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INDUSTRIAL AND ENGINEERING CHEMISTRY
does not allow sufficient time for complete interaction. Thus these two influences tend to nullify each other. All of the tests in this investigation were made with the same binary mixture and in the same apparatus. Therefore the results can yield no information as to the effect of other combinations of substances and apparatus construction. It is known that the efficiency of a bubble plate varies with the properties of the mixture being distilled, so it is likely that the efficiency of a dephlegmator varies in an analogous fasyion. Although numerous dephlegmators of unusual design have appeared in the past, most modern equipment of this type has the same general design-a tube bundle and shell. The arrangement, size, and spacing of tubes and baffles, as well as proportion of length to number of tubes for the same surface area, might be expected to influence the performance of the dephlegmator
1233
Literature Cited (1) Hauabrand, E., (tr. by E. H. Tripp), “Principles and Practice
of Industrial Distillation”, London, Chapman and Hall,1925. (2) McCabe, W. L., and Thiele, E. W., IND. ENG. CHEM., 17, 605 (1925). (3) Murphree, E. V., I b i d . , 17,960 (1925). (4) Perry, J. H., Chemical Engineers Handbook, New York, McGraw-Hill Book Co., 1934. ( 5 ) Peters, W. A., J. IND. ENG.CHEM.,14, 476 (1922). (6) Robinson, C. S., and Gillilrtnd, E. R., “Elements of Fractional Distillation”, New York, McGraw-Hill Book Co., 1939. (7) Underwood, A. J. V., Trans. Inst. C h e w Engrs., 10, 141 (1932). (8) Webber, H. A., and Bridger, G. L., IND.ENQ.CHEM.,30, 315 (1938). (9) Young, S., “Distillation Principles and Processes”, London, Macmillan and Co., 1922. PR~DSENTED before the Division of Industrial and Engineering Chemirtry at the lOlat Meeting of the American Chemical Society, St. Louis, M o .
EVALUATING FILTER AIDS.
..
RICHARD N. COGGER AND
HARVEY M. MERKER Parke, Davis & Company, Detroit, Mich.
EXPERIMENTAL APPARATUS
T
HE ordinary run of commercial work in the field of filtration as a chemical engineering unit process can be placed in one of two categories. One class of industrial filtra-
tion is carried out with the main purpose of recovering the solid material or the sludge and discarding the filtrate. Another class is concerned solely with clarification, in which the sludge is the waste product and a clear filtrate is the desired object. Dewatering or sludge filtrations are usually characterized by the formation of filter cakes which are homogeneous and relatively incompressible, with a tendency toward being reproducible from batch to batch. It is seldom advisable or even permissible to add foreign materials as aids to filtration. On the other hand, the fluid products which require Eltration to produce clarity are usually complex mixtures or vegetable extracts and have formed organic
sediments which are nonhomogeneous, decidedly oompressible, and seldom reproducible from batch to batch. The usual prerogative of good practice in these cases is to add inert materials to aid in filtration. The difficulties which are met in attempting to analyze a given filtration problem are multiplied by the lack of uniformity of conditions in clarification filtration as compared with the characteristics of sludge filtration. Dictates of practice and utility, combined with the many variable individual operating conditions of processing work, have resulted in the use of a great variety of filters throughout the field of filtration engineering. Individual designs of leaf and drum filters, operating under pressure and vacuum, have been numerous and excellent and are continually undergoing improvement and change. It is still
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true, however, that the well-known plate-and-frame filter press enjoys the widest scope of application in filtration because of a remarkably wide range of adaptability and simplicity of operation. The purpose of this paper is to describe the application and use of certain methods of investigating batch filtration operntions on plate-and-frame presses which have proved successful and practical in the analysis of widely varied clarification filtrations. This work has covered a wide field of changing filtration conditions, many types of difficult sludges, and over sixteen separate filter aid products. Several approaches to the analysis of each filtration problem were made and thoroughly tested under practical plant conditions. The result had been the formation of a systematic means of approach to filtration problems, based entirely on a practical rather than a theoretical foundation; and this has been a valuable help in decreasing costs and increasing rates of production.
Need for Practical Approach The preponderance of more recently publishcd work on filtration ( 2 , 3, 6) has been concerned with the improvement and development of theoretical aspects. Important new equations have been proposed, some of which are elaborate and involve a large number of empirical constants. Filtration equations are all developed from a simple theory proposed originally by Lewis ( 5 ) in 1923. The instantaneous rate of flow of filtrate from a filter is proportional to the driving force or pressure drop across the filter, and inversely proportional to the resistance to flow of the press units and the a t e r cake. From this point the theory has bcen developed in several directions, the important variations in the different equations proposed lying mostly in the choice of units, the number of empirical constants, and the scope which the equation is set up to cover. The practical use in manufacturing operations of any of these equations, even the best and most recent ones, has been greatly retarded, however, by many phenomena which are encountered in industrial filtration and which are too inconsistent to evaluate in an equation. The types of sludges usually encountered vary so greatly that the resistance to flow of any one is extremely sensitive to conditions of preparation or formation (4),and it is difficult to duplicate results within several hundred per cent. Thus, even though different batches may be prepared under apparently identical conditions, the results of experiments will be so widely divergent that the “constants” derived from solution of equations will prove not to be constants a t all. Also, the further the theoretical equation is developed in attempting to account for the inconsistent properties of various sludges, the more complicated and difficult of application i t becomes. This lack of practical utility of the theoretical filtration equation is evident to anyone who has attempted to apply it to the rcsults of test filtrations run on fluid products, extracts, etc., which must be clarified by removal of colloidal, highly compressible, slimy materials. As a result, new and different methods of attack are necessary in running practical filtration experiments when the sludges to be removed are of this difficult type and theoretical equations are not applicable. A large number of test filtrations were run with the primary purpose of providing answers which could be duplicated whenever necessary. A general test procedure was adopted which closely approached that used in large-scale plant operation. A specific filtration problem involves determination of the best operating conditions for filtering a particular product in regular production. Assuming that the equipment is a constant, either on hand or decided upon, the most important variables to be studied are the range of pressures, the filter aid product, the amount of filter aid, and the division be-
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tween precoat and body treatment of the filter aid used. Each one of these controllnble variables contributes measurably to the success of a filtration and should be determined at its optimum point.
Clarification Filtration on Plate-and-Frame Presses The products with which this work is concerned are solutions of materials of vegetable origin that contain large amounts of slimy, extremely compressible precipitates. The methods are applicable to the study of any filtration problem which is not amenable to successful attack by eqriations or mathematical means. The general procedure for the investigation of each given variable is to collect data from filtration runs, keeping all variables constant exccpt the particular one under study. The data are then assembled in as simple a form as possible and analyzed for the desired results or conclusions.
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The equipment and experimental setup for the test filtrations need not be complicated or elaborate. It is most important, however, that thc experimental unit closely approximate in type and setup the large production filters to which the results of the tests are to be applied. T o violate this rule is to jeopardize the cntire result of the tests, as the test runs may not be satisfactorily duplicated on production. I n the particular case of using large plate-and-frame filter presses, it is good practice to use for test runs a plate-and-frame press of smaller plate dimensions which can be cut down to a fraction of the filtering surface of the production units. These smaller presses are usually available and can be easily and inexpensively set up and equipped for filtration tests. As an illustration we can consider a 30-inch plate-andframe press of approximately 400 square feet of filtering surface as the production unit, a widely used size. Test runs for data on this equipment could be made successfully on 8 12-inch filter press of corresponding construction, using three or four plates and about 5 square feet of filtering area. The small samples required for runs on the 12-inch press make it possible to obtain a large number of comparisons on one lot of material. This particular ratio of sizes was used for a great deal of our work and was found satisfactory. Several units of auxiliary equipment are required on the test filter to provide sharp control over the filtration and accurate gathering of suitable data. A pump is required to deliver the slurry to the press. This should be coupled as closely as possible to both the slurry tank and the press, to be certain of delivering a uniform mixture at all times. The pump should also be of the same design as that used in the production unit; i t should be equipped on the discharge line to the filter with a pressure gage that is accurate and easily read; and it should be fitted with a pressure relief bypass and valve of sufficient size so that the pressure can be set and controlled within close limits. Diaphragm or plunger type pumps, installed with a discharge air cushion to darnpen out pulsations, are usually preferred for clarification filtration rather than the higher speed rotary and centrifugal designs. High-speed pumps have a churning action which tends to break down filter aid particles and precipitates; and by increasing the colloidal nature of the solids, they make the solution more difficult to filter. Other requirements for the experimental work include an accurately calibrated slurry tank with an efficient mixer, a stop watch for timing the operation, and graduates or other suitable measuring vessels for collecting and measuring the filtrate. The correct technique must be used in obtaining proper samples of liquid to be filtered, if this factor is to be kept constant throughout a given series of comparative runs. It is beat to put aside in a separate tank a representative amount of liquor from a regular production lot in sufficient quantity to supply a series of test runs. This lot is mixed thoroughly just before each portion is drawn off for a test run. It is also essential that all the test runs of a series be made with as little time lag as possible between runs. If more than two or three days elapse between the first and last run of a series subsequent precipitation in the main body of the liquid sometimes changcs its nature and introduces serious errors. T o ensure that the equipment itself will not add any unpredictable variables, it is necessary to clean the press thoroughly after each run. Each plate must be set up with new and unused cloths before a run, whether papers are used or not, because the shrinkage of the cloths upon washing is almost impossible to control. I n selecting the cloth, it is important only to choose one that is heavy enough to withstand the pressures tending to squeeze it into the filtrate channels on the plates and thus block the flow. The same style of cloth should be used consistently on both the experimental and production filters.
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Both the precoat slurry and the main batch of liquor to be filtered should be prepared ahead of each experimental run. It is common practice to use an amount of liquor equivalent to approximately one and a half times the calculated volume of frame space in the experimental filter press to carry the precont. The precoat is almost invariably used in clarification filtration; it is a thin coating of filter aid built up on the cloths to carry the filter bed, which is the real filtering medium, and also to protect the cloths. I n experimental work i t is essential that an identical preparation of precoat be made for each run in a series, and that the amount of precoat used in the experimental runs be equal to the product of the ratio of experimental filter surface divided by production unit filter surface and the amount of precoat used in the production unit. The precoat slurry is prepared in this manner in a separate tank and drawn into the filter when the run is first started. The main body of liquor for the experimental run is given body treatment as desired. A sufficient quantity of liquor for the entire run is drawn off and placed in the slurry tank, where filter aid is added in an accurately measured concentration of pounds per pint. The optimum body treatment will vary considerably, depending upon the physical characteristics of the product filtered and the quality and quantity of sludge filtered out. While the exact technique used in carrying on experimental filtrations is flexible, depending upon the operator and the equipment at his command, the procedure used in the first run of a series must be strictly duplicated in every subsequent run of the series. I n general, however, the following operations are best applied: The precoat slurry is pumped to the press first, followed immediately and without a break by the body of the liquor. Recording of data should start aa soon as any filtrate is observed. Simultaneous readings are taken at set intervals of the pressure on the press. total volume of filtrate collected, and elapsed time. This is continued for the duration of the run, or until the pressure has been raised to the highest point considered practicable and filtration has dwindled to the zero point. Pressure Cycle One of the most critical factors in test filtrations, particularly in the comparison of filter aids, is the use of a properly controlled pressure cycle for each run. Most filtration tcsts are still made at constant pressure because of the simplifications introduced into the analysis of results. Practically all production work in clarification filtration, however, is operated over pressure cycles. The filter is started at low pressure, and the pressure is raised at intervals as filtration proceeds and the cake thickness and resistance gradually increase. Test results can be applied more successfully to production when the experimental runs are made using a pressure cycle similar to that which will be used in practice. It is also possible to obtain a great deal of information regarding the characteristic behavior of the different sludges and filter beds under pressure gradients when this method of testing is in use. Consequently, a stepwise pressure gradient, of the general type shown in Figure 1, is used for experimental filtrations. Care is taken that each run in a given series is made over the same pressure gradient and that the pressure increments are taken at the same point of progress of the filtration in each individual test. I n this manner we are enabled to study the effect of pressure and yet not introduce it as an uncontrolled variable to hinder the results of comparisons. The clearest and most efficient way of setting up for inspection the data collected during a given run is shown by the simple graphical diagrams of Figures 1 and 2. Figure 1
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is the usual straight filtration curve of cumulative filtrate volume plotted against elapsed time. Figure 2 is the differential of the curve of Figure 1 and is a plot of rate of filtration against elapsed time. The corresponding pressure gradient curves are shown in each case in order to express them as pressure cycle filtrations and t o simplify them. These curves represent a n actual test filtration made on a product containing a particularly troublesome precipitate.
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The beginning of a filtration, as shown from the origin t o point A , is characterized by high rates of flow and a rapid dropping off of rate as the filter bed forms and beconm set. The bulk of the filtering is done during a more orless constant rate period, as from A to 13 on the graphs; during this time the pressure is raised gradually to compensate for the increasing resistance of the thickening filter bed. Then, wlien the critical point B is reached, a sudden large decrease in rnte occurs which slows down the filtration for the rest of the cycle. The pressure at which this critical point develops is one of the most important facts to be drawn from experimental clarification filtrations, since it defines practically the limits of the filtration production. There are two generally observed causes for the appearance of critical pressures in experimental filtration runs. The primary cause, observed most frequently, is what can he termed “compressibility”. The chief function of a filtcr aid is to give a filter cake a porous nature and hence a high permeability. Positively charged particles of colloidal dispersion are probably adsorbed on the negatively charged surfaces of most siliceous filter aids to a small extent; but even though this effect may tend to increase somewhat the efficiency of the filter aids, its contribution is minor compared t o the mechanical effect of change of texture (1). These mechanical mixtures of sludge and filter aid are more or less nonrigid and therefore have variable degrees of compressibility. Most filter beds built up with filter aid-sludge mixtures remain porous until the critical pressure is reached
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and the cake compresses. This destroys the permeability of the cake and filtration practically ceases. The other important cause of appearance of a critical pressure is frequently observed when using filter aids of high flow rate with slimy precipitates. In these cases filtration proceeds satisfartorily until a critical pressure is exceeded, beyond which the coarse open-teuturcd filter bed no longer retains the precipitate and the slimes are pushed through t o the filter cloth, where they rapidly block off the flow and stop the filtration. Both of these phenomena are easily observed and discovered by a close scrutiny of the filter cake and cloths at the completion of a given run. The appearance of a definite critical pressure seems to be a gil-en characteristic of each mixture of filter aid and sludge. While different batches of a product usually produce insoluble materials which do not always filter out at the same rate with a particular filter aid, the critical pressure seems to occur at just about the same point. Obviously, in running a production filter, the experimentally determined critical pressure is of the utmost importance in any given case, and the pressure increments should be applied so as to build up gradually to the critical pressure at the end of the cycle. Using this pattern of pressure grndient is the only possible means of obtaining maximum efficiency from a given filter aid, no matter what product is being clarified. A plot of comparative test filtrations made t o determine the best of a group of different filter aids on a given solution is shown in Figure 3. These curves were laid out from actual test data from four experimental filtrations made with every condition constant except the filtcr aid used. The manufacturers of diatomaceous filter aids have been actively improving their products, and n o w offer a large variety of different particle sizes. The intelligent selection of the best grade of filter aid for each operation is now possible, and this factor alone can be important in the solution of difficult filtration problems, if properly applied. It is now possible, by using carefully organized filtration test data, to learn a great dcal about the specific properties of any mixtures of sludge and filter aid. The shape of these pressure cycle filtration curves are always similar (Figure 3), and almost invarialjly show the various controlling points discussed in connection with Figure 1. The critical points occur at different pressures with each filter aid. As a result, Pome of the filter aids of a coarse gritty nature, which form more open filter beds, start the cytle with excellent flow ratcs, but the flow drops off sharply as the critical pressures are reached at low pressures. This is true of filter aids 2 and 3, Figure 3. However, although filter aid l starts out more slowly, it has such superior pressure charactrristics with this particular sludge that it delivers much greater total flow. Obviously, filter aid 1, although probably rated by the manufacturer as a slower filter aid than either 2 or 3, will give superior performance in production and should be chosen for use with the particular product under test. Sometimes cases occur where the results of tests on different filter aids must he modified because the best filter aid in flow characteristics does not give the clarity of product required. S:imples should always be taken sometime during the individual test runs for clarity examination, and all filter aids which fail to deliver a product meeting the required standards should be dropped from consideration. After the selection of the proper filter aid and pressure cycle, similar methods of making a series of test runs can be applied to discover the exact amount of body treatmmt necessary to obtain the best efficiency. Runs can easily be made with the same filter aid and pressure cycle, varying only the amount of body treatment in the liquor, and the data can be plotted in comparative curves.
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The use of the proper filter aid, added in the proper amount, is then applied to production runs. The maximum pressure for satisfactory results has also been noted as the critical pressure and should not be exceeded on the production filters. The application of this information is then checked on production filtration and should result in the maximum efficiency. This system of testing has been applied many times to difficult problems in clarification filtration and has proved highly successful and flexible as tool for attacking annoying types of sedimentation. This description is presented with the purpose of providing a simple, practical means of ap-
1237
proach to filtration difficulties in the specific field of clarification.
Literature Cited Carman, p. c,, ENa. cHnaa., 31, 104, (1939). (2) Ruth, Ibid., 27, 708 (1935). (3) Ruth, Montillon, and Montonna, Ibid., 25, 153 (1933). (4) Sperry, Ibid., 20, 892 (1928). (5) Walker, Lewis, and McAdams, “Principles of Chemical Engineering”, 2nd ed., pp. 363-72 (1923). (6) Walker, Lewis, McAdams, and Gilliland, Ibid., 3rd ed., PP. 346. 358 (1937).
Heat Transfer Coefficients for the Condensation of Mixed Vapors of Turpentine and Water on a Single Horizontal Tube E. L. PATTON AND R. A. FEAGAN, JR. Naval Stores Station, Bureau of Agricultural Chemistry and Engineering, U. S. Department of Agriculture, Olustee, Fla. INCE the beginning of the naval stores industry in this
be calculated, the data taken in these experiments will permit the calculation of over-all coefficients of heat transfer and the country, turpentine gum has been subjected to a crude size of the condenser required for a given gum-processing form of steam distillation in a direct-fired still. The plant. This should encourage the use of the more efficient condenser used at these fire stills consists of a copper coil tubular condenser to replace the old-fashioned tub and worm. surrounded by water in a wooden tub. The present tubs are usually 10 feet in diameter and 10 to 12 feet high although the size varies throughout the industry. This type Apparatus of condenser has certain inherent advantages for use on small The apparatus used in these experiments was similar to stills in the woods in that it is cheap and will permit some that of Baker and Mueller (1) and of Baker and Tsao (2) use of the still over a considerable period without the addition with the exception of the method of tube-wall thermocouple of more cooling water. This is an important consideration in installation. localities where the water supply is limited during dry seasons. Condensation of mixed vapors of turpentine and water Moreover, within limits, this type of condenser as used in the took place on a measured length of extra-heavy copper pipe naval stores industry serves as a cooler as well as a condenser. enclosed in a wellThe present trend of t h e i n d u s t r y , 0 i n s u l a t e d comer however, is toward jacket. This jacket centralized processwas equipped with ing and hence more sight glasses to perefficient plants. m i t c o n s t a n t obThe purpose of this s e r v a t i o n of t h e formation of coninvestigation was to densate on the tube. determine the coefThe rate and temcient of heat transp e r a t u r e of t h e fer on the vapor water flowing side of a single-tube t h r o u g h t h e tube horizontal conwere carefully condenser handling trolled during each mixed v a p o r s of turpentine and experiment, a n d PARTIAL FINAL water. Since the equilibrium vapors CONDENSER CONDENSER of turpentine and coefficients for the cooling medium can w a t e r w e r e conFIG^ 1. TESTCONDE~NSER AND AUXILIARY EQUIPMENT
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