www.seipub.org/ijepr International Journal of Engineering Practical Research (IJEPR) Volume 3 Issue 4, November 2014 doi: 10.14355/ijepr.2014.0304.03
A Study on CO2 Absorption in Bubble Column Using DEEA/EEA Mixed Solvent Pao Chi Chen*1, Chang‐Cheng Liao2 Department of Chemical and Materials Engineering, Lunghwa University of Science and Technology 300, Sec. 1, Wanshou Rd., Guishan Shing, Taoyuan County, Taiwan, R.O.C. *1
chenpc@mail2000.com.tw; 2jonecool21@yahoo.com.tw
Abstract Absorption of carbon dioxide in aqueous blends of alkanolamines prepared from renewable resources (DEEA/EEA), which can be produced from green materials, was explored. In addition, the scrubbing factor of CO2‐gas in alkaline solution was found to be excellent in a bubble‐ column, as compared with another scrubber. Thus, a continuous bubble‐column scrubber was used in this study. Therefore, a continuous bubble‐column scrubber absorbing CO2‐gas, using a DEEA/EEA mixed solvent as an absorbent under a pH‐stat condition, was used to search for the optimum process parameters by means of Taguchi’s analysis. The process variables included the pH of the solution, gas‐flow rate, and concentration of solvent. From measured outlet CO2‐gas concentrations, absorption rate, removal efficiency, and overall mass‐transfer coefficient could be determined, with the aid of steady‐state material balance equation, as well as a two‐film model. Additionally, the liquid‐flow rate of DEEA /EEA was observed automatically through the action of a pH controller. The data obtained were in the range of 1.26x10‐4‐11.80x10‐4(mol/s‐L), 0.0728‐0.8395(1/s), and 28‐98.66% for absorption rate, mass‐ transfer rate, and removal efficiency, respectively. According to S/N ratio, the significance sequence influencing the absorption rate was D(pH)>C (gas flow rate)>A(concentration of DEEA)>B(concentration of EEA), while the significance sequence for overall mass‐transfer coefficient was A>B>C>D. Keywords Amine; Taguchi’s analysis; Absorption rate; Overall mass‐transfer coefficient
Introduction In recent studies, CO2 absorption technologies have become more diversified, among which chemical absorption effect is the best (Bai and Yeh, 1997; Yeh et al., 1999; Chen et al., 2005; Yang et al., 2005; Lin et al., 2003; Petrov, et al., 2006). Internationally, thousands of factors use amines as absorbent to absorb CO2 (Versteeg et al., 1996; Xiao et al., 2000). Although the
78
amines has good absorption effect (Karpe and Aichele, 2013; Rinprasertmeechai et al., 2012), it is mostly fossil fuel derivatives, and is not environmentally‐friendly. Hence, it is necessary to develop energy‐friendly absorbent. DEEA and EEA can be synthesized by CO2 (Vaidya and Kenig, 2007). This study aims to discuss the importance of CO2 in the bubble column for absorption efficiency and overall mass‐transfer coefficient when DEEA/EEA mixed solvent prepared from renewable materials is used as an absorbent. The results can provide references for bubble column design. The bubble column features simple structure, easy operation, cost efficiency and excellent scrubbing factor as compared with other scrubbers (Chen et al., 2008; Lin et al., 2003; Sauer and Hempel, 1987; Aroonwilas et al., 1999), without mechanical heat production. Thus, recovery of CO2 using bubble columns can reduce costs and save energy. This experiment used bubble columns as an absorber for absorption. This study used DEEA/EEA mixed solvent as an absorbent and controlled pH value to explore the capture of CO2 in the bubble column scrubber. It explored the effect of relevant operation variables, including the pH of the solution, gas flow rate, EEA concentration and DEEA concentration on the removal efficiency, absorption rate and overall mass‐transfer coefficient. In addition, this study applied Taguchi’s analysis to determining importance of operating variables, providing reference for operation and design. Experimental Experimental Procedure The experiment equipment is shown in Figure 1. The inside diameter of the column was 5 cm, and the gas distributor was a perforated plate designed with 4‐ holes per square centimeter, each hole being 1mm in
International Journal of Engineering Practical Research (IJEPR) Volume 3 Issue 4, November 2014 www.seipub.org/ijepr
diameter. First, the buffer solution with pH=7 and 10 was used to correct pH electrodes and to set the required pH, and then DEEA/EEA mixed solvent was formulated. The mixed gas was adjusted to the required concentration, and the heater was started to heat the mixed gas to operate temperature at 50℃. The cooling system was started to control the liquid temperature at 25 ℃. The gas was fed to the tower bottom gas chamber, and entered into the column through perforated boards. Next, pH electrodes, digital thermometer and condensation tubes were inserted into the bubble column. After that, DEEA/EEA mixed solvent was poured into the column. The gas passing through the perforated boards generated bubbles and DEEA/EEA mixed solvent for contact and absorption. Meanwhile the DEEA/EEA mixed solvent feed system was started on. The mixed solvent was fed into the column automatically through the action of pH controller when the pH value was lower than the setting point. Next, CO2 meter was used to detect CO2 concentration every 5 min. The concentration value was recorded, including feed amount of mixed solvent. The sampling was conduced once every 30min. Total Organic Carbon (TOC) analyzer was used to measure carbon content of the solution, and a refractometer to measure refractive index (RI). After the test, the rest of mixed solvent in the column was recorded, and the test data were collected and calculated.
minimize the experimental number, Taguchi’s method was used for the optimal experiment design. The experimental operation variables included DEEA concentration, EEA concentration, pH value and gas flow rate, each one having three levels. In general, the number of experiments was 34=81 without experimental design, while with experimental design, the experimental number reduced to 9 (L9(34)) according to orgonal arrays. Among those, a simulated flue gas from coal‐fired plant was used, i.e., the concentration of CO2 was 15% and the temperature of gas was 50 ℃. The detailed design is shown in Tables 1 and 2. TABLE 1 CONDITION FACTOR LEVEL
Parameter level DEEA (M) EEA (M) Gas flow rate (L/min) pH
A B C D
1 1.0 0.1 3 10
2 2.0 0.2 5 10.5
3 3.0 0.3 7 11
TABLE 2 ORTHORGONAL ARRAYS
NO.
DEEA (M)
No.1 No.2 No.3 No.4 No.5 No.6 No.7 No.8 No.9
1.0 1.0 1.0 2.0 2.0 2.0 3.0 3.0 3.0
EEA (M) 0.1 0.2 0.3 0.1 0.2 0.3 0.1 0.2 0.3
Gas‐flow rate (L/min) 3 5 7 7 3 5 7 3 5
pH 10.5 10 11 11 10.5 10 10 11 10.5
Determination of Absorption Rate and Overall Mass-transfer Coefficient Absorption Rate
1. Nitrogen
8. pH controller
2. CO2
9. Feed pump
3. Float gas flow meter
10. Temperature gauge
4. Mixed gas heater
11. pH electrodes
5. Digital pressure gauge
12. CO2 detector
6. Bubble column
13. Mixed DEEA/EEA
7. Cooler
14. Overflow solvent
In this absorption system, after the contact occurred between mixed gas of A (CO2) and B(N2) fed from the bottom of the bubble column and DEEA/EEA mixed solvent, the gas was discharged from top of the column, and the mixed solvent was continuously fed from the column top. The liquid and gas contacted each other in a reverse direction. It was assumed that B was not dissolved in the liquid, and the liquid would not volatilize into primary gas. After contact occurred between mixed gas and liquid, it was assumed that the gas in the column flowed in multi‐ plug flow pipes (Chen et al., 2005). The absorption rate was determined.
FIG. 1 EXPERIMENT DEVICE DIAGRAM
Design of Experiments In order to obtain the optimal experiment data and
RA
FA1 1 y1 y [1 ( )( 2 )] VL y1 1 y2
(1)
where FA1(mol/s) is molar flow rate of CO2 inlet, VL(L)
79
www.seipub.org/ijepr International Journal of Engineering Practical Research (IJEPR) Volume 3 Issue 4, November 2014
is volume (end volume) of solution in the absorber, and yA1 and yA2 are CO2 concentrations at inlet and outlet respectively. Determination of Mass‐transfer Coefficient In order to further determine the mass‐transfer coefficient, it was assumed that two‐film model was used to describe CO2 mass transfer (Chen et al., 2008). Thus, the overall mass‐transfer coefficient can be expressed as follows: KG a
Qg VL
ln
C A1 C A2
(2)
where Qg(L/min) is volumetric feed rate of gas, VL(L) is solution volume (solution end volume), CA1(M) and CA2(M) are CO2 molar concentrations at inlet and outlet respectively. Eq. (2) can be used to determine overall mass coefficient at steady state.
In addition, the removal efficiency obtained in here was in the range of 28‐98.66%, which varied significantly depending on the conditions. In general, the removal efficiency was controlled at 90% for commercial scale (Karpe and Aichele, 2013). Therefore, NO. 8 and NO.9 could be candidates in here. Effect of (RA) S/N Ratio by Absorption Rate After implementation of the genetic algorithm (GA) using the orthogonal arrays, combinations of every four singnals and noises forming S/N ratio based on 9 groups of the experimental results are shown in Table 4 and FIG. 2, where A is DEEA concentration, B is EEA concentration, C is Gas flow rate and D is pH value. TABLE 4 S/N RATIO ANALYSIS FOR RA
Level 1 2 3 Delta Rank
Results and Discussion TABLE 3 EXPERIMENTAL DATA
KGa (1/s)
RA (10‐4mol/s‐L)
E (%)
TOC (M)
RI
NO.1
0.1083
3.5869
77.3
0.6075
1.3558
NO.2
0.1427
5.9057
62.7
0.5802
1.3581
NO.3
0.1847
7.7714
61.3
0.6103
1.3562
NO.4
0.0728
3.8006
41.3
1.1292
1.3779
NO.5
0.5684
4.1630
28.0
1.2002
1.3814
NO.6
0.0364
5.5433
40.0
1.1263
1.3744
NO.7
0.8395
11.8004
89.3
1.4426
1.3948
NO.8
0.3040
4.2737
98.7
1.0506
1.3915
NO.9
0.3373
2.4703
28.0
1.1403
1.3976
pH 17.25 10.45 14.01 6.80 1
EEA
16 14 12 10 1
2
3
0.1
Gas-flow rate
18
0.2
0.3
pH
16 14 12
(3)
TOC and RI are determined by analysis of the samples. The values showed the carbon dioxide dissolved in the solution. The absorption rate obtained was in the range of 1.26x10‐4‐11.80x10‐4(mol/s‐L), which was closed to that obtained by the capture of CO2 using NaOH solution (Chen et al., 2005). The obtained overall mass‐transfer coefficient was in the range of 0.0728‐0.8395(1/s), which was comparable with data for different solvents and different scrubbers that were reported in the literature (Chen et al., 2008).
80
Gas‐flow rate 12.03 12.72 16.95 4.92 2
DEEA
18
Table 3 illustrates the calculated data of this study, including KGa, RA and E. RA and KGa are determined by using Eqs. (1) and (2). E represents absorption efficiency, as determined by Eq.(3). y1 is CO2 inlet concentration and y2 is CO2 outlet concentration. y y2 E 1 100% y1
EEA 14.71 13.48 13.51 1.23 4
Main Effects Plot (data means) for SN ratios
Mean of SN ratios
DEEA 14.78 12.95 13.97 1.82 3
10 3
5
7
10.0
10.5
11.0
Signal-to-noise: Larger is better
FIG. 2 S/N RATIO FOR RA
With reference to the table for S/N ratio by absorption rate, it can be found that importance priority of absorption rate parameters is D>C>A>B. The results obtained from analysis of S/N ratio using Taguchi’s methods show the optimal combination of absorption rate is A1, B1, C3 and D1. The conditions are optimal when DEEA concentration is 1M, EEA concentration is 0.1M, gas flow rate is 7 L/min and pH is 10. Effect of (Kga) S/N Ratio by Overall Mass Transfer Coefficient Similarity, using the orthogonal arrays, combinations of the S/N ratio based on 9 groups of the experimental results are shown in Table 5 and FIG. 3.
International Journal of Engineering Practical Research (IJEPR) Volume 3 Issue 4, November 2014 www.seipub.org/ijepr
Main Effects Plot (data means) for SN ratios
TABLE 5 S/N RATIO ANALYSIS FOR KGA
DEEA ‐16.963 ‐18.814 ‐7.101 11.713 1
EEA ‐14.528 ‐10.720 ‐17.629 6.909 2
Gas‐flow rate ‐11.519 ‐18.376 ‐12.983 6.857 3
pH ‐15.736 ‐11.218 ‐15.924 4.706 4
Main Effects Plot (data means) for SN ratios DEEA
EEA
-10
DEEA
EEA
36 34
Mean of SN ratios
Level 1 2 3 Delta Rank
32 1
2 Gas-flow rate
3
0.1
0.2 pH
0.3
3
5
7
10.0
10.5
11.0
36
Mean of SN ratios
34 -15
32
-20 1
2 Gas-flow rate
3
0.1
0.2 pH
0.3
Signal-to-noise: Larger is better
-10
FIG. 4 S/N RATIO FOR E
-15 -20 3
5
7
10.0
10.5
11.0
Signal-to-noise: Larger is better
FIG. 3 S/N RATIO FOR KGA
With reference to the S/N ratio by overall mass transfer coefficient, it can be seen that importance priority of overall mass transfer coefficient is A>B>C>D. In addition, the results obtained from analysis of S/N ratio using Taguchi’s analysis show the optimal combination of overall mass transfer coefficient is A3, B1, C1 and D2. The conditions are optimal when DEEA concentration is 3M, EEA concentration is 0.1 M, gas flow rate is 3 L/min and pH is 10.5. Effect of (E) S/N Ratio by CO2 Removal Efficiency Using the Orthogonal Arrays, the S/N ratio based on 9 groups of the experimental results are shown in Table 6 and FIG. 4. With reference to the table for S/N ratio by the removal efficiency, it can be observed that the importance priority of removal efficiency is A>B>D >C. The results obtained from analysis of S/N ratio using Taguchi’s analysis show the optimal combination of CO2 removal efficiency is A1, B1, C3 and D3. The conditions are optimal when DEEA concentration is 1M, EEA concentration is 0.1M, gas flow rate is 7 L/min and pH is 11. TABLE6 S/N RATIO ANALYSIS FOR E
Level 1 2 3 Delta Rank
DEEA 36.49 31.10 35.95 5.38 1
EEA 36.37 34.92 32.24 4.12 2
Gas‐flow rate 35.53 32.31 35.70 3.39 4
pH 35.67 31.88 35.98 4.10 3
Conclusions A continuous bubble‐column scrubber using DEEA/EEA mixed solvent to capture CO2 gas is successfully used to search for optimal conditions by using Taguchi’s methods. Due to this, the paper provides an effective method to CO2 capture experimental since the experimental design can reduce a large amount experiments from 81 runs to 9 runs. In addition, the test of DEEA/EEA mixed solvent demonstrates that the mixed solvent can capture a large amount of CO2 gas as compared with other solvents. On the other hand, the absorption rate and overall mass‐transfer coefficient obtained in this work are comparable with previous study and higher than that obtained in packed bed. The data obtained in this work vary obviously depending on the operating conditions. This indicates that the system can be adjusted to obtain desired outcome data. The relevant operating variables are DEEA and EEA, while gas flow rate and pH are minor. The following conclusions were reached based on the Taguchi’s analysis. (1) It can be found that importance priority according to absorption rate is D>C>A>B. The optimal condition is A1, B1, C3 and D1. (2) It can be found that importance priority according to overall mass‐transfer coefficient is A>B>C>D. The optimal condition is A3, B1, C1 and D2. (3) It can be found that importance priority for removal efficiency is A>B>D >C. The optimal conditions is A1, B1, C3 and D3.
81
www.seipub.org/ijepr International Journal of Engineering Practical Research (IJEPR) Volume 3 Issue 4, November 2014
ACKNOWLEDGEMENT
We thank the financial support from NSC Taiwan under the grant of NSC‐99‐2221‐E‐262‐024. REFERENCES
Aroonwilas, Adisorn, Veawab A. and Tontiwachwuthikul P. “Behavior of the Mass‐Transfer Coefficient of Structured Packings in CO2 Absorbers with Chemical Reactions.” Ind. Eng. Chem. Res. 38( 1999):2044‐50.
Kuprathipnja S. “Carbon Dioxide Removal from Flue Gas Using Amine‐Based Hybrid Solvent Absorption.” World Academy of Science, Engineering and Technology 64 (2012: 410‐4. Sauer T., Hempel D. C. “Fluid dynamics and mass transfer in a bubble column with suspended particles.” Chem Eng Technol 10 (1987):180‐89. Vaidya P.D. and Kenig E.Y. “Absorption of CO2 into aqueous blends of alkanolamines prepared from
Bai, H. and Yeh A. C., “Removal of CO2 Greenhous Gas by
renewable resources.” Chem. Eng. Sci. 62 (2007): 7344‐50.
Ammonia Scrubbing.” Ind. Eng. Chem. Res., 36 (1997):
Versteeg G. F., Van Dijck L. A. J. and Van Awaaij W. P. M.
2490‐293. Chen, P. C., Shi W, Du R, Chen V. “Mass Transfer and Absorption of Acidity Gas in an Alkaline Solution
“On the kinetics between CO2 and alkalinoamines both in aqueous and non‐aqueous solutions: An Overview.” Chem. Eng. Comm. 144 (1996): 113‐58.
Containing Fine Crystals Using a pH‐Stat Contiuuous
Xiao, J, Li C. C. and Li M. H. “Kinetics of absorption of
Bubble‐Column Scrubber.” J. Chin. Inst. Chem. Engrs., 36
carbon dioxide into aqueous solutions of 2‐amino‐2‐
(2005): 223‐33.
methyl‐1‐propanol+monoethanolamin.” Chem. Eng. Sci.
Chen, P. C., Shi W, Du R, Chen V. “Scrubbing of CO2
55 (2000):161‐75.
Greenhouse gases, accompanied by precipitation in a
Yang H., Xu Z., Fan M. and Cupta R. “Progress in carbon
continuous bubble‐column scrubber.” Ind. Eng. Chem.
dioxide separation and capture: A review.” J of Envir Sci
Res. 47( 2008):6336‐43.
20 (2007):14‐27.
gas emissions.” The Science of the Total Environment. 228 (1999):121‐7.
Yeh, A. C. and Bai H. “Comparison of ammonia and monoethanolamine solvents to reduce CO2 greenhouse.
Karpe, Prakash and Aichele Clint P. “Amine Modeling for CO2 Capture: Internals Selection.” Enviromental Science & Technology 47 (2013: 3926‐32. Lin C.C., Liu W.T., Tan C.S. “Removal of carbon dioxide by absorption in a rotating packed bed.” Ind Eng Chem Res 42 (2003):2381‐86. Petrov, P., Ewert G. and Rohm H. J. “Chemisorptive removal of carbon dioxide from process streams using a reactive bubble column with simultaneous production of usable materials.” Chem. Eng. Technol., 29 (2006), 1084‐90. Rinprasertmeechai, S., Chavadej S., Rangsunvigit P. and
82
Dr. Pao Chi Chen, Engineering Doctor (Ph.D.‐Engineering), now is a professor, Department of Chemical and Materials Engineering, Lunghwa University of Science and Technology. He got Bachelor Degree in Chemical Engineering, Chung‐Yuan University, M Sc and Engineering Doctor’s degree (Ph.D.) in Chemical Engineering, Department of Chemical Engineering, National Taiwan University. Dr. Chen got a research award from Ministry of Education, Taiwan, 2010. Currently, Dr. Chen focus on the biotechnology, such as peptides and Nano‐structured lipid carriers, capture of carbon dioxide, nanotechnology, and technology education.