Study On Cold Oil Absorption Process For High Nitrogen Gas

Natural gas is recognized as a clean energy source in the world,and its consumption is increasing.However,some conventional and unconventional natural gas reservoirs contain higher concentrations of nitrogen and are of lower quality.The presence of nitrogen reduces the calorific value of natural gas,fails to meet pipeline requirements,and also reduces pipeline capacity,increase power and fuel consumption.Therefore,the removal of nitrogen from natural gas is of great significance.The nitrogen removal process of cold oil absorption method is studied.It mainly includes the following contents:(1)According to the principle of similar compatibility,the C5?C9 solvent which is similar in structure and properties to methane is selected to study.The differences of absorption,selection and regeneration performance of various pure solvents are calculated and compared.At the same time,the absorption performance of mixed solvent is studied.Finally,C5 solvent is selected as absorbent.(2)In order to obtain the required cooling capacity of the process,the difference in performance between the single-stage compression refrigeration system and the two-stage compression refrigeration system is calculated and compared.The results show that,under the condition of obtaining the same cooling capacity,compared with the single-stage compression refrigeration system,the two-stage compression refrigeration system saves about 15%of power consumption and is more economical.Therefore,a two-stage compression refrigeration system with incomplete cooling in the middle of the first stage is selected,and the refrigerant is selected from propane.(3)According to the literature research,four cold oil absorption methods for nitrogen removal are proposed.With the help of HYSYS process simulation software,the calculation model of each scheme is established.The optimal nitrogen removal scheme is determined by technical,economic and adaptive comparisons of various options.(4)Perform parameter sensitivity analysis on the selected nitrogen removal scheme to determine the range of key parameters in the process.With the help of orthogonal experiment design,the influence degree of each parameter on the index is explored.By analyzing the data by range analysis and variance test,three factors that have significant influence on the power consumption are determined:absorption pressure,the first stage desorption pressure and lean liquid temperature.Then,based on the results of the orthogonal test,the three factors that have obvious influence on the power consumption are further analyzed.With the response surface analysis method,the parameters are continuously optimized,and the refrigeration system temperature range is considered.The optimal parameter level combination is determined:the absorption pressure is 2.0MPa;the lean liquid temperature is-35?;the first-stage desorption pressure is 1.9MPa.It has been verified that the specific power consumption of the new parameter combination is 0.166kW·h/m3,the methane recovery rate is 97.3%,and the high heating value of the product gas is 35.71MJ/m3.Compared to pre-optimization,the optimized process reduces the power consumption by 8.3%.(5)The process adaptability is analyzed by analyzing the composition of the absorbent,the nitrogen concentration and the ethane concentration in the feed gas.The results show that when the composition of the absorbent or the concentration of ethane in the feed gas is changed,the process effect is achieved by adjusting the circulation amount of the lean liquid.When the nitrogen concentration in the feed gas reaches 40%or higher,the high methane recovery rate should not be pursued.In order to make the product gas calorific value reach the standard,the requirement for methane recovery should be relaxed appropriately.