problem. Capacity, or production rate, is a function of the filtration rate and the area of the filter. The filter employed in this pilot plant work was a drum 18 inches in diameter by 12-inch face, with a nominal rating of 4 square feet. When the filter is operated as a precoat unit, the thickness of the precoat increases the effective di-
Figure 2.
General arrangement of equipment in pilot plant area
MAIN BLEACH T4NK 60,000 LB.
n
maximum rate expected was 2 l / 2 gallons per minute from the pilot plant. Sizing of the pumps and auxiliary equipment was based on a 3-gallon-per-minute flow of liquid, and a maximum gas flow of 10 cubic feet per minute measured a t atmospheric pressure per square foot of in-
ameter which makes a larger effective filtering area. For example, with 1-inch precoat, the effective filtering area is 5 square feet. The filtration rates predicted, based on tests of preliminary pressure bomb, ranged from 10 to 30 gallons per hour per square foot, so the
SPECIAL B L E 4 C H TANK 30,000 La.
. p MIST T R A P
AUTOMATIC 4 O V l N C E FOR PRECOAT K N I F E
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CONVEYOR
PRECO4T T4NK 55 G A L ORUY
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C 4 K E RECEIVER
a,c.ca I , T T
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HlO" HEAD TRANSFER PUYP
0 l l
n*m #*.D"I*
B L i l l *
1
1.h"
Cb*l
D,IC*l*ii
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'DRY' SECTION RECEIVER
'PICK.UP'SECTION RECEWER
FILTRATE PUMP
10
Figure 3.
SO^"^,
,.*"I
Flowsheet of continuous pressure filtration pilot plant applied to vegetable oils VOL. 49, NO. 1 1
NOVEMBER 1957
1865
stalled area. This latter figure is substantially in excess ‘of the normal requirement, but was utilized to permit testing under a wide variation in conditions. Special Design Features
The filter design permits operation either as a high or low submergence filter, and can be used for either the precoat operation or normal cake filtration on filter media. A removable plate built into the end of the filter tank is in place for high submergence operation. This plate can be removed and a lower scraper blade mounted at a different angle when a low submergence cycle is to be used. This feature, together with the washing headers (which were provided but not used in this test work), permit any type of drum-filter cycle to be performed on this unit. All of the rotating items in the filter can be varied in speed. The filter drum can be operated by a variable speed drive between 1 and 10 minutes per revolution. T h e slurry agitator can be set to three different strokes by a linkage arrangement. The screw conveyor is driven by a variable speed drive which permits the rotation to be varied according to the amount of cake produced. Another feature promoting flexibility is the piping arrangement to the feed pump, which allows the slurry feed to enter the filter from any of three tanks. One of these is the regular production bleach tank, which operates a t about a 3-hour cycle per batch. Another is a special, smaller tank which can be set up for a continuous feed to the filter for studying special conditions. The third is a small drum in which the precoat slurry can be prepared. Liquid-level control in the filter tank, which maintains the proper degree of submergence of the filter drum, presents a special problem because of the closed construction, pressure condition, and solids content of the slurry. Most buoyancy-operated liquid-level sensing devices are too large and bulky for the pilot plant unit, and the nonconducting nature of the oil prevented the use of conductivity type of liquid-level control. The feed solids content varies from 0.5 to 170 during filtration, up to about 10% during precoating. Because of the solids content, a throttling type of control valve with a pneumatic controller was considered and rejected, particularly as the low flow rates which occur under certain operating conditions might result in solids piling up a t the control apertures, The problem was overcome by use of a direct-acting lever type of solenoid valve controlled by a liquid-level sensing float. This system maintains the level within 0.75 of 1 inch and typical frequency of solenoid action is about 3 to 5 seconds. No overheating due to high
1866
inrush current has been noted, even with solenoid frequencies of about 1 second. The same type of sensing element is used for liquid-level control in the filtrate receiver tanks-namely, a float-operated controller which functions by varying the magnetic field of a coil \vithin the tank. The float motion shifts the position of a steel-core piece, riding in the coil which changes the characteristics of the coil. The variation in magnetic strength operates a relay system which starts and stops the motor of the filtrate pump. Because of the narrow band of the liquid-level controller and because extremely close control of liquid level is not required in the filtrate receivers, the control circuit has been modified with a time-delay relay in the motor-control loop. Thus, each time the pump starts, the time-delay relay maintains the power circuit for a set period regardless of liquid level. This prevents rapid on-off cycling of the pump motor and possible overheating. The inert gas recycle system is based on a gas compressor with a rating of 46 cubic feet of gas per minute at suction conditions, delivered a t 40 pounds per square inch gage. As the compressor does ordinarily not require full capacity, a bypass has been provided so that cooled compressed gas can be bled back into the suction side of the compressor. Normally, the suction of the compressor and the return line from the filter maintains a positive pressure a t 1 or 2 pounds per square inch to prevent any possible inleakage of air. Air is detrimental in this particular application. The gum formed by excessive oxidation causes maintenance problems in the compressor and on the fine wire screening which supports the precoat. The make-up rate is low, being limited to the leakage which might occur through the stuffing boxes which seal the rotating shafts passing through the filter shell. The filter is provided with a cake receiver which can be isolated from the pressure in the shell, so that the cake can be sampled periodically and removed from the system. This arrangement is shown in Figure l B , the large gate valve serving to isolate cake receiver, item 15. One of the problems encountered was the tendency of high gas rates to entrain oil as the gas comes through the precoat layer. High gas rates cause an excessive carry-over of fine oil mist into the compressor, with consequent breakdown of the lubricating qualities of the compressor lubricating oil. T o counteract this, a mist trap was installed, consisting of a vertical column packed with Grade 2 steel wool. This trap proved satisfactory in the subsequent operation of the pilot plant. Examination of the compressor after several months of intermittent operation indicated no serious amount of gumming of the compressor
INDUSTRIAL AND ENGINEERING CHEMISTRY
valves or piston. Also an antioxidant was added to the compressor lubricating oil to minimize gum formation of any vegetable oil which might carry over. Design of the mist trap, based on a gas velocity of 10 feet per second, called for a diameter equivalent to 4-inch pipe. The gas rates are measured by a rotameter in the gas-pressure line to the filter. These readings are corrected for pressure and temperature of the flowing gas to determine actual gas flow in terms of compressor-suction conditions--the basis for rating compressor size.
Operating Procedure
Present Practice. I n the existing process, a batch of oil is dropped to the bleach tank and heated prior to addition of bleaching clay. The temperature of the oil is raised to 175’ F. The bleaching clay is added as a heavy slurry, prepared by mixing clay and a small quantity of oil. After a short holding time to allow adequate dispersion, the slurry is filtered in recessed-plate presses. Filtration of a batch normally requires 3 to 4 hours. As many as 3 to 10 batches can be run through a given press depending on the amount of clay required for bleaching; a typical value is 5. After the cake has built up to full thickness in the press, it is blown with air and steam, the cake is dischargpd, and the cycle i s repeated. Preliminary Studies. Kormally, two engineers operate the unit and collect the data although one man can handle both providing no changes are required in the bleaching clay concentration or in the temperature of the oil supplied to the filter. I n most experimental runs, the continuous pressure filter is supplied with oil from the main process bleach tank. Although this limited the duration of the runs, it allowed test work to be done on the oil from the actual process. T o evaluate the effect of variations in bleaching clay concentration and oil temperature, it is necessary to use an adjacent bleach tank for preparing special batches. The filtration method was investigated first. Three possible approaches are 1. Normal cake filtration on filter media, with blow discharge. valved for “cloudy port” operation. 2. Normal cake filtration on filter media with only partial cake removal, no blow-back. 3. Precoai filtration using a diatomaceous earth precoat, with continuous knife advance. The reason for the cloudy port operation is the high clarity requirements. At present, the recessed-plate filters produce a very brilliant filtrate. Hence, it is expected that a filtration process to replace the presses should also deliver a
C O N T I N U O U S PRESSURE F I L T R A T I O N brilliant filtrate. The cloudy port method is one wherein the first filtrate of each cycle, normally the dirtiest, is separated from the rest of the filtrate and recycled to the slurry tank. I t is anticipated in cloudy port operation that once a cake is laid down, the remaining filtrate will be clear. It is also necessary that flow of clear filtrate is sufficient to purge all the passages. Thus some clear filtrate must be “wasted” as flushings and still a reasonable flow maintained. As the latter condition could not be obtained in this case, an off-quality product was produced and the first approach was abandoned. The next possibility, using part of the cake on the media, gave very good clarity. However, migration of fines within the cake and partial blinding of the “heel” caused the filtration rate to be very poor. This approach also was abandoned. In the third method the filter was designed so that it would be relatively easy to convert to precoat operation. The precoat material used in most of the test work is Dicalite (Brand No. 372) Filteraid (Great Lake Carbon Corp., Los Angeles, Calif.). This material used in other parts of the process gave satisfactory results. Using the precoat approach, it is possible to produce very clear oil a t high filtration rates. Apparently, the precoat filtration of vegetable oils is a method of clarification after bleaching which offers lower direct costs and continuous and sanitary operation. Precoat Filtration. When the unit is to be operated, preparation of the precoat is started about 1 hour before the plant batch is scheduled to start. Approximately 45 minutes are required to heat the oil and to disperse the filter aid in the precoat mixing tank. A precoat batch consists of about 40 gallons of filtered oil and 30 to 35 pounds of Dicalite (No. 372), for a slurry concentration of about 10%. This is prepared in an agitated, heated tank constructed from a 55 gallon drum. The precoat slurry can be heated by a steam coil in this tank. I t is good practice to precoat a t the same temperature as the regular filtration process; therefore, the temperature of precoat slurry is increased u p to 170’ to 180” F. When the precoat slurry is ready, the transfer pump is started and the liquid level is allowed to build up in the filter tank until the liquid-level control takes over. At this point, the gas compressor is started and the pressure in the system is increased gradually by adding nitrogen from the make-up until the desired pressure is reached in the shell. The full volume capacity of the gas compressor is used a t the start of the precoat operation, in order to develop a pressure differential. As the precoat is
deposited, the resistance across the precoat layer is increased, and it gradually becomes necessary to bleed part of the gas from the compressor discharge back into the suction side. This maintains a slight positive value on the low pressure side of the system and avoids exctssive pressure in the shell. This bleed-off is taken after the gas has passed through the cooler to reduce the work load on the compressor. As soon as the full precoat layer is formed, the feed to the filter is switched to bleached oil slurry. The precoat knife is operated manually until the blade shaves across the full width of the drum face. The “breathing” phenomenon sometimes experienced in starting a precoat cycle is not a particular problem in this case. Collection of Data. Data are collected as soon as all of the precoat slurry in the filter tank is removed and operating conditions are stabilized. For each operating condition investigated, data are recorded to give or calculate the following points. Bleach concentration, weight per cent bleaching clay Slurry temperature, O F. Drum speed, minutes per revolution Knife advance, thousandths of an inch per drum revolution Gas flow rate, cubic feet per minute a t compressor suction conditions Pressure differential across pickup and across dry section, pounds per square inch Precoat thickness, inches Filtrate rate, gallons per minute and gallons per hour per square foot Filtrate quality, observed clarity Oil content in the filter cake, weight per cent Filtrate quality is obtained by comparison with a standard for clarity or against the product from the filter presses. In addition, samples are sent to the laboratory for color determinations, free fatty acid, and peroxide value. Details on these properties and cake oil content have not been released for publication. The test data indicated a potential for satisfactory operation. The precoat slurry concentration is noted to be substantially higher than the 1 to 2% range normally employed in precoating. This lower concentration is used because the water slurry of precoat material, such as diatomaceous earth, deposits so rapidly that it tends to develop shrinkage cracks when used a t higher concentrations. However, the oil slurry used in this work deposits more slowly and a much higher concentration can be used without any indication of these shrinkage cracks. I n some runs, concentrations as high as 12y0are employed without any trouble. Normally 10% is used as the 12% level appeared to
be a little difficult for the high head centrifugal pump to pump both the feed and precoat slurry. This pump represents a compromise. A high head centrifugal pump of this type normally is not used for handling a thick precoat slurry due to the possible degradation of the precoat slurry by the close clearances. However, the pump is suitable iol 1,umping the 1% solids of bleached oil slurry, and is also used for precoating as a more suitable design is not available a t present for handling the high pressure required. This high pressure delivery is needed because in1 the finishing stages of precoating, the precoat slurry must come into the filter against back pressure of 40 pounds per square inch, and in this particular case also must be elevated about 30 feet. Results. This pilot plant work has indicated the feasibility of filtering the bleached oil on a precoat filter of the continuous pressure drum type. Owing to oil viscosity, the filtration rate varied with thickness of the precoat in a marked manner that operation at a constant pressure drop over the entire precoat cycle is not desirable. Instead, the best approach is to vary the pressure drop by reducing it as precoat thickness is reduced, so that the required flow of gas is minimized, and also so that the tendency of high gas velocities to break down precoat bridging is no problem.
Optimum Conditions 170 to 175
Oil temperature, O F. Drum speed, minutesirevolution Knife advance, inch/revolution Precoat material Precoat thickness, inch Maximum Minimum Pressure drop, Ib./sq. inch Maximum Minimum Gas rate, cubic feet/minute/sq. foot” Maximum Minimum Gas temperature, O F.
2
0.001 Dicalite no. 372 11/2 1/2
40 20 6
3 180 to 200’ F.
a Cubic feet/minute/sq. f t . of installed area, gas measured a t compressor suction conditions of 170’ F. and 2 lb./sq. inch gage.
The oil temperature shows a very marked effect on filtration rate, indicating a straight line increase in rate with an increase in temperature (Figure 4). As cooling to 50’ F. is necessary in the next stage of the process to precipitate wax, it is undesirable to raise the temperature too high in view of the over-all operating costs. Present plant practice shows that 175’ F. is a satisfactory temperature in the bleach step. The pressure drop also has a very VOL. 49, NO. 1 1
NOVEMBER 1957
1867
30’1
Figure 4. Effect of oil temperature on filtration rate
160
170
180
200
190
OIL TEMPERATURE,
210
O F
CONDITIONS PRECOAT 30
THICKNESS
I”
162OF
OIL TEMPERATURE DRUM SPEED
25
Figure
5. Effect
of pressure drop on filtration rate
10
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I
c’
2
(0
2I
> -I a a
20
w-
tLT w
Figure 6. Effect of drum speed on
5a
2
filtration rate
U
IO
fi MINUTES PER REVOLUTION
1868
INDUSTRIAL AND ENGINEERING CHEMISTRY
marked effect on filtration (Figure 5). Higher pressure drops give higher filtration rates, but there are some disadvantages-eg., as the precoat layer decreases in thickness, the gas rate a t a high pressure drop becomes very excessive. This results in both poor oil quality, due to destruction of precoat bridging, and also high operating costs, due to higher than necessary gas circulation rates. T h e recommended operating conditions are based not only on pressure drop and its effect on filtration rate, but also on the initial investment, operating costs, and the effect of gas velocity on oil clarity. Another factor is that a t a given diameter of a drum filter, precoating increases the length and available filtering area, and is relatively slight in cost. Therefore, a filter sizing which will handle the required flow a t lower operating costs is more desirable than a smaller filter, which is not much lower in cost, but has higher operating costs. Drum speed has less effect on filtration rate in this case than is normally found because the precoat layer is a substantial factor in total resistance, coupled with the high viscosity of the corn oil. Figure 6 indicates the effect of drum speed on filtration rate, while Figure 7 shows the effect of precoat thickness. From these curves, variation in drum speed is not nearly as significant as variation in precoat thickness. For example, Figure 7 shows that at given operating conditions a decrease in the precoat thickness from 2 inches to 0.5 inch results in an increase in filtration rate of 12391,. This finding was important, because in the precoat filtration of an aqueous slurry, the deposited cake resistance is generally the dominant factor and variation in precoat thickness is much less significant. Operation a t a low drum speed gave a longer precoat life and lower precoat cost per gallon of oil produced, with only a moderate reduction in filtration rate. However, a t low drum speeds a substantial portion of the precoat layer in the drying zone was completely purged of oil, increased the gas permeability, and required a materially higher gas flow to maintain the pressure differential across the filter. This results in higher power costs and greater compressor capacity requirements. Under these circumstances, deeper submergence, or the use of misting nozzles or a drip bar to keep the precoat from drying excessively, would reduce the gas flow. Knife advance, or crcut,’swas established by gradually decreasing the depth of cut and observing the effect on filtration rate. The proper operation was lost a t or below a knife cut of about 0.0007 inch per revolution (Figure 8). Deeper cuts did not materially increase the filtration rate. For normal operation the value of 0.0010 inch allows a certain safety factor. In actual full-scale operation this can probably be reduced. The
C O N T I N U O U S PRESSURE F I L T R A T I O N knife advance is determined by the amount of penetration of the precoat by fine solids from the cake. If the cut is not deep enough to remove these solids, the surface becomes less permeable and the precoat shrinks from the knife due to compression by the gas atmosphere. When this occurs filtration slows down for several revolutions until suddenly the knife engages the precoat again. As it begins to cut and to let the gas through, the precoat expands, resulting in a thick cut and an increase in rate. Then gradually the surface closes after a few drum revolutions and the cycle repeats itself. This phenomenon is termed "breathing" and represents unsatisfactory operation. The effect of changes in the percentage of solids in the slurry was very small, as the slope of the curve in Figure 9 is practically zero. The range covered in this chart is the normal range of variation and plant use. According to Figure 9 the precoat layer contributes the major resistance to filtration. As a result, changes in deposited cake thickness exert only a minor effect on over-all rate.
0
\
\
Figure 7.
Effect
of precoat thick-
\
ness on filtration rate
CONDITIONS 30 P S I it MPR
PRESSURE DROP DRUM S P E E D OIL TEMP.
170 " F
2"
I" THICKNESS
OF PRECOAT
Discussion The pilot plant step in developing the process for continuous pressure filtration of corn oil was necessary to prove the feasibility and possible economic savings of continuous processing. Pressure bomb leaf tests performed duplicated the fullscale filter cycle over a wide range of operating conditions and permitted the investigation of temperature effects, pressure drop, cake forming and drying time, and feed solids concentration. From these data, full-scale filtration rates were calculated and later were verified in the pilot plant work. However, the leaf tests did not show the effect of operating time on precoat material, minimum required knife cut per revolution, precoat life, and operating requirements for a continuous precoat subjected to high temperatures, pressures, and organic viscous liquids. The pilot plant study was justified by the company specifications on product quality. The filtered oil must have a brilliant clarity which must be maintained throughout the precoat life cycle with the normal supervision given any continuous processing equipment. The continuous pressure filtration must meet this requirement which could be established only by a pilot plant operation over a long duration. During the pilot plant work, it was determined that excessive gas velocities through the precoat resulted in particles of diatomaceous earth passing into the filtrate yielding an off-specification corn oil. Accordingly, by maintaining constant gas rate and thus decreasing the
Figure 8.
Effect
of depth of precoat shave on filtration rate
0
-
PR EC OAT " B R E A T HI N G",
+.
.001
KNIFE
.DO3
.002
ADVANCE,
INCHES
PER
REVOLUTION
r
.30
-1
a
Figure 9. Effect of slurry concentration on filtration rate
c3
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-
CONDITIONS PRESSURE
DROP
29 p s i
DRUM SPEED la- MPR OIL TEMP. 168 O F PRECOAT THICKNESS
IF
VOL. 49,
NO. 11
NOVEMBER 1957
1869
pressure drop as cake thickness decreased, both filtration rate and filtrate clarity can be obtained. The pilot plant operation showed that the maximum precoat cake thickness for this viscous liquid is approximately 1.5 inches instead of the normal 3 inches. This maximum is yet to be verified in a full-scale installation, but is considered a reasonable place for starting up a fullscale unit. As the deposited filter cake resistance is small compared to the precoat, filtrate rate a t any temperature and cycle time is a direct function of the pressure drop and precoat thickness. Greater maximum precoat cakes result in lower design filtration rates and greater precoat consumption per 1000 gallons of corn oil. Lesser thicknesses yield less time per precoat which would also lower design filtration rates. Besides the automatic control on the pressure drop across the filter, the pilot plant work demonstrated the need for filtrate receiver and filter tank liquidlevel controls and temperature control of the compressed gas. Liquid-level control in the filter tank prevents overflow in case the filtration rate is lower than feed rate. This should not happen in normal operation, but it is desirable to install a high level control to shut off the feed pump if the liquid level should build u p too high. In addition, a system of back-pressure control for drying gas can be used to operate by liquid level in the filter tank rather than the shell pressure. A liquid-level controller which operates by the buoyancy of the partly submerged float, and which develops a pneumatic signal proportional to this buoyancy, should be installed. This pneumatic signal can then be used to operate a pneumatically controlled tlirottling valve in the gas stream leaving the receiver. At the high level set point, this float type of controller would also shut off the feed pump. Liquid-level control on the filtrate receivers is required to maintain a liquid seal and consists of the same float type of liquid-level sensing device--the pneumatic signal for operating a pneumatically controlled throttling valve in the filtrate outlet line. A high level alarm, fitted to this sensing device, could act as an alarm or cut in an auxiliary pump or any other emergency procedure. This is desirable in actual operating to prevent the carry-over of oil into the compressor. Temperature control on the high pressure gas is necessary, for the precoat layer would be cooled if the gas entered a t a temperature lower than slurry temperature. This would increase liquid viscosity and decrease filtrate rate. The heat of compression is partly removed in a gas cooler, and the desired outlet temperature from the cooler is controlled by throttling the cooling water flow. Care must be exercised in sizing and design of the aftercooler so that
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.cooling water capacity will not be too high, as this would increase a lag in the process. Also, a gas precooler should be used before the cornpressor to reduce the temperature of the gas entering the compressor suction and thus minimize the power requirement. This would require a heat exchanger with a manually set cooling water flow. In the pilot plant tests, it was established that the diatomaceous earth requirement for continuous pressure filtration is approximately the same as that for the present recessed-plate filters. However, less labor is required for operation of continuous pressure filters than for cleaning the filter presses. Additional savings are the filter cloth requirements. Other advantages of continuous pressure filtration which are not easily compared economically are-prevention of contact of corn oil with air and consequent oxidation, elimination of a difficult housekeeping problem, and removal of a fire hazard. Application of continuous pressure filtration, both precoat and cake filtration, lie primarily in seven categories : 1. Batch filtration is presently employed and economics would greatly be improved by continuous filtration. 2. The liquid vehicle possesses a vapor pressure at the processing temperature which could result in either serious volatile losses or low filtration pressure drops if continuous vacuum filtration is employed. 3. The feed slurry requires a higher pressure drop than can be obtained through vacuum filtration to achieve favorable filtration rates. 4. Liquid viscosities can be greatly reduced and filtration rates significantly increased by employing higher temperatures which make continuous vacuum filtration impossible. 5. Crystallization or “setting-up” of liquids resulting in stoppage of the filter should be eliminated. This filter stoppage can be caused by vaporization of the liquid and resultant cooling, and can be prevented by using above atmospheric pressures at all points. 6. Explosion or flammability hazards should be eliminated by substituting an inert gas such as nitrogen. 7. Economic utilization of process pressures and temperatures existing prior to the filtration step. Many of’ these categories will be found in the fields of chemicals, petroleum, foods, and pharmaceuticals. A few particular applications, already investigated or installed, are filtration of sodium sulfate from acetone and phenol obtained from cumene, removal of lime from petroleum tar, deashing of coal tar, filtration of Irish moss extracts (a viscous and gelling agent), dewatering of yeast, filtration of fatty acid derivatives, and filtration of such hazardous materials as metallic sodium products. Devised methods have proved to be accurate so
INDUSTRIAL AND ENGINEERING CHEMISTRY
that scale-up can be made from pressure bomb leaf tests in most cases as is done with vacuum leaf tests in continuous vacuum filtration. However, in many instances pilot plant tests will also be required in order to determine economics more closely or where the operation is so complex that the flow sheet cannot be completely determined through leaf tests. In this present investigation, the product quality is so strict that the entire flowsheet must be tested on the pilot plant level. If the pilot plant is to be employed, then the unit must be flexible enough to observe the effects of 1. Temperature 2. Pressure 3. Cycle time 4. Submergence
In addition, such factors as feed solids concentration, filter media or type of precoat, cake washability, and particularly the type of flowsheet will probably need to be investigated. In the pilot plant it should be possible to study all of the following flowsheets: 1. Automatic or manual control of filtration rate. 2. Control at maximum filtration rates at all times so that feed rate is controlled only by slurry level of the filter tank. 3. In cake filtration, it should be possible to permit any desired pressure drop across the cake formation part of the filter q-cle while controlling back pressure on the cake drying part at a different level. This would allow the use of a minimum amount of compressed gas to dewater the cake to the required degree. 4. Control of compressed gas temperature at any desired level so as to maintain filtration rates or eliminate cooling of the filter cake. The unit employed in this test work would allow the investigation of all of these variables. Control is rapid and can be easily changed by the operator. In addition, the unit is mounted on a platform so that the only outside items required are power, cooling water, a fwd line, and a precoat mix tank whrn needed. Thus. the set-up time is minimized and floor space requirements are nominal. Acknowledgment The authors express their sincere thanks to the Corn Products Refining Co. for permission to publish these data and this information. Special thanks are given to the many people in that organization who cooperated in this test work and to Paul Sheffer of the corn oil refinery and associated facilities.
RECEIVED for review October 30, 1956 ACCEPTED May 8, 1957 Division of Industrial and Engineering Chemistry, 132nd Meeting, ACS, New
York. N. Y.. September 1957.
