The production of third-generation bioethanol from eucheuma cottonii using dowex (TM) DR-G8
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Date
2015-09-01
Authors
Tan Inn Shi
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Abstract
The red macroalgae, Eucheuma cottonii (EC) is a third-generation biomass,
and it contains large amount of carbohydrate that can be readily fermented into
bioethanol. Up to now, the use of the liquid acid-catalyst have been reported for the
hydrolysis of carbohydrates from various biomass to fermentable sugar. However, the
need of easily separable and reusable solid acid catalyst is considered essential in the
hydrolysis process. In this study, for the first time, Dowex (TM) Dr-G8 was explored
as a potential solid catalyst to hydrolyze carbohydrates from dried raw EC or
macroalgae extract (ME) and pretreatment of macroalgae cellulosic residue (MCR), to
simple reducing sugar prior to the fermentation process. The reaction condition for
hydrolysis of EC (6 % w/v Dowex (TM) Dr-G8, 120 ºC, 1 h) and ME (8 % w/v Dowex
(TM) Dr-G8, 120 ºC, 1 h) resulted to a galactose yield of 43.2 % and 49.4 %,
respectively. This result was slightly lower compared with the feedstock by using ME.
However as for MCR, the solid acid catalyst (Dowex (TM) Dr-G8) was used in the
pretreatment process to enhance enzymatic conversion of MCR to reducing sugar.
Reusability of Dowex (TM) Dr-G8 was also investigated in this study, the galactose
yield maintained at around 46.7 % till the fifth run. This shows that Dowex (TM) Dr-
G8 was not significantly deactivated even after repeated used. The pretreatment
condition for MCR is 10 % (w/v) Dowex (TM)-Dr G8, 120 oC, and 30 min. An
optimum sugar yield of 99.8 % was attained when pretreated MCR (P-MCR) was used
as substrate for enzymatic hydrolysis after 30 h. Catalyst recyclability study were
performed and a sixth-times reuse was accomplished without any loss of catalytic
activity. In addition, a novel concept for the synthesis of stable polymer hybrid matrix
was developed. In this study, glutaraldehyde crosslinked κ-carrageenan was used for
the immobilization of β-glucosidase using the covalent method via polyethyleneimine
and glutaraldehyde. The immobilized β-glucosidase was then used to hydrolyze PMCR
for the production of reducing sugar and a hydrolysis yield of 73.4% was
obtained. When a solid acid hydrolysate containing 35 g/L of galactose were fermented
with an inoculums amount of 16.0 g/L, an optimum bioethanol production of 11.63
g/L was achieved (64.6 % of the theoretical value) after 72 h. Bioethanol production
by PSSF (prehydrolysis and simultaneous saccharification and fermentation) process
was observed to be more effective than the SHF (separate hydrolysis and fermentation)
process, producing 5.80 mg/mL of bioethanol, with a theoretical yield of 91 %. Scale
up of SHF of solid acid hydrolysate was carried out in a 5 L fermenter resulting to
61.6 % of bioethanol yield, while scale up study of PSSF process was carried out in a
5 L fermenter conducted with optimized conditions resulting to 87.1 % of bioethanol
yield, which is almost the same as in shaking flasks. This result indicated that the
fermentation process using macroalgae biomass could be easily scaled-up to large
fermenter without compromising its performance. On the other hand, the estimated
bioethanol production cost using macroalgae was 0.77 USD $/L. Compared to other
feedstocks, bioethanol production cost from macroalgae are competitive and
economically viable. A consequential life cycle assessment (LCA) and exergy analysis
of macroalgae-based bioethanol were performed. Results suggested that the
purification of bioethanol (S6) is found to have the highest impact in all the impact
categories considered. The proposed technology in this study using solid acid catalyst
was found feasible for the production of bioethanol from EC.