238
The Chemistry and Technology of Petroleum
The most practical method of decomposition of the adduct is by solution, generally with hot water.
The adducted hydrocarbons form an immiscible layer on top of the aqueous urea solution and may
be readily separated. Volatile reactants may be recovered by heating the adduct, either dry or with
steam, and collecting the liberated hydrocarbon as it is released. Actually, a crude fractional dissociation may be accomplished in this manner or by partial solvent extraction. The least stable adduct
formers are released first and may be collected. Separations of compounds from mixtures are carried
out, in effect, on the basis of their stability. The most desirable results are generally obtained if all the
possible material is first precipitated as the adduct; the recovered adducted material is then fractionally re-adducted. For such fractionation, the reprecipitation may be performed (1) by using a quantity
of reagent insufficient to adduct all the material first adducted, (2) at a different temperature using a
more optimum reagent to reactant ratio, and (3) by applying both techniques at the same time.
In a system containing essentially only one homologous series, such as the n-paraffins in paraffin
waxes, the use of a fractional adduction technique furnishes a separation on the basis of molecular
weight.
In a narrow molecular weight distillation fraction containing several adductible hydrocarbon
types, separation can be accomplished as a result of the difference in stability of the adducts of
the different hydrocarbon homologous series. With urea the n-paraffins react first, followed by the
slightly branched iso-paraffins and by the more highly branched iso-paraffins and the cyclic structures last. However, working with a wide molecular weight fraction, such as petroleum waxes, the
stability of the urea adducts of lower n-paraffins present are of the same order of magnitude as those
of the adducts of higher iso-paraffins and monocyclic compounds; a mixture of types is of different
molecular weights. Hence, fractional distillation of the hydrocarbon samples either before or after
adduction allows better analysis of the various fractions.
Fractionation may also be effected by selective decomposition of the adduct or by selective
replacement. In the former method, the adduct is extracted using a solvent of relatively poor
dissociating power whereby the least stable adducts tend to dissociate first. In selective replacement, the hydrocarbons of the less stable adducts are gradually displaced by slightly more stable
adductors, and these in turn are displaced by more stable adduct formers until the most stable
n-paraffins are employed.
The relative instabilities of the thiourea adducts renders the method less useful than the urea
method. In addition, the less selective nature of thiourea adduction and the fact that the large differences in stability within a homologous series with the urea adducts do not apply to the thiourea
adducts.
9.6 USE OF THE DATA
In the simplest sense, petroleum can be considered composites of four major operational fractions.
However, it must never be forgotten that the nomenclature of these fractions lies within the historical
development of petroleum science and that the fraction names are operational and are related more
to the general characteristics than to the identification of specific compound types. Nevertheless,
once a convenient fractionation technique has been established, it is possible to compare a variety
of different feedstocks varying from a conventional petroleum to a propane asphalt (Corbett and
Petrossi, 1978).
It is noteworthy here that, throughout the history of studies related to petroleum composition,
there has been considerable attention paid to the asphaltene fraction and the resin fraction. This is
due in no small part to the tendency of the asphaltenes to be responsible for high yields of thermal
coke and also for shortened catalyst lifetimes in refinery operations (Speight, 2000). In fact, it is
the unknown character of the asphaltenes that has also been responsible for drawing the attention
of investigators for the last five decades (Speight, 1984, 1994). Residua contain the majority of all
of the potential coke-forming constituents and catalyst poisons that were originally in the crude oil
because the distillation process is a concentration process. Most of the coke-formers and catalyst
The Chemistry and Technology of Petroleum
The most practical method of decomposition of the adduct is by solution, generally with hot water.
The adducted hydrocarbons form an immiscible layer on top of the aqueous urea solution and may
be readily separated. Volatile reactants may be recovered by heating the adduct, either dry or with
steam, and collecting the liberated hydrocarbon as it is released. Actually, a crude fractional dissociation may be accomplished in this manner or by partial solvent extraction. The least stable adduct
formers are released first and may be collected. Separations of compounds from mixtures are carried
out, in effect, on the basis of their stability. The most desirable results are generally obtained if all the
possible material is first precipitated as the adduct; the recovered adducted material is then fractionally re-adducted. For such fractionation, the reprecipitation may be performed (1) by using a quantity
of reagent insufficient to adduct all the material first adducted, (2) at a different temperature using a
more optimum reagent to reactant ratio, and (3) by applying both techniques at the same time.
In a system containing essentially only one homologous series, such as the n-paraffins in paraffin
waxes, the use of a fractional adduction technique furnishes a separation on the basis of molecular
weight.
In a narrow molecular weight distillation fraction containing several adductible hydrocarbon
types, separation can be accomplished as a result of the difference in stability of the adducts of
the different hydrocarbon homologous series. With urea the n-paraffins react first, followed by the
slightly branched iso-paraffins and by the more highly branched iso-paraffins and the cyclic structures last. However, working with a wide molecular weight fraction, such as petroleum waxes, the
stability of the urea adducts of lower n-paraffins present are of the same order of magnitude as those
of the adducts of higher iso-paraffins and monocyclic compounds; a mixture of types is of different
molecular weights. Hence, fractional distillation of the hydrocarbon samples either before or after
adduction allows better analysis of the various fractions.
Fractionation may also be effected by selective decomposition of the adduct or by selective
replacement. In the former method, the adduct is extracted using a solvent of relatively poor
dissociating power whereby the least stable adducts tend to dissociate first. In selective replacement, the hydrocarbons of the less stable adducts are gradually displaced by slightly more stable
adductors, and these in turn are displaced by more stable adduct formers until the most stable
n-paraffins are employed.
The relative instabilities of the thiourea adducts renders the method less useful than the urea
method. In addition, the less selective nature of thiourea adduction and the fact that the large differences in stability within a homologous series with the urea adducts do not apply to the thiourea
adducts.
9.6 USE OF THE DATA
In the simplest sense, petroleum can be considered composites of four major operational fractions.
However, it must never be forgotten that the nomenclature of these fractions lies within the historical
development of petroleum science and that the fraction names are operational and are related more
to the general characteristics than to the identification of specific compound types. Nevertheless,
once a convenient fractionation technique has been established, it is possible to compare a variety
of different feedstocks varying from a conventional petroleum to a propane asphalt (Corbett and
Petrossi, 1978).
It is noteworthy here that, throughout the history of studies related to petroleum composition,
there has been considerable attention paid to the asphaltene fraction and the resin fraction. This is
due in no small part to the tendency of the asphaltenes to be responsible for high yields of thermal
coke and also for shortened catalyst lifetimes in refinery operations (Speight, 2000). In fact, it is
the unknown character of the asphaltenes that has also been responsible for drawing the attention
of investigators for the last five decades (Speight, 1984, 1994). Residua contain the majority of all
of the potential coke-forming constituents and catalyst poisons that were originally in the crude oil
because the distillation process is a concentration process. Most of the coke-formers and catalyst