Scholarly article on topic 'Absorption of carbon dioxide in aqueous ammonia'

Absorption of carbon dioxide in aqueous ammonia Academic research paper on "Chemical engineering"

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Abstract of research paper on Chemical engineering, author of scientific article — Jinzhao Liu, Shujuan Wang, Bo Zhao, Huiling Tong, Changhe Chen

Abstract Aqueous ammonia can be used to capture CO2 from flue gas of coal-fired power plant with quick reaction rate, high removal efficiency, and high loading capacity of CO2. It is thought to be a promising technology. However, the CO2 absorption rate, diffusion rate and VLE (Vapor liquid equilibrium) in aqueous ammonia have still been rarely researched. More research is necessary for these characters of ammonia solution as CO2 absorbent. The paper will focus on the fundamental characteristics of CO2 in aqueous ammonia. A series of tests were conducted in a semi-batch reactor that has been developed in this paper. CO2 removal efficiencies at different concentrations of aqueous ammonia have been studied in the similar operation conditions to compare their basic characters in the CO2 absorption process. And a small wetted wall column (WWC) with a contact area of about 41.45 cm2 was also built for the study of the absorption rate, diffusion, and solubility of carbon dioxide in the aqueous solution. In the paper, reaction rate and overall gas transfer coefficient were studied. The concentration of ammonia ranges from 1, 5, 10 to 15. CO2 and N2 are used to simulate the flue gas. The concentration of CO2 ranges from 5, 10, 15, to 20%. The reaction temperatures are 20 ∘C and 40 ∘C. It is indicated in this paper that the best concentration of aqueous ammonia should be selected from 5% to10%. And aqueous ammonia has a high Flux in WWC, which is three times higher than that of MDEA+PZ, under the same condition. It is also found that K G is appropriate to be used in the mass transfer process of the carbon dioxide absorption by aqueous ammonia. Alls of these experiments established foundation for the further study of CO2 removal by ammonia.

Academic research paper on topic "Absorption of carbon dioxide in aqueous ammonia"

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Energy Procedia

ELSEVIER

Energy Procedia 1 (2009) 933-940

www.elsevier.com/locate/procedia

GHGT-9

Absorption of carbon dioxide in aqueous ammonia

Jinzhao Liu, Shujuan Wang*, Bo Zhao, Huiling Tong, Changhe Chen

Key laboratory for thermal science and power engineering ofMinistry of Education Tsinghua University, Beijing 100084, China

Abstract

Aqueous ammonia can be used to capture CO2 from flue gas of coal-fired power plant with quick reaction rate, high removal efficiency, and high loading capacity of CO2. It is thought to be a promising technology. However, the CO2 absorption rate, diffusion rate and VLE (Vapor liquid equilibrium) in aqueous ammonia have still been rarely researched. More research is necessary for these characters of ammonia solution as CO2 absorbent. The paper will focus on the fundamental characteristics of CO2 in aqueous ammonia.

A series of tests were conducted in a semi-batch reactor that has been developed in this paper. CO2 removal efficiencies at different concentrations of aqueous ammonia have been studied in the similar operation conditions to compare their basic characters in the CO2 absorption process. And a small wetted wall column (WWC) with a contact area of about 41.45 cm2 was also built for the study of the absorption rate, diffusion, and solubility of carbon dioxide in the aqueous solution. In the paper, reaction rate and overall gas transfer coefficient were studied. The concentration of ammonia ranges from 1,5, 10 to 15. CO2 and N2 are used to simulate the flue gas. The concentration of CO2 ranges from 5, 10, 15, to 20%. The reaction temperatures are 20 oC and 40oC. It is indicated in this paper that the best concentration of aqueous ammonia should be selected from 5% to10%. And aqueous ammonia has a high Flux in WWC, which is three times higher than that of MDEA+ PZ, under the same condition. It is also found that KG is appropriate to be used in the mass transfer process of the carbon dioxide absorption by aqueous ammonia. Alls of these experiments established foundation for the further study of CO2 removal by ammonia. © 2009 Elsevier Ltd. All rights reserved.

Keywords: Carbon dioxide; Removal efficiency; Aqueous ammonia; Absorption rate; Mass transfer

Carbon dioxide is the major greenhouse gas in the world that needs to be reduced. There are various technologies used to separate CO2 from flue gas. These include chemical solvent methods, physical absorption methods, cryogenic methods, membrane systems, biological fixation, and the O2/CO2 combustion process.

Compared with the chemical plant, power plant has a large flue gas flow, different ingredients, relatively low CO2 concentration and other characteristics. So the chemical solvent methods are generaslly recognized as the most effective technologies at present. This requires that the researchers developed a relatively low-cost, low-energy requirement CO2 capture technology.

Among the conventional CO2 chemical removal processes, the monoethanolamine (MEA) process has been comprehensively studied and successfully used in chemical plants for CO2 recovery. Although the MEA process is a promising system for the control of CO2 emissions from massive discharging plants, it is an expensive option since the cost of CO2 separation may range from US$40 to 70/ton of CO2 removed [1](Chakma (1995)). In addition, it has several major problems including a slow absorption rate, a small solvent capacity, etc[2]. (Molburg et al. (1994)).

Previous research shows that aqueous ammonia have a higher absorption capacity than that of monoethanolamine (MEA) at same temperatures and pressures. The CO2 removal efficiency of aqueous ammonia can reach 95~99%, even 100%, and MEA is

1. Introduction

* Corresponding author. Tel.: +96-10-62799669; fax: +96-01-62770209. E-mail address: wangshuj@tsinghua.edu.cn.s

doi:10.1016/j.egypro.2009.01.124

generally 90%[3], An absorption capacity of aqueous ammonia can be higher than 1.0kgCO2/kg solvent, and MEA is only 0.36kg CO2/kg MEA[4].

A series of tests was conducted in a semi-batch reactor that has been developed in this paper. CO2 removal efficiencies of aqueous ammonia have been studied. And a small wetted wall column (WWC) was also built for the study of the absorption rate, diffusion rate, and solubility of carbon dioxide in solutions.

2. Experimental apparatus and methods

2.1 semi-batch reactor

First of all, a semi-batch reactor has been used in this paper. The factors influencing CO2 removal efficiency in CO2 loading process has been got in this paper, this is aqueous ammonia concentration. And the ammonia solution have been compared in the similar condition to study their basic characters as loading solution in CO2 loading process, this established foundation for the further study of CO2 loading by aqueous ammonia.

Fig.l.The experiment system of the semi-batch reactor

However many results have been got by semi-batch reactor so far, the principle of the absorption process such as the CO2 | absorption rate, diffusion rate and VLE (Vapor liquid equilibrium) in aqueous ammonia have still been rarely researched. For this reason, a small wetted wall column (WWC) with a contact area of about 41.45 cm2 has been built for the study. This paper focuses on the relevant work of the reactor construction and the system commissioning, and a preliminary measurement of aqueous ammonia and carbon dioxide apparent reaction rate and mass transfer coefficient were also introduced.

These works laid an important foundation for the further study of the aqueous ammonia removal of carbon dioxide removal by aqueous ammonia.

2.2 The wetted-wall column

Mshewa, M. M. (1995) in Austin first built and used a wetted-wall column (WWC), which can hold a pressure from 1to 8 atm to test the carbon dioxide desorption/absorption with aqueous mixtures of methyldiethanolamine (MDEA) and diethanolamine (DEA)[5]. Then a lot of researches have been done on the amines in the same apparatus by Gary.T Rsochelle et al. | There are also some researchers from India[6] (India Institute of Technology) ,Taiwan[7] (Chung Yuan Christan Universi-ty)and Korea[8](Y onsei University) used their WWC to do many experiments to study the carbon dioxide desorption/absorption with a variety of amines.

A wetted wall column with a contact area of about 41.45 cm2 has been built in this paper at The Department of Thermal Engineering in Tsinghua University in China, and the details shows in Fig.2.

The main reactor of the wetted-wall column, depicted in Fig.2 , was used as the gas-liquid contactor throughout the equilibrium and rate experiments. The contactor was constructed from a stainless steel tube with a well-defined surface area (41.45 cm2) and a characteristic liquid film mass transfer coefficient similar to that of a packed tower. The stainless steel wetted-wall, measuring 11.0 cm in height and 1.2 cm in diameter, is a tube extending from the liquid feed line into the column housing. The gas-liquid contact region is enclosed by a 31.0 cm thick-walled glass tube, separating it from a water-bath.

According to the problems of the past wetted-wall column, many of the new design have been used to improve the structure of the reactor. Firstly, Fluent Software is used to simulate the gas flow inside the reactor to find the best way for the inlet gas diffuse-ion. A baffle-board is welded at the gas inlet to change the direction of the gas flow and make the liquid film not be disturbed. This change has been proved by a series of experiments to be necessary. Secondly, to solve the problem of the outlet liquid accumulated at the bottom of the column, a swallow-tailed groove on the bases of the reactor was processed. This can make the gas inlet and the liquid outlet to be completely separated and be non-interfering.

Fig.2. Schematic of the wetted-wall column

2.2The experiment system

The chemical solution of interest is pumped through the inside of the tube, overflows, and is evenly distributed across the outer surface of the tube. After collection at the base of the column, the liquid is pumped back to a liquid reservoir. Gas enters near the base of the column, counter-currently contacting the liquid as it flows up into the gas outlet.

Fig.3. Overall experimental flowsheet of the wetted-wall column

The water/oil-bath, with circulation of the water/oil inside, is used to control the temperature of the inlet gas, liquid and the reactor. Two reservoirs are used in this system, one of them is used for the amine solution storage, and another one is used to holdup the waste solution out of the reactor. A Cole-Parmer micro-pump pushes the solution from the reservoir through a coil submerged in the water/oil-bath, flowing through a rotameter for flowrate determination. The liquid flow rate was 2-3 cm3/s. After heating, the solution flows into the wetted-wall column. After contacting the gas stream, the solvent returns to the reservoir.

The total pressure used in this work was around 1 to 2 atm. To minimize gas film resistance, the gas flow rate was approximately 4.5 to 5.5 SLPM. A 10 SLPM mass flow controller is used to meter nitrogesn flow. The carbon dioxide flowrate is metered with a 1 SLPM mass flow controller in this paper. Other 3 mass flow controllers of CO2 are 0-5ml, 0-50ml and 0-0.2SLPM. All the valves in the gas-line are used to adjust the pressure in the reactor from 1to100Psi. The metered gases were mixed prior to entering the wetted-wall column. After exiting the contactor, excess water was removed by passing the stream through a condenser. A drying column filled with Silica was used to remove the remaining moisture. A carbon dioxide analyzer was used to measure the CO2 concentration of the dry gas with infrared spectroscopy.

3. Theory

3.1 Theory ofNH3-CO2 Reactions

Possible reactions between ammonia and CO2 are as following[9-13] (Brooks and Audrieth (1946), Brooks (1953), Hatch and Pigford (1962), Shale et al. (1971), Koutinas et al. (1983)):

CO2 (g)+ 2NH3(g) ^ NH2COONH4(s)

NH2COONH4(s)+ H2O(g) ^ (NH4)2 CO3(s) (2)

CO2(g)+ 2NH3(g) ~ CO(NH2)2(s)+H2O(g) (3)

CO2(g)+ 2NH3(aq) ^ NH2COO"(aq)+NH4+(aq) (4)

2NH3(g) + CO2(g)+ H2O(g) ~ (NH4)2 CO3(s) (5)

NH3(g) + CO2 (g)+ H2O(g) ~ NH4HCO3(s) (6)

2NH3(l) + CO2 (g)+ H2O(l) ~ (NH4)2CO3(s) (7)

NH3(l) + CO2(g)+ H2O(l) ^ NH4HCO3(s) (8)

The above reactions proceed at various temperatures and operation conditions. Ammonium carbamate (NH2COONH4) is formed by the reaction of carbon dioxide and ammonia in the dry condition under room temperature and a pressure of 1 atm. It is very soluble in water; therefore, under moist air the hydration product of ammonium carbonate ((NH4)2CO3) is produced under room temperature'9' (Brooks and Audrieth (1946)).

At room temperature and atmospheric pressure, reaction equations (4)-(8) also possibly occur. The formation of ammonium (NH4+) and carbamate (NH2COO-) ions is very fast, and reaction equation (4) is irreversible'11' (Hatch and Pigford (1962)), and the reaction equations (5)-(8) are reversible'10,12' (Brooks (1953), Shale et al. (1971)).

The explosive limit for NH3 gas is 15-28% (v/v)'14' (Merck (1996)). Therefore, in order to be safety and simplicity, the wet method is used in this study. Reaction equations (7) and (8) are the most probable in this study for CO2 removal by aqueous ammonia scrubbing. | 3.2 squeous Ammonia and CO2 Reactions

The total reaction of aqueous ammonia absorb carbon dioxide can be described as the equation (9):

CO2 (g)+ NH3 (aq)+ H2O(l) ^ NH4HCO3(aq) (9 )

The actual process of the reaction is more complicated, that can be described as step-by-step reactions.

First of all, the (4) equation (described in 3.1) occur, and CO2 and NH3 react to generate NH2COONH4, then NH2COONH4 | hydrolyzes in solution instantaneous.

CO2 (g)+ 2NH3(aq)^NH2COO"(aq)+NH4+(aq) (4 )

| Then, NH4+and NH2COO"_have an irreversible reaction_(10) in solution:

NH2COO"(aq)+NH4+ (aq)+ 2H2O^NH4HCO3(aq) + NH3 • H2O(aq) (10)

At the same time, the complex balances of solute ionizing and ion reactions are happening in the solution, and the reaction equations are (11)-( 15) as follows:

NH3(aq)+H2O(l)^NH4+(aq)+OH" (aq) (11)

NH4HCO3(aq)^ NH4+(aq) +HCO3"(aq) (12)

(NH4) 2CO3(aq)^2NH4+ (aq)+ CO3 2+(aq) (13)

OH-(aq)+ HCO3"(aq) ^ CO3 2+(aq)+ H2O (14)

CO3 2+(aq) +CO2 (g)+ H2O (l)~ 2 HCO3- (aq) (15)

4. Experimental Section

4.1 CO2 removal efficiency

Bai (1997) indicate a high potential of CO2 scrubbing with a fast absorption rate by the 28% aqueous ammonia'4'. The CO2 removal efficiency may be higher than that by the conventional MEA process (90%) under proper operation conditions.

James T. Yeh et al (NETL,2005) found in the experimental system, the kinetics was the most important factor to control the reaction, and another factor was the impact of the gas-liquid contact. James T. Yeh also indicated that a high concentration of

ammonia should be used. To consider the practical issues such as ammonia volatilization, however, the appropriate concentration must be selected. And a low-temperature reaction was also recommended[15].

... 100%

£ 60%

'4—1

^^ 10%

.........................Ç.Q4.........

1% , J/S

0:28 0:57 Operation t ime

1 : 55

Fig.4. Removal efficiency lines in different concentration of aqueous ammonia in the semi-batch reactor

A semi-batch reactor as the above-mentioned has been used in this paper, 1%,5%,10%and15% concentrations of aqueous ammonia has been compared for the removal efficiency, shows in fig.4.The initial removal efficiency and the duration of above 80% removal efficiency in the same reaction conditions shows in fig.5-6. The experimental conditions were as following : temperature: 20.0°C (room temperature), pressure: 1atm, gas flow: 540 ml / min; and CO2 concentration: 15%.

Fig.5. The duration of above 80% removal efficiency

Concentrations of NH3(Aq)

Fig.6. The initial removal efficiency

4.2 CO2 absorption rate

A number of experiments have been done on different temperatures, concentrations of ammonia and CO2 to test the carbon dioxide absorption with aqueous ammonia in the WWC, and some of the data shows in Table1-3:

Table.l.Absorption of CO2 in 10%NH3(aq),, Total pressure:1.4-1.7 Bar, Liquid flow rate:2-3 cm3/s

CO2(g) Set Inlet Pco2,in Pco2,out Pco2,b FluxXlO7 KGx1010

Concentration (kPa) (kPa) (kPa) (mol/cm2 • s) (mol/pacm2s)

293 5% 7.84 6.58 7.19 7.59 1.054

293 10% 16.64 14.12 15.35 15.01 0.978

293 15% 23.39 20.17 21.74 21.11 0.971

293 20% 31.84 27.04 29.37 32.40 1.103

313 5% 7.74 5.84 6.75 11.76 1.743

313 10% 15.4 11.93 13.61 22.38 1.644

313 15% 22.83 18.27 20.46 30.83 1.507

313 20% 31.31 24.8 27.92 44.87 1.607

Table.2. Absorption of CO2 at 293K, Gas flow rate: 5SLPM/min, Total pressure:1.4-1.7 Bar, Liquid flow rate:2-3 cm3/s

NH3(Aq) concentration CO2(g) Set Inlet Concentration PCO2,IN (kPa) PCO2,OUT (kPa) PCO2,b (kPa) FluxX 107 (mol/cm2 • s) KGx1010 (mol/pacm2s)

1% 10% 15.32 14.74 15.03 3.66 0.243

5% 5% 7.55 6.75 7.14 4.78 0.669

5% 10% 15.82 14.18 14.98 10.47 0.698

5% 15% 26.27 23.84 25.04 14.32 0.572

10% 5% 7.84 6.58 7.19 7.59 1.054

10% 10% 16.64 14.12 15.35 15.01 0.978

10% 15% 23.39 20.17 21.74 21.11 0.971

10% 20% 31.84 27.04 29.37 32.40 1.103

15% 10% 16.85 13.27 14.99 20.86 1.392

Table. 3 Absorption of CO2, C02:10%(set),Gas flow rate: 5SLPM/min, Total pressure: 1.4-1.7 Bar, Liquid flow rate:2-3 cm3/s

T(K) NH3(aq) concentration PCO2,IN (kPa) PCO2,OUT (kPa) PCO2,b (kPa) FluxX 107 (mol/cm2 • s) KGx1010 (mol/pacm2s)

293 1% 15.32 14.74 15.03 3.66 0.243

293 5% 15.82 14.18 14.98 10.47 0.698

293 10% 16.64 14.12 15.35 15.01 0.978

293 15% 16.85 13.27 14.99 20.86 1.392

313 1% 15.39 14.59 14.98 5.26 0.351

313 5% 15.41 12.83 14.08 16.77 1.191

313 10% 15.40 11.93 13.61 22.38 1.644

5. Result and discussion

5.1 Removal efficiency in aqueous ammonia

As shown in Fig.4, absorption capacity can be compared among different concentrations of ammonia.Fig.5 shows that the initial removal efficiency can reach 90% when the concentration of ammonia is higher than 5%, and the initial removal efficiency increases gradually with the increased concentration of ammonia,. As the absorbent, how long can it have a high and stability removal efficiency is to be concerned as an important parameter. And Fig.5 shows the time that the removal efficiency is more than 80% in different concentration of solutions.

Phenomenon of the experiments shows that if the concentration of the aqueous ammonia is more than 15%, a lot of ammonia will volatilize from the solution, and low concentration of ammonia can also have higher rates of removal. Therefore, the concentration of aqueous ammonia should be selected from 5% to10%.

5.2 Absorption rate and mass transfer in WWC

For the Hatta number Ha>2 '16'(Versteeg and van Swaaij,1988), the specific rate becomes:

NCO2 = 'CO2'^DCO, kOV = kOV

HCO2 (16)

N is the same as Flux in all the formulas. The Flux of CO2 into or out of the solution can be characterized by mass transfer coefficients such as the overall gas transfer coefficient.

NCO2 ~ KGPCO2,b (17)

The overall mass transfer coefficient, KG, is calculated as the slope of the flux versus the log mean pressure'17'. P - P

P _ CO2 ,in 1 CO2,out

CO2,b Ln(P / P )

CO2,in CO2,out J (18)

| So the overall gas transfer coefficient KG can be calculated by the Flux and PCO2jn and PCO2out._At the same temperature and concentration of ammonia, the value of KG is fixed and can be calculated by the Flux- CO2 partial pressure line. The KG lines at | 293K in 5%, 10% NH3(aq) and 313K in 10% NH3(aq) shows in Fig. 8. The lines used to calculate and compare the value of KG,

at different temperatures and concentrations of aqueous ammonia. The influence of aqueous ammonia concentration to KG shows inFig.9.

30 20 10

T=313MJH-;{A<|)10 ..........................J^......... KyKc-1.586

^-'f=293|(, N1 lj(Aq)lC %,KG= 1.044

W '^lL........

T=293K,NII3(Aq)* ¡6, =0.609

♦ A KG = Ksxl0lu(mo /pa-cm'-sj

P™.b №Pal

Experim >nt Tempe ature 29c K

_■*■ ' '

...........*

-- - - - - - -¡-

NH,[aq) concentration ( % )

Fig.8. Kg calculated by Flux- CO2 partial pressure lines

Fig.9.KG-NH3(aq)concentration curve at 293K

Fig.8 shows that KG curves have a very good linearity, that indicates it is appropriate to use KG in the mass transfer process of the carbon dioxide absorbed by aqueous ammonia. As shown in Fig.8 and Fig.9 , with the temperature and the concentration of ammonia increased, the mass transfer coefficient KG also increased, which is consistent with the basic principles of the reaction.

However, the curves and the law of KG changing with different temperatures and ammonia concentrations need to be got by further experiments. The mass transfer model and the measurement of some basic mass transfer and kinetic parameters such as H, D, and kov etc. need further research.

Compared with the 0.6M PZ,plus 4M MDEA solution at low loading as 0.095 mol CO2/mol Amine [18](Bishnoi,2000), 10%NH3 (5.8M), have a higher Flux at the same temperature of 313K, that shows in Table4.

Table 4 Flux comparison between NH3(aq) and MDEA+PZ

Temperature AWrhent PCO2,b Flux^*10

_(K)_Absorbent_(kPa)_(mol/cm2 • s)

313 10% NH3 (5.8M) 6.75 11.76

313 MDEA(4M)+PZ(0.6M)[18] 5.25 3.61

It can be observed clearly that the ammonia Flux is three times higher than MDEA + PZ under the same conditions. And further research of aqueous ammonia as CO2 absorbent seems to be possible and necessary.

6. Conclusions

For 5% or higher concentration of ammonia, the initial removal efficiency of CO2 can reach 90%. This shows that low concentration of ammonia can also have a higher rate of removal. Phenomenon of the experiments shows that if the concentration of the aqueous ammonia is more than 15%, a lot of ammonia will volatilize from the solution. Therefore, the concentration of aqueous ammonia should be selected from 5% to10%. Aqueous ammonia has a high Flux in WWC, which is three times higher than that of MDEA+ PZ, under the same conditions.

KG curves got in WWC have a very good linearity, that indicates it is appropriate to use KG in the mass transfer process of the carbon dioxide absorption by aqueous ammonia. However, the curves and the law of KG changing with different temperatures and ammonia concentrations need to be got by further experiments. The mass transfer model and the measurement of some basic mass transfer and kinetic parameters such as HCO2, DCoi, and kov in aqueous ammonia et al need further experiments to study. The paper shows that aqueous ammonia is promising as an absorbent for CO2 removal and needs to be further studied.

Notation

D diffusion coefficient, m2/s

H Henry's law constant, m3Pa/kmol

Nx Flux of species x, kmol/m2 s

Px partial pressure of x, Pa

T temperature, K

KG overall gas transfer coefficient, mol/pa cm2 s

Kqv overall pseudo-first-order reaction rate constant,S-1 Ha Hatta number

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