Experimental and computational analysis of coated milli-structured bioreactor with immobilized nitrilase for continuous production of nicotinic acid

Abstract

In complement to or as a replacement for environmentally-costly chemical transformations, biocatalytic synthesis is attracting increasing interest. To make it competitive, basic research on process engineering with continuous operations using immobilized enzymes must be pursued. Micro-fluidic reactors are generally operated with wall-immobilized enzymes, but their implementation at industrial scale requires both parallelization and characteristic dimension increases. As part of this scale-up, research is required on millimeter-scale reactors to assess their bio-performance and potential limitations. Here, we present the first study of a pillar-structured milli-reactor. We studied this in-flow bioreactor with immobilized nitrilase and numerically characterized it by CFD modeling. After 5 days of continuous operation, a mean space-time-yield of 0.80 mM.min^-1 and a turnover number of 148 mg_product.mg_enz^-1 were reached. These promising performance results and the model validation were then used in a numerical study to determine how the reactor’s performance could be optimized. Under strict laminar conditions, strategies like increasing the surface-to-volume ratio or distribution of the enzyme all over the developed reactor surfaces are the main characteristics contributing to conversion improvement. Pillar reactors have a greater scale-up potential than zigzag reactors, requiring lower pumping energies for a given conversion rate. Finally, we hypothesize that going to hydrodynamic conditions with instabilities combined with more active enzymes would be an interesting avenue for future investigation to reach higher levels of process intensification.

Statement_of_novelty_and_significance

Micro-scale devices with wall-immobilized enzymes intensify mass transfers but the low productivity per unit requires complex parallelization to meet industrial throughput. This study proposes an experimental characterisation of a milli-size reactor coated with commercial polymethylmethacrylate beads on the surface. Compared to batch assays, the productivity has been intensified by 14-fold along with an increase of the space-time-yield of 27-fold. Reactor design engineering has been performed with CFD screening. Room for optimization has been elucidated with parameters including surface-to-volume ratio, enzyme distribution and liquid flow with instabilities using more active enzymes.