Packed Columns

Tanveer Ahmad(M07-110)

Fahad Ali Rabbani(M07-126)

Packed Column

It is Used to bring two phases in contact with one another, as a consequence a strong interaction occurs between the fluids

Packed columns are filled hollow packing elements

Fluid flow through packed column is at high Reynolds number.

Packed Column Vs Plate column

Contact patternDiameterPressure dropLiquid hold upSize and cost

Schematic Diagram of Packed Column

Liquid Distributor Hold-down plate Packing Packing Support Gas injection system

Material of Construction of Column

• The shell of the column may be constructed from:

Ceramics Glass plastics material Metal with a corrosion-resistant lining

• Note: Column should be mounted truly vertically

Types of Liquid Distributors

• Simple Orifice type Distributor

Gives very fine distribution

It must be correctly sized for a particular duty

Should not be used where there is any risk of the holes plugging

Types of Liquid Distributors

• Notched Chimney type Distributor

It has a good range of flexibility for the medium and upper flow rates

It is not prone to blockage

Types of Liquid Distributors

• Notched Trough Distributor

It is suitable for the larger sizes of tower

It is also suitable for the higher gas rates

Types of Liquid Distributors

• Perforated Ring Type Distributor

It is used where: High gas rates and

relatively small liquid rates are encountered

pressure loss must be minimum

Installation through manholes is necessary

Pressure Drop

• Increase in Gas and Liquid flow rate increases the pressure drop.

• “X” is loading point.

• “Y” is flooding point.

Characteristics of Packing

• The Principal Requirement of a Tower packing are:

Chemically inert Strong Cause less liquid hold up or pressure drop Provide good contact between the phases Reasonable in cost Corrosion resistant Non porous Easy to wet Easy to fabricate

Fahad Ali Rabbani

M07-126

Types of Packing

• Packings can be divided into four main classes

Broken solidsShaped packingGridsStructured packing

Broken Solids

Cheapest formused in sizes from about 10 mm to100 mmform a good corrosion-resistant materialthey are not as satisfactory as shaped

packings

Shaped Packing

• In case of Shaped Packings:

Channelling is much less marked Resistance to flow is much less Effective surface per unit volume is more Film flow is much improved compared with

broken solids. The voidage obtainable with these packings

varies from about 0.45 to 0.95. But shaped packings are more expensive,

particularly when small sizes are used

Grid Packing

• Grid packings are:

relatively easy to fabricate usually used in columns of square section frequently used in cooling towers made from wood, plastics, carbon, or ceramic

materials give low pressure drops ability to accept fluids with suspended solids ease of wetting even at very low liquid rates

Shaped Packings

Structured Packings

Common Packings

• Berl Saddle• Intalox Saddle• Raschig rings• Lessing rings• Pall rings • Nutter rings • Intalox • Intalox metal saddles • Hy-Pak • Mini rings

Packings used in Laboratory

1. Dixon packings (Lessing rings) 2. fine wire mesh packing

Properties:

• In case of these packings: • interfacial areas are high• transfer characteristics are very good even at low liquid

rates• the volume of liquid hold up in such a packing is low • pressure drop is also low

Effects of Packings on Tower Design

• The size of packing used influences:

height of column diameter of column pressure drop cost of packing mass transfer efficiency

Wetting

• Complete wetting is needed

• Wetting rate is given by

Lw = volume liquid rate per unit cross-sectional area of column

Packing surface area per unit volume of column

Design Consideration

1. Entering gas or liquid flow rate, composition

2. Desired degree of recovery of one or more solutes

3. Choice of solvent

4. Operating pressure and temperature, and allowable gas pressure drop

5. Minimum solvent (stripping agent) flow rate and actual absorbent (stripping agent) flow rate

6. No. of equilibrium stages

7. Heat effects and need for cooling or heating

8. Type of packing and its size

9. Height of column

10. Diameter of column

General Applications

• Absorption

• Distillation

• Liquid-liquid Extraction