Chemical and Physical Properties

Crude oil is simply a supersaturated mixture of long chain alkanes of various length. Light hydrocarbons act as the solvent, whereas heavier hydrocarbons act as solutes. Straight chain hydrocarbons constitute roughly 80 to 90% of the crude oil composition, whereas the balance is compose of branched iso-paraffin or aromatic cycloparaffins (Becker 1997).



Appearance

Pure paraffin waxes are colorless, odorless, and tasteless solids. The yellow colour that precipitates out of solution to foul pipelines is usually due to other impurities and other deposits . High molecular weight paraffin waxes can be hard and brittle, whereas lower molecular weight paraffin waxes are generally and malleable (Becker 1997).

Paraffin_Wax2.jpg
Figure 1:Fully refined paraffin wax (DIY Trade [date unknown])



Boiling point and melting point

The physical state of paraffin wax is highly dependent upon factors such as temperature and pressure, where multiple physical states of wax can coexist under specific . Its physical properties are also strongly determined by the length of the longest, uninterrupted, linear alkane constituent.

Paraffin waxes are solid at room temperature and typically start melting between 50°C and 70°C (Schmidt 2010). However, much greater melting points are observed for waxes with heavier content. Generally, the boiling and meting points of paraffins increase with increased content of longer, heavier alkanes, whereas the boiling point decreases with increased branched alkane content.

The intermolecular forces between paraffin molecules are London Dispersion forces (Van der Waal force). Bigger molecules exhibit greater Van der Waal forces, which increase the boiling point of the molecules (See Table 2). Branching decreases the magnitude of Van der Waal forces by preventing the attainment of optimal proximity of the molecules to each other, and lowers the boiling point (Becker 1997).

Because the melting point of paraffins mostly depend on how well the molecules fit into a crystal lattice, less regularity in the trend is observed (Becker 1997), as seen in Figure 2.

Table 1: Structural Isomers of C5H12 and melting point (Wikipedia: Alkanes... [updated 2010])
Name
Chemical Structure
Melting Point (oC)
n-pentane
CH3-CH2-CH2-CH2-CH3
-130
iso-pentane
CH3-CH(CH3)-CH2-CH3
-160
neo-pentane
CH3-(CH3C(CH3)-CH3
-16

In Table 1, note that the melting point of n-pentane is higher than the branched iso-pentane, but lower than the fully branched neo-pentane. This is due to how well the molecules fit together to form the crystal lattice.


AlkaneProperties.gif
Figure 2: Melting Points and Boiling Points of unbranched hydrocarbons, temperature in oC (KCPC [updated 2010])

It is important to note that minor variations to the general upward trend exist between even and odd numbers of carbon atoms as well. The even numbered carbon chains pack closer together in a crystal lattice than odd numbered carbon chains, and therefore have a higher melting point. The melting points of branched paraffins can be higher or lower than its unbranched counterpart depending on its stability to form crystal lattice (Wikipedia: Alkanes... [updated 2010])

Table 2: Melting Point, Boiling Point and Density of n-Paraffins (Wikipedia: Alkanes... [Update 2010])
Alkane
Formula
Boiling point [°C]
Melting point [°C]
Density [g/cm3] (at 20°C)
Methane
CH4
-162
-183
gas
Ethane
C2H6
-89
-182
gas
Propane
C3H8
-42
-188
gas
Butane
C4H10
0
-138
gas
Pentane
C5H12
36
-130
0.626(liquid)
Hexane
C6H14
69
-95
0.659(liquid)
Heptane
C7H16
98
-91
0.684(liquid)
Octane
C8H18
126
-57
0.703(liquid)
Nonane
C9H20
151
-54
0.718(liquid)
Decane
C10H22
174
-30
0.730(liquid)
Undecane
C11H24
196
-26
0.740(liquid)
Dodecane
C12H26
216
-10
0.749(liquid)
Hexadecane
C16H34
287
18
0.769(liquid)
Icosane
C20H42
343
37
solid
Triacontane
C30H62
450
66
solid
Tetracontane
C40H82
525
82
solid
Pentacontane
C50H102
575
91
solid



Density and Solubility

The density of paraffin wax generally increases with increasing molecular mass (see Figure 3), but remains below 1.0 g/cm3, which is the density of water. Paraffin waxes are also non-polar; therefore, they are insoluble in water but are soluble in other non-polar petroleum based solvents of similar structure. Examples of such solvents are benzene, ether, and certain esters.

AlkaneDensity.gif
Figure 3: Density of normal hydrocarbons, density in g/cm3 (KCPC [updated 2010])

Some conclusions on the effect of melting point, viscosity and temperature on the solubility of paraffin wax are as follows (Weber and Dunlap 1928):
  1. Solubility of paraffin wax decreases as melting point increases
  2. Solubility decreases as viscosity increases
  3. Solubility increases as temperature increases

Effects of melting point on solubility

When a molecular substance dissolves in water, the intermolecular forces within the substance need to be broken. The higher the melting point, the stronger the intermolecular forces and the more stable the crystal lattice structure (Becker 1997). Paraffin waxes with a higher melting point will need more energy to dissolve in water than paraffin waxes with a lower melting point. Therefore, solubility decreases as melting point of paraffin wax crude increases.

Effects of temperature on solubility

For the effects of temperature on solubility, there are two cases:
  1. As the temperature of a liquid increases, the solubilities of the gases in that liquid decrease.
  2. As the temperature of a liquid increases, the solubilities of solid in that liquid increases.

Paraffin wax deposits are solid at standard conditions. The addition of more heat to wax containing crude will facilitate the dissolving reaction by providing energy to break bonds in the solid. Therefore, adding heat to the oil makes it easier for the particle to move around between the solution and the solid, and causes the wax to melt by providing the necessary energy to break the bonds in the solid and enter into solution (Becker 1997). The Second Law of Thermodynamics also predicts that entropy always increase in a spontaneous process, in that particles favours disorder. Therefore, equilibrium will favor a solution state with increase temperature.

Effect of viscosity on solubility

Viscosity refers to a fluid's resistance to flow. Increased polymerization (addition of CH2) increases the molar weight of the fluid. The heavier the fluid, the more resistance it will have to flow. An increase in the length of carbon chains also leads to an increase in intermolecular attractive forces, as well as an increase in the extent to which nearby molecules become entangled when they have an extended shape (Wikipedia: Polymers... [updated 2010]). This means that the boiling point and melting point of the fluid increase as more energy is needed to overcome the intermolecular forces. Thus, solubility decreases as viscosity increases.

Effects of pressure on solubility

Liquids and solids show very little change in solubility with changes in pressure, if any. Pressure is not as influential on paraffins as temperature, but does impact its solubility due to the change in composition. Light hydrocarbons (C16-) act as solvents in crude and will come out of the solution when pressure drops. This decreases the concentration of the solvent. As a result, waxy molecules (C16+) precipitate sooner than they would if the pressure had not dropped (Becker 1997).



Structure

Typical paraffin waxes in crude oil have a clearly defined crystal structures composed of macroscopic crystals. The crystal form of paraffin wax is called the macrocrystalline (individual crystals visible to the unaided eye C18 to C30). Macrocrystalline paraffins are usually hexagonal or needle shaped (orthorhombic), as seen in Figure 4. Under the most favorable conditions, n-paraffins form clearly defined orthorhomic crystals (the orthorhombic crystal structure is one of the seven lattice point groups), but unfavorable conditions and presence of impurties lead to hexagonal and/or amorphous crystallisation (Becker 1997).

Note: In addition to n-paraffin, microcrystallene paraffins contain a large amount of iso-paraffins and a small amount naphthenes (C40 to C55)

ortho.jpghex.jpg
Figure 4: Hexagonal Crystal Structure(Left), Orthorhombic Crystal Structure (Right) (SAAU c2007)



Viscosity

Viscosity of the crude is affected by the concentration and average molecular weight of the wax molecules. The greater the molecular weight of the wax, the more viscous the oil becomes, which thickens the crude (Vulk and Sarica 2003).


viscosity.png
Figure 5: Viscosity vs. Number of Carbons (Vulk and Sarica 2003)



Stability and Reactivity

Paraffin wax stable and non-reactive under ordinary conditions of use and storage and is unaffected by most common chemical reagents; however, it does burn readily if ignited . Paraffins will also react with halogens under UV, light or heat. When heated at high temperatures in the absence of air, paraffin wax can also crack and break up into smaller, lighter paraffin molecules.




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