Essay: What is a Distillation Column? Understanding the Components



Separating mixtures has been a primary need of scientists and common folk alike. In many cases, mixtures that require separation can be done so through basic processes. These processes are often mechanical, and allow the person seeking to separate a mixture to physically pick pieces out of the mixture. Other cases though, can require more advanced processes involving the use of heat, pressure, and chemicals. One such process is distillation, a process in which a mixture of chemicals is separated into its individual components through the addition of heat and control of pressure. To complete a separation through distillation, a special piece of equipment known as a distillation column is required. Through this document, the general operations and purpose of a distillation column will be examined, and the major components of a distillation column will be explained.

What is a Distillation Column?

A distillation column is a piece of equipment used throughout chemical engineering industries and chemistry labs alike. Distillation columns are used to separate mixtures of chemicals, normally a mixture of two chemicals but occasionally more, by heating and pressurizing the mixture. Through the addition of heat and pressure, the mixture is separated into its components, one is removed as a vapor and the other is a high temperature liquid. Columns can come in a variety of sizes and levels of complexity, depending on the mixture being separated and the amount that needs separation. For example, a distillation in a lab may only need a column around a foot tall, but a commercial separation process may require a column two stories tall. While the size may change, the core components of these columns remain mostly unchanged, and so they can be explained generally and applied broadly.

What are the Components of a Distillation Column?

Distillation columns are composed of four major components, each of which plays a vital role in the separation of chemical mixtures. The four major components are also supplemented by three major streams that allow a distillation column to function properly. In order to understand how these streams and components work together, it is first important to understand the function of each and where in the system each is located.

Trays

In the most general terms, trays are large metal plates within a distillation column that are full of holes or valves. Trays facilitate an important part of distillation, the movement of heat, via hot vapor, through condensed liquid. The holes and valves on the tray are placed in such a manner to allow even transfer of heat through the liquid that has collected on its surface. Within columns there can be as little as two trays, or as many as thirty depending on the application of the process. Trays are coupled with downcomers and are spaced evenly along the height of the column, between the reboiler and condenser. In industry, there are three major types of trays used within distillation columns: sieve trays, bubble cap trays, and valve trays.

Sieve Trays

Sieve trays are the least complex type of tray used in distillation columns as they are metal plates with holes punched in them. The complexity of these plates comes in their design, since it will vary depending on the composition and amount of the mixture that requires separation. The number, arrangement and size of holes are all design parameters determined during the preliminary design process. This process considers the composition and amount of the mixture that needs separation. In practice, vapor passes through the holes on the plate and into liquid on the tray. If the tray is properly designed, and the column is operated correctly, no liquid will pass through the holes.

Bubble Cap Trays

Bubble cap trays are slightly more complicated than sieve trays. While a bubble cap tray does feature holes throughout the surface of the metal plate, the holes have a riser, sometimes referred to as a chimney, fitted over them. Fitted atop these risers are caps that allow vapor to pass through. In practice, the vapor passes into the hole, up through the riser and into the cap. The cap redirects the vapor downward through slots in the cap, allowing the vapor to bubble into and through the liquid on the tray. Like sieve trays, if designed and operated properly, no liquid should ever enter the bubble caps and pass through the holes.

Valve Trays

Valve trays are a special type of tray, as they feature moving valve components. Valve trays are studded with holes, but these holes are covered by caps that can be lifted and lowered depending on vapor pressure. Additionally, these valves direct vapor through liquid differently than sieve or bubble cap trays, in a horizontal manner rather than vertically or randomly through bubbles. The major advantage to this is better mixing in comparison to sieve trays. In operation, vapor would rise through the column, and push against the valves on the tray, opening the valve and allowing vapor to pass through. Like the other two tray designs, proper design and operation will prevent any liquid from passing through valves.  

Downcomers

Built onto most trays, downcomers are the spaces next to trays that allow the movement of condensed liquid from one tray to another. Shown in Figure 1, liquid flows down through a space between the wall of the distillation column and the tray. In this example, the wall, and resulting space, is the downcomer. The downcomer is placed on alternating sides of the tray, changing from left to right as you go up the column, so that liquid falls onto another tray below it. The only exceptions to this are when vapor passes into the condenser and liquid passes into the reboiler.

Reboiler

The reboiler is essential to maintaining the temperature and pressure of the column. The reboiler is located at the bottom of a distillation column. As liquid condenses, and falls through the column, it cools and must be reheated to maintain equilibrium. The reboiler contains a heating element and brings the mixture to a boil, sending vapor back into the column to pass through trays and mix with condensing liquid. Additionally, product can be removed from the reboiler in the form of a “bottoms product.”

Condenser

Opposite the reboiler, the condenser cools down vapor that reaches the top of the column. Normally, the condenser takes the form of a “partial condenser” and so part of the vapor stream entering is returned to the column as a liquid and a portion is removed from the column as the “distillate product.” The removal of these products, and reintroduction of some of the mixture into the system keeps the column operating in a state of equilibrium.

Distillation Streams

There are three major streams to note when looking at how a distillation column is constructed and operated. Two of these streams are considered “product streams” and the other is a feed stream. An understanding of the role of each stream in the column is essential in designing a column that is both functional and efficient.

Feed Stream

The feed stream is the most important stream in the design and operation of a distillation column. Feed streams vary case by case and can be in the form of a liquid or vapor. The feed stream will also vary in composition, temperature, and flow rate depending on the operations of the facility using the column. To design a column, these three variables will need to be identified and will in turn affect the products produced, the design and number of trays, and the operating conditions of the reboiler and condenser.

Bottoms Stream

The bottoms stream, referred to as the “bottoms” or “bottoms product”, is a stream that is purged from the reboiler. This stream usually has an almost pure composition of the least volatile component (the one with the highest boiling point). The stream exists to maintain equilibrium with the vapor stream that is reintroduced to the distillation column. The products produced here can be further processed by the company to increase their purity.

Distillate Stream

The distillate stream, referred to as the “distillate” or in some cases “distillate product”, is a stream purged from the condenser at the top of the column. The stream can be cooled immediately off of the column and be processed as a liquid into further cooling tanks. The distillate stream normally contains the component of the mixture that the company or individual is looking to extract. Inverse to the bottoms stream, the distillate stream usually has a pure composition of the most volatile component (the one with the lowest boiling point). Normally these products will be cooled and then refined through other processes. From there companies often sell or use the extracted compounds to create other products.

Conclusions

Distillation is a core process within the field of chemical engineering. While it can be done through simple or complex means, the distillation column is a standard piece of equipment found in chemical processing plants and research labs around the world. While the size and structure of a distillation column may vary with the mixtures and intent of use, the underlying components and process remain the same. As a means of separation, distillation is one of the most common industrial practices and vital to the society we live in today.