PORTEFEUILLE

The synthetic refrigerants impact on the environment and newly imposed safety measures force the cooling industry to seek for solutions to replace certain gases or to decrease their content in numerous different systems. In order to be free from the synthetic refrigerants, industries are unceasingly searching for environmentally friendly and suitable new technologies to enable energy savings. Today, CO2 hydrate slurry is obtained by CO2 gas injection into the water under certain temperature and pressure conditions.
The mean objective of the project consist to study and to propose a new approach to simplify the creation of CO2 hydrates slurry.

CO2 hydrate slurry is a natural, non pollutant, non toxic, relatively inexpensive fluid with higher heat-storage capacity. According to the available literature and our own measurements, the dissociation enthalpy of a mixture of CO2 hydrate and ice is higher than that of melting of ice alone (500 kJ/kg of water in contrast to 333 kJ/kg of water). The phase-change temperature of CO2 hydrate slurry is adjustable and can be applied to positive temperatures (1 to +4 Grad Celsius) that are suitable for air conditioning systems where there are currently no commercialized phase change fluids being used. CO2 hydrates can be also employed to cold storage.

A new pilot CO2 hydrate slurry system was successfully completed. An extended loop based on this system to study slow crystallization of pure CO2 hydrate for cold storage is under construction.
Based on previous studies, a comprehensive kinetic study of CO2 hydrates formation and growth was conducted. A general expression is derived for the supersaturation for crystallization of one-component gas hydrates in aqueous solutions. The supersaturation is the driving force of the process, since it represents the difference between the chemical potentials of a hydrate building unit in the solution and in the hydrate crystal. Expressions for the supersaturation are obtained for solutions supersaturated in isothermal or isobaric regime. The results obtained are applied to the crystallization of hydrates of one-component gases. The kinetics of nucleation of one-component gas hydrates in aqueous solutions are analyzed. The size of the hydrate nucleus and the work for nucleus formation are determined as functions of the supersaturation ∆µ. Expressions for the stationary rate J of hydrate nucleation are derived. These expressions describe the J (∆µ) dependence for homogeneous nucleation and for heterogeneous nucleation at the solution/gas interface or on solid substrates and nucleation-active micro-particles in the solution.
The solid mass fraction of CO2 hydrate slurry was determinated in this study.
Prior to study CO2 hydrate slurry, water tests were performed on the system in the aspects of density, dynamic viscosity, pressure drop, energy balance and heat transfer coefficients. Water test results on the new test rig have shown the measurement device were very accurate and reliable. We find the heat transferred between coolant and water in the double tube heat exchanger is well energy balanced and heat loss is within 3 %.
By using CCD camera, we confirmed that CO2 hydrate formation process takes place within a few seconds at positive temperature when suitable formation conditions are satisfied. Fast hydrate formation means an efficient energy-saving is available considering CO2 hydrate slurry as a kind of refrigerant.
The experimental results have shown that the apparent viscosity and fluid density change can also be good indicators for knowing the hydrate formation besides pressure and temperature. We argue in the induction period and the beginning of hydrate nucleation, solution viscosity will increase while the growth of the hydrate crystals will slightly decrease the apparent viscosity. The apparent dynamic viscosity of CO2 hydrate slurry was about 3.4 mPa.s when the solid mass fraction reached about 45%. We find that CO2 hydrate solid contributes only slightly to the viscosity increase resulting in excellent pump ability regarding power consumption even for a very high density fluid.
The results have shown that hydrate creation through the heat exchanger by super cooling of the saturated CO2 solution is feasible. Continuous CO2 hydrate slurry formation and dissociation by heat exchanger was proved to be feasible. Higher solid fraction is expected to obtain if we reduce the reaction water amount and increase the injected gas amount. CO2 hydrate slurry displayed very good stability at steady state during 11.5 hours running test. Longer stability period should be expected if the running conditions are maintained.
The pressure drop over the entire heat exchanger was investigated. The pressure drops increase with the increasing of mean velocities and mass concentrations. For high solid mass fractions, the pressure drop curves are observed to start from a high value at the lowest test velocity.
Base on energy balance, we estimated the heat transfer coefficient of CO2 hydrate slurry at steady state. We argue that the heat transfer capacity of the hydrate slurry was greatly enhanced due to the formed hydrate solid particles. Experimental results have confirmed the local heat transfer capacity of the CO2 hydrate slurry was greatly promoted by the formed hydrate solid particles

O. Sari, J. Hu, F. Brun, N. Erbeau, P. Homsy, J.-C. Logel: In-situ Study of the Thermal Properties of Hydrate Slurry by High Pressure DSC, Keynote paper, 22nd International Congress of Refrigeration – Refrigeration creates the future, Beijing, August 2007;
O. Sari, J. Hu, S. Eicher, P. Egolf , P. Homsy: Thermo physical and Flow Properties of CO2 Hydrate Slurry, International Refrigeration and Air Conditioning Conference at Purdue, July 14-17, USA, 2008;
O. Sari, J. Hu, F Brun, S. Eicher, P. Homsy: Le coulis de glace, Symposium du Froid, Yverdon-les- Bains, Switzerland, 2008;
J. Hu, O. Sari, S. Eicher, J. Forchelet, J. Bettex , P Homsy: Online and Real-time Monitoring of Corrosion Rates of Stainless Steel, Low Carbon Steel and Copper in CO2 Hydrate Slurry with Coupled Multielectrode Array Sensors, Proceedings of the 17th international Corrosion Congress, Paper #3129, USA, 2008;
O. Sari, J. Hu, S. Eicher, P. Homsy CO2 Hydrate Slurry, XIII European Conference, Milano 12th-13th June 2009;
J. Hu, O. Sari, C. Mahmed, R. Cereghetti, P. Stéphane, P. Homsy: Experimental Study on Flow Characters of CO2 Hydrate Slurry, 9th International Conference on Sustainable Energy Technologies, August 24-27, Shanghai, China 2010;
J. Hu, O. Sari, C. Mahmed, R. Cereghetti, P. Homsy: Flow behaviours of CO2 Hydrate Slurry, 9th IIR Conference on Phase-Change Materials and Slurries for Refrigeration and Air Conditioning, 29 September - 1 October, Sofia, Bulgaria, 2010.