What is distillation and distillation
column?
Distillation is one of the most popular separation technique of separating
chemical substances depending on differences in their volatilities in a
boiling liquid mixture. It is a process in which a mixture of liquid or
vapor with two or more substances is separated into its desired purity
component fractions by the application and removal of heat. It is usually a
part of a larger chemical process and hence also referred to as a unit
operation.
A
distillation column is used widely in various industrial
applications, especially in gas plants, to make the process of distillation
more efficient. Distillation columns are specially designed columns in the
shape of a tall metal cylinder internally fitted with perforated horizontal
plates to achieve this separation efficiently.
Types of Distillation Column
- Batch column: Here the column feed is introduced batch-wise.
In other words once the column is charged with a 'batch', the
distillation process starts and when one batch is completed, the
distillation of the next feed starts.
- Continuous column: Continuous columns process on the other
hand involves a continuous feed stream. No interruptions occur unless
and until there is a problem with the surrounding process units.
Continuous column is the most common type of distillation column and can
handle high throughputs.
Types of Continuous Columns
Continuous columns can be further classified as follows:
Depending on the nature of the feed, there are two types of distillation
columns:
- Binary column: feed contains only two components
- Multi-component column: feed contains more than two
components
Depending on the number of product streams, we have
- Multi-product column: It has more than two product streams.
Depending on the extra feed that exits in the column, we have
- Azeotropic distillation: Where the extra feed is seen at the
top product stream
- Extractive distillation: Where the extra feed is seen at the
bottom product stream. Depending on the type of column internals
- Tray column: In this type of continuous column, trays of
various designs are there to hold up the liquid. This leads to a better
contact between vapor and liquid, hence better separation
- Packed column: Here packings are used , instead of trays, to
enhance contact between vapor and liquid.
Distillation column process description
As already mentioned, distillation means a process to separate a mixture of
two pure liquids with different boiling points and this is attained by
heating the mixture to a temperature between their respective boiling
points. A distillation column is required to undergo this process, which we
are explaining below:
A liquid feed containing the mixture of both liquids enters at one or more
points in the distillation column made up of a series of stacked plates. As
the liquid flows over the plates and travels down the column, due to the
existence of multiple plates vapor comes in contact with it quite an number
of times. This is one of the critical process in distillation columns. The
liquid and vapor phases comes in contact as one molecule of higher boiling
material is transformed from vapor to liquid phase by energy release,
another molecule of the low boiling material make use of the free energy to
convert from liquid to vapor phase.
The base of the distillation column consists of a large volume of higher
boiling point liquid. Out of the base, some of this liquid flows , some of
which is heated and then returned to the column. This is called the boil up.
Some vapor escapes from the column's top, which is returned to a liquid
state in the condenser. Some of this liquid is returned to the column and
the remainder is the top distillate product.
Distillation Column Design
Distillation columns are designed on the basis of the properties of the
boiling point of the components in the mixtures being separated. This is
done using the vapor-liquid equilibrium (VLE) data for the mixtures to be
separated. The VLE characteristics of the mixture will determine the number
of stages, and also the number of trays required for the separation. In
other words, the sizes, particularly the height of distillation columns and
the overall design are determined by the VLE data for the mixtures.
Factors Affecting Distillation Column Performance:
- Feed conditions: The state of the feed composition and feed
mixture has an influence on the operating lines, thereby determining the
number of stages required for separation. In other words, feed
conditions depend on Fluid flow conditions.
- Fluid flow conditions: Flow of vapor in the wrong
direction can lead to weeping/dumping, foaming, flooding etc.
- State of packings or trays: Any factors that lead to a
decrease in tray efficiency will also change the performance of the
distillation column.
- composition of feed
- state of feed
- race elements that can severely affect the vapor-liquid
equilibrium of liquid mixtures.
- Weather conditions: Change in the weather conditions can
greatly affect distillation column operation.
Diagrammatic Representation of application:
A typical sectional picture of what a distillation column looks like with
its applications.
Distillation
Column
