Abstract
Supercritical fluids are attractive solvents for heterogeneous processes, including catalysis and adsorptive separation. However, adsorption processes in the nearโcritical region are poorly understood and exhibit unique behavior where the adsorption of the supercritical solvent plays an important role in the solute adsorption. The behavior of supercritical fluids in confined pores has been studied theoretically, but there are few experimental data on their behavior in industrially important microporous materials. The adsorption of carbon dioxide on Calgon F400 activated carbon over a wide range of pressures (0โ20 MPa) at temperatures near the critical point of carbon dioxide (30 to 45ยฐC) was studied. Nearโcontinuous adsorption and desorption isotherms were measured with a new flow gravimetric apparatus with precise control over pressure and temperature. As pressure is increased, the excess adsorption increases sharply at low pressures; then a broad maximum is observed. At temperatures greater than the critical temperature, there is a sharp drop in excess adsorption near the critical region where the density of the bulk fluid increases sharply. A crossover is observed near the critical region where, below a certain pressure, the excess adsorption decreases with temperature, while above the crossover point the trend is reversed. When analyzed as a function of solvent density, the crossover disappears, revealing an anomalous maximum in total adsorption near the critical point similar to the enhanced local density or โcharismaโ observed in binary soluteโsupercritical fluid systems. A 2โD EOS model using the 2โD PengโRobinson EOS was able to qualitatively describe the adsorption behavior over the entire pressure range, but the quantitative agreement was poor in the nearโcritical region.