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 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.
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 set of notes is to expose you to the terminology used in distillation practice and to give a very basic introduction to:
• types of columns
• basic distillation equipment and operation
• column internals
• distillation principles
• vapor liquid equilibria
• distillation column design and
• the factors that affect distillation column operation
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
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.
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
BASIC DISTILLATION EQUIPMENT AND OPERATION
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:
• a vertical shell where the separation of liquid components is carried out
• column internals such as trays/plates and/or packings which are used to enhance component separations
• a re boiler to provide the necessary vaporization for the distillation process
• a condenser to cool and condense the vapour leaving the top of the column
• 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
The vertical shell houses the column internals and together with the condenser and re boiler, constitutes 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 re boiler.
Heat is supplied to the re boiler 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 vapor raised in the re boiler is re-introduced into the unit at the bottom of the column. The liquid removed from the re boiler is known as the bottoms product or simply, bottoms.
The vapor 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.
Separation of components from a liquid mixture via distillation depends on the differences in boiling points of the individual components. Also, depending on the concentrations of the components present, the liquid mixture will have different boiling point characteristics. Therefore, distillation processes depends on the vapour pressure characteristics of liquid mixtures.
Vapour Pressure and Boiling
The vapour pressure of a liquid at a particular temperature is the equilibrium pressure exerted by molecules leaving and entering the liquid surface. Here are some important points regarding vapour pressure:
• energy input raises vapour pressure
• vapour pressure is related to boiling
• a liquid is said to ‘boil’ when its vapour pressure equals the surrounding pressure
• the ease with which a liquid boils depends on its volatility
• liquids with high vapour pressures (volatile liquids) will boil at lower temperatures
• the vapour pressure and hence the boiling point of a liquid mixture depends on the relative amounts of the components in the mixture
• distillation occurs because of the differences in the volatility of the components in the liquid mixture
The Boiling Point Diagram
The boiling point diagram shows how the equilibrium compositions of the components in a liquid mixture vary with temperature at a fixed pressure. Consider an example of a liquid mixture containing 2 components (A and B) - a binary mixture. This has the following boiling point diagram.
The boiling point of A is that at which the mole fraction of A is 1. The boiling point of B is that at which the mole fraction of A is 0. In this example, A is the more volatile component and therefore has a lower boiling point than B. The upper curve in the diagram is called the dew-point curve while the lower one is called the bubble-point curve.
The dew-point is the temperature at which the saturated vapour starts to condense.
The bubble-point is the temperature at which the liquid starts to boil.
The region above the dew-point curve shows the equilibrium composition of the superheated vapour while the region below the bubble-point curve shows the equilibrium composition of the sub cooled liquid.
For example, when a sub cooled liquid with mole fraction of A=0.4 (point A) is heated; its concentration remains constant until it reaches the bubble-point (point B), when it starts to boil. The vapours evolved during the boiling have the equilibrium composition given by point C, approximately 0.8 mole fractions A. This is approximately 50% richer in A than the original liquid.
This difference between liquid and vapour compositions is the basis for distillation operations.
Relative volatility is a measure of the differences in volatility between 2 components, and hence their boiling points. It indicates how easy or difficult a particular separation will be. Thus if the relative volatility between 2 components is very close to one, it is an indication that they have very similar vapour pressure characteristics. This means that they have very similar boiling points and therefore, it will be difficult to separate the two components via distillation.