Number of Theoretical Stages excluding condenser and reboiler

Figure 4.8 - Effect of Fractionation on ETBE Purity and Conversion 4.2.2.4 MTBE Columns

The effect of fractionation on MTBE columns was considered by repeating the above analyses with methanol in the feed. Interestingly, somewhat different results were observed. The benefit of increasing the reflux ratio was still evident (the predominant effect of this is to recycle reactive components) but no advantage was found from reducing the fractionation from the base case of 30 ideal stages The different behaviour is again caused by the methanol-C4 azeotropes. Provided the internal ratio of methanol to C4 is lower than the azeotropic composition, the effect of increasing the stripping section separation is to recycle methanol to the reactive section and promote further reaction. Unfortunately, a similarly useful azeotrope does not exist at the top of the column so that the rectifying separation must again be optimised. In fact, as with the ETBE system, the reactive distillation of MTBt is infeasible with perfect separation (infinite stages and infinite reflux).

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