PSA Experiment And Simulation Based On Upgrading And Nitrogen Rejection Of Low-quality Natural Gas

Natural gas is considered to be the most widely used low-carbon and clean fossil fuel in the world,and can be used in many industries and fields such as city gas,industrial fuel,chemical industry and power generation.Due to natural existence or acquired exploitation,natural gas forms a mixture of various gases.In addition to the main component methane,nitrogen is a common inert component,which greatly reduces the calorific value of natural gas,increases its transportation cost,and limits the scope of application,therefore,it is necessary to remove nitrogen to achieve the purpose of purifying methane.Pressure swing adsorption（PSA）technology has low energy consumption,high operating flexibility and low investment cost,which started in the 1960s and has been widely used in air separation,hydrogen purification and biogas upgrading.For PSA,the selection of adsorbents and the design of process flow are very critical parts,and the combination of experiment and mathematical modeling is usually used in the research of these two parts.Because the mathematical model can provide a good window for understanding the dynamic adsorption and desorption behavior in the PSA process,it is convenient to evaluate the effect of the adsorbent and design and optimize the PSA process flow,finally achieving the purpose of guiding practice in theory.Aspen Adsorption is a simulation software specially used for adsorption and separation,with rich physical data and mathematical equations,easy and controllable modular operation,which greatly saves the cost of experiments.Some of the low-quality natural gas extracted has a high N₂ content,even reaching 25%of the total volume.Therefore,we selected a mixture of 75%CH₄/25%N₂ as the research object,and used pressure swing adsorption technology to purify the CH₄.Regarding the adsorbent for CH₄/N₂ separation,there are mainly two options,one is CH₄/N₂ type adsorbent with CH₄ as the heavy component,and the other is N₂/CH₄ type adsorbent with N₂ as the heavy component.In this work,two typical adsorbents in two categories:molecular sieve Silicalite-1 and metal-organic framework material MIL-100（Cr）will be selected respectively,and a suitable PSA process will be designed for them.The main research contents and conclusions are as follows:（1）In previous work,our research group found that the CH₄/N₂ type separation adsorbent,Silicalite-1 molecular sieve,has excellent CH₄/N₂ separation ability and high stability,and has been applied to the study of low-concentration CH₄ pressure swing adsorption purification,and so far there is no PSA study of CH₄/N₂ molecular sieves enriching methane from high ratio CH₄/N₂ mixtures.Therefore,Silicalite-1 will continue to be used as the adsorbent to remove N₂from low-quality natural gas,but CH₄,as the heavy component,can only be obtained from the bottom of the adsorption column,and the purity improvement is limited,needing to be designed an efficient PSA process.First,a mathematical model was established by Aspen Adsorption to simulate the breakthrough experiment curves of 75%CH₄/25%N₂ under different adsorption pressures（100 k Pa,200 k Pa and 300 k Pa）and feed rates（300 m L/min,500 m L/min and 700m L/min）,then obtain the dynamic adsorption isotherms of the adsorbent,and verify their accuracy through experiments by experimental comparison.Then,on the basis of the mathematical model,the three process parameters of adsorption pressure,feed rate and bed height in pressure swing adsorption were studied.The increase of the bed height,the increase of the adsorption pressure and the decrease of the feed rate of the feed gas will increase the bed utilization rate,and the improvement of the bed utilization rate is very beneficial to the improvement of the CH₄ purity.We designed a four-bed and eight-step PSA process,which can realize almost full utilization of the bed,enrich CH₄ in 75%CH₄/25%N₂ to 95%,and have a 99%recovery rate.（2）MIL-100（Cr）is a kind of MOFs material adsorbent.Due to the high concentration of its unsaturated metal site Cr（III）,it forms a feedbackπbond with N₂ molecules,which is very beneficial to the adsorption of N₂,and CH₄ as a light component,so it is very suitable for the purification of CH₄ in high ratio CH₄/N₂.MIL-100（Cr）is still in the laboratory scale,and this thesis is the first time to use this kind of CH₄/N₂ equilibrium separation adsorbent to carry out the research of PSA separation of high proportion CH₄/N₂.When the temperature is 283 K and the pressure is 100 k Pa,the N₂/CH₄ selectivity is about 8,and the N₂ equilibrium adsorption amount is as high as 1.64 mmol/g.Using a simple single-bed and three-step PSA process（pressurization,adsorption,and vacuuming）,CH₄ product gas with a purity of 98%and a recovery rate of 58%can be obtained from the raw material gas of 75%CH₄/25%N₂.Through research,it is found that the N₂ displacement process can replace the adsorbed CH₄,greatly improving the CH₄ recovery rate,and at the same time,it will not affect the purity of CH₄,but it should be noted that the too long N₂ displacement time will make the adsorption column impure,indirectly affects the purity of CH₄ in the PSA cycle.Finally,a three-step PSA with feed gas pressurization,N₂ replacement and vacuuming was designed,which can enrich CH₄in 75%CH₄/25%N₂ to 97%with a recovery rate of 92%.Based on upgrading and nitrogen rejection of low-quality natural gas,the pressure swing adsorption process was designed for two different types of adsorbents,and the effective purification of CH₄ was realized,which has high theoretical significance and experimental value.