Research On The Influence Of Lubricant Oil On Mixed Refrigerant Liquefaction Natural Gas Process

Of all processes producing liquefied natural gas(LNG),mixed refrigerant cycle is considerably appealing for its high efficiency.In a small scale mixed refrigerant cycle,utilizing injected screw compressors makes the whole compression unit more compact.However mixed refrigerant may dissolve in lubricant oil existing in injected screw compressors.It is pretty essential to understand the detailed knowledge of mixed refrigerant absorption in refrigerant-lubricant oil mixtures,as the change in compositions of mixed refrigerant can directly influence some important operation parameters such as process energy consumption,heat exchangers performance,etc.To determine the optimized components of mixed refrigerant in single mixed refrigerant(SMR)liquefaction process,without considering the absorption of refrigerant in oil,the Knowledge Based Optimization(KBO)method was adopted.Then refrigerant components in SMR LNG process were optimized for both four and five compositions by adjusting sub-flow of the components from low to high boiling point according to the KBO method.At the end of the optimization process,the optimal constitution was gained with methane 21%,ethylene 14%,propane 58%,nitrogen 7% in the four-composition mixed refrigerant and methane 19%,ethylene 30%,propane 4%,i-butane 39% and nitrogen 8% respectively.These optimal results were then used as basis of follow-up studies on the effect of single refrigerant component change on operation parameters as well as experimental investigation on refrigerant absorption in lubricant oil.After the optimal constitution of mixed refrigerant was gained,composition change due to dissolving in lubricant oil was considered.In order to investigate the impact of the change of each component on the operation parameters referred to above,a variable-controlling approach based on HYSYS was adopted.More precisely,the influence of methane,ethylene,propane and isobutene was investigated respectively.Firstly,we investigated the effect of each component change on the specific power consumption.Reducing the same amount of each component,the specific power consumption increased in a different degree—reducing propane had the greatest impact when the refrigerant was made up of four components and iButane had the greatest influence in the five-composition refrigerant process.The liquefaction rate decrease share the same trend.Although five-component refrigerant process consumed less energy and need smaller refrigerant flow than four-component refrigerant process,it had greater influence on the process performance when take the i-Butane absorption in lubricant into consideration,so gas-liquid equilibrium calculation of i-Butane-lubricating oil mixture system were studied.As well as single component analysis based on HYSYS,experiments focusing on overall absorption of mixed refrigerant in lubricant oil were also conducted.During this experiment process,we studied the absorption of four-component refrigerant and five-component refrigerant in three different lubricant oil(CP-1010-100,CP-1005-100,CP-1516-100).The composition change of mixed refrigerant was obtained by the experiment.Because of absorption,Propane and i-Butane decreased greatly when the refrigerant was made up of four and five components respectively.When the calculation was completed,the results were then used to investigate the impact of mixed refrigerant absorbed in lubricant oil on operation parameters change in the LNG liquefied process through HYSYS.If refrigerant was made up of four components,CPI-1005-100 had the greatest impact on liquefied process.In order to reach the same liquefaction ratio with the optimized one,the specific power consumption would increase 20% and refrigerant flow rate need to increase 17.5%?The other two kind of lubricant oil changed the liquefied process to a relatively small extend,with specific power consumption increasing 10% and refrigerant flow rate growing 9%.If refrigerant was made up of five components,CPI-1516-100 had the greatest impact on liquefied process.In order to reach the same liquefaction ratio with the optimized one,the specific power consumption and refrigerant flow rate need to increase 42.9%.The other two kind of lubricant oil changed the liquefied process to a relatively small extend,with specific power consumption increasing17.8% and refrigerant flow rate growing 18.7%.