Propane Pre-cooled Mixed Refrigerant Process Research Articles

High ethane content enables efficient CO2 capture from natural gas by cryogenic distillation

Cryogenic CO2 capture technique is superior to other CO2 capture methods in terms of potential environmental protection, because it uses no chemical solvent or membrane materials that need regular replacement. However, the freezing problem of CO2 makes it difficult to achieve CO2 capture through simple gas–liquid separation, and the existing cryogenic CO2 capture techniques often require a specially designed separation equipment. In addition, cryogenic CO2 capture is generally considered a method with high energy consumption due to the need for refrigeration. This study proposes a cryogenic distillation-based CO2 capture method for natural gas with high ethane content, such as shale gas and oilfield associated gas. This method utilizes the azeotropic characteristics of CO2 and ethane to avoid CO2 freeze-out and the energy consumption for CO2 capture is minimized through process integration with natural gas liquefaction and ethane recovery. The proposed system uses the propane precooled mixed refrigerant process to provide the required refrigeration, and it is simulated in Aspen HYSYS and optimized by a combined method of a genetic algorithm and sequential optimization. The results show that the proposed process can remove up to 17 mol% of CO2 to less than 50 ppm. With extractive distillation, 99.50 % of the ethane in the feed gas is recovered with a purity of 99.50 mol%. The optimal results corresponding to the maximum allowable CO2 content to avoid its freeze-out (with a solubility margin of 500 ppm) show that when ethane content is 2–20 mol%, the specific power consumption of the system is about 0.43 kWh/Nm3(NG) with an exergy efficiency higher than 50 %. The proposed process achieves energy-saving and efficient CO2 capture method for natural gas with high ethane content.

Sustainability evaluation of C3MR natural gas liquefaction process: Integrating life cycle analysis with Energy, Exergy, and economic aspects

In a transition towards a clean energy future, natural gas serves as an interim fuel due to its lower emissions whereas the C3MR (propane pre-cooled mixed refrigerant process) is widely recognized for high efficiency and commercial scale application converting natural gas to LNG. Previously, numerous studies have been conducted to optimize the energy efficiency and performance of C3MR process but very little contribution has been made towards its sustainability analysis. To address this, a life cycle analysis approach is established to evaluate the carbon footprint of C3MR process. In order to showcase its link with energy consumption and put forward the groundwork of analyses, the research focuses on using simulation – assisted optimization to reduce the specific energy consumption (SEC) of C3MR process by optimizing design variables. The study highlights the use of mixed refrigerant (MR) as a process variable and validates that its optimal implementation has decreased SEC to 0.2195 kWh/kg LNG, while achieving a minimum inlet temperature approach (MITA) of main cryogenic heat exchanger (MCHX), less than 3 °C. The knowledge − based optimization approach applied in this research, developed comparative case studies on which multiple analyses are performed to verify scientific findings. Energy and exergy analyses are conducted to determine thermal efficiency of the process where the optimized case exhibits a reduced difference in its composite curves and an 8.65 % improvement in exergetic performance compared to the base case. The economic indicators assessment reveals that optimized case demonstrates lower capital and operating costs, establishing economic viability. Furthermore, the energy consumption parameters are utilized for evaluation of ecological impact ofLNGsupply chain through life cycle assessment (LCA) which reflects reduced greenhouse gas (GHG) emissions for optimized case. Hence, the current study fills existing research knowledge gaps in terms of providing a comprehensive process system engineering by evaluating energy synergistic point, thermodynamic irreversibility, process viability and sustainability using life cycle assessment.

A novel propane pre-cooled mixed refrigerant process for coproduction of LNG and high purity ethane

For high ethane-containing natural gas, such as shale gas, separation and purification of ethane during liquefaction can improve economic benefits. A novel process of propane pre-cooled mixed refrigerant natural gas liquefaction integrated with cryogenic distillation for ethane separation is introduced and analyzed, which produce both liquefied natural gas (LNG) and liquefied ethane at the same time. The chemical engineering software HYSYS is used to simulate the process, and the liquefaction operation parameters of the system are optimized by genetic algorithm to reduce energy consumption. The results show that the proposed process can effectively achieve ethane separation, with the purity of ethane product over 99.5% and ethane recovery rate higher than 99.5%. As the ethane content in the feed gas increases, the specific power consumption and exergy efficiency of the system decrease slightly. When ethane content is in the range of 10%–40%, the optimized specific energy consumption is around 0.44 kWh/Nm3(natural gas), and the exergy efficiency is around 47%.

Thermodynamic analysis of natural gas liquefaction process with propane pre-cooled mixed refrigerant process (C3MR)

Liquefaction processes plays an important role for transmission of natural gas in long distances. Production of liquefied natural gas (LNG) is classified as an energy consuming process. LNG is a complicated thermodynamics process and the thermodynamics analysis for the key points of the system seems necessary. The goal of this study is to enhance the overall performance and heat recovery from outlet streams of LNG process by changing the process thermodynamics variables. Simulation and sensitivity analysis for two objective functions; specific energy and compressor power consumption were done by Aspen HYSYS process simulator. The investigated variables in this study are four pressure levels, inner and outlet pressures of main compressor and the flow rate of propane in precooling cycle. By considering the process limitations, the possible range for each variable was obtained and then sensitivity analysis was done for each parameter. The obtained results show more improvement on objective functions compared to the other investigations; specific energy for this process was obtained 845 kJ/kg, which shows 30% improvement compared to initial model and the total energy consumption reached to 62.7 MW while it was 71 MW for initial model.

Design and comprehensive optimization of C3MR liquefaction natural gas cycle by considering operational constraints

Liquefied natural gas (LNG) plays a key role in gas transits. Hence, attempts to improve the current liquefaction technology to increase its efficiency and economic advantages have earned a place in the master plan of many countries. Propane precooled mixed refrigerant process (C3MR) is one of the most successful cycle in the liquefaction industry. In this study, an enhanced process including a three stage propane precooling cycle and two cryogenic heat exchangers with mixed refrigerant (MR) has been modeled in ASPEN-HYSYS software by 1 MTPA production capacity. Saturated temperatures of Precooling stages, outlet temperatures of heat exchangers (HE) and aftercoolers will determine the quality and efficiency of the precooling stage. Minimization of energy consumption per 1 kg LNG production (specific energy consumption) has been defined as the main objective function. Optimization of the cycle variables has been performed, intended to minimize the specific energy consumption. Subsequently, comprehensive and integrated optimization has been developed. Mixed refrigerant composition has been recognized as the most effective parameter in the performance of the cycle. Therefore, mixed refrigerant composition has been optimized by two methods. HYSYS optimizer functions and a self-initiated method entitled as “Observation of governing trend in mixed refrigerant cooling curve behavior by an approach to maximization of possible fit in cryogenic heat exchangers composite curve”. Although these methods will cause a great amount of energy saving, their results are not practically achievable. Therefore the cycle has been optimized by considering operational constraints. The ultimate results of the optimization are not only theoretical, but also practical. The optimized point introduced here is obtainable in real performance, and cycle results are constant despite perturbation. Thus specific energy consumption decreases from 1028.94 kJ/kg at initial condition to 973.93 kJ/kg at sustainable optimized condition, by 5.35 percent of specific energy consumption saving.

Advanced exergoeconomic analysis of the multistage mixed refrigerant systems

Advanced exergoeconomic analysis is applied on three multi stage mixed refrigerant liquefaction processes. They are propane precooled mixed refrigerant, dual mixed refrigerant and mixed fluid cascade. Cost of investment and exergy destruction for the components with high inefficiencies are divided into avoidable/unavoidable and endogenous/exogenous parts. According to the avoidable exergy destruction cost in propane precooled mixed refrigerant process, C-2 compressor with 455.5 ($/h), in dual mixed refrigerant process, C-1 compressor with 510.8 ($/h) and in mixed fluid cascade process, C-2/1 compressor with 338.8 ($/h) should be considered first. A comparison between the conventional and advanced exergoeconomic analysis is done by three important parameters: Exergy efficiency, exergoeconomic factor and total costs. Results show that interactions between the process components are not considerable because cost of investment and exergy destruction in most of them are endogenous. Exergy destruction cost of the compressors is avoidable while heat exchangers and air coolers destruction cost are unavoidable. Investment cost of heat exchangers and air coolers are avoidable while compressor’s are unavoidable.

Risk-based shutdown management of LNG units

Global demand for natural gas may double by 2030, with Liquefied Natural Gas (LNG) growing perhaps fivefold – driven by continued cost reduction. Propane Pre-cooled Mixed Refrigerant (PPMR) of Air Products and Chemicals Inc. (APCI) currently holds 78% of the liquefaction plants on the market. Since the PPMR process has the largest production capacity, its operational reliability needs to be high as any failure may result in catastrophic consequences. Therefore, operational reliability has a key importance in LNG plants. To achieve an acceptable reliability, usually, a constant interval of a preventive maintenance method is used. The limitation of such a method is that the shutdown strategy is not optimum. This study focuses on determining a risk-based shutdown management strategy for APCI plants with capacity of 4.5 million tons per annum. To achieve the minimum risk for the expected life of an LNG plant, a combination of preventive maintenance, active redundancy and standby redundancy is considered. Results of this study reveal that such a combination can significantly reduce the operational risk. This combination improves the plant reliability and maintains it above a minimum operational reliability.

Second law analysis in cryogenic processes

The energy crisis has generated great interest in applying the second law of thermodynamics to many energy related sectors of our society. Thermodynamic analysis based on the second law concept of exergy (availability) is a powerful tool for process scheme selection and improvement, especially in cryogenic processes. This paper will present the results of second law analysis for two cryogenic processes in the natural gas liquefaction of baseload LNG plants. A comparison is made between a single pressure mixed refrigerant process and a propane-precooled mixed refrigerant process. It is shown that the propane-precooled mixed refrigerant process is more efficient in producing LNG. Further comparisons of the process scheme variations for the propane-precooled mixed refrigerant process are presented. Effects of feed pressure on the process efficiency are also discussed. The concepts of effectiveness and irreversibilities are illustrated by these applications. This paper then briefly discusses some previous applications of the second law analysis in other cryogenic processes. It concludes by discussing the future applications of the second law analysis in cryogenic processes.