Carbon Monoxide Mass Transfer for Syngas Fermentation in a Stirred Tank Reactor with Dual Impeller Configurations

Abstract

This study compares the power demand and gas-liquid volumetric mass transfer coefficient, kLa, in a stirred tank reactor (STR) (T = 0.211 m) using different impeller designs and schemes in a carbon monoxide-water system, which is applicable to synthesis gas (syngas) fermentation. Eleven different impeller schemes were tested over a range of operating conditions typically associated with the "after large cavity" region (ALC) of a Rushton-type turbine (D/T = 0.35). It is found that the dual Rushton-type impeller scheme exhibits the highest volumetric mass transfer rates for all operating conditions; however, it also displays the lowest mass transfer performance (defined as the volumetric mass transfer coefficient per unit power input) for all conditions due to its high power consumption. Dual impeller schemes with an axial flow impeller as the top impeller show improved mass transfer rates without dramatic increases in power draw. At high gas flow rates, dual impeller schemes with a lower concave impeller have kLa values similar to those of the Rushton-type dual impeller schemes but show improved mass transfer performance. It is believed that the mass transfer performance can be further enhanced for the bottom concave impeller schemes by operating at conditions beyond the ALC region defined for Rushton-type impellers because the concave impeller can handle higher gas flow rates prior to flooding.

Commercial Biomass Syngas Fermentation

“…The key objective of bioreactor design is thus to provide a high gas-liquid mass transfer efficiency of syngas into the fermentation broth, while allowing process scale-up and low operation costs. A range of reactor designs was recently reviewed [29], and approaches to enhancing gas solubility include increased pressure, specific fluid flow patterns, the use of microbubbles, and impellers designed to enhance bubble break up [192]. Designs which provide the highest volumetric mass transfer rates are generally not the most efficient due to their need for increased power consumption.…”

“…Designs which provide the highest volumetric mass transfer rates are generally not the most efficient due to their need for increased power consumption. Thus, mass transfer performance is more usefully described as the volumetric mass transfer coefficient per unit power input (k L a P g −1 ) [192]. Munasinghe & Khanal compared the volumetric mass transfer coefficient (k L a) over eight different reactor configurations, reporting the highest k L a in an air-lift reactor combined with a 20 µm bulb diffuser [193].…”

Kinetics of CO conversion into H2 by Carboxydothermus hydrogenoformans

“…The cell culture density is limiting the CO conversion rate only when the cell concentration is low just after inoculation (Henstra 2006). However, because of the low aqueous solubility of CO, gasliquid mass transfer turns out to be the limiting step of CO conversion once the growth of biomass has been sufficient, as the volumetric bioactivity potential of the C. hydrogenoformans culture becomes limited by the dissolved CO available in the liquid (Cowger et al 1992;Kapic et al 2006;Riggs and Heindel 2006;J o n e s2007; Ungerman and Heindel 2007). Hence, for a thermophilic bioprocess to perform at a continuous maximal volumetric activity rate, it is essential for the cell concentration of the microorganism to be optimized to avoid the CO gas-liquid mass transfer limitation.…”

A review of conversion processes for bioethanol production with a focus on syngas fermentation

“…However, the use of high agitation speed increases the power requirement for large reactors. Ungerman and Heindel (2007) reported a dual impeller scheme with axial flow impeller at the top and lower concave impeller that resulted in a similar kLa and less power requirement compared to Rushton impellers. Bredwell and Worden (1998) used a microsparger that was shown to be energy efficient and increased the kLa by six fold compared to conventional gas sparging.…”