Mana Source

7/21/2019 Mana Source

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1

Abstract



2

Contents Page

Introduction 3

Theoretical Background 4

Experimental Procedure 9

Results and Analysis 16

Discussion 22

Error Analysis 24

Suggestions for Improvement 25

Experimental Safety Analysis 26

Conclusion 27

References 28

Appendix A – Raw Data 29



3

Introduction

1 page (jia Ming)



4

2.Theoretical Background

2.1 Packed Bed

The packed bed essentially consists of a vertical cylinder that is packed with packing

material. In most cases, liquids flow downwards from the top of the bed due to gravitational

forces while the vapors/gases travel upwards through contacting with the wetted packing

which is present to increase the surface area for reactions to take place. Optimum recovery

of the solute is achieved when the liquids present are non-volatile and the gases do not

react with the liquids present.

Packed towers are mostly utilized in stripping/ gas-liquid absorption processes as they are

relatively cheaper as compared to other processes such as and because it is very feasible to

change the geometrical parameters of the bed such as the height if the bed, the type of

packing present and such. However, the drawbacks of such processes are that although

efficient stripping occurs at low flow rates, flooding can also occur at higher flow rates and

this may hinder the performance of the bed. In addition, it is relatively difficult to allow

efficient heat exchange with the surroundings; it would not be possible to use this process if

the temperature of either the gas or liquid is too high. The dissolving capacity of the gas in

the liquid also decreases with increasing temperature.

In this particular experiment, the packing used is the Raschig rings that are small cylindrical

structures and hollow. They are inert, affordable and most importantly increase the surface

area for chemical reactions and absorption to take place. The liquid and the gas are

introduced in the counter directions to maintain the concentration gradient between both

streams to increase the driving force of mass transfer.

The stripping process in the packed beds can be improved by allowing a chemical reaction to

occur concurrently to reduce mass transfer resistance and this enhancement of the process

is often referred to as chemisorption.

For instance, in this experiment, the following reaction takes place to improve the stripping

of CO2 gas:

CO2 + 2NaOH Na2CO3 + H2O

Therefore, there are two processes occurring concurrently. The carbon dioxide from the gas

steam is reacting with sodium hydroxide as well as being absorbed by diffusion into the

liquid stream. The latter process is taken to be instantaneous.

2.2 Different Zones in a packed column

In every packed bed in which counter current absorption is taking place together with a

chemical reaction, three zones of absorption can be identified. They are the surface reaction

zone, the interior reaction zone and the physical absorption zone.



5

Where

Y: mole ratio of solute in gaseous phase

X: mole fraction of solute in liquid phase

H: height of each zone

A: Carbon Dioxide Gas

B: Sodium Hydroxide

G: Gas phase

L: Liquid phase

2.2.1 Surface Reaction Zone

This zone is present in the uppermost section identified in the column where the

concentration of liquid sodium hydroxide is the highest as compared to the concentration ofcarbon dioxide gas that is the lowest. Thus, the reaction takes place at the gas-liquid

interface and as mentioned before, is taken to be instantaneous. An assumption is made

here that sodium hydroxide is non-volatile and that increasing its concentration does not

affect the rate of diffusion significantly. The rate of the reaction is governed by the mass

transfer rate of carbon dioxide from the air because the mass transfer resistance is usually

the highest in any gas phase.

Surface Reaction Zone

Interior Reaction Zone

Physical Absorption Zone

H3

H2

H1

L, XB1=0

LinGout

G, YA1

YA2

YA3 XB3

XB2=0

XB4 YA4



6

At the liquid-gas interfaces: PAi =YAi Pinert =0

Then, using the governing equation for plug flow reactor,

∫

or

And it is known that

So.........

∫

or ∫

Defining ,

Equations for surface reaction zone:

Ainert GA

A

T AGA

AGA

A

As Y P k

Y

P Y k P k

dt

dN

S

r

1

1

And

)(ln

)1(43

4

33

3

4

3

4

A A

A

A

T GA

Y

Y A

A A

T GA

Y

Y As

AY Y

Y

Y

aP k

G

Y

dY Y

aP k

G

r

dY

a

Gh

A

A

A

A

An assumption that the solution is dilute is made.

So if YA << 1, pA= PTYA and

=

4A

3A

gA

3Y

Yln

aPk

Gh

Legend:

F A: molar feed rate of A (mol/s)

Differing concentrations ofNaOH



7

xA: fractional conversion of A

Ax: cross sectional area of the reactor (m2)

h3 : height of surface reaction zone (m)

Y A : mole ratio of reactant A to inert in gas bulk

G : molar flow rate of inert gas (mol/(m2.s))

P A: : partial pressure of gas (N/ m2)

PT : total pressure of gas (N/ m2)

k GA : mass transfer coefficient of A in gas

with chemical reaction (mol/m3s.Pa)

a : interfacial area per volume packing (m-1

)

2.2.2 Interior Reaction Zone

The interior reaction zone can be found in the middle of the packed column. Since the

location of this section is lower than the section observed in the previous part, it can be

observed that the sodium hydroxide concentration decreases while the carbon dioxide

concentration increases. This change in concentrations is the reason as to why the reactionlocus shifts into the liquid film as shown above. The reaction locus will continue to shift

deeper into the liquid film until the concentration of sodium hydroxide reaches zero but will

not move into the liquid bulk. In this section, the mass transfer resistance is governed by

both the liquid and gas film resistances.

Using the Hatta equation, the overall rate equation for this section is,

AGA LAo

A

B B

A

A

A

H k k

b D

C D

H

P

r 11

Using the same basis for reaction rate (interfacial mass transfer area, S),

Reaction locusGas-Liquid interface



8

o

LA

A

GA

B

B

T

A

A

B

A

A

T

A

As

k

H

k

X

X C

b

H

D

D

Y

Y P

dt

dN

S r

1

111

Using material balance over differential area in the interior reaction zone and integrating,

3

2

2 ln

2

3

2

3 A

A

T GA

Y

Y

B

A

B

A

A

T GA

Y

Y As

A

Y

Y

aP k

G

X b D

m DY

dY

aP K

G

r

dY

a

Gh

A

A

A

A

Where L

mG

D

D

A

B1 and

3 A

A

B

Bc

A

BY

L

mG

D

D X

b

m

D

D

h2 = height of interior reaction zone [m]

H A = Henry’s law constant* [N.m/mol]

K GA = overall mass transfer coefficient (with chemical reaction)[mol/m

3s.Pa]

DA, DB = diffusivities of A and B respectively [m2/s]

C, CT = partial and total molar concentration in the liquid [mol/m3]

b = stoichiometric coefficient of NaOH in reaction equation

(*Note that this can only apply if solution is dilute such that Y A << 1; XB << 1, then PA = PTYA

and CB = CTXB)



9

Experimental Procedures

Apparatus

2. 1 X Vertical packed bed column (Internal Diameter (ID): 0.05m, Height (H): 0.490m) along

with the following:

a. Raschig Rings as packing material (ID: 0.006m, Outer Diameter (OD): 0.008m and

Length (L): 0.009m),

b. Various valves, connecting pipes and flow meters,

c. Inlet Distributor,

d. By-passes for extraction of inlet and outlet gas samples,

e. Drainage Facility.

3. CO2 and fresh air supply system:

a.

1 X Compressed CO2 gas tank,b. 1 X Air supply,

c. 1 X CO2 pump and air pump,

d. 2 X Flow meter with control valve,

e. Connecting pipes.

4. One large reservoir feed tank, greater than 25 L (to store NaOH) along with:

a. 1 X Fluid pump,

b. 1 X Flow meter with control valve,

c. Connecting pipes,

d.

1 X Large stirring stick,e. 1 X Big funnel.

5. Tap Water supply system:

a. 1 X Water supply,

b. 1 X Flow meter with control valve,

c. Connecting pipes,

d. 1 X Filtration system.

6. Gas Chromatography analyser machine along with:

a.

1 X 1.0 mL Gas Syringe.

7. Acid-base Titration Apparatus:

a. 1 X 50.00 mL burette,

b. 1 X 10 mL,

c. 1 X 20 mL pipette,

d. 2 X Conical Flasks,

e. 1 X Small Funnel,

f. 2 X Beakers,

g. 1 X Retort Stand.



10

Reagents and Indicators

8. The following reagents and indicators were provided:

a. Deionised water,

b. 0.1084M HCl solution (titrant),

c.

0.1095M NaOH,

d. 2M NaOH solution (to be diluted for feed tank),

e. Methyl Orange indicator,

f. Phenopthalein indicator.

Experimental Set-up

9. Experiment R1 is set up as follows:

Figure 1: Schematic diagram of experimental set up

Experimental Procedure

10. The expertimental procedure can be summarised in Figure XXX.



11

Figure ??? – Flow Chart of Experimental Procedure

Dilute and

prepare NaOH

Find concentration of NaOH

through backward titration

Configure set-

up for Run 1

Sample and find composition of

inlet and outlet gas

Start Run 1

Backward titrate to getcomposition of liquid outlet

Adjust flow rates to Run 2

values

End of experiment

Inconsistent

Inconsistent

Consistent

Consistent

Continue withnext Run

Completed all

6 Runs



12

11. Preparation of diluted NaOH feed to approximately 0.2 M:

a. The 2 M NaOH solution was diluted with deionised water in the ratio of 1 L NaOH

to 19 L deionised water to make up approximately 20 L of NaOH feed.

b. Solution was stirred thoroughly with long stirrer to ensure homogeneity.

c.

20 mL of diluted NaOH solution was titrated with 0.1084M HCl solution using

phenolphthalein indicator to determine the exact NaOH concentration. The exact

concentration of NaOH was determined to be 0.180 M.

12. Configuring experimental set-up prior to experimental runs:

a. The large reservoir feed tank was filled up with 20 L of 0.180 M using the big

funnel.

b. Valves were ensured to be in their appropriate positions.

c. Compressed CO2 supply tank valve was adjusted to its open position.

d.

Air supply was turned on.e. Flow meter of CO2 and air was adjusted to attain 0.2 L/min and 1 L/min

respectively.

f. Water supply was turned on.

g. Water flow meter was adjusted to 0.7 L/min.

h. Water level in column was monitored to ensure that liquid fills the bulb at the

bottom of the column but does not overflow into the gas inlet. This was

continuously monitored by adjusted various valves.

i. The set-up was left untouched for 20 minutes to allow steady state to be reached.

j. Inlet gas and outlet gas was sampled using the gas syringe and composition of

inlet and outlet gas was identified using gas chromotography analyser system.The was done repeatedly.

k. Once consecutive values of inlet and outlet gas mole fraction were consistent

(within 1 %), gas inlet/outlet sampling ceased and the consecutive readings were

recorded.

l. Liquid outlet was sampled by obtaining about 40 mL with a beaker.

m. 10.00 mL of outlet sample was each pipetted into 2 conical flasks.

n. Titration was conducted on both samples. If readings were consistent (within

0.20 mL), tritration readings were recorded. Otherwise, the same process was

repeated.

o.

Steps g to n constituted Run 1 of Experiement R1. The steps were repeated intheir entirety for Runs 2 to 6 (except inlet gas concentration was no longer

sampled) by varying NaOH and water flow rates. The pump was switched on to

effect NaOH flow for Runs 2 to 6.

13. Adjust Flow rates:

a. Refer to the table below for the controlled flow rates for each experimental runs.

The first run used pure water and is known as the physical absorption run.

Subsequent runs involving NaOH are termed chemical absorption runs. The flow

rates of the CO2 – Air mixture were kept constant and the concentration of NaOH

entering the packed bed varied.



13

Table 3.1: System flow rates for different runs

Flow Rate (L/min) CO2(g) Air Water NaOH(aq)

Run 1 (Physical

Absorption)0.20 1.00 0.60 0.00

Run 2 (Chemical

Absorption) 0.20 1.00 0.55 0.05

Run 3 (Chemical

Absorption)0.20 1.00 0.50 0.10

Run 4 (Chemical

Absorption)0.20 1.00 0.45 0.15

Run 5 (Chemical

Absorption)0.20 1.00 0.40 0.20

Run 6 (Chemical

Absorption)0.20 1.00 0.35 0.25

14. Back titrate liquid outlet sample:

a. 5 mL of NaOH (0.1095M) was pipetted into 2 conical flasks in preparation of

titration.

b. 10 mL of liquid outlet sample was pipetted into the 2 conical flasks.

c. The mixure was swirled to ensure good mixing.

d. The solution mixtures were first titrated against HCl (0.1084M) using

phenolphthalein as the indicator. Two drop os phenolpthalein was added. End

point was reached when the solution colour changes from purple to colourless.

The amount of HCl used was recorded.

e.

Next, the resulting solution continued to be titrated against HCl using methylorange as the indicator. 2 drops of methyl orange was added. End point was

reached when the solution changes colour from yellow to orange red. The

amount of HCl used was recorded again.

f. The readings from both conical flasks were compared. Consistency was taken to

have been achieved if readings were within 0.2 mL of each other.

g. This constituted the titration readings for Run 1 of experiment R1. The same

steps were repeated for Runs 2 to 6.



14

Results and Analysis

Raw Experimental Data

Flow Rate (L/min) CO2(g) Air Water NaOH(aq)

Run 1 (Physical

Absorption)0.20 1.00 0.60 0.00

Run 2 (Chemical

Absorption)0.20 1.00 0.55 0.05

Run 3 (Chemical

Absorption)0.20 1.00 0.50 0.10

Run 4 (Chemical

Absorption)0.20 1.00 0.45 0.15

Run 5 (ChemicalAbsorption)

0.20 1.00 0.40 0.20

Run 6 (Chemical

Absorption)0.20 1.00 0.35 0.25

Table 4.1: Experimental Inlet Flow Rates

RunInlet (Vol %)

Outlet (Vol %)

Sample 1 Sample 2 Average

CO2(g) Air CO2(g) Air CO2(g) Air CO2(g) Air1 18.5 81.5 13.7 86.3 14.1 85.9 13.9 86.1

2 18.5 81.5 7.07 92.9 6.77 93.2 6.92 93.1

3 18.5 81.5 2.40 97.6 2.70 97.3 2.55 97.5

4 18.5 81.5 0.783 99.2 0.738 99.3 0.761 99.2

5 18.5 81.5 0.254 99.8 0.239 99.8 0.247 99.8

6 18.5 81.5 0.118 99.9 0.0953 99.9 0.107 99.9

Table 4.2: Composition of CO2(g) and Air measured using Gas Chromatography

Run

Volume of HCl(aq) used for titration

with phenolphthalein (mL)

Volume of HCl(aq) used for titration

with methyl orange (mL)

Sample 1 Sample 2 Average Sample 1 Sample 2 Average

1 0.00 0.00 0.00 0.00 0.00 0.00

2 5.10 5.10 5.10 1.20 1.30 1.25

3 6.00 6.00 6.00 1.30 1.40 1.35

4 7.00 7.10 7.05 1.50 1.40 1.45

5 8.60 8.50 8.55 1.50 1.50 1.506 9.60 9.50 9.55 1.80 1.60 1.70



15

Table 4.3: Volume of HCl(aq) used for Titration with liquid sample drawn from Absorption

Column

Volume of liquid sample taken from absorption column outlet for back-titration = 10mL

Volume of NaOH(aq) added for back-titration = 5mL

Known Physical Properties and Experimental Information

Density of air, ρair (kg/m3) 1.177

Average molecular weight of air, Mair (g/mol) 28.85

Density of CO2(g), ρCO2 (kg/m3) 1.788

Molecular weight of CO2(g), M

CO2 (g/mol) 44.01

Density of water, ρH2O (kg/m3) 1000

Molecular weight of water, MH2O (g/mol) 18.02

Henry’s constant, HA (atm.L/mol) 29.41

Henry’s constant, HA (m3.Pa/mol) 2979.97

Diffusivity of CO2(g) in water, DA (m2/s) 1.77 x 10

-9

Diffusivity of NaOH(aq) in water, DB (m2/s) 2.17 x 10

-9

System temperature, T (K) 298

System pressure, PT (Pa) 101325

System pressure, PT (atm) 1.00

Concentration of de-ionised water, CT (mol/L) 55.56

Table 4.4: Physical Properties

Height of absorption column, HC (m) 0.49

Inner diameter of absorption column, dC (m) 0.05

Inner diameter of Raschig Ring, dR (m) 0.006

Outer diameter of Raschig Ring, DR (m) 0.008

Length of Raschig Ring, LR (m) 0.009

Table 4.5: Absorption Column Data

[HCl(aq)]titration (mol/L) 0.1084

[NaOH(aq)]stock (mol/L) 2.00

[NaOH(aq)]feed tank (mol/L) 0.180

[NaOH(aq)]titration (mol/L) 0.1095

Table 4.6: Concentration of Reagents

Determining the concentration of NaOH(aq) for the feed tankAcid-base titration of NaOH(aq) with HCl(aq) in the reaction:



16

Titration: 5mL of NaOH(aq) sample is withdrawn from the feed tank and titrated against

0.1084M of HCl.

Average volume of HCl(aq) used to fully titrate the 5mL NaOH(aq), VHCl = 8.3mL

Since the reaction occurs in a stoichiometric ratio of 1:1,

Concentration of NaOH(aq) in feed tank = 8.3 x 0.1084 / 5

= 0.18M

Determining the inlet and outlet molar ratios

(i) Molar ratio for inlet CO2(g), YA,in

Since the inlet air remains the same for all the runs, the average inlet gas concentrations will

be used for calculations.

Inlet molar ratio of CO2(g),

YA,in = (average inlet CO2(g) conc.) / (average inlet air conc.)

= 18.5 / 81.5= 0.227

(ii) Molar ratio for outlet CO2(g), YA,out

Sample calculation using Run 6:

Outlet molar ratio of CO2(g),

YA,out = (outlet CO2 concentration)/(outlet air concentration)

= 0.107 / 99.9

= 0.00107

The sample calculations are then repeated for Runs 1 to 5:

Run Outlet molar ratio of CO2(g), YA,out 1 0.161

2 0.0743

3 0.0262

4 0.00767

5 0.00247

6 0.00107

Table 4.7: Outlet Molar Ratios of CO2(g)

(iii) Molar ratio for inlet NaOH(aq), XB,in

Sample calculation using Run 6:

Inlet molar flowrate of NaOH(aq) = 0.25 L/min x 0.18 mol/L

= 0.045 mol/min

Inlet molar flowrate of water = 0.35 L/min x 55.56 mol/L

= 19.45 mol/min

Inlet molar ratio of NaOH(aq),

XB,in = inlet molar flowrate of NaOH(aq) / inlet molar flowrate of water

= 0.045 / 19.45

= 0.00231




17

Run Inlet molar ratio of NaOH(aq), XB,in

1 0

2 0.000294

3 0.000648

4 0.001085 0.00162

6 0.00231

Table 4.8: Inlet Molar Ratios of NaOH(aq)

(iv) Molar ratio for outlet NaOH(aq), XB,out

Volume of outlet solution collected = 10mL

Volume of 0.1095M NaOH(aq) added = 5mL

no. of moles of NaOH(aq) added = 0.000548 mol

Sample Calculation using run 6:No. of mol of unreacted NaOH(aq) = (9.55 – 1.70)/1000 x 0.1084

= 0.000851 mol

Concentration of NaOH(aq) in outlet liquid = (0.000851 – 0.000548) / (20/1000)

= 0.0151 M

Molar flow rate of NaOH(aq) in outlet liquid = 0.0151 M x 0.35 L/min

= 0.00530 mol/min

Molar ratio of NaOH(aq) in outlet liquid,

XB,out = molar flowrate of NaOH(aq) / molar flowrate of water

= 0.00530 / 19.45

= 0.000273Note that Run 1 is conducted without any NaOH(aq) in the inlet for pure physical absorption,

thus there will not be any NaOH(aq) present in the outlet liquid.


Run Outlet molar ratio of NaOH(aq), XB,out

1 -0.000493

2 -0.000118

3 -0.0000395

4 0.0000531

5 0.0001956 0.000273

Table 4.9: Outlet Molar Ratios of NaOH(aq)

Negative values of XB,out indicate that all the NaOH(aq) have been reacted (amount of

NaOH(aq) left after reaction with dissolved CO2 is less than the amount added in), therefore

the negative values can all be taken to be zero.

Determining the mass transfer coefficients

(i) Overall gas mass transfer coefficient without reaction, KGAoa

To perform this calculation, only data from Run 1 is required (region of physical absorption).



18

Assume ideal gas behaviour for air at room temperature and pressure,

̇

Taking Run 1 to consist only of the physical absorption zone,

( )

(ii) Mass transfer coefficient in gas phase, kGAa

As the flow rate of NaOH(aq) used in Run 6 was the highest, it can be assumed that the

whole column was operating under the surface reaction zone.

(iii) Mass transfer coefficient in liquid without reaction, kLAoa

Determining the enhancement factor, E

CT = 1000 g/kg x 1000 m3/kg / 18g/mol = 55560 mol/m

3

PT = 1 atm

Sample calculation using run 6:

CB,in = XB,in x CT

= 0.00231 x 55560

= 128.34 mol/m3



19

CB,out = XB,out x CT

= 0.000273 x 55560

= 15.17 mol/m3

PA,in = PT x YA,in

= 0.227 atm

Enhancement factor,

Calculating E at the top and bottom of the column and taking the average as the overall

enhancement factor:

Eaverage = 0.5 x (11.19 + 2.20) = 6.70

(i) Determining the mass transfer coefficient in liquid phase, kLAa

kLAa = E x

= 6.70 x 0.0122

= 0.0817 s-1

(ii) Determining the overall gas mass transfer coefficient, KGAa


Run PA (atm)CB,in

(mol/m3)

CB,out

(mol/m3)

Etop Ebottom Eaverage

1 0.227 0 -27.39 1.00 -1.17 -0.085

2 0.227 16.33 -6.56 2.30 0.479 1.39

3 0.227 36.00 -2.19 3.86 0.826 2.34

4 0.227 60.00 2.95 5.76 1.23 3.50

5 0.227 90.00 10.83 8.15 1.86 5.01

6 0.227 128.34 15.17 11.19 2.20 6.70

Table 4.10: Calculations for C B,in , C B,out , E



20

RunK°GAa

(mol/m3.atm.s)

kGAa

(mol/m3.atm.s)

k°LAa (1/s) kLAa (1/s)KGAa

(mol/m3.atm.s)

1

0.375 3.80 0.0122

-0.00104 -0.0357

2 0.0170 0.502

3 0.0285 0.7724 0.0427 1.05

5 0.0611 1.34

6 0.0817 1.61

Table 4.11: Summary of Mass Transfer Coefficients Values

Determining the critical mole ratio, XBC

XBC is the critical mole ratio where the system is at the end of the surface reaction zone and

at the beginning of the interior reaction zone.

According to Table XXX, the types of reaction zones present are as follows:

Run XBC Types of reaction zones present

1 0.000311 Physical Absorption

2 0.000376 Physical Absorption, Interior Reaction

3 0.000562 Physical Absorption, Surface Reaction

4 0.000867 Surface Reaction, Interior Reaction



Table 4.12: Calculations for X BC and Types of Reaction Zones Present

Heights of various reaction zones



21

Figure 4.1: Visual Illustration of the 3 Absorption Zones

Calculation of YA,2, YA,3, XB,2, XB,3:

(i) Physical absorption zone only (Run 1):

The entire column height is h1 = H = 0.49 m

(ii) Physical absorption and interior reaction zones (Run 2):

Consider control area A, material balance gives:

YA,3 = YA,out = 0.0743

XB,3 = XB,in = 0.000294

XB,out = XB,2 = 0 (physical absorption zone present)

(iii) Surface reaction and physical absorption zones (Run 3):

Consider control area B, material balance gives:

YA,out = 0.0262

XB,in = 0.000648

XB,2 = 0 (physical absorption zone present)

This value of YA,2 is not possible as it has to be lower than Y A,in (i.e. 0.227) after physicalabsorption has taken place. This anomalous result may have occurred due to errors which



22

would be discussed below in the “error analysis” section. In the meantime, Run 3 will be

treated as though it does not possess a physical absorption zone in the further calculations.

Consider control area C, material balance gives:

XB,3 = XBC = 0.000562

(iv) Surface reaction and interior reaction zones (Runs 4, 5, 6):

Consider control area B, mass balance gives:

Consider control area C, mass balance gives:

XB,2 = XB,out

YA,2 = YA,in

XB,3 = XBC

Height of physical absorption zone,

( )

Height of interior reaction zone,

Height of surface reaction zone,

Run 1 2 3 4 5 6

XBC 0.000311 0.000376 0.000562 0.000867 0.00128 0.00182

XB,in 0 0.000294 0.000648 0.00108 0.00162 0.00231

XB,2 - 0 0 0.0000531 0.000195 0.000273

XB,3 - 0.000294 0.000562 0.000867 0.00128 0.00182

XB,out 0 0 0 0.0000531 0.000195 0.000273

YA,in 0.227 0.227 0.227 0.227 0.227 0.227

YA,2 - 0.194 0.290 0.426 0.583 0.831



23

YA,3 - 0.0743 0.0612 0.0945 0.141 0.201

YA,out 0.161 0.0743 0.0262 0.00767 0.00247 0.00107

- 0.559 0.713 1.10 1.63 2.32

h1 0.49 0.173 - - - -

h2 - 0.233 0.236 0.158 0.107 0.0894h3 - - 0.0775 0.230 0.370 0.479

Total 0.49 0.406 0.314 0.388 0.477 0.568

Error (%) - 17.1 35.9 20.8 2.65 -15.9

Table 4.13: Summary of Final Experimental Values

Run 3 is taken to consist of the surface reaction zone only due to the unfeasible Y A,2

calculated previously above.



24

Discussion

The main aim of this experiment was to study the various reaction zones for a continuous,

counter-current gas-liquid absorption process carried out in a packed bed column. In

addition to that, the heights corresponding to the various reaction zones, as well as the mass

transfer coefficients pertaining to the entire experimental process were determined. In this

section, the assumptions made throughout the course of the experiment will first be stated,

followed by a discussion regarding the results of our experiment and whether they agree

with theoretical principles.

Assumptions

The following assumptions were made for this experiment:

1. The entire experiment was carried out under isothermal, isobaric, and continuous

steady state conditions. 20min-intervals were given after the settings were calibrated

for each run so as to ensure that the conditions were reached before the collection

of any samples.

2.

Phase boundary occurs at thermodynamic equilibrium.

3. The solution in the absorption column was dilute enough (<0.05moldm-3

) such that

Henry’s law is valid and can be applied.

4. Reaction between the CO2(g) and NaOH(aq) was instantaneous and irreversible, such

that both the reactants cannot coexist at any point, and thus the process could be

considered as one that was mass-transfer controlled.

5. NaOH(aq) was non-volatile, no vaporisation of NaOH(aq) into the gaseous film or gas

bulk occurred.

6. The diffusion of CO2(g) from the bulk gas through the gas-liquid interface and into the

film was purely by mass transfer.7. Run 6 had a sufficiently high NaOH(aq) concentration such that only the surface

reaction zone existed.

8. There was no other mutual exchange of materials between the gas phase and the

liquid phase except for the component (i.e. CO2(g) in this case) to be removed.

Effect of chemical reaction on the rate of absorption

Run

NaOH(aq)

flow rate

(L/min)

Eaverage kLAa (1/s)

1 0.00 -0.085 -0.00104

2 0.05 1.39 0.0170

3 0.10 2.340.0285

4 0.15 3.500.0427

5 0.20 5.010.0611



25

6 0.25 6.700.0817

Table 5.1: Values for NaOH(aq) Flow Rates, E average , k LAa

The enhancement factor, E can be used to measure the extent at which absorption rate is

improved by chemical reaction. A large E signifies a higher ratio between the liquid film mass

transfer coefficients of CO2(g) with chemical reaction to that without chemical reaction (i.e.

with a larger E value, there is a greater extent of mass transfer that occurs with a chemical

reaction involved).

As shown from Table 5.1, increase in NaOH(aq) concentration in the liquid inlet stream

corresponds to the increase in enhancement factor, E, which also reflect similar

improvements in absorption rate. The reason for this is because the chemical reaction acts

as the main driving force for CO2(g) absorption, by continuously removing CO2(g) and thus

creates a concentration gradient at the interface. Hence, the efficiency of the absorption

column improves as the concentration of NaOH(aq) in the inlet feed increases.

However, it must be noted that the rate of absorption will not be able increase indefinitely.

When there is an excess amount of NaOH(aq), the liquid film resistance will approach to

zero. The overall resistance will consequently be controlled by the gas film resistance; and

we know that mass transfer of CO2 occurs within the gas film. Since the gas film resistance is

constant, the rate of absorption will correspondingly reach a limit. Further increasing

NaOH(aq) flow rate after that point will not increase the removal of CO2(g) from the gas

stream as the entire column will be in the surface reaction zone.

Determining the heights of various zones

Run 1 2 3 4 5 6

h1 0.49 0.173 - - - -

h2 - 0.233 0.236 0.158 0.107 0.0894

h3 - - 0.0775 0.230 0.370 0.479

Total 0.49 0.406 0.314 0.388 0.477 0.568

Error (%) - 17.1 35.9 20.8 2.65 -15.9

Table 5.2: Values for Heights Corresponding to the Various Zones and the % Errors Involved

As shown in Table 5.2, all the derived total heights deviate from the actual absorption

column height of 0.49m. This may be due to a number of reasons and sources of errors

which will be discussed under the “error analysis” section below.

As we have stated earlier, it is important to note that we have initially assumed Run 6 to

have a sufficiently high NaOH(aq) concentration such that only the surface reaction zone is

present. In view of that, we used the results from Run 6 to determine kGAa, and in turn

derived the other various heights from that.

However, the assumption was clearly just an approximation, because of that we would have

suffered some losses of accuracy in our results. In order to improve on the situation, another

run could be carried out with an even higher NaOH(aq) concentration to further ensure that

indeed only the surface reaction zone is present.



26

Nonetheless, we would still expect the calculated height to be less than the actual height

since the experimental absorption column itself is not 100% efficient. Apart from that, the

complexity of the calculations might also lead to large errors of uncertainty.



27

Error Analysis

Throughout the experiments, several assumptions made coupled with experimental errors

could have lead to the aberrations that are observed in the observed data.

1. The flow rates of some of the variables in the experiment such as air, water and

sodium hydroxide had to be manually controlled by the experimenter. Constant

fluctuations were observed in the flow rates of these variables. Everytime the water

level in the packed bed had to be adjusted, it was observed that the flow rates had to

be re-adjusted to desired values. Thus everytime a manual re-adjustment had to be

made, time may be required to establish a new equilibrium reading and therefore

the concentration of carbon dioxide recorded may not have been that at equilibrium.

2. During many steps of the experiment such as reading off carbon dioxide calculations

and titration results, the steps were repeated a few times to ensure the credibility

and therefore the average values were taken. Any random errors present may be

aggravated due to this averaging and might have led to larger deviations from the

ideal values.

3.

The sodium hydroxide solution in the feed tank is assumed to be perfectly mixed.However, when the experimenter manually diluted the stock solution of sodium

hydroxide, he might not have mixed the resulting solution with the stirrer for an

adequate time period. This would mean that there might be fluctuations in the

concentration of sodium hydroxide entering the packed column at any one time.

4. When taking the sample of carbon dioxide for testing its concentration, the sample

had to be taken from the end of a tube where it is possible for the sample and air to

intermingle. This suggests that although an accurate gas chromatography is utilized

to give us the concentration, the sample itself may be contaminated in the first place.

5. It was accepted that steady state had been attained when the carbon dioxide

concentrations with an interval of 5 minutes had less than a percent difference intheir values. However, this criteria may not be representative of a steady state

having been reached.

6. The reaction between sodium hydroxide and carbon dioxide is assumed to be

instantaneous. However, in reality, this assumption may not hold.

7. The usage of Henry’s constants in calculations may cause a problem if the solutions in

question are not dilute enough as Henry’s Law is only applicable for dilute solutions.



28

Suggestions for Improvement

1. The following areas of experiment design could be enhanced to increase the accuracy of

experimental results:

a. Usage of water mains (tap) as the source of water supply results in experimental

error. This is because water from water mains tend to have fluctuating flow rates.

Moreover, potable water contained ions that may potentially interfere with

experimental results. Instead, a pump can be used to draw water from another

large water reservoir containing pure deionised water. This essentially replicates

the set-up for the NaOH inlet.

b. The experiment assumes that concentration profile within the liquid and gas film

is a linear one. This assumption is valid if the Raschid rings have high diameter to

length ratio and hence can approximate a flat surface. Experimental accuracy can

be improved if Raschid rings with larger diameter to length ratio were used.

c. As mentioned in the error analysis, manual adjustment of flow meters is required

to maintain the flow rates at their designated values. This process can bemechanised using process control knowledge to minimise human error such as

time lag before deviation from set points are noticed.

d. The NaOH reservoir can be enhanced to include a stirring device to reduce spatial

heterogeneity in the solution concentration during the experiment.



29

Experimental Safety Analysis

3.5.1 Carbon Dioxide

Handle gas cylinders with utmost care. If in doubt, ask the lab demonstrator. Do not open

the main valves too quickly as it may cause an explosion.

Emergency: If inhaled, conscious persons should be moved to uncontaminated area with

fresh air. Unconscious persons should be moved to uncontaminated area and given

resuscitation or supplemental oxygen.

3.5.2 Sodium hydroxide

Concentrated sodium hydroxide is caustic. Avoid direct contact by wearing gloves and safety

goggles.

Emergency: Upon contact with skin or eyes, wash with copious amount of water.

For disposal, dilute sodium hydroxide are washed down a sink with a large quantity of water.

Larger amounts should be neutralised or diluted before disposal.

3.5.3 General precautionsi. One group member should monitor the flow rates at all times to ensure that the

fluctuations are minimized. If the pump becomes very noisy, switch off the power

and inform the lab demonstrator immediately.

ii. The packed bed flow rate must also be monitored and the outlet flowrate adjusted

accordingly to prevent excessive flooding

iii. If water is spilled around the electrical sockets, inform the lab demonstrator

immediately. Do not attempt to touch the area.

iv. Be properly attired in personal protective equipment (PPE) like lab coat, goggles and

covered shoes during the experiment.

v.

Handle glassware with care. In the event of breakage, sweep the broken piecesimmediately and report to lab demonstrator. Do not attempt to touch the broken

pieces with bare hands.



30

Conclusion

1 page – Jia Ming



References

Wolfgang Bertsch. (2000). Two-dimensional gas chromatography. concepts,

instrumentation, and applications – part 2: Comprehensive two-dimensional gas

chromatography. 23(3), 167.

W.E. McCabe, J.C. Smith, and P.Harriott 2001. Unit Operations of Chemical Engineering,

McGraw Hill, New York

Perry’s Chemical Engineers Handbook, 7th

Edition. 1997 (Ed: R.H. Perry, D.W. Green, and J.O.

Maloney), McGraw-Hill, New York.

P.V.Danckwerts. (n.d.). The absorption of gases in liquids. In Retrieved from

http://pac.iupac.org/publications/pac/pdf/1965/pdf/1004x0625.pdf

Hans T. Karlsson. (2010). Absorption with chemical reaction. Unpublished raw data, Energy

and Environment KET010, Retrieved from

http://www.chemeng.lth.se/ket010/Arkiv/Absorption_Reaction.pdfM.A. Olutoye and A. Mohammed. (2006). Modelling of a gas-absorption packed column for .

(Master's thesis)Retrieved from

http://www.journal.au.edu/au_techno/2006/oct06/journalTechV10N2_aticle11.pdf

Annex A – Raw Data

Documents

Mana Source