The worldwide transport sector depends almost totally on fossil fuels. While the production of electricity is more diversified, vehicles require gasoline, diesel, or natural gas for the mobilization. Only in specific cases, has the total substitu­tion of gasoline with liquid biofuels been achieved, as in the case of the ethanol obtained from energy-rich crops such as sugarcane. The advantage of liquid bio­fuels compared to solid ones consists in their ease of transport and in the utiliza­tion of the supply chains of fossil fuels. Biodiesel and bioethanol are the two main liquid biofuels.

1.1.2.1 Biodiesel

Biological diesel is an oxygenated fuel obtained as a result of the transesterifica­tion of vegetable oils and animal fats with an alcohol in the presence of a cata­lyst (Figure 1.1). In general, the employed alcohol is methanol or ethanol. Thus, the biodiesel is a mixture of methyl or ethyl esters of fatty acids from oil and fats. Usually, rapeseed oil (in central Europe), sunflower oil (in southern Europe), and palm oil (in tropical countries of Southeast Asia and South America) are employed as a source of triglycerides. Used frying oil of animal or vegetable origin is employed for their conversion into biodiesel as well. For biodiesel pro­duction, acid, basic, and biological (enzymes) catalysts are used, with KOH and NaOH being the most utilized catalysts worldwide.

CH2 — OOC —Ri Rj— OOC— C2H5 CH2 — OH

KOH

CH2 — OOC —R2 + 3C2H5OH < > R2 —OOC —C2H5 + CH — OH

I I

CH2 — OOC —R3 R3— OOC— C2H5 CH2 — OH

Triglyceride Ethanol Ethyl esters Glycerol

FIGURE 1.1 Transesterification reaction of triglycerides.

Biodiesel is employed in heating systems and as an oxygenating additive (oxygenate) in diesel engines. For these purposes, blends containing 10% (B10) or 20% (B20) of biodiesel and the remaining fossil diesel are being utilized. In some cases and depending on its purity, the biodiesel is directly used in internal combustion engines (Ma and Hanna, 1999). The main advantages offered by the diesel usage are the reduction of polluting gases (mostly CO) and particulate mat­ter emissions, as well as the net equilibrium in the balance of atmospheric CO2. Unlike the oil-derived diesel, the biodiesel does not contain sulfurs, making its combustion much cleaner.

Worldwide biodiesel production achieved about 7.70 billion liters in 2007. The production of biodiesel in the United States was about 1.82 billion liters in 2007, whereas the production in the EU was about 5.71 billion liters (European Biodiesel Board, 2008). Germany is the world leader in biodiesel utilization. In this country, pure biodiesel is used in adapted vehicles, while in France, 30% and 50% biodiesel blends with fossil diesel are employed (Demirbas and Balar, 2006). In Colombia, the agro-industry of oilseeds is being developed through the production of biodiesel to be used as an oxygenate of local diesel in B5 blends. Thus, the productive chain of the oil palm can be boosted.

During cane milling, the soluble fraction is separated from the rest of the plant achieving up to 97% efficiencies for industrial milling and 50% efficiencies for traditional milling (Gonzalez and Gonzalez, 2004). Cane juice is a viscous, greenish liquid. For South American cane varieties, the juice contains 15 to 16% soluble solids, from which 85% is sucrose yielding an approximate content of this sugar in the fresh juice of 12 to 13% (Lindeman and Rocchiccioli, 1979). The average composition of raw cane juice (before its clarification) is presented in Table 3.5. As in the case of the whole sugarcane, an example of the specifica­tion corresponding to the average composition of cane juice is shown in this table using data from India, Venezuela, Colombia, and South Africa (Bhattacharya et al., 2001; Gonzalez and Gonzalez, 2004; Sanchez and Cardona, 2008a; Seebaluck et al., 2008). This average composition was used during most simulations con­ducted for process synthesis purposes.

The clear beet juice obtained in sugar production plants can be used for etha­nol production as well. In general, the beet juice is used during the harvest season when there is a high availability of beets. Out of season, the distilleries produce ethanol mostly from beet molasses or beet syrup (Decloux et al., 2002). The com­position of beet juice does not differ substantially from the composition of cane juice, as reported by Ogbonna et al. (2001).

A significant number of papers and works related to cellulose hydrolysis have been published. For instance, the reader can consult the reviews of Lynd et al. (2002) and Zhang and Lynd (2004) on this theme. The description of cellulose degradation, in particular, the kinetics of cellulose hydrolysis, is difficult due to the influence of factors such as (Bernardez et al., 1993; Kadam et al., 2004; Meunier-Goddik and Penner, 1999; Philippidis and Hatzis, 1997; Philippidis et al., 1993; Zhang and Lynd, 2004):

• Adsorption of the enzymes to the substrate particles.

• Presence of interactions between the lignin and enzymes that do not lead to sugars formation.

• Decrease of cellulose hydrolysis as substrate conversion progresses.

• Enzyme dosage.

• Synergistic action of endoglucanases and cellobiohydrolases.

• Need of supplementing the cellulases with P-glucosidase to diminish cellulase inhibition by cellobiose formed.

• P-glucosidase inhibition by the glucose formed.

• Solids load to the reactor, among others.

The importance of cellulose hydrolysis description is recognized considering that the mathematical modeling of this process allows for developing appropriate simulation tools to be used during process synthesis procedures. Such institutions as the NREL have funded projects regarding the modeling of cellulose break­down, sugar formation, and ethanolic fermentation in order to assess several alter­native technological configurations of the biomass-to-ethanol process. This was the case of the work of South et al. (1995) dealing with the hydrolysis of cellulose contained in pretreated wood that, in turn, used the experimental data obtained in a previous work by these same authors using commercial fungal cellulase (South et al., 1993). In this study, a kinetic model, considering the cellulose conversion and the formation and disappearance of cellobiose and glucose, was developed. In addition, a Langmuir-type model taking into account the adsorption of cel — lulases on the solid particles of cellulose and lignin and expressions describing the dependence of cellulose conversion on the residence time of nonsoluble solid particles of biomass were considered.

The adsorption of cellulases to the particles present in the solid fraction of pretreated lignocellulosic biomass should be modeled in a suitable way to obtain results useful during the design of cellulosic ethanol production. This is not the case for starch saccharification considering that the amylases attack their substrate in a soluble form after starch cooking. In contrast, during the first steps of ligno — cellulosic hydrolysis, the cellulases are added to a suspension of cellulose and lignin particles. The Langmuir model is widely used for description of adsorption processes involving cellulases taking into account the good adjustment to experi­mental data in most cases (Cardona and Sanchez, 2007). In addition, it represents a simple mechanistic model that can be used to compare kinetic properties of various cellulase-cellulose systems. Kadam et al. (2004) employed a Langmuir — type isotherm to describe the enzymatic hydrolysis of cellulose for the case of dilute-acid pretreated corn stover. In this model, the inhibition effect on cellulases of other sugars present in the biomass hydrolyzate as xylose was considered as well as the effect of temperature (through the Arrhenius equation) and the dosage of P-glucosidase. In an early work, Bernardez et al. (1993) studied the adsorption process of complexed cellulase systems (cellulosomes) released by the anaerobic thermophilic bacterium Clostridium thermocellum onto crystalline cellulose, pretreated wood, and lignin employing the Langmuir description. Nevertheless, some experimental data indicate that the negative effect of lignin content in the hydrolyzate is not principally due to the enzyme partitioning between cellulose and lignin, suggesting that lignin hinders saccharification by physically limiting the enzyme accessibility of the cellulose (Meunier-Goddik and Penner, 1999). Hence, more structure-oriented modeling is required to gain insight on biomass hydrolyzate’s hydrolysis and its optimal operating conditions. Other models have been proposed since the union of the cellulases to the cellulose does not meet all the assumptions inherent to the Langmuir model. To this end, two-site adsorption models, Freundlich isotherms, and combined Langmuir-Freundlich isotherms have been proposed (Zhang and Lynd, 2004). Lynd et al. (2002) present in their wide review about the microbial cellulose utilization, a compilation of values of adsorption parameters for cellulases isolated from different microorganism and for diverse substrates. In that work, the kinetic constants for cellulose utilization by different microorganisms are reported as well. On the other hand, it has been shown that the intensity of the agitation in batch reactors has little effect over cel­lulose hydrolysis when cellulose particles are suspended. Based on the analysis of the kinetic constants and on experimental data, it was concluded that the external mass transfer is not a limiting factor of the global process of hydrolysis. However, when the internal area is much greater than the external one, as in the case of most cellulosic substrates, it is probable that cellulases can remain entrapped in the pores provoking lower hydrolysis rates (Zhang and Lynd, 2004). These con­siderations are essential when mathematical representations of cellulose saccha­rification are developed. On the other hand, some kinetic studies for cellulose hydrolysis highlight the significant effect the enzyme dosage has on glucose yield. In particular, Schell et al. (1999) accomplished the saccharification of dilute-acid pretreated Douglas fir and obtained data from which was derived a useful empiri­cal model to calculate the glucose yield as well as to formulate a kinetic cellulose hydrolysis model.

Zhang and Lynd (2004) reviewed in detail the works concerning the model­ing of cellulose hydrolysis and point out that most of proposed models for the

design of industrial systems fall in the category of semimechanistic models, i. e., models taking into account the substrate concentration or one of the enzymatic activities as a state variable. These models meet the requirement of including the minimum of necessary information for the description of the process (Cardona and Sanchez, 2007). These authors emphasize that most kinetic models do not consider the changes in the hydrolysis rate during the course of the reaction, and that those models that do this, are based mainly on empirically adjusted param­eters and not on a mechanistic approach. For instance, the model of a simultane­ous saccharification and fermentation (SSF, to be analyzed in Chapter 9) process developed for the case of nonpretreated wastepaper using commercial cellulases and S. cerevisiae for both batch and two-stage continuous regimes (Philippidis and Hatzis, 1997) made use of an exponential decay term to describe the time — dependent decline in the rate of cellulose hydrolysis. With the help of an exhaus­tive sensitivity analysis, the model showed that further improvements in the fermentation stage do not have great influence on ethanol yield. In contrast, the digestibility of substrate (as a result of pretreatment), cellulase dosage, specific activity, and composition have a great effect on ethanol yield. This confirms that major research efforts should be oriented to the development of more effective pretreatment methods and the production of cellulases with higher specific activ­ity (Cardona and Sanchez, 2007).

Half of the mechanistic models cited by Zhang and Lynd (2004) are based on the Michaelis-Menten model, which is valid when the limiting substrate is in excess relative to the enzyme. In addition, competitive inhibition is the mecha­nism most found in the literature, although a combination of both noncompeti­tive and competitive mechanisms for different inhibition effects can be found, as are analyzed in the work of Philippidis et al. (1993). Due to the importance of modeling, these authors highlight the need for developing functional models that include the adsorption process, several state variables for substrate besides the concentration (e. g., polymerization degree or amount of amorphous cellulose), and multiple enzymatic activities.

To achieve the separation of an azeotropic mixture by using pressure-swing dis­tillation, the manipulation of the column pressure is required, e. g., by utilizing a second distillation column working under vacuum conditions (Figure 8.2). This type of distillation makes use of the change of the vapor-liquid phase equilibrium at lower pressures than atmospheric (vacuum) leading to the disappearance of the azeotrope. The pressure required to eliminate the azeotrope in an ethanol-water mixtures is less than 6 kPa. But to obtain a high purity product, distillation columns with a large number of plates (above 40) and a high reflux ratio are needed. These conditions imply significant capital costs (large column diameters) and increased energy costs due to the maintenance of vacuum in distillation towers with many plates. This configuration has no fluxes or refluxes connecting the two columns. In general, pressure-swing distillation cannot always be employed; its utilization is limited to mixtures with azeotropes susceptible to be displaced with small changes of pressure, which is not exactly the case in ethanol-water systems.

The stillage obtained from the process employing sugarcane molasses can be centrifuged to collect the yeast debris, which has a significant nutritive value, remaining the thin stillage. This stillage is later evaporated leaving up to 50 to 65% solids to increase its stability against the microbial action. In this way, it can be commercialized for animal feed. However, its high salts content, especially potassium, has limited its use to only up to 10% of the ruminant diet (less than 2% in swine diet) to avoid laxative effects (Nguyen, 2003). The stillage derived from other types of sucrose-containing feedstocks, such as cane juice or beet molasses, can be used in the same way.

Tanzania is representative of an underdeveloped country that does not produce or consume bioethanol, but which has a high interest in having other countries using its fertile, virgin lands for bioenergy purposes. Tanzania is a sugarcane producer, but not a fuel ethanol producer as of yet. A Swedish company is among seven foreign firms that want to buy or rent large chunks of fertile land along the Rufiji Delta. The firm is looking for about 400,000 hectares for the production of etha­nol from sugarcane. It has already put 20,000 hectares into the crop and a further 50,000 hectares are yet to be developed (Cardona et al., 2009).

Sugarcane is an important commercial crop in Tanzania. It is the main source of sugar produced for both export and domestic consumption. Tanzania is well situated for the sugarcane production in East Africa. The country has a wide variety of climate and weather and an area of 945,087 km2. Rainfall may be con­sidered the limiting factor for most crops, sugarcane inclusive. About 21% of the
country can expect 90% probability of receiving slightly higher than 750 mm of rainfall and only about 3% can expect more than 1,250 mm.

The yields and technology for sugarcane growing are very low. Consequently, the feedstock prices are very high. The production of ethanol in Tanzania from sugarcane juice or molasses can be economically competitive with global produc­tion once the costs of feedstock, namely, sugarcane, are reduced by 70 to 80% of the current cost (Cardona et al., 2009).

Actually, this country is under very serious analysis by FAO (a BEFS project with the participation of the authors of this book) regarding the biofuels potential and food security risks. Some considerations about this country will be discussed in detail later in this chapter.

The utilization of ethanol as an oxygenate has many benefits: higher oxygen content (lesser amount of required additive), high octane number (see Table 1.1), greater reduction in carbon monoxide emissions, and nonpollution of the water sources. Compared to methanol, ethanol is less hygroscopic, has a higher combus­tion heat, and has less heat from evaporation, and, most important, it is much less toxic. In addition, the acetaldehyde formed during ethanol oxidation is much less dangerous than the formaldehyde formed during methanol combustion. In fact, acetaldehyde predominates in comparison to formaldehyde in exhaust gas from vehicles using ethanol-gasoline blends (Rasskazchikova et al., 2004). Ethanol — gasoline blended fuels increase the emission of formaldehyde, acetaldehyde, and acetone 5.12 to 13.8 times more than gasoline. Although the aldehyde emissions will increase when ethanol is used as a fuel, the damage to the environment by the emitted aldehydes is far less than that caused by the polynuclear aromatics emitted from burning gasoline (Yuksel and Yuksel, 2004).

From the viewpoint of combustion properties, the autoignition temperature and flash point (temperature at which the liquid generates sufficient vapor to form a flammable mixture with the air) of ethanol are higher than those of gasoline, which makes it safer to transport and store. Ethanol has a latent heat of evaporation 2.6 times greater than that of gasoline, which makes the temperature of the intake manifold lower and increases the volumetric efficiency of the engine. Nevertheless, this property causes the engine’s cold start ability to be reduced because the alco­hols require more heat to vaporize than does gasoline in order to form an appropri­ate air-fuel mixture that can be burned. Ethanol heating value is also lower than that of gasoline and, therefore, it is necessary to have 1.5 to 1.8 times more fuel ethanol to release the same amount of energy if it is used in a pure form rather than in gasoline blends. On the other hand, the stoichiometric air-fuel ratio of ethanol is about two thirds to one half that of gasoline, hence, the required amount of air for complete combustion is less for ethanol (Yuksel and Yuksel, 2004).

From a socioeconomic point of view, the utilization of fuel ethanol presents important advantages as well (Chaves, 2004; Sanchez and Cardona, 2008a). Bioethanol contributes to the decrease of imports of gasoline or oil in consuming countries through the partial substitution of these fossil fuels. Thus, this biofuel has the potential of compensating and reducing the impact of periodical rises of oil prices in the context of exhausting national reserves. Therefore, significant currency savings can be achieved that otherwise would have to be directed to fos­sil fuel imports. In each country, ethanol usage favors the economic utilization of raw materials and renewable resources, such as sugarcane, cassava, corn, and sor­ghum, as well as a great amount of lignocellulosic residues having the potential to be converted into ethyl alcohol. The use of fuel ethanol boosts the economic and productive reactivation of many rural communities through the increase in demand for agricultural production. By means of the development of productive projects for obtaining fuel ethanol, the base for creating and expanding actual agro-industrial chains where several links are integrated with the participation of private and public sectors is provided. This integration spreads benefits to dif­ferent segments of the economy such as the energy, agricultural, industrial, and financial sectors. This makes possible the development of commercial relation­ships as well as the creation of jobs in depressed rural areas avoiding the migra­tion of population to urban centers, especially in developing countries. In addition, the large-scale utilization of ethanol will promote the scientific and technological development of many countries in the biofuel field even when turnkey technology is acquired. Once installed, this type of technology generates new challenges, such as increase in productivity, improvement of the different crops varieties that are used as feedstocks, enhancement of process efficiency, and reduction of the environmental impact caused in production facilities, as well as many others.

From the environmental point of view, the utilization of ethanol as a gasoline oxygenate offers net reductions in the amount of greenhouse gas emissions per traveled mile of 8 to 10% in gasoline blends containing 10% ethanol by volume (known as E10 blends). For blends containing 85% ethanol (E85 blends), this reduction can reach up to 68 to 91% (Wang et al., 1999). In general, it is considered that the greater the percentage of ethanol in gasoline blends, the better the envi­ronmental benefits mostly expressed through the net reduction of greenhouse gas emissions during the entire life cycle of ethanol. Numerous studies have proved the environmental benefits of fuel ethanol usage in terms of its impact on the emission of the combustion production from ethanol-gasoline blends. According to data compiled by the Canadian Renewable Fuels Association (2000), the gaso­line oxygenated with 10% ethanol reduces the levels of carbon monoxide by 25 to 30% as well as the net CO2 emission by 10%. The main benefits of using ethanol as an oxygenate are presented in Table 1.2. One of the features of ethanol lies in the fact that it can be utilized as a feedstock for ETBE production using isobutene obtained from the petrochemical industry. This duality converts the ethanol to a very promising product in the international energy market, especially if the leg­islation of different countries continues to be aimed at using renewable gasoline oxygenates (Ancillotti and Fattore, 1998) as in the case of the EU.

The sugarcane bagasse is one of the most produced lignocellulosic materials in the world and represents the residue of cane stems after crushing and juice extrac­tion. It is a by-product of the sugar industry and is almost exclusively utilized in sugar mills as a fuel for steam generation. In the past few years, the research on the economic utilization of bagasse to produce electricity, paper and paper pulp, and fermentation products has intensified. For instance, the Colombian sugar sec­tor already produces paper from bagasse at an industrial level and commercializes 15 MW of electricity obtained from this material in the national interconnected electric network (Asocana, 2006). The technologies for production value-added products by fermentation employing bagasse are currently under development. In general, these technologies are oriented to the application of solid-state fermen­tation to produce animal feed, enzymes, amino acids, organic acids, and phar­maceuticals, among others (Pandey et al., 2000). The pretreated bagasse can be used as a substrate in submerged fermentations for production of xylitol, flavors, single-cell protein, cellulases, ligninases, and xylanases (Aguilar et al., 2002; Cardona and Sanchez, 2007; Pandey et al., 2000). In addition, sugarcane bagasse can be the feedstock for producing activated carbon with an exceptionally high adsorptive capacity (Cardona and Sanchez, 2007; Lutz et al., 1998).

The percentage of the main biopolymers in cane bagasse is quite similar to that of hardwood (see Table 3.11). In chemical terms, cane bagasse contains about 50% a-cellulose, 30% pentosans, and 2.4% ash. Due to its low ash content, the bagasse presents a better performance during fermentation than other residues, such as rice straw and wheat straw, which contain 17.5% and 11.0% ash, respec­tively. Moreover, the bagasse can be considered as a valuable means for accumu­lating solar energy due to its high yield (80 ton/ha) compared to wheat (1 ton/ha), grasses (2 ton/ha), and trees (20 ton/ha; Pandey et al., 2000). In general, great amounts of energy can be obtained from cane bagasse. By burning the bagasse in co-generation systems, the total amounts of process steam and electricity required to adequately cover the energy needs of sugar mills can be obtained, with an energy surplus that can be sold to the grid. Nevertheless, different energy and economic analyses indicate that more benefits can be achieved if the bagasse were used for ethanol production (Cardona and Sanchez, 2006; Moreira, 2000). To this regard, it is necessary to take into account that the electricity can be obtained from a great number of primary fuels, while the liquid fossil fuels for transporta­tion can be substituted only by a reduced amount of renewable fuels (bioethanol and biodiesel; Moreira, 2000).

The global production of cane bagasse can be estimated at 373 to 416 million tonne per year considering data of 2004 and yields from 280 kg bagasse per tonne of cane (Moreira, 2000) up to 312 kg/ton (Kim and Dale, 2004). The ethanol yields of bagasse depend on the conversion technology employed, which will be discussed in the following chapters. In a preliminary way, a yield of 140 L EtOH/ ton bagasse can be assumed based on the efficiency reported by different trials carried out in the National Renewable Energy Laboratory in the United States (Golden, CO), which has been used by Kim and Dale (2004). Considering the mentioned yield, the global potential for producing ethanol from cane bagasse reaches 58.2 million L/year, an amount greater than all the ethanol produced in the world in 2007.

In this chapter, main fermentation technologies for ethanol production are discussed. The analysis of these technologies is oriented to the description of the different ways by which different sugar solutions resulting from conditioned and pretreated feedstocks are converted into ethyl alcohol. The different regimes for ethanologenic fermentation are reviewed. The role of mathematical modeling of fermentation dur­ing the design of ethanol production processes is highlighted and related illustrative examples are presented. Finally, some simulation case studies are disclosed in the framework of process synthesis procedures for fuel ethanol production.

One of the most important advances in the bioethanol industry is the development and implementation of processes in which the hydrolysis of the glucan (starch, cellulose) and the conversion of sugars into ethanol are carried out simultaneously in the same single unit. This process is known as simultaneous saccharification and fermentation (SSF) and has been successfully implemented in the production of ethanol from corn, especially in dry-milling plants.