Absorption process design

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Absorption process design
Prof. Dr. Marco Mazzotti - Institut für
Verfahrenstechnik
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1. Variables
An absorption problem is usually presented as
follows. There is a polluted gas stream coming
out from a process. The pollutant must be
recovered in order to clean the gas or because
it's a valuable compound. In general, the
conditions of the gas stream are known, the
compositions can be measured, and the gas flow
rate can be set.
The temperature and the pressure at which the
process takes place must be chosen. The
thermodynamics of the process at different
conditions must be studied in order to decide the
most convenient. The solvent can be used pure or
can come from a recycle, and thus contain some
pollutant. The initial composition of the solvent
(xo) must be obtained by analytical methods, or
can be set by mixing pure and recycled solvent.
Process
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Once temperature, pressure and initial
composition of the solvent are set we can
consider them as data. This is a list of known
variables
Data and set values
Specifications
Unknowns
G
L
y1
x0
Temperature, T
Gas final composition, y1
Solvent flow rate, L
Pressure, p
Number of stages, n
Gas flow rate, G
Solvent final composition, xn
Gas initial composition, yn1
Solvent initial composition, x0
T
p
Equilibrium data, ym x
Y m x
n
xn
yn1
G
L
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2. Diagram
We have already seen in the counter-current
cascade of ideal stages that a graphical
representation can help to visualize and solve
the problem.
Considering the initial compositions of the gas
and solvent flows and the specification made for
the gas outlet composition, we can draw in the
diagram
y1
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Now, there are infinite number of possible
segments passing by the point (xo, y1) and ending
in a point with y coordinate yn1. We need to
choose one of those segments as operating line.
The operating line is the equation of all those
lines
The slope of the operating line depends on the
solvent flow-rate (L/G). When one of the lines is
chosen as operating line, then the slope is set
and thus, the required solvent flow-rate can be
calculated.
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3. Choosing an operating line
There is a special operating line among the
infinite number we can choose. This is the one
that touches the equilibrium line at the point
(yn1/m, yn1)
yn1
The mass exchange is maximum because the
equilibrium is reached, but the required number
of stages is infinite.
y m x
Lmin /G
y1
x
x0
The slope corresponding to this line is the
smallest possible slope, because the process
cannot go beyond he equilibrium line. The minimum
slope can also be written in terms of the
fractional absorption for a linear equilibrium
In the practice, the slope for the operating line
is taken as 1 to 2 times the minimum slope
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4. Graphical construction
Once the operating line is set, we can proceed
with the construction seen in the counter-current
stage configuration. This gives us the number of
stages, n.
yn1
y4
L /G
y3
y2
y1
xo
x1
x2
x3
x4
We already know all the initial unknowns L, n
and xn.
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5. Kremser equation
The next two concepts are very important. You can
review them in 4.3 Ideal equilibrium stage
1. Absorption factor
2. Fraction of absorption
As we saw before, in the case of a
counter-current configuration, the fraction of
absorption can also be written in terms of the
absorption factor, A. This gives us the Kremser
equation for the case in which A is not equal to
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Therefore, the number of stages required, n, can
be calculated using the two forms the fraction
of absorption definition and the Kremser
equation. The number of stages obtained by this
method should be equal to the one obtained by the
graphical construction.
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Have a look to the design Flowsheet