Investigation of hydrate growth rate on the interface between liquid and solid film

Hydrate slurry has been reported to be a suitable secondary fluid for refrigeration and air-conditioning systems. The latent heat of CO2 hydrate is 387 kJ/kg under phase equilibrium condition of 7 °C and 30 bar. The utilization of CO2 hydrate slurry in air-conditioning systems is promising in improving the energy efficiency and shifting energy supply and demand load as well as relieving greenhouse effect caused by normal refrigerants like CFCs, HCFCs etc.
The production of CO2 hydrate slurry in a coil heat exchanger is investigated in this study. Crystals are supposed to firstly form on the wall of the tube, generating a solid layer. The appearance of the solid layer increases the heat resistance from liquid to the refrigerant. Type-III antifreeze proteins have been added to the solution to better control the crystallization process of hydrate formation since AFPs have proven to be an effective hydrate formation preventer which is environment friendly. A kinetic model is developed based on the gas hydrate growth model of Skovborg and Rusmussen (1994), taking the mass transfer process to be the rate-control step. The influence of pressure, temperature and concentration of the hydrate formation preventer on the diffusion coefficient of dissolved gas into the solid interface is investigated and mainly the concentration of hydrate formation preventer appears to have a large impact.
Results show that the growth rate decreases with the increase of the concentration of AFPs. Higher concentrations of AFPs move the equilibrium line to slightly higher temperatures.