Absorption / Stripping Flashcards
(24 cards)
QDefine absorption and stripping
Absorption: Transfer of solute from gas to liquid phase.
ie. clean the GAS
Stripping: Removal of solute from liquid to gas phase.
ie. clean the LIQ
define CMO
constant molar overflow: molar flow rates of liq and cap remain the same
ie. eqimolar counter diffusion - equal mols vap and liq transferred across every stage
What is Henry’s Law?
At equilibrium: yi = mi * xi
where mi is Henry’s constant
does a smaller Henry’s constant m indicate more or less soluble?
smaller constant m = MORE soluble
x=y/m
as m is smaller, x increases (more moles dissolved in liquid)
henry’s law vs Raoult’s law
Henry’s describes solute-solvent interactions (ie. dissolved GAS in LIQ), usually for DILUTE conc
Raoult’s describes solvent-solvent interactions (between liquids), for higher conc esp when solution is ideal
Derive the operating line for absorption (single solute, counter-current).
IN=OUT material bal for single stage.
Lxf + Vyf = Lx1 + Vy1
rearrange for y1 = …
in terms of yf and xf entering, y1 and x1 leaving the stage
define the absorption factor (A)
A = L / mG
in absorption (cleaning a gas w a liquid), if the liquid used to clean is initially very clean, take…
xf = 0
simplifies single stage expression into
yi / yf = 1 / 1+A
very general method of approach for multi-stage absorption/stripping
mass balance and eqm over each stage.
for many stages, material balance over a CONTROL VOLUME
describe the set up of a two stage CROSS-flow cascade
G stream goes from 1 to 2 with yf, y1, y2.
L stream is SEPARATED across the two stages, with β being the fraction of L going to stage 1
xf, βL in to stage 1
xf, (1-β)L in to stage 2
still aim to express outlet gas frac, y2 in terms of yf
the EL and OL relate which streams in a multi-stage cascade?
EL relates x(n) and y(n) leaving the stage
OL relates stream of y(n-1). G into the stage with x(n). L out of the stage
ie. relates compositions of passing streams
gradient of the OL
slope L/G
from a material balance over a control vol for multi-stage cascade
How do you graphically determine the number of stages in absorption/stripping?
- Plot equilibrium line y = f(x), 2. Plot operating line, 3. Use step-off method between lines starting from known outlet composition.
What is a pinch point in absorption/stripping?
A point where operating and equilibrium lines touch; the driving force is zero, making further separation difficult or impossible.
how to determine point of pinch for absorption on McCabe-Thiele construction
fixed yf (also yB), slope L/G decreases until OL touches EL.
x value is xB*, which is the hypothetical stage BELOW the bottom stage
how to find minimum L/G ratio for absorption
min L/G is minimum slope of OL, hence corresponds to when a pinch is occurring.
How does stripping differ from absorption in terms of graph and direction?
Operating line slope is still L/G, but flow direction and compositions flip. Start at bottom liquid composition and step upwards.
pinch occurs at (yB, xB)
hence find slope:
L/G = yB-yT / xB-xT
what if the CMO assumption is NOT valid (ie. high rates of solute trasnfer?)
this means G and L are no longer constant
introduce new coordinates G’ and L’ for gas/solvent ONLY flow rate
and MOLE RATIOS, X and Y
results in an identical system.
define L’ , G’ and X, Y used for high rates of transfer of solute
L’ = L - xL and G’ = G - yG
X = x/1-x and Y = y/1-y
such that XL’ = YG’
What is the Kremser-Brown-Souders (KSB) equation used for?
To calculate the number of ideal stages for linear equilibrium (y = mx) and constant L/G in counter-current operation.
When is the KSB equation valid?
Linear equilibrium, constant L/G, dilute solute, no reaction.
How can multicomponent absorption/stripping be simplified?
Use a pseudo-component with effective K or H: K_eff = sum(z_i K_i), then apply single-component methods.
What happens if the operating line crosses the equilibrium line?
Physically infeasible—no driving force for mass transfer. Separation cannot be achieved under given conditions.
