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Random Tower Packing Department Editor: Kate Torzewski
Chemical Engineering©

Packing Tower

Packed columns[1]
A packed column is a vertical, cylindrical pressure vessel containing one or more sections of a packing material over whose surface the liquid flows downward by gravity, as a film or as droplets, between packing elements.

Vapor flows upward through the wetted packing, contacting the liquid and facilitating absorption of the vapor into the liquid.

Packings are offered in either random or structured designs. Here, we will focus on random packings, which are separate pieces of packing that have a uniform geometric shape. Instead of being arranged in a structured way, they are dumped or randomly packed into the column shell.

Design considerations[2]
Size. Random packings are typically available in diameters of 1–3.5 in. Generally, as packing size increases, mass-transfer efficiency and pressure drop decrease.

By this correlation, for a given column diameter, an optimal packing size can be determined that represents a compromise between achieving low pressure drop and high mass-transfer rates. A rule of thumb that must also be taken into account is to choose a packing diameter that is less than one-eighth of the column diameter, which minimizes liquid channeling.

Material. Metal packings are usually preferred because of their superior strength and good wettability. Ceramic packings, on the other hand, have superior wettability but inferior strength, and are used only in situations at elevated temperatures where corrosion resistance is needed and plastics would fail. Plastic packings, usually made of polypropylene, are inexpensive and have sufficient strength; however, they may experience poor wettability, especially at low liquid flowrates.

Packing Objectives[1]
1. Maximize the specific surface area. Increasing the surface area per unit volume maximizes the vapor-liquid contact area, and, therefore, efficiency. Efficiency generally increases as the random packing size is decreased.

2. Spread the surface area uniformly. This improves vapor-liquid contact, and therefore, efficiency. For instance, a Raschig ring and a Pall ring of identical size have identical surface areas per unit volume, but the Pall ring has a superior spread of surface area and therefore gives much better efficiency.

3. Maximize the void space per unit column volume. This minimizes resistance to gas upflow, thereby enhancing packing capacity. Capacity increases with random packing size. This poses a trade off, however, in that the ideal size of packing is a compromise between maximizing efficiency and maximizing capacity.

4. Minimize friction. An open shape minimizes friction, providing good aerodynamic characteristics.

5. Minimize costs. Packing costs, as well as the requirements for packing supports and column foundations, generally increase with the weight per unit volume of packing. Packings generally become cheaper as the size of random packing increases.

Packing structures[3,2]
Raschig Rings are hollow cylinders with a height that is equal to the ring diameter. This structure is the oldest form of random packing.

The original saddle-shaped packings, Berl Saddles, have a smaller free-gas design than Raschig Rings. However, they are often a more preferable choice, as they offer a lower pressure drop and higher capacity.

The invention of the Intalox Saddle marked the start of the second generation of random packings. When packed together, they prevent significant portions of wetting liquid from being blocked off, thus avoiding pools of liquid, trapped gas and violent directional changes of gas. They offer higher capacity, higher efficiency and lower pressure drop than Berl Saddles.

The Intalox Saddle was further improved into the Super Intalox Tower Packing, which has scalloped edges and holes in the material. This allows further liquid drainage, the elimination of stagnant pockets, and more open area for vapor rise, thus providing higher capacity and efficiency.

Pall rings are modified Raschig Rings that have windows cut and bent inward. This lowers friction while improving packing area distribution, wetting and liquid distribution. This design allows higher capacity and efficiency than all previously developed packings.

The next generation of packings features through-flow structures of a lattice-work design. The Metal Intalox IMTP offers the best features of packings that preceded it, combines the high void fraction and the well-distributed surface area of the Pall ring with the low aerodynamic drag of the saddle shape.

Similar in structure to the Pall Ring is the Cascade Mini-Ring, which has a height to diameter ratio of 1:3 compared to 1:1 in the Pall Ring. This allows the individual packing components to be oriented with their open side facing vapor flow, thus reducing friction and exposing more surface to mass transfer.

The latest generation of random packings features a very open, smooth and wave-like geometry that promotes wetting, but still promotes recurrent turbulence. This allows a decreased pressure drop while sustaining mass-transfer efficiency that may be independent of column diameter, and may allow a greater depth of packing without a liquid redistributor.

References
1. “Perry’s Chemical Engineers’ Handbook,” 8th ed. McGraw Hill, New York, 2008.
2. Seader, J. D. and Henley, E. J., “Separation Process Principles,” 2nd ed., John Wiley and Sons, Inc., New Jersey, 2006.
3. Schweitzer, P., “Handbook of Separation Techniques for Chemical Engineers,” 3rd ed., McGraw Hill, New York, 1997.

3 comentarios:

Silence Matthews dijo...

Thanks for posting this! I had no idea what tower packing was until today, but this was very helpful. Great explanations and stuff!

Marvin Jaisalmer dijo...

Thank you sharing
If are looking Ceramic Tower Packing company in Madya Pradesh must visit MBC Tower

Neha sane dijo...

Nice post. Useful information shared.
Distillation Column

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