|Title:||Removal of methyl mercaptan from foul gas by in-situ production of ferrate(VI) for odour control|
|Subject:||Sewage disposal plants -- Odor control.|
Sewage -- Purification -- Oxidation.
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Civil and Environmental Engineering|
|Pages:||xxi, 174 p. : ill. (some col.) ; 30 cm.|
|Abstract:||Offensive odour from sewage treatment works and other municipal waste management facilities generally cause nuisance to the adjacent neighbourhood. The growing awareness of people to odorous emissions has led governments set limits of odour exposures of the neighbouring residents. Methyl mercaptan (CH₃SH) is a representative member of sulphur species with a very low odour threshold of around 0.4 ppb/v. Compared with most dry processes, some wet processes are more suitable for treating these wet odorous gases in sewage treatment works. Ferrate(VI) as a powerful oxidant has a significant selectivity to oxidize sulphur-containing compounds rapidly. Using in-situ produced ferrate(VI) not only saves time and money on ferrate(VI) purification, but also saves steps in the transportation and storage of ferrate(VI) chemicals. This study demonstrated a new odorous gas treatment approach in a wet scrubbing/oxidation system with in-situ generation of ferrate(VI). In this approach, gaseous CH3SH can be quickly absorbed by aqueous alkaline solution and rapidly oxidized by liquid ferrate(VI) generated through electrochemical synthesis in-situ. Firstly, the electrochemical generation of ferrate(VI) in aqueous NaOH solution was studied. The related experiments demonstrated that the maximum current efficiency to generate ferrate(VI) occurred in 14 M NaOH solution, with the applied current density of 14.06 mA cm⁻². The self-decomposition of ferrate(VI) in such strong alkaline solutions was then studied. The results showed that the behaviour of ferrate(VI) was more stable in the stronger alkaline solution. This was the first time that the reactivity of ferrate(VI) with CH₃SH in the highly-concentrated NaOH solution had been investigated under different reaction conditions. The experimental results confirmed that CH₃SH can be fully oxidized by ferrate(VI) to sulphate ion (SO₄²⁻) as a final product. It has been confirmed that the second-order reaction model is suitable to describe the kinetics of ferrate(VI) reaction with CH3SH in the strong alkaline solution. Additionally, stoichiometry of ferrate(VI) reaction with CH₃SH in aqueous solution was determined with a minimum molar ratio of 2.20:1 (Fe(VI):CH₃SH) to fully destruct CH₃SH effectively and a higher molar ratio of 4.53:1 to completely convert CH₃SH to SO₄²⁻.|
Secondly, research effort was further focused on the kinetic models of the process named "In-situ Ferrate(VI) Oxidation (IFO)". Two kinetic models to describe the in-situ ferrate(VI) generation and its reaction with CH₃SH were established mathematically by considering three main reaction mechanisms of ferrate(VI) electrochemical generation, ferrate(VI) self-decomposition and CH₃SH degradation in aqueous strong alkaline solution. The effects of three key factors: (i) NaOH concentration, (ii) applied current density, and (iii) initial CH₃SH concentration on the performance of the IFO process were investigated in three sets of experiments, and the kinetic models were validated by fitting the experimental data. The goodness of the fittings demonstrated that the new models could well describe both the kinetics of ferrate(VI) generation reaction and CH₃SH degradation reaction. The experimental results confirmed that the higher NaOH concentration and current density applied would be beneficial to ferrate(VI) electrochemical generation and also the elimination of its self-decomposition. The experiments also demonstrated an optimum NaOH concentration to achieve the best CH₃SH degradation performance at 10 M. This is a breakeven balance between the ferrate (VI) oxidation potential and ferrate(VI) generation rate in such an IFO system. Finally, to evaluate the performance of this wet scrubbing/oxidation process with in-situ electro-generation of ferrate(VI) for removing odour from foul gas, a series of experiments were conducted. In this process, CH₃SH in synthetic gas is removed through absorption and oxidation by contacting the gaseous stream with aqueous NaOH electrolyte that offers in-situ generation of ferrate(VI). The process parameters including electrolyte concentration, applied current density, initial CH₃SH concentration, gas flow rate and gaseous CH₃SH compositions were investigated to conclude on the best operating conditions and design parameters for a scale-up design. Under all the experimental conditions, gaseous CH3SH was completely removed by the wet scrubbing/oxidation process at room temperature and atmospheric pressure. The process proved to be quite effective for gaseous CH₃SH removal under the optimum experimental conditions at NaOH concentration of 6 M in electrolyte, current density of 2.22 mA cm⁻² and CH₃SH loading below 30 g m⁻³ h⁻¹. A very short contact time of 0.06 s between the gas and the liquid phases was achieved in this in-situ ferrate(VI) generation reactor. In summary, this research optimized the electrochemical production of ferrate(VI) and studied the reactivity of ferrate(VI) with CH₃S- in details. Furthermore, kinetic models were successfully established and validated by the results of CH₃S- degradation experiments. Based on the above work, gaseous CH₃SH removal was achieved by this wet scrubbing/oxidation process. From this study, we had better understanding of the removal of methyl mercaptan from foul gas by in-situ production of ferrate(VI) for odour control.
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