| The amount of wastewater from urea plant is very large, which not only contains some ammonia and carbon dioxide, but also contains 0.5 wt%-2.0 wt% urea. If the wastewater which can not meet the emission standard is discharged, it will poison aquatic organisms and cause eutrophication. The ammonia and carbon dioxide can be removed by a simple stripping (desorption) process, but the urea in wastewater has to be removed by chemical decomposition. Removal of urea is the difficulty and key to this wastewater treatment. Thermal hydrolysis process, as a common treatment method in present urea plants, is carried out at an elevated temperature and pressure. In this method, the requirement for equipment material is relatively high and a large amount of medium pressure steam is supplied to heat the wastewater fed into hydrolyser. Moreover, there are some other methods reported for treating urea wastewater, such as biological, enzymatic and electrochemical methods, those are still under investigation and currently have some drawbacks which hinder the industrialization. Therefore, looking for a new efficient approach that is low energy consumption and investment is an important and urgent task. The experiments on heterogeneous catalytic hydrolysis of urea were conducted by using solid catalysts, aimed at finding a suitable catalyst that can be applied in urea wastewater treatment and exploring the mechanism of the catalytic hydrolysis of urea.Firstly, the catalytic hydrolysis of urea in wastewater was studied by using η-Al2O3 and α-Al2O3 as catalysts respectively. The effects of hydrolysis temperature, reaction time and catalyst dosage on the removal rate of urea were investigated, and the results show that η-Al2O3 exhibits obviously higher urea removal rate. Using η-Al2O3 catalyst, the urea concentration in wastewater is reduced to less than 1mg/L at 165℃ after 150 min. The reaction kinetics study shows that the hydrolysis of urea behaves as a pseudo first order reaction. From the calculated rate constants,η-Al2O3 exhibits an excellent catalytic activity for urea hydrolysis, whereas the activity of α-Al2O3 is considerably weak. In addition, the η-Al2O3 catalyst possesses good recycling ability. The slight loss of activity of η-Al2O3 should be attributed to the generation of a small amount of hydrated aluminas, and the activity can be recovered by calcination.Secondly, a series of catalysts were prepared by calcining gibbsite at different temperature (300,600,900,1200℃), and used to catalyze the hydrolysis of urea in wastewater. X-ray diffraction, N2-BET and FT-IR techniques were applied to characterize the crystal structure and surface properties of catalysts, and back titration and Hammett indicator method were used to respectively characterize surface hydroxyl amount and basicity of catalysts. The activity evaluation results reveal that the catalyst prepared at 300 ℃ (A1300) shows the highest activity. The activity of catalyst is closely related to its surface basicity, the larger the amount of base site is, the higher the catalytic activity is. In a certain extent, the basicity of catalyst is influenced by the surface hydroxyl amount. A1300 catalyst has a good reusability. The loss of activity should be attributed to the decrease of surface basicity. After use, the degree of crystallinity becomes higher, and the hydroxyl amount and the specific surface area decrease. This may be the reason why the surface basicity of A1300 decreases slowly with the increase of frequency of use. Furthermore, the degree of crystallinity becomes higher, the grain size increases and the specific surface area decrease should be attributed to the hydrothermal environment.Finally, varying the initial c(Mg2+), the MgAl-layered double hydroxides (MgAl-LDHs) catalysts were synthesized in the pores of η-Al2O3. The XRD shows that the crystal structure of MgAl-LDHs (c(Mg2+)=2.0 mol/L) is near perfect. When the initial c(Mg2+) is higher than 2.0 mol/L, an impurity phase of MgCO3 appears. The surface basicity increases with the increase of c(Mg2+), and reaches the maximum when the c(Mg) is 2.0 mol/L. The activity evaluation results show that the MgAl-LDHs with the largest amount of base sites exhibits the highest catalytic activity. During repeated use of catalyst, the activity of MgAl-LDHs firstly decreases and then trends to remain unchanged, and the surface basicity has the same trend. Furthermore, according to experimental and characterization results, a plausible reaction mechanism about catalytic hydrolysis of urea is proposed. MgAl-LDHs shows superior catalytic activity for urea hydrolysis compared to the other catalysts used in this research. |
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