Licensed Processes

Propylene Glycol

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Traditional propylene glycol (PG) production routes involve complex conversions of oil-derived feedstocks via propylene and propylene oxide intermediates.

By contrast, JM Davy’s simple PG process employs glycerol – a by-product of our biodiesel process – as its primary feedstock. In this way, our flowsheet offers PG production based on a 100% renewable feed while simultaneously adding value to an otherwise low-value biodiesel by-product.

Our PG flowsheet can also accommodate alternatively-sourced glycerol feeds.

Process Flowsheet

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hydrogenolysis ora hydrogenolysis ro polishing ora refining ora
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Key Reactions

An overall hydrogenolysis reaction converts glycerol to PG as follows:

PG big equation1

Process Description

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.

Process feedstock

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):

PG small equation1

A subsequent hydrogenation step then completes the overall hydrogenolysis process:

PG small equation2

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.

The JM Davy Advantage

JM Davy’s vapour-phase PG flowsheet offers an improved production route over the more conventional process. Find out more below:

+Improved biodiesel economics:

  • Our propylene glycol (PG) process employs glycerol, the Davy biodiesel by-product, as a feedstock.
  • Consequently these two Davy flowsheets complement each other perfectly. The clean glycerol stream from our biodiesel process can feed an adjoining PG plant, reducing overall project capital and operational costs by sharing ancillary systems and reducing tankage.
  • Utilising both Davy processes in this way would enable a single producer to ensure the most efficient use of resources and process outputs.

+Hydrogenolysis reactor - ideal mixing, high conversion and low residence times:

  • Vapour-phase operation ensures the organic reactants blend well with the abundant H2 and achieve ideal catalyst contact.
  • This in turn delivers high conversion rates and temperature uniformity with minimal residence times, generating high throughput.

+Hydrogenolysis - low operating pressures:

  • Using a large excess of H2 gas gives it a high partial pressure, which eliminates the need for a high overall operating pressure to drive the hydrogenolysis reaction.
  • H2 gas in a vapour-phase reactant requires only low pressures compared with using a liquid reactant.

+Low material and equipment costs:

  • Mild operating conditions and non-acidic feedstock allows construction of the hydrogenolysis system from carbon steel.
  • Operating at low pressure also reduces pipe and vessel wall thickness, further reducing equipment cost. Lighter equipment also reduces the installation, piling, structural and civil costs of the plant.

+Economic and sustainable process:

  • Our process is sustainable, yielding high-value propylene glycol from 100% renewable resources.
  • This is because the PG flowsheet’s glycerol feed is produced by conversion of oils, fats and greases from non-edible vegetation to biodiesel.

+Flexible output:

  • The process can be optimised to achieve the desired PG product grade – from technical grade to the assay requirements for pharmaceutical grade if required.

+High hydrogenolysis selectivity - high quality product:

  • The mild operating conditions of JM Davy’s hydrogenolysis technology, along with the uniform temperature profile and short residence time of the reactants and products, deliver a very selective process that results in minimal by-product formation and a very clean, high-quality product.

+Hydrogenolysis - plant safety and operability:

  • The use of low-pressure hydrogenolysis and the simplicity of the fixed bed vapour phase loop significantly improve plant safety.
  • The hydrogenolysis system can be started up and shut down very quickly without any consequences to the catalyst.

+No catalyst separation required:

  • As hydrogenolysis proceeds in the gas/vapour phase, the solid catalyst does not mix with any reactants or products, eliminating any need for catalyst separation prior to downstream processing.
Core Technologies & Related Processes

Learn more about how our well-established hydrogenolysis technology transforms of glycerol to propylene glycol:

Our biodiesel process generates the feedstock for Davy PG production. Learn more here:

Our butanediol (BDO) and natural detergent alcohols (NDA) processes also rely on hydrogenolysis to yield their final products:

Core Technologies

Learn more about how our well-established hydrogenolysis technology transforms of glycerol to propylene glycol:

Related Processes

Our biodiesel process generates the feedstock for Davy PG production. Learn more here:

Our butanediol (BDO) and natural detergent alcohols (NDA) processes also rely on hydrogenolysis to yield their final products: