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

The recovery of propylene glycol from dilute aqueous solutions via rea ction with formaldehyde to form 4-methyl-1,3-dioxolane or with acetald ehyde to form 2,4-dimethyl-1,3-dioxolane was studied experimentally. T he equilibrium and kinetics of the reaction with formaldehyde were stu died in systems catalyzed by Amberlite IR-120 ion exchange resin. The equilibrium constant ranged from 5.9 to 8.7 in the temperature range f rom 25 to 85 degrees C, with no obvious trend with respect to temperat ure. The kinetics was found to be first-order in the concentrations of propylene glycol, formaldehyde, and Amberlite IR-120, with an activat ion energy of 102 kJ/mol. In the reaction with acetaldehyde, the equil ibrium constant decreased from 18.1 at 40 degrees C to 8.5 at 83 degre es 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, aro matic hydrocarbons exhibited the highest distribution of 2,4-dimethyl- 1,3-dioxolane from the aqueous into the organic phase. Recovery of pro pylene glycol by reactive distillation with formaldehyde or acetaldehy de 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, i n comparison with a triple-effect evaporation process.