The key conversion step of glycerol to propylene glycol (GPG) proceeds by vapour-phase hydrogenolysis. Downstream polishing and refining follow to yield product-grade PG.
The key process feedstock is glycerol, the quality of which varies considerably both on the open market and from biodiesel production.
Our GPG process requires clean glycerol with low sulphur, chloride and ash content – a level of purity offered by our biodiesel process. Otherwise, a standard glycerol refining unit can achieve the necessary quality.
In addition, the inclusion of a refining unit gives the operational flexibility to use both high- and low-quality glycerol feeds or to supply the glycerol market when prices are advantageous.
The liquid glycerol feed enters the hydrogenolysis system, where it first vaporises before undergoing a two-step reaction over a solid catalyst to produce propylene glycol.
The glycerol initially dehydrates to hydroxypropanone (acetol):
A subsequent hydrogenation step then completes the overall hydrogenolysis process:
The resulting hot process stream, a vapour, passes through cooling and condensation steps before proceeding to the polishing stage as a liquid.
Inside the polishing reactor, the liquid process stream flows downwards over a fixed-bed of solid catalyst. Here, residual acetol converts to propylene glycol by reaction with hydrogen.
The liquid stream exiting the polishing reactor comprises mixed glycols and mixed alcohol by-products, water and glycerol.
Multi-stage distillation separates these components, with the water removed as waste and recovered glycerol recycled to hydrogenolysis.
The high-grade PG liquid stream exiting the process is of sufficient purity for such markets as unsaturated polyester resins and functional fluids, but can be further refined to pharmaceutical grade if required.
The relatively small by-product streams of mixed alcohols and mixed glycols are of high quality and are suitable for use as solvents or (in the case of the mixed glycols) functional fluids.