(88b) Heavy Hydrocarbons in Mixed Refrigerant Used in Natural Gas Liquefaction Process

When lubricant is used in a compressor for mixed refrigerant (MR) used in natural gas liquefaction process, butane and pentane, the heavy hydrocarbons in MR, are more likely to dissolve in the lubricant, comparing to other hydrocarbon components. In such liquefied natural gas plants, MR containing less or even no heavy hydrocarbons may be adopted. In this paper, the single mixed refrigerant (SMR) liquefaction process with different butane or pentane content in MR was optimized. In order to compare the influence of MR composition, first set the suction and exhaust pressure of MR compressor, then change the percentage of MR composition sequentially, and automatically adjust the MR flow to guarantee the minimum temperature difference of LNG cold box no less than 3°C. Knowledge based algorithm (KBO) is applied to simulate the process. Based on the work, the refrigerant flow and specific power consumption of the processes are investigated. The result shows that both MR flow and specific power consumption first decrease, and then increase with the increase of pentane content. The optimal value of specific power consumption is 0.33kWh/kg(LNG). For butane content, similar results are obtained and the optimal value is 0.35kWh/kg(LNG). When the content of pentane or butane is changed, the variation of propane content is the largest, and that of nitrogen content is the least. This phenomenon is related to the boiling point and latent heat of each composition. Thermodynamic analysis is applied to explain the results.