Toluene removal from waste air stream using MgO/GAC and MnO/GAC catalysts in catalytic ozonation process

Toluene is one of the volatile organic compounds and used extensively in industrial processes. Because of toluene effects on human health and environment, it is necessary to implement methods for toluene removal from contaminated air stream. Catalytic ozonation process (COP) is one of the emergent technologies for removal of volatile compounds from air streams. Catalytic ozonation process is an efficient, cost-effective and easy to operate for elimination of organic compounds. One of the main factors directly affecting the VOC removal efficiency and ozone destruction rate is the type of catalyst. In this study the potential of MnO/GAC and MgO/GAC catalysts was investigated for toluene elimination from air stream in the presence of ozone in a lab scale.
The MnO/GAC and MgO/GAC catalysts were prepared by Sol-gel method and then characterized by BET, XRD and SEM analysis. Specific concentrations of toluene in the air were produced by using an air pump and a syringe pump. Phocheck was used to measure the concentration of toluene in gas phase. The effect of retention time (0.5-4 Sec), inlet toluene concentration (100-400 ppmv) and air temperature (25-100˚C) were investigated on catalysts efficiency in the presence of ozone. Ozone dosage (0.1 g/h) kept constant in all experiments. The efficiency of MnO/GAC and MgO/GAC were determined from the breakthrough time and removal capacity and the results were compared statistically.
The BET surface area derived from N2 adsorption- desorption isotherms. From the results the BET surface area of MnO/GAC (1103 m2/g) was greater than that of MgO/GAC catalyst (1082 m2/g). XRD patterns clearly illustrate formation of MgO and MnO crystals on GAC surface. From XRD patterns the peaks at 2θ degrees of 50.3° and 73.8° were related to the MgO crystals. The peaks at 2θ degrees of 42.1°, 51.8° and 70.9° reveal the formation of Mn3O4. The crystallite phase of MgO and MnO was hexagonal and tetragonal respectively. The average size of MgO and MnO crystals was 10 and 12 nm, respectively, calculated using Debby-Scherrer equation. Results of experiments in different retention times showed that breakthrough time of MnO/GAC and MgO/GAC catalysts increased 11.3 and 13.9 times, respectively, by increasing retention time from 0.5 to 4 Sec. When inlet toluene concentration increased from 100 to 400 ppmv, the breakthrough time of MgO/GAC and MnO/GAC columns decreased 65% and 62.2%, respectively. In contrast, removal capacity of MgO/GAC and MnO/GAC was increased 39.1% and 50.4% by increasing inlet toluene concentration from 100 to 400 ppmv, respectively. The efficiency of MgO/GAC and MnO/GAC catalysts was increased 78.3% and 31.5% by increasing air temperature from 25 to 100 ˚C, respectively. These results demonstrates that increase of retention time and bed temperature could positively effect on the performance of MgO/GAC and MnO/GAC catalysts in toluene removal from waste air stream in the presence of ozone.
It can be concluded that MgO/GAC and MnO/GAC catalysts had high potential in VOCs removal from air stream in the presence of ozone. The difference between the efficiency of MgO/GAC and MnO/GAC catalysts was significant (P ≤ 0.05) and MgO/GAC catalyst had higher efficiency than MnO/GAC for toluene removal from waste air in the presence of ozone.