Separation Process
There are different techniques mainly used in petroleum industries for
the separation process. The
petroleum refining industry uses the same traditional chemical engineering
separation technologies as the chemical industry, including distillation,
crystallization, adsorption, membrane processes, absorption and stripping, and
extraction. Like the chemical industry, the petroleum refining industry would
benefit from separation technologies with improved energy efficiency, raw
materials efficiency, and cost effectiveness.
The
main separation process take placing in petroleum industries are:
Fractional
distillation
- Vacuum distillation
- Super fractionation
- Absorption
- Solvent Extraction
- Adsorption
- Crystallization
The primary process for separating the hydrocarbon
components of crude oil is fractional distillation. Crude oil distillers
separate crude oil into fractions for subsequent processing in such units as
catalytic reformers, cracking units, alkylation units, or cokers. In turn, each
of these more complex processing units also incorporates a fractional
distillation tower to separate its own reaction products.
Crude oil is
withdrawn from storage tanks at ambient temperature and pumped at a constant rate
through a series of heat exchangers in order to reach a temperature of about 120° C (250°
F). A controlled amount of fresh water is introduced, and the mixture is pumped into a desalting drum, where it passes through an electrical field and
a saltwater phase is separated. (If the salt were not removed at this stage, it
would be deposited later on the tubes of the furnace and cause plugging.) The
desalted crude oil passes through additional heat exchangers and then
through steel alloy tubes in a furnace. There it is heated to a temperature
between 315° and 400° C (600° and 750° F), depending on the type of crude oil and the end products desired. A mixture of vapour and
unvaporized oil passes from the furnace into the fractionating column, a vertical cylindrical tower as much as 45 metres (150 feet) high containing 20 to 40
fractionating trays spaced at regular intervals. The most common fractionating
trays are of the sieve or valve type.
The
oil vapours rise up through the column and are condensed to a liquid. A small
amount of gas remains uncondensed and is piped into the refinery fuel-gas
system. A pressure control valve on the fuel-gas line maintains fractionating column pressure at the desired figure, usually near atmospheric pressure (about 1 kilogram per square centimetre, or 15 pounds
per square inch). Part of the condensed liquid, called reflux, is pumped back into the top of the
column and descends from tray to tray, contacting rising vapours as they pass
through the slots in the trays. The liquid progressively absorbs heavier
constituents from the vapour and, in turn, gives up lighter constituents to the
vapour phase. Condensation and reevaporation takes place on each tray.
Eventually an equilibrium is reached in which there is a continual gradation of
temperature and oil properties throughout the column, with the lightest
constituents on the top tray and the heaviest on the bottom. The use of reflux
and vapour-liquid contacting trays distinguishes fractional distillation from
simple distillation columns.
Typical
boiling ranges for various streams are as follows: light straight-run naphtha
(overhead), 20°–95° C (70°–200° F); heavy naphtha (top side stream), 90°–165° C
(195°– 330° F); crude kerosene (second side stream), 150°–245° C (300°–475° F);
light gas oil (third side stream), 215°–315° C (420°–600° F).
Vacuum distillation
The principles of vacuum distillation resemble those of fractional
distillation (commonly called atmospheric distillation to distinguish it from
the vacuum method), except that larger-diameter columns are used to maintain
comparable vapour velocities at reduced operating pressures. A vacuum of 50 to
100 millimetres of mercury absolute is produced by a vacuum pump or steam ejector. The primary
advantage of vacuum distillation is that it allows for distilling heavier
materials at lower temperatures, thus avoiding thermal cracking of the
components.
Super fractionation
An extension of
the distillation process, superfractionation employs smaller-diameter columns
with a much larger number of trays (100 or more) and reflux ratios exceeding
5:1. With such equipment it is possible to isolate a very narrow range of
components or even pure compounds. Common applications involve the separation
of high-purity solvents such as isoparaffins or of individual aromatic compounds for
use as petrochemicals.
Absorption
Absorption
processes are employed to recover valuable light components such
as propane/propylene and butane/butylene from the vapours that leave the top of
crude-oil or process-unit fractionating columns within the refinery. These
volatile gases are bubbled through an absorption fluid, such as kerosene or
heavy naphtha, in equipment resembling a fractionating column. The light
products dissolve in the oil while the dry gases—such
as hydrogen, methane, ethane, and ethylene—pass through undissolved. Absorption
is more effective under pressures of about 7 to 11 kilograms per square
centimetre (100 to 150 pounds per square inch) than it is at atmospheric
pressure.
Solvent Extraction
Solvent
extraction processes are employed primarily for the removal of constituents
that would have an adverse effect on the performance of the product in use. An
important application is the removal of heavy aromatic compounds from
lubricating oils. Removal improves the viscosity-temperature relationship of
the product.
Adsorption
Certain
highly porous solid materials have the ability to select and adsorb specific
types of molecules, thus separating them from other materials. Silica gel is
used in this way to separate aromatics from other hydrocarbons, and activated
charcoal is used to remove liquid components from gases. Adsorption is thus
somewhat analogous to the process of absorption with an oil, although the
principles are different. Layers of adsorbed material only a few molecules
thick are formed on the extensive interior surface of the adsorbent; the
interior surface may amount to several hectares per kilogram of material. Molecular sieves are
a special form of adsorbent. Such sieves are produced by the dehydration of naturally
occurring or synthetic zeolites (crystalline alkali-metal aluminosilicates).
The dehydration leaves intercrystalline cavities that have pore openings of
definite size, depending on the alkali metal of
the zeolite. Under adsorptive conditions, normal paraffin molecules
can enter the crystalline lattice and be selectively retained, whereas all
other molecules are excluded. This principle is used commercially for the
removal of normal paraffins from gasoline fuels, thus improving their
combustion properties.
Crystallization
The
crystallization of wax from lubricating oil fractions is essential to make oils
suitable for use. A solvent (often a mixture of benzene and methyl ethyl ketone) is first added to the oil,
and the solution is chilled to about −20° C (−5° F). The function of the
benzene is to keep the oil in solution and maintain its fluidity at low
temperatures. 
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