Ammonium ion exchange in fluidised bed reactors: effects of expansion ratio & layer composition

This study evaluated natural clinoptilolite as a sorbent for ammonium removal in fluidised-bed ion-exchange systems, with emphasis on the comparative performance of monodispersed single-size beds and polydispersed mixed-size beds. Clinoptilolite was wet-sieved into four particle-size fractions between 250 and 600 µm and converted to the sodium form by NaCl conditioning. Fifty-six breakthrough experiments were conducted in a 21.9 mm × 100 cm column using a fixed bed mass, an influent concentration of 25 mg/L ${\mathrm{NH}}_4^{+}$-N, and inlet flow rates of 50-456 mL/min. Performance was assessed using the breakthrough/total capacity ratio, breakthrough capacity, bed usage rate, and overall mass-transfer coefficient. Under equal bed mass and comparable flow conditions, polydispersed beds generally outperformed monodispersed beds. The median breakthrough/total capacity ratio increased from 0.30 to 0.46, while breakthrough capacity increased from 138 to 249 mg. Polydispersed beds also showed an approximately 40% lower bed usage rate, indicating more efficient sorbent utilisation. Overall mass-transfer coefficients ranged from 5.0 to 10.1 × 10^-5 m/s and did not differ significantly between bed configurations. This suggests that the improved removal performance was mainly attributable to increased external particle surface area and more favourable packing, rather than to changes in intrinsic kinetics. The Thomas and Adams-Bohart models described the breakthrough curves well and produced comparable exchange-capacity estimates for both bed types. Overall, the results show that particle-size distribution and fluidisation conditions strongly influence ammonium removal in clinoptilolite fluidised beds and provide practical guidance for design and scale-up, especially where compact and operationally efficient nutrient-removal units are required.