PRESSURIZED CENTRIFUGES AND FILTERS C . L. A M E R O
ty years ago, continuous centrifuges were splashFproof f units’ at best, and rotary drum filters were limited to filtering pressure drops approaching, but never exceeding, one atmosphere. During the last 15 to 20 years, volatile and toxic slurries have been processed in vapor-tight machines sealed against gas flow into or out of the system. This equipment seldom operates with case pressures above 5 p.s.i.g., since vapor pressures involved are less than atmospheric. Now, however, continuous centrifuges and rotary drum filters are available in enclosures which meet the requirements of coded vessels for operation up to 150 p.s.i.g. Process engineers need no longer restrict these valuable units to low pressure operations, or suffer the serious energy loses resulting from pressure and temperature reduction for solid-liquid separations. Recent demand for pressurized machines stems from two basic sources. Equilibrium vapor pressure often exceeds one atmosphere at the optimum processing temperature; rotary drum filtration requires much higher pressure to do the necessary job. Higher processing temperatures may prevent precipitation of soluble solids from saturated filtrate solutions, thus retarding or eliminating cloth blinding from this ource. It mav keep soluble impurities dissolved in the other liquor so that good cake purity can be obtained ’th displacement washing rather than repulp washing. igh temperature or pressure may be dictated by the ’ etica of the process reaction or as aids in overcoming desirable high liquid viscosities.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Continuous centrifuges are now in service at temperatures and pressures up to 350’ F. and 150 p.s.i.g. Key to their successful use in high p m u r e service. is suitable shaft seals, properly adapted to centrifugal equipment. High speed pumps, agitators, and turbines have used pressure seals for many years. But pressurized centrifuges present dflerent seal problems: Negligible barrier fluid leakage around large diameter shafts; use of dw, inert gas as a sealing fluid; and a minimum continuow operating l i e of some 6000 hours. Centrifuge seals are usually “balanced” to ensure that the force e x e d on the primary sealing face is independent of the contained preSsure. With this feature, their life expectancy and proper functioning are not affected by operating pressures up to the ultimate strength of the sealing elements. In some instances optically flat, eight-inch diameter seals are operatin successfully with shafts turning at more than 2500 r.p.m The only leakage to and from the machine is bam fluid, when a conventional double seal is used. Clarification rate of continuous solid bowl centrifug depends primarily on solids settling velocity. Te settling velocity for sinal1 particles is controlled by viscou
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L. Amno, a regis6md professional mgincar I Massachusetts, is Laboratory Director fm Bird Mmhiw Co. Soufh Wa[pole, Mass.
AUTHOR
*
resistance to settling and the net effective centrifugal force applied. At equilibrium, centrifugal force equals the force of viscous resistance (7) :
*
The left-hand side of Equation 1 shows that the net effective centrifugal force acting on a suspended particle at a given radial acceleration varies only with the density difference between solid and liquid. Terminal settling velocity can be calculated from Equation 1 by rearranging (2) :
mounted, rotating valve which separates discharging fluids. Single cell filters do not use internally divided drums, thus minimizing restriction to gas and liquid flow, contamination of mother liquor with wash, and total pressure drop between filter medium and fluids discharge. Filtration relationships are usually described by the modified Poiseuille equation in which volume or filtrate collected per unit area and time are related to pressure drop, liquid viscosity, weight of cake deposited, average suecik. cake resistance. and the resistances of the filter medium and the filtrate handling system: P
d _V -
-
High temperature $erefore improves performance only to the extent that liquid density and viscbsity are minimized. There is no change in efficiency of centrifugal separation due to high pressure in itself. Prorrurizd Filters
High pressure, continuous filters with dry solids discharge are rotary drum units. Mechanical difficulties and stresses met from enclosing horizontal or traveling belt-types make them impractical for pressurized operation. Rotary drum filters may be either the conventional multicompartment or single cell units. Liquid and gas in multicompartment units are piped to an externally
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Eauation 3 remesents the instantaneous rate of -1 filtration per unit area as a ratio of pressure to the product of viscosity and the sum of cake and other resistances. High pressure drop filtration of low vapor pressure slumes can be justified, therefore, only if the greater driving force more than overcomes the increased specific cake resistance of compressible solids and/or the resistance produced by high liquid viscosity. For solids which require a high filtering pressure drop, line and filter medium resistances can be made negligible compared to that of the cake. (Contimud on mxrpogc) VOL 55
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Pressurizing sharply increases machine cost; savings must come from lower There are many inskinces in which pressurized filtration is advantageous even though high ~ u r drop e is not necessary or particularly deirable for good operation. When prcasurization is required because of equilibrium vapor pressure at the optimum processing temperature, adequate total preasure must be kept on the entire system to prevent excessive vaporization, filtrate cooling, and severe l i e resistance to flow which could occur at the lower pressure level. For example, separation of synthetic polymers from organic liquids often must be accomplished with a imall temperature drop or a minimum final temperature IO prevent precipitation of soluble impurities. Filtering ire drop in this type of application seldom exceeds .g. while rates of 140 pounds per hour of solids and 300 gallons per hour of feed per square foot of filter area r e common. Filter cake pomsity usually permits gas flow rates averaging 60 cubic feet per minute per square foot of total drum area. A liquid l i e hexane with an atmospheric boiling point of 64" C. can be filtered at 75' C. in equipment designed to hold only 7 p.s.i.g. total pressure. With filtering pressure drop of 5 p.s.i.g., receiver pressure level is adequate to prevent leakage of air into the system. Under these conditions, however, temperature drop exceeds 8" C. and vapor flow rate increases more than 60%. A 50-square-foot filter discharging 7000 pounds per hour of washed cake would be required to
handle approximately 350 gallons per minute of wash and mother liquors in addition to 4800 cubic feet per minute of vapor. If pressure level on the entire system were iucreased with inert gas to permit 30 p.s.i.g. in the filter case, the same feed, feed temperature, and filtering pressure drop could be handled with less than a 3" C. temperature drop. Vapor flow rate would increase only 15%, keeping it well within the volumetric handling abilities of existing commercial filter deaign. And need for oversize piping and auxiliaries would be eliminated. Approximate flow conditions around the filter drum can be calculated at the above temperature and prgsure levels by assuming that the perfect gas laws apply and that the separation occuls adiabatically. As mother liquor passes through the cake and filter medium its sensible heat will be used to vaporize liquid until equilibrium conditions at the lower pressure and a new temperature are satisfied. Actual operating flows will be less than the calculated values to the extent that nonequilibrium conditions exist. Pressurized filters for low pressure drop differ from vapor-tight varieties only in the design of case and seal components. Strength and sometimes configuration of the case are altered so that ASME coda for pressurized vessels are met. Wide latitude is possible for sealing mechanisms since filter speeds rarely exceed 20 r.p.m. so that stuffing boxes with shafts up to 18 inches in
Figure 3.
Prcssmud J h r inskdlalion shom'q locatim and purof conlrols. Point of lowest absolute plcrsurt--or the id41 to
Jiltrat# lrmfcr pUntpir th# b m from which other prcrnrrcs and
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INDUSTRIAL AND ENGINEERING CHEMISTRY
FEED
energy needs or faster throughputs diameter can be used. Face contact or circumferential seals are available when specified but hemme quite expensive in large diameters because of the manufacturing precision needed to make them operate with low leakage rates. Generol Considerations
Even for a moderate 50 p.s.i.g. operation, pressurized centrifuges cost almost 50% more than vaportight and approximately twice as much as unpressurized units. Normal piping sizes can be used to transport fluids discharging from centrifuges, and case pressure can be maintained close to the equilibrium vapor pressure because operation does not depend on pressure drop. A nominal purge rate is usually required to prevent accumulation of inert seal gas which bleeds into the system, but even this can be overcome when a liquid compatible with the process is specified as the barrier fluid. Seal controls and main motor are the only auxiliaries ,required, and high capacity is possible with compact equipment. Total cost of a pressurized rotary drum installation can be minimized only if the maximum possible processing rate per square foot of filtering area is used. Since, as Equation 3 shows, filtrate rate (and thus wash liquor rate) varies inversely with the weight per unit area (or thickness) of deposited cake, it follows that thin cake filtration is most desirable for high unit area capacity.
Figure 4 is a typical filtration curve relating total filtration volume and dry solid pickup per unit area to active filtering (or cake form) time at a given pressure drop. Cake thickness varies with form time which in turn can be varied substantially with drum speed and to a lesser extent with drum submergence. According to Figure 4, a filtrate rate of 3.3 gallons per minute and a dry solids rate of 1.42 pounds per minute would result per square foot of filter area at a drum speed of 1.5 r.p.m. ifa lo-second form time were 25% of the total cycle. With a three-second form time, however, drum speed would increase to 5 r.p.m., filtrate rate to 6 gallons per minute per square foot, and dry solids rate to 2.6 pounds per minute per square foot with the same cycle. Change in drum speed alone would permit the same machine to prcduce at an 83% greater rate. For slurries such as these it is essential that a filter handle high liquid and gas flow rates with negligible line resistance 80 that the major pressure drop is expended across cake solids only. To ensure clean separation of wash liquor from mother liquor, common piping for both liquids must be avoided, and practically unrestricted passage for filtrates provided between filtering medium and discharge. The unit must be able to discharge thoroughly on each revolution the thin cakes which normally r w l t at drum speeds of 5 or 10 r.p.m., but mechanical contact with the filtering medium for this purpose is undesirable because of rapid cloth degradation which occurs a t high peripheral speeds. One problem common to both iypes of pressure separator concerns the method of handling discharged cake prcduct. If repulping is contemplated, no difficulty exists since cake can fall into a v e d with liquid and be transported from it in slurry form. When repulping is not desirable, cake transport, especially to lower pressure levels, is a more complex situation requiring selection of equipment compatible with the nature of the wet solids. Various methods of accomplishing this such as variable pitch screws, seal lock valves to intermediate vessels, and reversing screw conveyors feeding alternately pressurized and open containers are now in successful operation. NOMENCLATURE
Ana of Filtering Surface PartideDiamcter Gravitational Constant TotalFksaunDmp Radius of Curvature of Path r r~ Line and Media Resistance V Volume of Filtrate V. Tcrminal or Equilibrium Settling Velocity w Weight of Dry Cake Solids per Unit Volume of Filtrate 01 Average Spsisc Cake Resistance 0 Angularvelocity e Time p Liquid Viscosity w DmsityofSolids p~ Density of Liquid A
d g P
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REFERENCES (1) Perrv. J~hn".."Chemical EneinmsHandbook." 3rd edition. ~I
page'995, Medraw-Hill, New-York, 1950. (2) Ibid., page 965. VOL 5 5
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