Week 6 Flashcards
objective to to remove material in the gas phase from liquid phase, resulting in a state of equilmolar counter diffusion
stripping
objective to to add material in the gas phase from liquid phase, resulting in a state of equilmolar counter diffusion
absorption
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.
absorption
can also be called gas washing, has scrubbing or gas absorption
absorption
Absorption and stripping can be conducted in:
- Tray towers
- Packed column
- spray towers
- Bubble columns
- Centrifugal contactors
General design considerations for strippers/gas absorbers
- Entering gas
- Desired degree of recovery
- Choice of stripping agent
- Operating pressure and temperature
- Minimum absorbent flow rate compared to actual absorbant flow rate
- Number of equilibrium stages
- Heat effects
- Type and dimensions of absorber
The ideal absorbent should be
- Highly soluble for solute (low usage)
- Low volatility (low losses)
- Be stable (low losses)
- Be non-corrosive
- Have low viscosity (low pressure drops with heat and mass transfer rates)
- Be non foaming
- Non toxic and non flammable
- Be cheap
- Be available
The degree to which a as is absorbed is ….
a fraction of its partial pressure at any temperature and pressure
Three types of gas can be considered for Henry’s Law for solubility of gas
1) Very soluble
2) Moderately soluble
3) Sparingly soluble
Henry’s law for solubility of gas equation
Pa = H * Xa
Pa = H * Ca
For dilute concentrations of most gases the equilibrium relation is given by Henry’s law
What solubility is significantly affected by
temperature & pressure
Equation for molecular diffusion
NA = - DA (dC/dz)
Na = diffusional flux
D = diffusivity of A with respect to B
C = concentration of A
z = diffusional path function
used in gas absorption to describe the mass transfer of a solution from a has into a liquid
Whitman Two film theory
https://www.oxfordreference.com/abstract/10.1093/acref/9780199651450.001.0001/acref-9780199651450-e-3194
Main assumptions of Whitman Two Film Theory
- Bulk gas & liquid phases are in turbulent flow and are therefore perfectly mixed, have uniform concentration and there is no resistance to mass transfer
- 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
- There is no resistance to mass transfer at the interface and conditions of local equilibrium occur
Film gas transfer coefficients for gas film
N’A = Kg (p-pi)
Kg = gas film transfer coefficient
pi = partial pressure solute A in gas at interphase
Film gas transfer coefficients for liquid film
N’A = Kl (xi-x)
Kl = liquid film mass transfer coefficient
xi = concentration solute A in iquid at interphase
Under steady state conditions the mass flux through each film is the same
N’A = Kg(p-pi) = Kl(x-xi)
(p-pi) is known as…
gas film driving force
(xi-x) is known as…
liquid film driving force
Whitman Two Film Theory: equations for gas film
N’A = Kg(p-pi) = kg(y-yi)
N’A = KG(p-pi) = KG(y-ye)
Whitman Two Film Theory: equations for liquid film
N’A = kL(xi-x)
N’A = KL(xe-x)
Slope of equilibrium line H
[pi-pe]/[xi-x]
In soluble gas, the value of H is small therefore…
1/KG ≈ 1/kg
for a soluble gas system the bulk of the resistance is within the gas phase
Gas film control
In insoluble gas, the value of H is large therefore…
1/KL ≈ 1/kL
Operating line equation
Mass balace on the solute between any point of composition
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
Lm and Gm are treated as ________ at low concentrations
low
On a driving force diagram, the area between the operating line and the equilibrium line represents
driving force for mass transfer
On a driving force diagram, the top line is
Operating line
Slope: L/G
On a driving force diagram, the bottom line is
Equilibrium line
Slope: H
total area of contact between two liquids in a liquid-liquid operation
interfacial area
https://www.collinsdictionary.com/dictionary/english/interfacial-area
Over a small volume, the amount transferred per unit time dQ =
dQ = KG a (p-pe) dv
a = effective interfacial area per unit volume of packing
