## 2 Mineral Acid Fractionation
### Conditions and Yields
#### 2.3.1 Low Concentrations of Acid and High Temperatures  
Dilute acid fractionation processes usually use an acid concentration range of 0.5 to 8% (_w/w_), a S:L ratio range of 1:5 to 1:20 (_w/v_), a temperature range of 100 to 200 \({}^{\circ}\)C, and a reaction duration range of 5 to 300 min. Dilute sulfuric acid fractionation generally leads to the solubilization of the hemicelluloses and a small fraction of the lignin, the hydrolysis of the solubilized hemicelluloses, and the decrease in cellulose crystallinity [1; 22]. Therefore, dilute acid fractionation eliminates or reduces the need for hemicellulose enzyme mixtures for hemicellulose saccharification [22]. In parallel, during dilute acid fractionation, the majority of lignin remains as a solid residue, and only some ether and ester linkages are cleaved, generating low molecular-weight lignin fragments with increased hydroxyl group content [43]. When dilute acid hydrolysis is run at high temperatures or for a long period of time, it can break down cellulose as well [30; 44]. During the acid hydrolysis of lignocellulosic biomass at a given acid concentration, S:L ratio, temperature, and the reaction duration monitoring is important. Indeed, under acidic conditions, cellulose is first hydrolys into glucose, which in turn can be converted into HMF, and HMF can finally be degraded into levulinic acid and formic acid (Figure 1). In parallel, hemicelluloses are also hydrolys into monomeric sugars (xylose, arabinose, galactose, mannose, glucose, etc. ), and acetate groups attached to the hemicelluloses are released, as well as uronic groups producing acetic acid, glucuronic acid, and galacturonic acid [45]. Monomeric C5 sugars are then converted into furfural, which in turn can be converted into formic acid and other degradation products [44; 46]. There is an optimum duration after which the hydrolysis of more hemicelluloses and celluloses into monomeric sugars does not compensate the loss of monomeric sugars being converted into furan degradation products [22]. The variation of one of the three parameters--acid concentration, S:L ratio, or temperature--affects the optimum duration of the hydrolysis. For instance, at 1% (_w/w_) sulfuric acid, the higher the temperature, the faster the maximum yield of glucose was reached, from 50 min at 170 \({}^{\circ}\)C to 2 min at 220 \({}^{\circ}\)C [41]. Longer reaction durations led to a decrease inthe concentration of glucose due to its degradation [41]. Similarly, with all other hydrolysis variables constant, an increase in the acid concentration led to a faster rate for the hydrolysis of the hemicelluloses [37]. Figure 1: **Formation of degradation products from lignocellulosic biomass under dilute acid pretreatment at high temperatures [44]. HMF, hydroxymethylfurfural. **Inspired by the pulp and paper industry, a combined severity factor (CSF) was developed by [PERSON] et al. (1990) for dilute acid treatment taking into account temperature, acid concentration, and duration of the reaction as detailed in Equation (1) [47]. On corn stover, a CSF in the range of 1.4-1.8 was optimal for the xylose yield in the acid hydrolysate, and lower or higher CSF reduced this yield due to incomplete solubilization and hydrolysis or due to the degradation of monomeric sugars, respectively [48]. \[\text{CSF}=\log\Bigl{\{}\text{t}\.\ \exp\Bigl{(}\frac{\text{T}_{\text{H}}- \text{T}_{\text{Ref}}}{14.75}\Bigr{)}\Bigr{\}}-\text{pH} \tag{1}\]  
where t is the reaction duration in min, T\({}_{\text{H}}\) is the reaction temperature in \({}^{\circ}\)C, T\({}_{\text{Ref}}\) is the reference temperature, most often 100 \({}^{\circ}\)C, and pH is the initial pH value (calculated from the mineral acid concentration). The efficiency of the fractionation has sometimes been defined by the sum of the monomeric sugar concentrations (glucose, xylose, arabinose) divided by the sum of the fermentation inhibitor concentrations (furfural, HMF, acetic acid) [34; 35; 49].