# Analysis of Sequential Pretreatments to Enhance the Early-Stage Biorefinery Designs
## 3 Results
### Techno-Economic Analysis

Table 4: Mass and energy yields of the proposed sequential pretreatments. The analysis of the utility requirement is crucial, as it will affect the operating costs of the processes. Regarding steam demands, their use will increase as the pretreatment thermal conditions increase. However, this work involved an energy integration analysis to decrease these demands. Therefore, thermal energy from the hot process streams was considered. It was evidenced that the schemes involving kraft as the first pretreatment demanded more steam consumption, especially the medium-pressure steam (48.8-59.9 tons per ton of raw material). Similarly, the organosolv processes demand steam by also involving solvent recovery through a distillation system, such as in the sequential scheme of organosolv and DA for hemicellulose. In fact, this was the scheme that most demanded cooling water both for the exchanger downstream of each reactor and for the condenser of the separation columns, as well as electricity for the reflux pumps. A low I\({}_{\text{WD}}\) was identified for the DA sequences as the first pretreatment. On the other hand, the electrical consumption was variable as the volume of the reactors increased due to the power demand. Although the WAO pretreatment requires an air compressor, it did not show a significant electrical increase compared to the other processes. Table 5 summarizes the investment and operating costs of the sequential pretreatments. The CapEx may increase or decrease depending on the equipment sizing regarding capacities, transfer areas, and residence time. Additionally, the direct cost will depend on the type of equipment used, which must be specified within the Aspen Process Economic Analyzer computational tool. For example, the pretreatment reactors were mapped with stirring, heating jacket, and enclosed, while the heat exchangers were shell and tube with TEMA-type design (see Supplementary Materials Table S1). Regarding CapEx, all schemes were within the range of 1.3-3.0 M-USD, which mainly involved the use of a threshing machine, sieving, a reagent mixing tank for both the first and second pretreatment, hydrolysis or pretreatment reactors, two heat exchangers downstream of each reactor, two filters for the separation of the WIS and hydrolyzed liquors, and a washing filter for the first WIS. The organosolv and DA sequence showed the highest investment costs due to the high capacity of the organic solvent separation equipment since it requires the use of the column, reboiler, condenser, pump, and collector tank for reflux. Although solvent recovery and recirculation, ethanol drastically increases the raw material costs and operating costs. In general, the average OpEx of the cellulose schemes was the lowest compared to hemicellulose and lignin. The labor cost for all the schemes was 59,057 USD year\({}^{-1}\) since they involved the same number of workers. Steam as raw material for pretreatments involving SE was also identified as a key factor in increasing the total OpEx of the process. It was mentioned previously that, in the cellulose pretreatments, on the other hand, the accessibility decreased because the WIS has remaining lignin fractions, as in the DA and alkali sequence, with a content of 26.3%. Therefore, an alternative would be to include a third pretreatment stage where much of the lignin is removed but with limited cellulose fractionations, especially for bioprocesses selective to hexoses only. WAO could be a favorable option based on operational indicators, whose removals have been reported to be 7.1% cellulose, 75.5% hemicellulose, and 97.3% lignin [32]. However, including this third step in the sequence would increase CapEx and OpEx costs by 92.3% and 42.6% (3.08 M-USD and 10.9 M-USD year\({}^{-1}\)). These cost increases would prevent the inclusion of a third step from the biorefinery design concept since its aim considers technical but also economic aspects. The CapEx and OpEx may vary in economic feasibility depending on the technological complexity and processing scale.