Refinery Cooling Towers Run Hot and Dirty. This Is the Chemistry That Keeps Them Efficient.

Refinery cooling towers move staggering volumes of water—often more than 90% of a plant’s makeup water—so even small gains in cycles‑of‑concentration (COC, the ratio of dissolved solids in circulating water to makeup) translate into outsized water and cost savings (www.prochemtech.com). But refinery service also means very high temperatures and trace contamination—hydrocarbons, process resins, amines—that concentrate and tend to deposit.

Industry studies show plant cooling waters often carry positive Langelier Saturation Index (LSI, a measure of calcium carbonate saturation) values—supersaturated with CaCO₃ and prone to scaling (www.scirp.org). The same waters can host active microbes, including sulfate‑reducing bacteria (SRB), that form biofilms and corrosion products (www.scirp.org).

The fix is coordinated: carefully controlled blowdown; scale inhibitors and dispersants; corrosion inhibitors; and a rigorous biocide program—all to maintain heat‑transfer efficiency and meet effluent standards. In refinery practice, that includes both chemicals and control hardware, for example accurate feed via a dosing pump to hold setpoints steady under load.

Alkalinity, pH and LSI targets

Bulk pH is held mildly alkaline—typically pH 7.5–8.5—to minimize corrosivity without exceeding scale limits. Industry guidance puts an ideal operating window around neutral to slightly alkaline, ≈7.0–9.0 (beta.co.id). Acid feed (sulfuric or hydrochloric acid) neutralizes bicarbonate alkalinity so carbonates do not precipitate; ASHRAE recommends acidifying cooling water and adding inhibitors and dispersants as the primary scale‑control strategy (handbook.ashrae.org).

In practice, targets are a Saturation Index (LSI) near 0 to +0.5. Negative LSI causes steel attack, while strongly positive LSI signals imminent CaCO₃ scale (www.chemtreat.com) (www.chemtreat.com). With pH and inhibitor adjustment, most systems can be held at LSI ≈ +0.2–+0.5.

Threshold inhibitors and dispersants

Threshold‑type scale inhibitors—organophosphonates and polyacrylates—interfere with crystal nucleation and growth. Phosphonates such as HEDP, PBTC, and ATMP adsorb to nascent crystals and sequester calcium. ATMP was the first phosphonate and was shown to effectively “raise” allowable saturation by about +1.5 LSI units (www.chemtreat.com)—roughly tripling safe calcium and alkalinity levels. Modern programs use HEDP or PBTC instead of ATMP because they tolerate oxidizing biocides better (www.chemtreat.com).

Dispersants (polyacrylates or polyacrylamides) keep any precipitated particles suspended—important in high‑silica or phosphate waters that form tough films. ASHRAE and water‑treatment handbooks warn that silica scale is extremely tenacious and a strong insulator (www.chemtreat.com), so systems with >100–150 ppm silica may need dedicated silica inhibitors or higher bleed. Practical implementation leans on purpose‑formulated blends, e.g., scale inhibitors for cooling towers paired with dispersant chemicals to sustain cleanliness at high heat load.

COC and blowdown control

Blowdown—removing a portion of circulating water and replacing it with makeup—controls dissolved solids and is metered by conductivity (or fixed intervals). Removing a portion is “essential to prevent scale formation and corrosion” (beta.co.id). Maintaining 4–6× concentration cycles by bleeding at 16–25% of circulation is common in heavy‑duty service.

Without treatment, hardness and alkalinity would limit cycles to ~3–4×, but with modern phosphonate/polymer programs 2–4 cycles are readily achieved (www.prochemtech.com). ProChemTech notes that basic phosphonate/polymer treatments in hard water typically only achieve ~2–4× cycles (www.prochemtech.com), whereas advanced technologies—such as softened makeup plus specialty inhibitors—can push 6–10×. Each additional cycle yields large water savings; doubling COC roughly halves blowdown. Raising COC from 3 to 6 reduces blowdown from ~33% to ~17% of flow, saving on the order of 25–30% of makeup water. In practice, the program routinely tests cycles, LSI, and limits hardness below ~400–500 ppm as CaCO₃ (beta.co.id). Where makeup hardness is the pinch point, plants commonly pair chemistry with a softener to open room for higher cycles.

Film‑forming corrosion control

Carbon steel dominates tower metallurgy, so film‑forming inhibitor programs build a passive layer on metal surfaces. Common choices include organophosphonates, zinc, molybdate, or silicate salts (handbook.ashrae.org). Metal‑ion inhibitors like chromate are not used because of toxicity and regulations (handbook.ashrae.org).

Zinc salts (at a few mg/L) can co‑precipitate as a protective film, while phosphonates complex iron and build a thin phosphate layer on steel. Silicates or polyamines can also be used for passivation. ASHRAE notes the key is consistent feed and control of water chemistry (handbook.ashrae.org). Well‑managed programs target very low corrosion rates—much less than 0.1 mm/yr for steel. Commercial formulations such as corrosion inhibitors are selected to match metallurgy and water chemistry.

pH, oxygen and metal protection

Corrosion is further minimized by maintaining near‑neutral pH and reducing dissolved oxygen (DO). A small dose of sodium nitrite or polyhydroxylamine is often used alongside film‑formers to scavenge residual O₂, which also helps protect copper alloys. Closed circuits—such as boiler condensate—require additional oxygen scavengers (e.g., hydrazine or sulfite) and filming amines, though the open cooling loop still benefits from low DO and neutral pH. Proper chemical selection depends on feed water quality, system design, and operating parameters (beta.co.id). In integrated plants, boiler side scavenging is handled with targeted programs, for example oxygen scavengers matched to condensate return conditions.

Oxidizing biocides and shock protocol

Bulk microbial control relies on oxidizers—chlorine, bromine, or chlorine dioxide—applied via periodic shocks and continuous feed. For shock (e.g., at startup or when biofilm is detected), ASHRAE recommends dosing ~10 ppm free halogen (typically by sodium hypochlorite or bromine) to achieve ~4–5 ppm residual for about 5 hours (handbook.ashrae.org). After shock, a low steady residual is maintained; operating guidance suggests free chlorine ~0.2–0.5 ppm as a continuous biocide (beta.co.id), with trials showing this range suppresses bacteria without overfeeding or corrosion.

If organics are very high, chlorine dioxide—effective at 0.2–0.8 ppm and pH‑insensitive—may be preferred, though it is usually generated on‑site (handbook.ashrae.org). Refinery programs often formalize these routines through dedicated biocide packages calibrated to heat load and contaminant levels.

Non‑oxidizing biocides and rotation

Hydrocarbons and high COD can consume oxidizers, so non‑oxidizing biocides (glutaraldehyde, isothiazolines like DBNPA, THPS or similar) are used continuously or in pulses. Typical dosing levels are in the single‑digit ppm range; for example, 5–10 ppm glutaraldehyde continuously. These agents penetrate biofilms and kill bacteria without oxygen and are often rotated to prevent resistance.

Microbial monitoring and cleaning

Effectiveness is verified by routine microbiological tests. Uncontrolled bacterial growth leads to biofilm that reduces heat transfer and accelerates corrosion (beta.co.id). Programs target very low plate counts; for example, maintain total aerobic bacteria <10^4 CFU/mL (colony‑forming units per milliliter) (beta.co.id). On‑site tests—dipslides and ATP analyzers (rapid microbial activity checks)—are done weekly or more often to adjust biocide feed, and periodic drain‑and‑clean remains necessary (beta.co.id). When fouling accumulates, operators typically schedule a cooling tower cleaning service to reset surfaces.

Instrumentation, performance and compliance

Instrumentation closes the loop. pH and conductivity sensors automate acid feed and bleed, while oxidation‑reduction monitors gauge oxidizer residual. Weekly lab or field tests—hardness, metal coupons, LSI, bacteria counts—provide feedback. Such a program typically raises permissible COC by 50–100% compared to untreated water. Moving from 4× to 6–8× cycles (via proper chemistry) can cut blowdown by ~20–30% (www.prochemtech.com), while corrosion rates are driven to near‑zero (well below 0.1 mm/yr) and heat‑transfer surfaces stay clean for long campaigns. Targets like <10 ppm iron release, <1 ppm copper release, and heterotrophic plate count (HPC) <10^4 CFU/mL become achievable metrics.

Compliance is addressed by minimizing effluent strength. The treatment minimizes organics and particulates, and any blowdown is collected and treated—oil/water separation and pH neutralization—so parameters like oil & grease and BOD meet local discharge limits, including Indonesian Permen KLHK No. 5/2014. Refinery wastewater systems often pair this step with targeted equipment such as oil removal units when hydrocarbons are present.

Evidence base and field targets

Industry literature and case studies affirm the approach. An Aramco study noted produced water with high LSI (>0) and microbial content—underscoring the tandem threats of scaling and biofouling (www.scirp.org). ASHRAE and water‑treatment handbooks describe the acid + inhibitor strategy and specific biocide protocols (handbook.ashrae.org) (handbook.ashrae.org). Field notes (Beta Pramesti, ProChemTech) and engineering guidelines present achievable targets—e.g., hardness <500 ppm (beta.co.id), pH ~7–9 (beta.co.id), free chlorine 0.2–0.5 ppm (beta.co.id), aerobic count <10^4 CFU/mL (beta.co.id)).

References: see inline citations (www.prochemtech.com) (www.scirp.org) (beta.co.id) (www.chemtreat.com) (handbook.ashrae.org) (www.chemtreat.com) (www.chemtreat.com) (beta.co.id) (handbook.ashrae.org) (handbook.ashrae.org) (handbook.ashrae.org) (beta.co.id).