Heat exchanger fouling is estimated to cost the global refining industry billions of dollars annually through reduced energy efficiency, increased fuel consumption, shortened run lengths, and cleaning costs. In refinery crude preheat trains—the network of heat exchangers that recover heat from hot process streams to preheat the incoming crude oil—fouling is particularly impactful. Every degree of preheat lost to fouling must be compensated by additional fired heater duty, directly increasing fuel consumption and operating costs.
Fouling Mechanisms
Heat exchanger fouling in refineries results from several mechanisms, often acting simultaneously. Chemical reaction fouling occurs when thermally unstable hydrocarbons polymerize on hot surfaces, forming gums and coke. This is the dominant mechanism in crude preheat trains, particularly at tube wall temperatures above 250°C. Corrosion fouling involves the accumulation of corrosion products (iron sulfide, iron oxide) on heat transfer surfaces. Particulate fouling results from the deposition of suspended solids—asphaltenes, sediment, and process-generated particles. Crystallization fouling occurs when dissolved inorganic salts precipitate on cooler surfaces.
Antifoulant Chemistry
Dispersants keep insoluble materials (asphaltenes, coke precursors, particulates) suspended in the bulk fluid, preventing their deposition on heat exchanger surfaces. Polymeric dispersants and surfactant-based products are used, with selection guided by the specific fouling mechanism and the crude oil properties.
Antioxidants interrupt the free-radical polymerization reactions that generate coke. By scavenging reactive radical species, antioxidants slow the formation of high-molecular-weight deposits. Hindered phenol-based and amine-based antioxidants are common in refinery antifoulant programs.
Metal deactivators chelate dissolved metals (iron, copper, vanadium) that catalyze polymerization reactions. By neutralizing these catalytic species, metal deactivators reduce the rate of chemical reaction fouling.
Program Design and Application
Antifoulant programs are designed based on crude slate analysis, operating conditions, and historical fouling data. The injection point is typically in the crude feed upstream of the preheat train, ensuring that the antifoulant is present throughout the exchanger network. Injection rates are typically in the range of 5–20 ppm based on crude throughput, though the optimal rate depends on the specific application.
For multi-mechanism fouling, combination products that address both organic and inorganic fouling may be more effective than single-mechanism treatments. The antifoulant must be compatible with other refinery chemicals—desalter demulsifiers, overhead corrosion inhibitors, and caustic injection—to avoid unintended interactions.
Monitoring Fouling and Program Effectiveness
Heat exchanger fouling is monitored through several metrics. Fouling resistance (calculated from exchanger duty, flow rates, and temperature data) quantifies the thermal resistance of the deposit layer. Pressure drop across the exchanger increases as fouling restricts flow. Crude inlet temperature to the fired heater (coil inlet temperature, or CIT) is a direct measure of preheat train effectiveness—declining CIT indicates increasing fouling.
Trending these parameters over time, with and without antifoulant treatment, provides the data needed to evaluate program effectiveness and optimize the treatment.
Economic Value
The economics of antifoulant programs are driven by the cost of the energy lost to fouling. In a large refinery, even a modest improvement in preheat train effectiveness—a few degrees of CIT improvement—can save millions of dollars per year in fuel costs. Additionally, extending run lengths between cleaning shutdowns avoids the direct costs of cleaning, the opportunity cost of lost production, and the safety risks associated with opening and cleaning fouled equipment. For most refineries, antifoulant programs deliver returns that far exceed their cost.
