Utilisation of bagasse for the production of C5- and C6- sugars.
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Date
1982
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Abstract
Surplus sugarcane bagasse, estimated at a maximum of 0,9x106
tons/year, represents an annual renewable resource which is
readily available at the mill site and is a suitable potential
source of alternative fuels and chemical feedstocks.
This work contains an extensive literature survey which covers
the production of C5- and C6- sugars from lignocelluloses by
chemical hydrolysis and the pretreatment of cellulosic materials
for enzymatic hydrolysis of the cellulose fraction. This survey
was then used to determine the final direction of this research
into the utilisation of bagasse for the production of fermentable
sugars.
It was decided that research should be directed at the dilute
acid hydrolysis of the bagasse hemicellulose fraction to determine
whether this fraction could be selectively hydrolysed from
the complex lignocellulose structure and to obtain xylose yields
under different hydrolysis conditions.
Acids, especially acetic acid, are liberated from bagasse by
steaming at elevated temperatures. In this acid medium the
hemicelluloses are hydrolysed and become soluble. Autohydrolysis
tests on whole bagasse indicate that hemicellulose hydrolysis
becomes significant at temperatures above 140°C. However, the
autohydrolysis liquor would still require dilute mineral acid
hydrolysis to convert the pentose oligomers to their monomeric
forms. Dilute sulphuric and batch hydrolysis of whole bagasse hemicellulose
has thus been investigated at a solid to liquid ratio
of 1:15 over the following temperature and acid concentrations
ranges : 80° to 150°C and 3 to 40 g/l acid. Xylose, glucose,
furfural and acetic acid formation and sulphuric acid consumption
were monitored during these hydrolyses. Hemicellulose hydrolysis to produce mainly xylose is readily achieved over the entire range of acid hydrolysis conditions
tested with little removal of the other bagasse components
(lignin and cellulose). At the upper end of the temperature
range acid concentrations below 20 g/l are sufficient for
hemicellulose hydrolysis due to the effect of temperature
on reaction rate.
The bagasse hemicellulose consists of two fractions, an easily
hydrolysable portion containing 165 mg of potential xylose/g bagasse and a resistant fraction containing 105 mg of potential xylose/g bagasse. A first order reaction model has been
developed using the batch acid hydrolysis results. It is
based on two hemicellulose fractions reacting simultaneously
to give a common product (xylose) and predicts total xylose
yield as a function of hydrolysis time for a given set of
hydrolysis conditions.
The encouraging xylose yields obtained during the batch hydrolyses
led to the design of a continuous hydrolysis reactor to process
bagasse at low liquid to solid ratios to determine whether
xylose yields similar to the batch hydrolysis yields could be
obtained at the same hydrolysis conditions.
The continuous hydrolyses showed that for the conditions tested
the xylose yields are unaffected by the decrease in liquid to
solid ratio (down to 3,6:1) and it would appear that reactor
performance is still controlled by reaction kinetics.
A number of reactor configurations for the industrial production
of pentoses from bagasse hemicelluloses are also proposed.
Description
Thesis (M.Sc.)-University of Natal. Durban, 1982.
Keywords
Bagasse., Sugar--Manufacture and refining--By-products., Waste products as fuel., Theses--Chemical engineering.