1. Absorption and Stripping

2. Some important definitions
In distillation, heat drives the separation of the more volatile from the less volatile component; this unit op is always counter-current.
In stripping/absorption, separation is induced by addition of a third component; these unit ops can be either counter-current or co-current.
stripping: a volatile component of a liquid stream vaporizes into a carrier gas stream
absorption: a soluble component of a gas stream dissolves in an extracting liquid stream
- physical absorption: the desired component is soluble in the extracting liquid
- chemical absorption: the desired component reacts with the extracting liquid
• irreversible chemical absorption: generates product/waste
• reversible chemical absorption: solvent is recycled by stripping
Stripping and absorption are often used together.

3. Ex.: Integrated system for removing CO2 from syn gas
solvent cooler
absorber
stripper
H2NCH2CH2OH
(MEA)
hot feed gas
H2, CO, CO2
syn gas
N2 + CO2
MEA
H2, CO
MEA + CO2
MEA + CO2 N2
heat exchanger
Gases in at the bottom.
Liquids in at the top. (Why?)

4. Key simplifying assumptions
stripping gas/carrier gas is insoluble in solvent
solvent is non-volatile
- therefore all streams are either pure or binary
columns are isothermal and isobaric
heat of absorption is negligible
- therefore energy balance is automatically satisfied
Degrees of freedom analysis:
D.o.F. = C – P = – = 3
(A,B,C) (V,L)
When T, P are fixed (assumption 3), can specify only one more variable: xB or yB

5. Labeling streams
Ln-1
xn-1 Vn yn Ln xn
Vn+1
yn+1
Packed columns are used more often than tray (plate) columns in absorption/stripping, because of low mass transfer efficiencies.
HETP ≡ height (of packing) equivalent to a theoretical plate.
nth stage
HET
HETP
Fractional stages are possible with packed columns.
xn, yn: mole fractions of solute A at equilibrium leaving the nth stage
Vn: total gas flow rate = (moles solute A + moles carrier gas B) / time
Ln: total liquid flow rate = (moles solute A + moles solvent C) / time
Since A is transferred in one direction only (liq → gas, or gas → liq), V and L are not constant. Therefore CMO is not valid.

6. Using mole ratios
what is constant? G ≡ carrier gas flow rate, moles B/time since B is presumed insoluble, Gn = Gn+1 = G S ≡ solvent flow rate, moles C/time since C is presumed non-volatile, Sn = Sn-1 = S
vapor mole ratio:
liquid mole ratio:

7. McCabe-Thiele analysis of stripping
CMB: GYj SX0 = GY1 + SXj
operating line equation:
Yj+1 = (S/G)Xj + [Y1 – (S/G)X0]
slope = S/G Yint = [Y1 – (S/G)X0]
analogous to operating line for stripping section of distillation column
usually specified: X0, YN+1, S/G, XN
fast plotting of operating line:
• the point (XN, YN+1) lies on the operating line
• calculate Y1 from CMB
• the point (X0, Y1) also lies on the operating line
feedS, X0
stripping gas
G, YN+1
G, Y1
S, XN
stage 1
stage j
G, Yj+1
S, Xj
stage N

8. Ex.: Analysis of counter-current stripper
VLE (may be curved)
Given X0, XN, YN+1 and S/G, find N.
1. Plot VLE data as mole ratios (unless x0 < 0.05)
Note: y = x line has no use here.
•X0
(S/G)max • 1
•(X0,Y1)
2. Plot (XN, YN+1) and (X0, Y1) and draw operating line.
It will be below the VLE line.
operating line
slope = S/G • • 2 •
3. Step off stages (use Murphree efficiencies if available). • 3
N = 3
•(XN,YN+1)
To find minimum stripping gas flow rate (Gmin):
1. Plot X0 on VLE line (watch for earlier pinch point, if VLE is curved).
2. Calculate Gmin = S / (S/G)max
Rule-of-thumb: (S/G)opt ≡ 0.7 (S/G)max

9. Estimating fractional stages
VLE Y
op. line
(X3, Y4)
(X4, Y4) • X3 XN X4 • • XN X
fractional stage requirement

10. McCabe-Thiele analysis of absorber
CMB: GYN SXk-1 = GYk + SXN
operating line equation:
Yk = (S/G)Xk [YN+1 – (S/G)XN]
slope = S/G Yint = [YN+1 – (S/G)XN]
analogous to operating line for rectifying section of distillation column
usually specified: X0, YN+1, S/G, Y1
fast plotting of operating line:
• the point (X0, Y1) lies on the operating line
• calculate XN from CMB
• the point (XN, YN+1) lies on the operating line
Solvent
S, X0
feed
G, YN+1
G, Y1
S, XN
stage 1
G, Yk
S, Xk-1
stage k
stage N

11. Ex.: Analysis of counter-current absorber
Given X0, Y1, YN+1 and S/G, find N.
1. Convert VLE data to mole ratios (unless x0 < 0.05)
Note: y = x line has no use here.
VLE (may be curved)
(XN,YN+1)•
YN+1 •
(S/G)min
operating line
slope = S/G
2. Plot (X0, Y1) and (XN, YN+1) and draw operating line.
It will be above the VLE line (because mass is transferred in opposite direction, gas → liq). • • 2 1 3 • •
3. Step off stages (use Murphree efficiencies if available).
(X0,Y1)• •
N = 3
Find minimum extracting solvent flow rate (Smin) for given G:
1. Plot YN+1 on VLE line (watch for earlier pinch point, if VLE is curved).
2. Calculate Smin = G • (S/G)min
Rule-of-thumb: (S/G)opt ≡ 1.4 (S/G)min

12. Multiple non-interacting solutes
Multiple soluble components (A, D, E…) in solvent C, to be stripped using gas B, OR Multiple components (A, D, E…) in carrier gas B, to be absorbed using solvent C.
If streams are dilute and components do not interact with each other, assume VLE for each component is independent.
Treat each as a single-component problem, and solve sequentially.
For dilute streams, Yi = yi / (1 - yi) ≈ yi
Xi = xi / (1 - xi) ≈ xi
S/G ≈ L/V

13. Ex.: 2-component absorber
Specify yA,N+1, yD,N+1, xA,0, xD,0
Specify L/V and yA,1. Find N and yD,1
(xD,N,yD,N+1)•
yA,1
yD,1
xA,0
xD,0
slope = L/V
Separation of A requires N = 3. 1
VLE(A)
(xA,N,yA,N+1)•
operating line
slope = L/V y • •
VLE(D) • • 2 3 1 2 3 1 N • • •
Separation of D must also use N = 3 and same L/V.
Trial-and-error: guess yD,1 •
yA,N+1
yD,N+1
xA,N
xD,N
(xA,0,yA,1)• •
(xD,0,yD,1)• • x
Probably a good idea to use a different graph for each component…

14. Irreversible absorption
Add reagent R to solvent. R reacts essentially irreversibly with solute A to form non-volatile products R + A(g) → R•A(l) e.g., NaOH + H2S(g) → Na2S + H2O
Equilibrium lies far to the right: xA ≅ and yA ≅ 0
Equation of the VLE line: yA = 0

15. Ex. Irreversible absorption
Specify yN+1, x0, L/V.
Required: xN = y1 = 0 B
y1 = 0
C + R
x0 = 0
Only one theoretical equilibrium stage required … 1
operating line
slope = L/V
yN+1 • y N
A + B
yN+1
C + R•A
xN = 0
VLE
(x1,y1) •
(x0,y1)• x
(A + R•A)

16. Ex.: Irreversible absorption with low efficiency
Specify yN+1, x0, L/V, y1 ≠ 0 B
y1 ≠ 0
C + R
x0 = 0
More than one actual equilibrium stage required … 1
operating line
slope = L/V
yN+1 •
pseudo-VLE • •
EMV = 0.25 2 6 5 4 3 1 y • • • • N • •
A + B
yN+1
A + R•A
xN = 0 • •
(x0,y1)• •
VLE x
(A + R•A)

17. Co-current cascade
• can use higher vapor velocity to increase mass transfer rate • can use smaller diameter column without risk of flooding • generally used for irreversible absorption
Specify y0, x0 = 0, xN, yN = 0
L, x0
V, yN
V, y0
L, xN j
Only one theoretical equilibrium stage required, if the reaction is irreversible and mass transfer is fast …
(x0,y0)•
operating line
slope = -L/V y
L, xj
V, yj
(x1,y1) •
VLE x
(A + R•A)

18. VLE for dilute streams
When streams are dilute, VLE data can be approximated by a straight line.
y = mx
Obtain the slope, m, from Henry’s Law: PB = HB xB where yB = PB/Ptotal PB is the partial pressure of B, and HB is the Henry’s Law constant. Note: HB = HB(T), like an equilibrium constant.

19. Analytical solution, when both VLE and op. line are straight
change in vapor composition between adjacent stages:
Op. line
(x2,y3) •
VLE: y = mx
(Δy)2
CMB:
(x1,y2) •
•(x2,y2)
(Δy)1 = y2 - y1
VLE:
(x0,y1)•
•(x1,y1)
general case: L/V ≠ m, then (Δy)j ≠ (Δy)j+1
special case: if L/V = m, then (Δy)j = Δy.
Δy1 + Δy2 + Δy3 + … = yN+1 – y1 = NΔy OR

20. Kremser equation: L/V ≠ m
use VLE:
where A = L/mV ≡ absorption factor
Kremser equation: ➠
where y0 = mx0

21. Other forms of Kremser equation
For gas phase compositions (absorber columns):
where S = mV/L ≡ stripping factor, and xN+1 = yN+1/m
For liquid phase compositions (stripper columns):
solve for N:
include Murphree vapor efficiency:
More forms shown in Wankat, chapter 12.4

22. Counter-current column sizing
Height: 1. measure HETP 2. measure EMV 3. obtain N Diameter: 1. key parameter is V, total gas flow rate (not constant) 2. Vj is largest at the top of a stripper column, or at the bottom of an absorber column 3. calculate D using same procedure as distillation column