375
Instability and Incompatibility
Such interactions are not always easy to define, and, thus, the measurement of instability and
incompatibility has involved visual observations, solubility tests hot filtration sediment (HFS), and
gum formation. However, such methods are often considered to be after-the-fact methods insofar as
they did not offer much in the way of predictability. In refinery processes (Chapter 15), predictability is not just a luxury, it is a necessity. The same principle must be applied to the measurement of
instability and incompatibility. Therefore, methods are continually being sought to aid in achieving
this goal.
In addition to the gravimetric methods, there have also been many attempts to use crude oil and/
or product characteristics and their relation to the sludge and deposit formation tendencies. In some
cases, a modicum of predictability is the outcome, but, in many cases, the data appear as preferred
ranges and are subject to personal interpretation. Therefore, caution is advised.
The purpose of this chapter is to document some of the more prominent methods used for determining instability and incompatibility. No preference will be shown, and none will be given to any
individual methods. It is the choice of the individual experimentalist to choose the method on the
basis of the type of fuel, the immediate needs, and the projected utilization of the data. As elsewhere, it the use of the data that often detracts from an otherwise sound method.
14.2 GENERAL ASPECTS
The instability/incompatibility of crude oil and of crude oil products is manifested in the formation
of sludge, sediment, and general darkening in color of the liquid (ASTM D1500; IP 17).
Sludge (or sediment) formation takes one of the following forms: (1) material dissolved in the
liquid; (2) precipitated material; and (3) material emulsified in the liquid. Under favorable conditions,
sludge or sediment will dissolve in the crude oil or product with the potential of increasing the viscosity. Sludge or sediment, which is not soluble in the crude oil (ASTM D96; ASTM D473; ASTM
D1796; ASTM D2273; ASTM D4007; ASTM D4807; ASTM D4870), may either settle at the bottom
of the storage tanks or remain in the crude oil as an emulsion. In most of the cases, the smaller part
of the sludge/sediment will settle satisfactorily, the larger part will stay in the crude oil as emulsions.
In any case, there is a need of breaking the emulsion, whether it is a water-in-oil emulsion or it is the
sludge itself, which has to be separated into the oily phase and the aqueous phase. The oily phase can
be then processed with the crude oil, and the aqueous phase can be drained out of the system.
Phase separation can be accomplished by either the use of suitable surface active agents allowing
for sufficient settling time, or by use of a high voltage electric field for breaking such emulsions after
admixing water at a rate of about 5% and at a temperature of about 100°C (212°F)
Emulsion breaking, whether the emulsion is due to crude oil–sludge emulsions or crude oil–
water emulsions, or breaking the sludge into organic (oily) and inorganic components is of major
importance from operational as well as commercial aspects. With some heavy fuel oil products
and heavy crude oils phase separation, difficulties often arise (Ruzicka and Nordenson, 1990; Por,
1992; Mushrush and Speight, 1995). Also, some crude oil emulsions may be stabilized by naturally
occurring substances in the crude oil. Many of these polar particles accumulate at the oil–water
interface, with the polar groups directed toward the water and the hydrocarbon groups toward the
oil. A stable interfacial skin may be so formed; particles of clay or similar impurities, as well as
wax crystals present in the oil may be embedded in this skin and make the emulsion very difficult
to break (Schramm, 1992).
Chemical and electrical methods for sludge removal and for water removal, often combined
with chemical additives, have to be used for breaking such emulsions. Each emulsion has its own
structure and characteristics: water-in-oil emulsions where the oil is the major component or oil-inwater emulsions where the water is the major component. The chemical and physical nature of the
components of the emulsion plays a major role in their susceptibility to the various surface-active
agents used for breaking them.
Instability and Incompatibility
Such interactions are not always easy to define, and, thus, the measurement of instability and
incompatibility has involved visual observations, solubility tests hot filtration sediment (HFS), and
gum formation. However, such methods are often considered to be after-the-fact methods insofar as
they did not offer much in the way of predictability. In refinery processes (Chapter 15), predictability is not just a luxury, it is a necessity. The same principle must be applied to the measurement of
instability and incompatibility. Therefore, methods are continually being sought to aid in achieving
this goal.
In addition to the gravimetric methods, there have also been many attempts to use crude oil and/
or product characteristics and their relation to the sludge and deposit formation tendencies. In some
cases, a modicum of predictability is the outcome, but, in many cases, the data appear as preferred
ranges and are subject to personal interpretation. Therefore, caution is advised.
The purpose of this chapter is to document some of the more prominent methods used for determining instability and incompatibility. No preference will be shown, and none will be given to any
individual methods. It is the choice of the individual experimentalist to choose the method on the
basis of the type of fuel, the immediate needs, and the projected utilization of the data. As elsewhere, it the use of the data that often detracts from an otherwise sound method.
14.2 GENERAL ASPECTS
The instability/incompatibility of crude oil and of crude oil products is manifested in the formation
of sludge, sediment, and general darkening in color of the liquid (ASTM D1500; IP 17).
Sludge (or sediment) formation takes one of the following forms: (1) material dissolved in the
liquid; (2) precipitated material; and (3) material emulsified in the liquid. Under favorable conditions,
sludge or sediment will dissolve in the crude oil or product with the potential of increasing the viscosity. Sludge or sediment, which is not soluble in the crude oil (ASTM D96; ASTM D473; ASTM
D1796; ASTM D2273; ASTM D4007; ASTM D4807; ASTM D4870), may either settle at the bottom
of the storage tanks or remain in the crude oil as an emulsion. In most of the cases, the smaller part
of the sludge/sediment will settle satisfactorily, the larger part will stay in the crude oil as emulsions.
In any case, there is a need of breaking the emulsion, whether it is a water-in-oil emulsion or it is the
sludge itself, which has to be separated into the oily phase and the aqueous phase. The oily phase can
be then processed with the crude oil, and the aqueous phase can be drained out of the system.
Phase separation can be accomplished by either the use of suitable surface active agents allowing
for sufficient settling time, or by use of a high voltage electric field for breaking such emulsions after
admixing water at a rate of about 5% and at a temperature of about 100°C (212°F)
Emulsion breaking, whether the emulsion is due to crude oil–sludge emulsions or crude oil–
water emulsions, or breaking the sludge into organic (oily) and inorganic components is of major
importance from operational as well as commercial aspects. With some heavy fuel oil products
and heavy crude oils phase separation, difficulties often arise (Ruzicka and Nordenson, 1990; Por,
1992; Mushrush and Speight, 1995). Also, some crude oil emulsions may be stabilized by naturally
occurring substances in the crude oil. Many of these polar particles accumulate at the oil–water
interface, with the polar groups directed toward the water and the hydrocarbon groups toward the
oil. A stable interfacial skin may be so formed; particles of clay or similar impurities, as well as
wax crystals present in the oil may be embedded in this skin and make the emulsion very difficult
to break (Schramm, 1992).
Chemical and electrical methods for sludge removal and for water removal, often combined
with chemical additives, have to be used for breaking such emulsions. Each emulsion has its own
structure and characteristics: water-in-oil emulsions where the oil is the major component or oil-inwater emulsions where the water is the major component. The chemical and physical nature of the
components of the emulsion plays a major role in their susceptibility to the various surface-active
agents used for breaking them.