(2) The material removed with the solvent is called extract and is sent out of the lubricating oil
system for other uses.
(3) Many different solvents are employed in the petroleum industry, the choice of the specific solvent
depends upon the compounds to be extracted.
7.
Conversion Processes.
Most of the components separated by the distillation of crude oil must undergo certain conversion processes
before they become marketable products. In the conversion processes, the ratio of carbon to hydrogen is changed
and the size and arrangement of the hydrocarbon molecules are altered. Desired conversion may be accomplished
by employing one or more processes. These processes include:
a. Cracking. Cracking is the process of subdividing large hydrocarbons molecules into smaller ones.
When molecules are heated, the effect is to weaken the grasp that one atom has on another. If the temperature is
sufficiently high and is maintained in that condition long enough, it is possible to break some of the mutual bonds
between the carbon atoms. This results in the breaking up or cracking of the original molecule into two or more
smaller molecules.
(1) Commercially, the purpose of cracking is to increase the yield of gasoline per barrel of crude by
subdividing large hydrocarbon molecules to form the lighter hydrocarbon molecules of gasoline and to improve
the quality of gasoline.
(2) Cracking is accomplished by means of heat. The rate and severity of cracking is a direct function
of the temperature to which the oil is raised by application of heat and the time allowed for the reaction. The
reaction may be accelerated by the use of a catalyst. Pressure is used in cracking units to confine the volume of
oil and vapors to manageable size. Pressure does not accomplish cracking; in fact, it has a slight tendency to
suppress cracking and a definite tendency to promote polymerization (the opposite of cracking) of the products of
the cracking reaction. By careful control of the temperature at which the cracking reaction is carried out, the
maximum of desirable constituents can be produced. Extremes of temperature and time would convert any oil to
light gases and coke.
b. Thermal Cracking. A cracking process using only temperature and time is called Thermal Cracking.
When a catalyst is added, the process is called Catalytic Cracking. A catalyst is a substance which will influence
a chemical reaction without being changed itself.
(1) In thermal cracking, the charge stock is subjected to intense heat and pressure for a period of
time. Normally, this process operates at a temperature of approximately 900ƒF and under a pressure of 200 to 750
PSI.
(2) Thermal cracking produces a mixture of hydrocarbons that closely resembles crude oil with
respect to boiling range. The mixture is distilled right in the cracking unit, and those parts of the mixture which
are not desired for finished products are returned to the cracking zone so that the net products are:
(a) Dry gases (methane and ethane).
(b) Liquid petroleum gases (essentially propane and propylene).
(c) Cracked gasoline.
(d) Either heavy fuel oil or coke.
20-5
QM5200
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