Studies Of Adsorption Equilibrium And Dynamic Process Of Charge And Discharge Of Methane On Activated Carbon

The natural gas is popular among people because of its high calorific value, less pollutantsand use safety. Currently, the long distance pipeline transport of natural gas is widely used instorage and transportation of natural gas for civil use. However, for some small andmedium-sized cities far away from the natural gas origin, the pipeline transport is not economical.Natural gas adsorption on activated carbon （ANG） is being considered as an efficient method forstorage of natural gas domestically used, but development of the suitable adsorbent and efficientmeasure for inhibiting the thermal effect from adsorption and compression is still beingemphasized for further application.For development of adsorbent for adsorption of natural gas, Setaram PCT Pro E&E wasemployed to measure the isotherms of methane adsorption on the activated carbon SAC-02withspecific surface area of2074m²·g^-1over a temperature range of263.15K-313.15K and a pressurerange of0-9MPa. Dubinin and Astakhov’s equation （D-A equation）, Clausius-Clapeyronequation and Virial plot were used to fit and analyze the adsorption data. Results showed thatwhen the pressure is higher than0.08MPa, the modified D-A model could much accuratelypredict the adsorption data with a mean relative error of less than5%; the isosteric heat ofmethane adsorption on the activated carbon is13.99kJ·mol^-1-17.57kJ·mol^-1, which revealed theadsorption of methane on energy heterogeneous surface; the limit isosteric heat of methaneadsorption linearly changes with temperature, and the average value was19.43kJ·mol^-1.For development of storage vessel of natural gas domestically used, a storage vessel involume of1260ml with a ratio of length to diameter about33:20and a U shaped heat exchangetube in central region was designed. Charge and discharge tests of methane on the storage vesselwere carried out at ambient temperature and pressure3.5MPa under the flow rate5L·min^-1,10L·min^-1,15L·min^-1and20L·min^-1, comparison studies were then undertaken for analyzing thethermal effect from adsorption on the adsorptive performance of the vessel. Results show that,during the charge and discharge processes, the temperature of the adsorbent bed changes rapidlyand reaches the maximum/minimum within a short period in correspondence with about10%ofthe equilibrium time; the maximum increase/drop of the temperature within the central region isabout56.3℃/40.9℃; the axial temperature gradient is smaller than that of the radial due to thepoor heat conductivity of adsorbent bed packing with the activated carbon. Results also reveal that circulation of running water along the U shaped pipe can significantly prolong the dischargeduration and increase the discharge amount of the methane confined within the adsorption vessel.For accurately setting a model to predict the influence of the structural parameter andpacking density of the adsorbent as well as the discharge rate of the natural gas on theperformance of the storage system, a simplified2d heat and mass transfer model was developedin consideration of boundary conditions determined by the actual discharge process at flow rate15L·min^-1, and the CFD commercial software Fluent was then used to calculate the temperatureprofile at typical locations within the adsorbent bed. Results show that the results fromcalculation can meet those from experiments fairly well. Conclusions are drawn that the setmodel can further be used to study the thermal effect of the adsorbent bed during practicaldischarge process.