Seven recent sign$cant developments have been observed in this important area
TRENDS I N DISTILLATION TECHNOLOGY JAMES R. F A I R
ractional distillation is an old and faithful friend
F of ;he hydrocarbon and chemical processing indus-
f
tries. To mention it in the same breath with glamor terms such as solid state science and operations research may be to assign it a rather prosaic nature. But in these days of the cost-price squeeze, distillation may be one of the most exciting terms, economical[y, in the engineer’s vocabulary. It still represenis a major pan of the new plant capital investment. As a separating technique it has never had any real competition, and it is not likely to have any in the future. Distillation technology is not static, and in fact is incorporating some applicable glamor terms itself. This article will emphasize some of the new developments and trends in this important technological area and provide additional background leading to these developments. General reviews of distillation have been appearing from time to time summarizing imponant historical developments in the field. Walsh (15) presents brief annual summaries of the current literature in I&EC. Texts on the subject have appeared within the last three years (a),supplementing the valuable text by Robinson and Gillidand and the useful reference section in Perry’s Handbook. Through references such as these and the
Choice of distillation type depends on process or research problem
34 cited in the Background Literature section of this article, the reader may quickly gather a fairly complete bibliography on the subject of distillation. Here, we have selected those references which contribute definitely to new trends in distillation technology. When one applies distillation to a particular separation problem, he usually carries out thoughts or efforts according to the following sequence : Type of distillation Vapor-liquid equilibria Equilibrium stages Mass transfer Equipment and control These steps are self-explanatory to the practitioner. T o others they will become clearer in the following text. Type of Distillation
Distillation may be batch or continuous. It may be modified for unidirectional mass transfer such as absorption or stripping. It may incorporate special vaporliquid equilibrium features : azeotropic, extractive, inert gas, or stream distillations. The chemist or engineer usually assigns a distillation type according to the process or research problem at hand. Since there have been no new discoveries in distillation type, only trends in new applications can be mentioned. Extractive distillation continues to be an early consideration for close-boiling pairs to be separated, although new plants using this method are being built infrequently. It is appreciably more expensive to produce a separation by extractive distillation than to produce a separation by distillation without a third component, provided separation without the solutions is possible at all. Extractive distilIation is not used when simple techniques are available and more practical. Industrial and academic research continues in the search for new or improved solvents. An interesting case history has recently been presented (14)which shows the economic advantages of switching from one solvent to another in order to increase throughput or recovery. It would appear that all companies now employing extractive distillation would be well advised to look toward such upgrading. Azeotropic distillation has long been hampered b>-the problem of separating the entrainer. Heterogeneous azeotropes have been exploited, but for separating homogeneous azeotropes a new membrane device has been developed (2). The search for azeotropes has been materially aided by extensive data compilation and CHEMICAL SOCIETY current efforts by the AMERICAN will result in a supplementary table of azeotropes.
AUTHOR J a m e s R. Fair is Leader, Engineering Research Section, at the Research Center of Monsanto Chemical Co. 54
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Vapor-Liquid Equilibria
Measurement and correlation of VLE (vapor-liquid equilibria) represent a major current effort in distillation technology. It is well recognized that inaccurate VLE often constitute the major uncertainty in design of new distillation equipment. When faced with the problem of obtaining VLE, the chemist or engineer resorts to one or more of the following : Simple calculation Direct measurement Indirect measurement Complex prediction Simple calculation refers to readily available handbook data. Such data may be only pure component vapor pressures (if Raoult’s law applies), tabulated x - y data, measured activity coefficients, or simple K values for hydrocarbons (usually limited to paraffin-olefin systems). A great many problems can be solved by this step alone, being careful to observe the limitations of the source material. Here, some background in the thermodynamics of solutions is essential. Direct measurement of VLE is often expeditious, especially if it is combined with proper theory to avoid the labor of covering all probable ranges of temperature, pressure, and composition. Circulating stills or agitated bombs are generally used, with measurement of phase compositions being facilitated by such new instruments as vapor-phase chromatographs. There are many variations of the circulating equilibrium stills, at least two of which are worthy of mention here. One is a metal still for high pressure. The other, called an ebulliometer, has been revived in recent years. One advantage of the ebulliometer is that composition measurements are not required. The device is inexpensive to construct, and it is not unusual for a laboratory to have a number of ebulliometers lined up and operating simultaneously. For obtaining VLE in dilute regions, the total pressure method, ebulliometric technique, is limited only by the ability t o measure small differences in temperature and pressure. Indirect measurement of VLE is often practiced crudely in the laboratory-. The mixture to be separated is distilled in a calibrated column and the theoretical stage requirements estimated directly. The column is usually packed. The obvious difficulty is that the calibration is not really known. A new trend here is to use a plate column such as the Oldershaw which approximates the efficiency of its commercial scale counterpart. When operated at total reflux, the relative volatility may be calculated from the Fenske equation, with suitable correction for temperature effects. Any laboratory that can afford this type of equipment can, with a little experience, predict VLE rather precisely. An added advantage is that certain practical problems are demonstrated-the actual distillation is carried out.
Complex prediction of V L E is misleading, but hcre it refers to the use of thnmodynamics of nonideal solutions lo predict V L E w to extend limited VLE meannmmts. The rcadn should consult thc newn texts such as Edmistn (7) fw background and modern dsuclopmmts in this segment of thnmodynamics. Of particular importance is the increased application of Hildcbrand theory in treating nonidealitics in the liquidphase. Through the use of a solubility parameter, simpb calculated from heal of vaporization and molal density, it is possible to estimate the amount of nonidsalily likely to be present in the liquid phase. 'Vapor-phase nonidealities are normally handled by gcnnalired fugacity c h t s (7) or calculated from equations of state. An enipirical equation of state expressly for dstcrrnining V L E has bem developed. A series of papers on the application of Hildebrand theory by Prausnitz and Coworkers (7.2, 73) and by workers at California Research Corp. has led to a recent gencral correlation of hydrocarbon R values. (4) which applies to aromatics and naphthenes as well as to paraffins and olefins. This represents a major advance in the technology of hydrocarbon processing. Of key importance in the prediction of VLE for various chemical systems is the infinite dilution activity coefficient-a mqsure of the nonideality of the solute when its concentration in the solvent is infinitely small. Deal and his coworkers (5, 7.7)have p u b l i e d methods for predicting these coefficientsfor several classes of chemical systems. Null (70)shows how dilute concentration data may be extrapolated to infinite dilution and,discusm the use of solutions of the Gibbs-Duhem equation in calculating the full concentrationdependence of the activity coefficient. This same matter has been.treated in great detail by Black, who covers the subject of extending binary data to multicomponent systems.
.
Work in VLE of interest to the dintillation field might be summarized a#: Continuing measurement and dasdcation of n.onideal systems Continuing gensraliaation of data Developing of predictive methods Equilibrium Stages
Rigorous ,methods for calculating equilibrium stages have been known for years; thus in principle there is nothing new in this category. There is continuing interest, however, in mathematical modiication or approximations which permit rapid designs with the slide rule or desk calculator. In the preface to his book published some 12 years ago, Gilliland suggested that we had passed a point of diminishing r e m s on developing new approaches to calculating equilibrium stages. Yet, during the past 12 years there has been a proliferation of papers along such lines. Whether the effort has been justified has not been answered. The important new trend in equilibrium stage calculations is, of course, application of high-speed com-
?d understanding of liquid-phase nonideolities, will ting improved correlations of vapor-liquid equilibrio computers for carrying out rigorous stogewise tal-
>f mass transfer theory to the more ac I
.
,. .
puting methods. Programs for such calculations have been developed by pmcessing and desrgn companies and by computer manufacturers. There have been several pertinent publications, and a program manual has been compiled by the AIChE (7). Thus, if the designer has accem to a computer, he can with little dfficulty develop a suitable program which will encompass complete trayto-tray calculations. But it must be stressed that he will always be limited by t h e validity of his input physical property data. The advent of the computer now permits a more quantitative treatment of separations involving boiling range materials. Such complex mixtures may be handled by splitting their true boiling point curves into several pseudo components, estimating vapor pressure variations from generalized methods, and then carrying out the hitherto laborious calculations on the computer. Product-cut boiling point curves are readily reconstructed from the split in the pseudo componenis. Mars Tmnrhr
Conversion of equilibrium to actual stages involves mass transfer rates. Perhaps the m a t important mass transfer work in recent years was that carried out under sponsonhip of the AIChE Research Committee. This work was summarized in final reports from three participating universities and certain practical aspects were presented in a design manual. Gordon and Davies are elaborating on this work i n a current series. The AIChE work was based on the two-film model and treated a relatively few simple binary mixtures. Extension of the work to more complex systems and conditions is being undertaken by private industry, but little is being published. It is known that the film model is unrealistic, and theoretical publications are leading the way to the adoption of the more realistic surface-renewal or penetration models. In a technological sense, however, the film model is still likely to be the standard of design for several yean to come. The AIChE method leads to the prediction of the VOL 5 4
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efficiency at one point on the tray. This efficiency is equal to or lower than the over-all tray efficiency and the correction process involves liquid mixing effects on the tray. For systems with well defined physical properties and understandable liquid mixing effects, full advantage can be taken of the enhanced value of the over-all efficiency. It is not uncommon now for a designer to use an over-all tray efficiency of 100%. He understands better how properties and flow conditions can be right for such a high efficiency. Of course, he may have the opposite case at hand : circumstances require designing for, say, an efficiency of 25%. In many instances physical properties under operating conditions cannot be predicted or measured conveniently. Other reasons, too, may lead to a lack of confidence in the AIChE method. Then scale-up studies must be made and here again mention must be made of the Oldershaw column. This small sieve tray column can give directly scalable mass-transfer data and in the bargain give useful information on potential control and recycle problems. There is a distinct trend
toward scaling directly from Oldershaw equipment, and an interesting case history of this was included in the description of the new Sohio acrylonitrile unit. Mass transfer in packed columns is continuing tc receive attention, and a related distillation research program is under way a t the University of Houston. The second thesis on this work deals with the benzenehexane system at various pressure levels ( 7 7 ) . Other distillation data in large equipment are being released periodically (6). It would appear that for special distillation services-e.g., corrosive system or hiqhvacuum operation-the packed column is very much alive today.
BACKGROUND L ITERATU RE
Vapor-Liquid Equilibrium Hala, E., Pick, J., Fried, V., Vilim, O., Pergamon, London, 1958. Solubility of Nonelectrolytes Hildebrand, J. H., Scott, T. L. (3rd ed.), Reinhold, 1950. Azeotropic Data Horsley, L. H., American Chemical Society, Washington, 1952. Chemical Process Principles. Part 11. Thermodynamics Hougen, 0. *4., Watson, K. M.: Ragatz, R. A. (2nd ed.), Wiley, New York, 1959. Use of Field Test Data for Control System Analysis Hoyt, P. R., Stanton, B. D., Instruments26, 1180 (1953). Distillier-und Rektifiziertechnik Kirschbaum, E. (3rd ed.): Julius Springer, Berlin; 1960. Computer Control of Distillation Reflux Lupfer, D. E.?Berger, D. E., ISA Journal 6 , 34 (June 1959). Versatile Vapor-Liquid Equilibrium Still Orr, V., Coates, J., IND.ENG.CHEM.52, 27 (1960). Chemical Engineer’s Handbook Perry, J. H. (3rd ed.), McGraw-Hill, New York, 1950. Activity Coefficients a n d Molecular Structure Pierotti, G. L.: Deal, C. H., Derr, E. L., IND. EKG.CHEM.51, 95 (1959). Hypothetical Standard States and the Thermodynamics of High-pressure Phase Equilibria Prausnitz, J. M., AIChE Journal 6 , 78 (1960). Thermodynamics of Solvent Selectivity in Extractive Distillation of Hydrocarbons Prausnitz, J. M.; Edminster, LV. C., Chao, K. C., Ibid., 6, 214 (1960). Thermodynamics of Solutions Prengle, H. W., Palm, G. F., IND. END.CHEM.49, 1769 (1957). T h e Properties of Gases a n d Liquids Reid, R. C., Sherwood, T. K., McGraw-Hill, h-ew York, 1958. Elements of Fractional Distillation Robinson, C. S., Gilliland, E. R. (4th e d ) , McGraw-Hill, Kew York, 1950. Distillation Literature Rose, A , , Rose, E., Applied Science Labs., State College, Pa., 1955. New Route to Acrylonitrile Veatch, F., Callahan, J. L., Idol, J. D., Milbcrger, E. C . , Chem. Eng. Progr. 56, S o . 10, 65 (1960).
In addition to the items of recent import cited in the text, the reader will find the following references invaluable in rounding out his understanding of the past, current, and possible future trends in distillation technology. Bubble T r a y Design Manual AIChE, New York, 1958. Tray Efficiencies in Distillation Columns Ibtd., Final Report, Univ. of Delaware, 1958. Tray Efficiencies in Distillation Columns Ibid., Final Report, North Carolina State Univ., 1959. Tray Efficiencies i n Distillation Columns Zbid., Final Report, Univ. of Michigan, 1960. Experience i n Controlling a Large Separation Column with the Continuous Infrared Analyzer Borger, D. E., Campbell, G. G., Chem. Eng. Progr. 51, 348 (1955). Now Separate b y Membrane Permeation Binning, R. C., James, F. E., Petrol. Refine? 37, No. 5, 214 (1958). Vapor Phase Imperfections i n Vapor-Liquid Equilibria Black, C., IND.END.CHEM.50, 391 (1958). Phase Equilibria i n Binary a n d Multicomponent Systems Ibid., 50, 403 (1958). Multicomponent Vapor-Liquid Equilibria from Binary Data Ibid., 51, 211 (1959). Vapor-Liquid Equilibrium Data Chu, J. C., Wang, S. L., Levy, S. L.. Paul, R., Edward,, Xnn Arbor, 1956. Improved Oldershaw Glass Bubble Plate Columns Collins, F. C., Lantz, V., IND.ENG.CHEM.(ANAL.ED.) 18, 673 (1946). Pressure-Volume-Temperature Relationships of Organic Compounds Dreisbach, R. R., Handbook Publishers. Cleveland, 1952. Performance of the Oldershaw Column a t Reduced Pressure Ellis, S. R. M.. Contractor, R. M., J . Znst. Petrol. 45, 147 (1959). Progress i n Fractional Distillation Geddes, R. L., Chem. Eng. Progr. Symp. Ser. 25 5 5 , 87 (1959). Distillation Theory a n d Fundamentals Gerster, J. A., IND. ENG.CHEm 52, 645 (1960). Latest Advances i n Extractive Distillation Gerster, J. A,, Petrol. Refiner 39, No. 6, 146 (1960). Distillation-In View of Modern Developments Gordon, K. F., Davies, J. A., PetrolChem. Eng. 33, No. 7, C-7 (1961). 56
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Equipment and Control
Commercial equipment for distillation may be classified as packed columns, counterflow tray columns, and crossflow tray columns ( 7 ) . All categories are used widely, although the third category dominates. Trends in packed column design are toward the use of metal pall ring packings, and ceramic saddle modifica-
tions. Metal rings provide increased contact surface at high void fractions and hence give favorable ratios of theoretical plates to pressure drop. There is limited interest in the various mesh and grid-type packings. Counterflow tray devices have perforated trays occupying the full tower cross section; vapor and liquid compete for the same openings. These devices had a wave of popularity five to ten years ago, but have since faded. It was found that they are oversensitive to variations in vapor and liquid flow rates. They are still specified for services where their self-cleaning features are valuable. Controlled cycling of vapor flow to such equipment is claimed to increase its capacity ( 3 ) . Crossflow tray devices show a marked trend away from the classical bubble-cap tray. Perforated trays are specified almost exclusively, and a great many of these are the valve or variable-orifice type. Use of valves permits a wide operating range of vapor rates without the deleterious weeping of liquid through the openings. For many services, however, the simple sieve tray, fixed opening, is both inexpensive and efficient. Perhaps the most important development in equipment design is the flourishing operation of Fractionation Research, Inc. This company tests large-scale contacting equipment but maintains the results as confidential. Some data on bubble-cap trays have been released amd proprietors of valve trays have released FRI test data on their equipment. Mention here is made of FRI because of the fairly widespread, though indirect, dissemination of the knowledge through operating companies and design firms. Periodically there are reports on basically different types of distillation columns. These are usually gadgets designed for special needs and in general purporting to give either high theoretical stages per unit volume or low pressure drop per theoretical stage. T o crowd a great many stages into a single column requires that throughput be extremely small. And to get vapor dispersion without pressure drop usually requires that some form of vapor pumping be used. Although these new devices merit careful attention, it is clear that in the chemical and hydrocarbon processing industry they are likely to have little if any general application. In the area of distillation column control there are several important trends. To appreciate these the reader must remember that traditionally columns have been designed on a steady-state basis. Allowances for extra trays and/or extra reflux have then been added to accommodate upsets likely to cause the column to The control system has been go off specification. added as a final thought, with instruments and control methods based on empirical-type experience. The net result of this approach has been costly additional equipment, for trays and reflux, without real assurance of maintaining specification products. Interest is now developing in understanding better the dynamic characteristics of columns and their auxiliaries. Studies mentioned in connection with mass transfer and equipment design have led to methods for predicting liquid and gas holdup on trays. A few papers (76) have
presented information on the rate of propagation of disturbances in columns. In general, however, attention to column dynamics has followed theoretical or mathematical lines leading to performance simulation by compute‘r. There is still insufficient commercial-scale performance data 09 the transient response of columns. The AIChE is considering a sponsored research project to alleviate this shortage to some extent. Ultimate design of the complete system will require time constants for condenser, reboiler, piping, and column in addition to elements of the control circuit. Many of these constants remain to be evaluated experimentally. The payoff will be, of course, improved performance by smaller equipment. To aid in designing optimum control systems, new and improved stream analyzers are being developed. No longer is temperature always considered adequate for indicating composition variations on distillation trays. Notable new developments in instrumentation include short-cycle chromatographs and boiling-range analyzers. Other instruments replacing simple temperature measurement include refractometers, infrared analyzers, viscometers, and hydrometers. In a limited way, computers are finding their way into distillation column control. Analog computers are used regularly to correct external reflux conditions and thus to permit control of the more important internal reflux flow (9). A recent paper describes the use of an optimizing computer for correcting the set points of controllers on a de-isobutanizer (76). Other work by process companies and computer manufacturers is known to be under way. In all cases, however, there is the continuing need for a better understanding of column dynamics. LITERATURE CITED (1) American Institute of Chemical Engineers, “Computer Propram Manual for ComDlex-Tower Distillation,” AIChE. New York, 1962. (2) Binning, R. C., Lee, R. J., Jennings, J. F., Martin, E. C., IND.ENG.CHEM.53, 45 (1961). (3) Cannon, M. R., Zbid., 53, 629 (1961). (4) Chao, K. C., Seader, J. D., AZChE Journal 7, 598 (1961). (5) Deal, C. H., Derr, E. L., Papadopoulos, M. N., IND.ENG. CHEM.FUNDAMENTALS 1,17 (1962). (6) Eckert, J. S., Chem. Eng. Progr. 57, No. 9, 54 (1961). (7) Edmister, W. C., “Applied Hydrocarbon Thermodynamics,” Gulf Publishing Co., Houston, Tex., 1961. (8) Hengstebeck, R. J., “Distillation Principles and Design Procedures,” Reinhold, New York, 1961. (9) Lupfer, D. E., Oglesby, M. W., IND. ENG. CHEM.53, 963 (1961). (10) Null, H. R., “Direct Evaluation of Infinite Dilution Activity Coefficients for Calculating Vapor-Liquid Equilibria.” Presented, Baltimore AIChE Meeting, May 1962. (11) Phillips, G. L., M.S. thesis, University of Houston, August 1961. (12) Prausnitz, J. M., Anderson, R., AIChE Journal 7 , 9 6 (1961). (13) Prausnitz, J. M., Shair, F. H., Ibid., 7, 682 (1961). (14) Sagenkahn, M. L., “Extractive Distillation with Acetonitrile to Improve the Profitability of Butadiene Manufacture,” Lake Placid AIChE Meeting, September 1961. (15) Walsh, T. J., IND.ENC.CHEM.53, 248 (1961). (16) Weiss, E. A., Archer, D. H., Burt, D. A., Petrol. Rejner 40, No. 10, 169 (1961). (17) Wilson, G . M., Deal, c. H., IND.ENG.CHEM.FUNDAMENTALS 1, 20 (1962). VOL. 5 4
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