It is a mass transfer operation in which one or more gas solutes is removed by dissolution in a liquid.

The inert gas in the gas mixture is called “carrier gas”.

In the absorption process of ammonia from air-ammonia mixture by water, air is carrier gas, ammonia is „solute” and water is absorbent.

An intimate contact between solute gas and absorbent liquid is achieved in a suitable absorption equipment, namely, tray tower, packed column, spray tower, venture scrubber, etc.

Desorption or stripping operation is the reverse of absorption. Absorption operation is of two types; physical and chemical.

𝑆𝑜𝑙𝑢𝑡𝑒 + 𝐶𝑎𝑟𝑟𝑖𝑒𝑟 𝑔𝑎𝑠 𝐴𝑏𝑠𝑜𝑟𝑏𝑒𝑛𝑡 = 𝑆𝑜𝑙𝑢𝑡𝑒 𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑑 𝑖𝑛

𝑎𝑏𝑠𝑜𝑟𝑏𝑒𝑛𝑡 + 𝐶𝑎𝑟𝑟𝑖𝑒𝑟 𝑔𝑎𝑠.

Absorption and stripping (as well as distillation) operations are usually carried out in vertical, cylindrical columns or towers in which devices such as plates or packing elements are placed.

The gas and liquid normally flow countercurrently, and the devices provide the contacting interfacial surface through which mass transfer takes place (cocurrent flow is also possible from economic standpoint).

Operations in which one or more components of a gas phase are absorbed into a liquid phase are common throughout the chemical process industries and frequently serve to achieve desired reactions among components in the two phases (Lee & Tsui, 1999).

Such operations are often called reactive absorption because of the combination of reaction and absorptive mass transport.

As an example, in the absorption of carbon dioxide by caustic soda, the carbon dioxide reacts directly with the caustic soda.

An advantage of absorption plus reaction is the increase in the mass-transfer coefficient.

This may be due to a greater effective interfacial area.

The process hydrodynamics can also be directly involved via correlations for the hold-up, pressure drop, and mass transfer coefficients, etc.

ADVANTAGE :

Generally , the gas which to be absorbed in the liquid which is sparged to the bottom of the column with the help of carrier gas.

The carrier gas which has insoluble in the liquid solvent.

The liquid solvent is entered at the top side countercurrently.

At the some height,the packing material is set for mass transfer operation where absorption occurs.

The packing either be Regular or Random based on industrial requirement.

Abbility of absorption mainly depends on choice of solvent however it will be good prefered for gases or not.

Gas solubility Volatility Corrosiveness Cost Viscosity Miscellaneous

The basic expressions for material balances and their graphical interpretations will be presented for countercurrent flow (a similar method may be used for cocurrent flow).

The gas stream at any point in the tower consists of G total moles/hr, made up of diffusing solute A of mole fraction yA, partial pressure pA, or mole ratio YA, and non- diffusing, essentially insoluble gas GS moles/hr. The relationships among these are:

Similarly the liquid stream consists of L total moles/hr, containing xA mole fraction soluble gas, or mole ratio XA, and essentially non-volatile solvent LS moles/hr. The relationships among them are:

Thus the material balance (for y and x subscript A is omitted):

which is rearranged for the gas phase mole fraction of y as:

It is easy to prove that the second parts of the last two equations are equal, by writing a material balance on the whole equipment which can be rearranged as follows:

The liquid and gas compositions in a packed tower changes continuously throughout the height.

The composition of solute from entry to exit of the packed tower is represented by operating line and every point indicates some location in the packed tower.

On the other hand, in tray tower, few points (number of trays) in the operating line represents the conditions in the trays.

Height Equivalent to Theoretical Plate::::::

For a specified separation job, in packed tower, the height of packing is to be determined and in tray tower, numbers of ideal trays are determined.

The ratio between packing height to number of trays required for the same separation is called height equivalent to theoretical plate (HETP)

• Relatively low pressure drop • Capable of achieving relatively high mass-

transfer coefficient and high mass transfer rate • Increasing the height and/or type of packing or try numbers capable of improving mass transfer without purchasing a new piece of equipment • Relatively low capital cost• Relatively small space requirements

• Fiberglass-reinforced plastic construction permits operation in highly corrosive atmospheres.

• Ability to collect particles as well as gases• Collected substances may be recovered

by distillation

• Possibility of creating water (or liquid) disposal problem (remark: IPPC)

• Product collected wet • Particulates deposition possibly causing

plugging of the bed or the plates • Fibreglass-reinforced plastic is sensitive

to temperature • Relatively high maintenance cost • Must be protected from freezing (if

operated in outside)