Optimization Of Mixed Refrigerant Liquefaction Process For FLNG And Study On Offshore Adaptability

Due to the special requirements of operations at sea, criteria of selecting liquefactionprocess is different from the land’s. Due to its simple process, less equipment, compactstructure, less investment and so on, the mixed refrigerant cycle (MRC) can be goodadaptation to offshore liquefaction plants. This paper focuses on propane pre-cooled mixedrefrigerant process (C3MR),double mixed refrigerant process(DMR) and single mixedrefrigerant process(SMR),a systematic thermodynamics analysis is done to the threeprocesses, with steady state simulation and parameters optimization, sensitivity analysis andevaluation of offshore adaptability. These studies provide the important theory to the offshorenatural gas liquefaction device of MRC.The paper provides an overview on the researched actualities on the domestic andoverseas liquefied technology of offshore natual gas and typical mixed refrigerantliquefaction process. At the same time, sea conditions of main FLNG project in foreigncountries are summed up, and the application database of different liquefaction processwhich has different treatment capacity and sea state conditions is established. When choosinga liquefaction process, factors should be considered including project scale, processingcapacity, refrigerants storage and sea conditions.From the thermodynamic perspective,this paper analyzes the influence of key parameterson process performance including refrigerant flow, refrigeration capacity and compressorpower consumption. From the analysis, there are complex interactions between variousparameters. By adjusting the parameters in the liquefaction process, natural gas liquefactionprocesses tend to be less energy consumption and be more feasible.In this paper,the numerical method is used for the steady state simulation andparameters optimization of the C3MR,DMR and SMR. Optimization includes Refrigerantcomposition optimization and cycle parameters optimization. First of all，the best fittingrefrigerants are selected for the processes, Based on reasonable constrains, taking theminimum energy consumption as thetarget function, the process variables are computed bypattern search. The optimized parameters for the three kinds of mixed refrigeration cycles are obtained.On the basis of process parameters optimization, sensitivity analysis and offshoreadaptability evaluation are Carried out for C3MR, DMR and SMR. From the analysis, DMRand SMR are more suitable for offshore FLNG liquefaction plants than C3MR,because oftheir less sensitivity to changes of the raw gas, simple processes, less equipment, higherliquefaction rate and lower power consumption.In order to solve the problem of complex refrigerant composition,large amount ofrefrigerant storage, non-uniform mixing of refrigerant in mixed refrigerant cycle, mixedrefrigerant-expanding liquefaction process is presented.Components of refrigerant in thisprocess contain methane and nitrogen which are components of BOG material gas. From thesensitivity analysis, feed flow has wide adjustment range,process is not sensitive to changesin feed gas composition. Therefore, the process has better adaptability to FLNG. Thedisadvantage of mixed refrigerant-expanding liquefaction process is larger power.