Bio-diesel has been promoted by a number of governments around the world and major programmes for its manufacture have been announced. From each unit of vegetable oil that is converted to bio-diesel approximately 10% by weight will be recovered as by-product glycerine. The planned massive increase in the production of bio-diesel will inevitably lead to a large oversupply of glycerine. Conventionally, the commercial propylene glycol production route is propylene-based, through propylene oxide, and therefore it is sensitive to the price of petroleum and associated products. There is clearly an attraction to de-coupling propylene glycol from petroleum and exploiting the predicted surplus of glycerine is one way of achieving this.
In the new Davy process, glycerine is reacted over a heterogeneous catalyst with hydrogen at moderate conditions. Fresh glycerine together with a recycle stream is vaporised in a recirculating stream of hydrogen, with a suitable quantity of make-up hydrogen, typically from a pressure swing adsorption unit. Per-pass glycerine conversion is around 99% and byproducts are removed by distillation. The advantage of the Davy scheme is high selectivity to the desired product. The refining scheme recovers high purity propylene glycol, whilst water produced in the reaction is of suitable quality to be passed to a biological treatment plant. Propylene glycol product specification meets the requirements of the target markets, namely unsaturated polyester resins and functional fluids, although pharmaceutical grade material can be produced if required. The relatively small by-product streams are of high quality and can be used as solvents or (in the case of the mixed glycols) functional fluids.
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