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Distillation - General comments

 For short, distillation refers to a kind of separation in which a liquid volatile component is evaporated from a liquid mixture, by evaporation. Many other factors affect the distillation and there are several types of distillation. This is what we will discuss in this post.

 

CONTENTS

 1 Distillation as a separation technique

 1.1 Factors affecting the distillation process

 2 Some concepts on distillation

1 Distillation as a separation technique

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Well, the separation techniques are vast and many times several of these may be required for a complete separation, such as evaporation, crystallization, etc. In distillation, the components of a liquid mixture are all volatile. Being all components of a mixture, volatile introduces a difficulty in the separation since when heat is supplied to the liquid, both liquid and resulting vapor phases could have barely the same composition, so that no separation have virtually occured (the  distillation is useless in such cases). If such is the case, the mixture needs a previous treatment such as: the addition of another component so that the properties of the mixture changes. At this point, you shluld notice that the separation techniques is an art.

Fig. 01 A general representation for the distillation column. $N$ represents the total number of plates counting from the top. This distillation column inluding rectifying and a stipping sections is termed fractionation column.


In general, distillation operations are carried out in a column, also called distillation tower. The plates technology were first introduced in 1830. The distillation column is formed by a number of plates or sieve trays which in turn will represent a stage of the distillation process. A column always works in countercurrent mode, this is: the liquid enters at the top and a vapor flows upwards from the bottom.

There is another approximation for a distillation column. Columns are also designed to use packing. In this cases, the treatment is quite different since the separation occurs in bulk rather than in stages. The packing technology, in distillation, was introduced in 1820.

In distillation, ideally, when the mixture is heated the component with the lowest boiling temperature (say component A) evaporates so that there is a higher concentration of A in the vapor than in the liquid. Also, if the vapor is cooled the component that condensates first (or in larger quantities) is that with the highest boiling temperature.

1.1 Factors affecting the distillation process

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As we it has been mentioned previously, distillation is not perfect and some comments against can be summarized. For example, according to Kister [Kister HZ. Distillation. Design. (1992)] distillation separations have low efficiency: around 10% (although improvements can be made). This is due to energy consumption in the reboiler, the condenser and mixing in every plate, and because heat losses.

Components relative volatility to each other

The components in a liquid mixture have a volatility of their own but if two or more of these components have very close volatility the separations could possibly not be done by distillation. For successfull distillation it is preferable that the difference of volatility be reasonable.

Corrosion or fouling

The mixture needs to be well known so that there is compatibility between this and the construction materials of the column and peripherial equipment (pumps, valves, seals, etc.). Otherwise, corrosion and early  wearing may occur. Fouling may also occur but it is related to films on surfaces. Heat transfer through surfaces  with fouling may be negatively affected.

Thermal instability of the component

Since distillation requires heating and evaporation of the liquid mixture, the component to be separated should be stable when subject to such conditions. Temperature can be reduced if pressure in the distillation chamber is reduced too (vaccuum) but again, this component should be stable under those conditions. For example, chemical degradation may occur if temperature is increased.

The component to be separated have high boiling point

This means that the component to separate should not have the highest boiling point among all components in the mixture. Otherwise, you would need to evaporate the whole mixture to get your product. This scenario would not be recommended for distillation.

2 Some concepts on distillation

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Simple distillation

It to the partial vaporization of a mixture so that the vapor phase has higher concentration of the most volatile components than the initial mixture. There are to kinds of simple distillation:

  • closed or equilibrium distillation 
  • open or differential distillation
In any of the options above, no column nor plates are used as part of the equipment for the distillation process.

Partial condensation

It refers to the process oposite to distillation. This is, a vapor mixture is cooled so that a liquid phase made of the components of lower volatility.

Rectification

This operation occurs in a distillation column and it consists of passing in countercurrent mode the vapor of the mixture and the condensate produced from the same vapor. The vapor product is later condensate to get a high concentrated liquid made of components of high volatility.

The rectification column can be a standalone column or be part of one single, very tall,  column. If the rectification takes part in a section of column, this process is located at the top of it (see Fig. 01, for example).

Stripping

A stripping process takes place in a distillation column usually called strip. This distillation method is used separate the less volatile components in a mixture. In the end you get a liquid product with high concentration of components with the lowest volatility. As you can imagine stripping is used to work in the bottom section of a distillation column (see Fig. 01, for example). A full distillation column separating all components would look like that presented in Fig. 01.

Ideal stages

Also known as the plates in the column. Are counted from the top to the bottom. There are several designs but these are estimated for steady conditions. As such, a stream of liquid fills a plate while another stream, of vapor this time, passes through it. These two streams travel in countercurrent mode.

Stages calculations consider that both liquid and vapor mix as well as possible during the time these are in contact. In other words, both phases are in intimate contact. Another interesting fact is that you must assumed that both phases reach equilibrium during the time these get in contact. If equilibrium is not reached, then mass transfer calculations may need to be done by a non-traditional method (that I do not know if such a thing exist).

The stages or plates, and its nature, will depend on the section of the column which is known to have a rectifying section and a stripping section.

Non ideal or real stages

Non ideal stages are those of real life. These plates do not match the theoretical calculations. However, there are modern methods that help to get better estimations of the plate. Real plates can be very different from ideal plates so that attention to the precision of the calculation technique should be paid.

Stage efficiency

The efficiency of the plate connects ideal stages to non ideal stages. As you may imagine, the efficiency will depend on the method used for your calculations. This is in general not a big problem since modern methods are property of some companies, are used for special cases and may need a special training for correct use. 

However, efficiency of the stage may depend on the section of the column and the kind of column. This is not an estimated data but something readily available according to what is available in the market.

The stage efficiency is defined as,

$Stage\,\, efficiency=\dfrac{Number\, \, of\,\, ideal\,\, stages}{Number\,\, of\,\, non\,\, ideal\,\, stages}$        Eq. (01)

This is the end of the post. I hope you find it useful.

Any question? Write in the comments and I shall try to help.

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