Studies On Adsorption Behavior Of CO₂ On Porous Solids Near The Critical Temperature

Starting from the classical adsorption theories, the state of research on supercritical adsorption was reviewed in detail. It was pointed out that only few work had been done on adsorption near the critical region ;however it was important for both theoretical and practice purposes. So studies of adsorption behavior near the critical region of carbon dioxide were carried out on porous materials presently.As was observed, little fluctuation in temperature or pressure would cause significant effect on the experimentally measured isotherms in the critical region. To provide the reliability of measurements , the precisions of temperature and pressure measurements in our adsorption apparatus was among the highest levels reported in the literature. The adsorption equilibria of CO2 crossing the critical temperature were measured on three kinds of adsorbents , microporous activated carbon (YN26), mesoporous silica gel (SG1) and macroporous silica gel (SG2). The temperature range covered 253.15K~360K, and pressures up to18MPa. Characterization of adsorbents were conducted by using nitrogen adsorption at 77k and the adsorption of carbon dioxide at 273.15k Adsorption isotherms of CO2 on microporous adsorbent showed type I feature at temperatures both below and above the critical. The maximum was observed in isotherms at 298.15k and 292.15k which were below the critical temperature. Although multi-layer adsorption seemed possible near the critical region, mono-layer adsorption was dominating. The absolute adsorption was determined by the method proposed by zhou et al, as consequence both the volume and density of the adsorbed phase were evaluated. The adsorption equilibrium data of CO2 spanning critical temperature were successfully predicted by two models. One was the valid strategy proposed by zhou et al recently, the other was the Simplified Local Density (SLD) model which could deal with the excess adsorption obtained by experiment directly, without determining the absolute adsorption.Isotherm transition from type IV to type I on crossing the critical temperature was experimentally observed for the first time at the adsorption of carbon dioxide on mesopore adsorbent. The adsorption mechanisms of carbon dioxide on SG1 at sub-critical temperature would pass three phases: mono-layer, multi-layer and capillary condensation consecutively. Near the critical temperature, mono-layer wasoccurred at low pressure ,and followed by multi-layer adsorption as the pressure increased. However only mono-layer adsorption existed at temperature much above the critical. Comparing the adsorption behavior of CO2 on SG1 and SG2 for the critical region, it was found that the wider the pore diameter the larger the number of layers in the adsorbed phase, and the absolute adsorption and the volume of the adsorbed phase decreased sharply with increasing temperature .It was believed that such phenomenon was related to the special property of fluid near the critical temperature and pore structure of adsorbents.Enlightened by Dubinin-Polanyi theory and the zhou's strategy, a new method basing on Generalized Adsorption Isotherm (GAI) was put forward to predict the adsorption equilibrium data. Basing on theoretical calculation of solid-gas potential energy and the experimental adsorption isotherm at only one temperature, this method allowed the adsorption equilibrium data of CO2,CH4,H2 on different kinds of carbon be predicted for pressure up to 2Mpa and covering a temperature region of 100 K.