Operating Conditions in Petrochemical Cooling Water
Process media temperatures are high, so cooling water return temperature commonly sits in the 40–45 °C band, and some units — FCC regenerator heat-removal systems, hydrocracker reactors — run hotter still.
Make-up water hardness is frequently high. Groundwater in parts of North and East China often runs 300 mg/L total hardness or more, and coastal sites using desalinated or brackish sources face their own chemistry.
High alkalinity and high chloride compound the problem. Alkalinity typically falls in the 200–500 mg/L range as CaCO3; at elevated temperature bicarbonate decomposes to carbonate, accelerating scaling. High chloride raises corrosion risk and narrows the usable metallurgy.
Where Chemical Treatment Struggles
Chemical consumption rises sharply under these conditions. Scale inhibitor dosing that would be 5–10 mg/L in an ordinary system may need substantially more at high temperature and hardness, raising operating cost and effluent load.
Blowdown volume is correspondingly large. For a 5000 m³/h circulation rate at four cycles of concentration with 2% evaporation loss, the discharge stream is substantial and continuous.
Corrosion and acid cleaning form a cycle. Chlorinated biocides such as sodium hypochlorite aggravate corrosion in water that already carries a high chloride background, and even aggressive dosing does not fully prevent scaling — so periodic acid cleaning returns, with its own corrosion and waste-handling burden.
Why ECD Suits High-Hardness Water
ECD has a useful property here: the higher the Ca2+ concentration, the greater the driving force for electrochemical crystallisation and the more efficient the cathode precipitation becomes. This inverts the behaviour of chemical inhibition, which becomes less economical as hardness rises.
Engineering Case: A Steel Works Captive Power Plant
A captive thermal power plant at a steel works provides a representative high-hardness, high-alkalinity case. Its circulating system had long suffered scaling under those water conditions, with the associated cycle of chemical dosing and periodic cleaning. Installing bypass ECD moved the scaling reaction to the cathode and reduced dependence on dosing.
Typical Application Scenarios
Crude and vacuum distillation units carry large and fluctuating heat loads on overhead condensers, with high return-water temperature. Ethylene plant quench systems impose severe thermal shock on quench water and cracked-gas coolers; ECD can pair with closed-circuit cooling towers to run at higher concentration ratio. Aromatics units (PX/PTA) combine high circulation rates with many exchangers, so both chemical consumption and blowdown are high. Captive cogeneration condensers and oil coolers are sensitive to water quality and benefit similarly.
Technical and Economic Assessment
Assessment should cover capital cost of the bypass skid, energy consumed by the electrochemical cells and scraping mechanism, savings from reduced chemical purchase and handling, savings from reduced make-up water and blowdown, and avoided cost from extended cleaning intervals and improved heat rate.