Fouling

This section discusses principal fouling mechanisms, and services and conditions where they may occur. In most cases, fouling conditions can be avoided by appropriate process and exchanger design. The principal fouling mechanisms are:
• Particulate fouling
• Salt precipitation fouling
• Chemical reaction fouling
• Filming amine fouling
• Biological fouling
• Corrosion fouling

Particulate fouling is possible for streams that contain a few parts per billion of solids. Particle sizes between 0.001 and 1 micron contribute most to the deposit. Particles over about 100 microns usually erode smaller particles and inhibit fouling. Simple particulate deposits are weak and yield readily to fluid shear. Exchangers that operate with fluid shear stresses greater than about 0.001 psi are usually not subject to simple particulate fouling.

Economic liquid velocities discussed in Section 220 result in nearly four times the shear stress (or twice the velocity) needed to prevent simple particulate fouling. Economic velocities for most two-phase exchangers are also above the threshold for particulate fouling, except at high vapor fraction. Economic gas velocities are not adequate to keep small particles moving. For example, in FCC flue gas coolers the process and/or exchanger design must be adjusted to control gas side fouling.

Salt precipitation fouling usually involves liquid-to-solid phase transition at the heat transfer surface. It occurs where an aqueous phase contacts the heat transfer surface and the aqueous phase is supersaturated with respect to one or more of the issolved salts. Most salt deposits can not be eroded at economic velocities. A few salt deposits are erodible at economic liquid velocities, and these are candidates for aqueous phase corrosion inhibitors (See Section 310). Salt precipitation fouling may occur alone or in combination with particulate fouling.

Gas-to-solid phase transition (sublimation) is a less common type of salt precipitation that can occur in overhead condensers and effluent streams in hydroprocessing plants. NH4Cl is usually the salt involved.

Addition of an aqueous phase and/or control of its composition is the normal method of eliminating salt precipitation conditions in heat exchangers. This method is practiced in cooling tower water exchangers, sea water coolers, crude oil preheat exchangers, crude unit atmospheric overhead condensers and some other services.

Chemical reaction fouling involves chemical bonding between thermally unstable organic compounds to the extent that liquid-to-solid phase transition occurs at the heat transfer surface. This is a slow process in liquids and usually does not cause fouling at economic velocities in the absence of particulate matter. However, chemical reaction fouling can occur in combination with particulate fouling and affect fouling by increasing the strength of predominantly particulate deposits. Chemical reaction fouling increases exponentially with temperature above a certain threshold temperature. It is controlled by maintaining heat transfer surface temperatures below the threshold. The threshold fouling temperature for naturally occurring hydrocarbons is about 600°F for the heaviest components and higher for lighter components. Residuum fouling is the most common type of chemical reaction fouling in refineries.

Filming amine fouling usually occurs with liquids in combination with particulate fouling and affects fouling by increasing the strength of predominantly particulate deposits. Economic liquid velocities will not stop filming amine fouling. Filming amines can cause fouling in two other ways. Filming amines dissolved in light hydrocarbon will “deposit” as a liquid (like tar) where the light hydrocarbon is evaporated to extinction. Most filming amines also decompose to solids above a certain temperature, usually between 300°F and 500°F.

Filming amines are marketed as corrosion inhibitors, dispersants, antioxidants, metal deactivators and antifoulants. They are commonly injected into crude unit atmospheric overhead systems and dispersed throughout the refinery.

Biological fouling occurs spontaneously in oxygenated waters between 32°F and 120°F, unless significant toxic material is present. Cooling tower water systems and sea water cooling systems are subject to biological fouling, usually in combination with particulate fouling. Chlorination is the most common method of biological fouling control. Redundant equipment and/or frequent cleaning is an alternative.

Corrosion fouling involves irregular loss of metal and accumulation of corrosion products. Increased surface roughness improves convective heat transfer and compensates for added thermal resistance of corrosion products that remain on the surface. Pressure drop increases initially due to the roughness but may decrease if metal loss is significant. These effects are usually small in heat exchangers and are usually ignored in design.


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