Comparison of Extractive Distillation and Pressure-Swing Distillation for Acetone−Methanol Separation

Abstract

Two of the most common methods for separating a binary homogeneous azeotrope are pressure-swing distillation and extractive distillation. The former is effective if the composition of the azeotrope changes significantly with pressure. The latter method is effective if a suitable solvent can be found. This paper compares the steady-state design and the dynamic control of these two methods when applied to the acetone-methanol binary system. The minimum-boiling azeotrope at 1 atm contains 77.6 mol % acetone at 328 K. At 10 atm the azeotropic composition is 37.5 mol % acetone at 408 K, so pressure-swing separation is feasible. Extractive distillation is also feasible using water as the solvent. Both systems require two distillation columns. Purities of the two products are set at 99.5 mol %. Results show that the extractive distillation system has a 15% lower total annual cost. However, a third component (water) is introduced that appears as trace impurities in both the acetone and methanol products. It is also much more difficult to attain higher purities in the extractive distillation system than in the pressure-swing system because of ternary vapor-liquid equilibrium constraints. The dynamic controllabilities of the two alternative processes are quite similar. Steady-state designs and control structures are also developed for the two methods when the columns are heat integrated. Heat integration is straightforward in the pressure-swing system because the condenser temperature in the high-pressure column is 60 K higher than the base temperature in the low-pressure column. In the extractive distillation system, the pressure in the second solvent recovery column must be increased from 1 to 5 atm to provide the necessary temperature differential driving force.