| In order to optimize the energy consumption structure and reduce the environmental pollution in China,the government would like to widely use natural gas in China's long-term energy plan.There are a large amount of remote gas fields and associated gases which are not developed because of the lack of mature and economic technologies.Small-scale LNG plant with skid-mounted packages is considered to be the most suitable technology to utilize the remote gas fields.However,the lack of designing liquefaction processes for small-scale LNG plant with skid-mounted packages restricts the utilization.Thus,it is meaningful to design the liquefaction processes for small-scale LNG plant with skid-mounted packages.In this thesis,the study of several key problems were investigated to develop the liquefaction processes and cold box for small-scale LNG plant with skid-mounted packages.The details research is as follows:Firstly,in order to overcome the disadvantages of traditional nitogen expansion process,a small-scale parallel nitrogen expansion liquefaction process(PNEC)was proposed and designed.To improve energy efficiency,an optimization model with genetic algorithm method was developed to optimize the process using unit energy consumption as objective function.Exergy analysis was adopted to the optimized process.The results indicated that the unit energy consumption of the optimized process was 0.5113 kWh/Nm~3 which was 6.88%lower compared with the base case.The exergy efficiency of the optimized case was 0.323 which was increased by 7.55%compared with the base case.Moreover,the unit energy consumption of PNEC was 17.6%lower than that of traditional nitrogen expansion liquefaction process.PNEC also could overcome the large temperature difference disadvantages in heat exchangers of traditional nitrogen expansion liquefaction process.Then,the flexibilities of the process were tested by three different feed gases.The results showed that PNEC was flexible for gas conditions with different pressure and flowrate.Based the traditional mixed refrigerant liquefaction process,a small-scale single mixed refrigerant liquefaction process(SMRC)was designed.An optimization model with genetic algorithm method was developed to optimize the key parameters using unit energy consumption as objective function.Then,the advantages and disadvantages of SMRC and PNEC were compared.The results showed that the unit energy consumption of optimized case was 0.3724kWh/Nm~3 which was 9.17%lower compared with the base one.The exergy efficiency of the optimized case was 0.4417 which was increased by 10%compared with the base case.The designed process was flexible for different gas conditions,especially when the flowrate,pressure and composition of natural gas varied.Finally,the comparison of SMRC and PNEC was conducted.The results indicated that SMRC had lower unit energy consumption,higher exergy efficiency and better flexibility.Thus,SMRC was selected as the process for small-scale LNG plant with skid-mounted packages.A novel mixed refrigerant cycle(MRC)integrated with NGL recovery process for small-scale LNG plant is proposed and optimized to improve the energy efficiency and economics of small-scale LNG plant.The proposed process can be used to produce LNG and NGL with low energy consumption.Genetic algorithm is chosen as the optimization method for the proposed MRC-NGL process.The unit energy consumption as an objective function is optimized with key parameters.Mixed refrigerant cycle provided the refrigeration for the condenser of two distillation towers in appropriate temperature ranges.The heat of the reboilers were provided by the high temperature gas which was discharged by the compressor.The inlet temperature of demethanizer had a great influence on the process performance.The unit energy consumption decreased firstly and then increased with the increase of the inlet temperature of demethanizer.Then,based on the optimization results of single mixed refrigerant liquefaction process,a dynamic model of single mixed refrigerant process(SMRC)for small-scale LNG plant in skid mount packages was proposed.Firstly,three control structures were compared.The results showed that the control structure by controlling LNG and NG-3 temperature simultaneously was the most robust control structure for this process.The variations of natural gas temperature,composition,pressure,flow rate and water-cooler temperature were selected as disturbances to test the stability and dynamic responses of the process.The dynamic responses of LNG temperature and total compressor energy consumption were the criteria to investigate the influences of disturbances on the process.Finally,the dynamic responses of disturbances were obtained and discussed.The simulation results showed that LNG temperature could go back to its set point value after a few minutes,while compressor duty varied with the disturbances.It indicated that the mixed refrigerant liquefaction process for small-scale LNG plant in skid mount packages could overcome some disturbances and operate at stable state.Finally,a novel cold box with brazed plate heat exchangers was designed based on the optimization results of SMRC.The size of the cold box was very compact,which made it easy to be a skid-mounted packge.To obtain the maximum tolerance of carbon dioxide in plate heat exchanger,an experimental appraratus with plate heat exchanger was designed and constructed.The frozen experiments with different carbon dioxide concentrations in methane were conducted.The results showed that the maximum tolerance of carbon dioxide in plate heat exchanger was higher than 5000 ppm when the pressure was 3MPa.The equipment number and cost of the purification system in LNG plant would be reduced when the carbon dioxide concentration was less than 0.5%.The solid-liquid separator could separate the frozen carbon dioxide and liquied natural gas in an appropriate temperate.The experimental results provided an important guaidance for engineering design of novel liquefaction-purification cold box. |
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