(354b) Optimization of Semi-Batch Reactive Distillation for the Synthesis of Isopropyl Acetate and Ethyl Acetate

Batch reactive distillation (BRD) is an integrated process which combines reaction and distillation in a batch process. Semi-batch reactive distillation (SBRD) is a alternative design from BRD in which reactant or entrainer is added as a side feed continuously. Besides the advantages of BRD, SBRD can circumvent limitations due to azeotropes.

In the synthesis of isopropyl acetate (IPAC) by BRD, the lowest-boiling stationary point is a ternary azeotrope located in a two-liquid region. After liquid-liquid separation the organic phase still contains 3% reactant and the aqueous phase contains only 95% water. Qi and Malone (2011) showed that the concentration of isopropyl alcohol in the distillate can be reduced efficiently by introducing acetic acid continuously as a side feed. A second non-reactive inverted batch distillation (IBD) can be employed to separate the product IPAC from water with high purity. Hence, SBRD has the potential to improve the process significantly.

The synthesis of ethyl acetate (ETAC) is similar, but the low-boiling ternary azeotrope is located outside of the two-liquid region. Therefore, conventional BRD is not feasible at all. However, if HAC is introduced as a side-feed, the composition of alcohol in the distillate can again be dramatically reduced so that the liquid-liquid split becomes feasable (Tang et al, 2003). Again a second non-reactive IBD comlumn is required to separate the product ETAC from water.

For both systems a tradeoff is encountered between the operation of the SBRD and the subsequent IBD. Specifically, if the constraint on the ammount of alcohol in the SBRD product is relaxed, the operation of the SBRD becomes easier but the operation of the subsequent IBD becomes more difficult. Therefore in order to determine the most economical operating policy, it is necessary to consider both steps. In this work we study the optimization of both processes (IPAC and ETAC) to find the operating policy that minimize the energy consumption (closely related to the operating cost). Because the constraint on the alcohol impurity is the most important variable affecting both columns, designs are optimized for different values of the constraint and then the best overall design is determined.

References:

Qi, W.; Malone, M. F., Semibatch Reactive Distillation for Isopropyl Acetate Synthesis. Industrial & Engineering Chemistry Research 2011, 50, (3), 1272-1277.

Tang, Y. T.; Huang, H. P.; Chien, I. L., Design of a complete ethyl acetate reactive distillation system. Journal of Chemical Engineering of Japan 2003, 36, (11), 1352-1363.