Unlike the design of fuel ethanol production processes from sucrose — or starch — containing feedstocks, the use of lignocellulosic materials entails the analysis of multiple alternative variants for accomplishing the conversion of biomass into eth­anol. This means that process synthesis procedures play a significant role during the design of processes based on biomass. In fact, there exists a significant variety of biomass conversion technologies proposed worldwide to produce fuel ethanol. This variety should be thoroughly assessed in order to select the most suitable technological configuration considering the local conditions and the process per­formance in terms of its technical, economic, and environmental effectiveness.

Many efforts have been put forth worldwide to improve the efficiency of bio- mass-to-ethanol conversion. In the United States, the production of ethanol from lignocellulosic biomass is being studied intensively. Ingram et al. (1999) have car­ried out significant research on the development of recombinant strains of enteric bacteria to use for cellulosic ethanol production. Current technology allows the use of a genetically engineered Escherichia coli strain with the natural ability of assimilating both pentoses and hexoses found in the liquid fraction resulting from the dilute-acid pretreatment of lignocellulosic biomass. The main Zymomonas mobilis genes encoding the ability for homofermentative production of ethanol have been integrated into the bacterial chromosome in this strain. The solid frac­tion from this pretreatment that contains cellulose and lignin undergoes SSF using a recombinant strain of Klebsiella oxytoca with the ability to ferment cellobiose and cellotriose, eliminating the need for supplementing Trichoderma reesei cel — lulases with P-glucosidase. This strain also has the genes to encode the produc­tion of ethanol. The proposed overall process can be observed in Figure 11.11a (Cardona and Sanchez, 2007). At present, research efforts are oriented toward the development of a single microorganism capable of efficiently fermenting both hemicellulosic and cellulosic substrates that will make possible the development of the direct conversion of biomass into ethanol.

The model process designed by the NREL comprises a previous hydrolysis of wood with dilute acid followed by a simultaneous saccharification co-fermenta­tion (SSCF) process using cellulases produced in situ by genetically engineered Z. mobilis with the ability to transform both glucose and xylose into ethanol (Figure 11.11b; Cardona and Sanchez, 2007). The process is energetically inte­grated using the heat generated during the combustion of methane formed in the anaerobic treatment of wastewater from pretreatment and distillation steps (Wooley et al., 1999). In addition, the burning of lignin allows the production of energy for the process and a surplus in the form of electricity. The production of one liter of ethanol by this process is calculated at US$0.396, whereas the ethanol production cost from corn is US$0.232 (McAloon et al., 2000). A pilot plant designed for conversion of lignocellulosic biomass into ethanol was built and operated with the aim of supporting industrial partners for the research and development of biomass ethanol technology (Nguyen et al., 1996). In this plant, tests in continuous regime

(a)

Hydrous EtOH

(b)

(c) Steam, power

Biomass

Cogeneration

(d) (e)

FIGURE 11.11 Some proposed flowsheet configurations for ethanol production from lignocellulosic biomass. (a) Process based on utilization of enteric bacteria (Ingram et al., 1999). (b) NREL model process (Wooley et al., 1999). (c) Iogen’s process (Tolan, 2002). (d) Process proposed by Reith et al., (2002). (e) IIT Delhi process (Ghosh and Ghose, 2003). Main stream components: C = cellulose, L = lignin, G = glucose, P = pentoses, I = inhibi­tors, Cel = cellulases, EtOH = ethanol, Sol = solvent, SCP = single cell protein. LF = liquid fraction, SF — solid fraction, HHZ = hemicellulose hydrolyzate, Rec = recombinant, IE = ion exchange. (From Cardona, C. A., and O. J. Sanchez. 2007. Bioresource Technology 98:2415-2457. Elsevier Ltd. With permission.)

for the utilization of lignocellulosic residues of low cost and great availability, such as corn fiber, were carried out (Schell et al., 2004). The objective of these tests was the assessment of the operation of the integrated equipments and the generation of data concerning the process performance. This type of plant allows the acquisition of valuable experience considering the future implementation of the industrial process, the same as the feedback of the models utilized during the design step. In addition, feasibility studies carried out by NREL help industrial partners make decisions about the potential implementation of these technologies for fuel ethanol production (Kadam et al., 2000; Mielenz, 1997). Future trends for costs reduction in the case of the NREL process include more efficient pretreat­ment of biomass, improvement of specific activity and productivity of cellulases, the possibility of carrying out the SSCF process at higher temperatures, improve­ment of recombinant microorganisms for a greater assimilation of all the sugars released during the pretreatment and hydrolysis processes, and further develop­ment of co-generation system (Cardona and Sanchez, 2007). Nagle et al. (1999) proposed an alternative configuration involving a total hydrolysis of yellow poplar using a three-stage countercurrent dilute-acid process validated at experimental level. The obtained hydrolyzate is co-fermented by the recombinant strain of Z. mobilis. In this case, the lignin is recovered prior to the fermentation. Aspen Plus was utilized for generating the needed information to evaluate the economic performance of the whole flowsheet configuration through a spreadsheet model. Optimized values of the key process variables obtained from the simulation are utilized as target values for bench-scale research to design an advanced two-stage engineering-scale reactor for a dilute-acid hydrolysis process.

Some commercial firms have also invested funds in the development of an etha­nol production process employing the lignocellulosic biomass. Iogen Corporation (Ottawa, Canada) developed an SHF process comprising a dilute-acid-catalyzed steam explosion and the removal of the major part of the acetic acid released during the pretreatment, the use of S. cerevisiae as a fermenting organism, distil­lation of broth, ethanol dehydration, and disposal of stillage in landfill (Tolan, 2002). Later modifications involve the co-fermentation of both hexoses and pen­toses using genetically modified strains of microorganisms, such as yeasts or bac­teria (Figure 11.11c). Using the recombinant Z. mobilis strain patented by NREL, Lawford and Rousseau (2003) tested two configurations for ethanol production using the conceptual design based on SHF developed by Iogen. These authors demonstrated that a configuration involving the continuous pentose fermentation using the recombinant Z. mobilis strain, and the separate enzymatic hydrolysis followed by continuous glucose fermentation using a wild-type strain of Z. mobi­lis is the most appropriate in comparison to the use of the co-fermentation process after the enzymatic hydrolysis or the use of an industrial yeast strain during the glucose fermentation (Cardona and Sanchez, 2007).

Reith et al. (2002) have reviewed different processes for production of biomass ethanol and concluded that verge grass, willow tops, and wheat milling residues could be potential feedstock for fuel ethanol production under the regulations of the Netherlands. These authors constructed a model using Microsoft™ Excel™

for the system description of generic biomass-to-ethanol process. This process involves the evaporation of the stream from the saccharification step in such a way that the sugar concentration allows a final ethanol concentration of at least

8.5 vol.% in the fermentation broth. In addition, pretreatment using Ca(OH)2 was included in the analysis (Figure П. Ш). The advantage of using this type of pre­treatment is that inhibitors are not formed making the detoxification step unneces­sary. The evaluation showed that currently available industrial cellulases account for 36 to 45% of ethanol production costs, and therefore, a 10-fold reduction in the cellulase costs and a 30% reduction in capital costs are required in order to reach ethanol production costs competitive with starch ethanol. These evaluation approaches indicate the need for developing processes that contribute to improv­ing one or all of the four critical areas related to cellulase research mentioned in Chapter 5, Section 5.2.3.1.

Ghosh and Ghose (2003) report on the model process for bioethanol production proposed by the Indian Institute of Technology (IIT) in Delhi (India). This process involves two pretreatment steps: steam explosion for xylose production followed by solvent pretreatment for delignification of biomass. The released pentoses are utilized for single cell protein production, while the cellulose undergoes simulta­neous saccharification and fermentation. The SSF reactor is coupled with vacuum cycling and has a stepwise feeding of cellulose (Figure 11.11e). The process has been tested in a pilot plant using rice straw as a feedstock. However, the obtained product is hydrous ethanol (95% v/v) and the production costs (US$0.544/L) are higher than those expected for the production of dehydrated ethanol through the NREL model process (US$0.395). The consideration of adsorption separation stage (instead of distillation) increases the cost of ethanol by about 50%. The possibility of using alkali pretreatment was also assessed, but the costs increased due to lower by-product credits (low quality of obtained lignin as a fuel; Cardona and Sanchez, 2007).

Pan et al. (2005) report the preliminary evaluation of the so-called Lignol pro­cess for processing softwoods into ethanol and co-products. This configuration makes use of the organosolv process for obtaining high quality lignin allowing the fractionation of the biomass prior to the main fermentation. For this, the pro­cess utilizes a blend of ethanol and water at about 200°C and 400 psi. For etha­nol production, SHF and SSF have been tested. Streams containing hemicellulose sugars, acetic acid, furfural, and low molecular weight lignin are also considered as a source of valuable co-products. Until now, the Lignol process has been oper­ated only in a three-stage batch mode, but simulation studies indicate an improved process economics by operating the plant in continuous mode (Arato et al., 2005). Gong et al. (1999) report a fractionation process employing corncob and aspen wood chips as feedstocks and utilizing alkaline pretreatment with ammonia that favors the separation of lignin and extractives. After this step, the hemicellulose is hydrolyzed with dilute acid and released sugars are fermented by xylose-assim­ilating yeast. Finally the cellulose is converted into ethanol by batch SSF using a thermotolerant yeast strain. So and Brown (1999) performed the economic analy­sis of the Waterloo Fast Pyrolysis process comprising a 5% acid pretreatment, fast pyrolysis, levoglucosan hydrolysis, and the use of two cultures, S. cerevisiae and P. stipitis, to ferment hexoses and pentoses, respectively. These authors also analyzed the SSF process of dilute-acid pretreated feedstock that comprises the pentose fer­mentation by recombinant E. coli for xylose fermentation, and the SHF process of dilute-acid pretreated feedstock using a strain of C. shehatae for hexose and pentose fermentation. The evaluation indicates that the cost of the fast pyrolysis process is comparable to the other two processes in terms of capital costs, operat­ing costs, and overall ethanol production costs (Cardona and Sanchez, 2007).

Due to the high costs of the feedstocks accounting for more than 20% in the case of the lignocellulosic biomass (Kaylen et al., 2000), the optimization of cel­lulose conversion is of great importance, especially if it is accompanied with the appropriate handling and utilization of all process streams. Although many related works can be found, the tendency is the optimization of separate process units. This implies that the integration of such separately studied units optimized at different scales does not always provide the correct information on the global process. This situation is particularly important in the case of the integration of the pretreatment step with the biological transformations. De Bari et al. (2002) undertook this prob­lem emphasizing the scale-up features and the potential of produced by-products in the case of steam-exploded aspen wood chips. The pretreatment step was carried out in a continuous steam explosion pilot plant fed with 0.15 ton/h of dry matter that was coupled with the extraction step in order to separate the lignin and the hemicel — lulose and carry out the detoxification. The subsequent conversion to ethanol was made by SSF. The process was completed with a packed distillation column with a maximum reboiler capacity of 150 L working batch-wise. The experimentation allowed the definition of the best combinations of operation parameters, the selec­tion of the best detoxification procedure, the determination of yields and operation conditions of SSF, the analysis of the distillation for its conversion to hydrogen or ethanol, and the determination of the chemical oxygen demand (COD) of the liquid stream from the distillation step. However, this work does not report if any analysis of the process was carried out from the viewpoint of thermodynamic and kinetics fundamentals of the studied system, or if process synthesis procedures were used for the definition of the selected configuration of the process. These tools may help predict the behavior of experimental systems. Similarly, pilot-plant data can pro­vide feedback to the mathematical models used for the analysis of the system, the same as for the study of its stability and operability. For this point, the complemen­tation with simulation tools is invaluable (Cardona and Sanchez, 2007).

In general, it is thought that reductions in processing or conversion costs of lignocellulosic biomass offer the greatest potential for making biobased products like ethanol competitive in the market place in comparison to oil-based products for which high raw material costs are characteristic (Dale, 1999). Therefore, the fundamental research on the development of cost effective processes for biomass processing represents the key for attaining the mentioned competitiveness. Lynd et al. (1999) argue that oil refineries are unlikely to have significant economies of scale advantages in comparison with the expected mature biomass refineries. In this way, the challenges associated with the biomass conversion are related to the recalcitrance of cellulosic biomass (conversion into reactive products like fer­mentable sugars) and to the product diversification (conversion of reactive inter­mediates into valuable products; Cardona and Sanchez, 2007).

In general, process synthesis procedures can be significantly enhanced using process simulation packages. These simulators have allowed the analyzing of sev­eral technological options and the gaining of insight about the process improve­ments (Cardona and Sanchez, 2007). Almost all the approaches for carrying out process synthesis can rely on simulation tools to evaluate different process alter­natives. This is illustrated in the following case study.