Conceptual Design

In Fig. 3.3 the vapor-liquid equilibrium and the chemical equilibrium of the ternary system isobutene-MTBE-methanol are illustrated. There are two minimum azeotropes in the non-reactive mixture, the first one between the high-boiling MTBE and the intermediate-boiling methanol, and the second one between methanol and the low-boiling isobutene. A border distillation line between the two azeo-tropes divides the entire concentration space into two distillation regions. The curve of the chemical equilibrium extends between the two products isobutene and methanol. It approaches the MTBE/methanol binary edge in the methanol corner and the MTBE/isobutene binary edge in the isobutene corner. According to Frey and Stichlmair [7], reactive azeotropes can arise wherever the concentration change due to distillation is co-linear to the concentration change due to reaction. In the present system, this consideration gives rise to two curves on which reactive azeotropes may exist. The first locus curve of reactive azeotropes passes between MTBE and the minimum azeotrope of MTBE/methanol and the second one between isobutene and the minimum azeotrope of isobutene/methanol. At a pressure of 500 kPa, the equilibrium curve does not intersect either of the two locus curves of reactive azeotropes anywhere in the entire concentration space. Accordingly, no reactive azeotrope exists in the system under these conditions.

A feasible column design can be devised from thermodynamic principles, as illustrated in Fig. 3.4. The reactive section is located in the center between the

Loci of reactive azeotropes

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