II / / High and Low Pressure Distillation 1035

High and Low Pressure Distillation

A. R. Jose and J. Lopez-Toledo, mole fraction in the gas phase and the mole fraction Instituto Tecnologico de Celaya, of the same component in the liquid phase. Eqn [1] Celaya, Gto. Mexico shows that the equilibrium constant and relative vola- Copyright ^ 2000 Academic Press tility depend on the ratios of vapour pressure, liquid- phase activity coefRcients () and vapour-phase fugacities ( ). Introduction The relative volatility  is a measure of the ease of the separation. Engineers and scientists frequently have the problem of separating a mixture into its components or need to purify a speciRc product. To solve the problem they E If "1.0 the separation is not possible by distilla- usually apply a heuristic procedure, such as the fol- tion. lowing McMaster Rve-stage method: E If 51.2, distillation will probably be a good alternative, and should be chosen as the next 1. Identify and deRne the problem. step. 2. Propose and develop several alternatives to solve the problem. In step 4, the distillation system should be de- 3. Based on available resources, choose one or two of signed. The design and analysis of these columns the best alternatives. operating at high and low pressure form the core of 4. Work on the chosen alternatives in greater detail this manuscript and are presented in the next sec- and compare them before selecting the most ap- tions. propriate solution. Finally, in step 5, the scientist or engineer debates 5. Evaluate and decide if the problem has been whether the problem have been solved satisfactorily. solved. If this is the case, the problem is Rnished; if not, it is necessary to go back and start again. Let us assume that, for step 1, the problem is identiRed as the recovery of one or more organic compounds from a mixture, or the puriRcation of Distillation a speciRc chemical. In order to apply the second step, the differences in Distillation is based on diffusion of one or more physical properties are taken into account. Sometimes components through a mixture operating at a temper- the physical properties of the components are very ature, pressure and composition that assures the pres- different, so that, if two phases already exist in the ence of liquid and vapour phases. In distillation, the mixture, mechanical separations such as Rltration or mass transfer is due to a concentration difference decanting may then be used. But more often the moving from a place of high concentration to one of components of the mixture form a single phase and low concentration; it is not bulk movement as a result other differences in physical properties need to be of a pressure gradient, like pumping liquid through found. a pipe. When a homogeneous mixture is formed with components of different vapour pressure or Relative Volatility different boiling points, then distillation may be The key separation factor in distillation is the relative one of the several alternatives proposed in step 2. As volatility, deRned by eqn [1]. As the value of relative  will be seen later, the relative volatility ( )ofa volatility increases, the easier it is for components to mixture is: be separated by distillation. The number or theoretical stages required to separ- K y /x p0   " i" i i+ i i j [1] ate two species to a desired degree is strongly depen- i,j / 0  Kj yj xj pj j i dent on the value of . The variation of this parameter with pressure is } where Ki and Kj are the equilibrium constant for shown in Figure 1 for the system ethane propane. As components i and j respectively. These K values pro- seen,  is greater at low pressure than at high vide for each component a linear relationship for the pressure. Therefore, at low pressure (e.g. 1 atm), for 1036 II / DISTILLATION / High and Low Pressure Distillation

The design or sizing of distillation equipment re- quires the calculation of diameter and height of the column. Diameter depends on volumetric Sow rates of liquid and vapour inside the column, and these are functions of the total amount of the mixture feed to the column and the reSux or boil-up ratios. The desired purity of the components at the top and bot- tom of the tower dictates the height of the column. First, the theoretical or ideal plates are calculated, then efRciency is estimated to convert from ideal to real stages. By specifying the distance between plates (30}60 cm), and providing space for about four plates at the top of the column and six for the bottom for disengagement of the phases, the total height of the shell is determined. Figure 4 shows a block diagram for a typical design of a distillation column. Distillation at high and low pressures involves Figure 1 Variation of relative volatility with pressure. special characteristics, summarized in Table 2.

With Internal Devices of Distillation Columns There are three types of internal devices which a speciRed separation, the number of theoretical provide the intimate contact between phases in a stages is less than at high pressure (e.g. 10 atm). In distillation column. These are tray, random packing this case, why not use low pressure for this separ- and structured packing. ation? As will be seen later, the temperature at the top and bottom of the column plays an important role in this decision. At low pressure (0.05 atm) Table 1 Different classifications of distillation equipment or top temperature !1303C, while at high operation pressure (30 atm) top temperature is 103C. Therefore, distillation at high pressure is used since it is much Amount of compound to be separated Solute recovery Fractionation easier and more economical to reach 103C than ! 3 Mode of operation Steady-state 130 C. Unsteady-state Batch Semibatch \ Classi cation, Equipment and Design of Distillation Start and shut-down Columns Mixing between phases Stagewise contact Distillation is the most used in Continuous contact Internal device used None the chemical and petrochemical industry; as shown R Plates in Table 1, its operation is classi ed into several Bubble cups forms. Sieve Most of the time, distillation is carried out in verti- Valve cal columns or towers (like the packed column shown Packing in Figure 2) where the liquid descends while the va- Random pour ascends to the top of the column. The vapour Structured System characteristics Flash left at the top of the tower is condensed and at least Fractionation a fraction is returned back to the top of the tower as Azeotropic liquid reSux. Part of the liquid leaving the bottom of Extractive the column is vaporized in a reboiler and returned to Operating pressure Low the column as boil-up. High vacuum How the distillation equipment operates and how Medium vacuum it is calculated has been modiRed over the years. Low vacuum Medium Figure 3 shows some developments related to distilla- High tion. II / DISTILLATION / High and Low Pressure Distillation 1037

Factors favouring trays E High liquid rate (this occurs when high column pressures are involved); E large diameter (packing prone to maldistribution); E complex columns with multiple feed/take-offs; E feed composition variation; E scale-up less risky; E columns equipped with tray weigh less than those equipped with some packings.

Factors favouring packings E vacuum conditions; E low pressure drop required; E in smaller diameter columns (where trays are more difRcult to install, diameters 0.6}0.9 m or less); E corrosive system (more construction materials available); E foaming; E low liquid hold-up.

High Pressure Distillation As seen from Table 3, distillation at high pressure cover a wide range of applications that have some of the following characteristics:

1. The compounds have low molecular weight (like

C2,C3,C4 hydrocarbons). 2. Cooling water can be used in the condenser. 3. A change in pressure could change the azeotropic point by more than 10% (in mole fraction). 4. Energy is integrated between condensers and re- boilers of different columns.

Sometimes the increase in pressure is limited by the heat sensitivity of the bottom product (it could poly- merize or degrade) or by its critical temperature or pressure. As is known, at the critical conditions only one phase exists. If the mixture reaches this point two phases cannot exist anymore and therefore separation is not possible. It is necessary, therefore, to be careful Figure 2 Distillation column equipped with structured packing. when one is thinking of the operating column pres- sure. The upper pressure could be limited by economical Each has advantages and disadvantages. Trays conditions. If water is used as cooling medium at have been used for many years. Random packings the top, the pressure required may be too high have also been used over three generations of design. and therefore it would be necessary to use a refri- Structured packings have replaced trays, especially in gerant. Some authors recommend a upper pressure applications at low and atmospheric pressures. At value of 1.48 MPa using a total condensation with high pressure, trays perform better than packings. water, 2.52 MPa using partial condensation with For any applcation one must determine whether water, and if the pressure required is higher than tray or packing is the most appropriate. The follow- this value it is recommended to Rx the top pressure ing factors are an indication of when trays or pack- at 2.86 MPa using partial condensation with a refrig- ings are favoured. erant. 1038 II / DISTILLATION / High and Low Pressure Distillation

Figure 3 Paradigm shifts related to distillation. (With permission from Chemical Engineering Process (1972) 68 (8): 16). II / DISTILLATION / High and Low Pressure Distillation 1039

Figure 4 Procedure for designing a distillation column.

The upper pressure also could be limited by cost. If tionship exists for other equipment and materials of pressure is increased above 1 MPa, all equipment construction. costs begin to go up. For instance, a stainless steel Some speciRc applications of high pressure distilla- column with sieve trays could double in cost when the tion are given in Table 3. Most applications are in the pressure is raised from 0.1 to 3 MPa. A similar rela- petrochemical industry, but another very important 1040 II / DISTILLATION / High and Low Pressure Distillation

Table 2 Characteristics of high and low distillation operation

Feature Low pressure High pressure

When is it used? Products sensitive to temperature (thermal Gas feeds (C2/C3/C4 separation, air separation degradation) and ammonia recovery) Avoid using high pressure steam Allow water to be used in the condenser Break and allow separation of the system; also when a significant change occurs at the azeotropic composition (about 5%) with change in pressure When trains of columns with energy integration are desired allow energy of overhead vapour partially to vaporize the bottom of the other column) High molecular weight of components Low molecular weight of components Liquid density No significant change Decreases slowly when pressure increases Vapour density Low; decreases if pressure decreases Increases quickly if pressure increases Liquid diffusivity Low; increases slowly if pressure increases Increases quickly if pressure increases Vapour diffusivity High; decreases if pressure increases Low; decreases quickly if pressure increases Liquid viscosity High; decreases quickly if pressure increases Low; decreases quickly if pressure increases Vapour viscosity High; no significant change at low or high High; no significant change at low or high pressure pressure Relative volatility Increases if pressure decreases Decreases if pressure increases L  " G (0.1 '0.3 Flow parameter Fp  G L

application is cryogenic air separation. This is based signing a new one. High pressure distillation begins on a low and high pressure distillation column. The about 1 MPa, where liquid density, liquid viscosity reboiler of the low pressure upper column (0.13 MPa) and surface tension are unusually low, while vapour cools the condenser of the high pressure lower col- density is high. Trays are recommended as internal umn (0.60 MPa). As can be seen, the high pressure devices, especially above 2 MPa; below this value, column does not have a high pressure value but nei- trays and packings must be evaluated. Many prob- ther is it at atmospheric pressure. lems in high pressure distillation could be avoided The Further Reading section cites a good report on with an appropriate design. Available design high pressure distillation by Brierley which gives methods for low pressure can be applied to high some tips on improving an existing column and de- pressure but with some corrections, speciRcally in downcomer design because this is where liquid Sood- ing usually starts in high pressure distillation, while in Table 3 Specific applications of high pressure distillation low pressure, the bottleneck is the vapour Sow through the active area. Downcomer design should Application Pressure Range consider both downcomer back-up and downcomer Ethylene plant velocity. S Demethanizer 32 MPa Three ow regimes may exist in industrial columns: Deethanizer 27 MPa spray, froth and emulsion (see Further Reading). In C-2 splitter 16 MPa high pressure distillation, tray operation is usually in Depropanizer 19 MPa the emulsion regime, where liquid loads are high and Debutanizer 5 MPa vapour velocities comparatively low. However, in Dimethyl ether (DME) production small diameter column (less than 1.5 m) at low liquid (via the dehydration of methanol) DME column 1.60 MPa loads or at the low end of the high pressure range Water column 0.75 MPa (about 1 MPa), froth and spray regimes can be found. Production of heptenes from propylene In the spray regime, Sooding is caused by excessive and butenes: entrainment of liquid from the active area to the tray # "/ C3 C3 mixture separation 0.60 MPa above. It increases the tray pressure drop and the Miscellaneous hydrocarbons entrained liquid recirculates to the tray below. The / Propylene pronane separation 2.1 MPa larger liquid load in the downcomer and the increased Cryogenic air separation Lower column 0.6 MPa tray pressure drop together cause the downcomer to overRll, so that the tray Soods. In the emulsion II / DISTILLATION / High and Low Pressure Distillation 1041 regime, there is little entrainment of liquid by the equipment to obtain better and lower vacuum has vapour; instead, the high liquid load causes the down- been maintained. The cost of carrying out these comer to overRll and the tray to Sood. In the froth operations is of course more expensive than at regime, which is between the spray and the emulsion around atmospheric pressure. The increase in cost ones, Sooding may be by either mechanism, depend- is inversely proportional to the absolute operating ing on tray spacing and the particular combination of pressure. vapour and liquid loads. Distillation at low pressure is used for special cases A new model for the design or analysis of sieve with one or more of the following characteristics: tray columns has been developed at the University of Texas at Austin, in its Separation Research 1. heat sensitive products; Program. This model applies to both low and high 2. liquid feeds or liquid residue with high viscosity; pressure columns. The model was adjusted with 3. liquids with fouling and/or foaming tendencies; a wide experimental database from the open litera- 4. low operating pressure (medium and high vac- ture and also from the Separation Research Program uum); facilities. 5. low residence time. The equipment used to perform distillation at high pressure is basically the same as that used for close to The applications may be classiRed into four groups: atmospheric pressure, but the following special con- siderations should be taken into account. E distillation or evaporation of sensitive organic chemicals; 1. The physical properties move in the direction in- E concentration of foods, chemicals, polymers and dicated in Table 2 (liquid density, liquid viscosity biological compounds; and surface tension are low and vapour density is E recovery of organic solvents; high). E desolventing, devolatilization and Rnishing of 2. The thickness of the shell and column peripheral polymer solutions. equipment must be greater. 3. The advantage in capacity and/or efRciency of Table 4 shows the levels of vacuum used and also structured packing over random packing and lists representative equipment. Some advantages of plates decreases as the pressure increases. vacuum and molecular distillation are: 4. At very high pressure the efRciency of distillation decreases, due to back-mixing. E low residence time; 5. At very high pressure the use of plates (standard E high selectivity due to the higher values for relative and high capacity) is more reliable than packing. volatility; E cheaper heating requirements. The design equations for packed columns are basi- cally the same, with one possible correction to the Some of these special kinds of distillation are dis- height of a transfer unit (HTU) value, to take into cussed below. account the deviation from plug Sow:

Agitated Thin-\lm or Wiped-\lm (WFE) " # HTUtotal HTUplug flow HTUback-mixing [2] and Short Path Distillation Equipment For medium , thin-Rlm evapor- Remember that at high pressure the density of ators are used with or without agitation, but evapor- gases increases by several orders of magnitude; the ators with scraping blades provide better perfor- opposite is true for the diffusivity of the gas phase, mance and Sexibility. Where there are heat-sensitive and the surface tension decreases to very low values. substances, thermal decomposition may occur during evaporation. Decomposition increases exponentially Low Pressure Distillation with temperature and linearly with duration of ther- mal load. A gentle distillation method therefore re- Modern society is becoming ever more demanding in duces the evaporation temperature and the residence the quality of the products it uses and for health and times at high temperature. environmental reasons a better removal of some com- Since the evaporation temperature depends on ponents is required. This better puriRcation of many pressure, evaporation is performed under vacuum at products sometimes requires operating at very low considerably lower temperatures. If, in addition to pressure. Fortunately, the development of industrial applying vacuum, the thickness of the material on the 1042 II / DISTILLATION / High and Low Pressure Distillation

Table 4 Classification of low pressure distillation

Approximate pressure Typical equipment

Low vacuum 760 to 1 Torr (1000 to 1.315789 mbar) Vacuum column internals Plates Random packing Structured packings Medium vacuum 1 to 10\3 Torr (1.315789 to 0.013157 mbar) Thin-film evaporators Agitated thin-film evaporators High vacuum 10\3 to 10\7 Torr (0.013157 to 0.0000013 mbar) Short path distillation Molecular distillation Rotary Falling-film stills Wiped-film stills Centrifugal stills

Adapted with permission from Eckles AJ (1997) Difficult to process? Vacuum it! Chemical Engineering 94}100. wall is reduced, lower evaporation operate at the lowest pressure and provide the lowest temperature and shorter residence times can be pressure drop. achieved. The double-walled evaporator jacket is heated con- Most WFE are vertical cylinders where the feed tinuously with a heating medium. A vacuum system material is distributed to the inner surface; as the (often a combination of several individual pumps) liquid Sows downward, axially arranged blades or reduces the pressure in the distillation chamber. roller wipers distribute the liquid as a thin Rlm which is constantly mixed. In Figures 5 and 6, two types of WFE are shown. Figure 5 illustrates a WFE with rotating blades, while Figure 6 shows a unit with roller wipers and condenser. The last feature is a dis- tinct characteristic of short path evaporators and mo- lecular distillation stills. These types of equipment

Figure 5 Agitated thin- or wiped-film evaporator (WFE) with Figure 6 Wiped-film evaporator or short-path distillation equip- rigid blade rotor. ment, with roller wiper system. II / DISTILLATION / High and Low Pressure Distillation 1043

Depending on the temperature and the pressure in the Advantages of the process of WFE and short path distillation chamber, vapours leave through the va- distillation Thin-Rlm evaporators offer a number of pour discharge and travel to an external condenser. advantages. If higher operating pressure and temper- Involatile substances are discharged at the lower end ature are required for economic reasons, thermally of the evaporator. sensitive materials can be processed because of The cylindrical evaporation chamber is externally the short residence times utilized in WFE equipment. heated with hot pressurized water, steam or heat The thin liquid Rlm and turbulent mixing of the transfer oils. Rlm result in very quick attainment of equilibrium. Essential parts of a WFE are the rotating blades or This is especially important if a complete separation roller wiper system, which are axially arranged in the of a low boiling, volatile component out of the resi- evaporator. The blades (or roller wiper system) inSu- due is required. This is why thin-Rlm evaporators ence the following aspects: are successfully used as reboilers for rectiRcation columns. E Film thickness: the components to be evaporated Thin-Rlm evaporators are excellent degassers. If, are more easily separated from a thin Rlm. for example, small quantities of a volatile component E Uniformity of distribution: uniform distribution of have to be removed down to only a few p.p.m., the the feed material on the evaporator surface pro- evaporation capacity is not very important. The main motes excellent heat transfer and avoids overheat- goal in this case is to transfer the portions to be ing. separated as completely as possible to the surface of E Mixing of the Rlm: optimal mixing within the the Rlm. The transport is achieved by the roller wiper liquid Rlm increases the complete separation of the or blade system. components, thus enhancing evaporation. E Residence time: if gentle distillation is required, Molecular Distillation heat-sensitive materials can be heated in the evap- Molecular distillation may be considered as a special orator for only a short period of time. With a well- version of evaporative distillation in which the liquid constructed blade or wiper system, the residence is evaporated without boiling, but in such circum- time can be considerably reduced. stances that the evaporating molecules reach the con- R denser surface without obstruction. Three conditions Thin- lm evaporators can be totally vacuum- for molecular distillation are: sealed, thus avoiding any oxidation of product caused by penetration of air. 1. Pressure must be lower than 0.001 mmHg. This Discharge of the distillate vapours takes place low pressure is required to ensure that the mole- above the feed nozzle. This is why undistilled liquid S cules do not collide with each other. droplets, which may occur during ash evaporation, 2. The distance between evaporation and condensa- are trapped in the head of the evaporator and then S tion surfaces is of the same order of magnitude ow back to the evaporator surface. as the mean free path of the molecules and the McKenna (see Further Reading) provides equations free motion of the molecule is not mechanically to design or analyse operation of a WFE. Some of the R hindered. important equations to calculate the nal concentra- 3. The temperature of the condenser surface should tion CF of the residual liquid are: be between 50 and 1003C lower than that of the evaporation surface to prevent re-evaporation of ! H molecules. " C0 C # H CF C [3] (1#)ntotal The mean free path  is given by:

w0.5d2 tan  " 1.5 0.5 e 1 n 2 Dif 0.5 " [6] QNb (2N

0.5  0.5  2NbQ de where is the diameter of the molecule in centimetres ; 1# !l sin [4] 3  2.5 2 0.5  b   and N is the number of molecules in 1 cm . By reduc- dew tan Nb ing the pressure to very low values, N decreases, giving values for  of 1}3 cm, in the range of "Hede" HeNb the distance between evaporator and condenser ntot  [5] deHs de tan surface. 1044 II / DISTILLATION / High and Low Pressure Distillation

In molecular distillation the maximum or theoret- equipment or molecular distillation are shown in ical rate of evaporation was proposed by Langmuir in Table 5. 1916:

M Technologies to Improve Distillation W "0.0583P [7] e mmT Processes There are several ways of improving the separation of where M is the molecular mass and T is the absolute mixtures into the desired products. Some of these temperature. combine distillation with other processes (like ad- The relative volatility is given by: sorption, stripping, pervaporation, , membranes, etc.). Others are improvements to the p0  (M internal devices (such as high efRciency trays or high  " 1 1  [8] 1,2 0  efRciency packings). There are also the so-called en- p2 2 (M  hanced distillation methods (like extractive distilla- tion, homogeneous and heterogeneous azeotropic dis- From these two equations it may be seen that tillation, , heat integration, high molecular weight of the compounds involved is an gravity distillation, distillation, mech- important consideration. anical vapour recompression, and pressure swing dis- Equipment for molecular distillation and medium tillation). Further information about these technolo- R vacuum is expensive, but it is economically justi ed gies can be found in the texts cited in the Further for the separation of high value products such as Reading section. vitamins, fats, essential oils and hormone concentra- tion. Typical unit operations and applications for me- Conclusion dium and high vacuum using WFE, short path In the 21st century, distillation at high, medium and low pressure, will continue to be a much used method for the separation of components from homogeneous Table 5 Specific applications of low pressure distillation mixtures. Unit operations where wiped-film evaporators (WFE) are used Evaporation See also: I/Distillation. II/Distillation: Historical Develop- Concentration Degassing ment; Theory of Distillation. Membrane Separations: Distillation Reactions Stripping Deodorizing . Heating

Distillation Concentration Further Reading

Application of WFE for distillation and concentration at pressure Billet R (1979) Distillation Engineering. New York: of 1}1000 mbar Chemical. Acrylonitrile Extract solutions Brierley RJP (1994) High-pressure distillation is different! Essential oils Insulin Chemical Engineering Progress 90 (7): 68}77. Amines caprolactam Peroxides Dean JA (1995) Analytical Chemistry Handbook. New Quinoline derivatives Phospholipids York: McGraw-Hill. Dioctyl phthalate Pyrethrum Hewitt GF, Shires GL and Polezhaev YV (eds) (1997) Ency- clopedia of Heat and Mass Transfer. Boca Raton, FL: WFE for distillation and concentration of viscous products Fatty alcohols Resins CRC Press. H H Waxes Honey Hollo J, Kurucz E and Borodi (1971) The Applications Glue of Molecular Distillation. Budapest: Akademiai Polymers KiadoH . Lubricant oils Humphrey JL and Keller GE II (1997) Separation Process Silicone fluids Technology. New York: McGraw-Hill. Kister HZ (1990) Distillation Operation. New York: WFE for distillation and concentration of products liable to encrust McGraw-Hill. the heated surface Kister HZ (1992) Distillation Design. New York: Used oils Contaminated effluents McGraw-Hill. Solvents containing impurities Residue products from rectifi- cation and evaporation plants Lockett MJ (1986) Distillation Tray Fundamentals. Cam- bridge: Cambridge University Press. II / DISTILLATION / Historical Development 1045

McKenna TF (1995) Design model of a wiped Rlm Seader JD and Henley EJ (1998) Separation Process evaporator. Application to the devolatilization of poly- Principles. New York: Wiley. mer melts. Chemical Engineering Science 50 (3): Treybal RE (1980) Mass-Transfer Operations, 3rd edn. 453}467. New York: McGraw-Hill.

Historical Development

M. S. Ray, Curtin University of Technology, searchers. Another useful reference source is a series Perth, Western Australia of annual update bibliographic papers on ‘Equilib- Copyright ^ 2000 Academic Press rium-staged Separations’ (e.g. Separation Science and Technology 32(18): 3067}3083, 1997). Patent searches can also be performed on the web, e.g. Introduction www.ibm.com/patents, and www.uspto.gov (the & R Distillation is one of the oldest and most widely website of the US Patent Trademark Of ce). Sev- studied unit operations in chemical engineering. It is eral handbooks and monographs (and CD-ROMs) familiar as a separation technique to chemical, pro- containing property data useful for distillation sys- cess and petroleum engineers and to chemists. The tems have also been compiled, e.g. C. L. Yaws (trans- common techniques, design methods and numerous port properties data and thermodynamic diagrams); applications have been extensively documented in J. Gmehling and co-workers (including VLE data, monographs and in the journal literature (and confer- heats of mixing, azeotropic data); and the American ence proceedings) over many decades. Specialist texts Institute of Chemical Engineers’ Design Institute for and more general handbooks should be familiar to Physical Property Data (DIPPR) publications. those working in related Relds, therefore these stan- dard sources are not documented here. The import- Prediction of Vapour^Liquid Equilibria ance of distillation and its future directions have been (VLE) Data discussed by Kunesh et al. (1995) and by Porter (1995) (see Further Reading). This article presents SigniRcant advances in the interpretation and predic- a state-of-the-art overview of distillation by concen- tion of vapour}liquid equilibria (VLE) data have been trating on recent advances and possible future devel- made since the 1970s. These advances developed opments. from the publication of a range of equations of state (EOS) based upon the application of traditional ther- Sources of Information and Data modynamic principles and relationships. The EOSs provide interpretation or evaluation of available ex- For a subject as old as distillation, and with such perimental VLE data. The Wilson model (1964) is a wide range of applications, there is an extensive probably the most popular for dealing with liquid- collection of published information. With the advent phase activity coefRcients because it has only two of electronic databases and online web-based re- adjustable parameters, and it works well for both sources it has become much easier to perform litera- binary and multicomponent systems. The prediction ture searches on particular topics and to keep abreast of nonidealities in binary mixtures using the of the current literature and recent developments. For UNIQUAC model (1975) is rather more complex. this reason an extensive reference list to journal arti- Subsequent and related studies led to the develop- cles is not provided in the Further Reading at the end ment and use of the Group Contribution Methods of this article. Selected papers are included that can be such as ASOG and UNIFAC for the prediction of used to locate related references. Author or subject VLE data. The latter is widely used when actual searches of the journal literature (from 1956 to the system data are not available, provided that an ap- present, with six-monthly updates) can easily be proximate nonideality correction is acceptable. New performed by using the CHERUB2+ Chemical Engin- methods are being developed and probably the one eering Database (complied by M. S. Ray) which is showing most promise and of general applicability is included on the Engineering & Applied Science CD- known as: A Generalized Approach to Phase Equilib- Rom Database. The ability to easily search the chem- ria (AGAPE, 1995). ical engineering literature is a recent development, There are many EOS models described in the litera- and is an important advantage for distillation re- ture but only a few have wide use for engineering