Selection Of Co₂ High-efficient Adsorbent And Adsorption Characteristics Investigation In LNG Pretreatment Project

With the rapid economic growth,my country’s demand for LNG has increased sharply.The CO₂in the feed gas will corrode equipment and pipelines.Therefore,it is necessary to ensure that the CO₂concentration in the natural gas before liquefaction is less than 50 ppm.The adsorption method has the advantages of high decarburization depth and simple process,and is considered to be the most potential pretreatment method for offshore LNG plants.The adsorption capacity of the adsorbent directly determines the purification effect.Therefore,the development of an adsorbent with high adsorption capacity and good separation performance is the key to solving the pretreatment of CO₂removal by adsorption.In this paper,13X molecular sieve is used as the basic adsorbent,the 13X molecular sieve is modified by a combination of simulation and experiment,the adsorption performance of the adsorbent before and after the modification is studied,and a high-efficiency adsorbent is finally screened out.Simulate the pressure swing adsorption of feed gas in the modified adsorbent under the engineering background,and discuss the influence of bed packing density,mass transfer coefficient,pressure and other conditions on natural gas purification and CO₂removal to guide engineering applications.In this paper,7 metal ions such as Li⁺,K⁺,Ca²⁺,Cu²⁺,Ni²⁺,Mg²⁺,Ag⁺are used to modify13X molecular sieves.First,Monte Carlo method is used to simulate the CO₂and CO₂/CH₄mixture in the metal ion modified molecular sieve.Static adsorption,comprehensively compare the adsorption capacity and selectivity coefficient of the modified molecular sieve,it is concluded that the Li X molecular sieve has the highest CO₂adsorption capacity,and the influence of the metal ion exchange degree on the CO₂adsorption capacity is discussed,comprehensively considering the adsorption capacity and the feasibility of ion exchange,Confirm that the ion exchange degree of Li⁺is 70%.In order to further improve the selectivity coefficient of the adsorbent for CO₂,MEA,MDEA and DEA were introduced to modify the Li X molecular sieve for the second time.The results are as follows:After the introduction of amine groups,the selectivity coefficient of the molecular sieve increases,and the adsorption capacity of the MEALi X molecular sieve for CO₂And the highest selectivity coefficient.Then study the effect of MEA loading on the adsorption effect.When the MEA loading is 8%,the molecular sieve has the highest CO₂selectivity coefficient and good adsorption capacity.Therefore,it is determined that the MEA loading is 8%and the Li⁺exchange degree is 70%MEALi X molecular sieve is the best modified adsorbent.Then the best modified molecular sieve was synthesized through experiments,and CO₂static adsorption and dynamic penetration experiments were carried out on the molecular sieve before and after modification.When the pressure is 5MPa,the static adsorption capacity of CO₂in the MEALi X molecular sieve is 1.55 mmol/g,and the adsorption capacity is increased by 63%compared with that before the modification.Study the dynamic penetration of mixed gas in 13X molecular sieve and MEALi X molecular sieve at different temperatures.As the temperature increases,the initial penetration time of MEALi X molecular sieve gradually increases.At 30°C,the initial penetration time of the modified molecular sieve adsorption bed is 30 minutes.After the temperature rises to 40℃,the CO₂concentration of the MEALi X molecular sieve bed reaches50ppm after 37 minutes at the outlet of the adsorption tank,and the CO₂separation factor is as high as 8.77,which is about twice that of the 13X molecular sieve.Through experimental research,it is found that MEALi X molecular sieve has good recycling performance.Use Aspen Adsorption software to fit the dynamic penetration experimental data with a flow rate of 100ml/min to obtain adsorption isotherm parameters and mass transfer coefficients,establish a dynamic penetration adsorption model,and simulate the dynamics of 300ml/min mixed gas in the adsorption bed.Through penetration,it is found that the simulation results are basically consistent with the experimental results,which provides a basis for the establishment of a large flow adsorption model.The process parameters of the adsorption tower under the engineering background were calculated,and the pressure swing adsorption model was established.It was found that the product gas CO₂concentration reached 50ppm after 307min when the gas processing capacity was 1×104Nm³/d,indicating that the modified adsorbent can meet the practical engineering application.Finally,the influence of pressure,mass transfer coefficient and bulk density on the adsorption effect is explored to provide effective guidance for the adsorption process.