Wet Scrubbing Oxidation Of Methyl Mercaptan By Peroxymonosulfate

Methyl mercaptan (CH3SH) is a representative acidic odorous compound.Because of its toxicity, irritation and corrosivity, it has become one of environmentalproblems to be solved urgently. There are many ways to remove odors. Chemicalscrubbing oxidation has been developed into a reliable process of removing odors.In our preliminary studies, hydrogen peroxide (H2O2), peroxomonosulfate (PMS)and persulfate (PS) were used in wet scrubber to remove CH3SH in strong alkalinesolution (pH=12,12.5or13). It was proved that the removal of CH3SH maintainedhigh levels of>95%by PMS in strong alkaline solution. However, CO2absorptionmay result in much more NaOH consumptions, which the higher the pH of aqueoussolution is, the higher the consumptions become.Therefore, the aim of this study is to find a method which removes CH3SH byPMS under weakly alkaline or acidic conditions. Effect of pH on CH3SH removal byPMS in a wet scrubber is discussed. Under weak alkaline condition, the influences ofexperimental parameters, buffer solution on CH3SH removal by PMS are also studiedand the application condition of this system is discussed. PMS is used to removeCH3SH in wet scrubber under acidic condition for the first time. The study comparesits efficiency of PMS alone with that of Co2+/PMS for the removal of CH3SH. Thesestudies will give supports to remove CH3SH under weakly alkaline or acidic condition.Specific studies and results are as follows:(1) Effect of pH on CH3SH removal by absorption liquid is critical. Theabsorption of CH3SH is more than75%at pH13. The absorption liquid with PMS canremove CH3SH effectively and more than95%of CH3SH is removed under strongalkaline condition (pH=11,12,13). When the pH value is≤12, there is a high outletof CH3SH gas. More than50%of CH3SH is removed by PMS wet oxidation underweakly alkaline condition (pH=8,9,10). CH3SH removal under acidic condition (pH=2,6) is poor. Compared with PMS, H2O2absorption liquid hardly remove CH3SH under weakly alkaline or acid condition.(2) Effects of experimental parameters on CH3SH removal by absorption liquidunder weakly alkaline condition (pH=9) are studied. The concentration of PMS orthe rate of liquid spray would make CH3SH removal increase. However, increasingthe gas flow rate leads to CH3SH removal decrease. When the gas flow rate is0.5L·min-1, CH3SH removal is100%. Besides, the greater the inlet CH3SHconcentration is, the higher the outlet CH3SH concentration is. When the gas flowrate is low and the gas residence time is long enough, PMS can remove CH3SHeffectively in a wide range no matter the inlet CH3SH concentration is low or high.(3) Under weakly alkaline condition, the control of pH is necessary. When thereis no buffer solution, the pH value will decrease from9to3.5in the process ofoxidation reaction and the removal ratio of CH3SH quickly reduces to60%. However,when the absorption liquid is instead of H3BO3-NaOH buffer solution, the pH valuechanges little and at the same time, more than91%of CH3SH is removed.CO32-/HCO3-may favor CH3SH removal by PMS oxidation. Compared withH3BO3-NaOH or phosphates buffer solution, CO32-/HCO3-buffer solution improvesCH3SH removal obviously at pH9which98%of CH3SH is removed in120min.However, there is only65%CH3SH removal in H3BO3-NaOH buffer solution.(4) The CH3SH odor removal by PMS or Co2+/PMS based wet scrubbingoxidation under acid condition (pH=2) is feasible and efficient. When the gas flow is0.5L min-1, PMS alone can remove94%of60ppm/v CH3SH in120minutes. CH3SHremoval efficiency can be drastically increased by reducing the gas flow rate toprolong the contact time between CH3SH(g)and the PMS liquid. Co2+concentrationinfluences the removal of CH3SH. Trace Co2+favors CH3SH removal. However,excess Co2+can lead to scavenger effect.