## 2 Mineral Acid Fractionation
### Conditions and Yields
#### 2.3.2 High Concentrations of Acid and Low Temperatures  
The concentrated acid hydrolysis process appeared to be an interesting process for saccharification of lignocellulosic biomass, as it leads to high yields of monomeric sugars, low levels of production of fermentation inhibitors, and good flexibility regarding the different raw material treated compared to the dilute acid fractionation [18]. Low temperatures (typically under 60 \({}^{\circ}\)C) and high S:L ratios (from 1:2.5 to 1:10 (_w/v_)) are generally used, improving significantly the cost effectiveness of the pretreatment [21; 42; 61]. No enzymatic saccharification is required to reach the same yields of monomeric sugars as with dilute acid hydrolysis. For instance, rice hulls treated with concentrated H\({}_{2}\)SO\({}_{4}\) 67% (_w/w_) at 25 \({}^{\circ}\)C for 3 h gave higher yields of monomeric sugars (62%) than dilute acid treatment (H\({}_{2}\)SO\({}_{4}\) 1% (_v/v_), 121 \({}^{\circ}\)C, 15 min) followed by enzymatic saccharification (60% yield) [51]. Concentrated mineral acids such as sulfuric acid and hydrochloric acid are widely used for treating lignocellulosic materials because they are powerful agents for both hemicellulose and cellulose hydrolysis. The major drawback of these concentrated acids is their corrosive nature and the need to recycle acids to lower the cost of fractionation [21; 42]. Special acid resistant materials for the vessels are still to be investigated, ceramic or carbon-brick lining having great potential, for example [8].  
As for dilute acid treatment, the four main parameters (acid concentration, S:L ratio, temperature, and hydrolysis duration) have to be selected carefully in order to maximize the yield of solubilization and hydrolysis of the polysaccharides. For instance, from corn stover treated under H\({}_{2}\)SO\({}_{4}\) 70% (_w/w_) at 50 \({}^{\circ}\)C for 20 min, 90% yield for the monomeric sugars was achieved with an S:L ratio of 1:50 (_w/v_), but when the S:L ratio was increased to 1:10, then the yield of monomeric sugars dropped to 65% [20]. With the same sulfuric acid concentration, an intermediate S:L ratio of 1:20 (_w/v_) and an increased temperature to 70 \({}^{\circ}\)C, total conversion into monomeric sugars was achieved [20]. Using a Taguchi experimental design, [PERSON] et al. (2012) determined that the concentration of sulfuric acid followed by the S:L ratio were the most influential parameters for the recovery of glucose and xylose [61]. It was also shown that the production of sugar degradation products (furans and aliphatic carboxylic acids) was more influenced by the temperature.  
In order to be competitive with fossil fuels, the main challenge for the use at industrial scale of the concentrated acid fractionation of lignocellulosic biomass to produce ethanol is the recycling of the acid, which has risen from 80 to 97% in the process in the last 50 years, but the cost of the recycling process remains high [4; 18]. In theory, sulfuric acid acts during lignocellulosic biomass hydrolysis as a catalyst, so in principle, no sulfuric acid should be consumed during this process [62]. However, some acid still has to be reintegrated in the process in addition to the recycled acid due to unavoidable losses (for instance, through absorption by the biomass or salification of inorganic cations) [62].