The molecular sieve is a more energy-efficient method than azeotropic distillation. Furthermore, this method avoids the occupational haz­ards associated with azeotropic chemical admixtures. In molecular sieve drying, 95% ethanol is passed through a bed of synthetic zeolite with uniform pore sizes that preferentially adsorb water molecules. Approximately three-fourths of adsorbed material is water and one — fourth is ethanol. The bed becomes saturated after a few minutes and must be regenerated by heating or evacuation to drive out the adsorbed water. During the regeneration phase, a side stream of ethanol/water (often around 50%) is produced, which must be redistilled before return­ing to the drying process [82].

Gerhard Knothe

5.1 Introduction

Biodiesel is an alternative diesel fuel (DF) derived from vegetable oils or animal fats [1, 2]. Transesterification of an oil or fat with a monohy — dric alcohol, in most cases methanol, yields the corresponding mono­alkyl esters, which are defined as biodiesel. The successful introduction and commercialization of biodiesel in many countries around the world has been accompanied by the development of standards to ensure high product quality and user confidence. Some biodiesel standards are ASTM D6751 (ASTM stands for American Society for Testing and Materials) and the European standard EN 14214, which was developed from pre­viously existing standards in individual European countries.

The suitability of any material as fuel, including biodiesel, is influenced by the nature of its major as well as minor components arising from pro­duction or other sources. The nature of these components ultimately deter­mines the fuel and physical properties. Some of the properties included in standards can be traced to the structure of the fatty esters in the biodiesel. Since biodiesel consists of fatty acid esters, not only the structure of the fatty acids but also that of the ester moiety can influence the fuel proper­ties of biodiesel. The transesterification reaction of an oil or fat leads to a biodiesel fuel corresponding in its fatty acid profile with that of the parent oil or fat. Therefore, biodiesel is largely a mixture of fatty esters with each ester component contributing to the properties of the fuel.

Properties of biodiesel that are determined by the structure of its component fatty esters and the nature of its minor components include [9]

ignition quality, cold flow, oxidative stability, viscosity, and lubricity. This chapter discusses the influence of the structure of fatty esters on these properties. Not all of these properties have been included in biodiesel standards, although all of them are essential to proper func­tioning of the fuel in a diesel engine.

Generally, as the least expensive alcohol, methanol has been used to produce biodiesel. Biodiesel, in most cases, can therefore be termed the fatty acid methyl esters (FAME) of a vegetable oil or animal fat. However, as mentioned above, both the fatty acid chain and the alcohol functionality contribute to the overall properties of a fatty ester. It is worthwhile to consider the properties imparted by other alcohols yield­ing fatty acid alkyl esters (FAAE) that could be used for producing biodiesel. Therefore, both structural moieties will be discussed in this chapter. Table 5.1 lists fuel properties of neat alkyl esters of fatty acids. Besides the fuel properties discussed here, the heat of combustion (HG) of some fatty compounds [3] is included in Table 5. 1, for the sake of under­scoring the suitability of fatty esters as fuel with regard to this property.

The major problem with methanol is high levels of formaldehyde emis­sion, which is negligible with conventional fuels. Formaldehyde emis­sion levels with and without an electric heater are shown in Fig. 7.13. The level with an electric heater is considerably lower compared with its absence.

The performance characteristics compared with petrol engine are considered as brake thermal efficiency versus air fuel (A:F) ratio, the effect of speed power output and specific heat consumption. In addi­tion, the performance characteristics also include the effect of A:F ratio on exhaust emission. The effects of A:F ratio and speed on brake power are shown in Figs. 7.14a and 14b. Another important charac­teristic is the effect of speed on volumetric efficiency, which is shown in Figs. 7.15 and 7.16.

Both alcohols, as well as their blends, are studied as alternative fuels for IC engines. The power can be increased from 6 to 10% with alcohols or their blends. The use of a leaner mixture provides more O2, which reduces the emission. Because of the high heat of vaporization of these fuels compared to petrol, greater cooling of the inlet mixture occurs, which gives higher thermal efficiency, less specific heat consumption, and smooth operation. At higher speeds, the specific heat consumption is lower than that of petrol. Methanol dissociates in the engine cylinder forming H2. This H2 gas helps the mixture to burn quickly and increases the burning velocity, which brings about complete combustion and makes a leaner mixture more combustible. In a petrol engine, misfiring

Formaldehyde emission
with electric heater

Figure 7.16 Effect of speed on volumetric efficiency.

0 1000 2000 3000 4000 5000 6000 —— ► Speed

occurs while operating at a lean A:F ratio, whereas in an engine using alcohol, the engine can manage to handle leaner mixtures without any misfire. Important objectionable emissions are CO, HC, NOx, and alde­hydes. The effect of equivalence ratio on all these emissions for petrol and methanol are shown in Figs. 7.17 through 7.20.

For all the above graphs, the engine details and compression ratio are as follows:

1. Full throttle rpm = 2500

2. Compression ratio

Methanol Petrol

Rc = 9 Rc = 9

Rc = 12.6

Regarding emission, ethanol and methanol are considered as clean fuels, as emissions of CO, HC, and NOx are reduced by nearly 10-15% compared with a petrol engine. The flame speed of alcohol mixtures is higher than a petrol A:F mixture, and this helps in making the com­bustion more complete without misfiring.

Regarding the production of formaldehyde, its percentage in exhaust is much higher, which is a great problem to extract methanol in pure form

as a replaceable fuel. To avoid this, blends (15-25%) of both alcohols are preferred over pure ethanol or methanol. The properties of blends and their effect lie in between pure alcohol and petrol. As we know, methanol blends have lower stoichiometric air requirements compared to petrol. Therefore, if we use a methanol-petrol blend without any modification in the carburetor, we get more air for combustion, which will reduce the emission of CO and HC as well as NOx as the engine works cooler with the blend compared with a petrol engine.

Oxidizing catalytic devices can control aldehyde emissions. Platinum-rhodium and platinum-palladium catalysts are considered the most effective in tackling aldehyde emissions of methanol fueling. Concerning the alcohol fuels, the following conclusions can be drawn:

1. Alcohol is potentially a better fuel than gasoline for SI engines.

2. Its use improves the thermal efficiency as a higher compression ratio (12:16) can be used.

3. It can avoid knocking even at a higher compression ratio because of the high octane number.

4. It provides better fuel economy and less exhaust emissions.

5. High latent heat of alcohol reduces the working temperature of the engine.

6. It gives more power, specially when used as a blend.

7. Easy availability of raw materials.

8. Cost of production is low because of the price hike in crude petroleum.

In agricultural countries like India, we can get ethyl alcohol easily from vegetables, agricultural material, and sugarcane waste at a much lower cost compared with the cost of petrol today. Therefore, replacing petrol with alcohol in a SI engine has a good future.

Hydrogen production by photoheterotrophic bacteria is principally sim­ilar to that of blue-green algae, capable of fixing nitrogen and produc­ing hydrogen. The microbes are capable of converting large varieties of organic compounds to carbon dioxide and hydrogen up to 50 kg/(m2 • yr). Practical applications of these bacteria are more of an engineering problem than one of scientific “know-how.” The scope of newer research exists on the noncyclic hydrogen production by these microbes unin­hibited by nitrogen. Dilute wastes can be utilized by the photosynthetic bacteria, which is an added advantage over those of the methane fer — mentors. The conventional fermentation of organic substrates to methane or hydrogen is theoretically limited to 80% and 20%, and prac­tically to 65% and 15%. The difference is accounted for by the synthesis of ATP and cell biomass. ATP is produced in presence of light and reac­tions are driven at its expense, if hydrogen is produced by nitrogenase.

Enzymatic hydrolysis of cellulose and hemicellulose can be carried out by highly specific cellulase and hemicellulase enzymes (glycosyl hydro­lases). This group includes at least 15 protein families and some sub­families [15, 27]. Enzymatic degradation of cellulose to glucose is generally accomplished by synergistic action of three distinct classes of enzymes [2]:

■ 1,4-^-D-glucan-4-glucanohydrolases or Endo-1,4-^-glucanases, which are commonly measured by detecting the reducing groups released from carboxymethylcellulose (CMC).

■ Exo-1,4-^-D-glucanases, including both 1,4-^-D-glucan hydrolases and 1,4-^-D-glucan cellobiohydrolases. 1,4-^-D-glucan hydrolases liberate D-glucose and 1,4-^-D-glucan cellobiohydrolases liberate D-cellobiose.

■ ^-D-glucoside glucohydrolases or |3-D-glucosidases, which release D — glucose from cellobiose and soluble cellodextrins, as well as an array of glycosides.

There is a synergy between exo—exo, exo—endo, and endo—endo enzymes, which has been demonstrated several times.

Substrate properties, cellulase activity, and hydrolysis conditions (e. g., temperature and pH) are the factors that affect the enzymatic hydroly­sis of cellulose. To improve the yield and rate of enzymatic hydrolysis, there has been some research focused on optimizing the hydrolysis process and enhancing cellulase activity. Substrate concentration is one of the main factors that affect the yield and initial rate of enzymatic hydrolysis of cellulose. At low substrate levels, an increase of substrate concentration normally results in an increase of the yield and reaction rate of the hydrolysis. However, high substrate concentration can cause substrate inhibition, which substantially lowers the rate of hydrolysis, and the extent of substrate inhibition depends on the ratio of total sub­strate to total enzyme [12].

Increasing the dosage of cellulases in the process to a certain extent can enhance the yield and rate of hydrolysis, but would significantly increase the cost of the process. Cellulase loading of 10 FPU/g (filter paper units per gram) of cellulose is often used in laboratory studies because it provides a hydrolysis profile with high levels of glucose yield in a reasonable time (48-72 h) at a reasonable enzyme cost. Cellulase enzyme loadings in hydrolysis vary from 5 to 33 FPU/g substrate, depend­ing on the type and concentration of substrates. ^-glucosidase acts as a limiting agent in enzymatic hydrolysis of cellulose. Adding supplemental ^-glucosidase can enhance the saccharification yield [28, 29].

Enzymatic hydrolysis of cellulose consists of three steps [12]: (1) adsorp­tion of cellulase enzymes onto the surface of cellulose, (2) biodegrada­tion of cellulose to simple sugars, and (c) desorption of cellulase. Cellulase activity decreases during hydrolysis. Irreversible adsorption of cellulase on cellulose is partially responsible for this deactivation. Addition of surfactants during hydrolysis is capable of modifying the cel­lulose surface property and minimizing the irreversible binding of cel — lulase on cellulose. Tween-20 and Tween-80 are the most efficient nonionic surfactants in this regard. Addition of Tween-20 as an additive in simultaneous saccharification and fermentation (SSF) at 2.5 g/L has several positive effects in the process. It increases the ethanol yield, increases the enzyme activity in the liquid fraction at the end of the process, reduces the amount of enzyme loading, and reduces the required time to attain maximum ethanol concentration [30].

Crop description. Linum usitatissimum L.—commonly known as lin­seed, flaxseed, lint bells, or winterlien—belongs to the family Linaceae (see Fig. 4.8). This annual herb can grow up to 60 cm in height in most temperate and tropical regions. This plant is native to West Asia and the Mediterranean [96]. The seeds contain 30-40% oil, including palmitic

acid (4.5%), stearic acid (4.4%), oleic acid (17.0%), linoleic acid (15.5%), and linolenic acid (58.6%).

Main uses. Medicinal properties of the seeds have been known since ancient Greece. It is used in pharmacology (antitussive, gentle bulk lax­ative, relaxing expectorant, antiseptic, antiinflammatory, etc.) [97]. As the source of linen fiber, it was used by the Egyptians to make cloth in which to wrap their mummies. However, today it is mainly grown for its oil [98, 99], which is used in the manufacture of paints, varnishes, and linoleum. Linseed oil is used as a purgative for sheep and horses. It is also used in cooking. There is a market for flaxseed meal as both animal feeding and human nutrition [96].

Lang et al. transesterified linseed oil by using different alcohols (methanol, ethanol, 2-propanol, and butanol) and catalysts (KOH and sodium alkoxides). Butyl esters showed reduced cloud points and pour points [100]. Some authors have found that biodiesel from linseed oil presents a lower cold filter plugging point (CFPP) than biodiesel from rapeseed oil, due to large amounts of linolenic acid methyl ester and their iodine value [101]. Long-term endurance tests have been carried out with methyl esters of linseed oil, showing low emission characteristics. Wear assessment has shown lower wear for a biodiesel-operated engine [102]. Experimental investigations on the effect of 20% biodiesel blended with diesel fuel on lubricating oil have shown a lubricating oil life longer while operating the engine on biodiesel [103]. Oxidation stability have shown better results compared with methyl esters of animal origin [104]. Lebedevas et al. have suggested the use of three-component mixtures (rapeseed-oil methyl esters, animal methyl esters, and linseed oil methyl esters) to fuel the engine. These three-component mixtures reduced exhaust emissions significantly, with the exception of NOx that increased them up to 13% [105].

Enzymatic transesterification of TG by lipases (3.1.1.3) is a good alter­native over a chemical process due to its eco-friendly, selective nature and low temperature requirement. Lipases break down the TAG into FFA and glycerol that exhibits maximum activity at the oil-water inter­face. Under low-water conditions, the hydrolysis reaction is reversible,

i. e., the ester bond is synthesized rather than hydrolyzed. Scientists are interested in the development of lipase applications to the inter­esterification reactions of vegetable oils for production of biodiesel.

Nag has reported [43] celite-immobilized commercial Candida rugosa lipase and its isoenzyme lipase 4 efficiently catalyzed alcoholysis (dry ethanol) of various TG and soybean oil (see Fig. 6.14). This process has many advantages over chemical processes such as (a) low reaction tem­perature, (b) no restriction on organic solvents, (c) substrate specificity on enzymatic reactions, (d) efficient reactivity requiring only the mixing of the reactants, and (e) easy separation of the product.

Kaieda et al. have developed [44] a solvent-free method for methanol — ysis of soybean oil using Rhizopus oryzae lipase in the presence of 4-30 wt%

water in the starting materials. Oda et al. [45] have reported methanol — ysis of the same oil using whole-cell biocatalyst, where R. oryzae cells were immobilized within porous biomass support particles (BSP). Kose et al. [46] have reported the lipase-catalyzed synthesis of alkyl esters of fatty acids from refined cottonseed oil using primary and secondary alcohols in the presence of an immobilized enzyme from C. antarctica, commercially called Novozym-435 in a solvent-free medium. Under the same conditions, with short-chain primary and secondary alcohols, cot­tonseed oil was converted into its corresponding esters.

Alcoholysis of soybean oil with methanol and ethanol using several lipases has been investigated. The immobilized lipase from Pseudomonas cepacia was the most efficient for synthesis of alkyl esters, where 67 and 65 mol% of methyl and ethyl esters, respectively, were obtained by Noureddini et al. [47]. Shimada et al. [48] have reported transesterifi­cation of waste oil with stepwise addition of methanol using immo­bilized C. antarctica lipase, where they have successfully converted more than 90% of the oil to fatty acid ME. They have also implemented the same technique for ethanolysis of tuna oil.

Dossat et al. [49] have found that hexane was not a good solvent as the glycerol formed after the reaction was insoluble in n-hexane and adsorbed onto the enzyme, leading to a drastic decrease in enzymatic activity. Enzymatic transesterification of cottonseed oil has been stud­ied using immobilized C. antarctica lipase as catalyst in t-butanol sol­vent by Royon et al. [50].

Sometimes, gums present in the oils used inhibit alcoholysis reactions due to interference in the interaction of the lipase molecule with sub­strates by the phospholipids present in the oil gum. Crude soybean oil cannot be transesterified by immobilized C. antarctica lipase. So, Watanabe et al. [51] have used degummed oil as a substrate for a trans­esterification reaction, in order to minimize this problem, and have effec­tively achieved conversion of 93.8% oil to biodiesel.

Methanol is insoluble in the oil, so it inhibits the lipases, thereby decreasing its catalytic activity toward the transesterification reaction. Du et al. [52] transesterified soybean oil using methyl acetate in the presence of Novozym-435 (see Fig. 6.15). Further, glycerol was also insol­uble in the oil and adsorbed easily onto the surface of the immobilized lipase, leading to a negative effect on lipase activity. They have suggested that methyl acetate was a novel acceptor for biodiesel production and no glycerol was produced in that process, as shown below:

CH,-OOC-R1 . RrCOOCH3 CH2-OOCCH4

j Lipase | “

CH-OOC-R2 + ЗСН3СООСН3 R2-COOCH3 + CH-OOCCH3

CH — OOC-R R3-COOCH3 CH2OOCCH3

They found 92% yield with a methyl acetate-oil molar ratio of 12:1, and methyl acetate showed no negative effect on enzyme activity.

The comparison of biodiesel production by acid, alkali, and enzyme is given in Table 6.2.

Cottonseed oil has been thermally decomposed at 450oC using 1% Na2CO3 as a catalyst [42]. Pyrolysis produced a yellowish-brown oil with 70OC yield. The fuel properties of original and pyrolyzed cottonseed oil are sum­marized in Table 8.7. Results of ASTM distillation compared to diesel are given in Table 8.8 showing a higher volatility for the conversion product.

Rapeseed oil was pyrolyzed in the presence of about 2% calcium oxide up to a temperature of 450OC [43]. An oil was obtained with a heating

value of 41.3 MJ/kg, a kinematic viscosity of 5.96 mm2/s, a cetane number of 53, and a flash point of 80°C. When tested on a diesel engine, the thermal efficiency (^th) and brake specific fuel consumption were improved. The concentration of nitrogen oxide in the exhaust gas was less than diesel. The absence of sulfur in the pyrolytic oil was seen as an advantage to avoid corrosion problems and the emission of polluting sulfur compounds from combustion.

Triolein, canola oil, trilaurin, and coconut oil were pyrolyzed over acti­vated alumina at 450°C and atmospheric pressure [44]. The products were characterized by IR spectrometry and decoupled 13C-NMR spec­troscopy. The hydrocarbon mixture contained both alkanes and alkenes. These results are significant for the pyrolysis of lipid fraction in sewage sludge as well as for wastes from food-processing industries [44].

Pyrolysis of rapeseeds, linseeds, and safflowers results in bio-oil con­taining oxygenated polar components. Hydropyrolysis at medium pres­sure in the presence of 1% ammonium dioxydithiomolybdenate (NH4)2MoO2S2 can remove two-thirds to nine-tenths of the oxygen pres­ent in the seeds to generate bio-oils in yields up to 75% [45]. In addi­tion, extraction with organic solvents including diesel oil gave yields up to 40%.

The potential of liquid fuels from Mesua ferrea seed oil [46], Euphorbia lathyris [47, 48], and underutilized tropical biomass [49] has been inves­tigated in the search for “energy farms” involving the purposeful culti­vation of selected plants to obtain renewable energy sources.

In photosynthesis, CO2 from the atmosphere and water from the earth combine to produce carbohydrates, which are the components of bio­mass and solar energy that drive this process. When biomass is effi­ciently utilized, the oxygen from the atmosphere combines with the carbon in plants to produce CO2 and water (see Fig. 2.1). Typically, pho­tosynthesis converts less than 1% of the available sunlight to be stored as chemical energy.

The advantages of using plants for renewable energies (fuels and chemicals) are listed follows:

■ Advances in agriculture and forestry technologies have resulted in increased utilization of land resources for cultivation of energy crops.

■ By increasing harvesting of solar energy, there is effective usage of biomass-based resources.

■ Multiple economic benefits can be derived—for example, sugar can be used as such for fermentation to alcohol—depending on the market.

■ Biomass combustion, unlike fossil fuels, does not contribute to increased CO2 levels in the atmosphere [2].

■ Increased employment opportunities resulting from the above.

While the advantages of using biomass-based energies are apparent, it is important to note that biomass cannot by itself provide complete replace­ment of fossil fuels. Hence, it is one of the solutions toward achieving energy efficiency. Further factors, such as competition for biomass between energy production and human nutritional needs, as well as the possible environ­mental effects, must be kept in mind. There are several factors that should be considered in using plants for the generation of energy; efficiency of solar energy absorption and conversion, quality of biomass produced, plant growth, growth under marginal conditions, soil characteristics, and cost — effectiveness of production of energy and conversion. We will focus on the utilization of terrestrial plants for production of renewable energies.

Process design studies of molasses fermentation have shown that the investment cost was considerably reduced when continuous rather than batch fermentation was employed, and that the productivity of ethanol could be increased by more than 200%. Continuous operations can be classified into continuous fermentation with or without feedback control. In continuous fermentation without feedback control, called a chemostat, the feed medium containing all the nutrients is continuously fed at a con­stant rate (dilution rate D) and the cultured broth is simultaneously removed from the fermentor at the same rate. The chemostat is quite useful in the optimization of media formulation and to investigate the physiological state of the microorganism [71]. Continuous fermenta­tions with feedback control are turbidostat, phauxostat, and nutristat. A turbidostat with feedback control is a continuous process to maintain the cell concentration at a constant level by controlling the medium feeding rate. A phauxostat is an extended nutristat, which maintains the pH value of the medium in the fermentor at a preset value. A nutristat with feedback control is a cultivation technique to maintain nutrient con­centration at a constant level [71].

When lignocellulosic hydrolyzates are added at a low feed rate in con­tinuous fermentation, low concentration of bioconvertible inhibitors in the fermentor is assured. In spite of a number of potential advantages in terms of productivity, this method has not developed much yet in fermentation of the acid hydrolyzates. One should consider the following points in continuous cultivation of acid hydrolyzates of lig — nocelluloses:

■ Cell growth is necessary at a rate equal to the dilution rate in order to avoid washout of the cells in continuous cultivation.

■ Growth rate is low in fermentation of hydrolyzates because of the presence of inhibitors.

■ The cells should keep their viability and vitality for a long time.

The major drawback of the continuous fermentation is that, in contrast to the situation in fed-batch fermentation, cell growth is necessary at a rate equal to the dilution rate, in order to avoid washout of the cells in continuous cultivation [21]. The productivity is a function of the dilu­tion rate, and since the growth rate is decreased by the inhibitors, the productivity in continuous fermentation of lignocellulosic hydrolyzates is low. Furthermore, at a very low dilution rate, the conversion rate of the inhibitors can be expected to decrease due to the decreased specific growth rate of the biomass. Thus, washout may occur even at very low dilution rates [18]. On the other hand, one of the major advantages of continuous cultivation is the possibility to run the process for a long time (e. g., several months), whereas the microorganisms usually lose their activity after facing the inhibitory conditions of the hydrolyzate. By employing cell-retention systems, the cell-mass concentration in the fermentor, the maximum dilution rate, and thus the maximum ethanol productivity increase. Different cell-retention systems have been inves­tigated by cell immobilization and encapsulation, and cell recirculation by filtration, settling, and centrifugation. A relatively old study [72] shows that the investment cost for a continuous process with cell recir­culation has been found to be less than that for continuous fermentation without cell recirculation.

Biostil® is the trade name of a continuous industrial process for ethanol production with partial recirculation of both yeast and waste­water. The fermentor works continuously; the cells are separated by using a centrifuge, and a part of the separated cells is returned to the fermentor. Most of the ethanol-depleted beer including residual sugars is then recycled to the fermentor. In this process, besides providing enough cell concentration in the fermentor, less water is consumed and a more concentrated stillage is produced. Therefore, the process has a lower wastewater problem. However, the process needs a special type of centrifuge (which is expensive) in order to avoid deactivation of the cells [47, 73].

Application of an encapsulated cell system in continuous cultivation has several advantages, compared to either a free-cell or traditionally entrapped cell system, e. g., in alginate matrix. Encapsulation provides higher cell concentrations than free-cell systems in the medium, which leads to higher productivity per volume of the bioreactor in continuous cultivation. Furthermore, the biomass can easily be separated from the medium without centrifugation or filtration. The advantages of encap­sulation, compared to cell entrapment, are less resistance to diffusion through beads/capsules, some degree of freedom in movement of the encapsulated cells, no cell leakage from the capsules, and higher cell con­centration [74].