(660a) An Electric Drive for Biorefineries: Electrochemical pH-Shift Liquid-Liquid Extraction of Succinic Acid

The
progress in industrial biotechnology provides new bio-based routes for the
sustainable production of platform chemicals.[1]
Biotechnological production of succinic acid (SA) is a promising example where bio-based
processes now compete with established petrochemical production routes.[2] Succinic acid is a four carbon (C4) di-carboxylic acid which makes it a versatile building
block for bio-based polyester and nylon-type polymers or a precursor for
solvents like 1,4-butanediol and tetrahydrofuran.[3]
The development of well-performing bio-catalysts shifted the bottleneck of bio-based
succinic acid production from upstream to downstream operations. In the
manufacturing of carboxylic acids, separation and product purification can
account for up to 40% of total production costs.[4]
The high separation costs originate from a mismatch between the optimal pH for
fermentation and product recovery. While most bio-catalysts work best at
neutral pH, acidic pH is advantageous for product recovery.[5]

However,
pH control by inorganic bases and acids comes with the drawback of producing
large amounts of inorganic salt waste like gypsum.[6]
Hence, new separation technologies which overcome the excessive application of
inorganic acids and bases for pH adjustment are a key element for improving the
competitiveness of bio-based production routes for succinic acid.

In
this work we propose electrochemically driven pH-shift liquid-liquid extraction
as a new method for the recovery of succinic acid from pH neutral aqueous
solution. The proposed approach features the generation of spatially separated
regions of acidic and alkaline pH by water electrolysis. The anodic reaction of
water electrolysis produces protons, which create an acidic pH near the anode.
This acidic pH shifts the aqueous acid-base equilibrium towards the protonated
succinic acid, which is then extracted from the aqueous solution by an organic solvent.[5] 
Hydroxide ions produced at the cathode create an alkaline catholyte, which can
be used for pH control in succinic acid fermentation. The water electrolysis coupled
to liquid-liquid extraction creates an integrated buffer-recycle between
fermentation and product separation and overcomes the need for adding inorganic
acids and bases for pH-control. In contrast to electrodialysis where membrane
stability and high mass transport resistance reduce separation efficiency[4]
no ion selective separation of anolyte and catholyte is required in our
approach.

In
order to prove feasibility, we conducted batch experiments of electrochemical
extraction in a 140ml setup. Thereby separation of succinic acid from aqueous
solution at initial pH of 7 was achieved with tri-n-octylamine and 1-octanol as
organic solvent. The change of succinic acid concentration in the aqueous phase
was measured by HPLC and follows farady’s law of electrolysis. Based on the
concentration change, we determined a faradaic efficiency of 30-60% for
separation in the non-optimized laboratory setup. The mass balance of succinic
acid holds within an experimental error of 3-5%. In addition, no byproducts or
indications of succinic acid degradation were found after electrolysis.

Furthermore,
we developed a dynamic model of the electrochemically operated liquid-liquid
extraction. A set of differential and algebraic equations (DAE) for mass
balances, aqueous acid- base-equilibrium as well as mass transport kinetics of
liquid-liquid extraction was implemented in MATLAB. The model was validated
with the experimental results and can predict the experimentally observed
change in concentration. In order to investigate its application to control the
pH of succinic acid fermentation, the dynamic model for electrochemical
pH-shift extraction was also implemented as s-function in the MATLAB Simulink
environment and coupled to a dynamic fermentation model based on Monod-kinetic
type reaction rates considering pH-Inhibition of the succinic acid producing
microorganism A. succinogenes.[7]

Simulation
results indicate that pH-management by electrochemical liquid-liquid extraction
seems feasible and the simulated fermenter achieved a space time yield of 2.15
g l^-1h^-1 which is comparable to reported experimental values
of 1.8-3.9 g l^-1h^-1.[8]
A case study for flexible utilization of electricity was conducted to investigate
the impact of intermitted electricity supply from renewable sources on the
overall process performance. The results indicate that kinetics of bio-reaction
are much slower than the kinetics of electrolysis and liquid-liquid extraction.
Thus, a fluctuating electricity supply does not compromise the process. In a
sensitivity study we set a target for faradaic efficiency, cell voltage and
electricity price at which electrochemical pH-shift extraction becomes a
competitive option for succinic acid downstream processing.

References

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[2]  N. Nghiem, S. Kleff, S.
Schwegmann, Fermentation 2017 3 (2), 26. DOI: 10.3390/fermentation3020026.

[3]  J. Becker, A. Lange, J.
Fabarius, C. Wittmann, Current Opinion in Biotechnology 2015 36,
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[4]  C.S. López-Garzón, A.J.J.
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[5]  T. Kurzrock, D. Weuster-Botz, Biotechnology
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[6]  A.A. Kiss, J.-P. Lange, B. Schuur,
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[7]  S.K.C. Lin, C. Du, A. Koutinas,
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[8]  C.D. van Heerden, W. Nicol, Biochemical
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