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Petrochemical engineering: Ammonia Absorption

1

Summary
The aim of this experiment was to understand the effect of flowrate on mass transfer
coefficient
...

The experiment was conducted using an ammonia absorption system
...
Gas absorption systems are widely used in industrial applications
...
The apparatus consisted of an ammonia absorption
packed column and absorption train used to measure the ammonia concentration in the air
...
Raschig rings are ideal as they are not packed tightly and so leave space for
the air to pass through, and also have a large surface area where the liquid gets trapped
...
In order to determine the effect of flowrate on mass transfer coefficient, water
containing absorbed ammonia and sulphuric acid containing absorbed ammonia had to be
collected, observed, and then titrated and back-titrated
...

Key results:
: At an airflow rate of 14 L/min mass transfer coefficient = 0
...
427 mol/sbar
: At an airflow rate of 10 L/min mass transfer coefficient = 0
...
270 mol/sbar
: Having plotted the log-log graph of mass transfer coefficient against the flow rate of air,
the gradient (n) was found to be 0
...
So as the airflow rate increases so does the mass transfer coefficient
...
There is a slight anomaly at the airflow rate of
8 L/min, this van be put down to human errors associated with carrying out the experiment
...
The value gradient was 0
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83
...

There are a number of key skills that I have learnt over the course of this experiment and
report
...
I have become familiar with the fundamental and detailed operation of
an ammonia absorption column, and have the confidence to carry out this process again
...
If this experiment were to be repeated again I would experiment the effect that
water flowrate has on ammonia absorption as well as the airflow rate
...


2

Introduction
The first objective of the experiment was to determine the overall mass-transfer coefficient
(Kga), in a small packed column for the air/ammonia/water system at several different
airflow rates
...

For this particular experiment a laboratory ammonia absorption system consisting of an
ammonia absorption packed column was used
...
Gas absorption is an operation in which gas mixture is contacted with a liquid for
the purpose of preferentially dissolving one or more components of the gas mixture and to
provide a solution of them in the liquid 1
...
In this particular system
a counter-current system is used, the gas enters the column from below and leaves the top,
while the liquid enters from the top and flows in the opposite direction and exits the bottom
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The purpose of
the packing is to provide multiple surfaces where the vapour rising meets the liquid
cascading down the column
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They offer the best combination of low weight per unit volume, free
volume, free cross section and total surface 3
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A packed column is simple to use and
as long as the diameter is no too large, usually a packed column is relatively cheap to
construct and maintain
...
The disadvantages of
packed columns include the lifetime of the column is uncertain, and occasionally the column
will need to be repacked if channels form in the column 4
...
Gas absorption can be used fro removing
contaminants or impurities from a gas stream, for example the removal of hydrogen
sulphide from a fuel gas by liquid amine
...
It is also used in air pollution
control where the various oxides of nitrogen can be removed with the application of water,
sulphuric acid and other organic solutions
...
The purpose of the experiment
is to investigate the effect of flowrate on mass transfer coefficient
...

Where: Fw = Flow rate of water
: FA = Flow rate of the air
: yi = Concentration of ammonia in inlet air
: yo = Concentration of ammonia in outlet air
: y = Concentration of ammonia in air in equilibrium with solution of ammonia
: xi = Concentration of ammonia in inlet water
: xo = Concentration of ammonia water out

2

1

absorption

safety

3 way

valve

yo

xi

water

FW

Ammonia
absorption
column

Adjustable
U-trap

Ammonia bubbler
FA

xo
yi

drain

In order to achieve the aim of the experiment we carried out two objectives
...

: Volume of 0
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01 N of H2SO4 = B ml
: Volume of 0
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1 N solution contains 1 mol of NH3 per litre of solution
: Therefore C ml of 0
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001 x 0
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001 x 0
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001 x 0
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: 10g of absorption water was titrated against D ml of 001 N of H2SO4
: Moles of NH3 in 10g = D x 0
...
01 mol
: Therefore the concentration of NH3 in the water leaving the column
: xo = D x 10-5/(10 x 0
...

: N = FA * (yi – yo)
= FW * (xo – xi) = FW * xo ; since xi = 0
∴ FA * yi – FA * yo = FW * xo

∴ yi = (FW * xo + FA * yo)/ FA

In the air-ammonia-water system, the gas film controls the rate of mass transfer according
to the equation:
N = kg * a * (y – y*)
Where (y – y*) is the difference between the ammonia concentration in the bulk gas flow
and the ammonia concentration at the water/gas interface,
y* = is the equilibrium value for the ammonia concentration in the liquid phase, x
...
0012x
: Where: y = concentration of ammonia in air in moles per litre
x = concentration of ammonia in water in moles per litre
The average value of the rate of mass transfer is given by: N = kg * a * ∆(y)mean
In order to find the mass transfer coefficient we found the arithmetic mean difference
: ∆(y)mean = ½ * (∆ytop + ∆ybottom)
: ∆ytop = (yo – y*i) = yoi since y*I = 0 ; when xi = 0
: ∆ybottom = (yi – yo)
Using equation the experimental data obtained in the test, kga, the overall mass transfer
coefficient, can be determined
...
607 mol/sbar

5

A graph of log kga against log FA is plotted and the gradient is compared with the literature
value of 0
...


2
...
The apparatus consisted of an
ammonia absorption packed column and absorption train used to measure the ammonia
concentration in the air
...
In order to determine the effect of flowrate on mass
transfer coefficient, water containing absorbed ammonia and sulphuric acid containing
absorbed ammonia had to be collected, observed, and then titrated and back-titrated
...
The numbers labelled 1-14 on
figure 1 (shown below) represents the different instruments and parts that an ammonia
absorption packed column system unit is made up of
...


6

Figure 1: The ammonia absorption packed column used to conduct the experiment
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Bottle 1 (containing 5ml H2SO4)
8
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Bottle 2 (containing 25ml H2SO4) 9
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5cm diameter absorption column 10
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Raschig Rings
11
...
Water-flow manometer
12
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Air-flow manometer
13
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Three-way tap
14
...


7

1

2

7

3

10

4

5

6

9
8

Where:
1
...
01 N NaOH)
6
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Tank 2 (containing 0
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Suction tube
3
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Phenolthalein pH-indicator
4
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Measuring Cylinder
5
...
Round bottom flask
Description
An ammonia absorption system is made up of a 5cm diameter absorption column packed
with 1cm raschig rings to a height of approximately 12cm
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Bottle 1 contains 5ml of H2SO4 and 45ml of distilled water, bottle 2
contains 25ml of H2SO4 and 25ml of distilled water, and both bottles also contain a few
drops of pH indicator
...
A conical flask containing ammonia is present so that the
air can bubble through, and a safety bottle containing dilute sulphuric acid to remove any
unabsorbed ammonia
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When the airflow rate has been set, air bubbles through the conical flask containing the
concentrated solution of ammonia
...
From
the top of the column the air passes through to the absorption train, which is accessed
through the open three-way valve
...
During
the experiment while the test stabilises, the three-way valve is turned to direct the air
leaving the top of the column to the safety bottle containing diluted sulphuric acid
...
At the end of the experiment the drained water
containing absorbed water, and bottle 1 and 2 are collected
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The first burette is
used to back-titre the collected sample from bottle 1 and bottle 2 using 0
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The second burette is used to titre the collected sample of water containing
absorbed ammonia with 0
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Before titrating both samples a pH buffer is
added to the samples so that a colour change can be observed
...


3
...
This was because the provided stock solution of sulphuric acid is a
potential irritant and also highly flammable
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In this dummy run no water was passed down the
packed column and the air was set at a flowrate of 4 L/min
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Firstly the water flowrate was set at 1
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2
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01 N sulphuric acid was collected
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3
...
Approximately 6-7 drops of
phenolthalein was then added to each flask, before the bottles were placed in the
absorption train
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4
...


5
...

The clock was started the same time as the valve was turned
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The experiment was over as soon as the acid in the absorption bottle had been
exhausted
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7
...

Test 2:
1
...
0 N NaOH in order to calculate the excess H2SO4 used in the absorption
bottles
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A 10ml sample of water containing the absorbed NH3 is titrated with 0
...

Test 3:
1
...

The water flowrate was kept at 1 L/min for each of the tests
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Before the experiment
begins it can be observed that the packed column is made up of 1cm Raschig rings
...
The
absorption train is made of two bottles, the first containing 5ml of sulphuric acid and the
second containing 25ml
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The experiment is over when this indicator turns pink, this indicates
that an alkali is present within the solution
...
As the air passes through the conical flask
containing concentrated ammonia bubbling can be observed
...

The experiment was carried out at a water flowrate of 1 L/min and at various different air
flowrates
...
From the results gained it can be seen that as the airflow rate was
increased, the duration of the test decreased showing a negative correlation
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01 N sodium hydroxide
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From the results gained it could be observed that
generally as the airflow rate increased, the volume back titrated decreased
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01 N sulphuric acid
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Once the sample had been fully
titrated it resembled a pale pink colour showing the volume of water absorbed
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10

4
...
017
0
...
017
0
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233
0
...
167
0
...
01
0
...
01
0
...
0
20
...
1
21
...
7
1
...
9
1
...
930E-04
1
...
025E-05
1
...
790E-07
1
...
450E-07
2
...
700E-03
1
...
000E-04
1
...
070E-01
4
...
020E-01
2
...

x

y

3
...
280E-06
1
...
440E-06

1
...
580E-04
9
...
500E-04

Table 4: Data calculated showing the values of xi, xo, yo, yi, y*-y and the rate of mass
...

xi

xo

yo

yi

0
0
0
0

2
...
900E-03
9
...
200E-03

1
...
750E-07
2
...
500E-07

1
...
585E-04
9
...
503E-04

(y-y*)

1
...
580E-04
9
...
500E-04

Rate of mass
transfer, N (mol/s)

4
...
170E-05
1
...
000E-05

11

Figure 3: A graph to show the operating line against the equilibrium line for the ammonia in
both the inlet/outlet air and water at an airflow rate of 14 L/s
...


12

Figure 5: A graph to show the operating line against the equilibrium line for the ammonia in
both the inlet/outlet air and water at an airflow rate of 10 L/s
...


13

Figure 7: A graph to show the log of the flow of air against the log of the mass transfer
coefficient
...

Firstly the airflow rate had to be converted from L/min into L/s
An airflow rate of 14 L/min = 14/60 = 0
...

Volume of 0
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01 N of H2SO4 = 20ml
Volume of 0
...
2 N solution contains 1 mol of NH3 per litre of solution
Therefore 5ml of 0
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001 x 0
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17 x 10-8 mol/s
Therefore the concentration of NH3 in the air leaving the column
yo = 4
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233 = 1
...

10ml = 10g
10g of absorption water was titrated against 2
...
7 x 0
...
01 = 2
...
7 x 10-5/(10 x 0
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7 x 10-3 molNH3/L of water
In order to calculate yi, concentration of ammonia in inlet air the following equation is used:
yi = (FW x xo +FA x xo)/ FA
yi = (0
...
7 x 10-3 + 0
...
7 x 10-3)/ 0
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93 x 10-4 molNH3/L of air
In order to calculate the rate of mass transfer, N, the following equation was used:
N = FA x (y-y*) = 0
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93 x 10-4 = 4
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5 x (avg yo + avg yi)
y = 0
...
12 x 10-7+ 1
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41 x 10-5
In order to calculate the mass transfer coefficient, kga, the following equation was used:
kga = y x N = 7
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50 x 10-5 = 0
...
786,
which was compared with the literature value of 0
...


5
...
It consists of the
volume of back titration using 0
...
01 N sulphuric acid recorded at the various different airflow rates
...
From the results it can be
noticed that as the airflow rate decreases the duration of the test increases
...
However at the airflow rate of 10 L/min there is a slight anomaly because the
volume has decreased from the previous 12 L/min
...
As the airflow rate decreases it can be seen that the volume of the titration of the
water sample containing ammonia decreases, showing a positive correlation
...
This anomaly may have occurred due to the lack of
pH buffer added to the water sample
...
From the table of results we can see that the as the airflow rate decreases the
concentration of ammonia in the inlet air decreases
...
As the airflow rate decreases, the concentration of ammonia in
the air outlet increases
...
Therefore the packing
contained within the column absorbs less ammonia and more ammonia is present in the air
outlet
...
This is because at a higher airflow rate more ammonia is absorbed in the air and
brought into the packed column
...
From

15

the table we can see that most ammonia is absorbed at the air inlet when compared to he
air outlet and water outlet
...
There’s no ammonia absorbed in the water inlet
...
The operating line was obtained by joining the
points (xi, yo) and (xo, yi)
...
Separate graphs were plotted for each different
airflow rate
...
Figure 4
represents an absorption graph at the airflow rate of 12 L/min
...
Figure 3 represents an absorption graph at
the airflow rate of 8 L/min
...
As the concentration of ammonia in the air entering and leaving the air
increases the concentration of ammonia exiting the water also increases
...

Table 4 once again shows the concentration of ammonia in air and water
...
From the table we can see that as the airflow rate
decreases the rate of mass transfer decreases as well
...
Therefore once we had found the rate of
mass transfer we were then able to find the mass transfer coefficient
...
The calculated
mass transfer coefficients can be seen in table 2
...
This was consistent with the
theory, which suggested that as flowrate increased so did the mass transfer coefficient
...
The value obtained could then be compared with the
literature value of 0
...
Figure 7 consists of a graph to show log kga against log FA
...

From
the
graph
the
gradient
‘n’
was calculated to be 0
...
This value is very close to the literature value of 0
...
There are
a number of different reasons for the gradient value being slightly less than the literature
value
...
Other errors are errors associated with measurements
read of instruments, or human errors
...






At an airflow rate of 14 L/min mass transfer coefficient = 0
...
427 mol/sbar
At an airflow rate of 10 L/min mass transfer coefficient = 0
...
270 mol/sbar

16

From these results it can be educed that as the airflow rate increases the mass transfer
coefficient also increases
...

Having plotted the log-log graph of mass transfer coefficient against the flow rate of air, the
gradient (n) was found to be 0
...
When comparing this value to the literature value of
0
...

Any of the anomalies that occurred whilst the experiment was carried out or within the
results recorded can be due to human errors associated whilst undertaking the titration, or
due to the percentage errors associated with the instruments used to record the results
...

If this experiment were to be repeated again the number of airflow rates experimented with
would be increased and the increments between each of the airflow rates would be
decreased in order to gain more accurate readings
...
SM’s e-Home, Separation Processes 2008, Accessed on 11th April 2013, Available from
http://www
...
com/Absorption/GA_Chp03
...
John Stone, A professional guide for amateur distillers, The column Feb 2001, Accessed
on 12th April 2013, pp 35-38
 3 
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anadolu
...
tr/~lakyalci/sunular/Absorbtion_flue_gas
...
RPW Scott, Preparative Chromatography, Preparative-columns  2000  , Online book
Accessed
on
12th
April
2013,
http://www
...
org/topics/packed/column
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NH3
...
s-1)
(L
...
NH3
...
NH3
...
NH3
...
NH3
...
s-1

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