Treatment of wastewater containing methanol and H2S
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There is a stream of methanol-washing wastewater containing approximately 0.14% methanol, 0.13% CO2, and 0.23% hydrogen sulfide, in a volume of about 2 to 5 tons per hour; this wastewater needs to be treated before it can be discharged into the sewage treatment system. Because H2S is highly toxic, it needs to be pre-treated. Are there any good technologies or manufacturers in China that have production facilities? Please recommend them.
1. Stirring is used to remove and concentrate methanol, carbon dioxide, and hydrogen sulfide. It’s easy to do simulations for distillation; as for actual application examples, one would need to turn to experts for those. But it must be easy. The steam consumption is not high; it depends on the feed temperature, with a steam usage of 0.05 – 0.15 tons per ton of wastewater. If hydrogen sulfide and carbon dioxide are in dissociated form, sulfuric acid or hydrochloric acid must also be added to adjust the value to 5, or even below 4. 2. Using a support gas membrane process (membrane desorption-chemical absorption): Wastewater flows on one side of the membrane, while an aqueous solution of 10% MDEA flows on the other side; H2S is selectively removed, concentrated, and purified. Methanol is hardly removed; if 99% of hydrogen sulfide is removed, it is estimated that 30–50% of carbon dioxide will be removed. MDEA can be regenerated by thermal methods. Concentrated hydrogen sulfide can be fed into a Claus reactor to produce elemental sulfur. Sodium hydroxide, MEA, and DEA can also be used as absorption fluids, but their selectivity for H2S/CO2 is slightly lower than that of MDEA. This type of membrane device is used to recover ammonia or amines from water. The investment required for membrane devices designed to recover hydrogen sulfide is much lower than that for ammonia removal, as the relative volatility of hydrogen sulfide is much higher than that of ammonia. We can immediately provide membrane equipment for on-site demonstrations of selective removal and recovery of hydrogen sulfide. If you are interested, please contact: yjqin1@yahoo.com or yqin@chembrane.com. This post was last edited by yjqin1 on 2009-2-17 at 15:47.]
The best method for treating hydrogen sulfide is to add ferrous salts, which forms a precipitate that can be removed through solid-liquid separation. Its ionic state can also be altered through oxidation, thereby making it non-toxic. Methanol can be recovered through stripping concentration, but the cost is high. With membrane treatment, there is some concern that organic contamination may shorten the membrane’s lifespan. Methanol can be processed without issues through anaerobic decomposition and aerobic treatment.
There are generally three methods for treating sulfur-containing wastewater: (1) Alkali absorption method: Under acidic conditions, sulfide ions are converted into hydrogen sulfide gas, which is then absorbed by sodium hydroxide solution to produce sodium sulfide for reuse. By adjusting the pH to 2–3 with hydrochloric acid, the removal rate of sulfides is 50–60%. The equipment has high sealing requirements and suffers from severe corrosion. (2) Precipitation method: Ferrous sulfate is used as a precipitant to convert sulfide ions into insoluble sulfide precipitates, which are then removed. Or sulfites can be added to produce elemental sulfur. 1. A large amount of ferrous sulfate is added, resulting in fine precipitates; moreover, the amount of sediment generated is substantial, making it difficult to handle. 2.2S2- + SO32- + 6H+ = 3S + 3H2O; this method requires the presence of corresponding sulfite wastewater in order to achieve waste-to-waste utilization and reduce costs. (3) Oxidation method: This includes air oxidation under heating conditions, catalytic oxidation, and electrochemical oxidation. Currently, catalytic oxidation is the most commonly used method, which involves using oxygen from the air to oxidize sulfide ions into thiosulfates and sulfates under the action of a catalyst. Commonly used catalysts include quinone compounds and metal salts such as manganese, copper, iron, and cobalt. 1. Wet oxidation method: This method requires high temperatures and pressures, making it impractical to use in reality. (265℃ ； 7MPa ； Gas/water = 200) 2. Electrochemical method, whose process is still the reaction to produce FeS precipitate. 3. Catalytic oxidation method: 2S2- + 2O2 + H2O = S2O32- + 2OH-, generally carried out under alkaline conditions with manganese sulfate as a catalyst.
The support gas membrane method I’m talking about (membrane desorption-chemical absorption) represents a revolutionary improvement over the alkali solution absorption method discussed on the 4th floor; it uses just one piece of equipment to achieve the function of two towers (one for desorption and one for absorption), eliminating the need for gas circulation and thereby reducing the size of the equipment. Since plastic equipment is used, there are no corrosion issues. The contaminants of this type of membrane are mainly oxidants, surfactants, suspended solids, oils, and nearly saturated salts. Organic solvents in dilute aqueous solutions, such as methanol, have almost no effect on membrane operation.