219
Fractional Composition
However, when the added compound is relatively nonvolatile, it exists almost entirely in the liquid phase, and the process is actually extractive distillation.
Therefore, extractive distillation is distillation in the presence of a miscible, high boiling, relatively nonvolatile component—the solvent, which forms no azeotropes with the other components
in the mixture. It is widely used in the chemical and petrochemical industries for separating azeotropic (close boiling) and other constituents in a mixture.
In extractive distillation, the solvent is specially chosen to interact differently with the components of the original mixture, thereby altering their relative volatility. Because these interactions
occur predominantly in the liquid phase, the solvent is continuously added near the top of the
extractive distillation column so that an appreciable amount is present in the liquid phase on all
of the trays below. The mixture to be separated is added through second feed point further down
the column. In the extractive column, the component having the greater volatility, not necessarily
the component having the lowest boiling point, is taken overhead as a relatively pure distillate. The
other component leaves with the solvent via the column bottoms. The solvent is separated from the
remaining components in a second distillation column and then recycled back to the first column.
The separation of petroleum by distillation into fractions results in a concentration effect in
which the heteroatom constituents (metals included) occur for the most part in the residua (Long
and Speight, 1990). Sulfur, because of its ubiquitous molecular nature, is often an exception to this
generalization and occurs in most distillation fractions.
Finally, a comment about distillation and its use in the fractionation of petroleum: the issues
that arise when distillation is employed relate to the composition of the nonvolatile residue. For the
purpose of illustration, if it is assumed that atmospheric residua (boiling range >345°C, >650°F)
have a >C 20 cutoff and vacuum residua (boiling range >565°C, 1050°F) have a >C 35 cutoff, distillation can leave as much as 60% w/w of the original oil unfractionated. It is because of this limitation
that other methods of fractionation have been sought.
In summary, and as already noted, the distribution of a component between vapor and solution
can be expressed as a function of temperature and pressure. Thus, at a specific temperature, the
vapor phase in equilibrium with the liquid phase at this temperature is relatively richer in the more
volatile components. The component with higher vapor pressures, the liquid phase is relatively
richer in the less volatile components; the components with lower vapor pressures, and separation
of components may be achieved.
9.3 SOLVENT TREATMENT
The use of solvents invokes the concept of the solubility (or insolubility) of a solute in the chosen
solvent. The solubility of a solute is the maximum quantity of solute that can dissolve in a certain
quantity of solvent or quantity of solution at a specified temperature. The main factors that have an
effect on solubility are (1) the nature of the solute and solvent, (2) temperature, and (3) pressure.
The rate of solution is a measure of how fast a substance dissolves. Some of the factors determining the rate of solution are (1) size of the particles, (2) stirring, (3) the amount of solute already
dissolved, and (4) temperature.
In order for a solvent to dissolve a solute, the particles of the solvent must be able to separate the
particles of the solute and occupy the intervening spaces. Polar solvent molecules can effectively
separate the molecules of other polar substances. This happens when the positive end of a solvent
molecule approaches the negative end of a solute molecule. A force of attraction then exists between
the two molecules. The solute molecule is pulled into solution when the force overcomes the attractive force between the solute molecule and its neighboring solute molecule. Ethyl alcohol and water
are examples of polar substances that readily dissolve in each other. Polar solvents can generally
dissolve solutes that are ionic.
Fractionation of petroleum by distillation is an excellent means by which the volatile constituents can be isolated and studied. However, the nonvolatile residuum, which may actually
Fractional Composition
However, when the added compound is relatively nonvolatile, it exists almost entirely in the liquid phase, and the process is actually extractive distillation.
Therefore, extractive distillation is distillation in the presence of a miscible, high boiling, relatively nonvolatile component—the solvent, which forms no azeotropes with the other components
in the mixture. It is widely used in the chemical and petrochemical industries for separating azeotropic (close boiling) and other constituents in a mixture.
In extractive distillation, the solvent is specially chosen to interact differently with the components of the original mixture, thereby altering their relative volatility. Because these interactions
occur predominantly in the liquid phase, the solvent is continuously added near the top of the
extractive distillation column so that an appreciable amount is present in the liquid phase on all
of the trays below. The mixture to be separated is added through second feed point further down
the column. In the extractive column, the component having the greater volatility, not necessarily
the component having the lowest boiling point, is taken overhead as a relatively pure distillate. The
other component leaves with the solvent via the column bottoms. The solvent is separated from the
remaining components in a second distillation column and then recycled back to the first column.
The separation of petroleum by distillation into fractions results in a concentration effect in
which the heteroatom constituents (metals included) occur for the most part in the residua (Long
and Speight, 1990). Sulfur, because of its ubiquitous molecular nature, is often an exception to this
generalization and occurs in most distillation fractions.
Finally, a comment about distillation and its use in the fractionation of petroleum: the issues
that arise when distillation is employed relate to the composition of the nonvolatile residue. For the
purpose of illustration, if it is assumed that atmospheric residua (boiling range >345°C, >650°F)
have a >C 20 cutoff and vacuum residua (boiling range >565°C, 1050°F) have a >C 35 cutoff, distillation can leave as much as 60% w/w of the original oil unfractionated. It is because of this limitation
that other methods of fractionation have been sought.
In summary, and as already noted, the distribution of a component between vapor and solution
can be expressed as a function of temperature and pressure. Thus, at a specific temperature, the
vapor phase in equilibrium with the liquid phase at this temperature is relatively richer in the more
volatile components. The component with higher vapor pressures, the liquid phase is relatively
richer in the less volatile components; the components with lower vapor pressures, and separation
of components may be achieved.
9.3 SOLVENT TREATMENT
The use of solvents invokes the concept of the solubility (or insolubility) of a solute in the chosen
solvent. The solubility of a solute is the maximum quantity of solute that can dissolve in a certain
quantity of solvent or quantity of solution at a specified temperature. The main factors that have an
effect on solubility are (1) the nature of the solute and solvent, (2) temperature, and (3) pressure.
The rate of solution is a measure of how fast a substance dissolves. Some of the factors determining the rate of solution are (1) size of the particles, (2) stirring, (3) the amount of solute already
dissolved, and (4) temperature.
In order for a solvent to dissolve a solute, the particles of the solvent must be able to separate the
particles of the solute and occupy the intervening spaces. Polar solvent molecules can effectively
separate the molecules of other polar substances. This happens when the positive end of a solvent
molecule approaches the negative end of a solute molecule. A force of attraction then exists between
the two molecules. The solute molecule is pulled into solution when the force overcomes the attractive force between the solute molecule and its neighboring solute molecule. Ethyl alcohol and water
are examples of polar substances that readily dissolve in each other. Polar solvents can generally
dissolve solutes that are ionic.
Fractionation of petroleum by distillation is an excellent means by which the volatile constituents can be isolated and studied. However, the nonvolatile residuum, which may actually