1. (7)Effect of pressure
When total pressure P increases, or temperature T decreases, the solubility of soluble components increases. Increasing P and decreasing T are Favorable conditions for absorption.吸收( 高压、低温有利)
If the gas and liquid rates are kept constant so the operating line is not changed, for given ZT=Hoy•Noy =Constant,
For given V, yb, ya, P x*b=yb/m liquid flow rates Lmin.

2. (8)Temperature variations in packed towers
When rich gas is fed to an absorption tower, the temperature in the tower varies appreciably from bottom to top.
Heat of absorption of the solute temperature T;
Evaporation of the solvent temperature T;
Usually, overall effect: T. Tp*(equilibrium partial pressure) Driving force. The heat of absorption of the solute is disadvantageous to absorption Factors influencing the temperature profile: 1)rates of solute absorption; 2)evaporation or condensation of solvent; 3)heat transfer between the phases.

3. When the gas inlet temperature is close to the exit temperature of the liquid and the incoming gas is saturated, there is little effect on solvent evaporation, and the rise in liquid temperature is roughly proportional to the amount of solute absorbed. [Fig.18.15a, p.567]
When the gas enters the columns 10 to 20 0C below the exit liquid temperature and the solvent is volatile, evaporation will cool the liquid in the bottom part of the column, and the temperature profile may have a maximum, as shown in Figure 18.15b.
*E.g.(18.4)

4. (9)Supplementary: Stripping factor method for calculating the number of transfer units
Assume
Operating-line equation:
That is:

5. Substituting x from Eq.(b) into Eq.(a) and rearrangement give
Integration(综合) of above Eq. gives

6. Slope of equilibrium line Slope of operating line
脱吸因数
[Similar to Eq.(3-84) in Chinese textbook]脱吸因数法

8. Absorption efficiency A:
When

9. Two ways of increasing absorption efficiency A:
1)Increasing the mass transfer driving force: (a)Increasing the liquid-gas ratio液气比L/V driving force; (b)Improving the equilibrium relationship: P or T driving force; (c)xa driving force.
2)Decreasing the resistance to mass transfer (1/Kya): (a)V (Gas-film controls);L (liquid-film controls).

10. Noy or Nox reflects the difficulty of absorption process
Noy or Nox reflects the difficulty of absorption process. The greater the value of Noy is, the more difficulty is the absorption process.
Hoy or Hox reflects the performance of the absorption equipment.

11. (10)Desorption or stripping(pp.81~85) 解吸
1)Definition of Desorption/stripping: the reverse of gas absorption: recover valuable solute from the absorbing solution and regenerate the solvents.
In stripping, the transfer is from the liquid to the gas phase.
2)Characteristics (comparing with absorption):
Identical points: (a)The equipment used is the same as in absorption(plate or packed tower); (b)Principles are the same, both are of one way diffusion; (c)Methods and equations for calculating ZT(height of a stripping column) are the same as for absorption.

12. Different points:
(a)Purpose of absorption: Separating components from gas mixtures; Purpose of stripping: Recover the solute and regenerate the solvents.
(b)Mass transfer direction: Gasliquid (in absorption); Liquidgas (in stripping).
(c)Mass transfer driving forces: y>y*,(y-y*)in absorption; y<y*,(y*-y)in stripping.
(d)Favorable conditions: P and T for absorption; P and T for stripping.
(e)Positions of equilibrium line and operating line: see the following figures.

13. y* Driving force: y – y* Operating line Equilibrium curve
Equilibrium And Operating Lines for gas absorption y*
Driving force:
y – y*

14. Operating line
Equilibrium curve
For stripping
Driving force:

15. Calculations for stripping
Stripping media: Inert gas or superheated steam(condensable).
1)Molal flow rate of Inert gas or superheated steam V=?
Higher conc. end
Lower conc. end
Figure. packed column for stripping.
Inert gas or steam

16. Comparing absorption:
By balancing the operating costs against the fixed costs of the equipment,
That is,
For stripping,
Vmin is determined by calculating (V/L)min, i.e., (L/V)max:
Equilibrium curve
Operating line

17. For case (a),
Equilibrium curve
Operating line
(a)
(b)

18. 2)Calculating ZT(height of a stripping column) [Equations are the same as for absorption]
Gas film:
Liquid film:
Overall gas:
Overall liquid:

19. Or,

20. [Example 18.5.]

21. *Other absorption processes（自学）
1)Multicomponent（多组分） absorption(pp.81)
Definition: Absorption process in which more than one solute is absorbed by a solvent from a gas mixture.
Characteristics: Separate equilibrium and operating lines are needed for each solute, but the slope of the operating line, which is L/V, is the same for all the solutes.

22. Usually the packed height ZT and the liquid flow rate L are determined according to the absorption requirement of some key component A, then the fractional removal of other components are calculated from the given ZT and L.

23. 2)Absorption from rich gases (pp.574~580) （自学）
Solute concentration is moderate or high in the gas.
(a) V and L must be accounted for in the material balance; (b)The correction factor for one-way diffusion should be included; (c)Ky or Kx will not be constant; (d)There may be an appreciable temperature gradient in the column, which will change the equilibrium line.

24. 3)Absorption with chemical reaction (pp.588~589)（自学）
Absorption followed by reaction in the liquid phase is often used to get more complete removal of a solute from a gas mixture.
Characteristics: (a)Equilibrium partial pressure of the solute p* and p*0; Driving force (p-p*) . (b)Liquid film resistance to mass transfer , that is kL (gas film controls). (c)The solvent is hardly saturated by the solute. Therefore, absorption with chemical reaction can be used to substitute some difficult physical absorption processes.

25. *[Mass transfer correlations
Empirical correlations for mass transfer coefficients in absorption
1)For NH3 absorption in water:
The absorption of NH3 in water is often cited as an example of gas-film control, since the gas film has about 80 to 90 percent of the total resistance.

26. 2)For CO2 absorption in water at 1 atm:
Liquid film controls: KL kL.
3)For SO2 absorption in water:
With gases of intermediate solubility both resistances are important.