Advantages and Disadvantages of Biocatalysis
When compared to the metal-based chemical catalysis route, the fermentation method has its advantages as well as some disadvantages.
Advantages
In contrast to chemical catalysts, biocatalysts operate at moderate temperatures close to ambient temperature, and pressures close to atmospheric pressure, which result in substantial energy savings in large industrial-scale operations. Additionally, the reactor designs are simpler and no high-temperature, pressure-resistant special materials are required in the fabrication of the reactors. Moreover, operation at ambient temperature avoids the thermodynamic equilibrium relationship and causes the irreversibility of biological reactions, which consequently should result in high conversion efficiencies. High reaction specificity is achieved in fermentation — based methods in comparison to chemical catalytic processes due to the high enzymatic specificity. Biocatalysts are known to have higher tolerance for sulfur-containing gases like hydrogen sulfide (H2S) and carbonyl sulfide (COS), and also for smaller amounts of mercaptans or organic sulfur compounds, as well as for chlorine and chlorine-containing compounds. Furthermore, most microorganisms are even capable of adapting to contaminants like tar within certain limits. It is interesting to note that the growth of some anaerobic bacteria can be stimulated in the presence of sulfur compounds, as sulfur acts as reducing agent which reduces the redox potential of the medium. Even though most microorganisms can tolerate these impurities, the syngas requires some clean-up before the fermentation process to maintain the maximum bacterial activity. With metal catalysts, even a trace amount of sulfur gases present in the syngas can poison the chemical catalytic conversion; therefore, elaborate gas cleaning techniques are required in the chemical catalysis process, which contribute to the high cost of ethanol. Furthermore, biocatalysts are less sensitive to the composition of syngas and usually do not require a fixed CO/H2 ratio, whereas metal — based chemical catalysts need a specific ratio of gas components to yield a desired product.
Disadvantages
There are major drawbacks in this technology as well, like intrinsic poor solubility of CO and H2 components of syngas in aqueous broths, which will result in low substrate uptake by microbes, thus, leading to poor conversion efficiencies and low ethanol yields. For example, Kuniyana et al. have reported their findings on a pilot plant-scale experiment conducted in a 100 L fermenter, and they have indicated that the conversion efficiency of CO and H2 from the gaseous phase is only 20% at a continuous gas flow rate of 0.9 L per minute (LPM) at 37°C [5]. Additionally, Henstra et al. have shown that increasing the temperatures has a negative impact on the solubilities of CO and H2 and will result in a decrease in the mass transfer rate of these gases to cells [6]. Dissolution of a gas in a liquid phase is a complex process, and then there are several intermediate steps involved in transporting syngas components into the microbial cells. These steps include the diffusion through the bulk gas to the gas-liquid interface, moving across the gas-liquid interface, transport into the bulk liquid surrounding the microbial cells, and the diffusive transport through the liquid-solid boundary. In an assessment of various steps Klasson has identified that gas-liquid interface mass transfer is the major resistance for gaseous substrate diffusion [7].
The solubility of a gas mixture in the liquid phase is often quantified by means of the volumetric mass transfer coefficient (kLa). Klasson and Ackerson have proposed the following equation to calculate the mass transfer coefficient (kL) in the liquid phase:
(12.1)
Where Ng (mol) is the molar substrate transferred from the gas phase, VL (L) is the volume of the reactor, Pg and Pg (atm) are the partial pressures of the gaseous substrate in gas and the liquid phases, H (L atm/mol) is Henry’s law constant, and a (m2/L) is the gas-liquid interfacial area for unit volume. The difference in the partial pressures of the gaseous substrate (Pg — Pg )is the driving force for mass transfer and thus controls the solubility of the substrate. High-pressure operation improves the solubility of the gas in aqueous phase. However, at higher concentrations of gaseous substrates, especially CO, anaerobic microorganisms are inhibited after a threshold concentration.
The cost of fermentation media is also an important factor in a large-scale operation of a fermenter. For instance, Kundiyana et al. have reported that morpholinoethanesulfonic acid (MES) used as a buffering agent in syngas fermentation media accounts for approximately 97% of the cost of "C. ragsdalei" standard media. The buffering of the media is an essential feature of the fermentation broth, as pH of the media controls the balance of acidogenesis (acetic acid production) to solventogenesis (ethanol production). Therefore development of economical buffering systems is also a central issue in scaling up the process.