Study Of Flow,Mass And Heat Transter In The Adsorber And Separation Performance During RVPSA Oxygen Production Process

In this paper,based on the experiments and numerical simulation,the flow,mass and heat transter in the adsorber and separation performance during rapidvacuum pressure swing adsorption(RVPSA)oxygen production process werestudied.An one-dimensional model has been used to study the RVPSA processwith consideration of adsorption and heat effect,axial diffusion,and gas-solidconvection heat transfer.The RVPSA experimental apparatus was established,where the pressure,concentration and temperature were collected for the modelvalidation.Based the mathematical model and experiments,the effects ofadsorption and desorption pressure,pressurization and depressurization metholdson the flow,mass and heat transter in the adsorber and separation performancewere deeply analyzed.According to the study of the flow,mass and heat transter characteristicsduring the RVPSA process,the distributions and evolutions of velocity,concentration and gas temperature were obtained.The dynamic relationshipsbetween these parameter with time and gas pressure were also investigated.Itshowed that the response of velocity and gas temperature to pressure change wasfaster than the concentration.Moreover,there were independent velocity wave,concentration wave,and temperature wave in the adsorber.In these waves,thevelocity wave has the highest speed,and adsorbed phase nitrogen concentrationwave was ahead of the temperature wave during the adsorption step.As theadsorption progresses,the distance between concentration wave and temperaturewave gradually reduced,but there was still a certain distance at end of the step.Compared with the traditional pressure swing adsorption(PSA)oxygenproduction process,the velocity of concentration wave and temperature wavewere increased by 7.3%and 35.59%,respectively.And it was found that theresults were different from the PSA process,where the concentration wave andtemperature wave would be overlapped during the adsorption process.According to the analysis of the effects of adsorption pressure and desorptionpressure on the flow,mass and heat transter during the RVPSA process,theevolutions of velocity,concentration and gas temperature distribution withadsorption pressure and desorption pressure chagc were obtained.Moreover,the up-flow velocity was far larger than the allowable velocity during the feedpressurization step.There were sharper oxygen concentration wave,shorter masstransfer zone and higher temperature gradient with bigger ratio of adsorption anddesorption pressure.According to the analysis of the effects of pressurization anddepressurization metholds on flow,mass and heat transter in the adsorber andseparation performance during the RVPSA process,the evolutions of velocity,concentration and gas temperature distribution with different pressurization anddepressurization metholds were obtained.It was found that applying linearpressurization can effectively reduce local maximum velocity,while the feedpressurization metholds have little influence on the concentration and temperaturedistribution.When the blowdown time was long,the gas velocity was less than thelimit value except exponential depressurization.And the downward hyperboladepressurization method was recommended as the best choices for controllingtransient velocity at short blowdown time.The bed size factor(BSF)could beeffectively reduced by decreasing the vacuum desorption time.With decreasing ofthe vacuum desorption time,the gas velocity gradually increased.However,thegas velocity was less than the limit value except the exponential depressurization.The depressurization metholds have ob-vious effect on the concentration andtemperature distribution only at short desorption time(≤1s),and the evacuationdepressurization methold has little influence on the energy consumption.Whenthe blowdown and evacuation desorption was accomplish,applying pressurizationwith intermediate gas from product end and two-step pressurization withintermediate gas can push nitrogen concentration wave to the feed end,and theneffectively improve the oxygen purity of the product.Morever,the RVPSAprocess with two-step pressurization with intermediate gas has better separationperformance.The pressure and oxygen purity of intermediate gas beforepressurization were key parameters that affected the concentration andtemperature distribution.Based on the study of flow,heat and mass transfer characteristics in theadsorbers,the RVPSA process with intermediate gas pressurization from productend and the RVPSA process with two-step intermediate gas pressurization wereproposed to improve the separation performance.The oxygen purity of theproduct by the cycle with intermediate gas pressurization improved by 15.85%compared with the cycle without intermediate gas pressurization.And the oxygenrecovery by the RVPSA process with two-step pressurization with intermediategas improved approximately by 5%compared with the process with intermediategas pressurization from the product end.According to the analysis of the effect of process parameters on the performance,the relationships bewteen separation performance with cycle time and adsorption pressure and desorption pressure were obtained.Results of the current research will provide valuable references for understanding flow,mass and heat transfers inside the RVPSA adsorbers and process intensification.