Xiangan Li – Michael Henson Lab
Integrated Metabolic and Process Modeling of Bubble Column Reactors for Synthesis Gas Fermentation
Xiangan Li and Michael Henson
Gas fermentation has emerged as a technologically and economically attractive option for producing renewable fuels and chemicals from carbon monoxide (CO) rich waste streams. LanzaTech has developed a proprietary strain of the gas fermentating acetogen Clostridium autoethanogenum as a microbial platform for synthesizing ethanol, 2,3-butanediol and other chemicals. Bubble column reactor technology is being developed for large-scale production, motivating the investigation of multiphase reactor hydrodynamics. In this study, we combined hydrodynamics with a genome-scale reconstruction of C. autoethanogenum metabolism and multiphase convection-dispersion equations to compare the performance of bubble column reactors with and without liquid recycle. For both reactor configurations, hydrodynamics was predicted to diminish bubble column performance with respect to CO conversion, biomass production and ethanol production when compared to bubble column models in which the gas phase was modeled as ideal plug flow plus axial dispersion. Liquid recycle was predicted to be advantageous by increasing CO conversion, biomass production, and ethanol and 2,3-butanediol production compared to the non-recycle reactor configuration. Parametric studies performed for the liquid recycle configuration with two-phase hydrodynamics showed that increased CO feed flow rates (more gas supply), smaller CO gas bubbles (more gas-liquid mass transfer) and shorter column heights (more gas per volume of liquid per time) favored ethanol production over acetate production. Our computational results demonstrate the power of combining cellular metabolic models and two-phase hydrodynamics for simulating and optimizing gas fermentation reactors.
Eric Hernandez – Friederike Jentoft Lab
Elucidating the Mechanism of Deactivation during Zeolite-Catalyzed Alcohol Dehydration via in situ Diffuse Reflectance Spectroscopy
Eric Hernandez and Friederike Jentoft
Zeolites represent a vital class of microporous catalysts used for a variety of industrially relevant acid-catalyzed chemical reactions. In addition to their shape-selective pore architectures, the versatility of these aluminosilicate materials as catalysts can be attributed to their high temperature stabilities and tunable acid site densities. Alcohol dehydration and methanol-to-olefins (MTO) reactions are arguably some of the most important zeolite-catalyzed industrial processes for the production of unsaturated hydrocarbons . Unfortunately, rapid catalyst deactivation is encountered during these alcohol transformations. This phenomenon has been attributed “coking”, the formation of carbonaceous deposits which block the active sites and/or pore channels of the zeolite . The nature of these deposits has been extensively studied and they are reported to be both monoenyl and dienenyl carbenium ions . Furthermore, the presence of coke ultimately leads to production of undesired saturated hydrocarbons. Still, the mechanism of coke formation remains unanswered and the relationship between coke and formation of alkanes remains ambiguous. Rational design of more stable catalysts can only be achieved through a better understanding of this mechanism.
A combination of in situ diffuse reflectance Fourier transform infrared (FTIR) and ultraviolet-visible light (UV-vis) spectroscopies coupled with online gas phase product analysis was applied to investigate the mechanism of coke formation during 1-butanol temperature-programmed dehydration on H-ZSM-5. In the present study, evidence for the direct relationship between the formation of alkyl-substituted cyclopentenyl cations and alkane production via a hydride transfer mechanism is provided. The evolution of several bands (both transient and persistent) during both IR and UV vis experiments were correlated with the characteristic desorption of gas phase products, mainly isobutane and branched alkanes. The identities of the surface species were obtained through additional probe molecule adsorption spectroscopy experiments. Furthermore, the acid site density of the zeolite was tuned by varying the Si/Al ratio and a correlation between Brønsted acid site concentration and coke formation is presented.
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