(450d) Experimental Measurements and Modelling of the Dissociation Conditions of Clathrate Hydrates for (Refrigerant + NaCl + Water) Systems

Abstract

 Globally, gas hydrates have been proposed as a potential means for many positive applications, e.g. wastewater treatment and desalination, CO₂ capture and separation, separation of close-boiling point compounds, hydrogen/methane storage, refrigeration and air conditioning industry, food industry^1,2,3,4,5. Javanmardi and Moshefeghian⁴; Chun et al.⁶; Seo and Lee,⁷ and Eslamimanesh et al.⁸ have reported that the use of refrigerants for the formation of gas hydrates in water desalination processes is promising when compared to traditional desalination processes. The use of refrigerants to form gas hydrates is attractive because hydrates as host molecules may occur at ambient conditions. According to Eslamimanesh et al.⁹, the dissociation of gas hydrate results in the production of pure water and the release of refrigerant which can be recycled.  However, there is a lack of research regarding the use of hydrate technology for the treatment of industrial wastewater and desalination processes, particularly using fluorinated refrigerants as the hydrate former.

The focus of this study is to generate accurate hydrate dissociation data which will be used to design a wastewater treatment and desalination processes using gas hydrate technology. In this study, three refrigerants 1,1,1,2-tetrafluoroethane (R134a), (0.5 mass fraction difluoromethane + 1,1,1,2,2-pentafluoroethane) (R410a), and (0.5 mass fraction 1,1,1-trifluoroethane + 1,1,1,2,2-pentafluoroethane)  (R507)were investigated in the absence and presence of a NaCl aqueous solution at various salt concentrations. Hydrate dissociation data was measured using an isochoric pressure-search method.

 The binary systems, in this study, which consisted of {R134a, or R410a, or R507} + water were measured in the temperature range between (276.2 to 291.8) K and pressures ranging from (0.114 to 1.106) MPa. The ternary system R134a + water + NaCl, at three salt concentrations of (5, 10, and 15) %, was measured in the temperature range between (268.1 to 280.6) K and pressures ranging from (0.086 to 0.340) MPa. For the ternary systems comprising of {R410a or R507} + water + NaCl, at two salt concentrations of (5 and 10) %, measurements were undertaken in the temperature range between (275.1 to 290.3) K and pressures ranging from (0.269 to 1.170) MPa.

Modeling of the measured data was undertaken using a combination of the solid solution theory of van der Waals and Platteeuw¹⁰ for the hydrate phase, the Aasberg – Petersen et al.¹¹ model for the electrolyte aqueous system, and the Peng-Robinson¹²equation of state with classical mixing rules for the liquid and vapour phases. The correlated results show good agreement with the experimental dissociation data.

The results of this study show that the presence of the NaCl in the aqueous solutions has a thermodynamic inhibition effect on refrigerant gas hydrates. The {R134a or R507}  Hydrate–Liquid water–Vapour (H–L_w–V) equilibrium phase boundary is shifted to lower dissociation temperatures compared to the dissociation temperatures and pressures for R22 + water + NaCl systems measured by Chun et al.⁶. Also, it is higher than propane + water + NaCl systems measured by Chun et al.¹³. The hydrate dissociation temperatures for R410a + water system are close to ambient conditions. In addition, quadruple points at which the four phases (H–L_w–L_R134a–V) coexist were determined for the water + R134a + NaCl system. Further research is being investigated to identify suitable promoters with the systems discussed to determine feasible operating conditions the desalination processes. 

Keywords:Gas hydrate, Refrigerant, Salt, Dissociation data, Model, Desalination.

*Corresponding authors: Deresh Ramjugernath, e-mail: ramjuger@ukzn.ac.za,

Amir H. Mohammadi, e-mail:  a.h.m@irgcp.fr

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