A Recycle Model of Spent Liquor in Pre-treatment of Lignocellulosic Biomass

The pre-treatment of lignocellulosic biomass to produce valuable products is one of the key steps of the process, due to the high costs involved. In most typical methods, biomass is treated with a water solution of either mineral or organic acids, caustic soda or organic solvents. The high costs associated with supply of these chemicals and energy requirements make the pre-treatment step a bottleneck in the development of cost-effective bioprocesses. In order to save water, chemicals and energy, a strategy that decouples liquid and solid residence time is proposed. A sequential re-use strategy of the spent liquor has been proposed recently by many authors, independently. It consists in retaining part of the liquor for the subsequent batch, instead of utilizing a fresh liquid mixture. This causes the conditions in every batch cycle to be different, which is a hindrance to effective overall process integration, because it is difficult to include such operation in a general process flowsheet. However, we observe that after several batches have occurred, cycles reach a stationary condition in the sense that initial and final conditions do not change. The presented model describes the behaviour of the cyclic re-use of spent liquor as a function of decision variables such as removed spent liquor (r) and cycle time (t(c)). For a series of first order reactions, it will be shown that the cyclic condition can be obtained analytically without needing to simulate every preceding stage.