Simulation of a cross-flow shrinking-bed reactor for the hydrolysis of lignocellulosics

An idealized model is developed for the case in which biomass slurry is conveyed through an annulus, with water or steam entering through an inner porous wall and liquid product leaving through an outer porous wall. It is assumed that the ratio of occluded liquid to solid in the slurry is a constant, Rws, and that non-occluded water is immediately removed from the reactor. The goal of >90% sugar yield with >10% sugar in the product is almost reached (88% glucose yield, 91% xylose yield, 47 g/l glucose and 45 g/l xylose) at 240 °C, 1% acid, Rws=1 and a radial wash water flow of three times the initial mass flow of solids to the reactor per meter of reactor length per g/l of sugar concentration in the occluded water. If Rws is limited to 3, the yield falls to 85% and the total sugar concentration to 61 g/l. Even without cross-flow wash, the yields can be increased by about 16 percentage points, compared to plug flow, by extracting excess liquid through the outer wall as it is formed. At 200 °C, where one might prefer to operate for ease of control and concern about the possibility of making fermentation inhibitors at higher temperatures, the maximum glucose yield in a plug-flow reactor is low (12–13%) whereas in a cross-flow reactor, at a high cross-flow wash rate, it can still be quite high (60–83%) but at a very low concentration (0.57–1.47%). In these simulations it is assumed that one-half of the inerts is solubilized. The formation of oligomers is neglected.