Distillation Principles

Definition of distillation, Types of columns, Simple Distillation methods (Flash, batch, Steam), Basic distillation Equipment and operation, Column internal, Reboilers, Distillation principles, Vapor `liquid equilibria, Distillation column design, Effect of the number of trays or stages, Factors affecting distillation column operation, Introduction to multi-component Distillation

Definition of Distillation

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.

• 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:

◘ distillation is the most common separation technique

◘ it consumes enormous amounts of energy, both in terms of cooling and heating requirements

◘ 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 optimization and control. To achieve this improvement, a thorough understanding of distillation principles and how distillation systems are designed is essential.

The purpose of this introduction is to expose you to the terminology used in distillation practice and to give a very basic introduction to:

Types of columns
Simple distillation methods (Flash, batch and Steam distillation)
Basic distillation equipment and operation
Column internals
Distillation principles
Vapor liquid equilibria
Distillation column design
Effect of the number of trays or stages
Factors affecting distillation column operation
Introduction to multi-component distillation


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

extractive distillation – where the extra feed appears in the bottom product stream

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

 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 vapor and liquid, hence better separation

packed column – where instead of trays, ‘packings’ are used to enhance contact between vapor and liquid

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