Process and catalytic considerations for producing propylene glycol from glycerin-derived biomass sources are compared to current conventional sources, while economic factors and potential markets are discussed.
Introduction
Propylene glycol (PG) is a basic petrochemical used primarily in polyurethane polyols and unsaturated polyester resins, and as an anti-freeze agent in aircraft de-icing applications, as well as an automotive coolant. Total annual global consumption is 1.5 million metric tonnes. Historically, PG has been made commercially by simply hydrating propylene oxide (PO). PO, in turn, has been made historically by the peroxidation of propylene using co-feeds of either ethylbenzene (EB) or isobutene (iC4).
When EB is used, PO is produced with styrene monomer co-product. When iC4 is used, PO is produced with tertiary butyl alcohol (or sometimes methyl tertiary butyl ether [MTBE]) co-product. An older technology produces PO from propylene via chlorohydrin chemistry. Recently, a new plant has been commissioned by SK Chemicals to avoid co-product formation by using hydrogen peroxide as the oxidising agent. Whichever historic route is taken, PG production economics are dominated by the cost of the basic feedstock propylene. Produced from either natural gas liquids or refinery naphtha, propylene prices have averaged $1000/mt over the past three years, and PG market prices have been $1500/mt during the same period. During the mid-2008 crude oil price run-up, with naphtha prices exceeding $1000/mt, market prices for both ethylene and propylene averaged approximately $1500/mt, driving PG market prices to $2000/mt.