September 2, 2013
Make “green” propylene glycol from glycerol. The conversion of biodiesel byproduct glycerol to conventional petrochemicals is akin to making a silk purse out of a sow’s ear—that is, upgrading a low-value material to a more valuable product. This is a particularly good time to leverage this route to propylene glycol for two reasons:
- The conventional route to propylene glycol starts with propylene, which is expensive because steam-cracker operators want to use inexpensive ethane from shale gas as the cracker feedstock. Ethane-based steam crackers make very little propylene, so propylene prices are rising.
- Propylene glycol made from bioglycerol can be labeled as “green” because it is made from a renewable resource. This feature is valuable to personal-care product producers because propylene glycol is often used as the moisturizing agent in skin creams, and personal-care companies like to advertise their products as environmentally friendly.
C. R. K. Rabello and co-inventors disclose process conditions in which hydrogenolysis is used to convert biodiesel-derived glycerol to propylene glycol. The process gives high yields in a commercially feasible fixed-bed reactor.
In an example, purified glycerol and hydrogen in a 1:120 mol ratio were admitted to a fixed-bed reactor that contains 557 mL copper chromite catalyst at 225 ºC and a space velocity of 0.5 h–1 (based on glycerol). Product analysis showed that glycerin conversion was 99% and the selectivity to propylene glycol was 89.8%.
In an improvement to this reaction scheme, the recycled hydrogen stream was passed through a methanation reactor containing a Ni/Al2O3 catalyst. The methanation reactor converted any CO2 or CO byproducts to methane. (CO and CO2 act as catalyst poisons.) With the methanation reactor in place, propylene glycol selectivity improved to 93.5%. (Petroleo Brasiliero S.A.-Petrobras [Rio de Janeiro]. US Patent 8,492,597, July 23, 2013; Jeffrey S. Plotkin)
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