Research On Natural Gas Liquefaction Process Technology Of Mixed Refrigerant

In the development of natural gas liquefaction process,a large number of liquefaction processes have been formed,of which the most widely used is the mixed refrigerant liquefaction process.During the operation of the process,the composition ratio of the mixed refrigerant directly affects the overall energy consumption of the liquefaction plant and is a major problem faced by the liquefaction process of the mixed refrigerant.In addition,natural gas liquefaction is a process of high energy consumption at low temperatures.It has special requirements for equipment and needs to accurately determine the energy saving potential in the process,and provide a reliable basis for process improvement.This paper firstly analyzes the characteristics of the mixed refrigerant liquefaction process,and then uses the C3-MR process as an example to analyze the key parameters of the process and obtain the effect of various key parameter changes on the process performance.It is found that the distribution ratio of the mixed refrigerant group has the greatest influence on the process performance,and that the key parameters in the process and the composition ratio of the mixed refrigerants correlate with each other.For the SMR process,C3-MR process and DMR process,the optimization model of mixed refrigerant composition ratio was established using the HYSYS internal optimizer,and the sequential quadratic programming(SQP)method was chosen to solve the optimization model.The study firstly analyze the liquefaction process performance before and after optimization to determine the feasibility of the optimization model,then analyze the changing rule of composition ratio of mixed refrigerant under different feed gas composition and different feed gas pressure conditions.It found that with the reduction of the CH4 content in the feed gas,that is,as the feed gas becomes richer and richer,the required coldness in the low-temperature zone during liquefaction decreases,and the required refrigeration in the middle-high temperature zone increases.The sum of light components in the mixed refrigerant gradually decreases,and the sum of heavy components gradually increases.With the raw gas composition unchanged,as the pressure of the feed gas increases,the required coldness in the low-temperature zone during liquefaction increases,and the required coldness in the medium-high temperature zone decreases,making the light refrigerant in the mixed refrigerant.The sum of the contents gradually increased and the sum of the heavy component contents gradually decreased.This study provides guidance for regulating the composition ratio of mixed refrigerants when the temperament conditions of the raw gas are changed.Through the analysis of the mixed refrigerant liquefaction process,it was found that the crucible loss of the equipment in the process was in descending order of compressor,heat exchanger,cooler,throttle valve,gas-liquid separator and flow mixer.The crucible loss of the compressor and the heat exchanger accounts for more than 70%of the entire process.To reduce the crucible loss of the compressor,it is recommended to use a three-stage compression in the mixed refrigerant cycle to maximize the heat insulation efficiency of the compressor.For reducing the loss of the heat exchanger,it is recommended to keep the minimum approach of the heat exchanger at 3～5 ℃ by adjusting the distribution ratio of the mixed refrigerant groups.By analyzing and analyzing the example of the natural gas liquefaction peaking device of JD oilfield,it is recommended to adjust the outlet pressure of the raw gas compressor group to 4.5 MPa,and adjust and optimize the mixed refrigerant cycle parameters and composition ratio.The total compressor power consumption can be reduced by 1175kW compared with the design report,and the annual energy saving can be 763.75×104kW·h,the annual output of LNG is increased by about 200t from the designed output,the specific power consumption is reduced by 0.141kW·h/kg,and the energy saving effect is the most obvious.This paper systematically analyzes the mixed refrigerant natural gas liquefaction technology and has certain guidance and reference significance for the development and optimization of natural gas liquefaction technology in China.