Hydrolysis of Cellulose in Supercritical Water: Reagent Concentration as a Selective Factor

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Figure 1. C-6 sugars and glycolaldehyde yields depending on residence time

Biomass is mainly composed by cellulose, hemicellulose and lignin. These components can be separated and then used as starting materials to produce interesting compounds via thermo-chemical processes such as hydrolysis. In the recent years there is an increasing interest in using supercritical water (SCW) as hydrolysis medium due to its suitability as environmentally friendly, non-toxic and inexpensive media for chemical reactions. Also, supercritical water allows fast reaction rate, high selectivity and high conversion yield of many biomass feedstocks.

The hydrolysis of cellulose was carried out at 400 ºC and 25 MPa in a continuous plant (see FastSugars Project). As supercritical water behaves as a selective reaction media for the synthesis of selected chemicals from biomass, it was possible to enhance the production of sugars or derived compounds as glycolaldehyde from cellulose, just by changing the residence time in the micro-reactor (see Figure 1). It was demonstrated that low reaction times enhanced the production of C-6 sugars (glucose and soluble oligosaccharides up to six units of glucose). On the other hand, an increment in the reaction time favored the degradation of glucose into other products such as glycolaldehyde. In addition, a flash separation step was added to the process, making it possible to increase the concentration of valuable products after the reaction.

Existing models to describe the conversion rate of cellulose in SCW are based on the hypothesis that hydrolysis of cellulose particles mainly takes place at their surface, and therefore the particle size is considered as the key parameter for the conversion rate. Carrying out the hydrolysis of cellulose in SCW with different inlet concentrations, it was possible to prove that also reagent concentration had an effect over the conversion rate of cellulose in SCW. It was observed that beyond 4 % w/w of cellulose, the solubility of cellulose in supercritical water decreased and the reaction occurred in a heterogeneous media where the mass transfer resistances limited the reaction rate. Moreover, these mass transfer resistances showed a strong dependence on cellulose concentration, since when increasing the cellulose concentration the kinetic constant was lower, as it can be seen in Figure 2.

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Figure 2. First order kinetic for 5, 15 and 20 % w/w of cellulose (1.5, 4.5 and 6 % at the reactor inlet)

Celia Martínez – Project CTQ2013-44143-R

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