Bioresource Technology 101 (2010) 5394–5401 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech A two-step continuous ultrasound assisted production of biodiesel fuel from waste cooking oils: A practical and economical approach to produce high quality biodiesel fuel Le Tu Thanh a,b,*, Kenji Okitsu c, Yasuhiro Sadanaga a, Norimichi Takenaka a, Yasuaki Maeda a, Hiroshi Bandow a a Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan Faculty of Environmental Sciences, University of Science, Vietnam National University, Ho Chi Minh City, 227 Nguyen Van Cu St., Dist 5, Ho Chi Minh City, Vietnam c Department of Materials Science, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan b a r t i c l e i n f o Article history: Received 28 December 2009 Received in revised form February 2010 Accepted 11 February 2010 Available online March 2010 Keywords: Biodiesel production Transesterification Waste cooking oils Ultrasonic reactor a b s t r a c t A transesterification reaction of waste cooking oils (WCO) with methanol in the presence of a potassium hydroxide catalyst was performed in a continuous ultrasonic reactor of low-frequency 20 kHz with input capacity of kW, in a two-step process For the first step, the transesterification was carried out with the molar ratio of methanol to WCO of 2.5:1, and the amount of catalyst 0.7 wt.% The yield of fatty acid methyl esters (FAME) was about 81% A yield of FAME of around 99% was attained in the second step with the molar ratio of methanol to initial WCO of 1.5:1, and the amount of catalyst 0.3 wt.% The FAME yield was extremely high even at the short residence time of the reactants in the ultrasonic reactor (less than for the two steps) at ambient temperature, and the total amount of time required to produce biodiesel was 15 h The quality of the final biodiesel product meets the standards JIS K2390 and EN 14214 for biodiesel fuel Ó 2010 Elsevier Ltd All rights reserved Introduction Biodiesel, a liquid fuel consisting of mono-alkyl esters of longchain fatty acids derived from vegetable oils or animal fats, can be used as a substitute for diesel fuel (Hu et al., 2004; Veljkovíc et al., 2006) Some of the advantages of using biodiesel fuel are its renewability, easy biodegradability, non-toxicity and safer handling due to its higher flash point compared to those of fossil fuels (Wang et al., 2006) In addition, biodiesel fuel is also primarily free of sulfur and aromatics, producing more tolerable exhaust gas emissions than conventional fossil diesel (Demirbas, 2009) Biodiesel produced from virgin vegetable oils costs much more than petro-diesel; this is a major drawback to the commercialization of biodiesel in the market Therefore, it is necessary to find the ways to minimize the production cost of biodiesel In this context, methods that can reduce the costs of raw materials as well as the energy consumption are of special concern The use of waste cooking oils (WCO) is one of the more attractive options to reduce the raw material cost (Encinar et al., 2005; Kulkarni and Dalai, 2006) * Corresponding author Address: Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan Tel./fax: +81 72 254 9326 E-mail address: lethanh@chem.osakafu-u.ac.jp (L.T Thanh) 0960-8524/$ - see front matter Ó 2010 Elsevier Ltd All rights reserved doi:10.1016/j.biortech.2010.02.060 Biodiesel is synthesized by the transesterification of triglycerides (TG), the main components of vegetable oils and animal fats, with mono-alcohol in the presence of a catalyst, into fatty acid alkyl esters The TG is converted stepwise to diglycerides (DG), monoglyceride (MG) intermediates and finally to glycerin (GL) (Darnoko and Cheryan, 2000) The transesterification can be carried out in batch or continuous reactors (Meher et al., 2006a; West et al., 2008; Zhang et al., 2003) The batch transesterification process requires large reactors and longer reaction and separation times because the reaction and the separation stages are usually carried out in the same tank In contrast, the reactor for the continuous process can be smaller than that of the batch process for the same production capacity Several types of continuous reactors have been studied and applied for biodiesel production (Lertsathapornsuk et al., 2008; Zhang et al., 2010) On the laboratory scale, continuous reactor systems assisted by microwave have been demonstrated Other continuous-flow processes using a rotating packed bed, supercritical methanol or gas–liquid reactor have been found to be more effective for the transesterification (Chen et al., 2010; He et al., 2007; Behzadi and Farid, 2009) It is believed that the transesterification of TG with methanol is an equilibrium reaction system Therefore, the equilibrium can be shifted to the right, i.e., the formation of FAME, by performing a multi-step transesterification processes To minimize the influence 5395 L.T Thanh et al / Bioresource Technology 101 (2010) 5394–5401 of glycerin on the back reaction, the glycerin in the reaction mixture should be taken out after each step Over the past two decades, applications of sonochemistry have been widely developed in many areas of chemical technologies Ultrasound energy is well known as a useful tool to make fine emulsions from immiscible liquids Owing to this aspect, the transesterification reaction of vegetable oil and alcohol can reach equilibrium in a short reaction time with a high yield of alkyl esters even at low temperatures (Stavarache et al., 2003, 2006; Hanh et al., 2008; Georgogianni et al., 2008a, 2009) In the previous work, a pilot plant using ultrasound irradiation method for biodiesel production from canola oil and methanol was developed by our group (Thanh et al., 2010), in which the transesterification was carried out by a circulation process at room temperature The high yield of FAME can be obtained even in a short reaction time under the molar ratio of methanol to oil 5:1, and potassium hydroxide (KOH) catalyst, at 0.7 wt.% In this work, the transesterification of WCO with methanol in the presence of KOH catalyst was carried out in the continuous ultrasonic reactor by a two-step process The effects of the residence time of reactants in the reactor, molar ratio of methanol to WCO and separation time of glycerin from the reaction mixture in each step were investigated The objective of this work is to produce biodiesel of high quality meeting the specifications of the standard for B100 (pure 100% biodiesel) fuel with minimal costs of materials and energy Methods 2.1 Materials The WCO used were those after domestic use, collected by municipal activities, and then filtered and settled in a drum to remove particles remaining in the oils The physical and chemical properties of WCO are shown in Table KOH (grade 95.5%) and methanol (grade 99%) were purchased from Wako Pure Chemical Industries, Osaka, Japan, and used without further purification Chemical standards such as methyl oleate, methyl linoleate, methyl linolenate, methyl palmitate, methyl stearate, monoolein, diolein, and triolein, were obtained from Sigma–Aldrich, Tokyo, Japan 2.2 Apparatus The major units of the pilot plant include the liquid pumps, flow meters and ultrasonic reactors with a working volume of 0.8 L, and separation and purification tanks An ultrasound source was a horn type transducer generating low-frequency ultrasounds of 20 kHz with an input capacity of kW The experimental setup for the transesterification and purification of crude biodiesel using the pilot plant is schematically depicted in Fig This system was described in more detail in the previous paper (Thanh et al., 2010) 2.3 Procedures KOH was pre-mixed with a known amount of methanol adapted to each experiment and kept at ambient temperature (20–25 °C) In the first step of the transesterification, 120 L of WCO was fed with methanol, in the desired molar ratios 2.5:1, 3:1, 3.5:1 or 4:1, to the reactor The feeding of WCO and methanol was carried out by piston and peristaltic pumps, respectively, and both were connected to flow meters to control the mixing ratio of the reactants accurately The flow rate of the reaction mixture was set in the range of 0.5–2.5 L minÀ1 After passing through the reactor, the reaction mixture was transferred to the separation tank, where the transesterification and phase separation of glycerin from the reaction mixture proceeded simultaneously It took h to complete the phase separation The lower layer, containing glycerin, catalyst and excess methanol, was drained from the separation tank On the other hand, the upper layer, mainly FAME, TG and small amounts of DG and MG, was used for the second-step transesterification Table Chemical and physical properties of WCO used in this study (five samples were analyzed, n = 5) Properties À3 Density Acid value Iodine value Water content Oleic acid (C18:1)a Linoleic acid (C18:2)a Linolenic acid (C18:3)a Palmitic acid (C16:0)a Stearic acid (C18:0)a Other fatty acids Mean molecular weight of WCO a b Average ± SDb Unit g cm mg KOH/g oil g I2/100 g oil mg gÀ1 wt.% wt.% wt.% wt.% wt.% wt.% g molÀ1 0.918 ± 0.002 1.07 ± 0.10 112.5 ± 0.5 0.15 ± 0.03 47.02 ± 0.51 31.42 ± 0.48 10.21 ± 0.18 7.42 ± 0.44 2.77 ± 0.21 1.15 ± 0.16 876.60 ± 15.76 Carbon atoms number: double bond number SD: one standard deviation of five samples V US2 US1 S1 P V S2 F P V W1 P’ F M2 M1 V V O P V F G1 P V F V V V P G2 B O: Oil tank; M1, M2: Methanol and catalyst tanks; P: Liquid pumps; V: Valves; F: Flow meters US1, US2: Ultrasonic reactors; S1, S2: Separation tanks; G1, G2: Glycerin tanks P’: Purification tank; B: Biodiesel product tank; W1, W2: fresh and waste water tanks Fig Flow diagram of ultrasound assisted continuous reactor for biodiesel production at the pilot plant W2 L.T Thanh et al / Bioresource Technology 101 (2010) 5394–5401 The second-step transesterification was performed in the same manner as the first step, except that the molar ratios of methanol to initial WCO that is 1:1, 1.5:1 or 2:1 for the second step After the transesterification and the phase separation were completed, the crude FAME was transferred to the purification tank Here, the KOH catalyst, excess methanol and glycerin remaining in the crude FAME were removed by washing three times with tap water of the ratio of 20% by weight to crude FAME for each washing After washing, the water content in the FAME was effectively eliminated by heating the FAME to 70 °C under reduced pressure around 500 torr while flushing with a small amount of dried air for h All experiments were performed at ambient temperature of 20– 25 °C After passing through the reactor, the temperatures of the reaction mixtures were in the range of 30–32 °C and 27–29 °C for the first and second steps, respectively, due to the heating effect of the ultrasound 2.4 Analysis A 200 mL sample of the reaction mixture was withdrawn from the pipe connecting the ultrasonic reactor and the separation tank, and the sample was stored in a 250 mL beaker The time zero of the reaction was defined when the reactants, including WCO, methanol and KOH were introduced to the ultrasonic reactor Five milliliters of samples were taken from the beaker in prescribed time intervals and were immediately neutralized by the addition of mL of 5% phosphoric acid aqueous solution to stop the reaction The samples were left to settle for h for phase separation before analysis of the samples The concentrations of the reactants such as TG, DG, MG and FAME, were quantified by a high performance liquid chromatograph connected to a refractive index detector The analytical method employed in this study is described in more detail in the previous paper (Thanh et al., 2010) The FAME yields of each transesterification step were calculated from the weight of FAME in the FAME phase and the theoretical material balance of the transesterification reaction, as shown in Eq (1): wFAME =MFAME FAME yield %ị ẳ 100; 3wWCO =MWCO ð1Þ where wFAME and wWCO are the weight of FAME in the FAME phase and the weight of WCO used, respectively, MFAME and MWCO are the average molecular weights of the FAME and the WCO, respectively, and the factor indicates that one mole of triglyceride yields three moles of FAME The amount of the glycerin phase obtained from phase separation was determined by the gravity method and was calculated by Eq (2): GL wt:%ị ẳ wGL 100; wm (FFA) content and water, the acid-catalyst transesterification process is preferable However, this process requires higher temperatures and longer reaction times, in addition to causing undesired corrosion of the equipment Therefore, to reduce the reaction time, the process with an acid-catalyst is adapted as a pretreatment step only when necessary to convert FFA to esters, and is followed by an alkaline-catalyst addition for the transesterification step to transform triglycerides to esters (Leung and Guo, 2006) In contrast, when the FFA content in the oils is less than wt.%, many researchers have recommended that only an alkaline-catalyst assisted process should be applied because this process requires fewer and simpler equipment than that mentioned above (Meher et al., 2006b; Freedman et al., 1984) Among alkaline-catalysts, sodium and potassium hydroxide have most often been used in industrial biodiesel production, both in the concentration range from 0.4 to wt.% of the oil (Meher et al., 2006b) Encinar et al (2005) studied the effects of alkaline-catalyst types, such as sodium hydroxide, potassium hydroxide, sodium alkoxide and potassium alkoxide, on the methanolysis of WCO They concluded that the best yield of methyl esters was obtained at KOH concentration of wt.% In our previous work, the transesterification of canola oil, containing 0.4 wt.% of FFA, with methanol, was assisted by ultrasound irradiation in the circulation process The optimal FAME yield was observed at a KOH concentration of 0.7 wt.% (Thanh et al., 2010) In another previous study, the transesterification of WCO containing 1.7 wt.% of FFA was conducted with the same system mentioned above The best yield of FAME was attained when the amount of KOH catalyst was 1.0 wt.% (Thanh et al., 2008) Generally, as noted above, KOH is an effective catalyst for the transesterification, and as such, it was chosen for this study As shown in Table 1, the acid value of the WCO was 1.07, corresponding to FFA 0.54 wt.% Based on the previous work, the total KOH concentration of 1.0 wt.%, i.e 0.7 and 0.3 wt.% for the first and the second steps, respectively, was conservatively used for all of the transesterification experiments on the WCO 3.2 Effect of flow rate In the continuous reactor, the flow rate is one of the most important parameters affecting the reaction yield Lower flow rates lead to longer residence times of the reaction mixture in the reactor One could expect a low flow rate to enhance the emulsification efficiency of the reactants, resulting in increased FAME yield In the present study, with the reactor volume of 0.8 L, the flow rates were 100 95 ð2Þ where wGL and wm were the weights of the glycerin phase and the reaction mixture, respectively The weight of the reaction mixture was the sum of the weights of the raw materials, including the WCO, methanol and catalyst used for the transesterification In this study, each experiment used 120 L of WCO; thus, a limited number of experiments were performed in triplicate, and the results are shown as average values with one standard deviation Results and discussion FAME yield/ % 5396 90 85 80 75 70 0.0 First-step Second-step 0.5 1.0 1.5 2.0 2.5 -1 Flow rate/ L 3.1 Choice of type and amount of catalyst The choice of a catalyst for the transesterification depends on the quality of raw materials If the oils have high free fatty acid Fig Effect of flow rate on the FAME yields for methanolysis of WCO in the continuous ultrasonic reactor The molar ratio of methanol to WCO and the amount of KOH catalyst were (2.5:1 and 0.7 wt.%), and (1:1 and 0.3 wt.%) for the first and second steps, respectively 5397 L.T Thanh et al / Bioresource Technology 101 (2010) 5394–5401 90 80 70 FAME yield/ % varied from 0.5 to 2.5 L minÀ1, corresponding to residence times of the reactants in the reactor from 1.60 to 0.32 In the first step, the molar ratio of methanol to WCO and the catalyst amount were 2.5:1 and 0.7 wt.%, respectively When the first step was completed and phase separation accomplished, the FAME phase was used for the second step In the second step, the molar ratio of methanol to initial WCO and the catalyst amount were 1:1 and 0.3 wt.%, respectively, added to the FAME phase As shown in Fig 2, the FAME yield value increased, from 72.3% to 81.0% and from 95.3% to 97.5% for the first and second steps, respectively, as the flow rate decreased from 2.5 to 0.5 L minÀ1 The maximum FAME yields were 81.0% and 97.5%, which were obtained at the flow rates less than 1.5 and 2.0 L minÀ1 for the first and second steps, respectively Even with a short residence time of 0.93 and a small molar ratio of methanol to WCO of 3.5:1, for the sum of the two steps, the FAME yield was 97.5% As demonstrated in the literature (Stavarache et al., 2007; Georgogianni et al., 2008b; Ramachandran et al., 2006), ultrasonic irradiation is a tremendously useful tool for forming fine emulsions of immiscible liquids 60 50 40 30 20 10 2.5:1 3:1 3.5:1 4:1 0 10 20 30 Time/ 40 50 Fig 3a Effect of molar ratio of methanol to WCO on the FAME yield in the presence of KOH catalyst 0.7 wt.% for the first step of the transesterification 3.3 Effect of the molar ratio of methanol to WCO 3.3.1 The first transesterification step The first step of transesterification was conducted with molar ratios of methanol to WCO in the range from 2.5:1, 3.0:1, 3.5:1 or 4:1 in the presence of KOH 0.7 wt.% of WCO The flow rate of the reactants was fixed at 1.5 L minÀ1, corresponding to a residence time of 0.53 After passing through the reactor, the reaction mixture became a fine emulsion, and thus the reaction proceeded efficiently After 10 of reaction time, the yield of FAME reached about 80% for all cases, and thus the reaction mixture had become homogeneous To determine the amount of time required to reach equilibrium and the yield of FAME during the experiments, the reaction mixture was analyzed for FAME content at every sampling interval As shown in Fig 3a at the initial of the reaction time, the conversion rate of FAME was found to be faster at the lower molar ratios of methanol to WCO This can be explained by FAME Product composition/ wt.% The molar ratio of methanol to oil is also other the important factor affecting the yield of FAME Although the molar ratio of methanol to oil necessary to complete the transesterification is 3:1, an excess amount of methanol is helpful to shift the reaction toward the FAME formation Thus in practice, the molar ratio of methanol to oil used is usually more than 6:1 Because methanol and oil are immiscible liquids, the transesterification reaction occurs on the interface between the oil and the methanol As a result, only methanol on the surface of droplets is effective for the transesterification reaction if there is droplet formation in the reaction mixture, whether the reaction takes place using the conventional stirring method or the ultrasound assisted method such as the present study Additionally, glycerin is also formed as a by-product Because glycerin and methanol are polar compounds, they can dissolve each other at any ratio Therefore, the presence of glycerin absorbs significant amounts of methanol, requiring large amounts of methanol for the transesterification However, the use of large amounts of excess methanol has adverse effects on the phase separation of glycerin and FAME, and increases the energy and time consumption for the recovery of excess methanol Moreover, as mentioned above, using the ultrasonic reactor reactants form a fine emulsion, which increases the interface area between methanol and oil Therefore, in this case, the rate of the transesterification can be enhanced, and it can reduce the amount of excess methanol required Overall, to enhance the effectiveness of methanol, the transesterification was carried out by a two-step process A proper amount of methanol was used, and the glycerin and excess methanol were removed after each step MG DG TG 90 80 70 60 50 40 30 20 10 2.5:1 3:1 3.5:1 4:1 Molar ratio of methanol to WCO Fig 3b Composition of products in the FAME phase of the first step of the transesterification of WCO with various molar ratios in the presence of KOH catalyst 0.7 wt.% after the reaction and phase separation were completed the fact that the concentration of catalyst was lower in the cases with larger amounts of methanol because the same amount of catalyst was used based on the amount of WCO As reported by Vicente et al (2004), the transesterification is initiated by attacks of methoxide ions (CH3OÀ) on the carbonyl carbon atoms of TG, DG and MG molecules Because the KOH catalyst is a strong base, its dissociation constant is very large Therefore, higher concentrations of methoxide ions on the surface of the methanol droplets were obtained when lower molar ratios were used As a result, the lower molar ratios of methanol to WCO increased the reaction rate during initial stage of the reaction This result agrees with the previous work, where the same reactor was used in the circulation process (Thanh et al., 2010) However, after of the reaction, higher conversion of FAME was achieved when higher molar ratios were employed The equilibrium state of the reaction was reached at 25, 30, and 40 with the molar ratios of 2.5:1, 3:1, and 4:1, respectively As shown in Fig 3a, when the molar ratio of methanol to WCO increased from 2.5:1 to 4:1, the yield of FAME increased from 81.0% to 90.1% Although the addition of methanol was increased significantly by 60%, the yield of FAME increased only by 10% This result can be explained as follows: methanol and glycerin are structurally similar molecules, containing hydroxyl groups, which can easily stimulate the intermolecular H-bonding between glycerin and methanol, and thus dissolve each other well Therefore, even 5398 L.T Thanh et al / Bioresource Technology 101 (2010) 5394–5401 though excess methanol is added to the reaction mixture, larger proportions of the excess methanol could be removed from the reaction zone by dissolution into the glycerin phase once the glycerin phase has formed during the transesterification reaction In other words, a very limited portion of the methanol added could act as the reactant for the transesterification This phenomenon may be the reason why the mechanical stirring method applied for the transesterification needs a higher molar ratio i.e., at least 6:1, a higher temperature, and a longer reaction time to enhance the effect of methanol Fig 3b shows the composition of products of the first step of transesterification after the phase separation was completed The concentrations of TG, DG, and MG changed insignificantly when the molar ratio increased from 2.5:1 to 4:1 The concentrations of MG and DG were in the range from to wt.%, obtained at the molar ratio from 4:1 to 2.5:1, and the concentration ratios of DG and MG changed slightly in all the molar ratios used The concentration of TG remaining in the FAME phase was 7.6, 6.9, 4.5, and 4.1 wt.%, acquired at the molar ratios of 2.5:1, 3:1, 3.5:1, and 4:1, respectively This result agrees with the conversion of FAME described above Consequently, the molar ratio of 2.5:1 between methanol and WCO has been judged as the best compromise point between the acceptable FAME conversion for the first step and the least methanol usage Therefore, the crude FAME consisting of FAME 81 wt.%, TG 7.6 wt.%, and the rest of TG and DG was used for the second transesterification step 3.3.2 The second transesterification step The molar ratio of methanol to initial WCO was in the range from 1:1; 1.5:1 or 2:1, and the amount of KOH catalyst was 0.3 wt.% of initial WCO The flow rate of reactants was fixed at L minÀ1, corresponding to the residence time of 0.4 in the reactor After of reaction time, the reaction mixture attained homogeneity by emulsification As shown in Fig 4a, when the molar ratio was increased from 1:1 to 2:1, the yield of FAME was increased from 97.2% to 99.3% It should be noted that the conversion of FAME became extremely high, and the equilibrium was almost reached after around 20 of reaction in all cases Because the average concentration of TG in the crude FAME was 7.6 wt.%, the molar ratios of methanol to initial WCO of 1:1, 1.5:1 and 2:1, corresponded to the ratios of methanol to TG in the crude FAME phase of 12.7:1, 19.1:1 and 25.5:1, respectively These ratios are much higher than the theoretical molar ratio of methanol to TG, i.e., 3:1 Furthermore, the starting material containing 81 wt.% of FAME has low viscosity Therefore, methanol can easily diffuse in the 100 FAME phase to facilitate the reaction between the methanol and the TG as well as the DG and MG remaining in the FAME phase These effects may be the main cause of the outstandingly high yield of FAME Fig 4b shows the composition of the products in the FAME phase of the second step of the transesterification To demonstrate the changes in the concentrations of the products more clearly, the concentrations of TG, DG, and MG shown in Fig 4b are plotted along a scale multiplied by 10 The concentrations of TG, DG, and MG were 1.1, 1.0, and 0.7 wt.%, respectively, at the molar ratio of methanol to initial WCO 1:1 At the molar ratios from 1.5:1 to 2:1, TG was not detected in the FAME phase, indicating that TG was converted completely to the products, and the concentrations of DG and MG were also less than 0.2 and 0.8 wt.%, respectively Compared to the biodiesel standard, JIS K 2390 and EN 14214, the concentrations of TG, DG, and MG should be less than 0.2, 0.2, and 0.8 wt.%, respectively Therefore, the optimal molar ratio for the second step of the transesterification was 1.5:1 3.4 Glycerin separation Separation of glycerin is an important factor to determine the final product quality and FAME recovery, as well as the time necessary for the full process of the biodiesel production Therefore, the separation of glycerin was investigated intensely In this discussion, the time for the separation of the glycerin phase was defined as zero when the reaction mixture of the first step of the transesterification was completely transferred to the separation tank The glycerin phase was drained from the bottom of the separation tank every 0.5 h As shown in Fig 5, at the initial stage of separation within h, the higher the molar ratios of methanol to WCO, the faster the glycerin separation took place This fact can be elucidated as follows: at higher molar ratios, higher FAME yield could be attained In this case, the amount of excess methanol remaining in the reaction mixture was large As a result, the viscosity of the reaction mixture was reduced Furthermore, as mentioned in the effect of molar ratio, when a larger amount of methanol was used, the gathering probability of methanol in the glycerin phase was large Therefore, methanol and glycerin easily encounter each other to form a large droplet, resulting in the faster separation of glycerin and methanol from the reaction mixture This phenomenon agrees with the conversion of FAME, which only rose by 10% when the molar ratio increased from 2.5:1 to 4:1 On the other hand, in the case of settling for more than h, the time would be long enough for methanol and glycerin dissolving each other; the smaller the amount of methanol, the faster the glycerin separation took place Because glycerin has a much higher density Product composition/ wt.% FAME yield/ % FAME 95 90 85 1:1 1.5:1 2:1 MGx10 DGx10 TGx10 100 80 60 40 20 80 10 20 30 40 50 Time/ Fig 4a Effect of molar ratio of methanol to initial WCO on the FAME yield in the presence of KOH catalyst 0.3 wt.% for the second step of the transesterification 1:1 1.5:1 2:1 Molar ratio of methanol to initial WCO Fig 4b Composition of products in the FAME phase of the second step of the transesterification of WCO with various molar ratios in the presence of KOH catalyst 0.3 wt.% after the reaction and phase separation were completed 5399 L.T Thanh et al / Bioresource Technology 101 (2010) 5394–5401 12 WCO 100 ± 2.0 Glycerin phase/ wt.% 10 Methanol First-step KOH 9.13 ± 0.10 transesterificationm 0.70 ± 0.01 2.5:1 3:1 3.5:1 Glycerin separation Glycerin phase 9.75 ± 0.30 4:1 0 Time/ h Fig The amount of glycerin phase separation in the first step of the transesterification with the molar ratios of 2.5:1, 3:1, 3.5:1, and 4:1 and the KOH catalyst 0.7 wt.% (d20 = 1.26 g cmÀ3) than methanol (d20 = 0.79 g cmÀ3), the density of the glycerin phase decreases as the amount of methanol in the glycerin phase increases Therefore, a slower acceleration of phase separation between the FAME layer (typical density is ca 0.885 g cmÀ3 for WCO in this study) and the glycerin layer takes place owing to the larger difference in the densities of the two layers Due to the presence of excess methanol in the glycerin phase, the weight of glycerin phase separated increased from 8.3 to 12.5 wt.% when the molar ratio increased from 2.5:1 to 4:1 Practically, phase separation could be completed within h after settling the reaction mixture in the separation tank The behavior of the glycerin separation in the second step was the same as in the first step However, the time required for entire separation in the second step was h Table shows the product compositions and the distribution of methanol in both phases at different molar ratios of five runs When the total molar ratios of methanol to WCO used for the two steps were all higher than 4:1, the concentrations of the impurity components TG, DG and MG in the FAME phase of the second step met the biodiesel standards, JIS K 2390 or EN 14214 In these cases, of course, the lowest total molar ratio of 4:1 would be favorable for biodiesel production, and this ratio was applied in runs #2 and #3 The molar ratios for the first and second steps of runs #2 Methanol FAME Methanol 0.16 ± 0.10 98.76 ± 1.8 0.26 ± 0.10 Methanol Second-step KOH 5.48 ± 0.10 transesterificationm 0.30 ± 0.01 Glycerin separation Glycerin phase 5.81 ± 0.25 Methanol FAME Methanol 1.50 ± 0.22 96.23 ± 2.0 1.71 ± 0.10 Tap water 60 ± Wastewater Purification 63 ± FAME product 93.83 ± 2.1 Fig Material balance for the full process under the optimal conditions (The materials are shown in proportions by weight, over three runs, n = 3) Table Methanol content in the FAME and glycerin phases from each step after phase separation Run Step First Second First Second First Second First Second First Second Molar ratio CH3OH:WCO 2.5:1 1:1 2.5:1 1.5:1 3:1 1:1 3.5:1 1:1 4:1 1:1 Product composition (wt.%) of FAME phase FAME phase FAME MG DG TG Weight (kg) CH3OH (wt.%) CH3OH (kg) Weight (kg) CH3OH (wt.%) CH3OH (kg) 80.52 97.10 81.61a ± 1.52b 98.65 ± 1.30 83.25 98.73 86.07 98.91 89.30 99.20 6.31 1.16 5.93 ± 1.00 0.63 ± 0.15 5.55 0.75 5.13 0.16 3.32 0.40 5.45 1.02 4.85 ± 0.70 0.20 ± 0.10 4.83 0.21 4.31 0.11 3.52 0.09 8.72 0.72 7.61 ± 0.90 ND 6.47 ND 4.49 ND 3.81 ND 106.5 104.3 105.3 ± 1.8 104.9 ± 1.3 106.9 103.7 107.1 106.3 106.3 104.8 0.15 0.87 0.17 ± 0.10 1.61 ± 0.32 0.94 1.2 1.23 1.41 1.77 1.59 0.16 0.91 0.18 ± 0.10 1.65 ± 0.34 1.00 1.24 1.32 1.50 1.88 1.67 10.06 4.35 9.85 ± 0.27 6.40 ± 0.24 11.02 4.39 12.93 4.9 14.58 4.85 3.06 18.85 2.91 ± 0.55 29.35 ± 1.73 8.65 16.44 18.17 21.98 20.37 25.81 0.31 0.82 0.29 ± 0.05 1.88 ± 0.11 0.95 0.72 2.35 1.08 2.97 1.25 Notes: ND: not detectable a Average value of three runs b One standard deviation of three runs Glycerin phase 5400 L.T Thanh et al / Bioresource Technology 101 (2010) 5394–5401 Table Properties of biodiesel produced from WCO under the optimal conditions (2.5:1 and 1.5:1 of molar ratio of methanol to initial WCO for the first and second steps, respectively; KOH catalyst 1.0 wt.%) Test parameter Unit Total ester Density (15 °C) Viscosity (40 °C) Flash point Sulfur 10% carbon residue Cetane number Sulfated ash Water content Particulate Copper corrosion (3 h, 50 °C) Oxidation stability Acid value Iodine value Methyl linolenate Methanol Total glycerin Free glycerin Monoglyceride Diglyceride Triglyceride Total Na + K Total Phosphorus Pour point CFPP Result mass% g cmÀ1 mm2 sÀ1 °C mg kgÀ1 mass% À mass% mg kgÀ1 mg kgÀ1 À hours mg KOH gÀ1 I2/100 g mass% mass% mass% mass% mass% mass% mass% mg kgÀ1 mg kgÀ1 mg kgÀ1 °C °C JIS K2390 98.2 0.8845 4.452 180 0.03 57.2