A distillation column is crucial in separating liquid or vapour mixtures into individual components in a process called distillation. This process applies the difference in boiling points for the different parts in the mixture. Therefore, it involves heat application to vapourise the components and condense them into individual components. Thus, distillation columns are designed to make the process more efficient and enhance the purity of the ingredients. Even though it requires huge amounts of energy, it’s the most common separation technique in the manufacturing industry. Here’s an overview of distillation columns and their operation.
What is a Distillation Column?
This is a tall vessel/tower used to heat and separate vapour or liquid mixtures with different boiling points, such as crude oil. It comprises stacked plates that provide surfaces on which vapourisation and condensation occur. This tower is also known as a fractional distillation column and has several components, including:
- A vertical shell where the separation of the mixtures occurs.
- Column internals could be trays, plates, or packings that enhance component separation. These are inside the vertical shell.
- A reboiler where vapourisation occurs.
- A condenser for cooling and condensing the vapour as it escapes through the top of the column.
- A reflux drum where the condensed vapour collects before it’s recycled back into the column.
How the Distillation Column Operates
The mixture being separated is called the feed and is introduced at the middle of the column where there’s a tray called a feed tray. This tray splits the column into the top and bottom sections. The top section is called the enriching or rectification section, while the bottom is known as the stripping section. Once the feed is introduced, it flows through the bottom section and into the reboiler.
In the reboiler, heat is applied to vapourise the feed. Usually, the heat source is fluid, but most refineries use output streams of other columns. As the feed heats, the vapour goes into the column through the bottom and rises to the top. Additionally, some liquid leaves the reboiler and is called the bottoms product or bottoms.
The vapour reaches the top of the column and is condensed in the condenser. The resulting liquid is collected in a container called a reflux drum. Some fluid is channeled back to the top of the column and is called reflux. The remaining liquid goes out the reflux drum and is called the distillate or the top product. Thus, there’s the flow of vapour and liquids within the column.
The Distillation Principle
Distillation applies the difference in the boiling points of the mixture’s components. Additionally, the elements may have different boiling point characteristics depending on their concentrations. Therefore, the success of this process depends on the vapour pressure of the liquid mixtures.
There are different distillation columns with different designs to cater to various uses in engineering and manufacturing companies. Some mixtures require complex columns, while others require basic ones. Usually, classifying columns depends on how they operate. These include:
- Batch columns
This column involves batch operation, and the feed is introduced in batches. Thus a column works with one batch of feed at a time.
- Continuous columns
This column works with a continuous stream of feed. For this reason, there are no interruptions unless the column or the other units have a problem. They are the most common and can handle throughputs. The continuous columns include binary, multi-component, multiproduct, extractive, tray, and packed.
Factors Affecting Operation
The performance of a distillation column can be affected by the following:
- Feed Conditions: The feed’s state and composition determine the number of cycles necessary for separation. It also influences the tray’s location.
- Reflux Conditions: increase in reflux ratio means better separation; hence fewer trays are used, and vice versa.
- Column Diameter: the column diameter should be well-sized to enhance performance. Otherwise, it’s impossible to achieve the desired separation.
- State of Trays and Packings: The plate/packing efficiency determines the number of trays used.
- Weather Conditions: Wind and cold can affect the amount of vapour from the reboiler. Therefore, the reboiler should be well-sized.
A distillation column separates liquid or vapour mixtures whose components have different boiling points. It consists of a vertical shell with internals, a reboiler, a condenser, and a reflex drum. Some factors that affect its performance include the state of the feed, column diameter, state of trays, and weather conditions.