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Packed Column & Packings

Besides tray column, distillation (as well as other unit operations such as gas absorption, liquid-liquid extraction, etc.) can also be carried out using packed column filled with packings. Various types of packings made of different types of materials of construction are available, and both random and structured packings are commonly used. Examples of random packings as shown in the Figures below - left and right - are Raschig rings, Pall rings, Berl saddles, etc.

Packing Example 1Packing Example 2

Random vs. Stacked

Random packings, as the name implied, are dumped into a column during installation and allowed to fall in random. Small packings poured randomly into a vessel is certainly the more popular and commonly employed form of packed-tower design. However, in certain instances where exceptionally low pressure drop and very high flowrates are involved, stacked or oriented packings have also been used. See the Figure below. However, only those packings of cylindrical shape and with a diameter larger than 3-inch would be practical to install in a stacked form. Two types of arrangement are possible: triangular (diamond) pitch or square pitch.

Random vs. Stacked packing

The different packings has several basic characteristics that make them suitable for gas-liquid contacts. Click here for more information.

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Dry vs. Wet Random Packing

In dry packing application, the packings are allowed to drop into the column via the (a) chute-and-sock method, or (b) rope-and-bucket method.

Dry packing avoids high hydrostatic liquid head, and prevents the introduction of water into a dry process. It is also quicker and less expensive than wet packing, and it minimises rusting of metal packings. In any case, it is not suitable for plastic packings, as plastic typically floats on water.

Wet packing applications are preferred when the packings are constructed of breakage-prone materials, such as ceramic or carbon. The column is first filled with water and the packings are gently poured down the column. The water cushions the fall and promotes randomness of settling. This tends to increase column capacity and improve the column pressure drop characteristics. Wet packing also minimises compression and mechanical damage to packing materials. The main disadvantage is the need to remove the water after loading and dry the packings.

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Structured Packing

Structured packings are considerably more expensive per unit volume than random packings. They come with different sizes and are neatly stacked in the column. Structure packings usually offer less pressure drop and have higher efficiency and capacity than random packings.

2 examples are shown in the Figures below - left and right.

Structured Packing 1Structured Packing 2

 

Besides packings, there are other column internals.

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