Biodiesel is a renewable fuel obtained from vegetable oil or animal fat. It can be used either as direct substitute, extender or as an additive to fossil diesel fuel in compression ignition engines. It is an extremely environmentally friendly fuel source which can help in easing the pressure off the non-renewable sources of energy.
The biodiesels are made from vegetable oils like soya and corn. It is a clean fuel, since it does not contain petroleum and other toxic substances and has fairly cost effective preparation. It is thus helpful in reducing the carbon footprint and prevents global warming. Environmental Protection Agency (EPA) has passed this renewable source of energy as a clean and non polluting fuel and has accredited it to meet the standards of California Air Resources Board (CARB).
Methanol and chemical processes that separate glycerin and methyl esters from fats or vegetable oils are used to prepare biodiesel. Glycerin being a common byproduct is used ensuring optimum utilization and cost effective production.
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Biodiesel is an important source of renewable energy and plays an extensive role in various sectors of industrial growth. It is cost effective and hence has great potential for use as a alternate biofuel.
Biodiesel is the oil derived from vegetable sources. Both edible and non-edible soil sources can be used to derive vegetable oil. The non-edible oils commonly used for industrial purposes are produced from linseed, castor and oilseeds of some trees. The direct use of vegetable oils in diesel engines can pose problems because these oils are too viscous. Thus the vegetable oils are converted into their methyl esters which have low viscosity. For decreasing viscosity, vegetable oils are allowed to react with methyl alcohol in the presence of a catalyst to form methyl esters.
In addition to having lower viscosity, biodiesel has a cetane number of 58 which is quite high and an indicator of fuel ignition efficiency. There is no sulphur or benzene present which leads to much lower environment pollution. The biodiesel is also biodegradable, non toxic and has high oxygen content for efficient combustion.
The general process is depicted below. A fat or oil is reacted with an alcohol,
Vegetable oils are triglycerides of fatty acids. For preparation of biodiesel, alcoholesters of fatty acids are prepared by transesterification of glycerides. In this process, linear monohydroxy alcohols react with the vegetable oils in presence of catalysts to produce alcohol ester vegetable oils and glycerin as a by-product. These alcohol ester vegetable oils are then used as biodiesel.
Transesterification is the process in which an ester reacts with alcohol and R group of the ester is exchanged with R group of the alcohol. This gives a new ester as a product and an alcohol.
In this process, triglycerides (1) undergo reaction with an alcohol like ethanol (2) to produce ethyl esters of fatty acids (3) and a byproduct glycerol (4):
The reaction by itself may proceed exceedingly slow or at worst may not take place at all. Acids, bases and heat are generally used to catalyse the reaction to proceed more quickly.
Natural oils are preferably used in this process. The alkyl groups present on the triglyceride are not necessarily the same. Following reaction distinguishes it:
R1, R2, R3 : Alkyl group.
Triglyceride is reacted with alcohol in presence of catalyst (NaOH, KOH, or Alkoxides). This is done primarily in production of biodiesel to estimate amount of alkaline that is required to neutralize completely any free fatty acid present which more or less ensures 100% transesterification.
The catalyst generally used is sodium hydroxide or potassium hydroxide dissolved in methyl alcohol. Reaction mix is kept above the boiling point of alcohol in contention. Time that is prescribed is around 1 to 8 hrs.
A special catalyst Sodium Methylate has been developed by Degussa which is widely used for biodiesel production. During production process, 30 % sodium methylate solution in methanol is used to catalyse the reaction of feedstock oil to biodiesel and glycerol. 17 -18 kgs of catalyst are required for one tonne of raw material. Methyl ester and glycerol are separated at the end.
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These catalysts are marketed in ready to use form by Degussa. The catalyst are added directly to the production process through a storage tank and can be used in facilities having a an annual capacity of around 50k to 100k tones.
The primary advantage of use of Degussa Catalyst is that high glycerol yield having satisfactory quality is produced at very optimum costs. Around 2/3 of reputed biodiesel facilities are designed to cater to this special catalyst.
Potassium methylate, is also used for preparation of biodiesel from old cooking fat.
Production of Biodiesel
There are two basic methods for production of biodiesel:
Base catalyzed transesterification of feedstock oil
Acid catalyzed transesterification of feedstock oil
Base Catalyzed Transesterification Of Feedstock Oil
Most of the biodiesel being produced is by the use of base catalyzed reaction process. The primary reason for use of this process are as follows:
Low temperature (150o F) and presssure (20 psi) required.
High conversion (98%) with min reaction time.
Direct conversion to methyl ester involving no intermediate steps.
Expensive materials of construction are not required.
The process is carried out by mixing of biodiesel and catalyst. The catalyst is mixed with the agitator and the resulting mix is then charged in a closed reaction container. Oil or fat is subsequently added in required portions into the mixture. The container is sealed to ensure no evaporation.
About 1 to 8 hours of reaction time is required for biodiesel production. The levels of alcohol and water are monitored closely to maintain right proportions. On completion of the reaction, biodiesel is separated from excess glycerin and methanol which are the byproducts of the reaction.
The Base Catalysed Method
A strong deprotonating base is required (like KOH, NaOH, Sodium methoxide), to mix with alcohol to disperse the solid catalyst into oil. A dry ROH is required. This is because any moisture will initiate saponification. This would lead to formation of soaps and consumption of base.
The alcohol mixture is then dissolved in the triglyceride. Alkyl group on the triglyceride is replaced in a number of steps depicted below:
At the carbonyl group RO group gets attached through this mechanism.
The reaction mechanism has a few demerits. RO- has to attack where there is a +ve charge density, ie on C=O. MeO- is efficient because of its small size. As the chain of RO- increases in length, due to the stearic hindrance, its effectiveness decreases considerably, as the reaction rate decreases. Thus due to this very reason use of short chained alcohols like methanol, ethanol is preferred to increase the reaction rate.
Acid Catalyzed Transesterification Of Oil Process
The reaction kinetics of acid-catalyzed transesterification of waste frying oil in excess methanol to form Fatty Acid Methyl Esters (FAME), for possible use as biodiesel have been studied in recent times.
There is no significant difference in the yield of FAME when the rate of mixing is in the turbulent range 100 to 600 rpm. The oil : methanol : acid molar ratios and the temperature are the most significant factors affecting the yield of FAME. At 70 °C with oil : methanol : acid molar ratios of 1:245:3.8, and at 80 °C with oil : methanol : acid molar ratios in the range 1:74:1.9-1:245:3.8, the transesterification is essentially a pseudo-first-order reaction as a result of the large excess of methanol which drives the reaction to completion (99±1% at 4 h). In the presence of the large excess of methanol, free fatty acids present in the waste oil are very rapidly converted to methyl esters in the first few minutes. Little or no monoglycerides are detected during the course of the reaction, and diglycerides present in the initial waste oil are rapidly converted to FAME.
The reaction is as follows:
The basic steps for industrial production of biodiesel are as follows:
Determination and treatment of free fatty acids
The feedstock ie waste vegetable oil is first filtered to remove dirt and other non-oil material. Water is thus removed since it initiates the hydrolysis of triglycerides; hence it gives salts of the fatty acids (soaps). Therefore it does not undergo the process of transesterification which is essential to produce biodiesel.
Determination and Treatment of the Free Fatty Acids
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Sample of pure feedstock oil is titrated with a standardized base solution to evaluate concentration of the free fatty acids (carboxylic acids) which is contained in vegetable oil sample. Esterification of diesel oil is thus carried out, which is turned into biodiesel henceforth. Once esterified, the bound glycerides are removed, typically through neutralization.
Products of the reaction include biodiesel and byproducts like soap, glycerin, excess of alcohol, and traces of water. The biodiesel produced is thus required to be separated from the other byproducts.
The density of glycerin is found to be greater than of biodiesel. Hence this particular property of glycerin is used to filter the bulk of the glycerin formed as byproduct. The residual methanol is removed through the process of distillation and reuse. Soaps are converted or reused as acids.
Methods of Biodiesel Production
A number of methods are used for biodiesel production. These include the batch method, supercritical method, ultrasonic shear method and the microwave method. These have been discussed in the following paragraphs:
Batch Process Method In this process, catalyst and alcohol are dissolved together using a agitator. The alcohol catalyst mixture is then placed into a closed reaction container and the bio-lipid is added. Hereafter a closed system is maintained to avoid any losses to atmosphere. The reaction mixture is brought to a temperature above boiling point of alcohol. The may take upto 8 hours and then both the resultant biodiesel and by product glycerin are allowed to settle down. Glycerin is thereafter separated from the biodiesel by use of centrifuge technique.
Super Critical Method This is a continuous catalyst-free process for trans-esterification using supercritical methanol at very high temperatures and pressures. The 'oil' and 'methanol' being in single phase ensure the reaction occurs almost instantaneously. The process is not affected by water in the feedstock, as free fatty acids are converted during the reaction to methyl esters rather than soap, Hence a wide variety of feed stocks can be used, however energy costs of production are similar to catalytic method.
Ultra Shear And High Shear Biodiesel Production Method, This process uses a number of sets of rotors and stators that convert mechanical energy to high tip speed, high shear stress and high shearing frequencies. These Shear mixers are used for the pre-treatment of crude feedstock for the trans-esterification. The shear mixers quickly and intimately blend the water or acid, allowing a continuous trans-esterification process. This reduces production time considerably and volume of production is increased.
Ultrasonic Reactor Biodiesel Production Method The ultrasonic waves are used in this method to produce and collapse bubbles constantly in the reaction mixture. This provides both the mixing and heating which are required to carry out the trans-esterification process.
The trans-esterification process can thus be run inline in this method, rather than batch processing method which is time consuming. Large sized ultrasonic devices permit the processing of several thousand liters of biodiesel per day.
Research is being focused upon using commercial microwave ovens for biodiesel production to provide the heat needed for the trans-esterification process. The microwaves are capable of providing very high temperatures. A continuous flow process which can produce upto 6 liters/minute of biodiesel at a 99% conversion rate has been innovated and proved to consume only 25% of the energy required as in case of batch process.
Although it is still in its development stage, the microwave method has undoubtedly great potential to be a highly efficient, cost effective method for the biodiesel production commercially.
There have been various advances in production of biofuels. Various researches have been carried out which are aimed at better utilization of bio-fuels that are cost effective too.
Radhakrishna Sureshkumar, Professor and Chair of biomedical and chemical engineering at L.C. Smith College of Engineering and Computer Science, Syracuse University, and a Ph.D. student S Wani have discovered a process that which holds a lot of promise in the biodiesel production.
A new bioreactor developed by the SU team can enhance algae growth. This was accomplished by making use of nanoparticles, selectively scattering blue light which promote algae metabolism. The team was able to achieve 30 percent greater growth of an algae sample as compared to a control with the use of optimal combination and confined nanoparticle suspension configuration.
Green algae (Chlamydomonas reinhardtii) was placed in a Petri dish, which is put on top of another petri-dish containing silver nanoparticles in suspension form, to serve backscattering blue light into algae culture was created for this process. By changing the size and concentration of the silver nanoparticle solution, it was discovered that the frequency and intensity of the light source could be manipulated, hence achieving an optimal wavelength for algae growth.
As per Sureshkumar, engineering optimal algae strains for optimal production of biofuels is dependent upon enhancing their phototropic growth rate. Use of nanoparticles for wavelength specific backscattering will have a substantial impact on efficient harvesting of phototropic microorganisms.
Thus far, this is one of the breakthroughs into utilizing nanobiotechnology to promote growth of micro-algae. The increase in the growth enhancement of algae also had various advantages outside the area of biofuel production.
The importance of biodiesel as a renewable and economically viable alternative to fossil diesel for applications in compression ignition (CI) engines has led to intense research in the field over the last two decades. This is predominantly due to the depletion of petroleum resources, and increasing awareness of environmental and health impacts from the combustion of fossil diesel. Biodiesel is favoured over other biofuels because of its compatibility with present day CI engines, with no further adjustments required to the core engine configurations when used in either neat or blended forms.
The biodiesel is non toxic, bio-degradable and free of sulphur. The cancer causing potential of biodiesel particulate matter is 94 % less than that of the diesel emission. Blending of just 1 % biofuel into diesel can also increase lubricity by upto 65 % which would lead to much higher engine life. Biodiesel has already shown commercial success as an oxygenated lubricity additive.
However there are differences in opinion about the health effects of using biodiesel as some of the studies have hinted at the carcinogenic properties of some blends. According to a study, rapeseed oil (Brassica napus) can produce 10 times more cancer causing emissions and pollutants than diesel, because the oil produces alkenes, 1.3 butadiene and benzene.
However there are both promises as well as doubts over the effectiveness of biofuels in meeting the environmental objectives till the time detailed studies are carried out.