Research On Optimal Design And Dynamic Characteristic Of Mixed Rfrigerant Process

The liquefied natural gas will contribute to the gas transportation problem and enables the nature gas international trade and the exploiture of the scattered small natural gas fields. In this paper, the current nature gas liquefaction processes are analyzed and compared. The single mixed refrigerant process (SMR) is designed for small scale liquefaction devices. The double mixed refrigerant process (DMR) is designed for large scale liquefaction systems. At the same time, a systematic thermodynamics analysis is done to the both processes, with steady state simulation and parameters optimization, dynamic simulation and optimal operation, exploring the optimal design and operation methods of the natural gas liquefaction process of the mixed refrigerant cycle (MRC). These studies provide the important theory to the natural gas liquefaction device of MRC and have great actual practice values.In the paper the process selection rules of natural gas liquefaction devices for different scales are analyzed. The two liquefaction processes are designed, SMR and DMR. The SMR is suitable to small scale nature gas device and the new type DMR is suitable to large scale liquefaction devices. The evaluation and selection to thermodynamics parameters computing methods of natural gas and mixed refrigerants are presented. The thermodynamics models for the devices used in natural gas liquefaction process of MRC are built, including compressor, cooler, heat exchanger, throttle valve, mixer, and gas-liquid separator. The evaluation standards are presented for mixed refrigerant cycle, which are energy consumption, refrigeration coefficient and exergy efficiency. Based on the above, the process parameters in the single mixed refrigerant process, propane pre-cooled mixed refrigerant process (C3/MR) and double mixed refrigerant process are analyzed, obtaining the exergy loss, compressor shaft power, refrigeration coefficient and exergy efficiency. The analysis of exergy loss in the different equipments helps find the measure of improving refrigeration performance. At last, by comparing the thermodynamics performances of DMR and C3/MR, DMR is recommended to be applied in large scale natural gas liquefaction system.The numerical method is used for the steady state simulation and parameters optimization of the SMR and DMR. The best fitting refrigerants are selected for the both processes. With the reasonable constrains, the process structures of the mixed refrigeration cycles are optimized. Taking the minimum energy consumption as the target function, the process design variables are computed by pattern search. The optimized design parameters for the two kinds of mixed refrigeration cycles are obtained.The dynamic models of the main equipments in the mixed refrigeration cycles, including compressor, heat exchanger, throttle valve, etc are built. The control scheme of the main process parameters in the DMR is designed. The actual probable disturbances in natural gas liquefaction process are analyzed. When the natural gas pressure, temperature, flow, components, mixed refrigerant cooling temperature, deep refrigerant composition and pre-cooling refrigerant composition change suddenly, by the dynamic simulation the change tendency of the process parameters and response time are obtained. The dynamic simulation results show that, the system can adjust and adapt automatically when DMR is disturbed, and regain its balance within very short time. The control scheme designed for DMR is also proved. The dynamic simulation takes an important role in optimizing design and its practice implementation for the DMR.Based on the steady state simulation and dynamic simulation of DMR and the actual device parameters, the research to its optimal operation is done. The operation degree of freedom in the mixed refrigerant cycle is discussed. By analyzing the operation variables, the suitable control variables and control models are selected. Establishing the two possible optimal operation modes, which are the minimum energy consumption per unit product and the maximum LNG outputs. The optimal operation is done separately with and without disturbances. When natural gas pressure, flow and cooling temperature change, by adjusting the refrigerant composition and compressor rotating speed, the refrigeration process is still maintained to run with high efficient manner. When the deep and pre-cooling refrigerant makeup deviate from their optimum compositions, the system will adjust the compressor speed and the same LNG output is maintained. The optimal operation result shows: the DMR is well in the adaptation and operability, which provides the theory guidance to its actual debugging and operation.Based on the theoretical study in this paper, the actual natural gas liquefaction devices are designed. The low concentration coal bed methane liquefaction device of 5000 Nm~3 (LNG5) and the miniature natural gas liquefaction device of 2000Nm~3 (MR3) are tested successfully. The parameters of the refrigeration processes in the LNG5 and MR3 are tested with different refrigerant compositions. By comparison between experimental result and the design parameters, the actual parameters of the devices are close to the design parameters. This shows the theoretical simulation is reliable。The research results in this paper are helpful to the application of SMR in the small scale natural gas liquefaction devices. These also provide the technical support for developing the large scale natural gas liquefaction system domestically.