influence of the acid value on biodiesel fuel production using a two

biomass & renewablesInternational Journal of

A two-step batch process was optimized for the production of biodiesel fuel by the transesteri�cation of used vegetable oils containing the free fatty acids. The optimum reaction conditions for reducing unreacted glycerides to less than 1 wt% were determined. Potassium hydroxide dissolved in methanol was used as a catalyst. The e�ect of the quantity of free fatty acid on the conditions required for e�cient conversion was clari�ed. In addition, the optimum ratio of KOH to methanol based on the overall cost of producion was determined.

Keywords : Biodiesel, used oil, methanol, potassium hydroxide, two step

INFLUENCE OF THE ACID VALUE ON BIODIESEL FUEL PRODUCTION USING A TWO-STEP BATCH PROCESS WITH A HOMOGENEOUS CATALYST

Takami Kai1, Atsushi Kubo1, Tsutomu Nakazato1, Hirokazu Takanashi2, Yoshimitsu Uemura31Department of Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan2Department of Chemistry, Biotechnology, and Chemical Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan3Universiti Teknologi Petronas, Bandar Seri Iskandar, 31750 Tronhoh, Perak, Malaysia

*Corresponding author. E-mail address : [email protected]

Abstract

In Japan, biodiesel fuel is produced mainly by the transesteri�cation of used vegetable oils with methanol. Some production methods have been proposed for the transesteri�cation of used frying oils, such as homogeneous acid or base catalysis, heterogeneous acid or base catalysis, enzyme catalysis, and supercritical methods [1, 2, 3]. Homogeneous base catalysis is still the primary method used for producing biodiesel from fresh oils worldwide.

Many researchers have developed a two-stage acid and base-catalyzed transesteri�cation to avoid the problems associated with the independent use of acid and base catalysts [2]. In the �rst stage of this process, free fatty acids are converted to methyl esters using an acid catalyst. Although sulfuric acid [4] is most commonly used, ferric sulfate o�ers the advantages of high e�ciency, low equipment costs, nonacidic waste and easy recovery of the catalyst [5]. In the second stage, transesteri�cation is performed out using a base catalyst.

It is reported that the base-catalyzed transesteri�cation process is most e�ective when the level of free fatty acids is lessthan 1% [3]. At this concentration, the acid value (AV) is approximately 2 mg KOH/g oil at this concentration. Although the AV of oils used in delicatessens should be less than 2.5mg KOH/g oil according to food sanitation standards in Japan, the AV of the free fatty acids in collected used oils is not always below this value. Therefore, the raw materials used for the base-catalyzed transesteri�cation process are sometimes unsuitable.

The biodiesel fuel product should satisfy the Japan Industrial Standards (JIS) regulation K 2390. The fuel satisfying the standard is considered to be an appropriate fuel for diesel engines when

blended with petrodiesel fuel. The maximum values of unreacted glycerides required by JIS K 2390 are 0.2 wt% for triglycerides, 0.2 wt% for diglycerides, and 0.8 wt% for monoglycerides. Although the standard has no legal power, customers of small-scale plants using a neat biodiesel product expect a high-quality fuel that meets the standard.

In a one-step batch process, more than three times the stoichiometric amount of methanol is required to decrease the amount of unreacted glycerides to an acceptable level. The ratio of methanol/oil must be increased to prevent soap formation, particularly in the treatment of used oils. Methanol is sparingly soluble in triglycerides, and is easily captured by the glycerol phase formed as a byproduct of transesteri�cation. During the reaction, the glycerol phase is dispersed into the oil phase, which consists mainly methyl esters and triglycerides. The methanol and catalysts dissolved in the glycerol phase cannot e�ectively participate in the reaction because of interface mass transfer resistance. Therefore, to e�ciently utilize the methanol and catalysts, it is important to reduce the fraction of the glycerol byproduct in the reaction mixture.

In the case of homogeneous base-catalyzed transesteri�cation, the two-step batch process is known to be an e�ective method for increasing the yield of methylesters [6, 7]. The reaction conditions for the one-step process have been extensively studied. Although many industrial processes use the two-step batch process, there is little information regarding the base-catalyzed step of this process [8-11]. In the two-step batch process, the glycerol byproduct is removed from the reaction system after the �rst step reaction. Therefore, the dissolution of the methanol is suppressed, thus

1. Introduction

1 (2012) 15 - 20

5


allowing its e�ective participation in the reaction. This method not only achieves a high level of conversion but also a reduction in the total amount of methanol used.

In large-scale processes, an excess amount of methanol is used, and unreacted methanol is subsequently distilled and recycled. However, the distillation of methanol is not suitable for small-scale plants because of the high initial and running costs. Therefore, the amount of methanol required for optimum conversion in small-scale plants should be limited.

In previous studies, we have reported the minimum amount of the KOH catalyst and methanol required to maintain the concentration of the unreacted glycerides below 1.0 wt% in fresh [10] and used vegetable oils [11]. Because used vegetable oils contain higher amounts of free fatty acids and water compared with fresh oils, higher amounts of KOH and methanol are required. In the present study, the two-step batch process was used for the production of biodiesel fuel from used vegetable oils. We studied the e�ect of AV on the reaction conditions required to maintain the concentration of unreacted glycerides below 1.0wt% without increasing the amount of methanol. In addition, we examined the in�uence of the KOH/methanol ratio on the yield of biodiesel fuel. Finally, the optimum ratio of KOH/oil was determined on the basis of these results, the cost of reagents (methanol and KOH), and the overall production cost.

2. Experimental

Fresh commercial canola oil and used rice oil were used as the raw materials. The moisture content measured by Karl Fischer coulometric titration was 0.03wt% and 0.04wt% for fresh canola oil and used rice oil, respectively. Simulated used vegetable oils were prepared by controllingits AV by adding oleic acid to fresh canola oil. Transesteri�cation was performed in a 100-mL glass batch reactor using KOH as the catalyst. The mass ratio of methanol/oil was changed from 0.145 to 0.29 kg/kg. When the ratio was 0.145kg/kg, the amount of methanol was approximately 1.34 times the stoichiometric value. The mass ratio of KOH/methanol was changed from 0.02 to 0.12 kg/kg. The �rst and second step transesteri�cation runs in a two-step experiment were carried out with the same ratio of KOH/methanol. In the �rst step reaction, 80% of total amounts of KOH and methanol were used. This ratio was found to be the optimum value to increase the conversion [10]. In both the �rst and second batch processes, the reaction time was 1 h and the reaction temperature was maintained at 310 K using a constant temperature water bath. The reactant mixture was stirred using a magnetic stirrer during the reaction. The methyl ester phase was separated from the glycerol phase after 1 h of settling using a separating funnel after each reaction step. Since the neutralization of the catalyst was not performed after the reaction steps, the yield

of methyl ester increased by about 0.4wt% during the second settling step.

After the �rst step reaction, the products were separated, and the upper ester phase was used as the raw material for the second step reaction. The amounts of KOH and methanolused in this step were 20% of the total dosage. After the second step reaction, the separated ester phase products were washed with water. The yield of methyl esters was obtained by measuring the weight of the ester phase. The composition of the ester phase was analyzed using a high performance liquid chromatograph (HPLC, Shimadzu Corp.) equipped with a gel permeation chromatography (GPC) column (GPC-801, Shimadzu Corp.). Several methods have been developed for analyzing the methyl ester phase obtained by the transesteri�cation of vegetable oils and some researchers have selected GPC [12–16]. We also used GPC because of its rapidity, while each method has advantages and disadvantages.

3. Results and Discussion

Fig.1 shows the relationship between the mass ratio of KOH/oil, WKOH, and the concentration of unreacted glycerides when the fresh canola oil was used. Since the ratio of KOH/methanol was �xed to 0.020 kg/kg, the mass ratio of methanol/oil, WMeOH, increased linearly with WKOH. Unreacted glycerides consisted of triglycerides, diglycerides, and monoglycerides. Their concentrations decreased with increasing amounts of KOH and methanol. We determined the critical concentration at which the glyceride concentration was less than 1.0 wt%. Under these conditions, the concentrations of triglycerides and diglycerides were below 0.2 wt%, and the concentration of monoglycerides was below 0.8 wt%.

Figure 1 : Decrease in unreacted glyceride concentration with increasing addition of catalyst

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In Fig. 2, WMeOH required for suppressing the unreacted triglycerides below 1.0 wt% is plotted against WKOH with various AV values. The broken line shows the stoichiometric amount of methanol when tri-olein is used as a raw material. The acid values of the methyl esters produced under these conditions were below 0.04 mg KOH/g. Although it was not con�rmed what kind of acid components a�ected the AV, almost all the free fatty acid contained in the raw materials were consumed by the reactions. The concentration of glycerides was less than 1.0 wt%, when the amounts of KOH and methanol were higher than the values represented by the lines in the �gure. The values indicated by the curves were the sum of the dosage in the �rst and second step reactions. By increasing WKOH, the amount of methanol required was decreased. However, the methanol concentration was inevitably higher than the stoichiometric value shown by the broken line in the �gure.

Figure 2 : Critical conditions required to suppress unreacted glycerides below 1.0 wt%

Although the results when AV was 2.0 mg KOH/g oil were not given in this �gure, they were almost the same as those of the fresh oil (AV = 0.02 mg KOH/g oil). Therefore, the critical linewas not a�ected by the AV when it was less than 2.0 mg KOH/g oil. The amounts of KOH and methanol required increased with the concentration of thefree fatty acid when the AV was above 2.0 mg KOH/g oil. However, the amount of methanol required did not increase when WKOH was high with an increase in the AV, whereas it was signi�cantly increased when WKOH was low.

The reaction conditions for the used rice oil (URO) having AV=3.4 mg KOH/g oil are also shown in Fig. 2. This oil required higher amounts of KOH and methanol than the simulated used oil having AV=4.0 mg KOH/g oil. This is because the water content of used rice oil was higher than that of fresh canola oil. The saponi�cation of triglycerides is generally enhanced in the presence of water.

Therefore, conditions for used vegetable oil were in�uenced not only its AV but also the water content in the oils [9, 11].

The amount of KOH used for the neutralization of the free fatty acids contained in the simulated used oils can be calculated when the AV is known. The straight line in Fig. 3 shows the additional amount of KOH required to neutralize the free fatty acid in the simulated used oils as a function of AV when the additional KOH is consumed by only neutralization. The amount of KOH is proportional to the AV. The experimental data are also plotted in the �gure. The results show that the addition of KOH was not necessary when the AV was low, as described above. Fig. 4 also shows clearly that the amount of KOH required was reduced when the ratio of KOH/oil was increased.

Figure 3 : Increase in additional amount of KOH required to suppress unreacted glycerides

Figure 4 : E�ect of KOH/oil ratio on additional amount of KOH required to suppress unreacted glycerides

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The neutralization of the free fatty acids by KOH occurs in competition with transesteri�cation catalyzed by KOH. The saponi�cation of free fatty acid is very fast reaction. From the experimental results shown in Fig. 3, however, it is supposed that excessive KOH was not simultaneously consumed by free fatty acid, but KOH played a role of the catalyst before it was consumed by neutralization. This suggests that the addition of a stoichiometric amount of KOH is not necessary to neutralize all the fatty acid contained in used vegetable oils.

Because the measured AV of the methyl ester phase was low, the free fatty acid in the simulated used oil was converted to soap by reaction with KOH. Therefore, the yield of biodiesel fuel after washing decreased with increasing soap formation. If the raw material is assumed to be composed of triolein and oleic acid, the concentration of the free fatty acid will be 2.0 wt% when the AV is 4.0 mg KOH/g oil. In this case, the yield of biodiesel fuel will be decreased by at least 2%.

Fig. 5 shows the experimental values of the yield of biodiesel fuel. Triglycerides were also converted to soap when the KOH concentration was high, and methyl esters taken into the glycerol phase increased with the soap concentration. Therefore, the yield of biodiesel was not 100% even though fresh oil was used. The yield of biodiesel fuel for the raw material of AV=4.0 mg KOH/g oil decreased by 3%–7% because of the increase in AV. When AV=4.0 mg KOH/g oil, the yield, YBDF, can be correlated by

YBDF = 0.93–5.3WKOH　　(1)

where WKOH is the mass ratio of KOH/oil. The yield was lower when higher amount of the free fatty acid was present in the raw materials.

Figure 5 : E�ect of reaction conditions on yield of biodiesel fuel

In order to enhance the yield of biodiesel fuel, the ratio of KOH/oil should be decreased and the ratio of methanol/oil should be increased. However, unreacted methanol is not recovered in the case of small scale plants. Therefore, the optimum reaction conditions for these plants would be in�uenced by the costs of KOH, methanol, and the product. If the operating costs are independent of the change in reaction conditions, the optimum ratios, WKOH and WMeOH, can be determined to maximize the following bene�t function:

B = YBDFPBDF– (WKOHPKOH+ WMeOHPMeOH) (2)

where PBDF, PKOH, and PMeOH are the costs of 1 kg of biodiesel fuel, KOH, and methanol, respectively. As given by Eq. 2, the bene�t function, B, is obtained by subtracting the costs for purchasing the catalyst and methanol from the price of produced biodiesel fuel when 1 kg of oils is used as the feedstock. The cost for purchasing oils is not considered because this function is based on 1 kg of oils.

The relationship between WKOH and WMeOH for the simulated used oil having AV=4.0 mg KOH/g oil in Fig.2 is given by the following equation:

WMeOH= 0.108 + 1.8 ×10–5WKOH–1.9 (3)

Because the yield of biodiesel fuel and the amount of methanol required are functions of WKOH, Eq.2 is converted to Eq.4 using Eq.1 and 3 as follows:

B=(0.93 –5.3WKOH) PBDF–WKOHPKOH–( 0.108 + 1.8×10-5WKOH-1.9) PMeOH (4)

The relationship given by Eq. 4 is shown in Fig. 6 when PKOH and PMeOH are 4.0 $/kg and 1.2 $/kg respectively. When PBDF = 1.0 $/kg, it is found that the optimum ratio, WKOH is 0.012kg/kg required to maximize the value of function B. For each value of PBDF, the value of B is not in�uenced by WKOH when WKOH is higher than 0.01 kg/kg. When the ratio is higher than the optimum value, B decreases because of the decrease in the yield of biodiesel fuel. On the other hand, B decreases because of the increase in the costs of methanol due to the increase in methanol consumption when WKOH is lower than the optimum value.

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Figure 6 : E�ect of KOH/oil ratio on bene�t function B

When the cost of biodiesel fuel is 1.0 $/kg, the optimum ratio of KOH/oil is plotted against the costs of KOH and methanol in Figs. 7 and 8, respectively. The optimum ratio not only satis�es the reaction conditions required for decreasing the unreacted component concentrations, but also gives the maximum value of B. The cost of methanol has a strong in�uence on the optimum reaction conditions, whereas the in�uence of the cost of KOH is insigni�cant. If the cost of methanol increases, increasing the ratio of KOH/oil and decreasing the ratio of methanol/oil would be bene�cial. Because the function B takes account of the decrease in the yield of biodiesel fuel due to the soap formation, the value of B is maximum even though the yield of biodiesel fuel decreases. The optimum ratio of KOH/oil deviates 4.3% for the 10% deviation of PMeOH, while it deviates only –1.5% for the 10% deviation of PKOH.

Figure 7 : Relationship between the cost of KOH and optimum ratio of KOH/oil

Figure 8 : Relationship between the cost of methanol and optimum ratio of KOH/oil

4. Conclusions

This two-step batch process o�ers high conversion when biodiesel fuel is produced by KOH catalyzed transesteri�cation of used vegetable oils. The optimum reaction conditions to reduce the concentration of unreacted glycerides to less than 1 wt% were determined by the ratios of KOH/oil and methanol/oil. The ratio of KOH/oil should be increased when the ratio of methanol/oil is decreased. However, the yield of biodiesel fuel decreased with increasing ratio of KOH/oil. The optimum ratio of KOH/methanol is in�uecned by the costs of KOH, methanol, and produced biodiesel fuel. The price of methanol would be most important for the determination of the optimum reaction condition.

Ackowledgement

This work was supported by an Adaptable and Seamless Technology Transfer Program through Target-driven R&D, Japan Science and Technology Agency (JST).

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influence of the acid value on biodiesel fuel production using a two

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