C H A P T E R
15 Production of Biodiesel Using Palm Oil Man Kee Lam, Keat Teong Lee* School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, Seri Ampangan, 14300 Nibong Tebal, Pulau Pinang, Malaysia. *Corresponding author: E-mail:
[email protected]
1 INTRODUCTION The world is gradually heading toward severe energy crisis due to limited availability of fossil fuels, such as petroleum oil, natural gas, and coal. These fossil fuels are categorized as nonrenewable energy resources that cannot be replaced in a relatively short time after being utilized. Nevertheless, it is an undeniable fact that man is still heavily dependent on fossil fuels for electricity generation, transportation, and development. In addition to that, over-exploiting the usage of fossil fuels by human beings has raised severe environmental issues and directly caused negative impacts on the Earth. One of the most critical examples is climate change due to excessive emission of green house gases caused by the burning of fossil fuels. Global warming and extreme weather changes such as sudden drought, flash flood, windstorms, and heat waves are the evidences of climate change. Therefore, the search of an alternative and renewable energy source has emerged as one of the key challenges in this century in order to protect the environment and creating a sustainable world for future generation. There are indeed a lot of renewable energy sources that have been explored, including solar, hydropower, wind, wave, geothermal, and nuclear energy. However, most of these options are not economically feasible due to the requirement of high capital and operating cost that has limited its usage in many countries over the world that would likely to diversify their energy sources. Furthermore, availability of those renewable energies is highly dependent on regional or local condition that can be very unpredictable and inconsistence. For example, solar collector would require clear sky and plenty of sunshine to generate a sufficient amount of energy and, therefore, it is certainly not an appropriate choice for temperate countries. However, a hybrid energy conversion system can be recommended to overcome the problem and to achieve satisfactory energy conversion efficiency. Nevertheless, developing
Biofuels: Alternative Feedstocks and Conversion Processes
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2011 Elsevier Inc. All rights reserved.
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a hybrid system is not an easy task as the technology know-how to integrate the operations of the whole process is still at an infancy stage. As a consequence, it is not feasible to introduce the renewable energy integrated systems in the third world and underdeveloped countries. Recently, biodiesel has emerged as a spark of hope in the field of renewable energy. This is because biodiesel has close similarity with conventional fossil diesel in terms of chemical structure and energy content. Apart from that, modification of a diesel engine is not required as biodiesel is compatible with existing engine and has been commercially blended with diesel as transportation fuel in many European countries (Lam et al., 2009b). Besides, significant reduction in greenhouse gases emission has been proven by burning biodiesel, and this result directly reflects the unique benefit of using biodiesel (Basha et al., 2009). Furthermore, biodiesel is a nontoxic alternative fuel and easily biodegradable in freshwater and soil, making it unquestionably good for the environment (Pasqualino et al., 2006). In general, biodiesel can be produced through transesterification reaction, in which triglyceride from vegetable oil is reacted with short-chain alcohol (e.g., methanol) in the presence of catalyst as shown in Equation (1). Soybean, rapeseed, sunflower, and palm oils are among the common vegetable oils that are used in biodiesel production. However, since these oils are edible resources, many nongovernment organizations in the world have raised the “food versus fuel” feud and, therefore, biodiesel production has shifted to other alternative feedstock such as waste frying oil (WFO) and nonedible oil (e.g., jatropha curcas, karanja, pongamia pinnata, and microalgae). The use of WFO and nonedible oil has its fair share of problem, mainly due to the exceptional high free fatty acid (FFA) content that complicates the overall biodiesel processing steps. Soap is easily formed (saponification reaction) if a base catalyst is used and consequently increases the difficulty in final product purification. O
O
CH2-O-C-R1 O CH-O-C-R2 O
CH3O-C-R1 O + 3CH3OH
CH2-O-C-R3 Triglyceride
CH3O-C-R2 O CH3O-C-R3
Methanol
Methyl Ester
CH2-OH +
CH-OH
ð1Þ
CH2-OH Glycerol
In this chapter, focus will be given toward biodiesel production from palm oil. Lately, oil palm plantation has been criticized to cause several serious environmental issues such as deforestation and habitat destruction of endangered species (specifically orangutan). Fortunately, with various researches and scientific findings, these accusations were found to be baseless (Lam et al., 2009b). Up to date, oil palm still remains as the most efficient edible oil-producing crop as shown in Table 1 (Malaysian Palm Oil Council (MPOC)). Oil palm plantation area only accounted for less than 5% of the world’s agriculture land in year 2007, but yet it is able to supply 25% of the global oils and fats (Lam et al., 2009b). Hence, if the intention is to optimize land usage to meet the food and fuel demand simultaneously, oil palm will be the outstanding option as large quantity of oil can be produced with minimum land requirement. In addition to that, new breeds of oil palm cloned by Applied Agricultural Resources Sdn. Bhd. are able to produce 10.6 tonne/ha/year of crude palm oil (CPO), almost double of
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TABLE 1
Oil Yield and World Plantation Area for Major Edible Oils
Oil Crop
Average Oil Yield (Tonne/Ha/Year)
Planted Area (Million Hectare)
% of Total Planted Area
Soybean
0.4
94.15
42.52
Sunflower
0.46
23.91
10.8
Rapeseed
0.68
27.22
12.29
Oil palm
3.62
10.55
4.76
the current yield (Lam et al., 2009b). Apart from that, palm oil production has the highest energy efficiency factor (energy output to energy input) of 9.6 compared to rapeseed of 3.0 and soybean of 2.5 (Lam et al., 2009b). This is because less fertilizer and diesel (machinery and agrochemical usage) are required to produce 1 tonne of palm oil. Apart from the positive contributions toward the environment, sustainable oil palm plantation program can also leverage poverty by helping the poor farmers and rural dwellers to improve their living standards. The successful story of Malaysian palm oil industries in transforming the rural communities to have access to their basic needs for a healthy life reflects the significant outputs of the strategy. In fact, even the Food and Agriculture Organization (FAO) does agree that new demand for biofuels production from sustainable agricultural feedstock can indeed generate a new income opportunity for farmers, leading to increased food production and poverty eradication.
2 PALM BIODIESEL CONVERSION TECHNOLOGY 2.1 Overview on the Existing Process and Technology Currently, commercial-scale palm biodiesel production is usually carried out in a batchtype continuous stirred tank reactor. Initially, CPO is pretreated to increase its oxidative stability and to minimize the FFA content in the oil. A series of pretreatment steps are adopted such as degumming, neutralization by caustic soda, pigment removal using bleaching earth and, finally, high-temperature vacuum deodorization (Lim and Teong, 2010). The refined, bleached, and deodorized (RBD) palm oil in the presence of excess methanol and base catalyst is then heated to certain reaction temperature to produce biodiesel. Normally, multistage batch reactors are used in series to drive the reaction toward completion (Lim and Teong, 2010). After each stage of reactions, glycerol (byproduct) is withdrawn to push the reaction forward to attain higher biodiesel conversion within a minimum reaction time (Lipochem (M) Sdn Bhd and MPOB). After the reaction is completed, excess methanol is recovered through flashing in a flash vessel and further purified in a structured packing distillation column (Lipochem (M) Sdn Bhd and MPOB). The purified methanol can be recycled and use as reactant in the subsequent reactions. Apart from that, glycerol will also go through a few purification steps and is stored in a storage tank as crude glycerol. Meanwhile, the crude biodiesel is subjected to water-washing stages in cyclones to remove the remaining catalyst as well as to purify the biodiesel. Finally, the water is discharged at the bottom of the cyclone as wastewater, and the washed biodiesel is dried under vacuum condition to reduce its water content
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15. PRODUCTION OF BIODIESEL USING PALM OIL
within the specified limits of biodiesel standards. Figure 1 illustrated the overall process involved in palm biodiesel production. Biodiesel derived from palm oil has been reported to have similar fuel properties to petroleum diesel as shown in Table 2 (Lim and Teong, 2010). In addition, the palm biodiesel meets the international biodiesel specification as underlined by EN 14214 and ASTM D 6751. It was reported that pure palm biodiesel (without blending with petroleum diesel) can be directly used as fuel in a diesel engine without prior modification (Lipochem (M) Sdn Bhd and MPOB). Alternatively, it can also be blended with petroleum diesel at any proportion to initiate the implementation of biodiesel at national level and subsequently promote the advantages of using biodiesel toward environmental sustainability. Exhaustive test on the performance of palm biodiesel as an alternative fuel on diesel engine has also been conducted, including on 36 Mercedes Benz engines mounted onto passenger buses (Choo et al., 2005).
Methanol & base catalyst Refined, bleached and deodorized (RBD) palm oil
Transesterification
Glycerol phase
Purification
Biodiesel phase
Crude glycerol
Methanol recovery
Wastewater treatment plant
Water washing
Drying
Normal grade palm biodiesel Fractional distillation
Winter grade palm biodiesel (Mixed C18:1 and C18:2)
Suitable for cold climate countries
C16:0 and C18:0
Carotenes, vitamin E, squalene, sterols
Oleochemical industry
Pharmaceuticals, nutraceuticals, foods and cosmetics industry
FIGURE 1 Overview on the existing palm biodiesel production process
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TABLE 2
Properties of Palm Biodiesel (Normal and Winter Grade) Palm Diesel
Property
Unit
Petroleum Diesel
Normal Grade
Winter Grade
EN 14214
ASTM D6751
Ester content
% mass
–
98.5
98.0-99.5
96.5 (min)
–
Free glycerol
% mass
–
<0.02
<0.02
0.02 (max)
0.02 (max)
% mass
–
<0.25
<0.025
0.25 (max)
0.24 (max)
kg/L
0.853
0.878
0.87-0.89
0.86-0.89
–
Viscosity at 40 C
cSt
4
4.4
4.0-5.0
3.5-5.0
1.9-6.0
Flash point
C
98
182
150-200
120 (min)
130 (min)
Cloud point
C
–
15.2
18 to 0
–
–
Pour point
C
15
15
21 to 0
–
–
Cold filter plugging point
C
–
15
18 to 3
–
–
Sulfur content
% mass
0.1
<0.001
<0.001
0.001 (max)
0.0015
Carbon residue
% mass
0.14
0.02
0.02-0.03
0.3 (max)
0.05 (max)
53
58.3
53.0-59.0
51 (min)
47 (min)
Total glycerol
Density at 15 C
Cetane index Acid value
mg KOH/g
–
0.08
<0.3
0.5 (max)
0.8 (max)
Copper strip corrosion
3 h at 50 C
–
1a
1a
1
3 (max)
Gross heat of combustion
kJ/kg
45,800
40,135
39,160
–
–
The engines were able to successfully complete for over 30,000 km mileage, the expected performance of the engine. Apart from that, no technical problem was reported throughout the trial period, provided the engines were maintained according to their service manual (Choo et al., 2005). On the other hand, the fluidity of fuel in an engine is a crucial factor to ensure its efficient performance. When starting up an engine especially during cold weather, it is vital that the fuel can be pumped into the engine and mechanical parts are able to move freely. Otherwise, this may result in engine malfunctioning associated with long-term use (Choo et al., 2002b). In this regard, unfortunately, palm biodiesel has a relatively high pour point of þ15 C which limits its usage in cold climate countries. Pour point is defined as the lowest temperature for an oil to pour or flow freely under a specified condition (Lee et al., 1995). If the surrounding temperature approaches or becomes lower than the pour point temperature of palm biodiesel, the fuel will be solidified and cause cold flow-related problems such as blockage to the flow pipes and filters (Chen et al., 2010). One of the possible ways to overcome this limitation is adding chemical additives such as pour point depressants, flow improvers, paraffin inhibitors, or
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15. PRODUCTION OF BIODIESEL USING PALM OIL
wax modifiers to the biodiesel (Soriano et al., 2006). However, it was reported that these commercially available chemical additives which were developed for petroleum diesel may not be suitable for biodiesel application and the results are not satisfactory (Soriano et al., 2006). Table 3 shows the composition of fatty acid in palm oil (Ma and Hanna, 1999). From the table, saturated fatty acid such as lauric (C12:0), myristic (C14:0), palmitic (C16:0), and stearic (C18:0) contributed nearly 50% of the overall palm oil fatty acid composition. Because of this, the pour point of palm biodiesel is relatively higher than biodiesel derived from other feedstock such as soybean (0 C) and canola oil (9 C). However, it should be noted that palm oil constitutes various high-valued phytonutrients, namely, carotenes, vitamin E, squalene, and sterols (Harrison Lau et al., 2009). These phytonutrients bring great benefits, especially to pharmaceuticals, nutraceuticals, foods and cosmetics industry, as well as an ample health source for human consumption. After years of research and development, Malaysian Palm Oil Council (MPOC) had designed an integrated technology to recover these phytonutrients and to produce winter grade biodiesel simultaneously that was filed under Malaysian Patent PI 20021157 (Choo et al., 2002a). The integrated process starts with esterification and transesterification of CPO to produce biodiesel. Under mild reaction conditions, the phytonutrients are not completely destroyed and thus can be recovered before the palm biodiesel is burnt as fuel. After purification with water washing and drying, the palm biodiesel is further processed using short path distillation to produce three product streams—distilled palm biodiesel, carotene (vitamin A), and vitamin E. The distilled palm biodiesel will then be fed into a fractional distillation column to separate saturated methyl ester (C16 and C18) and to produce winter grade biodiesel (mixed C18:1 and C18:2); carotene will be subsequently concentrated to obtain a carotene concentrate; and finally vitamin E will be further processed, polished and solvent fractionation to obtain a concentrate vitamin E (Toh and Koh, 2008). It was reported that the expected recovery rate of carotene and vitamin E concentrate is 50 and 100 kg/day (Toh and Koh, 2008). Consequently, with the attractive market price for carotene at RM 760/kg ($ 217/kg) and vitamin E at nearly RM 1900/kg ($ 543/kg), this definitely offers a great opportunity for international business investors to gain valuable monetary return (Harrison Lau et al., 2009). Apart from that, the separated C16 and C18 methyl ester can
TABLE 3
Fatty Acid Composition of Palm Oil
Fatty Acid
Composition (%)
Lauric (12:0)
0.1
Myristic (C14:0)
1.0
Palmitic (C16:0)
42.8
Stearic (C18:0)
4.5
Oleic (C18:1)
40.5
Linoleic (C18:2)
10.1
Others
1
Total
100
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be used as oleochemical feedstock for the production of white soap as well as used as active ingredients in detergent formulations (Choo et al., 2002c). The production of all these diversified byproducts simultaneously with palm biodiesel certainly enhances the economic viability of using palm oil as feedstock for biodiesel production. Specifically, the relatively higher prices of biodiesel compared to petroleum diesel may be offset by the revenue obtained from selling those phytonutrients byproducts.
2.2 Catalysis Process for Palm Biodiesel Conversion 2.2.1 Homogeneous Base Catalyst Base catalysts such as sodium hydroxide (NaOH) and potassium hydroxide (KOH) are the most commonly used catalysts in industrial biodiesel production plant. The reasons for this are: (1) relatively low cost compared to heterogeneous and enzymatic catalysts, (2) easily available in the market and (3) able to accelerate transesterification effectively under a mild reaction condition. Furthermore, base-catalyzed transesterification was 4000 times faster than acidic catalysts (Fukuda et al., 2001; Kulkarni and Dalai, 2006). NaOH and KOH are available in pellet form but highly soluble in alcohol. Therefore, these catalysts are normally premixed with alcohols before transesterification reaction takes place. However, homogeneous base catalysts suffer a serious drawback in the biodiesel industry due to their high sensitivity toward FFA content in oil. FFA content in oils needs to be kept as low as possible (0.5-1%) to hinder saponification reaction from occurring because it will react with the base catalyst to produce soap as the byproduct. Excessive soap formation inhibits the biodiesel-glycerol phase separation and thus reduces biodiesel yield drastically. Generally, RBD palm oil is used to produce biodiesel due to the low FFA content (0.1-0.5%) and thus minimize the impact of saponification reaction. Darnoko and Cheryan (2000) studied the transesterification of RBD palm oil with methanol catalyzed by KOH. From the study, the highest biodiesel concentration attained was 90% with the following reaction conditions: reaction temperature of 60 C, methanol to oil molar ratio of 6, catalyst concentration of 1%, and reaction time of 40-60 min. The result was in accordance to other feedstock catalyzed by KOH or NaOH, such as soybean (Dias et al., 2008), sunflower (Rashid et al., 2008), and rapeseed oil (Jeong et al., 2004). However, since a series of refining processes are required to convert CPO to RBD, the additional processing cost has increased the overall RBD palm biodiesel production cost and making the whole process not economic viable. Feedstock cost has been reported to contribute the most in the whole biodiesel production chain, nearly 80% of the overall biodiesel production cost (Lam et al., 2009b). Although CPO is cheaper than RBD palm oil, it has high FFA contents, ranging from 3% to 6.5% (Che Man et al., 1999). Normally, preesterification step is required to reduce the FFA content in the CPO before base-catalyzed transesterification reaction takes place. 2.2.2 Homogeneous Acid Catalyst Apparently, homogeneous acid catalysts are preferred for feedstock that contains high FFA in biodiesel production. Sulfuric acid (H2SO4) and hydrochloric acid (HCl) are the most widely used due to their strong acidic properties and low cost. Nevertheless, it was reported that H2SO4 can give better performance than HCl in transesterification of
360
15. PRODUCTION OF BIODIESEL USING PALM OIL
waste frying palm oil (Al-Widyan and Al-Shyoukh, 2002). Apart from that, the advantage of using acidic catalysts is insensitivity to FFA content in the oil and thus elimination of side reaction such as saponification (Kulkarni and Dalai, 2006). Furthermore, acidic catalysts are able to perform esterification and transesterification simultaneously. Esterification occurs when FFA reacts with alcohol in the presence of acidic catalysts to form ester as the reaction product. This is the most common method to reduce the FFA content in jatropha (Lu et al., 2009), waste cooking oil (Wang et al., 2006), pongamia pinnata (Sharma et al., 2010), and kusum (Sharma and Singh, 2010). However, high alcohol to oil molar ratio is required to accelerate acid-catalyzed transesterifcation (e.g., molar ratio of methanol: oil ¼ 20-30:1) with reaction temperature ranging from 65 to 99 C, H2SO4 loading ranging from 1 to 4 wt% (referred to weight of oil) and reaction time ranging from 20 to 70 h (Freedman et al., 1984; Wang et al., 2006). On the other hand, homogeneous acid catalysts posed several disadvantages, such as (1) strong acidic properties caused serious corrosion to reactor wall, pipelines, and valves, (2) slow reaction rate, (3) difficulty in catalyst separation. Therefore, homogeneous acid catalysts are not favored for commercial biodiesel production, but they appear as a suitable choice in esterification reaction rather than transesterification due to the simple molecular structure of FFA as compared to triglycerides (Wang et al., 2006). It was recommended that homogeneous acid catalysts are used initially to reduce the FFA content in the oil to a lower content and then only followed by transesterification reaction catalyzed by homogeneous base catalysts (Canakci and Van Gerpen, 2003). This combined two-step process gives better advantages than the individual single step, such as relatively less energy requirement, minimizing saponification reaction, and resulting in easy separation of biodiesel and glycerol. Perhaps, the main problem associated with this combined process is the difficulty in catalyst separation that requires multiple water washing steps, resulting to huge amount of wastewater that is not environmental friendly. Up to now, research on this two-step process is still limited for palm biodiesel. Nevertheless, it holds an important key to be easily incorporated into the existing palm biodiesel plant (only homogeneous base catalysts are utilized) when there is a need to change the feedstock from RBD palm oil to CPO. 2.2.3 Heterogeneous Catalysts Due to the severe difficulty in separating homogeneous catalyst after reaction and also the huge amount of wastewater generated, heterogeneous catalysts appear as an excellent solution to this problem. Heterogeneous catalysts in the form of powder or pellet can be easily separated out after the reaction is completed, and the catalysts have the potential to be recycled, regenerated, and reused. This approach is more environment friendly and indirectly reduces the overall biodiesel production cost. In fact, the use of heterogeneous catalysts in transesterification is not a new phenomenon as in the past few years; extensive researches have been carried out to explore its potentials. However, high reaction temperature, high alcohol to oil molar ratio, and long reaction time are generally required due to mass transfer limitation of oil-alcohol-heterogeneous catalyst (three-phase system) in the initial stage of the reaction. Therefore, utilization of heterogeneous catalysts for commercial biodiesel production is still not attractive. The following sections depict various types of heterogeneous base and acid catalysts used in transesterification of palm oil.
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2.2.3.1 HETEROGENEOUS BASE CATALYSTS
Heterogeneous base catalysts such as basic zeolites, alkaline earth metal oxides, and hydrotalcites which exhibited alkaline properties on its surface are being identified as a good option in replacement of current homogeneous base catalysts for biodiesel production. Nevertheless, it should be noted that heterogeneous base catalysts are still very sensitive to FFA content in the oil; only oil with less than 1 wt% of FFA is favored. Thus, the major challenges facing the development of heterogeneous base catalysts are their ability to withstand high FFA oil at a mild reaction condition and their reusability. 2.2.3.1.1 CALCIUM OXIDE (CAO) CaO is one of the most favorable heterogeneous base catalysts due to their relatively high basic sites, nontoxic, low solubility in methanol and can be prepared from cheap resources like lime stone and calcium hydroxide. The catalytic activity among alkaline earth metal oxides catalyst in transesterification is according to the following order: BaO > SrO > CaO > MgO (Yan et al., 2008), which also represents the order for the amount of basic sites. Although BaO and SrO have the highest number of basic sites and catalytic activity, it was found that both BaO and SrO are soluble in methanol. Therefore, BaO and SrO are more inclined toward homogeneous system rather than heterogeneous system. However, BaO and SrO are not used as homogeneous catalyst in the industry as NaOH or KOH is much cheaper with higher efficiency. Thus, among heterogeneous base catalysts, only CaO had been extensively tested for transesterification of various vegetable oils such as soybean, rapeseed, and sunflower oils. The results reported were indeed promising with high biodiesel yield at mild reaction conditions. Nevertheless, the active sites on CaO can be easily poisoned by contact with air due to adsorption of CO2 and H2O on the surface of the catalyst as carbonates and hydroxyl groups (Hattori, 1995). Therefore, activation of CaO by calcinations at >700 C is generally required to revert CO2 poisoning (Granados et al., 2007). The activated CaO shows complete decarbonation in which all CaCO3 are converted to CaO. Apart from that, activated CaO is also covered with several layers of Ca(OH)2 and thus minimizing H2O adsorption on its surface. Nevertheless, care must be taken on the activated CaO to avoid further contact with ambient air that can cause reoccurrence of carbonation and hydration especially if exposed for long period. A recent study on the use of CaO as the catalyst in transesterification of palm oil was reported by Yoosuk et al. (2010). It was revealed that the activated CaO can be subjected to hydrationdehydration method in order to further improve its physical and chemical properties such as surface area, pore volume, number of basic sites, and basic strength. Consequently, the catalyst was used in transesterification of palm olein and the optimum yield attained was 93.9% at the following reaction conditions: reaction temperature of 60 C, methanol to oil molar ratio of 15, catalyst loading of 7 wt%, and reaction time of 45 min. Furthermore, the catalyst can be reused for up to five cycles with minimum drop in biodiesel yield. The decrease in the activity of the catalyst was attributed to active site blockage by adsorbed impurities or product species (monoglyceride, diglyceride, triglyceride, and glycerol) and leaching of active sites into the reaction media. Apart from hydration-dehydration method, catalytic activity of CaO can also be enhanced by the addition of catalyst support. Among various catalyst supports available in the market, alumina (Al2O3) has been identified as a cheap and effective support for various catalytic chemical reactions such as steam reforming and hydrogenation. This is because Al2O3 has
362
15. PRODUCTION OF BIODIESEL USING PALM OIL
high specific surface area, large pore volume, mesopore size, high thermal stability, and mechanical strength (Arzamendi et al., 2007; Komintarachat and Chuepeng, 2009; Zabeti et al., 2010). These physical characteristics are important in heterogeneous catalysts that are to be used in transesterification reaction in order to minimize mass transfer limitation. Zabeti et al. (2010) optimized the reaction parameters in transesterification of palm oil using CaO supported with Al2O3 as the catalyst. Respond surface methodology (RSM) coupled with Central Composite Design (CCD) was used to identify the correlation and interaction between the reaction parameters. It was found that the optimum biodiesel yield attained was 98.6% at the following reaction conditions: 65 C of reaction temperature, methanol to oil molar ratio of 12:1, catalyst loading of 6 wt%, and reaction time of 5 h. In addition, the catalyst was reused for two cycles with sustained catalytic activity. Although CaO appears to have the potential to replace current homogeneous base catalysts, several important issues must still be addressed. One of them is the loss of active sites that can be leached out during transesterification reaction (Granados et al., 2007; Kouzu et al., 2008a,b). This does not only cause catalyst deactivation but also results in product contamination and thus extra purification steps (water washing) are required. Due to the additional purification step, most of the active sites will be lost and therefore limit catalyst recovery. Apart from that, sensitivity of CaO toward FFA content in oil is another problem that needs to be underlined. This is because FFA will react with the basic sites of CaO to form soap and cause serious difficulty in product separation (Kouzu et al., 2008a,b). In the case of using unrefined palm oil as biodiesel feedstock, the FFA content needs to be reduced before CaO can be used as the heterogeneous catalyst. 2.2.3.2 OTHER METAL OXIDES
Apart from CaO, there are several other alkaline metal oxides reported to have good performance in transesterification reaction. Bo et al. (2007) revealed the potential of aluminasupported potassium fluoride (KF/Al2O3) as an alternative heterogeneous base catalyst to produce biodiesel. KF/Al2O3 is regarded as a low-cost, commercially available, reusable, and environment-friendly catalyst in various organic processes, such as preparation of amides from nitriles, conversion of aldehydes to nitriles, and hydrothilation of alkynes (Bo et al., 2007; Zare et al., 2009). In their study, KF/Al2O3 was prepared through impregnation method with 0.33 as the optimum load ratio of KF to Al2O3. In order to achieve a good interaction between KF and Al2O3, the impregnated catalyst was subjected to calcinations at 600 C for 3 h. The resulted catalyst was applied for transesterification of palm oil and the optimum reaction condition were as follows: reaction temperature of 65 C, methanol to oil molar ratio of 12:1, catalyst loading of 4 wt%, and reaction time of 3 h. The optimum biodiesel yield attained was almost 90%. Nevertheless, the active sites of the catalyst were lost after the first cycle of reaction and regeneration study is promptly required to further strengthen the feasibility of the catalyst to be applied in the industry. Apart from that, KF loaded to ZnO was also identified as an active and promising heterogeneous base catalyst in transesterification. When 15 wt% KF is loaded on ZnO and calcined at 600 C for 5 h, the resulting catalyst contains a very high number of basic sites (1.47 mmol/g; Xie and Huang, 2006). It was observed that when calcination temperature was increased further, the KF active sites were decomposed and consequently lower down the number of basic sites. Application of KF/ZnO in biodiesel production from palm oil was reported
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by Hameed et al., (2009). From the study, optimum yield of biodiesel attained was 89.2% at reaction temperature of 65 C, methanol to oil molar ratio of 11.4, catalyst loading of 5.5%, and reaction time of 9.7 h. However, further improvement in catalyst preparation steps is required to obtain a higher number of basic sites on KF/ZnO and to reduce the catalyst deactivation rate. Besides, direct impregnation of potassium (K) onto SBA-15 also exhibited high catalytic activity in transesterification. SBA-15 is selected as a good support due to the following reasons: (1) high surface area (600-1000 m2/g), (2) tunable mesopore size (5-30 nm), and (3) high thermal stability (Abdullah et al., 2009; El Berrichi et al., 2007). Generally, microporous structure is preferred for a heterogeneous catalyst in transesterification reaction since triglycerides are categorized as large molecules with average molecules size of 2 nm (Lam et al., 2010). Therefore, SBA-15 posed a superior advantage compared to zeolite (microporous solid) as support in which mass transfer resistance and diffusion limitation can be significantly reduced. Abdullah et al. (2009) synthesized high catalytic activity of K/SBA-15 through impregnation method and calcined at 350 C for 3 h. The resulted catalyst exhibited high surface area of 539 m2/g and average pore diameter of 5.63 nm. Subsequently, optimization on the transesterification reaction variables using palm oil and the synthesized catalyst were carried out using design of experiment (DOE). The optimum biodiesel yield attained was 87.3% at the following optimum reaction conditions: reaction temperature of 70 C, methanol to oil molar ratio of 11.6:1, catalyst loading of 3.91 wt%, and reaction time of 5 h. 2.2.3.3 WASTE MATERIAL
In line with the world’s sustainability concept, reutilization of waste material has emerged as a new trend in order to reduce the accumulation of waste and to protect the environment. As such, researchers are now looking for potential waste materials to be converted to catalyst for various applications. One of the potential sources of waste material is agricultural waste such as biomass from oil palm industries in Malaysia. In the year of 2005 alone, it was reported that Malaysia produces about 55.73 million tonnes of oil palm waste biomass in the form of empty fruit bunches (EFBs), shell, fiber, palm kernel, frond, and trunk (Shuit et al., 2009). The synthesis of catalyst from oil palm waste biomass for transesterification reaction has been reported. Initially, oil palm biomass such as palm shell must be converted to activated carbons through pyrolysis and steam activation process. Then, active compounds are impregnated on the surface of the activated carbon. Activated carbon produced from oil palm biomass has numerous applications typically in wastewater treatment as an effective adsorbent (Hameed et al., 2008, 2009; Tan et al., 2008). High adsorption capacities attributed by oil palm-activated carbons are always correlated to their physical properties, such as high surface area, pore volume, and pore diameter. Baroutian et al. (2010) reported that by depositing KOH on palm shell-activated carbon, the activated carbon can act like a catalyst for the transesterification of palm oil. Optimum biodiesel yield of 98% was attained at the following reaction condition: reaction temperature of 64 C, methanol to oil molar ratio of 24:1, catalyst loading of 30.3 wt%, and reaction time of 1 h. However, leaching of the active sites into the reaction media was observed, but at minimum level with 0.98 and 0.80 ppm, respectively, in the first and second cycles of reaction. Nevertheless, the presence of KOH (due to leaching) in the product mixture does not affect the quality of the biodiesel as it still meets the basic standard of biodiesel in which concentration of mineral matter should be below 200 ppm (Kouzu et al., 2008a,b).
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More study on the utilization of activated carbon (derived from oil palm biomass) as the catalyst for transesterification could be found in the study by Chin et al. (2009). Besides oil palm biomass, waste eggshells have also been identified as a low-cost catalyst for biodiesel production. It was reported that chicken eggshells consist high content of calcium carbonate (CaCO3) which can be converted to CaO after calcinations at 700 C (Cho and Seo, 2010; Wei et al., 2009). The product after the calcinations process can act as a catalyst for heterogeneous transesterification due to their high number of basic sites. Cho and Seo (2010) reported that by introducing hydrochloric acid (HCl, weak acid) solution to quail eggshells, this can further enhance its catalytic activity in transesterification of palm oil. The purpose of the weak acid treatment is to remove the dense cuticle outer layer of the eggshells which are not porous and therefore facilitating the diffusion of triglycerides to the porous palisade middle layer of the eggshell (Cho and Seo, 2010). The acid-treated quail eggshell catalyst was able to maintain its catalytic activity with 98% palm biodiesel conversion even after five cycles of reaction at temperature of 65 C, methanol to oil molar ratio of 12:1, catalyst loading of 1.5 wt%, and reaction time of 2 h. Similar reports on utilization of waste mud crab shell for transesterification of palm oil could be found in the study by Boey et al. (2009). 2.2.4 Heterogeneous Acid Catalysts The development of different types of heterogeneous acid catalysts lately has widened the choice of feedstock for biodiesel production including CPO that has high FFA. The advantages of using heterogeneous acid catalysts in transesterification are: (1) insensitive to FFA content in the oil, (2) catalyzed esterification and transesterification simultaneously, (3) easy catalyst recovery from reaction media, (4) have potential to be recycled and regenerated, (5) minimize the number of washing steps required and (6) less corrosion toward reactors wall, pipelines and valves compared to homogeneous acid catalyst (Jitputti et al., 2006; Kulkarni and Dalai, 2006; Suarez et al., 2007). The potential and performance of various heterogeneous acid catalysts in transesterification of different oil sources have been explored extensively in the past few years such as sulfated zirconium oxide (SO42-/ZrO2; Furuta et al., 2004; Jitputti et al., 2006; Park et al., 2008), sulfated titanium oxide (SO42-/TiO2; de Almeida et al., 2008; Peng et al., 2008), sulfated tin oxide (SO42-/SnO2; Furuta et al., 2004; Lam et al., 2009a), sulfonic ion-exchange resin (Dos Reis et al., 2005; Heidekum et al., 1999), sulfonated carbon-based catalyst (Lou et al., 2008; Takagaki et al., 2006), and heteropolyacids (Cao et al., 2008; Zhang et al., 2009). Nevertheless, the main challenges of commercializing these catalysts are their relatively high cost; complicated synthesis procedures and extreme reaction conditions are generally required for transesterification (e.g., high reaction temperature). Jitputti et al. (2006) reported that SO42-/ZrO2 has high catalytic activity in the transesterification of palm kernel oil (Jitputti et al., 2006). The catalyst exhibited extremely strong acid strength on its surface and therefore it is suitable to be used in transesterification of oil with high FFA. The palm kernel oil used in the experiments contains FFA value of 1% (as lauric acid) which is not favorable for base catalysts. The yield of palm kernel biodiesel attained was more than 90% at reaction temperature of 200 C, methanol to oil molar ratio of 6:1, and catalyst loading of 3 wt% in a nitrogen-pressurized reactor at 50 bars. They also studied the recycling and regeneration of the SO42-/ZrO2 catalyst. After the first cycle of reaction, the catalyst was recovered, dried at 100 C, and used in the subsequent reaction. It was found that the catalytic activity of SO42-/ZrO2 dropped tremendously to give only 28% biodiesel yield.
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This may be due to several factors such as catalyst leaching and active sites blockage by reactants or products. Nevertheless, the catalyst can be easily regenerated and gave the same activity as fresh catalyst. More research studies on palm oil conversion to biodiesel using heterogeneous acid catalysts can be found in the studies by Yee et al. (2010), Melero et al. (2010), Aderemi and Hameed (2009), and Kansedo et al. (2009). 2.2.5 Enzymatic Catalyst More recently, enzymatic catalyst (especially lipase) has revealed its potential as a catalyst in transesetrification reaction to produce biodiesel. Different from chemical catalyst, enzymatic catalyst offers several advantages such as: (1) not energy intensive because reactions normally occur at room temperature, (2) insensitive to FFA content, (3) easy recovery of catalyst and glycerol, and (4) minimize water-washing step that consequently reduce wastewater treatment cost. Mucor miehei (Lipozym IM 60), Pseudomonas cepacia (PS 30), C. antarctica (Novozym 435), and Bacillus subtilis are the examples of enzymes that have shown good catalytic activity in transesterification. In addition to that, enzymatic catalyst has attained another significant milestone with the introduction of immobilization technology. The purpose of immobilization is to provide a more rigid external backbone for lipase so that it can maintain its high stability, easily recycle and reuse for the subsequent reactions (Jegannathan et al., 2008; Knezevic et al., 1998). Lipase can be immobilized into ion-exchange resin, photocrosslinkable resin, silica beads, alumina and activated carbon through adsorption, covalent bonding, entrapment, encapsulation, and cross-linking (Tan et al., 2010). However, the main limitation of using enzymatic catalyst in commercial scale is the high cost of lipase and slow reaction rate. In transesterification using enzymatic catalyst, solvent is added into the reaction media to ensure homogeneous phase between oil and alcohol (reducing mass transfer limitation) and thus enhancing lipase catalytic activity (Fjerbaek et al., 2009). Generally, hexane is preferred due to low cost and easily availability in the market. However, it was found that solubility of methanol and glycerol in hexane is low and resulted to lipase deactivation (poisoned by methanol or glycerol; Royon et al., 2007). After years of research, tert-butanol was discovered as a superior solvent than hexane. Methanol and glycerol are easily soluble in tert-butanol which minimizes the poisoning rate caused by methanol and also reduces the heavy deposition of glycerol on the immobilized lipase (Nielsen et al., 2008; Watanabe et al., 2000). The importance of tert-butanol in enzymatic transesterification using unrefined palm oil is more prevalent as the oil contains a high level of phospholipids (major components of oil gum), which increases viscosity and mass transfer limitation (Talukder et al., 2009). Talukder et al. (2009) reported that the catalytic activity of Novozym 435 increased nearly 358% due to the positive effect of adding tert-butanol into the reaction mixture. A similar observation was also discovered by Halim and Harun Kamaruddin (2008) in transesterification of waste cooking palm oil using Novozym 435. Nevertheless, several issues must still be addressed due to the use of solvent in transesterification, such as: (1) extra reactor volume is required to accommodate the additional volume of solvent, (2) plant safety requirement (toxicity of solvent), (3) extra production cost (extra solvent recovery steps and the loss of solvent; Nielsen et al., 2008). Unlike chemical catalyst, enzymatic transesterification requires certain amount of water in the reaction media to maintain the enzyme catalytic activity. Generally, the availability of interfacial area is one of the factors that influence enzyme activity. The presence of water
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will facilitate the formation of oil-water droplet which will increase the interfacial area between the reactants that will subsequently accelerate the transesterification reaction rate (Noureddini et al., 2005). However, it should be noted that if too much water is added, this could cause the hydrolysis of oil that leads to low biodiesel conversion. In addition, it is also important to ensure the support used to immobilize lipase does not adsorb water or otherwise it could inhibit the penetration of oil to the lipase layer (Samukawa et al., 2000; Talukder et al., 2009). The minimum and also optimum amount of water added into a specific reaction mixture is strongly dependent on the type of lipase used. For example, lipase PS (Burkholderia cepacia) immobilized within k-carrageenam (biopolymer) requires an optimum water to palm oil volumetric ratio of 0.085:1 (v/v) (Jegannathan et al., 2010), whereas C. rugosa requires 1:1 (v/v) (Talukder et al., 2010). However, if the feedstocks used for biodiesel do contain water such as CPO and waste cooking palm oil, this important factor must be considered in determining the optimum amount of water to be used during transesterification reaction. Theoretically, in transesterification, 3 mol of alcohol is required to produce 3 mol of biodiesel and 1 mol of glycerol. However, since the reaction is reversible, excess alcohol is generally preferred to drive the reaction toward completion within a minimum reaction time. In homogeneous and heterogeneous base catalysts, methanol to oil molar ratio of 6-15:1 is generally used. However, the same ratio cannot be applied to enzymatic catalysis as this will cause the poisoning of lipase and results in exceptionally low biodiesel yield. Therefore, in enzymatic catalysis, alcohol is normally added to the reaction mixture in stepwise order to minimize lipase poisoning by the alcohol and thus prolong its durability (Watanabe et al., 2001). Using this strategy, high yield of biodiesel can be attained (Chen et al., 2006; Ying and Chen, 2007) and the lipase has a higher possibility to be reused in the subsequent reaction. Nevertheless, there is very limited research being carried out on the development of stepwise addition of alcohol on transesterification of palm oil. Apart from the effect of water and alcohol concentration in the reaction mixture, reaction temperature also plays a significant role in enzymatic transesterification. It is well reported that in transesterification, high reaction temperature will reduce mass transfer limitation due to the decrease in oil viscosity and therefore accelerates the transesterification rate. Nevertheless, this is not true for enzymatic catalysis. Enzyme is extremely sensitive to the surrounding temperature; once the surrounding temperature exceeds a certain limit, the lipase will deactivate immediately and perhaps permanently. Therefore, the control of temperature in enzymatic catalysis is a very important factor that does not only affect the biodiesel yield, but also indirectly determines the survival of lipase and its reusability. On the other hand, immobilized enzyme has a higher temperature resistance compared to free enzyme due to the binding of the enzyme within a carrier material that gives it a higher stability and therefore decreases the effect of thermal deactivation (Fjerbaek et al., 2009). In general, reaction temperature between 30 and 40 C is favorable, depending on the type of lipase used in the reaction (Sim et al., 2010a,b; Talukder et al., 2009). Sim et al. (2010b) had reported on the effect of temperature in enzymatic catalysis toward biodiesel production from palm oil (Sim et al., 2010a). In that study, Lipozyme TL IM was used as catalyst and CPO as feedstock. The optimum biodiesel yield attained was 85% at the following reaction conditions: reaction temperature of 30 C, enzyme loading of 6.67 wt%, agitation speed of 150 rpm, and reaction time of 6 h. Thus, from the result of this study, it proves that it is indeed a plausible option to produce palm biodiesel at room temperature and hence reduce energy requirement in biodiesel production.
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2.3 Other Technologies 2.3.1 Noncatalytic Transesterification: Supercritical Alcohol Method Supercritical fluid technology has opened up a new dimension in various reaction, extraction and separation processes. Under supercritical condition, thermophysical properties of fluid such as dielectric constant, viscosity, and specific polarity are drastically changed depending on the temperature and pressure (Lee and Saka, 2010). For transesterification reaction, supercritical alcohol (typically methanol) has been introduced as an alternative means to produce biodiesel without the need of a catalyst. More importantly, supercritical alcohol technology poses several advantages over catalysis method, such as: (1) no further purification step (water washing) required and indirectly reduce the wastewater treatment cost, (2) insensitive to water and FFA level in the oil and (3) fast reaction rate and thus, has a higher possibility to produce biodiesel on a continuous basis. During transesterification reaction under supercritical conditions, triglyceride becomes miscible in alcohol due to the decrease in dielectric constant of alcohol. Consequently, the reaction mixture becomes a single phase rather than a heterogeneous system, and this seems to be the key factor that accelerates transesterification reaction. Up to now, supercritical alcohol technology has attracted considerable attention by researchers in this field, but the true potential of this technology in the commercial scale is yet to be revealed. On the other hand, although supercritical alcohol enjoys enormous advantages over the catalysis method, there are a few imperative issues that need to be addressed urgently upon scaling the technology up. Because high temperature and pressure are the major factors to reach the supercritical state, the energy output by burning biodiesel may not counter the extensive energy input. A proper life-cycle assessment (LCA) should be studied to further justify the possibility of using supercritical alcohol technology in biodiesel production. Despite the overall energy balance, the decomposition of biodiesel was found to be the most severe problem. When the reaction temperature was over a certain limit, the decomposition of unsaturated fatty acids was observed, which deteriorated the biodiesel conversion seriously (Imahara et al., 2008). Apart from thermal decomposition, isomerization of polyunsaturated methyl ester from cis-type carbon bonding into trans-type carbon bonding was detected through Fourier transform infrared spectrometry (FT-IR; Imahara et al., 2008). Trans-type fatty acids are naturally unstable, which degrades the cold flow properties of biodiesel (Gui et al., 2009; Imahara et al., 2008). Furthermore, the ultra high alcohol to oil molar ratio in the supercritical method is also a crucial factor in determining biodiesel conversion. Consequently, the energy consumption for alcohol separation from biodiesel and glycerol (alcohol recycling process) is expected to be exceptionally high at the industrial scale. The transesterification of palm oil using supercritical methanol technology was investigated by Song et al. (2008). From the study, more than 90% of the biodiesel content was obtained at the following optimum reaction conditions: a reaction temperature and pressure of 350 C and 40 MPa, respectively, a methanol to oil molar ratio of 30, and a reaction time of 5 min. The reaction rate with supercritical methanol was much faster than the reaction catalyzed by a homogeneous base catalyst. This promising result has escalated further development and research using palm oil as the feedstock to produce biodiesel. Continuous biodiesel production through supercritical methanol, which was carried out by Bunyakiat et al. (2006), indicated the special advantage of the technology (Bunyakiat et al., 2006). Palm kernel oil was tested as one of the feedstocks with the optimum conversion of 95% at the
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following reaction conditions: a reaction temperature and pressure of 350 C and 19 MPa, respectively, and a molar ratio of methanol to oil of 42 at a time of 400 s. The result attained was similar to that of Song et al. (2008), although the FFA level in the palm kernel oil was exceptionally high, 31 mg KOH/g, which is far beyond the limit if a homogeneous base catalyst is utilized. Because supercritical methanol is highly tolerant to the FFA content in the oil, unrefined palm oil and waste frying palm oil are an attractive choice to further reduce the overall biodiesel production cost. However, care should be taken that the supercritical temperature should not exceed 375 C as low biodiesel content was observed due to thermal decomposition (Song et al., 2008). Overall, biodiesel is not a totally green biofuel if methanol is used as one of the reactants because the methanol available on the market is generally derived from fossil fuels, such as petroleum and natural gas. Therefore, ethanol has emerged as an alternative to methanol because ethanol can be derived from agricultural renewable biomass through pretreatment, hydrolysis, and fermentation processes. In fact, ethanol is more soluble in oil compared to methanol and, consequently, minimizes the mass transfer limitation of the transesterification reaction. Nevertheless, findings from various studies have proven that ethanol has lower reactivity than methanol (Meneghetti et al., 2006) due to (1) steric hindrance effects of the larger alkyl chains in ethanol (Suwannakarn et al., 2008) and (2) the stable emulsification created by ethanol during transesterification, resulting in the difficulty of ethyl ester separation from glycerol (Encinar et al., 2007). The potential of supercritical ethanol transesterification from palm oil was first explored by Gui et al. (2009). It was possible to produce palm biodiesel through supercritical ethanol; however, relatively lower yield of biodiesel was attained (79.2%) with a long reaction time (30 min). This finding may be attributed to the type of oil used because palm oil contains high-saturated fatty acids compared to other types of oil reported, and the saturated fatty acid indirectly increases the viscosity of the oil. Nonetheless, the environmental advantages of using ethanol to produce greener biodiesel should not be ignored and should be explored through intensive research. 2.3.2 Ultrasonic-Assisted Transesterification Currently, ultrasonic technology is on the frontier of improving the mass transfer rate between the immiscible liquid-liquid phase within a heterogeneous system (Ji et al., 2006). Ultrasound is defined as sound with a frequency beyond the response of the human ear. The normal sound frequency that can be detected by the human ear lies between 16 and 18 kHz, but the frequency for ultrasound generally lies between 20 kHz and 100 MHz (Vyas et al., 2010). This high-frequency sound wave compresses and stretches the molecular spacing of a medium through which it passes. Thus, molecules are continuously vibrated, and cavities are created. As a result, microfine bubbles are formed through the sudden expansion and collapse, generating energy for chemical and mechanical effects (Colucci et al., 2005). Furthermore, the collapsed bubbles disrupt the phase boundary and impinging of the liquids to create microjets, leading to intensively emulsification of the system (Ji et al., 2006). Subsequently, the mixing effect due to emulsification increases the interfacial area between the immiscible reactants and thus facilitated reaction kinetics (Kalva et al., 2009). The positive effect of introducing ultrasound in transesterification has been reported recently. Transesterification is well recognized as a slow reaction process; generally, 40-60 min of reaction are required for the reaction to go to completion if a homogeneous base
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catalyst is used under favorable reaction conditions because alcohol (typically methanol) has a very low solubility value in oil at room temperature. Therefore, a two-phase system is created in the initial reaction. This two-phase system has a limited mass transfer rate between the reactants and, thus, slows down the overall reaction rate. This phenomenon becomes even worse if heterogeneous catalysts are used. A three-phase system is created and directly interferes with the reaction rate significantly. Ultrasound technology is relatively a new method to improve the mass transfer rate in transesterification rather than mechanical agitation. A shorter reaction time and better energy efficiency for transesterification were observed with ultrasound (Colucci et al., 2005; Ji et al., 2006; Singh et al., 2007). In addition, the reaction rate constants were enhanced by a factor of 3-5 higher than those for the mechanically agitated process (Kalva et al., 2009). Currently, research on using ultrasound technology in transesterification of palm oil is relatively limited compared to the research on other vegetable oils such as rapeseed, soybean, and sunflower oils. Inevitably, there is a knowledge gap between the effects of ultrasound on the palm biodiesel conversion efficiency under various reaction conditions and different types of catalysts. Mootabadi et al. (2010) demonstrated a significant improvement in the palm oil transesterification reaction rate catalyzed by heterogeneous base catalysts via ultrasound technology(Mootabadi et al., 2010). Refined palm oil was used in the study, and high biodiesel yields were attained after performing optimization. The reaction catalyzed by BaO and enhanced with ultrasound attained the highest biodiesel yield, which was 95.2%. In comparison, only a 67.3% biodiesel yield was attained if the reaction was enhanced with magnetic stirring (mechanical mixing). Furthermore, lower methanol and catalyst concentrations were used in the reaction due to the ultrasound effect. All of the evidence proves the efficiency of ultrasound technology in enhancing the transesterification of palm oil. In addition, Deshmane et al. (2009) demonstrated the positive effect of ultrasound in the esterification of palm fatty acid distillate (PFAD). Normally, PFAD is generated as a byproduct during the refinement of palm oil and has a lower market value compared to palm oil. Due to the high FFA value of PFAD, acid catalysts are preferred. Nevertheless, acid catalysts suffer from very slow reaction rates, and long reaction times are generally required. With the introduction of ultrasound into the reaction media, the reaction time was reduced by half due to the turbulence created intensively by ultrasound through cavitation, which resulted in excellent mixing between the two phases (oil and alcohol; Deshmane et al., 2009).
3 CONCLUSIONS Although oil palm has been severely questioned regarding its sustainability and environmental issues, the oil yield attained annually is still far superior than other edible oil bearing crops. Therefore, palm oil should be considered as an alternative and promising feedstock to further diversified the biodiesel production in the global market. In addition, palm oil contains various phytonutrients that can be separated out prior to biodiesel production. These phytonutrients have a high market value and can thus offset the overall palm biodiesel production cost. Indeed, this benefit has not been foreseen for other edible oil crops. To date, palm biodiesel conversion technologies have been well researched, especially the catalysis method. Homogeneous base catalysts are the most common but pose severe
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problems when high FFA appears in CPO. Other methods, such as heterogeneous (base and acid), enzymatic and supercritical technologies, have emerged as an alternative route to produce palm biodiesel in a greener manner with excellent biodiesel yield. However, these new methods have not been readily available at the commercial scale because the catalysts are easily poisoned and deactivated, a high energy input is required, and there are safety-related issues. Extensive research is still required to produce a breakthrough for these technologies in palm biodiesel conversion.
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