1.0 Introduction
Fractional
distillation is the process by which oil refineries
separate crude oil into different, more useful hydrocarbon products
based on their relative molecular weights in a distillation tower. This is the
first step in the processing of crude oil, and it is considered to be the main
separation process as it performs the initial rough separation of the
different fuels. The different components that are separated out
during this process are known as fractions.
Fractions that are separated out include gasoline, diesel, kerosene,
and bitumen. Fractional distillation allows a lot of useful products
to be made from crude oil, with many environmental consequences for the use of
those useful products
1.1 Process
The process of fractional
distillation is fairly simple, but is powerful in the way that it separates all
the different, complex components of crude oil. First, the crude oil is heated
to vapourize it and is fed into the bottom of a distillation tower. The
resulting vapour then rises through the vertical column. As the gases rise
through the tower, the temperature decreases. As the temperature decreases,
certain hydrocarbons begin to condense and run off at different levels. Each
fraction that condenses off at a certain level contains hydrocarbon molecules
with a similar number of carbon atoms. These boiling point 'cuts' allow
several hydrocarbons to be separated out in a single process. It is this
cooling with the height of the tower that allows for the separation.
After this rough
refinement, individual fuels may undergo more refinement to remove any
contaminants or undesirable substances, or to improve the quality of the fuel
through cracking.
1.1.1 Fractions of petroleum
There are several ways of
classifying the useful fractions that are distilled from crude oil. One general
way is by dividing into three categories: light, middle, and heavy fractions.
Heavier components condense at higher temperatures and are removed at
the bottom of the column. The lighter fractions are able to rise higher in the
column before they are cooled to their condensing temperature, allowing them to
be removed at slightly higher levels. In addition to this, the fractions
have the following properties:
- Light distillate is
one of the more important fractions, and its products have boiling
points around 70-200°C. Useful hydrocarbons in this range include
gasoline, naphta (a chemical feedstock), kerosene, jet fuel, and paraffin.
These products are highly volatile, have small molecules, have
low boiling points, flow easily, and ignite easily.
- Medium distillate are
products that have boiling points of 200-350°C. Products in this
range include diesel fuel and gas oil - used in the
manufacturing of town gas and for commercial heating.
- Heavy distillates are
the products with the lowest volatility and have boiling points above
350°C. These fractions can be solid or semi-solid and may need to be
heated in order to flow. Fuel oil is produced in this fraction. These
products have large molecules, a low volatility, flow poorly, and do not
ignite easily.
However, there are two
major components that are not accounted for in these three categories. At the
very top of the tower are the gases that are too volatile to condense,such as
propane and butane. At the bottom are the "residuals" that contain
heavy tars too dense to rise up the tower, including bitumen and other waxes.
To further distill these they undergo steam or vacuum distillation as they are
very useful.
1.2 Locations of petroleum refinery in Nigeria
The downstream industry in
Nigeria is well established. NNPC has four refineries, two in Port
Harcourt (PHRC), and one
each in Kaduna (KRPC) and
Warri (WRPC). The
refineries have a combined installed capacity of 445,000 bpd. A comprehensive
network of pipelines and depots strategically located throughout Nigeria links
these refineries.
Cracking
of hydrocarbons
The process of breaking
higher hydrocarbons with high boiling points into a variety of lower
hydrocarbons that are more volatile (low boiling), is called cracking (or
pyrolysis). For example, a higher hydrocarbon C10H22 splits
according to the reaction.
The process of cracking,
increases the relative amounts of the lower hydrocarbons. During cracking,
carbon-carbon bonds get broken in a random manner, leading to various kinds of
products being formed.
Types of cracking
Thermal
cracking
Breaking down large
molecules by heating at high temperature and pressure is termed as thermal
cracking. Thermal cracking is further classified into the following classes.
- Liquid phase thermal cracking process:
The higher boiling fractions e.g., fuel oil, lubricating oil are converted
into low boiling fractions by heating the liquids at a temperature of 750
K, under a pressure of about 10 atmosphere.
- Vapor phase thermal cracking process:
Low boiling fraction e.g., kerosene is cracked in the vapor phase at a
temperature of about 875 K and under a pressure of 3 atmosphere.
Catalytic
cracking
Higher hydrocarbons can
also be cracked at lower temperature (600 - 650 K) and lower pressure (2 atm)
in the presence of a suitable catalyst. Catalytic cracking produces gasoline of
higher octane number and therefore this method is used for obtaining better
quality gasoline. A typical catalyst used for this purpose is a mixture of
silica (SiO2), 4 parts; alumina (Al2O3), 1
part, and manganese-dioxide (MnO2), 1 part.
Steam
cracking
Here, higher hydrocarbons
are mixed with steam in their vapor phase and heated for a short duration to
about 900°C, and cooled rapidly. This process is suitable for obtaining lower
unsaturated hydrocarbons.
1.3 Reforming
Reforming,
in chemistry, processing technique by which the molecular structure of a
hydrocarbon is rearranged to alter its properties. The process is frequently
applied to low-quality gasoline stocks to improve their combustion characteristics. Thermal
reforming alters the properties of low-grade naphthas by converting the
molecules into those of higher octane number by exposing the
materials to high temperatures and pressures. Catalytic reforming uses
a catalyst, usually platinum, to produce a similar result. Mixed with
hydrogen, naphtha is heated and passed over pellets of catalyst in a series of
reactors, under high pressure, producing high-octane gasoline.
Reforming or aromatisation involves the conversion of open chain (aliphatic) hydrocarbons and/or cycloalkanes in the presence of a catalyst, into aromatic hydrocarbons (arenes) containing the same number of carbon atoms. Aromatisation involves reactions of the type, dehydrogenation, cyclisation, and isomerisation.
In reforming (or aromatisation), cyclic and acyclic alkanes containing six to eight carbon atoms are heated at about 670 K in the presence of palladium, platinum or nickel as catalyst. Platinum seems to be the best catalyst and so the process is sometimes called platforming.
