Ammonia nitrogen separation performance and kinetics from biogas slurry using vacuum distillation method

Abstract: Due to high ammonia nitrogen concentration, biogas slurry can contribute to air and water pollution through volatilization of free ammonia. Additionally, high ammonia nitrogen concentration may result in high phytotoxicity to plant germination and growth when biogas slurry is put into agricultural or horticultural applications. So ammonia nitrogen in biogas slurry should be removed. Conventional methods for removing ammonia nitrogen are based on gas or thermal stripping. However, these methods have low ammonia nitrogen removal rate and are time-consumed. Due to the higher ammonia mass transfer driving force ascribed to the lower ammonia partial pressure in gas phase, vacuum distillation has the advantages of high ammonia nitrogen removal kinetics constant and short time required. Furthermore, ammonia can also be recovered and enriched to act as the valuable carbon dioxide (CO2) absorbent to upgrade biogas. Therefore, in this study, ammonia nitrogen separation from biogas slurry by using vacuum distillation method was conducted in a rotary evaporator. And the key operating parameters including sodium hydroxide (NaOH) dosage, temperature and pressure were investigated and optimized. The first-order rate constant (k), ammonia nitrogen removal time constant (τ), ammonia nitrogen removal efficiency (η) and ammonia nitrogen separation factor (St) were adopted to evaluate the ammonia nitrogen removal performance. Results showed that when raw biogas slurry without pH adjustment was vacuumed, CO2 loading of biogas slurry reduced from 0.15 to 0.08 mol CO2/L under the conditions of 45 °C and 5 kPa. But the first-order rate constant and ammonia nitrogen separation factor were low and ammonia nitrogen removal time constant was very high. Increasing NaOH dosage to elevate the initial pH value of biogas slurry was positive for enhancing the first-order rate constant and ammonia nitrogen separation factor. Additionally, if high first-order rate constant value was targeted, high removal temperature and low operating pressure should be required. However, it will lead to the decrease of ammonia nitrogen separation factor value. So, the orthogonal tests were conducted in this study to optimize the operating parameters in order to obtain high the first-order rate constant and ammonia nitrogen separation factor values simultaneously. The results showed that the importance order for ammonia nitrogen removal performance was ranked as pH value > pressure > temperature. And when NaOH dosage was 15 g/L (pH=13.04), removal temperature was 35 ?C and pressure was 15 kPa, the comprehensive ammonia nitrogen removal performance was the best among all the experiments in the orthogonal tests. Under these optimal operating conditions, the first-order rate constant was 0.97 h-1, and ammonia nitrogen separation factor was about 395.96 when 90% of the total ammonia nitrogen was removed. It implied that when biogas slurry with high pH value was vacuumed, relatively high ammonia nitrogen removal performance may be achieved, and aqueous ammonia solution with higher concentration may be recovered as well to act as the potential CO2 absorbent to upgrade biogas. In addition, compared with the conventional ammonia nitrogen removal methods like thermal stripping (maximum first-order rate constant was 1.21 h-1) and gas stripping (maximum first-order rate constant was 0.86 h-1), vacuum distillation had a higher first-order rate constant value at the similar conditions. It meant that vacuum distillation method may achieve a relatively higher ammonia nitrogen removal rate and lower time required to reach the same removal efficiency. It should be noted that the first-order rate constant could be enhanced through adjusting temperature and pressure. These results reported in this study may provide a reference for the future research aiming at ammonia nitrogen separation from biogas slurry with high removal efficiency and low energy consumption.