Simulation And Optimization Of VPSA Process For N₂/CH₄Separation

Separation of CH4/N2was considered as a challenge due to the similarity of thetwo gases in physicochemical properties. Among the several available methods, PSAmethod with activated carbons as adsorbent is suggested to perform the best. In thispaper, this process was researched in both simulation and optimization. On one hand,the models of PSA were improved to be more close to the actual. On the other hand,the PSA operating conditions were optimized to achieve energy saving and raise therecovery of product.In the study of the mathematic model of PSA, most of the simplifications inprevious literatures were abandoned. Sub-models were establish for all the PSA units.Strict mathematical model with auto-switching boundary conditions were establishedfor fixed-bed adsorption bed as well as auxiliary devices of PSA system. Experimentsusing a three-bed PSA pilot plant were carried out to verify the model. By changingthe outlet flow rate, a series of experiments data with different purity and recovery ofmethane were obtained. Comparing with the simulation results, the experiment resultshowed little difference, which proved the accuracy of the model.Solution and sensitivity analysis strategies of PSA process were studied. In thesolving of models, finite difference method was used to discretize the partialdifferential equations, and method of lines (MOL) was used to solve the differentialalgebraic equations. Sensitivity analysis is a key part of the optimization process,which can analyze the impact of various parameters on the results. In this paper, thesub-models of each unit were established accompanying with the sensitivity analysismodel and solved simultaneously. The mathematical derivation proved its feasibility.On the basic of the sub-models and sensitivity analysis, reduced sequentialquadratic programming (rSQP), was used to optimize a three-bed VPSA process forN2and CH4separation. The optimization goal is to maximize the recovery rate ofmethane (R) and minimize of the energy consumption of the process (W) on thecondition of meeting the required product purity (P). The raw gas component is set toN2/CH4=70%/30%. Under initial conditions, the purity of methane: P=78.8079%,the objective function: R=83.3499%，W=0.163417Kwh/(m3raw gas). After9iterations, Purity of methane in the product gas reached80%, and the objective function value was also relatively stabilized after20iterations, and gradually changed.The value of objective function was R=95.0725%，W=0.188026Kwh/(m3raw gas).After28iterations, the process reached the optimal conditions. The optimizationresults was P=80.0657%，R=97.0049%，W=0.186949Kwh/(m3raw gas). The resultshows that this method is suitable for the optimization of PSA process. The purity ofCH4reaches80%with14%improvement of recovery. Study the impact of variousparameters on the process according to the sensitivity of the data at the same time,provides a theoretical basis for the pressure swing adsorption process.