Comparison of sorbents and isotherm models for NH3-gas separation by adsorption

Adsorption equilibrium isotherms of ammonia gas were measured at temperatur es between 298 and 393 K on 13X zeolite, 4A zeolite, alumina, silica gel, a nd activated carbon. The applicability of these sorbents to ammonia gas sep aration was compared based on equilibrium data. In the pressure range of I to 100 kPa activated carbon has its highest working capacity (5.5 mmol . g( -1)) at 298 ii: and the working capacity drops rapidly with temperature rea ching its lowest point at 393 K: The two zeolites provide almost the same w orking capacity, 3.0-3.5 mmol g-l, over the entire temperature range. Silic a gel and alumina showed low working capacities. The experimental equilibri um data were fitted to 16 different isotherm models, with accuracy and reli ability statistically evaluated The Langmuir-Freundlich model with the van' t Hoff equation for the equilibrium constant and with a thermal expansion e quation for the saturation sorbate concentration provided the most accurate fit for the 13X and 4A zeolites. This model was also very accurate for the alumina and silica gel data, even though the Dubinin-Astakhou model gave s lightly higher predictions. The Henry and vacancy solution models provided the best fit for activated carbon.