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’
how is the operating line equation for an absorber obtained
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
what are the given values when doing a design problemm and what do you have to solve for
gas flow V’, inlet gas composition YN+1, desired outlet gas composition Y1, inlet liquid composition X0
use these to solve for minimum solvent flow rate L’
how do you find the minimum liquid flow rate for absorption?
there is a minimum liquid flow rate corresponding to a pinch point at the bottom of the column
how does the pinch point occur?
the pinch point occurs when L’ is reuced to the point which the operating line intersects the equilibrium curve at Y=YN+1
can the pinch point be found graphically
yes
what is a typical liquid flowrate?
L’ (1.1 to 2) xL’ min
what is the equation for minimum solvent flow rate for a dilute system
Y=K’X
Y=Y1+(L’/V’)(X-X0)
hence L’min can be found by equating X from the operating line and equilibrium line equations as Y=YN+1 leading to
L’min=V’(YN+1-Y1)[(YN+1/K’)-X0]^-1
if X0 =0 this becomes
‘min=V’K’(1-Y1/YN+1)
what are the specified design variables in a stripper
top and bottom liquid stream and inlet gas compositions
X1, XN+1, Y0, YN
what is the operating line for a stripper and how is it obtained?
V’Y0 +L’. XN+1= V’.YN +L’ X1
Y=[Y0-(L’/V’)X1]+(L’/V’)X
what does the operating line for a stripper relate
compositions of passing streams like Y1 and Xn+1
where does the operating line for a stripper lie in respect to the equilibrium curve
as the process is stripping the operating line lies below the equilibrium curve
what are the given values in the design problem of a stripper and what do they solve
given values;
liquid flow L’, inlet gas composition Y0, inlet liquid composition XN+1, desired outlet liquid composition X1
solving for minimum gas flowrate V’
what does the minimum gas flow rate correspond to?
the minimum gas flowrate corresponds to a pinch point at the top of the column
when does a pinch point occur in stripping
the pinch point occurs when V’ is reduced to the point at which the operating line intersects the equilibrium curve
can the pinch point be found graphically for strippingyw
yes, the pinch point is at the top of the graph where X=XN+1
what is a typical actual gas flow rate
a typical actual gas flow rate is (2.2 to 2) x V’min
what is the minimum gas flow rate for a dilute system?
Y=K’X
Y=Y0+(L’/V’)(X-X1)
hence V’min can be found analytically by equating Y from the operating lineand the equilibrium equations atX=XN+1 leading to:
V’min=l’(XN+1-X1)[K’XN+1-Y0}^-1
if Y0=0 this becomes:
V’min =(L’/K’)[1-X1/XN+1]
when do analytical solutions exist
if in X,Y coordinates the equilibrium relationship is linear
and
if the solvent or stripping gas entering the process is solute free then an analytical solution exists
this requiresK’ to be constant over the operating range of X and in sufficiently dilute systems this is the case
how dilute do dilute systems need to be be/
how dilute is ‘dilute’ entirely depends upon the system
the key criterion is the linearity of the equilibrium relationship
what is the analytical solution called
the analytical solution is called the kremser method
what is the absorption coefficient
A=L’/K’V’
what is the stripping coefficient
S=K’V’/L’
what is the fraction of solute absorbed or stripped f?
f=1-Y1/YN+1 absorber
f=1-X1/XN+1 stripper
what is the analytical solution for f given N and theta (theta = A or S)
f=theta(N+1)-theta/ theta (N+1) -1
what is the inverse relation for N given f and theta?
N=ln[(theta-f)/(1-f)]/ln(theta) -1
what does the inverse relation tell you about theta and f
because ln(theta -f0….
theta must be equal to or greater than f because ln(0) DNE
what is the limit of feasibility?
thetamin=f is the limit of feasibility
N tends to infinity as theta tends to f
what may cause a pinch point to occur/
if A or S is less than 1 then a pinch point may occur
what causes the limit of feasibility
if A and S are at the minimum possible values (Amin and Smin) we ahve the limit if feasibility where N tends to infinity
smaller values of A or S lead to an infeasible process
when is the operating line parallel to the equilibrium line?
if A or S is greater than 1 the operating line is parallel to the equilibrium line
when does the operating line slope away from the equilibrium curve?
if A or S is greater than 1 then the operating line slopes away from the equilibrium curve
what trend is noticed ona graph of number of theoretical tages N against the fraction not absorbed/stripped
for any A or S greater than 1, the fraction not absorbed or stripped decreases monotically with increasing N
how is the overall efficiency estimated for stripping/ absorption
empirical correlations of overall efficiency are available based on the liquid viscosity and other properties
a method of correlation was first developed by O’Connell Eo
the number of trays required in the column is N/Eo where N is the numbr of equilibrium stages
can murphree efficiency factors be used
as in distillation, , murphree vapour efficiency factors can be specified for each stage of a plate absorber or stripper
what is the differrence between the linear operating lines for stippers and absorbers
for an absorber, the operating line is above the equilibrium curve
for a stripper, the operating line is below the equilibrium curve
what two ways can the minimum flow rates of solvent or stripping gas be obtained
minimum flow rates of solvent or stripping gas obtained graphically or (for constatn K’) analytically
when are analytical solutions available?
analytical solution are available for dilute systems where the solvent/ stripping gas is solute free
