Multiunit process

Because of the computational complexities associated with dynamic process simulation for multiunit processes, there is still much to be done before simulators of this type become available for general application. Another problem complicating their development is that process models for even individual separation units are usually for steady-state cases this is the result of both incomplete understanding of the chemical and physical nciples involved and computational difficulties. This is one of the rmun reasons why process control considerations are difficult to incorporate into chemical process simulation and thesis and why on-line plant optimization is still far away in most instances. 

Such examples abound in chemical engineering, The unsteady-state material and energy balances of multiunit processes, without chemical reaction, often yield linear differential equations. 

Jeff Siirola reports that this single reactive column replaced a conventional multiunit process that consumed 5 times more energy and whose capital investment was 5 times that of the reactive column. The methyl acetate reactive distillation column has become the prize example of the application of reactive distillation. It provides an outstanding example of innovative chemical engineering. 

Steady-state designs of reactive distillation columns are developed that are economically optimum in terms of total annual cost, which includes both energy and capital costs. The economics of reactive distillation columns are quantitatively compared with conventional multiunit processes over a range of parameter values (chemical equilibrium constants,... 

In the next three chapters we will explore various aspects of the ideal quaternary chemical system introduced in Chapter 1. This system has four components two reactants and two products. The effects of a number of kinetic, vapor-liquid equilibrium, and design parameters on steady-state design are explored in Chapter 2. Detailed economic comparisons of reactive distillation with conventional multiunit processes over a range of chemical equilibrium constants and relative volatilities are covered in Chapter 3. An economic comparison of neat versus excess-reactant reactive distillation designs is discussed in Chapter 4. 

This chapter presents detailed economic comparisons of two alternative flowsheets. In the first, a single reactive distillation is operated in neat mode. In the second, a conventional multiunit process with independent reaction and separation sections is designed. Both flowsheets are optimized in terms of their TACs, which reflect both energy and capital costs. 

There are 12 design degrees of freedom for this multiunit process. Subtracting the number of specifications and assumed heuristic relationships from the degrees of freedom gives the number of design optimization variables. 

A wide range of (7Teq)366 values is explored in this section. Optimum economic steady-state designs of both the reactive distillation process and the conventional multiunit process are developed and compared in terms of TAC. 

Direct comparisons of the conventional multiunit process with the reactive column process at their economic optimum steady-state designs are given in Table 3.5 for five different kinetic cases. The results indicate that the TACs of both design configurations decrease as the value of (ATeq)366 increases. The results also show that the reactive distillation column configuration has lower capital cost and energy cost than the conventional configuration for all kinetic cases. These costs result in lower TAC for the reactive distillation columns compared to the reactor/column/recycle systems. 

The design objective is to obtain 95% conversion for fixed fiesh feed flowrates (Fqa and Fob) of 12.6 mol/s and product purilies of both components C and D of 95 mol%. The assumptions, specifications, and steady-state design procedures used for both process flowsheets are the same as used earlier in this chapter. There arc three optimization variables for the conventional multiunit process molar holdup in the reactor Vr, composition of reactant B in the reactor zb. and reactor temperature Tr. 

Comparison. The top graph in Figure 3.20 gives a direct comparison of the TACs of both processes for the temperature-dependent cases. There is a small increase in TAC for the conventional multiunit process as the relative volatilities decrease, but there is a very rapid increase for the reactive distillation process. 

The economics of reactive distillation have been quantitatively compared with those of conventional multiunit processes with separate reaction and separation sections. With favorable chemistry and relative volatilities, reactive distillation is less expensive than a conventional process. However, if a mismatch occurs in the temperatures conducive for good reaction kinetics and the temperatures conducive for good vapor-liquid equilibrium, reactive distillation is not an attractive alternative. 

In this chapter we take a look at an important example of a reactive distillation column operating in a plantwide environment. The reactive column is part of a multiunit process that includes other columns for recovery of one of the reactants. The process may give the impression that the reactive column is not operating in neat mode because of the need for reactant recovery. We will show that this is really not the case. The recovery of reactant is made necessary by the presence of azeotropes that unavoidably remove one of the reactants from the reactive column. 

In Chapter 8 we explored the steady-state design of the TAME reactive distiUalion system. The reactive column is part of a multiunit process that includes other columns for recovery of the methanol reactant. The recovery is necessary because the presence of methanol/C5 azeotropes unavoidably removes methanol from the reactive column in the distillate stream. The economics of two alternative methanol recovery systems were evaluated in Chapter 8. In this chapter the dynamic control of the process is studied, and an effective plantwide control structure is developed. The process has three distillation columns one reactive column, one extractive distillation column, and one methanol/water separation column from which methanol and water are recycled. 

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. 

The purpose of this book is to present a comprehensive treatment of both steady-state design and dynamic control of reactive distillation systems using rigorous nonlinear models. Both generic ideal chemical systems and actual chemical systems are studied. Economic comparisons between conventional multiunit processes and reactive distillation are presented. Reactive distillation columns in isolation and in plantwide systems are considered. There are many parameters that affect the design of a reactive distUlation column. Some of these effects are counterintuitive because they are different than in conventional distillation. This is one of the reasons reactive distillation is such a fascinating subject. 

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