Mechanisms of fouling

It is of great importance to understand the fouling mechanisms in principle, as they will indicate the causes and conditions of fouling and hence give clues how to minimize it.

Fouling can be generally classified into six types:
1. Particulate fouling
2. Reaction fouling
3. Corrosion fouling
4. Precipitation fouling
5. Biological fouling
6. Solidification fouling

1. Particulate Fouling

Particulate fouling may be defined as the accumulation of particles suspended in the process streams onto the heat-transfer surfaces. This type of fouling includes sedimentation of settling under gravitation as well as deposition of colloidal particles by other deposition mechanisms on to the heat transfer surfaces. Various forms of particulate fouling are:

1. Fouling that occurs in once-through cooling-water systems using sea, river, and lake water containing mud, silt, and sediments. They are capable of depositing in low-flow areas, forming a physical barrier, and preventing oxygen from reaching the metal/solution interface. The deposit buildup will promote localized corrosion.

2. Gas-side fouling: Gas-side fouling is mostly by particulate fouling by dirty gas streams, airborne contaminants, and the leakage and mixing of one process stream with the other, in addition to combustion gases and exhaust gases. In some cases, gas-side fouling may also be accompanied by corrosion, particularly when condensations of corrosive acids take place from combustion gases.

2. Chemical Reaction Fouling (Polymerization)

Deposits formed by chemical reactions at the heat-transfer surface in which the surface material itself is not a reactant are known as chemical reaction fouling. Polymerization, cracking, and coking of hydrocarbons are prime examples of reaction fouling. The factors likely to affect reaction fouling include the following:

- Temperature is the most sensitive variable. It is usual that below a certain surface temperature polymerization does not initiate, but increases rapidly above that.

- The presence of most sulfur compounds, nitrogen compounds, and the presence of trace elements (metallic impurities) such as MO and Va in hydrocarbon streams significantly increases the fouling rates.

Composition of the process stream, including contaminants and, especially, oxygen ingress will affect reaction fouling.

3. Corrosion Fouling

Corrosion fouling is due to the deposition of corrosion products on heat-transfer surfaces. In this category of fouling process, the heat-transfer surface material itself reacts to produce corrosion products, which foul the heat-transfer surface. The most common forms of this type of fouling are material loss due to general thinning, iron oxide on carbon steel tubes in cooling water systems, and fouling of soldered radiator tube ends on the water side by solder bloom corrosion. Corrosion fouling is highly dependent upon the choice of material of construction and the environment. Hence, it is possible to overcome corrosion fouling if the right choice of material has been made to resist the environment. Measures such as the use of inhibitors, cathodic protection, and surface treatment such as passivation of stainless steel will minimize corrosion and hence corrosion fouling.

4. Crystallization or Precipitation Fouling

This type of fouling mostly takes place in cooling-water systems, when water-soluble salts, predominantly calcium carbonates, become supersaturated and crystallize on the tube wall to form scaling. Such scaling occurs because many of the dissolved salts in water exhibit inverse solubility effects, a condition that reverses the normal solubility (increasing with temperature) into one that decreases with temperature. Thus an inverse solubility solution will crystallize when heated (e.g., cooling water), while normal solubility salts will crystallize when cooled.

Chemical additives can be helpful to reduce fouling problems due to crystallization and freezing in a number of ways. Broadly, there are four groups of chemicals to control crystallization: distortion agents, dispersants, sequestering agents, and threshold chemicals.

5. Biological Fouling

The attachment of microorganisms (bacteria, algae, and fungi) and macroorganisms (barnacles, sponges, fishes, seaweed, etc.) on heat-transfer surfaces where the cooling water is used in as drawn condition from river, lake, sea and coastal water, etc., is commonly referred to as biological fouling. On contact with heat-transfer surfaces, these organisms can attach and breed, sometimes completely clogging the fluid passages, as well as entrapping silt or other suspended solids and giving rise to deposit corrosion. Concentration of microorganisms in cooling-water systems may be relatively low before problems of biofouling are initiated. Corrosion due to biological attachment to heat transfer surfaces is known as microbiologically influenced corrosion.

6. Solidification Fouling or Freezing Fouling

The freezing of a liquid or of higher-melting constituents of a multicomponent solution on a subcooled heat-transfer surface is known as solidification fouling. Some examples include frosting of moisture in the air, freezing of cooling water in low-temperature processes, and paraffin wax deposition during cooling of hydrocarbon streams.