Biodiesel separation and purification: A review

Renewable Energy 36 (2011) 437e443 Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene Review Biodiesel separation and purification: A review I.M. Atadashi, M.K. Aroua*, A. Abdul Aziz Chemical Engineering Department, Faculty of Engineering, University Malaya, 50603 Kuala Lumpur, Malaysia a r t i c l e i n f o a b s t r a c t Article history: Received 4 January 2010 Accepted 28 July 2010 Available online 23 August 2010 Biodiesel as a biodegradable, sustainable and clean energy has worldwide attracted renewed and growing interest in topical years, chiefly due to development in biodiesel fuel and ecological pressures which include climatic changes. In the production of biodiesel from biomass, separation and purification of biodiesel is a critical technology. Conventional technologies used for biodiesel separation such as gravitational settling, decantation, filtration and biodiesel purification such as water washing, acid washing, and washing with ether and absorbents have proven to be inefficient, time and energy consumptive, and less cost effective. The involvement of membrane reactor and separative membrane shows great promise for the separation and purification of biodiesel. Membrane technology needs to be explored and exploited to overcome the difficulties usually encountered in the separation and purification of biodiesel. In this paper both conventional and most recent membrane technologies used in refining biodiesel have been critically reviewed. The effects of catalysts, free fatty acids, water content and oil to methanol ratios on the purity and quality of biodiesel are also examined. Ó 2010 Elsevier Ltd. All rights reserved. Keywords: Biodiesel Transesterification Separation Purification Membrane technology 1. Introduction Increased demand for energy, price hike of crude oil, global warming due to emission of green house gases, environmental pollution, and fast diminishing supply of fossil fuels are the major key factors leading to search for alternative sources of energy. Some of the most notable alternative sources of energy capable of replacing fuels include amongst others: water, solar and wind energy, and biofuels. Currently, 86% of the energy being consumed worldwide and nearly 100% of energy desired in the transportation sector is provided by non-renewable fossil fuels [1]. Biofuels production is being supported by the European Union (EU) with the objective of increasing fuel supply sources, boosting decarbonisation of fuels for transportation, decreasing hazardous gaseous emission which causes global warming effects, providing more earning opportunities in rural communities and developing long term plan for finite fossil fuels replacement. Presently several countries such as United States, Germany, Australia, Italy, and Austria are already using biofuels such as biodiesel and bioethanol. Table 1 presents volume of biodiesel production in different countries. This trend is expected to continue worldwide with more countries to use biofuels as source of energy [2]. Biodiesel (fatty acid alkyl esters), a substitute to diesel fuel, is produced from renewable natural sources such as vegetable oils, * Corresponding author. Tel.: þ60 3 79674615; fax: þ60 3 79675319. E-mail address: [email protected] (M.K. Aroua). 0960-1481/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2010.07.019 animal fats and microalgal oil. It is biodegradable, sustainable, and also environmentally beneficial, thereby providing lower gas emission profile. Biodiesel is considered to be carbon neutral, as biodiesel yielding plants such as jatropha curcas, rape plant and palm trees absorb carbon-dioxide to a greater extent than that contributed to the atmosphere when used as fuel in diesel engines. Also, biodiesel has similar physicochemical properties to that of diesel produced from crude oil and can be used directly to run existing diesel engines without major modifications or as a mixture with petroleum diesel and produces less harmful gas emission such as sulfur oxide. However, the direct use of vegetable oils as fuel in compression ignition engines is problematic due to their high viscosity (about 11e17 times greater than diesel fuel) and low volatility. These oil types do not burn completely and form carbon deposits in the fuel injectors of diesel engines. The viscosity of vegetable oils can be better improved with transesterification reaction, a process which seems to insure very good outcomes in terms of lowering viscosity and enhancing other physicochemical properties [3]. Transesterification is a chemical reaction involving triglycerides and an alcohol of lower molecular weights using homogeneous or heterogeneous substances as catalyst to yield biodiesel and glycerol, as presented in Fig. 1. Even though transesterification reaction catalyzed by alkali homogeneous catalyst such as sodium and potassium hydroxides yields higher conversion of vegetable oil to methyl esters in short time, the reaction has several drawbacks: it is energy intensive; recovery of glycerol is difficult; the catalyst has to be removed from 438 I.M. Atadashi et al. / Renewable Energy 36 (2011) 437e443 Table 1 Volume of biodiesel production in different countries [2]. Country Volume potential (Ml) Production cost (US$ per liter) Malaysia Indonesia Argentina USA Brazil Netherlands Germany Philippines Belgium Spain 14,540 7595 5255 3212 2567 2496 2024 1234 1213 1073 0.53 0.49 0.62 0.7 0.62 0.75 0.79 0.53 0.78 1.71 the product; alkaline wastewater requires treatment and free fatty acids (FFA) and water interfere with the reaction [4]. The presence of water lowers the activity of catalyst, while FFA react with the catalyst to produce saponified product [2]. The formation of soap reduces the biodiesel yield, and causes significant difficulty in product separation and purification. Thus, biodiesel and byproduct, glycerol have to be refined by washing with hot deionized water two to three times, leading to high waste of time, energy and water [3,4]. The major limiting factor to biomass use is the technology development for the separation, purification, and transformation of it into biochemicals and biofuels. Currently, “down-stream processing” alone accounts for 60e80% of the process cost [5]. Ineffective biodiesel separation and purification causes severe diesel engines problems such as plugging of filters, coking on injectors, more carbon deposits, excessive engine wear, oil ring sticking, engine knocking, and thickening and gelling of lubricating oil [6]. Several different separation and purification of biodiesel techniques have been studied. This paper reviews the technologies employed with emphasis on the most suitable practice for effective separation. Membrane separation seems to be the most suitable for this purpose and is the focus of this study. 2. Conventional biodiesel separation techniques Several researchers had studied extensively numerous conventional techniques for the separation of biodiesel. A brief review for the biodiesel separation techniques is presented here. Most of the researchers reported that high-quality biodiesel that is economically viable can be achieved when suitable biodiesel separation process is employed. After transesterification, separation of biodiesel and by-product, glycerol is usually first carried out. This process of biodiesel separation is based on the facts that the biodiesel and glycerol produced are typically sparingly mutually soluble, and that there is palpable difference in density between biodiesel (880 kg/m3) and glycerol (1050 kg/m3, or more) phases respectively. More so, this difference in density is sufficiently enough for the application of simple techniques such as gravitational settling or centrifugation for the separation of biodiesel and glycerol phases. In addition, the separation rate of biodiesel O || CH2 - O - C - R1 | | O | || CH - O - C - R2 + 3 CH3OH | | O | || CH2 - O - C - R3 Triglyceride Methanol ↔ (Catalyst) O || CH3 - O - C - R1 | | O | || CH3 - O - C - R2 | | O | || CH3 - O - C - R3 + Esters Fig. 1. Transeterification of vegetable oil with methanol. CH2 - OH | CH - OH | CH2 - OH glycerin mixture is influenced by numerous factors such as intense mixing, formation of emulsion, solubility of biodiesel in glycerol, and glycerol in biodiesel [7]. Demirbas [6] reported that supercritical methanol process is non-catalytic, involves a much simpler purification of transesterified products, has a lower reaction time, is more environmentally friendly, and requires lower energy use. Though, the reaction requires temperatures of 525e675 K and pressures of 35e60 MPa. Another advantage with supercritical methanol is that the conversion gets 95% complete in 10 min [8]. Siti et al. [9] revealed the problems encountered in the use of chemical catalysts to be high energy and methanol consumptions, and large amount of alkaline wastewater. The use of enzymes such as lipase has recently received a wider attention and considered to be an effective way to overcome such problems. Particularly, the separation of glycerol without complicated treatment. Even though cost is the major bottle neck associated with enzymatic catalysis. Sharma et al. [8] stated that methanol has polar hydroxyl group which can act as an emulsifier causing emulsification and rendering severe difficulties in the separation of the methyl ester layer from water. 2.1. Effects of catalyst The high consumption of energy and costly separation of the homogeneous catalyst from the reaction mixture have drawn to the need of development of heterogeneous catalysts for transesterification reaction, which is easily separated from the reaction mixture and recyclable. Several authors used heterogeneous catalysts with the aim of eliminating neutralization and washing steps needed for processes using homogeneous catalysts but were faced with major problems such as higher temperature of transesterification reaction, longer reaction time and lower yield of esters [1]. Helwani et al. [10] have discussed the advantages and disadvantages of using homogeneous catalysts (alkali and acid) and heterogeneous catalysts (solid and enzymes) in the industrial production of biodiesel. Homogeneous catalysts are significant for industrial biodiesel production because of their easy conversion at moderate temperatures (40e65  C), but are faced with refining problems. However, heterogeneous catalyst was introduced in the biodiesel production to avoid several neutralization and washing steps needed for processes using homogeneous catalysts. The authors revealed purity of methyl esters to exceed 99%, with yield close to 100%, glycerol as by-product with purity of more than 98% compared to about 80% from homogeneous process. The overall production economy is improved through the utilization of the byproduct, glycerol [1]. Several researches are also currently going on with the enzymatic methanolysis using lipases for biodiesel production with view of overcoming issues involved in recovery and treatment of the by-product that requires complex processing equipment. The major problem of enzyme catalyzed process is the high cost of the lipases as catalyst. In order to reduce the cost, enzyme immobilization was introduced for ease of recovery and reused [2]. They stated that although transesterification reaction catalyzed by lipase provides an attractive alternative, the industrial use of this technology has been retarded as a result of feasibility aspects and some technical challenges. Biodiesel production assessment has shown that, homogeneous base-catalyzed reaction is still much favorable in spite of the difficulties encountered in the product separation and purification. The major reason has been that the kinetic rates of homogeneous reaction are much faster than heterogeneously catalyzed transesterification reaction and is economically viable. Nestor et al. [11] stated that higher amount of FFA has resulted in excessive soap formation as FFA react with the catalyst, which is normally sodium and potassium hydroxides via saponification 439 I.M. Atadashi et al. / Renewable Energy 36 (2011) 437e443 reaction. Soap renders biodiesel purification and catalyst removal more challenging. However, homogeneous lewis acids such as AlCl3, ZnCl3, and excess methanol were tested at higher temperatures and demonstrated that AlCl3 irrespective of the feedstock type can give higher conversion of esters with better product that can be easily refined. Boey et al. [12] concluded that use of classical homogeneous catalysts results in higher yield loss through saponification as well as from the complicated separation process due to soap formation. Calcium oxide (CaO) catalyst was employed, though the reaction time was longer but the elimination of few purification processes and less wastewater generation compensates the delay, in addition to higher yield and the possibility of catalyst reusable. Masato et al. [13] noted massive wastewater discharged due washing of alkali-hydroxide from biodiesel. The authors tested different heterogeneous catalysts such as H-Y zeolites, sulfated titanium oxides and cation-exchange resin in the esterification of FFA and made use of calcium oxide and higher methanol ratios in the transesterification and achieved biodiesel yield of 93%. Yomi et al. [14] reported the use of chemical processes to give high conversion of triglycerides to their corresponding methyl ester in short reaction time but accompanied with several drawbacks such as being energy intensive, difficulty in recovering glycerol, the need for removal of alkaline catalyst, treatment of wastewater, and the interference of reaction by FFA and water. They observed that, enzymatic methods can overcome these problems but have not been industrialized because of the high cost of enzymes. A lipase continuous three-step flow reaction process was developed with the aim of reducing cost of enzymes. Thiam and Subhash [1] stated the removal of homogeneous catalyst to be sometime difficult and bring extra cost to the final product. Marchetti and Errazu [15] reported that feedstock with large amount of FFA is catalyzed using heterogeneous acid catalyst, solid resins, enzymes or in supercritical process. Transesterification reaction with basic homogeneous catalysts will promote soap formation and render product separation difficult. In order to improve the separation of the phases, a centrifuge was used for 20 min. Hideki et al. [16] found that biodiesel is best produced by using homogeneous alkaline catalyst but the process is followed with several drawbacks including difficulty in glycerol recovery and catalyst removal, in particular several stages such as evaporation of residual methanol, removal of soap and neutralization etc. The authors developed enzymatic process using extracellular and intracellular lipases to overcome the drawbacks and acknowledged the production cost of lipase to be very high. The significance of different catalysts in the purification of biodiesel is shown in Table 2. O || CH2 - O - C - R1 CH3 - OH | | | O | O O | || | || || CH - O - C - R2 + H2O → CH3 - O - C - R2 + HO – C - R1 | | | O | O | || | || CH2 - O - C - R3 CH3 - O - C - R3 Water Diglyceride Fatty acid Triglyceride Fig. 2. Hydrolysis of a triglyceride to form free fatty acids. its cost. Van Gerpen et al. [7] stated that due to low solubility of glycerol in alkyl esters, separation usually takes place rapidly and is accomplished with settling tank, and that addition of water to the reaction mixture after transesterification reaction can enhance the separation of alkyl esters and the by-product, glycerol. In his work, he revealed that unreacted methanol tends to act as a stabilizer and can lengthen the separation and that it is advantageous to remove the unreacted methanol before phase separation. 2.3. Effects of water and free fatty acids Water and FFA in oils and fats can pose a great problem during transesterification. When water is present, especially at elevated temperatures, it can hydrolyze the triglycerides to diglycerides and form an FFA (Fig. 2). However, the presence of water at average temperatures leads to formation of excessive soap formation. When an alkali catalyst such as sodium or potassium hydroxides is present, the FFA will react to form saponified product (Fig. 3). The saponified product formations of saturated fatty acids tend to be strengthened at ambient temperatures and the reaction mixture may gel and form a semi-solid substance that is very difficult to recover. The negative effects of excessive soap formation include amongst others; consumption of the catalyst, reduction of catalyst effectiveness, difficulty in glycerol separation, and prevention of crude biodiesel purification [7]. Demirbas [6] reported that even a little amount of water (0.1%) in the transesterification reaction will reduce the methyl ester conversion from vegetable oil. At the same time the presence of water has a significant effect in the yield of methyl esters when methanol at ambient temperature was replaced by supercritical methanol [8]. In conventional catalyzed methods, the presence of water has bad effects on the yields of methyl esters. 3. Conventional biodiesel purification techniques 2.2. Effects of oil to alcohol ratio 3.1. Biodiesel purification Oils to alcohols ratios play a vital role in determining the purity of biodiesel. The lower the ratios the lesser the complexity of the separation and purification processes vice versa. Sharma et al. [8] reported that increased in molar ratio of methanol to oil ratios beyond 6:1 neither increase the product yield nor the ester content, but rather makes the ester recovery process complicated and raises The main objective of biodiesel washing is to remove free glycerol, soap, excess alcohol, and residual catalyst. The drying of alkyl ester is needed to achieve the stringent limits of biodiesel specification on the amount of water content in the purified biodiesel product. However, there are other treatments used to reduce Table 2 Comparison of the different technologies to produce biodiesel [10]. Variable Base catalyst Acid catalyst Lipase catalyst Supercritical alcohol Heterogeneous catalyst Reaction temperature ( C) Free fatty acid in raw materials Water in raw materials Yields of methyl esters Recovery of glycerol Purification of methyl esters Production cost of catalyst 60e70 Saponified products Interfere with reaction Normal Difficult Repeated washing Cheap 55e80 Esters Interfere with reaction Normal Difficult Repeated washing Cheap 30e40 Methyl esters No influence Higher Easy None Relatively expensive 239e385 Esters e Good e e Medium 180e220 Not sensitive Not sensitive Normal Easy Easy Potentially cheaper 440 I.M. Atadashi et al. / Renewable Energy 36 (2011) 437e443 O || + KOH HO - C - (CH2)7 CH=CH(CH2)7CH3 Oleic Acid Potassium Hydroxide O || → K+ -O - C - (CH2)7 CH=CH(CH2)7CH3 + H2O Water (Soap) Fig. 3. Formation of soap. biodiesel colour, remove glycerides, sulfur and phosphorus, from the fuel. Water has the ability to provide a means for addition of acid to neutralize the unreacted alkaline homogeneous catalyst. This process simplifies immediate removal of the salt products. The unreacted methanol after transesterification reaction should be removed before the washing stage to minimize the presence of alcohol in the wastewater effluent. However, some processes remove the excess methanol after washing with water Von Gerpen et al. [7]. The authors prevented precipitation of saturated fatty acid esters using deionized water (120e140  F). Formation of emulsions is retarded when gentle water washing is applied fostering rapid and complete phase separation. Calcium and magnesium contamination is eliminated with the help of softened water (slightly acidic) which has the capability of neutralizing the remaining unreacted alkali catalysts. Similarly, iron and copper ions removal get rid of a source of catalysts that reduces the fuel stability [7]. Sharma et al. [8] reported that simple biodiesel purification process and recovery of high-quality glycerin are the key factors to be considered to reduce the price of biodiesel fuel and make it competitive to the conventional diesel fuel. Transesterification reaction is a reaction that is widely considered and mostly adopted to produce commercial biodiesel [11]. The transesterified products undergo different purification techniques in order to purify biodiesel from glycerol and other by-products. However, the neutralization of alkali catalyst and the purification stage performed using a larger amount of water create additional wastewater which are the major roadblocks of such process. Downstream from the reactor, the recovery of the residual alcohol, the purification of fatty acid alkyl esters from the catalyst and the separation of glycerol as a major secondary product are generally desirable. In the case when vegetable oils or waste vegetable oil contains an appreciable amount of FFA, the catalyst during transesterification reaction is consumed due to neutralization of acids causing decrease of the reaction rate, and rendering separation and purification difficult. Harding et al. [17] conducted experimental studies to compare the simplicity of purification and energy saving of biological process and that of inorganic chemical process. The authors stated that the conversion rate are generally slower in the enzymatic catalyze process but the process has eliminated the restriction imposed on water content or level of free fatty acid, and avoided saponification reaction and alcohol recovery. This process made separation of esters from glycerol, recovery of glycerol and purification of esters much more easier and economical compared to the conventional chemical process whose separation and purification processes proved too complicated and energy consuming. The major conventional biodiesel purification techniques are discussed below. 3.1.1. Washing with distilled water Water washing is generally carried out to remove soap, catalyst, methanol and other contaminants from biodiesel, using deionized water. Chongkhong et al. [18] conducted an experimental neutralization technique instead of distillation to purify the transesterified product. The process was carried out using 3 M of sodium hydroxide in water. Then 2%wt of sodium chloride was dissolved in the solution to remove the soap formation. Water at 60e80  C was used to wash the ester phase which was allowed to settle and then heated to evaporate residual water. Ferella et al. [3] stated that formation of soap due to neutralization of FFA in the vegetable oil or triglyceride by potassium hydroxide decreases biodiesel yield and quality. In addition separation of biodiesel and glycerol underwent simple centrifugation and washing product leads to high utilization of time and water. Jaya et al. [19] demonstrated simple filtration of cation-exchange resins catalyst in the production of biodiesel. The ester layer separated was washed with hot deionized water and dried over anhydrous sodium sulfate (Na2SO4). Haq et al. [20] noted the application of tetrahydrofuran (THF) as a co-solvent to enhance the homogeneity of oil and methanol and to promote more transesterification reactions increases biodiesel cost of purification. Removal of the co-solvent requires extra processing equipment. The washing processes conducted consist of acid neutralization followed by water washing. This was done to remove sodium hydroxide and other impurities such as excess methanol, triglyceride, diglyceride and monoglyceride. However, the glycerol separation and purification at the end of transesterification reaction were complicated and costly. Suprihastuti and Aswati [21] investigated biodiesel washing with water extraction. The process showed that washing biodiesel by water extraction in a single stage stirred tank reduced the glycerol content from 0.9331% to at least 0.09% for 20 min washing time by adding 50% water of biodiesel volume. When the water was 300% of biodiesel volume the glycerol content was less than 0.05% and the pH was 7.3. To achieve the standard requirement of glycerol content in biodiesel to be less than 0.02%, the washing was carried out in multistage process. It was also noted that the rate of mass transfer of glycerol from the biodiesel into water was affected by the temperature of extraction and the volume ratio of solvent to biodiesel. Higher mass transfer rate was achieved on higher biodiesel to water volume ratio as well as higher temperature. The more water added the larger the mass transfer area, so the higher volumetric mass transfer coefficient. The higher washing temperature gave higher diffusivity of glycerol from biodiesel to water phase, then the mass transfer coefficient was higher. Praveen et al. [22] stated that the reaction mixture was separated into an upper layer of methyl esters and lower layer of glycerol. The methyl esters are neutralized and vacuum distilled to remove excess methanol. They introduced an alternative method of glycerol recovery through conversion of crude glycerol to its mono and diesters using triglycerides (glycerolysis). Neutralization and water washing are necessary for total removal of catalyst from the product [22]. 3.1.2. Washing with acids Acids are added to transesterified product to neutralize the catalyst and decompose the soap formed. This process is followed with water washing to purify biodiesel from contaminants such as catalyst, soap, methanol and free glycerol. Sharma and Singh [23] washed biodiesel with 10% H3PO4 by bubble wash method after separation from glycerol, and then purified it further by passing air by aquarium stone for at least 24 h. It was furthermore, treated by washing with hot distilled water to remove the dissolved impurities such as catalysts, alcohol, etc. Silica gel was used for the removal of catalyst from the biodiesel product. Meher et al. [24] reported the difficulty in the separation and purification of glycerol and esters when higher alcohol to oil ratios is used. The use of triglyceride of relatively low FFA was emphasized to avoid soap formation. The products formed were separated using sedimentation, and the ester phase was distilled at 80  C to remove excess methanol, followed by numerous washes with distilled water and treatment with Na2SO4 and filtration. 3.1.3. Washing with solid adsorbent, ether, and water Absorbents are another means of treating transesterified product. Absorbent such as Magnesol has the potential of selectively I.M. Atadashi et al. / Renewable Energy 36 (2011) 437e443 absorbing hydrophilic materials such as glycerol and mono- and diglycerides. This treatment, followed by an appropriate filter, has been shown to be effective in lowering glycerides and total glycerol levels. Some vegetable oils and many yellow greases and brown greases leave an objectionable colour in the biodiesel. Activated carbon bed is an effective way to remove excessive biodiesel colour [7]. Dube et al. [25] mentioned the use of a solid absorbent, such as activated clay, activated carbon, activated fiber, etc. to purify the resultant biodiesel, and also use glycerine as a solvent to wash impurities. Hot deionized water washing at 50  C was considered to be the best way to separate and purify biodiesel but with challenging wastewater treatment problem in the wastewater stream. Meher et al. [26] adopted overnight separation of biodiesel from glycerol layer. The catalyst and unused methanol were in the lower glycerol phase, whereas little amounts of catalysts, methanol and glycerol were in the upper biodiesel phase. The upper phase was collected for further purification, and to obtain pure biodiesel, method of washing with hot distilled water and petroleum ether (1:1) was performed in the refining process. The biodiesel after separation was washed using same amount of hot distilled water (60  C) to remove the unreacted catalysts and glycerol. The moisture from washed biodiesel was removed by boiling at 120  C for 1 h. Sharma and Singh [23] stated that the distillation of esters can be assumed to be completed when the temperature reached 240  8  C (40  5 mmHg). The biodiesel separated after acidic transesterification reaction was washed with petroleum ether and then with hot distilled water (50  C) until the washing reached a neutral pH. The authors gave the relationship for the calculation of biodiesel yield as follows. Product yield ¼ Weight of product Weight of raw oil 4. Biodiesel membrane separation and purification The membrane equipments mostly produced for the separation and purification of crude biodiesel seem to exhibit several advantages over the conventional ones such as eradication/minimization of higher capital cost and other related costs of production, and provide high specific area of mass transfer. Membrane equipments for biodiesel refining are usually made from inorganic microporous ceramic membranes and generally have a lot of applications in biotechnology. These membranes hold some hope in their use for biofuels. Some of the most effective devices used for the separation and purification of crude biodiesel include: Membrane reactor and separative ceramic membrane. 4.1. Biodiesel separation and purification using membrane reactors Operations involving membrane technologies in the last years have shown their potentialities in the rationalization of production systems [27]. Membrane performance is usually governed by: selectivity or separation factor and permeability. In the absence of defects, the selectivity is a function of the material properties at given operating conditions. The productivity is a function of the material properties as well as the thickness of the membrane film, and the lower the thickness, the higher the productivity [28]. Permeability; Selectivity; Lp ¼ a ¼ Qfiltrate ADP Flux of impurity Flux of product (1) (2) 441 Membrane reactors intrinsic characteristics of efficiency, operational simplicity and flexibility, relatively high selectivity and permeability, low energy requirements, good stability under a wide spectrum of operating conditions, environment compatibility, easy control and scale-up have been confirmed in a numerous variety of applications and operations, as molecular separations, fractionations, concentrations, purifications, clarifications, emulsifications, crystallisations, etc [27]. Dube et al. [25] developed a membrane reactor that removed unreacted vegetable oil from the fatty acid methyl esters (FAMEs) product after transesterification, yielding high-purity biodiesel and shifting the reaction equilibrium to the product side. The authors stated that the novel membrane process was particularly useful in removing unreacted canola oil from the FAME product yielding a high-purity biodiesel. Additionally, a novel refining method using membrane extraction was developed by He et al. [29]. The authors compare membrane extraction and the traditional extraction methods of biodiesel refining. The membrane separation proved to be more effective and efficient over the conventional dispersed phase separation, in the following ways: no emulsification formed, no density difference noticed between fluids for hollow fiber membranes, and the interfacial area was high. The use of hollow fiber membrane leads to avoidance of emulsification of water and 99% purity of methyl esters was recorded. Peigang et al. [30] investigated high-purity FAME production using vegetable oils and fats with lower and higher FFA contents such as canola, soybean, palm, brown grease and yellow grease by means of a membrane reactor. The membrane used in the reactor system had a 300 kDa MWCO. This property was instrumental in providing excellent means of retaining emulsion. Highquality FAME was achieved which was ascertained by Gas Chromatography (GC) analysis based on the ASTM D6584 standard and the glycerin content of FAME produced was significantly lower than that produced via a conventional batch transesterification reaction. FAME quality was reported to be reasonably affected by the fatty acid composition of the lipid feedstock. The authors noted absence of glycerin after phase separation and recorded FAME purity of 79.07e86.36% prior to hot distilled water washing. All the glycerin was distributed in the methanol/glycerin rich phase because methanol and glycerin are all hydrophilic. Meanwhile diglyceride was detected in the FAME-rich phase because of Diglyceride’s hydrophobicity. To meet American standard of testing materials (ASTM), FAME-rich phase from the permeate stream was subjected to six water washes at one-third the volume of the FAME-rich phase for each wash. They also stated the potentials and challenges of high temperature membrane reactors. Peigang et al. [31] studied methanol recycling in the production of biodiesel in a membrane reactor. The authors reported that microporous inorganic membrane reactor could selectively remove FAME, methanol, and glycerol during transesterification reaction from triglycerides. The reactor is based on the general principle that oil particles form droplets in hydrophilic environment. When the reactants are mixed the oil droplets with larger pore sizes are formed. The smallest calculated oil droplet size in the membrane reactor was 12 mm and this greatly exceeds most of the membrane pore sizes (0.01e0.04 mm) employed. This distinct characteristic enables unreacted vegetable oil to be retained in the retented stream and permit the removal of the product. A significant reduction in the amount of water washing for the treatment of EAME for higher purity was noticed. The authors stated that in the same physical enclosure, membrane reactors can be employed to carry out a transesterification reaction as well as separation simultaneously. Li-Hua et al. [32] tested membrane separation using the ceramic membrane combined with liquideliquid extraction for the continuous cross flow rejection of triglycerides 442 I.M. Atadashi et al. / Renewable Energy 36 (2011) 437e443 Table 3 GC results according to ASTM D6584 [30]. Feedstock Soybean Canola Palm Yellow grease Brown grease Canola with methanol recycle a Biodiesel from membrane reactor (w%) Biodiesel from batch reaction (w%) Total glycerin Free glycerin Total glycerin Free glycerin 0.0685 0.0712 0.0124 0.0989 0.104 0.0929 0.00763 0.00654 0.0117 0.00735 0.0138 0.00749 e 0.131 e 0.685a 0.797a e e 0.0124 e 0.0234a 0.0171 e Does not meet the ASTM standard. from fatty acid methyl esters mixture. The oil-rich phase was rejected by the membrane, but the methanol-rich phase permeated and tested to be free of triglycerides. The membrane performance was found to be influenced by temperature because it has an impact on liquideliquid extraction (LLE). The risk of permeating oil through the membrane increases with increase in the feed temperature up to 60  C. The authors discovered modified UNIFAC model not to be capable of simulating the boundary of the LLE at different temperatures but suggested that the experimental results obtained to be used for the regression of appropriate model for oileFAMEemethanol system. Table 3 compares the quantity of glycerin left in biodiesel during membrane biodiesel purification and the conventional biodiesel purification [30]. The results show clearly that the product obtained through membrane purification contained less glycerin, indicating high biodiesel purity. 4.2. Effect of membrane pore size for biodiesel separation and purification The membrane pore size plays a significant role in the separation and purification of crude biodiesel. It is important to estimate the minimum particle sizes in the vegetable oilealcohol emulsion for efficient refining process. Peigang et al. [33] investigated the effect of membrane pore size on the performance of a membrane reactor for biodiesel production. The average pore size for an oil emulsion was determined to be 44 mm with lower and upper limits of 12 mm and 400 mm respectively. The oil droplet was found not to pass through the membrane pores because of their large molecular size relative to membrane pore size. The membrane provides a barrier to the passage of oleophilic substances in lipid feedstock. This introduced inherent reliability in the production of biodiesel that parallels the use of distillation in petroleum processes. 4.3. Separative ceramic membrane for biodiesel purification Today, purification of transesterified product posed a great challenge in commercial production and application of biodiesel fuel. The development of membrane reactor technology in the production of biodiesel has reasonably reduced the complicated separation and purification of crude biodiesel. This technology has led to the successful separation of the unreacted emulsified oil from the transesterified products which is a key factor in the production of biodiesel. Further attempts have been made to purify crude biodiesel without necessarily using water washing process. Water washing process proved critical to the production of economically viable biodiesel. Yong et al [34] investigated refining of biodiesel by ceramic membrane separation. Different membrane sizes of 0.1 mm, 02 mm and 0.6 mm, temperatures of 30, 40, 50 60, and 70  C, and transmembrane pressures of 0.05 and 0.2 MPa were tested respectively. The results of the content of potassium, sodium, calcium, magnesium and free glycerol recorded were far better than those obtained when water washing was employed. The authors stated that the size of the reverse micelle formed by glycerol and soap molecular weight with the mean of 2.21 mm was larger than that of biodiesel molecular size, and was easier to be removed during membrane separation. This clearly demonstrated that application of membrane technology in the purification of biodiesel will ease the difficulty encountered in conventional biodiesel purification. Additionally, the recent use of membranes by Maria et al. [35] has further demonstrated great efficiency of membrane technology for the separation and purification of crude biodiesel. 5. Conclusion and recommendation Based on the foregoing, the following conclusions and recommendations were made: 1. Even though homogeneous catalyst such as sodium and potassium hydroxides indicated faster rates in the commercial biodiesel production. The transesterification reaction involving these catalysts generates soap leading to great difficulties in the separation and purification of biodiesel from product mixture. 2. The development of heterogeneous catalysts of lower cost for the production of biodiesel should be encouraged to overcome the effects of soap formation. This will immensely contribute in lowering the cost of biodiesel separation and purification processes. 3. It is observed that application of water for the purification of biodiesel leads to higher cost of wastewater treatment, appreciable energy and time consumptions and low biodiesel yields. 4. The use of higher oil to methanol ratios in the transesterification reaction was found to contribute significantly to the higher cost of biodiesel separation and purification. 5. The use of raw materials with water content above the standard specification resulted in the deactivation of the catalyst and in some cases promotes soap formations. 6. 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