A Study on the Production of Biodiesel from Rubber Seed Oil (Hevea Brasiliensis)
The demand for energy around the world is continuously increasing, specifically the demand for petroleum-based energy. Petroleum is the largest single source of energy which has been consuming by the world’s population, exceeding the other energy resources such as natural gas, coal, nuclear and renewable. 90% of energy consumption of the world is from petroleum fuels. The demand and the price of these fuels are increasing at an alarming rate. The world consumption for petroleum and other liquid fuel will grow from 83 million barrels/day in 2004 to 97 million barrels/day in 2015 and just over 118 million barrels/day in 2025 [1]. Under these growth assumptions, approximately half of the world’s total resources would be exhausted by 2025. Also, many studies estimating that the world oil production would peak sometime between 2007 and 2025. Therefore the future energy availability is a serious problem for us.
A country like Bangladesh is heavily dependent on import of fossil fuel and coal. Such dependency makes economy of Bangladesh more vulnerable to external price shocks in the international energy market. Price of fuel in the international market has been showing rising trend since last few years. Bangladesh annually imports about 3.5 million tons of different fuel oils. Of them, some 1.3 million tons are crude oil, 1.45 million tons diesel, 380 tons kerosene, 215 tons jet fuel and 155,000 tons petrol and octane [2]. The search for alternatives of fossil fuels is a major environmental and political challenge also.
Another major global concern is environmental concern or climate change such as global warming. Global warming is related with the greenhouse gases which are mostly emitted from the combustion of petroleum fuels. In order to control the emissions of greenhouse gases, Kyoto Protocol negotiated in Kyoto City, Japan in 1997 and came to effect since February, 2005. Now, Kyoto Protocol covers more than 160 countries globally and targeting to reduce the greenhouse gas emission by a collective average of 5% below 1990 level of respective countries. The Intergovernmental Panel on Climate Change (IPCC) concludes in the Climate Change 2007 that, because of global warming effect the global surface temperatures are likely to increase 1.1C to 6.4C between 1990 and 2100 [3]. Recent environmental and economic concerns have prompted resurgence in the use of biodiesel throughout the world. In 1991, the European Community, proposed a 90% tax reduction for the use of biofuels, including biodiesel
To solve both the energy concern and environmental concern, the renewable energies with lower environmental pollution impact should be necessary. Nowadays several new and renewable energies have been emphasized and biomass energy is one of the renewable energies among them. Biomass energy includes liquid biofuels and which are promising as alternative fuels with low environmental pollution impact, to replace petroleum based fuels. Some of the well known liquid biofuels are ethanol for gasoline engines and biodiesel for compression ignition engines or diesel engines.
In recent years, systematic efforts were under taken by many researchers to determine the suitability of vegetable oil and its derivatives as fuel or additives to the diesel [4-6]. Biodiesel is a renewable and environmental friendly alternative diesel fuel for diesel engine. It can be produced from food grade vegetable oils or edible oils, nonfood grade vegetable oils or inedible oil, animal fats and waste or used vegetable oils, by the transesterification process. Transesterification is a chemical reaction in which vegetable oils and animal fats are reacted with alcohol in the presence of a catalyst. The products of reaction are fatty acid alkyl ester and glycerin, and where the fatty acid alkyl ester is known as biodiesel.
Fig:1.1 Biodiesel as a source of renewable energy.
Biodiesel is an oxygenated fuel and which containing 10% to 15% oxygen by weight. Also it can be said a sulfur-free fuel. These facts lead biodiesel to more complete combustion and less most of the exhaust emissions from diesel engine. But, comparing the fuel properties of biodiesel and diesel fuel, it has higher viscosity, density, pour point, flash point and cetane number than diesel fuel. Also the energy content or net calorific value of biodiesel is about 12% less than that of diesel fuel on a mass basis.
Using biodiesel can help to reduce the world’s dependence on fossil fuels and which also has significant environmental benefits. The reasons for these environmental benefits are: using biodiesel instead of the conventional diesel fuel reduces exhaust emissions such as the overall life circle of carbon dioxide (CO2), particulate matter (PM), carbon monoxide (CO), sulfur oxides (SOx), volatile organic compounds (VOCs), and unburned hydrocarbons (HC) significantly.
Methyl esters of vegetable oils or biodiesel have several advantages and optimum blend can be used in any diesel engine without modification. The use of vegetable oil based fuels is not a recent development. Rudolf diesel, the inventor of diesel engine, used peanut oil as a fuel for his diesel engine at the world exhibition at Paris in 1900. But the interest in vegetable oils decreases due to cheap and abundant supply of petroleum based fuels. But the shortage of petroleum based fuels their rising prices and harmful emissions have accelerated the research in biodiesel.
The rubber tree (Hevea brasiliensis) is a perennial plantation crop, indigenous to South America and cultivated as an industrial crop since its introduction to Southeast Asia around 1876. Rubber plantations yield from 100 to 150 Kg/ha rubber seeds. Rubber seeds are composed of about 43% oil [7-8]. Rubber seed oil (RSO) is a semi-drying type oil [9-10] that does not contain any unusual fatty acids, but is a rich source of polyunsaturated fatty acids C18:2 and C18:3 that make up 52% of its total fatty acid composition [11]. RSO has already been shown to have many applications for industrial purposes, including possible uses for the manufacture of fatty acids, paint, alkyd resin, soap making, surface coatings, and water-reducible alkyds, as well as in the production of biodiesel and for use in fuel compression ignition engines.
To date, no studies have been conducted on the properties of Bangladeshi rubber seed oil (BRSO), particularly those properties relevant to RSO’s industrial uses, such as the types of triacylglycerols (TAG) present, its thermal profile and its solid fat content.
This paper is aimed to study the optimized condition of methanol, catalyst molar ratio of alkali catalysed transesterification reaction of crude rubber seed oil (CRSO) from Hevea brasiliensis sp. on the biodiesel quality and study the CRSO-biodiesel on the diesel machine performances. The effects of reaction temperature and time on the conversion, yield of FAME and composition of the reaction product also investigated. In this study required physicochemical properties of crude oil, produced methyl esters, functional groups of TAG, thermal properties of BRSO were also evaluated.
2.1 Background
Over 100 years ago Rudolf Diesel invented the cycle of diesel engine using the compression-ignition method. The diesel engine was originally made to run on peanut oil, and only later did petroleum become the standard fuel.
Rudolf Diesel said,
“The use of vegetable oils for engine fuels may seem insignificant today, but such oils may become, in the course of time, as important as petroleum and the coal tar products of the present time.”
Fig: 2.1 Portrait of Rudolf Diesel.
With the advent of cheap petroleum, appropriate crude oil fractions were refined to serve as fuel and diesel fuels and diesel engines evolved together. In the 1930s and 1940s vegetable oils were used as diesel fuels from time to time, but usually only in emergency situations. Recently, because of increases in crude oil prices, limited resources of fossil oil and environmental concerns there has been a renewed focus on vegetable oils and animal fats to make biodiesel fuels. Continued and increasing use of petroleum will intensify local air pollution and magnify the global warming problems caused by CO2.
Today, each country in the world is seriously involved in active search for substitutes for petroleum derivatives such as “biodiesel”. There are many conceptual definitions of biodiesel. It can be defined as “Biodiesel is the mono alkyl esters of long chain fatty acids derived from renewable feed stocks, such as vegetable oil or animal fats, for use in compression ignition (CI) engine”.
Technically speaking, biodiesel is the alkyl ester of fatty acids, made by the transesterification of oils or fats, from plants or animals, with short chain alcohol such as methanol and ethanol. Glycerol is, consequently, a by-product from biodiesel production.
Fig: 2.2 Simplified representation of fatty oil to biodiesel conversion.
2.2 Biodiesel as alternative to fossil fuel
Biodiesel is an alternative fuel similar to conventional or ‘fossil’ diesel. Biodiesel can be produced from straight vegetable oil, animal oil/fats, tallow and waste cooking oil. The process used to convert these oils to Biodiesel is called transesterification. This process is described in more detail below. The largest possible source of suitable oil comes from oil crops such as rapeseed, palm or soybean. In the UK rapeseed represents the greatest potential for biodiesel production. Most biodiesel produced at present is produced from waste vegetable oil sourced from restaurants, chip shops, industrial food producers such as Birdseye etc. Though oil straight from the agricultural industry represents the greatest potential source it is not being produced commercially simply because the raw oil is too expensive. After the cost of converting it to biodiesel has been added on it is simply too expensive to compete with fossil diesel. Waste vegetable oil can often be sourced for free or sourced already treated for a small price. (The waste oil must be treated before conversion to biodiesel to remove impurities). The result is Biodiesel produced from waste vegetable oil can compete with fossil diesel. [1]
2.3 Feedstock
Biodiesel is derived from biological sources, such as vegetable oils or fats, and alcohol. Commonly used feedstock is shown in Table 2.1.
Table 2.1: Feedstock used for biodiesel manufacture.
| Vegetable oils | Animal Fats | Other Sources |
| Ø Soybeans
Ø Rapeseed Ø Canola oil (a modified version of rapeseed) Ø Safflower oil Ø Sunflower seeds Ø Yellow mustard seed Ø Rubber seed oil Ø Algae [28] |
Ø Lard
Ø Tallow Ø Poultry fat Ø Fish oil |
Ø Recycled Restaurant Cooking Oil (Yellow Grease)
Ø Rice bran oil[25] |
2.4 Methyl esters of fatty acids suitable as diesel fuel
The analogy to hexadecane as “ideal” petro-diesel component shows why biodiesel is suitable as an “alternative” diesel fuel. The fatty acids whose methyl esters are now used as biodiesel also are long-chain compounds similar to long-chain alkanes such as hexadecane which make good petro-diesel.
Petro-diesel consists of many components. Besides hydrocarbons, petro-diesel often contains significant amounts of compounds known as aromatics. Aromatics are cyclic compounds such as benzene or toluene.
Aromatic compounds have low cetane numbers and therefore are undesirable components of petro-diesel. However, they have high densities and thus help elevate the energy contained in a gallon of the fuel. Polyaromatic hydrocarbons (PAHs) [29] are found in exhaust emissions of petro-diesel and, in reduced amounts, of biodiesel fuel. Biodiesel’s lack of aromatic compounds is often cited as an advantage.
Fats and oils are primarily water-insoluble, hydrophobic substances in the plant and animal kingdom that are made up of one mole of glycerol and three moles of fatty acids and are commonly referred to as triglycerides. Fatty acids vary in carbon chain length and in the number of unsaturated bonds (double bonds). The oil and fatty acids composition found in different vegetable oils and fats are summarized in following table. [4]
Table: 2.2 Fatty acid composition of different oil (on % basis) [12]
| Fatty acid | Soybean | Cottonseed | Palm | Lard | Tallow | Coconut |
| Lauric (C12:0) | 0.1 | 0.1 | 0.1 | 0.1 | 0.1 | 46.5 |
| Myristic (C14:0) | 0.1 | 0.7 | 1.0 | 1.4 | 2.8 | 19.2 |
| Palmitic (C16:0) | 10.2 | 20.1 | 42.8 | 23.6 | 23.3 | 9.8 |
| Stearic (C18:0) | 3.7 | 2.6 | 4.5 | 14.2 | 19.4 | 3.0 |
| Oleic (C18:1) | 22.8 | 19.2 | 40.5 | 44.2 | 42.4 | 6.9 |
| Linoleic (C18:2) | 53.7 | 55.2 | 10.1 | 10.7 | 2.9 | 2.2 |
| Linolenic (C18:3) | 8.6 | 0.6 | 0.2 | 0.4 | 0.9 | 0.0 |
2.5 Vegetable oils and biodiesel
The major components of vegetable oils are triglycerides. The term triacylglycerols is being used more and more, but we will use the classical term in this discussion. Triglycerides are esters of glycerol with long-chain acids, commonly called fatty acids.
Tables: 2.3, lists the most common fatty acids and their corresponding methyl esters. The trivial names of fatty acids and their esters are far more commonly used than their rational names. It is to be noted that fatty acids have higher melting points than their corresponding methyl esters. It is extremely important to realize that vegetable oils are mixtures of triglycerides from various fatty acids. The composition of vegetable oils varies with the plant source. Often the terms fatty acid profile or fatty acid composition are used to describe the specific nature of fatty acids occurring in fats and oils.
Table: 2.3 Characteristics of Common Fatty Acids and Their Methyl Esters [14]
| Fatty acid
Methyl ester |
Formula | Molecular weight | Melting point
(ºC) |
| Palmitic acid
Methyl palmitate |
C16H32O2
C17H34O2 |
256.428
270.457 |
63-64
30.5 |
| Stearic acid
Methyl stearate |
C18H36O2
C19H38O2 |
284.481
298.511 |
70
39 |
| Oleic acid
Methyl oleate |
C18H34O2
C19H36O2 |
282.465
296.495 |
16
-20 |
| Linoleic acid
Methyl linoleate |
C18H32O2
C19H34O2 |
280.450
294.479 |
-5
-35 |
| Linolenic acid
Methyl linolenate |
C18H30O2
C19H32O2 |
278.434
292.463 |
-11
-52 / -57 |
2.6 Nonconventional vegetable oils as feedstock for biodiesel
In most developed countries, biodiesel is produced from soybean, rapeseed, sunflower, groundnut, sesame, palm oil which are essentially edible oils and thus face high demand and more expensive than diesel fuel. A country like Bangladesh is not in a position to compromise its food producing landsd or edible vegetable oil to produce bio-diesel. In this perspective non-edible sources are the only option.
Azam et al. [15] has studied on 75 species of indigenous oil seed bearing plants. Fatty acid compositions, IV and CN were used to predict the quality of fatty acid methyl esters of oil for use as biodiesel. Fatty acid methyl ester of oils of 26 species were found most suitable for use as biodiesel and they meet the major specification of biodiesel standards of USA, Germany and European Standard Organization. Some of these indigenous Bangladeshi non-edible oil seed plants are, Jatropha (Jatropha curcas), Karanja (Pongamia pinnata), Royna (Aphanamixis polystachya), Rubber (Hevea brasiliensis), Castor (Ricinus communis), etc.[15].
2.6.1 Rubber seed oil as a non-conventional source:
Large area of land for rubber plantation is already allotted and we have over 92 000 acres of rubber plantation under BFIDC and non-governmental organization.[16]
Rubber seed oil currently solely has the highest potential for biodiesel production. Bangladesh already existing rubber estates produce more than 2,000 tons of seeds/year, approximately 150 kg/acre [6]. Currently, it has no economic use, rather considered as a waste and can yield more than 500 tons (25%) of RSO annually.
There are 16 governmental rubber estates in three different zones of Bangladesh, i.e. 7 are located in Chittagong Zone, 4 in Sylhet Zone and 5 in Madhupur Zone of Tangail District. The Table: 2.4 and 2.5 shows the rubber plantation in Bangladesh.
Table:2.4. Rubber estates under BFIDC Lists of Rubber Estates under BFIDC. [16]
| Name and place | Total area (acres) | Year
started |
|
| 1. | Ramu Rubber Estate, Rumu, Cox’s Bazar | 2131.00 | 1961 |
| 2. | Raojan Rubber Estate, Raojan, Chittagong | 1378.00 | 1961 |
| 3. | Dabua Rubber Estate, Raojan, Chittagong | 2120.00 | 1969 |
| 4. | Holudia Rubber Estate, Raojan, Chittagong | 2246.00 | 1983 |
| 5. | Kanchannagor Rubber Estate, Ftikchachari, | 2371.00 | 1983 |
| 6. | Tarakho Rubber Estate, Ftikchachari, Chittagong. | 2436.00 | 1983 |
| 7. | Dantmara Rubber Estate, Ftikchachari, | 3965.00 | 1970 |
| 8. | Rupichora Rubber Estate, Bahubol, Hobigonj | 1832.00 | 1977 |
| 9. | Satgaon Rubber Estate, Srimongol, Moulovibazar | 1744.00 | 1971 |
| 10. | Shajibazar Rubber Estate, Madhobpur, Hobigonj | 2040.00 | 1980 |
| 11. | Bhatere Rubber Estate, Kulaura, Moulovibazar | 2467.00 | 1966 |
| 12. | Pirgacha Rubber Estate, Madhupur , Tangail | 2906.00 | 1987 |
| 13. | Chadpur Rubber Estate, Madhupur, Tangail | 2379.00 | 1989 |
| 14. | Sontoshpur Rubber Estate, Madhupur, Tangail | 1036.00 | 1989 |
| 15. | Komolapur Rubber Estate, Madhupur, Tangail | 994.00 | 1989 |
| 16. | Karnajhora Rubber Estate, Madhupur, Tangail | 620.00 | 1994 |
| Total | 32635.00 |
Table: 2.5 Overall land distribution for rubber plantation in Bangladesh
| Name of the organization | Area of garden in acres | |
| 01 | BFIDC | 32 635 |
| 02 | Rubber garden (Private, standing committee) | 32 550 |
| 03 | Development board of Chittagong Hill Tract | 12 000 |
| 04 | Duncun Brothers | 7 500 |
| 05 | James Finley | 5 000 |
| 06 | Messrs. Ragib Ali | 2 500 |
| 07 | Ispahani Neptune | 800 |
| Total | 92 985 |
2.6.2 Exploitation of rubber plant:
2.6.2.1 Plant profile of rubber plant:
Kingdom : Plantae
Division : Magnoliophyta
Class : Magnoliopsida
Order : Malpighiales
Family : Euphorbiaceae
Subfamily : Crotonoideae
Tribe : Micrandreae
Subtribe : Heveinae
Genus : Hevea
Species : H. brasiliensis
Binomial name : Hevea brasiliensis.
Rubber plantations mainly consist of only one species, Hevea brasiliens, a variety of plants of the genus Hevea (Euphorbiaceae family), and native to Brazil. Commonly known as the rubber tree, Hevea brasiliensis is a tall erect tree with a straight trunk and bark which is usually fairly smooth and grey in colour. The plant, grows up to over 40 meters (m) in the wild. The rubber tree is a perennial (lasting for over 100 years) plant.The rubber tree flourishes in the tropics with annual rainfall of 2,000-4,000 mm evenly spread throughout the year, and temperatures ranging between 24-28°C. Rubber (hevea brasiliensis) tree starts to bear fruits at four years of age. Each fruit contain three or four seeds, which fall to the ground when the fruit ripens and splits. Each tree yields about 800 seeds (1.3 kg) twice a year. A rubber plantation is estimated to be able produce about 800-1200 kg rubber seed per ha per year [18], and these are normally regarded as waste.
2.6.3 Toxicity studies of Rubber seed oil:
However, many studies of rubber seeds have indicated that the use of RSO for nutritional purposes faces various vital challenges, one of which is the presence of toxins in RSO. It is well known that some concentration of poisons will always be found in the seeds of all types of plants, including the seeds of the rubber plant. Rubber seeds known to contain linamarin[26,27].
A linamarin is a cyanogenic glucoside. The hydrolysis or cyanogenesis of linamarin by the endogenous enzyme linamarase (?-glucosidase) results in the formation of glucose and acetonecyanohydrin, which later decomposes into hydrogen cyanide (HCN) and acetone[27]. Linamarin has been demonstrated to protect the plant from herbivores, both animals and generalized insect feeders
The presence was confirmed in this study (18.6 mg/100 g). There have also been reports that fresh rubber seeds and its kernel contain about 63.8 to 74.9 mg ofHCN per 100 g (George et al., 2000), as well as about 200 mg /100 g of seeds [26].
Heat treatment (roasting at 350°C for 15 minutes), soaking in hot water or in a 2.5% ash solution for 12 hours could work in detoxification (UNIDO, 1987), or storage at room temperature for a period of 2 to 4 months has been shown to be effective in reducing the hydrogen cyanide (HCN) content of rubber seeds [26].
2.6.4 Potential of Rubber seed oil:
Christopher Columbus is believed to have first found rubber in tropical South America around 1500. Hevea brasiliensis, the common variety of rubber tree produces 99% of world’s natural rubber. The seed contains an oily endosperm. Generally 37% by weight of the seed is shell and the rest is kernel. The oil content of air-dried kernel is 47%. The seed fall season in India is July September. Rubber seed oil is a non-edible vegetable oil. The increase in the price of non-edible oil in recent years generated interest in the collection and processing of rubber seeds. According to a study conducted by the rubber board, on an average, a healthy tree can give about 500 g of useful seeds during a normal year and this works out to an estimated availability of 150 kg of seeds per hectare. The price of rubber seeds is around one Indian rupee per kg. Rubber seeds are produced mostly in kerala (southern most state of India), the processing of rubber seeds is concentrated in Tamilnadu (another southern state).
Table 2.6 Physicochemical properties of Rubber seed oil [17]
| Fuel Property | Diesel oil | Rubber seed oil | Biodiesel |
| Density (gm/cc3) | 830 | 930 | 860 |
| Specific gravity | 0.830 | 0.930 | 0.860 |
| Viscosity (cSt) | 3.55 | 66 | 6 |
| Flash point (0C) | 55 | 198 | 72 |
| Calorific value(MJ/Kg) | 43 | 37.5 | 35 |
Table 2.7 Fatty acid composition of rubber seed oil [17]
| Fatty acid composition (%) | Rubber seed oil |
| Palmitic (C16/0) | 10.2 |
| Stearic(C18/0) | 8.7 |
| Oleic(C18/1) | 24.6 |
| Linoleic(C18/2) | 39.6 |
| Linolenic(C18/3) | 13.2 |
2.7 Process overview of biodiesel production
Different methods for using vegetable oil as alternative to diesel:
- Direct use and Blending, which is the use of pure vegetable oils or the blending with diesel fuel in various ratio.
- Micro emulsions with simple alcohols,
- Thermal Cracking (Pyrolysis) to alkanes, alkenes, alkadienes etc
- Transesterification (alcoholysis);
2.7.1 Direct use and blending
The direct use of vegetable oils in diesel engines is problematic and has many inherent failings. It has only been researched extensively for the past couple of decades, but has been experimented with for almost a hundred years. Although some diesel engines can run pure vegetable oils engines, turbocharged direct injection engines such as trucks are prone to many problems. For short term use ratio 1:10 to 2:20 oil to diesel has been found to successful. [12]
There have been many problems associated with using it directly in diesel engine. [12] This includes:
- High viscosity of vegetable oil interferes with the injection process and leads to poor fuel atomization.
- The inefficient mixing of oil with air contributes to incomplete combustion, leading to high smoke emission.
- The high flash point attributes to lower volatility characteristics.
- Lube oil dilution.
- High carbon deposits.
- Ring sticking.
- Scuffing of the engine liner.
- Injection nozzle failure.
- Types and grade of oil and local climatic conditions.
- Higher cloud and pour points may cause problems during cold weather.
These problems are associated with large triglycerides molecule and its higher molecular mass, which is avoided by chemically modified to vegetable oil in to biodiesel that is similar in characteristics of diesel fuel.
2.7.2 Micro emulsion
Micro emulsion is defined as a colloidal equilibrium dispersion of optically isotropic fluid microstructures, with dimensions generally in the 1-15 nm range. They are formed spontaneously from two normally immiscible liquids and one or more ionic or non-ionic amphiphiles.[13]A microemulsion is designed to tackle the problem of the high viscosity of oils with solvents such as simple alcohols. The performance of ionic and non-ionic microemulsions where found to be similar to diesel fuel, over short term testing. They also achieved good spray characteristics, with explosive vaporization of the low boiling constituents in the micelles, which improved the combustion characteristics. In longer term testing no significant deterioration in performance was observed, however significant injector needle sticking, carbon deposits, incomplete combustion and increasing viscosity of lubricating oils were reported.
2.7.3 Thermal cracking
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 450-8500C. In some situations this is with the aid of a catalyst leading to the cleavage of chemical bonds to yield smaller molecules. 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 SiO2 and Al2O3. [18] The chemical compositions of diesel fractions were similar to fossil fuels.
2.7.4 Transesterification
Ramesh et al, 2002 [20]stated that there are three stepwise reactions in transesterification resulting in the production of 3 moles of methyl esters and one mole of glycerol from triglycerides. The overall reaction is as follows:
Fig: 2.5 Transesterification reaction
The overall process is normally a sequence of three consecutive steps, which are reversible reactions. In the first step, from triglycerides diglyceride is obtained, from diglyceride monoglyceride is produced and in the last step, from monoglycerides glycerol is obtained. In all these reactions esters are produced. The stoichiometric relation between alcohol and the oil is 3:1. However, an excess of alcohol is usually more appropriate to improve the reaction towards the desired product:
Triglyceride (TG) + ROH ? Diglycerides (DG) + RCOOR1
Diglycerides (DG) + ROH ? Monoglycerides (MG) + RCOOR2
Monoglycerides (MG) + ROH ? Glycerol (GL) + RCOOR3
There are several generally accepted ways to make biodiesel. Some are more common than others, e.g. blending and transesterification, and several others that are more recent developments e.g. reaction with supercritical methanol. An overview of these processes is as follows:
Different methods for production of biodiesel by Transesterification (alcoholysis):
(a) Homogenous acid/alkali catalyzed,
(b) Heterogeneous acid/alkali catalyzed,
(c) Microwave assisted transesterification,
(d) Ultrasound assisted transesterification,
(e) Bio/Enzyme catalyzed,
(f) Catalyst free/ Supercritical and subcritical fluid
2.7.4.1 Acid catalyst esterification
The transesterification process is catalyzed by Bronsted acids, preferably by sulfonic and sulfuric acids [28]. These catalysts give very high yields in alkyl esters, but the reactions are slow, requiring, typically, temperatures above 100 °C and more than 3 h to reach complete conversion.
The mechanism of the acid-catalyzed transesterification of vegetable oils is shown in Scheme 5. Acid-catalyzed transesterification should be carried out in the absence of water, in order to avoid the competitive formation of carboxylic acids which reduce the yields of alkyl esters.
Figure: 2.6. Homogeneous acid-catalyzed reaction mechanism for the transesterification of triglycerides: (1) protonation of the carbonyl group by the acid catalyst; (2) nucleophilic attack of the alcohol, forming a tetrahedral intermediate; (3) proton migration and breakdown of the intermediate. The sequence is repeated twice.
2.7.4.2 Base catalyst transesterification
The base-catalyzed transesterification of vegetable oils proceeds faster than the acid-catalyzed reaction [28]. Alkaline catalysts are less corrosives than acidic compounds, such as alkaline metal alkoxides and hydroxides as well as sodium or potassium carbonates. The mechanism of the base-catalyzed transesterification of vegetable oils is shown in Scheme 6.
Alkaline metal hydroxides (KOH and NaOH) are cheaper than metal alkoxides, but less active. Even if a water-free alcohol/oil mixture is used, some water is pro- duced in the system by the reaction of the hydroxide with the alcohol. The presence of water gives rise to hydrolysis of some of the produced ester, with consequent soap formation. This undesirable saponification reaction reduces the ester yields and considerably difficults the recovery of the glycerol due to the formation of emulsions. Potassium carbonate, used in a concentration of 2 or 3 mol% gives high yields of fatty acid alkyl esters and reduces the soap formation [30]. This can be explained by the formation of bicarbonate instead of water, which does not hydrolyse the esters.
Figure: 2.7. Homogeneous base-catalyzed reaction mechanism for the transesterification of TGs: (1) production of the active species, RO-; (2) nucleophilic attack of RO- to carbonyl group on TG, forming of a tetrahedral intermediate; (3) intermediate breakdown; (4) regeneration of the RO- active species. The sequence is repeated twice.
2.7.4.3 Supercritical Methanol
The study of the transesterification of rapeseed oil with supercritical methanol was found to be very effective and gave a conversion of >95% within 4 min. A reaction temperature of 3500C, pressure of 30 MPa and a ratio of 42:1 of methanol to rapeseed oil for 240s were found to be the best reaction conditions. The rate was substantially high from 300 to 5000C but at temperatures above 4000C 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 thermophysical properties such as dielectric constant, viscosity, specific weight and polarity .[15]
2.7.4.4 Biocatalysts
Biocatalysts are usually lipases; however conditions need to be well controlled to maintain the activity of the catalyst. Hydrolytic enzymes are generally used as biocatalysts as they are readily available and are easily handled. They are stable, do not require co-enzymes and will often tolerate organic solvents. “Their potential for regioselective and especially for enantioselective synthesis makes them valuable tools”. [15]
2.7.4.5 Catalyst free transesterification
Transesterification will occur without the aid of a catalyst, however at temperatures below 3000C the rate is very low. It has been said that there are, from a broad perspective, two methods to producing biodiesel and that is with and without a catalyst.
The technical tools and processes for monitoring the transesterification reactions like TLC, GC, HPLC, GPC, H NMR and NIR should be noted. In addition, biodiesel or fuel properties and specifications by different countries should be noted.
2.8 Variables affecting transesterification reaction
The process of transesterification is affected by various factors depending upon the reaction condition used. The effects of these factors are described below.
2.8.1 Effect of free fatty acid and moisture
The free fatty acid and moisture content are key parameters for determining the viability of the vegetable oil transesterification process. To carry the base catalyzed reaction to completion; a free fatty acid (FFA) value lower than 2% is needed [xxxx]. The higher the acidity of the oil, smaller is the conversion efficiency. Both, excess as well as insufficient amount of catalyst may cause soap formation [32].
The triglycerides should have lower acid value and all material should be substantially anhydrous. The addition of more sodium hydroxide catalyst compensates for higher acidity, but the resulting soap causes an increase in viscosity or formation of gels that interferes in the reaction as well as with separation of glycerol [34]. When the reaction conditions do not meet the above requirements, ester yields are significantly reduced.
2.10.2 Catalyst type and concentration
Catalysts used for the transesterification of triglycerides are classified as alkali, acid, enzyme or heterogeneous catalysts, among which alkali catalysts like sodium hydroxide, sodium methoxide, potassium hydroxide, potassium methoxide are more effective [37]. Sodium methoxide causes formation of several by-products mainly sodium salts, which are to be treated as waste. In addition, high quality oil is required with this catalyst [38]. Although chemical transesterification using an alkaline catalysis process gives high conversion levels of triglycerides to their corresponding methyl esters in short reaction times.
If the oil has high free fatty acid content and more water, acid catalyzed transesterification is suitable. The acids could be sulfuric acid, phosphoric acid, hydrochloric acid or organic sulfonic acid. The rate is comparatively much slower.
Enzymatic catalysts like lipases are able to effectively catalyze the transesterification of triglycerides in either aqueous or non-aqueous systems, the by-products, glycerol can be easily removed without any complex process, and also that free fatty acids contained in waste oils and fats can be completely converted to alkyl esters. On the other hand, in general the production cost of a lipase catalyst is significantly greater than that of an alkaline one.
2.10.3 Molar ratio of alcohol to oil and type of alcohol
One of the most important variables affecting the yield of ester is the molar ratio of alcohol to triglyceride. Transesterification is an equilibrium reaction in which a large excess of alcohol is required to drive the reaction to the right. For maximum conversion to the ester, a molar ratio of 6:1 should be used. However, the high molar ratio of alcohol to vegetable oil interferes with the separation of glycerol because there is an increase in solubility. When glycerol remains in solution, it helps drive the equilibrium to back to the left, lowering the yield of esters.
2.10.4 Effect of reaction time and temperature
The conversion rate increases with reaction time. Transesterification can occur at different temperatures, depending on the oil used. For the transesterification of refined oil with methanol (6:1) and 1% NaOH, the reaction was studied with three different temperatures. After 0.1 h, ester yields were 94, 87 and 64% for 60, 45 and 32.80C, respectively. After 1 h, ester formation was identical for 60 and 45 OC runs and only slightly lower for the 32.80C run. Temperature clearly influenced the reaction rate and yield of esters.
2.10.5 Mixing intensity
Mixing is very important in the transesterification reaction, as oils or fats are immiscible with sodium hydroxide–methanol solution. Once the two phases are mixed and the reaction is started, stirring is no longer needed. Initially the effect of mixing on transesterification of beef tallow was study by Ma et al. No reaction was observed without mixing and when NaOH–MeOH was added to the melted beef tallow in the reactor while stirring, stirring speed was insignificant. Reaction time was the controlling factor in determining the yield of methyl esters. This suggested that the stirring speeds investigated exceeded the threshold requirement of mixing.
2.10.6 Effect of using organic co-solvents
In order to conduct the reaction in a single phase, cosolvents like tetrahydrofuran, 1,4-dioxane and diethyl ether were tested. Although, there are other cosolvents, initial study was conducted with tetrahydrofuran. At the 6:1 methanol–oil molar ratio the addition of 1.25 volume of tetrahydrofuran per volume of methanol produces an oil dominant one phase system in which methanolysis speeds up dramatically and occurs as fast as butanolysis. In particular, THF is chosen because its boiling point of 67.80C is only two degrees higher than that of methanol. Therefore at the end of the reaction the unreacted methanol and THF can be co-distilled and recycled.
2.11 Purification of biodiesel:
Purification of biodiesel is necessary because of:
• Corrosion of fuel injectors (water, catalyst)
• Elastomeric seal failures (methanol)
• Fuel injector blockages (glycerin, soaps etc)
• Increased degradation of engine oil
• Pump seizures due to high viscosity at low temperatures
• Corrosion of fuel tanks (excess water, catalyst)
• Bacterial growths and clogging of fuel lines/filters
Purification of biodiesel includes:
(a) Separation of biodiesel
Once the reaction is complete, two major products exist: glycerol and biodiesel. Each has a substantial amount of the excess methanol that was used in the reaction. The reacted mixture is sometimes neutralized at this step if needed.
Glycerol separation: The glycerol phase is much denser than biodiesel phase and the two can be gravity separated with glycerol simply drawn off the bottom of the settling vessel. In some cases, a centrifuge is used to separate the two materials faster.
Alcohol Removal:Once the glycerol and biodiesel phases have been separated, the excess alcohol in each phase is removed with a flash evaporation process or by distillation. In others systems, the alcohol is removed and the mixture neutralized before the glycerol and esters have been separated. In either case, the alcohol is recovered using distillation equipment and is re-used. Care must be taken to ensure no water accumulates in the recovered alcohol stream.
(b) Washing of biodiesel
Once separate major by-product then the methyl esters are not classified as biodieseluntil the proper specifications are met because of impurities andcontaminants include free glycerin, soap, metals, excess methanol, catalyst, moisture, FFA etc are not properly removed.
There are many processes for washing biodiesel. These are:
i) The Wet Wash Process:
Generally in this process, once separated from the glycerol, the biodiesel is sometimes purified by washing gently with warm water to remove residual catalyst or soaps, dried, and sent to storage.
Fig: 2.7 Washing methyl ester using wate
ii) The Dry Wash Process:
In this process, magnesol used as a washing agent to wash methyl ester successfully.
2.12 Fuel properties and specification of biodiesel
The properties of fuel briefly in the following description:
Density:
The density of a material is defined as its mass per unit volume. The symbol of density is ? (the Greek letter rho). A common laboratory device for measuring fluid density is a pycnometer. The SI unit for density is kilograms per cubic meter (kg/m³), Metric units outside the SI kilograms per litre (kg/L), kilograms per cubic decimeter (kg/dm³), grams per millilitre (g/mL), grams per cubic centimeter (g/cc or g/cm³).
Viscosity:
Viscosity refers to the thickness of the oil, and is determined by measuring the amount of time taken for a given measure of oil to pass through an orifice of a specified size. Viscosity affects injector lubrication and fuel atomization. Fuels with low viscosity may not provide sufficient lubrication for the precision fit of fuel injection pumps, resulting in leakage or increased wear. Fuel atomization is also affected by fuel viscosity. Diesel fuels with high viscosity tend to form larger droplets on injection which can cause poor combustion, increased exhaust smoke and emissions. Kinematic viscosity: The resistance to flow of a fluid under gravity”. The kinematic viscosity = viscosity/density. The kinematic viscosity is a basic design specification for the fuel injectors used in diesel engines. Dynamic viscosity: Ratio between applied shear stress and rate of shear of a liquid.
Flash point:
The flash point is defined as the “lowest temperature corrected to a barometric pressure of 101.3 kPa (760 mm Hg), at which application of an ignition source causes the vapors of a specimen to ignite under specified conditions of test.” This test, in part, is a measure of residual alcohol in the B100.The flash point is a determinant for flammability classification of materials. The typical flash point of pure methyl esters is > 200 ° C, classifying them as “non-flammable”. However, during production and purification of biodiesel, not all the methanol may be removed, making the fuel flammable and more dangerous to handle and store if the flash point falls below 130ºC. Excess methanol in the fuel may also affect engine seals and elastomers and corrode metal components.
Pour point:
The pour point of a liquid is the lowest temperature at which it will pour or flow under prescribed conditions. It is a rough indication of the lowest temperature at which oil is readily pumpable. Also, the pour point can be defined as the minimum temperature of a liquid, particularly a lubricant, after which, on decreasing the temperature, the liquid ceases to flow.[1]
Acid value:
Acid value (or “neutralization number” or “acid number” or “acidity”) is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance. The acid number is a measure of the amount of carboxylic acid groups in a chemical compound, such as a fatty acid, or in a mixture of compounds. In a typical procedure, a known amount of sample dissolved in organic solvent is titrated with a solution of potassium hydroxide with known concentration and with phenolphthalein as a color indicator.
The acid number is used to quantify the amount of acid present, for example in a sample of biodiesel. It is the quantity of base, expressed in milligrams of potassium hydroxide, that is required to neutralize the acidic constituents in 1 g of sample.
Veq is the amount of titrant (ml) consumed by the crude oil sample and 1ml spiking solution at the equivalent point, beq is the amount of titrant (ml) consumed by 1 ml spiking solution at the equivalent point, and 56.1 is the molecular weight of KOH.[1]
Carbon residue:
In petroleum products, the part remaining after a sample has been subjected to thermal decomposition…” is the carbon residue. The carbon residue is a measure of how much residual carbon remains after combustion. The test basically involves heating the fuel to a high temperature in the absence of oxygen. Most of the fuel will vaporize and be driven off, but a portion may decompose and pyrolyze to hard carbonaceous deposits. This is particularly important in diesel engines because of the possibility of carbon residues clogging the fuel injectors.
Caloric value:
The heating value or calorific value of a substance, usually a fuel or food, is the amount of heat released during the combustion of a specified amount of it. The calorific value is a characteristic for each substance. It is measured in units of energy per unit of the substance, usually mass, such as: kcal/kg, kJ/kg, J/mol, Btu/m³. Heating value is commonly determined by use of a bomb calorimeter. The heat of combustion for fuels is expressed as the HHV, LHV, or GHV:
Sulfur content:
The percentage by weight, of sulfur in the fuel Sulfur content is limited by law to very small percentages for diesel fuel used in on-road applications.
Biodiesel generally contain less than 15ppm sulfur. ASTM D 5453 test is a suitable test for such low level of sulfur. ASTM D 2622 used for sulfur determination of diesel fuels gives falsely high results when used for biodiesel. More work is needed to assess suitability of ASTM D 2622 application to B20 biodiesel blend. The increase in oxygen content of the fuel affects precision of this test method.
Water content:
Biodiesel and its blends are susceptible to growing microbes when water is present in fuel. The solvency properties of the biodiesel can cause microbial slime to detach and clog fuel filters.
Cetane number:
The cetane number is “a measure of the ignition performance of a diesel fuel obtained by comparing it to reference fuels in a standardized engine test.” Cetane for diesel engines is analogous to the octane rating in a spark ignition engine – it is a measure of how easily the fuel will ignite in the engine.
Cetane number of a diesel engine fuel is indicative of its ignition characteristics. Higher the cetane number better it is in its ignition properties. Cetane number affects a number of engine performance parameters like combustion, stability, drivability, white smoke, noise and emissions of CO and HC. Biodiesel has higher cetane number than conventional diesel fuel. This results in higher combustion efficiency and smoother combustion.
Ash content:
Ash Percentage – Ash is a measure of the amount of metals contained in the fuel. High concentrations of these materials can cause injector tip plugging, combustion deposits and injection system wear. The ash content is important for the heating value, as heating value decreases with increasing ash content.
Ash content for bio-fuels is typically lower than for most coals, and sulfur content is much lower than for many fossil fuels. Unlike coal ash, which may contain toxic metals and other trace contaminants, biomass ash may be used as a soil amendment to help replenish nutrients removed by harvest.
Table: 2.8 Fuel properties of commercial diesel and biodiesel. Fuel Standard ASTM D975 ASTM D6751
| Fuel Property | Diesel | Biodiesel |
| Lower Heating Value, Btu/gal | ~129,050 | ~118,170 |
| Kinematic Viscosity, at 400C | 1.3-4.1 | 4.0-6.0 |
| Specific Gravity kg/l at 600F | 0.85 | 0.88 |
| Density, lb/gal at 150C | 7.079 | 7.328 |
| Water and Sediment, vol% | 0.05 max | 0.05 max |
| Carbon, wt % | 87 | 77 |
| Hydrogen, wt % | 13 | 12 |
| Oxygen, by dif. Wt% | 0 | 11 |
| Sulfur, wt% | 0.05max | 0.0 to 0.0024 |
| Boiling Point, 0C | 180 to 340 | 315 to 350 |
| Flash Point, 0C | 60 to 80 | 100 to 170 |
| Cloud Point, 0C | -15 to 5 | -3 to 12 |
| Pour Point,0C | -35 to -15 | -15 to 10 |
| Cetane Number | 40-55 | 48-65 |
| Lubricity SLBOCLE, grams | 2000-5000 | >7000 |
| Lubricity HFRR, microns | 300-600 | <300 |
Sulfur content for on-road fuel will be lowered to 15 ppm maximum in 2009.
2.13 Advantages of biodiesel
Key Advantages of Biodiesel:
1. Biodiesel is the only alternative fuel that runs in any conventional, unmodified diesel engine.
2. Cetane number is significantly higher than that of conventional diesel fuel.
3. Biodiesel can be used alone or mixed in any ratio with petroleum diesel fuel. The most common blend is a mix of 20% biodiesel with 80% petroleum diesel, or “B20.”
4. The lifecycle production and use of biodiesel produces approximately 80% less carbon dioxide emissions, and almost 100% less sulfur dioxide. Combustion of biodiesel alone provides over a 90% reduction in total unburned hydrocarbons, 75-90% reduction in aromatic hydrocarbons and significant reductions in particulates and carbon monoxide than petroleum diesel fuel.