Distillation columns are one of the most often used types of separation equipment in industry.
Distillation is one of the most common liquid-liquid separation processes, and can be carried out in a continuous or batch system.
Distillation works by the application and removal of heat to exploit differences in relative volatility. The heat causes components with lower boiling points and higher volatility to be vaporized, leaving less volatile components as liquids. Mixtures with high relative volatilities are easier to separate. This makes separations of close-boiling and azeotropic feeds difficult, so special distillation techniques have to be used to separate these mixtures.
Distillation can be used to separate binary or multi-component mixtures. Many variables, such as column pressure, temperature, size, and diameter are determined by the properties of the feed and the desired products. Some specialized columns perform other functions, such as reactive distillation columns, which combine reaction and separation of products into a single unit.
Column Types/Conventional Distillation
Although packed bed columns are used most often for absorption, they are also used for the distillation of vapor-liquid mixtures. The packing provides a large surface area for vapor-liquid contact, which increases the column’s effectiveness.
The feed mixture contains components of different volatilities, and enters the column approximately at the middle. The liquid flows downward through the packing, and the vapor flows upward through the column.
Differences in concentration cause the less-volatile components to transfer from the vapor phase to the liquid phase. The packing increases the time of contact, which increases the separation efficiency. The exiting vapor contains the most volatile components, while the liquid product stream contains the least volatile components.
After the feed mixture enters the column, as the green arrows in the animation below demonstrate, liquid flows down the column through the packing countercurrently and contacts the rising vapor stream. The liquid at the bottom, which is highlighted in yellow in the animation, enters a reboiler. Two streams exit the reboiler; a vapor stream, which returns to the column, and a liquid product stream. The vapor stream flows upward through the packing, picks up the more volatile components, exits the column, and enters a condenser. After the vapor condenses, the stream enters a reflux drum, where it is split into an overhead product stream, known as the distillate, and a reflux stream that is recycled back to the column.