Countercurrent operation in the dilute-acid hydrolysis of cellulose for ethanol production

Sammanfattning: Fuel ethanol can be produced from lignocellulosic materials to replace gasoline and diesel. A crucial step in the production process is the hydrolysis (saccharification) of the material, which can be achieved with dilute acid at high temperature. Unfortunately, sugar is degraded under these severe conditions, and the sugar yield is generally low. This thesis deals with countercurrent operation, which is a way of minimizing sugar degradation, while keeping the sugar concentration high. A mathematical model of a countercurrent reactor was developed, which considers the porosity of the pulp, and the continual collapse of the pulp (so-called bed shrinkage). Simulations revealed that bed shrinkage is advantageous, but that the porosity is not significant as long as the total solids loading in the reactor is constant. In an experimental study, a 250 l batch reactor was employed to investigate the importance of the number of hydrolysis stages. The results, although ambiguous, could support the idea that sugar survival is enhanced when the hydrolysate is removed from the increasing severity that affects the solid, as is the case in a countercurrent process. Since it has been reported that a countercurrent reactor is difficult to operate when using a hardwood feedstock, a comparative study of hardwood and softwood was made. The two substrates were hydrolysed in a lab-scale, shrinking-bed flow-through reactor, and the solid residues were analysed. Structural differences were observed that indicate that softwood is more manageable in a countercurrent reactor. The heterogeneous aspects of dilute-acid hydrolysis were studied in both experiments and mathematical models. Especially, the importance of structural effects such as hydrogen bonding was considered.