Introducing a novel hybrid system for cogeneration of liquefied natural gas and hot water using ejector-compression cascade refrigeration system (energy, exergy, pinch and sensitivity analyses)

Abstract

• A novel integrated structure for natural gas liquefaction was developed. • Integrated cascade ejector refrigeration & low-temperature organic Rankine cycles. • This hybrid system generated 38.39 kg/s LNG as main product & 575.5 kg/s hot water. • The COP of the two stages cascade ejector refrigeration system was 0.8635. • Total exergy efficiency & specific power of the cycle were 36.4% & 0.386 kWh/kg LNG. Multi-component refrigerant compression refrigeration cycles have a high coefficient of performance and difficult controllability. In these systems, it is not easily possible to keep the ratio of stream compositions constant in the event of a leak. Also, due to high power consumption in compression refrigeration cycles, the use of compression-ejector refrigeration systems can reduce energy consumption. In this paper, a novel integrated system for cogeneration of liquefied natural gas (LNG) and hot water using two stages ejector refrigeration system (ERC) and low-temperature organic Rankine cycle (ORC) is developed. The feasibility of using ejector refrigeration systems in the novel integrated structure as the compression refrigeration cycle alternative is investigated. The novel integrated structure produces 38.39 kg/s LNG as the main product and 575.5 kg/s hot water as a byproduct. In the novel integrated structure design, due to the elimination of some equipment in the compression refrigeration systems, high energy consumption is reduced in these units. HYSYS software and MATLAB programming are used to simulate the hybrid system. The specific power consumption and exergy efficiency of the present system are 0.3868 kWh/kg LNG and 36.42%, respectively. The results of exergy analysis illustrate that the most exergy destruction belongs to the heat exchangers, which alone accounts for 60.19% of the total destruction. The heat exchanger network related to each of the multi-stream heat exchangers used in the system is extracted through the pinch method. The sensitivity assessment illustrates that the specific power consumption decreases up to 0.3809 kWh/kg LNG and produced LNG mass flow rate increases up to 39.81 kg/h, respectively when the mole fraction of methane in natural gas increases from 83% to 90%.