Supercritical difluoromethane

Critical points vary widely. Table 6.1 shows a representative sample of critical parameters and it is immediately obvious why carbon dioxide is widely used. With a critical temperature just above room temperature and a critical pressure that is relatively low, the amount of energy needed to render carbon dioxide supercritical is comparatively small. Fluoroform (CHF3) and difluoromethane also have easily attainable critical parameters, but they are much more expensive than carbon dioxide. Despite its high critical temperature and pressure, supercritical water (SCH2O) is used widely as a destructive medium since it is highly acidic. 

The dipolarity/polarizabiUty parameter 7t, introduced by Kamlet and Taft, was measured for difluoromethane (HFC 32) and pentafluoroethane (HFC 125) and compared with those previously reported for 1,1,1,2-tetrafluoroethane (HFC 134a). This was carried out as a function of temperature and pressure, to cover the liquid and supercritical states over the range 30-130°C and 40-300 bar (Abbott and Eardley, 1999). 

A global electrophilicity index of common benzylating and acylating agents has been established from MO calculations and it shows a quantitative linear correlation with the experimental substrate selectivity index from a series of benzylation and acylation reactions.21 The values of relative rate coefficients predicted from the index may be accurate to within 10%. The reaction of /-butyl chloride with anisole catalysed by /Moluenesulfonic acid in supercritical difluoromethane has been subject to kinetic analysis.22 The proportions of substitution at the ortho -position and disubstitution increase at lower pressures, attributed to the decrease in the hydrogen-bonding ability of the solvent. 

To illustrate the development of a transport property correlation, discussion is focused here on the relevant example of binary mixmres of two new alternative refrigerants. A global correlation, based on theory where possible, is desired for the transport properties of mixtures of difluoromethane (R32) and pentafluoroethane (R125). This activity represents a portion of a project currently under way at National Instimte of Standards and Technology (NIST). Viscosity data for R125 have been published (Diller Peterson 1993), and the thermal conductivity data surface for R125 is shown in Figure 7.1. Primary data will be available for kinematic viscosity, thermal conductivity and thermal diffusivity from 180 to 400 K in the vapor, liquid and supercritical phases as well as... 

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