Numerous publications and patents are describing various routes for the production of biodiesel from different feedstocks. An overview is given in Mittelbach and Remschmidt (2004) and Mittelbach (2009).

Alkaline transesterification is the traditional process. Acid catalyzed transesterification is not industrially applied. However, it is seldom used in

combination with alkaline transesterification and the catalyst can be homogeneous and heterogeneous. The new developments are non-catalyzed interesterification but not industrially operational. 5.4.1 Homogeneously catalyzed production of biodiesel Today the most commercially available biodiesel production plants are using homogeneous alkaline catalyst. The reaction is a nucleophilic addition of an alkoxide anion to the carbonyl function followed by an expulsion of the glyceroxide anion (Fig. 5.4). The catalysts used are sodium and potassium methoxides or hydroxides. The advantage of using sodium and potassium methoxides is that no water is formed
ROH + В > SOR + BH® Nucleophile
CH2—Oe
CH2—OH HC—— O—C——- R2 + в I II о CH2—О—C——— R3 II о
Reverse reaction: removal of glycerine Rate limiting step: first step leading to DG
CH2—OH
CH2—O-
“OR
HC—— О—C—— R + OR	HC—— О—C—— R + RCOOR’
CH2—OH
5.4 Reaction scheme for the base-catalyzed reaction.

and no saponification is occurring. The use of hydroxide involves the formation of water resulting in hydrolysis of the acylglycerides or alkyl esters with formation of soaps (Fig. 5.5).

Potassium catalysts are favorable compared to sodium catalyst due to the acceleration of the phase separation (higher density of the glycerol layer) and lower soap formation. Furthermore, salts can be used as fertilizer after neutralization with an acid. However, the price of the potassium-derivatives is higher.

In addition, due to the fact that during the reaction glycerol is separated out (glycerol has a limited solubility in lipids and biodiesel), water is also removed, shifting the equilibrium of the reaction towards alkyl ester formation. Kinetic studies of this multiple phase reaction show that the formation of the diglyceride is the slowest, whereas the next steps are much faster (Mittelbach and Trathnigg, 1990). The standard conditions for the alkaline transesterification are 6:1 molar ratio of methanol to oil, concentration of catalyst in the range of 0.5-1.5% (depending on the FFA content of the feedstocks) and temperature of 60°C.

Reaction times can be shortened using a two-step procedure or a continuous reaction with simultaneous separation of the glycerol. A typical reaction scheme involving a two-step reaction is depicted in Fig. 5.6 using potassium hydroxide as catalyst.

In the alkaline catalyzed process, it is important that the feedstock is as much as possible water-free as well as with low FFA content in order to prevent

RCOOH + M+OH

M+=Na+, K+

5.5 Reaction scheme for soap formation.

hydrolysis. FFAs are not converted into esters but are transformed into soaps which cause problems in separating the glycerol layer and the water washing due to the formation of emulsions. In addition, FFAs are deactivating the catalyst with the soap formation. A feedstock with a high FFA content needs a higher concentration of catalyst. Preferably the FFA content should be less than 0.5% to ensure a complete conversion and efficient post-treatment. The glycerol layer separated from the biodiesel contains methanol, catalyst and soaps. After acidifying, the FFAs can be separated, the methanol evaporated and the sodium or potassium salts separated, purification steps which result in crude glycerol.

Biodiesel is then dried and used as such without post-treatment (if the feedstock is a refined lipid) after recovering of the excess of methanol and water washing.

Biodiesel can also be produced by transesterification of the oil in situ. In this procedure there is no need for the extraction of the oil from seeds. The liquid phase and solid oil containing feedstock are mixed and stirred. The disadvantage of this process is that large quantities of methanol and high concentration of catalyst are required. Furthermore, soaps are formed and additional solvent is needed to wash the seeds in order to ensure the complete separation of the oil and the transesterification reaction (Haas et al., 2007; Georgogianni et al., 2008; Qian et al., 2008).

At 60°C, the highest yield of esters is obtained using a molar ratio 226:1:1.6 of methanol:oil:NaOH. The FFA content in the biodiesel was lower than one per cent and contains no acylglycerols. By applying a drying step of the flakes prior to the reaction, the amount of methanol and catalyst can be reduced by 50% (Haas and Scott, 2007). If the feedstock contains more than three per cent FFAs, a combination of esterification and transesterification can be used (see Section 5.4.3).

In the ‘Alcohol Refining’ process (Fig. 5.7) developed by Westfalia Separator (Harten, 2006), feedstocks with high FFA content can be transformed into

biodiesel by extraction of the FFA in the glycerol layer (still containing alkaline catalyst) separated from the alkaline transesterification step. The extracted FFAs are converted into soaps. The glycerol layer is acidified and the FFAs separated. The advantages of alcohol refining are that it provides degumming and that during the transesterification less fouling and emulsion formation is observed. The disadvantages are that the FFAs are not converted into esters (loss of yield) and that the glycerol layer (which cannot be purified economically) has to be discarded.

The addition of solvents such as tetrahydrofuran and methyl t-butyl ether can accelerate the transesterification due to an increased solubility of methanol in the oil (Boocock et al.,1998). Another alternative to prepare biodiesel involves the application of microwave irradiation (Breccia et al, 1999) and ultrasonic irradiation (Stavarache et al, 2003).

Microwave heating looks very promising for a continuous flow preparation using a commercially available scientific microwave apparatus. The methodology allows for the reaction to be run under atmospheric conditions with flow rates up to 7.2 L/min, using a 4 L vessel. It can be utilized with methanol 1:6 molar ratio of oil:alcohol (Barnard et al., 2007).

The apparatus can be also used with butanol and sulphuric acid or potasium hydroxide as catalyst (Leadbeater et al., 2008).

A novel laminar flow biodiesel reactor/separator has been developed achieving high conversion rate while simultaneously allowing glycerol to separate and to settle from the reaction flow. At 40-50°C, a feed of 1.2 L/min (1:6 oil:methanol molar ratio) and 1.3% potassium hydride, a 99% conversion of waste canola oil was achieved with the removal of 70-99% of produced glycerol (Boucher et al., 2009).

Transesterification can also be performed in the presence of strong acid catalysts such as sulfuric acid and p-toluene sulfonic acid, normally for feedstocks with high FFA (>0.5%) (Fig. 5.8).

Methanesulfonic acid has been introduced by BASF (Lutropur®) as an efficient catalyst for esterification of low quality oils. This acid is less corrosive, non­oxidizing and more environmentally friendly than sulfuric or phosphoric acid. It is also used as a neutralizing agent in this base-catalyzed transesterification.

The advantage of this process is that FFAs are simultaneously converted into esters. Therefore, acid-catalyzed transesterification can be used for feedstocks which are containing high amounts of FFAs such as crude palm oil (up to 8%), used frying oils (3-7%), animal fats (up to 30%), grease and side-streams from oil refining (10-90%).

The mechanism involves protonation of the carbonyl function giving rise to a carbonium ion which is attached by the nucheophilic alcohol followed by splitting of the diglyceride and the aliphatic ester. The reaction is repeated with the diglyceride and monoglyceride. Acid transesterification has a number of disadvantages:

• Acid catalyzed transesterification is much slower than alkali catalysis (Canada 1999).

• Due to the slow reaction rate higher temperatures and pressure have to be applied (100°C/5 bar) which can also result in formation of by-products (formaldehydes and glycerol ethers).

• Feedstocks containing 0.5% water give rise to a yield decrease of one to five per cent (Canacki 1999).

• During the esterification water is formed which causes hydrolysis of the triglycerides

• The most employed acid catalyst (sulfuric acid) is very corrosive and causes dark colouring of the produced biodiesel.

The most economical conditions involve the use of molar ratio of 20:1 of methanol:oil, three per cent of sulfuric acid at 65°C for 48 hours. A comparison of base and acid catalyzed transesterification with methanol (with KOH and H2SO4 as catalysts) has also been reported (Nye et al., 1983).

The reaction kinetics of acid catalyzed transesterification of used frying and cooking oils has been studied by Zheng et al, (2006). The oil:methanol molar ratio and the temperature were the most significant factors influencing the conversion. Using a large excess of methanol and oil:methanol:acid ratio of 1:245:38 at 70°C and respectively a ratio of 1:75:1.9 at 80°C, gave a conversion of 99% in ca. 4 hours (pseudo-first-order kinetics). However, large scale production may not be economically feasible due to the large excess of methanol employed. It was reported that waste palm oil has also been transesterified in acid conditions (Al-Widyan and Al-Shyoukh, 2002).

Other homogeneous catalysts which can be used are phosphoric acid, hydrogen chloride, sulfonic acids and Lewis acids (BF3, SnCl2, AlCl3, FeCl3, CaCl2) (Mittelbach and Trathnigg, 2004).

Other promising catalysts are acetates and stearates of calcium, barium, magnesium, mangane, cadmium, bad, zinc, cobalt and nickel. A ratio of oil:alcohol of 1:12 at 200°C for 200 minutes is used. Stearates gave better yields due to higher solubility in the lipophilic phase. The advantages of these catalyst in comparison with Brpnsted acids are the lower alcohol used and a less sensitivity towards the content of water in the feedstock (Di Serio et al., 2005).

In addition in situ acid catalyzed transesterification were successfully performed. Using sulfuric acid in situ transesterification of homogenized sunflower seeds, an esters yield up to 20%, greater than with extracted oil, was reported due to the transesterification of the seed with lipids (Harrington and D’Arcy-Evans, 1985; Siler-Marinkovic and Tomasevic, 1998).

Similar processes have been used for rice bran oil (Ozgul-Yucel and Turkay, 2002). Recently, an acid catalyzed in situ transesterification of soybean oil in carbon dioxide expanded methanol was published (Wyatt and Haas, 2009). A 1.2 N sulfuric acid solution in methanol containing 50% mol fraction CO2 resulted in a 90% conversion of the triacylglycerol within 10 hours. Introduction of CO2 into the system increases the rate of reaction 2.5-fold. Alkaline transesterification in gas expanded methanol was unsuccessful.

Tin(Sn2+) complexes using the liganol 3-hydroxy-2-methyl-4-pyronate (maltolate) have been used to convert various vegetable oils into FAME at 80°C using a molar ratio of 400:100:1 of methanol:oil:catalyst. Yields up to 90% can be obtained but the methanolysis is dependent on the nature of acid chain favoring the presence of unsaturation and chain length. Technological potential is rather low as the complexes remain dissolved in the reaction medium. Attempts have been made to immobilize the complexes (Suarez et al., 2008).

A single acid or alkaline catalysis is not efficient to produce alkyl esters meeting the EU and US biodiesel standards if crude oils, fats and waste oils are being used. Therefore, a combined process with both acidic and alkaline catalyst in a two step reaction is required in which the acid treatment convert soaps and FFA into esters while the alkaline catalyst converts the acylglycerides into esters.

A dual process (Fig. 5.9) has been developed by Canacki and Van Gerpen (2003). A batch reactor was used to produce biodiesel from crude soybean oil, yellow grease (9% FFA) and brown grease (40% FFA). The high FFA feedstocks are firstly esterified with H2SO4 to reduce the FFA content to 1%, followed by transesterification with methanol and KOH.

The reaction rate is dependent upon the concentration of methanol and is increased if higher concentrations of H2SO4 are used. A ratio of methanol:oil (40:1) is used for feedstocks with high FFA content in comparison with the 6:1 ratio in alkaline catalysis. Replacing methanol by ethanol shows faster acidic esterification. Similar processes have been developed by Issariyakul et al. (2007) using a mixture of methanol/ethanol.

A more efficient procedure to convert high acidic feedstocks using short chain alcohols and a combination of acidic esterification and an alkaline process in the presence of ethylene glycerol and glycerol (temperature lower than 120°C and pressure lower than 5 bar) was described by Lepper and Friesenhagen (1986). Due to the immiscibility with the lipophilic layer, the water formed is entrained.

5.9 Production of biofuels from recovered oil and from frying oils (Canacki and Van Gerpen, 2003).

The acid treated oil containing fatty acid esters and mono — and diesters of ethylene glycol and glycerol is treated with alkaline catalyst for the transesterification. The advantage of the process is a short reaction time due to the continuous removal of the water and no soap formation is taking place. The disadvantage is the removal of the catalyst which requires two separations if the acidic catalyst has not been neutralized by an additional amount of alkaline catalyst. Canacki and Van Gerpen (1999) stated that using in feedstocks like crude and waste oil can result in a 25% reduction in cost relative to fully refined edible oils. However, Zhang et al. (2003) explained that it seems not to be clear. The traditional alkaline process requires the lowest fixed capital cost, but a much higher feedstock cost. The acid-alkaline process has a lower manufacturing cost, an attractive tax return (green certificates) and a lower biodiesel breakeven price. On the contrary, the corrosive nature of the acid catalysts should be included in the economic evaluation.

Biodiesel from low grade animal fat mixed with soybean oil has been synthesized in a combined esterification and transesterification process. A mixture of 50% of both raw materials has been selected and a computer simulation of the production process using Aspen Plus software has been carried out to evaluate the industrial feasibility. The acid esterification was performed with p-toluene sulfonic acid instead of sulfuric acid. The use of the latter one can cause a high concentration of sulfur in the biodiesel. However, the reaction rate is much faster than the reaction rate with p-toluene sulfonic acid (Canoira et al., 2008).

A variant of the acidic alkaline process is an alkaline transesterification followed by an acidic esterification on condition that the feedstock is containing maximum ten per cent FFAs (Verhe et al., 2009). Higher acid concentrations deactivate the alkaline catalyst in a too large extent. Both reactions are carried out in one reactor without intermediate separation of the layers (Fig. 5.10).

Glycerol 5.10 Schematic representation of the process for the conversion of crude palm oil (CPO) to biodiesel (Verhe et al., 2009).

Reaction steps:

1. Neutralization of FFA and alkaline transesterification

2. Acidic esterification

3. Evaporation of methanol

4. Separation of glycerol layer

5. Washing of biodiesel with water.

Reaction conditions:

• Alkaline transesterification:

— catalyst 0.6% KOCH3 (33% in CH3OH) + calculated amount of KOCH3 in order to neutralize the FFA

— methanol: 20%

— temperature: 65°C

— reaction time: 90 minutes

• Acidic esterification:

— catalyst: 2% H2SO4 + calculated amount to neutralize the excess of KOCH3 and split the soaps

— temperature: 65°C

— reaction time: 180 minutes

The oil is heated at 65°C and mixed with the alkaline catalyst and methanol. Without separation of the glycerol that calculated amount of H2SO4 is added. After 3 hours, the methanol is stripped off and the crude biodiesel is transferred to a separator for glycerol removal. Water is added and the glycerol/water layer is separated. The biodiesel is washed with water until neutral and dried at 65°C under vacuum.

The main advantage of this process is that the feedstock does not require a pre­refining for the removal of the phospholipids, gums, traces of proteins and carbohydrates. During the esterification and work-up in acidic conditions at 65°C, a degumming is occurring, pigments are decomposed and are discarded with the aqueous layers. Separation of the glycerol and aqueous layers is much easier in acidic medium as no emulsions are formed. During the acidic esterification the residual acylglycerols are also transesterified into FAMEs resulting in higher conversion rates.