Biodiesel is unique among biofuels in not being a single, defined chemical compound but a variable mixture, even from a monoculture crop source. The triglycerides in any plant oil are a mixture of unsaturated and saturated fatty acids esterified to glycerol; fatty materials from land animals have much higher contents of saturated fatty acids (table 6.2).6 This variability has one far reaching implication: reducing the content of saturated fatty acid methyl esters in biodiesel reduces the cloud point, the tempera­ture below which crystallization becomes sufficiently advanced to plug fuel lines; a diesel suitable for winter use may have a cloud point below -11°C, and “winteriza­tion” (treatment at low temperature and removal of solidified material) of biodiesel generates a product with similar improved operability and startup characteristics.7,8

The idiosyncratic fatty acid content of canola seed oil, with its preponderance of the very long chain erucic acid (table 6.2), has a quite different significance. Erucic acid has been known since the 1950s to stimulate cholesterol synthesis by animals.9 The potential adverse health effects (increased risk of circulatory disease) led to leg­islation on the erucic acid content of edible oils and the development of low-erucic acid cultivars, whereas, by contrast, high-erucic acid oils have a market (estimated to be more than $120 million in 2004) because erucic acid and its derivatives are

Saturated Unsaturated

a Erucic (canola), C14 monoethenoic (soybean)

feedstocks for the manufacture of slip-promoting agents, surfactants, and other spe­cialized chemicals.10 High-erucic acid oils would be either desirable or neutral for biodiesel production, but low-erucic cultivars are higher yielding — and, in any case, legal requirements were in place in the European Union by 1992 to geographically separate the two types of “oilseed rape” cultivation to minimize cross-pollination and contamination of agricultural products intended for human consumption.4

The majority of the biodiesel producers continue to employ a base-catalyzed reaction with sodium or potassium hydroxide (figure 6.3).11 This has the economic attractions of low temperatures and pressures in the reaction, high conversion efficiencies in a single step, and no requirement for exotic materials in the construc­tion of the chemical reactor. The liberation of glycerol (sometimes referred to as “glycerine” or “glycerin”) in the transesterification reaction generates a potentially saleable coproduct (see section 6.3). The generation of fatty acid methyl esters is the same reaction as that to form volatile derivatives of fatty acids before their analysis by gas liquid chromatographic methods, and the key parameters for optimization are reaction time, temperature, and the molar ratio of oil to alcohol, but choices of

the type of catalyst used and the short-chain alcohol coreactant can also be made.12 Different oil types of plant origin have been the subject of process optimization studies; five recent examples, exemplifying the global nature of R&D activities with biodiesels, are summarized in table 6.3.13-17 More subtle factors include differential effects on product yield and purity; for example, temperature has a significant posi­tive effect on biodiesel purity but a negative influence on biodiesel yield, and the alcohol:oil molar ratio is only significant for biodiesel purity (with a positive influ­ence).17 Although the biodiesel yield increased at decreasing catalyst concentration and temperature, the methanol:oil ratio did not affect the material balance.18

A variety of novel catalysts have been explored, partly to avoid the use of caustic materials but also to facilitate catalyst recovery and reuse:

• Sulfonated amorphous carbon19

• Ion-exchange resins20

• Sodium ethoxide21

• Solid acid catalysts (e. g., ZnO)14

Indeed, the requirement for a catalyst can be eliminated if high temperatures and pressures are used to generate “supercritical” fluid conditions, under which alcohols can either react directly with triglycerides or (in two-stage procedures) with fatty acids liberated from triglycerides.22-24

Far greater attention has, however, been paid to developing a biotechnological approach to biodiesel production, employing enzyme catalysts, usually lipases, and employing their catalytic abilities to carry out transesterification (or alcohololysis) rather than straightforward hydrolyses of triglycerides to liberate free fatty acids and glycerol.25 The principal process advantage of the enzyme-based approach is the ability to use low to moderate temperatures and atmospheric pressure in the reaction vessel while ensuring little or no chemical decomposition (i. e., a high product purity); the main drawback is the much longer incubation times to achieve more than 90% conversion of the triglycerides, that is, up to 120 hours.26 The barrier to full commer­cialization is maintaining the (relatively expensive) enzyme active during repeated

batch use. Rival enzyme products show differing stability, and methanol appears to induce a faster loss of activity than does ethanol.27 Examples of enzyme-catalyzed processes using oils of plant origin and with prolonged survival of the lipases are summarized in table 6.4.26-29 Lipase from Pseudomonas cepacia was used in an immobilized form within a chemically inert, hydrophobic sol-gel support; under optimal conditions with soybean oil, high methyl and ethyl ester formations were achieved within a 1-hr reaction, and the immobilized lipase was consistently more active than the free enzyme, losing little activity when subjected to repeated uses.30 Stepwise addition of methanol to delay inactivation of the enzyme is another pos­sible strategy.31 Reversal of methanol-mediated inactivation of immobilized lipase has been demonstrated with higher alcohols (secondary and tertiary butanols).32 In the long term, molecular evolution technologies will develop lipases with reduced sensitivity to methanol and increased specific activities; in the short term, whole-cell biocatalysis has obvious potential for industrial application, offering on-site genera­tion of lipase activity in cell lines that could be selected to be robust for oil trans — esterification.33-35 As with cellulases (chapter 2, section 2.4.3), investigation of newly discovered microbes or extremophiles may reveal enzymes with properties particu­larly well suited for industrial use. A lipase-producing bacterium strain screened from soil samples of China, identified as Pseudomonas fluorescens, contains a novel psychrophilic lipase (with a temperature optimum of only 20°C); this may represent a highly competitive energy-saving biocatalyst because lipase-mediated biodiesel production is normally carried out at 35-50°C.36

Commercially available lipases and lipases identified in a wider spectrum of microbial enzyme producers can efficiently use different low-molecular-weight alcohols as substrates for transesterification. Substituting higher alcohols for metha­nol can maintain active lipase for much longer periods of continuous batch opera — tion.37 A research group in Italy has also exploited this lax substrate specificity to produce fatty acid esters from a mixture of linear and branched short-chain alcohols

that mimics the residual fusel oil left after ethanol; not only can this utilize a waste product from bioethanol production but the fatty acid esters are potentially impor­tant for biodiesels because they improve low-temperature properties.38

Crude oils can give poor transesterification rates because of their contents of free fatty acids and other components lost during refining; the free fatty acids (up to 3% of the oils) react with the alkaline catalysts and form saponified products during the transesterification. Crude soybean oil could be converted into methyl esters as well as refined oil if a lipase process was used, although a lengthy incubation period was again required.39 With an Indonesian seed oil (from Jatropha curcas) exhibiting very high free fatty acids (15%), a two-step pretreatment process was devised: the first step was carried out with sulfuric acid as catalyst in a reaction at 50°C and removing the methanol-water layer; the second step was a conventional transesterification using an alkaline catalyst to produce biodiesel at 65°C.40 Rice bran stored at room tempera­ture can show extensive (>75%) hydrolysis of triglycerides to free fatty acids; the suc­cessful processing of the oil fraction also required a two-step methanolysis process (but both steps being acid-catalyzed), resulting in a 98% methyl ester formation in less than 8 hr, and the coproduction of residue with high contents of nutraceuticals such as y-oryzanol and phytosterols.41 Supercritical methanol treatment (without any catalyst) at 350°C can generate esters from both triglycerides and free fatty acids, thus giving a simpler process with a higher total yield of biodeisel.42

Other innovations in biodiesel production have included the following:

• A six-stage continuous reactor for transesterification of palm oil in Thai­land, claimed to produce saleable biodiesel within residence time of six minutes in a laboratory prototype with a production capacity of 17.3 l/hr43

• A Romanian bench-scale continuous process for the manufacture of biodiesel from crude vegetable oils under high-power, low-frequency ultrasonic irradiation44

• A two-phase membrane reactor developed to produce biodiesel from canola oil and methanol (this combination is immiscible, providing a mass-trans­fer challenge in the early stages of the transesterification); this Canadian design of reactor is particularly useful in removing unreacted oil from the product, yielding high-purity biodiesel and shifting the reaction equilib­rium to the product side45

• A novel enzyme-catalyzed biodiesel process was developed to avoid the liberation of glycerol from triglycerides, maximizing the carbon recovery in the product; methyl acetate replaced methanol, and the resulting triacetylglycerol had no negative effect on the fuel properties of the biodiesel46

After biodiesel production, the fuel’s thermal properties have been improved — in this case, to reduce the onset of volatilization (table 6.1) of soybean-derived biodiesel to below that of conventional diesel — by ozonolysis; the onset freezing temperature of ozonated methyl soyate was reduced from -63°C to -86°C.47

The most radical development in biodiesel production has, however, been in Brazil where PETROBRAS has combined mineral and biological oils in the H-BIO
process; few details have been made public, but the essential step is to add a vegetable oil to the straight-run diesel, gasoil, and coker gasoil fractions from the refining pro­cess, the total streams then being catalytically hydrogenated.48 The triglycerides are transformed into linear hydrocarbon chains, similar to the hydrocarbons in the petro­leum oil streams; the conversion of triglycerides is high (at least 95%), with a small propane coproduct. Because the process takes advantage of the existing infrastruc­ture of an oil refinery, the potential exists for an orderly transition from conventional diesel to biodiesel blends, with a gradual increase in the “bio” input if (as widely predicted) oil reserves dwindle (chapter 5, section 5.6).