Factors+Influencing+Fouling

When it comes to fouling in crude oil heat exchangers in refineries, there are many different factors of the preheating process that determine the rate of fouling that occurs.

The most important factors influencing fouling in the crude oil industry:
 * Surface Temperature
 * Bulk Velocity
 * Bulk Temperature
 * Crude Type
 * Crude Blending
 * Bio film Plugging


 * Threshold Fouling Models **

In crude oil heat exchangers, we generally find that a lower crude inlet velocity and high temperatures of the surface wall will accelerate the rate of fouling. Studies have been done to determine some threshold fouling models to see the effects of temperatures on fouling for shell-and-tube heat exchangers. A threshold condition is the boundary between a fouling and no-fouling zone, and many models have been developed based on the Ebert and Panchal model to determine crude oil fouling behavior. These models however are usually conducted in a laboratory situation, thus the reliability is limited. The complexity of the physical and chemical mechanisms that are encountered in the industrial situation hinders to a certain extent better developed threshold models for the crude oil heat exchangers.





The use of the film temperatures or surface temperatures will depend on the location of where the chemical reactions between the crude oil and heat exchanger occur. If the occurrence is on the thermal boundary layer, film temperature is to be used, however if the reaction is on the heat transfer surface, the choice is surface temperature.




 * Velocity Effects **

The fouling in the crude oil industry, specifically its preheat trains, have a fouling rate that decreases or increases with changes in velocity. When the fouling is reaction controlled for a given bulk temperature and heat flux, the fouling rate will decrease as the flow velocity increases. When the fouling is controlled by a mass transfer from the fouling species of the bulk fluid onto the surface of the preheat trains, then a increase in velocity leads to a higher fouling rate. When the fouling rate decreases with increasing velocity, this can be concluded based on these reasons:
 * Having a weak deposit layer is more prone to the erosion effects of the fouling layer, offsetting the depositions of foulant. The erosion effects arise from the shear stress at the wall, which is proportional to the velocity of the fluid.
 * If the foulant material is found to be formed on the thermal boundary layer, where deposition rate is highest, then the foulant has the ability to diffuse back into the bulk fluid, and when velocity is increased, the mass transfer from the boundary layer back to the bulk fluid increases, resulting in reduced fouling rates.


 * Crude Type/Blending **

When it comes to the crude types, there tend to be a large mixture of hydrocarbons in the crude oil. The oil consists mostly of molecules such as paraffins, naphthenes, asphaltenes, and aromatic hydrocarbons. When classifying the oil according to its API gravity (measure of how heavy or light a petroleum liquid is compared to water), we can determine if it is a light, medium, or heavy mixture. Again the molecule asphaltene plays a role in its fouling rate, as heavy oils have much higher proportions of asphaltenes, ultimately fouling at a faster rate compared to the medium and light oils. asphaltenes form a solubility class and contains many different components such as polar aromatics, naphthenic molecules, hetereoatoms, and trace metals. They are high molecular weight entities, whose weight and structure depends on the source of the crude oil and the n-alkane used to precipitate them. If different crudes are blended together, this could cause a unstable mix, ultimately precipitating out molecules such as asphaltene and lead to rapid fouling rates. This is all due to crude oil incompatibility, and for this reason a compatibility model and tests were made to predict the proportions and order of blending of oils to avoid this situation.




 * Surface/Bulk Temperature Effects **

The effects of bulk fluid temperatures on deposit formation has had contradicting results reported, however generally in the crude oil industry a specific molecule called asphaltenes is a large, complex ring structure that is insoluble and a major cause of fouling. The solubility of asphaltene in the crude oil will increase with a rise in temperature. The maximum solubility of asphaltene has been found to be at 140°C, having lower solubility at decreasing temperatures. When the bulk fluid are at high temperatures, the asphaltene is in the form of a solution having a low fouling rate. Otherwise at low bulk temperatures, asphaltene tends to precipitate out from the crude oil onto the preheat trains resulting in a higher fouling rate.This graph, //see reference 4//, shows the correlation of the increase in fouling resistance and bulk temperatures.

Temperature is considered a very important variable in organic fluids flowing. Most fouling systems show a dependence that correlates to the surface temperature used, as chemical reactions produce deposits on the walls. Deposition begins as soon a the increase in temperature begins. With increased surface temperatures or heat flux, the fouling resistances generally increase as well. This graph, //see reference 4//, shows the correlation of the increase in fouling resistance and surface temperatures.




 * Biofilm Plugging **

When plugging occurs, there is a loss of production as there will not be enough conductivity into the well, which is a result of the biofilm growing, causing slime formations and encrustation. In the oil and gas industry, wells can start to lose their production capacity soon before the supplies in a well is exhausted. Although some extreme environments can seem unsuitable for bacteria growth, biofilm still manages to produce in harsh conditions. This is because the bacteria has the ability to adapt and survive in harsh conditions, leading to a stablized colony. In plugging, liquid water has to be available to bond into the complex slime structures, and when this happens the bacteria is able to stabilize. Even if the oil has a 0.1% water content, the bacteria can utilized this to their advantage and grow within the oil in complex structures. Bacteria is also able to use the paraffins and anthracenes from oil to coat the complex structures. When these compounds accumulate, the production will either slow or shut down.