One of the main problems during the pretreatment and hydrolysis of lignocel — lulosic biomass lies in the big differences found in its content of both lignin and hemicellulose. This content depends not only on the plant species from which the lignocellulosic materials were obtained, but also on crop age, method of harvest­ing, etc. This means that none of the pretreatment methods could be applied in a generic way for the great amount of potential feedstocks (Claassen et al., 1999). This justifies the need for a detailed analysis of all pretreatment options for dif­ferent materials, conditions, and regions. With this aim, process synthesis can provide the necessary tools for discarding, in a preliminary way, the less promis­ing pretreatment options and considering new procedures, schemes, and alterna­tives proposed during the conceptual design involving all the steps of the biomass processing as well.

The significant variety of pretreatment methods of biomass has led to the devel­opment of many flowsheet options for ethanol production (Cardona and Sanchez, 2007). Von Sivers and Zacchi (1995) analyze three pretreatment processes for etha­nol production from pine: concentrated acid hydrolysis, two-stage hydrolysis by steam explosion using SO2, and dilute acid and steam explosion using SO2 followed by the enzymatic hydrolysis. Through sensitivity analysis, these authors show that

TABLE 4.5 Biological Methods for Pretreatment of Lignocellulosic Biomass for Ethanol Production
examples of Pretreated Materials Corn stover, wheat straw
Remarks Fungi produces cellulases, hemicellulases, and lignin-degrading enzymes: ligninases, lignin, peroxidases, polyphenoloxidases, laccase and quinone-reducing enzymes Very slow process: Pleurotus ostreatus converts 35% of wheat straw into reducing sugars in 5 weeks Brown-rot fungi degrades cellulose White- and soft-rot fungi degrade cellulose and lignin Fungi decompose the lignin network Ethanol action allows hemicellulose hydrolysis Biological pretreatment can save 15% of the electricity needed for ethanolysis Ethanol can be reused; environmentally friendly process	References Sun and Cheng (2002); Tengerdy and Szakacs (2003)
Methods ProcedureMgents Fungal pretreatment Brown-, white — and soft-rot fungi Cellulase and hemicellulase production by solid-state fermentation of biomass
Ceriporiopsis subvermispora for 2-8 weeks followed by ethanolysis at 140-200°C for 2 h
Beechwood	Itoh et al (2003)
Bioorganosolv pretreatment
Source: Adapted from Sanchez, O. J., and C. A. Cardona. 2008. Bioresource Technology 99:5270-5295. Elsevier Ltd.

none of the processes can be discarded as less reliable. Milling has been suggested as a sole pretreatment method before the cellulose hydrolysis since the required equipment is less expensive than the equipment needed for other pretreatment methods such as steam explosion or ammonia fiber explosion (AFEX) process, which can account for 6 to 20% of the capital costs of the process. In contrast, milling equipment accounts for about 1% of these costs. However, it is considered that milling has elevated energy costs. Alvo and Belkacemi (1997) point out that milling of perennial grasses requires much less energy that milling of wood. These authors consider that milling as a unique pretreatment method should not be dis­carded as an option taking into account the advantages of this configuration: toxic products of degradation are not formed, soluble carbohydrates of the initial biomass are not destroyed, and many rural communities can acquire an easier way to mill in comparison to other expensive pretreatment equipment. This alternative should be evaluated in depth, utilizing simulation and optimization tools in the design step.

Dilute-acid pretreatment is the most studied method in the world along with steam explosion since they have a major probability of being implemented at an industrial scale in the near future. In fact, the utilization of dilute acids is con­sidered one of the most mature technologies compared to the rest of biomass pretreatment methods. The NREL (National Renewable Energy Laboratory) of the U. S. Department of Energy, which is one of the institutions leading the research and industrial development of technologies for fuel ethanol production from lignocellulosic materials in the United States, has chosen the dilute-acid pretreatment as the best model process to have been developed in the past years and offered to the industry (Aden et al., 2002; Wooley et al., 1999). The main advantage of this method compared to steam explosion is the higher recovery of sugars derived from the hydrolyzed hemicellulose. In the case of hardwood, about 80% of sugars can be recovered using dilute sulfuric acid while this recovery does not reach 65% when steam explosion is used (Lynd, 1996). The higher the sugars recovery, the greater the monosaccharide content in the liquid fraction resulting from the pretreatment. This liquid fraction can be employed as a culture medium for pentose-assimilating yeasts, or can be added to the bioreactor where the fer­mentation of cellulose hydrolyzates is accomplished as an additional sugar source (see the following chapters).

Another prospective method is the pretreatment by LHW. In particular, steam explosion and LHW processes have been compared in the case of poplar biomass obtaining better results for the latter method (Negro et al., 2003). In general, it is considered that the most efficient and promising methods are dilute-acid pretreatment, steam explosion with the addition of acid catalysts, and the LHW method (Ogier et al., 1999). To this regard, the evaluation of the global process for fuel ethanol production from lignocellulosic materials has been performed by Hamelinck et al. (2005) and the above-mentioned promising pretreatment methods should be noted. These authors selected one of the three pretreatment methods and diverse biological conversion technologies according to three differ­ent stages of technological maturity and development of the operations involved. For this, they employed spreadsheets and commercial simulators to study the

Process Synthesis for Fuel Ethanol Production

configuration of the process flowsheet corresponding to each one of three ana­lyzed scenarios (short-, mid-, and long-term variants). For the pretreatment step and short-term scenario (five years), the dilute-acid pretreatment was selected considering that this method is the technology offering the highest efficiency and reliability at the moment, while for the mid-term scenario (10 to 15 years), the steam explosion was chosen. For the long-term scenario, the LHW method was analyzed due to its comparative advantages and considering that this technology, for a period of time greater than 15 years, will be completely developed and that the current drawbacks will be overcome. Undoubtedly, the evaluation through the simulation of these alternatives will provide more insight for the selection of the best configuration of the overall process flowsheet. This will allow, in turn, the definition of the main research and development directions for the design of more effective pretreatment methods.

Process simulation requires suitable models for describing the studied pro­cesses. Considering process synthesis procedures, the mathematical modeling can allow a deeper insight into the pretreatment methods and make possible the defi­nition of operating parameters for which the system attains a better performance. In particular, mathematical modeling is a valuable tool for planning and execut­ing different trials at pilot and industrial scales (Cardona and Sanchez, 2007). For instance, through a kinetic model of cane bagasse pretreatment using nitric acid, the best conditions for increasing sugar yields were predicted. In this spe­cific case, the model considered the formation of inhibitory compounds (furfural and acetic acid). The results obtained were better than when sulfuric acid was used (Rodriguez-Chong et al., 2004) or when no acid was used (Jacobsen and Wyman, 2002). This type of kinetic study also has been done for poplar wood, switchgrass, and corn stover treated with sulfuric acid (Esteghlalian et al., 1997), as well as for wheat straw using hydrochloric acid (Jimenez and Ferrer, 1991). The kinetic model of corn stover pretreatment at pilot scale was also used to determine the process conditions leading to the maximization of xylose yield (Schell et al., 2003). Malester et al. (1988) studied the kinetics of dilute-acid pretreatment of municipal solid waste (MSW) since the cellulosic materials are the main com­ponent of this potential feedstock for ethanol production. For this case, the major difficulty lies in the resistance of cellulose to be converted into fermentable sug­ars in the presence of acids. This is explained by the neutralizing capacity of the MSW over the acid that imposes an additional difficulty to the measurement of kinetic parameters of the process. Therefore, these authors proposed to measure this effect based on pH and not on the concentration of the employed acid. This capacity is inherent to other lignocellulosic materials like corn stover and saw­dust, so the acid concentration should be adjusted slightly in order to maintain the efficiency of this type of pretreatment (Esteghlalian et al., 1997).