235
Fractional Composition
The method was accepted by the American Society for Testing and Materials as a standard method
of test for Characteristic Groups in Rubber Extender and Processing Oils by the Precipitation
Method (ASTM D2006). However, there is still some doubt regarding the applicability of the
method to a wide variety of petroleum, petroleum residua, and other bituminous materials.
In the present author’s experience, application of the method to conventional crude oil, heavy oil,
and bitumen has led, in some instances, to emulsion formation and hence to difficulties in the separation procedure. It must also be remembered that the use of the strong grades of sulfuric acid can,
and presumably does, lead to sulfonation of the constituents in many instances, and regeneration of
these constituents to their natural state may be difficult if not impossible. Thus it is not feasible to
compare the sulfuric acid method to the relatively simple precipitation of the basic constituents of
petroleum (as their hydrochlorides) by passage of dry hydrogen chloride gas through a solution of
the material in a solvent, such as carbon disulfide. It therefore appears that a number of refinements
of the sulfuric acid method are desirable since it is apparent that not all carbonaceous liquids react
in the same manner with sulfuric acid.
9.5.2 moleCulAr ComPlex FormAtIon
The formation of crystalline molecular complexes between urea (H 2 N . C=O . NH 2 ) or thiourea
(H 2 N . C=S . NH 2 ) and hydrocarbons has been known since the 1940s, and it is not surprising that
the technique has received considerable attention with respect to its use in petroleum chemistry.
The previously described techniques of adsorption and distillation differentiate molecules by
class and volatility (or size within any one homologous series), respectively, whereas adduct formation separates on the basis of molecular shape and, to a lesser extent, by size and class. When
combined with the older fractionation methods, adduct formation can often be useful for solving
separation problems, provided the limitations of the method are realized. Urea and thiourea adduction are not completely selective, as was first supposed, and there is an overlapping of structural
types that adduct, especially among the higher molecular weight hydrocarbons.
Adduct formation may be achieved merely by bringing together the adduct-former and reagent
under a wide range of reaction conditions. Preferably the reactant is an inert hydrocarbon solvent,
a reagent solvent, referred to as the activator, present in varying amounts to increase the rate of
reaction, and the crystalline product that precipitates can be conveniently separated by filtration.
Furthermore, the adduct may be decomposed easily and the adducting material recovered by one
of several procedures.
9.5.2.1 Urea Adduction
For a hydrocarbon to form an adduct with urea, it is mandatory that the compound contain a long,
unbranched chain, usually a chain of at least six carbon atoms, under the conditions most often
employed, such as ambient temperature and atmospheric pressure. As side chains or ring structures
are added to the molecule, the chain must be lengthened if adduction is to occur. Generally, the
larger the size or number of the substituents or the farther the substituent is from the terminal carbon
atom, the longer must be the unbranched portion of the chain. Olefinic unsaturation usually has little
effect on adduct formation, but the stability of the adduct of n-olefins are somewhat less than those
of the corresponding n-paraffins. Thus, n-pentane does not form an adduct under normal conditions.
However, an adduct can be obtained at low temperature with some pressure, and n-hexane is the first
member of the n-alkane series that forms an adduct at room temperature and atmospheric pressure;
under similar conditions, 1-octene is the lowest olefin reported to adduct. There is usually no upper
limit to chain length other than that imposed by the solubility of the hydrocarbon; however, as the
reaction temperature is increased to meet the requirements of hydrocarbon solubility and preferred
reaction rate, the stability of the urea lattice in the adduct decreases.
In any given homologous series, the ease of adduct formation and adduct stability increases with
increasing straight-chain length. Single methyl side chains, such as those present in compounds
Fractional Composition
The method was accepted by the American Society for Testing and Materials as a standard method
of test for Characteristic Groups in Rubber Extender and Processing Oils by the Precipitation
Method (ASTM D2006). However, there is still some doubt regarding the applicability of the
method to a wide variety of petroleum, petroleum residua, and other bituminous materials.
In the present author’s experience, application of the method to conventional crude oil, heavy oil,
and bitumen has led, in some instances, to emulsion formation and hence to difficulties in the separation procedure. It must also be remembered that the use of the strong grades of sulfuric acid can,
and presumably does, lead to sulfonation of the constituents in many instances, and regeneration of
these constituents to their natural state may be difficult if not impossible. Thus it is not feasible to
compare the sulfuric acid method to the relatively simple precipitation of the basic constituents of
petroleum (as their hydrochlorides) by passage of dry hydrogen chloride gas through a solution of
the material in a solvent, such as carbon disulfide. It therefore appears that a number of refinements
of the sulfuric acid method are desirable since it is apparent that not all carbonaceous liquids react
in the same manner with sulfuric acid.
9.5.2 moleCulAr ComPlex FormAtIon
The formation of crystalline molecular complexes between urea (H 2 N . C=O . NH 2 ) or thiourea
(H 2 N . C=S . NH 2 ) and hydrocarbons has been known since the 1940s, and it is not surprising that
the technique has received considerable attention with respect to its use in petroleum chemistry.
The previously described techniques of adsorption and distillation differentiate molecules by
class and volatility (or size within any one homologous series), respectively, whereas adduct formation separates on the basis of molecular shape and, to a lesser extent, by size and class. When
combined with the older fractionation methods, adduct formation can often be useful for solving
separation problems, provided the limitations of the method are realized. Urea and thiourea adduction are not completely selective, as was first supposed, and there is an overlapping of structural
types that adduct, especially among the higher molecular weight hydrocarbons.
Adduct formation may be achieved merely by bringing together the adduct-former and reagent
under a wide range of reaction conditions. Preferably the reactant is an inert hydrocarbon solvent,
a reagent solvent, referred to as the activator, present in varying amounts to increase the rate of
reaction, and the crystalline product that precipitates can be conveniently separated by filtration.
Furthermore, the adduct may be decomposed easily and the adducting material recovered by one
of several procedures.
9.5.2.1 Urea Adduction
For a hydrocarbon to form an adduct with urea, it is mandatory that the compound contain a long,
unbranched chain, usually a chain of at least six carbon atoms, under the conditions most often
employed, such as ambient temperature and atmospheric pressure. As side chains or ring structures
are added to the molecule, the chain must be lengthened if adduction is to occur. Generally, the
larger the size or number of the substituents or the farther the substituent is from the terminal carbon
atom, the longer must be the unbranched portion of the chain. Olefinic unsaturation usually has little
effect on adduct formation, but the stability of the adduct of n-olefins are somewhat less than those
of the corresponding n-paraffins. Thus, n-pentane does not form an adduct under normal conditions.
However, an adduct can be obtained at low temperature with some pressure, and n-hexane is the first
member of the n-alkane series that forms an adduct at room temperature and atmospheric pressure;
under similar conditions, 1-octene is the lowest olefin reported to adduct. There is usually no upper
limit to chain length other than that imposed by the solubility of the hydrocarbon; however, as the
reaction temperature is increased to meet the requirements of hydrocarbon solubility and preferred
reaction rate, the stability of the urea lattice in the adduct decreases.
In any given homologous series, the ease of adduct formation and adduct stability increases with
increasing straight-chain length. Single methyl side chains, such as those present in compounds