Kinetic model for hydrothermal fractionation of lignocellulosic biomass in a lab-scale biorefinery

A novel physico-chemical model for hydrothermal fractionation of lignocellulosic biomass in a packed bed reactor is proposed in this work. The objective of this work is to reproduce the experimental behaviour with this model, including the effect of temperature, water flow and biomass properties. To do so, a novel reaction pathway based on two types of hemicellulose and cellulose is used to consider structural effects in biomass cleaving.

Besides, populations of 60 oligomer are defined for each biopolymer to also include the molecular weight role.  The initial length of hemicellulose and cellulose oligomers is fixed in 200 and 10,000 monomers, respectively. Samples of 5 g of holm oak (one of the most common trees in southern Spain) were treated at different temperatures and flows. The role of the operating temperature was assessed by two different ranges of temperature (140-180ºC and 240-280ºC) at constant flow (6 mL/min). These two ranges were selected to study the hemicellulose and cellulose extraction, respectively. The effect of the volumetric flow was taken into account by experiments at a constant temperature range (180-260ºC) and three different flows: 11.0, 15.0 and 27.9 mL/min.  Analysing the experimental results, it was observed that temperature is the main variable in the process and flow only affected when soluble compounds are released. The model was validated with these results, obtaining absolute deviations around 33% (Image 1.a) for the instant total organic carbon (measured at the reactor outlet). In addition, the whole set of physico-chemical phenomena observed in the process (porosity and pH variations, biomass solubilisation and mass transfer) was also reproduced. Finally, the model was able to calculate successfully the biopolymers depolymerisation, simulating oligomer distribution of decreasing molecular weight (Image 1.b).

Figure 1: TOC fitting (a) and oligomer molecular weight evolution (b).
Figure 1: TOC fitting (a) and oligomer molecular weight evolution (b).

Álvaro Cabeza – Proyecto nacional: ENE2012-33613; Programa FPU: FPU2013/01516

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