Conventional Multiunit Process

The temperature in the reactor Tr can be set completely independent of the conditions in the two distillation columns, so the best reactor temperature can be used that is most favorable for the reaction. The pressures and temperatures in the two distillation columns can be set at their best values that are favorable for vapor-liquid phase equilibrium conditions. 

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. 

An approximate heuristic design procedure is used for the columns in the conventional process. The optimum number of frays is assumed to be equal to 2 times the minimum, and the optimum reflux ratio is assumed to be 1.2 times the minimum. The assumptions and specifications use up 9 of the 12 design degrees of freedom, leaving 3 that can be used to optimize the process. Three design optimization variables were selected  

The chemical equilibrium constant at 366 K [(Feq)366] and the relative volatilities (constant or temperature dependent) are specified for each case. Equimolal overflow is assumed in the distillation columns, which means that neither energy balances nor total balances are needed on the trays for steady-state calculations. Other assumptions are isothermal operation of the reactor, theoretical trays, saturated hquid feed and reflux, total condensers, and partial reboilers in the columns. Additional assumptions and specifications are the following  

ECONOMIC COMPARISON OF REACTIVE DISTILLATION WITH A CONVENTIONAL PROCESS 

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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. 

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. 

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. 

In the previous section, the optimum economic steady-state designs of reactive distillation columns were quantitatively compared with conventional multiunit systems for a wide range of chemical equilibrium constants. Relative volatilities (a = 2) were assumed constant. Reactive distillation was shown to be much less expensive than the conventional process. In this section we explore how temperature-dependent relative volatilities affect the designs of these two systems. 

Most chemical processes involve two important operations (reaction and separalion) that are typically carried out in different sections of the plant and use different equipment. The reaction section of the process can use several types of reactors [continuous stirred-tank reactor (CSTR), tubular, or batch] and operate under a wide variety of conditions (catalyzed, adiabatic, cooled or heated, single phase, multiple phases, etc.). The separation section can have several types of operations (distillation, extraction, crystallization, adsorption, etc.), with distillation being by far the most commonly used method. Recycle streams between the two sections of these conventional multiunit flowsheets are often incorporated in the process for a variety of reasons to improve conversion and yield, to minimize the production of undesirable byproducts, to improve energy efficiency, and to improve dynamic controllability. 

Economic and environmental considerations have encouraged industry to focus on technologies based on process intensification. This is an area of growing interest that is defined as any chemical engineering development that leads to smaller inventories of chemical materials and higher energy efficiency. Reactive distillation is an excellent example of process intensification. It can provide an economically and environmentally attractive alternative to conventional multiunit flowsheets in some systems. 

The reactive distillation process was described in Chapter 2, and the multiunit conventional process is described in this chapter. 

The lower gr h in Figure 16.7 compares the economic optimum steady-state design of the column/side reactor process with those of the reactive distillation column and the multiunit conventional process. The reactive distillation column is the most economical alternative for the a39o = 2, where thoe is noreaction/separation temperature mismatch. The column/ side reactor process becomes more attractive as the mismateh of reaction/separation temperatures becomes more severe. The distillation column with a side reactor is economically superior for reference relative volatilities that arc smaller than 1.5 for this case study. 

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