FILTRATION EQUIPMENT
APPLICATIONS The practical plant pararn eters con troll in g selection, change, a n d operation of filtration equipment, a n d some suggestions as to how they can b e applied by R. H. VAN NOTE and F. T. WEEMS, The Eimco corp., New York, N . Y.
TL
FINANCE
/n I
\
OPERATIONS
By establishing a merit system for each of the dimensions of consideration which enter into the selection of a filter, and b y reducing this merit rating system to numerical values, even i f only comparative, it is possible to approach a numerical evaluation for each of four basic dimensions which enter into equipment selection. These can be plotted so that they can be balanced one against the other and an optimum solution obtained. It i s suggested here that the four basic dimensions should be process and product, maintenance, operation, and finance. Under normal conditions, it is felt that if these parameters can be assigned numerical dimensions and plotted as shown, the optimum point would be equidistant from the four apexes. If for any reason a selection is made a t a point other than this point, it must be done with the realization that the maximum profitable return available to the processor i s in jeopardy.
546
INDUSTRIAL AND ENGINEERING CHEMISTRY
O N L Y JuSTIFIABLE REASOX for a piece of process equipment, filtration or otherwise, being installed in and operated by a chemical processing plant is to enable the owner to conduct business at maximum profit. During the present intense economic and technological strain. it becomes increasingly important to establish and evaluate the parameters which determine success in achieving this purpose. This article is concerned with appl>-ing for practical uses, the principles of filtration, and presumes that the equipment manufacturer can supply filtration equipment which satisfies completely the prescription developed through the precise balance of the requirements of management, labor, maintenance, process, and product considerationu. A four-dimensional diagram could be developed in which the parameters would be :
0 Process and Product 0 iwaintenance 0 Operation 0 Finance Process and Product refers to all aspects which are concerned with stoichiometry, conditions, yield, and product quality. Maintenance shall include all items of replacement and prevention as well as the labor associated with these items. Operations pertain to those people who are concerned with conducting the process and the tools which they have available to them for either intelligence or function. Finance shall be used to establish all items of accounting and revenue both into and out of the owner’s organization. Thus, finance would cover increased revenues which would result from improved product quality, as well as irems
Use These Contributing Dollar Costs to Establish Priorities on Available Effort Item
Cost
Deprec.
DRUM FILTER 30,000 VACUUM PUMP 6,000 FILTRATE PUMP 1,200 PIPING 8, VALVES 15,000 RECEIVERS 1,000 CONTROLS 2,000 ERECTION 12,500
Services Operating
500 2,500 500
5,000 1,000 500
3,000
-
-
200 400 2,500
-
6,000
2,000 400
of depreciation, interest, and taxes resulting from capital expenditures. The perfect solution to the filtration problem would be defined by a point equidistant from the four apexes of the diagram. Selection of a filter not defined by this point should be made only under extremely special circumstances. Even then, the basic purpose of maximum profitable return from the equipment is in jeopardy. Further, it is highly improbable that the perfect solution can be reached without careful analysis of the significant factors, and recognition of their relative value. I n this evaluation, care must be taken to give each item its proper weight in relation to its over-all contribution. The literature offers suggestions for assigning numbers to these factors and certain basic rules may be applied, vendors may be asked to submit proposals, or extrapolations may be made from previous experience. Broadly, the cost break-out on a new filter installation would consist of four approximately equal components :
-
-
100
Maint.
Total
1,000
12,500 6,000 1,900
500 500 500
3,500 200
100
700 2,500
-
100
-
-
Process and Products In analyzing the profitable selection, the function of the filter must be carefully considered. There are several basic layouts in simplified form which fall into three categories:
0 Layout controlled by feed or product stream 0 Layout controlled by a foreign
liquid stream 0 Layout controlled by the gas stream Feed or Product Controlling. In the simplest of all the layouts, a slurry is bulk separated into two components. The valuable product may be either the liquid or the solid, or both. Normally,
this bulk separation results in some filtrate being left with the filter cake, and some feed solids being retained or suspended in the filtrate. By precoating the filter, however, a brilliant filtrate could be obtained from this layout. The problem suggested is one of extracting the menstruum from a slurry in its purest form. The first filtrate will contain suspended solids: hence, it is collected in a separate receiver and is recast to the feed tank for recycling. This technique is based on the fact that the filtrate becomes progressively less contaminated with suspended solids as the filter cake becomes thicker. After a comparatively thin cake is formed over the filter medium, the filtrate will be essentially clear, and may be collected in a separate receiver as product. These layouts are shown in Figure 1. Liquid Controlling. Where the solids product is to be as free as possible of mother liquor-either because the liquor represents a contaminate to the cake or because the liquid represents the valuis normal to wash able product-it the cake which has been formed during the filtration operation with a foreign liquid which is miscible with the filtrate, but not necessarily of the same chemical make-up as the filtrate. The drum filter shown is a typical machine used where cake must be washed (Eigure 2). This same notion has been extended to include counter-current use of the wash
I
c---------L
BULK
SEPARATION
G$S
I 10--J I
I
n
Equipment
VACUUM PUMP
Building and Services Piping and Wiring Engineering and Construction
4
In making the selection and installation of a new piece of equipment, consideration must be given to each item contributing to the profitable production of goods. For example, the items contributing to the cost of a simple drum filter station are given in the table above. The relative over-all importance of the various items demands that the available effort be allotted on a nonuniform basis proportional to the contribution of each. It is also important to note that the allotment within any given category does not necessarily follow that of any other; therefore, subdistributions must be made when assigning responsibilities and when making selection.
CAKE
I
I
CLOUDY
FILTRATE
I Figure 1.
F---T---
Gf S
RECYCLE
i
I
RECYCLE
-- 4 0 - i VACUUM PUMP
I
FILTRATE
Where feed or product is controlling, these layouts may b e used VOL. 53,
NO. 7
JULY 1961
547
Figure 2. A typical drum filter installation, used where mother liquor must be washed out of the cake
liquid for better washing of the cake, and if necessary, leaching of material from the solids of the cake. This technique may also be used in the introduction of surface finishes to the solid product, as in catalyst operation. Base exchange may be practiced as a means of eliminating certain ions from the solids components of the filter cake and replacing these ions with others more desirable (Figure 3). Gas Controllinrz. I n certain cases, the entire layout L u s t be studied with
I _
I
primary regard for a gas stream, rather than for the solid or liquid stream (Figure 4). One situation which is common is that of high condensables in the gas stream. This can result either from the gas stream being preheated before passing through the filter cake, or from the liquid components having exhibited a high vapor pressure. I t is often necessary or desirable to install a condenser in the gas stream to collect the condensable va-pors.
.
I
WASH
+CAKE
COUNTERCURRENT
WASHING
01
LEACHING
CF
SollPS
Figure 3. Where the liquid is controlling, this layout provides for washing or leaching of the cake
548
INDUSTRIAL AND ENGINEERING CHEMISTRY
In some cases, the vapor could be either toxic or explosive. Here thc gas stream is recycled from the exhaust of a vacuum pump back to the hood, which completely encloses the filter. I t is normal to use a n inert gas under these conditions and to install a condenser before the vacuum pump or an after-cooler after the vacuum pump. I t is normally necessary to use a system for removing the heat of compression from the vacuum pump. I t is common to use as the inert gas in a system such as this either flue gas or nitrogen. Another modification of the basic filtration technique is to preheat the gas stream to introduce enough heat into the filter cake to evaporate the residual liquors present. I n this case, normally about lOy0 of the filter surface is devoted to cake formation, and about 80% of the filter surface is devoted to drying the filter cake with hot gas. The cake, in some cases. is washed between the formation zone and the drying zone. Normally, a unit of this type is applicable only to very granular materials which will pass fairly large quantities of hot gas. This system offers the advantage over separate dewatering and drying device by being lower in installed cost and much more gentle as concerns product or crystal degradation. The foregoing indicates that broad generalizations concerning filtration equipment must be used with caution. Obviously, the degree of control required for proper operation of these various layouts, and the relative cost of the filter as compared with the auxiliaries will vary widely. O n the other hand, any given layout after having been carefully analyzed with a view to its exact function would lend itself to a diagram of the type shown previously.
F/E A P P L I C A T I O N S
CAKE
Figure gases
4. Where the gas is controlling, these layouts provide for recycling dangerous fumes, and for drying the cake with hot
The selection and design of filtration equipment will be strongly influenced by certain details of the operation as concerns the chemistry and yields from the process. This can vary from the problems of materials of construction due to such operating requirements as pH, solvents, the presence of halogens, and temperature. But even more specifically, certain direct influences can be the result of equipment selection and application. The rotary vacuum drum filter is probably the most familiar of the continuous filters. This type of unit however, cannot be expected to serve all applications. In the table, at right, this and other available types of filtration equipment are shown, each identified according to their driving force. Maintenance I n selecting filtration equipment, the application of any given design must be viewed very carefully as to the over-all maintenance requirements. Of primary concern is the local condition of labor and machines available within the plant. For remote locations, it is often necessary to eliminate the more complicated designs of filtration equipment, even though they might be attractive from other points of consideration. Where small maintenance staffs and comparatively small shop facilities are located, one must immediately recognize that either large supplies of expensive spare parts must be kept on hand, or the risk of expensive shutdowns must be considered. I n any case, these two items should be considered in the accounting to exactly the same extent that the first cost of equipment would be considered. The cost of changing the filter medium on the operation machine must also be weighed. For example, on the continuous belt drum filter, it is possible for an operator to change a filter cloth in 10 minutes by the use of plastic zippers to attach the ends of the belt. O n the corresponding counterpart, the drum filter, it is normally necessary for two or three men to devote.as much as a full
FILTER CLASSIFICATION Driving force, relative washing ability, and normal range of solids in the feed should b e considered when chosing a particular filter design
VACUUM
WASH 0
CONTINUOUS DRUM SCRAPER DISCH.
100
FEED SOLIDS I 5 IO 20 +
STRING ROLL BELT TOP
FEED
HOPPER DEWATER. CONTINUOUS
HORIZONTAL
SCROLL
DISCH.
TILTING
PAN
BELT CONTINUOUS
DISK
CON TIN UOUS PR ECOAT BATCH
LEAF
PRESSURE CONTINUOUS
DRUM
CONTINUOUS
PRECOAT
BATCH
P L A T E & FRAME
BATCH
V E R T I C A L LEAF
BATCH
TUBULAR
HORIZONTAL
ELEMENT
EDGE
CARTRIDGE BATCH
HORIZONTAL
LEAF
CENTRIFUGAL shift to making a cloth change. Consideration must be given to the frequency with which the cloth would be changed. This maintenance labor must be brought down to a matter of dollars and cents and properly credited as the cost item in making a comparison or selection. I n many cases, maintenance considerations represent items of detail design of the basic filter. I n other cases, the maintenance consideration goes
far beyond detail design of the individual machine and becomes a n inherent characteristic of the filter. High maintenance cost need not of itself be the deciding factor in the selection of the machine, but must be given its proper weight. Much of the maintenance around a filter station can be attributed to the auxiliaries. I n the case of vacuum filtration equipment the use of water YOL. 53, NO. 7
JULY 1961
549
Continuous belt drum filter-the
-PROCESS
filter cloth can be changed in only a few minutes
flushed vacuum pumps can materially reduce, if not eliminate, the major maintenance problems associated with carryover of corrosive gases, liquids, or solids in the gas stream. I n many applications, the decrease in maintenance costs more than outweighs the increase in equipment costs, the additional power consumption, and the somewhat lower vacuum level obtainable with this type of equipment. Similarly: the use of filtrate pumps designed on the basis of good “hot well” practice eliminates many of the problems associated with extracting filtrates from vacuum receivers. Cavitation and vapor binding can be eliminated by the use of slow speed centrifugal pumps having large suction inlets and designed for extremely low NPSH characteristics. I n addition, a pump can be essentially self priming and much less sensitive to the state of repair of the seal or packing- on the rotating drive shaft.
REQUIREMENTS AFFECTING DESIGN
,Clarity of Filtrate Required. T h e selection of the filter medium, and the basic design of the filter influences both the clarity of the filtrate and the production rate. Many times this results in a compromise between clarity and production rates. Where brilliant filtrates are required, precoating is usually carried out either by the introduction of filter aids such as diatomaceous earth or by the preliminary deposition of cake solids. ,Cake Purity. Where the cake is to be freed of mother liquor to avoid contamination of the cake solids, it is essential that the residual moisture clinging to the particles be displaced by washing with an acceptable liquid. Where extreme purity is required. it is often necessary to conduct the washing operations through several stages of filtration, either by repulping external to the filter or internal with the filter. ,Recovery of Values. Where the valuable product is in solution in the mother liquor, washing of the cake to displace the values and make them available for recovery becomes essential. This is sometimes done in countercurrent fashion to minimize the over-all dilution of the valuable stream.
Tilting pan filter i s designed for maximum recovery of valuable mother liquor
,Product Degradation. The physical nature of the final if a fragile crystal-can be materially product-particularly influenced by the physical discharge technique employed in the filter cycle.
,
Vapor Considerations. Where vapors from process are toxic or otherwise hazardous, it is necessary to confine the equipment, during the complete filtration cycle. This will usually result in complete enclosure of the filtration equipment to confine the vapors and possibly to eliminate oxygen from the system. Once equipment must be totally enclosed, the over-all design and selection of the filtering element becomes critical. The geometry of the unit, its ability to discharge cake, and the degree to which the device lends itself to automation all become important considerations.
,
Sanitation and Housekeeping. Equipment selection is often dictated bp the degree to which the equipment can be maintained in a completely sanitary condition if the end p r o d u a is to be used for food. Similarly where the material being handled is a toxic solid, it is necessary to provide enclosures and often provide complete automation to eliminate exposure of human beings to the solids.
This hydrocarbon clarifier shows a totally enclosed installation
F E / A P P L I CAT1 0 NS
This filtrate pump is mounted in a large-diameter flange at the base of the filtrate receiver
Operation The labor relations of today require that special attention be given to the human being required to operate the selected filter. T h e items to be conconsidered normally fall into four categories : 0 Amount of attentive time required 0 Level of concentration or intelligence involved 0 Physical strength or endurance required from the operator 0 Toxicity or discomfort and the general housekeeping of the station
Most of these can be related either directly or indirectly to an out-of-pocket cost which must be assigned against the processing cost. Care must be taken to assign properly the amount of time which is actually required in the operation of this equipment. The level of intelligence and degree of concentration will usually establish the scale required of the laborer and becomes an important factor in establishing the over-all cost. Another important consideration in level of intelligence must involve the likelihood
of this intelligence being available. This is often influenced by local conditions and availability of intelligent labor. I t can be an extremely important factor in dictating the degree of automation applied to equipment or even the acceptability of equipment for the purpose intended. Physical strength and endurance, and the degree to which they can be made available on a dependable basis, can influence the selection markedly. Overoptimistic evaluations of what can be accomplished on a continued day-inand-day-out basis might be materially less than would be indicated for short periods as, for example, during test work. It is, of course, essential that materials which are toxic be kept away from the human being operating the machine. In many cases, the presence of unpleasant, though nontoxic vapors and fumes can materially hamper the progress of a workman. Where these problems of nuisance exist, the equipment must be isolated from the operator, or the operator must be protected through proper ventilation, clothing, etc., if he is to be effective. Housekeeping can be a n important
These rules are useful for estimating costs For Capital Equipment: Cost is proportional to t h e 0.6 p o w e r of capacity over fairly small ranges.
For Power Operating Horse Power: Cost normally ranges from $80 to $1 15 per horsepower year.
Pumping Cost: N o t including first cost, costs a r e approximately $10 per gallon p e r m i n u t e for p o w e r and $10 p e r gallon p e r m i n u t e for maintenance, calculated on an a n n u a l basis. For specific industries and applications, much more accurate values are available either in the literature or within general information easily available in each industry.
consideration since it represents an outof-pocket cost through labor, and normally it is of great necessity. Where corrosive splash and drippage are inherent in a design or technique, the cost of maintaining the area around the equipment must be properly assigned to the equipment. Where a special painting, shielding, or protective coating is inherent with the filter, these must be reduced to numerical values.
Finance The usual problem of first cost and the distribution of the first cost enters very strongly into the selection of filtration equipment. T h e first cost must be distributed over the expected life of the equipment or the process. O n the basis of this distribution, and consistent with the local laws, the items of taxes must be applied. Depending on the money market over the life of the machine, proper assignment of interest rates and other money charges must be made. None of this is unique, nor different from normal accounting procedures; but it does require diligence and careful consideration since depreciation charges and tax rates are not necessarily the same for all components of a filter station. T h e cost of housing the equipment, and the erection of the equipment including design and layout must be properly assigned. Since the accounting techniques are more easily reduced to numerical values, the financial balance sheet will probably be the easiest to obtain, and is apt to be the most accurate and clearly defined dimension in pinpointing the equipment to be selected. RECEIVED for review April 15, 1961 ACCEPTEDApril 20, 1961 Division of Industrial and Engineering Chemistry, 139th Meeting, .4CS, St. Louis, Mo., March, 1961. VOL. 53, NO. 7
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JULY 1961
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