Petroleum and Crude oil quality

1. Petroleum Quality
By : Khawar Nehal
5 Nov 2013

2. 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.

4. Crude Quality Issues
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.

5. Crude Quality Issues
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.

6. 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.

8. 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 nonbonding interactions.

9. 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.

10. 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.

11. 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.

12. Elemental Composition
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.

13. Elemental Composition
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?)

14. Elemental Composition of
Typical Crude:

15. 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:

16. 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).

17. 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.

18. 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

19. Methane

20. Ethane

21. Propane

22. Butane

23. Pentane

24. Hexane

25. Paraffin - Straight &
Branched
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.

26. 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.

27. 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 <−30

28. Cetane number
Cetane number or CN is a measurement of the
combustion quality of diesel fuel during
compression ignition.
It is a significant expression diesel fuel.
A number of other measurements determine
overall diesel fuel quality - measures of diesel
fuel quality include density, lubricity, cold-flow
properties and sulfur content.

29. Cetane number
Cetane number or CN is a measure of a fuel's
ignition delay, the time period between the start
of injection and the first identifiable pressure
increase during combustion of the fuel. In a
particular diesel engine, higher cetane fuels will
have shorter ignition delay periods than lower
cetane fuels.

30. Cetane number
Cetane numbers are only used for the relatively
light distillate diesel oils.
For heavy (residual) fuel oil two other scales are
used CCAI and CII.

31. Cetane number
In short, the higher the cetane number the more
easily the fuel will combust in a compression
setting (such as a diesel engine).
The characteristic diesel "knock" occurs when
the first portion of fuel that has been injected into
the cylinder suddenly ignites after an initial delay.

32. Cetane number
Generally, diesel engines operate well with a CN
from 40 to 55.
Fuels with higher cetane number have shorter
ignition delays, providing more time for the fuel
combustion process to be completed.
Hence, higher speed diesel engines operate
more effectively with higher cetane number fuels.

33. Cetane number
Minimizing this delay results in less unburned
fuel in the cylinder at the beginning and less
intense knock.
Therefore higher-cetane fuel usually causes an
engine to run more smoothly and quietly.
This does not necessarily translate into greater
efficiency, although it may in certain engines.

34. Octane rating
Octane rating or octane number is a standard
measure of the performance of a motor or
aviation fuel.
The higher the octane number, the more
compression the fuel can withstand before
detonating.

35. Octane rating
In broad terms, fuels with a higher octane rating
are used in high-compression engines that
generally have higher performance.
In contrast, fuels with lower octane numbers (but
higher cetane numbers) are ideal for diesel
engines.
Use of gasoline with lower octane numbers may
lead to the problem of engine knocking.

36. Paraffin - Straight & Branched
Or you can use the formula:
Cetane has 16 carbon atoms (but if we used
decane you would know how many carbons it
contained, right?) so C16H(2 x 16)+2 OR
C16H34 and the Mw is = (12 x 16) + (1 x 34) =
226 amu (atomic mass units).

37. Paraffin - Straight & Branched
The paraffins' are the desired contents of the
crude oil.
Long chains (> 60 carbon atoms are wax) used
to be used extensively for the production of
candles.

38. Isomer
In chemistry, isomers from Greek isomerès; isos
= "equal", méros = "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.

39. 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).

40. Paraffin - Straight &
Branched
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.

41. 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.

42. 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.
This is soot from a diesel engine.

43. 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.

44. 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.
This is soot from a diesel engine.

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

46. 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.

47. 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.

48. 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).

49. 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.

50. 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).

51. 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.

52. 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%.

53. 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.

54. 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.

55. Summary
We have discussed some of the ways to
measure the quality of crude oil and petrol
products.
Contact : Khawar@atrc.net.pk
http://atrc.net.pk
http://dubai-computer-services.com