Mixing and mass transfer studies of the fermentation product (poly)-gamma-glutamic acid

The mixing and mass transfer problems encountered in the viscous fermentation of (poly)-gamma-glutamic acid (gamma-PGA) have been studied. The overall objective of this research was to enhance the productivity of this viscous fermentation through bioreactor design and operation.; It was believed that inadequate bulk mixing was the major limitation to gamma-PGA production, when conducted in a conventional Rushton turbine reactor. The active well mixed core formed in the vicinity of the turbines occupied less than 40% of the total volume at a gamma-PGA concentration of 30 g/L. In order to increase the efficient use of the reactor volume, mixing time studies were carried out with various agitator configurations in a 1.25 L fermenter containing 3% gamma-PGA. These impeller designs included various sizes of Rushton turbines and paddle configurations. For a series of Rushton turbines with similar geometries, the lowest agitation speed necessary to create satifactory mixing decreased with increasing impeller diameter. Two major problems were encountered during this mixing study with the Rushton turbines. First of all, the well mixed core extended only radially and not vertically from the impeller blades. Secondly, aeration severely hampered the mixing action in the fermenter.; Many configurations of paddle-turbine impellers (D/T = 0.87) were also tested in the same system. Straight and twisted paddles showed a slight improvement over Ruston turbines with almost the same power requirement. The most promising result was obtained with a new configuration which combined a bottom Rushton turbine (D/T = 0.87) for gas dispersion with a paddle impeller (D/T = 0.87) above it for bulk mixing. Furthermore, aeration helped to reduce not only the mixing time, but also the required power input.; The enhanced oxygen transfer capacity of the new impeller configuration was further confirmed by oxygen transfer studies performed in a 3% gamma-PGA solution. By extending the well mixed zones this impeller improved oxygen transfer and bulk mixing. Unfortunately this resulted in a low volumetric productivity of 0.10 g gamma-PGA/L hr (compared to 1.1 g gamma-PGA/L hr in the conventional fermenter). The conventional turbines were then reinvestigated. A gamma-PGA concentration of 1.35 g/L hr was achieved with oxygen enriched air with a maximum agitation rate of 600 RPM and the specific gamma-PGA biopolymer production was found to be directly proportional to agitation rate. Agitation rates about 600 RPM were found to significantly degrade the polymer with high shear rates. These results showed that a well mixed environment and ample oxygen supply did not favor gamma-PGA production. Inadequate mixing (circulation time of 2 minutes) favored gamma-PGA productivity over growth and resulted in the best gamma-PGA production rate (1.3 5 g/L hr).