The selective chemical transformation of cellulose into different products by hydrothermal treatment under controlled conditions has been achieved in the labs of the High Pressure Process Group of the University of Valladolid.
Using a continuous facility with instantaneous heating and cooling and with reaction residence times as low as 20 milliseconds, high selectivities of different cellulose hydrolysis compounds have been obtained:
– At 400ºC and 23 MPa cellulose was hydrolyzed into soluble sugars with a selectivity of 96% on carbon basis in residence time of 30 milliseconds. In those conditions (400ºC and 23 MPa), the 5-HMF selectivity was lower than 0.01% and 0.1% on carbon basis at 0.03 s and 20 s of residence time, respectively.
– At 400ºC and 23 MPa and 20 s of residence time, cellulose was hydrolyzed into glycolaldehyde with a selectivity of 60% on carbon basis .
– At 400ºC and 27 MPa lactic acid was achieved by using sodium hydroxide as catalyst obtaining a product selectivity of 60% on carbon basis.
These studies have been carried out using a novel system that makes it possible to achieve a controlled, immediate interruption of the reaction by a sharp temperature reduction caused by a sudden depressurization of the effluent. Compared with a conventional cooling by quenching, this method avoids an unnecessary dilution of products that could hamper subsequent product recovery and processing steps.
Using this facility the kinetic data of some reaction of cellulose hydrolysis reactions have been obtained, and a reactor mathematical model has been developed.
Using this reactor the heat integration of the process and the influence of the heating-cooling times has been studied. Employing the model, it has been determined that for obtaining a sugar rich effluent at 400ºC from cellulose with a reaction time of 20 ms, selectivity would not be affected is cellulose suspension is preheated in a heat exchanger till 250ºC, then heated by supercritical water injection till 400ºC, and after 20 ms, and suddenly depressurized to decrease temperature till 250ºC. At that temperature the reaction rate it is slow enough to allow recover the heat contained in the stream in a heat exchanger. Nevertheless if both heating and cooling from 400ºC to room temperature are performed in a heat exchanger the selectivity of the process would be spoiled.