| Summary: | Sugar production from spruce wood chips, the presence of
fermentation inhibitors (carbohydrate degradation products,
extractives and phenolic compounds), their removal and the
fermentability of ACOS (&cid catalyzed Qrganosolv
Saccharification) hydrolysates were extensively studied.
In percolation type runs 92.75% of the theoretical
sugars were recovered in 30 to 40 mm runs at 180° C with
80:20 acetone:water solutions containing 0.04 N sulfuric acid
as the hydrolysis catalyst. This hydrolysis process was not
optimized. 96% of the wood charge was dissolved. 4.0 %
of the
total dissolved sugars were in the form oligomers, mainly
cellobiose and cellotetraose, in crude ACOS hydrolysate.
Furfural was not found but trace amount of
hydroxylmethylfurfural (HF) was detected in the hydrolysate.
Recovery of the solvent precipitated the bulk of the waterinsoluble
lignin. The lignin precipitate removed the
dissolved extractives (resin- and fatty acids) almost
completely making the hydrolysate free of extractives.
Although the untreated ACOS hydrolysates contained 1.269
mg/rnL phenolic materials, the sugars were fermentable to
ethanol without hydrolysate pretreatment in 24 h by
Saccharomyces diastaticus, a flocculating yeast, when 25 g
dry-weight cell mass/L was employed. This demonstrated that
with high cell mass the inhibition could be alleviated or
removed. The ACOS hydrolysate can be considered free of
inhibitors since extractives and furans were absent and those
of lignin origin could be easily overcome.
Secondary hydrolysis for lh at 1200 C at 4% sulfuric
acid catalyst concentration improved the monomeric sugar
yield by as much as 57% mainly by conversion of suspected
isopropylidenes and some oligomers to their monomeric sugars.
Among the various hydrolysate clarification treatments
extraction with diethyl ether, treatment with charcoal, IRN
150 mixed ion exchange resin, XAD-16 adsorption resin were
tried. All these treatments improved the fermentation rate
and ethanol yield from the sugars by the removal of phenolic
materials. In the alcoholic fermentation only glucose and
mannose were utilized. Residual color of the hydrolysate
after treatment seemed to have little effect on the
fermentation rate since in all treated hydrolysates (6% sugar
solids) 88% of the fermentable glucose and mannose was
consumed in 1 to 2 h and maximum ethanol yield (about 0.4228
- 0.4729 g/g sugar consumed) was achieved in 3 to 6 h. The
XAD-l6 plus charcoal polishing treatment removed watersoluble
phenolic materials in the hydrolysate up to 79%.
Combined treatment of the hydrolysate with XAD-l6 resin
and charcoal (polishing) resulted in a water-clear
hydrolysate and produced up to 0.4729 g
ethanol/g sugar
consumed at 6% sugar solid fermentation in 3 h and 0.4416 g
ethanol/g sugar consumed at 15% sugar solid in 24 h reaching
6.12 % ethanol concentration in the beer. Arabinose,
galactose and xylose did not ferment to ethanol but some
xylitol was detected by HPLC (High Performance Liquid
Chromatography).
Combination of proper pretreatment and high cell density
seemed to be the most desirable way of removing inhibition in
fermentation of the ACOS wood hydrolysate.
Based on these results the ACOS process can be projected
to produce 354.98 L
of ethanol per ton of unextracted spruce
wood analyzed at 57.46% hexoses (glucose and mannose only)
Additional ethanol can be expected on second fermentation of
xylose to xylitol and conversion of residual sugars left in
the still bottoms.
|
|---|
