The Base Catalysed Transesterification Mechanism Biology Essay
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Published: Mon, 5 Dec 2016
The transesterification reaction is base catalyzed. Any strong base capable of deprotonating the alcohol will do (e.g. NaOH, KOH, Sodium methoxide, etc.). Commonly the base (KOH, NaOH) is dissolved in the alcohol to make a convenient method of dispersing the otherwise solid catalyst into the oil. The ROH needs to be very dry. Any water in the process promotes the saponification reaction, thereby producing salts of fatty acids (soaps) and consuming the base, and thus inhibits the transesterification reaction. Once the alcohol mixture is made, it is added to the triglyceride. The reaction that follows replaces the alkyl group on the triglyceride in a series of steps.
The carbon on the ester of the triglyceride has a slight positive charge, and the carbonyl oxygens have a slight negative charge. This polarization of the C=O bond is what attracts the RO- to the reaction site.
Polarized attraction |
RO- —————-> C=O
This yields a tetrahedral intermediate that has a negative charge on the former carbonyl oxygen:
RO-C-O- (pair of electrons)
These electrons then fall back to the carbon and push off the diacylglycerol forming the ester.
Then two more RO groups react via this mechanism at the other two C=O groups. This type of reaction has several limiting factors. RO- has to fit in the space where there is a slight positive charge on the C=O. MeO- works well because it is small in size. As the chain length of the RO- group increases, reaction rates decrease. This effect is called steric hindrance. This effect is a primary reason the short chain alcohols, methanol and ethanol, are typically used.
There are several competing reactions, so care must be taken to ensure the desired reaction pathway occurs. Most methods do this by using an excess of RO-.
The acid-catalyzed method is a slight variant that is also affected by steric hindrance.
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, was studied. Rate of mixing, feed composition (molar ratio oil:methanol:acid) and temperature were independent variables. There was no significant difference in the yield of FAME when the rate of mixing was in the turbulent range 100 to 600 rpm. The oil:methanol:acid molar ratios and the temperature were 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 was essentially a pseudo-first-order reaction as a result of the large excess of methanol which drove 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 were very rapidly converted to methyl esters in the first few minutes under the above conditions. Little or no monoglycerides were detected during the course of the reaction, and diglycerides present in the initial waste oil were rapidly converted to FAME.
Preparation: care must be taken to monitor the amount of water and free fatty acids in the incoming biolipid (oil or fat). If the free fatty acid level or water level is too high it may cause problems with soap formation (saponification) and the separation of the glycerin by-product downstream.
Catalyst is dissolved in the alcohol using a standard agitator or mixer.
The alcohol/catalyst mix is then charged into a closed reaction vessel and the biolipid (vegetable or animal oil or fat) is added. The system from here on is totally closed to the atmosphere to prevent the loss of alcohol.
The reaction mix is kept just above the boiling point of the alcohol (around 70 °C, 158 °F) to speed up the reaction though some systems recommend the reaction take place anywhere from room temperature to 55 °C (131 °F) for safety reasons. Recommended reaction time varies from 1 to 8 hours; under normal conditions the reaction rate will double with every 10 °C increase in reaction temperature. Excess alcohol is normally used to ensure total conversion of the fat or oil to its esters.
The glycerin phase is much denser than biodiesel phase and the two can be gravity separated with glycerin simply drawn off the bottom of the settling vessel. In some cases, a centrifuge is used to separate the two materials faster.
Once the glycerin and biodiesel phases have been separated, the excess alcohol in each phase is removed with a flash evaporationprocess or by distillation. In other systems, the alcohol is removed and the mixture neutralized before the glycerin 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.
The glycerin by-product contains unused catalyst and soaps that are neutralized with an acid and sent to storage as crude glycerin (water and alcohol are removed later, chiefly using evaporation, to produce 80-88% pure glycerin).
Once separated from the glycerin, the biodiesel is sometimes purified by washing gently with warm water to remove residual catalyst or soaps, dried, and sent to storage.
An alternative, catalyst-free method for transesterification uses supercritical methanol at high temperatures and pressures in a continuous process. In the supercritical state, the oil and methanol are in a single phase, and reaction occurs spontaneously and rapidly. The process can tolerate water in the feedstock, free fatty acids are converted to methyl esters instead of soap, so a wide variety of feedstocks can be used. Also the catalyst removal step is eliminated. High temperatures and pressures are required, but energy costs of production are similar or less than catalytic production routes.
Ultra- and high-shear in-line and batch reactors
Ultra- and High Shear in-line or batch reactors allow production of biodiesel continuously, semi- continuously, and in batch-mode. This drastically reduces production time and increases production volume.
The reaction takes place in the high-energetic shear zone of the Ultra- and High Shear mixer by reducing the droplet size of the immiscible liquids such as oil or fats and methanol. Therefore, the smaller the droplet size the larger the surface area the faster the catalyst can react.
In the ultrasonic reactor method, the ultrasonic waves cause the reaction mixture to produce and collapse bubbles constantly. This cavitation provides simultaneously the mixing and heating required to carry out the transesterification process. Thus using an ultrasonic reactor for biodiesel production drastically reduces the reaction time, reaction temperatures, and energy input. Hence the process of transesterification can run inline rather than using the time consuming batch processing. Industrial scale ultrasonic devices allow for the industrial scale processing of several thousand barrels per day.
Current research is being directed into using commercial microwave ovens to provide the heat needed in the transesterification process.The microwaves provide intense localized heating that may be higher than the recorded temperature of the reaction vessel. A continuous flow process producing 6 liters/minute at a 99% conversion rate has been developed and shown to consume only one-fourth of the energy required in the batch process.Although it is still in the lab-scale, development stage, the microwave method holds great potential to be an efficient and cost-competitive method for commercial-scale biodiesel production.
Large amounts of research have focused recently on the use of enzymes as a catalyst for the transesterification. Researchers have found that very good yields could be obtained from crude and used oils using lipases. The use of lipases makes the reaction less sensitive to high FFA content which is a problem with the standard biodiesel process. One problem with the lipase reaction is that methanol cannot be used because it inactivates the lipase catalyst after one batch. However, if methyl acetate is used instead of methanol, the lipase is not in-activated and can be used for several batches, making the lipase system much more cost effective.
The project funded by a federal grant, aims at finding a production system that is affordable.
Steve Bond, Blue Sun Energy’s marketing manager CLAIMS that it costs about $20 a gallon to produce biodiesel out of algae at the present and the com[any’s aim is to get the costs down to under $2 a gallon.
The company believes that it has already made advances in biodiesel production that makes it greener and more versatile than other production methods on the market.
The company says its product reduces emissions of pollutants including global warming gases like nitrogen oxide. According to the company, many biodiesels products actually increase nitrogen oxide emissions.
Blue Sun Energy also claims its additive helps boost fuel economy by seven per cent, reduce wear in fleet vehicles and even improve performance in cold-weather conditions.
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. Studies conducted to date on various CI engines fuelled with varying biodiesel types and blends under numerous test cycles have shown that key tailpipe pollutants, such as carbon monoxide, aromatics, sulphur oxides, unburnt hydrocarbons and particulate matters are potentially reduced. The effects of biodiesel on nitrogen oxides emission require further tests and validations. The improvement in most of the diesel emission species comes with a trade-off in a reduction of brake power and an increase in fuel consumption. Biodiesel’s lubricating properties are generally better than those of its fossil diesel counterpart, which result in an increased engine life. These substantial differences in engine-out responses between biodiesel and fossil diesel combustion are mainly attributed to the physical properties and chemical composition of the fuels. Despite the purported benefits, widespread adoption of biodiesel usage in CI engines is hindered by outstanding technical challenges, such as low temperature inoperability, storage instabilities, in-cylinder carbon deposition and fuel line corrosion. It is imperative that these issues are addressed appropriately to ensure that long-term biodiesel usage in CI engines does not negatively affect the overall engine durability. Possible solutions range from biodiesel fuel reformulation through feedstock choice and production technique, to the simple addition of fuel additives. This calls for a more strategic and comprehensive research effort internationally, with an overarching approach for co-ordinating sustainable exploitation and utilisation of biodiesel. This review examines the combustion quality, exhaust emissions and tribological impacts of biodiesel on CI engines, with specific focus on the influence of biodiesel’s physico-chemical properties. Ongoing efforts in mitigating problems related to engine operations due to biodiesel usage are addressed. Present day biodiesel production methods and emerging trends are also identified, with specific focus on the conventional transesterification process wherein factors affecting its yield are discussed.
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