Absorption of CO

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Institutionen för teknik och design, TD

Absorption of CO 2 - 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.

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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.

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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, K_G; 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.

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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.

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3. CO

2

-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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10 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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11 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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12 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].

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4. CO

2

-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)

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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^-isabout 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.

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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 (N_t) 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].

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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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17 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].

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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·k_L· a_v), 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

𝐾_𝐺∙𝑎_𝑣

=

_𝑘 ¹

𝐺∙𝑎_𝑣

+

_𝐸∙𝑘^𝐻

𝐿∙𝑎_𝑣 ^(5.0)

This can be simplified to 1

𝐾_𝐺

=

_𝑘¹

𝐺

+

_𝐸∙𝑘^𝐻

𝐿 (5.1)

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

𝐸 =

^{𝑘∙[𝑁𝐻}³^]∙𝐷

𝑘_𝐿 (5.2)

(5.2) inserted into (5.0) gives:

1

𝐾𝐺∙𝑎_𝑣

=

¹

𝑘𝐺∙𝑎_𝑣

+

𝑘∙[𝑁𝐻 3]∙𝐷^𝐻

𝑘𝐿 ∙𝑘𝐿∙𝑎_𝑣

↔

¹

𝐾𝐺∙𝑎_𝑣

=

¹

𝑘𝐺∙𝑎_𝑣

+

_𝑎 ^𝐻

𝑣∙ 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

=

_𝑎 ^𝐻

𝑣∙ 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

=

_𝑘 ¹

𝐺∙𝑎_𝑣

↔ 𝐾

_𝐺

∙ 𝑎

_𝑣

= 𝑘

_𝐺

∙ 𝑎

_𝑣

^(5.5)

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

𝑁_𝐴

=

𝑘

_𝐺

∙ 𝑎

_𝑣

∙ 𝑝

_𝑆𝑂

2

^(5.6)

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

𝑁 ∙

_𝐴

𝑎

_𝑣

∙ 𝑑𝑍 = −

_𝑝^𝐺

𝑡𝑜𝑡

∙ 𝑑𝑝

_𝑆𝑂₂

^(5.7)

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

𝑘_𝐺

∙ 𝑎

𝑣

∙ 𝑑𝑍

= −_𝑝^𝐺

𝑡𝑜𝑡

∙

^𝑑𝑝_𝑝^𝑆𝑂2

𝑆𝑂2

^(5.8)

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

𝑘

_𝐺

∙ 𝑎

_𝑣

𝑑𝑍

₀^𝑧

= −

_𝑝^𝐺

𝑡𝑜𝑡

𝑑𝑝_{𝑆𝑂 2} 𝑝_{𝑆𝑂 2} 𝑜𝑢𝑡

𝑖𝑛 (5.9)

Finally Equation (5.10) is obtained

𝑘

_𝐺

∙ 𝑎

_𝑣

=

_𝑝 ^𝐺

𝑡𝑜𝑡∙𝑍

∙ 𝑙𝑛

_𝑝^𝑝^{𝑆𝑂 2,𝑖𝑛}

𝑆𝑂 2,𝑜𝑢𝑡 (5.10)

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

gives:

𝐾

_𝐺

∙ 𝑎

_𝑣

⁼

^{𝐺 𝑘𝑁𝐻}_𝑝 ³^𝐷^{𝐶𝑂 2}

𝑡𝑜𝑡∙𝑍

∙ 𝑙𝑛

^𝑝^{𝐶𝑂 2,𝑖𝑛}

𝑝_{𝐶𝑂 2,𝑜𝑢𝑡}

(5.11)

See further Equation (8.2) to (8.4).

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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.

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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.

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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)

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 CO₂concentration 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)

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

η

CO2

%

L (l/h)

13% CO

²

G=1,8 m

³

/h

1M NH3 0,5M NH3 0,5M K2CO3 H2O

(25)

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

η

CO2

%

L (l/h)

5% CO

2

G=1,8 m

³

/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. NH

3

, 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)

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/m³ 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% CO

₂

abs. by H

₂

O

G=1,8 (m3/h)

0 20 40 60 80 100

0 10 20 30 40



^CO2%

L (l/h)

Abs. 13% CO

2

by 0,5M K

2

CO

3

G=1,8 (m3/h) G=0,9 (m3/h)

(27)

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% CO

₂

by 0,5M NH

₃

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% CO

2

by 1M NH

3

(28)

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% CO

₂

by 0,5M NH

₃

0,00 20,00 40,00 60,00 80,00 100,00

0 10 20 30 40



CO2

%

L (l/h)

Abs. 5% CO

₂

by 1M NH3

1,8 m3/h 0,9 m3/h Back to start

(29)

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

Appendix B Table A.B.14.

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 shown in Figure 7.10 below.

A gas consisting of 5% CO₂ was absorbed by a 3M 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 shown in Figure 7.11 below.

0,00 20,00 40,00 60,00 80,00 100,00

0 10 20 30 40



^{CO2 %}

L (l/h)

5% CO

₂

abs. by 2M NH

₃

0,00 20,00 40,00 60,00 80,00 100,00

0 10 20 30 40



CO2 %

L (l/h)

5% CO

₂

abs. by 3M NH

₃

(30)

30 Test results in absolute numbers as well as molar flows in and out of the system are presented in Appendix B Table A.B.7.

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

Test results in absolute numbers as well as molar flows in and out of the system are presented in Appendix B Table A.B.5.

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

CO

2

%

L (l/h)

5% CO

₂

abs. by 5M NH

₃

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)

5% CO

₂

abs. by 1M NH

₃

G=1,8(m3/h) G=0,9 (m3/h) Back to start

(31)

31 Test results in absolute numbers as well as molar flows in and out of the system are presented in Appendix B Table A.B.12. Test 3.a can be found in Appendix B Figure A.B.2 and

Appendix B Table A.B.15.

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)

5% CO

₂

abs. by 2M NH

₃

0,00 20,00 40,00 60,00 80,00 100,00

0 10 20 30 40 L (l/h)

5% CO

₂

abs. by 3M NH

₃

(32)

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)

5% CO

₂

abs.

^by

5M NH

₃

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 SO

₂

abs. by 0,5M K

₂

CO

₃

G=1,8 m3/h G=3,6 (m3/h)

(33)

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.

𝐺_𝐶𝑂2 = 𝐺 ∙ 𝑣𝑜𝑙%_𝐶𝑂2 100

𝐺_𝐶𝑂2 = 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.

h t mol

n_co

/ 4 4

, 22 09 10 , 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

in co

abs CO

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.

) / ( )

/

( 3

3 ,

3 l h mol l

t

L C

n

NH NH

in

NH  

h t mol

n

^NH³^,ⁱⁿ _₂₀_₁ _/



 

h t mol

n

^CO ^abs

/ 25 , 100 2

44

, 4

2  





(34)

34 Then the mole flow of NH3 is calculated as follows:

 

^mol ^h

t

n n

n

abs CO in NH out

NH 2 /

, 2 ,

3 ,

3   

h t mol

n

^NH³^,^out _₂₀_₍₂_,₂₅_₂₎_₁₅_,₅ _/



The average concentration of NH3 is calculated as follows:

 

l Lmol out

NH in

NH

n

C

NH a age

n

3, 3, 1 /

var

2

,

3 





l

C

^NH³^,^a^var^age^ ²⁰^₂¹⁵^,⁵^₂₀¹ ^⁰^,⁸⁹mol^/



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

°C.

Absorption of CO

Absorption of CO 2 - by Ammonia

Diploma work

Filip Sjöstrand & Reza Yazdi

Abstract

Foreword

Contents

1. Objective

2. Background

3. CO

-removal techniques

4. CO

-absorption process

5. Procedures for process design of absorption columns

=

+

=

+

𝐸 =

=

+

↔

=

+

=

=

↔ 𝐾

∙ 𝑎

= 𝑘

∙ 𝑎

𝑘

∙ 𝑎

∙ 𝑝

𝑁 ∙

𝑎

∙ 𝑑𝑍 = −

∙ 𝑑𝑝

∙ 𝑎

∙ 𝑑𝑍

∙

𝑘

∙ 𝑎

𝑑𝑍

= −

𝑘

∙ 𝑎

=

∙ 𝑙𝑛

𝐾

∙ 𝑎

=

∙ 𝑙𝑛

6. Laboratory experiment

7. Results

η

%

13% CO

G=1,8 m

/h

η

%

5% CO

G=1,8 m

/h



Conc. NH

, L/G = 11,1



13% CO

abs. by H

O



Abs. 13% CO

by 0,5M K

CO



Abs. 13% CO

by 0,5M NH



Abs. 13% CO

⁼