Study On Influence Of BOG Recycle In LNG Process

As the low boilling point of nitrogen in natural gas, the existence of boil off gas(BOG) isinevitable in the LNG process. The generation ratio and components of BOG are changedwith different LNG process parameters. In this paper, the natural gas component wassimplified into nitrogen and methane, and some process parameters affect the generation ratioand components of BOG, which were calculated quantitatively. The calculation resultsindicate that nitrogen concentration of feed gas and the temperature in the inlet of throttlingvalve are sensitive factors of BOG problem. As for the generation ratio of BOG, it increasesby3.4%while the temperature in the inlet of throttling valve increases by5℃, and1.12%while nitrogen concentration of feed gas increases by1%In this paper, a model was set to send BOG back into coldbox, where BOG mixed withthe feed gas at the same time. When the natural gas in the coldbox was liquefied, BOG wasrecycled. Different BOG recycle ratios were set to discuss the influence of recycle BOG onthe LNG process. It shows that, the recycle BOG improve nitrogen concentration of NG in thecoldbox and LNG. While the process works at its full load, BOG recycle will reduce its feedgas handing capacity. More over, the process works at under load, BOG recycle will improveLNG output without cutting down the feed gas handing capactiy. In particular,() the mass flowand the component of LNG output are the same as the feed gas while all BOG is recycled.The component of mixture refrigerant should be adjusted with the change of NGcomponent in the coldbox. Therefore, genetic algorithm was applied to optimizing themixture refrigerant component and mole flow of BOG recycle conditions in SMR process.The refrigerant components after optimizing can ensure the safety of process operation, andthen the optimizing result has been fitted. The opimizing results indicate that the BOG recycleslightly decreases process exergy efficiency：exergy efficiency is cut down by0.9%while thetemperature before throttling(TN3) is-160℃，and1.6%while TN3is-155℃, which can beconsidered as changeless in engineering. Additionally, Simple economic accounting ismeasured, which shows that the recycle BOG can increase the plant profit.Plate-fin heat exchanger is widely aaplied in LNG process. In this paper, manufacturingdata of SMR process was analysed, with the heat transfer correlation for hydrocarbons, the heat transfer coefficient of plate-fin heat exchanger was calculated in condensing and boilingregion. The condensing heat transfer coefficient is about ten times of the boilling heat transfercoefficient. With Bell-Ghaly formula correlation, the condensing heat transfer cofficient ofnatural gas was determined to indicate how condensing heat transfer cofficient change withthe nitrogen concentration of NG. BOG recycle expands the temperature glide interval of NG,and condensing heat transfer shifts to lower temperature.