ABSORPTION

What is Absorption?

• Absorption is the transfer of one or more materials from the gas phase to a liquid solvent

• It is typically a physical phenomenon and often involves no change in the chemical species present in the system

• It is an important mitigation method

Terminology • Solute:- It is the material that is transferred from the gas

to the liquid phase• Liquid desiccants:- Liquid solutions that have the

property of absorbing moisture from air, e.g., lithium chloride and triethylene glycol

• Scrubbing:- Another term for absorption• Salting out:- Liquid desiccants contain significant

amount of water in which the pollutants are dissolved, and presence of certain chemicals like lithium chloride can decrease the solubility of pollutants in water, this phenomenon is known as salting out.

Fundamentals • Prediction of performance of absorbers and

absorption equipmentThe first approach makes use of mass

transfer coefficients and depends on the molecular diffusivities of the solute for the equipment in which the operation is being carried out

The second technique is a graphical solution method

The third method is described as the absorption factor or overall approach

Packed tower

Vb

Lb

La

VaDemister

packingLiquid distributor

Material balance equation

Lb + Va = La + Vb

• V – moles of gas phase per unit time

• L – moles of liquid phase per unit time

• a,b – top and bottom of the contactor, respectively.

Desirable properties of Tower packing

Large void volume to decrease pressure drop Chemically inert to the fluids being processed Large surface area per unit volume of packing Lightweight but strong Good distribution of fluids Good wettability

Tower packing

• Tower packings are usually available in a variety of materials, including ceramic, metal, plastic and carbon

• The size of an ideal tower packing must not be greater than one-eighth of the tower diameter

Packing height

• The rate of mass transfer and packing height depend on the area of contact between the phases

• The height of a packed tower depends on the properties and flow rates of the contacting streams as well as on the type of packing being used

Application of absorption to dehumidification and pollutant control

• Temperature and humidity are the key factors in determining the comfort of an environment

• A significant reduction in pollutant concentrations can be achieved by controlling the humidity of the space to be air conditioned

• Dehumidification and pollutant control can be achieved by absorption using spray columns, fine tube surface type dehumidifier, packing column, liquid desiccant based HVAC systems and absorber-stripper air conditioning systems

• Liquid desiccant systems are in use in large commercial buildings only and not in homes or small buildings