Refinery Sulfides Oxidation

?????????????Sulfide is found throughout the environment as a result of both natural and industrial processes. Most sulfides found in nature are produced biologically (under anaerobic conditions) and occurs as free hydrogen sulfide (H2S). Manmade sources of H2S typically occur as a result of natural materials containing sulfur (e.g., gas and oil) being refined into petroleum products. For a variety of reasons - aesthetics (odor control), health (toxicity), ecological (oxygen depletion) and economic (corrosion of equipment) - sulfide laden wastewater must be handled carefully and remediated before being released to the environment. Typical discharge limits for sulfide are < 1 mg/L. ????????????????????????????????There are dozens of alternatives for treating sulfide-laden waters, ranging from simple air stripping to elaborate sulfur recovery plants (used to treat several tons per day at refineries). There are processes based on chemistry (oxidation, precipitation, absorption and combination) and physics (adsorption, volatilization and incineration). Each process occupies a niche that is often defined by the scale and continuity of treatment, whether the sulfide is in solution or is a gas, the concentration of sulfide involved and the disposition of the sulfide-containing medium. However, for reasons relating to convenience and flexibility, chemical oxidation (using hydrogen peroxide) continues to grow in its scope of application. ??????????????????????????????????H2O2 can control sulfides in two ways, depending on the application: Destruction… by oxidizing sulfide to elemental sulfur or sulfate ion; and Prevention… by providing dissolved oxygen that inhibits the septic conditions that lead to biological sulfide formation. This technical note focuses on the oxidation chemistry of odor control with H2O2, particularly as it is applied to wastewaters containing moderate to high levels of sulfide (50 - 10,000 mg/L). Oxidation of sulfide with H2O2 proceeds differently depending primarily on the pH of the wastewater. SULFIDE OXIDATION USING HYDROGEN PEROXIDE Refinery Industry The US Peroxide Advantage The following advantages of H2O2 should be considered when evaluating treatment options: Rapid, complete oxidation of H2S H2O2 decomposition liberates oxygen which helps maintain aerobic conditions, inhibiting H2S regeneration No capital expenditures for storage and handling equipment when using US Peroxide’s H2O2 dosing system (11/09) Refinery – Sulfide Oxidation Using H2O2 - Page 2 Neutral - Slightly Acid Conditions H2S + H2O2 ? S0 + 2H2O The product of the oxidation is predominately elemental sulfur, which appears as a yellow colloid (if under dosed) or a white colloid (with complete oxidation). If clarity of the effluent is needed, the sulfur may be removed by flocculation with an anionic polymer followed by filtration. The stoichiometry calls for 1.0 lb. H2O2 per lb. H2S and it is not unusual for efficiencies to approach 100%, particularly when the concentrations of other oxidizable substances (e.g., thiosulfate) are low. There is very little heat generated in the reaction, even when sulfide levels are several thousand mg/L. Alkaline Conditions S 2- + 4H2O2 ? SO4 2- + 4H2O The above reaction predominates at pH > 9.2 and yields soluble sulfate as the reaction product. The stoichiometry calls for 4.25 lbs. H2O2 per lb. S 2- and again it is not unusual for reaction efficiencies to approach 100%, provided that the H2O2 is added in a controlled fashion and the reaction medium is thoroughly mixed. This is due to the much faster reaction brought about by the increased reactivity of H2O2 at alkaline pH. Consequently, as the pH increases above 9 - 10, there is generally little benefit to catalyzing the reaction. Slightly Alkaline Conditions In moving from pH 7 to pH 9, both of the above reactions may occur with the following results: The reaction products transition from elemental sulfur to sulfate The H2O2 requirement transitions from 1:1 to 4.25:1 The rate of reaction speeds up To some extent, catalysts may be used to push the reaction one way or the other. Catalysts, such as iron, favor sulfate formation. These catalysts may be used to economize H2O2 use or to produce a clear effluent. In both cases, the speed of reaction is greatly accelerated. U.S. Peroxide, LLC 500 Bishop St. NW, Suite C-3 Atlanta, GA 30318 www.h2o2.com ?Inquiries call 877-346-4262 or email: info@h2o2.com About US Peroxide US Peroxide is the leading supplier of hydrogen peroxide-based technologies and services for environmental applications. We have been serving the water, wastewater, refinery and remediation markets for over 15 years and have offices and field service locations throughout North America. Our consultative approach to problem solving includes application assessment, technology selection (which may include lab scale testing, if necessary) and development of a tailored treatment approach. Our full-service programs successfully integrate storage and dosing equipment systems, chemical supply, inventory and logistics management, and ongoing field and technical support. This approach provides cost-effective “hands-off” solutions to our customers. Getting Started We look forward to supporting your sulfide control needs, whatever the scale of your chemical management requirements. To obtain a streamlined sulfide control solution tailored to your specific project, just give us a call. SULFIDE OXIDATION USING HYDROGEN PEROXIDE