|Irika Devi Anggraini||Department of Chemical Engineering, Faculty of Industrial Technology Institut Teknologi Bandung, Jl. Ganesa 10, Bandung 40132, Indonesia|
|Keryanti||Department of Chemical Engineering, Faculty of Industrial Technology Institut Teknologi Bandung, Jl. Ganesa 10, Bandung 40132, Indonesia|
|Made Tri Ari Penia Kresnowati||Department of Chemical Engineering, Faculty of Industrial Technology Institut Teknologi Bandung, Jl. Ganesa 10, Bandung 40132, Indonesia|
|Reiji Noda||Division of Environmental Engineering Science, Graduate School of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515 Japan|
|Tomohide Watanabe||Graduate School of Science and Technology, Division of Environmental Engineering Science, Gunma University, Kiryu City, Japan|
|Tjandra Setiadi||- Faculty of Industrial Technology Institut Teknologi Bandung - Centre for Environmental Studies (PSLH), Institut Teknologi Bandung|
The production of ethanol via syngas fermentation obtained from lignocellulose gasification provides a method for completely utilizing all of the carbon content from lignocellulosic feedstock. The low mass transfer rate of less soluble CO and H2 gas to liquid has been considered a major bottleneck in the overall process; however, microporous membrane has been proposed as a gas diffuser to improve gas-to-liquid mass transfer. In this study, a liquid batch of syngas fermentation employing Clostridium ljungdahlii with continuous gas supply was obtained using the configuration of a bioreactor connected to microporous hydrophobic polypropylene hollow fiber membrane (HFM) as a gas diffuser. Liquid recirculation between the fermentation vessel and membrane module was applied to enhance the gas–liquid contact as well as cell-recycle. The fermentation performance with and without HFM was compared and evaluated by cell growth, CO utilization, ethanol yield, and productivity. A higher ethanol yield, 0.22 mol/mol, was achieved using the system of an HFM-supported bioreactor with a higher ethanol titer of 1.09 g/L and an ethanol-acetate molar ratio of 1.43 mol/mol. The obtained result demonstrates that an HFM-supported bioreactor is the best fermentation system compared to stirred tank reactor (STR) without a membrane.
Clostridium ljungdahlii; Ethanol; Hollow fiber membrane; Mass transfer; Syngas fermentation
Syngas fermentation has been widely studied as an alternative means of completely obtaining and further converting all of the carbon content from lignocellulosic feedstock generated from the gasification process. It is possible to generate a cleaner and higher quality of syngas from gasified lignocellulose compared to the gas product derived from coal and other fossil fuels (Sulaiman et al., 2012). Most of the acetogenic species from more than 20 genera as the employed biocatalyst on syngas utilization are identified to produce solely acetate via the Wood–Ljungdahl pathway with acetyl-CoA as the intermediate product, such as Acetobacterium woodi, Alkalibaculum bacchi, Butyribacterium methylotrophicum, and Eubacterium limosum (Kopke et al., 2011). Some strains of Clostridium sp. have been studied as CO-fixing microorganisms able to further convert acetate into solvent products such as ethanol, butanol, and 2,3-butadienol, otherwise known as the carbodixotrophic microorganism, through the syngas fermentation process.
Syngas fermentation presents several advantages, such as greater biocatalyst specificity, lower energy costs, and no fixed ratio requirements in respect of CO: H2 (Camacho et al., 2014). However, the efficient mass transfer of sparingly soluble gases such as CO and H2 is a known bottleneck in the overall fermentation performance since CO acts as the main substrate for the growth of bacteria (Mohammadi et al., 2011).
Studies on bioreactor configuration aimed at addressing this mass transfer limitation have been conducted by evaluating the volumetric mass transfer coefficient (kLa) of the system as the key parameter. Such studies have been conducted in various bioreactor configurations, including the Continuous Stirred Tank Reactor (CSTR) (Riggs & Heindel, 2006), Bubble Column Reactor (BCR) (Datar et al., 2004), Gas Airlift Reactor (Munasinghe & Kanal, 2010), Trickle Bed Reactor (TBR) (Orgill et al., 2013), and Hollow Fiber Membrane Bioreactor (HFMBR) (Shen et al., 2014). High CO mass transfer in CSTR is normally achieved by increasing the agitation speed to increase the interface area (Orgill et al., 2013). Microbubble sparging has been used as one tool to enhance the dispersion of gas in the CSTR system, although no significant difference in the kLa values compared to a conventional bubble system has been demonstrated (Munasinghe & Khanal, 2010). Meanwhile, TBR offers the potential for a more efficient mass transfer system without mechanical agitation by applying a low gas and liquid flow rate to maintain the liquid holdup and retention time in packed columns. However, compared to other systems, low productivity fermentation is possible when a high liquid flow rate, if applied, is accompanied by decreasing CO mass transfer due to the bubble regime (Lee et al., 2012).
The Stirred Tank Reactor (STR) is a conventional and the most commonly used type of fermenter in microbial bioethanol production. The approach frequently used to attain high gas–liquid mass transfer is by increasing the agitation speed; however, this is economically infeasible due to the high-power consumption required for upscale fermentation. The application of hollow fiber membrane (HFM) as an external gas–liquid contactor connected to the bioreactor as the reservoir is able to significantly increase the CO mass transfer rate since it provides a large ratio of surface area to volume as the syngas flowing through the lumen of the membrane diffuses through the microporous membrane without forming bubbles (Orgill et al., 2013). The high gas–liquid mass transfer offered by this system enables the application of a low gas flow rate for higher gas conversion. The membrane could also serve as the biofilm support as the microbes grown on the outer wall of the membrane pass by the liquid stream (Shen et al., 2014).
Despite the potential of HFM for syngas fermentation application, no comparative studies on an HFM-supported bioreactor and conventional STR have been undertaken using the same strain of Clostridium sp. and the same scale of reactor. Therefore, the objective of this study is to evaluate the performance of both systems by investigating the profile of cell growth, carbon conversion, ethanol yield, and productivity. Two experiments will be carried out using the same strain of bacteria, working volume fermentation, and applied operating condition to enable a direct comparison of the performance of both systems with no effect from other factors.
The design of HFM as a gas–liquid contactor and biofilm platform has demonstrated a significant improvement in the amount of ethanol production with the achievement of a high ethanol-to-acetate molar ratio (1.43) and a threefold-higher maximum ethanol concentration (1.09 g/L) than that produced in an STR without HFM support (0.35 g/L). The application of HFM to bioethanol production via syngas fermentation is an innovative approach that offers a reduction of the CO mass transfer barrier observed in other conventional reactor designs.
Financial support for this research was provided by the SATREPS project of JICA and JST: “Project for Development of a Model System for Fluidized Bed Catalytic Gasification of Biomass Wastes and Following Liquid Fuel Production in Indonesia,” Gunma University, BPPT, Yayasan Dian Desa and ITB, 2013 – 2019. The first, second, and last authors (I.A., K., and T.S.) would like to acknowledge the partial funding obtained from the World Class University - 2018 Program of Institut Teknologi Bandung No 021/WCU-ITB/LL/II/2018.
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