Abbreviations

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1. Introduction

The U.S Energy Information Administration, in its International Energy Outlook 2016 report, indicated that the world total energy consumption is significantly increasing. In this report, the worldwide energy consumption is projected over the 28 year period from 2012 to 2040 as shown in Figure 1.

This projection specified that much of the growth in energy consumption is expected from non-OECD countries, where strong economic growth and expanding populations lead the increase in world energy use.

As countries develop and living standards improve, energy demand grows rapidly. For instance, in nations experiencing fast-paced economic growth, their life style changed and more economic activities emerge demanding more and more energy.

Crude oil, coal and gas are the main dominant resources for world energy supply ^[2]. However, most argue that demand for renewables would increase owing to limited reserve of the conventional fuels, which jeopardize the energy security issue, as well as for the environmental benefit of using renewables as alternative energy supplies.

According to the IEA Medium Term Renewable Energy Report 2015, the renewable energy share in the total world energy consumption is expected to have at least 26% increment by 2020 ^[3]. And the International Energy Agency, world energy outlook 2013 ^[4], particularly showed that, for the next two decades, world fuel oil demand is concentrated in transport sector and in which, diesel fuel demand is expected to dominate by 5.5 million barrel per day as shown in Figure 2.

As it can be seen from Figure 2, diesel fuel use is expected to be the main to get the highest score in increment in oil demand for the years to come. This indicates there is more practical opportunity in working towards substituting the conventional diesel fuel with biodiesel so that attaining the required demand without causing negative consequence to our environment.

In accordance with this, the awareness of energy issues and environmental problems associated with burning fossil fuels has globally encouraged many researchers to investigate the possibility of using alternative sources of energy instead of oil and its derivatives. Among them, biodiesel seems very interesting for several reasons.

The invention of the vegetable oil fueled engine by Sir Rudolf Diesel dated back in the 1900s, however, full exploration of biodiesel only came into light in the 1980s as a result of renewed interest in renewable energy sources for reducing greenhouse gas (GHG) emissions, and alleviating the depletion of fossil fuel reserves. Biodiesel is defined as mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats and alcohol with or without a catalyst ^[5,6,7,8,9].

Biodiesel is highly biodegradable and has minimal toxicity. It has almost zero emissions of aromatic compounds and other chemical substances that are destructive to the environment. It has a small net contribution of carbon dioxide (CO₂) when the whole life-cycle is considered (including cultivation, production of oil and conversion of oil to biodiesel); and its production can be decentralized so that it could have significant potential for improvement of rural economy ^[5,10].

Compared to diesel fuel, biodiesel produces no sulfur, less carbon monoxide, less particulate matters, less smoke and hydrocarbons emission and more oxygen. More free oxygen leads to the complete combustion and reduced emission ^[11,12].

Vegetable oil and/or animal fat can be converted to fuel for diesel engine through four major possible ways: direct use or blending of oils, micro-emulsion, thermal cracking or pyrolysis and transesterification reaction. Among these methods, the most preferred one is transesterification reaction. Transesterification reactions enables the use of diverse feedstock types to produce a fuel highly resemble to conventional diesel in quality. Through this method, oils and fats (triglycerides) are converted to their alkyl esters with viscosity similar to diesel fuel.

Transesterification reaction can be catalyzed or non-catalyzed. The catalysis of transesterification is usually either chemically like base catalyzed transesterification and acid catalyzed transesterification, or using enzyme catalysts like lipase-catalyzed transesterification. However, there are also some less investigated but efficient ways to produce biodiesel through esterification of oils and fats such as those using Nano catalysts and ionic liquid catalysts. The non-catalyzed transesterification is carried out without any catalyst only by using an alcohol at supercritical conditions where the alcohol, usually methanol, is at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist ^[13,14,15]. In the supercritical state, the dielectric constant of alcohol is decreased so that two-phase formation of vegetable oil/alcohol mixture is not encountered and only a single phase is found favoring the reaction ^[16].

Each transesterification technique requires different feedstock character. For example, some can handle feedstock with high FFA content where as others are very sensitive to even small amount. Some esterification techniques are more advantages than the others at least with respect to cost of production, or minimum waste generation, or high productivity and the like. In addition, there are some very important reaction conditions, which should always be optimized for efficient production of biodiesel. Among them the very commonly studied are: molar ratio of alcohol to oil, type and amount of catalyst, reaction temperature, reaction time, reaction medium, type and relative amount of solvents.

Accordingly, in this paper, more emphasis is given on reviewing the effect of the main reaction conditions for an efficient production of biodiesel from different feedstock types as well as on summarizing the advantages and disadvantages of these major transesterification techniques.

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2. Oil and Fat as Fuel in Diesel Engines and Biodiesel Production Technologies

The dominant technologies, which enable us to use oil and fat feedstock types as fuel in diesel engines, are usually described as direct use or blending of oils, micro-emulsion, pyrolysis and transesterification. Transesterification being currently mentioned by various researchers as the most preferable due to better quality of fuel produced ^[17,18,19].

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2.1. Pyrolysis

Pyrolysis refers to a chemical change caused by the application of thermal energy in the absence of air or oxygen, or by the application of heat in the presence of a catalyst, which results in cleavage of bonds and formation of a variety of small molecules. Pyrolysis is conducted at temperature range of 400–600 ℃. The process produces gases, bio-oil, and a char depending on the rate of pyrolysis. Based on the operating conditions, the pyrolysis process can be divided into three subclasses: conventional pyrolysis, fast pyrolysis and flash pyrolysis ^[20] as shown in Table 1. Fast pyrolysis is the one used for production of bio-oil.

This liquid fraction of the thermally decomposed vegetable oil, bio-oil, is likely to approach diesel fuel properties and characteristics. Ma et al. ^[21] mentioned that the chemical compositions (heavy hydrocarbons) of the diesel fractions produced by catalytic cracking of copra oil and palm oil stearin were similar to fossil fuels. The process was simple and effective compared with other cracking processes according to them.

According to Ma et al. ^[21] pyrolytic chemistry is difficult to characterize because of the variety of reaction paths and the variety of reaction products that may be obtained from the reactions that occur. The pyrolyzed material can be vegetable oils, animal fats, natural fatty acids and methyl esters of fatty acids.

In another study, Mahanta et al. ^[22] mentioned that pyrolyzate (product of pyrolysis) from any feedstock type has lower viscosity, flash point, and pour point than petroleum diesel fuel and equivalent calorific values. In addition, the cetane number of the pyrolyzate is lower. According to them, the pyrolyzed vegetable oils contain acceptable amounts of sulfur, water and sediments and give acceptable copper corrosion values but unacceptable quantities of ash, carbon residual and pour point.

Abbaszaadeh et al. ^[23] also reported that biodiesel fuel produced through a pyrolysis process or known as bio-oil is suitable for diesel engines; however, low-value materials are produced due to the elimination of oxygen during the process. Undesirable properties that sometimes restrict the application of biodiesel produced through this process are low heating value, incomplete volatility, and instability ^[24]. But, in another view, Singh and Singh ^[25], mentioned that thermal pyrolysis of triglycerides has several advantages such as lower processing cost, simplicity, less waste, and no pollution.

Another disadvantage of pyrolysis is the need for distillation equipment for separation of the various fractions. Also the product obtained is similar to gasoline containing sulphur which makes it less ecofriendly ^[26].

The equipment for thermal cracking and pyrolysis is expensive for modest throughputs. In addition, while the products are chemically similar to petroleum-derived gasoline and diesel fuel, the removal of oxygen during the thermal processing also removes any environmental benefits of using an oxygenated fuel. It produces some low value materials and, sometimes, more gasoline than diesel fuel ^[21].

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2.2. Micro-emulsification

Among the physical properties of raw vegetable oil, which makes it to be not directly used as fuel, is its viscosity. Ma et al. ^[21] pointed out that, the formation of micro-emulsion is one of the potential solutions for solving the problem of vegetable oil viscosity.

According to IUPAC definition, micro-emulsion is dispersion made of water, oil, and surfactant (s) that is an isotropic and thermodynamically stable system with dispersed domain diameter varying approximately from 1 to 100 nm, usually 10 to 50 nm ^[27].

The components of a biodiesel micro-emulsion include diesel fuel, vegetable oil, alcohol, and surfactant and cetane improver in suitable proportions. Alcohols such as methanol and ethanol are used as viscosity lowering additives, higher alcohols are used as surfactants and alkyl nitrates are used as cetane improvers ^[28].

Mahanta et al. ^[22] reported that micro-emulsion can be made of vegetable oils with an ester and dispersant (co-solvent), or of vegetable oils, and alcohol and a surfactant and a cetane improver, with or without diesel fuels. All micro-emulsions with butanol, hexanol and octanol met the maximum viscosity requirement for diesel fuel. The 2-octanol is an effective amphiphile in the micellar solubilization of methanol in triolein and soybean oil ^[29].

Micro-emulsions can improve spray properties by explosive vaporization of the low boiling constituents in the micelles. Micro-emulsion results in reduction in viscosity, increase in cetane number and good spray characters in the biodiesel. According to Srivastava and Prasad ^[30], short term performances of both ionic and non-ionic micro-emulsions of aqueous ethanol in soybean oil was nearly as good as that of NO. 2 diesel fuel, in spite of the lower cetane number and energy content. NO. 2 diesel fuel is a fuel with distillation temperature of 640 degrees Fahrenheit at the 90% recovery point and meets the specifications defined in ASTM Specification D 975 ^[31].

However, as indicted by Parawira ^[32], continuous use of micro-emulsified diesel in engines causes problems like injector needle sticking, carbon deposit formation and incomplete combustion.

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2.3. Dilution/Blending

Direct uses of vegetable oils have generally been considered not satisfactory and impractical for both direct and indirect diesel engines. The high viscosity, acid composition, free fatty acid content, as well as gum formation due to oxidation and polymerization during storage and combustion, carbon deposits and lubricating oil thickening are obvious problems. In another view, Ma et al. ^[21], pointed out that oil deterioration and incomplete combustion are the two severe problems associated with the direct use of vegetable oils as fuels.

In such cases, it is helpful to dilute vegetable oils with such materials as diesel fuels, solvent or ethanol. Dilution results in reduction of viscosity and density of vegetable oils. Bilgin et al. ^[33] indicated that the addition of 4% ethanol to diesel fuel increases the brake thermal efficiency, brake torque and brake power, while decreasing the brake specific fuel consumption. They also argued that since the boiling point of ethanol is less than that of diesel fuel, it could assist the development of the combustion process through an unburned blend spray.

In their review of biodiesel production methods, Ma et al. ^[21] mentioned that, the viscosities of 50/50 (winter rapeseed oil and diesel) and 70/30 (whole winter rapeseed oil and diesel) blends were much higher (6–18 times) than NO. 2 diesel. According to them, a blend of 70/30 winter rapeseed oil and NO. 1 diesel fuel (A light distillate fuel oil that has distillation temperatures of 550 degrees Fahrenheit at the 90% recovery point and meets the specifications defined in ASTM Specification D 975 ^[31]) was used successfully to power a small single-cylinder diesel engine for 850 h. No adverse wear and no effects on lubricating oil or power output were noted.

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2.4. Transesterification

Transesterification is the main convenient method to produce biodiesel from oil and fat feedstock types, which chemically resembles petroleum diesel. Through this method, oils and fats (triglycerides) are converted to their alkyl esters with reduced viscosity to near diesel fuel levels. This product is thus a fuel with properties similar to petroleum based diesel fuel, which enable it be used in existing petroleum diesel engines without modifications. Generally, transesterification is a reversible reaction, which simply proceeds essentially by mixing the reactants usually under heat and/or pressure. However, if some kind of catalyst is added to the reaction, it will be accelerated. The simplest chemical reaction for transesterification of triglycerides is presented in Figure 3.

There are a number of ways to produce biodiesel through transesterification. The general schematics diagram for these possible ways is shown in Figure 4.

All of the catalytic transesterification processes involve the reaction of a triglyceride (fat or oil) with an alcohol in the presence of some catalyst to form esters and glycerol. A triglyceride has a glycerin molecule as its base with three long chain fatty acids attached. The characteristics of the oil/fat are determined by the nature of the fatty acids attached to the glycerin. The nature of the fatty acids can in turn affect the characteristics of the biodiesel ^[34].

A successful transesterification reaction for efficient biodiesel production is signified by easy and effective separation of the ester and glycerol layer after the reaction time. The heavier, co-product, glycerol can be purified for use in other industries, e.g. the pharmaceutical, cosmetics etc.

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2.4.1. Homogeneous acid catalyzed transesterification

Acid catalyzed transesterification was the first method ever in history to produce biodiesel (ethyl ester) from palm oil using ethanol and Sulfuric acid ^[35].

The acid catalyzed process is due to the reaction of a triglyceride (fat/oil) with an alcohol in the presence of acid catalyst to form esters (biodiesel) and glycerol. Specially, this method is convenient and economically viable in producing biodiesel from oil or fat resources with high free fatty acid content. However, the acid catalyzed reaction requires a longer reaction time and a higher temperature than the alkali catalyzed reaction ^[36].

Acid catalyzed transesterification starts by mixing the oil directly with the acidified alcohol, so that separation and transesterification occur in single step, with the alcohol acting both as a solvent and as esterification reagent ^[23].

The acid catalyzed transesterification should be carried out in the absence of water, in order to avoid the competitive formation of carboxylic acids which reduce the yields of alkyl esters ^[22]. Park et al. ^[37] did an investigation of the effect of water on transesterification of oleic acid with methanol in the presence of sulfuric acid as a catalyst. In their work, the yield of fatty acid methyl ester (FAME) was studied at oil to methanol molar ratios of 1:3 and 1:6 and reaction temperatures of 60 ℃ and 80 ℃. According to the result of their study, the rate of esterification of oleic acid significantly decreased as the initial water content increased to 20% of the oil ^[37].

Since transesterification is an equilibrium reaction, there should always be more alcohol than the oil to favor the forward reaction for complete conversion of the oil to alkyl ester. It is also known that the temperature and the amount of acid catalyst affect the transesterification rate and the yield of alkyl ester. However, more alcohol beyond the optimum will also cause some extra cost on separation of more produced glycerol from the alkyl ester and that is why there should always be an optimization of the ratio for efficient production. Different studies have been conducted to investigate how the molar ratio of oil to alcohol to acid as well as how temperature ranges affect the transesterification and thus the alkyl ester yield.

Zheng et al. ^[38] showed that, with convenient molar ratios and temperature ranges, methyl ester conversion of waste cooking oil in acid catalyzed transesterification can reach up to 99%. By their study, they concluded that, the oil: methanol: acid molar ratios and the temperature were the most significant factors affecting the yield of fatty acid methyl ester (FAME). According to their study result, at 70 ℃ with oil: methanol: acid molar ratios of 1:245:3.8, and at 80 ℃ with oil: methanol: acid molar ratios in the range 1:74:1.9–1:245:3.8, the transesterification was essentially a pseudo-first-order reaction as a result of the large excess of methanol which drove the reaction to completion (99 ± 1%) at 4 hours. In the presence of the large excess of methanol, free fatty acids present in the waste oil were very rapidly converted to methyl esters in the first few minutes under the above conditions ^[38].

Sulphuric acid, sulfonic acid, and hydrochloric acid are the usual acid catalysts but the most commonly used is sulphuric acid. There are also various studies done to see the yield effect of using alternative acids.

Soriano et al. ^[39] demonstrated that AlCl₃ could be used to catalyze the esterification of stearic acid suggesting that it is a potential alternative catalyst for biodiesel preparation using cheaper vegetable oil containing high amount of FFA. In their study, optimum conditions to afford 98% conversion of canola oil to FAME is with the use of methanol to oil molar ratio of 24:1 and reaction time of 18 h at 100 ℃ in the presence of 5% AlCl₃ as catalyst ^[39].

Marchetti et al. ^[40] mentioned that one of the drawbacks of producing biodiesel using acid catalyzed transesterification is having more amount of free glycerol in the biodiesel higher than the maximum value allowed to satisfy the international standard ASTM. However, they also mentioned in their study that this could be improved either by adding equipment for purification of the product stream or by modifying some of the process variable such as residence time in the reactor ^[40].

The use of co-solvents to overcome the mass transfer resistance due to immiscibility of alcohol with oil as well as the use of different acids usually help to get different alternatives for efficient production of biodiesel from low value feedstock. In this regard, Miao et al. ^[41] studied the effectiveness of trifluoroacetic acid catalyzed transesterification of soybean oil to produce biodiesel. The results from their study showed that the oil could be converted to biodiesel directly by one-step trifluoroacetic acid catalyzed process without extreme temperature and pressure conditions. The optimum process combination was 2.0 M (M is for Molarity, which is defined as the number of moles of solute dissolved per liter of solution) catalyst concentration with 20:1 molar ratio of methanol to oil at temperature of 120 ℃ ^[41]. According to the authors this procedure represents a simple and mild method for biodiesel production with short reaction time and with high conversion rate, which would offer potential for an industrial process^[41].

Despite its relatively slow reaction rate, the acid-catalyzed process offers benefits with respect to its independence from free fatty acid content and the consequent absence of a pretreatment step. These advantages favor the use of the acid-catalyzed process when using feedstock types like waste cooking oil as well as most non-edible plant oil, which are usually associated with higher content of fatty acid ^[42]. Table 2 indicates some selected research results about the effect of process variables on acid catalyzed transesterification of different feedstock types.

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2.4.2. Homogeneous alkaline catalyzed transesterification

The alkaline catalyzed transesterification process is the reaction of a triglyceride (fat/oil) with an alcohol in the presence of alkaline catalysts such as alkaline metal alkoxides and hydroxides as well as sodium or potassium carbonates to form esters (biodiesel) and glycerol. Alkali catalyzed transesterification is much faster than acid catalyzed transesterification and is less corrosive to industrial equipment and therefore is the most often used commercially ^[26,46]. However, presence of water and high amount of free fatty acid in a feedstock gives rise to saponification of oil and therefore, incomplete reaction during alkaline transesterification process with subsequent formation of emulsion and difficulty in separation of glycerol ^[32]. The saponification reaction is represented by the equation shown below:

The main disadvantage resulted due to saponification reaction is the consumption of catalyst and increased difficulty in separation process, which leads to high production cost. In addition to that, formation of water in the product will also inhibit the reaction. In this case, water generated either from vegetable oil (due to its high water content) or formed during saponification reaction will hydrolyze triglyceride to form more free fatty acid as shown in the equation below.

Generally, base catalyst manifest much higher catalytic activity than acid catalysts in the transesterification reaction, but are selectively suitable for deriving biodiesel only from refined oils having low content of free fatty acids (FFA) usually less than 0.5% ^[48]. This makes base catalyzed transesterification confined to use only best quality refined oil like vegetable cooking oil for input, which in turn makes it expensive way to produce biodiesel while creating food versus energy controversy. Here we can consider esterification as additional step to decrease the free fatty acid content of feedstock with greater than 0.5% FFA. This will enable us to choose among different feedstock types with higher FFA content. However, this additional process usually makes it more complex in the instrumentation (because of the addition of esterification unit) than the sole alkaline-catalyzed process, thereby resulting in an increase in equipment and operating costs.

The efficient production of biodiesel using base catalyzed transesterification is not only dependent on the quality of the feedstock, it is also dependent on the crucial reaction operation variables such as alcohol to oil molar ratio, reaction temperature, rate of mixing, reaction time, type and concertation of catalyst and also on the type of alcohol used ^[19,25,49].

Even though in theory, the stoichiometric ratio of alcohol (usually methanol) to oil is 3:1, in order to assist the forward reaction so that to get more conversion, the concentration of the methanol has to be increased. This is because; lower amount of methanol means slower forward reaction and less percentage of yield. In contrary, high methanol amount beyond the optimum, interfere with the separation of glycerin because of an increase in solubility; the glycerin remaining in the solution drives the equilibrium back to the left side of reaction, resulting in the lower yield of esters. This is due to the fact that methanol, with one polar hydroxyl group, can act as an emulsifier that enhances emulsions ^[19].

Sodium hydroxide, potassium hydroxide and sodium methoxide are catalysts usually used in base catalyzed transesterification. Sodium hydroxide is mostly preferable owing to its intermediate catalytic activity and a much lower cost ^[50]. Lueng et al. ^[51] evaluated the effect of catalyst on transesterification by comparing the maximum ester content and yield percentage attained using three base catalysts while other determinant variables were kept the same for all conditions during the base catalyzed transesterification processes. The result of their study is shown in Table 3.

This study revealed that sodium hydroxide is better in attaining purity percentage (ester content) than the others whereas sodium methoxide is good in providing higher yield percentage.

The relative concentration of catalysts required for maximum production is dependent on the type of feedstock used. Dias et al. ^[52] compared the performance of three alkali catalysts for transesterification of virgin and waste soybean and sunflower oil and they reported that, the optimum conditions which ensured that the attainment of the final product being in agreement with the European biodiesel standard were: 0.6 (wt.%) CH₃ONa for both virgin oils; 0.6 (wt.%) NaOH for sunflower oil and 0.8 (wt.%) for soybean oil and; 0.8 (wt.%) using both sodium based catalysts for waste frying oils. They also reported that under these optimum conditions, a purity of 99.4 (wt.%) could be obtained.

In the study carried out by Hossain and Boyce ^[53] in spite of higher yield, using NaOH as catalyst during biodiesel synthesis from waste sunflower cooking oil, causes more emulsion than KOH and makes separation of biodiesel from glycerin complicated, as they reported. The solution of alkaline catalyst in methanol is recommended to be prepared freshly in order to avoid the moisture absorbance and to maintain the catalytic activity ^[54].

Leung et al. ^[51] reported that the conversion of waste cooking oil using sodium hydroxide catalysts was approximately 86%. As presented in Table 4, Ojolo et al. ^[55], explained that the increase of the catalyst concentration influences the ester yield in a positive manner up to 0.80% NaOH for Jatropha oil and then after that it decreases.

Parawira ^[32] reported that, the alkaline catalyst concentration in the range of 0.5–1% by weight yield 94–99% conversion of most vegetable oils into esters. There are several disadvantages in using an alkaline catalysis process although it gives high conversion levels of triglycerides to their corresponding methyl esters in short reaction times. According to Parawira ^[32], the process is energy intensive, recovery of glycerol is difficult, the alkaline catalyst has to be removed from the product, alkaline wastewater generated requires treatment and the level of free fatty acids and water greatly interfere with the reaction. The risk of free fatty acid or water contamination results in soap formation that makes the separation process difficult ^[56].

Table 5 summarizes the results of some researches done to study the effect of different process variables on base catalyzed transesterification of different feedstock types.

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2.4.3. Heterogeneous catalyzed transesterification

The use of homogeneous catalysts, especially base catalysts, are associated with some amount of difficulty in purification of by-product glycerol as well as in the requirement of wastewater treatment. To alleviate these problems, the use of heterogeneous catalysts usually solid base catalyst is recommended. Solid base catalysts have many advantages, such as having mild reaction condition, easy separation, and high activity and less contaminant ^[66].

Many researchers argued that the use of heterogeneous catalysts both in acid and base form brought about the advantage of having easy and less costly separation as well as possibility of reusing the catalyst. Parawira ^[32] mentioned that, the heterogeneous catalyst eliminates the additional cost associated with the homogeneous sodium hydroxide to remove the catalyst after transesterification. In addition, the heterogeneous catalyst offers a wide option for the catalytic selection because of its high selectivity and reusability characteristics ^[67]. Dell'Anna et al. ^[68] investigated transesterification of polyunsaturated compounds catalyzed by a recyclable polymer supported palladium catalyst. They found out that the heterogeneous solid catalyst, palladium exhibited a remarkable activity and was reusable for eight consecutive cycles.

Heterogeneous solid catalysts are usually categorized as acid solids capable to catalyze free fatty acids esterification reaction; base solids, which are able to catalyze triglycerides transesterification reaction; and bifunctional solids (acid-base character) which show ability to simultaneously catalyze esterification and transesterification reaction ^[69].

There are various efforts made to find effective solid catalysts in both acid and basic form for heterogeneous catalyzed process. Bournay and Casanave ^[70] investigated the use of new solid catalyst for continuous transesterification process. They mentioned that this new solid catalyst consists of a mixed oxide of zinc and aluminum, which promotes the transesterification reaction without catalyst loss. Actually using this new catalyst, the reaction has to be performed at higher temperature and pressure than homogeneous catalysis processes, with an excess of methanol, which can finally be removed by vaporization and recycled to the process ^[70]. In contrary, however, there are some solid metal oxides, such as oxides of tin, magnesium, and zinc, which are well known as catalysts, and perform like homogeneous catalysis and end up as metal soaps or metal glycerates ^[70].

Bournay and Casanave ^[70] claimed also that, while using this new solid catalyst, neither catalyst recovery nor aqueous treatment steps are required. The purification steps of products could then be much more simplified so that very high yields of methyl esters, close to the theoretical value, could be obtained. In addition, the glycerin can directly be produced with high purity levels (at least 98%) without any salt contaminants.

Heterogeneous catalysts such as amorphous zirconia, titanium and potassium zirconias have also been used for catalyzing the transesterification of vegetable oils. Furuta et al. ^[71] evaluated amorphous zirconia catalysts, titanium-, aluminum-, and potassium-doped zirconias, in the transesterification of soybean oil with methanol at 250 ℃, and the esterification of n-octanoic acid with methanol at 175–200 ℃. They reported that, titanium-and aluminum-doped zirconias are promising solid catalysts for the production of biodiesel fuels from soybean oil because of their high performance, with over 95% conversion in both of the reactions.

In another study, Huaping et al. ^[72] demonstrated the potential of preparing biodiesel from Jatropha curcas oil catalyzed by solid super base of calcium oxide. When treated with ammonium carbonate solution and calcinated at high temperature, calcium oxide becomes a solid super base, which shows high catalytic activity in transesterification. They reported that, under the optimum conditions, the conversion of Jatropha curcas oil can reach 93%.

Sánchez et al. ^[73] studied the influence of the reaction temperature, the alcohol:oil ratio and the catalyst percent on the methanolysis of Jojoba oil using CaO as a catalyst, which was particularly derived from mussel shells. According to their study, the variables which had the higher positive effect on the methanolysis of Jojoba oil, in a pressurized environment (with approximate 10 bars in Parr reactor), are the methanol:oil ratio and the temperature, whereas the catalyst percent had a slight negative impact on the process. They reported that, using this catalyst, the reaction time could be reduced by half, from 10 to 5 hours and the Jojoba oil conversion reached a maximum of 96.3% with a pressurized environment in the reactor.

Avhad et al. ^[74] investigated the catalytic activity of glycerol-enriched calcium oxide for ethanolysis of avocado oil. The calcium oxide catalyst was derived from Mytilus Galloprovincialis shells through thermal and glycerol (with glycerol dosage of 10% with respect to catalyst weight) treatment before using for the ethanolysis reaction. This shell is simply a waste generated from the fish industry. In this study, they examined the influence of temperature, ethanol-to-oil molar ratio, and the catalyst amount on the variation in the concentration of triacylglycerols and biodiesel with reaction time. They also determined the interaction between the reaction variables (ethanol-to-oil molar ratio and catalyst amount), their influence on the ethanolysis process, and the optimum variables affecting the process through the response surface methodology. According to their conclusion, both catalyst amount and ethanol-to-oil molar ratio significantly affected the described ethanolysis process. They also reported that, temperature of 75 ℃, ethanol-to-oil molar ratio of 9:1, and 7 wt.% catalyst amount was taken to be suitable for the studied glycerol-enriched CaO assisted avocado oil ethanolysis process.

The sensitivity of the base catalyzed transesterification to the FFA and water content of the feedstock still persist as the main problem in case of heterogeneous base catalyzed transesterification reaction too. Again, to solve such feedstock quality problem, solid acid catalyst for simultaneous esterification of FFAs and transesterification of triglycerides can be good alternative for biodiesel production from feedstock with higher FFA and water content. Moreover, Melero et al. ^[75] mentioned that a heterogeneous acid catalyst if incorporated into a packed bed continuous flow reactor, can simplify product separation and purification and reducing waste generation. Solid acid catalyst can be recycled, easily removed and can simultaneously catalyze esterification and transesterification ^[76].

Peng et al. ^[76] characterized and studied the activity of a solid acid catalyst comprising SO₄²⁻/TiO₂–SiO₂ for the production of biodiesel from several low cost feedstocks with high FFAs. They studied the influence of reaction parameters and found out that optimum yield could be attained at reaction temperature of 200 ℃, molar ratio of methanol to oil 9:1 and catalyst concentration of 3wt.%. They finally concluded that, the solid acid catalyst SO₄²⁻/TiO₂–SiO₂ is inexpensive and environment friendly, has high catalytic activity, and is stable for biodiesel production from cheap raw feedstocks with high FFAs.

In another study, Juan et al. ^[77] carried out transesterification of refined and crude vegetable oils with a sulfonic acid-modified mesostructured catalyst. According to them the catalyst could enable to yield fatty acid methyl ester (FAME) with purity over 95 wt.% and oil conversion close to 100% under best reaction conditions of temperature 180 ℃, methanol/oil molar ratio 10, and catalyst loading 6 wt. %. They reported that, regardless of the presence of free fatty acids, the sulfonic acid-modified mesostructured catalyst showed high activity towards simultaneous esterification and transesterification.

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2.4.4. Lipase catalyzed transesterification

The other way of transesterification of oils and fats for biodiesel production is using enzymes in which there is no problem of saponification, purification, washing and neutralization so that it is always a preferred method from these perspectives. Enzymatic catalysts can also be applied on a feedstock with high FFA and can convert more of the oil into biodiesel. However, the problems associated with enzyme catalysts are their higher cost and longer reaction time ^[19]. Usually because of these two drawbacks, enzyme catalyzed transesterification method is not very frequently used.

In another view, it is more frequently pointed out that enzymatic transesterification has currently attracted much attention for biodiesel production as it produces high purity product (esters) and enables easy separation from the by-product, glycerol ^[26,78]. The enzymes that are usually found to be capable of catalyzing transesterification are the lipases.

The lipase catalyzed transesterification process is the reaction of a triglyceride (fat/oil) with an alcohol in the presence of lipase enzyme as a catalyst to form esters (biodiesel) and glycerol. Mahanta et al. ^[22] mentioned that, in a lipase catalyzed process no complex operations are needed not only for the recovery of glycerol but also in the elimination of catalyst and soap. This is an environmentally more attractive option to the conventional process. However, Mahanta et al. ^[22] again argued that the reaction yields as well as the reaction times are still unfavorable compared to the alkaline catalyzed reaction systems.

Lipases for their transesterification activity on different oils can be found from different sources. Ability to utilize all mono, di, and triglycerides as well as the free fatty acids, low product inhibition, high activity and yield in non-aqueous media, low reaction time, reusability of immobilized enzyme, temperature and alcohol resistance are the most desirable characteristics of lipases for transesterification of oils for biodiesel production ^[79].

Some also argue that, biocompatibility, biodegradability and environmental acceptability of the biotechnological procedure when using lipase as a catalyst are the desired properties in this alternative biodiesel production method ^[46,78]. However, the use of extracellular lipase as a catalyst requires complicated recovery, purification and immobilization processes for industrial application ^[80]. Consequently, the direct use of whole cell biocatalyst of intracellular lipases has received considerable research efforts ^[78]. For the industrial transesterification of fats and oils, Pseudomonas species immobilized with sodium alginate gel can be used directly as a whole cell bio-catalyst ^[78].

Devanesan et al. ^[78] reported maximum yield (72%) of biodiesel from transesterification of Jatropha oil and short chain alcohol (methanol on hexane) using Pseudomonas fluorescens immobilized with sodium alginate gel at the optimum conditions of 40 ℃, pH 7.0, molar ratio of 1:4, amount of beads of 3 g and reaction time of 48 h.

According to Parawira ^[32], in all the work in literature on lipases, the enzymes or whole cells are immobilized when used for catalysis. It is usually mentioned that, the advantage of immobilization is that the enzyme can be reused without separation. In addition, the operating temperature of the process is low (50 ℃) compared to other techniques which operate at harsh conditions. However, the cost of enzymes remains a barrier for its industrial implementation ^[81].

Most of the time, in order to tackle this cost barrier of the process, the enzyme (both intracellular and extracellular) should be reused by immobilizing in a suitable support particle, which is usually associated with considerable increase in efficiency.

Enzymes are usually immobilized for better enzyme loading, activity and stability. Selecting and designing the support matrix are important in enzyme immobilization ^[82]. With this respect, there are a number ways to immobilize enzymes. These include cross-linked enzyme aggregates, microwave-assisted immobilization, click chemistry technology, mesoporous supports and most recently nanoparticle-based immobilization of enzymes ^[83]. Recently, the use of nanoparticles has emerged as a versatile tool for generating excellent supports for enzyme stabilization due to their small size and large surface area ^[82], which results in better stability and activity of enzymes immobilized on such materials. In addition, Nanoparticles strongly influence the mechanical properties of the material like stiffness and elasticity and provide biocompatible environments for enzyme immobilization ^[82].

However, during transesterification, the activity of immobilized enzyme is inhibited by methanol and glycerol which are always present in the reacting mixture. The use of tert-butanol as solvent, continuous removal of glycerol, stepwise addition of methanol are some of the ways to reduce the inhibitory effects thereby increasing the cost effectiveness of the process ^[26].

Guang Jin et al. ^[81] examined the use of whole-cell biocatalysts to produce biodiesel at room temperature (25 ℃). They used Rhizopus oryzae (ATCC^® 10260^TM) to catalyze the conversion of virgin and waste oils (triglycerides) into biodiesel fuel in the presence of 15% water. Their research results indicated that the whole-cell biocatalyst could produce about a 90% yield of fatty acid methyl ester from virgin oil, and nearly complete conversion of the remaining oil into free fatty acid, using a 96-hour reaction at room temperature (25 ℃). They also reported that, in a 72 hour reaction, fatty acid methyl ester yields were about 75% for virgin oil, 80% for waste vegetable oil, and 55% for brown (trap) grease ^[81], which implies, whole-cell biocatalysts may be an effective way to trans esterify waste oils or greases that are high in FFA and difficult to dewater ^[81].

Du et al. ^[84] developed methyl acetate, a novel acyl acceptor, for biodiesel production and carried out a comparative study on Novozym 435 catalyzed transesterification of soybean oil for biodiesel production with different acyl acceptors. They reported that methanol has a serious negative effect on enzymatic activity as for example a molar ratio of methanol to oil of above 1:1 leads to serious inactivation of the enzyme. In their study, they used methyl acetate as the acyl acceptor, and reported that, a yield of 92% of methyl ester could be obtained with a molar ratio of methyl acetate to oil of 12:1, and methyl acetate showed no negative effect on enzymatic activity. They also mentioned that, with crude soybean oil as the oil source and methanol as acyl acceptor, a much lower methyl ester yield was obtained than that with refined soybean oil, while with methyl acetate as acyl acceptor, an equally high yield of methyl ester (92%) was achieved for both soybean oils. Lipase loses its activity very rapidly during repeated experiments with methanol as the acyl acceptor, however, according to their report, there was almost no detected loss in lipase activity, even after being continuously used for 100 batches, when methyl acetate was used for biodiesel production ^[84].

During enzymatic transesterification for biodiesel production, it has been demonstrated that excessive methanol present in the reaction medium would cause significant deactivation of the lipase. However, effective methanolysis using extracellular lipase has been reported to improve by stepwise addition of methanol (usually, a three-step addition of methanol in solvent-free medium) through which, according to Watanabea, 90–95% conversion can be achieved even after 50 and 100 cycles of repeated operation ^[85].

In another study, Li et al. ^[86] used tert-Butanol, as a novel reaction medium, for lipase-catalyzed transesterification of rapeseed oil for biodiesel production, with which, they claimed, both the negative effects caused by excessive methanol and by-product glycerol could be eliminated. They reported the highest biodiesel yield of 95% achieved under the optimum conditions of tert-butanol/oil volume ratio 1:1; methanol/oil molar ratio 4:1; 3% Lipozyme TL IM and 1% Novozym 435 based on the oil weight; temperature 35 ℃; 130 rpm, and 12 hours. According to them, there was no obvious loss in lipase activity even after being repeatedly used for 200 cycles with tert-butanol as the reaction medium ^[86].

Shah et al. ^[87] also worked on three different lipases (Chromobacterium viscosum, Candida rugosa, and Porcine pancreas) for a transesterification reaction of Jatropha oil in a solvent-free system to produce biodiesel. They reported that, only lipase from Chromobacterium viscosum was found to give appreciable yield. Immobilization of lipase (Chromobacterium viscosum) on Celite-545 enhanced the biodiesel yield to 71% from 62% yield obtained by using free tuned enzyme preparation with a process time of 8h at 40 ℃. Further addition of water to the free (1%, w/v) and immobilized (0.5%, w/v) enzyme preparations enhanced the yields to 73% and 92%, respectively. They mentioned also that, immobilized Chromobacterium viscosum lipase can be used for ethanolysis of oil. According to their conclusion immobilization of lipases and optimization of transesterification conditions resulted in adequate yield of biodiesel in the case of the enzyme-based process.

Some authors also argued that using convenient reaction medium would help increase conversion of oil to biodiesel in lipase-catalyzed transesterification reactions. In this respect, Ha et al. ^[88] demonstrated production of biodiesel through immobilized Candida antarctica lipase-catalyzed methanolysis of soybean oil in 23 different ionic liquids. They reported that, the highest fatty acid methyl esters (FAMEs) production after 12 h at 50 ℃ was achieved in EmimTfO (1-Ethyl-3-methylimidazolium trifluoromethanesulfonate). They also pointed out that around 15% higher free fatty acid production could be achieved using this ionic fluid as a reaction medium than the production system using tert-butanol as an additive. Table 6 summarizes results of some selected researches done to optimize production of biodiesel through lipase-catalyzed transesterification of different feedstock types.

According to Marchetti et al. ^[46], the use of lipase is a great viable method for production of ester from different sources of oil or grease even though, research on this topic is still in progress due to the enzyme flexibility and adaptability to new process.

However, when we compare enzymatic production of biodiesel with conventional chemical processes, the major obstacles repeatedly mentioned are the cost of lipases, the relatively slower reaction rate and lipases inactivation caused by methanol and glycerol. Some main advantages and disadvantages of using lipases as catalyst are summarized in Table 7.

In general, process optimization in lipase-catalyzed transesterification, can be done at least in the following: screening of various commercial lipase preparations; pH tuning; immobilization; adjusting water content in the reaction media; adjusting amount of enzyme used; and adjusting temperature of the reaction ^[90].

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2.4.5. Nano catalyzed transesterification

There a number of recent developments in catalytic conversion of oils and fats to biodiesel. Among them biodiesel production using Nano catalyst and Ionic liquid catalysts are more promising in terms of few advantages over the conventional acid/base catalysts.

Nano catalysis involves the use of nanomaterials as catalysts for a variety of homogeneous and heterogeneous catalysis applications. Nanoscale catalysts have high specific surface area and surface energy resulting in high catalytic activity. Generally, Nano catalysts improve the selectivity of the reactions by allowing reaction at a lower temperature, reducing the occurrence of side reactions, higher recycling rates and recovery of energy consumption ^[91]. With this respect, Nano catalysts are promising alternatives for efficient production of biodiesel from oils and fats as they have high specific surface area and high catalysis activities eliminating the specific problem of mass transfer resistance associated with conventional catalysts.

Wen et al. ^[92] studied that the solid base Nano catalyst KF/CaO can be used for biodiesel production with yield of more than 96%. This catalyst can efficiently be used to convert the oil with higher acid value into biodiesel ^[92,93]. It is porous with particle sizes of 30–100 nm. Wen et al. ^[92] could show, using X-ray powder diffraction analysis, that the Nano catalyst KF/CaO has new crystal KCaF₃, which increases catalytic activity and stability. The high specific surface area and large pore size are favorable for contact between catalyst and substrates, which effectively improve efficiency of transesterification ^[93].

Few have studied on the possible optimum conditions for production of biodiesel from different oil inputs using different Nano catalysts. Sidra et al. ^[94] investigated the production of biodiesel from Jatropha oil through transesterification process by using CaO-Al₂O₃ Nano catalyst. According to them, the optimization results for production of biodiesel from the transesterification of Jatropha curcas oil catalyzed by CaO-Al₂O₃ nanoparticles showed maximum yield of 82.3% at 5:1 methanol to oil molar ratio ^[94]. Similarly, Sivakumar et al. ^[95] did comprehensive study of smoke deposited Nano sized MgO as a catalyst for biodiesel production. They studied the transesterification reaction to determine the optimum conditions for different parameters like catalyst quantity, methanol oil molar ratio, reaction temperature and reaction time. A maximum conversion of 98.7% was obtained at 1.5 wt.% catalyst; 5:1 methanol oil molar ratio at 55 ℃, achieved after 45 min. The conversion was three to five times higher than those reported for laboratory MgO in literature ^[95]. According to this study, the higher conversion was mainly due to the enhancement of surface area of the catalyst. The other advantage is that the catalyst can be easily recovered and reused up to eight times with easy regeneration steps ^[95]. Table 8 shows some of optimization studies on biodiesel production using Nano catalysts.

In another two separate studies Mookan et al. ^[96,97] conducted an investigation on the fuel quality of biodiesel produced from castor oil as well as Pongamia pinnata oil with methanol using iron nanoparticles as catalyst. The fuels properties that they investigated are specific gravity, kinematic viscosity, flash point, cloud point, water content, carbon residue, refractive index, copper corrosion and calorific value. They pointed out that, these properties of resulting biodiesel both from castor oil and Pongamia pinnata oil agree well with the specifications of biodiesel standards ASTM D6751 except for specific gravity and kinematic viscosity. According to them, most of the physico-chemical properties of castor oil biodiesel match well with the normal diesel. They also concluded that the use of iron nanoparticles as catalyst showed more advantages than the conventional acid/base catalyst for the production of biodiesel in terms of shorter reaction time as well as less amount and reusability of the catalyst.

More recently, Gupta et al. ^[98] did a research on the preparation and characterization of CaO nanoparticle for biodiesel production from Soybean oil. They synthesized Nanoparticle of CaO from calcium Nitrate (CaO/CaN) and Snail shell (CaO/SS) so as to investigate the performance of the catalysts in terms of biodiesel yield. According to their conclusion, the Nano catalyst from snail shell exhibits excellent catalytic activity and stability for the transesterification reaction, which suggested that this catalyst would be potentially used as a solid base Nano catalyst for biodiesel production ^[98].

The catalytic activity of such Nano catalysts are usually affected by calcination temperature during catalyst preparation with calcination. This is because in the preparation process of the catalyst, calcination treatment of catalyst at high temperature is favorable for the interaction between support and active ingredient, which generates new active sites for the catalyst ^[99]. With this respect Hu et al. ^[99], did an investigation on the optimum calcination temperature for the preparation of three Nano catalysts, KF/CaO–Fe₃O₄, KF/SrO–Fe₃O₄ and KF/MgO–Fe₃O₄. And they found out that the fatty acid methayl ester yield reaches the maximum value at calcination temperature of 600 ℃, 600 ℃, and 500 ℃ for KF/CaO–Fe₃O₄, KF/SrO–Fe₃O₄ and KF/MgO–Fe₃O₄, respectively ^[99]. Thus, calcination temperature can be taken as additional parameter to optimize biodiesel production using Nano catalysts prepared through calcination.

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2.4.6. Transesterification using ionic liquids as catalysts

Ionic liquids are organic salts comprising of anions and cations that are liquid at room temperature. The cations are responsible for the physical properties of ionic liquids (such as melting point, viscosity and density), while the anion controls its chemical properties and reactivity ^[101]. Their unique advantage is that while synthesized, they can be moderated to suit required reaction conditions.

Another great advantage of using Ionic Liquids specifically to catalyze transesterification for biodiesel production is the formation of a biphasic system at the end of the reaction. This biphasic system occurs because the ionic liquid, insoluble in the organic phase, remains in the aqueous phase along with alcohol, the catalyst used and glycerol produced during the reaction ^[102]. This makes it very easy to separate the final products, because most of the top phase is biodiesel with very little amount of methanol. Pure biodiesel can then be isolated by simple vacuum evacuating of this very little amount of methanol ^[103]. The bottom phase contains methanol, glycerol and Ionic Liquids. This bottom phase can then be rinsed with water for 3 to 4 cycles to separate glycerol with high purity ^[104], or pure glycerol can be obtained simply by distillation ^[103], which leaves the pure Ionic Liquid behind for further direct use for another reaction.

Among the different possible types of ionic liquids for catalysis of transesterification reaction for biodiesel production, Ionic liquids composed of the 1-n-butyl-3-methylimidazolium cation are the most widely studied and discussed compounds ^[105].

Very few researches are done to investigate the optimum reaction conditions for ionic liquid catalyzed transesterification with respect to temperature, molar ratio, catalyst amount, etc. Guo et al. ^[106] did a research to find out such optimum conditions for ionic liquid catalyzed transesterification of soybean oil with methanol by using ultrasound (24 kHz, 80 W). They found out that, at 60 ℃ under ultrasonic irradiation and a molar ratio of 14:1 methanol to oil, a biodiesel yield in excess of 96% can be achieved in a remarkably short time duration of 20 minutes or less in comparison to 5h or more using conventional method. They concluded that ionic liquid catalyzed transesterification is proved to be efficient and time saving for the preparation of biodiesel from soybean oil. They also mentioned the ionic liquid had a good reusability and can be easily separated from the biodiesel by simple decantation ^[106].

In another study, Ren et al. ^[103] investigated the influence of some reaction conditions, such as the amount of morpholine alkaline basic ionic liquid 1-butyl-3-methyl morpholine hydroxide ([Hnmm]OH) as a catalyst, the molar ratio of methanol to soybean oil, reaction temperature and time. The optimum reaction conditions to get the maximum biodiesel yield (97.0%) were found as 4% (mass fraction) of [Hnmm]OH, the methanol to soybean oil molar ratio of 8, temperature of 70 ℃ and reaction time of 1.5 hours. They pointed out that, the reaction exhibited high stability upon recycling, and the biodiesel yield remained more than 90% even after being reused for five times ^[103]. Table 9 shows the results of some few researches done on optimized reaction conditions for ionic liquid catalyzed transesterification.

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2.4.7. Supercritical transesterification

One of the approaches to overcome problems associated with poor immiscibility between the reactants and at the same time, technical problems caused by catalysts is to use supercritical method. Supercritical alcohol transesterification reaction takes place under extremely high temperature and pressure. When a gas or liquid is under high pressure and temperature beyond its critical point, unusual phenomena are exhibited on its properties. In this case, liquid and vapor phase are no longer confined under these conditions and single supercritical fluid phase is generated ^[19]. In the supercritical transesterification method, methanol and oil, which are immiscible liquids at room temperature, will form a homogenous fluid. This is due to the sharp drop in the solubility of methanol and reduction in dielectric constant, which makes methanol a non-polar substance ^[19]. In this case, the reaction will be accelerated, as there is no mass transfer limitation under such conditions.

When we consider specific application of the mothed for biodiesel production, super critical methanol is usually used to speed up the transesterification reaction. Using this technique, the conversion of vegetable oils into biodiesel is done in about 4 min but extremely high pressure and temperature is required for this method, which makes it highly sensitive and costly ^[9]. A lot of energy is required to build such a high pressure and temperature. Some authors recommend use of co-solvent to improve the conversion efficiency.

The simple chemical catalyzed transesterification processes mentioned above (Acid catalyzed and Base catalyzed) are confronted with two problems, i.e. the processes are relatively time consuming and need separation of the catalyst and saponified impurities from the biodiesel. The first problem is due to the phase separation of the oil/alcohol mixture, which can be alleviated by vigorous stirring. These problems are totally mislaid in the supercritical method of transesterification. This is perhaps due to the fact that the tendency of two phase formation of vegetable oil/alcohol mixture is not encountered and a single phase is found due to decrease in the dielectric constant of alcohol in the supercritical state ^[9]. In general, the supercritical methanol process, which is non-catalytic, is simpler in purification, takes lower reaction time and lower energy use than the common commercial process^[16,51].

It is argued that supercritical transesterification, as an alternative technology, satisfies all the requirements to produce biodiesel suitable to be used on normal engines. In addition, it produces more than a kilo of fuel per kilo of oils used ^[40]. According to a study done by Marchetti et al. ^[109], the techno-economic analysis of the supercritical process shows that, although the supercritical alternative appears as a good technical possibility to produce biodiesel, today, it is not an economic alternative due to its high operating cost. However, there is still a possibility in reducing the operating cost and making the method more economically advantageous too. Kasteren and Nisworo ^[110] have proved this by using one reaction step in the process as well as propane as a co-solvent in supercritical biodiesel production plant so as to decrease operating cost.

In another view, Atabania et al. ^[111] pointed out that, supercritical methanol method uses lower energy and completes in a very short time (2–4 minutes) compared to catalytic transesterification. According to them, since no catalyst is used, the purification of biodiesel and the recovery of glycerol are much easier, trouble free and environment friendly. However, the method has a high cost in reactor and operation (due to high pressures and high temperatures), and high methanol consumption (e.g., high methanol/crude-oil molar ratio of 40/1) ^[9].

Similarly, Marulanda ^[13] carried out a lab scale experiment on biodiesel production process by supercritical transesterification in a continuous reactor working at a 9:1 methanol to triglycerides molar ratio and 400 ℃. The results of this study showed that for a specified biodiesel production plant capacity set at 10, 000 tons/year the total energy consumption of this specific process (573 kW) was considerably lower than another supercritical process working at a 42:1 molar ratio and 300 ℃ (2407 kW), and the conventional base catalyzed process (2326 kW).

Different studies done on investigation of optimum condition for supercritical transesterification process agree that among the determinant variables temperature has the highest impact on yields, followed by reaction time and pressure. Kiss et al. ^[112] have done a series of experiments with ethyl alcohol to the effect of temperature, time and pressure. They found that, by increasing the reaction time at 350 ℃ and 12 MPa, yield increases during the whole range (from 63.36% to 93.22%). After reduction of pressure at 350 ℃ temperature, the maximum yield (80.1%) was reached within 30 min. They concluded that, in general, lowering the pressure would result in yield decrease. By reducing the reaction temperature (350–250 ℃), the yield decreases which can in general be compensated with prolonging the duration of reaction. They attained lowest yield at a minimum temperature, minimum pressure and short reaction time (250 ℃, 8 MPa, 7 min; the yield is 14.8%) ^[112]. Table 8 summarizes the results of some selected researches done on optimization of reaction conditions for supercritical transesterification of different feedstock types.

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2.4.8. Comparison between transesterification techniques

As it has been tried to clearly put in the different research works reviewed in this paper, the different transesterification techniques do have their own advantages and disadvantages. These advantages and disadvantages can be seen with respect to cost of input material, degree of waste generation, cost of production, product purity, yield percentage, environmental and health hazard and the like. Table 9 shows a summary on advantages and disadvantages of these major transesterification techniques as well as required character of suitable feedstock for each method.

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3. Conclusion

Even though biodiesel is a good alternative over petroleum diesel in various aspects, it is always jeopardized by the high cost of feedstock and absence of economically and technically viable technology for its efficient production from any feedstock type.

Different researchers recommend different methods of biodiesel production, which are usually dependent on properties of the feedstock. Among the methods to change fat and oil to biodiesel, the most convenient one is the transesterification or also called alcoholysis reaction. There are a number of techniques used to carry out transesterification of fat/oil for biodiesel production, each of which requiring specific feedstock property and optimum operating condition for efficient production of biodiesel.

From the review of different works, it can be concluded that:

(1) Homogenous acid catalyzed transesterification is relatively insensitive to FFA content of the feedstock, is relatively less energy intensive but requires higher temperature operation and the biodiesel produced usually has more amount of free glycerol (low purity percentage).

(2) Homogeneous base catalyzed transesterification is very sensitive to FFA and water content and thus is very selective in feedstock type. The reaction is fast, the catalysts are relatively cheap, and thus it is usually applied at industrial scale for biodiesel production.

(3) Heterogeneous acid catalyzed transesterification avoids the problem of product separation and purification and enable reuse of the catalyst. However, it relatively requires high alcohol to oil molar ratio and long reaction time.

(4) Heterogeneous base catalyzed transesterification reduces process stages and wastes, and enables easy catalyst separation and reuse. However, catalyst might possibly be poisoned when exposed to ambient air so that not environmentally friendly.

(5) Lipase catalyzed transesterification is insensitive to FFA and water content, carried out at low temperature, and convert more amount of the feedstock to biodiesel. Nevertheless, it is costly due to expensiveness of the enzymes and takes longer time to have good yield.

(6) Nano catalyzed transesterification is insensitive to FFA and water content, carried out relatively at low temperature and takes short time. The catalyst can be reused many times providing cost benefits. However, it requires more alcohol for effective yield and in some cases preparation of appropriate catalysts is expensive.

(7) Ionic Liquids catalyzed transesterification enables easy separation of final products due to formation of biphasic thus reducing process cost and be efficient and time saving. In addition, it enables modulating desired properties of catalysts while preparing them. Catalysts have high catalytic activity, excellent stability and can also be easily separated and reused many times. However it requires relatively more alcohol for effective yield and usually expensive to have ionic liquids.

(8) Supercritical transesterification is insensitive to FFA and water content of feedstock and thus enable to use wider feedstock types, usually takes shorter time and produces more fuel amount per feedstock mass. However, it requires higher temperature and pressure and consumes more methanol so that it is not economically profitable due to its high operating cost.

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Acknowledgements

The authors would like to thank Norwegian University of Life Sciences, NORAD and NORHED for their financial support.

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Conflict of Interest

All authors declare no conflicts of interest in this paper.

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