Fouling+Mechanisms

There are several types of fouling existing in oil and gas industry heat exchangers, which are chemical reaction fouling, crystallization of inorganic, and corrosion fouling.
__** Chemical Reaction Fouling **__ When the process fluid enters the heat exchanger, the chemicals in this fluid would react, and the deposits would form at the heat transfer surface. In this case of fouling, the wall of heat exchanger would not involve in the reaction but would act as a catalyst to catalyze the reaction process. The general steps involving in chemical reaction fouling are:



In oil and gas chemical reaction fouling, autoxidation may be the main source of unwanted deposits in heat exchanger. The autoxidation is initiated by the thermal decomposition of hydrocarbons. Under the presence of excess oxygen, the free radicals generated by these hydrocarbons would be rapidly oxidized and form hydroperoxide or polyhydroperoxide, which leading to the deposit formation. The prime examples of chemical reaction fouling would be polymerization. · Under moderate temperature, the hydrocarbons in the processing fluid would be polymerized by free radical chain reaction. In the free radical chain reaction, oxygen, traces of sulphur, and nitrogen would participate along with unsaturated species in the processing fluid. The polymerization reaction could be simplified by the following reaction scheme:
 * ** Polymerization **

Asphaltene is a common back and carbonaceous substance in crude oils in a form of colloidal, suspended, solid particles, and asphaltene precipitation has been one of the major causes for crude oil production loss during the crude oil operation by plugging the heat exchangers. The insolubility of asphaltene is being one of the major causes of asphaltene precipitation.
 * __** Asphaltene precipitation **__

Table 2: The amount of asphaltene and resin in crude oil around the world
 * **Crude** || **Resin wt%** || **Asphaltene wt%** ||
 * Canada, Atabasca || 14.0 || 15.0 ||
 * Venezuela, Boscan || 29.4 || 17.2 ||
 * Canada, Cold Lake || 25.0 || 13.0 ||
 * Mexico, Panucon || 26.0 || 12.5 ||
 * USA, MS, Baxterville || 8.9 || 17.2 ||
 * Russia, Kaluga || 20.0 || 0.5 ||
 * USA, TX, Hould || 12.0 || 0.5 ||
 * Brazil, Campos, Atabasca || 21.55 || 2.8 ||
 * USA, CA, Huntington Beach || 19.0 || 4.0 ||
 * Canada, Alberta || 8.5 || 5.3 ||
 * USA, LA, Brookhaven || 4.6 || 1.65 ||
 * Russia, Balachany || 6.0 || 0.5 ||
 * Russia, Bibi-Eibat || 9.0 || 0.3 ||
 * Russia, Dossor || 2.5 || 0.0 ||
 * Russia, Surachany || 4.0 || 0.0 ||
 * USA, TX, Mexia || 5.0 || 1.3 ||
 * USA, OK, Ok. City || 5.0 || 0.1 ||
 * USA, OK, Tonkawa || 2.5 || 0.2 ||
 * France, Lagrave || 7.5 || 4.0 ||

Aaphaltenes are normally held in a suspension state by polar molecules under static reservoir conditions (i.e. the crude oil has not yet been processed), and these type of polar molecules is called resin; however once the fluid is being processed, the temperature and pressure of the fluid would change, and these two parameters would highly associated with the flocculation of asphaltenes, which potentially lead to asphaltenes precipitation. It is generally believed that the incompatibility of asphaltene and the remainder of oil lead to asphaltene precipitation. The precipitated would then adhere to the hot exchanger surface and be carbonized to insoluble coke.



__** Crystallization Fouling **__

During crude oil processing operations, some inorganic substances (inorganic impurities in crude oil) are dissolved in the processed fluid. Once the inorganic substance reaches its supersatuation, crystallization would be likely to occur either on the heat exchanger surface or in the bulk solution. The solubility of the inorganic substance is temperature dependent. Generally, as temperature increase, the solubility would decrease; however, in some cases such as calcium sulphate, it solubility increases as processed temperature increases.

Table 3: Solubilty of various substances vs. temperature change

__** Corrosion Fouling **__

Corrosion fouling occurs when the surface of the heat exchanger reacts with the dissolved oxygen in the processed fluid (i.e. the surface is being oxidized to produce corrosion products). The corrosion products would then form deposits on the heat exchanger surface and promote the attachment of other foulants. Another cause of corrosion fouling might be the presence of acidic substances in the fluid and the surface would be oxidized by the acidic substances. Corrosion fouling would result in a rough surface and create sites of nucleation, which potentially lead to crystallization and particulate fouling. A prime example of corrosion fouling will be rusting of carbon steel.