Carbon Monoxide Mass Transfer for Syngas Fermentation in a Stirred Tank Reactor with Dual Impeller Configurations Andrew J. Ungerman and Theodore J. Heindel* Department of Mechanical Engineering, Iowa State University, Ames, Iowa 50011 This study compares the power demand and gas-liquid volumetric mass transfer coefficient, k L a, 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 k L a 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. Introduction Biological conversion of synthesis gas (syngas), derived from the gasification of biomass, among other carbonaceous materials, has the potential to replace petroleum-based products and produce a variety of fuels and chemicals including polyhy- droxyalkanoates (PHAs), a feedstock for biopolymers. The syngas approach gasifies fiber-rich (lignocellulosic) materials into a gaseous mixture of carbon monoxide (CO) and hydrogen (H 2 ), which can be anaerobically metabolized by microorgan- isms specifically for the production of PHAs. However, because of the low aqueous solubilities of CO and H 2 (e.g., the solubility of CO is 77% that of oxygen (1)), syngas fermentations are limited by the gas-liquid mass transfer rates in the bioreactor when the cell concentration is high (2). If the cell concentration is too low, the system yield will be low and the conversion process will be kinetically limited (3). The design of industrial- scale bioreactors requires high cell concentrations, high volu- metric mass transfer coefficients (k L a), and homogeneous mixing of the multiphase system. Stirred tank reactors (STRs) are commonly used in industrial- scale fermentation processes because of their good mixing ability and scale-up characteristics (4). A common approach to increas- ing the gas-liquid mass transfer rate in a STR is to increase the speed of the impeller, which ultimately increases the gas- liquid interfacial area. This works well for laboratory-scale fermentors but creates problems in industrial-scale fermentors because of the large increase in power requirements. Addition- ally, in culture media, increased power input could damage shear-sensitive microorganisms (5). For this reason, alternative STR configurations must be investigated to enhance carbon monoxide gas-liquid mass transfer and increase mass transfer performance. The hydrodynamics of stirred bioreactors have been exten- sively studied, especially in the past two decades. This has led to advancements in impeller design for gas-liquid dispersions claimed to improve bioreactor performance (6-8); these changes modify flow patterns, aerated power efficiency, and mixing time. In addition, there has been recent work on sparger (9, 10) and baffle (11, 12) design, as well as microbubble production and dispersion (5) for gas-liquid mass transfer. However, comparing past research has produced inconsistencies on how gas-liquid mass transfer is affected by these changes. The need for improved k L a rates for syngas fermentation is a result of the low solubility of the syngas in the fermentation broth. For syngas fermentation to become commercially successful, alternative STR geometries must be further investigated, especially impeller design and configuration in actual CO-liquid systems. The six-blade Rushton impeller has traditionally been the most popular choice for gas-liquid dispersions, and studies on its hydrodynamics and cavity formation have been extensive (13-15). However, several disadvantages exist for the standard Rushton impeller including a significant drop in power draw upon gassing, high shear stress around the impeller, nonuniform distribution of the energy dissipation, and low gas holdup near the bottom of the stirred tank, which can all lower mass transfer potential (16). Concave or hollow-blade turbines tend to form smaller cavities on the impeller backside, which leads to a smaller power demand while gassing. Therefore, gas dispersion is higher for this impeller at a given speed and impeller diameter when * To whom correspondence should be addressed. Ph: 515 294-0057. Fax: 515 294-3261. E-mail: theindel@iastate.edu. 613 Biotechnol. Prog. 2007, 23, 613-620 10.1021/bp060311z CCC: $37.00 © 2007 American Chemical Society and American Institute of Chemical Engineers Published on Web 02/28/2007