Process
intensification of CO consumption by C. ljungdahlii in a low power input
biocomposite gas absorber

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 Mark
J. Schulte and Michael C. Flickinger

Chemical
and Biomolecular Engineering

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line-height:115%;font-family:"Times New Roman",serif'>Single carbon gases are
inexpensive sources of carbon (CO, CO₂, CH₄) for
conversion to commodity chemicals but there currently are no efficient bioprocessing
methods operating beyond pilot scale to convert these low water soluble
compounds to useful multi-carbon compounds using engineered microbes.
Biocatalytic processing of gaseous substrates is limited by assimilation rate -
lagging chemical catalysts specific reactivity by 10³ to 10⁵.
Our investigation is a new approach to increasing the rate and decreasing the
power input for large scale gas phase biocatalysis through using cellular
biocomposite materials where concentrated cells are immobilized within an
inexpensive porous matrix in contact with the gas phase through a thin liquid
film. Process Intensification (PI) in a cellular biocomposite concentrates cells,
reduces mass transfer resistance using thin liquid films, decreases water
consumption and can significantly reduce power input for carbon recycling into
fuels and chemicals. Our model system, C. ljungdahlii OTA1, takes up
CO/H₂ and produces ethanol/acetate. The cells adhere to
chromatography paper without the use of an adhesive at 10¹² CFU/m².
These batch biocomposites are tested in horizontal Balch tubes, hydrated with
growth limiting media, and flushed with an H₂/N₂/CO
mixture (45%/10%/45%). The biocomposite is hydrated by the media moving through
the paper pores while the coating remains in the gas phase with only a thin
liquid film (thickness ~30µm) limiting mass transfer to the cells. Using an
extrusion coating method, at 100rpm in horizontal tube reactors the CO specific
uptake rate of OTA1 is ~10% faster in a biocomposite than in suspension. However,
at 25rpm (97% power reduction) the biocomposite is ~300% faster. This is the
result of the paper maintaining a high interfacial area without vigorous mixing
which is required for high CO uptake in a sparged stirred tank reactor
configuration using suspended cells. A specific CO consumption PI of more than
an order of magnitude is demonstrated (69.8 mmol CO/m²/h) over
previous biocomposite researchers (Gosse et al, 2012). The k_La_apparent
(a minimum value) of this unoptimized system is ~40 h^-1at
<10 W/m³ power input which compares favorably with a CSTR (k_La
is ~100h^-1 at ~100W/m³). For example, our maximum specific
CO uptake of ~25mmol CO/g_DCW/h with low power input is comparable to
the specific uptake for C. ljungdahlii of ~35mmol CO/g_DCW/h
in a CSTR (Richter et al, 2014). Further reactor optimization (e.g.
porous substrate characteristics, biocomposite surface area/reactor volume,
cell loading) will enhance biocomposite performance. This batch system is being
used to develop design parameters using a computation fluid dynamics model for
a continuous low power biocomposite gas absorber falling film system. A low
power system is necessary for remote gas capture and reforming using biocatalysts
to access untapped commodity chemical markets using cheap single carbon gasses.

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