Ethanol has emerged as a promising alternative to fossil fuels. Ethanol is produced primarily through two different processes. The first process produces ethanol by fermentation of simple sugars. The second process uses syngas (H2, CO, and CO2) produced from gasification of biomass. Many different reactors can be used for syngas fermentation. These include, but are not limited to: stirred tank bioreactor, bubble column reactor, gas spargers, membrane bioreactor, and trickling bed reactor. Of the membrane bioreactors, there are also several subclasses including: hollow fiber reactor, stacked array bioreactor, modular membrane supported bioreactor, and horizontal array bioreactor.
Initial choices for an effective reactor can be based solely upon mass transfer characteristics. In syngas fermentation, mass transfer is important because of the use of sparingly soluble gases like CO and H2. One reactor that has specifically high mass transfer rates is the hollow fiber reactor. This type of reactor usually has high mass transfer rates because of its typically large surface area to volume ratio. Conveniently, many hollow fiber reactors that can be commercially purchased include mass transfer coefficients for various species through the fiber. However, if these values have not been measured by the manufacturer for a specific hollow fiber unit, then it must be measured experimentally.
Besides high mass transfer rates, there are specific advantages to a membrane system. One such advantage is the ability for cells to be immobilized. For example, Clostridium ragsdalei, an ethanol producing bacteria, may be immobilized in membrane systems under certain conditions. Because ethanol production predictions rely on accurate cell density approximations, if an accurate model is to be designed for hollow fiber syngas fermentation, the biolayer must be described in full.
In this study, mass transfer rates for several different hollow fiber reactors were measured experimentally and compared. The highest overall mass transfer coefficients on a per volume basis were the PDMS non-porous hollow fiber reactor. However, other membranes had higher intrinsic mass transfer coefficients, but larger fibers resulted in smaller overall coefficients. In addition to mass transfer rates, one of the hollow fiber reactors (Optiflux) was studied in order to determine the conditions necessary for the immobilization of C. ragsdalei.