Selective production of difluorodimethyl ether from chlorodifluoromethane - a kinetic study using a well-mixed batch absorber.

Abstract

The gas-liquid reaction between chlorodifluoromethane (R-22) and methanol, in the presence of sodium hydroxide, was investigated in an isothermal, stirred, semi-batch reactor. The objective of the study was to develop a model for the reaction and to identify the kinetic parameters. Reactor temperature was varied from 283 to 303 K, with inlet R-22 partial pressures between 40.5 and 60.8 kPa (absolute). Solutions containing sodium hydroxide concentrations of between 1.5 and 2.5 mol·dm-3 were charged into the reactor prior to each experiment. Preliminary investigations using the R-22-methanol system revealed that stainless steel was an inappropriate choice of material for the reactor as it displayed catalytic tendencies toward trimethyl orthoformate formation. Consequently, the reactor was constructed from glass and was equipped with an internal cooling coil, a single heating jacket and a temperature control unit. Liquid samples that were withdrawn from the reactor were degassed under vacuum to remove residual chlorodifluoromethane, and thereby inhibit further reaction. Spectrophotometry was used to analyze the liquid samples to determine the concentration of chloride ions in solution. The products obtained were difluorodimethyl ether (major product) and trimethyl orthoformate (by-product) as well as sodium chloride and sodium fluoride salts. Difluorodimethyl ether is a potential replacement for ozone depleting CFC refrigerants. A Box-Behnken experimental design was used to investigate the effect of reaction conditions on the product distribution. Variations in the reaction temperature, initial concentration of sodium hydroxide and inlet partial pressure of R-22 were considered. The modeling of the gas-liquid reactions was based on the -dehydrohalogenation mechanism. Since gas solubility in a liquid decreases in the presence of dissolved salts, the "salting-out" effect on mass transfer was included in the reactor model. Sechenov coefficients for sodium chloride and sodium fluoride were combined to give a salt Sechenov coefficient Ksalt . It was known from the literature that the presence of precipitated salts causes inefficient mixing and inhibits mass transfer, particularly in this system due to the relatively low salt solubilities in methanol. This mixing effect was also included in the appropriate mass transfer terms of the reactor model. The experimental data was fitted to a proposed kinetic scheme. Kinetic parameters for each of the proposed reactions, the Sechenov ‘salting out’ coefficients and the mixing parameter were obtained through the use of a non-linear, least-squares optimization algorithm. For the kinetic study, activation energies of 89.12 and 45.83 kJ·mol-1 were obtained for the difluorodimethyl ether and trimethyl orthoformate formation reactions, respectively, with a Sechenov salt coefficient of 0.712 and a mixing parameter of 22.43.