Distillation 1979–1987

Review Distillation

19794987

K.E. Porter and J.D. Jenkins Separations

Research

Group, Aston

University,

Birmingham

B4 7ET. UK

Received 8 July 1987 This article is to be published at the time of ‘Distillation and Absorption ‘87’. This is the fourth in a series of distillation conferences, all organized bythe Institution of Chemical Engineers, at more or less regular intervals since the first Brighton conference in 1960. It is then perhaps appropriate to review work in this field which has been done since the previous conference in 1979. Indeed, at the end of that conference we presented a paper entitled ‘The inter-relationship between industrial practice and academic research in distillation and absorption” which reviewed the conference papers in the context of previous work. In this paper we comment on what has happened since then, but excluding the work to be presented at ‘Distillation and Absorption ‘87’. The review considers recent work under the headings (a) Packed columns, (b) Trayed columns and (c) Distillation Process Design (Sequencing). Keywords:

distillation;

packed columns;

trays; scale-up;

Packed columns It is appropriate to start with packed columns because the

last five years have seen significant changes in industrial practice, both by the much-increased use of packed columns of a large diameter and by the introduction of new structured packings. Although it is still true that a large diameter trayed tower is less costly than a packed column for the same duty, the demand for large diameter packed columns has increased in recent years, often associated with revamping for energy-saving schemes. Nevertheless, until recently there was a widespread lack of confidence in packed column design methods, a lack of confidence related to scale-up failures, so that packed distillation columns larger than 1.2 m in diameter were rare. At Distillation ‘79, Bolles and Fai? reviewed all the previously published work (mostly in small or pilot scale columns) and concluded that even for small diameter columns the reliable design of packed distillation columns required safety factors as large as 100%. (It may be noted that Porter and Jenkins’ also reviewed the Fair and Belles data bank and concluded that safety factors of 19%might be more appropriate: this was also the conclusion of a later paper by Vital et a1.3). It is now accepted that an unexpected loss in efficiency and thus scale-up failures are a result of vapour- or liquid-maldistribution, due to a badly designed distributor, and that properly designed columns will produce a predicted separation even for large diameter columns. It is believed that this increased confidence is due in part to unpublished work by Fractionation Research Incorporated in parallel with work by the packing manufacturers on improving distributor designs. Successful experience of large packed column operation has been reported by Strigle et aL4. Further, it has been reported recently, by the Norton Company, that as many as 20 theoretical plates were

sequencing

achieved in one packed bed. The examples given were in high pressure distillation. (A C-2 splitter, 8.5 m in diameter and at 8.5 bar pressure, and demethanisers of 1.45 and 2.45 m diameter at 31 bar pressure5.) We note in passing that the practical experience quoted above at last confirms the older theoretical models of maldistribution and scale-up failure, in particular that of Huber and Hiltbrunner published as far back as 19606.It is no longer necessary to seek for theoretical models that will predict that large diameter columns will always fail. This much increased use of large-diameter packed columns has been accompanied by the development of new packings. At the 1979 Conference, Strigle and Porter’ described a new random packing, Intalox Metal Tower Packing (IMTP) which combined the characteristics of both ring and saddle packings and was shown to have a significantly lower pressure drop than other random ring packings of the same mass transfer efficiency. Since then IMTP has established itself in the industry and inspired the creation of other random packings of a similar saddle shape (e.g. Levapack and Nutter rings). Of equal, if not greater significance has been the introduction and w-idespread use of structured packings, i.e. packing made of corrugated vertical sheets. These packings have a lower pressure drop and greater throughput than random packings of the same HETP. The use of structured packings has been pioneered by the Sulzer and Koch companies who introduced their metal sheet Flexipacs which was derived from the much older Sulzer BX structured packing made from wire gauze. Other companies, notably Glitsch (Grid-Pack), have also provided structured packings’ and very recently the Norton Company”. In concluding this report on packed columns we note papers which report new experimental results or theoretical descriptions of packed column performance. Hock, Wesselingh and Zuidenveg have reported careful measure-

%50-4214/87/01011-05$03.00 0 1987

Butteworth

8 Co (Publishers) Ltd.

Gas Separation

and Purification

1987

Vol 1 September

11

Reviews

ments of the liquid flow-pattern in randomly-packed columns” which confirms a previous description of the flow in terms of rivulets’*. Hoek et al. show that the liquid flows through a limited number of channels which mix at intervals and they obtained reasonable agreement with a random walk theory (cf. Albright”). They also investigated the rate of build-up of the wallflow and reach conclusions about the design of liquid distributors. It is of interest to read these papers and compare them with the recent experimental work of Spedding et al. ‘6 l7 who made detailed observations of point-to-point variations on liquid concentration produced in a packed column absorbing ammonia into water. They were able to interpret their observations in terms of the rivulet model of Porter’2-14. Fair has continued work on packed columns and published both an improved method of calculating HETP18 and a correlation of pressure drop” in structured packings. Another interesting paper by Koshi and Rukovena” reports experimental measurement of HETP for two mixtures of high relative volatility. They show a variation of HETP with concentration and point out this may be expected from theoretical considerations. This is an interesting confirmation of ‘the easy separation effect’, i.e. HETP is often unusually large for easy separations or empirical rules of ‘never use less than 2 m of packing’ 4 thumb). Also of interest is a pape? which interprets packed columns performance in terms of a simplified maldistribution model reminiscent of that of Mullin**, and a paper on heat transfer in packed beds based on observations on a vacuum crude oil pump-around application23.

Trays Whereas the recent emphasis in packed column technology has been in new applications and new packings, most of the recently published work on trays has been concerned with theory and experiments which may permit a better understanding of tray performance. Nevertheless a new tray design was described by Owen Jenkins24 the Paraflow tray where the two halves of a tray are displaced vertically from each other. The author reports an increased separation in a column fitted with trays and that this is because the design permits more trays to a column, but an alternative explanation is that this is another arrangement which permits parallel flow, and consequent enhancement of tray efficiency (Lewi~*~). The arrangement sends all the liquid across a weir length half that of conventional trays, thus it is probably more suitable for distillation at a reduced pressure, (i.e. low liquid rate duties). Lemieux has published data on baffle tray performance26. Little has been published previously on baffle trays. The period between the distillation conferences of 1969 and 1979 was one in which significant advances were made in understanding flow-regimes and flow patterns and their likely influence on tray performance. This was reviewed in some detail by Porter and Jenkins’. At the Distillation ‘79 meeting Zuidenveg27 introduced the definition of another regime, that of emulsified flow, in which the vapour is dispersed as small bubbles in a continuous liquid phase. The regime of emulsified flow is expected to occur at high weir loads and low vapour velocities (typical of high pressure distillation) and may be contrasted with the spray regime in which drops of liquid appear in a continuous high velocity vapour

12

Gas Separation

and Purification 1987

(typical of vacuum distillation). Following Zuiderweg’s interpretation, inbetween the spray regime and the regime of emulsified flow, lies the mixed regime, part bubbly and part spray. A number of workers have presented equations to distinguish between the spray regime and the bubbly , but most of this work considers (or mixed) regime 28-30 liquid hold-up and a no-liquid cross-flow situation such as the experiment of Porter and Wong3’. At Distillation ‘79, Porter and Jenkins’ suggested that the boundary between the spray and bubbly regimes on an actual liquid cross flow tray might be identified by the minima in rates of liquid entrainment plotted against weir load. Since then no new regimes have been suggested but several papers provide valuable observations which support the previous work. In a recent paper Fai2* and co-workers describe experiments to determine the transition from spray to bubbling on a cross flow sieve plate using a technique which established how many holes were bubbling vapour through a continuous liquid and how many were producing continuous gas jets. They show that the transition from a bubbly dispersion to spray is a gradual process. The transition determined by light beam3’ was shown to be equivalent to 60% of the holes jetting. The work included measurements of bubble sizes (large and small) as an introduction to workon point efficiency mass transfer. Lockett and Plaka33 have used different bubble sizes to interpret their measurements of the liquid film resistance in distillation (but see also Garner and PorteG4, Porter, Davies and Wong35, Haselden36). In some work from Beijing reported by Ye, Shi and Zhou37 the transition was identified by the entrainment minimum method as suggested by Porter and Jenkins. Their results include more than one size of hole and confirm previous ideas that small holes produce a bubbly mixture (and less entrainment) under conditions for which large holes produce a spray. Dulesia38 measured the transition (spray to bubbly) for a valve tray (all previous works had used sieve trays). He showed that a tray with the usual size of valve (1% ins) behaves more like a small hole sieve tray, i.e. more bubbly then spray. Zuidenveg has continued with his studies of the regime of emulsified flow and has suggested that the tray efficiency of high pressure distillation columns operating in this regime, would be reduced by much increased vapour backmixing, i.e. a significant mass of vapour is expected to be carried down through the downcome+‘. This concept forms part of his interpretation and prediction of tray efficiency in terms of flow regimes given at the 2nd World Congress of Chemical Engineeringa. Of interest also is his use of much simplified equations for gas film and liquid film mass transfer coefficients in terms of but one physical property. This approach is based on the ideas behind the ‘Total Reflux Flow Parameter’ correlations of Porter and Jenkins’ (i.e. for the conditions of practical distillation all physical properties and flowrates correlate one with another). Other simplified correlations have been developed, e.g. that of the Norton Co. for HETP with IMTP (Ref. 78). Turning now to the effect of the liquid flow pattern on tray efficiency, a recent paper by Solari and Bell”’ reports measurements (using tracers) of the flow-pattern on a sieve plate 1.2 m diameter (i.e. air-water simulation of distillation). They were able to make detailed measurements of the direction of liquid flow for different gas and liquid flow rates. They concluded that severe liquid

Vol 1 September

Reviews

channelling accompanied by circulating zones at the sides of the tray occurred at low gas velocities, but that this disappeared at high gas velocities. They did not state in which flow regime the plate operated, but their conclusion is similar to that predicted by Porter and Locket& i.e. expect channelling for bubbly regime operation where liquid momentum dominates, but expect no channelling in the spray regime where vapour momentum dominates and the liquid flows by ‘spray diffusionV4*.However, much more work is required on the liquid flow pattern, including variations in tray and hole diameter, and with other types of tray than sieve trays. At present theories are based on hypothetical flow patterns rather than on experimental ones. The effect of the flow pattern on mass transfer is being studied by workers at Essen43 and at Aston44.4s by the technique o f feeding hot water on to a tray and cooling it by air used to simulate the vapour. This permits investigation of commercial scale trays in the laboratory. An interesting paper by Raper, Pincewski and Fell& presents experimental results to show that back-mixing in the spray regime (determined by the spread of a forwardflowing pulse tracer) is less than that previously assumed on the basis of back mixing from the outlet-weir. The authors suggest the previous work is misleading because of unusually high back mixing near the weir due to circulation of liquid there. In brief, they calculate that spray moves liquid in essentially plug flow. This increases even more uncertainty about the nature of the spray regime, what it is and how it flows (for a discussion of this problem see Porter, Lockett and Safekourdi4’). No recent papers appear to exist on the theory of point efficiency but two groups have looked at the use of a small laboratory Oldershaw column to provide an experimental ‘point’ efficiency which is then used in a ‘plug flow’ model to predict tray efficiency for a commercial-scale tray. Thus Fair47 reported a successful prediction of published FRI data for a 1.2 m diameter column. Biddulph48 evaluated this technique in the laboratory and concluded that the Oldershaw point efficiency may be conservative. A theoretical model to predict the effect of liquid entrainment on tray efficiency is presented by Lockett49. Their discussion included the effect of stagnant zones at the sides of the plate as described in previous work on tray efficiency5’. They conclude that in some cases the simple will produce an overdesign of equation of Colbur# about 10% in the number of plates required. A paper by Thorogood”, based on Air Products experience, confirms the importance for maintaining tray efficiency of crossmixing to overcome liquid (or vapour) maldistribution. It is perhaps worth noting that the same explanation may be used for scale-up failures in both trays and packed columns. Maldistribution reduces separation and crossmixing can correct maldistribution, but cross mixing is only effective over a limited distance (about 0.5 m) so the effect of maldistribution in small columns is eliminated, and small diameter columns work when large diameter columns fail. A revolutionary development

-

Higee

About five years ago a quite new device was announced by ICI. This was the centrifugal ‘Higee’ rotating distillation column53. By increasing the rate of separation of vapour and liquid in countercurrent flow a much increased

throughput is obtained, together with increased rates of mass transfer. It is claimed that a rotating distillation column 5 ft tall and 3 ft in diameter is equivalent to a conventional column 100 ft tall and 3 ft in diameter. The Higee column works at 1500 to 3000 revolutions per minute, i.e. at centrifugal pump speeds rather than at centrifuge speeds. It is the type of device which could conceivably make trays and packing obsolete overnight. However, despite the fact that Higee was developed by a large chemical company, as far as is known it has never been used on a large scale. Why this is so is not known, other than the conservation inherent in the chemical process industries or maybe it costs too much. Floating distillation columns

A possible application for the Higee column will be on floating chemical plants, subject to wave motion, so that the conventional tall distillation column swings about. The commercial interest in floating plant fluctuates with the price of oil, nevertheless it has been such as to promote some research. Two papers describe the effect of a rockin motion on the efficiency of both trayed and packed 4 columns. Both papers to the same conclusion, i.e. first, that a column which rocked about a truly vertical position suffered only a small-to-moderate loss in efficiency, but second, the effect of a non-vertical mean position is to produce a considerable loss in efficiency. Distillation

process design

(sequencing)

Of particular interest is the design of the optimum energyintegrated sequence. The cost of distillation is reduced very significantly by using waste heat from the process of which it forms a part, or by exchanging heat between the condenser of one column and the reboiler of another. The problem is notorious for the amount of calculation required to evaluate all possible designs. For example, a six-component mixture may be separated by any of 42 different sequences each of five columns. Even more sequences may be proposed if complex column designs are used (see for example Refs. 56 and 57). Thus the earliest solutions consisted of ‘rules of thumb5s. 59and with the development of computing technology a number of optimization procedures have been proposed, based on either algorithmic60 or evolutionary procedure&‘. These produced answers but added little to our understanding of process design. It is encouraging to find in the more recent literature, work which is used to identify and illustrate some fundamental principles. To be noted are the works of Westerberg and co-workers on energy-integrated sequence#** 63 and those of Doherty and co-workers on multicomponent azeotropic distillation (see for example Refs. 64 and 65). Both introduce new concepts. Another paper directed towards increasing understanding by simplified approaches is that of Fiaskowski and Krolikowski66 who compare the energy requirement in alternative thermally-coupled sequences by means of equations based only on total vapour load. A question of some importance for energy-integrated columns (the condenser of one column heats the reboiler of another) concerns changes in the composition of the feed. This may so change column temperatures that a design becomes unworkable. The problem has been tackled by Westerberg6z*63who introduces and defines the concept of ‘structural flexibility’ to guide the choice of

Gas Separation

and Purification 1987 Vol 1 September

13

Reviews

a satisfactory integrated process. The similar problems of choosing the best integrated column design from the point-of-view of control is considered by Levien and Mora&‘. Multiple-effect distillation and the advantages of a vapour feed has been analysed by Lynn and Hanson68. For some years now studies have been published on energy-saving by means of complex column designs. These are sequences based on multiple feeds and sidestreams. For example the ternary mixture ABC may be first split into an AB top product and a BC bottom product, and both products are fed by different feed points into a second column which produces Aat the top, C at the bottom and sidestream of B. A general calculation procedure for sequences of complex columns has been described by Sargent64 New complex column arrangements have been described by Glinos et al.56. As well as rigorous computer-based calculations of energy usage and cost, they provide approximate design methods to estimate the range of feed compositions where there is likely to be an economic advantage in using the new designs. These should be compared with previous work in this field which is usually based on case studies. For example, Cheng and Luyben5’ compare several energy-integrated process designs for ternary mixtures to show that some of those containing complex columns require 20-30% less energy than the conventional sequences. The theoretical advantages of complex columns have been known for many years, but little has been published about the practical application of this technology. The calculation of the number of stages in a single column for a multicomponent mixture is by now usually straight-forward by methods such as that of Napthali and Sandholm”. Nevertheless several recent papers have described alternative calculation procedures which avoid the convention of associating the number of calculation steps with the number of theoretical plates, i.e. the calculation proceeds by groups of plates (see Sivasbranian er al.” and Benallou et al. 72). Sivasbranian et al. relate their work to packed column design and Benallou et al. to column control strategies. In a ‘collocation approach to distillation column design’ due to Swartz and Steward73, the number of stages in a column is treated as a continuous variable so that distillation problems are reduced to standard non-linear programming problems. When the number of components in the feed and products of a column is very large and the streams are defined by their range of boiling points, the mixture is said to be complex. In general empirical design procedures are used to size columns for complex mixtures, or alternatively tray-to-tray calculations are carried out based on a smaller number of ‘pseudo components’. A very interesting but mathematically lengthy alternative procedure based on continuous thermod namic functions is described in Kehlen and Tazsch’ 2. There is at present a renewed interest in column design in general which may be illustrated by briefly mentioning some recent work. Arising from their work on azeotropic mixtures, Levy and Doherty75 present a method for calculating tangent pinch points for multicomponent mixtures. The choice of minimum reflux ratio for a multifeed, multiproduct column is considered by Chou and Yaws76, column design to ensure controllability by Levien and Morar?’ and an analysis of batch distillation optimization by making and recycling an intermediate ‘waste cut’ by Christensen and Jorgensen7’.

14

Gas Separation

and Purification 1987

References 1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16 17 18 19

Porter, ICE. and Jenkins, J.D. Distillation ‘79 ZChemE.Symp. Ser. (1979) 56 5.1/l-5.1/47 Bolles, W.L. and Fair, J.R. Distillation ‘79ZChemE. Symp. Ser. (1979) 56 3.3/35-3.3/89 Vital, J.J., Grossels, S. and Olsen, P. AJChE National Meeting, Cleveland (1982) Strigle, R.F., Rukovena, F. and Koshy, D. Proc. 2nd World Congr. Chem. Eng. Montreal (1981) 4 247 Robinson, K. Advanced Distillation Short Course. Aston University. IChemE (1987) private communication Huber, M. and Hiltbrunner, R. Chem. Eng. Sci. (1966) 21 819 Strigle, R.F. and Porter, K.E. Distillation 79 ZChemE. Symp. Ser. (1979) 56 3.3119-3.3133 Lieberman, N.P. Hydrocarbon Processing (1984) 63 (4) 143 Chen, G.K., Kitterman, L. and Shieh, J.E. Chem. Eng. Prog. (1983) 79 (11) 49 chern; En; (fnt. Edn.) (1987) 94 (3) 14 Hoek, P.J., Wesslingh, J.A. and Zuidenveg, F.J. Chem. Eng. Res. Dev. (1986) 64 43 1 Porter, K.E. Trans. ZChemE. (1968) 46 T69 Porter, K.E., Bamet, V.D. and Templeman, J.J. Trans. ZChemE. (1968) 46 T74 Porter, K.E. and Templeman, J.J. Trans. ZChemE. (1968) 46 T86 Albright, M.A. Hydrocarbon Processing (1984) 63(9) 173 Spedding, P.L., Munro, P.A. and Jones, M.T. Chem. Eng. J (1986) 32 65 Spedding, P.L. and Jones, M.T. Chem. Eng. .I (1987) 33 1 Bravo, J.L. and Fair, J.R. Znd. Eng. Chem. Proc.Des. Dev. (1982) 21(l) 163 Bravo, J.L., Rocha, J.A. and Fair, J.R. Hydrocarbon Processing (1986) 65(3), 45

20 21 22 23 24

Koshi, J.D. and Rukovena, F. Hydrocarbon Processing (1986) 65(5) 65 Spiegel, L. and Yuan, H.C. Proc. 2nd World Congr. Chem. Eng., Montreal, (1981) 4 274 Mullin, J.W. Znd. Chemist (1957) 33 408 Kulbe, B., Hoppe, K. and Keller, J. Znt. Chem. Eng. (1985) 25 474 Jenkins, A.E.O. Cost savings in distillationZChemE. Symp. Ser. (1981) 61 73

25 26 27

Lewis, WK. Znd. Eng. Chem. (1936) 28 399 Lemieux, E.J. Hydrocarbon Processing (1983) 62(9) 106 Hotbuis, P.A.M. and Zuidenveg, F.J. Distillation ‘79ZChemE. Symp. Ser. (1979) 56 2.2/l-2.2/26 28 Payne, G.J. and Prince, R.H. Trans. ZChemE. (1977) 55 266 29 Barber, AD. and Wijn, E.F. Distillation ‘79 ZChemE. Symp. Ser. (1979) 56 3.1/15-3.1/35 30 Locket& M.J. Trans. ZChemE. (1981) 59 26 31 Porter, K.E. and Wong, P.F.Y. Distillation ‘69ZChemE.Symp. Ser. (1969) 32 2122 32 Prado, M., Johnson, K.L. and Fair, J.R. Chem. Eng. Prog. (1987) 83(3) 32 33 Locke& MJ. and Plaka, T. Chem. Eng. Res. Dev (1983) 61(2) 119 34 Gamer, F.H. and Porter, K.E. Int. Symp. on Distillation, IChemE., Brighton, England (1960) 43 35 Porter, K.E., Davies, B.T. and Wong, P.F.Y. Trans. ZChemE. (1967) 45 T265 36 Haselden, G.G. and Ashley, M.J. Trans. ZChemE. (1973) 51 188 37 Ye, Y., Shi, J. and Zhou, Z Znt. Chem. Eng. (1985) 25(l) 176 38 Dulesia, H. Trans. ZChemE. (1983) 61 329 39 Zuiderweg, F.J., Hofhuis, P.A.M. and Kuzniar, J. Trans. ZChemE. (1984) 62 39 40 Zuiderweg, F.J. Proc. 2nd World Congr. Chem. Eng., Montreal (1981) 4 268 41 Solar-i, R.B. and Bell, R.L.AIChE. J. (1986) 32 640 42 Porter, ICE., Safekourdi, A. and Locke& M.J. (1977) 55 190 43 Stichlmair, J. and Wesshuhn, E. Chem. Zng. Techn. (1973) 45(5) 242 44 Porter, K.E., O’Donnell, K.A. and Latifipour, M. ZChemE. Symp. Ser. (1982) 73 L33 45 Audu, B. PhD Thesis Aston University (1986) 46 Raper, J.A., Pinczewski, W.V. and Fell, C.J.D. Chem. Eng. Res. Dev. (1984) 62(l) 111 47 Fair, J.R., Null, H.R. and Bolles, W.C. Znd. Eng. Chem. Z’roc. Des. Dev. (1983) 22 53

Vol 1 September

Reviews Biddulph, M.W. and Dribika, M.M., AZChE.1. 1986, 32 1864 49 Locke& M.J., Rahman, MA. and Dulesia, HA. Chem. Eng. Sci. (1983) 38 661 50 Porter, K.E., Locket& M.J. and Lim, C.T. Trans. ZChemE. (1972) 50 91 51 Colbum, A.P. Znd. Eng. Chem. (1936) 28 526 52 Thorogood, R.M. Cost savings in distillation ZChemE. Symp. Ser. (1981) 61 95 53 New mass transfer find is a matter of gravity Chem. Eng. (1983) Feb 2lst 54 Svensson, V. Chem. Eng. Pmg. (1982) 78( 11) 43 55 Hoemer, B.K., Wiesner, F.G. and Berger, E.A. Chem. Eng. Rag. (1982) 78(11) 47 56 Glinos, K.N., Nikolaides, I.P. and Malone, M.F. Znd. Eng. Chem. Proc. Des. Dev. (1986) 25 694 51 Cheng, H.C. and Luyben, W.L. Znd. Eng. Chem. Z%c. Des.Dev. (1985) 24 707 58 Lockhart, F.J. Petroleum Refiner (1947) 26 104 Harbert, V.D. Petroleum Refiner (1957) 36 169 2 Hendy, J.E. and Hughes, R.R. Chem. Eng. Z+og (1972) 68 69 61 Westerberg, AW. and Stephanopoulos, G. Chem. Eng. Sci. (1975) 30 %3 62 Bingzhen, C. and Westerberg, A.W. Chem. Eng. Sci. (1986)41 355 48

63 64 65 66 67

68 69 70 71 72 73 74 75 76 77 78

Westerberg, A.W. and Bingzhen, C. Chem. Eng. Sci. (1986) 41 36.5 Knight, J.R. and Doherty, M.F. Ind. Eng. Chem. Fund. (1986) 25 279 Doherty, M.F. and Perkin, J.D. Distillation ‘79 ZChemE. Symp. Ser. (1979) 56 4.2/21 Fiaskowski, Z. and Krolikowski, L. AZChE. J. (1986) 32 537 Levien, K.L. and Morarl, M. AZChE.Z. (1987) 33 70 Lynn, S. and Hanson, D.N. Znd. Eng. Chem. Ptrx. Des. Dev. (1986) 25 936 EIiceche, A.M. and Sargent, RW.H. Cost Savings in distillation ZChemE. Svmo. Ser. 11981) 61 1 Naphthali, L.M. and-Sandholm, D:P.AZChE. .Z(1971) 17 148 Sivasubramanian, M.S., Taylor, R. and Krishnamurthy, R. AZChE. J. (1987) 33 325 Bemallou, A.. Seborrz.D.E. and MeBichamns. _ I DAAZChE. J (1986) 32 l&i -. Swartz, O.L. and Stevard, W.E. AZChE.J (1986) 32 1832 Kehlen. H. and Ratsch. M.T. Chem. Ena. Sci. (1987142 221 Levy, S;G. and Doherty, M.F. Chem. E& Sci. (1986j 41 3155 Chou, S.M. and Yaws, C.L. Hydrocarbon Processing (1986) 65(12) 41 Christensen, F.M. and Jorgensen, S.B. Chem. Eng. J (1987) 34 57 Stringle, R.F. Random packings and packed towers Gulf Publ. Co., Houston, Texas, 1987

Gas Separation and Purification 1987

Vol 1 September

15