Dr Saad Al-ShahraniChE 334: Separation Processes Absorption and Stripping of Dilute Mixtures In absorption (also called gas absorption, gas scrubbing,

Dr Saad Al-ShahraniChE 334: Separation Processes

Absorption and Stripping of Dilute Mixtures

In absorption (also called gas absorption, gas scrubbing, and gas washing), a gas mixture is contacted with a liquid (absorbent or solvent) to selectively dissolve one or more components by mass transfer from the gas to the liquid.

The components transferred to the liquid are referred to as solutes or absorbate.

Absorption is used for:

a) separate gas mixtures.

b) remove impurities, contaminants, pollutants.

c) recover valuable chemicals. The opposite of absorption is stripping (also called desorption), where

in a liquid mixture is contacted with a gas to selectively remove components by mass transfer from liquid to the gas phase.


A typical absorption operation is shown in Figure 6.1 (Seader):

(absorption of acetone by water)

For acetone, the K-value is calculated by the modified Raoult's law:

= 6.7 for a dilute solution of acetone in water at 25°C and 101.3 kPa.

K = Psat/P


89.2

L= 1943 kmol/h, V= 703 kmol/h


For oxygen and nitrogen, K-values are based on the use of Eq. (6) of Table (2.3) (Seader) Henry's law:

K = H/P

For water, the K-value is obtained from Eq. (3) of Table 2.3, (Seader) Raoult's law:

K = Psat/P

A = absorption factor = L/KV, L= liquid flow rate

V= vapor flow rate.



If the value of A >1, any degree of absorption can be achieved. The larger the value of A, the fewer the number of stages required to absorb a desired fraction of the solute.

From an economic standpoint,

1.25 ≤ A ≤ 2.0

For a stripper, the stripping factor, S = 1IA = KVIL is crucial.





There are two types of absorption:

1. Physical absorption.

2. Chemical absorption (reactive absorption)

Examples of chemical absorption type is absorption of CO2 in NaOH (irreversible chemical absorption)

More complex example of chemical absorption is CO2 or H2S in monoethanol amine (reversible reaction takes place in liquid phase)

Chemical reaction can increase:

1. Rate of absorption.

2. The absorption capacity of the solvent,

3. Selectivity to preferential dissolve only certain components of the gas,

4. Convert a hazardous chemical to a safe compound.



EQUIPMENT

1.Trayed towers (plate columns). 2.Packed columns.



3. Spray towers. 4. Bubble columns. 5.Centrifugal contactors.



Details of a contacting tray in a Trayed tower.



Froth regime:

The most common and favored regime is the froth regime, in which the liquid phase is continuous and the gas passes through in the form of jets or a series of bubbles.

The spray regime,

In which the gas phase is continuous, occurs for low weir heights (low liquid depths) at high gas rates.


Vapor liquid regimes for sieve tray tower, when the openings are holes, are one of the following:

bubble regime

For low gas rates, in which the liquid is fairly quiescent and bubbles rise in swarms.

Emulsion:

At high liquid rates, small gas bubbles may be undesirably emulsified.



The liquid carries no vapor bubbles to the tray below.

The vapor carries no liquid droplets to the tray above, and there is no weeping of liquid through the openings of the tray.

With good contacting, equilibrium between the exiting vapor and liquid phases is approached on each tray.

Cellular foam

bubble coalescence is hindered, an undesirable foam forms.

Ideally




Types of trays

a) Perforated (sieve) tray

b) Valve cap tray

c) Bubble cap tray

Comparison of type of trays




valve cap bubble capperforated



Tray with valve caps.


Tray types are compared on the basis of cost, pressure drop, mass transfer efficiency, vapor capacity, and flexibility in terms of turndown ratio (ratio of maximum to minimum vapor capacity).

At the limiting vapor capacity, flooding of the column occurs because of excessive entrainment causing the liquid flow rate to exceed capacity of the down comer and, thus, go back up the column.

At low vapor rates, weeping of liquid through the tray openings or vapor pulsation becomes excessive.


Sieve trays are preferred because of their low relative cost.


Packed column.



Centrifugal contactor,

As shown in Figure 6.2

(Seader), consists of a

stationary ringed housing,

intermeshed with a ringed

rotating section. The liquid

phase is fed near the center

of the packing, from which it

is caused to flow outward by

centrifugal force.


The vapor phase flows inward by a pressure driving force.


1. Very high mass transfer rates can he achieved with only moderately high rotation rates

Advantage:

2. It is possible to obtain the equivalent of several equilibrium stages in a very compact unit.



Types of packing materials

1. Random Packing materials



2. Structured Packing materials





GENERAL DESIGN CONSIDERATIONS

Design or analysis of an absorber (or stripper) requires consideration of a number of factors, including:

1. Entering gas (liquid) flow rate, composition, temperature, and pressure

2. Desired degree of recovery of one or more solutes.

3. Choice of absorbent (stripping agent).

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

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

(stripping agent) flow rate as a multiple of the minimum rate needed to

make the separation

6. Number of equilibrium stages



7. Heat effects and need for cooling (heating)

8. Type of absorber (stripper) equipment

9. Height of absorber (stripper)

10. Diameter of absorber (stripper)



(a) have a high solubility for the solute (s) to minimize the need for

absorbent,

(b) have a low volatility to reduce the loss of absorbent and facilitate

separation of absorbent from solute (s),

(c) be stable to maximize absorbent life and reduce absorbent makeup

requirement,

(d) be non corrosive to permit use of common materials of construction,

(e) have a low viscosity to provide low pressure drop and high mass and

heat transfer rates,

The ideal absorbent should



(f) be non foaming when contacted with the gas so as to make it unnecessary to increase absorber dimensions,

(g) be nontoxic and nonflammable to facilitate its safe use.

(h) be available, if possible, within the process, to make it unnecessary to provide an absorbent from external sources, or be inexpensive.

The most common absorbents:

1. Water

2.Hydrocarbon oil.

3. Aqueous solutions of acids and bases.



The most common stripping agents are:

1. Water vapor.

2. Air.

3. Inert gases.

4. Hydrocarbon gases.

For absorber, operating pressure should be high and temperature low:

1.To minimize stage requirements and/or absorbent flow rate.

2.To lower the equipment volume required to accommodate the gas flow.

Unfortunately, both compression and refrigeration of a gas are expensive. Therefore, most absorbers are operated at feed-gas pressure,



For a stripper, operating pressure should be low and temperature high to minimize stage requirements or stripping agent flow rate.

However, maintenance of a vacuum is expensive. Also a high temperature can be used, but it should not be so high as to cause undesirable chemical reactions.


Documents

Dr Saad Al-ShahraniChE 334: Separation Processes Absorption and Stripping of Dilute Mixtures In absorption (also called gas absorption, gas scrubbing,