Introduction
Distillation Columns use a heating and cooling process to separate two (or more) mixed components (liquids and/or gas) into their respective separate or original states. In simple terms, this process is achieved by heating the mixed liquid components to a temperature that is in-between their respective boiling points, thereby boiling off the lower temperature liquid in order to separate them by evaporation.
Refinery Distillation Columns
History
The Distillation process is one of the oldest and most commonly used separation and purification methods. Its historical links date back to the Romans, Arabs, Chinese, Indians, and Babylonians of Mesopotamia more than 5000 years ago, where evidence of perfume and alcohol Distillation was recorded on the Akkadian tablets (circa. 1200BC), on through to the medieval period, and subsequently to the 19th century Industrial Revolution where the more contemporary applications were developed.
Whiskey Still
Continuous Distillation Column
There are many types of distillation column designs, such as the ‘batch’ column or ‘continuous’ column. A basic batch column is where the input feed (liquid/gas component) is distilled entirely and then another batch of feed is added to the column and processed in turn. A continuous column has a constant input and output stream which facilitates a much higher throughput, and the process does not stop unless maintenance or a repair is required.
Both types of columns have specific application relating to the separation of two or more liquid/gas components, and both incorporate varying levels of complexity and associated equipment to facilitate the overall distillation process.
What are the main parts of a distillation column?
A typical Continuous Distillation Column has several components to facilitate the overall process. These four main components are:
- Column (incorporating the enriching and stripping sections), where the liquid/gas phase separation takes place.
- Reboiler (heat exchanger), which provides the heat energy to partially vaporise the columns input feed (liquid) for the separation (distillation) process to take place. The input feed flows into the centre of the column and down through the stripping section to the base, before entering the reboiler where the feed liquid is heated to generate the feed vapour (through evaporation). This feed vapour travels up through the column by convection, to continue the feed components separation process.
- Condenser, which cools and condenses the separated and enriched feed vapour at the top of the column before it enters the reflux drum.
- Reflux drum, which collects the condensed feed vapour (as a liquid), and splits it into two streams, 1. the distillation process output (distillate), and 2. the reflux drums recycled liquid (reflux), which is fed back into the top of the column, to further enrich and purify the distillate.
General Distillation Column Main Components
In order to make this process more efficient, the distillation column usually has a series of stacked plates or trays inside the column. These plates have circular perforations, and/or corrugated metal packings, to improve the liquid/vapor contact surface area and therefore the feed liquid evaporation process efficiency. This type of column is called a fractionating distillation column and supports the removal/separation of several components (liquid or gas) at different levels in the column.
Fractionating Distillation Column
Basic Distillation Column Operation and Terminology
The feed components that are to be processed in the distillation column are introduced near to, or at, the centre of the column, into a central tray/plate known as the feed tray (plate).
Distillation Column Process Flow Diagram
This central feed tray divides the column into a top (enriching) section and a bottom (stripping) section. The feed liquid flows into the column and across the tray/plate, then down through the column (by gravity) through to the next tray/plate via its downcomer conduit, where the enriched feed liquid ‘vapour component’ is stripped out through the distillation process as it rises through the feed liquid. The remaining feed liquid falls down the column and collects at the column’s base, then flows back into the reboiler which vaporises the lighter component into the feed liquid vapour component. This vapour then continues the distillation process as it rises to the top of the column, through the column tray plates where it comes into contact with the feed liquid, stripping the lighter component (vapour) out and becoming more enriched as it does so. At the base of the column the ‘heavy’ (not vapourised) feed liquid product is removed from the column as the ‘bottoms’ component.
Distillation Column Liquid and Vapour Flow Paths
The reheated feed vapour, flowing back into the base of the column from the reboiler, is enriched as it rises up through the column and ‘contacts with’ the falling feed liquid, before exiting the column at the top, into the cooling condenser. This condensed liquid then enters a holding vessel known as the reflux drum.
Distillation Column and Reflux Drum
A proportion of this reflux drum liquid i.e. the reflux ratio, is recycled back into the top of the column, further enriching and purifying it through contact with the feed vapour component that is rising up through the column plates. Finally, the condensed feed vapour component which is now cooled into liquid form by the condenser and reflux drum system, is the column output (distillate) that is recovered through the overall distillation process.
Distillation Column Operating Profile
There are certain operating parameters and trends which are common to the distillation column process, and an appreciation of these aspects will assist in gaining a general understanding of the process. In simple terms the column design (diameter and height, number of trays/plates, packers etc.) and the operating parameters (temperature, pressure, feed liquid input stage etc.) are optimised by the designers to achieve an overall internal vapour liquid equilibrium (balance), based on the volatility (boiling points) of the feed liquid components. Distillation Column Operating Profile.
Temperature and Pressure
The general temperature profile within the column is that it is hotter at the top and cooler at the base. For example, in a simple process that has a feed liquid with two components, the temperature at the base of the column is slightly cooler than the boiling point of the heavier feed component, and the temperature at the top is slightly higher than the boiling point of the lighter component, as it is desired to create an environment for a vapour - liquid balance within the column. At the base of the column, we require the heavy feed component to remain as a liquid and the lighter component to remain as a gas until it reaches the top of the column, before entering the condenser and reflux drum. To control these temperatures, the reboiler heat exchanger temperature is adjusted to control the bottoms heavy feed component temperature; the reflux drums ‘return’ flow rate (flow back to the top of the column) is used to control the lighter components temperature at the top of the column.
The columns overall pressure gradient is ‘higher’ pressure at the base and ‘lower’ at the top. This gradient is determined by the feed stage liquid (input) falling down through the column tray plates and impeding the rising vapour flowing up through the feed liquid.
In general, for a steady state distillation process the pressure gradient of the column is kept as constant as possible, using the reboiler temperature and reflux drum flow to control the composition of the light and heavy components streams.