The Research Of Adsorption Model For Methane/Hydrogen And Thermal Effects On Storage Process

Storage by adsorption of hydrogen/methane on carbon-based material such as activatedcarbon has a potential application while comparing with other storage methods. However, inpractice, adsorption storage still faces such critical issues as the low storage density and thermaleffect of the storage system on the adsorption process. This paper carries out the research infollowing aspects:(1) Analysis of adsorption equilibrium model of supercritical gas on activated carbon. AjaxActivated carbon which has a specific surface area about1200m2g1was selected as anadsorbent, isotherms of excess adsorption amount of methane were measured at temperaturefrom268.15K-338.15K and pressure up to12.5MPa. Comparisons were made betweenexperimental data and those predicted by Langmuir, Langmuir-Freundlich and Toth models.Isosteric heats of methane adsorption on Ajax activated carbon were determined by adsorptionisosteres on the absolute amounts. Results show that Toth equation performs well in predictingthe equilibrium data in the whole pressure range0-10MPa, but the results fromLangmuir-Freundlich equation have higher prediction accuracies while the pressure gets higher.The adsorbed phase densities of supercritical methane on the activated carbon vary withequilibrium temperatures and pressures, the mean value of isosteric heat of adsorption set byabsolute amounts is about15.72kJ mol1, which is smaller than that plotted from the excessamounts.(2) Analysis of the influence of temperature variation on the isosteric heat of hydrogenadsorption on activated carbon. A kind of coconut shell-type SAC-02activated carbon, whichhas a specific surface area about2074m2g1, was selected as an adsorbent. Setaram PCT ProE&E was employed to measure adsorption isotherms over a temperature region respectivelyfrom77.15K-293.15K for pressure up to11MPa. The superheated liquid assumption presentedby Ozawa was used to calculate the density of hydrogen molecules confined within the adsorbedphase. Adsorption isosteres of hydrogen were plotted by the determined absolute adsorptionamount to set the isosteric heat of hydrogen adsorption. Results show that the mean value of theisosteric heat of hydrogen adsorption on the SAC-02activated carbon in the whole temperatureregion was5.62kJ mol-1. Conclusions were drawn that adsorption equilibrium data in the sametemperature range as that of the hydrogen adsorption storage system should be used to determine the isosteric heat of adsorption for analyzing the thermal effect during the adsorption process.(3) Analysis of dynamic characteristic of ANG system under typical flow rate for chargeand discharge test. For the development of suitable family environment adsorption gas (ANG)storage tanks, a cylindrical tank in volume of1.5L was designed and equipped with a spiral heattransfer tube and radial perforated tube. SAC-02activated carbon was also selected as anadsorbent. Tests were undertaken under ambient temperature and equilibrium pressure3.5MPaby the flow rate15L/min. Comparisons were made of the effect of application of perforated tubeand circulation of water within the tube on the performance of ANG system in terms oftemperature distribution of adsorption bed, the total amount of charge and discharge and therecycling efficiency of charge and discharge of the storage system.Results show that the effects of adsorption heat are easily conducted by using perforatedtube for charging and discharging the gas. The thermal effect are also obviously lessened bycirculating water along the spiral heat transfer tube; the lower temperature of the circulatingwater, the greater amount of in-charged methane; the higher temperature of the heating water, thelarger amount of discharged methane.(4)Analysis of energy balance and Simulink simulation of the charging process. Energybalance of the storage system was evaluated based on the assumption on isothermal adiabaticprocess of the charging process, an ideal gas state of methane, uniformly continuous medium ofactivated carbon. It shows that, under the flow rate15L/min at temperature303K and pressure3.5MPa, the contribution of the adsorption heat amounts to94.6%of the total amount of energyobtained in the system, and that from the compression among the charge-in methane is only5.4%.In simulation, the total amount of charge-in methane is0.11006kg which is from simulationalso greater than0.07681kg from experiment. Results show that the tank pressure and meantemperature variations agrees well with those from the experiments. Simulink can be in someway used to simulate the charging process of ANG adsorption tank.