(11a) Combined Pre-Concentrator/Recovery Column Design for Ethanol Dehydration Process

Ethanol dehydration process is important in producing high-purity ethanol from renewable biomass resources. In a diluted stream from fermentation after filtration or centrifuge to remove heavy phase materials, the ethanol content is typically no more than 10 mol%. Taking a feed stream to the ethanol dehydration system with 5 mol% of ethanol and 95 mol% of water as an example, this paper will investigate the optimal design flowsheet to produce high-purity ethanol via heterogeneous azeotropic distillation. A classical example of ethanol dehydration system is to use benzene as entrainer to aid the separation. However, because benzene is a known carcinogen, this work will demonstrate the design concept using cyclohexae or n-hexane as entrainer.

An obvious design for the overall process is to include a pre-concentrator column in the upstream of the heterogeneous azeotropic column to concentrate the fresh feed composition to a point near the azeotropic composition of ethanol and water. After the pre-concentrator column, with the help of the entrainer entering into the heterogeneous azeotropic column, the top vapor of this second column will be close to the lowest boiling point of the ternary system (ternary azeotrope with b.p of 62.39 oC using cyclohexane as entrainer). The bottom product of this heterogeneous azeotropic column is pure ethanol. The top vapor after condensation can naturally separated into two liquid phases with the organic phase containing mostly entrainer which can be recycled back to the heterogeneous azeotropic column. The aqueous phase still contains significant amount of ethanol will enter into recovery column. The bottom stream of the recovery column is pure water while the distillate can be designed to recycle back to heterogeneous azeotropic column. This conventional overall process will contain three distillation columns (pre-concentrator column, heterogeneous azeotropic column, and a recovery column).

An alternative simpler process design is proposed in this paper with a column (C-2 in Figure 1 below) acting as both the pre-concentrator column and the recovery column. This design flowsheet can be seen in Figure 1 with material balance lines shown in Figure 2 using cyclohexane as entrainer. The Total Annual Cost (TAC) will be used as the objective function to be minimized for the above two alternative process design flowsheets (conventional three-column system and proposed two-column system). A comparison of the TACs will be made for the optimized two systems. Because feed composition of the waste IPA stream can vary widely, the overall control strategy for these two alternative design are critically important in rejecting this feed composition disturbance. The operability of these two alternative process systems under proper overall control strategy will also be compared. Aspen Plus and Aspen Dynamics will be used in the above design and control studies.

Keywords: Ethanol Dehydration, Heterogeneous Azeotropic Distillation, Pre-concentrator column, Recovery Column, Design and Control.