Capture of Carbon Dioxide Using Aqueous Ammonia in a Lab-Scale Stirred-Tank Scrubber

Transcription

1 Advanced Materials Research Online: ISSN: , Vols , pp doi:0.4028/ 204 Trans Tech Publications, Switzerland Capture of Carbon Dioxide Using Aqueous Ammonia in a Lab-Scale Stirred-Tank Scrubber Pao-Chi Chen a and L. C. Lin b Department of Chemical and Materials Engineering Lunghwa University of Science and Technology 300, Wan-Shou Rd., Sec., Kueishan, Taoyuang, Taiwan, 333 a chenpc@mail2000.com.tw, b stan@mail.uturn.com.tw Keywords: Aqueous ammonia, Carbon dioxide, Scrubber, Mass-transfer coefficient Abstract. A -stat stirred-tank scrubber for capturing carbon dioxide using aqueous ammonia was used to explore the effects of process variables on the absorption of carbon dioxide. In order to maintain the value of the solution, aqueous ammonia was automatically introduced into the tank through the action of a -controller. The process variables were the of the solution, gas-flow rate, gas concentration and stirring speed. The absorption rate and mass-transfer coefficient could be determined by means of mass balance at a steady-state. It was found that the liquid-flow rate was ml/min; the removal efficiency was in the range of % and the loading of CO 2 was in the range of mol-co 2 /mol-nh 3. The results also showed that the absorption rate was in the range of 5.4x0-5 to 6.27x0-4 mol/s-l, while the mass-transfer coefficient was in the range of 0.05 to 0.4 /s. The effects of mixing on the absorption rate, mass-transfer coefficient and loading of CO 2 were also discussed in this work. Introduction Removal of greenhouse gas, such as carbon dioxide, is an important issue from the viewpoint of environmental protection in the modern world. Due to this, there are several technologies being used to capture CO 2 -gas from flue gas in fossil fuel fired power plants, in which the MEA absorption process is the major technology for the control of carbon dioxide emissions from massive discharging plants. However, this has several shortcomings, including a slow absorption rate, low solvent capacity and high energy cost []. In order to improve the drawbacks, several authors used ammonia as a solvent to capture carbon dioxide [-4]. A novel approach that may provide another route for reducing CO 2 -gas from the flue gas of fossil fuel fired power plants is capturing by ammonia solution scrubbing. It was found that an aqueous ammonia absorbent has a lot of advantages, including higher absorption capacity, cheaper cost, lower energy required from the viewpoint of regeneration, no corrosion problems and safety, compared with the MEA process [5]. Initially, it is one step of the processes in synthesis of ammonia production, which is widely applied to produce fertilizer in the Chinese chemical industry, and also, its by-product is ammonia bicarbonate (ABC). ABC will decompose into NH + 4 and HCO - 3 after being sprayed into the soil. The bicarbonate remaining in the soil might react with metal oxides or hydro-oxides to form stable carbonates such as calcium carbonate and magnesium carbonate for a permanent capture of carbon All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, (ID: , Pennsylvania State University, University Park, USA-09/04/6,09:9:46)

2 928 Advances in Environmental Technologies III within the soil. Due to this, many authors investigated the absorption of CO 2 -gas by aqueous ammonia. A preliminary study for the removal of carbon dioxide by ammonia scrubbing was fiund in the literatures [2,4,6-9]. In these investigations, they put emphasis on the removal efficiency, absorption capacity and regeneration processes. Some of them studied the power of absorbents. In general, absorption of CO 2 gas in an alkaline solution depends on the temperature, ionic strength and the of the solution [0]. It shows the carbonate distribution fraction during absorption. The fractions are α 0, α, and α 2, for H 2 CO 3 *, HCO 3 - and CO 3 2-, respectively. The dissolved CO 2 gas dissociates into bicarbonate and carbonate ions. At higher values, more CO 2 gas is absorbed and more carbonate ions are formed. On the other hand, a high solution would cause the ammonia ion to be lost as ammonia gas, resulting in lower absorbent concentrations. The distribution of NH 3 -NH 4 + is shown in literature. Therefore, how to control the operating condition becomes significant. The effects of the of the solution, CO 2 gas concentration, and gas flowrate on the mass transfer coefficient, absorption rate, removal efficiency and loading of CO 2 are significantly for the capture of CO 2. In this manner, the stirred-tank scrubber is a good selection since the features simple structure, easy operation and efficiency as compared with other scrubbers [-5, 9]. In this work, a continuous stirred-tank scrubber using aqueous amminia solution as an absorbebt under a controlled operation was used to study the effects of process variables, including the of the solution, CO 2 concentration, gas-flow rate and stirring rate, on the removal efficiency, absorption rate, mass-transfer coefficient and loading of CO 2. The absorption rate and mass-transfer coefficient could be simultaneously determined at a steady state by means of material balance. Determination of absorption rate and mass-transfer coefficient In a scrubber, a simulated gas mixture containing A (carbon dioxide) and B (nitrogen) flowing into a stirred-tank scrubber at the bottom comes into continuous contact with an aqueous ammonia solution flowing into the scrubber. Two streams come into contact in the scrubber with stirring. If we assume that nitrogen gas is essentially insoluble in the liquid phase and that liquid does not vaporize to the gas phase, the gas phase is a binary A-B system. Using the material balance, the absorption rate of carbon dioxide can be determined. At fast chemical reaction kinetics, the material balance for carbon dioxide at a steady state is: ( F F 2 ) R V 0 () A A A L = where R A is the mean absorption rate of carbon dioxide under stirring, which cannot be obtained directly. Eq. shows that the absorption rate, R A, is equal to the consumption of carbon dioxide through the scrubber. However, Eq. can be rewritten as: R A FA y y2 = [ ( )( )]. (2) V y y L 2

3 Advanced Materials Research Vols Therefore, the absorption rate, R A, can be determined with measurable quantities. According to a two-film model [], the overall volumetric mass-transfer coefficients can be written as: K G a R [ C HC ] = A (3) g L av where H is Henry s Constant, which is a function of temperature and ionic strength. In addition, [ C ] is the average value between the inlet and outlet. For the NH 4 OH/CO 2 system, g HC L av chemical species can be estimated by using the ionic strength approximation method [2]. Due to this, the ionic strength, H and C L can be determined, and hence K G a. Experimental section The experimental device is shown in Fig.. The inside diameter of the tank was 2 cm, and the gas distributor was a perforated plate under the bottom designed with 4-holes per square centimeter, each hole being mm in diameter. Initially, a known amount of distillated water was introduced into the column, in which a electrode was inserted as well. The solution was adjusted to the desired value by adding a known concentration of NaOH solution. The experiment was started when the mixed N 2 -CO 2 -gas, in the range of 0-30% of CO 2 gas, began bubbling through the column. The gas-flow rates were set in the range of 3 to 6 L/min. The stirring speeds were set within the range of 500 to 500rpm. During the operation, the of the solution decreased due to absorption in the scrubber. In order to hold the value of the solution at a desired value, NH 3 solution was introduced into the tank through the action of the -controller. Aqueous ammonia acted as an absorbent and also as a adjusting agent. A digital gauge pressure meter measured the inlet gas pressure. The overflow slurry solution flowed out to the reservoir. A Gas Data PCO 2 Meter was used to measure the concentration of the CO 2 -gas at the outlet of the scrubber. The experiments were conducted at 25 in most of the runs, and the operational values were kept within the range of 9 to 2. The experimental conditions are shown in Table. A total of twenty-two runs were conducted for the absorption of CO 2 gas to study the effects of value, stirring rate, CO 2 -gas concentration, and gas-flow rate on the absorption rate, removal efficiency, liquid-flow rate, and overall mass-transfer coefficient. Gas-flow rate Table Experimental conditions 3-6 (L/min) CO 2 -gas concentration 0-30 (%) value 9-2 Operating temperature 25 ( ) Stirring rate (rpm) Concentration of NH 3 solution 28 (%)

4 930 Advances in Environmental Technologies III Fig. Removal of carbon dioxide using aqueous ammonia as an absorbent in a continuous -stat scrubber..n 2-gas tank 6.Heater. Metering Pump 2.CO 2-gas tank 7. Gas mixer 2. solution vessel 3.Gas flow meter 8. Thermal stat 3.NH 3 Storage vessel 4.CO 2-gas meter 9. Scrubber 4. NH 3-gas meter 5.Motor controller 0.-controller 5. Cooling coil Results and Discussion Experimental data obtained in this work Experimental data, including the liquid-flow rate, removal efficiency, absorption rate and overall mass-transfer coefficient, obtained in this work is shown in Table 2. The results show that the liquid-flow rate was ml/min; the removal efficiency was in the range of %; the absorption rates were in the range of 5.4x x0-4 mol/s-l; the overall mass-transfer coefficients were in the range of /s and the loading of CO 2 was in the range of mol-co 2 /mol-nh 3. The values were quite different depending on operating conditions. Table 2 Experimental data obtained in this work NO Q g N Q L V L y y 2 E R A a K G ψ (L/min) (rpm) (ml/min) (L) (%) (%) (%) (mol/s-l) (/s) mol/mol E E E E E

5 Advanced Materials Research Vols E E E E E E E E E E E E E E E E E Removal efficiency At a steady state, removal efficiency vs. at different input gas concentrations is shown in Fig. 2. The removal efficiency varies from 30% to 00%, depending on operating conditions. The results show that E increased steeply with an increase in value up to a of 0 and then increased slowly with the. However, at a of 2, the removal efficiency was close to 00% for most input gas concentrations. The results also found that gas concentration has a weak effect on the removal efficiency E(%) y =30% y =20% y =0% Fig. 2 Removal efficiency of carbon dioxide vs. value in aqueous ammonia solution at different gas inlet concentrations.

6 932 Advances in Environmental Technologies III Absorption rate and mass-transfer coefficient From measured quantities, y, y 2, F A and V L, the absorption rate could be calculated by using Eq. 2. Calculated values obtained in this work were within the range of 5.3x x0-4 mol/s-l. Substituting these values and C g into Eq. 3, the overall volumetric mass-transfer coefficient could be determined. Values obtained in this manner were in the range of /s. Fig. 3 shows a plot of R A vs. at different gas-inlet concentrations. The results show that the absorption rate increases with an increase in value up to a of 0 and then the absorption rate becomes attenuate as the is increased further. This can be explained as follows: at higher values, the addition rate of aqueous ammonia was higher. For example, at y =0%, at a of 2 the addition rate was ml/min, while at a of 0 the addition rate was only.08 ml/min. This shows that at higher values more ammonium was required to compensate the consumption of ammonia, on one hand, and to keep a constant value, on the other hand. Additionally, Fig. 4 is a plot of K G a vs. at different y values. The results show that K G a increased with an increase in value, while the gas-inlet concentration has little effect on K G a. E-2 E- R (mol/s-l) A E-3 E-4 y =30% y =20% y =0% K a(/s) G 8E-2 6E-2 4E-2 y =30% y =20% y =0% 2E-2 E E Fig. 3 A plot of R A vs. at different y Fig.4 A plot of K G a vs. at different y Effects of mixing on R A and K G a The influence of mixing on the absorption rate and mass-transfer coefficient was also investigated by changing the gas-flow rate and stirring speed. Fig. 5 shows that the K G a increases with an increase in stirring speed up to 800 rpm, and then becomes alleviated when the stirring speed is increased further. The same trend is also found in absorption rate, as shown intable 2. This was due to more small bubbles being formed at a higher stirring speed, which favors the absorption of carbon dioxide. The effect of the gas-flow rate was found and shown in Fig. 6. The results show that K G a is nearly linear in proportion to Q g. The effect of mixing resulted in more aqueous ammonia being added in the scrubber, which enhanced the absorption rate and mass-transfer coefficient.

7 Advanced Materials Research Vols E K a(/s) G E- 5E-2 K a(/s) G E N(RPM) Fig.5 Effect of stirring rate on R A and K G a (Q g =3L/min, =0 and y =30%) Q g (ml/min) Fig.6 A plot of K G a vs. Q g Loading of CO 2 The loading of CO 2 (φ) was affected by the value, gas concentration, stirring speed and gas flow rate, as shown in Fig. 7, by a plot of φ vs. at various y. Obviously, the loading decreased with an increase in the value for a gas concentration of 0%, while the loading had maximum values for gas concentrations of 20% and 30%. The maximum shifted from =9.5 (0.5079mol/mol) to =0.0 (0.3538mol/mol), showing that the loading is not only affected by the but also by the gas concentration. Fig. 8 shows that the loading increased with N to a maximum value (N=000RPM; φ=0.566 mol/mol) and then decreased with N further. In addition, the removal efficiency is higher than 90% when N is higher than 800 RPM. In addition, Nos. 3, in Table 2 show the effect of the gas-flow rate on the loading, showing a slight change in loading ( mol/mol). Here, we concluded that No. 8 has the best operating conditions (=0, N=000 RPM, Q g =3 L/min, Q L =4.7 ml/min, y =30%) obtaining the highestφ=0.566 mol/mol having a removal efficiency of 96.2%, R A of 5.8x0-4 mol/s-l and K G a of 0.09 /s. Loading of CO (mol-co /mol-nh ) y =0% y =20% y =30% Loading of CO (mol-co /mol-nh ) % 85.3% 89.9% 96.2% 92.0% N(RPM) 600 Fig. 7 Effect of on the loading of CO 2 Fig. 8 Effect of stirring rate on the loading of CO 2 at different y

8 934 Advances in Environmental Technologies III Conclusions The capture of CO 2 gas using aqueous ammonia in a continuous stirred-tank scrubber under a controlled value was successfully investigated in this study. The effects of process variables on the removal efficiency, absorption rate, overall mass-transfer coefficient and loading of CO 2 were explored. The results show that the of the solution plays an important role on the removal efficiency, absorption rate, mass-transfer coefficient and loading of CO 2, while the effect of gas concentration on R A and K G a are minor. The effects of the stirring speed on the R A and K G a both are moderate and become attenuate when the stirring speed is higher than 800 rpm, showing that the stirring speed has a limiting effect. The liquid-flow rate of aqueous ammonia could act as an indicator in the absorption of carbon dioxide in this study. Additionally, the higher loading of CO 2 (0.566) and E (96.2%) can be obtained when effectively adjusting the operating conditions, such as stirring rate and value. The results also show that no crystallization of ABC could be found in this study. This is due to the effect of the value on the concentration of bicarbonate, which is insufficient to precipitate ammonia bicarbonate (ABC). The conditions in the precipitation of ABC can be investigated further in the future. Acknowledgements The authors acknowledge the financial support of National Science Council of the Republic of China (NSC ET). References []Y. F. Diao, X. Y. Zheng, B. S. He, C. H. Chen and X. C. Xu: Energy Conversion and Management Vol. 45 (2004), p [2]H. Bai, and A. C. Yeh: Ind. Eng. Chem. Res. Vol 36 (997), p [3]J. T. Yeh and H. W. Pennline: Energy Fules Vol 5(200), p.274. [4]J. T. Yeh, K. P. Resnik, K. Rygle and H. W. Pennline: Fuel Processing Technology Vol 86 (2005), p.533. [5]G. F. Versteeg, L. A. J. Van Dijck and W. P. M. Van Awaaij: Chem. Eng. Comm. Vol 44(996), p.3. [6]A. C. Yeh and H. Bai: The Science of the Total Environment Vol 228(999), p.2. [7]H. Huang and S. G. Chang: Energy & Fuels Vol 6 (2002), p.904. [8]X. Li, E. Hagaman, C. Tsouris and J. W. Lee: Energy & Fuels 7(2003), p.69. [9] Y. F. Diao, X. Y. Zheng, B. S. He, C. H. Chen and X. C. Xu: Vol 45(2004), p [0]P. C. Chen, C. C. Chen, M. H. Fun, O. Y. Liao, J. J. Jiang, Y. S. Wang and C. S. Chen: Chem. Eng. Tech. Vol 27(2004), p. 59. []P. C. Chen, W. Shi, R. Du and V. Chen: J. Chin. Inst. Chem.. Engrs. Vol 36(2005), p.223. [2]G. H. Nancollas, Interactions in Electrolyte Solutions, Elsevier Publishing Company, Amsterdam, Netherlands (966).

9 Advances in Environmental Technologies III / Capture of Carbon Dioxide Using Aqueous Ammonia in a Lab-Scale Stirred-Tank Scrubber / DOI References [] Y. F. Diao, X. Y. Zheng, B. S. He, C. H. Chen and X. C. Xu: Energy Conversion and Management Vol. 45 (2004), p /j.enconman