Topic 5: Absorption and Stripping in Plate Columns Flashcards
how does distillation differ to absorption and stripping processes
distillation is a process of separation by phase creation whereas, absorption and stripping operations are processes of separation by phase addition
why are stripping and absorption processes used
stripping and absorption processes are normally used to recover components that are present in low concentrations
what is absorption
absorption is the removal of a volatile solute from a gas stream by means of contact with a liquid solvent
what is stripping
stripping is the removal of a volatile solute from a liquid stream by means of contact with a stripping gas
what are some simplified assumptions for stripping and absorption processes
there are 3 components
- liquid component (carrier liquid or solvent)
- gas component (carrier or stripping gas)
- single volatile solute
only the volatile solute can transfer between phases
isothermal and isobaric throughout
no vaporization of solvent
no dissolution of carrier gas
constant molar overflow
what is L’ in stripping and absorption processes
L’ is the liquid molar flowrate of the solute- free solvent
what is V’ in stripping and absorption processes
V’ is the molar flowrate of solute-free carrier gas
how can we assume constant molar overflow in stripping and absorption processes
we can assume constaqnt molar overflow if L’ and V’ are constants throughout the column.
this will happen is we assume no vaporization and no dissolution of the carrier gas
define variable X
X is the mole ratio of solute to solvent in the liquid phase, X=x/1-x where x is the mole fraction in the liquid phase
define the variable Y
Y is the mole ratio of solute to carrier gas in the gas phase, Y=y/1-y where y is the mole fraction in the gas phase
explain the absorber diagram
liquid flows in from the top (L’, x0) and flows out the bottom (L’, xN)
gas flows in from the bottom (V’, yN+1) and exits from the top (V’, y1)
the stages are labelled from the top (stage 1) to the bottom (stage N)
what is observed about X and Y in dilture solutions
in dilute solutions, X is approx. = x , and Y is approx. = y
when x and y are4 much much smaller than 1, meaning that they are dilute
what do we need to design absorption/ stripping equipment by the equilibrium stage method
Y(X) OR X(Y)
what is the general equilibrium ratio relation expressed in?
for a single solute the general equilibrium relation is expressed in terms of the k factor and mole fractionm of the solute; k=y/x
what is the inverse relation between mole fractions and mole ratios?
y=Y/1+Y
and
x=X/1+X
using inverse relations between mole fractions and mole ratios, express Y(X) and its inverse X(Y) in terms of K
Y=KX/1+X(1-K)
X=Y/K(1+Y)-Y
what is the new equilibrium factor in terms of mole ratios?
K’=Y/X
if we do not have necessary Y(X) OR X(Y) data to construct the equilibrium curve, what do we do
we would calculate the equilibrium curve through other methods
we would need to calculate it from a thermodynamic model for the k factor
appropriate thermodynamic models for dilute systems are;
K=Y1-P1*/P modified raoult’s law non-ideal dilute volatile liquid solute
K=H1/P dilute gaseous state
for dilute solutions, what is the relationship between K’ and K
for dilute solutions
X=x and Y=y approx.
therefore
K’= K approx.
what are the basis for the mccabe thiele graphical analysis for absorption/stripping
countercurrent flow of gas and liquid
carrier liquid/ solvent molar flowrate is L’
carrier/ stripping gas molar flow rate is V’
transfer of solute only between plates
hence L’ and V’ are constant throughout the cascade and we have constant molar overflow
mole ratios X and Y are the composition variables
linear operating lines follow mccabe thiele graphical analysis
discuss the operating line for the absorber
- top and bottom gas stream and inlet solvent compositions are specified as design variables
- to determine the operating line for the process, a solute balance is carried out down to an arbitrary stage n
V’. Yn+1 +L’.X0 = V’.Y1 +L’,Xn
-thus thus the equation of the operating line is
Y=[Y1-(L’/V’)X0] +(L’/V’)X
this equation relates the compositions of the passing streams such as Yn+1 and Xn
the slope is L’/V’ c.f. L/V
as the process is absorption, the operating line lies above the equilibrium urve
where does the operating line lie with respect to the equilibrium curve for an absorber
as the process is absorption, the operating line lies above the equilibrium curve
what is the equation of the operating line
Y=[Y1-(L’/V’)X0] +(L’/V’)X
what is the slope of the operating line for absorption
L’/V’