petroleum and crude oil quality

Petroleum Quality

By : Khawar Nehal5 Nov 2013

Crude Quality Issues

Crude oil is a highly variant natural resource. The quality ranges are similar to coal and depending on the maturation of the crude the quality can be high or low (younger crude's are of lower quality). One of the first indications of quality is color. The variations in oil color can be dramatic, and very indicative of the quality of that crude. Not all crude oil is black - higher quality oils can be a golden or amber in color.


All the quality measures here are based on the ability to produce the desired products. In the U.S., about 50% of the oil is converted into gasoline.

So an oil that produces a higher % of gasoline "cuts" is more desirable and have a higher quality oil.


Take note, we have used much of the higher quality crude oil already!

Now we need to use the lower quality oils too and the general trend is to use increasingly lower quality crude's.

This quality reduction has an impact on how we refine the crude into the desirable products.

Viscosity

Viscosity is the resistance to flow.

Do not use the term "Thickness" which is a length measurement.

The higher the viscosity the slower the liquid will flow and the lower the quality.

Viscosity

The viscosity process is a measure of quality, because the chemical structure of the crude influences its flow ability.

Longer chain molecules for example are harder to flow than short chains because of non-bonding interactions.

Viscosity

If you have had any chemistry you will recall ionic (type of bonding in salt crystals) and covalent bonding (the type of bonding between 2 carbon atoms).

Those are bonding interactions.

Viscosity

There are several non-bonding interactions that occur which attract (and repel) molecules.

It is the relative strength of these non-bonding interactions that influences the resistance to flow.

Elemental Composition

For coal we used the correct terminology, which was ultimate analysis.

For crude, that terminology we use is Elemental Analysis.

Crude oil is complex, it contains C, H, N, S, O, and metals too.


But the bulk of the composition is C and H, the rest being the N, S, O and metals.

S is a good indication of the quality of the crude because as the oil is heated underground the weak S-C bond can break producing H2S (hydrogen sulfide gas).

So, older crudes - higher quality - will have lower S content.


Higher S crudes also cost more to process as S is a catalyst poison it has to be removed or the extensive catalysts used in the petrochemical industry would be damaged, as would your catalytic converter.

The atomic H/C ratio is also an indicator of quality (why?)

Elemental Composition of Typical Crude:

Chemical Structures

Hydrocarbons are molecules that contain only the elements of carbon and hydrogen. These are the bulk of the crude oil. We find 4 types of chemical structure of hydrocarbon in crude oil:

Paraffin - Straight & Branched

We have seen normal (for example n-heptane) and branched (2,2,4 iso-octane) examples of the paraffin's.

The all have the same formula: CnH2n+2 (n is the number of carbon atoms).

Linear alkanes (straight)

Straight-chain alkanes are sometimes indicated by the prefix n- (for normal) where a non-linear isomer exists. Although this is not strictly necessary, the usage is still common in cases where there is an important difference in properties between the straight-chain and branched-chain isomers, e.g., n-hexane or 2- or 3-methylpentane.

Linear alkanes (straight)

The members of the series (in terms of number of carbon atoms) are named as follows:

methane, CH4 - one carbon and four hydrogen ethane, C2H6 - two carbon and six hydrogen propane, C3H8 - three carbon and 8 hydrogen butane, C4H10 - four carbon and 10 hydrogen pentane, C5H12 - five carbon and 12 hydrogen hexane, C6H14 - six carbon and 14 hydrogen

Methane

Ethane

Propane

Butane

Pentane

Hexane


For example, in the cetane molecule, to determine the molecular weight (Mw) you can count the carbons (x 12 the amu of a carbon atom) and count the hydrogen atoms (x 1 amu) and add the numbers together to obtain the molecular weight.

Hexadecane

Hexadecane (also called cetane) is an alkane hydrocarbon with the chemical formula C16H34. Hexadecane consists of a chain of 16 carbon atoms, with three hydrogen atoms bonded to the two end carbon atoms, and two hydrogens bonded to each of the 14 other carbon atoms.

Hexadecane

Cetane is often used as a shorthand for cetane number, a measure of the detonation of diesel fuel. Cetane ignites very easily under compression; for this reason, it is assigned a cetane number of 100, and serves as a reference for other fuel mixtures.

It has one of the lowest octane ratings, at 60 carbon atoms are wax) used to be used extensively for the production of candles.

Isomer

In chemistry, isomers from Greek isomers; isos = "equal", mros = "part") are molecules with the same molecular formula but different chemical structures.

That is, isomers contain the same number of atoms of each element, but have different arrangements of their atoms in space.

Isomer

Isomers do not necessarily share similar properties, unless they also have the same functional groups.

There are many different classes of isomers, like positional isomers, cis-trans isomers and enantiomers, etc.

There are two main forms of isomerism: structural isomerism and stereoisomerism (spatial isomerism).


Now we use the shorter chains produce gasoline, diesel and jet fuel (and many other products). Note that each molecule might have many structural isomers, for example a molecule containing 10 carbon atoms has 75 structural isomers.

Aromatics

Aromatics are found in both crude oil and coal. In crude oil they are now undesirable because of soot production during combustion.

This is soot from a diesel engine.

Aromatics

This soot picture was taken with a scanning electron microscope so we can see the very small (>1 micron) spherical soot particles. These sphere join together to form chains of spheres. To give you some idea of the scale: 80 microns is about the width of human hair.


Aromatics

In organic chemistry, aromaticity is a chemical property describing the way in which a conjugated ring of unsaturated bonds, lone pairs, or empty orbitals exhibits a stabilization stronger than would be expected by the stabilization of conjugation alone.

Aromatics

The earliest use of the term was in an article by August Wilhelm Hofmann in 1855.

There is no general relationship between aromaticity as a chemical property and the olfactory properties of such compounds.


Naphthenes

These are cyclo-paraffins Cyclohexane has an interesting boat or chair configuration.

Classification

In a similar manner to coal as the source rock is buried deeper the temperature increases with increasing depth.

Thus looking at quality indicators allows for a classification system similar to that of coal rank.

Because "old deep" oil provides the highest quantity of gasoline, it is the higher quality crude oil.

Classification

Most graphs you are used to seeing or plotting have just 2 axes.

This works fine if you're just comparing 2 components, but as you see below, we're comparing 3 general classifications for crude oil compound types.

Classification

It is the ratio of these compound types (aromatics, paraffin's and naphthenes) that impacts the quality of the crude (in addition to S content, especially when the S is within the aromatic portion, which makes it much harder to remove during refining).

This ternary diagram is used to illustrate the relative percentage of three components in crude oil. The patterned band represents the mixture found in the majority of crude oils.

Classification

So, to plot 3 items on a single graph we use ternary diagrams like the one you see above.

At the three apexes, the composition would either be pure (100%) aromatics, pure naphthenes or pure paraffins (clockwise from top).

Classification

Along any of the borderlines of this triangle, you're looking at a mixture of just 2 of these components (aromatics - naphthenes, or naphthenes - paraffins, or paraffins - aromatics).

At any point within the triangle, the crude contains all three components, in varying degrees.

Classification

Take the example of 50 % aromatics to begin with. To plot this point on the graph, you'd create a drop a horizontal line about half way between the apex (100 %) and the base of the triangle opposite of that apex (0%), representing 50%.

Classification

You repeat this process to locate the other %'s of the compound types on the graph, and the point you're after is the convergence of those three lines.

Classification

Thus, the center of the triangle is: 33 %, 33 %, and 33 % of aromatics, naphthalenes and paraffin's, crude oil that would generally fall into the "old shallow" classification.

Summary

We have discussed some of the ways to measure the quality of crude oil and petrol products.

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