Neat Operation Versus Using Excess Reactant

The reactive distillation columns considered in previous chapters were all operated in neat mode. The two reactants are fed in exactly the correct amounts to satisfy the stoichiometry of the reaction. The control system must be able to detect any imbalance, which will inevitably result in a gradual buildup of one of the reactants and a loss of conversion and product purities.

An alternative to operating neat is to operate the reactive column with an excess of one of the reactants. This eliminates the need to perfectly balance the reactant feeds in the reactive column itself, which makes the control of the reactive column easier. However, this mode of operation may have the disadvantage of requiring the recovery and recycle of the reactant that is in excess. The flowsheet typically consists of a two-column system: a reactive column and a recovery column.

We would expect that a single reactive column that is operated neat will have lower capital investment and energy costs than a two-column system. The purpose of this chapter is to give a quantitative comparison of these two alternative processes.

The neat one-column system may require a composition analyzer to detect the inventory of one of the reactants so that the fresh feed can be adjusted. Because composition analyzers are expensive and sometimes unreliable, the economic advantages of the one-column system need to be assessed.

The process considered is the ideal quaternary system with the reversible reaction A + B , C + D. The relative volatilities are favorable for reactive distillation, that is, the reactants are intermediate boilers between light product C and heavy product D. Relative volatilities are constant at 2 between adjacent components.

We study two-column systems in which an excess of either A or B is fed to the reactive column. If there is an excess of B, the distillate from the reactive column is product C. The bottoms stream is a mixture of components B and D, which are separated in the second

Reactive Distillation Design and Control. By William L. Luyben and Cheng-Ching Yu Copyright # 2008 John Wiley & Sons, Inc.

column. The bottom of the recovery column B2 is product D. The distillate of the second column D2, which is mostly component B but also contains some component D, is recycled back to the first column and mixed with fresh feedstream F0B.

If there is an excess of A, the bottom of the reactive column is product D. The distillate stream is a mixture of components A and C, which are separated in the second column. The distillate of the recovery column is product C. The bottom of the second column, which is mostly component A but also contains some component C, is recycled back to the first column and mixed with fresh feedstream F0A.

4.1 INTRODUCTION

Many industrial reactive distillation systems do not use stoichiometric amounts of reactants. An excess (10-20% above the stoichiometric amount) of one of the reactants is fed to the reactive column. There may be kinetic reasons for using an excess in some systems. These include suppressing undesirable side reactions, reducing catalyst requirements, and increasing conversion. However, even in the absence of kinetic reasons, the use of an excess of one of the reactants makes the control problem easier because the fresh feed flowrates of the components do not have to be precisely balanced in the reactive column. Achieving this exact balance may require the use of expensive and high maintenance on-line composition analyzers in some systems. In addition, the variability of product quality may be larger in the neat operation process because there are fewer manipulated variables available and there is only one column to contain disturbances.

One example of an industrial system with excess reactant is the ETBE system. A 10 -20% excess of ethanol is fed to the column. If the excess ethanol can be included in the ETBE bottoms product from the column and blended into gasoline with the ETBE, there may be no economic penalty and no need for recovering the excess ethanol. However, in other systems, the excess reactant must be removed from the product and recycled. This involves an additional separation step, so capital investment and energy costs are increased.

In the two-column process, the recovery column acts conceptually as an on-line analyzer: a higher recycle flowrate means that more of the excess reactant is leaving the reactive column, so the fresh feed flowrate of that reactant must be decreased. An effective control structure for the two-column system is to flow control the sum of the recycle stream and the appropriate fresh feedstream. When the recycle flowrate increases, the fresh feed flowrate is decreased to keep the total constant. Thus, the scheme changes the fresh feed flowrate to accommodate changes in the component inventory of the reactant. From a steady-state economic perspective, the two alternative processes (one column and two columns) have different capital investments and different operating costs. From a dynamic perspective, the two processes show different dynamic behavior and require different control structures. The economic design differences are quantitatively explored in this chapter. The control of these types of systems is discussed in Chapter 11.

4.2 NEAT REACTIVE COLUMN

The process considered is the ideal quaternary system with the reversible exothermic reaction occurring on the reactive trays A + B , C + D with constant and favorable relative volatilities.

Reactants A and B are intermediate boiling between the products. Therefore, fresh feedstream F0A containing reactant A is fed at the bottom of the reactive zone, and fresh feedstream F0B containing reactant B is fed at the top of the reactive zone. The reactive section contains NRX trays. The rectifying section above the reactive section contains NR trays and the stripping section below the reactive section contains NS trays. The optimum economic design found in Chapter 3 for the single reactive column operating in neat mode is used (see Table 4.4).

Figure 4.1 shows the one-column reactive distillation column flowsheet with stream information and equipment sizes. Specification products are produced at both ends of the column. Conversion is 95%, and product purities are 95 mol%. Fresh feeds are 12.6mol/s. Note that the production rate of both products is 12.6mol/s with equal amounts of the two reactants lost in the two products.

There are five stripping trays, five rectifying trays, and nine reactive trays. The column operating pressure is 8 bar, and the holdup on the reactive trays is 1000 mol. The vapor boilup is 28.91 mol/s, and the column diameter is 0.805 m. Figures 4.2 and 4.3 give composition and temperature profiles in the column, respectively.

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