Mass Transfer Column

8/7/2019 Mass Transfer Column

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STUDY AND DESIGN OF PACKED TOWER FOR

ABSORBING GASES WITH LOW SOLUBILITIES

Heat and Mass Transfer Laboratory project by

Vishal Surana

Vivek Nagar

Vivek Nigam

under the guidance of

Dr. A. Kannan and Dr. R. Ramnarayan

Department of Chemical Engineering

Indian Institute of Technology Madras

Chennai - 600 036


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Contents

Topic Pages

1 Acknowledgements 1

2 Introduction 2

2 Brief outline of issues tackled in this project 4

3 Theory of gas absorption 5

4 Determination of HTU and NTU for various packing

Experimental Procedure

Precautions to be followed

Experimental Observations

Sample Calculations

Error Analysis

7

5 Design of packed tower using different packing

Introduction

Theoretical Predictions of model


16

6 Design of optimum packed

Introduction Matlab Code for finding out the optimal packing


18

7 Results and discussion 24

8 Suggestions for improvement 24

9 References 25


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Acknowledgements

First of all, we would like to thank Dr. A. Kannan and Dr. R. Ramnarayan for giving us

an opportunity to design our own experiment. And at the end of it, we have to accept the

fact that it was a very challenging experience and but for their constant guidance and

support the project couldnt have been completed. To have come up with not only an

experiment but to also have put forth a general strategy to design packed towers has

indeed been a very thrilling and satisfying experience. We would also like to that Dr.

Krishnamurthy and Mr. Meghanathan of the Chemical Engineering Workshop

Laboratory who were very patient and helpful throughout the course of this project.


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Introduction

Gas absorption is one of the major mass transfer unit operations used in the separation or

purification of gas mixtures. The operation is carried out by contacting the gas with a

liquid solvent, usually in a packed or plate column. The dissolved solute is frequently

recovered by thermal "desorption" or "stripping", and the regenerated solvent is recycled

to the absorption column.

Among major industrial uses are the absorption of SO3 in oleum in the production of

H2SO4, and of HCl and NO2 in water in hydrochloric and nitric acid manufacturing.

Another major application is the purification of various process streams to prevent

pollution, corrosion, catalyst poisoning or condensation in subsequent low temperature

treatment. Examples of these applications are the large scale removal of CO2 from air or

natural gas prior to liquefaction, absorption of sulphur compounds from natural gas and

CO2 removal from ammonia synthesis gas.

A packed bed is a hollow tube or pipe that is filled with a packing material. The packing

can be randomly filled small objects like Raschig rings or else it can be a specifically

designed structured packing.

The purpose of a packed bed is typically to improve contact between two phases in a

chemical or similar process. Packed beds can be used in a chemical reactor, distillation

process, or a scrubber, but packed beds have also been used to store heat in chemical

plants. In this case, hot gases are allowed to escape through a vessel that is packed with a

refractory material until the packing is hot. Air or other cool gas is then fed back to the

plant through the hot bed, thereby pre-heating the gas feed.

Distillation columns with packing are often called packed columns. Columns used in

certain types of chromatography consisting of a tube filled with packing material can also

be called packed columns and their structure has similarities to packed beds.


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A packed column is usually favorable, when

1. Only a small pressure drop is allowed in the column (for example in vacuum

columns).

2. The components are corrosive.

3. The diameter of the column is small (below 1 m).

4. The hold-up must be small (for example due to thermal decomposition).

5. The liquid foams.


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Brief outline of issues tackled in this project

1. Study the mass transfer from gas to liquid phase in a tower filled with random

packing.

2. Explore how changing the water flow rate affects absorption. Carbon dioxide gas

flow rate is fixed.

3. What are the HTU and NTU for this condition?

4. Compare the changes in gas absorption arising due to using different kind of

packing.

5. Study the performance of a tower having different packing of various heights.

6. Optimize the tower packing so as to reduce the cost of packing used.


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Theory of Gas Absorption

The simpler theoretical equations for gas absorbers serve three main purposes:

They are used to "design" the column, i.e., to find the height of a column necessary to

achieve a given separation.

They are used in adapting an existing column to a new process or to new operating

conditions. The problem might then be: what flow rates should be used to achieve a given

separation, knowing the mass transfer and equilibrium characteristics of the system?

Finally, they can be used to evaluate the mass transfer characteristics themselves from

measured concentrations and flow rates. The experiment described here falls in this

category.

In all the three cases, what is needed is an equation relating all the system variables, i.e.:

Concentration=f(height of column, flow rates, mass transfer rate, feed concentration)

Since CO2

is sparingly soluble in water, operation of the scrubbing column requires large

water to air flow rate (L/V) ratio. If the L/V ratio is too small, the flow rate of CO2

in the

effluent gas stream will be almost the same as in the incoming gas stream and the

operating line will appear to be almost horizontal. To achieve a large L/V ratio, operate at

low gas flow rates and CO2

concentrations. Under these conditions, the concentration of

CO2

in the effluent gas stream will be significantly smaller than in the incoming gas

stream.

Let

a is the interfacial area per unit volume (m2/m

3)

c* is the solubility of CO2 in water at 250C (mol/l)

c is the bulk concentration in the differential element (mol/ l)

dZ is the differential length of the column taken for consideration (in m)

S is the cross sectional area (m2)

Co is the concentration of CO2 in the output liquid stream (mol/ l)


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L is the liquid flow rate (m3/hr)

Ka is the liquid phase mass transfer coefficient (in m/hr)

L dC= S a dZ Ka (c - c*)

co

a cc

dc

aKS

LZ

0*

Z = HTU X NTU

NTU = co

ccdc

0*

and HTU =aKaS

L**

HTU values generally vary with gas and liquid flow rates, going through a minimum and

rising again as flooding conditions are approached. In order to reduce the tower cost, it is

advantageous to operate near this minimum, and one of the objectives of the present

experiment is to study the variation of HTU with L to establish the region of optimum

operation. In addition, the experiment provides an opportunity to test various mass

transfer concepts and design procedures taught in mass transfer courses.


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DETERMINATION OF HTU AND NTU FOR VARIOUS PACKINGS

Experimental Procedure

Start up

1. Switch on the CO2

heater and heat it for about an hour or so so as to prevent cold

carbon dioxide from damaging the pipes.

2. Prepare 0.05N NaOH solution in distilled water. For preparing a 0.05N solution,

weigh out NaOH (1g for 500 ml) in a beaker and add distilled water (500 ml) in it

and stir the beaker to get a uniform solution.

Building the Column

1. Disconnect all the pipes connecting the tower to the water and the gas supplies.

2. Unscrew the nuts and bolts using a spanner.

3. Empty the earlier packing, and then fix the tower back to its original place.

4. Fill the tower with the desired packing. Use a wire gauze to make the flow of

fluids more uniform.

5. Screw back the nuts and bolts using a spanner.

6. Connect the supply lines, and test the tower for any leakages. If there are no

leakages then we can go ahead with the next stage of the experiment.

7. Before starting the experiment, the water flow is turned on full to allow air

bubbles to escape, and to clean the column.

Mass transfer process

1. Open the appropriate valves to start CO2

gas flow to the system. Control the CO2

flow rate using the valve.

2. Similarly adjust the liquid flow meter.


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3. Meanwhile when the system reaches steady state take a sample of the output

liquid stream.

4. Titrate known volume of the sample against 0.05N NaOH solution using

phenolphthalein as indicator.

5. Repeat the procedure for different liquid flow rates.

Shut down

1. Turn off water flow to the system by closing the water valve

2. Close the CO2

gas cylinder. Check that the pressure gauges on the cylinder read

zero.

3. Close all valves on the CO2

inlet line, starting from the gas cylinder and working

up to the flow meter.

4. Unplug the CO2

gas heater.

Precautions to be followed

1. All the apparatus should be rinsed with distilled water and dried before every

titration.

2. System should be allowed to reach steady state for every reading.

3. Liquid flow rates tend to fluctuate quite a lot, and so care must be taken to ensure

that it is reasonably constant.

4. During titration, the indicator must be added in minimal quantities.

5. Do not leave pressure in lines after shut down.


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Gas Rate=2.5m3/hr

Height Z=0.9m

C*=0.0329

Data for rashcig rings

Water Flow

Rate(LPH)

Initial

Volume(ml)

Final

Volume(ml)

Volume of base

required(ml)

Concentration

of CO2 (M) NTU HTU(m)

30 0 3.2 3.20.0020 0.0566 15.8958

50 8 13 50.0031 0.0937 9.6045

70 13 16.2 3.20.0020 0.0566 15.8958

Variation of NTU with liquid flow rate

0.0000

0.0200

0.0400

0.0600

0.0800

0.1000

0 20 40 60 80

Liquid flow rate(l/hr)

NTU(m)


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Data for spherical packing (7 mm dia)

Water Flow

Rate(LPH)

Initial

Volume(ml)

Final

Volume(ml)

Volume of base

required(ml)

Concentration


30 16.3 19.3 30.0019 0.0526 17.1162

50 19.3 24.8 5.50.0034 0.1043 8.6325

70 24.8 29.3 4.50.0028 0.0833 10.8088


0.0000

0.0500

0.1000

0.1500

0 20 40 60 80


NTU(m)

Data for spherical packing (4 mm dia)

Water Flow

Rate(LPH)

Initial

Volume(ml)

Final

Volume(ml)

Volume of base

required(ml)

Concentration

of CO2 (M) NTU HTU(m)30 29.3 33.5 4.2

0.0026 0.0771 11.680350 33.5 39.8 6.3

0.0039 0.1214 7.415170 39.8 44.6 4.8

0.0030 0.0895 10.0540


0.0000

0.0500

0.1000

0.1500

0 20 40 60 80


NTU(m)


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Data for plastic pall ring

Water Flow

Rate(LPH)

Initial

Volume(ml)

Final

Volume(ml)

Volume of base

required(ml)

Concentration


30 1 2.9 1.90.0024 0.0688 13.0759

50 3.4 4.2 0.80.0010 0.0248 36.3356

70 4.8 5.8 10.0013 0.0326 27.5761


0.00000.0200

0.0400

0.0600

0.0800

0 20 40 60 80


NTU(m)


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Sample Calculations

Solubility of CO2 in water at 250C (c*) = 0.0329 mol/l

The conc. of CO2 in inlet water flow rate = 0.0002 mol/l

The following chemical reaction is assumed to take place without any side reactions:

CO2 + 2 NaOH Na2CO3 + H2O

Mole/litre of CO2 = oBB Cmlsample

NV

)(2

VB = 1.9 ml

NB = 0.05 N

Sample volume = 20 ml

Concentration of CO2 = 0.0022

NTU =*

C

Co

dc

c c =*

ln*

oc c

c c

= 0.0631


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SAMPLE ERROR ANALYSIS

1 1 2 2

101 1

1 1 10

1 1 2

1 1 2

,

N V N V

dVdN dW

N W V

dN dV dV dCThus

C N V V

Where C is Concentration of CO2

100dC

C =

0.0001 10 0.1 1100

1 1000 3.2 20

=9.135

ln

1

ln

CNTUC C

dNTU dC

NTU CC C

C C

100dNTU

NTU =

0.004 9.135 1

0.03290.0329 0.004ln

0.0329 0.004

=9.7535


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DESIGN OF A PACKED TOWER WITH MIXED PACKINGS

The above set of experiments gives us an idea of the absorption characteristics of a

packed tower when the tower is filled with one kind of packing only. In the next stage,

we study the effect of packing the tower with different kinds of packing and observing

whether the process of gas absorption follows the same relationship as earlier or whether

it is different. Differences could arise due to change in flow characteristics, etc.

Discounting any such effects, the theory behind such an experiment would follow

analogously.

1HTU

k

* /

1

* /

2

* /

1

* /

2

*

1 1 *

1

*

12 2 *

2

*

1

*

ln( )

ln( )

ln( )

ln( )

ln( )nn n

n

C Cz

HTU C C

C Ckz

C C

Ck z

C C

C Ck z

C C

C Ck z

C C

So if different packing materials were to be stacked upon each other in the tower, then thenet effect of the entire tower will be described as

* **11

1 1 2 2 * * *

1 2

..... ln( ) ln( )...... ln( )o nn n

n

C C C C C Ck z k z k z

C C C C C C

* **

11

* * *

1 2

ln( . ....... )o n

n

C C C C C C

C C C C C C

*

*ln( )o

n

C C

C C


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Additionally, we have a constraint on the height of packing:

1 2 3.....

nz z z z Z

Summarily,

1

*

*1

ln ( )

0

n

i

i

no

i i

i n

i

z Z

C Ck z

C C

z Z

Thus, if we choose zi such that they add up to the height of tower, then the final outlet

concentration can be obtained from the second relation. The values of HTU for each of

the packing were obtained in the first part of the project and with the assumptions that wehave made, their values can be used here too. In effect, packing a tower with different

materials is equivalent to passing it through towers of height zi. In order to verify our

model, we performed an experiment with different packing materials and the results of

that experiment are described in the next section.


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Gas Rate=2.5m3/hr

Height Z=0.9m

C*=0.0329 M

Individual heights of each packing = 0.225m

Predicted final concentration is given by the relation:

* **

111 1 2 2 * * *

1 2

..... ln( ) ln( )...... ln( )o nn n

n

C C C C C Ck z k z k z

C C C C C C

*

*ln( )o

n

C C

C C

Thus, Cn=0.0021

Actual experimental data:

Water Flow

Rate(LPH)

Initial

Volume(ml)

Final

Volume(ml)

Volume of base

required(ml)

Concentration of

CO2 (M) NTU HTU(m)

7016.5 18.6 2.1 0.0026 0.0771 11.6803

The difference between experiment and actual values of outlet concentrations of carbon

dioxide turn out be 12%. This may seem too large an error, but one has to take into

account the fact that an error of 10% is possible due to limit on the accuracy of the

apparatus itself. It is very much possible that with better apparatus, the theoretical and

experimental results would match very well. From the experiment, it can be seen that

packing a tower with various materials is equivalent to, with reasonable accuracy, a group

of tower filled with a single kind of packing material. This finding can be used to design

towers with different kinds of packing materials so as to minimize the total cost of

packing. This is exactly what is done in the section that follows.


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DESIGN OF AN OPTIMUM PACKED TOWER

Before we discuss the details of the experiment, it is necessary to understand the

interpretation of optimum for there are several ways in which a packed tower could be

optimized. In our case, we shall propose a strategy that can be used to absorb some

desired amount of gas while minimizing the total cost of the packing used. This is

because different packing materials have different costs and contacting properties and so

we can neither go in for the material which provides the best contacting, since the cost o f

the material may be prohibitive, nor the cheapest, since that may require a very long

tower, but settle for a compromise between the two. The basic theory comes from the

theory behind the design of packed tower using different packing materials. Let Ci denote

the cost per unit height of packing. By unit height we mean unit height of the same tower

that we worked on. This can be found out by filling the tower and measuring the mass of

packing taken and the height of the packing. Using this value of Ci, we will have an

additional equation. So now the problem is described as follows:

1HTU

k

1

*

*1

ln ( )

0

n

i

i

no

i i

i n

i

z Z

C Ck z

C C

z Z

The total cost of packing materials, C is given by:

1 1 2 2 3 3.....

n nC z C z C z C z C


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We have to find out that combination of heights of packing so that C, the total cost of

packing is minimized. As it can be seen, we have several equations and inequations, all of

which are linear. That is to say that the above set of equations form a linear program and

so we can use tools like Simplex to solve for the optimum cost.

We solved the above linear program using Matlab 7 and found out the optimum tower

heights. Then the tower was setup based on the output of this program and the absorption

experiment was carried out using in order to verify the model put forth by us.


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MATLAB CODE FOR FINDING OUT THE OPTIMAL PACKING

Z=0.9; %Total height of tower

cstar=0.0329 %Solubility

cf=0.0024 %Desired final concentration

ntu=log(cstar/(cstar-cf))

%Inverse of htu for a particular experiment

ihturaschig=0.1296

ihtuysphere=0.1875

ihtuwsphere=0.2013

ihtuclock=0.0387

%kg per metre of individual packing

draschig=1.55

dysphere=1.98

dwsphere=2.39

dclock=0.67

%There is no inequations

A=[]

b=[]

%Constraints due to effect of packing and total height of packing

Aeq = [ihturaschig ihtuysphere ihtuwsphere ihtuclock

1 1 1 1]

Beq = [ ntu

Z]

lb=zeros(4,1); %Lower bound on the individual heights of packing

ub=[Z; Z; Z; Z] %Upper bound on the individual heights of packing


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%cost per kg of individual packing

for craschig=1:100

for cysphere=1:100

for cwsphere=1:100

for cclock=1:100

%Objective function

f =[craschig*draschig cysphere*dysphere cwsphere*dwsphere cclock*dclock]

%The linear program

[x, fval, exitflag, output, lambda]=linprog(f, A, b, Aeq, beq, lb, ub)

end

end

end

end


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The final concentration was set to 0.0024 and the program was executed. It turns out that

the cost is minimized when 7mm dia spheres of height 0.76m and plastic pall ring of

height 0.14m.

Gas Rate=2.5m3/hr

Height Z=0.9m

C*=0.0329 M

Water Flow

Rate(LPH)

Initial

Volume(ml)

Final

Volume(ml)

Volume of base

required(ml)

Concentration


70 37.5 39.6 2.1 0.0026 0.0771 11.6803

The difference between experiment and actual values of outlet concentrations of

carbon dioxide turn out be 12%. This may seem too large an error, but one has to take

into account the fact that an error of 10% is possible due to limit on the accuracy of the

apparatus itself. It is very much possible that with better apparatus, the theoretical and

experimental results would match very well. From the experiment, it can be seen that

packing a tower with various materials is equivalent to, with reasonable accuracy, a group

of tower filled with a single kind of packing material. This finding can be used to design

towers with different kinds of packing materials so as to minimize the total cost of

packing. This is exactly what is done in the section that follows.


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Results and Discussions

1. The gas absorption process in a packed tower was studied for various packing

materials at constant gas flow rate but varying liquid flow rates were studied.

2. The NTU vs. liquid flow rate curve goes through a minimum and then rises for all

but one packing for which the trend is the opposite.

3. The tower was filled with packing of different kinds and the process of gas

absorption studied. It turn out that filling a tower with different packing is

equivalent to carrying the gas absorption in different towers but consisting of

single packing only.

4. Based on the results obtained, we proposed a method to fill the tower so as to

minimize the total cost of materials used. This is a simple linear program for

which numerous techniques exist and can readily be solved using tools such as

Matlab.

5. One must remember that these results were observed for carbon dioxide gas,

which is sparingly soluble in water. Had a more soluble gas been taken, then we

may not have obtained results that match with predictions so well.

Suggestions for Improvement

1. Keep all the flow rates constant. Explore how water temperature affects

absorption.

2. Using gases other than carbon dioxide, perhaps even a mixture of gases.

3. The effect of increasing Z on HTU, NTU, and outlet solute concentrations

4. Relationship between the height of the packed column and the rate of absorption

5. The characteristics of flooding point can be studied using this setup.


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References

1. Treybal R.E. (1981). Mass Transfer Operations (3rd

Edition). McGraw-Hill, New

York.

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