Batch Distillation: Hybrid Processes
Process simplification are often possible by replacing one of the distillation steps by a non-distillation separation operation.
For example, consider the separation of ethanol-water (a minimum-boiling azeotrope at 78.2 oC) using toluene (boiling point 110.6 oC). In a batch process as shown in the Figure, a cycle consists of 2 distillations and 1 decantation steps, performed at consecutive periods of time.
In the first part of the cycle, the ethanol-water mixture and the fraction S1 (a water-rich layer from the decanter from the previous cycle) are fed into vessel V1 located at the top of the distillation column. The mixture is then heated, and pure water is separated as bottoms of the column. The residue in V1 is a mixture of ethanol-water near the azeotropic composition.
At the beginning of the second part of the cycle, the residue from vessel V1 is transferred into vessel V2. The fraction S2 (a toluene-rich layer from the previous cycle) is also added into vessel V2. The mixture is then heated and a ternary azeotrope of ethanol-water-toluene (minimum-boiling, 74.6 oC) is removed as the overhead product from the column. The residue in vessel V2 becomes enriched in ethanol. The residue is removed as the ethanol product. The overhead product is condensed and split in the decanter into 2 liquid phases: a water-rich fraction S1 and a toluene-rich fraction S2. The 2 fractions are recycled to vessels V1 and V2 respectively for the next cycle.