Distillation is a process by which a liquid mixture is separated into fractions with higher concentrations of certain components by exploiting differences in relative volatility. In industrial settings such as oil refineries and natural gas processing plants this separation process is undertaken using a distillation column. This article describes the basic principles and operation of a distillation column and the equipment and terminology used when discussing distillation.
Objective of Distillation
The basic objective of distillation is to take a liquid mixture and separate it into two or more streams which have compositions different to the feed stream. In a basic distillation column a feed stream enters in the middle of the column and two streams leave, one at the top and one at the bottom. Components with lower boiling points are concentrated in the stream leaving the top while components with higher boiling points are concentrated in the stream leaving the bottom.
Separation is achieved by controlling the column temperature and pressure profiles to take advantage of differences in the relative volatility of the mixture components and therefore tendency to change phase. The lighter, lower boiling point components evaporate and travel up the column to form the top product and the heavier, higher boiling point components condense and travelling down the column to form the bottom product.
Discussion of distillation requires an understanding of some basic nomenclature. Listed below are terms that are commonly used to describe the main components of a distillation column complete with a diagrammatic representation of a tray column.
Fractionation - Another term for distillation, or fractional distillation.
Feed - The liquid and/or gas feed into the distillation column. The tray below the inlet nozzle is called the feed tray.
Heavy Component - The component with the lower relative volatility, for simple hydrocarbon this is the component with the higher molecular weight. Found in higher concentration in the bottom product of the column.
Light Component - The component with the higher relative volatility, for simple hydrocarbon this is the component with the lower molecular weight. Found in higher concentration at the top of the column.
Stripping section - The trays between the bottom of the column and the feed tray. In the stripping section the aim is to concentrate the heavier component in the liquid phase.
Rectifying section - The trays between the feed tray and the top of the column. In the rectifying section the aim is to concentrate the lighter component in the vapor phase.
Top Product - The product which leaves the top of the column, also called distillate. This product is usually passed through a heat exchanger and liquefied.
Bottom Product - The product which leaves through the bottom of the column.
Reflux - A portion of vapor from the top of the column which has been condensed to a liquid and returned to the column as a liquid above the top tray.
Reboiler - A heat exchanger at the bottom of the column which boils some of the liquid leaving the column. The vapor generated returns to the column at the bottom of the stripping section.
Vapor-Liquid Equilibrium (VLE) Curve - A plot of the actual composition of the lighter component in the vapor phase for a given composition in the liquid phase. Usually derived from thermodynamic data.
Principle of Separation
Distillation takes advantage of the difference in relative volatility of the feed mixture components. Generally for two or more compounds at a given pressure and temperature there will be a difference in the vapour and liquid compositions at equilibrium due to component partial pressure. Distillation exploits this by bringing liquid and gas phases into contact at temperatures and pressures that promote the desired separation. During this contact the components with the lower volatility (typically lower boiling point) will preferentially move into the liquid phase while more volatile components move into the vapor phase.
A distillation column may use either trays or a packed bed to bring the gas and liquid into contact. For a column using trays we can consider the changes to gas and liquid phase compositions as they both enter and exit a single tray. The liquid entering the tray will contact the gas exiting the tray. The hotter vapor phase will heat the incoming liquid phase as it bubbles through the tray, evaporating the light components which then leave the tray with the vapor phase. Conversely the cooling of the vapor phase by the liquid phase will cause the heavier components of the vapor phase to condense and exit the tray with the liquid phase.
For the liquid across the tray:
For the vapor through the tray:
Where is concentration and is temperature.
When a packing is used rather than trays the principle remains the same, in fact packing is often referenced in terms of height equivalent to a theoretical plate (HETP) i.e. what height of packing is equivalent to one theoretical plate. The packing is just an alternative method to bring the liquid and vapor phases into contact with the liquid generally flowing over the surfaces of the packing material, while the vapor passes up through the space between packing elements.
Typical Operating Parameters
There are some general trends common to the operation of distillation columns. By knowing these trends and why they occur we can improve our understanding of the distillation process.
The basic temperature profile of a distillation column is hotter at the bottom and cooler at the top. For a simple two component distillation the temperature at the bottom is just lower than the boiling point of the heavier component. The temperature at the top of the column is just above the boiling point of the lighter component.
At the bottom of the column we would like the heavy component to remain as a liquid and the lighter component to stay as a gas. So we set the temperature at the bottom to match this requirement. This temperature is set by adding heat via a heat exchanger called a reboiler. Typically the heat added to the bottom of the column is easy to control, via steam or hot oil flow rates.
At the top of the column the situation is reversed. We would like the light component to remain a gas while the heavier component is condensed to a liquid and falls back down the column. The top temperature is set just above the boiling point of the lighter component. The temperature control situation is different here to the bottom of the column, because we usually want the top product to be a liquid when we send it for storage. So we condense all of the gas coming out of the top of the column to liquid. This liquid stream is split with some returning to the column and some going to storage. The top temperature is often controlled by changing the reflux rate, i.e. the flow rate of liquid sent back to the top of the column. A higher reflux rate means more cooler liquid falling down the column against the rising warmer gas, and the top temperature is lower.
Overall heat is added at the bottom of the column and heat is extracted at the top of the column. Inside the column the temperature balance is created between the hot gas rising up the column and the cooler liquid falling down the column.
There is typically a pressure gradient across the column with the pressure being higher at the bottom of the column than the top. This pressure gradient occurs as the liquid coming down the column impedes the flow of vapor up the column and imposes a pressure loss on the flow. In steady state distillations the pressure in the column is held constant, and the temperature is varied to control the composition of the product streams.