Distillation binary systems

Fig. 2-31. Diffusional distillation, binary system isopropyl alcohol/water.

For a binary system, the two possible opportunistic distillations are essentially identical and can be combined to concentrate the feed up to about 68% 2-propanol and produce pure water. The feed and products of the two possible strategic separations are ill-defined, whereas those of the opportunistic separation are known as outlined in Table 6. 

For ease of presentation and understanding, the initial discussion of distillation processes involves binary systems. Fxamining the binary boiling point (Fig. la) and phase (Fig. lb) diagrams, the enrichment from Hquid composition Xj to vapor composition represents a theoretical step, or equiHbrium stage. 

McCabe-Thie/e Example. Assume a binary system E—H that has ideal vapor—Hquid equiHbria and a relative volatiHty of 2.0. The feed is 100 mol of = 0.6 the required distillate is x = 0.95, and the bottoms x = 0.05, with the compositions identified and the lighter component E. The feed is at the boiling point. To calculate the minimum reflux ratio, the minimum number of theoretical stages, the operating reflux ratio, and the number of theoretical stages, assume the operating reflux ratio is 1.5 times the minimum reflux ratio and there is no subcooling of the reflux stream, then ... 

In order to determine the packed height it is necessary to obtain a value of the overall number of transfer units methods for doing this are available for binary systems in any standard text covering distillation (73) and, in a more complex way, for multicomponent systems (81). However, it is simpler to calculate the number of required theoretical stages and make the conversion ... 

Heretofore, the reflux ratio has been defined as reflux/distillate, L/D. Another very useful molar ratio is reflux/feed, L/F. In binary systems, L/F for all practical purposes is unchanging for wide differences in feed composition, so long as the following hold ... 

In distillation work for binary systems with constant relative volatility, the equilibrium between phases for a given component can be expressed by the following equation ... 

Multicomponent distillations are more complicated than binary systems due primarily to the actual or potential involvement or interaction of one or more components of the multicomponent system on other components of the mixture. These interactions may be in the form of vapor-liquid equilibriums such as azeotrope formation, or chemical reaction, etc., any of which may affect the activity relations, and hence deviations from ideal relationships. For example, some systems are known to have two azeotrope combinations in the distillation column. Sometimes these, one or all, can be broken or changed in the vapor pressure relationships by addition of a third chemical or hydrocarbon. 

Multicomponent distillation is by far the common requirement for process plants and refineries, rather than the simpler binary systems. There are many computer programs which have been developed to aid in accurately handling the many iterative calculations required when the system involves three to possibly ten individual components. In order to properly solve a multicomponent design, there should be both heat and material balance at every theoretical tray throughout the calculation. 

A good understanding of the basic equations developed for binary systems is essential to the understanding of distillation processes. 

The distillation of binary mixtures is covered thoroughly in Volume 2, Chapter 11, and the discussion in this section is limited to a brief review of the most useful design methods. Though binary systems are usually considered separately, the design methods developed for multicomponent systems (Section 11.6) can obviously also be used for binary systems. With binary mixtures fixing the composition of one component fixes the composition of the other, and iterative procedures are not usually needed to determine the stage and reflux requirements simple graphical methods are normally used. 

The McCabe-Thiele Method is restricted in its application because it only applies to binary systems and involves the simplifying assumption of constant molar overflow. However, it is an important method to understand as it gives important conceptual insights into distillation that cannot be obtained in any other way. 

Batch distillation from a pot will not provide a good separation unless the relative volatility is very high. In most cases, a rectifying column with reflux is added to the pot, as shown in Figure 14.7. The operation of a batch distillation can be analyzed for binary systems at a given instant in... 

CONSTILL - Continuous Binary Distillation Column System... 

The azeotrope in the ethanol-water binary system has a composition of 89 mole per cent of ethanol(14). Starting with a mixture containing a lower proportion of ethanol, it is not possible to obtain a product richer in ethanol than this by normal binary distillation. Near azeotropic conditions exist at points marked in Figure 11.44. The addition of the relatively non-polar benzene entrainer serves to volatilise water, a highly polar molecule,... 

Example The location of the best temperature-control tray in a distillation column is a popular subject in the process-control literature. Ideally, the best location for controlling distillate composition xa with reflux flow by using a tray temperature would be at the top of the column for a binary system. See Fig. 8.9o. This is desirable dynamically because it keeps the measurement lags as small as possible. It is also desirable from a steadystate standpoint because it keeps the distillate composition constant at steadystate in a constant pressure, binary system. Holding a temperature on a tray farther down in the column does not guarantee that x will be constant, particularly when feed composition changes occur. 

Binary System. The first task is to examine the characteristics of the 2-propanol—water-phase equilibria (VLE, LLE, SLE) to determine the compositions of interest and any critical features. 2-Propanol forms a minimum boiling azeotrope with water (80.4°C at 101.3 kPa (760 torr), 68 mol % 2-propanol). The azeotrope is between the feed and the IPA product and is a distillation boundary, thus it is impossible to obtain both desired products from any single-feed... 

Since in an extractive distillation process based on this ternary system the extractive agent is nonvolatile and remains in the liquid phase, and since because of the similarity of the molar latent heats of nitric acid and water there is substantially constant molar liquid overflow, the mole fraction of magnesium nitrate remains almost constant throughout the process. It is appropriate to represent the equilibrium situation as a pseudo-binary system for each magnesium nitrate concentration, and Figure 7 shows vapor-liquid equilibria on a nitric acid-water basis at a series of magnesium nitrate concentrations from zero to 0.25 mole fraction in the liquid phase. 

The calculational base consists of equilibrium relations and material and energy balances. Equilibrium data for many binary systems are available as tabulations of x vs. y at constant temperature or pressure or in graphical form as on Figure 13.4. Often they can be extended to other pressures or temperatures or expressed in mathematical form as explained in Section 13.1. Sources of equilibrium data are listed in the references. Graphical calculation of distillation problems often is the most convenient... 

In the years from 1940 through the 1960s, several notable shortcut fractionation methods were published. Of these, one method that included several of these earlier methods has stood out and is today more accepted. Fenske, Underwood, and Gilliland [9-12] are the core of this proposed method. Yet one more entry is added, the Hengstebeck [13] proposed method to apply multicomponent distillation. As these earlier methods pointed out only two component separations (called binary systems), the Hengstebeck added contribution is most important for multicomponent applications. 

Hgure 1.6 Common types of equilibrium diagrams for binary systems. (From J, R Fair. Distillation, in R, W, Rousseau, Handbook of Separation Process Technology, copyright 1987 by John Wiley Sons, New York. Reprinted by permission of John Wiley 4 Sons Inc.)... 

The equilibrium distillation behavior of the model fuels is adequately covered in the fuel oil discussion. However, the case for the rapid droplet vaporization, which was not clearly seen for fuel oils, is more amenable to analysis for a binary system. The surface gradients are given by the following relationship,... 

Many industrial columns use temperatures for composition control because direct composition analyzers can be expensive and unreliable. Although temperature is uniquely related to composition only in a binary system (at known pressure), it is still often possible to use the temperatures on various trays up and down the column to maintain approximate composition control, even in multicomponent systems. Probably 75 percent of all distillation columns use temperature control of some tray to hold the composition profile in the column. This prevents the light-key (LK) impurities from dropping out the bottom and the heavy-key (HK) impurities from going overhead. 

Liquid-vapor phase diagrams, and boiling-point diagrams in particular, are of importance in connection with distillation, which usually has as its object the partial or complete separation of a liquid solution into its components. Distillation consists basically of boiUng the solution and condensing the vapor into a separate receiver. A simple one-plate distillation of a binary system having no maximum or minimum in its boiling-point curve can be understood by reference to Fig. 3. Let the mole fraction of B in the initial solution be represented by... 

For application to distillation (a nearly isobaric process) binary-mixture data are frequently plotted, for a fixed pressure, as y versus x, with a line of 45° slope included for reference, and as T versus y and X, as shown in Figs. 13-3 to 13-8. In some binary systems, one of the components is more volatile than the other over the entire composition range. This is the case in Figs. 13-3 and 13-4 for the benzene-toluene system at pressures of both 101.3 and 202.6 kPa (1 and 2 atm), where benzene is more volatile than toluene. 

Strictly speaking, Eqs. (13-69) and (13-70) are valid only for describing mass transfer in binary systems under conditions where the rates of mass transfer are low. Most industrial distillation and absorption processes, however, involve more than two different chemical species. The most fundamentally sound way to model mass transfer in multi-component systems is to use the Maxwell-Stefan (MS) approach (Taylor and Krishna, op. cit.). 

Extractive distillation is sometimes used to separate mixtures for which fractional distillation is difficult, such as for binary systems of... 

---