Distillation

Distillation

is defined as:

a

process in which a liquid or vapour mixture of two or more substances

is separated into its component fractions of desired purity, by

the application and removal of heat.

alt="Boiling and condensing" align="left">Distillation is based on the fact

that the vapour of a boiling mixture will be richer in the components

that have lower boiling points.

Therefore,

when this vapour is cooled and condensed, the condensate will contain

more volatile components. At the same time, the original mixture will

contain more of the less volatile material.

Distillation

columns are designed to achieve this separation efficiently.

Although

many people have a fair idea what “distillation” means,

the important aspects that seem to be missed from the manufacturing

point of view are that:














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distillation

is the most common separation technique






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it

consumes enormous amounts of energy, both in terms of cooling

and heating requirements






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it

can contribute to more than 50% of plant operating costs











The

best way to reduce operating costs of existing units, is to improve

their efficiency and operation via process optimisation and control.

To achieve this improvement, a thorough understanding of distillation

principles and how distillation systems are designed is essential.

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 and Continuous Columns







One

way of classifying distillation column type is to look at how

they are operated. Thus we have:







batch

and







continuous

columns.





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.

Types of Continuous Columns







Continuous

columns can be further classified according to:

the

nature of the feed that they are processing,









binary

column - feed contains only two components







multi-component

column - feed contains more than two components





the

number of product streams they have







multi-product

column - column has more than two product streams





where

the extra feed exits when it is used to help with the separation,







extractive

distillation - where the extra feed appears in the bottom product

stream







azeotropic

distillation - where the extra feed appears at the top product

stream





the

type of column internals







tray

column - where trays of various designs are used to hold up the

liquid to provide better contact between vapour and liquid, hence

better separation







packed

column - where instead of trays, 'packings' are used to enhance

contact between vapour and liquid