For reasons similar to liquid hot water percolation processes, dilute acid processes that employ a percolation mode of operation have also been investigated. Very high yields of monomeric and oligomeric xylose have been obtained in a two-stage percolation pretreat­ment of hardwoods, with high enzymatic hydrolysis yields of the cellulose in the pretreated solids (55). The high digestibility achieved in this approach has been attributed to signifi­cant lignin solubilization and removal from the pretreated solids in the continuously-flowing percolation process.

Kinetic modeling studies and associated experimental work have shown that extension of the percolation concept to a countercurrent contacting of biomass particles with the flowing dilute acid medium can further reduce hemicellulose-derived sugar degradation losses and also produce highly digestible pretreated solids. This is achieved by further reduction of the residence time of released sugars under reaction conditions based on the observed first-order hydrolysis reaction kinetics (56). This concept has also been extended to a full thermochemical hydrolysis of both hemicellulose and cellulose, with much higher sugar yields than traditional batch or co-current two-stage dilute acid hydrolysis processes (56). Such processes are highly attractive from a sugar yield standpoint, but will be difficult to apply commercially due to the high liquid volume requirements and complex large-scale reactor configurations.

Dilute acid percolation and countercurrent processes that use lower liquid volumes yet still achieve the highly digestible pretreated solids attributed to lignin solubilization, have also been investigated. In this approach, pretreatment is conducted in a batch mode, followed by a separation and a limited-volume washing of the pretreated solids prior to cooling below the lignin phase-transition temperature, where re-precipitation of solubilized lignin would be expected to occur. High enzymatic digestibility has been reported using this approach on a yellow poplar hardwood feedstock (57), but further work revealed limited benefit to this approach using corn stover as a feedstock.