Recent Advances in the Concentrated Acid Process
There are a number of recent literature reports on the saccharification of different types of biomass such as aspen (Populus tremula) [15], Scots pine (Pinus sylvestris) chips [15, 16], bamboo [17, 18], and corn stover [19] using concentrated acids. Janga and coworkers studied the influence of acid concentration, temperature, and time in the first stage (decrystallization) of a two-stage concentrated sulfuric acid hydrolysis of softwood (Scots pine) and hardwood (aspen) [15, 16]. The study focused on the multi-variable effects of the decrystallization stage and applied a statistical modeling with a central composite face (CCF) design of the experiment to systematically study and simulate the effect of decrystallization reaction conditions on hydrolysis products and degradation products. The models were statistically significant and showed that for both aspen and pine, the reaction temperature and acid concentration were the most influential variables on monosaccharide and total sugar yields compared to the reaction time [15, 16]. The sugar degradation products were much influenced by the decrystallization temperature on both aspen and pine. Furthermore, the models were validated by a test-set and showed a good agreement between the experimental and predicted values. The optimum predicted total sugar yields were 56 g/100 g dry weight for aspen (74% theoretical) and 64 g/100 g dry weight for pine (91% theoretical) [15, 16]. Although fermentation experiments indicated that furfural was the most significant inhibitor to fermentation performance, no adverse effects were observed at furfural levels below approximately 1 g/L; for all hydrolyzates glucose fermented totally to ethanol [15].
Several studies on saccharification of bamboo using concentrated sulfuric acid have recently been reported in the literature with high biomass loadings. In one study by Sun and coworkers [18], bamboo saccharification was carried out at an acid to substrate ratio of 1:4 (bamboo chips: 75 wt% H2SO4 = 2.4 kg:4.4 kg). Saccharification consisted of treating bamboo chips with 75 wt% sulfuric acid at 50°C for 30 min (solubilization step), and then 27 wt% sulfuric acid at 80°C for 60 min (hydrolysis step). The resulting slurry was pressed at 20 MPa for 20 min to separate solids from the liquid; glucose content in the hydrolyzate was measured after a ten-fold dilution. Furthermore, Sun and coworkers reported a 80.1% sugar recovery efficiency in a concentrated sulfuric acid bamboo saccharification experiment [18].