Asphaltene


Because asphaltene is typically talked about in the same context as paraffin wax when considering fouling of this nature, it is important to understand what asphaltene is and why it is also problematic to the oil industry.


Definition

Asphaltene is the material present in petroleum that is insoluble in n-paraffins but soluble in aromatic solvents (Ancheyta and Fernando 2009).
It is said to be among the least understood deposits, yet the most problematic since it is the main cause of catalyst deactivation and sediment formation. The oil industry has defined these materials mostly on an operation basis rather than a actual chemical structure, and is mostly known through its solubility (Ancheyta and Fernando 2009). Asphaltene is insoluble in n-paraffins and precipitates readily in n-heptane.


Chemical Structure

The actual structure of asphaltene is said to be extremely complex and is still considered hypothetical (Becker 1997). The elemental composition varies with the solvents used to cause precipitation but at least the composition of the molecule is mostly known. Asphaltenes are composed of aromatic rings containing sulfur, nitrogen and alkyl side chains up to C30. When they are in a colloidal state in crude oil they are surrounded by resins, which stabilize to form micelles (Ancheyta and Fernando 2009). Functional groups like ketones, phenols, and carboxylic acids are observed as elemental functional groups of asphaltene at times, and inorganics, such as vanadium and nickel, are also observed in crude with high asphaltene concentration. Figure 1 contains two hypothetical structures of asphaltene molecules.



VenezAsphaltene.GIF
Asph.Maya.gif

Figure 1: Hypothetical Structures of Asphaltene (Priyanto 2001)


Physical and Chemical Properties

Physical

Asphaltene has been generally defined as brown and black powdery material produced by the treatment of petroleum, petroleum residue, or bituminous materials with a low boiling liquid hydrocarbon (Pineda et al. 2007). From the data collected by Nazmul et al., the following are physical properties of asphaltene (2005):
  • Aggregate size ranges from 20 µm to 200 µm
  • Porosity of aggregates increase with aggregate size
  • Permeability rapidly increases as the aggregate size increases
  • Settling velocity ranges from 100 µm/s to 600 µm/s

However, due to the complex nature of asphaltene, the chemical properties of asphaltenes are solely dependent on the properties of the crude (Pineda et al. 2007).

Asphltene_Aggregate.jpg
Figure 2: Left) n-C7 Asphaltenes Right) n-C5 Asphaltenes (NMT [date unknown])

Composition of Hertoatoms

There is abundant experimental evidence on the composition of heteroatoms in asphaltene to primarily consist of oxygen, sulfur, nitrogen and also trace amounts metals. The following are functional groups found in the asphaltene structure (Diallo et al. 2000):
  • Oxygen has been found as carboxylic, phenolic, and ketonic groups
  • Sulfur is primarily found in groups as benzothiophenes, napthenobenzothiophenes, alkyl-alkyl sulfides, alkyl-aryl sulfides and aryl-aryl sulfides
  • Nitrogen is found scattered at various heterocyclic positions
  • Metals are found in trace amounts, primarily affecting the molecular weight of asphaltene, and tend to concentrate during the process of aggregation with increase temperature (Pineda et al. 2007)

Pressure

Asphaltene deposition has been found to increase with a decrease in pressure from the initial reservoir pressure to the bubble point pressure, and increase further below the bubble point (Pineda et al. 2007). Data collected by Narve and co-workers showed that the reversibility of the aggregation of asphaltene in crude oil is heavily dependent on two variables: time and pressure (2002).During the depressurization process of the crude oil, the decrease in molar volume of the light components is larger than for heavier components. As such, the relative volume of the components with the poorest asphaltene solubility power increases the amount of aggregation of asphaltene. By increasing the pressure of the compressible crude oil, ashphaltene aggregates begin to re-dissolve almost completely back into the natural crude. As a result, asphaltene aggregation is reversible but clearly time-dependent due to the increased pressure of the system (Narve et al. 2002).


Temperature

Traditionally, the prediction of asphaltene deposition has been related to the heat of the solution. As such, the solubility of asphaltene aggregates has a dependence on the Van der Waal interactions which reflect the heat of the crude oil (Buckley et al. 1998). The following are the properties of asphaltene aggregates that are affected by temperature:
  • asphaltene aggregates are responsible for the high viscosity of heavy crude oils, as they are the most polar components (Barre et al. 2008)
  • At atmospheric temperature, asphaltenes in solution are found in a solid phase (Pineda et al. 2007)
  • Results from Pineda et al. have found that asphaltene aggregation tends to decrease when temperature is increased (Pineda et al. 2007)



Problems associated with asphaltene

Asphaltene is almost always found in association with paraffin wax when they are retrieved from wells, storage tanks, or pipelines. Studies show that the amount of asphaltene precipiation decreases as the number of carbons forming straight chain paraffns increases (Ancheyta and Fernando 2009). In other words, treatment of paraffin wax could escalate the precipitation of heavier, problematic asphaltene compound. Then, as the petroleum becomes heavier, the content and complexity of asphaltenes present in it increases considerably as well. Therefore, treatment of paraffin and asphaltene deposition must both be considered because one can have a considerable affect on the.




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