A novel population balance model for the dilute acid hydrolysis of hemicellulose

Abstract

BackgroundAcid hydrolysis is a popular pretreatment for removing hemicellulose from lignocelluloses in order to produce a digestible substrate for enzymatic saccharification. In this work, a novel model for the dilute acid hydrolysis of hemicellulose within sugarcane bagasse is presented and calibrated against experimental oligomer profiles. The efficacy of mathematical models as hydrolysis yield predictors and as vehicles for investigating the mechanisms of acid hydrolysis is also examined.ResultsExperimental xylose, oligomer (degree of polymerisation 2 to 6) and furfural yield profiles were obtained for bagasse under dilute acid hydrolysis conditions at temperatures ranging from 110°C to 170°C. Population balance kinetics, diffusion and porosity evolution were incorporated into a mathematical model of the acid hydrolysis of sugarcane bagasse. This model was able to produce a good fit to experimental xylose yield data with only three unknown kinetic parameters ka,kb and kd. However, fitting this same model to an expanded data set of oligomeric and furfural yield profiles did not successfully reproduce the experimental results. It was found that a “hard-to-hydrolyse” parameter, α, was required in the model to ensure reproducibility of the experimental oligomer profiles at 110°C, 125°C and 140°C. The parameters obtained through the fitting exercises at lower temperatures were able to be used to predict the oligomer profiles at 155°C and 170°C with promising results.ConclusionsThe interpretation of kinetic parameters obtained by fitting a model to only a single set of data may be ambiguous. Although these parameters may correctly reproduce the data, they may not be indicative of the actual rate parameters, unless some care has been taken to ensure that the model describes the true mechanisms of acid hydrolysis. It is possible to challenge the robustness of the model by expanding the experimental data set and hence limiting the parameter space for the fitting parameters. The novel combination of “hard-to-hydrolyse” and population balance dynamics in the model presented here appears to stand up to such rigorous fitting constraints.

Evolution of Hemicellulose Molar Mass during Softwood Hydrolysis

“…Extensive studies were published exploring hemicellulose hydrolysis, particularly in hardwoods and grasses. − ,− Fewer studies examining the hydrolysis of softwood’s unique hemicelluloses exist. − The composition and structure of softwood hemicellulose differs substantially from xylose-based materials. The hemicellulose of softwoods is mainly arabino-4-O-methylglucurono-xylans (AGX, up to 10 wt % of o.d.…”

“…The first hemicellulose hydrolysis model was posed as a series of pseudohomogeneous first-order reactions with rate constants having Arrhenius-type temperature dependence . The next development was introduction of biphasic models to explain the high reactivity of a portion of hemicellulose. ,,, However, there is no physical or chemical standard to distinguish the two fractions. Identification of soluble xylooligomers in hydrolysate from corn stover and Eucalyptus globulus wood led to the incorporation of intermediate oligomers. , …”

“…Analytical molar mass characterization studies of oligomers are relatively rare, in part due to a lack of uniform calibration standards . HPLC has been used to characterize xyloligomers of DP1–6, and studies have been published exploring the mechanism of xyloligomers depolymerization and degradation. ,, In these studies, it was found that xylose formation is favored by low temperature (120–130 °C), high acid concentrations (0.6–1.0 v/v%), and low xyloligomer DP. , …”

“…The concept originates from comminution models describing size reduction during milling . Population balance models have been used to describe depolymerization, granulation, dissolution, carbohydrate hydrolysis, and oxidation treatment. ,,,,, The assumed mode of bond breaking is central to final model and varies by study (Table S1). Random bond breaking is the most common assumption for hemicellulose hydrolysis. ,, However, Basedow et al determined that chain ends of dextran during hydrolysis with 0.12 N sulfuric acid were more reactive .…”

“…Population balance models have been used to describe depolymerization, granulation, dissolution, carbohydrate hydrolysis, and oxidation treatment. ,,,,, The assumed mode of bond breaking is central to final model and varies by study (Table S1). Random bond breaking is the most common assumption for hemicellulose hydrolysis. ,, However, Basedow et al determined that chain ends of dextran during hydrolysis with 0.12 N sulfuric acid were more reactive . Lau et al reported that xyloligomer of DP4 tends to break at the middle bond during water hydrolysis at conditions of 160 °C and 200 °C, which agrees with Kumar and Wyman’s conclusion that low pH favors end-wise breakage of xyloligomers and neutral pH favors middle breakage. , Since bond breakage is related to oligomer polydispersity, which represents polymer uniformity, further discussion will be provided in Molar Mass Distribution Analysis.…”