Neat Operation Versus Excess Reactant

The reactive distillation column described in the previous section was designed to operate "neat" (precisely the correct amounts of reactants are fed to the column to satisfy the stoi-chiometry of the chemistry and there are only small amounts of unreacted reactants that leave in the streams leaving the column). Only a single column is required, so both capital investment and energy cost are minimized. However, it can be difficult to control a reactive column that operates in this neat mode. The problem is the need to feed in exactly enough of both reactants, down to the last molecule, to make sure that there is no excess of either reactant. If the balance is not absolutely perfect, the reactant that is in excess will gradually build up in the column, and it will not be possible to maintain product purities. This build-up may take hours or days, but eventually the column control structure will not be able to hold the products at their specified compositions.

One might think that this problem can be very easily overcome by simply ratioing the flowrates of the two fresh reactant feeds. This strategy works in computer simulations, but it does not work in a real plant environment. The reasons why ratio schemes are not effective are inaccuracies in flow measurements, which are always present, and/or changes in the compositions of the feedstreams. Either cause will result in an imbalance of the stoichiometry. Therefore, it is necessary to have some way to determine the amount of at least one of the reactants inside the column so that feedback control can be used to adjust a fresh feed flowrate. Sometimes temperatures or liquid levels can be used. Sometimes a direct composition measurement on a tray in the column is required. This issue is the heart of the reactive distillation control problem and will be quantitatively studied in detail in subsequent chapters.

An alternative flowsheet, which is more costly but easier to control, uses two distillation columns. As illustrated in Figure 1.5, the first is a reactive distillation column into which an excess of one of the reactants is fed (component B), along with the second fresh feed of component A. The total B fed to the reactive column is 10-20% in excess of the stoichiometric amount needed to react with the moles of A being fed. The amount of this excess is determined by the variability in the compositions of the two fresh feeds and by the flow measurement inaccuracies. Reactant A is the "limiting reactant" in this column and its conversion is high. The conversion of reactant B is not high in the reactive column. Because not all of the B is consumed by the reaction, the excess comes out of the bottom of the column with product component D. This binary mixture is fed to the second distillation column, the recovery column, which produces component D out the bottom and component B out the

Reactive Recovery

Column Column

Figure 1.5 Two-column process with excess reactant.

top. The distillate is recycled back to the reactive column, the fresh feed of B is added to the recycle stream, and the total is fed to the reactive column.

The control of this system is easy because the inventory of B in the system can be inferred from the liquid level in the reflux drum of the recovery column. If too much B is being fed into the system, it will accumulate in the reflux drum because the total B fed to the reactive column is fixed. Thus, a simple level controller on the recovery column reflux drum adjusting the flowrate of the fresh feed of B into the system can achieve the required balancing of the stoichiometry. Note that the overall conversion of B is high, considering the entire process, despite having a low "per pass" conversion in the reactive column.

These two alternative flowsheets (neat vs. excess) are quantitatively compared in Chapter 4 in terms of steady-state design and in Chapter 11 in terms of dynamic controllability.

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