Absorption

ABSORPTION

Falling Liquid solvent absorbs the gas in the absorption column & is then sent into any one of he following unit:- Distillation, stripping section, removal through precipitation & settling, neutralization, oxidation, reduction & hydrolysis.

Purpose: 

• Gas purification
• Gas separation
• Product recovery
• Solvent recovery

Solvent Properties:

• Solubility of gas should be high in selected solvent. ( If not, then absorption is a waste!! )
• Low volatility organic liquid ( but water is preferred in many cases, due to easy availability & removal of water soluble gases like, HF, HCl, SiF4 ).
• Low vapor pressure ( to reduce evaporative loss of solvent )
• non-toxic, non-flammable, non- corrosive 
• Low viscosity

Types of columns:

    Packed Column

• Smaller column diameter application.
• Large inter facial area for mass transfer
• Simple and cheap in construction
• Preferred for corrosive gases because of availability of ceramic / plastic MOC packing

Material & types of packing

                  

Internals of Packed tower

1. Packing support plate:                                                                                                                   It must bear the weight of the packings & allow unrestricted flow of down coming liquid. Drawback is that, that the packing blocks some holes, thus reducing the tower capacity. 
2. Liquid distributor:                                                                                                                      Placed 6-12 inch above packing for allowing gas disengagement from the bed. Absorption & stripper columns require only one distributor, whereas, Distillation column requires 2 (feed & reflux)
3. Liquid re-distributor:                                                                                                                        Some part of entering liquid flows through the wall without coming in contact with the gas flowing counter currently, thus we need a liquid re-distributor to collect the down coming liquid and distribute it uniformly throughout the bed and thus increase the efficiency of the tower.
4. Demisters / Entrainment separators: Generally installed in exit gas streams for arresting the liquid droplets entrained in the gases. If demisters are not installed they can corrode / choke downstream equipments like heat exchangers tubes, damage tube sheets, contaminate products etc.

Types of packing :

Material of construction:

1. Against all acid gases, metal tower, metal plates, metal packing or any other metal internals can have deleterious effect.
2. Same is the case with organic liquid and plastic packing.
3. Ensure internal cooling facility is available when materials and gases releases high heat of absorption.

Dry & wet random packing:

Packed & plate column: