Energy, exergy, and pinch analyses of a novel low-temperature cascade mixed refrigeration system for fuel gas and crude helium production by waste heat recovery
摘要
The growing demand for helium in recent years has attracted the attention of liquid natural gas (LNG) producers toward helium production units, resulting in lower energy consumption and prices. The helium production unit requires external cooling to extract pure helium from natural gas. This study introduces a unified system for the production of LNG and crude helium from natural gas using flash separation and a compression refrigeration cycle involving a dual mixed refrigerant (DMR) and the Kalina cycles. The system receives 942.1 MW of electricity and 1179.4 kg s−1 of natural gas, producing 998.3 kg s−1 of LNG, 1.14 kg s−1 of crude helium, and 179.9 kg s−1 of fuel gas. The novelty of this study relies on two points: first, designing an optimal cascade refrigeration system for helium production, and second, transforming waste heat from the helium production unit into electricity by a Kalina cycle to generate 2.856 MW of electricity. The coefficient of performance (COP) of the DMR cycle and the specific energy consumption (SEC) for the helium production cycle based on flash separation are 3.017 kWh kmol−1 and 364 kWh kmol−1 helium, respectively. The refrigeration system is integrated with the other units using pinch and exergy analysis methods. The efficiencies of the combined system in terms of exergy and SEC of the natural gas efficiency cycle are 57.60% and 0.2621 kWh kg−1 LNG, respectively. The devices causing the most exergy destruction rate are recognized as compressors and heat exchangers. The arrangement of the heat exchanger network is formulated through pinch analysis to achieve optimal energy recovery. Sensitivity analysis is conducted to examine the system’s performance under various conditions, leading to a reduction in SEC for helium and LNG production to 10.44 kWh kmol−1 helium and 0.2606 kWh kg-1 LNG, respectively.