Sugar beet is a biennial plant belonging to the family Quenopodiaceae. Its sci­entific name is Beta vulgaris L. It is believed that the beet originated in Italy. Beet flowers are not very appealing and are hermaphroditic. The roots are pivot­ing (they are sunk as a trunk prolongation), almost totally buried, with a yellow- greenish rough peel. The root is the organ where most sugar is accumulated in the plant. The seeds are adhered to the calyx. The different beet varieties are used for human food (table or red beet), animal feed (fodder beet), and sugar production (sugar beet). The latter variety (B. vulgaris var. altissima) is the most employed in temperate zones, especially in Europe and North America. To be cultivated, sugar beet requires a temperate, sunny, moist climate and deep soils with neutral pH, high water retention, and good aeration. Clay, sandy, calcareous, and dry soils are not adequate for this crop (Infoagro, 2002).

Sugar beet is an important crop in Europe, North America, and Asia. France is the major producer of sugar beet followed by Germany and the United States (FAO, 2006) as shown in Table 3.4. France had a cropped area with sugar beet of about 379,000 hain 2005, while Russia had about 780,000 ha cropped with sugar beet. These data give an idea on the yield differences between these two countries.

The microbial cellulolytic enzymes (cellulases) can overcome the disadvantages of acid hydrolysis of cellulose. Being specific biological catalysts, secondary products of degradation are not formed and working under milder conditions (temperatures up to 60°C, pH of 2.5 to 5.5), exigencies to the enzymatic bioreac­tors are not very strict. Nevertheless, the reaction times are more prolonged than in the case of acid hydrolysis.

A significant number of microorganisms have the ability to biosynthesize cel — lulases. In general, the anaerobic bacteria degrade the cellulose through high — molecular-weight complex systems with cellulolytic activity named cellulosomes, as in the case of Clostridium thermocellum. The cellulosomes are extracellular structures consisting of spherical polypeptidic complexes that include an enzy­matic package with cellulase activity, which group together the enzymes through the action of one polypeptide with no hydrolytic activity involved in the adhesion of cellulosomes to the substrate. The cellulosomes increase the magnitude of the enzymatic action on the plant particles rich in cellulose due to the close contact of the bacterium with its enzymes and such particles (Fondevila, 1998). C. ther — mocellum cellulosomes are distributed not only in the culture medium, but also on the surface of bacterial cells, though several bacterial strains do not release appreciable amounts of these cellulolytic complexes to the medium. In this case, the cellulosomes are concentrated in the cell wall surface or inside its structure. These complexes have a high effectiveness in the degradation of crystalline cel­lulose (Lynd et al., 2002). In particular, anaerobic thermophilic bacteria exhibit high growth rates on cellulose and have enzymes with a high stability. Moreover, their culture requires less agitation energy. These bacteria could be potential can­didates for the direct conversion of cellulose to ethanol (Lee, 1997).

In contrast, aerobic cellulose-degrading microorganisms, including bacteria and fungi, break down this polysaccharide through the production of significant amounts of individual extracellular cellulases, although some enzymatic com­plexes can be occasionally found on the cell surface. Due to their individual nature, these cellulases exhibit a synergic action on the cellulose (Lynd et al., 2002). At an industrial level, a great variety of cellulases are produced and employed in the formulation of detergents, in the textile and food industries, and during the pro­duction of paper pulp and paper (Bhat, 2000). Most of the commercial cellulases are obtained from Trichoderma reesei, though a small portion is obtained from A. niger. Several reports can be found on the features of cellulase aerobic produc­tion by T. reesei. See, for example, the work of Marten et al. (1996). T. reesei releases a mixture of cellulases, among which at least two cellobiohydrolases, five endoglucanases, P-glucosidases, and hemicellulases can be found. The two cellobiohydrolases and one of the endoglucanases represent approximately 92% of total production of cellulases (Zhang and Lynd, 2004). Cellobiohydrolases break down P(1,4) linkages from nonreducing or reducing ends of the cellulose chain releasing cellobiose or even glucose, whereas endoglucanases hydrolyze these same linkages randomly inside the chain. The action of cellobiohydrolases causes a gradual decrease in the polymerization degree, while endoglucanases cause the rupture of cellulose into smaller chains reducing rapidly the polymer­ization degree. Endoglucanases especially act on amorphous cellulose, whereas cellobiohydrolases are capable of acting on crystalline cellulose as well (Lynd et al., 2002), as illustrated in Figure 5.1.

Although T. reesei produces some P-glucosidases, which is the result of hydrolyzing formed cellobiose into two molecules of glucose, their activities are not very high. Unfortunately, cellobiohydrolases are inhibited by cellobiose. Therefore, P-glucosidase from other sources needs to be added in order to com­plement the action of the cellulases from this fungus (Sanchez and Cardona, 2008). Dekker and Wallis (1983) have determined a ratio of FPU (filter paper

FIGURE 5.1 Schematic representation of cellulose hydrolysis using individual cel­lulolytic enzymes.

units) to P-glucosidase units of enzymatic activity as the minimum required to achieve more than 80% conversion of cellulose into glucose. The combined action of these enzymes is synergic leading to the conversion of cellulose into glucose (Beguin and Aubert, 1994; Walker et al. 1993). Factorial optimization techniques have been applied to the design of mixtures of cellulases from different sources along with P-glucosidase in order to maximize the yield of glucose produced (Kim et al., 1998). It has been suggested to develop a multicellulase plasmid in which the different cellulase genes could be expressed to produce cellulases with an optimum ratio from a single cultivation.

Cellulases should be adsorbed onto the surface of substrate particles before hydrolysis of insoluble cellulose takes place. The three-dimensional structure of

these particles in combination with their size and shape determines if P-glycosidic bonds are or are not accessible to enzymatic attack (Zhang and Lynd, 2004). This makes the hydrolysis process slower in relation to the enzymatic degradation of other biopolymers. For comparison, the hydrolysis rate of starch by amylases is 100 times faster than the hydrolysis rate of cellulose by cellulases under indus­trial processing conditions. Apparently, this difference in hydrolysis rates can be explained to a greater extent by the higher accessibility of the enzymes to the substrate, which is more limited in the case of the cellulose, than by the fact that the P(1,4) bond of cellulose is more difficult to hydrolyze than the a(1,4) bond of starch. In the case of the pretreated lignocellulosic complex, the cellulases can bind in a reversible way not only to the cellulose particles, but also to the lignin that reduces their effectiveness. In addition, the cellulases can bind in an irreversible way to the substrate provoking a progressive loss of enzymatic activity. It has been postulated that the addition of surfactants to the reaction mixture can improve the effectiveness of enzymatic cellulose hydrolysis due to the reduction of enzyme loss through irreversible binding to the substrate. The surfactant increases the rate of hydrolysis as well as prolongs the enzyme life. This allows the reduction of enzyme dosage to 50%. It has been reported that the usage of Tween-80 improved sugar production for any given particle size of cellulose when milled newsprint was used as a feedstock. Similarly, the use of sophorolipid increases by 67% the hydrolysis of steam-exploded wood (Duff et al., 1995; Helle et al., 1993).

One nonconventional approach for saccharification that demonstrates the diversity of trends for research and development of cellulose hydrolysis is the enzymatic hydrolysis in biphasic media, by which higher glucose concentrations can be attained. The goal of replacing part of the water with organic substances is explained by the need of ensuring the necessary rheological properties to accom­plish saccharification using higher substrate loads taking into account that glu­cose does not migrate to the organic phase. Cantarella et al. (2001) employed this approach for saccharification of steam-exploded wheat straw employing a medium with 25% (by volume) aqueous phase and 75% organic phase (acetates). Higher glucose concentrations (measured in the aqueous phase) were obtained compared to the case when only the aqueous phase was used. These concentrations reached about 150 g/L. However, when the aqueous phase was fermented using S. cer — evisiae, an increase in the lag-phase and a small reduction in ethanol yield were observed (nearly 86 to 92% of the theoretical maximum). Another approach to cellulose hydrolysis consists in the design of hydrolyzing agents, which are dif­ferent from mineral acids (that degrade the formed glucose) or enzymes. Mosier et al. (2002) have systematically studied several organic acids in order to assess their cellulose hydrolysis and glucose degradation characteristics in comparison to sulfuric acid. Their results indicate that maleic acid presented the best char­acteristics. In particular, this acid does not degrade the formed glucose. These studies are aimed at developing a nonprotein catalyst that mimics the action of the cellulases. In this way, maleic acid is a suitable catalytic domain for the synthesis of such enzymatic mimicry.

In this chapter, main technologies for concentration of dilute aqueous solutions of ethanol obtained during the fermentation step are discussed. The main ethanol dehydration technologies, including the most promising, are disclosed as well. The main advantages and drawbacks of the discussed dehydration schemes are included as well. The utilization of process simulation tools is illustrated through the analysis of different technological schemes for ethanol separation and dehy­dration. The role of thermodynamic-topological analysis during the concep­tual design of separation and dehydration technological configurations is also highlighted.

The recovery of ethanol produced by different technological configurations and from diverse types of feedstocks is accomplished in a very similar way. The ethanol content in the culture broth resulting from fermentation processes oscil­lates between 2.5 and 10% (by weight). The utilization of fuel ethanol as a gaso­line oxygenate requires a high-purity ethanol, so it is necessary to concentrate the ethyl alcohol up to 99% to obtain the anhydrous ethanol, which is the suit­able form used for ethanol-gasoline blends. Because the presence of water in fuel ethanol can lead to failures in the engine during the combustion of such blends (Sanchez and Cardona, 2005). The first step of the ethanol recovery scheme is the concentration of ethanol contained in culture broths. This process is carried out in distillation columns achieving ethanol content of about 50%. The next step is the rectification of this concentrated stream to obtain a product with a composi­tion near to the azeotropic mixture of ethanol and water. In the following section, these two steps are discussed.

10.1.1 Residues Generated in the Process of Bioethanol Production

Fuel ethanol production generates solid wastes, atmospheric emissions, and liquid effluents. The atmospheric emissions correspond mostly to the gas outlet stream from fermenters, which are washed with water in the scrubbers in order to recover the volatilized ethanol. The gases exiting from the scrubbers contain mainly car­bon dioxide that is released into the atmosphere. The CO2 can be used for produc­tion of dry ice and beverages.

However, if these gases are not utilized, they should be considered in the calcu­lation of the environmental impact of ethanol-producing facilities. In this regard, it should be emphasized that the bioethanol presents net emissions of nearly zero CO2 because the plant biomass already fixed the CO2 during its growth and only this carbon dioxide is released during the combustion of fuel ethanol in the engines. In contrast, the burning of fossil fuels releases into the atmosphere additional amounts of carbon dioxide that was fixed by the plant biomass millions years ago.

The solid wastes formed during production of fuel ethanol are strongly linked to the raw material from which it is produced. When sugarcane is used, huge amounts of sugarcane bagasse are produced. Fortunately, this solid material has multiple uses and applications. Its most important utilization is as solid biofuel due to its high energy content. In fact, bagasse combustion allows the generation of the thermal energy (steam) required not only during the conversion of sugar­cane into ethanol, but also during cane sugar production. The bagasse also can cover the electricity needs if co-generation units are used (see Chapter 11). In the
corn-to-ethanol process by the dry-milling technology, most solid residues are concentrated in the so-called distiller’s dried grains with solubles (DDGS), so the generation of solid wastes is limited. If wet-milling technology is employed, the solids generated produce part of the different co-products in the framework of a corn biorefinery, as corn gluten meal and feed. When cassava roots are used, the solids produced correspond to the root peels that can be utilized as feedstock for production of mushrooms as well as the fibrous residue contained in the stillage stream. This fibrous residue is obtained after centrifugation of the whole stillage to obtain the thin stillage and this solid material. In addition, this residue can be used for animal feed or as a substrate in solid-state fermentations.

The production of ethanol from lignocellulosic biomass, in turn, generates lignin as the most important solid residue. This polymer can be isolated during the pretreatment step if some pretreatment methods like the pretreatment with sol­vents (organosolv process) or oxidative delignification are employed (see Chapter 4, Section 4.3.3). Nevertheless, most pretreatment methods allow the lignin to remain in the solid fraction resulting from this processing step along with the cel­lulose. After enzymatic hydrolysis using cellulases, the lignin remains in the liquid suspension until the end of the process where it can be recovered from the stillage stream. The lignin has a high energy value and, therefore, is used as a solid biofuel for feeding boilers or co-generation units, as in the case of sugarcane bagasse.

The stillage (or vinasses) is the major effluent of all flowsheets for ethanol production involving the submerged fermentation of streams containing sugars or carbohydrate polymers. This is valid for feedstocks such as sugarcane juice, molasses, starchy materials, and pretreated lignocellulosic biomass. The still­age represents the residual liquid material obtained after distillation of ethanol from the fermented wort (wine) and contains both solid and soluble matter. The elevated organic load of the stillage is most responsible for the high polluting properties of this burden, thus this stream should undergo treatment to reduce this load and minimize the environmental impact during its discharge into the water streams.

Among the bioenergy crops used for fuel ethanol production, sugarcane is the main feedstock utilized in tropical countries like Brazil and India. In North America and Europe, fuel ethanol is mainly obtained from starchy materials, especially corn. Different countries such as the United States and Sweden have defined strategic policies for the development of this technology in order to pro­duce large amounts of renewable biofuels and diminish their dependence on imported fossil fuels. However, the possible land competition between food and biofuels is not often regulated by the government, being considered the market law as the natural judge for this kind of competition. Different scenarios must be analyzed depending on the type of feedstock and participation of the country in the supply chain (producer or consumer)

12.1.1 Corn in the United States

The United States is a developed country, which will have a future dependence on bioethanol imports (the U. S. is going to become more of a consumer than a producer). This is explained by the fact that this country actually plants about 32 million hectares of corn, but this acreage is not going to be strongly increased in the coming years. The energy products in the United States are expected to be covered by imports from its neighbor, Latin America.

The production of bioethanol in the United States has increased from 4.16 billion liters in 1996 to 24.6 billion liters in 2007 (Renewable Fuels Association, 2008). But the target consumption for 2022 is 56.7 billion liters annually of etha­nol (U. S. Dept. of Agriculture, 2007). The country uses corn instead of sugarcane as a raw material (sugarcane is produced with agroecological limitations only in Florida). The energy balance concerning corn conversion into ethanol is negative (for each unit of energy supplied by ethanol, more energy is used to produce it). The bioethanol productivity per hectare of corn is three times less than in the case of sugarcane.

Consequently the bioethanol production costs for corn are 80% higher than for sugarcane. The U. S. ethanol industry is viable only because there are major subsidies for bioethanol production.

High-fructose corn syrup (HFCS) is an important food product from corn. It is more economical because the U. S. price of sugar is twice the global price and the price of corn is substantially low due to government subsidies. Overall analysis shows that, while the United States is a consumer of corn, it also is an exporter of this grain (Table 12.1).

The most important foods in the United States from corn are flakes and HFCS. Table 12.1 shows that these products are not greatly affected by biofuels produc­tion. However, the real concerns about food security in the United States are out­side the country. In past years, Mexico has become a net importer of genetically modified corn, absorbing the massive American surplus. As maize cultivation in Mexico becomes an economically impractical proposition, the farmers aban­don the land to migrate to Mexico City or to the United States. In this case, the consumers suffer the consequences. As an example, between 1994 and 2003, the price of tortillas (Mexican national food) quadrupled. The problem could increase for Mexico if American corn is used to produce fuel ethanol instead of tortillas. This, however, depends on the market and negotiations between the United States and producers in Latin America.

If this occurs, the price of corn will increase substantially. But an import complement to the food security discussion in corn-ethanol production is the existence of the by-product called dried distillers grains (DDGs), which is used

as livestock feed. DDGs are the high protein feed produced through distilling, vaporizing, and drying after fermentation in alcohol production from corn. DDGs contain abundant amino acid, citrine, and diversiform minerals, which are ben­eficial to the growth of the animals. Countries in Europe (especially Ireland), Mexico, Taiwan, Indonesia, Venezuela, Malaysia, and Israel are today importers of DDGs for animal feeding. Therefore, any change in the actual protectionist policies of the United States for corn and ethanol production could drastically affect the price of different food products inside and outside this country.

Di-isopropyl ether (DIPE) is not currently produced at an industrial level, but it represents an important option as an oxygenate due to its antiknocking properties and low volatility, among other features. However, it presents the disadvantage of the auto oxidation with the subsequent production of low solubility and explosive peroxides. To neutralize this effect, the addition of antioxidants in amounts as small as 20 ppm is enough, which does not allow the production of more peroxides than the ones formed during MTBE synthesis (Ancillotti and Fattore, 1998). DIPE is produced by the addition of water to isopropylene for producing isopropylalcohol, which is added to other molecules of isopropylene obtaining DIPE. Hydration reac­tion is carried out with catalysts of the type ZSM-5 or acid polysulfone resins at 700 to 24,000 kPa. The second reaction is accomplished using acid zeolites at 450 to 7,000 kPa (Harandi et al., 1992). In this case, the need for employing alcohols is eliminated because oxygen is supplied by the water. Like the ETBE, the production of DIPE by reactive distillation has been proposed (Cardona et al., 2000, 2002).

Ethers, as gasoline oxygenates, have demonstrated several advantages derived from their oxygen content, antiknocking properties, and compatibility with the gasoline, among other factors. Nevertheless, the concerns arising from the low biodegradability and high mobility of MTBE have imposed serious limitations to the production of this type of oxygenates in some countries, particularly in the United States where even the usage of ETBE and TAME has been restricted in California. A limiting factor is that the toxicological properties and the environ­mental impacts of these ethers are not sufficiently known. It is considered that due to the similarity in the molecular structure of the different ethers, their properties should be analogous to those of MTBE. For this reason, contamination problems in groundwater as a consequence of using oxygenates like ETBE, TAME, TAEE, or DIPE are expected (Graham et al., 2000; Nadim et al., 2001).

The lignocellulosic complex represents the most abundant biopolymer on Earth and constitutes the main component of a great variety of wastes and residues from domestic and industrial activities of man. Moreover, it is present in profuse biological materials, such as wood, grass, straw, and forage. Due to its immediate origin in a biological process, this biopolymer complex has received the name of lignocellulosic biomass. Being the most plentiful material in the biosphere, the use of the lignocellulosic biomass will allow the production of a valuable biofuel like bioethanol as well as the economic exploitation of a wide range of potential feedstocks resulting from domestic, agricultural, and industrial activities.

The directed modification of DNA through recombinant DNA technology, widely known as genetic engineering, represents a powerful tool not only for improv­ing strains of industrial microorganisms (e. g., by the increase of product yields), but also for transferring specific traits or properties from some species to oth­ers, which are very separated in the evolutionary chain. This technology implies the extraction of genes from organisms exhibiting a determined trait in order to introduce (recombine) them in the DNA of a host microorganism (or organism). When the modified microorganism is reproduced, the succeeding generations will inherit the newly acquired trait. In the case of bacteria, the most common vectors used for DNA transfer to the host organism are the plasmids, relatively short autonomous (nonintegrated to the bacterial chromosome) DNA sequences that are autoreplicable, i. e., can form copies of themselves especially during the cell reproduction. It is worth emphasizing that the recombination of the gene with the plasmid is carried out in vitro (Figure 6.4). The recombinant vector obtained is introduced into the bacterial cells through diverse techniques that include a temporal increase in the porosity of both cell wall and cell membrane for the vec­tor to be introduced into the cytoplasm attaining, in this way, the transformation of the host cells. The microorganisms having genes of other organisms are called recombinant microorganisms or, in a general way, genetically modified organ­isms or simply engineered organisms.

Ffinrifir ritnr fnr DMA chain hyrlrnlyr

Foreign DNA chaii DNA after cleavage

Vectors replication

Cell reproduction

_____ U_______

C C GGGC
GGGG G G G GG
C GG GGGG

FIGURE 6.4 Principle of recombinant DNA technology to develop genetically modified bacteria. A plasmid is the vector for transferring the foreign gene to the bacterium.

The introduction of recombinant microorganisms in the ethanol industry can lead to the development of industrial processes radically different from the conven­tional technologies based on the fermentation of molasses or starch hydrolyzates using native or improved yeast strains. Due to the specific and directed charac­ter of the modifications that can be done by genetic engineering, it is possible to “create” microbial strains capable of assimilating alternative feedstocks (first, lignocellulosic materials) or ones with the ability to perform simultaneous trans­formations in integrated process (e. g., the direct conversion of starch or lignocel — lulosic biomass). In this regard, when the microorganisms are genetically modified
to assimilate more substrates or to form new products, the integration of a series of very complex chemical transformations allowing the execution of several chemical processes in the same unit can be carried out. This kind of integration is verified at cell or even molecular level. These integrated processes represent a new approach in ethanol production and are the base for integral utilization of feedstocks in the so-called biorefineries (Cardona and Sanchez, 2007).

Process efficiency plays a crucial role when the performance of different tech­nological configurations are to be considered. To reach this improved efficiency, several conventional approaches may be used, which make possible to a certain degree the intensification of processes, a necessary condition for the design of technologies with enhanced performance. But for attaining a higher degree of process intensification, the application of new concepts within the framework of a new paradigm is required—a product and process engineering paradigm, according to Stankiewicz and Moulijn (2002). These authors define process intensification as the development of new equipment and procedures leading to a “dramatic improvement” in chemical processes through the reduction of the ratio between the equipment size and the production capacity, energy consump­tion, and waste production, resulting in cheaper and sustainable technologies, i. e., any chemical engineering development leading to a significantly smaller, cleaner, and more energy-efficient technology. Process intensification can be carried out by using new types of equipment and unconventional processing methods, such as integrated processes and processes using alternative energy sources such as light, ultrasound, and the like. It also can be done by implementing new process control methods, such as intentional unsteady-state operation. In this way, pro­cess intensification can be considered as a major headway toward the design of essentially more efficient technologies with much better performance comparing them to processes based on individual unit operations preferentially connected in a sequential mode (Cardona et al., 2008).

One of the main ideas of process intensification is to combine different process functions (separation, mixing, chemical reaction, biological transformation, fluids transport; Li and Kraslawski, 2004) and to utilize the energy flows of the same process in order to achieve better process performance. The combination of func­tions implies the physical combination of unit operation and processes through their simultaneous accomplishment in the same single unit or by their coupling (conjugation). Similarly, the combined utilization of energy flows allows a better exploitation of available energy sources. This physical combination of material and energy flows leads to the integration of processes oriented to their intensifi­cation. In this way, the possibilities of improving the performance of the overall process in terms of saving energy and reducing capital costs are greater when the integration of several operations into one single unit is carried out (Cardona et al., 2008).

Process integration offers many advantages in comparison to nonintegrated processes. Particularly in the case of reaction-reaction and reaction-separation processes, integration allows increasing the conversion of reactants and, conse­quently, the volumetric productivity. This increased conversion is explained by the fact that some key components formed during the chemical or biochemical transformation are removed from the reaction zone leading to the acceleration of the direct reaction in reversible reactions, or to the reduction of the inhibition effects in the case of some biological processes (Cardona et al., 2008). For inte­grated processes, the increased conversion makes possible a better utilization of the feedstocks, and the increased selectivity allows the reduction in the amount of nondesired products, which implies the reduction of the waste streams. In this way, the process integration approach contributes to the design of environmen­tally friendly technologies. From the viewpoint of production costs, the integra­tion allows the development of more compact processes due to the reduction in the amount and size of processing units. Therefore, capital costs may be reduced as well as energy consumption. The reduction of energy costs is related to the decrease in the size of processing units. Smaller units have lower steam and cool­ing water requirements. Moreover, the integration approach allows achieving a synergetic effect in the heat transfer leading to the reduction of energy consump­tion. The reduction of the energy needs leads to the decrease in the size of the heat exchangers, which contributes to the compactness of the technological configura­tion, one of the most important features of integrated processes. Furthermore, the compactness of some integrated schemes allows the reduction in the amount of external recycling streams, which are substituted by internal recycles.

However, integrated processes exhibit some disadvantages when compared to nonintegrated processes. First of all, the controllability of integrated processes is much more complex. Often, the integration leads to the existence of multiple steady-states in the system. For this reason, integrated processes require robust control loops, which are expensive and difficult to design. In addition, the use of third substances in some integrated schemes, such as extractive reaction where the addition of an extractive agent is necessary, indicates the need for using recov­ery units in order to decrease the operating costs of the process (Cardona et al., 2008). One of the most difficult issues during the design of integrated processes is related to the lack of appropriate models for describing this type of configura­tion. Most of the developed models correspond to short-cut methods where main phenomena taking place in the system are quite simplified. These methods are mostly based on equilibrium models. However, this kind of method has allowed the preliminary and conceptual design of many integrated processes as well as the assessment of the viability of their implementation.

In a previous work (Rivera and Cardona, 2004), the classification of integrated processes was provided. Such processes can be divided into two main classes depending on whether unit operation or unit process is being combined. The inte­grated process is homogenous when two or more unit operations or two or more reactions (unit processes) are combined and heterogeneous when the combination is carried out between one unit operation (physical process) and one chemical reaction. Each case can be accomplished through either simultaneous or conju­gated configuration. In the first case, the physical and/or chemical processes are simultaneously carried out in a single unit. In the second case, the processes are carried out in different apparatuses connecting them by fluxes or refluxes, i. e., by coupling two or more units (Cardona et al., 2008). In relation to the process steps that can be combined, integrated processes can be of the following types: reaction-reaction, reaction-separation, or separation-separation.

In this context, the design of technologies with improved performance accord­ing to technical, economic, and environmental criteria for producing fuel ethanol is required. The reduction of energy consumption along with the decrease in the capital costs through process integration offers promising opportunities for the improvement of the overall process for bioethanol production. Thus, this reduc­tion can contribute to the worldwide development of the biofuels industry with its inherent economic, social, and environmental benefits. The aim of this chapter is to study and recognize the vast possibilities of process integration during the con­ceptual design and development of high-performance technologies for production of fuel ethanol from different feedstocks.

Process integration, as a mean for process intensification, is a successful approach for designing improved technological configurations for fuel ethanol production in which energy consumption, production costs, and negative envi­ronmental impacts can be reduced. This fact is remarkably important taking into account that the main objective of using liquid biofuels, like bioethanol, is the progressive displacement of fossil fuels. This implies the sustainable exploitation of the huge biomass resources of our planet and the use of clean and renewable energy sources. Solutions provided by the process integration approach have to be proved at an industrial level in order to develop energy efficient, environmentally friendly, and even “politically correct” processes for fuel ethanol production. In fact, some technologies directly involving the principle of integration for bioetha­nol production have already been successfully implemented.

In a previous work (Sanchez et al., 2007), the environmental impacts of fuel ethanol production processes from sugarcane or lignocellulosic biomass were estimated. The evaluation of the environmental performance was performed for three techno­logical configurations employing sugarcane juice as feedstock (stand-alone facili­ties). The first one comprised the utilization of sugarcane bagasse for co-generation of process steam and power and included the production of concentrated stillage for fertilization purposes. The second option did not consider the co-generation though it did consider the production of the concentrated stillage as a fertilizer, thus the bagasse was accumulated as a solid residue. The third variant involved neither cogeneration nor production of concentrated stillage. The simulation data for these alternative flowsheets were taken from a preceding work (Cardona et al., 2005b). In addition, the production of fuel ethanol from lignocellulosic biomass through a process including the dilute-acid pretreatment of biomass, simultaneous saccharifi­cation and co-fermentation (SSCF) and ethanol dehydration using molecular sieves was also analyzed. The simulation data for production of biomass ethanol were taken, in turn, from a previously published work (Cardona and Sanchez, 2006). For the biomass process, two variants were considered: with and without cogenera­tion using the lignin recovered from the whole stillage. For all five technological configurations, the normalized values of the total impact expressed as PEI units per kilogram of ethanol produced were obtained (Figure 10.6).

The values of the total PEI were calculated from the weighted sum of the eight impact categories evaluated by the software WARGUI. The PEI for each category is presented in Figure 10.7. The weighted factors employed for six of the eight impact categories were taken from the work of Chen et al. (2002) as follows:

• Global warming 2.5 (equivalent to GWP in the WAR algorithm)

• Smog formation 2.5 (equivalent to PCOP)

• Acid rain 10.0 (equivalent to AP)

• Human noncarcinogenic inhalation toxicity 5.0 (equivalent to HTPE)

• Human noncarcinogenic ingestion toxicity 5.0 (HTPI)

• Fish toxicity 10.0 (equivalent to ATP)

The two remaining impact categories in the WAR algorithm methodology were assigned the following weights: TTP 2.5 and ODP 2.5.

The results obtained indicate that the use of sugarcane presents a higher envi­ronmental friendliness than the use of lignocellulosic biomass. This is explained by the complexity of the conversion process from biomass to ethanol. For such conversion, a pretreatment step involving the use of inorganic acids and high pres­sure is required. In addition, the hydrolysis of cellulose and fermentation of formed

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FIGURE 10.6 Total output rate of potential environmental impact (PEI) per mass of products for five process configurations for fuel ethanol production: Cane A = production of ethanol and fertilizer (concentrated stillage) from sugarcane employing bagasse for co-generation, Cane B = production of ethanol and fertilizer (concentrated stillage) from sugarcane without co-generation, Cane C = production of ethanol from sugarcane without co-generation, Biomass A = production of ethanol from lignocellulosic biomass without co-generation, Biomass B = production of ethanol from lignocellulosic biomass employ­ing the recovered lignin for co-generation.

FIGURE 10.7 Potential environmental impact (PEI) per mass of product streams for different ethanol production configurations according to the eight impact categories considered by the WAR algorithm (the denominations of the categories are presented in Figure 10.5): Cane A = production of ethanol and fertilizer (concentrated stillage) from sugarcane employing bagasse for co-generation, Cane B = production of ethanol and fer­tilizer (concentrated stillage) from sugarcane without co-generation, Cane C = production of ethanol from sugarcane without co-generation, Biomass A = production of ethanol from lignocellulosic biomass without co-generation, Biomass B = production of ethanol from lignocellulosic biomass employing the recovered lignin for cogeneration.

glucose should be accomplished. These processes imply higher energy expendi­tures. To supply this amount of energy, the combustion of lignin is considered lead­ing to the release of atmospheric emissions containing CO2, CO, particulate matter, and polycyclic aromatic hydrocarbons, which generate important environmental impacts. In fact, if lignin is not burned, the environmental impacts are appreciably reduced (see Figure 10.6).

In the case of the process using sugarcane, the co-generation and the production of concentrated stillage as a fertilizer (configuration Cane A in Figures 10.6 and 10.7) is a good option to diminish the burdens to the environment. However, the configuration considering the cane bagasse as a solid residue and the concentrated stillage as co-product (configuration Cane B in Figures 10.6 and 10.7) shows indi­cators slightly more favorable. The gases released during the bagasse combustion have a higher contribution to the aquatic toxicity calculated by the WARGUI soft­ware than the components of the bagasse itself (see Figure 10.7). This difference is mostly responsible for the better environmental performance of this configuration compared to the scheme involving the co-generation using the bagasse as a solid fuel. Nevertheless, the economic considerations do indicate the evident benefit of burning the bagasse because no money is spent for acquiring the fossil fuels needed to supply the thermal energy for the overall process. If the stillage is not treated and considered as a liquid effluent, the environmental impact potential remarkably increases, as shown in Figures 10.6 and 10.7. The total PEI per kilogram of products is increased by 430% related to the Cane A case. This fact is explained by the very high organic load of the stillage that raises the potential impacts corresponding to the four toxicological impact categories.

It should be noted that the apparent better environmental performance of the configuration Cane B is due in a higher degree to the weighting factors chosen. If equal weighting factors are selected for the four local toxicological impact catego­ries (using a value of 10) related to the global atmospheric impact categories (using a value of 2.5), a lower PEI/kg for the Cane A case is obtained: 0.43 for configura­tion Cane A and 0.74 for configuration Cane B. This selection of weighting factors favors the local effects on human health, flora, and fauna during the environmental analysis more than the effects on the biosphere, which is logical considering that the former effects are more significant in the short and middle terms.