Comparative Study of Biodiesel Preparation Methods

Transcription

1 SUST Journal of Science and Technology, Vol. 19, No. 5, 2012; P:19-26 Comparative Study of Biodiesel Preparation Methods (Submitted: June 10, 2012; Accepted for Publication: November 29, 2012) Kaniz Ferdous* 1, 2, M. Rakib Uddin 1, Maksudur R. Khan 1, 2, 3 and M. A. Islam 1,2 1 Dept. of Chemical Engineering and Polymer Science, 2 Centre for Environmental Process Engineering, Shahjalal University of Science and Technology, Sylhet, Bangladesh. 3 Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, Gambang, Kuantan, Pahang, Malaysia * engr_kaniz@yahoo.com Abstract Biodiesel is usually produced from food-grade vegetable oils using transesterification process. Base catalyzed transesterification reaction is widely used for biodiesel production from vegetable oil due to its faster kinetics than that of acid catalyzed process. But if free fatty acid (FFA) content in the oil is more than 2%, the base catalyzed process is not feasible. In the present paper biodiesel is prepared from non-edible oils, such as nahor seed oil (NSO) and rubber seed oil (RSO) by different methods. Oil was extracted by different method from both seeds. The FFA of NSO reduces from wt% to 2. wt% and kinematic viscosity reduces from mm 2 /s to 6.64 mm 2 /s. The corresponding value for RSO were 45 wt% to 1.85 wt% and 33 mm 2 /s to 4.5 mm 2 /s respectively. The difficulties and importance of each processes are discussed and the biodiesel properties of biodiesel produced from both NSO and RSO were measured and compared with standard value. Keywords: Biodiesel; Transesterification; Esterification; Saponification. 1. Introduction The vast majority of motor vehicles used around the world rely on four-stroke internal combustion engines run on petroleum-based fuel or biodiesel. Biodiesel, which is a new, renewable and biological origin alternative diesel fuel, has been receiving more attention all over the world due to the energy needs and environmental consciousness. In comparison with petro-diesel, vegetable oils have their own advantages: first of all, they are renewable as the vegetables which produce oil seeds can be planted year after year; secondly, they are available everywhere in the world; thirdly, they are greener to the environment, as they seldom contain sulfur element in them [1]. Use of biodiesel is catching up all over the world especially in the developed countries. Most developed countries are moving from voluntarily to obligatory legislations to increase the market share of biofuels within the transport sector (e.g. set up of a mandatory biofuel target of 10% by the European Commission for the European transport sector in 2020) [2]. The end cost of the biodiesel mainly depends on the price of feedstock. Although biodiesel is currently produced from high quality food-grade vegetable oils using methanol and an alkaline catalyst, but it can be made from any plant oils with over 350 oil-bearing crops being identified for the production of biodiesel [3]. The predominant feedstock in the Europe is rapeseed, while soybean is the most widely used feedstock in the United States (US). Feedstocks such as palm, jatropha and coconut are common in Malaysia, India and the Philippines, respectively [3]. Gui et al [4] made a thorough analysis on the production of biodiesel from edible and non-edible oils. Generally the cost of raw materials accounts about 70-% of the total production cost of biodiesel. Four major techniques (dilution, microemulsion, pyrolysis, and transesterification modification techniques) are used for biodiesel production, among them transesterification process has been widely used to reduce the high viscosity of the oil. Transesterification reaction can be catalyzed by both homogeneous (alkalies and acids) and heterogeneous catalysts. The most commonly used alkali catalysts are NaOH, CH 3 ONa, and KOH [5]. It has been found that the alkaline-catalyzed transesterification process is not suitable to produce esters from unrefined oils [6] where the FFA

2 20 Kaniz Ferdous, M. Rakib Uddin, Maksudur R. Khan and M. A. Islam content is higher. In order to prevent saponification during the reaction, FFA and water content of the feed must be below 0.5 wt.% and 0.05 wt.%, respectively. Because of these limitations, only pure vegetable oil feeds are appropriate for alkali-catalyzed transesterification without extensive pre-treatment [7]. Homogeneous acidcatalyzed reaction is about 4000 times slower than the homogeneous base-catalyzed reaction and hence is not popular for industrial production of biodiesel. A two-step process is developed recently for the biodiesel production from oil, where in the first step acidcatalyzed esterification is conducted to convert the FFA to fatty acid methyl ester (FAME) followed by the base catalyzed transesterification to convert the triglyceride (TG) to FAME. Another approach was reported, where FFA was produced from oil by saponification-acidification reaction and FAME is produced by acid catalyzed esterification reaction [8]. In the present paper biodiesel is prepared from non-edible oils, such as NSO and RSO by different methods. NSO and RSO have high oil content and these two oil seed are available in Bangladesh. For these reasons both NSO and RSO was selected for biodiesel production. The difficulties of each process are discussed and the biodiesel properties are measured and compared with standard value. 2. Materials and Methods 2.1. Chemicals Methanol (99-100%), ethanol (99-100%), sodium hydroxide pellets (96%), potassium hydroxide pellets (>84%), phenolphthalein (ph ), starch, acetone (99%), n-hexane (96%), hydrochloric acid (37%), isopropanol, iodine, sodium iodide, glacial acetic acid, bromine, carbon tetrachloride, s-diphenylcarbazide, potassium dichromate etc. were purchased from Merck, Germany Ltd. All the chemicals used were analytical reagent grade Extraction of oil Different methods were used to extract oil from the seed [9]. For mechanical press, a vertical, manually operated, cylindrical (4.3 cm ID) mechanical press was used. For cold percolation process, hexane was used as a solvent. A Soxhlet Extraction unit was used for oil extraction where hexane was refluxed for 6 h for a given amount of karnel mass Biodiesel preparation from oil Biodiesel from non-edible oil was prepared by several methods, which are discussed below: Single step method Biodiesel from non-edible oil was prepared by single step method i.e acid catalyzed transesterification reaction [10]. The reaction was carried out at 70 0 C and atmospheric pressure under reflux for 18 hours with vigorous stirring. Typically 50 gm of oil sample were placed in a three-necked 250 ml round bottom flask equipped with a reflux condenser. The flask was placed in an electric heater with a temperature controller and magnetic stirrer. Concentrated sulfuric acid (2 wt % of oil) was mixed in required amount of methanol. Methanol was used 9:1 molar ratio to oil. The methanesulfonic acid solution was transferred into the reaction flask. After 18 hours, the contents were cooled to room temperature. After the reaction period, the reaction product was allowed to stand 2-3 hours in a separatory funnel. Two separate layers were observed. Upper layer was methyl ester (Biodiesel) and lower layer were a mixture of crude glycerin and lye catalyst. The Biodiesel layer was separated and this layer was opaque as it contained some catalysts, methanol and TG. For purification of the biodiesel given hot water wash. For this purpose hot water and biodiesel mixed together in separatory funnel and were shaked. Then allow stand for phase separation. Water is heavier than biodiesel and absorbs the excess alcohol, catalyst and impurities. After washing and settling, the water and the impurities in the water were drained from the bottom of the separatory funnel. Several wash cycle were generally needed. The first washed water drained from funnel was milky, and the final washed water drained off was clear. The washed biodiesel was then dried at C under vacuum Two-step method A two-step acid-catalyzed methanolysis of higher FFA containing oil was adopted for the conversion of FFA and TG to FAME [11]. In brief, in the first step esterification was carried out at 60 0 C and atmospheric pressure with a molar ratio of FFA /methanol as 1/5. Sulfuric acid (2 wt% of oil) was used as catalyst and reaction was carried out for 2 h. After 2 h the contents were cooled to room temperature and reaction product was washed with hot water in a seperatory funnel until clear water found and dried under vacuum at C for 20 min. In the second step acid

3 Comparative Study of Biodiesel Preparation Methods 21 catalyzed transesterification oil was reacted with methanol in presence of sulfuric acid (2 wt% of oil) as catalyst. Oil/methanol molar ratio of 1/9 was used. The methanolysis was performed under vigorous stirring at C. After 6 h the contents were cooled to room temperature, and reaction product was washed with hot water until clear water found. The organic phase was collected and dried under vacuum at C for 30 min Three-step method In this method the raw seed oil was saponified, acidified and esterified sequentially. Saponification was carried out for 30 min with different stoichiometric amount of alcoholic sodium hydroxide solution at 60 0 C [8]. In further study saponification of oil was done by aqueous calcium oxide solution. The reaction time and molar ratio of oil to calcium oxide were optimized. After saponification, the soap solution were treated with different stoichiometric amount of concentrated hydrochloric acid at a temperature of C with vigorous stirring. After dissolving the soap the fatty acid contents were separated in separatory funnel. The fatty acid content was determined by titrimetric method. Esterification of FFA was carried out as similar the first step of the two-step method. The molar ratio of FFA to methanol, catalyst concentration and reaction temperature were optimised. The biodiesel was washed and dried under vacuum Analytical Methods for Oil and Biodiesel FFA in the oil and biodiesel samples was analyzed by the method described in AOCS Aa 6-38 [12]. Saponification value (SV) was determined by the method described by Jeffery et al. [13]. The iodine value (IV) were determined by titrating 0.01 N sodium thiosulfate to the mixture of tested fuel and chemical reagents until the disappearance of the blue color based on the analysis methods of American Oil Chemist s Society [12]. Physical properties such as color, moisture content, density and calorific value of the oil were determined by following ASTM D 1500, ASTM D 1744 (Karl fisher method), ASTM D 14/81 and ASTM D 240 respectively. Kinematic viscosity, flash point, pour point and cloud point were determined by standards ASTM D 445, ASTM D 93 (Pensky- Martens Flashpoint Apparatus, Lazer Scientific Inc, Germany), ASTM D 2500 and ASTM D 97 respectively. 3. Result and Discussion 3.1. Extraction of oil and Characterization The oil was extracted from the seed by different methods and the oil content in the seed presented in Table 1. Table 1: Oil contents in nahor and rubber seed by different methods Press Cold percolation Press with solvent Sohxlet Seed wt.% wt.% wt.% wt.% Nahor Seed Rubber Seed Properties of raw oil are given in Table 2. Table 2: Properties of raw oil. Properties NSO RSO Physical state Liquid Liquid Color Brown Brown Specific gravity at 15 0 C Viscosity (mm 2 /s) at 25 0 C FFA wt. % Average molecular weight of FFA (gm/mol) Molecular weight of oil (gm/mol) Saponification value (mg of KOH/gm of oil) Iodine value (g I 2 /100 g oil) Moisture content (wt. %)

4 22 Kaniz Ferdous, M. Rakib Uddin, Maksudur R. Khan and M. A. Islam 3.2. Properties of biodiesel prepared by different methods: Biodiesel prepared by single-step (transesterification) and two-step methods The properties of biodiesel for acid catalyzed transesterification reaction and two-step method are presented in Table 3. The final viscosity of the biodiesel by two-step method is lower than the single step acid catalyzed process. RSO yields poor results than the NSO due to its high FFA content in the oil. The acid catalyzed single step process is time consuming and even after 18 h the viscosity of the biodiesel is not comparable with the standard biodiesel viscosity. However, there are some drawbacks in two-step process also. The second step, transesterification requires 6 h instead of 2 h reported by Zullaikh et al [11]. And the final biodiesel viscosity is higher than the standard value reported by Balat and Balat, 2008 [1] (Table 3) Table 3: Properties of acid catalyzed transesterification and two-step method products Property Trannsesterificati on Two-step Biodiesel Standard NSO RSO NSO RSO Value [1] Color Black Dark Black Dark brown - brown Kinematic Viscosity (mm 2 /s) at 22 0 C Saponification value (mg KOH/gm oil) FFA (wt%) Trace Specific gravity Biodiesel prepared by three-step method FFA was prepared from oil by Saponification of oil followed by acidification mentioned above. The results are represented in Fig. 1. From Fig. 1, it can be seen that the optimum molar ratio of sodium hydroxide to oil was 6:1 for both NSO and RSO. Further increase in the molar ratio of NaOH to oil the conversion of oil to soap remain unchanged FFA, wt.% N S O R S O N a O H /O il (m o l ra tio ) Fig.1: Preparation of FFA from NSO and RSO [ Reaction temperature 60 0 C and time 30 min.]. In further study, saponification was carried out by aqueous calcium oxide solution and FFA prepared from NSO. The results are shown in Fig 2. From Fig. 2, it was seen the optimum molar ratio of oil to calcium oxide was 1:2 for NSO and time was 1 hour.

5 Comparative Study of Biodiesel Preparation Methods 23 Percent FFA O il: C a O = 1 : 3 O il: C a O = 1 : 2 O il: C a O = 1 : T im e (m in.) Fig 2: Preparation of FFA from NSO [Reaction temperature C]. FFA obtained from oil was subjected to esterification reaction to convert FAME. The reaction was carried out with different FFA/methanol molar ratio in presence of catalyst (HCl). Effect of methanol/ffa molar ratio was investigated at 60 0 C and HCl used 5 wt% of FFA as catalyst. The results are presented in Fig. 3. The maximum conversion of FFA to FAME was found for methanol/ FFA molar ratio of 6:1 after 120 minutes for both oil and conversions were 97.2% and 98.15% for NSO and RSO respectively. Further increase in molar ratio of alcohol to FFA the conversion of FFA and viscosity remain unchanged Conversion, % methanol/ff A, mol ratio F F A from N S O F F A from R S O Fig. 3: Effect of FFA/methanol molar ratio on esterification reaction [T= 60 C, Catalyst concentration 5.0% wt of FFA, Reaction time 120 min]. Effect of Catalyst Concentration was studied at methanol/ FFA molar ratio of 1:6 at 60 0 C and the results are presented in Fig. 4 (a) and (b). Fig 4 shows that around 98 % conversion of FFA can be achieved by catalyst concentration 5 wt% of FFA. Further increase in catalyst concentration has no impact on FFA conversion.

6 24 Kaniz Ferdous, M. Rakib Uddin, Maksudur R. Khan and M. A. Islam 100 (a) NSO 100 (b) RSO FFA wt% wt% HCl Catalyst 5 wt% HCl Catalyst 3 wt% HCl Catalyst FFA, wt.% Catalyst concentration = 3 wt.% of FFA Catalyst concentration = 5 wt.% of FFA Catalyst concentration = 7 wt.% of FFA Time (min) Time (min) Fig 4: Effect of catalyst concentration on esterification reaction [T = 60 C, FFA / methanol ratio 1:6, Reaction time 120 min]. Effect of Temperature on esterification reaction was studied with NSO derived FFA. The results are presented in Fig 5. It can be seen that the temperature has a significant effect on the conversion of esterification reaction. As the temperature increased, the conversion was also increased. 100 FFA wt% C 50 0 C 40 0 C 30 0 C Time (min) Fig. 5: Effect of temperature on esterification reaction for NSO [methanol/ FFA molar ratio 6:1, Catalyst (HCl) 5.0% wt of FFA, Reaction time 120 min] 3.4. Some Properties of biodiesel produced by three-step method Some of the important quality parameters of biodiesel (viscosity, density, flash point, cloud point, pour point, calorific value, FFA content, copper strip corrosion etc.) are shown in Table 4. Physical properties of biodiesel were compared with standard biodiesel properties. All of the measured values were in the range of standard biodiesel values except the FFA content which was found higher in the biodiesel prepared by three-step method. But as the copper strip corrosion test showed non corrosive nature of the biodiesel, it can be safely used in the diesel engine.

7 Comparative Study of Biodiesel Preparation Methods 25 Table 4: Some properties of biodiesel produced by three- step method Properties Biodiesel from NSO Biodiesel from RSO Biodiesel Standard Value [1] Specific gravity at 15 0 C Pour point ( C) to 10 Copper Strip Corrosion ( 3 hours at Non Non Non corrosive C) corrosive corrosive Flash point ( C) Fire point ( C) Kinematic Viscosity (mm 2 /s) at 38 C FFA wt% Trace Moisture content, % vol Calorific value (Gross Kcal/Kg) Saponification value (mg KOH/gm oil) Iodiene value Free glycerol, % wt. Nil Nil 0.02 max. Total glycerol, % wt. Nil Nil 0.24 max. 4. Conclusion Biodiesel has been prepared from NSO and RSO by single-step method, two-step method and three-step methods. Among these methods single-step (transesterification) is best because it requires fewer amounts of equipment and investment. But base catalyzed tranesterification is not used for the preparation of biodiesel from NSO and RSO because of their high FFA content. Acid catalysed transesterification reaction was done for the preparation of biodiesel from both NSO and RSO. The viscosity of oil reduces from mm 2 /s to mm 2 /s and FFA reduces wt% to 2.12 wt% for NSO and 33 mm 2 /s to mm 2 /s and FFA 45 wt% to 8.5 wt% for RSO after 18 h of reaction. The product was not used as biodiesel because of its higher viscosity and FFA and the process was time consuming. Acid catalysed two-step method was performed for the preparation of biodiesel from both oil. The viscosity of NSO reduces from mm 2 /s to 9.64 mm 2 /s which was slightly higher than biodiesel standard. FFA of the oil reduces from wt% to 0.64 wt% and for RSO Viscosity changes from 33 mm 2 /s to 14.5 mm 2 /s and FFA 45 wt% to 6.2 wt%. Three-step method, where FFA was produced by saponification and acidification of oil and thereafter biodiesel was produced by esterification of FFA, showed comparable biodiesel properties and similar optimum conditions for both oil. Hence three-step method can be used for biodiesel production from non-edible feed stocks. Biodiesel properties was measured by standard methods and compared with the standard biodiesel properties. 5. Acknowledgement The authors express their deep gratitude to University Grants Commission (UGC) for financial support and to Eastern Refinery Ltd. (ERL) for their laboratory facilities for conducting this research.

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