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Institutionen för teknik och design, TD

Absorption of CO2 - by Ammonia

Diploma work

Filip Sjöstrand & Reza Yazdi

Växjö, 4/6-09

15 Hp

Ämne/Kurs Energiteknik/BT9903

Supervisor: PhD. Ann-Charlotte Larsson, Alstom

Examiner: Prof. Sune Bengtsson, Växjö universitet, Institutionen för teknik och design

Examensarbete nr: TD 083/2009

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Organisation/ Organization Författare/Author(s)

VÄXJÖ UNIVERSITET Filip Sjöstrand & Reza Yazdi

Institutionen för teknik och design

Växjö University

School of Technology and Design

Dokumenttyp/Type of Document Handledare/tutor Examinator/examiner

Examensarbete/Diploma Work Ann-Charlotte Larsson Sune Bengtsson

Titel och undertitel/Title and subtitle

Absorption med CO2 – av Ammoniak

Absorption of CO2 – by Ammonia

Sammanfattning I detta examensarbete har absorptionseffektivitet av CO2 hos olika vätskelösningar undersökts genom gasabsorption i en slumpmässigt packad kolonn. För att karakterisera absorptionen absorberades även SO2 i några experiment. Rapporten är utförd med anledning av de stora mängder koldioxid som släpps ut i atmosfären, främst från fossileldade kraftverk. För att minska dessa utsläpp kan koldioxiden avskiljas från rökgaserna genom olika tekniker t.ex. genom CO2-absorption med ammoniak. Arbetet består av en teoridel och en laborativ del med mätningar och beräkningar. I den experimentella delen konstruerades ett system med en absorptionskolonn och tillhörande mätutrustning. Olika vätskelösningar bestående av rent vatten, kaliumkarbonatlösning och ammoniak i olika koncentrationer användes till att ta upp koldioxid genom motströms absorption. Även SO2 absorberades i kaliumkarbonatlösning för att bestämma gasfilmkonstanten. Absorptionsgraden av CO2 varierade från några få procent i försöket med vatten upp till 7 % med kaliumkarbonatlösningen. CO2-absorptionen av ammoniak varierade med koncentrationen och gav en avskiljning på mellan 12 och 94 %. Ammoniakförsöken gjordes med både vid 10 och 20 °C. Generellt erhölls en bättre CO2-avskiljning vid 20°C, vilket bekräftas av teorin.

Nyckelord Avskiljning av koldioxid, massöverföring, koldioxid, absorption, ammoniak, packad kolonn

Abstract In this diploma work, the absorption of CO2 in different liquid solutions was studied by gas absorption in a randomly packed column. To characterize the absorption a few experiments with SO2 absorption were made. The report has resulted due to the large amounts of carbon dioxide released into the atmosphere, mainly from fossil-fired power plants. To reduce these emissions, carbon dioxide can be separated from flue gas by different techniques such as CO2 absorption with ammonia. The work consists of a theoretical and a laboratory part of measurements and calculations. In the experimental part a system of absorption and associated test equipment was constructed. Different liquid solutions of pure water, potassium carbonate solution and ammonia in various concentrations were used to catch carbon dioxide by countercurrent absorption. Also SO2 was absorbed in the potassium carbonate solution to determine the gas film constant. The absorption efficiency of CO2 ranged from a few percent in the experiment with water to up to 7% with potassium carbonate solution. The CO2 absorption of ammonia varied with concentration and gave a separation of between 12 and 94%. Ammonia tests were made at both 10 and 20 °C. In general, a higher CO2-capture at 20 °C was obtained as confirmed by theory. Key Words Carbon capture, mass transfer, carbon dioxide, absorption, ammonia, packed column

Utgivningsår/Year of issue Språk/Language Antal sidor/Number of pages

2009 English 55

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Abstract In this diploma work, the absorption of CO2 in different liquid solutions was studied by gas

absorption in a randomly packed column. To characterize the absorption a number of

experiments with SO2 absorption were made.

Absorption of CO2 with ammonia is a technique used to reduce emissions of carbon dioxide

to the atmosphere created by fossil-fired power plants.

The work is laboratory-oriented and deals with an absorption column and measurement

equipment

Pure water, potassium carbonate solution and ammonia in various concentrations were used to

catch carbon dioxide by countercurrent absorption. Also SO2 was absorbed in the potassium

carbonate solution to determine the gas film constant.

The absorption efficiency of CO2 ranged from a few percent in the experiment with water to

up to 7% with potassium carbonate solution. The CO2 absorption of ammonia varied with

concentration and gave a separation of between 12 and 94%. Ammonia tests were made at

both 10 and 20 °C. In general, a higher CO2-capture at 20 °C was obtained as confirmed by

theory.

4

Foreword This diploma work completes our Bachelor of Education in Energy and the Environment at

the Department of Technology and Design at Växjö University.

The work has been carried out in cooperation with Alstom and is intended to provide an

additional basis for their research on CO2-capture.

We would like to thank our supervisor at Alstom, Ann-Charlotte Larsson, for her guidance

and consultation. We would also like to express our gratitude to the lab staff at Alstom for all

their help. Finally we would like to say a big thank you to our examiner, Sune Bengtsson, for

all his support, advice and guidance and being there to discuss and exchange information with

us.

5

Contents

1. Objective ................................................................................................................................ 7

2. Background ............................................................................................................................ 8

3. CO2-removal techniques ........................................................................................................ 9

3.1 Carbon capture and storage .............................................................................................. 9

3.2 Pre-combustion ............................................................................................................... 10

3.3 Post-combustion ............................................................................................................. 11

3.4 Oxy-fuel combustion ...................................................................................................... 12

4. CO2-absorption process ........................................................................................................ 13

4.1 CO2 in H2O ..................................................................................................................... 13

4.2 CO2 in K2CO3 ................................................................................................................. 13

4.3 CO2 in NH3 ..................................................................................................................... 14

4.4 SO2 in K2CO3 solution ................................................................................................... 14

5. Procedures for process design of absorption columns ......................................................... 15

5.1 Physical processes .......................................................................................................... 15

5.2 Number of transfer units in packed towers .................................................................... 16

5.3 Chemical absorption processes ...................................................................................... 17

5.3.1 The Two-film model ............................................................................................... 17

5.3.2 Higbie's penetration model ...................................................................................... 18

5.3.3 Gas film controlled system ...................................................................................... 18

6. Laboratory experiment ......................................................................................................... 20

6.1 Experimental apparatus .................................................................................................. 20

6.3 Experimental methodology ............................................................................................ 22

6.3.1 Test 1: H2O .............................................................................................................. 22

6.3.2 Test 2: K2CO3 solution ............................................................................................ 22

6.3.3 Test 3: NH3 .............................................................................................................. 23

6.3.4 Test 4: SO2 .............................................................................................................. 23

7. Results .................................................................................................................................. 24

7.1 CO2 absorption by H2O .................................................................................................. 25

7.2 CO2 absorption by K2CO3 solution ................................................................................ 26

7.3 CO2 absorption by NH3 .................................................................................................. 27

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7.3.1 13-vol% CO2 absorption by NH3 at 20 °C .............................................................. 27

7.3.2 5-vol% CO2 absorption by NH3 at 20 °C ................................................................ 28

7.3.3 5-vol% CO2 absorption by NH3 at 10°C ................................................................. 30

7.4 SO2 absorption by CO3 solution ..................................................................................... 32

8. Calculations .......................................................................................................................... 33

8.1 Mass balances ................................................................................................................. 33

8.2 Interfacial area of the packing ........................................................................................ 35

8.3 Dependency of CO2 removal efficiency on temperature ............................................... 37

8.4 Removal efficiency of CO2 versus ammonia concentration .......................................... 39

9. Discussion ............................................................................................................................ 40

10. Conclusions ........................................................................................................................ 41

11. Further work ....................................................................................................................... 42

12. Nomenclature ..................................................................................................................... 43

References ................................................................................................................................ 44

Appendix A .............................................................................................................................. 45

Appendix B .............................................................................................................................. 46

7

1. Objective

In this diploma work the absorption efficiency of CO2 and SO2 by different liquid solutions

through gas absorption in a randomly packed column have been investigated.

The aqueous solutions used are water (H2O), potassium carbonate (K2CO3) and ammonia

(NH3). It is interesting to compare the carbon capture efficiency in a carbonate solution with

the capture rate in an NH3 solution. Due to absorbent recirculation process liquor is always

partly carbonated, i.e. containing both free NH3 and CO32-

(carbonate ions) potentially

reacting with CO2 and immediately affecting the CO2 mass transfer in the absorber. It is then

be possible to say something about the role of carbonate in an ammonia solution.

By varying liquid flow, L, concentration of ammonia, C, and temperature, T, the influence of

important parameters can be measured. The parameters are mass transfer coefficient, KG; CO2

removal efficiency, η; reaction rate, r and the effective gas-liquid interfacial area, av of the

packed column.

How much these parameters influence the absorption efficiency of CO2 when changing the

operation conditions is of interest in order to draw conclusions about CO2 absorption in

ammonia solutions essential for Alstom Power’s “chilled ammonia” process.

The studies in the CO2-ammonia process for this diploma work are limited to experiments and

theoretical analyses of the absorption process. The recovery (desorption/stripping) of CO2 in

the process has not been part of this work. An extended version of this report will be available

for Alstom.

8

2. Background

The world's leading climate experts have agreed on the fact that the climate is changing

because of the increasing amount of CO2 in the atmosphere.

This increase of CO2-emissions is mostly caused by fossil fuel combustion. Most of the CO2-

contribution comes from coal-fired power plants. Since the coal reserves are very large they

will continue to be the main fuel for the energy sector.

One way of decreasing the amount of greenhouse gases in the atmosphere would be to capture

the CO2 in the flue gas and store it. Several techniques for the capture are under development,

among them: chilled ammonia process being investigated by Alstom.

9

3. CO2-removal techniques

3.1 Carbon capture and storage

Carbon capture and storage (CCS) is a method of removing carbon dioxide from flue gases by

burning fossil fuels (gas, oil and coal). Large power plants release significant amounts of CO2

annually. By installing CCS in new or existing plants, it will be possible to capture and store

large amounts of CO2 that normally would be released to the atmosphere. This contributes to

major environmental benefits since fossil fuels account for the largest source of current power

generation. According to investigations made by International Energy Agency (IEA), the

fossil fuel used for power generation will continue to increase during the next 20 years. IEA

hence proclaims an accelerating expansion of the CCS [1].

The three most common technologies for capturing CO2 are the following:

Pre-combustion

Post-combustion (Alstom Chilled Ammonia)

Oxyfuel

Different CCS techniques are described in Figure 3.1 below.

Figure 3.1: Possible processes for carbon capture [2].

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3.2 Pre-combustion

Pre-combustion is most suitable for gaseous fuels, such as natural gas. Carbon dioxide is then

reduced from the fuel before the combustion stage. Thus a high energy unburned gas (syngas)

is formed. This gas consists of carbon monoxide (CO) and water (H2O). The synthesis gas is

then purified from unwanted particles to avoid damage to the system components such as

steam turbines and other upgrading processes. The synthesis gas may react with water vapour

to form hydrogen as shown in the chemical formula below:

CO + H2O ↔ CO2 + H2

The method used is called Water Gas Shift (WGS). Carbon monoxide and water vapour reacts

under high pressure in the presence of a catalyst. The products now consist of hydrogen and

carbon dioxide and small amounts of water vapour.

Finally it remains to separate water vapour from carbon dioxide. After this the CO2 (g) is

pressurized to CO2 (l). Thereafter, carbon dioxide is transported and pumped into the bedrock,

for example. The hydrogen produced can now be consumed for power generation. When

hydrogen is incinerated only water vapour and heat are formed. The process is described in

Figure 3.2 below [2].

Figure 3.2: Pre-combustion [3].

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3.3 Post-combustion

The post-combustion method is based on CO2 separation after the fuel combustion step. This

means that the technology can be installed on existing power plants without excessive

interference due to the structure.

The method works for solid, liquid and gaseous fuels. Combustion occurs with air, which

contributes to a flue gas content of mainly nitrogen and 3-15 vol% CO2. In order to separate

CO2 from the flue gas organic solvents such as monoethanolamine (MEA) can be used.

Inorganic solvents such as ammonia have proved successful for the separation of CO2 as well.

In the separation stage a chemical reaction between gas and liquid occurs. The flue gas is

washed in a column with a suitable liquid that reacts with CO2 to form other compounds.

After the flue gas treatment, CO2 levels could be about 90% lower than the initial value,

depending on choice of reagent. All that remains now is to pressurize the disposal of carbon

dioxide.

Obviously this is done in a closed circuit in which no new medium replenished. This means

that the chemicals are used again, which is an environmental profit. In order to separate CO2

from the chemicals a process called "stripping" is used. This is done in a different column

where high pressure steam traps the chemicals [2]. Post-combustion works as shown in Figure

3.3 below.

Figure 3.3: Post combustion [3].

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3.4 Oxy-fuel combustion

The method aims to achieve complete combustion by firing fuel with only oxygen. High

concentrations of CO2 and small amounts of water vapour are formed during combustion. The

carbon dioxide is separated from the water solution in a condenser. Flue gas may undergo

superficial treatment to remove non-condensing gas, and to eliminate toxic components.

Separation of oxygen from the air is very energy-demanding. This corresponds to a 15%

effect loss from the total effect. This will generate increased spending on energy suppliers,

which will be reflected in higher electricity prices for customers.

However, there are more effective methods under development. One promising technique is

chemical looping combustion (CLC), where metal ions oxidize in contact with atmospheric

oxygen. The oxidized metal ions are transported into the combustion chamber. The fuel now

consumes oxygen and metal ions, which are recycled back and recharged with oxygen again.

With a well-functioning plant the flue gas could almost exclusively consist of water vapour

after treatment [2]. CLC system can be characterized as in Figure 3.4 below.

Figure 3.4: Chemical looping system [4].

13

4. CO2-absorption process

The following reactions were investigated in the laboratory tests:

CO2 reacting with:

1. Water, reaction (1.1) & (1.2)

2. Potassium carbonate (2.0)

3. Ammonia (3.0)

SO2 reacting with:

4. Potassium carbonate (4.0)

4.1 CO2 in H2O

Carbon dioxide is not very soluble in water and undergoes physical absorption rather than

chemical reaction in water.

The following two reactions occur when CO2 is absorbed by an aqueous alkaline solution:

CO2 + H2O ↔ HCO3- + H

+ (1.1)

CO2 + OH- ↔ HCO3

- (1.2)

Reaction (1.2) is much faster than reaction (1.1) at pH > 10. Therefore reaction (1.1) could be

considered as negligible when determining the rate of absorption for CO2 in alkaline solutions

[5].

4.2 CO2 in K2CO3

Alkaline solutions, however, do undergo chemical reaction with CO2 and are hence more

effective absorbents [6].

The following acid-base reaction can be written:

CO2 + CO32-

+ H2O → 2HCO3- (2.0)

In contrast to absorption of SO2 into carbonate solution the CO2 absorption is hampered by a

very slow reaction:

CO2 + H2O ↔ HCO3- + H

+ (1.1)

Further, the acid/base reaction with carbonate ions goes via OH-:

CO2 + OH- ↔ HCO3

- (1.2)

14

In a carbonate solution the OH- concentration is 10

-2 to 10

-3 mol/l and also not instantaneous

as the reaction between SO2 and OH-. The reaction rate constant at 20 °C for the reaction

between CO2 and OH- is

about 6000 (l/mol*s) [5].

4.3 CO2 in NH3

However, when CO2 reacts with an aqueous solution NH3 the following overall reaction

occurs:

CO2 + 2NH3 + H2O → 2NH4+ + CO3

- (3.0)

Reaction (3.0) can be divided into four step reactions, see below:

CO2 + NH3 → NH2COOH (3.1)

NH2COOH + NH3 → NH4+ + NH2COO

- (3.2)

NH2COO- + H2O → NH4CO3

- (3.3)

NH4CO3- → NH4

+ + CO3

- (3.4)

Carbon dioxide reacts with ammonia to form carbamic acid in reaction (3.1). Reaction (3.1)

determines the rate of absorption since reaction (3.2) is very fast and irreversible. Reaction

(3.2) involves carbamic acid in reaction with another ammonia molecule forming carbamate

(NH2COO-) and ammonium.

Reaction (3.1) and (3.2) shows that one CO2-molecule needs two NH3-molecules to form

carbamate, shown in reaction (3.5).

CO2 + 2NH3 → NH2COO- + NH4

+ (3.5)

The carbamate then slowly reacts with water forming ammonium bicarbonate ion pair in

reaction (3.3). This reaction may take minutes or hours, but can be catalyzed. Ammonium

bicarbonate ion (NH4CO3-) may be dissociated in reaction (3.4) to ammonium and carbonate

ions [7]. Ammonium bicarbonate can precipitate and form a salt at higher concentrations.

4.4 SO2 in K2CO3 solution

Sulphur dioxide (SO2) absorption by aqueous solution of carbonate is known to be very fast.

The reaction can be written:

SO2 + CO32-

→ SO32-

+ CO2 (4.0)

Sulphur dioxide is a stronger acid than the carbonic acid and therefore the weaker carbonate

ion is reacting – whereupon sulfite ions and carbon dioxide are formed.

15

5. Procedures for process design of absorption columns

5.1 Physical processes

Number of ideal stages in plate columns

In straight mass transfer processes without chemical reaction, such as distillation and

absorption processes in plate columns it is common practise to use the ideal (equilibrium or

theoretical) stage concept. In this case the gas component to be absorbed does not react in the

liquid but exhibit or build up an equilibrium pressure according to Henry’s law (or Raolt’s

law at high concentrations), often called “back pressure”.

An operating line defining inlet and outlet composition in gas and liquid phases in the column

is established and the number of ideal stages (Nt) graphically (McCabe-Thiele method) be

determined (see sketch below). Typically mass transfer coefficients are not used in this plate

or tray concept. The design is based on the establishment of partial equilibrium between the

gas and the liquid phase, called “stage efficiency”. The relative resistances to mass transfer

from gas or liquid side are not part of this concept. The number of ideal stages can be

calculated graphically as in Figure 4.1 below. The x unit describes component A's (CO2)

concentration in the liquid while unit y describes component A's (CO2) partial pressure in the

gas [9].

Figure 5.1: McCabe-Thiele construction (McCabe, Smith p. 737) [8].

16

5.2 Number of transfer units in packed towers

In packed towers there are no plates or visible stages, but a randomly placed packing, such as

Raschig rings, Berl saddles, Intalox saddles or structured packing. For simple physical

separation processes carried out in such towers the HTU-concept (Height of a transfer unit) is

often used. Also in this case graphical procedures can be used for design, establishing an

“operating line” in a diagram having gas phase concentration on the Y-axis and liquid phase

concentration of the component to be absorbed or desorbed on the X-axis. A graphical

procedure involves the film coefficients (kG and kL) and the equilibrium curve (Henry’s law

relationship between gas and liquid for the component to be transferred).

Using the driving force differences at absorption tower bottom and top the number of transfer

units (NTUs) can be calculated.

The HTU/NTU concept is not suitable or does not apply when there is a chemical reaction

taking place between the gas component and the liquid and is consequently not usable in

analysing absorption processes accompanied by chemical reaction, such as CO2 absorption

into ammonia solution. In such cases the gas component often reacts in irreversible reactions

and does not exhibit an equilibrium back pressure. The “equilibrium line” is the X-axis [9].

See Figure 5.2 below.

Figure 5.2: Design procedure for straight mass transfer without reaction [10].

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5.3 Chemical absorption processes

For mass transfer with chemical reaction the absorption rate expressions must account for

both the gas component and the liquid reactant, the chemical reaction rate and equilibrium,

Henry’s law and the gas and liquid phase (film) transfer (diffusion) resistances. The

conditions become much more complicated and a number of cases can be defined based on

the relative rates of these mechanisms.

However, the opportunity is also given to design the absorption experiments to isolate one or

two of these factors to develop a better understanding for the process and its different transfer

and reaction mechanisms. That was the aim of this diploma work: covering the absorption of

CO2 into ammonia solution [9].

5.3.1 The Two-film model

There are several models proposed and they all work in a similar way.

The film model describes the mass transfer between a gas and a liquid that is physically

absorbed or undergoes a chemical reaction. According to the film model an interfacial surface

exists, which is surrounded by two stationary films, one containing the liquid and one

containing the gas. The mass transfer occurs through molecular diffusion inside the films.

The film model provides a good accuracy even if it is a simplification of the reality.

In Figures 5.3 and 5.4 below the interfacial surface is presented as a y-axis, while the

horizontal continuous line is the x-axis. The left dashed line and the interfacial surface defines

the gas film while the right dashed line and the interfacial surface defines the liquid film. In

the Figures below p represents the partial pressure and C represents the concentration. A and

B represent substances, G represents gas and L represents liquid.

An instantaneous reaction may occur in a small zone at the interfacial surface as in Figure 5.2,

which is gas controlled absorption. SO2 (substance A) in CO32-

(substance B) is an example of

a gas film controlled absorption that can be characterized by Figure 5.3. The reaction between

CO2 and NH3 is fast and liquid film controlled. The reaction is described in Figure 5.4 where

the mass controlling transfer occurs in the liquid film [9].

Figure 5.3: Instantaneous reaction [11]. Figure 5.4: Fast reaction [11].

18

5.3.2 Higbie's penetration model

The total mass transfer between liquid and gas is characterized by 1/(KG·av). Moreover 1/(kG·

av) describes the mass transfer in the gas film. According to Higbie's model, the following

expression for the liquid film can be used: H/(E·kL· av), which characterizes the mass transfer

in the liquid film with accompanied chemical reaction.

The two-film theory using Higbie's model on the liquid side can be described by the following

formula:

1

𝐾𝐺 ∙𝑎𝑣=

1

𝑘𝐺 ∙𝑎𝑣+

𝐻

𝐸∙𝑘𝐿 ∙𝑎𝑣 (5.0)

This can be simplified to

1

𝐾𝐺=

1

𝑘𝐺+

𝐻

𝐸∙𝑘𝐿 (5.1)

For CO2 absorption with NH3, the chemical enhancement factor, E can be described by the

following formula:

𝐸 = 𝑘∙[𝑁𝐻3]∙𝐷

𝑘𝐿 (5.2)

(5.2) inserted into (5.0) gives:

1


1


𝐻

𝑘∙[𝑁𝐻 3]∙𝐷

𝑘𝐿∙𝑘𝐿 ∙𝑎𝑣

↔1


1


𝐻

𝑎𝑣∙ 𝑘∙[𝑁𝐻3]∙𝐷 (5.3)

When a reaction is liquid film controlled as in the case of CO2 in NH3, 1/(kG·av) can be

neglected. First CO2 dissolves in order to react with NH3 and the reaction then occurs by a

fast reaction in the liquid film. The total mass transfer, 1/(KG·av) is controlled by the reaction

in the liquid film and may be determined by H/(E·kL·av) equal to:

1


𝐻

𝑎𝑣∙ 𝑘∙[𝑁𝐻3]∙𝐷 (5.4)

5.3.3 Gas film controlled system

SO2 reacting with CO3 on the other hand occurs instantaneously at the interfacial surface and

is hence gas film controlled. H/(E·kL·av) in Equation (5.0) can then be neglected since E→

and the total mass transfer can be explained by eq. (5.5).

1


1

𝑘𝐺 ∙𝑎𝑣↔ 𝐾𝐺 ∙ 𝑎𝑣 = 𝑘𝐺 ∙ 𝑎𝑣 (5.5)

19

The specific absorption rate, NA can be described by eq. (5.6) below.

𝑁𝐴 = 𝑘𝐺 ∙𝑎𝑣 ∙ 𝑝𝑆𝑂2 (5.6)

Equation (5.7) is obtained through a differential mass balance over a column element.

𝑁𝐴 ∙ 𝑎𝑣 ∙ 𝑑𝑍 = −𝐺

𝑝𝑡𝑜𝑡∙ 𝑑𝑝𝑆𝑂2

(5.7)

Equation (5.6) and (5.7) give (5.8).

𝑘𝐺 ∙𝑎𝑣 ∙ 𝑑𝑍 = −𝐺𝑝𝑡𝑜𝑡

∙𝑑𝑝𝑆𝑂2𝑝𝑆𝑂2

(5.8)

Integration of formula (5.8) gives the following for a “dilute” system, where G can be

considered constant:

𝑘𝐺 ∙ 𝑎𝑣 𝑑𝑍𝑧

0= −

𝐺

𝑝𝑡𝑜𝑡

𝑑𝑝𝑆𝑂 2

𝑝𝑆𝑂 2

𝑜𝑢𝑡

𝑖𝑛 (5.9)

Finally Equation (5.10) is obtained

𝑘𝐺 ∙ 𝑎𝑣 = 𝐺

𝑝𝑡𝑜𝑡 ∙𝑍∙ 𝑙𝑛

𝑝𝑆𝑂 2,𝑖𝑛

𝑝𝑆𝑂 2,𝑜𝑢𝑡

(5.10)

The same procedure with a differential mass balance using KG·av and liquid film for CO2

gives:

𝐾𝐺 ∙ 𝑎𝑣 = 𝐺 𝑘𝑁𝐻3𝐷𝐶𝑂2

𝑝𝑡𝑜𝑡 ∙𝑍∙ 𝑙𝑛

𝑝𝐶𝑂2,𝑖𝑛

𝑝𝐶𝑂2,𝑜𝑢𝑡

(5.11)

See further Equation (8.2) to (8.4).

20

6. Laboratory experiment 6.1 Experimental apparatus

Figure 6.1: Experimental equipment scheme: 1. CO2-cylinder; 2. Flowmeter; 3. Packed column;

4. IR-CO2-analyzer; 5. Feed tank; 6. Pump; 7. Head tank; 8. Valve; 9. Product tank.

The experimental apparatus is shown in Figure 6.1.

To begin with, the CO2-cylinder (1) brought CO2/N2-gasmixture, through a flowmeter (2) and

then in through the bottom of the column (3). Thereafter the non-absorbed gas flowed out

from the top of the column, through a hose and was then ventilated by a fan.

A small amount of the outgoing gas was led through a refrigerator system to condense H2O

and then into an IR-CO2-analyzer (4) in which 0-20% CO2 could be analyzed.

The liquid solution was placed in a feed tank (5) and pumped upwards by pump (6) to a head

tank (7). The liquid solution in the head tank was held at a constant volume when steady state

was achieved. A constant liquid flow entered the top of the column and was regulated by a

valve (8). The liquid solution reacted with the incoming gas in the column and finally flowed

by gravity to a product tank (9).

A photo of the system can be found in Appendix B Figure A.B.3.

21

6.2 Experimental parameters

Experimental parameters were varied to resemble Alstom's chilled ammonia process in the

pilot plant in Växjö, Sweden.

The column was made of transparent plexiglass with a height of approximately 1 meter.

Column bed height was 0,62 meters and consisted a tower packing material of Ø 7 x 7 mm

Raschig rings of glass.

The liquid solutions had a temperature of either 20°C or 10°C, approximately.

Gas inlet temperature was isothermal at a temperature of 20°C.

The pressure in all experiments was 1 atm, i.e all experiments were carried out under

atmospheric pressure.

Three different liquid flows were used: 10, 20 and 30 l/h. The calibration curve for liquid flow

meter is available in Appendix A Figure A.A.1 and Table A.A.1.

In order to compare the system with Alstom’s pilot plant the relationship between gas and

liquid flow (L/G) were selected to 6, 11, 17 and 35 l/m3gas.

All parameters described above are summarized in Table 6.1 below.

Table 6.1: Column Parameters and experimental conditions.

Packed column Value

Column bed height 0,62 m

Column diameter 0,074 m

Temperature 5-25 ºC

Pressure 1 atm

CO2 concentration gas inlet 5% and 13%

Liquid flow rate 10-30 l/h

Gas flow rate

L/G

1,8 and 0,9 m3/h

6-35 l/m3

Table 6.2 below shows the volume of 22%-NH3 solution needed in order to mix a 10 l,

0,5 to 5 M NH3 liquid.

Table 6.2: Volume of 22% NH3-solution.

C [mol/l] V (22%), [l]

0,5 0,42

1 0,85

2 1,70

3 2,55

5 4,25

A similar table for preparation of carbonate solution can be found in Appendix B Table

A.B.17.

22

6.3 Experimental methodology

A water test series was run in order to calibrate the system and gas and liquid flows were

determined.

To begin with a gas cylinder with 13% CO2 and 87% of N2 was used.

The gas flow was adjusted between 1,8 and 0,9 m3/h by the valve on the gas cylinder. To read

the gas flow, a calibrated flow meter was used. A calibrated flow meter was used on the liquid

side as well, measuring flows appropriate for the system: 10, 20 and 30 l/h. To regulate the

liquid flow, a simple hose clamp as valve was installed and worked very satisfactory giving

quite stable flows.

When the head tank reached a constant level of 2 liters the liquid flow valve was opened to

the column top. After about 2-3 minutes when the packing material became wet, the gas flow

was opened and regulated to 1,8 m3/h. The system then reached a steady state, i.e. until the

CO2 analyzer reached a stable value, within five minutes. Thereafter the liquid flow was

regulated to 20 and 30 l/h and the CO2 outlet concentration was read at each steady state

condition.

In all experiments the gas flow was halved to 0,9 m3/h when the liquid flow was 30 l/h.

To check the measuring accuracy the first experiment of each series was repeated at the end

of the experiment. In all cases this last test repeated the first test with very good accuracy,

proving very good reproducibility.

During the NH3 tests made at 20 ºC the liquid temperature increased by about 1-2 ºC. In the

experiments carried out at 10 ºC the initial liquid temperature altered between 10 ± 1-2 º C

and changed with about 2-3 ºC during the tests.

The pH values of the liquid solution were measured between 11,6-12,3 before the absorption

and between 10,5-11,4 afterwards. The lower pH values are for 1M NH3 concentration and

the higher pH values are for 5M. Information about temperatures and pH values were taken

from the experiment logbook.

6.3.1 Test 1: H2O

In order to investigate the mechanical function of the column, water was used first in the

system. Liquid flows between 10 and 30 l/h were tested together with 13% CO2 gas. The

current temperature was 20°C.

6.3.2 Test 2: K2CO3 solution

Subsequent tests were made with potassium carbonate solution. Temperature and gas / liquid

flow for the experiment with K2CO3 was regulated in the same way as for the water

experiment. Experiments were performed as follows: K2CO3 powder were weighted and

mixed in tap water to an estimated concentration of 0,5 mol/l. The solution had a total volume

of 10 l. pH for the liquid solution was measured both at the column inlet and outlet.

23

6.3.3 Test 3: NH3

Experiments with NH3 were performed in a similar way as with pure water and K2CO3

solution. Titration was done to ensure that the concentration determination of ammonia was

correct. Three determinations were made for 2M NH3 at 20 °C, 1M NH3 10 °C and 3M NH3

at 10 °C as well. Results can be found in Appendix B Table A.B.18.

Test 3.1: The first test with ammonia was made with the same conditions as for K2CO3

solution in order to be able to compare their CO2 removal efficiency.

Test 3.2: In order to reduce the consumption of chemicals in the liquid a 5% CO2 gas was

used. This is a low but still relevant CO2 concentration in a flue gas.

Test 3.3: To investigate how the temperature affects the CO2 removal efficiency tests were

made at about 10 °C. All experiments were run with ammonia at 10 °C in the same way as the

experiments at 20 °C.

6.3.4 Test 4: SO2

To investigate the mass transfer coefficient, kG in the gas SO2 was absorbed by K2CO3

solution.

The analyzer for CO2 was replaced by an analyzer which detected SO2 in ppm range 400-

4000 ppm. The liquid solution used was 0.5 M K2CO3. The gas flow was adjusted between

1,8 and 3,6 m3/h by the valve. Liquid flows were varied between 5, 10 and 20 l/h. When the

liquid flow was 5 l/h the gas flow was doubled to 3,6 m3/h to check that the SO2 analyzer

"reacted" accordingly.

24

7. Results

The experiments yielded the following results (see below).

In Figure 7.1 and 7.2 the CO2 absorption efficiency is plotted against the liquid flow, L at 10,

20 and 30 l/h.

The 13% CO2 gas used gave a CO2 removal efficiency of at most 1,2% in H2O, 8% in 0,5M

K2CO3, 12% in 0,5 M NH3 and 23% in 1M NH3. The results are given in Figure 7.1 below

and in Appendix B Table A.B.1-4.

Figure 7.1: 13% CO2 absorbed by H2O, K2CO3 & NH3.

In Figure 7.2 NH3 at 20 ºC is presented in different shades of red while NH3 at 10 ºC is

presented in different shades of blue.

At 20 ºC the 5% CO2 gas gave a CO2 removal efficiency of at most: 22% in 0,5 M NH3, 42%

in 1M NH3, 74% in 2M NH3, 82% in 3M NH3 and 94% in 5M NH3.

At 10 ºC the 5% CO2 gas gave a CO2 removal efficiency of at most: 46% in 0,5 M NH3, 74%

in 1M NH3, 64% in 2M NH3, 76% in 3M NH3 and 92% in 5M NH3.

Results for NH3 at 10 ºC and 20 ºC can be found in Appendix B Table A.B.5-13.

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

ηCO2 %

L (l/h)

13% CO2 G=1,8 m3/h

1M NH3

0,5M NH3

0,5M K2CO3

H2O

25

Figure 7.2: 5% CO2 absorbed by NH3 at 10 ºC & 20 ºC.

Figure 7.3 shows how the CO2 removal efficiency varies with different concentrations of NH3

and CO2 along with different temperatures.

Figure 7.3: CO2 absorption at different NH3 concentration and temperature.

7.1 CO2 absorption by H2O

CO2 absorption into water was also investigated. With a gas consisting of 13% CO2 the

absorption efficiency was only about 1%. Thus, further tests were of low interest. The liquid

temperature was about 20°C. The absorption efficiency plotted against the liquid flow is

shown in Figure 7.4 below.

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

ηCO2 %

L (l/h)

5% CO2 G=1,8 m3/h

5M NH3, 20C

3M NH3, 20C

2M NH3, 20C

1M NH3, 20C

0,5M NH3, 20C

5M NH3, 10C

3M NH3, 10C

2M NH3, 10C

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6

CO2 %

C[NH3] (mol/l)

Conc. NH3, L/G = 11,1

5% CO2, 10C

5% CO2, 20C

13% CO2, 20C

5% CO2, 10C, test 3.a

5% CO2, 20C, test 2.a

26

Figure 7.4: CO2 absorption by H2O.

Test results for CO2 absorption by H2O in absolute numbers are presented in Appendix B

Table A.B.1.

7.2 CO2 absorption by K2CO3 solution

A gas consisting of 13% CO2 was absorbed by a 0,5M solution of K2CO3 and L/G was varied

from 6 to 35 l/m3. The blue curve in Figure 7.5 below shows a liquid flow changing between

10, 20 and 30 l/h while the gas flow remained constant at 1,8 m3/h. The CO2 removal

efficiency then varied between about 0 and 3 %. The red measuring point was obtained when

the liquid flow was 30 l/h and the gas flow was cut by half to 0,9 m3/h (L/G 34 l/m

3 gas). This

resulted in an absorption efficiency of about 8 %.

Figure 7.5: CO2 absorption by carbonate solution.

Test results for CO2 absorption of K2CO3 in absolute numbers are presented in Appendix B

Table A.B.2. The pH range for the test is presented in Appendix B Table A.B.15.

0,00

20,00

40,00

60,00

80,00

100,00

0,00 5,00 10,00 15,00 20,00 25,00 30,00 35,00

CO2 %

L (l/h)

13% CO2 abs. by H2O

G=1,8 (m3/h)

0

20

40

60

80

100

0 10 20 30 40

CO2%

L (l/h)

Abs. 13% CO2 by 0,5M K2CO3

G=1,8 (m3/h)

G=0,9 (m3/h)

27

7.3 CO2 absorption by NH3

7.3.1 13-vol% CO2 absorption by NH3 at 20 °C

Test 1: A 13-vol% CO2 gas was absorbed by a 0.5 M NH3 solution shown in Figure 7.6

below. The CO2 absorption efficiency by NH3 with L/G varying between 6-20 l/m3 were 5-

13%. With an L/G of 35 l/m3 an absorption efficiency of about 26% was measured. 10 l/h

gave a CO2 absorption efficiency of about 7%. The triangular point was measured at the end

of the test and was a repetition of the starting point. This is further analyzed in the discussion

part.

Appendix B Table A.B.3 shows the results for 0,5M NH3 in figures.

Figure 7.6: 13% CO2 absorbed by 0,5M NH3 solution.

CO2 removal efficiency of 1M NH3 for the liquid flow 10, 20 and 30 l/h was 5, 8 and 12%,

respectively. At halved gas flow 26% of the incoming CO2 was absorbed and the temperature

of the liquid was about 20 °C. See Figure 7.7 below.

Figure 7.7: 13% CO2 absorbed by 1M NH3 solution.

Appendix B Table A.B.4 shows the results for 1M NH3 in absolute numbers.

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

CO2 %

L (l/h)

Abs. 13% CO2 by 0,5M NH3

G=1,8 (m3/h)

G=0,9 (m3/h)

Back to start

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

CO2 %

L (l/h)

Abs. 13% CO2 by 1M NH3

G=1,8 (m3/h)

G=0,9 (m3/h)

Back to start

28

7.3.2 5-vol% CO2 absorption by NH3 at 20 °C

Test 2: Gas consisting of 5% CO2 was absorbed by 0,5M NH3 solution at liquid flows of 10;

20 and 30 l/h. This resulted in a CO2 removal efficiency of 16, 18 and 22% for the respective

liquid flow. When gas flow was halved to 0,9 m3/h a CO2 removal efficiency of 42% was

received. Current liquid temperature was about 20°C. Results are shown in Figure 7.8 below.

Figure 7.8: 5% CO2 absorbed by 0,5M NH3 at 20°C.

Test results in figures as well as molar flows in and out of the system are presented in

Appendix B Table A.B.13.

Test 2b1: A gas consisting of 5% CO2 was absorbed by 0,5M NH3 solution at liquid flows of

10, 20 and 30 l/h. That resulted in a CO2 removal efficiency of 38, 44 and 46% for the

respective liquid flow. When the gas flow was halved to 0,9 m3/h a CO2 removal efficiency

of 66% was received. Current liquid temperature was about 20°C. Results are shown in Figure

7.9 below.

Figure 7.9: 5% CO2 absorbed by 1M NH3 at 20°C.

1 Tests 2 & 3 for 1M NH3 were made twice in order to repeat the test series and secure the reliability of result.

Test a was carried out before test b. Test 2.b & 3.b are used for calculations.

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

CO2 %

L (l/h)

Abs. 5% CO2 by 0,5M NH3

G=1,8 (m3/h)

G=0,9 (m3/h)

Back to start

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

CO2 %

L (l/h)


1,8 m3/h

0,9 m3/h

Back to start

29

Test results in absolute numbers as well as molar flows in and out of the system are presented

in Appendix B Table A.B.11. Test 2.a can be found in Appendix B Figure A.B.1 and


A gas consisting of 5% CO2 was absorbed by a 2M NH3 solution resulting in a CO2 removal

efficiency of 70% at the liquid flow 10 l/h. The CO2 removal efficiency was 74% for the

liquid flow 20 l/h as well as for 30 l/h. When gas flow was halved to 0,9 m3/h a CO2 removal

efficiency of 84% was received. Current liquid temperature was about 20°C. Results are






efficiency of 70% at the liquid flow 10 l/h. The CO2 removal efficiency was 74% for the

liquid flow 20 l/h as well as for 30 l/h. When gas flow was halved to 0,9 m3/h a CO2 removal

efficiency of 84% was received. Current liquid temperature was about 20°C. Results are



0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

CO2 %

L (l/h)

5% CO2 abs. by 2M NH3

G=1,8 (m3/h)

G=0,9 (m3/h)

Back to start

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

CO2 %

L (l/h)


G=1,8 (m3/h)

G=0,9 (m3/h)

Back to start

30


in Appendix B Table A.B.7.


efficiency of 94% at the liquid flows 10, 20 and 30 l/h. When gas flow was halved to 0,9 m3/h

a CO2 removal efficiency of 98% was received. Current liquid temperature was about 20°C.

Results are shown in Figure 7.12 below.




7.3.3 5-vol% CO2 absorption by NH3 at 10°C

Test 3.b: The CO2 removal efficiencies of 1M NH3 for the liquid flow 10, 20 and 30 l/h were

44, 50 and 50%, respectively. At halved gas flow 72% of the incoming CO2 was absorbed.

The liquid temperature was about 10 °C. See Figure 7.13 below.


0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

CO2 %

L (l/h)


G=1,8 m3/h

G=0,9 m3/h

Back to start

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40 L (l/h)


G=1,8(m3/h)

G=0,9 (m3/h)

Back to start

31


in Appendix B Table A.B.12. Test 3.a can be found in Appendix B Figure A.B.2 and


The CO2 removal efficiencies of 2M NH3 for the liquid flow 10, 20 and 30 l/h were 62, 62

and 64%, respectively. At halved gas flow 78% of the incoming CO2 was absorbed and the

liquid temperature was about 10 °C. See Figure 7.14 below.




The CO2 removal efficiencies of 3M NH3 for the liquid flow 10, 20 and 30 l/h were 74, 76

and 76%, respectively. At a halved gas flow 90% of the incoming CO2 was absorbed and the

liquid temperature was about 10 °C. See Figure 7.15 below.




0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

CO2 %

L (l/h)


G=1,8 (m3/h)

G=0,9 (m3/h)

Back to start

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40 L (l/h)


G=1,8 (m3/h)

G=0,9 (m3/h)

Back to start

32

CO2 removal efficiency of 5M NH3 for the liquid flow 10, 20 and 30 l/h was 92, 92 and 92%.

At halved gas flow 98% of the incoming CO2 was absorbed and the liquid temperature was

about 10 °C. See Figure 7.16 below.




7.4 SO2 absorption by CO3 solution

A gas consisting of 1800 ppm SO2 was absorbed by 0,5M K2CO3 solution at liquid flows of 5,

10 and 20 l/h. That resulted in a SO2 removal efficiency of about 99% for the respective liquid

flow. When gas flow was doubled to 3,6 m3/h the absorption rate of SO2 was reduced with a

few tenths of a percentage. Current liquid temperature was about 20°C. Results are shown in

Figure 7.17 below.

Figure 7.17: 1800 ppm SO2 absorbed by K2CO3 at 20 ºC.



0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40 L (l/h)


G= 1,8 (m3/h)

G=0,9 (m3/h)

Back to start

0,00

20,00

40,00

60,00

80,00

100,00

0 5 10 15 20 25

SO2 %

L (l/h)

1800 ppm SO2 abs. by 0,5M K2CO3

G=1,8 m3/h

G=3,6 (m3/h)

33

8. Calculations

8.1 Mass balances

According to reaction formula, there are about twice as many moles of NH3 as CO2.

CO2 + 2NH3 + H2O → 2NH4+ + CO3

- (3.0)

Gas flow of 5 volume-% CO2 is known and can be used. Ammonia concentration and liquid

flow in this example are 1 mol/dm3 and 20 l/h.

𝐺𝐶𝑂2 = 𝐺 ∙ 𝑣𝑜𝑙%𝐶𝑂2

100

𝐺𝐶𝑂2 = 1,8 ∙ 0,05 = 0,09 𝑚3/ℎ

CO2 could be considered as an ideal gas at 20 ºC and 1atm pressure. The constant 22.4 l/mol

was used to determine the mole flow.

hmolt

nco /44,22

1009,0

3

2

The CO2 removal efficiency, is known and can be used to determine the absorption mole

rate of CO2.

100

2

,2

,2

CO

inco

absCO

t

n

t

n

Liquid flow, L used in the calculation below is 20 l/h. Mole flow rate of ammonia is

calculated as follows.

)/()/(33

,3lmolhl

tCL

nNHNH

inNH

hmolt

n inNH/120

,3

hmol

t

n absCO/25,2

100

444,2

34

Then the mole flow of NH3 is calculated as follows:

hmolt

nnn

absCOinNH

outNH/2

,2,3

,3

hmolt

n outNH/5,15)225,2(20

,3

The average concentration of NH3 is calculated as follows:

lmol

L

outNHinNH nnC ageaNH

/1,3,3

2var,3

lmolC ageaNH/89,0

20

1

2

5,1520var,3

Appendix B Table A.B.11 provides data for mass balance 1M NH3, 5-vol% CO2 at about 20

°C.

35

8.2 Interfacial area of the packing

Calculations were made to determine the effective gas – liquid interfacial area, av eq. (5.11)

[12].

G is considered to be constant for 5% CO2 gas since pCO2 << ptot. Here ptot = 1 atm. HCO2 is

taken from literature [13], NH3/CO2 reaction rate constant, k is taken from [14] and DCO2 from

[15]. HCO2, k and DCO2 all vary with temperature.

𝑁𝐴(𝑜𝑟 𝑅𝐴) =𝑝𝑡𝑜𝑡∙𝑝𝐶𝑂2 𝑘∙[𝑁𝐻3]𝑚∙𝐷𝐶𝑂2

𝐻𝐶𝑂2∙𝐺 (8.1)

The following estimation can be made for very low partial pressures:

𝑝𝐶𝑂2 ≈𝑝𝐶𝑂 2

1−𝑝𝐶𝑂2

Equation (8.2) is a mass balance in a column cross section with the height dZ.

𝑁𝐴 ∙ 𝑎𝑣 ∙ 𝑑𝑍 = −𝐺∙𝑑𝑝𝐶𝑂2

𝑝𝑡𝑜𝑡 (8.2)

Equation (8.1) and (8.2) and integration give:

𝑎𝑣 ∙ 𝑘 ∙ [𝑁𝐻3]𝑚 ∙ 𝐷𝑐𝑜2 ∙ 𝑍 ∙ 𝑝𝑡𝑜𝑡 = −𝐺 ∙ ln 𝑝𝐶𝑂 2𝑜𝑢𝑡

𝑝𝐶𝑂 2 𝑖𝑛

(8.3)

The CO2 removal efficiency, can be defined as:

= 𝑝𝐶𝑂2𝑖𝑛

− 𝑝𝐶𝑂2𝑜𝑢𝑡

𝑝𝐶𝑂2𝑖𝑛

inserted into eq. (8.3) gives the following expression:

𝑎𝑣∙ 𝑘∙[𝑁𝐻3]𝑚 ∙𝐷𝑐𝑜 2∙𝑍∙𝑝𝑡𝑜𝑡

𝐻𝐶𝑂 2∙𝐺= −𝑙𝑛 1 − (8.4)

From this the effective gas – liquid interfacial area, av can be calculated:

𝑎𝑣 = −𝑙𝑛 1− ∙𝐻𝐶𝑂2∙𝐺

𝑘∙[𝑁𝐻3]𝑚 ∙𝐷𝑐𝑜 2∙𝑍∙𝑝𝑡𝑜𝑡 (8.5)

36

By using formula (8.5) av was calculated for different mean concentration of ammonia and

plotted against the liquid flow. The following results were obtained (shown in Figure 8.1

below).

Figure 8.1: Effective gas – liquid interfacial area, av.

The effective gas – liquid areas in figures are presented in Table 8.1 below. A concentration

of 0,5M NH3 at a liquid flow of 10 l/h gave an av value of 0,492 cm2/cm

3.

Table 8.1: Effective gas – liquid interfacial area, av.

av (cm2/cm

3) 0,5M 1M 2M 3M 5M

a1 L=10 0,492 0,780 1,709 1,830 2,436

a2 L=20 0,542 0,996 1,843 1,896 2,388

a3 L=30 0,674 1,047 1,819 1,878 2,372

0,000

0,500

1,000

1,500

2,000

2,500

3,000

0 10 20 30 40

av

L (l/h)

Effective gas – liquid interfacial area

5M NH3

3M NH3

2M NH3

1M NH3

0,5M NH3

37

8.3 Dependency of CO2 removal efficiency on temperature

Equation (8.4) was used in order to determine how the CO2 removal efficiency, teor varies

when changing the temperature:

1 − 𝑒−𝑎𝑣∙ 𝑘∙[𝑁𝐻 3]𝑚 ∙𝐷𝑐𝑜 2 ∙𝑍∙𝑝𝑡𝑜𝑡

𝐻𝐶𝑂2 ∙𝐺 = 𝑡𝑒𝑜𝑟

(8.6)

Eq. (8.6) can be written in the form:

ln(1 − 𝑡𝑒𝑜𝑟

) = −𝑎𝑣∙ 𝑘∙𝐷𝑐𝑜 2∙𝑍∙𝑝𝑡𝑜𝑡

𝐻𝐶𝑂2 ∙𝐺 (8.7)

av, Z, ptot, [NH3] and G are parameters which are not affected by the temperature. These can be

separated from the formula (8.7) and set to a new constant Y to simplify calculation. After

further simplification we get the following formula:


) = − 𝑘 ∙ 𝐷𝑐𝑜2

𝐻𝐶𝑂2∙ 𝑌

Where Y can be calculated as:


)

− 𝑘 ∙ 𝐷𝑐𝑜2

𝐻𝐶𝑂2

= 𝑌

Now, the theoretical efficiency can be analyzed with the change of temperature by keeping

the value of, Y constant in formula (8.8) below.

𝑡𝑒𝑜𝑟

= 1 − 𝑒− 𝑘∙𝐷𝑐𝑜 2𝐻𝐶𝑂2

∙𝑌 (8.8)

The temperature dependent variables are k, HCO2, DCO2. Table (8.2) shows how the values of

HCO2, K and DCO2 vary with temperatures 5, 10, 15, 20, 30 and 40 ºC.

Table 8.2: Values of HCO2, K and DCO2 vary with temperature.

T (K) H (cm3*atm)/mol D (cm

2/s) k (liter/(mol*s))

278 16000 9,8E-06 107

283 18900 1,2E-05 155

288 22000 1,5E-05 221

293 26000 1,7E-05 313

303 35000 2,2E-05 603

313 45000 2,6E-05 1114

38

Formula (8.8) was used together with values from Table 8.2 resulting in Figure 8.2 below

where the theoretical capture in relation to the temperature is described. The graph shows that

the CO2 capture rate decreases when the temperature decreases. The starting points are at

20°C and the CO2 removal efficiency is set to 70, 80 and 90%. Thereafter the temperature

dependent parameters are changed.

Figure 8.2: Theoretical CO2 removal efficiency in relation to temperature.

0,0

10,0

20,0

30,0

40,0

50,0

60,0

70,0

80,0

90,0

100,0

0 10 20 30 40 50

teor.

t (°C)

changing with T

abs. 90% at 20C

abs. 80% at 20C

abs. 70% at 20C

39

8.4 Removal efficiency of CO2 versus ammonia concentration

The relationship between CO2 removal efficiency and the average ammonia concentration

was determined during the following conditions.

At a temperature of 20 ºC, L/G was 6, 11 and 17 l/m3 at constant gas flow of 1,8 m

3/h. A 5 %

CO2 gas was used and the average liquid concentration of ammonia varied according to Table

A.B.13, 11, 9, 7 and 5. Values for the CO2 removal efficiency were measured according to

previously mentioned tables.

Equation (8.4) can be written:

−𝑙𝑛 1 − =𝑎𝑣 ∙ 𝑘 ∙ 𝐷𝑐𝑜2 ∙ 𝑍 ∙ 𝑝𝑡𝑜𝑡

𝐻𝐶𝑂2 ∙ 𝐺∙ [𝑁𝐻3]𝑚

Equation (8.4) could then be used since –ln(1-) on the right side of the equation was known.

On the left side of the equation all parameters were constant at 20 ºC, except the average

concentration of ammonia. The relation between the CO2 removal efficiency and the average

ammonia concentration shown in Figure 8.3 could then be plotted.

Figure 8.3: CO2 removal efficiency as a function of the root of the average ammonia concentration.

0,00

0,50

1,00

1,50

2,00

2,50

3,00

0,00 0,50 1,00 1,50 2,00 2,50

-ln(1-)

([NH3] m)0,5

Operating line at 20 °C

L/G= 6

L/G 11

L/G 17

40

9. Discussion

The CO2 absorption in water was low, which is consistent to the theory presented in Section

4.1.

Absorption of CO2 with potassium carbonate was about four times lower than with 0.5 M NH3

at the same conditions. This shows that the reaction is very slow and that the residence time

between gas and liquid is too short. This means that K2CO3 is not suitable for CO2 absorption

in this type of process, and also probably does not significantly contribute in a carbonated

ammonia solution.

The concentration of NH3 in the absorbent had a very significant effect on the CO2 removal

efficiency. The CO2 absorption varied between 16 to 94%, simply by increasing the NH3

concentration from 0,5M to 5M. Nevertheless, the CO2 removal efficiency will never become

higher than those 99% achieved in the test with SO2 (Section 7.4) as the gas film will be

limiting at that level.

The CO2 removal efficiency was generally higher at 20 °C than 10 °C. Therefore it was

unexpected that 1M NH3 at 10 °C gave a higher CO2 removal efficiency than 1M NH3 at 20

°C. These tests were done twice and gave the same magnitude of the results both times.

Effective gas – liquid interfacial area, av – varies with the concentration of NH3 which is

surprising. At an increased liquid flow av remains almost constant. The maximum wetting of

the column was possibly already achieved at the liquid flow 10 l/h. An increased flow would

then only give a marginal change of the effective gas – liquid interfacial area.

The tests started at a particular measuring point at a given gas and liquid flow. The flows were

changed and then finally returned to the first measuring point again. The CO2 removal

efficiency was almost the same at both the start and end point. This increases the credibility of

results as being fairly accurate.

From the beginning there were some problems with the distribution of the liquid solution in

the cup at the column top. The holes in the cup were improved in order to get a better liquid

flow through the holes.

Liquid accumulation arose in the column bottom at an early stage. The holes in the column’s

bottom support plate were too small causing an accumulation of the liquid. This is a well-

known problem in column installations and was resolved by making the holes bigger.

41

10. Conclusions

We can first conclude that water is a poor CO2 absorbent. This is mainly because water has

low solubility for CO2 and also reacts slowly to hydrolyze CO2. The mass transfer takes place

through physical absorption.

When absorbing CO2 with K2CO3 solution a slightly better CO2 removal efficiency than that

of water was obtained. This is due to the slow chemical reaction between CO2 and carbonate.

Ammonia reacts rapidly with CO2, which means that ammonia is a good CO2 absorbent. The

challenge is to find optimal operating conditions for the CO2 absorption in a complete

absorption/stripping process.

The concentration of NH3 in the absorbent had a very significant effect on the CO2 removal.

For CO2 absorption into the tested NH3 solutions the CO2 removal could be varied in the

range of 16 to 94%, by increasing theNH3 concentration from 0,5M to 5M.

The impact of the liquid flow on the CO2 removal efficiency is diminished when there is an

excess of NH3 in relation to CO2.

In general the liquid flow rate only had a small influence on the CO2 removal. This seems to

be due to the effective gas – liquid interfacial area (av) – almost remaining constant when

increasing the liquid flow rate.

Most experiments were carried out at 20°C. When reducing the temperature to 10 ˚C, the

removal of CO2 was reduced by 3 to 13% units in NH3 solution of higher concentration then

2M. This is in accordance with the presented theory.

A good correlation of experimental CO2 absorption data to the accepted theory for mass

transfer accompanied by a fast chemical reaction was possible. This enables good modeling of

the process and accurate prediction when changing parameter values.

42

11. Further work

Process liquids: Determine CO2 removal efficiencies for different NH3-solutions

containing typical process concentrations of carbonate and carbamate.

Further tests with 13% CO2 gas with a higher NH3 concentration (2 to 5M). This is in

order to see how the CO2 removal efficiency changes with an excess of NH3.

Repeat some of the previous tests to gain a greater accuracy.

Test different column packing material.

Chemical analysis of the outgoing liquid from the absorber.

Perform physical tests on NH3-solution, such as viscosity and surface tension to find

influence on packing wetting (a).

Examine the advantages and disadvantages of the CO2 absorption at high and low

temperatures, from a technical and economic perspective.

For example investigate how much more NH3 departs through the outgoing gas at high

temperatures and compare it with the NH3 departure at low temperatures. Then take

the improved CO2 removal efficiency (in comparison with NH3 content lost to the

outlet gas) and investigate the entailed costs.

Different bed height for the packing material could be used in order to optimize the

CO2 removal efficiency.

Develop optimal operating data for the system based on calculations and test results.

Make absorption tests at two to three more temperatures and compare experimental

results with theoretical (10, 20, 30 and 40 °C) for 3 and 5 M NH3-solutions.

Investigate the removal efficiency (η) as a function of the CO2-concentration in the

range 5 to 15 % CO2.

Change dimensions of the column to achieve a greater difference of wetting when the

flow increases.

43

12. Nomenclature

av effective gas – liquid interfacial area, (cm2/cm

3)

DCO2 diffusion constant, (cm2/s)

E chemical amplification

G total gas flow, (m3/h) and mol/(cm

2·s)

HCO2 Henry´s constant for CO2

k reaction rate constant, l/(mol·s)

KG total mass transfer coefficient, mol/(cm2·s·atm)

kG gas side mass transfer coefficient, mol/(cm2·s·atm)

kL liquid side mass transfer coefficient, mol/(cm2·s·atm)

NA (or RA) specific absorption rate, mol/(cm2·s)

L liquid flow, l/h

L/G relationship between gas and liquid flow, l/m3

ptot atmospheric pressure, atm

t time, s

T temperature, ºC and K

Z column height, cm

[NH3]m average concentration of NH3, mol/l

η CO2 removal efficiency

44

References

[1] International Energy Agency. 2009. Cleaner Fossil Fuels. (09.04.10)

http://www.iea.org/Textbase/subjectqueries/keyresult.asp?KEYWORD_ID=4152

[2] Davidson, Ogunlade et.al. 2005. Carbon Dioxide Capture and Storage. Cambridge

University Press, New York, USA.

ISBN-13 978-0-521-86643-9

[3] IEA Greenhouse R&D Gas Programme 2007.

Storing CO2 underground. Stoke Orchard, Cheltenham, UK. ISBN: 978-1-898373-36-0

[4] Chemical looping (09.05.23)

http://2.gvt0.com/vi/Z0lLl1svrLI/0.jpg

[5] Danckwerts, P. V., 1967: Gas-Liquid Reactions. New York.

[6] W.B.S. Newling et al. The partial removal of oxides of carbon from flue gases. I.G.E.

Journal-March. I963.

[7] Hatch Jr., T. F. & Pigford, R. L. Simultaneous absorption of carbon dioxide and ammonia

in water. I & EC Fundamentals, 1. (1962) 209.

[8] McCabe, W. L. & Smith, J., Unit Operations of Chemical Engineering. McGraw-Hill.

1956

[9] Bengtsson, S., Växjö University. Personal Communication. 09.05.13

[10] Levenspiel, D.: Chemical Reaction Engineerring. John Wiley & Sons. 1972

[11] Bjerle, I., Berggren, J.C., Karlsson, H.: Kemisk processteknik - del 2: Heterogena

reaktionssystem av typ gas - vätskefas , Kemisk Tidskrift, 90 (12), 30, 1978

[12] Pei Cheng Luo, et al; Ind. Eng. Chem. Res. 42. 4861-4866 (2003)

[13] Perry. Chemical Engineer's Handbook, McGraw-Hill, 5th edition.1973

[14] Pinsent, B.R.W, et al; Trans. Faraday Soc. 52. 1512 (1956)

[15] Wales, C.E. A.I.Ch.E.Journal, 12. 1166 (1966)

http://www.iea.org/Textbase/subjectqueries/keyresult.asp?KEYWORD_ID=4152

http://2.gvt0.com/vi/Z0lLl1svrLI/0.jpg

Appendix A

Figure A.A.1: Liquid flow meter calibration curve.

Table A.A.1: Liquid flowmeter calibration values.

x-axis y-axis

Measurement Liquid flow [l/h] Gradation

1 5,4 6,2

2 7,2 7

3 7,8 7,4

4 15,6 12,3

5 19,2 14,0

6 26,4 18,2

Appendix B Table A.B.1: 13% CO2 absorbed by H2O at 20°C.

Test H2O G [m3/h] L [l/h] mol-% CO2 in mol-% CO2 out % Abs.eff

1 1.8 10 13 13.00 0.0

2 1.8 20.00 13 12.90 0.8

3 1.8 31.14 13 12.85 1.2

Table A.B.2: 13% CO2 absorbed by 0,5M K2CO3 at 20°C.

G [m3/h] L [l/h] % CO2 in % CO2 out L/G % Abs. Eff. mol/h CO2 in mol/h CO2 out

1,8 10 13,0 12,9 5,6 0,2 10,4 10,4

1,8 20 13,0 12,7 11,1 2,3 10,4 10,2

1,8 31 13,0 12,6 17,3 3,1 5,2 5,1

0,9 31 13,0 12,0 34,6 7,7 10,4 9,6

Table A.B.3: 13% CO2 absorbed by 0,5M NH3 at 20°C.

G [m

3/h] L [l/h] % CO2 in % CO2 out L/G % Abs.eff mol/h CO2 in mol/h CO2 out

1,8 10 13,00 12,4 5,6 4,62 10,4 9,96

1,8 20 13,00 11,9 11,1 8,46 10,4 9,56

1,8 31 13,00 11,4 17,2 12,31 10,4 9,16

0,9 31 13,00 9,6 34,4 26,15 5,22 3,86

1,8 10 13,00 12,1 5,6 6,92 10,4 9,72

Table A.B.4: 13% CO2 absorbed by 1M NH3 at 20°C.

G [m3/h] L [l/h] % CO2 in % CO2 out L/G % Abs.eff mol/h CO2 in mol/h CO2 out

1,8 10 13,00 11,3 5,6 13,08 10,4 9,08

1,8 20 13,00 10,3 11,1 20,77 10,4 8,28

1,8 31 13,00 10,0 17,2 23,08 10,4 8,04

0,9 31 13,00 7,3 34,4 43,85 5,22 2,93

1,8 10 13,00 11,1 5,6 14,62 10,4 8,92

Table A.B.5: Mass balance 5M NH3, CO2 5-vol% ~20 °C.

L (l/h) Gtot

(m3/h)

G CO2

(m3/h)

n CO2 in

(mol/h) n CO2 out

(mol/h) n NH3 in

(mol/h) n NH3 out

(mol/h) η(%) n CO2 reacting

(mol/h) n NH3 reacting

(mol/h) C NH3 average

(mol/l)

10 1,8 0,09 4,02 0,24 50 42,45 94 3,78 7,55 4,622

20 1,8 0,09 4,02 0,24 100 92,45 94 3,78 7,55 4,811

30 1,8 0,09 4,02 0,24 150 142,45 94 3,78 7,55 4,874

30 0,9 0,045 2,01 0,04 150 146,06 98 1,97 3,94 4,934


L (l/h) Gtot

(m3/h)

G CO2

(m3/h)

n CO2 in

(mol/h) n CO2 out

(mol/h) n NH3 in

(mol/h) n NH3 out




(mol/l)

10 1,8 0,09 4,02 0,32 50 42,61 92 3,70 7,39 4,630

20 1,8 0,09 4,02 0,32 100 92,61 92 3,70 7,39 4,815

30 1,8 0,09 4,02 0,32 150 142,61 92 3,70 7,39 4,877

30 0,9 0,045 2,01 0,04 150 146,06 98 1,97 3,94 4,934


L (l/h) Gtot

(m3/h)

GCO2

(m3/h)

n CO2 in

(mol/h) n CO2 out

(mol/h) n NH3 in

(mol/h) n NH3 out




(mol/l)

10 1,8 0,09 4,02 0,80 30 23,57 80 3,21 6,43 2,679

20 1,8 0,09 4,02 0,72 60 53,41 82 3,29 6,59 2,835

30 1,8 0,09 4,02 0,72 90 83,41 82 3,29 6,59 2,890

30 0,9 0,045 2,01 0,16 90 86,30 92 1,85 3,70 2,938


L (l/h) Gtot

(m3/h)

G CO2

(m3/h)

n CO2 in

(mol/h) n CO2 out

(mol/h) n NH3 in

(mol/h) n NH3 out




(mol/l)

10 1,8 0,09 4,02 1,04 30 24,05 74 2,97 5,95 2,703

20 1,8 0,09 4,02 0,96 60 53,89 76 3,05 6,11 2,847

30 1,8 0,09 4,02 0,96 90 83,89 76 3,05 6,11 2,898

30 0,9 0,045 2,01 0,20 90 86,38 90 1,81 3,62 2,940


L (l/h) Gtot

(m3/h)

G CO2

(m3/h)

n CO2 in

(mol/h) n CO2 out

(mol/h) n NH3 in

(mol/h) n NH3 out




(mol/l)

10 1,8 0,09 4,02 1,21 20 14,38 70 2,81 5,63 1,719

20 1,8 0,09 4,02 1,04 40 34,05 74 2,97 5,95 1,851

30 1,8 0,09 4,02 1,04 60 54,05 74 2,97 5,95 1,901

30 0,9 0,045 2,01 0,32 60 56,63 84 1,69 3,38 1,944


L (l/h) Gtot

(m3/h)

G CO2

(m3/h)

n CO2 in

(mol/h) n CO2 out

(mol/h) n NH3 in

(mol/h) n NH3 out




(mol/l)

10 1,8 0,09 4,02 1,53 20 15,02 62 2,49 4,98 1,751

20 1,8 0,09 4,02 1,53 40 35,02 62 2,49 4,98 1,875

30 1,8 0,09 4,02 1,45 60 54,86 64 2,57 5,14 1,914

30 0,9 0,045 2,01 0,44 60 56,87 78 1,57 3,13 1,948

Table A.B.11: Mass balance 1M NH3, CO2 5-vol% ~20 °.

L (l/h) Gtot

(m3/h)

G CO2

(m3/h)

n CO2 in

(mol/h) n CO2 out

(mol/h) n NH3 in

(mol/h) n NH3 out




(mol/l)

10 1,8 0,09 4,02 2,49 10 6,95 38 1,53 3,05 0,847

20 1,8 0,09 4,02 2,25 20 16,46 44 1,77 3,54 0,912

30 1,8 0,09 4,02 2,17 30 26,30 46 1,85 3,70 0,938

30 0,9 0,045 2,01 0,68 30 27,35 66 1,33 2,65 0,956

Table A.B.12: Mass balance 1M NH3, CO2 5-vol% ~10 °.

L (l/h) Gtot

(m3/h)

G CO2

(m3/h)

n CO2 in

(mol/h) n CO2 out

(mol/h) n NH3 in

(mol/h) n NH3 out




(mol/l)

10 1,8 0,09 4,02 2,25 10 6,46 44 1,77 3,54 0,823

20 1,8 0,09 4,02 2,01 20 15,98 50 2,01 4,02 0,900

30 1,8 0,09 4,02 2,01 30 25,98 50 2,01 4,02 0,933

30 0,9 0,045 2,01 0,56 30 27,11 72 1,45 2,89 0,952

Table A.B.13: Mass balance 0,5M NH3, CO2 5-vol% ~20 °.

Gtot

(m3/h)

G CO2

(m3/h)

n CO2 in

(mol/h) n CO2 out

(mol/h) n NH3 in

(mol/h) n NH3 out




(mol/l)

10 1,8 0,09 4,02 3,38 5 3,71 16 0,64 1,29 0,436

20 1,8 0,09 4,02 3,29 10 8,55 18 0,72 1,45 0,464

30 1,8 0,09 4,02 3,13 15 13,23 22 0,88 1,77 0,471

30 0,9 0,045 2,01 1,17 15 13,31 42 0,84 1,69 0,472

Test 2a: Gas consisting of 5% CO2 was absorbed by a 1M NH3 solution resulting in a CO2 removal efficiency of 32, 40 and 41%. When gas flow

was halved to 0,9 m3/h a CO2 removal efficiency of 62% was received. Current liquid temperature was about 20°C. Results are shown in Figure

A.B.1 below.

Figure A.B.1: 5% CO2 absorbed by 1M NH3 at 20°C..

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

CO2 %

L (l/h)


G=1,8 (m3/h)

G=0,9 (m3/h)

Back to start

Test results in absolute numbers as well as molar flows in and out of the system are presented in Appendix B Table A.B.14 below



1,8 10 5,00 3,4 5,6 32,00 4,0 2,7

1,8 20 5,00 3,0 11,1 40,00 4,0 2,4

1,8 31 5,00 2,9 17,2 42,00 4,0 2,3

0,9 31 5,00 1,9 34,4 62,00 2,0 0,8

1,8 10 5,00 3,4 5,6 32,00 4,0 2,7

Test 3a: Gas consisting of 5% CO2 was absorbed by a 1M NH3 solution resulting in a CO2 removal efficiency of 32, 44 and 46%. When gas flow

was halved to 0,9 m3/h a CO2 removal efficiency of 66% was received. Current liquid temperature was about 10°C. Results are shown in Figure

A.B.2 below.

Figure A.B.2: 5% CO2 absorbed by 1M NH3 at 10°C.

Test results in absolute numbers as well as molar flows in and out of the system are presented in Appendix B Table A.B.14, see below.

0,00

20,00

40,00

60,00

80,00

100,00

0 10 20 30 40

CO2 %

L (l/h)

Abs. 5% CO2 by 1M NH3 10 C

G=1,8 (m3/h)

G=0,9 (m3/h)

Back to start

52



1,8 10 5,00 3,4 5,6 32,00 4,0 2,7

1,8 20 5,00 2,8 11,1 44,00 4,0 2,3

1,8 31 5,00 2,7 17,2 46,00 4,0 2,2

0,9 31 5,00 1,7 34,4 66,00 2,0 0,7

1,8 10 5,00 3,2 5,6 36,00 4,0 2,6

Table A.B.16: 1800 ppm SO2 absorbed by K2CO3 at 20 ºC.

G [m3/h] L [l/h] ppm SO2 in ppm SO2 out L/G % Abs.eff

1,8 10 1800 8,0 5,6 99,56

1,8 20 1800 4,0 11,1 99,78

1,8 5 1800 8,0 2,8 99,56

3,6 5 1800 12,0 2,8 99,33

Table A.B.17: K2CO3 test.

Units Values

M(K2CO3) [g/mol] 138,9

C(K2CO3) [mol/l] 0,5

Density(K2CO3)[g/l] 69,5

V(solution) [l] 10

m(K2CO3) [g] 694,5

t (ºC) 19,2

pH (tap water) 8,2

pH (K2CO3 before) 11,55

pH (K2CO3 after) 11,05

Table A.B.18: Titration results for NH3 solution.

Cteor NH3 (mol/l) Creal NH3 (mol/l) Temp. (ºC)

1 1,07 10

2 2,01 20

3 3,03 10

Figure A.B.3: Experimental apparatus.

Institutionen för teknik och design 351 95 Växjö

tel 0470-70 80 00, fax 0470-76 85 40

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