The previous section considered the case in which the fresh feedstreams of both reactants A and B are pure. In most of the real commercial reactive distillation systems, lighter reactant A is fed with other components that are inert in terms of the reaction but have volatilities that are quite similar to component A. We will assume that fresh feedstream F0A is a mixture of reactant A and an inert component I, which is not involved in the reaction. The volatility of I is assumed to be identical to that of A, so both of these components are lighter than the other reactant B and product C.
In the ternary reaction system without inerts, the column has only a bottoms product in which heavy product C is removed and has only stripping and reactive zones. In the ternary reaction system with inerts, the column has both distillate and bottoms streams. Figure 5.10 gives the flowsheet of the reactive column.
Heavy product C comes out the bottom. Because inert component I has the same volatility as low-boiling A, it is removed from the column in a distillate stream. The reactive distillation column has all three zones: a stripping zone to keep light components A, I, and B from dropping out the bottom; a reactive zone in which the reaction occurs; and a rectifying zone to keep heavier component B from escaping out the top.
It is important to note that any A that leaves the top of the reactive zone will go overhead with inert I. The reactive zone cannot keep A from escaping. It can only keep reactant B from being lost in the distillate.
The chemical kinetics are slightly modified from those used in the case without inerts. Because the presence of the inert decreases the concentrations of the reactants, the reaction rates will be smaller. Therefore, (Keq)366 is increased to 50 in the base case. The effect of changing this parameter is demonstrated in the next section.
The volatility of I is identical to A, so the vapor pressure constants used for I are the same as those used for A (see Table 5.1).
Nr Trays in Reactive Section ■
Nr Trays in Reactive Section ■
5.2.3 Design Parameters and Procedure
In the quaternary system without inerts, the flowrates of both fresh feeds and both product streams are fixed, and the reflux is changed to drive the purity of one of the products to the desired specification. In the ternary system without inerts and without a distillate stream, the method for converging the column is very different. The flowrate and the composition of product C of the bottoms are fixed. There is no distillate. The flowrates of the two fresh feeds are calculated to provide the amounts needed for the reaction plus that lost in the bottoms. The reflux flowrate is changed to drive the bottoms composition to 98 mol% C. The vapor boilup is changed to control the level in the base. Reflux-drum level is not controlled.
The presence of inerts in the ternary systems requires a different method for converging the flowsheet. The impurity levels in both the bottoms and distillate streams are not known and change as design parameters change. The bottoms, which are 98 mol% C, can have impurities of any of the other three components: A, I, or B. The distillate, which is mostly I, can have impurities of either A or B.
The convergence method used in this ternary case with inerts is outlined in the following list and is quite different that those used in the other systems.
1. Fix the flowrate of fresh feed F0B = 12.6mol/s, which is pure B. This fixes the throughput in the process but does not fix the production rate of C because varying amounts of B are lost in both the distillate and bottoms products.
2. Fix the flowrate of reflux R = 70 mol/s. The effect of this design parameter will be explored.
3. Manipulate the bottoms flowrate to control the base level.
4. Manipulate the distillate flowrate to control the reflux-drum level.
5. Manipulate the vapor boilup to drive the composition of the bottoms to 98 mol% C.
6. Manipulate the flowrate of fresh feed F0A to control the composition of A on the bottom tray in the reactive zone at 30 mol% A.
The last item requires some discussion. One of the important aspects of the design and/ or control of a reactive distillation column operating in the neat mode is the need to balance the stoichiometry. The correct amounts of each reactant must be fed into the system. This usually requires some way of detecting any imbalance by measuring something inside the process that is an indication of a gradual buildup or depletion of one of the reactants. One way to achieve this is to measure an internal composition inside the column. Because the composition of A is highest at its feed tray, this location is selected.
The base case conditions are supplied in Table 5.3. Note that the number of reactive trays in the base case has been increased to 15 from the 9 considered in the system without inerts. Likewise, the holdup on the reactive trays has been increased to 2000 mol. The effects of these design parameters are explored in the following paragraphs. The composition z0A( j) of the F0A fresh feed is 50 mol% A and 50 mol% I. This results in a much larger flow-rate of this stream. The distillate is 97.2 mol% inerts. The main impurity is B at 2.04 mol%,
TABLE 5.3 Steady-State Conditions and Design Parameters for Base Case With Inerts
Fresh feed flowrate of A F0A (mol/s)
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