Distillation design approach

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. 

Wave models were successfully used for the design of a supervisory control system for automatic start-up of the coupled column system shown in Fig. 5.15 [19] and for model-based measurement and online optimization of distillation columns using nonlinear model predictive control [15], The approach was also extended to reactive distillation processes by using transformed concentration variables [22], However, in reactive - as in nonreactive - distillation, the approach applies only to processes with constant pattern waves, which must be checked first. 

For single separation duty, Bernot et al. (1991) presented a method to estimate batch sizes, operating times, utility loads, costs, etc. for multicomponent batch distillation. The approach is similar to that of Diwekar et al. (1989) in the sense that a simple short cut technique is used to avoid integration of a full column model. Their simple column model assumes negligible holdup and equimolal overflow. The authors design and, for a predefined reflux or reboil ratio, minimise the total annual cost to produce a number of product fractions of specified purity from a multicomponent mixture. 

Using an entirely different approach to the modeling of multicomponent mass transfer in distillation (an approach that we describe in Chapter 14), Krishnamurthy and Taylor (1985c) found significant differences in design calculations involving nonideal systems. For an almost ideal system (a hydrocarbon mixture), pseudobinary methods were found to be essentially equivalent to a more rigorous model that accounted for diffusional interaction effects. 

Total reflux of type 1 may be approached in a continuous distillation column by approaching total reflux conditions in both the rectifying and stripping sections. The designer approaches this type of operation of a continuous distillation column as the reflux ratio is increased indefinitely at a fixed feed rate and nonzero product rates. In the limit, this type of operation is recognized as total reflux of type 1, continuous distillation columns at total reflux. The necessary conditions for an equivalence to exist between columns at the operation conditions of total reflux of types 1 and 2 are presented in Sec. 10-2. Total reflux of types 1 and 2 are of significant interest because columns at these types of operation produce the best possible separations. 

Hauan, S. and K.M. Lien, Phenomena based design approach to reactive distillation. Chemical Engineering Research Design, Transactions of the Institute of Chemical Engineers, Part A, 1998, 76(A3) 396 407. 

Hauan, S., Lien, K., 1998. A phenomena based design approach to reactive distillation. Chem. Eng. Res. Des. 76, 396- 7. 

For very complex mixtures, the entire distillation design can be done using the Older-shaw column by changing the number of trays and the reflux rate until a combination that does the job is found. Since the commercial column will have an overall efficiency equal to or greater than that of the Oldershaw column, this combination will also work in the commercial column. This approach eliminates the need to determine vapor-liquid equilibrium (VLE) data (which maybe quite costly), and it also eliminates the need for complex calculations. The Oldershaw column also allows one to observe foaming problems TKister. 19901. 

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. 

These books deal primarily with the steady-state design of reactive distillation columns. Conceptual approximate design approaches are emphasized. There is little treatment of rigorous design approaches using commercial simulators. The issues of dynamics and control... 

The effects of several important design and chemical parameters on the steady-state design of the ideal chemical system with four components are considered in this chapter. The impact of some parameters is similar to that experienced in conventional distillation. However, in some cases the effects are counterintuitive and unique to reactive distillation. The approach is to see the effect of changing one parameter at a time, while holding all other parameters at their base case values. The base case values of kinetic and vapor-liquid equilibrium parameters are given in Table 2.1. Table 2.2 gives design parameters and steady-state values of process variables for the base case. 

Several hundred papers and patents have appeared in the area of reactive distillation, which are too numerous to discuss. A number of books have dealt with the subject such as (1) Distillation, Principles and Practice by Stichhnair and Fair, (2) Conceptual Design of Distillation Systems by Doherty and Malone," and (3) Reactive Distillation— Status and Future Directions by Sundmacher and Kienle. These books deal primarily with the steady-state design of reactive distillation columns. Conceptual approximate design approaches are emphasized, but there is little treatment of rigorous design approaches using commercial simulators. The issues of dynamics and control stmcture development are not covered. Few quantitative eeonomic comparisons of conventional multiunit processes with reactive distillation are provided. 

Exploitation of Boundary Curvature A second approach to boundaiy crossing exploits boundaiy curvature in order to produce compositions in different distillation regions. When distillation boundaries exhibit extreme curvature, it may be possible to design a column such that the distillate and bottoms are on the same residue curve in one distillation region, while the feed (which is not required to lie on the column-composition profile) is in another distillation region. In order for such a column to meet material-balance constraints (i.e., bottom, distillate, feed on a straight hne), the feed must be located in a region where the boundary is concave. 

The problem presented to the designer of a gas-absorption unit usually specifies the following quantities (1) gas flow rate (2) gas composition, at least with respect to the component or components to be sorbed (3) operating pressure and allowable pressure drop across the absorber (4) minimum degree of recoverv of one or more solutes and, possibly, (5) the solvent to be employed. Items 3, 4, and 5 may be subject to economic considerations and therefore are sometimes left up to the designer. For determining the number of variables that must be specified in order to fix a unique solution for the design of an absorber one can use the same phase-rule approach described in Sec. 13 for distillation systems. 

Graphical methods at best are simply illustrative for the student today, but they are occasionally referenced by the process engineer. Extraction, like distillation can be viewed as a stage-wise operation, and hence metliods based on the McCabe Thiele approach briefly described in Chapter 4 have been applied to preliminary design cases. Indeed, both absorption and adsorption are stage-wise operations. 

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