Research On Frost Point And Pressurized Liquefaction Process Of Co₂-Contained Natural Gas

With offshore natural gas playing a significant role in the global natural gas market,the developemnt and production of offshore gas are under very serious consideration.FLNG（Floating liquefied natural gas）technology with multi-functions of liquefaction production,storage and offloading,is considered as the most promising technology for offshore natural gas production in the future because of its low capital cost,short construction period and flexible movability.Currently,the main chanlleges FLNG production face are space limits,followed by energy efficiency and safety concerns.To overcome those challenges,this work performed a study of liquefaction process,which is the heart of FLNG.Aimed at footprint reduction,this work designed several natural gas pressurized liquefaction processes.For safety concern,special attentions were paid to refrigerant selection,particularly,avoiding using those refrigerants with high danger potentials,such as C₃H₈ refrigerant.To improve energy efficiency,simulation based optimization were conducted for the designed processes using energy consumption as the objective function.Moreover,thermodynamic analysis and performance comparison with regard to footprint,energy efficiency and safety issues were performed for the processes to provide guidance for FLNG process selection.The detailed research is as follows:To provide support for process design,CO₂ frost data for CH₄-CO₂binary mixture,CH₄-CO₂-C₂H₆ and CH₄-CO₂-N₂ ternary mixtures were collected within the temperature range from 153.15 K to 193.15 K and the pressure range from 200 kPa to 3100 kPa by using a gas-solid phase equilibrium experimental apparutas.Results show that Temperature in higher region has more obvious effect on CO₂ frost content than that in lower region,and pressure in lower region has more obvious effect on CO₂ frost content than that in higher region.The addition of nitrogen and ethane to CH₄-CO₂binary mixture have little effect on CO₂ frost point except that the maximum pressure for CO₂ frosting decreases with C₂H₆ content but increases with N₂content.As an outward extension of measurements,calculations of CO₂ frost point in CH₄-CO₂ binary mixture,CH₄-CO₂-C₂H₆ and CH₄-CO₂-N₂ ternary mixtures were conducted in this work using ideal gas approach,PR equation of state approach and Hysys simulation approach.Comparison results of the calculated data and the experimental data show that ideal gas approach,with an average relative deviation of 0.89%,has lower accuracy than PR equation of state approach and Hysys simulation approach,of which the average relative deviation are 0.46%and 0.49%,respectively.Based on CO₂ frost point study,this work designed cascade PLNG（Pressurized liquefied natural gas）process.In the cascade PLNG process,a two-stage refrigeration cycle was used instead of the conventional three-stage refrigeration cycle by utilizing pressurized liquefaction technology.CH₄-C₂H₆,CH₄-C₂H₄ and C₂H₄-C₃H₈ cascade PLNG processes were designed for natural gas with a CO₂ content less than 0.5%,and CO₂ cryogenic removal was integrated with these processes for natural gas with a CO₂ content of0.5-30%.Taking the specific energy consumption as the objective function,one conventional three-stage cascade LNG process and six cascade PLNG processes were simulated with Hysys and optimized by sequential search method.Moreover,thermodynamic analysis and performance comparison were conducted for them.Results show that the designed PLNG processes not only eliminate CO₂ pretreatment facilities but also save heat exchange area by13-66%and save equipment count by 25-35%.Moreover,this work designed expander based PLNG process for natural gas with a CO₂ content less than 0.5%.To enlarge its application scope,CO₂ cryogenic removal was integrated with the expander based PLNG process for natural gas with a CO₂ content of 0.5-30%.Furthermore,precooling cycle was added to that process for the purpose of energy consumption reduction.In the designed processes,N₂,50%N₂+50%CH₄ and CH₄ were used as expansion refrigerants,and C₃H₈,R407C and CO₂ were used as precooling refrigerants,respectively.Taking the specific energy consumption as the objective function,AP-N process,a conventional expander based process which is applied in FLNG.,and nine expander based PLNG processes were simulated with Hysys and optimized by sequential search method.Moreover,thermodynamic analysis and performance comparison were conducted for them.Results show that expander based PLNG processes are more space-saving and energy-saving than AP-N process.The most compact process reduces equipment number as much as 65%and the most energy efficient process saves energy as high as 39%.CH₄refrigerant is more energy efficient than N₂ refrigerant and CO₂ refrigerant is the best candidate to substitute C₃H₈ refrigerant in the precooling cycle.In addition,the effect of CO₂ content in feed gas on PLNG process was investigated.It was found that the specific energy consumption increases with CO₂ content.In addition,this work designed SMR（Single mixed refrigerant）PLNG process,SMR PLNG process integrated with CO₂ cryogenic removal,and MR PLNG process integrated with CO₂ cryogenic removal and precooling cycle,respectively.For safety concern,mixed refrigerant with and without C₃H₈ composition were both studied.Taking the specific energy consumption as the objective function,PRICO process,a conventional SMR process which is applied in FLNG,and six MR PLNG processes were simulated with Hysys and optimized by sequential search method.Moreover,thermodynamic analysis and performance comparison were conducted for them.Results show that the designed processes have higher exergy efficiency than PRICO process.The specific energy consumption of the designed processes is22-46%higher than that of PRICO process,presenting an obvious advantage of high energy efficiency in FLNG application.In addition,it was found that mixed refrigerant with and without C₃H₈ composition are almost the same in energy efficiency and footprint,but mixed refrigerant without C₃H₈composition is safer,therefore,C₃H₈ refrigerant can be eliminated in MR processes for FLNG production.Based on the above study,PLNG processes show a huge advantage in space savings by eliminating CO₂ pretreatment facility.Moreover,it presents a great advantage in energy savings due to increased liquefaction temperature.In addition,PLNG processes have high security as special considertions were given to refrigerant selection.Therefore,PLNG processes have every reason to be the most promising solution to FLNG production in the future.