Electrocoagulation for Oil-Water Separation in Refinery ETP Plants
Oil refineries produce vast quantities of oily wastewater as a byproduct of crude oil processing that requires comprehensive treatment before discharge or reuse. While gravitational separation, dissolved air flotation, and multimedia filtration can address free and dispersed oil contaminants, they often struggle to remove tight oil-water emulsions containing sub-micron-sized droplets. Electrocoagulation has emerged as a robust solution to destabilise these stable emulsions by introducing highly charged metal hydroxide coagulants in situ.
We will examines how electrocoagulation systems function and why they represent an advantageous approach for refineries tackling tough oily waste streams.
The Electrocoagulation Process
Rather than dosing chemical coagulant salts, electrocoagulation reactors generate coagulating metal hydroxide species by electrically dissolving sacrificial aluminium or iron electrode plates directly into the oil-contaminated water flow. As current passes between the anode and cathode plates, aluminium (Al3+) or iron (Fe2+) ions are liberated, subsequently forming multi-charged hydroxide coagulants like Al(OH)3 or Fe(OH)3 with excellent coagulating properties. Additionally, generated hydrogen gas aids in contaminant flotation while the cathode helps remove emulsifying agents.
Advantages Over Chemical Coagulation
Conventional chemical coagulation encounters limitations in treating stable oil-water emulsions at refineries, with over-dosing yielding excessive sludge volumes. However, electrocoagulation employs in-situ dosing matched to effluent loadings, while the highly charged metal hydroxide coagulants prove extremely effective in destabilising oil emulsions. The energised process also efficiently separates solids and oil droplets without excessive chemical addition. Removing emulsifiers prevents re-emulsification downstream.
The electrode reactions also supply a unique disinfecting action by generating oxidants like ozone, chlorine, and hypochlorous acid capable of inactivating bacteria or killing live oil mussels that can promote fouling throughout treatment trains and cooling towers.
Compact and Automated Operation
Electrocoagulation reactors provide compact footprints due to high treatment rate capabilities, short residence times of around 20 minutes, and the elimination of requiring bulk coagulant shipments, storage and dosing facilities. Automated electrical control packages regulate energy input, electrode changes, and coagulant production. The small physical plant area required compared to settling ponds or clarifiers helpsminimise capital costs.
The concentrated sludges produced can be dewatered into dense, portable cakes, while treated discharges readily separate oil in downstream compact dissolved air flotation units primed with highly destabilised contaminants. Electrochemical oxidation cells help destroy residual petrochemicals with minimal additions prior to biological polishing.
Operational Factors and Innovations
To maximise electrocoagulation cell performance, refineries must carefully control factors like electrode gap width, residence time, current density, effluent conductivity, pH and temperature. Innovations like pulse power supplies, rotating electrodes, and specialised electrolyte flow configurations help optimise dosing and reduce fouling. Electrochemical reaction modelling aids in validating appropriate reactor scaling.
Conclusion
Amid increasingly stringent refinery effluent limits and the desire for water recycling, electrocoagulation provides an attractive solution for facilities struggling with hard-to-treat stable oil emulsions. The electrochemical process destabilises the tightest oil-water emulsions while compactly generating active hydroxide coagulants, oxidants and disinfectants. With automated control over treatment intensity and lower sludge volumes than chemicals, electrocoagulation is steadily gaining adoption across oil/gas production and refining applications. Continued enhancements around electrode materials, cell hydrodynamics and sludge management will further elevate electrocoagulation's oil-water separation performance.
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