Binary Distillation Principles

The equipartition principle is mainly used to investigate binary distillation columns, and should be extended to multicomponent and nonideal mixtures. One should also account for the coupling between driving forces since heat and mass transfer coupling may be considerable and should not be neglected especially in diabatic columns. 

The principles of binary distillation presented in Chapter 5 are applied in this chapter to study column performance under different operating conditions. The objective is to identify relevant column parameters and determine performance trends for a variety of applications. 

In this application, the column is designed with a computer simulation program and then the computer output is used for plotting the distillation diagram to check the design. This example, which is based on two articles by Johnson and Morgan (1985, 1986), also shows how the principles of binary distillation can be applied to multi-component mixtures. 

In Chapters 7 through 11, the performance of multistage separation processes was analyzed qualitatively on the basis of fundamental principles developed in Chapters 3 through 6. The objective was to gain an understanding of the different types of separation processes and columns and the factors that affect their performance. Chapters 5 and 6 employed graphical and semi-quantitative methods to represent a limited set of separation processes, namely binary distillation. Chapters 10 and 11 also used combinations of graphical and analytical methods applied to binary or ternary systems to represent speciflc classes of nonideal separations. 

The above mass transfer equations, although based on sound molecular diffusion principles, are limited in their applicability in a number of ways. A basic condition for their validity is the assumption of equimolar or dilute unimolar mass transfer. This limits the NTU and HTU approach to processes that are essentially either binary (distillation) or ternary (absorption or stripping) with only one component crossing the phase boundary. Another shortcoming of the transfer units technique is its exclusion of energy balances or temperature calculations. 

FIGURE 12.14 Ternary diagram of a system containing three binary azeotropes and one ternary azeotrope. Arrows denote direction the distillation will proceed in, based on starting mixtures. (J. Stichlmair, J. R. Fair, 1998. Distillation—Principles and Practices. New York Wiley-VCH.)... 

The alternative designs generated for the two test cases (heat exchanger and binary distillation) on basis of this TCA principle prove to be remarkably well aligned with the results of a black-box, input-output controllability analysis using the steady-state disturbance sensitivity approach. 

The design of a distillation column is based on information derived from the VLE diagram describing the mixtures to be separated. The vapor-liquid equilibrium characteristics are indicated by the characteristic shapes of the equilibrium curves. This is what determines the number of stages, and hence the number of trays needed for a separation. Although column designs are often proprietary, the classical method of McCabe-Thiele for binary columns is instructive on the principles of design. 

Even in the first publications concerning the copolymerization theory [11, 12] their authors noticed a certain similarity between the processes of copolymerization and distillation of binary liquid mixtures since both of them are described by the same Lord Rayleigh s equations. The origin of the term azeotropic copolymerization comes just from this similarity, when the copolymer composition coincides with monomer feed composition and does not drift with conversion. Many years later the formal similarity in the mathematical description of copolymerization and distillation processes was used again in [13], the authors of which, for the first time, classified the processes of terpolymerization from the viewpoint of their dynamics. The principles on which such a classification for any monomer number m is based are presented in Sect. 5, where there is also demonstrated how these principles can be used for the copolymerization when m = 3 and m = 4. 

If the components of a binary mixture are immiscible, the vapor pressure of the mixture is the sum of the vapor pressures of the two components, each exerted independently and not as a function of their relative concentrations in the liquid. This property is employed in steam distillation, a process particularly applicable to the separation of high boiling substances from non-volatile impurities. The steam forms a cheap and inert carrier. The principles of the process also apply to other immiscible systems. 

In this chapter, the fundamental principles and relationships involved in making multicomponent distillation calculations are developed from first principles. To enhance the visualization of the application of the fundamental principles to this separation process, a variety of special cases are considered which include the determination of bubble-point and dew-point temperatures, single-stage flash separations, multiple-stage separation of binary mixtures, and multiple-stage separation of multicomponent mixtures at the operating conditions of total reflux. 

Many of the distillations of industry involve more than two components. While the principles established for binary solutions generally apply to such distillations, new problems of design are introduced which require special consideration. An important principle to be emphasized is that a single fractionator cannot separate more than one component in reasonably pure form from a multicomponent solution, and that a total of C - 1 fractionators will be required for complete separation of a system of C components. Consider, for example, the continuous separation of a ternary solution consisting of components A, B, and C, whose relative volatilities are in that order (A most volatile). In order to obtain the three substances in substantially pure form, the following two-column scheme can be used. The first column is used to separate C as a residue from the rest of the solution. This residue is necessarily contaminated with a small amount of B and an even smaller amount of A. The distillate, which is necessarily contaminated with a small amount of C, is then fractionated in the second column to give nearly pure A and B. 

Kogan [66] has shown that upon addition of a third component to a binary mixture that component increases in relative volatility in which the third comjjonent is less soluble. Further principles applicable to the selection of additives have been discussed by Kafarov and Gordijewski [67] and Kogan [68]. The connections between gas-liquid chromatography and extractive distillation have been elucidated by Rock [69] and Porter and Johnson [70]. They have pointed out that the latter Tuethod provides a simple means of finding suitable additives for extractive distillation. 

The principle of distillation is the use of differences in volatiHties of the components to be separated. Distillation processes are usually carried out in countercurrent mode in multistage units. The differences that can be obtained in concentrations of the components in the vapor and liquid phases are determined by the vapor-liquid equihbrium (VLE). Until the 1970s reliable data for vapor-liquid equilibria could only be obtained by measurement, which, for a mixture containing more than two components, required a large number of time-consuming measurements. Advances in chemical thermodynamics have resulted in methods activity coefficient models (g models or equations of state) for the calculation of the phase-equihbrium behavior of multicomponent mixtures on the basis of binary subsystems. In the case that no information about the binary subsystems is available, predictive methods (group contribution methods) are available to allow estimation of the required phase equilibria. 

The addition of an external substance (called entrainer e) is a veiy effective means for fractionating azeotropic mixtures by distillation. In the multicomponent mixture generated by the addition of the entrainer the azeotrope is circumvented by distillation. This principle is explained in Rg. 11.3-4 at the example of a binary mixture a-b having a minimum azeotrope. 

Figure 3 Vapor-liquid diagram for a binary mixture of components P and O , illustrating the principles of distillation (see text for details).

As with distillation, the McCabe-Thiele analysis is strictly valid only for a binary system. To analyze the behavior of a multicomponent system it is, in principle, necessary to make plate-to-plate calculations. However, it is commonly found that only two components are present at significant concentrations within each individual section of the column. A preliminary analysis in which each section is considered as a pseudo binary McCabe-Thiele system can therefore provide useful guidance in the design of a multicomponent adsorption system. 

In industry many of the distillation processes involve the separation of more than two components. The general principles of design of multicomponent distillation towers are the same in many respects as those described for binary systems. There is one mass balance for each component in the multicomponent mixture. Enthalpy or heat balances are made which are similar to those for the binary case. Equilibrium data are used to calculate boiling points and dew points. The concepts of minimum reflux and total reflux as limiting cases are also used. 

What is the principle of fractional distillation With the help of boiling point -composition curve, explain the separation of an ideal binary liquid mixture. 

Geometric interpretation of distillation process of binary mixtures has been decisive in understanding the subject and the basic principles of the distillation units design development. Geometric interpretation of multicomponent mixtures distillation is also important for deep insight into the pattern of the multicomponent mixture distillation and better understanding of the methods of design of the units used for the separation of these mixtures. 

For simplicity, we ll only consider the theory for separating ideal solutions (Sec. 4.3) consisting of two volatile components, designated X and Y. Solutions containing more than two such components are often encoimtered, and their behavior on distillation may be understood by extension of the principles developed here for a binary system. 

Many of the distillations of industiy involve more than two components. While the principles established for binary solutions generally apply to such distillations, new problems of design are introduced which require special consideration. 

As an example of such an operation, consider the process of Fig. 9.54, The separation of toluene (bp 110.8 C) from paraffin hydrocarbons of approximately the same molecular weight is either very difficult or impossible, due to low relative volatility or azeotropism, yet such a separation is necessary in the recovery of toluene from certain petroleum hydrocarbon mixtures. Using isooctane (bp = 99.3°C) as an example of a paraffin hydrocarbon, Fig. 9.54a shows that isooctane in this mixture is the more volatile, but the separation is obviously difficult. In the presence of phenol (bp = 181.4 C), however, the isooctane relative volatility increases, so that, with as much as 83 mole percent phenol in the liquid, the separation from toluene is relatively easy. A flowsheet for accomplishing this is shown in Fig. 9.546, where the binary mixture is introduced more or less centrally into the extractive distillation tower (1), and phenol as the solvent is introduced near the top so as to be present in high concentration upon most of the trays in the tower. Under these conditions isooctane is readily distilled as an overhead product, while toluene and phenol are removed as a residue. Although phenol is relatively high-boiling, its vapor pressure is nevertheless sufficient for its appearance in the overhead product to be prevented. The solvent-recovery section of the tower, which may be relatively short, serves to separate the phenol from the isooctane. The residue from the tower must be rectified in the auxiliary tower (2) to separate toluene from the phenol, which is recycled, but this is a relatively easy separation. In practice, the paraffin hydrocarbon is a mixture rather than the pure substance isooctane, but the principle of the operation remains the same. 

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