Single Mixed Refrigerant Cycle Research Articles

Enhancing Energy Efficiency and Reliability in Floating LNG Operation: A Hydrofluoroolefin-Based SMR Cycle with Thermo-Economic Assessment and Uncertainty Analysis

Natural gas liquefaction is the most energy-intensive technique in floating liquefied natural gas (FLNG) operations. Hence, a hydrofluoroolefin-based single mixed refrigerant (SMR) technology was proposed to achieve energy-efficient and eco-friendly operations. The study was perfomed to enhance the SMR process by substituting expansion valves with hydraulic turbines to improve process reversibility and enhance energy efficiency. The proposed process was optimized using a sine–cosine concept-based algorithm that tuned the design variables which yielded an overall compression energy consumption of 0.2501 (kWh/kg-LNG), resulting in a 21.43% saving as compared to the conventionally used mixed refrigerant-based process. This resulted in a reduction of about 38.81% in overall exergy destruction. The economic analysis results draw curtains that the process saves about 26.02% of total annualized cost as compared to conventional study. Owing to the appropriate mixed refrigerant component mixture ratio and process design improvement. To determine the process reliability under uncertain conditions the optimal compression power and minimum approach temperature were determined. Further, uncertainty analysis reveals that the methane and butane refrigerant flow rates along with evaporation, and condensation pressure, were among the most influential parameters that influence the process most that may affect the whole operation in uncertain process inlet conditions. In summary, our approach presents immense promise for achieving energy efficiency, environmental responsibility, and economic viability in FLNG operations, offering a robust solution for the challenges of offshore plants.

Process design and optimization of single mixed-refrigerant processes with the application of deep reinforcement learning

Deep reinforcement learning approach is considered for the design and optimization of SMR (Single Mixed Refrigerant) cycles with which the refrigeration power is minimized. Deep Q-Network (DQN) agent is programmed with Python®, which is interacted with a process simulator UniSim Design®, as an environment, through the delivery of decision with MATLAB®. The optimization goal is achieved by designing a reward function such that specific power requirement is minimized, subject to the constraint of minimum temperature difference for the heat transfer. The case study focuses on the design of the SMR cycle for the liquefaction of lean and rich natural gas feeds. GA (Generic Algorithm) optimization framework is also constructed and applied for the case study. Almost the same or similar computational performance between the DQN and the GA methods is observed for finding optimal solutions. For the case study, the specific power required for the liquefaction process is reduced by 15.7% and 13.4% through DQN optimization for the cases of lean and rich feeds, respectively, compared to base cases. The case study clearly demonstrates the applicability of the reinforcement learning which can effectively deal with the optimization problem having complex interactions and high nonlinearities.

Development of an Energy-Efficient Single Mixed Refrigerant Cycle for Small-Scale LNG Production

Energy-intensive single mixed refrigerant (SMR) cycles are employed to produce liquefied natural gas (LNG) at small scale. The energy required for refrigerant compression (shaft work) dominates the overall operating costs of the LNG plant. While current methods to minimize shaft work demand (e.g., exergy analysis) focus on the PRICO cycle, this paper shows that structurally modifying the configuration of SMR cycles can yield significant savings in shaft work demand with low added complexity. The novel SMR cycle developed in this work is based on the CryoMan SMR cycle; its benefits, in terms of energy savings, are demonstrated through a case study for natural gas liquefaction at small scale. The shaft work demand of the PRICO, CryoMan, and novel SMR cycles are minimized using a genetic algorithm and nonlinear optimization. The structural modifications applied tailor the refrigerant composition and exploit complex trade-offs between the operating variables to enhance the energy efficiency of the SMR cycles. The results from the case study demonstrate that the novel SMR cycle achieves 10% savings in shaft work demand in comparison with the PRICO SMR cycle.

Comparing exergy losses and evaluating the potential of catalyst-filled plate-fin and spiral-wound heat exchangers in a large-scale Claude hydrogen liquefaction process

Detailed heat exchanger designs are determined by matching intermediate temperatures in a large-scale Claude refrigeration process for liquefaction of hydrogen with a capacity of 125 tons/day. A comparison is made of catalyst filled plate-fin and spiral-wound heat exchangers by use of a flexible and robust modeling framework for multi-stream heat exchangers that incorporates conversion of ortho-to para-hydrogen in the hydrogen feed stream, accurate thermophysical models and a distributed resolution of all streams and wall temperatures. Maps of the local exergy destruction in the heat exchangers are presented, which enable the identification of several avenues to improve their performances.The heat exchanger duties vary between 1 and 31 MW and their second law energy efficiencies vary between 72.3% and 96.6%. Due to geometrical constraints imposed by the heat exchanger manufacturers, it is necessary to employ between one to four parallel plate-fin heat exchanger modules, while it is possible to use single modules in series for the spiral-wound heat exchangers. Due to the lower surface density and heat transfer coefficients in the spiral-wound heat exchangers, their weights are 2–14 times higher than those of the plate-fin heat exchangers.In the first heat exchanger, hydrogen feed gas is cooled from ambient temperature to about 120 K by use of a single mixed refrigerant cycle. Here, most of the exergy destruction occurs when the high-pressure mixed refrigerant enters the single-phase regime. A dual mixed refrigerant or a cascade process holds the potential to remove a large part of this exergy destruction and improve the efficiency. In many of the heat exchangers, uneven local exergy destruction reveals a potential for further optimization of geometrical parameters, in combination with process parameters and constraints.The framework presented makes it possible to compare different sources of exergy destruction on equal terms and enables a qualified specification on the maximum allowed pressure drops in the streams. The mole fraction of para-hydrogen is significantly closer to the equilibrium composition through the entire process for the spiral-wound heat exchangers due to the longer residence time. This reduces the exergy destruction from the conversion of ortho-hydrogen and results in a higher outlet mole fraction of para-hydrogen from the process.Because of the higher surface densities of the plate-fin heat exchangers, they are the preferred technology for hydrogen liquefaction, unless a higher conversion to heat exchange ratio is desired.

Conceptual process design and optimization of refrigeration cycles for the liquefaction of boil-off gas

Improving thermodynamic efficiency for refrigeration cycles is one of the key elements in conceptual process design activities for achieving cost-effective and sustainable production of liquefied natural gas. Various structural and heat recovery options are available for the design of refrigeration cycles, with which shaft power required to drive compressors is attempted to be minimized. Choice of refrigerants to be employed is another important degree of freedoms in a process design of refrigeration cycles. Hence, systematic investigation in a holistic manner is required to screen different liquefaction technologies and evaluate their techno-economic impacts. In this study, the focus is made to gain conceptual understanding in the design of liquefaction processes for boil-off gas. The current study aims to provide guidelines for the selection of the most appropriate technologies for the application of boil-off gas liquefaction as well as for determining optimal design and operating conditions of refrigeration cycles. A heat-integrated design framework is adopted in this work to evaluate energy efficiency in a systematic manner, while optimization methods are applied to systematically determine the most appropriate operating conditions for the liquefaction cycles. From the case study with 1 ton/day of liquefaction capacity, it was found that the power consumption for a single mixed refrigerant cycle is about 12 % less than that of an N2 expander cycle.

Process development and exergy cost sensitivity analysis of a novel hydrogen liquefaction process

A novel hydrogen liquefaction process including a single mixed refrigerant cycle is proposed and investigated by energy, conventional and advanced exergy and exergoeconomic analyses methods. The proposed liquefaction process can produce 130 tons liquid hydrogen per day. Results of energy analysis reveals that 7.646kWh/kgLH2 power is consumed to liquefy the hydrogen from 21 bar to 25 °C. Exergy analysis indicates that exergy efficiency of the proposed hydrogen liquefaction process is almost 32%, which is higher than Ingolstadt operating hydrogen liquefaction plant with exergy efficiency of 21.28%. Also, the power consumption of proposed hydrogen liquefaction plant is less than its kind between other similar conceptual processes. Based on the advanced exergy analysis, most part of the exergy destruction rates of the compressors, heat exchangers and air cooler are endogenous, while the dominant part of irreversibilities of the turbo expanders is exogenous. Effect of mole fraction of the mixed refrigerant components on performance of the process is investigated. Also, sensitivity of exergy destruction cost rate and exergoeconomic factor to operating variables of the proposed hydrogen liquefaction process is investigated and discussed. Results of the analyses show that maximum exergy destruction cost rate of the process components is 2265.26 $/h, which most of it is endogenous and unavoidable.

Plant simulation and operation optimisation of SMR plant with different adjustment methods under part-load conditions

A plant simulation model of SMR (single mixed refrigerant cycle) plant which considers the performance variations of devices under different working conditions is established. It could be used to predict the performance variations of an SMR plant under part-load, and other working conditions. The operational optimisation based on plant simulation is conducted for SMR plant with different load adjustment facilities. The constraint condition uses a penalty function method and the Box method is applied for solution optimisation. The results show that the frequency conversion method could achieve the best adjustment performance and the composition optimisation is significant for the frequency conversion method, especially when the load is below 40%. The operational strategy giving the lowest energy consumption for SMR plant with different adjustment methods under part-load conditions was discussed and the efficiency of the MR compressor is regarded as the prime influence thereon.

A flexible and robust modelling framework for multi-stream heat exchangers

Heat exchangers are important units in most industrial processes. They involve physical phenomena such as condensation and evaporation including several boiling regimes. Different types of heat exchangers constructed for different applications may differ much in geometrical design. This work explains and demonstrates a modelling framework which is capable of handling a multitude of geometries and relevant physical phenomena affecting the performance of the heat exchangers. The data structure and governing equations are explained, before the framework is demonstrated for a particular challenging test case with a heat exchanger operating similar to the main heat exchanger in a single mixed refrigerant cycle. In the test case, both evaporation and condensation may happen simultaneously along the length of the heat exchanger. 1000 cases with random changes within predefined intervals in inlet temperatures, mass flows and pressures were used to test the robustness of the model framework. The solution scheme converged in 98.7% of the cases, and in the non-converging cases, the operating conditions exceeded the physical limits of the heat exchanger. The framework demonstrated may thus be used to create flexible and robust heat exchanger models for use in process simulations, optimization, or as a stand-alone model.