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

Adsorption equilibrium isotherms of ammonia gas were measured at temperatures between 298 and 393 K on 13X zeolite, 4A zeolite, alumina, silica gel, and activated carbon. The applicability of these sorbents to ammonia gas separation 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 reaching its lowest point at 393 K: The two zeolites provide almost the same working capacity, 3.0-3.5 mmol g-l, over the entire temperature range. Silica gel and alumina showed low working capacities. The experimental equilibrium data were fitted to 16 different isotherm models, with accuracy and reliability statistically evaluated The Langmuir-Freundlich model with the van't Hoff equation for the equilibrium constant and with a thermal expansion equation 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 slightly higher predictions. The Henry and vacancy solution models provided the best fit for activated carbon.