Investigation Of Efficient Formation For Hydrate Slurry

Tetra-n-butyl-ammonium bromide (TBAB) solution turns into clathrate hydrate slurry (CHS) when it is chilled in the temperature range of 0-12oC. This is well compatible with the temperature range of common air-conditioning operation. Thus, it is considered as an ideal heat-transport and cold-storage medium. Due to the phase change, TBAB slurry has far higher cold-loading density than water under the same temperature difference. Moreover, TBAB slurry behaves as fluid, so it can be easily transported through pipeline. Therefore, it is very promising to use TBAB slurry in air conditioning or district cooling system for its energy-saving advantages.In this paper, the physical properties and formation characteristics of TBAB CHS were investigated. First, the phase change diagram studied by Ma was supplemented by a cooling experiment. The application concentration range can be enlarged when 5wt% NaCl added to the TBAB solution. Moreover, the sedimentation phenomenon can be largely retarded due to the increased solution density after NaCl added to the solution. Researchers rarely measured the thermal conductivities of TBAB CHS, so this work was done in this paper. The thermal conductivities of 5-30 wt% TBAB aqueous solution varies in the range of 0.4-0.6 W·m^-1·K^-1, and it decreases with the concentration. The thermal conductivities of 10-40vol% TBAB clathrate hydrate slurry between 0.5 and 0.6 W·m^-1·K^-1, and it increases with the volume fraction. With the same volume fraction it was found that the thermal conductivities of type A slurry larger than type B slurry. Also, influence of latent heat on the thermal conductivity was analytically discussed to improve measurement accuracy. The modified values were about 3% smaller than the measurements.In this study, the crystallization and growing process of TBAB hydrates on the brass tube surface were investigated; also the temperature mutation due to the latent heat release during cooling process were interpreted. Moreover, the induction time and memory effect during TBAB crystallization was discussed. At last, the average thickness of hydrate adhere to the copper tube was measured.Based on the super-cooled principle, the TBAB CHS generation system was set up. Coaxial heat exchanger and straight copper tube heat exchanger (with visualization part) were used to generate slurry respectively. Lower the temperature of the cooling liquid can obtain larger volume fraction slurry, but more slurry adheres to the heat exchanger surface and form thermal resistance. When the coaxial heat exchanger was used to generate slurry, pressure drops of 15wt% TBAB solution (velocity0.8-1.6m/s) was about 1.2 times than the water; pressure drops of 8-24vol% TBAB CHS were between 4.0 and 24.0 kPa at a flow velocity range of 0.9-1.6m/s, which about 1.2-2 times of that 15wt% TBAB solution. When the straight copper tube heat exchanger substitutes the coaxial heat exchanger, more hydrates adhere to the surface due to the poor heat exchange capacity. In short, the initial solution during 10wt%-15wt% was reasonable to generate slurry used the above system, 15vol% TBAB CHS can obtain when the straight copper tube heat exchanger was implemented, the finally overall heat transfer coefficient was about 100W.m^-2.K^-1.In the contrast, the coaxial heat exchanger can obtain 25vol% TBAB CHS, the overall heat transfer coefficient was about 400W.m^-2.K^-1.