385
Instability and Incompatibility
The obvious disadvantage is the closeness of the indices, almost analogous to comparing crude oil
character by specific gravity only where most crude oils fall into the range d = 0.800–1.000. The API
gravity expanded this scale from 5 to 60, thereby adding more meaning to the use of specific gravity data.
In a similar manner, the correlation index, which is based on a plot of the specific gravity (d)
versus the reciprocal of the boiling point (K) in °K (°K = degrees Kelvin = °C + 273) for pure hydrocarbons adds another dimension to the numbers:
Correlation Index CI 473 7d 456 8
48 64
K
( )
.
.
,
=
-
+
0
In the case of a petroleum fraction, K is the average boiling point determined by the standard distillation method.
The line described by the constants of the individual members of the normal paraffin series is
given a value of CI = 0 and a parallel line passing through the point for benzene is given a value
of CI = 100 (Chapters 10). Values between 0 and 15 indicate a predominance of paraffinic hydrocarbons in the sample, and values from 15 to 20 indicate predominance either of naphthenes or of
mixtures of paraffins/naphthenes/aromatics; an index value above 50 indicates a predominance of
aromatics in the fraction.
14.5 EFFECT OF ASPHALTENE AND HETEROATOM CONSTITUENTS
The focus of incompatibility studies has usually been on the whole crude oil and specifically on
the asphaltene fraction. As already stated (Chapters 12 and 13), the asphaltene constituents are the
highest molecular weight and most polar fractions found in crude oil. The characteristics of the
asphaltene constituents and the amount in crude oil depend to a greater or lesser extent on the crude
oil source. During refining of crude oil, asphaltene constituents will end up in a high percentage
in the residual fuels as the light fractions (gasoline, jet fuel, etc.) are removed from the oil through
cracking and visbreaking.
The problems with the asphaltene constituents have increased due to the need to extract even
the heaviest crude oils as well as the trend to extract large amounts of light fractions out of crude
oil by among other methods such as cracking and visbreaking. Some examples of problems due to
flocculation and/or sedimentation of asphaltene constituents and reacted asphaltene constituents are
• Oil recovery—wellbore plugging and pipeline deposition
• Visbreaking and cracking processes—degraded asphaltene constituents are more aromatic
(loss of aliphatic chains) and less soluble
• Emulsion formation—if there is a high degree of water contamination emulsions may be
formed; asphaltene constituents are responsible for the undesired stabilization of emulsions,
since asphaltene constituents are highly polar and surface active
• Preheating—preheating of fuel oil prior to combustion encourages reaction and precipitation
of the reacted asphaltene constituents leading to coking
• Combustion of fuel oil—a high content (>6% w/w) of asphaltene constituents in the fuel oil
results in ignition delay and poor combustion further leading to boiler fouling, poor heat
transfer, stack solid emission, and corrosion problems
• Blending of fuel oils as well as blending of fuel oils with heavy crude oils—the change of
medium during mixing may cause destabilization of asphaltene (and reacted asphaltene)
constituents
• Storage problems of fuel oils and visbroken residua—sedimentation and plugging
can occur due to oxidation of the asphaltene constituents; increased polarity may cause
asphaltene aggregation and sludge/sediment formation can furthermore be accelerated if
there is bacterial infestation in the fuel
Instability and Incompatibility
The obvious disadvantage is the closeness of the indices, almost analogous to comparing crude oil
character by specific gravity only where most crude oils fall into the range d = 0.800–1.000. The API
gravity expanded this scale from 5 to 60, thereby adding more meaning to the use of specific gravity data.
In a similar manner, the correlation index, which is based on a plot of the specific gravity (d)
versus the reciprocal of the boiling point (K) in °K (°K = degrees Kelvin = °C + 273) for pure hydrocarbons adds another dimension to the numbers:
Correlation Index CI 473 7d 456 8
48 64
K
( )
.
.
,
=
-
+
0
In the case of a petroleum fraction, K is the average boiling point determined by the standard distillation method.
The line described by the constants of the individual members of the normal paraffin series is
given a value of CI = 0 and a parallel line passing through the point for benzene is given a value
of CI = 100 (Chapters 10). Values between 0 and 15 indicate a predominance of paraffinic hydrocarbons in the sample, and values from 15 to 20 indicate predominance either of naphthenes or of
mixtures of paraffins/naphthenes/aromatics; an index value above 50 indicates a predominance of
aromatics in the fraction.
14.5 EFFECT OF ASPHALTENE AND HETEROATOM CONSTITUENTS
The focus of incompatibility studies has usually been on the whole crude oil and specifically on
the asphaltene fraction. As already stated (Chapters 12 and 13), the asphaltene constituents are the
highest molecular weight and most polar fractions found in crude oil. The characteristics of the
asphaltene constituents and the amount in crude oil depend to a greater or lesser extent on the crude
oil source. During refining of crude oil, asphaltene constituents will end up in a high percentage
in the residual fuels as the light fractions (gasoline, jet fuel, etc.) are removed from the oil through
cracking and visbreaking.
The problems with the asphaltene constituents have increased due to the need to extract even
the heaviest crude oils as well as the trend to extract large amounts of light fractions out of crude
oil by among other methods such as cracking and visbreaking. Some examples of problems due to
flocculation and/or sedimentation of asphaltene constituents and reacted asphaltene constituents are
• Oil recovery—wellbore plugging and pipeline deposition
• Visbreaking and cracking processes—degraded asphaltene constituents are more aromatic
(loss of aliphatic chains) and less soluble
• Emulsion formation—if there is a high degree of water contamination emulsions may be
formed; asphaltene constituents are responsible for the undesired stabilization of emulsions,
since asphaltene constituents are highly polar and surface active
• Preheating—preheating of fuel oil prior to combustion encourages reaction and precipitation
of the reacted asphaltene constituents leading to coking
• Combustion of fuel oil—a high content (>6% w/w) of asphaltene constituents in the fuel oil
results in ignition delay and poor combustion further leading to boiler fouling, poor heat
transfer, stack solid emission, and corrosion problems
• Blending of fuel oils as well as blending of fuel oils with heavy crude oils—the change of
medium during mixing may cause destabilization of asphaltene (and reacted asphaltene)
constituents
• Storage problems of fuel oils and visbroken residua—sedimentation and plugging
can occur due to oxidation of the asphaltene constituents; increased polarity may cause
asphaltene aggregation and sludge/sediment formation can furthermore be accelerated if
there is bacterial infestation in the fuel