ANNUAL REVIEW
WILLIAM L. BOLLES JAMES R. FAIR
Distillation
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Distillation research a t the large scale and a renewed interest in design have been accompanied by attention to the potential benefits of
a
unsteady-state processing
a
I
w’
ith the apparent maturity of distillation technology,
one would not expect to find a continuing flow of literature on the subject. Such is not the case, however, and much of the ‘current literature is significant in its application to the improved understanding and development of distillation systems. I n his continuing search for greater efficiencies in this economically important unit operation, the chemical engineer will find much of the work noted here to be of value. This review covers the general period of July 1966 through June 1967, in sequence with the previous review (27). Some earlier references, not previously noted, are also included. Some overlap of coverage AND ENGINEERING CHEMISTRY with that of the INDUSTRIAL reviews of mass transfer (62) and process control (748) is included here to serve best the needs of the distillation practitioner. General Works
Two general reviews of the current literature on rectification, absorption, and extraction appeared, both in German. The one by Huebner (72) covers 108 references, largely for 1965, and the one by Hoppe (66) extends the coverage to September 1966 and lists 130 references. As might be expected, both authors tended to emphasize the European literature. The only distillation oriented book noted dealt with computer calculation of vapor-liquid equilibria (772). This worthwhile summary of theory, correlations, and prediction methods is restricted to “moderate” pressures and below; a forthcoming volume will cover the high pressure range. Books dealing expressly with distillation principles are likely to be few and far between in the future, because of adequate present coverage. There will be a continuing need for coverage of evolving distillation practice.
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A new book of great value to those working in the distillation field is worthy of mention-that of Reid and Sherwocd (716) which, in its second edition, represents the standard for estimating properties of gases and liquids. And further to the subject of physical properties, the compilation of the American Petroleum Institute (7) will be of value in the design and analysis of hydrocarbon fractionation processes. A special fourday symposium on distillation theory and practice was held a t Baku in May 1966 (77). The fact that 44 papers were presented indicates strong interest in distillation in the USSR.
MAJOR TRENDS
Research
Research in distillation per se appean to be a thing of the past in university laboratories. This is borne out by the absence of distillation titles in the annual listings of chemical engineering theses in the United States (26) and. Japan (10). In those compilations, about two dozen titles have a bearing on distillation technology, and most of them deal with the important supporting areas of gas-liquid fluid mechanics and mass transfer. However, another type of distillation research is emerging and is welcome indeed. Studies of distillation equipment are being made a t large scale and under true distillation conditions (as opposed to “simulated” conditions, usually with air and water). Mention in the past has been made of the work at Fractionation R e search, Inc. (FRI) using columns up to 8.0 feet in diameter. Some of the FRI work is being released for publication, and during the year a paper was published that dealt with the measurement of liquid mixing on crossflow trays ( 723). Another large scale research project is under way, in Europe, where a 3.3-foot column is used under distillation conditions (74). The first study reported dealt with the performance of Turbogrid trays in the methanolwater system (75). Studies of other devices are promised. Research at the large scale has been aided by the development of new or imprbved measuring devices. Included in the literature was the description of a special movable probe for measuring maldistribution in packed columns (70); of dye injection methods for mixing studies (103, 723); of radiation absorption methods for determining froth density and level (14,700); and of
AUTHORS William L. Bolles is Rim$al Enginening Specialist and James R. Fair is Managn of th Chcmical Enginening Section, Central Enginening Department, Monsanto Co., St. Louis, M o . The authors collaborated on last year’s review; &. Fair hns prepared the review since 7962. VOL 5 9
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special photographic methods for identifying the mechanics of gas-liquid contacting (30). These and other tools are facilitating performance tests of plant distillation systems and are a vital part of the new research development. Physical Properties
Vapor-Liquid Equilibria. Extensive studies of vaporliquid equilibria (VLE) continued, and it is outside the scope of this article to list the large number of resulting publications. hfany of the titles appear in the Journal of Chemical and Engineering Data, and in the Russian Journal of Applied Chemistry, which is available in English translation. One interesting series of Czechoslovakian articles on VLE describes, in its 37th installment (745),a computer-aided study to fit to the various VLE correlation models the large collection of data presented by Hala (57). A preliminary study of 30 binary systems showed the two- and three-constant Redlich-Kister equations to give the best fit. A similar study by the M. LV. Kellogg Co. produced the first of a planned series of articles on VLE model-fitting ; this article dealt with the application of the Wohl equation to ternary VLE ( 2 ) . The comprehensive review of data and experimental techniques for solubility of gases in liquids by Battino and Clever (72) extends the earlier work of Markham and Kobe (97); these two references form a relatilely coinplete coverage of the solubility situation. Diffusion Coefficients. For liquid diffusion coefficients, the modified Eyring rate theory model (57) is a preferred predictive method, especially for viscous systems, although more cumbersome to use than the semi-empirical method of Wilke and Chang (747). This model, with appropriate adjustment of coefficients, gave reasonable agreement for new diffusion coefficient data (749) and was also successfully extended to ternary systems (34). Stage Calculations
Binary Systems. There is naturally declining interest in stage calculation methods applicable only to binary systems because of the widespread use of rigorous multicomponent models implemented by solution with computers. However, engineers sometimes find themselves without ready access to a computer. Also, the special case of the binary system is particularly amenable for illustration and teaching purposes. Husain (76) demonstrated a simplified method for the design of distillation columns with side streams or multiple feeds to handle binary systems having unequal heats of vaporization. T h e new method provides the same accuracy as the Ponchon-Sal-arit method without the cumbersome graphical construction. Singh (725, 729) developed another new method proposed as an alternate to the Ponchon-Savarit method for calculating the number of stages when the operating lines are curved. The method is analytical and based on the calculus of finite differences, and is applicable to columns with multiple feeds and side streams. General Methods for Continuous Distillation. A great deal of attention is being devoted to improvement 88
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of generalized stage models for rigorous, plate-to-plate, multicomponent, continuous distillation of ideal and nonideal systems, assuming implcmentation by a computer. A significant amount of important work on this problem is being done by private firms that regard their accomplishments as proprietary. Hence not all of the significant developments find their way into the public literature. Kevertheless, many worthwhile publications did appear. Tierney and Bruno (739) reported on use of the Newton-Raphson iteration method. Billingsley ( 78) demonstrated the mathematical basis for the Holland e method of convergence, as well as introducing techniques based on Jacobian matrices. An algorithm with simplified requirements for initial starting values was developed by Yarnada et al. (753). I n another approach, Chen (27) applied the principle of the difference equation. Petryschuk and Johnson (707) compared the 8- and block-relaxation methods for convergence. Tridiagonal matrices were introduced by Wang and Henke (144). Wilhelm (746) developed an analytical solution to the differential equations describing open distillation of ideal mixtures. Takamatsu and Tosaka (735), proved that plate-column models can be rigorously employed in packed-column applications, provided the proper physical model transforniations are made. Finally, Platonov and Bergo (708) developed the role of the pair of key components in the calculation of multicomponent fractionation. Batch Distillation. General, rigorous models for batch distillation are more complicated than those for continuous processing by two orders of magnitude, one for the change of most of variables with time, and the other for the effect of column holdup on the separation. Hence the progress of model development for batch distillation has lagged behind that of the continuous process. -41~0,there is the feeling in many quarters that batch distillation is obsolete, on the supposition that anything which can be done by batch distillation can be done better and cheaper continuously. However, batch distillations continue to be employed commercially, particularly in small scale operations, and in multiproduct, variableproduct applications. And, happily, rigorous generalized models for batch distillation are now beginning to appear. There were four such articles reviewed last year (27), and another appeared this year. Sverchinskii and Popov (734) showed that the computer design of batch distillation may be carried out by a newly developed method based on calculating the mixture compositions along the height of the column before recovery of the distillate. The method is claimed to give results closer to experimental values than those obtainable by the relaxation method. Multicolumn Simulation. The rigorous generalized models for continuous fractionation of multicomponent mixtures are also compounded in the analysis of systems of columns with recycle streams, Maikov and Abramov (94) presented an analytical treatnlent of a fractionating unit, consisting of several columns, which is based on one of the forms of dynamic programming. This approach
takes into account the complications arising from large number of variables at the withdrawal points of system. Equations are derived for cases where columns are all connected in series, and where they connected both in series and in parallel.
the the the are
Hydrodynamics
There was a considerable increase, both in number and in depth, of articles on hydrodynamics research. This trend is felt to be due not only to more recognition of its importance, but also to the availability of techniques of experimentation and handling of mathematical models to make possible commerically important results. Hydrodynamic studies were noted on bubble-cap trays (754). Studies of sieve tray hydraulics included use of a simulation to study the effect of geometry ( 3 ) , experiments to test theoretical and empirical models (73, 738), and testing of commercial-scale vacuum columns (743). As previously noted, Huml (74)described a large scale research fractionator 3.3 feet in diameter erected in Czechoslovakia for the purpose of testing and conducting research on various types of trays. Given here is a description of the equipment, including a flow diagram, a photograph, and control and measuring procedures. I n a second article, Huml (75) reported on results of tests in the new unit of Turbogrid trays fractionating a methanol-water mixture at total reflux and atmospheric pressure. Data were presented on five sets of Turbogrid trays showing the effect of free area, slot width, tray spacing, and vapor velocity. Mass-transfer coefficients calculated on the basis of measured tray efficiencies were found to agree well with those determined previously by others. Also demonstrated was the practical working range of the Turbogrid tray. Also in the area of Turbogrid trays, Rylek and Standart (722) demonstrated, by comparisons with experimental data, the validity of a theoretical model for prediction of the lower critical load, Eduljee (43) compared Turbogrid and shower-deck trays, and Hiratsuka et al. (63) report on the effect of tray geometry. I n the field of fundamental fluid mechanics research, Collier and Hewitt (30),in the second of a three-part series, presented a very interesting and complete review of the use of advanced photographic techniques for developing an understanding of two-phase flow. Techniques discussed include, for example, photographing streams into which dyes have been injected, special problems of illuminating the vapor-liquid interface, multicolored guided lighting, three-dimensional techniques, exposures down to 50 microseconds, stroboscopic illumination, microsopic techniques, use of x-rays and gamma rays, and many others. An understanding of the froth layer (aerated vaporliquid mass) is of vital importance to developing tray hydrodynamics. Several papers dealt with froth density. Rodionov (779) measured froth densities on trays in columns of diameters ranging from 2 inches to 2.3 feet, including both sieve and grid trays. The froth density was measured by four different techniques : transmission
of gamma radiation, difference in static and dynamic heights, hydrostatic head, and photography. I t was reported that froth density was relatively constant throughout the aerated mass and changed only slightly with gas velocity. Also studied was the effect of liquid flow rate, liquid head on the plate, viscosity and surface tension of the liquid, tray geometry, and the presence of surface-active agents. The experimentally determined froth densities were compared with those calculated by previously published mathematical models, with the result that certain ones involving the Froude number were found to be superior. I n a similar vein, Bernard and Sargent (74) used gamma-radiation to measure froth density on a 16-inch diameter sieve tray. Graphs were presented on the variation of froth density with elevation, both on the tray and in the downcomers. Also measured and discussed were foam height, liquid holdup, plate pressure drop, and the liquid entrainment. Aleksandrov et al. (5) reported on experimental studies of the effect of operating pressure on froth density on sieve trays. Kim (83) demonstrated that the holdup in vapor-liquid contacting on a sieve tray is related to the cube root of the Froude number. Standart and Rylek (733), in Part 7 of a series, discussed the significance of rate of dissipation of energy on the creation of interfacial surface and the mixing of the fluids in the phases. I n the area of liquid mixing on the tray, Molokanov et al. (703) studied flow patterns on a bubble-cap tray by means of dye injection. I n a study of entrainment on sieve trays ( 4 0 A ) , it was reported that the amount of entrainment depends not only on the density and viscosity of the gas, but also on its solubility in the liquid. Experimental techniques for the measurement of entrainment were also reported (30). Research on wall wetted columns is also important, from a standpoint of developing fundamental understanding of hydrodynamics. Abdonin et al. ( 7 ) discussed experiments on the effect of properties, geometry, and operating conditions on liquid film thickness, and presented a mathematical analysis of this model. Considerable work appeared on packed columns. Huber and Hiltbrunner (77) developed a theory for a packed column with maldistribution of reflux on the basis of three simplified models. I t was shown that number of transfer units in such a column depends on the degree of maldistribution and on the amount of lateral mixing. De Waal and Van Mameren (36) reported on studies of pressure drop, liquid holdup, liquid distribution, residence time distribution, and interfacial area in a 1-foot diameter column packed with 1-inch Raschig rings. One interesting observation was that interfacial area was determined mainly by liquid flow rate. Huber (70)demonstrated a new method for determining maldistribution by measuring variations in composition by means of a movable probe. There also appeared (79) a correlation on capacity and loading. Three studies were published on the effect on hydroVOL. 5 9
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Mass transfer in distillation devices is strongly influenced by geometry and systematic studies will have to be developed before generalization can be successful dynamics of which might be called column and tray “abnormalities.” Kovshov (87) reported that inclination of sieve trays from to 3 degrees resulted in an unstable flow pattern which led to a very significant (30-4070) loss of plate efficiency. Similar studies (88) were made of sieve trays in which different floor sections were at slightly different elevations. Hausdoerfer (59) experimented with the effect of inclining a packed column. Also in the field of hydrodynamics is a report by Todd and Van Winkle (740) on experiments with the jet tra) in a n air-water system. Empirical correlations were developed from the experimental data. Mass Transfer and Plate Efficiency
Mass transfer in distillation devices is strongly influenced by geometry, and this fact is emphasized in the general review of mass transfer in gas-liquid systems by Sideman et al. (127). Systematic mechanistic studies will, therefore, have to be developed along equipment lines before any generalizations can be successful. Within broad equipment types some generalization appears to have been successful. Asano et al. developed general expressions for mass transfer on bubble trays (sieve, bubble cap) for the liquid phase ( 9 ) and the gas phase (70). Dytnerskii et al. (47) attempted the same thing for a variety of contacting devices, using Russian data only, with only fair success. Sharnia and Gupta (726) developed separate phase correlations for dual flow and Turbogrid trays. I n all cases the models require considerable validation before they- can be considered reliable for design. Studies of the liquid phase transfer coefficients were reported for a wetted wall column with rippling ( 7 77), for a packed column operated in the loading region (147),for a small sieve tray (38),for a variety of packing types a t low loading ( 7 7 4 ) , and for liquid surfaces containing surfactants (22, &, 709). Overall transfer rates were measured in supported, high-density froths (752); in a sieve tray column operated in the critical region, where efficiency dropped off, partly due to high entrainment (52, 5 3 ) : in packed columns ( 4 2 ) ; and on “vapor surge” trays, where for the ethanol-methanol-acetone system the vapor-phase resistance was about three times that of the liquid phase resistance (75). Also studied was the efficiency of a large (1.0 meter) Turbogrid tray, the methanol-water data being directly useful for commercial-scale work (75) so long as system 90
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variations can be handled. T h e influence of liquid distribution and packing wetting on overall transfer rate was studied by Gil’denblat et al. (54). Everitt and Hutchinson (46, 47) demonstrated that indicated influences of composition on tray efficiency can be attributed to the corresponding composition-dependence of the two-film model on physical properties. Finally, in an interesting article, Armstrong and Throne (8) experimented in small laboratory columns to determine scale-down characteristics of sieve trays, using published large scale data ; although their efforts were moderately successful the approach used shows promise for the reverse-Le., scale-up uses. There was considerable work on the influence of gas and liquid mixing as well as recycle of gas and liquid (entrainment and leakage) on effective tray efficiency. Hartman and Standart (58),in Part 19 of a continuing series on distillation technology, presented a general discussion of a diffusion mixing model that accounts for vertical as well as horizontal mixing on distillation trays. Ruckenstein (720) and Haddad and M:olf (56) also discussed the general effects of mixing on efficiency. Sakata (723) described equipment and techniques for studying mixing during fractionation conditions in large equipment. Liquid mixing results were reported for sieve trays (78, 87), downcomers (738), slat-tray columns ( 7 73), and packed beds (90). I t is usually assumed that the vapor above trays is completely mixed; the influence of partial mixing of vapor was discussed by Diener (37). Analytical expressions were developed to show the effects on efficiency of entrainment ( 4 ) , leakage from trays (705),and concentration driving force at the extremities of the equilibrium curve (29). Distillation Equipment
Comparisons of Equipment. Several articles appeared in which the various vapor-liquid contacting devices were compared. British Chemical Engineering (23) assembled a very complete review of the available hardware for vapor-liquid (and liquid-liquid) contacting. Accompanying diagrams and descriptions cover bubblecap trays, sieve trays, the Hy-Flex tray, the trickle film column, a fabric packing, cyclone contactors, mechanically aided devices, and pulsed columns. Solomakah and Chekhov ( 7 3 7 ) classified tray-type devices into bubble types and flow types. The former class was divided into crossflow and counterflow trays, and the latter into trays with horizontal and vertical
flow of the phases. Each subclass was further subdivided according to the type of tray or its operating characteristics. Billet and Raichle (17) compared mass transfer devices by techniques of engineering evaluation and cost optimization. The devices compared included bubble-cap trays, sieve trays, valve trays, tunnel-cap trays, and Kittle trays, plus Interpack and Pall ring packings. After describing the general performance characteristics of sieve plates and plates without downcomers, Hoppe (64)presented a n analysis of published performance data on the relative efficiencies of several types of plates. McQuinn and Furter (701) compared contacting trays based on new data for the liquid-phase-controlled absorption of carbon dioxide into water from air. T h e trays studied included bubble-cap, sieve, Turbogrid, and Ripple types. Packed Columns. Porter et al. ( 7 70) discussed the problem of selecting contacting devices for vacuum distillation in the range of 1-20 mm. H g abs., where packed columns are often preferred over plate-type devices. Their comparison criterion was the volume of packing required per theoretical plate at unit throughput, which combines the traditional criteria of H E T P and pressure drop per theoretical plate. Several new types of packings were described, including wire gauze, wire fabric, and coiled springs. T h e special application of mesh-type packing in the rectification towers of air separation plants was discussed by Alekseev et al. (6). Experimental data were presented on oxygen-nitrogen fractionation with three different types of corrugated mesh packings. New Devices. A new type of “tangential plate” was described by Slobodyanik (130), wherein the motion of the phases is so organized as to cause the phases to move concurrently within the contacting stages, but countercurrently between the various stages of the column. Experimental data were presented on the hydrodynamics and mass transfer during desorption of carbon dioxide from water by air in a 12-inch diameter column. Very high mass transfer coefficients were claimed. A “spinning band” column was described by King and Yates (ad), wherein the vapor phase, and in some cases the liquid phase, is agitated by a band centered in the column and rotated a t speeds u p to 3000 r.p.m. A systematic experimental study of band design was made on
two systems at atmospheric pressure and under vacuum. Malyusov et al. (95) presented a study of a “highspeed, concurrent-flow, multitube column.” Experimental data were presented for two-stage and threestage columns having several vertical tubes in each stage. Proprietary Devices. The present status of the Kittle tray was discussed by Hoppe e t al. (65). Various versions of the Kittle tray were depicted, all based on use of a n expanded metal of special design. Drew and Schilling (40) presented new experimental data on the Kloss column, including H E T P and pressure drop us. load for 11 systems at pressures ranging from 1.5 to 760 mm. H g abs. This column is said to be particularly applicable for high vacuum distillation because of its low HETP, low pressure drop per theoretical plate, and absence of scale-up problems (scale-up is achieved by connecting multiple standard units in parallel). de Groot (35) presented new experimental data on the hydrodynamics and efficiency of Glitsch Grid packing in columns 1.5 and 8 feet in diameter. Laboratory Columns. Laboratory columns are potentially important for the purpose of providing basic data for the design of commercial-scale columns. A number of articles appeared on the performance and operating techniques of laboratory columns. Unfortunately, only one dealt with the problem of translation to the commercial scale (8). A good state-of-the-art review of the Oldershaw column was given by Mead and Rehm (102). T h e topics discussed included the purpose of employing such columns, description of the apparatus, techniques of operation, and methods for evaluating experimental results. Included is a demonstration of Oldershaw tests on two systems under both continuous and batch operating modes. Based on the experimental results, it was concluded that the Oldershaw involves column holdup too great for effective batch separations. An improved Oldershaw column was developed by Cooke (32) with demonstrated improvement in both efficiency and capacity on one system. He also compared the performance of the Oldershaw with Cannon packing and Goodloe wire mesh. Various laboratory column packings were compared by Manovyan and Khachaturova (96). T h e experimental study included six atmospheric and three vacuum columns containing different types of packing bodies. VOL. 5 9
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Wetted wall columns have low AP per transfer unit and provide an ideal yardstick for distillation performance Butler and Pasternak (24) described a new still for obtaining distillation curves on small (5-ml.) samples boiling above 300" F. Distillation Processes
Azeotropic Distillation. I n an extension of earlier work, Berg et al. (73) presented new experimental data on azeotropic distillation of hydrocarbons on a laboratory scale in a glass column of 1-inch diameter packed to a height of 4 feet with '/*-inch stainless steel Fenske rings. The column was calibrated on toluene and methylcyclohexane. The experimental plan was to mix equal quantities of component 1, component 2, and entrainer. Runs were made at total reflux with and without entrainer, and the relative volatilities between the subject components were calculated with and without entrainer by the Fenske equation from overall column performance. The experimental data presented cover 11 binary systems, with 8 to 34 entrainers per system. For the more promising systems, hypothetical commercial designs were calculated with and without entrainers. I n these cases it was found that the required theoretical plates could be reduced 20-50yo by use of azeotropic distillation, although the use of entrainer raised the utilities requirements and also the amount of materials to be handled. Kudryavtseva et al. (89) reported new experimental data on the azeotropic properties of seven three-component systems. Three binary azeotropes as well as a ternary azeotrope were found. Extractive Distillation. Coogler (37) presented a new commercial process for application of extractive distillation to separation of butadiene from the C, stream of an olefin plant. T h e solvent employed was an aqueous solution of dimethylacetamide. Bock and Fariwar-Mohseni (20) reported on a study of extractive distillation wherein n-propyl acetate was added to a benzene-toluene mixture, acetophenone to a methyl caprylate-methyl pelargonate mixture, cis- and trans-decahydronaphthalene to a methyl enanthatemethyl caprate mixture, and ethyl propionate to a nheptane-methylcyclohexane mixture. Two references dealt with the use of an inorganic salt as the extractive agent. Furter and Cook (50) presented a complete literature review on the effect of salt in distillation. Le Bec and Le Goff (92) reported on a laboratory study of the separation of water-isopropanol mixtures assisted by the addition of calcium or lithium chlorides. Vacuum Distillation. Vacuum distillation is recognized of such importance that the European Federation of Chemical Engineering has formed a working party on the special problems of vacuum distillation. A 92
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report of this committee (2.5) pointed out that the wetted wall column forms an ideal yardstick for performance, having the lowest pressure drop per transfer unit and the highest vapor capacity. I t was stressed that an internationally agreed series of test mixtures with known physical properties should be used to evaluate distillation column internals. Several devices were tested, both plates and packings. Eichel (44) disagreed with the practice noted in some plants of reducing the size of packing from bottom to top of the column. For vacuum distillation, where the volumetric vapor flow at the top may be much larger than at the bottom, it was proposed that the size of the packing material be increased from bottom to top. I t was observed that the HTU in the upper section will be lower, but it was shown by analysis that the greater capacity and other favorable factors are more than compensating. The example was based on Raschig rings. Cryogenic Distillation. A very complete state-ofthe-art report on the distillation of air was authored by Latimer (97). Topics included commercial flow schemes, problems of removal of water, carbon dioxide, and hydrocarbons, effect of the rare gases, recovery of argon, various refrigeration cycles, stage calculations in cryogenic towers, vapor-liquid equilibria, prediction of column capacities, and plate efficiency. Of particular interest is a discussion of flow arrangements in which the liquid flows in the same direction on successive trays, with greatly increased plate efficiency. Also discussed were froth height, weep point of sieve trays, sieve tray pressure drop, and downcomer hydraulics. I n a paper on the thermodynamic efficiency of gas separation plants, Ruhemann and Charlesworth (727) discussed the economic factors involved in the recovery of the heavier constituents from gases by cryogenic distillation. Foam Distillation. Foam distillation continues to receive attention, this year in the areas of methods for designing foam distillation processes (742), calculation of plate efficiencies ( I 7 I ) , surface excess parameter and the effect of column diameter (750), and the effect of various feed arrangements ( 7 7 ) .
System Dynamics
Studies of controlled cycling of phase flows and compositions in distillation columns continue to be prominent. This unsteady-state processing method is designed to permit only one phase to flow at a given time and t o require that the phases use the same passages during their respective flow periods. Schrodt (724) reviewed basic concepts and described application of the technique
to a number of chemical engineering operations, including distillation. Robinson and Engel ( 1 78) demonstrated theoreticaliy the advantages of controlled cycling. A group from the Monsanto Company developed analytical solutions of the dynamic equations, showed analogies to conventional operation, and derived asymptotic expressions for plate efficiencies (28, 732). This same group then conducted plant scale distillation studies, which demonstrated high capacities and higher efficiencies for fewer than about 12 trays (725). I t would appear that this mode of operation has economic potential but that certain scale-up hurdles will have to be overcome. I n a series of eight articles on cyclic processing operations, two dealt directly with distillation. I n one, relatively high frequency vapor pulsing gave efficiency increases under certain loading conditions (99). T h e other paper dealt with the same problem studied by other workers-mathematical simulation of the periodic countercurrent process (68). A number of dynamic analyses of distillation columns were repeated. They dealt with the experimental transient response of small columns (39, 48, 98), the theoretical frequency response of multicomponent distillation columns (757), theoretical dynamic response of packed columns (67),and the mathematical analysis of the startup of a plate-type gas absorber (33). Generalized models for the dynamic behavior of a distillation column were developed by Tetlow et al. (736)and Osborne et al. (706); and methods for obtaining tran iient plate efficiencies from transient operating data were developed by Groves et al. (55). T h e importance of considering holdup in dynamic analyses was stressed by Howard (69). Related to the control problem was the development of a steady-state model for a predictive control system (79). Distillation Design
For the past decade there has been observed a general decline in the number of design oriented articles in the distillation area, accompanied by a shift to fundamentals of thermodynamics, fluid mechanics, and transport phenomena. This year, however, marks a strong comeback for papers on the strategy of design, based on theoretical but mechanistic mathematical models, frequently supported by computer solution. For example, in a two-part paper by Billet and Raichle (76),there was a lucid illustration of the design process as applied to optimization of vacuum distillation systems. In this case the optimization is accomplished entirely by graphical means without use of a computer. T h e objective function optimized is total cost (amortized capital plus operating cost) over the life of the project. The chief constraint is maximum allowable pressure drop. I n an illustrative example, the column design for each type of contacting device is optimized, and then the various optimized cases are compared, all for the process of separating styrene and ethylbenzene at 100-mm. H g abs. stillhead pressure. T h e contacting devices compared included metal Pall rings, Interpack packing, tunnel-cap trays, valve-ballast trays, and centrifugal trays.
T h e model coefficients required as basic design data on these various devices were determined experimentally in a pair of pilot plant columns, 1.64 feet in diameter for the packings, and 2.62 feet in diameter for the trays. Also on the subject of vacuum distillation design was which developed a n an article in two parts by King (85), analysis of the degree of degradation in the distillation of heat-sensitive materials. Two indexes were proposed to evaluate the degradation : “stability index’’ to relate the degradation to the vapor pressure and residence time and “degradation index” to relate the degradation to the type of equipment employed. Ellis (45) presented a discussion on the classification of separation methods and the factors affecting the choice among them. These processes included ordinary distillation, extractive distillation, azeotropic distillation, absorption, extraction, adsorption, and chromatography. Heckmann (60) reported on 10 years of research a t the Institut fiir Chemisches Apparatewesen of the Technische Hochshule Magdeburg on mass transfer during continuous distillation. This resulted in the development of general criteria for comparing seven types of columns. Kitterman and Ross (86) wrote on special column design problems, including liquid inlet arrangements, subcooled reflux, feed tray design, sidedraw sumps, bottom tray seal pans, and reboiler return. Thibodeaux and Murrill (737) compared packed and plate columns, and developed a comparison checklist consisting of 20 items. A special design problem arises in an absorption system where the circulating oil is of wide boiling range and includes the boiling points of some of the components in the feed. I n this situation, Maaruschkin (93) pointed out that the composition of the circulating absorption oil at equilibrium may not a t all be the same as that originally charged and added as makeup. He then went on to derive a realistic model relating the equilibrium oil composition to the makeup oil composition, the design geometry, and the plant operating conditions. Also on the subject of absorption process design, Kim and Molstad (82) presented a model for economic optimization of the absorption process. Another important design problem is rangeability, or turndown, the ratio of maximum to minimum allowable operating rates. T h e turndown of sieve trays was analyzed by Hoppe and Mittelstrass (67). Still another important design problem arises in connection with the design and optimization of systems of distillation columns. Nartker et al. (704) showed how existing separate computer building blocks, each simulating one column, may be combined, without modification, into a system process simulation by integration with a properly written executive routine. REFERENCES (1) Abdonin, Y. A,, et al., Khim. i Ne/L Maskinoslr. 12, 16 (1965). (2) Adler, S. B., Friend, L . , Pigford, R . L., A.I.Ch.E. J. 12, 629 (1966). (3) Aerov, M. E., Boyarchuk, P. G., Bystrova, T. A . , Pozin, L. S., Lagover, I. I., Intern. Ckem. Eng. 7,235 (1967). (4) Aleksandrov, I. A , , Khim. i Tekknol. TopZiv i Marel 11 (12), 37 (1966). (5) Aleksandrov, I. A., Gorechenkov, V. G., Khim. Prom. 42, 219 (1966). (6) Alekseev, V. P., Navrotskii, Y.D., Braun, V. M., Poberezkin, A. E., Chem. Pelrol. En,!. ( U J S R ) 9, 581 (1966).
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