Week 6

Question 1

objective to to remove material in the gas phase from liquid phase, resulting in a state of equilmolar counter diffusion

Answer 1

stripping

Question 2

objective to to add material in the gas phase from liquid phase, resulting in a state of equilmolar counter diffusion

Answer 2

absorption

Question 3

the feed is a gas that is added at the bottom of the column while the solvent is fed into the top of the column. The absorbed gas leaves at the bottom of the column while the unabsorbed gas leaves the top of the column.

Answer 3

absorption

Question 4

can also be called gas washing, has scrubbing or gas absorption

Answer 4

absorption

Question 5

Absorption and stripping can be conducted in:

Answer 5

1. Tray towers
2. Packed column
3. spray towers
4. Bubble columns
5. Centrifugal contactors

Question 6

General design considerations for strippers/gas absorbers

Answer 6

1. Entering gas
2. Desired degree of recovery
3. Choice of stripping agent
4. Operating pressure and temperature
5. Minimum absorbent flow rate compared to actual absorbant flow rate
6. Number of equilibrium stages
7. Heat effects
8. Type and dimensions of absorber

Question 7

The ideal absorbent should be

Answer 7

1. Highly soluble for solute (low usage)
2. Low volatility (low losses)
3. Be stable (low losses)
4. Be non-corrosive
5. Have low viscosity (low pressure drops with heat and mass transfer rates)
6. Be non foaming
7. Non toxic and non flammable
8. Be cheap
9. Be available

Question 8

The degree to which a as is absorbed is ….

Answer 8

a fraction of its partial pressure at any temperature and pressure

Question 9

Three types of gas can be considered for Henry’s Law for solubility of gas

Answer 9

1) Very soluble
2) Moderately soluble
3) Sparingly soluble

Question 10

Henry’s law for solubility of gas equation

Answer 10

Pa = H * Xa
Pa = H * Ca

For dilute concentrations of most gases the equilibrium relation is given by Henry’s law

Question 11

What solubility is significantly affected by

Answer 11

temperature & pressure

Question 12

Equation for molecular diffusion

Answer 12

NA = - DA (dC/dz)

Na = diffusional flux
D = diffusivity of A with respect to B
C = concentration of A
z = diffusional path function

Question 13

used in gas absorption to describe the mass transfer of a solution from a has into a liquid

Answer 13

Whitman Two film theory

https://www.oxfordreference.com/abstract/10.1093/acref/9780199651450.001.0001/acref-9780199651450-e-3194

Question 14

Main assumptions of Whitman Two Film Theory

Answer 14

1. Bulk gas & liquid phases are in turbulent flow and are therefore perfectly mixed, have uniform concentration and there is no resistance to mass transfer
2. On either side of the interface there are thin films where the flow is laminar or stagnant where the mechanisms of mass transfer is by molecular diffusion as described by Fick’s Law & where the whole of the resistance to mass transfer occurs
3. There is no resistance to mass transfer at the interface and conditions of local equilibrium occur

Question 15

Film gas transfer coefficients for gas film

Answer 15

N’A = Kg (p-pi)

Kg = gas film transfer coefficient
pi = partial pressure solute A in gas at interphase

Question 16

Film gas transfer coefficients for liquid film

Answer 16

N’A = Kl (xi-x)

Kl = liquid film mass transfer coefficient
xi = concentration solute A in iquid at interphase

Question 17

Under steady state conditions the mass flux through each film is the same

Answer 17

N’A = Kg(p-pi) = Kl(x-xi)

Question 18

(p-pi) is known as…

Answer 18

gas film driving force

Question 19

(xi-x) is known as…

Answer 19

liquid film driving force

Question 20

Whitman Two Film Theory: equations for gas film

Answer 20

N’A = Kg(p-pi) = kg(y-yi)
N’A = KG(p-pi) = KG(y-ye)

Question 21

Whitman Two Film Theory: equations for liquid film

Answer 21

N’A = kL(xi-x)
N’A = KL(xe-x)

Question 22

Slope of equilibrium line H

Answer 22

[pi-pe]/[xi-x]

Question 24

In soluble gas, the value of H is small therefore…

Answer 24

1/KG ≈ 1/kg

Question 25

for a soluble gas system the bulk of the resistance is within the gas phase

Answer 25

Gas film control

Question 26

In insoluble gas, the value of H is large therefore…

Answer 26

1/KL ≈ 1/kL

Question 27

Operating line equation

Mass balace on the solute between any point of composition

Answer 27

Lm[X-X2]=Gm[Y-Y2]

Lm = flow of solute free liquid
X,Y=mole ratio in liquid & vapour phase
Gm = flow of solute free gas

Question 28

Lm and Gm are treated as ________ at low concentrations

Answer 28

low

Question 29

On a driving force diagram, the area between the operating line and the equilibrium line represents

Answer 29

driving force for mass transfer

Question 30

On a driving force diagram, the top line is

Answer 30

Operating line

Slope: L/G

Question 31

On a driving force diagram, the bottom line is

Answer 31

Equilibrium line

Slope: H

Question 32

total area of contact between two liquids in a liquid-liquid operation

Answer 32

interfacial area

https://www.collinsdictionary.com/dictionary/english/interfacial-area

Question 33

Over a small volume, the amount transferred per unit time dQ =

Answer 33

dQ = KG a (p-pe) dv

a = effective interfacial area per unit volume of packing