The non-edible vegetable oil derived fuels will have wider use in near future due to their lower impact on environment in contrast to fuels derived from crude oil. However, neat vegetable oils display inferior properties, particularly extreme viscosity, and thus their use in fuels is limited.

Transesterification process, generally used for converting vegetable oils to biodiesel, is complex and expensive due to involvement of several energy intensive steps. This requires setting up of biodiesel producing unit for blending with diesel derived from fossil fuel. The separation of excess methanol and byproduct glycerol also requires significant energy. Further, low purity of glycerol poses disposal problem. There are steps of water washing to remove the caustic & this adds to the plant effluent load. Moreover, if the vegetable oil is rancid, an additional step of estrification is necessary. The process is suitable only for oils having low Free Fatty Acid (FFA) < 0.5%. Biodiesel has higher density of about 0.88 g/cc as compared to normal diesel (0.825 to 0.845 g/cc) and narrow boiling range 340 0C+ of which around 20% boils above T-95 specification of Euro-IV diesel i.e 360°C affecting refiner's profitability adversely due to requirement of production of lighter diesel for blending to meet final diesel specifications. The presence of oxygen in biodiesel also results in higher emissions of NOx. Also, biodiesel is not well accepted by auto industry in all proportions, as these are responsible for injector coking.

Co-processing of vegetable oils along with diesel feed in Diesel Hydrotreater Unit in Refineries offers several advantages as existing refinery infrastructure can be utilized with improved product properties and engine performance. In co-processing, vegetable oils are hydrodeoxygenated to form paraffins/ iso-paraffins and removal of oxygen in the form of water and carbon oxides. The diesel fuel produced by co-processing improves fuel properties w.r.t. cetane number, density, viscosity etc.

The present paper discusses various aspects related with co-processing in detail such as exothermicity of reactions, metallurgy for storage & handling of feed stock, chemical H2 consumption, metal content & their removal from vegetable oils, CO inhibition, oxidation and thermal stability of vegetable oils etc.

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