In situ product recovery of bio-based ethyl esters via hybrid extraction-distillation†
In situ product recovery (ISPR) of bio-based chemicals from microbial cultivation is a means to improve titer, rate, and yield because it alleviates end product inhibition. Metabolic engineering for the production of bio-based esters has recently received considerable attention because esters can serve as platform chemicals with applications as bio-based fuels, plastic monomers, fragrances, flavors, and solvents. The separation of ethyl esters from microbial cultures is generally achieved by condensing esters from the exhaust gas of a bioreactor. However, the separation of ethyl esters by in situ extraction rather than exhaust gas stripping may achieve a more energy efficient means to recover pure esters because extraction effectively decreases the energy requirements of downstream distillation. To that end, this study develops a hybrid extraction-distillation (HED)-ISPR process for the recovery of pure bio-based esters by determining extraction equilibrium and distillation energy requirements as a function of the ester bioprocess titer. This work shows that the energy requirements of the HED-ISPR process for volatile esters such as ethyl acetate is ∼30-fold less than the energy requirements of the conventional gas-stripping process due to several factors including the reduced water content in downstream distillation and the high driving force of ester extraction. Furthermore, the HED-ISPR system for esters is compared to a previously reported HED-ISPR process for carboxylic acids, which are chemical derivatives of esters. Several green chemistry benefits are predicted for bio-based ester recovery relative to carboxylic acid recovery. Namely, HED-ISPR allows for the use of more benign solvents, decouples the downstream impact of the bioproduction pH, and lowers the downstream energy requirement for ethyl acetate compared to its carboxylic acid derivative by 3.6-fold.