Valuable monomers and oligomers from hemicelluloses

G5 Doctoral dissertation (article)


Internal Authors/Editors


Publication Details

List of Authors: Andrea Pérez Nebreda
Publisher: Åbo Akademi
Publication year: 2019
ISBN: 978-952-12-3793-5
eISBN: 978-952-12-3794-2


Abstract

Current environmental problems such as global warming and climate change
related to greenhouse gas emissions have enhanced industrially relevant
research aimed towards a more sustainable future. The biorefinery
concept plays a key role in the development. Lignocellulosic biomass is
the most abundant renewable resource on the earth and it can be
potentially converted to different biofuels and biochemicals, which
diminishes the current dependence on fossil feedstocks. Lignocellulosic
biomass is composed of three major constituents: cellulose,
hemicellulose, and lignin. Hemicelluloses can account for up to 20-40%
of the total dry weight of lignocellulosic materials and this fraction
is very often underutilized in industry. However, hemicelluloses are
heteropolysaccharides which can be cleaved into their monomeric
constituents yielding different pentose and hexose sugars, which can be
further be transformed into fuels and chemicals.

Hemicelluloses can be obtained from biomass by different
pretreatment methods, in which the biomass is selectively fractionated
into its different constituents. Pressurized hot water extraction (PHWE)
is a method which can be utilized for extracting hemicelluloses without
degrading the extracted sugars. Water is used as a solvent, which makes
the process environmentally friendly. The extraction of hemicelluloses
by PHWE from stone pine, holm oak and Norway spruce was investigated in
the current work. The main experimental parameters were the effect of
temperature, raw material, and the solid-to-liquid ratio.

The hydrolysis of two structurally different hemicelluloses was
studied in different reactor configurations in the presence of acidic
catalysts. Oacetylgalactoglucomannan and inulin were selected as model
substrates as they differ significantly from each other; the former
being a large, branched and complex molecule while the latter is linear
and significantly simpler. The reaction products were mainly sugar
monomers.

Hydrolysis of the hemicelluloses using a homogeneous acid catalyst
(HCl) was studied in a continuous tubular reactor. The experimental
conditions influenced significantly the conversion and product
distribution, and the influence of the different parameters on the
reactor performance was systematically investigated. Continuous
production of sugar monomers without degradation to low-molecular
products was achieved in optimal conditions.

The reaction mechanisms and the kinetics of inulin hydrolysis to
fructose in the presence of a solid catalyst (cation-exchange resin)
were studied in a batch reactor. A detailed mathematical model was
successfully utilized for describing the intrinsic kinetics of the
hydrolysis.

The kinetic data obtained were subsequently used to design a
continuous reactor system of the packed bed type. The continuous reactor
developed in this work was equipped with intermediate sampling points
between the catalyst beds, which enabled following the progress of the
hydrolysis along the reactor. High fructose yields were obtained in the
hydrolysis of inulin. Detailed flow characterization was carried out,
taking into account mass transfer limitations. An advanced mathematical
model comprising kinetics, mass transfer phenomena, and flow
characteristics in the reactor was developed and solved numerically. The
model was well able to describe the performance of the continuous fixed
bed reactor.

Overall, the results obtained in the current work demonstrate the
feasibility of the efficient hydrolysis of hemicelluloses in a
continuous reactor using a heterogeneous catalyst and they contribute to
designing and evaluating the performance of the process.


Last updated on 2019-09-12 at 03:07