Why Deposit Control Polymer Selection Impacts Stressed Cooling Water System Phosphate-Based Treatment Program Performance
By Dr. Libardo A. Perez, Dr. Zahid Amjad, Robert W. Zuhl
Phosphate-based cooling water treatment programs are widely used to prevent corrosion on low-carbon steel heat exchangers and piping. Water scarcity and environmental issues have necessitated cooling system water consumption approaches, including operating at higher cycles of concentration (COC) and using treated municipal or industrial wastewater as makeup water. These water consumption reduction approaches increase potential fouling and microbiological activity, especially when using wastewater effluents containing phosphate, other inorganic ionic species (e.g., iron, aluminum, sulfate), and organic contaminants. Both system water chemistry (e.g., high phosphate, high suspended solids, high ionic species, high pH) and operating conditions (e.g., high temperature, low flowrates) create stresses that may increase potential cooling water system fouling and/or corrosion rates. This article presents a technological approach to treat cooling systems operating under stressed conditions. The article discusses treatment program efficacy for controlling metal-phosphate scale formation on equipment surfaces for several stressed conditions and compares results to commonly used industrial treatment programs.
Readily available fresh water sources for industrial uses have decreased dramatically. Accordingly, an industrial water conservation focus has been operating cooling systems at higher COC, using alternative makeup water sources (versus the use of fresh water), and in some cases operating systems at higher temperatures.
Operating at higher COC increases potential scale and deposit formation in at least two ways.
- Higher ionic species concentrations might cause scale formation that would otherwise not occur. For example, low levels of calcium and phosphate in the feedwater can cause a calcium phosphate scaling problem when operating at high COC.
- Increased system holding times (half-life) provide longer time periods for suspended solids to settle on system surfaces; this may occur during severe droughts and heavy rainfall seasons that may cause higher suspended solids in makeup waters.
Typically, 10 to 20 milligrams per liter orthophosphate, in combination with polyphosphate and/or zinc ions, is used in cooling water systems to control low carbon steel corrosion. Replacing fresh water makeup with plant wastewater, reused water, recycled water, or reclaimed water increases potential scaling as well as corrosion and microbial activity. Phosphate-based scales are typical when using alternative makeup water sources. Therefore, when using alternative water sources, higher phosphate levels may already be present, necessitating operating systems at much higher than normal phosphate levels.
Operating cooling systems under these conditions increases the need for robust cooling water treatment programs that prevent calcium phosphate scaling at higher supersaturation conditions as well as disperse suspended solids at holding times greater than 24 hours (hr). Deposit control polymers have dual roles: inhibiting inhibit metal-phosphate precipitation, and dispersing inorganic suspended solids thereby preventing deposition. This article presents performance data for several commercially available DCPs under simulated stressed cooling water operating conditions caused by high phosphate, high solids, and/or low-flow conditions.
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