Transesterification : Transesterification, on the other hand, is the displacement of the alcohol from an ester by another alcohol in a process similar to hydrolysis, except that an alcohol is used instead of water. This reaction, cleavage of an ester by an alcohol, is more specifically called alcoholysis and is represented by the general equation:
In this case, a new ester is formed. Generally, alkaline catalysts are used with sodium methylate said to be the most effective, although sodium hydroxide can also be used. Transesterification is an equilibrium reaction. To shift the reaction to the right, it is necessary to use a large excess of alcohol or to remove one of the products from the reaction mixture. The second option is preferred where feasible, as in this way, the reaction can be driven to completion. Transesterification is a general term. More specifically, if methanol is used, the reaction is termed methanolysis. Methanol is generally used because it is cheap, but other alcohols can be used.
Transesterification Processes: Transesterification of fats and oils is the most commonly used process for the manufacture of methyl esters, except in cases where methyl esters of specific fatty acids are needed. Triglycerides can readily be transesterified batchwise at atmospheric pressure and at a slightly elevated temperature of approximately 333-343 K with an excess of methanol and in the presence of an alkaline catalyst. The mild reaction conditions, however, requires the removal of free fatty acids from the oil by refining or pre-esterification before transesterification. This pretreatment is not required if the reaction is carried out under high pressure of 9000 kPa and high temperature of 513 K. Under these conditions, simultaneous esterification and Transesterification take place.
The mixture at the end of the reaction is allowed to settle. The lower glycerin layer is drawn off whereas the upper methyl ester layer is washed to remove entrained glycerine and is then processed further. The excess methanol is recovered in the condenser, sent to a rectifying column for purification, and recycled. Continuous transesterification is well suited for large capacity requirements. Depending on the quality of the feedstock, the unit can be designed to operate at high pressure and high temperature or at atmospheric pressure and slightly elevated temperature.
Hydrogenation : The equation for Hydrogenation reaction is given below.
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The fractionated methyl esters can be converted into fatty alcohols by the high-pressure hydrogenation process in the presence of a catalyst. Usually, copper chromite catalyst is used. Copper chromite catalyst also converts any unsaturated carbon double bonds so that only saturated fatty alcohols are formed. If unsaturated fatty alcohols are desired, a special zinc-bearing catalyst is employed. The hydrogenation process is carried out at 25,000–30,000 kPa and a temperature of 523–573 K in a tubular column. Depending on the method by which the catalyst is employed, the hydrogenation can be conducted using the suspension process or the fixed bed process.
Fixed Bed Process: The fixed bed process, as differentiated from the suspension process, has the catalyst ‘‘fixed’’ as a bed inside the reactor. The catalyst is either compact pelletized or supported on a carrier. The reaction is conducted in the vapor phase where a part of the organic feed is vaporized in an excess of hydrogen gas (20–25 moles) through a peak heater before passing through the fixed catalyst bed. The hydrogenation is carried out at 20,000– 30,000 kPa and 200–250_C. The reaction mixture leaving the reactor is cooled and is separated into the gas and liquid phases. The gas phase, mostly excess hydrogen, is recycled. The liquid phase is expanded into a flash tank to strip off the methanol from the fatty alcohol. The operating conditions are comparatively mild, so that the fatty alcohol produced does not require further processing. The overall yield is 99% with hydrocarbons and unreacted ester not exceeding 1.0%. Catalyst consumption is claimed to be below 0.3%.
Mono Alkyl Phosphates : Phosphate esters are prepared at reaction temperatures from 353 to 393 K at essentially atmospheric pressure. It has been claimed that temperature may be maintained at353 to 393 K. Lower temperature favors lower color of the product, whereas, at temperatures near 373 K, noticeable decomposition occurs . The phosphorous pentoxide is added portion wise to the alcohol at a rate such that the pentoxide dissolves and reacts without lumping. Lumping makes complete dissolution difficult and the P2O5 unreactive. The reaction between the alcohol and the P2O5 is carried out in the liquid phase and is basically exothermic. No catalyst is required. The addition of a small amount of hypophosphorous acid or its salt gives a colorless, color-stable product.
