Study of hemicelluloses composition of different tree species, extraction using subcritical water with a batch cascade reactor

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Hemicelluloses from 10 different tree species commonly growing in the Castilla y Leon region have been extracted with a batch cascade reactor. Raw materials came from tree branches, cut during the seasonal pruning. The aim was to correlate the yield of hemicellulose achieved through the extractions, with the total composition and the structure of the species, identifying the best species to obtain a high concentration of hemicelluloses, high yield and high molecular weight.

Experiments were carried out at the same operating conditions: temperature of 160°C, particle size between 1,25 and 2 mm, residence time of 80 min; the only variable was the species of the tree. Previous works indicates these conditions to be the best to obtain the highest yield of hemicelluloses, with the lowest amount of degradation products.

The system consisted in 5 Parr reactors (fig. 1) with a volume of 200 mL each one, where a dry weight of 25g of wood were laced (5 g per unit) and prewetted for 12h in distilled water. An amount of water was then preheated and, when the desired temperature was achieved, hot water was let to flow through every unit in recirculation mode, starting the extraction. After regular intervals of time, the water flow was deviated, and units were bypassed one by one and removed from the system. Samples of extracted liquid solution and extracted solid were taken at 5, 10, 20, 40 and 80 min from the beginning of the experiments.

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Figure 1. Cascade batch reactor setup.

The system allowed the sampling of both the solid and liquid phases during the reaction, which enables an accurate mass balance at the end of the experiments. Temperature and pressure were monitored and recorded online inside each reactor unit, enabling a good interpretation of the data.

The concentration of hemicellulose extracted and that remaining in the exhausted solid was measured by performing methanolysis of each sample, and then analyzing the composition by gas chromatograph. The yield of hemicellulose extracted from any species, during the time, was determined by comparing the concentration obtained in each sample, with the total in the raw material (fig. 2).

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Figure 2. Hemicellulose composition of the 10 raw materials.

The molecular weight of each of liquid sample was analyzed, allowing to obtain a distribution of the molecular weights of the hemicellulose extracted; Autohydrolitic capacity of each species was studied by comparing the molecular weight of the samples with their pH.

The total composition of every raw materials was determined, carrying out the acid hydrolysis of the solid, and identifying the total amount of cellulose, hemicellulose and lignin. The TGA of the exhausted solids was performed and, by means of a kinetic model developed by our group (http://hpp.uva.es/autocatalytic-modelling-of-biomass-thermogravimetric-analysis), we studied the relationship between the yield of hemicellulose of each species, obtained by the extraction and the composition and structure of the plant.

The work was performed at the Univerity Abo Akademi (Finland), in collaboration with Professors Henrik Grenman and Tapio Salmi.

Gianluca Gallina – BES-2013-063556 ; Project Reference: ENE2012-33613

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