Selective production of difluorodimethyl ether from chlorodifluoromethane - a kinetic study using a well-mixed batch absorber.
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Date
2013
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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 Liquid samples that were withdrawn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.
Description
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2013.
Keywords
Chemical kinetics., Cameras--Calibration., Spectrophotometry., Refrigerants., Theses--Chemical engineering.