Candela Rocío Barbisan
Chemical engineer
Distillation towers or columns are pressure vessels where the separation of the various components that make up the feed stream occurs. The operating principle is based on the difference between the evaporation temperatures of each of the substances and their volatility.
Theoretical fundament
Like all distillation, the essence of the process is based on the scheme:
A distillation tower has a heat source in the lower area, such as the Reboiler, which generates that, thanks to the delivery of caloric energy, part of the components (one or more) pass into the vapor phase. As it ascends through the column, it will meet the liquid that descends, therefore, when it reaches the top of the column, the vapor stream is fed by the most volatile components. Meanwhile, the heavier components were dragged by the liquid that descended in a countercurrent flow. That is why liquid streams are said to be at their bubble point, while vapor streams are said to be at their dew point.
When these currents exchange energy, they simultaneously exchange mass and that happens at each stage, that is, at each plate (it could also be a filling) symbolized as an inner horizontal line in the tower.
When the vapor fraction reaches the top of the tower, it is directed to a condenser, where the distilled product is obtained and where a portion of that current, known as reflux, returns to the tower.
It must be taken into account that in the previous image a binary type distillation is plotted, that is, only two components in the feed stream are separated, extracting one from the top and the other from the bottom. However, there are distillations of multiple components, where in each stage of the tower it is possible to separate a different component.
Applications
The industries that use distillation towers are many and, therefore, depending on their use will be the diameter and length, the manufacturing material and its type of plate or filling. For example, a distillation tower is the heart of the world’s crude oil refineries. When the oil arrives at a treatment plant, it first undergoes a desalting process and then high-temperature furnaces. It is then introduced into the tower itself, where the more volatile components rise through the stages and the higher boiling ones fall to the bottom of the tower and the condensates at the top are collected in buckets.
To maintain the temperature of the tower, there are various mechanisms, particularly here diesel and kerosene (components of the original mixture) are usually recycled, prior to their re-entry they are cooled in a heat exchanger. Whereas, at the top, the “reflux head” maintains the right temperature at the top.
To understand the significant size of these towers, between 60 and 80 m in length and 6 m in diameter, we must understand the importance of this in the process. From the initial mixture, it is possible to separate heavy diesel oil (at 340 °C), light diesel oil (at 280 °C), kerosene (at 210 °C) and naphtha (at 180 °C), from which derives the extensive length required to cover all stages of fractionation of the mixture. In turn, the heaviest component is obtained at the bottom of the tower: fuel oil.
In fractionation plants they are also used to sweeten the gas, for example, in amine contactor towers, the sweet gas is obtained at the top, while the amine stream with acid gas content is obtained at the bottom, a stream that is sent to another tower for regeneration. They are also used in other industries, such as in the production of organic compounds and polymers.
As expected, there is also a variety of manufacturing materials, from glass for laboratory scale or pilot scale distillation columns to large towers made of carbon steel or low alloy steel. On the other hand, if it is known that they will work with a highly corrosive fluid such as gases with CO2 and H2S content, they are usually made of stainless steel or other resistant alloys. It is also common to find towers with different materials depending on the sections, given that, for example, the critical zone for acid gas condensation, the top zone can be made of stainless steel or have a cladding while the lower section can be made of carbon steel. carbon.
Its design is made based on your service and must comply with rigorous quality, safety and manufacturing standards provided by design standards, such as ASME VIII.
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