J A M E S R.
ANNUAL REVIEW
F A I R
DEVELOPMENTS IN DISTILLATION TECHNOLOGY The current literature seems to reject a shtyt in emphasis-from fundamentals to optimization istillation continues to be a major factor in the de-
D velopment, design, and optimum operation of chemical and physical processes. Because it has such an
important bearing on plant capital and manufacturing costs, there is a never-ending search for methods to improve its economic efficiency. During the past 18 months there has been no shortage of publications in the field; however, there appears to be a shifting emphasis from fundamental to performance-improvement endeavors, no doubt stimulated by the high-tonnage payoff of optimizing existing equipment. This review covers the general period of February 1963 to June 1964. Trends noted in the previous review (37) continue, but under the influences noted above: --Improved understanding of liquid-phase nonidealities is making possible more accurate prediction of vapor-liquid equilibrium relationships. -The use of computers for stage calculations is now routine, both in industry and in the universities. --Improved models for interphase mass transfer are being developed, although understanding of fluidmechanical effects is far from adequate. -Contacting devices continue to attract efforts toward better understanding of hydrodynamic characteristics. This is particularly true for packed columns. Little is happening in the development of radically different devices. -Theoretical studies of distillation system dynamics are receiving support directly from experimental work, and models are sufficiently developed for computer control application. General
Few books and reviews have appeared recently. The new edition of Perry’s Handbook (87) has pertinent sections on distillation, gas absorption, and gas-liquid contacting equipment. The sections contain completely revised material and cover developments up to the 196061 period. The section on distillation equipment suffers somewhat from an academic point of view.
Other books include one on mass transfer calculations (90) and one which summarizes and evaluates laboratory distillation techniques (58). Features of the latter include a rather interesting illustrated history of distillation and a new coverage of laboratory methods used primarily in Europe. Still another book (32) gives detailed references to hydrocarbon vapor-liquid equilibrium data. General reviews have been published by Holdsworth (57) and Dieter (20). Reviews on distillation-related topics cover azeotropic separations (80), mass transfer fundamentals (49), tray efficiency research and prediction (37) and dynamics of mass transfer operations ( 7 77). The usual refresher articles in trade journals, as well as distillation sections in new textbooks, have also appeared. Academic research related to distillation appears to be in decline. Of 1963 titles for chemical engineering theses in this country (73), about 10 per cent may be associated with distillation. This is significantly lower than in previous years, and about two thirds the proportion in the United Kingdom (96). This trend toward decreased basic activity is supported by the general literature ; when duplication and “re-pioneering” articles are eliminated, the residue showing fundamentally new work is relatively small.
Basic Physical Data
Physical data of special interest to distillation practitioners are vapor-liquid equilibria (VLE) and diffusion coefficients for the gas (vapor) and liquid phases. Certainly there is no shortage of effort in the accumulation of VLE; in a recent 18-month interval data on 145 different systems were noted (51). However, the specific VLE demands of a process development often cannot be satisfied directly by published data ; thus emphasis on improved methods for screening and predicting VLE continues. Perhaps the most significant article to appear on VLE prediction is that of Black et al. (5). Support for this article rests in many years of experimental and theoretical work in phase equilibria at the Shell laboratories, and the recommended predictive methods are of interest to chemists and engineers in both process development and process design. A significant predictive method for hydrocarbon systems, published two years ago by Chao VOL
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and Seader (70) continues to be developed, and articles extending or qualifying the method have appeared (77, 40,79). Other articles on VLE concern their extension from minimum data (18, 1 0 Z ) , their prediction for systems containing hydrogen (57) and methane (15), and their general prediction (60, 701, 7 7 3 ) . The series by Wales and coworkers (708) offers refresher material in various aspects of phase equilibria. Other articles related to VLE are noteworthy. A detailed study of vapor pressure estimation methods (69) leads to recommended equations best suited to needs for accuracy, ease of calculation, and pressure range covered. Another article along similar lines (34) is concerned primarily with the extension of measured vapor pressure data. Thermodynamic relationships for vaporization are covered in a detailed study of methods for predicting latent heats (35). Finally, there were noted many articles on VLE presenting specific data, describing measurement techniques, or discussing consistency checks. Several articles giving binary diffusion coefficients have appeared. The correlation of such coefficients has been the subject of three papers. For gases diffusing through air, an empirical correlation has been developed (86) which takes into account polarity of the gases. Another study of gaseous diffusion (97)is directed toward the effect of temperature on rate. Finally, a new correlation for liquid phase diffusion (94) claims slight improvement over the Wilke and Chang (770) method, but likewise is restricted to dilute solutions.
Stage Calculations
Although the basic principles of mass transfer are reasonably well understood, this understanding has not led to significant synthesis of new and different countercurrent separation proce~ses. I n a n excellent review, Souders (98) re-emphasizes the principles of staged or differential separation techniques, and calls for imaginative use of these principles in the development of more efficient processes. At the present time, most distillation separations are defined by the equilibrium stage concept, and during the period of this review most activity was concerned with the use of computers to make stage calculations more accurately and for more complex systems. Articles of interest in the area of machine computation digital) computers cover the use of hybrid (analog (36),a Fortran program which includes improved computation of tray temperature (75), the calculation of batch distillation (68),and the calculation of absorberstripper combinations (76). A great many recent articles have dealt with minor modifications or rearrangements of well known graphical or analytical techniques for calculating stage requirements. Much of the work represents interesting exercises, of possible use to students, but is of little consequence in advancing the practice of distillation. The more rigorous design studies are carried out on computers, and existing approximate methods are adequate for preliminary work.
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Mass Transfer and Tray Efficiency
Interest continues in models for the correlation and prediction of mass transfer and tray efficiency. Because of the complex contacting operation, semiempirical or statistical models must suffice at present, but fundamental studies in mass transfer and gas-in-liquid dispersions may lead to more valid mechanistic models. Work in mass transfer fundamentals is reviewed in INDUSTRIAL & ENGINEERING CHEhlISTRY by Himmelblau and Bischoff (49),and a refresher in the subject is given by Holland (52). The 1964 review of the same topic by Himmelblau and Bischoff will be published in the December issue of this journal. One mass transfer research note of interest (89) mentions experimental evidence that superheating of the feed vapor to a column does not materially influence mass transfer rate on the feed tray, and another (56) defines limits within which phase resistances may be considered additive. The dispersion of gases in liquids has been reviewed by Miller (70),and needed research in dispersion discussed by Jackson (54). Froth structures on sieve plates have been related to process variables (48), and a simple relationship for predicting the heights of these structures has been established (I). The status of commercial tray efficiency research has been summarized by Gerster (37), and certain of its aspects also discussed by Thorogood (106). A different approach to determining phase transfer units has been described (29), and new equations for efficiency prediction have been developed (2, 21). Statistical treatment of efficiency data from a sieve tray simulator has been discussed (33), and a general statistical correlation for predicting tray efficiency presented (30). The limitations of such statistical approaches are well known. Mass transfer in foams is creating interest among separations research people. Foam fractionation has many similarities to conventional fractionation, and similar equipment can be used. Typical of fundamental studies is that of Weissman and Calvert (109) where volumetric mass transfer coefficients are shown to compare favorably with those measured in conventional equipment. Application of foam fractionation is reviewed by Eldib (27). Equipment
Developments in vapor-liquid contacting equipment are strongly influenced by the efforts of proprietors of special tray or packing devices. At present these efforts are mostly directed toward minor improvements of existing product lines. An excellent article summarizing the status of device development has appeared (72), and some new devices have also been described: a perforated tray utilizing centrifugal action (65); baffling arrangements to improve efficiency of existing trays ( 7 I ) ; and the new Linde tray which carries high performance AUTHOR James R. Fair is Manager, Chemical Engineering Section, Chemical Engineering Department, at the St. Louis Headquarters of Monsanto Co. He has prepared I g E C ’ s distillation review since 1962.
claims but no disclosure of details (72). A slight modification of a tunnel-type bubble-cap tray, called the Thormann tray, was described (85), and its average performance documented by large-scale experimental research data. The previous review ( 31) mentioned papers presented at the 1963 New Orleans AIChE meeting; these papers have now been published under the heading “A New Look at Distillation” (7,24, 67,234, 95, 99). More recently, the 1964 Pittsburgh AIChE meeting featured a symposium on “Applied Distillation-The Unusual Problems” which emphasized difficulties in start-up and operation of commercial equipment. The papers will be published in late 1964, and the symposium theme is to be continued a t succeeding meetings. Crossflow trays, primarily of the sieve and valve type, have greatest commercial acceptance. Downcomer hydraulics common to all crossflow trays have been elucidated by two articles on liquid capacity-froth buildup effects (55, 705). Although valve tray popularity continues. published articles deal primarily with sieve trays. For these devices valuable pressure drop data have been reported (79) and longitudinal mixing of the crossflow liquid studied (46). Variations in conventional sieve tray design have received most attention : use of corrugated metal for perforation-as a Ripple tray with downcomers (23); use of protruded lips on the underside of the tray, to decrease entrainment (703); use of slats to form long, narrow openings for vapor flow (38); and slight sloping of the trays to compensate for hydraulic gradient (87). As expected, all such articles report limited operating conditions for which the variations appear advantageous. Often, the conditions are so limited as to be impractical. For counterflow devices, the decline of interest in trays has been more than matched by increased interest in packings. The specification of counterflow trays (Turbogrid, Ripple, etc.) is now confined to special services involving fouling, unusual pressure drop-efficiency requirements, or increased throughput demands. Two pertinent articles on counterflow trays have appeared, one giving useful though not readily correlatable data for gas and liquid transfer rates (47), and the other showing that entrainment from such trays becomes significant if the froth level approaches the tray above closer than 8 inches (700). The many articles on packed columns confirm earlier reports on revived interest in such devices. I n contrast to efforts of a decade or more ago, emphasis is now being given to improved understanding of the physical mechanisms occurring in the packed bed. This is especially true in the general area of phase distribution. A careful study of liquid irrigation (50) differentiates between effective packing element wetting, uniform radial distribution, and wall discontinuities. For a typical operating condition, most of the liquid contacted only 5 per cent of the packing surface, and 50 per cent of the surface was unwetted! Radial maldistribution was further studied experimentally (9) and theoretically (83), with likely effects on mass transfer established. Other work
on distribution dealt with longitudinal mixing of gas and liquid phases; such work can correct prior impressions that the phases pass through the column in plug flow. An excellent review of longitudinal mixing effects is given by Miyauchi and Vermeulen (77),and additional experimental mixing data are available (22, 92). Packed bed mass transfer studies reflect renewed interest in evaluating the effective interfacial area for transfer; such studies are interdependent on the distribution work noted above. Packing surface wettability influences transfer area ; to elucidate this parameter several combinations of surface character and contacting systems have been investigated (28, 42, 45, 47, 59, 76). Also studied are the effects on mass transfer rate of liquid holdup (93), liquid viscosity and interpacking mixing (or stagnation) (77), and presence of chemical reaction ( I 14). Two articles present valuable rate data for moderate size columns: in one (26) over-all rates for several packings are given, with a minimum of interpretation; in the other (707), data for two sizes of Raschig rings are accompanied by valuable interpretive material on liquid-phase transfer mechanisms. Operating characteristics of packed columns are discussed in a series of papers ( 7 4 , and pressure dropcapacity data for a number of packing materials reported (704). Other papers of interest cover pulsing of vapor to packed columns (176), and spray column transfer rates (64)of interest directly or as applied to distribution sections above or below packings. The need for carefully measured performance data on commercial distillation systems continues. A review of methods for testing and evaluating distillation equipment has appeared ( 3 ) , and a complete case history on research, scale-up, and performance of a large vacuum fractionator was given (66). Some scale-up criteria for gas-liquid contacting were reviewed in an interesting European article (43). Advances in distillation equipment design continue to emanate from Fractionation Research, Inc. (78), but the information is available only to sponsors, or to others indirectly through member engineering firms. Column Dynamics and Control
Theoretical and experimental work with distillation systems continues at a rapid pace. Interest stems from several sources: the complexities of the systems offer a challenge to those wishing to develop further the techniques of mathematical simulation; the dynamic behavior of the over-all plant is often strongly dependent on that of the distillation systems; and direct computer control requires an understanding of such dynamic behavior. Distillation columns have always been attractive for teaching and research on control theory. Separate reviews in this journal cover automatic control (l72) and only selected references will be noted here. Development of mathematical models for distillation columns continues to be of interest. Williams ( 7 7 I ) and Sargent (88) have reviewed techniques for VOL. 5 6
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formulating the models. Theoretical model studies are presented for batch distillation ( 6 ) , continuous distillation (54, 7 7 5 ) and absorption (8). It is encouraging to see that some of these studies include experimental validation. I t would appear that on the basis of present knowledge, prediction of system dynamics can be made with sufficient accuracy for engineering design, including computer control, but as pointed out in the previous section, systematic experimental verification in large commercial equipment is still in short supply. Current research in column control has been reviewed (39), and general concepts have been discussed qualitatively (44). Feedforward control systems have been described (61-63) and evaluated from operating data. Computer-generated response data (72) have been used to develop approximation models for large columns (73). Rate of approach to equilibrium was studied experimentally in a small column (4). The special problems of controlling packed columns were discussed (25); these generally have to do with the differential features of bed-contacting and the different degree of phase mixing. Case studies of commercial control systems were published (14, 82). Finally, the dynamics of a column were excited in cyclic fashion to increase mass transfer rates (67). REFERENCES (1) Azbel, D. S., Intern. Chem. Eng. 3, 319 (1963). (2) Bakowski, S., Brit. Chem. Eng. 8, 384, 472 (1963). (3) Barker, P. E., Zbid., p. 306. (4) Barker, P. E., Jenson, V. G., Rustin, A., J.Znst. Petrol. 49, 316 (1963) (5) Black, C., Derr, E. L., Papadopoulos, M. N., ISD.ENG.CHEM.55 N-0. 8, 40; No. 9, 38 (1963). (6) Bowman, W. H., Clark, J. B., Chem. Eng. Progr. 59, No. 5: 54 (1963). (7) Brown, E. C., Von Rosenberg, D. V., Zbid., No. 10, 75 (1963). (8) Calvert, S., Coulman, G. A,, Chem. Eng. Progr. Symp. Ser. 59, No. 46, (1964). (9) Changez, X. D., Znd. Chemist 39, 181 (1963). (10) Chao, K . C., Seader, J. D., AIChE J . 7, 598 (1961). (11) Chem. Eng. News 41, K-0. 20, 64 (May 20, 1963). (12) Zbid., 41, No. 47, 46 (Kovember 25, 1963). (13) Chem. Eng. Progr. 60, No. 1, 71 (1964). (14) Crico, A,, Genie Chim. 91, 12 (1964). (15) Dastur, S. P.: Thodos, G., AZChE J . 9, 524 (19G3). (16) Davis, P. C., Sobel, B. A,, Chem. Eng. Progr. Symp. Ser. 59, No. 42, 95 (1963). (17) Delorme, M., Genie Chim. 91, 177 (1964). I Chem. . Eng. 41, 84 (1963). (18) Deshpande, A . K.: Lu, B., Con. . (19) Detman, R. F., HJdrocarbon Proc. Petml. Ref. 42, No. 8, 147 (1963). (20) Dieter, K., V D Z Z . 105, 1371 (1963). (21) Dieter, K., Hundertmark, F. G., Chem.-Zng.-Tech. 35, 620 (1963). (22) Dunn, W.E., Vermeulen T . M’ilke C. R . Word T. T. “Longitudinal Mixing in Packed Gas-Absorption Cblukm,i’ 6CRLil0394,’Officd Tech. Serv., JVashington, D. C. (1962). (23) Dytnerskii, Y. I., Aleksandrov, I. A, Khim. Prom. No. 1 :70 (1964). (24) Eckert, J. S., Chem. Eng. Progr. 59, KO.5, 76 (1963). (25) Eckert, J. S., Walter, L. F., Chem. Eng. 71, 79 (hlarch 30, 1964). (26) Cckert, J. S., Walter, L. F., Hydmnrbon Proc. P e t r d . Ref. 43, No. 2, 107 (1964). (27) Eldib, I. A,, “Foam and Emulsion Fractionation,” in “Advances in Petroleum Chemistry and Refining, Vol. 7,” Interscience, New York (1963). (28) Elli?, S. R . h?., Porter, K. E., Jones, ?.I. C., Trunr. Znst. Chem. Engrs. 41, 212 (1963). (29) Ellis, S. R. M., Rose, L. AX., Con. J . Ciiem. Eng. 41, 146 (1963). (30) English, G. E., Van Winkle, M., Chem. Eng. 70, KO.23, 241 (Xov. 11, 1963). (31) Fair, J. R., IND.ESG.C H m i . 5 5 , No. 5, 55 (1963). (32) Fenske, M. R., Braun W.G Holmes A. S., “Bibliography of Vapor-Liquid Equilibrium Data foi Hydrocarbon Syitems,” Am. Petrol. Inst., New York (1 963). (33) Finch, R . K., Van IVinkle, M., Znd. Eng. Chem. Process Design Develop. 3, 106 (1964). (34) Fishtine, S. H., Hjdrocarbon Proc. Petrol. Ref. 42, No. 10, 143 (1963). (35) Fishtine, S. H., INn. EXG.CHEM.55, No. 4, 20; K-0. 5, 49: No. 6, 47 (1963). (36) Frank, A , . Lapidus, L., Chem. E n g . Progr. 60, No. 4, 61 (1964). (37) Gerster, J, A,, Chem. Eng. Progr. 59, KO,3, 55 (1963). (38) Giles, B. D., Dryden, C . E., Ind. Eng. Chem. Process Design Deuelop. 2, 188 (1963). (39) Gould, L. A., Chem. Enp. P,ogr., Symp. Ser. 59, So. 46: 155 (1964). (40) Grayson, H . G., Streed, C. W., Sixth World Petroleum Congress, 1963, Sect. VII, Paper 20. (41) Grohse, E. W., Stark, J., Chem. Eng. Progr. 59, So. 11, 72 (1963). (42) Guenther, W., IVolf, F., Chem. Tech. (Berlin) 16, 26 (1964).
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