Paper No: 7 Energy and Environment Module: 26 Biodiesel production from lipids
Development Team
Principal Investigator
&
Co- Principal Investigator
Prof. R.K. Kohli
Prof. V.K. Garg &Prof.AshokDhawan Central University of Punjab, Bathinda
Paper Coordinator
Dr. Dhanya M.S.,
Central University of Punjab, Bathinda
Content Writer
Dr. A.K Sarma , Scientist E, Sardar Swaran Singh National Institute of Bio-Energy, Kapurthala Content Reviewer Prof. A.K Jain, Former Director, SSSNIRE
Anchor Institute Central University of Punjab
Description of Module
Subject Name Environmental Sciences Paper Name Energy and Environment Module Name/Title Biodiesel production from lipids Module Id EVS/EE-VII/26
Pre-requisites
Objectives
ï To learn about feedstocks for biodiesel production
ï To understand the properties of biodiesel
ï To study the transesterification process for biodiesel production
ï To understand different types of Transesterification
ï To learn about factors affecting transesterification process for biodiesel production
Keywords Biodiesel, transesterification, properties,feedstock
Bio-Diesel Module 26: Biodiesel Production from Lipids Objectives:
After studying this module, you will be able to know
ï· To learn about feedstocks for biodiesel production
ï· To study the transesterification process for biodiesel production
ï· To understand different types of transesterification
ï· To learn about factors affecting transesterification process for biodiesel production
ï· To understand the properties of biodiesel
1.0. Introduction
Vegetable oil is a simple lipid having tri-esters linkages of three fatty acyl groups in a glyceride chain. The three esters resemble in structures to petroleum derived hydrocarbons in the range C12- C22 and very closely matches with the chemical composition of diesel fraction, except that it has about 11% oxygen content. Thus, it can be used as a very good substitution of cetane or petroleum diesel if appropriately processed. The structural information of a molecule of lipid in resemblance to cetane can be easily understood from Fig.1.
a. Triglyceride composition of a typical vegetable oil (1-Stearoyl 2-oleoyl 3-linoleoyl glycerol)
Fig. 1 Structural information of hydrocarbon chain of a typical vegetable oil, analogous petroleum hydrocarbons and biodiesel
O CH3 O
Diesel (Cetane) Kerosene/Jet-A1 (n-Dodecane)
Gasoline (Iso-octane)
O (Stearic acid: C18:0) CH2â O
O (Oleic acid: C18:1) CH â O
O (Linoleic acid: C18:2) CH2â O
Bio-diesel is an alternative to petroleum-based fuels derived from vegetable oils, animal fats, and used waste cooking oil. Bio-diesel production is a very modern and technological area for researchers due to the relevance that it is winning every day because of the increase in the petroleum prices and the environmental advantages. It refers to a fuel made from biologically derived resources that has properties similar to those of petroleum- based diesel fuels. American Society for Testing and Materials (ASTM) defines biodiesel fuel as monoalkyl esters of long chain fatty acids derived from a renewable lipid feedstock, such as vegetable oil or animal fat which is the most suitable substitute to diesel. Transesterification is the most common method that leads to production of monoalkyl esters of vegetable oils and fats (called FAME).
Biodiesel can be used directly or blend with diesel. A blend of 20 % biodiesel with 80 % diesel, by volume, is termed âB20â which is the most common blend. A blend of 5 % biodiesel with 95% diesel, designated is âB5â. Similarly, B100 (100% biodiesel), B10 (10% biodiesel with 90% petroleum diesel), all other blends B2, B15, B30,etc.
2. History of biodiesel
The use of vegetable oil as alternative fuels was recorded more than one hundred years ago, when the inventor of the diesel engine Rudolph Diesel first tested peanut oil in his compression- ignition engine. During 1940âs (mostly 2nd world war period) scientists discovered that the viscosity of vegetable oils could be reduced by a simple chemical process after separation of acid unit of the triglyceride, and that it could perform as diesel fuel in modern engine. Catalytic processes of biofuels and fuel additive production using vegetable oil as feedstock were gained momentum during the second world war of the 1940âs, where triesters and fatty acid molecules obtained from rapeseed and other vegetable oil was successfully used for diesel and steam engine. The ready availability of petroleum based diesel at low cost and global co-operation after the world war-II changed the picture of vegetable oil processing.
During 1980âs, just after the second oil embargo of 1973, the requirement of alternative feed stock for petroleum based fuels were highlighted around the globe in R & D sectors.
3. Feedstocks for biodiesel production
Biodiesel can be obtained from edible, non-edible oils and animal fats which mainly comprise of triglycerides. Economically, biodiesel commercialization mainly depends on price of raw materials which occupies 70% of marketed biodiesel production cost. Edible oil such as palm oil, soybean oil, sunflower oil, coconut oil, rapeseed oil etc., was the main focus for biodiesel production in the United States and European countries.
Non-edible energy crops include Jatropha (Jatropha curcas), Mahua (Madhuca indica), Karanja (Pongamia pinnata), Polanga (Calophyllum inophyllum), Rubber seed (Hevea brasiliensis), Cotton seed, Jojoba (Simmondsia chinensis), Neem (Azadirachata
indica), Tobacco (Nicotiana tabacum), etc. were included in second generation biodiesel feedstocks. The promising indigenous non-edible biodiesel feedstocks in India are Melia azadirachta (Neem), Shorea robusta (Sal), Madhuca indica (Mahua) Schleichera oleosa (Kusum), Soapnut (Sapindus mukorossi), Kokum (Garcinia indica), Cheura (Diploknema butyracea) and Tung (Aleurites fordii).
Especially, Jatropha curcas (Ratan Jyot) was promoted within Jatropha mission. Jatropha curcas is a drought-resistant perennial, growing well in marginal/poor soil belongs to Euphorbiaceae. It has oil content of 37-40%.
Microalgae with high oil content like Scenedesmus dimorphus,Chlamydomonas rheinhardii, Chlorella vulgaris also be used for biodiesel production. Waste cooking oil (WCO) also called waste frying oil (WFO) is also reported to have the potential for biodiesel production.
Germany and France are the leaders in biodiesel production. The primary feedstock for biodiesel production is soy bean in United States, rape seed in Europe, and palm in South East Asia.
4. Biodiesel Production by Transesterification
The direct utilization of vegetable oil as fuel in engine creates coking of injectors, carbon deposition, fouling, and oil ring sticking. Several methods have been employed for processing of vegetable oils making it suitable for diesel engine applications such as direct blending, micro emulsion, pyrolysis and transesterification.
Industrially biodiesel is produced through transesterification preparation technology which includes chemical synthesis or biological transformation of triglycerides into esters using different catalysts.
Chemical techniques have mainly KOH and NaOH catalytic process while sulfuric acid (H2SO4) and hydrochloric acid (HCl) are acid catalytic processes (Felizardo et al., 2006;
Kulkarni and Dalai, 2006). Positive features of this technique are short reaction time, high yield and low cost of catalysts. However, high temperature involvement, pretreatment of raw material, high energy consumption, complex downstream process, difficulty in catalyst recovery are the major drawbacks of this process. Furthermore, single step chemical method can be used when the oil has low free fatty acid and water content, 0.5% and 0.1%, respectively (Lotero et al., 2005). These bottlenecks of chemical process can be easily overcome by deploying heterogeneous catalytic processes.
Heterogeneous catalyst includes solid catalysts which are immiscible with the oil, alcohol or in both and remain in different phase to catalyze the transesterification reaction.
Thermo catalytic conversion of vegetable oil to useful liquid fraction with or without external hydrogen under high temperature and pressure is another alternative route for production of liquid fuels from vegetable oil, waste cooking oil, animal fat etc. The product obtained can be refined into various fraction similar to the distillation of petroleum crude oils.
4.1. Chemical transesterification process
Transesterification, also called alcoholysis, is the reaction of an oil or fat with an alcohol to form esters and glycerol. The alcohols that can be used in the transesterification process are methanol, ethanol, propanol, butanol and amyl alcohol. Among all these alcohols methanol and ethanol are used most frequently, especially methanol because of its low cost, polar nature and shortest chain length. It can quickly react with triglycerides and sodium hydroxide easily dissolves in it.
The transesterification reaction can be catalyzed by alkalis, acids or enzymes. The alkalis include sodium hydroxide, potassium hydroxide, carbonates and corresponding sodium and potassium alkoxides. The acid catalysts include sulphuric acid, sulphonic acids, hydrochloric acids etc. Lipases can be used as biocatalysts.
The transesterification reaction is shown in Fig 3.
O
O R O C R1
+
O CH2 O C R1 Catalyst H2C OH O
R2C O CH + 3 ROH HC OH + R O C R2 +
CH2 O C R3 O
H2C OH O
R O C R3
Triglyceride Alcohol Glycerol FAME where, R, R1, R2 and R3 represent different fatty acid chains
Fig. 3 Transesterification of triglycerides with alcohol
Stoichiometric material balance yields the following simplified equation:
Fat or Oil + 3 Methanol ïźï 3 Methyl Ester + Glycerol
1000 kg 107.5 kg 1004.5 kg 103 kg
Scheme 1 Stoichiometric material balance for biodiesel production
The mass flows in the equation are for the case of complete conversion of stearic acid triglyceride.
Regardless of the use of different processes and catalysts, the two routes for biodiesel production have been discussed in detail. Biodiesel preparation technology (transesterification) includes different methods of production.
4.1.1 Homogeneous catalyzed transesterification 4.1.1.1Base-catalyzed transesterification
This process is conventionally used in industry using base catalysts such as alkaline metal alkoxide (sodium methoxide, CH3ONa) and hydroxides (NaOH & KOH). Alkaline catalysts are less corrosive than acidic compounds and gave faster reaction rate than acid-catalyzed reactions. Alkaline metal alkoxide are most common active catalysts as they gave greater yield in short reaction time but their requirement of absence of water makes them inappropriate for industrial use so hydroxides gain more importance. The most important merit of this process is its low temperature and low pressure conditions. But they are limited by the FFA content of the feedstock which on reacting with alkali forms soap and reduces the yield of production. The FFA content for this process should be within the range between 0.5% - 2% (Yan et al., 2009).
Mechanism: It involves four steps.
(a) Reaction of base with alcohol to form alkoxide with protonated catalyst
ROH + B RO- + BH+
(b) Nucleophilic attack at carbonyl carbon of triglyceride molecule by alkoxide ion to form tetrahedral intermediate
R'COO CH2 R"COO CH
+ -OR
R'COO CH2
R"COO CH OR
H2C OCR'"
O
H2C O C R'"
O
(c) Rearrangement of intermediate to give rise to alkyl ester and a diglyceride anion
R'COO CH2
R"COO CH OR
R'COO CH2
R"COO CH + ROOCR'"
H2C O C R'"
O
H2C O-
(d) Diglyceride anion deprotonates the catalyst forming active catalyst and diglyceride
R'COO CH2
R"COO CH H2C O-
+ BH+
R'COO CH2
R"COO CH + B H2C OH
Diglyceride and monoglycerides are transformed into alkyl esters and glycerol using the same mechanism.
4.1.1.2 Acid-catalyzed transesterification
This process uses Bronsted acids like BF3, HCl, H3PO4, H2SO4 and suphonic acids as catalysts. It is insensitive to FFA in feedstock which makes it suitable for low grade oils and simultaneously catalyzed esterification and transesterification process. But the darker side of the process is that the reaction rate is 4000 timeâs slower and low yield as compared to base-catalyzed transesterification (Zheng et al., 2006). Catalyst separation is difficult, requirement of high oil alcohol molar ratio, corrosion problem, neutralization and waste water treatment or high energy consumption depicts the major drawbacks of this process.
Mechanism: It involves three steps (Srivastava and Prasad, 2000) (a) Protonation of carbonyl group and results in carbon cation formation
O H+ +OH OH
R'
OR
"
R' OR"
+
R' OR
I II (b) Nucleophilic attack of alcohol to produce
tetrahedral intermediate
OH R
+ O
+ H
I OH
+
R' O
H
R' OR" OR"
III
(c) Rearrangement of tetrahedral intermediate to release an alkyl ester and proton catalyst
OH O
+
R' O
OR" H
-H+/R"OH
R' OR
III IV
4.1.1.2 Acid - base catalysed transesterification (Two-step process)
For oils having high amount of free fatty acids (FFAs) two step processes was recommended (Mc Cormick et al, 2001): first acid catalyzed esterification followed by alkali catalyzed transesterification. Attempts were made to use a combined effect of acid and base for synthesis of biodiesel from high FFA containing oils. This process involves two steps: first the acid catalyst was employed for esterification of FFAs to ester.
When FFA level reaches less than 0.5-1 wt.% , then base catalyst was employed for transesterification step. The major drawback of this process is the requirement of extra separation stages, washing and catalyst removal in both steps. The acid catalyst from the first step can be removed by base neutralization in second but it adds extra cost of base catalyst to the process, which makes this process economically not feasible to use (Canakci and Van Gerpen, 2003).
R
R
Step:1
R1OH
Triglyceride + RCOOH Triglyceride + RCOOR1+ H2O
H2SO4
Step:2
R1OH
Triglyceride + RCOOR1 RCOOR1 + glycerol
NaOH
4.1.2Heterogeneous catalyzed transesterification
The complex problems of homogeneous reactions were easily sorted out by using solid heterogeneous catalyst. Their characteristics like interconnected large pore system, high concentration of strong acid sites, hydrophobic surface etc. helps in making this process feasible for industrial application. These catalysts mainly include (Chouhan and Sarma, 2011):
ï Ion-exchange resins such as Nafion, Amberlyst
ï Transition metal oxides and their derivatives
ï Boron group based catalyst
Aluminium and Al2O3, Îł-Al2O3, Al2O3 supported on CaO and MgO, halides, alloys and nitrates were included in this category.
ï Alkaline earth metal oxides and derivatives
Mg, Be, Ca, Sr, Ba and Ra, their oxides and derivatives come in the category of alkaline earth metals. Mainly MgO, CaO and SrO are widely used for biodiesel production as heterogeneous catalyst.
ï Mixed metal oxides and derivatives
ï Alkali metal oxides and derivatives
ï Waste material based catalysts
ï Carbon based catalyst
Although chemical transesterification using an alkali-catalysis process gives high yield of esters in short reaction times, the reaction has several drawbacks: it is energy intensive, recovery of glycerol is difficult, the catalyst has to be removed from the product, free fatty acids and moisture interfere with the reaction, and alkaline waste water requires treatment before discharging to the environment
4.2 Bio-chemical transesterification process 4.2.1 Enzyme catalyzed transesterification process
Application of enzymes eliminates the aforementioned downstream processing problems of chemical catalysts and has the potential to provide easy way to settle down the bottlenecks of biodiesel production through chemical process. The large waste water generation and difficulty in glycerol recovery eventually increases the production cost of biodiesel and also it is not environmental benign. However, in contrast to this enzyme catalysis especially using lipase proceed without generating by- products, facilitates simultaneous esterification and transesterification of triglycerides and operates at low temperature and pressure, thus reducing energy consumption (Atadashi et al., 2011; Marchetti et al., 2008; Fukuda et al., 2008).
Enzyme catalyst (Lipases) offers the biological route of biodiesel production with a number of advantages both economically and environmentally viable.
ï Specific and selective towards substrates like triglycerides
ï Minimize side reactions and impurities, easy product separation and recovery
ï Eliminates treatment cost of catalyst through recovery process
ï Low reaction condition requirement
ï Biodegradable and environmentally acceptable nature
ï Provide opportunity for catalyst reuse with great stability through immobilization technique
The overall spectrum of catalysts employed for the transesterification process has been shown in Fig. 4.
Fig. 4 Classification of transesterification catalysts
It is therefore required to emphasize completely green technology for biofuel production using all required precursor derived from renewable resources and ecofriendly, as depicted in Fig.5. (Sarma et al, 2011).
Homogeneous Acid Catalyst
Transesterification Catalysts
Acid Catalyst Base Catalyst Bio-catalyst
Heterogeneou s Acid
Homogeneous Base Catalyst
Heterogeneous Base Catalyst
Enzyme Catalyst
Ion-exchange Resin Acid Catalyst
Functionalized Acid Catalyst of waste
Carbon Group Based Catalyst
Boron Group Based Catalyst
Waste Materials Based Catalyst
Transitional Metal Oxide Catalyst
Mixed Metal Oxide
& Derived Catalyst
Alkali Metal Oxide
& Derived Catalyst
Fig. 5 Flow chart of biodiesel production using all renewable resources
Table 1: Advantages & Disadvantages of different types of catalysts used for transesterification process (Lam et al., 2010)
S.
No. Type of catalyst Advantages Disadvantages
1. Homogeneous acid catalyst
Esterification &
transesterification occurs simultaneously
H2SO4 lead to corrosion on reactor and pipelines Insensitive to FFA and water Very slow reaction rate Useful method if low grade oil
is used Difficulty in separating catalyst Less energy intensive
Mild reaction conditions
2. Homogeneous alkali catalyst
Very fast reaction rate than acid catalyzed
Soap formation will reduce the biodiesel yield, increase viscosity
of the product
Mild reaction conditions FFA>1% lead to soap formation Less energy intensive Sensitive to FFA content in oil Catalysts like NaOH & KOH
are cheap & easily available
Product purification problem &
produce huge amount of waste water
3. Heterogeneous acid catalyst
Insensitive to FFA and water content of oil
Complicate catalyst synthesis lead to higher cost Useful method if low grade oil
is used
High reaction temperature &
oil/alcohol ratio Easy separation of catalyst Long reaction time & energy
intensive Esterification &
transesterification occurs simultaneously
Product contamination due to leaching of catalyst active sites High possibility to reuse &
regenerate the catalyst
4. Heterogeneous alkali catalyst
Relatively faster reaction rate than acid catalyzed
Soap formation will decrease the biodiesel yield & Problem in
product purification Mild reaction conditions Slow reaction rate than acid
catalyzed
Less energy intensive FFA>2% lead to soap formation Easy separation of catalyst
from product
Poisoning of catalyst when exposed to air High possibility to reuse &
regenerate the catalyst
Product contamination due to leaching of catalyst active sites Highly reactive at high
temperature and pressure
Energy intensive but the quality of product is good
5. Enzyme catalyst
Preferred method for low grade oil
Reaction rate is very low, High cost of enzymes Insensitive to FFA and water
content of oil
Sensitive to FFA content due to basicity property Reaction occurs at low
temperature,
& need Simple purification steps
Sensitive to alcohol mainly methanol as it deactivates the
enzyme
After production biodiesel, can be purified and separated and can be blended with diesel in any proportion or directly use in CI (diesel engine) without any engine modification. The diesel engine (CI engine) used for power production shows comparable engine efficiency, brake specific fuel consumption and mechanical efficiency if it is used in engine mixing with 20% by volume with petroleum diesel. This homogeneous mixture is called B20.
4.4 Factors affecting transesterification process for biodiesel production
4.4.1 Oil composition and free fatty acids (FFA)
Most of the commercial biodiesel is produced from plant oils (mainly colza, soybean and sunflower (Meher et al., 2006)). These oils are also classified as edibles oils that contain less than 1% free fatty acid, no toxic components and are called high grade oils. Recycled or waste oil and byproducts of the refining of vegetable oils, some non- edible oils, and animal fats contain higher levels of FFAs, and crude mahua oil and tobacco seed oil contain about 20% and 17% FFAs, respectively. Therefore, an esterification step is required using homogeneous acid-catalyzed, supercritical, enzymatic or heterogeneous catalyst processes (Ma and Hanna, 1999; Meher et al., 2006). The fatty acid profile of the typical oil used for transesterification should also be suitable.
4.4.2. Reaction temperature
The rate of transesterification reaction is greatly influenced by the temperature at which the reaction proceeds. A higher reaction temperature can be instrumental in reducing reaction time and decrease the viscosities i.e. increase in reaction rate. If the reaction temperature surpasses optimum temperature, the saponification reaction of triglycerides starts which decreases the yield of biodiesel product during homogeneous alkali catalyzed transesterification reaction (White et al., 2011).
4.4.3 Alcohol to triglyceride molar ratio
The molar ratio of alcohol to triglyceride is a very important factor for effective yield of biodiesel. Three moles of alcohol and one mole of triglyceride is required for the production of three moles of fatty acid alkyl ester and one mole of glycerol in stoichiometric terms. It is suggested by many researchers to use higher alcohol to triglyceride molar ratio so that maximum ester conversion is achieved (Lam et al., 2010).
The yield of biodiesel is enhanced if alcohol to triglyceride molar ratio is increased beyond 3:1 and reaches a maximum. Further, increasing the alcohol amount beyond the optimal ratio will not increase the yield, but will increase costs for alcohol recovery. In addition, the molar ratio is associated with the type of catalyst used and the molar ratio of alcohol to triglycerides in most investigations is 6:1; with the use of an alkali catalyst, alcohol to triglyceride ratio as high as 21:1 was suggested when the percentage of free fatty
acids in the oils or fats were high (Barakos et al., 2008).
1.4.3. Reaction time
In general, the rate of conversion of fatty acid esters increases with higher residence time. Initially the reaction proceeds with a slow rate due to the mixing and dispersion of alcohol into the oil. In general reaction is completed within 90 minutes and biodiesel yield remains constant until the start of backward reaction. Backward reaction results in loss of ester as well as may to initiate the saponification reaction (Ramosa et al., 2008; Huerga et al., 2014).The ester conversion rate increases with reaction time.
4.4.4 Catalyst type and loading
Homogeneous alkali-catalysed transesterification is much quicker than acid-catalysed transesterification. However, a large amount of water is used to transfer the catalysts from the organic phase to a water phase after completion of the reaction. Hence, it is considerably costlier to separate out the catalyst from the developed solution. (Ma and Hanna, 1999;
Chouhan and Sarma,2011). Heterogeneous base catalysts are noncorrosive, environmental friendly and have minute disposal issues. But they can be much easily separated from the biodiesel. They can be instrumental to give high activity, selectivity and longer catalyst lifetimes (Ebiura et al., 2005; Hak et al., 2005; Dorado, 2004).
4.4.5 Type of reactor
Different kinds of reactor were found instrumental for biodiesel production in laboratory scale and bench scale applications. The different operational parameters require different types of reactors based upon the feedstock used.
A microporous TiO2/Al2O3 membrane was packed with potassium hydroxide catalyst supported on palm shell activated carbon showed highest conversion of palm oil to biodiesel in the reactor at 70 ËC employing 157.04 g catalyst per unit volume of the reactor and 0.21 cm/s cross flow circulation velocity (Baroutian et al., 2011). The reactors reported for the biodiesel production are domestic microwave oven (DMO) with a circulating pump, micro-tube reactor, heterogeneous plug flow reactor with axial dispersion, pressurized fused- silica jet-stirred reactor, dynamic tubular loop reactor
4.4.6 Water content and acid value
Water causes problems not only during the production of biodiesel but also in purification, storage and in combustion. In alkali-catalyzed transesterification reaction, water causes decrease in biodiesel quality as it lowers heat of combustion, increased corrosiveness and catalyzes hydrolytic reaction (Atadashi et al., 2012).
For alkali catalyzed transesterification, the triglycerides and alcohol must be substantially anhydrous because water leads to partial saponification reaction (Wright et al.,
1944). The soap reduces the efficiency of the catalyst and increases the viscosity due to gel formation which causes problems in separation of glycerol. The acid value of a vegetable oil is defined as the Normally the free fatty acid content of the refined oil should be below 0.5wt.% (Ma et al., 1998).
Other factors
Speed of agitator or stirring speed is an important factor for transesterification reaction. High speed of agitation is suggested for complete reaction. Gum content in the oil sometime affects the transesterification due to oxidation and polymerization during reaction.
5. Chemical composition and properties of biodiesel
The chemical composition of fat and oil esters is dependent upon the length and degree of unsaturation of the fatty acid alkyl chains. The most important compositional differences between petroleum diesel and biodiesel are oxygen content.
Biodiesels contain 10-12 %wt oxygen, which lowers energy density and hence lowers the particulate emission. Acids may be saturated or unsaturated (contain one or more double bonds).
Depending on the nature of the fatty acids present in the source, the fuel properties of the biodiesels may differ to different extent.
5.1. Specific gravity
Biodiesel specific gravity is reported to vary between 0.86 and 0.90 g/cc.
5.2. Iodine number
Iodine number is a measure of the degree of unsaturation of the fuel. Unsaturation can lead to deposit formation and storage stability problems with fuels. Ryan et al. (1984) suggested that the maximum iodine number should be limited to 135.
5.3. Cetane number
The cetane number of a fuel as specified by ASTM D-613 is a measure of its ignition delay. A higher cetane number indicates shorter time between the initiation of fuel injection and ignition, a desirable property in diesel engine fuel. Petroleum diesel has a cetane (46-47) in general while biodiesel has a cetane value 48-55 as reported in several literatures.
The cetane number of biodiesels depends on the parent oil sources. Reported cetane numbers for soybean oil methyl esters ranges from 45.8 to 56.9 and that of rapeseed oil methyl esters ranges from 48 to 61.8. Highly saturated esters such as those prepared from tallow and used frying oil have the highest cetane numbers. Cetane increases with chain length, decreases with the number of double bonds, and decreases as double bonds and carbonyl groups move towards
the centre of the chain. Increasing cetane number of biodiesel has been shown to reduce nitrogen oxides (NOx) emissions (Ullman et al., 1990).
5.4. Flash point
Flash point as specified by ASTM D-93 is a measure of the temperature to which a fuel must be heated such that the mixture of vapor and air above the fuel can be ignited. The flash point of the neat biodiesels is always higher than those of petroleum diesel fuels, typically greater than 900C and thus neat biodiesel is much safer than diesel from a storage and fire-hazard point of view. However, because of the oxidative instability flash point of biodiesel prepared from unsaturated fatty acids may change during storage.
6.5. Distillation temperature
Fats and oil ester fuels have a narrow boiling point range relative to petroleum diesel and exhibit average boiling points ranging between about 3250 and 3500C (Graboski and McCormick, 1998).
6.6 Flow properties
Cloud point and pour point are the key flow properties of fuels for winter use. Cloud point, ASTM D-2500, is the temperature at which wax formation occurs to plug the fuel filter. It is the measure of the temperature of first formation of wax when the fuel is cooled.
All biodiesel fuels exhibit poor cold flow properties with cloud and pour point 200 - 250C, higher than those of petroleum diesel. The structural properties of biodiesel that affect freezing point are degree of unsaturation, chain length and degree of branching. Highly saturated tallow esters are poorer in freezing point than soybean and rapeseed esters.
6.7. Viscosity and surface tension
According to ASTM D-445 specification, the maximum viscosity of diesel fuel is 4.1 centistokes (cS or cSt) at 400C. High viscosity leads to poorer atomization of the fuel spray and less accurate operation of the injectors. Moreover, the viscosity of neat biodiesel and biodiesel blends increases more rapidly as temperature is decreased (EMA, 1995). Surface tension of a fuel affects the spray atomization and droplet during the spray. Very limited data on the surface tension of neat biodiesel are available. Stotler and Human (1995) reported a value of 34.9 dyne/cm at 600C for neat soybean oil methylester and Recce and Peterson (1993) reported 25.4 dyne/cm at 100 0C for rapeseed oil methylester.
6.8 Storage and stability
Stability includes thermal stability under both hot and cold conditions, resistance to oxidation, polymerization and microbial activity during storage and absorption of water. The main source of instability in biodiesel fuels is unsaturation in the fatty acid chain. If two or more double
bonds are present in the fatty acid chain, they have a mutually activating effect. The metals and elastomers, in contact with biodiesel during storage can also impact stability. Oxidation leads to the formation of hydro peroxide, which can polymerize to form insoluble gum.
Water present in biodiesel fuels can cause the formation of rust. Water is also a necessary ingredient for microbial growth.
6.9. Oxidative stability
Oxidation products formed in biodiesels affect fuel storage life and contribute to deposit formation in tanks, fuel systems and filters. Gum number is a measure of deposit formation.
Unsaturated fatty acid esters possess high gum numbers. Earlier workers determined the gum numbers of the methyl- and ethylesters of soybean oil were reported to be 16,400 and 19,200 respectively which were much higher than the gum number of petroleum diesel oil (Graboski and Mc Cormick, 1998).
Table-4 Key properties of biodiesel and diesel as compared to international standards
S.No. Property Jatropha oil
Jatropha
biodiesel Diesel
Biodiesel standards
ASTD 6751- 02
DIN EN 1421 1 Density (15 0C, 4
Kg/m3) 940 880 850 - 860-900
2 Viscosity (mm2/S) 24.5 4.8 2.6 1.9-6.0 3.5-5
3 Flash point ( 0C ) 225 135 68 >130 >120
4 Pour point ( 0C) 4 2 -20 - -
5 Water content (%) 1.4 0.025 0.02 <0.03 <0.02
6 Ash content (%) 0.8 0.012 0.01 <0.02 <0.02
7 Carbon residue (%) 1 0.20 0.17 - <0.30
8 Acid value
(mg KOH/g) 28 0.40 - <0.80 <0.50
9 Calorific value
(MJ/Kg) 38.65 39.23 42 - -
20
Environmental Sciences
Energy and Environment Biodiesel production from lipids 5. Advantages of Biodiesel
Biodiesel is
Biodiesel fuel is a renewable and biodegradable fuel, eco-friendly in nature and can be effectively used as the substitute of petroleum diesel.
B%0 are also used as reported in literature. It emits comparable NOx but negligible SOx as compared to petroleum diesel and lower number of aromatic compounds during combustion and has higher cetane values than diesel fuel.
ï· renewable,
ï· non-toxic,
ï· biodegradable,
ï· contains low sulphur and aromatic content,
ï· high cetane number,
ï· easy transportability,
ï· high combustion efficiency and
ï· clean burning substitute to petroleum based diesel fuels.
ï· The GHG emissions of biodiesel fuel are 4-5 folds lesser than the gasoline and 3 fold less than petrodiesel.
ï· It also helps in reducing the risk of global warming by decreasing the carbon emissions to the atmosphere.
7. Disadvantages of biodiesel But biodiesel has
ï· high viscosity
21
Environmental Sciences
Energy and Environment Biodiesel production from lipids
ï· high cloud and pour point and
ï· The plugging of filters, coking on injectors, more carbon deposits, excessive engine wear, oil ring sticking, engine knocking, and thickening and gelling of lubricating oil are some of the issues with biodiesel. So Biodiesel separation and purification are very essential
ï· slightly higher NOx emissions than petro diesel
ï· edible crops has forced many countries (mainly the Asian countries) to demolish the forest land which resulted ecological imbalance.
ï· Long time storage, low heating value of oil, oxidative stability, higher NOx emissions and especially the high feedstock prices were the major critics which suppresses the use of edible crops for biodiesel production.
Summary
If the processes for utilization of non-edible vegetable oil and blending practices are implemented inside the refinery, a major problem of utilizing biodiesel and blends can be sorted out. If these non- edible oils could be converted to petroleum like hydrocarbons (to obtain the different petroleum fractions and could be used for industrial scale production), it would be of immense help for the next generation. Further, large scale non-edible seed plantations can provide energy security to the country and create infrastructure facilities for biorefinery, oil extraction, bio-mass utilization and their commercialization from existing plantations/trees and also provide the bulk employment to
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Environmental Sciences
Energy and Environment Biodiesel production from lipids rural masses.
