Research On Calorific Value Adjustment Process Of Liquefied Natural Gas

The rapid development of the Liquefied Natural Gas(LNG)market has broken through the limitations of traditional pipeline gas transmission and promoted efficient circulation of natural gas in different markets around the world,which provides an opportunity for the rapid development of China's natural gas market,and natural gas is also converted from the traditional peak-regulating gas source to the main gas source.The international LNG trade generally adopts the calorific value measurement method,while the domestic LNG trade still adopts the volume measurement method,but the calorific value represents the value of LNG.The sale of LNG with different calorific values at a uniform price will affect the economic interests of natural gas users,and some special industries have stricter requirements for the calorific value of LNG,it can be seen that the difference in heat value leads to the contradiction between supply and demand,which in turn restricts the sustainable development of LNG.Due to the low temperature physical properties of LNG and the inevitable heat leakage of the storage tank,LNG absorbs heat and then generate boil-off gas(BOG),if there is too much BOG,it will affect the safe operation of the tank.If the BOG is directly discharged,it will cause waste of resources,therefore,it is necessary to consider recycling of BOG.Therefore,based on the concept of low calorific value or no calorific value fluid dilution LNG to reduce the calorific value,a jet mixer was used to realize the miscible mixing of LNG,BOG and liquid nitrogen medium,so as to design a experimental scheme of LNG calorific value adjustment and BOG reliquefaction combination,which realized the calorific value adjustment of high calorific value LNG and the recycling and reuse of BOG.The main work contents are as follows:(1)Process parameter calculation and process design of liquefied natural gas calorific value adjustment.Ensure that the nitrogen content of the finished LNG meets the national standard,when the raw LNG flow rate is 2687kg/h and remains stable,the liquid nitrogen medium and LNG mixture are simulated by Aspen Plus software,which concluded that the maximum flow rate of liquid nitrogen during mixing is 197kg/h.Through the calculation of process parameters,a combined process of LNG calorific value adjustment and BOG reliquefaction,an electrical system and an automatic control system for testing was designed.(2)The influence of structural parameters of jet mixer on the mixing performance of cryogenic liquid.A single-primary fluid inlet ejection mixing model of liquid nitrogen and LNG was established.The Fluent software is used to simulate the internalfluid of the mixer with different throat diameter and length,the distance from the throat to the nozzle and diffuser length.The internal flow field distribution of the mixer with different structure sizes was obtained and the mixing effect of LNG and liquid nitrogen was analyzed.A optimal mixer model with a throat diameter and length of 36 mm and 85 mm,the distance from the throat to the nozzle of 18 mm,and diffusion tube length of 170 mm was determined.On this basis,the ejection mixing model of the double fluid inlets was established.The mixing effect of the internal fluid of the ejection mixing is compared with the single-priming fluid inlet mixing model,The results show that the quantity of the secondary fluid inlets has little effect on the fluid mixing characteristics.(3)The influence of process parameters on LNG calorific value adjustment and BOG reliquefaction effect.The optimal mixing model of double fluid inlets was established.The mixing of LNG,liquid nitrogen and BOG are numerically simulated.Based on BOG condensation phase transition,the effects of BOG inlet velocity,temperature,pressure and low calorific value medium inlet pressure on fluid mixing and BOG condensation are studied.The main factor affecting the flow of mixed fluid and BOG condensation was BOG inlet velocity.When the BOG inlet velocity is between 1~10m/s,the gas phase of the finished LNG is increased with the increase of velocity and the temperature uniformity decreases;when the liquid nitrogen inlet velocity is between 0.03~0.135m/s,the gas phase fraction of the finished LNG decreases with the increase of velocity;the BOG inlet temperature is between120~150K,the jet mixer outlet pressure is between 0.05~0.2 MPa,the effect of fluid mixing performance and BOG condensation is small.