Syngas Fermentation Reactors
Several reactor designs can be used for the fermentation process, and reactor configuration is closely related to the product yield and plays an important role in syngas fermentation. The most important parameters that we are looking for in designing an efficient fermentation bioreactor system are: high mass transfer rates, high cell densities, low operation and maintenance costs, and easy scale-up to very large industrial scales [47]. In this section some of the common reactor designs that are in current use in syngas fermentation and undergoing further improvements will be discussed.
Continuous Stirred Tank Reactors (CSTR)
The continuous stirred-tank reactor (CSTR), also known as vat or backmix reactor, is the most commonly employed bioreactor in syngas fermentation. This reactor has a continuous flow of gas bubbling through the liquid which typically consists of a dilute solution of essential nutrients for the microorganism to grow and
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Figure 12.2 Schematic representation of the continuous stirred tank reactor (CSTR) used for fermentation of syngas to ethanol. |
survive on. A schematic representation of continuous stirred tank reactor (CSTR) used for fermentation of syngas to ethanol is shown in Figure 12.2. The liquid can be added and removed from this type of reactor while the reactor is under operation, and high agitation rate is needed to enhance the transfer rate of the CO, CO2, and H2 from the syngas to organisms [48]. Higher agitation speeds lead to a higher mass transfer rate between the substrate gases and the microbes. However, in very large industrial-scale fermenters, higher agitation speeds increase the agitator’s power consumption, thus increasing the operational cost of the plant. If the transfer is not fast enough, the production of cellular products will be limited to how fast the gas is transferred to the organism.
Microbial cell recycle systems can also be used in conjunction with the CSTR to increase cell density within the reactor. In such a system, the fermentation broth is pumped through a recycle filter and the retentate containing the microbial cells is separated from the permeate (cell-free media) and recycled back to the bioreactor. This process prevents loss of cell mass from the bioreactor during continuous operation and also allows the CSTR to be operated at dilution rates greater than the maximum growth rate of the microbial catalyst. Recycling has been shown to provide up to about a 2.6-fold increase in cell concentration [14,49].
Packed Bed Reactors (Immobilized-Cell Reactors)
Packed-bed reactors, or immobilized-cell reactors, are columns packed with biocatalyst particles to which the microorganisms are immobilized. These reactors are usually operated concurrently where the liquid and gas flow in the same direction [48]. Advantages of this reactor include high density of the microorganisms and easy separation of the microbial cells from the fermentation broth. However, the rate at which syngas components are transferred to the organism is usually slow.
Trickle Bed Reactors (TBR)
This is a vertical tubular reactor packed with solid material that the microorganisms can attach to as solid support. The term "trickle bed" entails the downward movement of a liquid and gas over a packed bed of catalyst particles. It is considered to be the simplest reactor type for performing catalytic reactions where a gas and liquid (normally both reagents) are present in the reactor, and accordingly it is extensively used in processing plants. The direction of fluid flow is normally counter current, with the liquid trickling downwards as the gases flow upwards [50].
Comparative studies on different types of reactors in syngas fermentation are rare in the open literature. However, Kundiyana and Wilkins have reported their work on a 100L pilot plant-scale syngas fermenter [5] as well as a two-stage continuous fermenter design on productivities during Clostridium ragsdalei syngas fermentation [32]. These pilot-scale 100L fermenter studies were conducted in strictly anaerobic conditions, the fermentation system was maintained in a batch mode with continuous syngas supply, and the impact of improving the syngas mass transfer coefficient on the utilization and product formation was studied. Results indicated a six-fold improvement in ethanol concentration compared to serum bottle fermentation and the formation of other compounds as well, such as isopropyl alcohol, acetic acid and butanol, which are of commercial importance [5]. The two-reactor configuration experiment was conducted using two stirred-tank fermenters of equal volume in series in the partial-cell recycle mode [32]. The operational strategy of this reactor scheme involved operating the first reactor as a "growth reactor" and the second reactor as a "product reactor." These studies clearly demonstrated the advantages of two-stage reactor design over the single-vessel design.