# Analysis of Sequential Pretreatments to Enhance the Early-Stage Biorefinery Designs
## 3 Results
### Heuristic Analysis
Therefore, either low composition or high hydrolyzed pentoses could affect soluble lignin valorization after a second pretreatment. (ii) Despite low lignin removal, acid-catalyzed pretreatments fractionate this heteropolymer, and small amounts affect future bioconversions [1]. Therefore, proposing schemes where lignin isolation is prioritized first is advisable. (iii) Lignin-based schemes could obtain a first hydrolysate rich in pentoses and a second one with soluble lignin. (iv) During the process scale-up, a high energy demand was observed in hemicellulose schemes, impacting operating costs. For lignin valorization processes, the sequences DA with kraft and SE with kraft obtained a better total score. The removal of hemicellulose decreases the integrity of the biomass, leading to a better attack of chemical reagents on the structure and a faster delignification. Some studies have shown complete hemicellulose removal and delignification  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c} \hline \hline \multirow{3}{*}{**Fraction**} & \multirow{3}{*}{**Sequential Scheme**} & \multicolumn{8}{c}{**Indicators**} \\ \cline{3-11}  & & **I\({}_{\text{R}}\)** & **I\({}_{\text{A}}\)** & **I\({}_{\text{CR}}\)** & **I\({}_{\text{S}}\)** & **I\({}_{\text{V}}\)** & **I\({}_{\text{V}}\)** & **I\({}_{\text{WD}}\)** & **I\({}_{\text{E}}\)** & **I\({}_{\text{CapEx}}\)** & **I\({}_{\text{OpEx}  
of up to 68% for SE and kraft sequential pretreatments [38]. A second pretreatment step also increased lignin removals due to eliminating physicochemical barriers exerted by the lignocellulosic matrix [14]. However, sequences considerably impact the overall biomass removals. Unlike acid-catalyzed pretreatments such as DA, hydrothermal pretreatments decrease the overall sequence delignification. At temperatures of 100-170 \({}^{\circ}\)C, lignin liquefies into small coalesced droplets that migrate out of the cell wall due to their hydrophobic character, and these droplets eventually attach to the biomass structure during the cooling steps (relocation of coalesced lignin droplets), decreasing delignifications [39]. However, pretreatments such as DA do not tend to form these coalesced droplets, as they are easily removed, together with hemicellulose solubilization [13]. Additionally, carrying out future alkaline pretreatments fractionates \(\beta\)-O-4 bonds within the lignin matrix, overcoming condensation issues [40]. The results of the heuristic analysis of sequential pretreatments are in accordance with the efficacy results of single-stage pretreatments, where technical, economic, environmental, and social indicators were assessed [9]. It was identified that the DA and WAO pretreatments correspond to a proposed sequence for cellulose isolation. In contrast, organosolv is not feasible due to hexose losses, as well as high costs and energy demands that will increase with additional pretreatment steps. The sequential schemes also included the most effective pretreatments (LHW, SE, and DA) for hemicellulose valorization. Meanwhile, for the removal and future use of lignin, only the kraft pretreatment was agreed to be the most effective one included in a sequence where a previous acid treatment is performed. Considering the results of the heuristic analysis and single-step efficacy, the DA with WAO sequential pretreatment seems to be the best overall scheme in biorefinery approaches since it would result in two hydrolysates for pentose and soluble lignin, as well as a WIS rich in cellulose. Figure 3 shows a schematic diagram of the interaction between the lignocellulosicics and the proposed sequential scheme for a biorefinery. After DA pretreatment, a hydrolyzed liquor with 82.9% pentose accessibility would be obtained, removing more than 98% of hemicellulose. Likewise, the pretreated solid subjected to WAO would yield a second hydrolyzed liquor rich in soluble lignin (92.2% composition) with accessibility of 96.1%. Finally, the WIS rich in hexoses would have a composition of 97.2% cellulose, achieving a recovery of 83.7%.