Absorption and Stripping ofDilute Mixtures

Learning Objective

Explain the difference between absorptionand stripping.

Explain the difference between physicaland chemical absorption.

Explain why absorbers are best operatedat high pressure and low temperature,while strippers are best operated at lowpressure and high temperature.

Definition

In absorption (also called gas absorption,gas scrubbing, and gas washing), a gasmixture is contacted with a liquid (theabsorbent or solvent) to selectively dissolveone or more components by mass transferfrom the gas to the liquid.

Definition

The components transferred to the liquidare referred to as solutes or absorbate.

Why using absorption?

Absorption is used to:

1- Separate gas mixtures;2- Remove impurities,3- Remove contaminants,4- Remove pollutants,5- Remove catalyst poisons from a gas6- Recover valuable chemicals.

Stripping / Regeneration

The opposite of absorption is stripping (alsocalled desorption), wherein a liquid mixtureis contacted with a gas to selectively removecomponents by mass transfer from theliquid to the gas phase.

Absorption/Regeneration

Typical Absorber

What is the purposeof absorber?

The purpose of the 30-tray (equivalent to 10equilibrium stages) absorber is to remove theacetone by contacting the gas with a suitableabsorbent, water.

By using countercurrent flow of gas and liquid ina multiple-stage device, the material balance,indicates that 99.5% of the acetone is absorbed.

Typical Absorber

The gas leaving the absorber contains only 143ppm (parts per million) by weight of acetonevapor and can be recycled to the dryer orexhausted to the atmosphere.

Typical Absorber

Although the major component transferredbetween phases is acetone, the materialbalance indicates that small amounts of oxygenand nitrogen are also absorbed by the watersolvent.

The exit gas is almost saturated with watervapor and the exit liquid is almost saturatedwith air.

Typical Absorber

The temperature of the absorbent decreases by3C to supply the energy of vaporization neededto strip the water, which in this example isgreater than the energy of condensationliberated from the absorption of acetone.

Typical Absorber

The fraction of a component absorbed in acountercurrent cascade depends on the numberof equilibrium stages and the absorption factor,A = L/(KV), for that component.

For the conditions of example, using L = 1943kmol/h and V = 703 kmol/h, estimated K-valuesand absorption factors, which range over manyorders of magnitude, are:

Typical Absorber

Typical Absorber

Typical Absorber

If the value of A is greater than 1, any degree ofabsorption can be achieved.

The larger the value of A, the fewer the numberof stages required to absorb a desired fractionof the solute.

Very large values of A can correspond toabsorbent flow rates that are larger thannecessary.

Typical Absorber

From an economic standpoint, the value of A,for the main (key) species to be absorbed,should be in the range of 1.25 to 2.0, with 1.4being a frequently recommended value.

Thus, the above value of 1.38 for acetone isfavorable.

Typical Absorber

For a given feed-gas flow rate and choice ofabsorbent, factors that influence the value of Aare:

absorbent flow rate, temperature, pressure.

Typical Absorber

Because A = L/(KV), the larger the absorbentflow rate is, the larger will be the value of A.

The required absorbent flow rate can bereduced by reducing the K-value of the solute.

Because the K-value for many solutes variesexponentially with temperature and is inverselyproportional to pressure, this reduction can beachieved by reducing the temperature and/orincreasing the pressure.

Typical Absorber

Increasing the pressure also serves to reducethe diameter of the equipment for a given gasthroughput.

However, temperature adjustment by feed-gasrefrigeration and/or absorbent refrigeration,and/or adjustment of the feed-gas pressure bygas compression can be expensive.

For these reasons, the absorber operates atnear-ambient conditions.

Typical Absorber

For a stripper, the stripping factor, S = 1/A =KV/L, is crucial.

To reduce the required flow rate of strippingagent, operation of the stripper at a hightemperature and/or a low pressure is desirable,with an optimum stripping factor in the vicinityof 1.4.

Commercial Applications of Absorption

Physical Absorption

When water and hydrocarbon oils are used asabsorbents, no significant chemical reactionsoccur between the absorbent and the solute,and the process is commonly referred toas physical absorption.

Chemical Absorption

When aqueous sodium hydroxide (a strongbase) is used as the absorbent to dissolve anacid gas, absorption is accompanied by a rapidand irreversible neutralization reaction in theliquid phase and the process is referred to aschemical absorption or reactive absorption.

Chemical Absorption

More complex examples of chemical absorptionare processes for absorbing CO2 and H2S withaqueous solutions of monoethanolamine (MEA)and diethanolamine (DEA), where a reversiblechemical reaction takes place in the liquidphase.

Chemical AbsorptionChemical reactions can:

increase the rate of absorption,

increase the absorption capacity of thesolvent,

increase selectivity to preferentially dissolveonly certain components of the gas,

convert a hazardous chemical to a safecompound.