annual review
JAMES R. FAIR
Distillation istillation has reached a maturity D not equaled by any of the other unit operations. This should not come as a suprise to the reader; theory and practice of this important operation have been reasonably well understood for some time, and fundamental workers have turned to less well charted territory. Yet, distillation technology is far from static-there are too many investment dollars at stake-with much of the important work kept under proprietary restrictions. This review covers work done from July 1964 through June 1965, in sequence with the previous review (24). During the present period the literature on distillation was as voluminous as it was unimpressive. Few important contributions appeared, and because of this situation an attempt has been made here to comment on selected papers and to list additional papers considered worthwhile. General Reviews
Few books and general reviews appeared during the year. The revised edition of “Distillation” in the Weissberger series (73) made its appearance. This volume covers the research/design interface and provides valuable reference material on the laboratory practice of distillation. In his task of scale-up, the process engineer would be greatly aided by this volume. Also appearing was a text on azeotropic and extractive distillation principles (42) which will be of primary value in academic circles. Conversely, the series by Ludwig (59) contains much practical, design-oriented material on fractionation. The only full journal review to be noted was that of Dieter (17), in German, covering selected articles u p to early 1964. Related reviews 128
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
dealt with mass transfer (5) and fundamentals of distillation (43). T h e annual listing of U. S. theses in chemical engineering ( 9 ) showed that, of 264 titles, only seven were classified as distillation and another 13 were included in distillationrelated areas. This decline is part of a continuing trend partly due to the maturing role of distillation and partly to the apparent lack of challenge in distillation research topics. The latter situation is unfortunate when the poorly understood fluid-mechanical and mass transfer aspects of distillation are considered. Industrial research in distillation is still prominent, much of it dealing with the analysis of pilot- and commercial-scale systems. T h e industry-sponsored project a t Fractionation Research, Inc., deals with characteristics of large equipment and continues to expand. Under-
standably, little of this work finds its way to the open literature. Distillation in columns or vessels wherein a foam is generated (foam fractionation) continues to be developed. Primarily used now for removal of deleterious surfactants from water (7), studies are being made of its more general usefulness (32, 34). Basic Physical Data
There continues to be much interest in the measurement, analysis, and general correlation of vaporliquid equilibria (VLE). References to VLE articles are too numerous to list here, but much of the work is reported in the Journal of Chemical and Engineering Data and in the physical chemistry journals. No significant generalized correlations of VLE, useful for design, appeared. Two articles were published (75, 56) dealing with methods for screening
Maior Trends in Distillation Development
-Distillation
research p e r se has practically died out at the universities
but has maintained good health in industry. -There
i s much interest in mathematical modeling and simulation
Stage Calculations
of
distillation systems to support both control and optimization studies. -Supporting
work in thermodynamics, mass transfer, and fluid mechanics
i s relatively active, but its influence on distillation technology is indirect
and gradual. -Improvements
in design and rating methods for distillation columns
are being developed by industry, but are being maintained on a proprietary basis. -Experimental
extractive distillation solvents. Both proposed dilute-concentration activity coefficient ratios for selection criteria even though the criteria might shift under concentrated conditions. Another article (36) pointed out effectively the influence of varying VLE on tray efficiency. Diffusion coefficients for both liquid and vapor phases are necessary to the prediction and interpretation of mass transfer efficiency in distillation. A significant review of liquid-phase diffusion was authored by Himmelblau (39). His discussions of experimental methods and semiempirical correlating techniques are of general interest, even though his primary concern is with dissolved-gas diffusion. An interesting communication on vapor-phase diffusion (87) emphasizes the need for better experimental data and covers the general problem of the unenlightened use of literature data for correlation purposes. Finally, two articles on diffusion in nonideal liquid solutions (58, 97) provide needed support for the development of correlations for diffusion in such solutions.
studies
of column dynamics are continuing, both for analy-
sis and for use of controlled dynamics to improve efficiency and capacity.
Evaluation of equilibrium stage requirements for separations continues to be an active facet of distillation. Although large computers are used routinely for the -stage calculations, there remain problems of interpreting distillation theory, especially for complex systems, and improving routines to minimize machine time. Papers on theory dealt with thermodynamic efficiency of distillation (25), degrees of freedom (70), treatment of thermal effects (83, 90), and a generalized approach to operating lines for cascade separations (47). Improvements in the formulation and convergence of disVOL. 5 7 NO. 1 1
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tillation calculations were also proposed (27). Appearing were several useful refinements of conventional graphical or analytical methods for stage calculations covering binary systems with unequal molal latent heats (72), ternary mixtures (96), columns with multiple feeds (27), and considerations of the intermediate fraction in batch distillation (69). One of the most important activities in distillation today has to do with simulation, or modeling, for control system studies, plant analysis, or optimization in design. As more work is done with overall process simulations it will be necessary to adapt existing distillation models to the time-cost restrictions of computing machinery. How distillation simulations fit into overall process optimization has been discussed many times; an interesting new discussion includes arguments for both analog and digital machines (26). Direct simulation studies were reported for a refinery light ends unit (57), a methanol column (ZS), a styrene/ethylbenzene column (703, and a multicomponent hydrocarbon fractionator (44). General treatments of distillation optimization also appeared (74, 7 9 , and there was indication of considerable optimization interest in Europe (77).
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Stagnant-film models for mass transfer in distillation are giving way to dynamic models, particularly the penetration and surfacerenewal models. These latter models are closer to reality but involve time parameters which must be approximated. A considerable amount of work, reviewed in I&EC (3,is current. Related to the types of models for transfer resistance within the contacting phases are the interactions of the resistances and the creation of separate bamers at the vapor-liquid interface. King (53) shows that the interaction problem usually exists and can explain variations between vaporization and absorption measurements in the same systems. The mechanism of interfacial resistance is discussed separately (74, 37). I t is clear that much challenging research is necessary before mass transfer mechanisms
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in distillation are adequately understood. Important aspects of liquid-phase mass transfer in commercial equipment were reviewed by Miller (66). Two papers (6, 702) dealt with liquid-phase transfer in bubble columns operated at high gas rates; one (6) showed a direct correspondence with data for bubble-cap trays and permitted further elucidation of liquid holdup effects. For the gas-phase, the Froude number, based on vapor velocity and weirheight, was proposed as an important correlating parameter (77). For packed columns, Japanese work led to modified correlations for liquidphase (67) and gas-phase (38) mass transfer; the correlations have not been checked for systems with distributed resistance. Prediction of tray efficiency from system properties, geometric variables, and flow parameters continues to challenge distillation technologists. Hughmark (46)presented a variation of the semiempirical AIChE model which gives an improved fit of laboratory data. However, it represents commercial data from Fractionation Research about the same as the AIChE model. Confirmation of the AIChE work was in studies of the methanol/water separation in a 12.5-inch bubble-cap column (48). Another empirical predictive method (54) correlated selected data but was not generally validated. Prediction of multicomponent system efficiency usually involves manipulation of values for the binary subsystems. An analytical treatment of this situation was given in two papers by Toor (93, 94). Finally, the fitting of multicomponent efficiency data from plant studies to stage calculation models was discussed ( 9 2 ) , with resulting development of vaporization efficiencies to account for mixing effects on trays. Equipment
Perhaps the most active area of distillation technology is that of equipment selection and design. This work is primarily associated with the specification of optimum column internal contacting devices; vendors of such devices are active in developing equipment-dated technology through their own experimental research. A general article appeared on contacting devices (23) 132
and rated trays and packings according to capacity, efficiency, flexibility, pressure drop, cost, and design reliability. The high ratings of sieve and valve trays are substantiated by their popularity; however, the advantages of high void packings for vacuum rectification were also apparent. Other papers gave general comparisons of devices (70) and comparative data for several devices in carbon dioxide absorption service (29), A significant paper on devices for commercial high vacuum work was presented (78) showing that packings such as Pall rings are competitive with devices that are niechanically aided to overcome pressure drop. The AIChE symposium on “Distillation-The Cnusual Problems,” mentioned in the previous review, resulted in several excellent papers. An analysis of tray flooding detection and consequences was made (37). I n one of the most interesting and informative case studies ever published, Martin (64) documented various problems with a solventwater vacuum fractionator. Other papers dealt with flooding and foaming in columns (76, 30) and trouble-shooting of columns (2, 79, 88). Another symposium on the same theme was held at the February 1965 AIChE meeting. Sieve tray columns received the most attention. Surface tensiongradient influences on sieve tray efficiency continued under investigation ( 3 ) . Tray w i t h small (3/32inch) holes remained clean in dirty service (20). Attention was also given to sieve tray hydraulics (4, 40, 72, 76, 85), mass transfer (82),and general design features (35). Counterflow trays (Turbogrid, dualflow) are still specified, particularly in Europe. A comprehensive review of the hydraulics of these trays was published ( 8 4 ) ; those concerned with details of counterflow tray design will want to study this paper carefully. Other papers on this subject dealt with mass transfer (50, 87) and performance using off-level trays (80). ~
~~
AUTHOR James R. Fair is Manager of the Chemical Engineering Section, Central Engineering Department, of the Monsanto Co. He hasprepared IMEC’S annual reuiew of dzstillation since 7962.
INDUSTRIAL A N D E N G I N E E R t N G CHEMISTRY
Because of their proprietary nature, valve trays had little mention. One article dealt with mixing on valve trays and their downcomers (TOO), with concern more for model development than for comparison with other tray types. Another article (78) reported experimental studies showing mass transfer coefficients for valve trays to be about the same as for sieve trays with the same hydrodynamic conditions. Bubble-cap tray flooding in commercial equipment was covered by a new correlation (2) which appears to need further testing. A new tunnel-cap tray, the Thormann, was described (77),and reported efficiencies for large column diameters were high. A general review of bubblecap tray design appeared (22). Liquid mixing on trays is one of the more challenging fluid-mechanical aspects of analysis and design. Control of this mixing gives the designer control of tray efficiency. Some Russian workers concluded that liquid rate influences mixing only as it influences tray holdup (73), while others from Russia (75) concluded that for Peclet numbers above six liquid rate does not influence mixing. Another study (700) showed that for conditions of good vapor dispersion, liquid rate has little influence on mixing. Heretofore, liquid rate has been thought to have a fairly strong effect on mixing. Packed columns continued to have a revival of popularity for large installations, although publications dealt primarily with studies in small equipment. Of importance were papers on flow and heat transfer characteristics of packings (98,9 9 ) ) wetted and effective packing areas (63), distribution of phases within the packed beds (51))and pressure drop (52). A worthwhile new correlation for packed bed flooding appeared (65),and another paper dealt with the general aspects of gas-liquid flow in countercurrent contacting equipment (703). A new packing of the grid-type, suitable for vacuum service, was introduced (89). Finally, more data w’ere published on spray tower mass transfer (621,useful for predicting end effects in certain packed columns. Other equipment-oriented papers covered design of baffle (showerdeck) columns (86) and characteristics of helically coiled columns (68). Circle No. 75
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The latter consist of open tubes and have purported advantages for vacuum distillations. Column Dynamics and Control
Books and general papers on process control usually treat in detail the classical problem of distillation column control. Two of the better such treatments during the past year were by Williams in the aforementioned book on distillation (73) and by Buckley in his book on process control (8). For complete references on this aspect of distillation the reader should consult Williams’ review of process control in next month’s issue of I&EC. Knowledge of the dynamic characteristics of distillation systems is important not only to designers of control systems but also to those interpreting the transport phenomena occurring in the equipment. For example, frequency response tests can be used to establish the mixing characteristics of column trays and downcomers (100). Many studies of distillation dynamics have been purely theoretical (97) and it is encouraging to note several recent papers which include experimental validation of theory. Studies reported dealt with the methanol-tert butyl alcohol system in a 12-tray column (45) ; the methanolwater system in a 10-tray column (49) ; the acetone-air-water absorption system in a small packed column (47); and the acetone-benzene system in a 10-tray column (60). The last-named study is part of a continuing investigation at the University of Delaware on the column previously used for efficiency studies in the AIChE program, and the paper cited here includes extrapolation of feed-forward control characteristics to a large-diameter 40-tray column. Large columns have slow response and thus impose difficult demands on corrective action for feed-forward control. Another paper dealt with the dynamic response of a large benzene-toluene fractionator (67) and, though theoretical, it promises experimental validation through plant tests. In the column dynamics category may be placed controlled-cycling operation of columns. In such operation, reflux and/or boilup are purposefully cycled to improve efficiency and capacity. There is evidence of considerable work in progress in the area, some of it now 134
reaching the literature. A small packed column was pulsed to obtain a twofold increase in capacity (104). Other packed columns were pulsed to obtain increases in mass transfer (55, 95). There are large variations in frequency-amplitude values used in such studies, and the problem of extension to large equipment still remains. More work in this area may be expected. REFERENCES (1) Andersen, A. E., Jubin, J . C., Chem. Eng. Propr. 60 (IO), 60 (1964). (2) Andersen A. E., Phillips, E. M., Hvdrocarbon Proc./Petrol.’Rej. 43 ( 8 ) , 159 (1964). (3) Bainbridge, G. S., Sawistowski, H., Chem. Eng. Sci. 19, 992 (1964). (4) Bene, T., Int. Chem. Eng. 4, 625 (1964). (5) Bischoff, K. B., Himmelblau, D. M., IND. ENC. CHEM.5 6 (12), 61 (1964). (6) BrauIick, W. J., Fair, J.R., Lerner,’B. J., A.I.Ch.E. J . 11, 73 (1965). (7) Brunner, C. A . , Stephan, D. G., IND. ENC. CHEM.57 (5) 40 (1965). (8) Buckley, P. S., “Techniques of Process Control,” Wiley, New York, 1964. (9) Chem. Eng. Progr. 61 ( l ) , 97 (1965). (10) C h m . Ing. Tech. 36, 976 (1964). (11) Chem. Tech. (Berlin) 16, 375 (1964). (12) Chen, N. H., Chem. Proc. Eng. 46,87 (1965). (13) Danilychev, I. A,: Planovskii, A. N., Chekhov, 0. S.,Khim. Prom. 6,461 (1964). (14) Davies, J., T., Kilner, A . A,, Ratcliff, G. A,, Chem. Eng. Scz. 19, 583 (1964). (15) Deal, C. H . Derr E. L., I N D . ENC. CHEhf. P ~ O C E S SD ~ S I G N ’ D E V E ~ 3,394 O P . (1965). (16) Dickinson, W. S., Chem. Eng. Progr. 60 (lo), 73 (1 964). (17) Dieter, K., VDI Z . 106, 1541 (1964). (18) Dytnerskii Yu. I. Kasatkin A. G., Kochergin, N. V., Gervitk, V. M!, Int. Chem: Eng. 5, 95 (1965). (19) Eagle, R. S., Chem. Eng. Progr. 60 ( l o ) , 69 (1965). (20) Eagle, R. S., Lemieux, E. J., Ibid., p. 74. (21) Eduljee, H . E., Brit. Chem. En,?. 9, 6 6 8 (1964). (22) Eduljee, H. E., Ibid., p. 820; 10, 103, 162, 251 (1965). (23) Fair, J. R., Chem. Eng. 72 (14): 107 (1965). (24) Fail, J. R., IND.ENC.CHKM.5 6 (101, 61 (1964). (25) Flower J. R Jackson, R., Trans. Inst. Chem. E n p . 42,’T249 ?1964). (26) Franks R. G. E,, Chem. Eng. Progr., Sym. Series 60, No. 56, 8 (1964). (27) Friday, J. R., Smith, B. D., A.I.C.h.E. J . 10, 698 (1964). (28) Gelbin, D., Bri2. Chem. Eng. 10, 301 (1965). (29) Gibson, G. H., Cribb, G. S., Trans. Inst. Chem. Engrs. 42, T140 (1964). (30) Glausser, W.E., Chem. Enx. Progr. 60,67 (1964). (31) Goodridge, F., Robb, I., IND.ENC.CHEM.FUNDAMENTALS 4, 49 (1965). (32) Grieves, R. B., Wood, R. K., A.I.Ch.E. J . 10, 456 (1964) ( 3 3 ) Haring, H . G., Knol, H . W., Ret. Trau. Chim. 83, 645 (1964). (34) Harper D. 0. Lemlich R., TNn. ENG. CHEM. &SIGN EVEL LOP. 13 (1965). PROCESS (35) Harris, I. J., Brit. Chem. Eng. 10, 377 (1965). (36) Haughton, C. O., Zbid., p. 237. (37) Hausch, D. C., Chem. Eng. Progr. 60 ( I O ) , 55 (1964). (38) Hikita H . Maeda, M., Umemura, M., Chem. Eng. Ju& (Ekgl. Ed.) I,178 (1964). (39) Himmelblau, D. M., Chem. Reu. 64, 527 (1964). (40) Hobler, T., Int. Chem. Eng. 4, 395 (1964). (41) Hoerner, G. M., Schiesser, W. E., Chem. Eng. Progr. Sym. Ser. 61, (55), 115 (1965). (42). Hoffman, E. J. “Azeotropic and Extractive Distillation,” Intersdience, Kew York, 1964. (43) Holland, F. A., Brit. Chem. Eng. 9,444 (1964). (44) Horn, J. A., Miller, D. G., Chem. Eng. 71 (201, 117 (1964). (45) Huckaba, C. E., Franke, F. R., May, F. P., Fairchild, B. T., Distefano, G. P., Chem. En
