(174f) Economic Comparison of Processes for the Recovery of Lactic Acid by Reactive Distillation with Different Alcohols

Poly lactic acid (PLA) is a biodegradable, environmentally-friendly polymer that can be made from renewable resources and has many potential applications. Production of lactic acid (LAC) from renewable resources is usually accomplished by fermentation which has the additional advantage that an enantiomerically pure product is produced. Therefore, separation and purification of lactic acid from fermentation broth is important. However, it is also challenging because LAC is relatively nonvolatile and tends to oligomerize at high temperature. The most attractive method for purification of LAC is a two-step reaction process in which the acid is first reacted with an alcohol to form a more volatile ester which can be more easily purified by distillation. Then pure LAC is recovered by hydrolysis of the ester (Kumar et al., 2006; Kumar and Mahajani, 2007).

            In this work we determine the optimal designs of esterification/hydrolysis lactic acid recovery processes using different alcohols and compare the cost of each process. We consider the presence of a heavy organic impurity and LAC oligomerization reactions in order to make the design problem more realistic. Processes are simulated using the commercial simulation package ASPEN PLUS. The processes are optimized to minimize the Total Annual Cost (TAC), with an overall lactic acid with lactic acid esterification and hydrolysis conversions of 99 mol%.

            Different alcohols lead to different process configurations due to different component characteristics (especially boiling point ranking). The methanol system includes one pretreatment column, two reactive distillation columns and one separation column. The advantage of the methanol system is that methyl-lactate is the lightest compound among all lactic esters, so it is easier to separate methyl-lactate from other species. The butanol system includes one pretreatment column, two reactive distillation columns, two decanters and two separation column. The advantage of the butanol system is that there is a two-liquid phase zone, which facilitates separation of the species. It also may reduce the energy consumption of the separation system if the system is well designed. Designs using ethanol and isopropanol are also briefly discussed.

            The butanol system has the lowest TAC among all alcohols due to low separation cost in the esterification and separation column because of the two-liquid-phase region. Furthermore, the methanol system has lower TAC than the ethanol and isopropanol systems due to the absence of an azeotrope between the alcohol and water.

References

Kumar R., Nanavati H., Noronha S. B. and Mahajani S. M. A continuous process for recovery of lactic acid by reactive distillation, Journal of Chemical Technology and Biotechnology, 81, 1767-1777, 2006.

Kumar R. and Mahajani S. M. Esterification of Lactic Acid with n-Butanol by Reactive Distillation, Ind. Eng. Chem. Res., 46, 6873-6882, 2007.