Bioprocess intensification : a study of rotating packed bed porous mesh impellers for enhancement of aerobic fermentation processes

Abstract

Oxygen transfer can be a rate limiting factor in aerobic fermentation systems. If oxygen concentration becomes insufficient productivity will decrease and the culture will ultimately die. Novel reactor technology such as the Rotating Packed Bed Reactor (RPB) may overcome this traditional limitation. The exploitation of high centripetal forces and highly porous packing material in the RPB in gas-liquid processes is proposed to enhance oxygen transfer by production of fine bubbles. The aim of the enhancement is to ensure that the Dissolved Oxygen (DO) concentration exceeds the demand of the microbial culture utilised. The purpose of the thesis is the development and characterisation of a new HiGEE Bioreactor (HBR) for application to fermentation systems. The work was undertaken in three stages. Initial experiments focused on the mass transfer characterisation of several porous packings intended for use in the HBR. The performance of the packings was evaluated by measuring their oxygen transfer capability, power input and air bubble size produced when employed as impellers in a conventional gas-liquid stirred tank reactor (STR). It was observed that the 11 cm stainless steel Knitted Wire mesh impeller (at a fixed airflow rate of 1.0 vvm, and agitation rate of 400 rpm) produced a KLa value of 0.0312 s-1 compared to KLa of 0.0334 s-1 for the double Rushton turbine at an agitation rate of 1000 rpm and aeration rate of 1.0 vvm but with a significant decrease of about 7000 W m-3 in power. Similarly in the bubble experiments performed, the 11 cm knitted wire impeller could produce bubble diameters as low as 0.15 cm compared to 0.28 cm for the Rushton impeller. Two fermentation systems studied (oxygen transfer optimisation with Escherichia coli K12, and product optimisation with Pseudomonas putida KT2442) further demonstrated that the knitted wire mesh packing could produce a higher biomass concentration due to the enhanced oxygen transfer rate. In the final set of experiments a new HBR was designed and commissioned. A set of hydrodynamic experiments focused on the flooding conditions and bubble sizes produced within the reactor. For both experiments the packing had a profound influence on the results, producing a very fine bubble diameter of 0.361 mm at 1200 rpm with packing compared to 2.50 mm at 1200 rpm without packing whilst also allowing higher throughputs of liquid and gas before flooding occurred. A series of transfer studies also illustrated the effect of packing, with a KLa value of 0.0025 s-1 (no packing) and 0.0030 s-1 (with packing) achieved for an experiment at 1200 rpm. The RPB was then tested to be utilised as a bioreactor by studying the fermentation of P.putida KT2442 to produce polyhydroxyalkonates (PHA).