| In order to solve the problem that the nitrogen content of gas source in some oil and gas fields in China is too high and seriously affects sales,this paper proposes to remove nitrogen by liquefying the raw material gas by means of cryogenic denitrification,and the natural gas liquefaction process of mixed refrigerant refrigeration The unique advantages will be used as the liquefaction process for the main optimization study in this paper.Based on this,this paper takes the gas source with 20%nitrogen content as the research object,and uses the optimization theory and the improved enthalpy analysis method to optimize and study the liquefaction process of high nitrogen natural gas by HYSYS software.The main research results are:1)Thermodynamic calculation of natural gas liquefaction process simulationThe PR equation is selected as the equation of state for physical property calculation.By establishing the thermodynamic model of the key logistics in the process,the phase equilibrium analysis of natural gas liquefaction process,the calculation of natural gas enthalpy and entropy value are completed.At the same time,the HYSYS model of three liquefaction processes is built.Lay the foundation for the optimization calculation of the liquefaction process below.2)Comparative analysis of high nitrogen natural gas liquefaction processBased on the three liquefaction processes established in Chapter 2,the effects of the nitrogen content of the feed gas on the liquefaction rate,specific power consumption,methane recovery,and nitrogen content of LNG in the liquefaction process were studied.The main parameters in the natural gas liquefaction process(A series of single-variable parameters were studied for raw gas pressure,temperature,natural gas pre-cooling temperature,cryogenic temperature,mixed refrigerant high pressure,low pressure,low pressure temperature,and the results were analyzed in detail.3)Optimization of natural gas liquefaction process based on traditional enthalpy analysis methodIn this chapter,the main parameters of the liquefaction process are optimized by calling the optimizer of HYSYS software with the minimum power consumption as the optimization goal.The parameters obtained are shown in Table 3-10?Table 3-12.The specific power consumption of the SMR liquefaction process is reduced by 13.8%,the specific power consumption of the C3/MRC liquefaction process is reduced by 27.5%,and the specific power consumption of the DMR liquefaction process is reduced by about 15.5%.The performance analysis of the heat exchanger shows that although the heat exchanger load matching degree is higher after the process optimization,the matching degree of the hot and cold load of most heat exchangers has not reached the optimal state,indicating that the process still has A large optimization space;based on the traditional enthalpy analysis method,the turbulence block diagrams of three liquefaction processes are drawn,and the enthalpy analysis model of the main equipment is established,and the damage of each equipment before and after optimization and the enthalpy efficiency of the overall process are calculated.The loss of SMR liquefaction process was reduced by 5.7%,the loss of C3/MRC liquefaction process was reduced by 25.33%,and the loss of DMR liquefaction process was reduced by 22.98%.4)Optimization of natural gas liquefaction process based on improved enthalpy analysis methodBased on the improved enthalpy analysis method,the avoidable/inevitable enthalpy analysis calculation model of the main energy consuming equipment(water cooler,compressor,heat exchanger)in the liquefaction process was established,and the endogenous and external sources of the equipment were classified by linear equations.Sexual damage,and based on the division of the two types of damage,the calculation model of the damage of each part of the equipment is established.Combined with the results of traditional enthalpy analysis,the damage of the main equipment in the liquefaction process is divided into four parts:avoidance of endogenous,avoidance of-exogenous,inevita5)Scheme optimizationBased on the multi-objective evaluation method of entropy weight,six evaluation indexes of specific power consumption,methane recovery rate,economic investment,net present value of finance,nitrogen content of products and payback period of dynamic investment are put forward for three liquefaction processes.The influence of each evaluation index is transformed into comprehensive judgment.The calculation program and input interface of multi-objective evaluation system of entropy weight are compiled by using MATLAB software,which greatly simplifies the calculation.Finally,the optimized DMR liquefaction process was determined as the final optimal scheme by comparing the evaluation values Z1(w)=0.2355,Z2(w)=0.1266,Z3(w)=0.6379.The specific power consumption of the scheme is 86450 kJ/kmol,the efficiency is 76.36%,the nitrogen content of the product is 8.41%,and the methane recovery rate is 88.45%.6)LNG quality control and case analysisCompared with throttling flash denitrification and distillation column denitrification,the distillation column denitrification process was selected after considering the quality and liquefaction rate of LNG products comprehensively.The actual case of Tahe Oilfield was introduced.The results show that the specific power consumption,liquefaction rate,nitrogen content of products,methane recovery rate and other evaluation indexes have obvious advantages af-ter introducing the optimized process into the case.The power consumption was reduced by about 16.96%,and the nitrogen content of LNG reached the denitrification standard.Finally,the preliminary economic evaluation of the case shows that the internal rate of return after tax is 15.6%and the net present value is 8.421 million yuan,which are larger than the industry standard.It shows that the improved process has better profitability and higher feasibility. |
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