Thermal design optimization analysis of an intermediate fluid vaporizer for liquefied natural gas

An intermediate fluid vaporizer (IFV) is the core heat transfer equipment in a liquefied natural gas (LNG) regasification system, particularly in an offshore floating LNG receiving terminal where more efforts are focused on improving the efficiency and structure size of the vaporizer for reducing the volume and weight. By considering the constraints of both the initial velocities of the working fluids and length of the heat transfer tubes, a new numerical model based on the distributed parameter method is developed to determine the heat transfer performance and required heat transfer area (HTA) of an IFV. The effects of the intermediate fluids and their saturation parameters, inlet temperature of the seawater, and temperature drop of the seawater in the thermolator are investigated. The results show that propylene exhibits the best heat transfer performance, but its higher saturation pressure would require an increase in the wall thickness of the IFVs and therefore, limit its application. The heat transfer performances of propane and dimethylether are better than the other intermediate fluids, and are promising to be used in IFVs. With increase in the saturation temperature of propane, the required total HTA of IFVs first decreases and then increases, and the optimal saturation temperature is in the range of 250-265 K. A higher seawater temperature is beneficial for reducing the HTA, and it is also indicative of a wider optimization saturation temperature range in which the required total HTA is not sensitive to the saturation temperatures. When the temperature drop of the seawater in the thermolator varies from 0.3 K to 0.8 K, the variation in the required area is not more than 5% compared to the lowest area, and the recommended range for the corresponding heat load ratio between the evaporator and condenser is recommended is 5-15.