Distillation columns are key unit
operations in traditional chemical
engineering, especially in the oil and gas
industry. They are usually tall structures filled with heated flammable fluids
usually under pressure, and are consequently unavoidably hazardous. Many
serious accidents have centred on columns and their secondary operations. Where
they are present, the layout of distillation columns should receive early
investigation since the layout of a number of other major items of equipment
usually depends upon their placement, and they can have a high potential for
initiating a situation in which one incident is sets off a chain of accidental
incident from fire, explosion or collapse.
Main Components of Distillation Columns
Distillation columns are made up of
several components, each of which is used either to transfer heat energy or
enhance material transfer. A typical distillation contains several major
components:
o
a
column internals such as trays/plates and packings which are used to enhance
component separations
o
a
vertical shell where the separation of liquid components is carried out
o
a
reflux drum to hold the condensed vapour from the top of the column so that
liquid (reflux) can be recycled back to the column
o
a
condenser to cool and condense the vapour leaving the top of the column
o
a
reboiler to provide the necessary vaporisation for the distillation process
The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column. A schematic of a typical distillation unit with a single feed and two product streams is shown below:
Basic Operation and Terminology
The liquid
mixture that is to be processed is known as the feed and this is introduced
usually somewhere near the middle of the column to a tray known as the feed
tray. The feed tray divides the column into a top (enriching or rectification)
section and a bottom (stripping) section. The feed flows down the column where
it is collected at the bottom in the reboiler.
Heat is supplied to the
reboiler to generate vapour. The source of heat input can be any suitable
fluid, although in most chemical plants this is normally steam. In refineries,
the heating source may be the output streams of other columns. The vapour raised in the reboiler is
re-introduced into the unit at the bottom of the column. The liquid removed
from the reboiler is known as the bottom product or simply, bottoms.
The vapour
moves up the column, and as it exits the top of the unit, it is cooled by a
condenser. The condensed liquid is stored in a holding vessel known as the
reflux drum. Some of this liquid is recycled back to the top of the column and
this is called the reflux. The condensed liquid that is removed from the system
is known as the distillate or top product.
Thus, there are internal flows of
vapour and liquid within the column as well as external flows of feeds and
product streams, into and out of the column.
Crude Oil Distillation
A crude oil refinery is a group of
industrial facilities that turns crude oil and other inputs into finished
petroleum products. A refinery's capacity refers to the maximum amount of crude
oil designed to flow into the distillation unit of a refinery, also known as
the crude unit.
The diagram presents a stylized version
of the distillation process. Crude oil is made up of a mixture of hydrocarbons,
and the distillation process aims to separate this crude oil into broad
categories of its component hydrocarbons, or "fractions." Crude oil
is first heated and then put into a distillation column, also known as a still,
where different products boil off and are recovered at different temperatures.
o Lighter products, such as
butane and other liquid petroleum gases (LPG), gasoline blending components,
and naphtha, are recovered at the lowest temperatures.
o Mid-range products include jet
fuel, kerosene, and distillates (such as home heating oil and diesel fuel).
o
The
heaviest products such as residual fuel oil is recovered at temperatures
sometimes over 1,000 degrees Fahrenheit.
The simplest refineries stop at this
point. Most refineries in the United States reprocess the heavier fractions
into lighter products to maximize the output of the most desirable products
using more sophisticated refining equipment such as catalytic crackers,
reformers, and cokes.
Types
of Distillation Columns
There are many types of
distillation columns, each designed to perform specific types of separations,
and each design differs in terms of complexity.
Batch Columns
In batch operation, the feed to the
column is introduced batch-wise. That is, the column is charged with a 'batch'
and then the distillation process is carried out. When the desired task is
achieved, a next batch of feed is introduced.
Continuous Columns
In contrast, continuous columns process a
continuous feed stream. No interruptions occur unless there is a problem with
the column or surrounding process units. They are capable of handling high
throughputs and are the most common of the two types. We shall concentrate only
on this class of columns.
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