Paraffin and Waxes


Paraffins can be described as organic, saturated hydrocarbon molecules with single bonds. Paraffins (or alkanes) are commonly found as hydrocarbon chains of varying length, and are defined by the formula CnH2n+2. The simplest paraffin is methane, which serves as the starting point for paraffinic chain expansion.

A wax, such as 'beeswax', is chemically defined as a long chain ester. It, along with paraffin waxes can vary from being malleable to brittle, depending on the average molecular weight. Typically, wax is colorless, tasteless, odorless, and possesses a smooth, slick surface that is water-resistant. At standard conditions, waxes are found in the solid state. In a practical sense, wax can be defined as anything with a 'waxy' feel which has a melting point between 40°C and 100°C (Becker, 1997).


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Figure 1-1 | Paraffin wax at room temperature and pressure [1]

Specifically, ‘paraffin wax’ is the term used to describe paraffin chains of lengths varying from C16 to C40. They are referred to as ‘wax’ because they are in the solid state at moderate temperatures. The variation in size of waxes leads to a significant variation of their physical properties (Bai and Bai, 2005). Naturally occurring paraffin waxes are found in mixtures of n-alkanes, isoalkanes, cycloalkanes, and alkane-substituted aromatic compounds. During the refining of crude oil, the produced waxes are categorized according to melting points, and go on to be used in a variety of products.



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Figure 1-1a | Paraffin wax molecules can be found as: simple long chains, branched long chains, or long chains with cyclical structures at room temperature and pressure [1a]

According to the organic theory on the formation of hydrocarbons, the decomposition of organic matter is the key source of all hydrocarbons. Sedimentation and compaction of subsequent layers of strata aids in increasing temperature and pressure until conditions are suitable for the decomposition of these carbon compounds (Chandra, 2006). Over time, the heat and pressure acting on the broken-down carbon molecules led to the formation of hydrocarbons, and in-turn, paraffin waxes.













Video 1-1 | An illustrative visual of a paraffin wax C25 (created using ChemDraw Pro 13 - White: hydrogen | Grey: carbon) *Note: may take a second to load and may need an updated version of QuickTime Player ©


Solubility


Solubility is a property of a substance that indicates how well it is able to dissolve in a solvent. It has many influencing factors which including temperature, pressure, viscosity, density, and intermolecular forces. Waxes are not soluble in water, but are soluble in non-polar, organic solvents such as ether, benzene, toluene, and xylene.

The problem of paraffin deposition arises mainly due to the waxes’ solubility properties. Since the solubility is relatively low at lower temperatures, precipitation of dissolved waxes in crude oil occurs. The Cloud Point, or Wax Appearance Temperature (WAT) are the terms used to describe the temperature at which waxes begin to precipitate and form crystals. After the wax initially precipitates, it continues to build up in the pipelines, reducing the overall flow of crude, among other negative effects. Many of the methods designed to remove wax deposition target the enhancement of this property in particular. Treatments involving the use of hot oil, hot water, surfactants, and solvents are intended to dissolve the deposited wax by circulating the higher temperature fluids or solvents through the pipelines (Becker, 1997).



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Figure 1-1b | Paraffin wax at room temperature and pressure [1b]

In crude oil, paraffin wax is readily precipitated because of the other components present in the oil. While paraffin makes up the bulk of crude oil, other compounds which contain atoms such as Oxygen and Nitrogen are still present. Therefore, the nature of the solvent becomes polar. The non-polar paraffin wax molecules therefore become insoluble in the polar crude oil. However, since the polar molecules are in significantly less quantities than the paraffin, the elevated temperatures and pressures of oil reservoirs result in paraffin waxes being dissolved in the crude.


Intermolecular Forces


Intermolecular forces that are present are predominantly London dispersion, or van der Waal’s forces. These forces arise due to the relative orientation of molecules near one another. Their electronic orbitals 'blend' together and result in an induced dipole which effectively attracts the molecules together (Becker, 1997) Since the paraffin molecules are long-chain alkanes, they do not tend to have a particularly strong dipole interaction. Also, due to the relative absence of strongly electronegative atoms such as nitrogen, oxygen, and fluorine in hydrocarbons, the hydrogen bonding interactions are also minimal. London dispersion forces treat the overall paraffin chain as an 'electron cloud' and attractive forces between molecules are induced.

With waxes such as 'beeswax', (long chained esters), oxygen molecules are present. This implies that H-Bonding forces will be present and relevant. The polar nature of this kind of wax completely changes it's solubility in polar solvents. The presence of the oxygen atoms in the ester make the wax molecule polar, resulting in Hydrogen-Bond interaction. With this interaction present, the London dispersion forces play a secondary role in interactions between molecules.

Specifically, a paraffin wax is a long chained hydrocarbon by nature. It is non-polar as it has no largely electronegative atoms, and consists predominantly of carbon and hydrogen. Therefore, they can be viewed as large clouds of electrons. When two of these clouds are lined up in a certain orientation, the electrons interact in such a way that a temporary induced dipole is formed. This force, despite being one of the weaker interactions is most commonly found in paraffins because the other forces are not present. Lack of Oxygen, Nitrogen, or Fluorine results in no H-bond interaction, and the lack of a permanent dipole eliminates dipole interaction between paraffin wax molecules. The H-bond interactions play the least significant role because Carbon is not sufficiently electronegative to attract the electropositive Hydrogen.




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Figure 1-2 | Intermolecular forces, descriptive model and the basis of attraction found to occur between molecules near one another [2]


Density


Solubility is a property that varies with respect to the average molecular weight of the wax. The longer the chains, the larger the molecules, and therefore the less soluble it is. Typically, the smaller-chain waxes tend to be more soluble in a given solvent due to their lower density. The van der Waals interactions increase the density by strengthening the attractive forces between molecules, making them harder to separate. Figure 1-3 depicts the increasing density with respect to the size of the wax molecules.


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Figure 1-3 | Density versus the number of carbons in a saturated hydrocarbon chain [3]

Viscosity


The increase of molecular weight by adding CH2 groups to the chain increases the ‘bulk’ of the wax. The heavier wax (in crude oil) is therefore denser and has a higher resistance to flow, or increased viscosity (Vulk and Sarica, 2003). Factors such as lower temperatures and the involvement of van der Waals forces both contribute to increasing the viscosity of the fluid. Intermolecular attraction accomplishes this by bringing the molecules closer together. Combined, all of these factors increasing the viscosity result in a decrease in the solubility of the wax.

Temperature


As with most solutes, paraffin waxes tend to become more soluble as temperatures increase. Increasing the temperature increases the kinetic energy of the molecules in the solvent. Therefore, the molecules of the solute are able to disperse throughout the solvent faster and more thoroughly at higher temperatures. Intermolecular forces hold the molecules closer together, therefore making it harder to separate them and induce melting or boiling. This increase in the required energy to separate the molecules implies a higher melting or boiling point for the wax (Becker, 1997). As shown in Figure 1-4, the heavier the paraffin chains become, more energy is required to induce melting and boiling.

Observing the temperature effects on the solubility, one can see that higher temperatures correspond to increased solubility. This is because waxes are solid at standard conditions, and solids become more soluble with respect to increasing temperature. Therefore an overall temperature increase is a possible solution, but proves to be very impractical. It is too inefficient and expensive to maintain high temperature liquids over very long distances such as in pipeline transport.

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Figure 1-4 | Temperature versus number of carbons in a saturated hydrocarbon chain [4]


Pressure


Compared to temperature and viscous effects, the relative effects of pressure are often considered to be negligible. When looking at the production and transportation of crude oil, in which the solubility is a weak function of pressure. Therefore, the pressure effects on solubility are often deemed negligible in industry. However, when looking at precipitation fouling, it can be seen that an overall decrease in pressure leads to a decrease in the solubility of the precipitate. Nonetheless, it remains that the solubility of paraffin wax in crude is a weak function of pressure compared to thermal and viscous effects, and can be excluded from the analysis.

However, when dealing with very high pressure variations, as with producing oil from a reservoir; pressure can play a significant role. The disparity between reservoir pressure and standard pressure often spans 2-3 orders of magnitude. Therefore, pressure can be instrumental to paraffin fouling at the near-wellbore region. Here, the pressure relief can cause the solutes to come out of solution and cause blockages in the wellbore during production (Bai and Bai, 2005).


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Figure 1-5 | The experimental and calculated relationship of WAT versus Pressure for paraffins [5]

Structure


Paraffin waxes have a tendency to be brittle and therefore not very useful for industrial purposes. It is commonplace to add heavier, more branched alkane chains (microcrystalline wax) in order to make the paraffinic more malleable for industrial use. The increase of isoparaffinic (branched) hydrocarbons generate finer, more defined crystals. The geometry of these crystals is dependent on temperature, pressure, mixing conditions, and solute-solvent interaction; the physical properties here are commonly grouped under the thermodynamic term known as entropy. These conditions go on to result in a wide variety of crystal structures bearing resemblance to needles, pyramids, cubes, rods, pentagons, hexagons, octagons, and rhomboids (Becker, 1997).