Refiners’ quiet battle inside alkylation exchangers: stop fouling early, dose smart, clean fast

Fouling is a slow choke. In alkylation units, polymerized olefins, carried‑over catalyst, and inorganic scale build on tube walls, driving up temperatures and pressures while cutting heat‑transfer and flow. One industry source is blunt: fouling “leads to less efficient plant operations due to reduced heat‑transfer capacity and lower fluid flow…[and] increased corrosion…resulting from metal oxide deposition and overheating” (chemicalprocessing.com).

Even high velocities only buy time. Engineers “size the exchanger tubes so speed through the tubes is as high as possible to increase the duration before fouling starts…however, fouling is an inescapable fact when heat exchangers are used” (epcmholdings.com).

Alkylation deposit mechanisms and hotspots

Alkylation‑specific foulants include organic polymeric deposits — such as HF red oil resins (a resinous by‑product formed via aldol condensation of hydrocarbons) — and inorganic salts (carbonates or sulfates after sulfuric acid service, fluorides from HF). A vendor case underscores where it bites hardest: “heat exchangers, reboilers, and acid separators have the heaviest fouling… Inorganic scaling can foul exchanger bundles so much that they are impossible to pull” (fqechemicals.com).

Mixed deposits (“grease, coke, asphaltenes and scales”) hammer overall heat‑transfer coefficients (U‑values). Modeling of a refinery network found that once overall U falls ~40% from clean, energy efficiency severely degrades — with optimal cleaning at a 40% U‑drop instead of waiting longer (journal.ugm.ac.id).

Mechanistically, fouling layers insulate tube surfaces, reducing effective heat‑transfer coefficient and increasing pressure drop (chemicalprocessing.com; epcmholdings.com).

Process control and feed pretreatment

Operators lean on basic discipline first: feed streams are pre‑treated to remove heavy polymers or particulates (e.g., DRI filters on alkylation feed), and stable conditions (temperature, acid strength, moderate olefin concentration) curb polymer formation. Continuous monitoring of pressure drop and inlet/outlet temperatures provides early warning of deposit buildup.

Cooling‑water chemistry and filtration control

On the water side, particulate/feedwater fouling is held back by filtration and chemical treatment. Cooling‑tower scale inhibitors (polyphosphates, polymers, biodispersants) are routinely dosed to prevent calcium/carbonate scale (watertreatment.qualichem.com). One expert flatly notes that keeping the cooling water “clean… is the best way to avoid particulate fouling” (swep.net).

Programs typically combine scale inhibitors with corrosion inhibitors and biocides to keep rust and biofilms from settling. In practice, many sites set cooling‑circuit limits (e.g., less than 200 ppm Ca) and schedule blowdown to avoid buildup. Where dispersancy is needed, dispersant chemicals help maintain particulates in suspension.

Chemical antifoulant injection in service

Beyond water chemistry, some units inject chemical antifoulants into process loops. These include dispersants (keep foulants soluble) and polymerization inhibitors. Although less common in alkylation than in crude preheat, some units use small‑dosed antioxidants or inhibitors (e.g., nitroxides or hindered phenols) to slow polymer formation in olefin feeds.

Heavy hydrocarbon fouling presents another target. Refinery operators have trialed nano‑scale surfactant‑dispersants; one magnesium‑sulfonated nano‑additive “appears to be effective on numerous types of fouling” in upgraders and refineries (digitalrefining.com). In continuous pilot tests mimicking refinery conditions, this additive reduced solid deposits by over 70% — one trial dropped new deposits from ~23 g (no additive) to ~6 g under injection, with exchanger pressure drop staying low (researchgate.net).

Real units show similarly striking gains. A crude preheat case achieved a 1.2× increase in overall U using an asphaltene dispersant, recovering about $60,000/day of throughput (fqechemicals.com). Capacity rose by 15,000 barrels/day with zero downtime for cleaning (fqechemicals.com). Reviews also highlight nanoparticle‑boosted dispersants as a notable trend in fouling control (digitalrefining.com; researchgate.net).

Shutdown chemical cleaning procedure

Chemical cleaning during a scheduled turnaround remains the fastest way to restore performance. A standard two‑stage sequence (alkaline then acid) is well established (epcmholdings.com):

• Isolate and drain. Disconnect tube‑ and shell‑side inlets/outlets, pressure‑test, and install blinds. “Valve leakage is common” once deposits are removed (seats clear), so robust blinds are essential (chemicalprocessing.com). Flush residual process fluids (HF, isobutane, caustic). In HF units, drain acid inventory to the regenerator or neutralize with caustic before access.
• Alkaline solvent wash. Circulate a hot alkaline detergent (dilute sodium hydroxide with surfactants and a corrosion inhibitor) to dissolve oils, tars, and soluble organics (epcmholdings.com). For stubborn paraffin/asphaltene fouling, sites use custom “decon” blends or high‑flash solvents; one vendor recommends an alkaline degreaser pass before descaling (fqechemicals.com). Typical circulation: 50–60 °C and sufficient pressure to sweep all tubes, monitored by effluent clarity. Where appropriate, an alkaline degreaser is used; some programs also incorporate a solvent‑based step.
• Acid descale. After organics are stripped, apply an acid chosen for the scale: hydrochloric, nitric, or sulfamic acids for calcium/magnesium carbonates or sulfates, often with chelants like EDTA or citric acid (epcmholdings.com). Alkylation specifics matter: for scaling from hydrofluoric acid, hydrochloric acid “will need to be added with the chemistry” to address fluoride scales such as CaF₂ or AlF₃ (fqechemicals.com). In H₂SO₄ units, magnesium sulfate scales may be targeted by hot HCl or proprietary chelants. Citric or formic acid cycles are common for iron oxides, while strong HCl attacks carbonates. Add corrosion‑inhibiting packages (e.g., nitrites, azoles) to protect steel. Circulate, often at ~40–60 °C, for several hours, tracking pH and acid consumption until effluent runs free of solids.
• Rinse, neutralize, passivate. Flush with fresh water, then circulate a neutralizer/passivation solution — for carbon steel, dilute sodium carbonate (soda ash) to neutralize residual acid and form a protective oxide (epcmholdings.com). Drain, dry, and return to service.

The two‑stage approach “removes the organic part of residues such as grease and oil” and then “disintegrate[s] scaling and soften[s] residues,” though severe cases may still need mechanical brushing (epcmholdings.com). Modern formulations can shorten outages (epcmholdings.com), and solvent‑based systems have yielded near‑complete deposit removal in the field; one case saw U‑values rebound by up to 120% after cleaning (fqechemicals.com).

Safety and compliance are central. Neutralize HF or caustic residues before discharge. In Indonesian refineries in particular, spent cleaning fluids are classified as hazardous (Limbah B3) and must be fully treated (neutralized, filtered, etc.) by on‑site waste systems per environmental regulations.

Scheduling thresholds and economic outcomes

Quantified schedules pay off. Huda et al. found that a planned cleaning schedule (125°C) executed when U had dropped 40% used significantly less fuel and energy than delaying to 90% decay, with the 40% decline point balancing energy recovery against cleaning cost (journal.ugm.ac.id; journal.ugm.ac.id). In day‑to‑day practice, many units clean exchangers whenever overall ΔT increases 20–30% or pressure drop doubles to avoid steeper losses.

Case studies show large upside. The FQE treatment cited earlier not only restored duty but “recovered $60,000 in lost production per day” (fqechemicals.com). Remesat’s pilots showed that continuous nano‑additive injection can virtually eliminate fouling: collected deposits fell to ~1 g (vs grams of coke with no treatment), with pressure drop near baseline (researchgate.net). Even partial treatment at 300 ppm dosing cut deposits from ~23 g to ~6 g — a ~75% reduction (researchgate.net).

Across the literature and field, the pattern is consistent: preventive additives curb fouling rates (digitalrefining.com; researchgate.net), and well‑planned chemical cleans fully restore exchanger performance. A combined program — water‑treat and antifoulant injection on‑line, plus periodic turnaround cleaning — is essential. Maintaining cleanliness saves energy (up to double‑digit percent fuel savings), reduces unscheduled outages, and supports sustained high octane output in alkylation. Even a single day’s throughput regained can justify the cost of additives or chemicals.

For related mechanisms, see the AIChE Annual Meeting abstract on asphaltene‑induced fouling (aiche.org). Additional vendor case histories are reprinted in industry journals (fqechemicals.com; fqechemicals.com).