GAS SOLUBILITY OF HFCs IN ORGANIC SOLVENTS

1. GAS SOLUBILITY OF HFCs IN ORGANIC SOLVENTS J.P.B. Almeida1, H.C. Fachada2, I. M. A. Fonseca1* 1 Chemical Engineering Department, University of Coimbra, Pólo II, Pinhal de Marrocos, 3030–290 Coimbra, Portugal. 2 Electrical Engineering Department, Institute Polytechnique of Coimbra , 3030-199 Coimbra, Portugal. * e-mail: fonseca@eq.uc.pt Results Abstract This work is inserted in a research program that consists mainly in the experimental and theoretical study of the effect of association [1] between solute and solvent molecules in the solubility of gases in liquids. To measure the solubility of gases in liquids an automated apparatus based on Ben-Naim-Baer and Tominaga et al designs has been implemented [2]. The accuracy of the experimental method was checked by measuring the solubility of carbon dioxide and nitrous oxide in water, being found to be 0.6%. The solubilities of hydrofluorocarbons, HFCs, (CH3F, CH2F2, CHF3 and CF4) in lower alcohols (methanol, ethanol, 1-propanol, 1-butanol) have been determined in the temperature range [285, 310] K, at atmospheric pressure. To represent the temperature dependence of the mole fraction solubilities, the equation Rlnx2 = A + B/T + ClnT was used. From this equation the experimental Gibbs energies, enthalpies and entropies of solution at 298 K were calculated [3]. Experimental Automated apparatus [2,3] (which embodies a volumetric method) Table 2 Comparison of solubility results with literature values Solubility apparatus: TB, thermostated bath; TC, temperature controller; T, thermometer; PT, pressure transducer; LA, linear actuator; PC, pressure controller; PE, Penning gauge; DIF, diffusion oil pump; TRAP, liquid N2 trap; EQ, equilibrium vessel with connector; GB, gas burette; V1,…V6, high vacuum Teflon stopcocks; AGIT, magnetic stirrer; E, elevator. AAD = (1 / N) 100 (x2 -x2lit / x2lit) Calculations Conclusions Table 1 Thermodynamic functions (J mol-1) obtained from Rlnx2 = A + B/T + ClnT, at 298 K. • The solubilities of HFCs in the alcohols decrease in the order: • CHF3 > CH2F2 > CH3F > CF4. This is corroborated by the values of Gº2 in Table 1. This can be explained by the formation of a complex between solute/solvent molecules by means of hydrogen-bonding. Actually the presence of the strongly electron-attracting halogen atom(s) on the carbon of the solute molecule looses the hydrogen(s) and makes it available for coordination to the donor atom (of the solvent molecule). • For each HFC the solubility increases with the C- content of the alcohol increases. This is related to H-bonding or association in the alcohol; i.e. solvents with strong H-bonding tendencies dissolve less the same gas those with weaker H-bonding tendencies. • The comparison of the solubility with literature values in Table 2 shows a satisfactory agreement. References [1] J.M. Prausnitz, R.N. Lichtenthaler, E.G. Azevedo, Molecular Thermodynamics of Fluid-Phase Equilibria, 3rd ed., Prentice Hall, Englewood Cliffs, 1999. [2] I.M.A Fonseca, J.P.B. Almeida, H.C. Fachada, Automated apparatus for gas solubility measurements, J. Chem. Thermodynamics 39 (2007) 1407-1411. [3] H.L. Clever, R. Battino, The Experimental Determination of Solubilities, edited by G.T. Hefter and R.P.T. Tomkins, Wiley, 2003, pp. 101-150. [4] M. Takenouchi, R. Kato, H. Nishiumi, J. Chem. Eng. Data 46 (2001) 746-749. [5] S. Bo, R. Battino, E. Wilhelm, J. Chem. Eng. Data 38 (1993) 611-616. [6] J. Pardo, M.C. Lopez, J. Santafe, F.M. Royo, J.S. Urieta, Fluid Phase Equilibria 109 (1995) 29-37. Acknowledgements This work was carried out under Research Project POCI/EQU 44056/2002 financed by FCT – Fundação para a Ciência e Tecnologia (Portugal) and FEDER