Extraction column

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Extraction columns are the most commonly used liquid-liquid extractors. In them the extraction is carried out in countercurrent . There are a large number of designs, but they are based on a common functional principle.

function

In the vertical column , the specifically heavier liquid (e.g. the liquid mixture to be extracted) is continuously introduced at the top. The specifically lighter liquid is introduced at the bottom (e.g. the extractant). The two liquids are introduced via distributor spirals in order to achieve fine distribution. While the heavier liquid sinks in the column, the lighter one diffuses through it upwards. There is an exchange of substances between the many small droplets and the extract passes from the carrier liquid into the solvent . Suitable devices and fixtures promote intensive mixing, which leads to multiple mixing and separation. Internals in the column are necessary since otherwise the liquid droplets rising upwards converge and the liquids would run past one another in countercurrent.

The lighter liquid (solvent loaded with extract) is taken off at the top of the column, while the heavy liquid (leached carrier) runs off at the bottom.

Types

In principle, extraction columns are divided into columns with fixed or moving internals.

Packed column

The packed column contains a bed of random packing in order to enlarge the exchange surface for the liquids and to cause the liquids to be divided several times. A wide variety of shapes and materials are used for packing elements (for example so-called Pall rings or Berl saddle bodies).

Sieve tray column

Sieve tray column

Sieve bottom

Sieve tray columns have a very good separation efficiency due to the multiple division and confluence of the liquids. Sieve trays are installed in the column. The light phase introduced from below flows through the heavy phase upwards, collecting under the sieve trays and pressing through the holes. The liquid is broken up into many small drops.

This is done using a pulsator, which is usually a piston pump. This sits at the lower end of the column, which causes the mixture of substances in the column to vibrate. This increases the separation efficiency, since the "bubbles" that are generated by the internal sieve are distributed smaller and finer. This in turn creates a larger surface for mass transfer.

Without such a pulsator, the separation efficiency of the sieve tray column would decrease significantly, since no large mass transfer of the extract from the extraction material (feed material) to the extractant would be guaranteed.