Distillation assisted purification, transport and delivery of liquefied natural gas

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Manek, Veera
Ghiaasiaan, S. Mostafa
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The removal of dissolved CO2 from natural gas is essential for the safe and reliable operation of liquefied natural gas (LNG) systems. The purification of natural gas (NG) from CO2 down to a concentration of 50 ppm by multi-stage distillation is theoretically investigated. A three-column distillation system is proposed that can purify NG to lower than 50 ppm concentration of CO2, while avoiding CO2 freezeout. The columns include a 30-stage Demethanizer, in which high purity methane is obtained in the distillate by separating the impurities from natural gas including CO2; a 50-stage extractive column where the azeotrope between CO2 and ethane is broken; and a 50-stage solvent recovery column that recovers a mixture of heavy hydrocarbons suitable for recycling as a solvent back into the extractive column. The proposed system avoids CO2 freezeout by utilizing a multi-component feed of some heavier hydrocarbons added to natural gas; propane, butane and pentane additives are injected into stage 20 of the Demethanizer column alongside the raw feed. Furthermore, arrangements are made to break the CO2-ethane azeotrope, which may occur in the bottoms stream of the Demethanizer by administering a solvent stream in the extractive column. The proposed system can operate in a closed-loop arrangement where the bottoms stream that leaves the recovery column can be recycled and injected into the extractive column for azeotrope prevention. Hydrodynamic and heat transfer characteristics of a double helically coiled tube confined in a cylindrical shell is experimentally studied using an instrumented test loop that represents a prototypical LNG fuel delivery system for natural gas-burning IC engines. The test loop comprises of a heat exchanger consisting of a double-helically coiled tube that carries liquid nitrogen (liquefied natural gas (LNG) in the real system), placed in a shell-confined secondary side through which a secondary coolant (a mixture of propylene glycol and water in the experiments, and engine oil in the prototype) flows. Experiments addressing liquid (water) and gas (nitrogen) single-phase flows, as well as two-phase flows (air-water), are performed. CFD simulations are carried out, and empirical correlations are developed for the frictional pressure losses and two-phase pressure multiplier for the double helically coiled heat exchanger.
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