Butanol

Equations and Calculations
for Fermentations of Butyric Acid Bacteria

Eleftherios Terry Papoutsakis
Department of Chemical Engineering, Rice University,
P.O. Box 1892, Houston, Texas 77251

Accepted for publication M ay 12, 1983

Saccharolytic clostridia grow anaerobically on a variety
of substrates, can produce a large number of useful produ c t ~ , and ~
~ - thus appear to be verypromising bacteria for
production of organic chemicals from mono-, oligo-, and
polysaccharides. Butyric acid bacteria (clostridia) in particular, can anaerobically ferment a variety of sugars (hexoses, pentoses, and o l i g o s a c ~ h a r i d e s )to~produce a
~ ~~~
variety of organic solvents (butanol, acetone, ethanol, isopropanol, acetoin), carboxylic acids (acetic, butyric, lactic, andformic acids) and h y d r ~ g e n . ~ ~ ~ ~ ' ~ - ~ ~
The economic feasibility of any such saccharolytic fermentation will be determined by the maximal possible:
1) conversion of the sugar(s), 2) product yield and selectivity, 3 ) reactor productivity, and 4) product separation
INTRODUCTION
and purification efficiency. Ethanol and acetic acid can be
Despite the currently falling prices andplentiful supproduced with an equal or better biochemical efficiency by
plies of crude oil, there is considerable evidence to suggest
yeast or other bacteria fermentations, as sole product^;^-^
that renewable resources (mono- and oligosaccharides
overall, thus, they can be produced more efficiently by
from biomass hydrolysis or from waste streams of manuthose other fermentations than by themixed-product ferfacturing activities) will provide a significant percentage
mentations of butyric acid bacteria. On t he other hand,
of the chemical industry's feedstocks in the future.'-4
butanol has a significantly higher market price per mole
than acetone, butyric acid, isopropanol, or acetoin. ThereEthanol, acetic acid, lactic acid, butanol, acetone, butanediol, and isopropanol are some of themain feedstocks
fore, economically, the most attractive fermentations of
which can be produced by saccharolytic fermentati~ns.'-~ butyric acid bacteria are those with the highest selectivity
for butanol, which thus optimize substrate utilization and
Which, how, and when any of the above or other chemicals
is or will be produced economically by fermentation dereduce separation costs.
T heideal fermentation would
pends upon a sequence of complex socioeconomical and
then be the one that could convert the sugar units to butatechnological factors,2 which change drastically with geno1 to the highest allowable degree, and thus produce none
ography and time. Very conservatively, we can say that
or very little of the other fermentation products. The maxthese fermentation processes appearin principle as atimal allowable butanol (or any other product) yield is
tractive alternatives to current petrochemical processes
determined by both thermodynamic constraints and the
biochemical topology. The establishment of the thermoand therefore deserve some decent investment for technodynamic and biochemical constraints which determine
logical improvements. The latter means thatfermentations must be switched from batch to continuous or semithe theoretically highest yield for each product and the
continuous, reactor configurations and instrumentation
calculation of these maximal yields would be of both fundamental and practical importance. They would allow us
should be advanced, product yields and tolerances of mito establish rationally the upper bounds for theproductivcroorganisms should be improved by recombinant DNA
ity of the fermentations, which in turn can be used as a
or classical genetic techniques, and more efficient separation processes must be employed. If t he case of ethanol
guide in feasibility studies, and experimentation for genetic and bioreactor-productivity improvements. Simifermentation can be a guiding example, even modest relarly, it would...