Experimental Studies of Reboiler Heat Duty for CO2 Desorption from

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Experimental Studies of Reboiler Heat Duty for CO2 Desorption from Triethylenetetramine (TETA) and Triethylenetetramine (TETA) + N-methyldiethanolamine (MDEA) Xiao Luo, KaiYun Fu, Zhen Yang, Hongxia Gao, Wichitpan Rongwong, Zhiwu Liang, and Paitoon Tontiwachwuthikul Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b00158 • Publication Date (Web): 03 Aug 2015 Downloaded from http://pubs.acs.org on August 24, 2015

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Industrial & Engineering Chemistry Research

1

Experimental Studies of Reboiler Heat Duty for CO2 Desorption from

2

Triethylenetetramine (TETA) and Triethylenetetramine (TETA) +

3

N-methyldiethanolamine (MDEA)

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Xiao Luo1#, Kaiyun Fu1#, Zhen Yang1, Hongxia Gao1, Wichitpan Rongwong1, Zhiwu Liang1, 2*,

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Paitoon Tontiwachwuthikul1, 2

6 7

1

Joint International Center for CO2 Capture and Storage (iCCS), Provincial Hunan Key Laboratory

8

for Cost-effective Utilization of Fossil Fuel Aimed at Reducing Carbon-dioxide Emissions, College

9

of Chemistry and Chemical Engineering, Hunan University, Changsha, Hunan, 410082, P.R. China

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2

Clean Energy Technology Institute at the University of Regina, Regina, Saskatchewan, S4S 0A2, Canada

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# Xiao Luo and Kaiyun Fu contributed equally to this work, are the co-first authors.

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*CORRESPONDING AUTHOR: Tel.: +86-13618481627; fax: +86-731-88573033; E-mail address:

18

[email protected] (Z. Liang).

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Abstract

1 2

Amine scrubbing is regarded as one of the most suitable technologies for post-combustion CO2

3

capture. However, data on regeneration energy and performance is limited in literature. In this study,

4

the reboiler heat duties of triethylenetetramine (TETA) and triethylenetetramine (TETA) + N-

5

methyldiethanolamine (MDEA) were experimentally evaluated in a bench-scale stripper packed with

6

Dixon ring random packing. The effects of various operating parameters on the desorption

7

performance (presented in terms of reboiler heat duty, Qreboiler) were investigated, including lean

8

loading, rich loading, amine concentration, solvent flow rate and amine type. The experimental

9

results showed that the Qreboiler was very sensitive to these process parameters. In addition, a

10

comparison of the Qreboiler of TETA, monoethanolamine (MEA) and diethylenetriamine (DETA) was

11

conducted to evaluate the potential for TETA’s application in the CO2 capture process. The results

12

obtained in this work showed that the Qreboiler of TETA, as a function of amount of CO2 released, is

13

lower than that of MEA and DETA.

14

Keywords: Regeneration, reboiler heat duty, triethylenetetramine, N-methyldiethanolamine

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1. Introduction

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The emission of carbon dioxide (CO2), a major contributor to global warming, has become a

3

severe environmental issue internationally. As a result, the research and development of technology

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options to reduce CO2 emissions have become a priority. Amine scrubbing is regarded as one of the

5

most suitable technologies for post-combustion CO2 capture. However, the fact that energy

6

consumption for solvent regeneration accounts for almost 80% of the operating cost, is the most

7

important problem to be solved for the amine scrubbing process.1

8

Monoethanolamine (MEA) is the most commonly used amine because of its fast reaction rate

9

for CO2 absorption and low price. However, its advantages are offset by the low absorption capacity

10

and relatively high energy demand for regeneration.2 Therefore, the investigation of new solvents as

11

alternatives to MEA that require less energy for regeneration concurrently possess good absorption

12

performance has become a priority for scholars and engineer.

13

Polyamines are a category of potential amine solvents. They exhibit high absorption capacity

14

and kinetics and are expected to be potential absorbents for CO2 absorption because of their special

15

structure of two or more amino groups within one molecule. Examples are 2-(2-Aminoethylamino)-

16

ethanol (AEEA),3 diethylenetriamine (DETA)4, 5 and triethylenetetramine (TETA).6

17

TETA, a polyamine, has four amine groups, two primary amine groups and two secondary

18

amine groups. Schaffer et al.6 compared the equilibrium CO2 loading and cyclic capacity of both

19

aqueous solutions of MEA and TETA for CO2 capture from flue gases. Their experimental results

20

showed that the equilibrium solubility of 15 kPa CO2 in 18 wt% (about 1.23 kmol/m3) TETA at 40°C

21

and 90°C are 1.71 and 1.20 mol CO2/mol TETA, respectively, while the corresponding equilibrium

22

solubility of 15 kPa CO2 in 18 wt% (about 2.55 kmol/m3) MEA are only 0.56 and 0.41 mol CO2/mol

23

MEA, respectively. They stated that TETA can be perceived as an alternative to MEA and can

24

achieve the same overall CO2 uptake and high reaction kinetics with less amine concentration. In

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addition, Kim et al.7 experimentally determined the heat of absorption of TETA and MEA and found 3 / 29



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1

that TETA has a slightly less heat of absorption (-84.91 kJ/mol CO2) than that of MEA (-88.91

2

kJ/mol CO2). Moreover, Wang et al.8 studied the regeneration behaviors of 15 different amine-based

3

absorbents with membrane vacuum regeneration (MVR) technology and recommended TETA for

4

CO2 capture due to its high average CO2 absorption rates, relatively high cyclic capacities and

5

significantly low regeneration energy consumption. Compared with the effort devoted to improving

6

the performance of CO2 absorption and investigating new solvents, the work on reboiler heat duty,

7

one of the most important parameters for CO2 capture using absorption method, is rather scarce and

8

very little data could be obtained from the literature. The study with respect to energy consumption

9

for TETA regeneration is missing, leading to the comprehensive evaluation of TETA performance

10

for CO2 capture incomplete. In addition, since TETA is very reactive with CO2, the blending of

11

TETA with N-methyldiethanolamine (MDEA), a tertiary amine, can reach a good compromise

12

between absorption rate and solvent regeneration.9 Thesefore, it is very essential to investigate the

13

energy requirement for CO2 stripping from single TETA and blended amines of TETA−MDEA.

14

The objective of this study is to examine the reboiler heat duty of the TETA at various process

15

parameters, including lean loading, rich loading, solvent flow rate and amine concentration, and the

16

blended amines of TETA−MDEA at two different mixing ratios (2:1 and 1:1). In addition, the

17

desorption performance of TETA was evaluated by comparing with that of MEA (a benchmark

18

solvent for comparison)2, 10 and DETA (another promising polyamines).4, 11, 12

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2 Experimental Section

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2.1 Chemicals

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Reagent grade TETA, DETA and MEA were purchased from Tianjin Kermel Chemical

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Reagent Co. Ltd., China, each with purity of ≥ 98.0%. Commercial grade CO2 was supplied by

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Changsha Rizhen Gas Co. Ltd., China, with a purity of ≥99.9%.

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2.2 Experimental apparatus and procedure

2

The CO2 desorption experimental apparatus for the desorption experiments is similar to our

3

previous work of Zhang et al.,5 as shown in Figure 1. The stripper (28.0 mm internal diameter and

4

0.50 m packed height) was made of a double-layer glass, with vacuum interlayer for heat insulation,

5

and packed with 316L stainless steel Dixon rings (random packing, Φ3×3mm, specific surface area

6

of 2275 m2/m3, provided by China Haohua, Tianjin, Univtech Cl., Ltd., China). The reboiler is a

7

triple-layer glass reaction kettle with the amine solution placed in the kettle, heating oil flowing

8

between the innermost and middle layers, and a vacuum between the middle and outermost layers. A

9

Friedrichs overhead condenser (0.25m jacket length) was placed on the top of the stripper. In

10

addition, some auxiliary equipments included: (i) two 10-L reservoirs for the rich and lean solutions,

11

respectively, (ii) two variable-speed liquid feed pumps to transport the rich solution and the lean

12

solution inlet and outlet the stripper, respectively, at the same flow rate, (iii) a water-bath heater to

13

heat the rich solution to a desired temperature before entering the top of the stripper, (iv) an oil-bath

14

heating to elevate the oil temperature, (v) an oil pump to transport the heating oil, (vi) a metal tube

15

rotameter with ±1.5% accuracy to measure the flow rate of the heating oil, (vii) a gas flow meter to

16

measure the amount of CO2 stripped from the solution.

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Prior to each experiment, the rich amine solution with a desired CO2 loading was prepared

18

and two liters of rich solution was introduced into the reboiler in which the rich solution was

19

preheated to a desired temperature by the heating oil. Once the reboiler reached the desired set point,

20

the rich amine solution was circulated continuously at a given flow rate and heated to a given inlet

21

temperature before being fed into the top of the regeneration column. The liquid level in the column

22

remained unchanged, indicating that the rich solution entered the stripper and the lean solution exited

23

the stripper were at the same flow rate. In the reboiler, the rich solution was heated to boiling and a

24

gaseous mixture of CO2, water and amine vapour was released from the solution. A condenser at the

25

top of the stripper ensured the gaseous water and amine returned back to the stripper with total 5 / 29



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1

reflux, discharging only the stripped CO2. In each experimental run, all the desired experiment data

2

were not recorded until the desorption operation to reach a steady state, which usually took at least 3

3

hours. When the operation was stable, all the temperatures, flow rates, and components remained

4

unchange. As for different runs, the temperature in the reboiler could be different. For instance, to

5

lower down the CO2 lean loading from 1.17 to 0.98 mol/mol, the required temperature in the reboiler

6

increased from 99.9 to 105.6 °C for the same rich TETA solution (with TETA concentration of 2.0

7

kmol/m3 and CO2 rich loading of 1.60 mol CO2/mol TETA) being fed into the stripper at a liquid

8

flow rate of 4.87m3/(m2h). All experiment runs were carried out at atmospheric pressure. The

9

pressure change between the reboiler and the top of the stripper was small and can be neglected.

10

Several samples of lean solution were then taken for the analysis of CO2 loading and amine

11

concentration. A mass balance error of the amount of CO2 was calculated for each experiment run to

12

ensure the results were accurate and reliable. The mass balance compared the amount of gas-phase

13

CO2 leaving from the top of the stripper, as measured by CO2 mass flow meter, with the amount of

14

CO2 removed from the amine solution, as measured by the Chittick apparatus.13 The mass balance

15

error obtained in the work was found to be less than 10%.

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To employ TETA for realistic post-combustion CO2 capture, selections of suitable

17

operational conditions such as the TETA concentration and the rich/lean CO2 loading are of great

18

importance. The equilibrium solubility of CO2 in 2.0 kmol/m3 TETA (at 40°C) at 1.0 and 15 kPa are

19

1.29 and 1.68 mol/mol, which were measured in this study using the experimental setup similar to

20

our previous work of Liang et al.14 Thanks to that the TETA solution has advantages of high

21

absorption capacity and kinetics6, 9, TETA is allowed to be employed to capture CO2 at low TETA

22

concentration and high CO2 loading. Note that the selected concentrations of TETA in this work are

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not high because TETA provides very high CO2 absorption capacity, which makes TETA can

24

achieve satisfactory CO2 absorption capacity at low TETA concentration. If the TETA concentration

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in the solution is too high, the viscosity of the solution is high and it will worsen the mobility and

2

distribution of the solution in the packed column, resulting in a lower desorption efficiency.

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3 Analysis of Reboiler heat duty

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In this study, heat transfer oil was chosen to supply the reboiler heat duty (Hreboiler, kJ/h) which

5

was calculated by the following equation. The calculation procedure is similar to the works of

6

Sakwattanapong et al.15 and Zhang et al.5

7

H reboiler = m oil c oil (Toil,in − Toil,out )

(1)

8

Where moil and coil are the mass flow rate (kg/h) and specific heat (kJ/(kg·°C)) of the heat

9

transfer oil, respectively. Also, Toil,in and Toil,out are the inlet and outlet temperatures of the heat

10 11

transfer oil from the reboiler (°C), respectively. The Qreboiler for solvent regeneration can be calculated from the ratio of effective reboiler heat

12

duty and the CO2 mass flow rate, as shown below:

13

Qreboiler =

H reboiler − H loss mCO2

(2)

14

mCO2 = namine (α rich − α lean ) M CO2

(3)

15

where Hloss is the system energy loss (kJ/h) which can be assumed to be zero in this work. This was

16

because the outermost layer of reboiler was vacuum insulation as well as both the reboiler and the

17

stripper were wrapped with insulation material. namine is the molar flow rate of amine solution

18

(kmol/h), MCO2 is the molecular weight of CO2 (kg/kmol). αrich and αlean are the CO2 loadings of the

19

rich and lean solutions (mol CO2/mol amine, for simplicity, the unit of CO2 loading in this paper

20

below was denoted as mol/mol), respectively.

21

The reboiler heat duty Qreboiler provided for the solvent regeneration generally consists three

22

terms, namely Qabs, Qsen and Qvap (kJ/mol CO2), as shown below:

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Qreboiler = Qabs + Qsen + Qvap

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where Qabs, Qsen, and Qvap can be estimated by the following equations:

(4)

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1

Qabs = ∆H abs,CO 2

(5)

2

Qsen = ρsolVcsol (Tsol,reboiler − Tsol,input ) / mCO2

(6)

3

Qvap = Qreg − Qabs − Qsen

(7)

4

where ∆Habs,CO2 is the heat of reaction (kJ/mol CO2), Tsol,reboiler and T sol,input are the temperatures of

5

the solution in the reboiler and that inlet the stripper (°C), respectively. ρsol and csol represent the

6

density of solution (kg/m3) and the specific heat of the solution (kJ/(mol·°C)), respectively. V and

7

mCO2 are the volume flow rate (m3/h) and CO2 mass flow rate (kg/h), respectively. The reaction heat

8

and the specific heat of solution for TETA, DETA and MEA can be obtained from the literature.7, 16

9

Based on the data available in the literature for single amines, the heat of reaction of blended

10

amines solution were calculated by the following equation:

11

H R ,blended =

m

Ci

∑C i =1

(8)

H R,i

T

12

where HR,i, Ci, and CT denote the heat of reaction (kJ/mol CO2), molar concentration of ith amine in

13

the blend and total molar concentration of amine (kmol/m3), respectively. HR,blended is the heat of

14

reaction of blended amine solution (kJ/mol CO2).

15

4 Results and Discussion

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4.1. Effect of lean-CO2 loading

17

In order to study the effect of lean CO2 loading of TETA, the condition of TETA solution for

18

the regeneration experiments was selected at TETA concentration of 2.0 kmol/m3 and CO2 rich

19

loading of 1.60 mol/mol, which is considered to be reasonable and acceptable for CO2 post-

20

combustion.

21

As shown in Figure 2, the Qreboiler reduces from 4603.4 to 2736.5 kJ/kg CO2 as the lean-CO2

22

loading increases from 0.78 to 1.17 mol/mol. Figure 2 also shows that, at first, the increase of lean-

23

CO2 loading results in a great decrease of the Qreboiler, however, a further increase of the lean-CO2

24

loading has a less effect on the Qreboiler. This behavior can be explained by the Qabs, Qsen and Qvap, as 8 / 29


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1

shown in Figure 3. Firstly, the change of the heat of reaction Qabs was found to be insignificant when

2

the lean-CO2 loading increased from 0.78 to 1.17 mol/mol since there were not much differences in

3

the desorption temperature. Secondly, the sensible heat Qsen increased from 508.2 to 597.7 kJ/kg CO2

4

when the lean CO2 loading dropped from 1.17 to 0.78 mol/mol. It should be noted that the sensible

5

heat took up only a small part of the Qreboiler (13.0% for CO2 lean loading of 0.78 and 17.8% for that

6

of 1.17), which indicates that the changes of sensible heat have small effect on the Qreboiler. Lastly,

7

the most important reason for the behavior in Figure 2 is that the lean CO2 loading directly affects

8

the equilibrium partial pressure of CO2. To get a lower lean CO2 loading (especially below

9

0.91mol/mol), the larger Qvap is required to evaporate water to be vapor phase, which could reach a

10

lower equilibrium CO2 partial pressure.

11

4.2. Effect of rich-CO2 loading

12

Rich CO2 loading is another important parameter which has a significant effect on the

13

Qreboiler. In order to examine the effect of CO2 rich loading on the Qreboiler, two rich loading of 1.60

14

and 1.40 mol/mol were tested in this work. As can be observed in Figure 4, the Qreboiler varies

15

significantly as the rich CO2 loading changes. That is, a rich solution of TETA containing 1.60

16

mol/mol loading requires a reboiler heat duty of 2898.6 kJ/kg CO2 to achieve a 0.98 mol/mol lean

17

loading, while that containing rich loading 1.40 mol/mol consumes at least 4458.2 kJ/kg CO2 to

18

achieve the same lean CO2 loading level. This effect is mainly attributed to the differences in

19

magnitude of equilibrium CO2 partial pressure at different rich-CO2 loadings, resulting in great

20

change of the heat of vaporization for different rich loading. Desorbing CO2 from lower rich

21

loadings to the same lean CO2 loading requires more stripping steam. As is illustrated in Figure 5,

22

the Qvap took up a smaller part of the Qreboiler for the 1.60 mol/mol (about 15.1%) rich solution,

23

whereas the Qvap for the 1.40 mol/mol one accounted for approximately 36.2% of the Qreboiler. It also

24

can be seen from Figure 5 that there are not much differences between the Qsen and the Qabs at

25

different rich CO2 loadings because their regeneration temperatures changed very little. Therefore, it 9 / 29



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1

is recommended for an efficient desorption operation at a higher rich CO2 loading. Meanwhile, the

2

absorption performance needs to be taken into account as well, getting a higher rich CO2 loading

3

implies that a higher packing absorber is required to provide longer contact time for CO2 absorbing

4

into the solvent, which will increase equipment investment.

5

4.3. Effect of amine Concentration

6

The concentration of the aqueous amine solution is another important parameter which

7

influences the Qreboiler for solvent regeneration. It is well known that the higher amine concentrations

8

can lead to the more CO2 being absorbed in the absorber and desorbed in the regenerator at the same

9

solvent flow rate. In this work, the effect of amine concentration on the Qreboiler was studied at TETA

10

concentration of 1.0, 1.5 and 2.0 kmol/m3.

11

As shown in Figure 6, an increase in TETA concentration from 1.0 to 2.0 kmol/m3 leads to a

12

reduction of the Qreboiler. This is because the higher the TETA concentration can lead to the more CO2

13

being absorbed in the per unit volume solution. In order words, to release the same amount of CO2 at

14

the same lean/rich CO2 loadings, a higher TETA concentration will require a less solvent flow rate,

15

resulting in a reduction of sensible heat. In addition, further study was performed to evaluate the

16

effect of absorption capacity per unit volume (C×∆a), i.e., amine concentration (C) times delta a

17

(∆a=arich‒alean), on the Qreboiler. As can be seen in Figure 7, at a given amount of (C×∆a), the Qreboiler

18

decreases significantly as the TETA concentration increases, indicating that to get a lower lean CO2

19

loading for the solution containing a lower TETA concentration, a sharp increase of Qvap is required

20

to evaporate water to vapor to reach a low equilibrium CO2 partial pressure. Therefore, the TETA

21

solution is not suggested to be employed at a too low lean CO2 loading.

22

4.4 Effect of solvent flow rate

23

In order to investigate the effect of solvent flow rate on the Qreboiler, desorption experiments

24

were carried out over solvent flow rate range of 2.92−11.7 m3/(m2h), with the rich loading be set at

25

1.60 mol/mol and two lean loading be fixed at 0.91 and 0.98 mol/mol, respectively. As can be 10 / 29


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1

observed in Figure 8, the Qreboiler is rather sensitive to the solvent flow rate, i.e., firstly decreased to a

2

minimum and then increased with the increase of solvent flow rate. This is because when the low

3

solvent flow rates is low, the increase of solvent flow rate increases the effective interfacial area in

4

the stripper for both mass and heat transfer performance for CO2 stripping, resulting in an decrease

5

of the Qreboiler. However, as the solvent flow rate further increase, the residence time of solvent in the

6

stripper for CO2 stripping reduces, which requires higher desorption temperature to provide a higher

7

heat transfer force to achieve the same ∆a, resulting in more Qreboiler requirement.Error! Bookmark

8

not defined. It can also be seen from Figure 8 that the optimal solvent flow rate was 4.87m3/(m2h)

9

for the lean loading of 0.91 mol/mol and 8.77m3/(m2h) for the 0.98 mol/mol lean loading. Since

10

TETA possesses very high absorption capacity, low solvent flow rate of TETA solution can provide

11

satisfactory CO2 absorption capacity. Thus, most of the experiments in this work were performed at

12

solvent flow rate of 4.87m3/(m2h).

13

4.5.Effect of amine type

14

The desorption performance of blended amines of TETA−MDEA with total amine

15

concentration of 2.0 kmol/m3, including two different mixing ratios of TETA:MDEA (2:1, 1:1 molar

16

ratio, respectively), were investigated and compared with that of single TETA solutionin with TETA

17

concentration of 2.0 kmol/m3. The desorption performance were compared in terms of the Qreboiler as

18

a function of ∆a. The solubility of total 2.0 kmol/m3 of blended amines (2:1, 1:1) at 40°C and CO2

19

partial pressure of 15 kPa were measured in this study using the experimental setup similar to the

20

work of Liang et al.14 Their equilibrium solubility were found to be 1.27 mol/mol and 1.12 mol/mol,

21

respectively. Since the solubility of CO2 in TETA-MDEA is lower than that of TETA, the CO2 rich

22

loading of three solution were selected at about 83% of their corresponding equilibrium solubility,

23

i.e., 1.40 mol/mol corresponding to about 83% of 1.68 mol/mol for TETA, 1.05mol/mol

24

corresponding to 83% of 1.27 mol/mol and 0.93 mol/mol corresponding to 83% of 1.12 mol/mol for

25

2:1 and 1:1 molar ratios of TETA-MDEA, respectively. 11 / 29



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1

As shown in Figure 9, an increase of the amount of MDEA results in a decrease of the Qreboiler.

2

The results can be explained by the three energy components contributing to CO2 regeneration: heat

3

of reaction, sensible heat, and heat of vaporization, as is presented in Figure 10. Since the

4

∆Habs,CO2 of MDEA and TETA are 61.17 kJ/mol CO2 and 84.91 kJ/mol CO2, respectively, by

5

increasing the ratio of MDEA, the ∆Habs,CO2 of the blended amines would be reduced. As shown in

6

Figure 10, the vaporization heat decreases from 2220.8 to 1191.4 kJ/kg CO2 as the ratio of MDEA

7

increases from 0% to 50%. Also, it was found that the sensible heat for this three solution is almost

8

the same, the average absolute deviation is less than 6%. These observations correspond well with

9

the work of Sakwattanapong et al.15 who studied the desorption performance of several blended

10

amines of MEA-MDEA, DEA-MDEA and MEA-AMP.

11

4.6. Comparative regeneration performances of TETA, MEA and DETA

12

In order to evaluate the regeneration performance of TETA, the desorption performance of

13

TETA were compared with that of MEA (the base case for comparison)2, 10 and DETA (another

14

promising polyamines)4, 11, 12 in terms of Qreboiler as a function of absorption capacity per unit volume

15

(C×∆a). The comparing conditions for these three solution were (i) TETA: 2.0 kmol/m3 TETA, CO2

16

rich loading of 1.60 mol/mol; (ii) MEA: 5.0 kmol/m3 MEA, CO2 rich loading of 0.50 mol/mol; (iii)

17

DETA: 2.0 kmol/m3 DETA, CO2 rich loading of 1.40 mol/mol, respectively.

18

The Qreg of TETA, MEA and DETA solution were compared as the function of the (C×∆a).

19

As can be seen in Figure 11, most of the reboiler heat duty of TETA is lower than those of both

20

MEA and DETA except that when the (C×∆a) is less than 0.90 kmol/m3, corresponding to the lean

21

loading of TETA is higher than 1.17 mol/mol. The reasons can be given as follows. Firstly, TETA

22

has a lower heat of reaction (-84.91 kJ/mol CO2) than those of both MEA (-89.48 kJ/mol CO2) and

23

DETA (-88.91 kJ/mol CO2).7 Secondly, the boiling point of TETA at 1 atm is the highest of the three

24

solvents, it refers to that the vapour pressure of TETA is the lowest, which makes TETA least

25

vulnerable to evaporate together with the water, thus requires less Qvap. 12 / 29


Page 12 of 29

Page 13 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

As can be seen in Figure 11, TETA provides very high CO2 absorption capacity, which makes

2

TETA operated at high CO2 lean and rich loadings can achieve satisfactory CO2 absorption capacity

3

and desorption efficiency and is competitive for post combustion process.

4 5

5. Conclusion

6

The reboiler heat duty (Qreboiler) of CO2 desorption from aqueous TETA and TETA+MDEA

7

were experimental investigated in a regenerator with Dixon rings random packing. The results from

8

this work showed that the Qreboiler decreases as the lean CO2 loading, rich CO2 loading and TETA

9

concentration increase; decreases first and then increases as solvent flow rate increases. The

10

introduction of MDEA into TETA solution is beneficial to the reduction of Qreboiler. The Qrebolier of

11

TETA was lower than that of MEA and DETA as a function of amount of CO2 released, indicating

12

that TETA can be considered to be a promising solvent for application in post-combustion CO2

13

capture.

14 15

Acknowledgements

16

Supported by the National Natural Science Foundation of China (NSFC-Nos. U1362112, 21406057,

17

21376067 and 21476064), the National Key Technology R&D Program (MOST-Nos.

18

2012BAC26B01 and 2014BAC18B04), Innovative Research Team Development Plan of the

19

Ministry of Education of the People’s Republic of China (IRT1238), Specialized Research Fund for

20

the Doctoral Program of Higher Education (MOE-No. 20130161110025), Key project of

21

international & regional scientific and technological cooperation of Provincial Hunan science and

22

technology plan (2014WK2037), and China Outstanding Engineer Training Plan for Students of

23

Chemical Engineering & Technology in Hunan University (MOE-No. 2011-40). The proof reading

24

and comments from Mr. Wilfred Olson in Canada as well as reviewer’s comments are greatly

25

appreciated. 13 / 29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1 2

Nomenclature arich CO2 loading of the rich solutions, mol CO2/mol amine αlean CO2 loading of the lean solutions, mol CO2/mol amine ∆a Difference between the rich and lean CO2 loading, mol CO2/mol amine C Amine concentration, kmol/m3 Ci Molar concentration of ith amine in the blend, kmol/m3 CT Total molar concentration of amine, kmol/m3 coil Specific heat of heat transfer oil, kJ/(kg·K) csolution Specific heat of the solution, J/(mol·K) ∆Habs,CO2 Heat of reaction, kJ/mol CO2 HR,blended Heat of reaction of blended amine, kJ/mol CO2 HR,i Heat of reaction of ith amine in the blend, kJ/mol CO2 Hreboiler Reboiler heat duty, kJ/h Hloss The system energy loss, kJ/h L Solvent flow rate, m3/(m2h) moil Mass flow rate of heat transfer oil, kg/h

mCO2

Mass flow rate of CO2, kg/h

M CO2

Molecular weight of CO2, kg/kmol

namine Molar flow rate of amine solution, kmol/h Qreboiler Regeneration heat duty, kJ/kg CO2 Qabs Absorption heat, kJ/kg CO2 Qsen Sensible heat, kJ/kg CO2 Qvap Vaporization heat, kJ/kg CO2

14 / 29


Page 14 of 29

Page 15 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Toil,in Temperature of the heat transfer oil inlet the reboiler, °C Toil,out Temperature of the heat transfer oil outlet the reboiler, °C Tsol,input Temperature ofthe solution at the stripper inlet, °C Tsol,reboiler Temperature ofthe solution in reboiler, °C ρsol Density of solution (kg/m3) 1 2

References

3

1. Aaron, D.; Tsouris, C., Separation of CO2 from flue gas: a review. Separation Science and

4

Technology 2005, 40(1-3), 321-348.

5

2. Kohl, A. L.; Nielsen, R., Gas purification. Gulf Professional Publishing 1997.

6

3. Ma'mun, S.; Jakobsen, J. P.; Svendsen, H. F.; Juliussen, O., Experimental and modeling study of

7

the solubility of carbon dioxide in aqueous 30 mass % 2-((2-aminoethyl)amino)ethanol solution.

8

Industrial & Engineering Chemistry Research 2006, 45(8), 2505-2512.

9

4. Fu, K.; Chen, G.; Sema, T.; Zhang, X.; Liang, Z.; Idem, R.; Tontiwachwuthikul, P., Experimental

10

study on mass transfer and prediction using artificial neural network for CO2 absorption into

11

aqueous DETA. Chemical Engineering Science 2013, 100, 195-202.

12

5. Zhang, X.; Fu, K.; Liang, Z.; Rongwong, W.; Yang, Z.; Idem, R.; Tontiwachwuthikul, P.,

13

Experimental studies of regeneration heat duty for CO2 desorption from diethylenetriamine

14

(DETA) solution in a stripper column packed with Dixon ring random packing. Fuel 2014, 136,

15

261-267.

16 17

6. Schäffer, A.; Brechtel, K.; Scheffknecht, G., Comparative study on differently concentrated aqueous solutions of MEA and TETA for CO2 capture from flue gases. Fuel 2012, 101, 148-153.

18

7. Kim, Y. E.; Moon, S. J.; Yoon, Y. I.; Jeong, S. K.; Park, K. T.; Bae, S. T.; Nam, S. C., Heat of

19

absorption and absorption capacity of CO2 in aqueous solutions of amine containing multiple

20

amino groups. Separation and Purification Technology 2014, 122, 112-118.

15 / 29



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1

8. Wang, Z.; Fang, M.; Pan, Y.; Yan, S.; Luo, Z., Comparison and selection of amine-based

2

absorbents in membrane vacuum regeneration process for CO2 capture with low energy cost.

3

Energy Procedia 2013, 37, 1085-1092.

4

9. Amann, J.-M. G.; Bouallou, C., Kinetics of the absorption of CO2 in aqueous solutions of N-

5

methyldiethanolamine+ triethylene tetramine. Industrial & Engineering Chemistry Research

6

2009, 48(8), 3761-3770.

7

10. Rochelle, G. T., Amine scrubbing for CO2 capture. Science 2009, 325(5948), 1652-1654.

8

11. Hartono, A.; da Silva, E. F.; Svendsen, H. F., Kinetics of carbon dioxide absorption in aqueous

9

solution of diethylenetriamine (DETA). Chemical Engineering Science 2009, 64(14), 3205-3213.

10

12. Fu, K.; Sema, T.; Liang, Z.; Liu, H.; Na, Y.; Shi, H.; Idem, R.; Tontiwachwuthikul, P.,

11

Investigation of Mass-Transfer Performance for CO2

12

(DETA) in a Randomly Packed Column. Industrial and Engineering Chemistry Research 2012,

13

51(37), 12058-12064.

14 15

Absorption into Diethylenetriamine

13. Horwitz, W., Association of official analytical chemists (AOAC) methods 12th ed. George Banta Company, Menasha, WI 1975.

16

14. Liang, Y.; Liu, H.; Rongwong, W.; Liang, Z.; Idem, R.; Tontiwachwuthikul, P., Solubility,

17

absorption heat and mass transfer studies of CO2 absorption into aqueous solution of 1-

18

dimethylamino-2-propanol. Fuel 2015, 144, 121-129.

19

15. Sakwattanapong, R.; Aroonwilas, A.; Veawab, A., Behavior of reboiler heat duty for CO2

20

capture plants using regenerable single and blended alkanolamines. Industrial and Engineering

21

Chemistry Research 2005, 44(12), 4465-4473.

22 23

16. Kim, I.; Svendsen, H. F., Comparative study of the heats of absorption of post-combustion CO2 absorbents. International Journal of Greenhouse Gas Control 2011, 5(3), 390-395.

24 25 26 16 / 29


Page 16 of 29

Page 17 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

FIGURE CAPTIONS

22

Figure 1.Schematic diagram of gas stripping and solvent regeneration system.

23

Figure 2. Reboiler heat duty of an aqueous TETA solution as a function of lean-CO2 loading

24

(CTETA=2.0 kmol/m3, αrich=1.60mol/mol, Tinput=90°C, L=4.87m3/(m2h)).

25

Figure 3. Distribution of energy of an aqueous TETA solution as a function of lean-CO2 loading

26

(CTETA=2.0kmol/m3, αrich=1.60mol/mol, Tinput=90°C, L=4.87m3/(m2h)).

17 / 29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Figure4. Effect of rich-CO2 loading on reboiler heat duty of 2.0 kmol/m3 TETA solution (CTETA=2.0

2

kmol/m3, Tinput=90°C, L=4.87m3/(m2h)).

3

Figure 5. Distribution of energy associated with reboiler heat duty for different rich loading.

4

(CTETA=2.0 kmol/m3, Tinput=90°C, L=4.87m3/(m2h), αlean=0.98mol/mol).

5

Figure 6. Effect of amine concentration on reboiler heat duty(αrich=1.60mol/mol, Tinput=90°C,

6

L=4.87m3/(m2h)).

7

Figure 7. The synergistic effect of concentration and CO2 cyclic capacity on the reboiler heat duty at

8

different amine concentration (αrich=1.60mol/mol, Tinput=90°C, L=4.87m3/(m2h)).

9

Figure 8. Effect of solvent flow rate on reboiler heat duty (CTETA=2.0 kmol/m3, αrich=1.60mol/mol,

10

Tinput=90°C).

11

Figure 9.Reboiler heat duty of TETA-MDEA blended solutions as a function of lean-CO2 loading.

12

(Camine=2.0 kmol/m3, Tinput=90°C, L=4.87m3/(m2h)).

13

Figure 10.Distribution of energy associated with reboiler heat duty for blended alkanolamines.

14

(Camine=2.0 kmol/m3, Tinput=90°C, L=4.87m3/(m2h), ∆a =0.48 mol/mol).

15

Figure 11. The synergistic effect of concentration and CO2 cyclic capacity on the reboiler heat duty

16

comparison of TETA, DETA and MEA (TETA: C of 2.0 kmol/m3 & αrich of 1.60 mol/mol & L of

17

4.87m3/(m2h) & Tinput of 90°C; DETA: C of 2.0 kmol/m3 & αrich of 1.40 mol/mol & L of 4.87m3/(m2h)

18

& Tinput of 90°C; MEA: C of 5.0 kmol/m3 & αrich of 0.50 mol/mol & L of 4.87m3/(m2h) & Tinput of

19

90°C).

18 / 29


Page 18 of 29

Page 19 of 29

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2

Figure 1. Schematic diagram of gas stripping and solvent regeneration system.

3 4 5 6 7 8 9 10 11 12

19 / 29



5000

Reboiler heat duty (kJ/kg CO2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 29

4000

3000

2000 0.70

1

0.80

0.90

1.00

1.10

1.20

Lean loading (mol CO2/mol TETA)

2

Figure 2. Reboiler heat duty of an aqueous TETA solution as a function of lean-CO2 loading

3

(CTETA=2.0 kmol/m3,αrich=1.60mol/mol,Tinput=90°C, L=4.87m3/(m2h)).

4 5 6 7 8 9 10 11 12 13

20 / 29


Page 21 of 29

2500


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2000

1500

1000

500

0 0.70 1

Reaction heat Vaporization heat Sensible heat

0.80

0.90

1.00

1.10


1.20

2

Figure 3.Distribution of energy of an aqueous TETA solution as a function of lean-CO2 loading

3

(CTETA=2.0 kmol/m3, αrich=1.60 mol/mol, Tinput=90°C, L=4.87m3/(m2h)).

4 5 6 7 8 9 10 11 12 13 14 15 16 21 / 29



7000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 29

αrich =1.40 mol CO2/mol TETA

6000

αrich =1.60 mol CO2/mol TETA

5000 4000 3000 2000 0.70

1

0.80

0.90

1.00

1.10

1.20


2

Figure4.Effect of rich-CO2 loading on reboiler heat duty of 2kmol/m3TETA solution (CTETA=2.0

3

kmol/m3, Tinput=90°C, L=4.87m3/(m2h)).

4 5 6 7 8 9 10

22 / 29


Page 23 of 29

6000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4500

3000

1500

0 1

Vaporization heat Sensible heat Reaction heat

1.40 mol CO2/mol TETA

1.60 mol CO2/mol TETA

TETA solution

2

Figure 5. Distribution of energy associated with reboiler heat duty for different rich loading.

3

(CTETA=2.0 kmol/m3, Tinput=90°C, L=4.87m3/(m2h), αlean=0.98 mol/mol).

4 5 6 7

23 / 29



7000 3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 29

1.0 kmol/m TETA 3 1.5 kmol/m TETA 3 2.0 kmol/m TETA

6000

5000

4000

3000

2000 0.70

0.80

0.90

1.00

1.10

1.20

Lean loading(mol CO2/mol TETA)

1 2

Figure 6. Effect of amine concentration on reboiler heat duty (αrich=1.60mol/mol, Tinput=90°C,

3

L=4.87m3/(m2h)).

4 5 6

24 / 29


Page 25 of 29

7000 3

1.0 kmol/m TETA 3 1.5 kmol/m TETA 3 2.0 kmol/m TETA


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6000

5000

4000

3000

2000 0.30 1

0.60

0.90

1.20

1.50

3

1.80

(C×∆α), Cyclic capacity of TETA solution (kmol/m )

2

Figure 7. The synergistic effect of concentration and CO2 cyclic capacity on the reboiler heat duty at

3

different amine concentration (αrich=1.60mol/mol, Tinput=90°C, L=4.87m3/(m2h)).

4 5 6 7

25 / 29



6000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 29

alean=0.91 mol CO2/mol TETA alean=0.98 mol CO2/mol TETA

5000

4000

3000

2000 0.0

3.0

6.0

3

9.0

2

12.0

Solvent flow rate (m /(m h))

1 2

Figure 8. Effect of solvent flow rate on reboiler heat duty (CTETA=2.0 kmol/m3, αrich=1.60mol/mol,

3

Tinput=90°C).

4 5 6 7 8 9 10 11 12 13 14 15

26 / 29


Page 27 of 29

7000 3

2.0 kmol/m of total concentration


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6000

TETA TETA:MDEA=2:1 TETA:MDEA=1:1

5000 4000 3000 2000 0.20

1

0.30

0.40

0.50

∆α (mol CO2/mol amine)

0.60

2

Figure 9.Reboiler heat duty of TETA-MDEA blended solutions as a function of lean-CO2 loading.

3

(Camine=2.0kmol/m3, Tinput=90°C, L=4.87m3/(m2h)).

4 5 6 7

27 / 29



6000 Vaporization heat Sensible heat Reaction heat


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

5000 4000 3000 2000 1000 0

1

TETA

TETA:MDEA=2:1 TETA:MDEA=1:1 3

Absorption solvent with 2.0 kmol/m amine

2

Figure 10.Distribution of energy associated with reboiler heat duty for blended alkanolamines

3

(Camine=2.0kmol/m3, Tinput=90°C, L=4.87m3/(m2h), ∆a =0.475mol/mol).

4 5 6 7 8

28 / 29


Page 28 of 29

Page 29 of 29

6000


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


5000

4000

3000 3

5.0 kmol/m MEA 3 2.0 kmol/m DETA 3 2.0 kmol/m TETA

2000 0.60

0.90

1.20

1.50

3

1.80

(C×∆α), Cyclic capacity of amine solution (kmol/m )

1 2

Figure 11. The synergistic effect of concentration and CO2 cyclic capacity on the reboiler heat duty

3

comparison of TETA, DETA and MEA (TETA: C of 2.0 kmol/m3 & αrich of 1.60mol/mol & L of

4

4.87m3/(m2h) & Tinput of 90°C; DETA: C of 2.0 kmol/m3 & αrich of 1.40mol/mol & L of 4.87m3/(m2h)

5

& Tinput of 90°C; MEA: C of 5.0 kmol/m3 & αrich of 0.50 mol/mol & L of 4.87m3/(m2h) & Tinput of

6

90°C).

7 8 9 10 11

29 / 29


Experimental Studies of Reboiler Heat Duty for CO2 Desorption from

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