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Energy & Fuels 2007, 21, 2998-3003
Optimization of the Transesterification Process for Biodiesel Production and Use of Biodiesel in a Compression Ignition Engine S. Sivaprakasam* and C. G. Saravanan Department of Mechanical Engineering, Annamalai UniVersity, Annamalainagar 608 002, Chidambaram, Tamil Nadu, India ReceiVed October 17, 2006. ReVised Manuscript ReceiVed June 2, 2007
The main objective of the work is to optimize the transesterification process for biodiesel production to yield biodiesel that has the best properties, such as viscosity, cetane number, flash point, fire point, and calorific value. Jatropha oil (nonedible vegetable oil) was used to produce biodiesel. Conventional diesel fuel was the base fuel for comparing the above properties of biodiesel. The effect of the quantity of methanol and sodium hydroxide and reaction time were studied to optimize the esterification process. Diesel, blends of biodiesel, and neat biodiesel were tested in a twin-cylinder, water-cooled, four-stroke direct injection diesel engine. Performance and emission of the engine with conventional diesel fuel was used as the basis for comparison.
1. Introduction The world petroleum situation because of the rapid depletion of fossil fuels and the degradation of the environment because of the combustion of fossil fuels have caused a resurgence of interest in finding alternative fuel. Internal combustion engines, which form an indispensable part in the transportation as well as mechanized agriculture systems, have been badly affected by the two crises. Thermodynamic tests based on engine performance evaluation have established the feasibility of using a variety of alternative fuels, such as hydrogen, compressed natural gas (CNG), alcohol, biogas, producer gas, and a host of vegetable oil. The process of utilizing vegetable oil in the internal combustion engines for transport as well as other applications is gaining momentum. The International Energy Agency has recognized vegetable oil as one of the alternative fuels for the transport sector. There are four ways to use vegetable oil in a diesel engine: (i) direct use or blending in diesel fuel, (ii) microemulsions in diesel fuel, (iii) thermal cracking (pyrolysis) of the vegetable oil, and (iv) transesterification to produce biodiesel. Of these, esterification appears to be the most popular and best way to use vegetable oil. A total of 100 years ago, Rudolf Diesel tested vegetable oil as a fuel for his engine. Recently, because of the increase in crude oil prices, limited resources of fossil oil, and environmental concerns, there has been a renewed focus on vegetable oil and animal fats to make biodiesel fuels. Reports on the use of biodiesel in diesel engines indicate a substantial reduction in SO2, CO, and polycyclic aromatic hydrocarbons (PAHs). In most developed countries, biodiesel is produced from palm oil, soybean oil, sunflower oil, coconut oil, rapeseed oil, and peanut oil, which are essentially edible in the Indian context. There is a vast resource of nonedible seeds, which can be grown in the wasteland of the country to produce biodiesel. Some effort has been made to utilize Jatropha as biodiesel as mentioned by Dhanda.3 * To whom correspondence should be addressed. Telephone: +9198423-11137. Fax: +91-4144-238275. E-mail:
[email protected]. (1) Barnwal, B. K.; Sharma, M. P. Prospects of Biodiesel Production from Vegetable Oils in India. Renewable Sustainable Energy ReV. 2005, 9, 363-378.
With the viscosity of vegetable oil being 5-10 times more than that of diesel fuel, the spray characteristics of these vegetable oils are different, which leads to a different heatrelease pattern and emission characteristics. High viscosity can cause poor atomization, large droplets, and high spray jet penetration. This results in poor combustion, accompanied by losses. The high molecular weight of vegetable oil results in low volatility as compared to the diesel fuel, which leads to the oil sticking to the injector or cylinder walls. Jatropha oil has long-chain fatty acids; oleic acid is its major component. Oleic acid is a straight-chain acid of 18 carbon atoms with one double bond. Linoleic acid is the second major component of the Jatropha oil, which has two double bonds.4 Thus, vegetable oil in its raw form cannot be used in engines. In this work, optimization of the transesterification process to produce biodiesel from Jatropha oil was carried out. Methyl ester of Jatropha oil was investigated for its performance, emission, and combustion characteristics in a diesel engine. Parameters such as brake thermal efficiency, fuel consumption, smoke, particulate matter, and oxides of nitrogen emissions were evaluated. The performance of the engine with diesel as a fuel was used as the basis for comparison. All tests were conducted at various load conditions at a constant speed of 1500 rpm. 2. Experimental Section A laboratory-scale biodiesel production setup was designed and fabricated in the laboratory as shown in Figure 1. It consists of a motorized stirrer, straight-coil electric heater, and stainless-steel containers. The system was designed to produce 5 kg of biodiesel at a time.3 The power required for heating the biodiesel from 30 to 70 °C in (2) Meher, L. C.; Vidya Sagar, D.; Naik, S. N. Technical Aspects of Biodiesel Production by TransesterificationsA Review. Renewable Sustainable Energy ReV. 2004, 9, 1-21 (3) Dhanda, K. S. Experiences and Expectations from Jatropha Plantations, Proceedings of the International Conference on Biofuels, India, May 2003. (4) Patterson, J.; Hassan, M. G.; Clarke, A.; Shama, G.; Heligardt, K.; Chen, R. Experimental Study of DI Diesel Engine Performance Using Three Different Biodiesel Fuels. SAE International 2006 World Congress; Detroit, MI, April 3-6, 2006.
10.1021/ef060516p CCC: $37.00 © 2007 American Chemical Society Published on Web 07/07/2007
Optimization of the Transesterification Process
Figure 1. Biodiesel plant.
10 min was estimated. The temperature of the biodiesel was maintained at about 70 °C.
3. Biodiesel Production from Vegetable Oil There are three basic routes to produce biodiesel by transesterification of vegetable oil, namely, (i) base-catalyzed transesterification, (ii) acid-catalyzed transesterification, and (iii) conversion of oil to fatty acids and then to esters by acid catalysis. These methods are employed depending upon the quality of vegetable oil (free fatty acid and moisture content) available. The base-catalyzed transesterification process is the most preferred process and is used in the present work because this is carried out at normal temperature and pressure and has a high conversion rate. The acid-catalyzed transesterification process takes a very long time (greater than 10 h) to complete at reflux temperature, which may not be commercially viable to use. The base-catalyzed transesterification is a commercially used process because of the following reasons: (i) low temperature (70 °C) and pressure (1 atm) are required; (ii) high conversion (greater than 98%) can be obtained; (iii) it involves direct conversion and no intermediate steps; and (iv) it requires ordinary material for the construction of the setup. The most commonly used method in the transesterification of vegetable oils and animal fat2 is by converting the triglyceride of oils to methyl (or ethyl) esters. In the transesterification process, alcohol reacts with oil to release three “ester chains” from the glycerin backbone of each triglyceride. The reaction requires heat and a strong base catalyst to achieve complete conversion of the vegetable oil into the separated esters and glycerol. 4. Transesterification Process Transesterification is the process in which fat or oil reacts with an alcohol to form esters and glycerol. A catalyst is used to improve the reaction rate and yield. Because the reaction is reversible, excess alcohol is used to shift the equilibrium to the product side. Among the alcohols that can be used in the transesterification process are methanol, ethanol, propanol, butanol, and amyl alcohol. Methanol and ethanol are used most frequently, especially methanol, because of its low cost and physical and chemical advantages (polar and shortest chain alcohol). It can quickly react with triglycerides, and KOH is easily dissolved in it. However, methanol is toxic, and its production depends upon fossil fuels. There has been a trend toward the use of ethanol, which can be produced from biomass, making it possible to produce biodiesel completely from renewable sources. To complete the transesterification stoichiometrically, a 3:1 molar ratio of alcohol/triglycerides is needed. Alkalis, acids, or enzymes can catalyze the reaction. The alkalis
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include NaOH, KOH, carbonates, and corresponding sodium and potassium alkoxides, such as sodium methoxide, sodium ethoxide, and sodium peroxide. Sulfuric acid, sulfonic acids, and hydrochloric acid are usually used as an acid catalyst. Alkali-catalyzed transesterification is much faster than acidcatalyzed transesterification and therefore is most often used commercially, as mentioned earlier. Low free fatty acid content in triglycerides is required for alkali-catalyzed transesterification. If more water and free fatty acids are in the triglycerides, acidcatalyzed transesterification can be used. After transesterification of triglycerides, the products are a mixture of esters, glycerol, alcohol, catalyst, and tri-, di-, and monoglycerides. The coproduct, glycerol, needs to be recovered because of its value as an industrial chemical, such as CP glycerol, USP glycerol, and dynamite glycerol. Gravitational settling or centrifuging separates glycerol. The catalysts generally used for base-catalyzed production of biodiesel are potassium hydroxide (Caustic potash) and sodium hydroxide (Caustic soda). In this study, potassium hydroxide and methanol in various proportions were used to study their effects on the transesterification process. The vegetable oil (Jatropha oil) was charged into a closed reactor and heated to 70 °C, and then a mixture of alcohol and sodium hydroxide was added into the reactor. The reaction temperature was maintained in the range from 65 to 70 °C. The recommended reaction time was varied from 1 to 2 h. Once the reaction was completed in the reactor, a mixture was formed. This mixture contained two major products, namely, biodiesel and glycerol. The mixture was transferred into a separating funnel. The two separate layers were formed, with the upper layer being the methyl ester and the rest being glycerol. The glycerol phase was much denser than the biodiesel phase, and these two phases would be separated by gravity. The glycerol could be simply drawn off from the bottom of the separating funnel. After the glycerol layer was discarded from the separating funnel, the methyl ester was mixed with distilled water, shaken gently, and allowed to settle for 10 min. The procedure was repeated 3-4 times. The remaining moisture from the purified ester (biodiesel) was removed by keeping it in an electric oven, which was maintained at 105 °C. The product obtained was the methyl ester of the Jatropha oil (biodiesel). 5. Test Engine A 10 hp, 1500 rpm, twin-cylinder, 4-stroke, water-cooled, direct-injection, vertical diesel engine as shown in Figure 2 was selected for the present research work because it has a wide application in the agricultural sector. It is connected with a swing-field electrical dynamometer. The engine was run on neat diesel, four different blends of biodiesel and diesel fuels, namely, B20, B40, B60, and B80, and neat biodiesel (B100). The engine was loaded to the maximum load condition. The brake-specific fuel consumption (BSFC) and brake thermal efficiency (BTE) were calculated. An AVL 444 five-gas analyzer was used to measure the oxides of nitrogen. An AVL 437 smokemeter was used to measure the opacity of exhaust gases. 6. Results and Discussion Experimental work was divided into two phases. The first phase involves the optimization of the production of biodiesel. The second phase involves the use of biodiesel in a twin-cylinder diesel engine.
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Figure 2. Experimental setup.
Figure 3. Effect of the potassium hydroxide (KOH) on biodiesel yield and viscosity.
Figure 4. Effect of the methanol quantity on biodiesel yield and viscosity.
7. Production of Biodiesel
value. On the basis of the yield of biodiesel and its viscosity considerations, the optimum quantity of KOH was taken to be 8 g. 7.2. Effect of the Amount of Methanol. For this study, 500 mL of vegetable oil, 8 g of potassium hydroxide as a catalyst, and a reaction time of 1.0 h in each sample were taken as the standard quantity and methanol was varied from 50 to 250 mL. Figure 4 represents the percentage yield of biodiesel and viscosity as a function of the methanol quantity. It was seen that there was no conversion at 50 mL of methanol. The percentage of biodiesel yield increased as the methanol quantity increased. As seen in Figure 4, when the methanol quantity was varied from 50 to 250 mL, the percentage of biodiesel yield became nearly constant beyond about 50 mL of methanol. Hence, one could conclude that the conversion of vegetable oil into biodiesel was almost completed with 100 mL of methanol. It is clear from the figure that, if the methanol quantity in the sample is increased, the viscosity variation does not follow any specific trend. The lowest viscosity was obtained for the sample using 150 mL of methanol. The highest viscosity was observed for the sample using 200 mL of methanol. On the basis of the consideration on the cost of methanol, it was decided that 90 mL of methanol was a suitable quantity. With this quantity of methanol, the yield is fairly high and viscosity is fairly low. 7.3. Effect of the Reaction Time. For this study on the effect of the reaction time on the transesterification process, 500 mL of vegetable oil, 90 mL of methanol, and 8 g of potassium hydroxide as a catalyst were taken. The reaction time was varied
The process of producing biodiesel by transesterification involved the following: mixing of alcohol and the catalyst, reaction in the reactor, separation, and methyl ester water washing. During the process of optimization, the following parameters were varied: (1) concentration of potassium hydroxide (KOH), (2) quantity of methanol, and (3) reaction time. The following section gives the details of the effects of the above parameters in producing biodiesel. 7.1. Effect of the Sodium Hydroxide (KOH) Concentration. For this study, 500 mL of vegetable oil, 90 mL of methanol, and a reaction time of 1.0 h for each sample as a standard quantity were taken and potassium hydroxide as a catalyst was varied from 6 to 8 g. A plot of the percentage of biodiesel yield and viscosity versus quantity of potassium hydroxide is given in Figure 3. It was found that the optimum catalyst amount works out to be about 7.5 g of catalyst per 500 mL of oil to get the maximum percentage of biodiesel yield as seen in Figure 3; otherwise, 7 g of KOH quantity was the best from the consideration of flash and fire points. It is clear from the figure that the viscosity was lowest (i.e., 4.33 cSt) for the sample using 8 g of potassium hydroxide and the highest viscosity (i.e., 4.51 cSt) was observed for the sample using 7.75 g of potassium hydroxide. Incidentally, the sample, which had the lowest viscosity, also had the lower calorific
Optimization of the Transesterification Process
Figure 5. Effect of the reaction time on biodiesel yield and viscosity.
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Figure 7. Effect of the brake power on fuel consumption. Table 1. Optimized Transesterification Process oil (mL) methanol (mL) NaOH (g) reaction time (min) reaction temperature (°C) yield (mL) viscosity (cSt) HHV (MJ/kg)
500 90 8 60 70 455 4.06 36.29
of biodiesel was more or less the same. On the basis of the above figure, one can conclude that the 60 min reaction time was sufficient to produce biodiesel.
Figure 6. Effect of the reaction time on biodiesel yield and viscosity for the optimized sample.
from 5 to 180 min. A plot of the percentage yield of biodiesel and viscosity versus reaction time is shown in Figure 5. The reaction with zero time was, of course, zero. The percentage yield increased drastically with time in the starting phase of 5-10 min; however, the quality of conversion was doubtful. After that, the yield became nearly constant but the quality improved with time. The best time could be about 90 min. It is noticed from the figure that the lowest viscosity was obtained for the sample whose reaction time was 30 min and the highest viscosity was obtained for the sample whose reaction time was 5 min. 8. Optimization of Transeserification The effect of the temperature on the transesterification process was studied earlier. The arrived conclusion was that the sample that consisted of 500 mL of vegetable oil, 90 mL of methanol, 8 g of potassium hydroxide, and a reaction time of 1.0 h had properties fairly close to diesel fuel. It was necessary to see the effect of the reaction time on the transesterification process on the optimized sample as mentioned above. Figure 6 gives the plot of the percentage yield of biodiesel and viscosity versus reaction time for the optimized sample. It is observed that the maximum biodiesel yield was obtained for the sample whose reaction time was 5 min and the lowest yield was obtained for the sample whose reaction time was 15 min. Another peak occurs at a reaction time of about 60 min. A third peak occurs at a reaction time of 150 min. For other reaction times, the yield
It is seen in the figure that the lowest viscosity was observed for the sample whose reaction time was 180 min and the highest viscosity was observed for the sample whose reaction time was 120 min. Samples whose reaction times were 30, 60, and 90 min had nearly the same viscosity. Therefore, one could select the sample whose reaction time was 60 min as a standard sample for biodiesel production in the laboratory. It has been observed that the transesterification process was optimal for the sample details, as given below in Table 1.
9. Engine Test Results Performance and emission tests were carried out using biodiesel from the optimized transesterification process. Pure diesel, blends of 20, 40, 60, and 80% (by volume) biodiesel in diesel, and pure biodiesel were used in a twin-cylinder 4-stroke diesel engine. The results of these tests are discussed below. 9.1. Effect of the Brake Power on Fuel Consumption. Figure 7 compares the fuel consumption of diesel, blends of biodiesel with diesel, and neat biodiesel at various brake powers in the range of 0-10 kW. It was observed that the fuel consumption of all of the types of fuels tested decreased with an increase in the brake power. However, for the B-20 blend, the fuel consumption was lower when compared to other blends of biodiesel and, with neat diesel fuel, when the applied load was greater than 1.5 kW. The B-100 fuel (neat biodiesel) had the highest specific fuel consumption. This is probably due to the lower calorific value of neat biodiesel compared to its blends and diesel. Very little difference can be seen even for all blends of biodiesel at higher loads; however, as the brake power of the engine increases, the specific fuel consumption decreases
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Figure 8. Effect of the brake power on thermal efficiency.
for all fuels, pure as well as blends. The results are comparable with those of Parmanik.6 9.2. Effect of the Brake Power on Brake Thermal Efficiency. The variation of brake thermal efficiency of the engine with all of the types of fuels tested is shown in Figure 8. The thermal efficiency of diesel fuel was the basis of comparison for other fuels. From the test results, it is observed that initially with an increasing brake power the brake thermal efficiencies of the diesel fuel, blends of biodiesel, and neat biodiesel increased and the maximum thermal efficiencies were obtained at maximum brake power. There was a considerable increase in efficiencies with the blends of biodiesel compared to the efficiency of diesel fuel alone. The maximum value of brake thermal efficiencies with B-20 blend and diesel were observed as 28.20 and 28.56%, respectively. Maximum brake thermal efficiencies of 25.63% (pure biodiesel) and 28.56% (neat diesel) were observed. The maximum thermal efficiency of 28.20% was achieved with B-20 blend, whereas, with B-100 fuel, thermal efficiency was 25.63%. The drop in thermal efficiency with an increase of the blend quantity may be attributed to poor combustion characteristics because of higher viscosity and lower volatility. From the above results, it can be seen that B-20 blend shows a lot of promise. 9.3. Effect of the Brake Power on Exhaust NOx Emission. Plots of NOx versus brake power are given in Figure 9. It was observed that NOx is a function of higher combustion temperatures and the availability of oxygen favors the formation of (5) Prasad, L. Optimization of Transesterification Process for Biodiesel Production and Use of Biodiesel in Compression Ignition Engine. 4th International Symposium on Fuels and Lubricants; New Delhi, India, October 27-29, 2004. (6) Pramanik, K. Properties and Use of Jatropha Curcas Oil and Diesel Fuel Blends in Compression Ignition Engine. Renewable Energy 2003, 28, 239-248. (7) Senthilkumar, M.; Ramesh, A.; Nagalingam, B. Investigation on the Use of Jatropha Oil and Its Methyl Ester as Fuel in a Compression Ignition Engine. J. Inst. Energy 2001, 74, 24-28. (8) Thaddeus, H.; Machacon, C.; Matsumote, Y.; Ohkawara, C. The Effect of Coconut Oil and Diesel Fuel Blends on Diesel Engine Performance and Exhaust Emissions. J. SAE, 2001. (9) Senatore, A.; Cardone, M.; Rocco, V.; Prati, M. V. A Comparative Analysis of Combustion Process in D.I. Diesel Engine Fuelled with Biodiesel and Diesel Fuel. Soc. Automot. Eng. 2000, paper no. 2000-01-0691. (10) Varaprasad, C. M.; Muralikrishna, M. V. S.; Prabhakar Reddy, C. Investigations on Biodiesel (Esterified Jatropha Curcas Oil) in Diesel Engines. The 15th National Conference on I.C. Engines and Combustion, Anna University, Chennai, India, 1997. (11) Verma, O. P. S.; Patel, K. L. Emerging Perspectives for Biodiesel in India. Soc. Automot. Eng. 2004, paper no. 2004-28-034. (12) Zhang, Y.; van Gerpen, J. H. Combustion Analysis of Esters of Soybean Oil in a Diesel Engine. Soc. Automot. Eng. 1996, paper no. 960765.
SiVaprakasam and SaraVanan
Figure 9. Effect of the brake power on NOx emission.
Figure 10. Effect of the brake power on smoke opacity.
NOx emission. Unlike diesel, biodiesel blends and neat biodiesel also contain more oxygen. This factor together with higher combustion temperatures favors the production of higher NOx with diesel combustion. The second reason for higher NOx emission for blends of biodiesel and neat biodiesel may be due to the presence of higher quantities of nitrogen in the fuel. This probably also explains why NOx levels are higher with neat biodiesel and diesel. This is in agreement with the results of Patterson et al.4 9.4. Effect of the Brake Power on Exhaust Smoke Density. The effect of brake power on the smoke density of exhaust gases is shown in Figure 10. At partial load, the smoke density for neat diesel fuel was the lowest as compared to blends of biodiesel and neat biodiesel fuels. The smoke density increased as the brake power increased, but at maximum brake power, the smoke density was nearly the same for all fuels, excluding diesel. The smoke density for blends of biodiesel and neat biodiesel is low because of less moisture in the fuel. This is agreement with the results of Prasad, L.5 This may be because optimizing injection timing may reduce the smoke density of blends of biodiesel. 10. Combustion The variation of the cylinder pressure, heat release rate, and peak pressure are shown in Figures 11-13. For all of the parameters, the value of B-20 Jatropha oil is the highest followed by diesel. The cylinder pressure of 70 bar for B20 Jatropha oil is followed by the diesel oil at 68 bar, at maximum load. The trends of the heat release rate are also the same. The peak
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increase in the pressure. This is due to high viscosity and poor volatility. This also leads to the poor atomization and poor air fuel mixture. Hence, the delay period has been improved. 11. Conclusions
Figure 11. Pθ diagram of neat diesel and Jatropha oil.
Figure 12. Heat release at neat diesel and Jatropha oil.
Figure 13. Maximum pressure at neat diesel and Jatropha oil.
pressure in a diesel engine depends upon the rate of combustion at the initial stage. As the diesel quantity in the blend is increased, the amount of fuel taking part in the uncontrolled combustion of the mixture is reduced, which results in an
Optimization of the transesterification process for biodiesel production depends upon various parameters such as methanol quantity, potassium hydroxide (catalyst), reaction time, reaction temperature, and stirrer speed. A few efforts were made to optimize the transesterification process by considering some of the above parameters. The main aim of this work was to optimize the transesterification process for biodiesel production, to yield biodiesel, which has the best properties, such as viscosity, cetane number, flash point, fire point, and calorific value. The pure diesel was the base fuel for comparing above combustion properties of biodiesel. The viscosity and cetane number are very important properties of the fuel for diesel engine operation, and other combustion properties are secondary. On the basis of the cost consideration, a sample consisting of 500 mL of vegetable oil, 90 mL of methanol quantity, and 8 g of potassium hydroxide was selected for process optimization based on the viscosity. The effect of the reaction time was studied for this sample. The optimized sample has the viscosity of 4.06 cSt that is close to the diesel fuel (viscosity of 3.08 cSt at 40 °C). It has been observed that the viscosity of optimized biodiesel is higher by 24% than that of diesel fuel and the calorific value of diesel was higher than biodiesel by 8%. The flash and fire points of optimized biodiesel were 185 and 217 °C, respectively, while, for diesel, the flash and fire points were 120 and 163 °C, respectively. This shows that biodiesel is safer for storage. The performance and emission tests were conducted on a diesel engine from no load to maximum load conditions at a constant speed. The following conclusions have been drawn while the engine was running on a full load condition. The comparisons of all of the results were performed on diesel fuel as the base fuel: (1) The fuel consumption (FC) was 7% lower for B-20 blend and 4% higher for B-100 fuel (neat biodiesel) as compared to the diesel fuel. The fuel consumption of other blends of biodiesel was seen with a small variation between the two fuels. (2) The brake thermal efficiency of B-20 blend was almost equal to that of diesel fuel. The brake thermal efficiency of other blends of biodiesel varied between B20 blend and diesel fuel. (3) The NOx emission in exhaust gases for B-20 blend was higher by 8%. The NOx emission of B-100 fuel (neat biodiesel) was decreased by 7% as compared to that of diesel fuel. The NOx emission in exhaust gases of other blends of biodiesel varied between these values. (4) The smoke density of exhaust gases for B-20 blend was much less than that of diesel fuel. The smoke density of exhaust gases of B100 fuel (neat biodiesel) and other blends was also lesser than that of diesel fuel. (5) The peak pressure and heat release of Neem oil were higher than sole fuel; however, a slight decrease in other blends was noticed. EF060516P
