Mercaptan Control with Hydrogen Peroxide
Thiols are found in many industrial wastes, both liquid and gaseous. In earlier literature, thiols were called mercaptans. Although thiols are toxic, they usually are found in very low concentrations and can be detected by smell in a concentration far below a toxic level. They are not considered hazardous, but rather a nuisance, since the odor detection levels are in the order of parts per billion (PPB).
Hydrogen peroxide has been shown to effectively oxidize thiols over a range of temperatures and concentrations. Catalysis is sometimes required for vigorous oxidation.
Sources of Thiols
In the Kraft pulping process, methane thiol is formed by the demethylation of lignin. In petroleum products thiols evolve from the decomposition of various sulfur containing compounds during the refining process.
Thiols find application in ore flotation and are used or formed as byproducts in many chemical manufacturing processes, i.e., pharmaceuticals, insecticides, wetting agents, plastics, rubber additives. Thiols are formed also by bacterial decomposition of organic materials typically found in the rendering industry and at sewage treatment plants.
Hydrogen peroxide is useful for the treatment of both liquids and gases which are contaminated with thiols.
Treatment of Liquid Effluents
If only higher thiols other than methane thiol are present, a mild oxidation according to the equation:
|2 RSH + H2O2 → RSSR + 2 H2O|
may be sufficient. The reaction occurs rapidly after mixing. The products are water insoluble disulfides which form an oily layer. This can be readily separated leaving behind only trace amounts of odorous material. The weight ratios of hydrogen peroxide to source thiols for the disulfide formation are:
|H2O2 / Thiol Wt. Ratio|
If methane thiol is present, or if a complete deodorization is desired, more rigorous oxidation is necessary. This requires all or some of the following: presence of catalyst, most conveniently in the form of a soluble iron salt (e.g., ferrous sulfate), excess hydrogen peroxide, and elevated temperatures.
Some experimentally determined conditions are described in the following examples:
Example 1 - A 1000 mg/L ethane thiol solution was treated at ambient temperature with an amount of hydrogen peroxide providing a 3% solution (30,000 mg/L). In the presence of 250 mg/L of iron catalyst deodorization of the mixture was complete in 40 minutes.
Example 2 - At 50C in presence of 250 mg/L of iron catalyst, thiol solutions are completely deodorized in less than 15 seconds with hydrogen peroxide in 5:1 H2O2: RSH molar ratio at pH 2. At pH 6 or above, deodorization is complete in less than one minute. The weight ratios corresponding to the 5:1 molar ratio for some common thiols are:
|H2O2 / Thiol Wt. Ratio|
Treatment of Gas Effluents
Treatments for contaminated gases are illustrated in the following pilot plant examples. Scaling up to commercial size can be done by standard engineering procedures.
Example 1 - Tests were made in a laboratory scrubber constructed of 2" diameter Pyrex pipe containing a 28" column of 1/4' ceramic Intalox saddles. The total gas flow was 15 L/min. The gas contained 1000 ppm by volume methane thiol. The liquid flowrate was 1.35 L/min. The liquid consisted of an aqueous solution containing 1.0 g/L or NaOH and 1 g/L H2O2. The pH of the solution was 11.9.
After liquid-gas contacting, the effluent gas contained 4 ppm methane thiol, a 99.6% reduction. The effluent liquid pH had dropped to 11.0 and the content of unoxidized sulfur compounds was below detection limits.
Example 2 - The same conditions were used as in the previous example, only the gas contained 8000 ppm by volume H2S and 200 ppm by volume methane thiol. The effluent gas contained no detectable H2S and 2 ppm by volume methane thiol. The effluent liquid had a pH of 11.2 and the content of unoxidized sulfur compounds was below detection limits.
Example 3 - The same conditions were again used only the gas contained 1000 ppm by volume ethane thiol, 1000 ppm by volume dimethylsulfide and 100 ppm thiophene. The effluent gas contained no detectable sulfur compounds. The effluent liquid had a pH of 11.9 and none of the original sulfur compounds were present. The effluent did contain approximately 5 mg/L of diethyldisulfide.
---FMC Technical Data, Pollution Control Release No. 10
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