CHAPTER 4 PRODUCTION OF BIODIESEL

Transcription

1 56 CHAPTER 4 PRODUCTION OF BIODIESEL 4.1 INTRODUCTION Biodiesel has been produced on a large scale in the European Union (EU) since 1992 (European Biodiesel Board 2008) and in the United States of America (USA) since 1993 (National Biodiesel Board 2008). Today, there are 120 plants in the EU and 165 plants in the USA, annually producing almost 6900 and 7100 million liters of biodiesel, respectively. The most common feedstock of biodiesel is rapeseed oil in Europe and soybean oil in the USA (Canakci 2007). The feedstock, alcohol and catalyst used in biodiesel production affect the biodiesel fuel properties. The fuel properties of biodiesel must meet EN specifications in Europe and American Society of Testing and Materials (ASTM) D-6751 specifications in the USA, shown in Table 4.1. No separate ASTM quality specifications currently exist for biodiesel when blended with fossil-derived fuels (Terry et al 2006). When these limits are met, biodiesel can be used in the most modern engines without any modification while maintaining the engine s durability and reliability (Gerpen 2005). 4.2 BIODIESEL PRODUCTION METHODS Neat vegetable oils are not suitable as fuel for diesel engines; hence they have to be modified to bring their combustion-related properties closer to those of mineral diesel. The fuel modification is mainly aimed at reducing the viscosity to get rid of flow and combustion-related problems. Considerable

2 57 efforts have been made so far to develop vegetable oil derivatives that approximate the properties and performance of hydrocarbon based fuels. In this section the various possible methods of producing biodiesel from vegetable oils are discussed. Table 4.1 Biodiesel standards, test methods and limits Property (units) ASTM 6751 test Methods ASTM 6751 Limits IS15607 test Methods IS15607 limits Flash point ( C) D-93 Min130 IS 1448 P:21 Min C (cst) D IS 1448 P: Sulfated ash (% mass) D-874 Max0.02 IS 1448 P:4 Max0.02 Sulfur (% mass) D-5453 Max0.05 ASTM D 5453 Max0.005 Cloud point ( C) D N.A IS 1448 P:10 N.A Cu corrosion D-130 Max 3 IS 1448 P: 15 Max 1 Cetane number D-630 Min 47 IS 1448 P:9 Min 51 Water, sediment (vol. %) D-2709 Max 0.05 D-2709 Max 0.05 CCR 100% (% mass) D-4530 Max 0.05 D-4530 Max 0.05 Neutralization value (mgkoh/g) D-664 Max 0.80 IS 1448 P:1 Max 0.50 Free glycerin (% mass) D-6584 Max 0.02 D-6584 Max 0.02 Total glycerin (% mass) D-6584 Max 0.24 D-6584 Max 0.25 Phosphorus (% mass) D-4951 Max0.001 D-4951 Max0.001 Distillation temperature C D % at 360 C Oxidation stability (hrs) N.A N.A EN Min 6h Thermal Cracking Thermal cracking or pyrolysis is the conversion of one substance into another by means of applying heat i.e. heating in the absence of air or oxygen with temperatures ranging from 450 to 850 o C. In some situations a catalyst is used as an aid to the reaction leading to the cleavage of chemical

3 58 bonds to yield smaller molecules. Unlike direct blending, fats can be pyrolysised (Demirbas 2000). The pyrolysis of fats has been investigated for over a hundred years, especially in countries where there is a shortage of petroleum deposits. Typical catalysts that can be employed in pyrolysis are SiO 2 and Al 2 O 3. Although, the products are chemically similar to pyro chemically based diesel, oxygen removal from the process decreases the products benefit of being an oxygenated fuel. This decreases its environment benefits and generally produces fuel more similar in properties to gasoline than diesel, with the addition of some low value materials Hydrolysis The hydrolysis of lipids forms a heterogeneous reaction system made up of two liquid phases. The dispersed aqueous phase consists of water and glycerol and the homogenous lipid phase consists of fatty acids and glycerides. The hydrolysis of glycerides takes place in the lipid phase in several stages via partial glycerides (di-glycerides and mono-glycerides). Zinc oxide in its soap form has been suggested to be the most active catalyst for hydrolysis reactions. Reaction without a catalyst is not economical below 210 o C, thus requiring the implication of high temperature, pressure techniques (Minami and Saka 2006). Modern continuous plants operate at pressures between 0.6 MPa and 1.2 MPa at 210 o C to 260 o C without a catalyst. This increased pressure allows the mutual solubility of the two phases Using Biocatalysts Biocatalysts are usually lipases; however, conditions need to be well controlled to maintain the activity of the catalyst (Rathore and Madras 2007, Modi et al 2007). Hydrolytic enzymes are generally used as biocatalysts as they are readily available and are easily handled. They are stable, do not

4 59 require co-enzymes and will often tolerate organic solvents, their potential for region-selective and especially for enantio-selective synthesis makes them valuable tools. Recent patents and articles have shown that reaction yields and times are still unfavorable compared to base-catalyzed transesterification for commercial application Catalyst Free Process Transesterification will occur without the aid of a catalyst; however, at temperatures below 300 o C, the reaction rate is very low (Ma et al 1998). It has been said that there are, from a broad perspective, two methods for producing biodiesel and that is with and without a catalyst Supercritical Methanol The study of the transesterification of rapeseed oil with supercritical methanol was found to be very effective (Hansen and Jasen 1997, Bala 2001, Saka and Kusdiana 2001, Demirbas 2003) and gave a conversion of about 95% within 4 minutes. A reaction temperature of 350 o C, pressure of 30 MPa and a ratio of 42:1 of methanol to oil for 4 min were found to be the best reaction conditions. The rate was substantially high from 300 to 500 o C but at temperatures above 400 o C it was found that thermal degradation takes place. Supercritical treatment of lipids with a suitable solvent such as methanol relies on the relationship between temperature, pressure and the thermo physical properties such as dielectric constant, viscosity, specific weight and polarity (Kusdiana and Saka 2001, Demirbas 2005, Bala 2005) Transesterification of Raw Oil Transesterification is the general term used to describe the important class of organic reactions, where the triglyceride reacts with an alcohol to

5 60 form esters and glycerol as shown in Figure 4.1, which is also called as alcoholysis. The transesterification is an equilibrium reaction and the transformation occurs by mixing the reactants. However, the presence of a catalyst accelerates considerably the adjustment of the equilibrium. The basic constituent of vegetable oil is triglyceride. Vegetable oils comprise of 90 to 98% triglycerides and small amounts of mono-glycerides, di-glycerides and free fatty acids. In the transesterification of vegetable oils, a triglyceride reacts with an alcohol in the presence of a strong acid or base, producing a mixture of fatty acid alkyl esters and glycerol. The overall process is a sequence of three consecutive and reversible reactions in which di-glycerides and monoglycerides are formed as intermediates. The stoichiometric reaction requires one mole of triglyceride and three moles of alcohol. However, an excess of alcohol is used to increase the yields of alkyl esters and to allow phase separation from the glycerol formed. CH 2 OCOR 1 CH 2 OH R 1 COOCH 3 CHOCOR 2 3CH 3 OH Catalyst CHOH R 2 COOCH 3 CH 2 OCOR 3 CH 2 OH R 3 COOCH 3 Triglyceride Methanol Glycerol Methyl Figure 4.1 Chemistry of transesterification process Three types of catalysts, such as a strong alkali, a strong acid or an enzyme, can be used in the manufacturing process of the transesterification method. Almost all biodiesel fuels are produced by using base catalyzed transesterification process, as it is a simple process requiring only low temperature (Maa and Hanna 1999, Kalpande and Vikhe 2008), shorter reaction time and lesser amount of required catalyst (Lin and Lin 2006).

6 61 Hence, the strong alkali catalyst is widely used in the transesterification process to produce biodiesel. NaOH, due to its low cost is widely used in large scale transesterification (Agarwal 2007). In contrast, a strong acid catalyst generally needs a longer reaction time, but is adaptable to more kinds of reactant mixtures. For example, reactants containing a small amount of water and free fatty acids can still be transesterified to form biodiesel if a strong acid catalyst is used. The important factor that affects the transesterification reaction is the amount of alcohol and catalyst, reaction temperature and reaction time. A molar ratio of 6:1 is normally used in Figure. 4.2 Flow chart of transesterification process industrial processes to obtain ester yields higher than 98% by weight, because lower molar ratio requires more reaction time. With higher molar ratios,

7 62 conversion increased but recovery decreased due to poor separation of glycerol (Mehar et al 2006). Mostly methanol is used in this chemical reaction due to its superior advantages of high solubility in oil, fast reaction rate, good physical and chemical properties, and low cost (Korbitz 2000, Nagaraja and Kumar 2004, Sinha and Agarwal 2005, Lin and Lin 2006). Figure 4.2 shows the flow chart of the biodiesel production using transesterification process. Most researchers have used 1 to 0.5% NaOH or KOH by weight of oil for biodiesel production. If acid value is greater than 1, more catalyst is required to neutralize free fatty acids (Bala 2001). Many researchers have reported that alkali-catalyzed transesterification is much faster than acid catalyzed one and is more often used commercially (Babu and Devaradjane 2003, Barnwal and Sharma 2004). Veljkovic et al (2006) depicted the biodiesel production from tobacco (nicotiana tabacum L.) seed oil by a two-step process in which the acid-catalyzed esterification was followed by the base-catalyzed methanolysis. The first step reduced the FFA level to less than 2% in 25 min for molar ratio of 18:1. The second step converted the product of the first step into methyl ester and glycerol. Malaya Naik et al (2008) used a two-step process to produce biodiesel from pungamia pinnata oil and studied the effect of FFA level on production of biodiesel. Ester content of pungamia methyl esters was determined by high performance liquid chromatography. Holser and Kuru (2006) investigated milkweed (asclepias) seed oil as an alternative feed stock for the production of a biodiesel fuel. The authors concluded that conversion of this highly unsaturated oil into methyl ester is an easier process than conversion into its ethyl ester.

8 PRODUCTION PROCESS OF BIODIESEL Considering the availability of jatropha, pungamia, neem and other vegetable oil in the local areas, biodiesel processor based on the transesterification process was designed and fabricated which is shown in Figure 4.3. The process employed is the base-catalyzed process, where the transesterification of vegetable oils is faster than the acid-catalyzed reaction, together with the fact that alkaline catalysts are less corrosive than acidic compounds and is most often used commercially. 1. Stirrer 2. Heater 3. Control unit 4. Separating funnel Figure 4.3 Transesterification mini plant Effect of different parameters like temperature, molar ratio of alcohol to oil, catalyst, reaction time have been investigated by several researchers and it was found that for base catalyzed transesterification at atmospheric pressure, 55 to 60 o C temperature, 45 min to 1 hour reaction time and 6:1 molar ratio of alcohol to oil the yield was maximum (Freedman et al 1984, Marshall et al 1995, Akasaka et al 1997, Diasakou et al 1998, Marinkovic and Tomasevic 1998, Murayama et al 2000, Agarwal and Das 2001, Saka and Kustiana 2001, Agarwal et al 2003). Based on this, in the present work a temperature of 60 o C, 0.8% of sodium hydroxide (NaOH) as

9 64 catalyst on mass basis, 50 min as reaction time, and 6:1 molar ratio of methanol to oil was used as optimized parameters and biodiesel yield of 88% to 90% was obtained. Because the acid numbers of the vegetable oils were less than 1 mgkoh/g, there was no necessity to perform a pretreatment to the vegetable oil (Alptekin and Canakci 2008). The molecular weight of jatropha oil and palm oil is g/mol and g/mol. Every kg of jatropha oil requires 220 g of methanol while palm oil requires 230 g of methanol (6:1 molar ratio to oil). 1 kg of raw oil was pre-heated to remove water contents at about 100 o C in an appropriate vessel. After that it was filtered to remove any suspended particles. Required quantity of methanol and 8 g of NaOH were then mixed in a separate vessel to prepare sodium methoxide. Sodium methoxide was added to the pre-heated raw oil. The oil/sodium methoxide mixture was then agitated for 50 min at 60 o C and allowed to settle under gravity in a separating funnel. The methyl ester formed the upper layer in the separating funnel and glycerol formed the lower layer as shown in Figure 4.4. About 320 g of glycerol was separated from the mixture. The remaining separated ester was purified by washing it twice gently with 250 g of warm water and allowed to settle under gravity for 8 hours. The catalyst got dissolved in water, formed the lower layer, and was separated. The biodiesel obtained from raw oil had some impurities which were then separated using bubble washing method which is shown in Figure 4.5. After this final washing the biodiesel was again heated to remove excess alcohol and water at about 100 o C (Alptekin and Canakci 2008). Thus around 900 g of purified ester was obtained finally. The JME and PME thus obtained were then blended with petroleum diesel in various proportions for preparing biodiesel-diesel blends to be used in CI engine which was described in the forthcoming section. The level of blending with petroleum diesel is referred as Bxx, where xx indicates the amount of biodiesel in the blend (i.e. J20 blend means 20% JME and 80% diesel).

10 65 Figure 4.4 Separating funnel Figure 4.5 Bubble washing method 4.4 SUMMARY study: The following conclusions were made from the biodiesel production Out of various methods discussed in this section for the production of biodiesel, transesterification is the simplest and the most effective method that was used to produce JME from jatropha curcas oil. NaOH was used as catalyst due to its cost effectiveness and minimum reaction time. Methanol was used in the chemical reaction due to its high solubility with oil, fast reaction rate and low cost. A molar ratio of 6:1, transesterification temperature of 60 o C, 0.8% NaOH, reaction time of 50 min was used to attain maximum yield of 90%. To ascertain quality of biodiesel produced the important properties were measured and presented in the chapter 5.