PRODUCTION OF BIODIESEL USING SUPERCRITICAL METHANOL

When Rudolf Diesel envisaged his Diesel engine it was intended to run on vegetable oils.  Since then, the availability of cheap petroleum diesel has resulted in the engines being designed to run on this form of fuel.  The major problem with vegetable oils in modern engines is high viscosity.  Vegetable oils consist mainly of glycerol tri-esters of fatty acids (triglycerides).  If the glycerol esters are converted to methyl esters (a process called transesterification), the viscosity is much closer to that of diesel, and the fatty acid methyl esters (FAMEs) can be used as a replacement for diesel.

 

The most common method for transestrification is to stir the oil with methanol and a caustic catalyst for several hours.  This produces a FAMEs layer and a glycerol layer.  The glycerol is tapped off, the FAMES washed and then are ready to use.  Free fatty acids and water are a problem in this method.  The free fatty acid produces soap, which hinders the separation of the layers, and also uses up catalyst.  The level of acid in the feedstock needs to be determined, and the level of catalyst adjusted.  If the level of acid is too high, the feedstock cannot be used.  Recycled cooking oils, the most environmentally friendly oil source, tend to contain a lot of free fatty acid, so this is a problem for the catalysed method.   Energy is needed to continuously mix the immiscible oil and methanol.  The method lends itself well to small scale batch production, although large scale continuous or semi-continuous commercial plants exist.

An alternative method is to mix the oil and methanol under supercritical conditions.  The two components are then miscible, and react spontaneously without a catalyst.  A schematic diagram of a system for carrying out the reaction is shown below. Work has so far been carried out in laboratories and on a pilot or small production scale.

After reaction, the FAMEs and glycerol are then separated without further need of washing.  Free fatty acids are converted to FAMEs also (instead of soap), increasing yield and allowing  recycled oil to be used with no extra consideration.  Fatty acid sources such as soap stock could also be used directly.  Yields are high (98%) reaction times fast (300 secs) and the process is easy to operate continuously (rather than batch).  The plant design is relatively simple, requiring a tubular reactor, pumps for oil and methanol and a separator.  Flash distillation recovers the methanol for re-cycling back to the process.

 

The critical temperature of methanol is 239°C, but the temperature needs to be higher than this as it is the critical temperature of the mixture that is required.  For a methanol : triglyceride mixture in a weight ratio of 1.5 : 1 the critical temperature is between 320 and 350°C and the pressure required is under 100 bar.  Lower temperatures can be used with higher ratio of methanol to oil, and using CO₂ as a co-solvent may also reduce the critical temperature of the mixture, and hence the reaction temperature.  Depending on the feedstock, some unsaturated fatty acids may be slightly degraded at the higher temperatures.  Hydrolysis of the oil to free fatty acids by superheated water, followed by esterification with supercritical methanol has also been suggested.  The temperature required in this two stage process is reduced to 275°C, so there is less risk of degradation. 

 

Currently, nearly all biodiesel is produced by the catalytic method.  With the increase in scale of production of biodiesel the supercritical route to production will deliver many advantages.

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