RECOVERY OF PROPYLENE-GLYCOL FROM DILUTE AQUEOUS-SOLUTIONS VIA REVERSIBLE-REACTION WITH ALDEHYDES

The recovery of propylene glycol from dilute aqueous solutions via reaction with formaldehyde to form 4-methyl-1,3-dioxolane or with acetaldehyde to form 2,4-dimethyl-1,3-dioxolane was studied experimentally. The equilibrium and kinetics of the reaction with formaldehyde were studied in systems catalyzed by Amberlite IR-120 ion exchange resin. The equilibrium constant ranged from 5.9 to 8.7 in the temperature range from 25 to 85 degrees C, with no obvious trend with respect to temperature. The kinetics was found to be first-order in the concentrations of propylene glycol, formaldehyde, and Amberlite IR-120, with an activation energy of 102 kJ/mol. In the reaction with acetaldehyde, the equilibrium constant decreased from 18.1 at 40 degrees C to 8.5 at 83 degrees C. The kinetics was faster than with formaldehyde. The volatilities of 4-methyl-1,3-dioxolane and 2,4-dimethyl-1,3-dioxolane relative to water were 100 and 33, respectively. Of several solvents screened, aromatic hydrocarbons exhibited the highest distribution of 2,4-dimethyl-1,3-dioxolane from the aqueous into the organic phase. Recovery of propylene glycol by reactive distillation with formaldehyde or acetaldehyde is hampered by unfavorable chemical and phase equilibria. A process combining reaction and extraction into an organic solvent appears to be more attractive and substantially reduces the energy requirement, in comparison with a triple-effect evaporation process.