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Fractional
distillation is one of the unit operations of
chemical engineering. Fractionating columns are
widely used in the chemical process industries
where large quantities of liquids have to be
distilled.[3][4][5] Such industries are the
petroleum processing, petrochemical production,
natural gas processing, coal tar processing,
brewing, liquified air separation, and
hydrocarbon solvents production and similar
industries but it finds its widest application
in petroleum refineries. In such refineries, the
crude oil feedstock is a very complex
multicomponent mixture that must be separated
and yields of pure chemical compounds are not
expected, only groups of compounds within a
relatively small range of boiling points, also
called fractions and that is the origin of the
name fractional distillation or fractionation.
It is often not worthwhile separating the
components in these fractions any further based
on product requirements and economics.
Industrial distillation is typically performed
in large, vertical cylindrical columns known as
"distillation towers" or "distillation columns"
with diameters ranging from about 65 centimeters
to 6 meters and heights ranging from about 6
meters to 60 meters or more.
Industrial distillation towers are usually
operated at a continuous steady state. Unless
disturbed by changes in feed, heat, ambient
temperature, or condensing, the amount of feed
being added normally equals the amount of
product being removed.
It should also be noted that the amount of heat
entering the column from the reboiler and with
the feed must equal the amount heat removed by
the overhead condenser and with the products.
Image 3 depicts an industrial fractionating
column separating a feed stream into one
distillate fraction and one bottoms fraction.
However, many industrial fractionating columns
have outlets at intervals up the column so that
multiple products having different boiling
ranges may be withdrawn from a column distilling
a multi-component feed stream. The "lightest"
products with the lowest boiling points exit
from the top of the columns and the "heaviest"
products with the highest boiling points exit
from the bottom.
Industrial fractionating columns use external
reflux to achieve better separation of products.
Reflux refers to the portion of the condensed
overhead liquid product that returns to the
upper part of the fractionating column as shown
in Image 3.
Inside the column, the downflowing reflux liquid
provides cooling and condensation of upflowing
vapors thereby increasing the efficacy of the
distillation tower. The more reflux and/or more
trays provided, the better is the tower's
separation of lower boiling materials from
higher boiling materials.
The design and operation of a fractionating
column depends on the composition of the feed
and as well as the composition of the desired
products. Given a simple, binary component feed,
analytical methods such as the McCabe-Thiele
method or the Fenske equation can be used. For a
multi-component feed, simulation models are used
both for design, operation and construction.
Bubble-cap "trays" or "plates" are one of the
types of physical devices which are used to
provide good contact between the upflowing vapor
and the downflowing liquid inside an industrial
fractionating column.
The efficiency of a tray or plate is typically
lower than that of a theoretical 100% efficient
equilibrium stage. Hence, a fractionating column
almost always needs more actual, physical plates
than the required number of theoretical
vapor-liquid equilibrium stages.
In industrial uses, sometimes a packing material
is used in the column instead of trays,
especially when low pressure drops across the
column are required, as when operating under
vacuum. This packing material can either be
random dumped packing (1–3" wide) such as
Raschig rings or structured sheet metal. Liquids
tend to wet the surface of the packing and the
vapors pass across this wetted surface, where
mass transfer takes place. Differently shaped
packings have different surface areas and void
space between packings. Both of these factors
affect packing performance.
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