Abstract
Syngas (mixture of CO, H2 and CO2) fermentation suffers from mass transfer limitation due to low solubility of CO and H2 in the liquid medium. Therefore, it is critical to characterize the mass transfer in syngas fermentation reactors to guide in delivery of syngas to the microorganisms. The objective of this study is to measure and predict the overall volumetric mass transfer coefficient, kLa for O2 at various operating conditions in a 7-L sparged and non-sparged continuous stirred-tank reactor (CSTR). Measurements indicated that the kLa for O2 increased with an increase in air flow rate and agitation speed. However, kLa for O2 decreased with the increase in the headspace pressure. The highest kLa for O2 with air sparged in the CSTR was 116 h−1 at 600 sccm, 900 rpm, 101 kPa, and 3 L working volume. Backmixing of the headspace N2 in the sparged CSTR reduced the observed kLa. The mass transfer model predicted the kLa for O2 within 10% of the experimental values. The model was extended to predict the kLa for syngas components CO, CO2 and H2, which will guide in selecting operating conditions that minimize power input to the bioreactor and maximize the syngas conversion efficiency.
Highlights
Liquid biofuels can be produced from lignocellulosic feedstocks using biochemical or hybrid thermochemical-biological processes, which alleviates the heavy dependence on crude oil and avoid competition with food crops [1,2]
The dissolved oxygen (DO) probe was the DI water and the continuous stirred-tank reactor (CSTR) exhaust was completely closed by the backpressure regulator valve calibrated at each tested pressure to 100% DO by saturating the DI water with sparging air at 1000 sccm (Figure 2)
(mol min−1 ), PHP is the hydraulic pressure above the microsparger, ρw is water density, g is gravitational acceleration (m s−2 ), h is the distance between the microsparger and liquid surface (m), PHS is the headspace pressure, Ptotal is the total pressure in the CSTR, Qg is the air volumetric flow rate, R is the ideal gas constant (L kPa mol−1 K−1 ), T310K is the temperature used (310 K)
Summary
Liquid biofuels (e.g., bioethanol and biobutanol) can be produced from lignocellulosic feedstocks using biochemical or hybrid thermochemical-biological processes, which alleviates the heavy dependence on crude oil and avoid competition with food crops [1,2]. Most mass transfer studies in syngas fermentation using CSTR investigated operating parameters such as gas flow rate and agitation speed at a fixed working volume and mostly in unpressurised reactors [15,28,29]. For large-scale syngas fermentation, the headspace pressure, gas flow rate, the power consumption, and reactor working volume are critical parameters for estimating the feasibility of fermentation process. The incorporation of these parameters into one model will help determine the operational parameters and meet the microbial kinetics requirement for syngas fermentation. No reports were found on the effect of headspace gas backmixing on mass transfer for syngas fermentation reactors. The k a for syngas components CO, CO and H2 were estimated from kLa for O2 based on the penetration or Lsurface renewal theory [33,34]. 2 from kL a for O2 based on the penetration or surface renewal theory [33,34]
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