Absorption

1. Introduction
The most common use of the mass transfer coefficients developed in Chapter 8 from
this book is the analytical description of large-scale separation processes like gas
absorption and liquid-liquid extraction. These mass transfer coefficients describe the
absorption of a solute vapor like SOz or NH3 from air into water. They describe the
extraction of waxes from lubricating oils, the leaching of copper from low-grade ores,
and the efficiency of the distillation of xylene isomers. Mass transfer coefficients are
useful because they describe how fast these separations occur. They thus represent a
step beyond thermodynamics, which establishes the maximum separations that are
possible. They are a step short of analyses using diffusion coefficients, which have a
more exact fundamental basis. Mass transfer coefficients are accurate enough to cone
late experimental results from industrial separation equipment, and they provide the
basis for designing new equipment. All industrial processes are affected by mass
transfer coefficients but to different degrees. Gas absorption, the focus of this
chapter, is an example of what is called "differential contacting" and depends directly
on mass transfer coefficients. Distillation, the focus of the next chapter, is an example
of what is called "stage-wise contacting" and is less influenced by mass transfer.
Extraction and adsorption, the subjects of Chapters 11-12, may be effected in either
differential or staged contactors. Understanding absorption is the key to all thus
operations. This understanding is usually clouded by presenting the ideas completely
in algebraic terms. All chemistry and all simple limits are implied rather than
explained. As a result, novices often understand every step of the analysis but have a
poor perspective of the overall problem. To avoid this dislocation, we begin, in Section
9.1, with a description of the gases to be absorbed and the liquid solvents that absorb
them. A few of these liquids depend only on the solubility of the gas; many more
liquids react chemically with the components of the gas. Once this chemical problem
is stated, we turn, in Section 9.2, to the physical equipment used. This physical
equipment is simple, but it is constrained by the fluid mechanics of the gas and liquid
flowing past each other. These flows are complicated, described largely by empirical
correlations. The best strategy may be to follow the turnkey procedure to solve the
fluid mechanics. In Section 9.3 we turn to the simple case of dilute absorption and in
Section 9.4 to the parallel case of concentrated absorption. For the dilute solution
case, we assume a linear isotherm, that is, that a solute's solubility in liquid is directly
proportional to its partial pressure in gas. This leads to a simple analytical solution.
For the concentrated case, we do not assume the isotherm is linear and we recognize
that the liquid and gas flows change within the equipment. Now the solution requires
numerical integration..(1 )

In addition to the distillation, there are other unit operations used for separating
substances Absorption is the unit operation in which one or more components of a gas
stream are removed from the gas mixture by being absorbed onto a non-volatile liquid
(called a “solvent”) In this case, the solvent is the separating agent 2 Stripping is the
operation that is in the opposite direction to the absorption, in which one or more
gaseous components in a liquid stream is removed from the gas-liquid solution by
being vaporized into an insoluble gas stream In the stripping operation, the insoluble
gas stream is the separating agent.
Absorption can be:-
1. physical, when the solute is dissolved into the solvent because it has higher
solubility in the solvent than other gases
2. chemical, when the solute reacts with the solvent, and the resulting product still
remains in the solvent Normally, a reversible reaction between the solute and the
solvent is preferred, in order for the solvent to be regenerated
Note:- Similar to the distillation, both absorption and stripping are operated as
equilibrium stage operations, in which liquid and vapor are in contact with each other
(3 )
.

Fig (1): chemical absorption.(10)

 2. Process Operation
Changing the conditions of the absorption column can influence the
effectiveness and efficiency of absorption. Some important controlled variables
are as follows:

1. Pressure of the column.

2. Temperature of entering liquid and gas streams.

3. Humidity of the gas stream.
4. Ratio of the liquid and gas stream rates.
Raising the total pressure of the column may increase the efficiency of the separation
because increasing the pressure decreases the liquid flow rate and increases the
concentration of the gas. The temperature of entering liquid affects absorption in that
it affects the flow rate of liquid required for the separation with a given number of
stages.
Inlet gases of the absorber with high humidity at a high temperature affect the
capability of the gas to consume latent heat hindering the absorption process.
Therefore, dehumidification of the inlet gas should be considered for absorbers with
large heat effects. The ratio of the liquid to gas stream rates has effect in that if the
ratio is too low, the solute builds in the upper portion of the column 2 causing a higher
temperature profile in the top of the column. As a result, internal cooling maybe
necessary for lower liquid to gas ratios.
Gas liquid absorption is a heterogeneous process, which involves the transfer of a
soluble component of a gas phase into a relatively non-volatile liquid absorbent..(11)
(2)
.(4)
 3. Principles of Absorption
In the packed absorption tower (which is a differential-contact equipment), there are no
sudden discrete changes in composition as in a stage-contact equipment (i.e.
distillation). Instead the variation in composition are continuous from one end of the
equipment to the other. Material balances for the portion of the column above on
arbitrary section as shown below by the dashed line are as follows:

Figure (3)
Manipulating equation (2) yields the operating-line equation for a differential-contact
equipment (this equation is analogous the equation for the distillation column/stage-
contact equipment).

The transfer of a soluble component from a gas mixture would cause a decrease in the
total gas rate V and an increase in the flow of liquid L. These changes make the
operating line slightly curved as shown below (Figure.4). But for dilute mixtures,
containing less than 10% of soluble gas, the effect of changes in the total flow is usually
ignored and the design is based on the average flow rates.
The average slope of the operating line is L/V , the ratio of the molar flows of the liquid
and gas. A reduction in L decreases the slope. Thus the minimum L/V, (L/V)min is the
slop when the bottom compositions are ( x b’, y b) which lie on the equilibrium line as
shown in the figure below (Fig.4). In any actual tower the liquid rate must be greater than
this minimum to achieve the specified change in gas composition. The L/V ratio is
important in the economics of absorption in a counter-current column. The driving force
for mass transfer is y - y', which is proportional to the vertical distance between the
operating line and the equilibrium line on Fig. 4 .For more information see the attached
reading material. .(12)

Figure (4)

4. OBJECTIVES
This research aims:
 1.To give details about absorption process and clarify the differences between
absorption and adsorption process.
2.To explain the process of gas absorption and the importance terms involved
in it.
3.To enumerate some equipment used to make the process and understand how
it works.
4.To show the importance of the operation and its application in everyday life
.(5 )

5. PROBLEMS INVOLVING GAS ABSORPTION:

1. Calculation of tower height
2. Calculation of number of equilibrium stages
3. Calculation of efficiency
4. Calculation of vapor-liquid handling capacity of the equipment involved.(2 )

6. Which Gases Are Absorbed

most gas absorption aims at separation of acidic impurities from mixed gas streams'
These acidic impurities include carbon dioxide (COz), hydrogen sulfide (H2S), sulfur
dioxide (SOz), and organic sulfur compounds. The most important of these are CO2
and H2S, which occur at concentrations of five to fifty percent. The organic sulfur
compounds include carbonyl sulfide (COS) and merceptans, which are like alcohols
with a sulfur atom in place of the oxygen. Merceptans stink: For example, butyl
merceptan is responsible for the stench of skunks.
Other impurities vary widely. One common impurity is water, which can be removed
by either absorption or adsorption. Another is ammonia (NH:), which is basic, rather
than acidic. Sulfur trioxide (SOr), prussic acid (HCN), and nitrogen oxides (NO*) are of
concem because of their high chemical reactivity. Oxygen must be removed from some
reagent streams, and nitrogen can be absorbed to upgrade natural gas.
The occurence of these streams and the targets for their removal are summarized in
Table.1. The ubiquitous presence of H2S reflects the fact that fossil fuels' especially
coal and petroleum, contain large amounts of sulfur. Moreover, as the world becomes
more industrialized, the targets will decrease. This is particularly true for SO2 in flue
gas, which is the source of acid rain..(1 )

1
7. Rate of absorption
The rate of absorption can be expressed in four different ways, either using individual
coefficients or overall coefficients based on the gas or liquid phases. Volumetric-based
calculations are generally used in order to determine the total absorber volume. For
this project, the following rate of absorption per unit volume was used.(7 )

8. DESIGN OF GAS-ABSORPTION SYSTEMS

The use of physical absorption processes is preferred whenever
1. Feed gases are present in large amounts at high pressure and the amount of the
component to be absorbed is relatively large.
2. He purified gas (the fraction of gas that is not absorbed) must be of high purity.
3. One or more selectively absorbed components are to be obtained in enriched or
pure form, possibly still at elevated pressure.
4. The absorbent is also the product of a subsequent synthesis, e.g., methanol
(Gas Production).
On the other hand, absorption processes with simultaneous chemical reaction
(chemisorption) are always preferred when
1. The components to be separated from feed gases are present in small
concentrations and at low partial pressures.
2. The purity requirements of the purified gas are not too high.
3. The components to be separated from feed gases are strongly acidic and
undergo a thermally reversible reaction with the chemical component of the
absorbent.
4. Low-cost waste heat is available for thermally regenerating the absorbent.
Absorption equipment for both types of absorption generally consists of a column with
internals for heat and material exchange (plate or packing) in which the feed gas is
brought into countercurrent contact with the regenerated absorbent. The purified gas
is taken off the head of this column. The absorbent loaded with the removed
component, the absorbate, leaves the bottom of the column and is regenerated by
desorption. The basic scheme thus consists of an absorption and a desorption stage
with the absorbent cycling between them as it takes up and again gives off the
absorbate (Fig. 5).(6 )

Figure (5)

8.1 SELECTION OF COLUMN:

1. Components are corrosive.
2. The liquid holdup is lower in packed columns.
3. Pressure drop is lower in packing as compared to
plates.
4. Low cost of packing compared to plates..(8 )
8.2 DESIGNING STEPS FOR ABSORPTION COLUMN
5. Determining the approximate dia of the column.
6. Selection of column.
7. Selection of packing and material .
8. Calculating the size of packing.
9. Calculating the actual dia of column .
10. Calculating the flooding velocity
11. Finding loading velocity with the knowledge
the flooding velocity.
a) Calculating actual dia of column.
1. Finding the no. of transfer units (NoG) .
2. Determining the height of packing –
3. Determining the height of the column.
4. Determining the pressure drop.

By equation in.water/ft of packing

(9)
.

8.3 Control Strategies of Absorption Column

Steady state process models have long been used to assist the control engineer
in designing control strategies for absorption columns. However, with the large
number of industrial columns still operating in manual or with ineffectual
controls. The control of absorption column has received a great interest
because absorbing operations are found in almost every phase of the chemical
industry.
They are also frequently used in the final step of purification of a product;
thus, the accurate control of product composition and/or column performance
may be very important.
A control for the amount of a gaseous fluid absorbed in a liquid flowing through
an absorption column comprises a gage in an outlet from the absorption column
for the liquid having the gaseous fluid absorbed therein, an auxiliary absorption
column of a substantially smaller capacity than that of the absorption column
and receiving a comparatively small amount of the gaseous fluid in addition to
the liquid is used, another gage in an outlet from the auxiliary absorption
column for the liquid having the gaseous fluid absorbed there, the gages
measuring respective parameters which are a function of the amount of the
gaseous fluid absorbed in the liquid in the respective outlets and producing
output signals corresponding to the respective parameters, a comparator is
connected to the gages for receiving the output signals and comparing them to
produce a control signal, and a flow control in the inlet for the gaseous fluid or
the liquid of the absorption column, the flow is connected to the comparator
and controlled by the control signal.
Ebru K. and Soner G., in 2003 (28) describe the technical specifications of the
"Computer Controlled Gas Absorption Column" to be procured for use in mass
transfers in liquid-gas systems and they showed that control panel should
provide the following:
a. There shall be a control panel between the gas absorption column
and the computer, and control of process parameters shall also be
possible over the control panel.
b. Indicators on the control panel shall be indicated schematically in
conformity with the actual system
c. All sensors and the related signal powers shall be suitable for
computer output.
d. The sensor connectors shall be of different form in order to
prevent any connection errors (number of pins, shape, color etc.)
e. Calibration of all sensors in the process shall be possible.
f. The control panel shall have an enclosure for protection against
any external interference and shall use filtered signals.
g. Real time on/off buttons shall be provided for pumps,
compressors, heaters, control valves etc.
h. Real time parameter controls shall also be realized by PID control.
There shall be proportional control, integral control (general,
primary) and derivative controls (secondary) based on real time
PID mathematic formula. Said controls shall be possible by
changing proportional, integral and derivative constants.

Najim K., in 2007 (29) describes the model and solution of the constrained
optimal control problem associated with a packed absorption column. The
control problem is solved using a learning automaton operating in a random
environment. On the basis of physical and chemical laws, a model has been
developed. It consists of three hyperbolic partial non-linear differential
equations. A solution of diethanolamine (the absorbent) is used to absorb the
CO2 contained in a gas mixture. The primary manipulated variables are the
flow rate of the absorbent and the concentration of CO2 in the gas mixture.
The control objective is to maintain the concentration of CO2 close to a
desired value, subject to control limit restriction, in order to avoid the flooding
of the column. It leads to a stochastic programming problem, the solution of
which is closely associated with the behavior of an automaton in a random
environment corresponding to the column. Detailed computer simulation
results which demonstrate the performance of this automaton controller are
presented.(11)
1. http://calliope.dem.uniud.it/CLASS/IMP-CHIM/C9-Cussler.pdf
2. https://www.scribd.com/document/325471625/GAS-ABSORPTION-pdf
3. http://prodpran.che.engr.tu.ac.th/AE335/08%20Absorption%20and
%20Stripping.pdf
4. https://www.cpp.edu/~tknguyen/che313/pdf/chap5-1.pdf
5. https://www.coursehero.com/file/17995356/GAS-ABSORPTION-REVISED/
6. https://mycourses.aalto.fi/pluginfile.php/160669/mod_folder/content/0/Absorptio
n%20design%20Ullmann.pdf?forcedownload=1
7. https://web.wpi.edu/Pubs/E-project/Available/E-project-042408-
133605/unrestricted/Modeling_Absorption.pdf
8. https://www.academia.edu/4216699/8_Absorber_Design
9. https://www.academia.edu/22699967/DESIGN_OF_ABSORBER
10. https://www.google.com/url?sa=i&url=https%3A%2F%2Flink.springer.com
%2F10.1007%252F978-1-4419-7991-
9_38&psig=AOvVaw3ESyRkCKNBC1UhWzR5gTRa&ust=1588360703218000&sour
ce=images&cd=vfe&ved=0CA0QjhxqFwoTCJDV0vDukOkCFQAAAAAdAAAAABAO
11. http://nahrainuniv.edu.iq/sites/default/files/thesis_22.pdf
12. https://classes.engineering.wustl.edu/2009/fall/che473/handouts/PackedBed.pdf