Percolation Reactors
While a variety of reactor designs have been evaluated, the percolation reactors are still the most reliable and effective, and have been tested in small — and pilot-plant-scale designs [24-27]. Percolation can be carried out in one or two stages; Kim et al. has investigated potential advantages of applying two different acid concentrations in a percolation reactor in the saccharification of hardwoods [24]. In this operation, a lower acid concentration was applied first to recover the easily hydrolyzable hemicellulose, followed by a higher acid concentration to recover the remaining hemicellulose. The reaction conditions applied were 160-180°C and 0.05-0.2% of sulfuric acid. The model predicted that the step change operation of acid concentration can increase the product yield by 3-4% over that of uniform acid concentration. They found that the difference in yield was not fully verifiable experimentally since it was barely over the experimental error range. However, a significant difference was found in sugar decomposition, the step change process showing much less furfural formation [24].
A renewed interest in this technology took hold in the United States in the late 1970s because of the gasoline shortages experienced during this period. The Department of Energy (DOE) and United States Department of Agriculture (USDA) took over sponsoring the research in the cellulosic ethanol process, and by the late 1980s most researchers recognized that these systems had reached the limits of their potential. Their comparatively high glucose yields (around 70%) were achieved at the expense of producing highly dilute sugar streams. Kinetic models, based on pseudo-first-order kinetics, and process design work showed that the most effective designs would require both high solids concentration and some form of countercurrent flow. The former is a consequence of equipment size and energy cost and the latter is a consequence of the reactor kinetics. In order to achieve these requirements significant improvements in the simple percolation method were necessary. Therefore, studies shifted to alternative designs, such as plug flow reactors [28, 29] and so-called progressing batch systems that mimicked countercurrent operation [30].