New Star Software Collection/NSS_Collection.iso/3-251 oo2 english program/1.ima/RR.ZIP/PREFINGI.SEQ

Text File | 1991-06-30 | 5.3 KB | 100 lines

Crude oil as it comes out of the ground is a mixture of
compounds called hydrocarbons. These consist of atoms of
hydrogen and carbon. Crude oil usually also contains some
impurities such as sulphur. The hydrogen and carbon can combine
to form many kinds of molecules. The simple molecules occur as
gases, the more complex ones as liquids, and the ones consisting
of very long chains of atoms occur as semisolids or solids. Each
of these separate constituents of crude oil is called a fraction
of the crude oil.

The purpose of the refining process is to separate the crude oil
into fractions, which become consumer products such as kerosene,
gasoline, and heating and lubricating oils. At the same time,
sulphur and other impurities are removed from the oil.

Refineries not only process crude oil but also handle gases
which come from the well along with the oil. Formerly, these
gases were simply "burnt off" at the well site, but now they are
more often recovered and processed to extract butane and propane
(liquefied petroleum gas).

The first process in oil refining is distillation of crude oil.
Distillation involves
boiling a liquid to turn it into vapour, and then condensing the
vapour on a cool surface. We can distill a single liquid, like
water, to purify it. Alternatively, we can distill a mixture,
like crude oil, to separate it into its different constituents.

This separation relies on the fact that different liquids have
different boiling points. If we cool their vapours gradually,
each constituent liquid condenses at the place where the
temperature is just below its boiling point.

In a refinery, crude oil is distilled into separate liquids in a
tall steel tower known as a fractionating column. It is called
this because each separate constituent of crude oil is known as
a fraction. The column is kept very hot at the bottom, but the
temperature gradually decreases towards the top. The inside of
the column is divided at intervals by horizontal trays with
holes in them. Going up the column, each tray is cooler than the
one below it, providing a sequence of decreasing temperatures at
which separate vapours can condense.

The crude oil is first heated by a furnace, and then passed into
the lower part of the column. Most of the fractions in the oil
are already boiling, so they vaporize and rise up the column
through the holes in the trays. They lose heat as they rise, and
when each fraction reaches the tray where the temperature is
just below its own boiling point, it condenses and changes back
into liquid. As the fractions condense on their separate trays,
they are drawn off by pipes. Distillation is continuous, with
hot crude oil flowing in near the base of the column, and the
separate fractions flowing out.

The fractions that rise highest in the column are called light
fractions, and those that condense on the lower trays are called
heavy fractions. The very lightest fraction, taken off at the
top of the column, is refinery gas, which remains a vapour, and
is used as a fuel in the refinery. Other light fractions are
liquefied petroleum gases, gasoline (petrol), and naphtha, a
major source of raw materials for the chemical industry. A
slightly heavier fraction is kerosene (paraffin oil).
Heavier again is gasoil, which is used as a fuel for diesel engines, and
for central heating. The heaviest fractions of all are drawn off
from the base of the column as fuel oils and bitumen, or asphalt.

Before the era of automobiles, kerosene was in greater demand
than gasoline. Now, however, refineries must produce much more
gasoline than kerosene. This is not possible through the process
of distillation alone, for a fractionating column can convert
only about 20% of the crude oil it receives into gasoline. Oil
companies asked their chemists to find ways of converting other
petroleum products into gasoline. So the chemists devised a way
of changing the chemical structure of the heavier fractions and
converting part of them to gasoline. This process, known as
cracking, breaks up the large molecules of the heavy fractions
into smaller molecules such as those in gasoline. As a result of
this process, the amount of gasoline that can be obtained from
crude oil has been increased to more than 80%.

Petrochemicals are perhaps one of the most exciting new
developments in oil refining. They are new substances formed by
altering the composition or shape of hydrocarbon molecules.
These molecules can be changed in an almost infinite variety of
ways: by adding or removing atoms, by changing the arrangement
of their atoms (from a cluster to a chain, for example), or by
combining them with atoms of other chemicals. The products of
petrochemistry are visible all around us. Our homes, cars, and
clothes contain synthetic substances which resemble cotton,
silk, wood, metal and rubber. On the farm, insecticides and
fertilizers made from petrochemicals have already been in use
for several decades.

It would be difficult to count the ways in which petroleum
refining has affected our lives. Few of us are aware of the
extent to which we are surrounded by petroleum products in one
form or another. In just a little more than a century, oil
products have not only assumed a central place in industrial
societies, but have changed the face of our world.