Binary Distillation Applications

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. 

As a learning tool, the binary model is useful for qualitatively studying the characteristics of multistage separation. The model is used in this chapter to answer such questions as what effect the reflux ratio or product rate or number of trays has on separation or what is the minimum number of trays or minimum reflux ratio required to achieve a given separation, or over what ranges column performance specifications are feasible. 

Moreover, the availability of computer programs provides an added impetus to the use of graphical methods, because these programs generally provide the data required for graphical representation. 

Multi-component separations can also be approximated with the graphical methods by selecting appropriate key components to define the separation. 

Using the graphical techniques developed in Chapter 5, Section 6.1 examines the effect on column performance of each parameter separately, and Section 6.2 considers the interactions of different parameters. Section 6.3 presents example applications where graphical methods are used in conjunction with simulation results to check the design data. 

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Part 3 of this book presents a number of major developments and applications of MINLP approaches in the area of Process Synthesis. The illustrative examples for MINLP applications, presented next in this section, will focus on different aspects than those described in Part 3. In particular, we will consider the binary distillation design of a single column, the retrofit design of multiproduct batch plants, and the multicommodity facility location/allocation problem. 

M. L. Luyben and C. A. Floudas. Analyzing the interaction of design and control, Part 1 A multiobjective framework and application to binary distillation synthesis. Comp. Chem. Eng., 18(10) 933, 1994a. 

Jafarey et al. (61) derived a simple, approximate equation for binary distillation by simplifying the solution to Smoker s equation. Their equation is powerful for predicting the effect of disturbances on column performance. This makes their equation particularly useful in computer and microprocessor control, where it can be applied to estimate the effect of disturbances and the control action needed to compensate for them. This application is highlighted in Examples 3.8 and 3.9. The Jafarey et al, equation is... 

As pointed out in the introduction to this chapter, it is helpful in understanding multistage separations to distinguish between two effects temperature and composition. (The other factors, such as pressure, number of stages, column configuration, etc., are assumed fixed in this context.) Of the two effects, the more predominant in distillation is temperature. In the case of binary distillation where the composition effect on the distribution coefficients may be neglected, the temperature is essentially the only factor. Under these conditions, the equations derived in Section 3.1.2 are applicable. 

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. 

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. 

S is given by Eq. (3.3). Equation (3.40) is stated (61) to predict N within plus or minus two to three st es. The effect of disturbances on column performance can be evaluated by combining Eq. (3.40) with the column overall mass and o>mponent balance equations [Eqs. (3.1), (3.2)]. Douglas et al. (62) illustrate several applications. Example 3.8 illustrates this for binary distillation. 

The Lewis method for binary distillation is easy to program on a spreadsheet using Visual Basic for Applications (VBA). If you are not familiar with VBA, read Appendix 4.B. Part 1. If you are familiar witii VBA, you can skip Part 1 and proceed to Part 2. 

Most engineering students are familiar with the use of spreadsheets, but they may not be familiar with the power of spreadsheets when they are coupled with VBA. VBA is an extremely useful programming language for controlling spreadsheets. Word, and other Microsoft programs. This short introduction focuses on use of VBA for spreadsheet calculations. The particular exanple is binary distillation, but the VBA programming method is applicable to many of the separation methods in this book. Readers interested in more information on VBA are referred to Microsoft (1999) or McFedries (2004). 

This section addresses the application of a dynamic optimization-based design approach to RD. The liquid-phase esterification reaction of C4 and methanol in the presence of inert nC4 in a staged RD column is used as tutorial example. Similar to the study on binary distillation (Bansal et al., 2000 Bansal, 2000) and on the synthesis of ethyl acetate by RD (Georgiadis et al, 2002), both spatial-related e.g. column diameter and heat exchanger areas) and control-related e.g. gain, set-point and reset time) design variables are optimized with respect to economic and dynamic performance in the presence of time-varying disturbances. 

Figure 6.6 Application of material balances to binary distillation analysis... 

The driving forces defined in Table 1 are based on a microscopic description. This should be extended to a macroscopic description in order to be directly applicable for typical process systems. This will be done for the two examples a heat exchanger and a binary distillation column. 

The benzene-alcohol-water system can produce an overhead vapor that will give two liquid phases on condensation which makes it possible to by-pass the azeotrope in a manner analogous to that which was shown for partly miscible binary distillations, and the same type of two-tower system is applicable. It should be noted that the system does not produce the ternary azeotrope as the overhead composition. 

To simplify the analysis, let us limit our attention to an ideal, binary distillation. This is somewhat restrictive, although the results will be applicable in a general way to multicomponent systems, particularly those that may be treated as quasibinary or pseudobinary systems. 

Here To is the surroundings temperature, discussed below. If the process in the box is an oil refinery, Eq, 2.57 will have dozens of terms for flows of material, heat and work in and out. If it is a simple binary distillation column it will have one material flow in and two out, with heat exchange at two different temperatures, and (generally negligible) work flows for some pumps. Chemical engineers often use Eq. 2.57 for simple applications like the distillation column, although there is no reason (except for complexity) that it could not be applied to an oil refinery. 

It took a little over half a century for the next major development to occur. G. Wilson in 1964 introduced the idea of local compositions which led to an expression for y,- bearing his name and later to the NRTL and UNIQUAC expressions by J. Prausnitz and his co-workers (1967 and 1975). The expressions have two main advantages over the van Laar model first, their parameters have a built-in temperature dependency making them very useful in distillation applications (why ) second, they provide successful prediction of multicomponent activity coefficients from binary ones. Finally, based on the UNIQUAC model, Fredenslund and his co-workers (1977) have developed a group-contribution model, UNIFAC, that can be used for the estimation of activity coefficients for a large variety of systems. 

The use of a ternary mixture in the drying of a liquid (ethyl alcohol) has been described in Section 1,5 the following is an example of its application to the drying of a solid. Laevulose (fructose) is dissolved in warm absolute ethyl alcohol, benzene is added, and the mixture is fractionated. A ternary mixture, alcohol-benzene-water, b.p. 64°, distils first, and then the binary mixture, benzene-alcohol, b.p. 68-3°. The residual, dry alcoholic solution is partially distilled and the concentrated solution is allowed to crystallise the anhydrous sugar separates. 

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. 

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