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CO2 capture from hot stove gas in steel making process
International Journal of Greenhouse Gas Control 4 (2010) 525–531
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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
CO2 capture from hot stove gas in steel making process Hsu-Hsiang Cheng a, Jui-Fu Shen b, Chung-Sung Tan a,* a b
Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC New Material Research & Development Department, China Steel Corp., Kaohsiung 81233, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2008 Received in revised form 15 July 2009 Accepted 12 December 2009 Available online 7 January 2010
The capture of CO2 from a hot stove gas in steel making process containing 30 vol% CO2 by chemical absorption in a rotating packed bed (RPB) was studied. The RPB had an inner diameter of 7.6 cm, an outer diameter of 16 cm, and a height of 2 cm. The aqueous solutions containing 30 wt% of single and mixed monoethanolamine (MEA), 2-(2-aminoethylamino)ethanol (AEEA), and piperazine (PZ) were used. The CO2 capture efficiency was found to increase with increasing temperature in a range of 303–333 K. It was also found to be more dependent on gas and liquid flow rates but less dependent on rotating speed when the speed was higher than 700 rpm. The obtained results indicated that the mixed alkanolamine solutions containing PZ were more effective than the single alkanolamine solutions. This was attributed to the highest reaction rate of PZ with CO2. A higher portion of PZ in the mixture was more favorable to CO2 capture. The highest gas flow rates allowed to achieve a desired CO2 capture efficiency and the correspondent height of transfer unit (HTU) were determined at different aqueous solution flow rates. Because all the 30 wt% single and mixed alkanolamine solutions could result in a HTU less than 5.0 cm at a liquid flow rate of 100 mL/min, chemical absorption in a RPB instead of a packed bed adsorber is therefore suggested to capture CO2 from the flue gases in steel making processes. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: Carbon dioxide Hot stove gas Chemical absorption Alkanolamine Rotating packed bed Height of transfer unit
1. Introduction CO2 is a major greenhouse gas. It contributes to the global warming more than 60% (Halmann and Steinberg, 1999). Though CO2 capture from power generation plants has received a lot of attention in the past years, little attention has paid to CO2 capture from the exhausted gases in iron and steel industry so far. Nearly 27 Gt of CO2 was emitted to atmosphere in 2007 and about 9% of the total emission came from iron and steel industry (International Energy Agency, 2007). It is therefore required to capture CO2 from the exhausted gases in steel making processes to cope with the worldwide CO2 reduction demanding. China Steel Corporation (CSC), established in 1971, is the biggest steel-making corporation in Taiwan. It produces 9.55 million tonnes of crude steel annually. Because steel production is a highly energy intensive process (Farla et al., 1995), about 20 million tonnes of CO2 is emitted to atmosphere annually from CSC. One of the emitted gases containing CO2 in CSC is the hot stove gas. Its temperature and pressure are at about 400 K and 0.1 MPa, respectively, and its flow rate is around 2.4 105 m3/h comparable to that emitted form power plants. This gas stream contains about 30 vol% of CO2, 68 vol% of N2, 2.0 vol% of O2, less than 30 ppm of
* Corresponding author. Tel.: +886 3572 1189; fax: +886 3572 1684. E-mail address: [email protected] (C.-S. Tan). 1750-5836/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2009.12.006
SOx, and 13 ppm of NOx, all in dry basis. The CO2 content is quite different from that in the flue gases of power plants varying from 3 to 15 vol% depending on fossil fuel used. In this situation, a CO2 capture process specifically suitable for treatment of the hot stove gas is needed to develop. There are several existing technologies such as chemical absorption (Mariz, 1998; Chakma, 1997; Singh et al., 2003), physical absorption (Kohl and Nielsen, 1997; Aaron and Tsouris, 2005; Yokoyama, 2006), physical adsorption (Kikkinides et al., 1993; Siriwardane et al., 2005; Ruthven, 1984; Yang, 1987), and membrane (Hyun et al., 1999; Kuraoka et al., 2000; Chen et al., 2000; Feron and Jansen, 2002; Zhang et al., 2002) have been proposed to capture CO2 from gases. In this study, chemical absorption was chosen to capture CO2 from the hot stove gas of CSC. For chemical absorption in a packed bed absorber, significant mass transfer limitations usually exist. A huge volume is consequently required. To reduce absorber volume and to enhance mass transfer rate between gas and liquid, a rotating packed bed (RPB) in which liquid and gas come into contact in the presence of a high centrifugal field is suggested (Ramshaw and Mallinson, 1981; Tung and Mah, 1985; Munjal et al., 1989a,b; Lin et al., 2003; Rao et al., 2004; Cheng and Tan, 2006). This technology has been denoted as Higee and was originally proposed by Ramshaw and Mallinson (1981). To capture CO2 from a gas containing 10% of CO2, the overall mass transfer coefficients in a RPB had been proved to be larger than those in a conventional packed bed absorber,
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implying that RPB is an effective gas-liquid contactor (Lin et al., 2003; Tan and Chen, 2006). Jassim et al. (2007) calculated the height of transfer unit (HTU) for chemical absorption of CO2 from a gas stream containing 3.5–4.5% CO2 in a RPB. The calculated HTU was found to lie in a range of 14–21 cm that is much lower than 340 cm in a packed bed absorber, indicating a significant volume reduction using RPB. This advantage is especially important for CSC, because there is limited land on the plant site to install CO2 capture equipments. The absorbents commonly used for capturing CO2 from a gas stream containing high CO2 concentrations such as 10 vol% in a packed bed absorber are alkanolamines including monoethanolamine (MEA), 2-(2-aminoethylamino)ethanol (AEEA), diethanolamine (DEA), methyldiethanolamine (MDEA), 2-amino-2-methyl1-propanol (AMP), and piperazine (PZ) (Tontiwachwuthikul et al., 1992; Aroonwilas and Veawab, 2004; Yeh and Pennline, 2001; Mandal et al., 2001; Ma’mun et al., 2007b; Aroonwilas et al., 1999; Lu et al., 2005; Akanksha et al., 2007; Paul et al., 2007). The reaction rate with CO2 of these alkanolamines follows the order of PZ > AEEA > MEA > DEA > MDEA > AMP (Bishnoi and Rochelle, 2000; Ma’mun et al., 2007a; Ramachandran et al., 2006; Saha and Bandyopadhyay, 1995; Hikita et al., 1979; Alper, 1990). Though PZ is the most effective absorbent, it can only be used as the promoter because of its solubility limitation in water, about 150 g/L at room temperature. Due to a short contact time in a RPB, alkanolamines with higher reaction rate with CO2 such as PZ, MEA, and AEEA were found to be more effective than those with lower reaction rate with CO2 for the alkanolamine concentrations less than 15 wt% (Lin et al., 2003; Tan and Chen, 2006). To capture CO2 from the flue gases of power plants, SOx and NOx scrubbers prior to a packed bed absorber and the addition of oxygen inhibitor into alkanolamine solutions are required in order to overcome the problems as the loss of alkanolamine by the reaction between alkanolamine and SOx/NOx and degradation of alkanolamine by oxygen. Because SOx, and NOx, and O2 contents in the hot stove gas are much lower than those in the flue gases of coal-fired power plant as 300–3000 ppm for SOx, 100–1000 ppm for NOx, and 5–15% for O2 (Chi and Rochelle, 2000; Chakravarti et al., 2001; Gielen, 2003), the problems caused by SOx and NOx may not be serious and less oxygen inhibitor is expected to add into alkanolamine solution in the treatment of the hot stove gas of CSC. The objective of this study is to verify the applicability of chemical absorption in a RPB for capturing CO2 from the CSC hot stove gas containing a CO2 concentration as high as of 30 vol%. The problems caused by SOx, NOx, and O2 are therefore not concerned in the study. Because of such a high CO2 concentration, 30 wt% of alkanolamine solutions containing the alkanolamines with high reaction rate with CO2 including MEA, AEEA, and PZ and their mixtures were chosen as the absorbents in this study. To assess the performance of the single and mixed alkanolamine aqueous solutions, CO2 capture efficiency, CO2 loading, and HTU were calculated and compared. The effects of rotating speed, temperature, liquid flow rate, and gas flow rate on CO2 capture efficiency, CO2 loading, and HTU were studied. From the obtained experimental data and the calculated regeneration energy after absorption, the appropriate alkanolamine solutions could then be chosen and the highest gas flow rate allowed to enter the RPB to achieve a desired CO2 capture efficiency and the correspondent HTU could also be determined. 2. Experimental MEA, AEEA, and PZ with a purity of at least 99% were purchased from Tedia, Sigma–Aldrich, and Acros Organics, respectively. They were used as received. The physical and chemical properties of
these absorbents can be found elsewhere (Cheng and Tan, 2006). CO2 with a purity of 99.5% and nitrogen with a purity of 99.99% were purchased from Boclh Industrial Gases (Taiwan). The experimental apparatus used in this study was the same as that used by Tan and Chen (2006) except that the larger tubing was used to allow higher gas flow rates. The RPB packed with stainless wire mesh with an opening of 5 mm 2 mm possessed an inner and outer diameter of 7.6 and 16 cm, respectively, and a height of 2 cm. The total volume of the RPB was of 311.4 cm3. The specific surface area and void fraction of the stainless wire mesh were 803 m2/m3 and 0.96, respectively. The inlet liquid and gas streams were heated prior to entering the RPB. The flow rates of CO2 and N2 gases were controlled to result in a CO2 gas stream containing 30 vol% of CO2. The total gas flow rate was varied from 12 to 33.3 L/ min. The fed alkanolamine aqueous solution was prepared by adding a predetermined amount of single or mixed alkanolamines into deionized water. In the operation, the gas stream flowed inward from the outer edge of the RPB and the liquid solution flowed outward from the inner edge of the RPB and left from the outer edge via a centrifugal force. The flow rates of the liquid solution were observed to be high enough to avoid entrainment in the effluent gas stream. All the ratios of gas flow rate to liquid flow rate were less than 1000 to satisfy the suggestion by Lin et al. (2003) to avoid flooding. The CO2 concentrations in the inlet and effluent gas streams were measured by a NDIR CO2 analyzer (Drager, Polytron Transmitter IR CO2). The measurement range is from 0% to 30% with a resolution of 0.01%.
3. Results and discussion The CO2 capture efficiency and HTU used for assessing the performance at different operating conditions were calculated from the experimental data using the following equations (Jassim et al., 2007), capture efficiency ¼
HTU ¼
ro ri lnðY i =Y o Þ
Yi Yo 100% Yi
(1)
(2)
In Eqs. (1) and (2), Yi and Yo are CO2 content in vol% in the inlet and effluent gas streams, respectively, and ro and ri are the outer and inner radii in cm, respectively. Obviously, a higher capture efficiency represents the better CO2 capture from gas feed and a smaller HTU represents the smaller volume of absorber needed to achieve a desired capture efficiency. The CO2 loading, representing the number of CO2 moles absorbed per mole of alkanolamine absorbent fed to the RPB, was also calculated in this study. The equilibrium CO2 loading of each alkanolamine depends on its thermodynamic limitation, for example, the CO2 loading is 0.5 mole of CO2 for one mole of MEA and 1.0 mole of CO2 for one mole of AEEA. A higher CO2 loading for an absorbent therefore represents the closer to the equilibrium loading. The reproducibility tests at almost all of the operating conditions were performed in this study. The CO2 concentration in the effluent gas stream, capture efficiency, and HTU were observed to reproduce with an average deviation of less than 2%, indicating the reliability of the measurement. To assure the superiority of chemical absorption to physical absorption, the latter using pure methanol as the absorbent (Rectisol process) to capture CO2 was also performed in this study. However, due to the pressure limitation of the present RPB, the operations could only be carried out at a maximum pressure of 0.31 MPa. Fig. 1 shows that the capture efficiencies for using pure methanol as the absorbent at
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Fig. 1. Comparison of capture efficiency between physical absorption using pure methanol and chemical absorption using the mixtures 6.1 wt% MEA + 10.4 wt% AEEA and 6.1 wt% MEA + 20.8 wt% AEEA in a rotating packed bed with a rotating speed of 1000 rpm.
283 K and 0.31 MPa were about 15% and nearly invariant with liquid flow rate. They were much lower than those for using the solutions 6.1 wt% MEA + 10.4 wt% AEEA and 6.1 wt% MEA + 20.8 wt% AEEA as the absorbents in the same RPB at 333 K and 0.1 MPa. Obviously, higher capture efficiency could be achieved if the total alkanolamine concentration in liquid solution was increased to 30 wt%. In this situation, CO2 capture efficiency by physical absorption cannot be comparable with that by chemical absorption using alkanolamine mixtures, possibly due to the lower absorption rate of CO2 in methanol. Because a 30 wt% MEA aqueous solution is commonly used in commercial chemical absorption processes such as Fluor Daniel Econamine FGSM, this solution was first used to examine its CO2 capture efficiency for a gas stream containing 30 vol% CO2 in the RPB operation. Fig. 2 shows the dependence of capture efficiency on rotating speed at different gas flow rates for a fixed liquid flow rate of 60 mL/min and at different liquid flow rates for a fixed gas flow rate of 33.3 L/min using the 30 wt% MEA aqueous solution as the absorbent at 333 K. When the rotating speed varied from 100 to 1600 rpm, the correspondent centrifugal acceleration varied from 7 to 1726 m s2. It is seen from Fig. 2 that the capture efficiency was very low at a rotating speed of 100 rpm, because the centrifugal force is not high enough to generate small liquid droplets and thin films over the packing. When the rotating speeds were equal to and higher than 700 rpm, a similar trend as that reported by Guo et al. (1997), capture efficiency was nearly invariant, was observed. These capture efficiency results indicate that small liquid droplets and thin films, favorable to mass transfer, could be formed at relatively higher rotating speeds. At a very high rotating speed, an opposite effect resulting from a reduction of contact time between liquid and gas was supposed to happen. However, this opposite effect was not observed in the present study, probably due to high gas flow rates used in this study compared with that in our previous studies (Lin et al., 2003; Tan and Chen, 2006). Because a rotating speed of 1000 rpm exhibits a higher capture efficiency, though not so significant, compared to others, this rotating speed was therefore chosen for the further study. Because higher temperature results in higher amine evaporation, thermal degradation of alkanolamine, and lower CO2 equilibrium absorption capacity, the temperature generally lies in a range of 313–333 K in a packed bed absorber for capture of CO2 from the flue gas of power plant (Chakravarti et al., 2001). With the
Fig. 2. Dependence of capture efficiency on rotating speed for the 30 wt% MEA solution at 333 K and different (a) gas flow rates, and (b) liquid flow rates.
same concern, the experiments in this study were performed at 303–333 K. Fig. 3 shows a notable decrease in capture efficiency with gas flow rate in the studied temperature range at a fixed liquid flow rate using 30 wt% MEA solution as the absorbent. The reduction in capture efficiency was mainly resulted from the more CO2 needed to remove and the less contact time at higher gas flow rates. It is therefore not surprised to see from Fig. 3 that the capture efficiency increased with increasing liquid flow rate, resulting from more MEA allowed to absorb CO2 and a reduction of liquid-side mass transfer resistance. Fig. 3 also shows an increase in capture efficiency with increasing temperature at a fixed liquid and gas flow rate. It is known that equilibrium absorption capacity decreases with increasing temperature due to exothermic absorption nature and an increase in reaction rate with increasing temperature according to the Arrhenius equation. An increase in capture efficiency with temperature, at a first glance, seemed to be caused more by an increase in reaction rate than a decrease in equilibrium capacity at higher temperatures. However, it should be noted that more CO2 was present in the gas stream with a fixed volumetric flow rate at lower temperatures because of the higher density at a lower temperature. In this situation, a decrease in capture efficiency with decreasing temperature does not simply imply that less CO2 was captured at lower temperatures. This can be seen from Fig. 4 that the CO2 loadings at different temperatures were nearly identical for the gas flow rates higher than 18 L/min at a
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Fig. 3. Temperature dependence of CO2 capture efficiency for the 30 wt% MEA solution at different gas flow rates (&&, 12 L/min; **, 18 L/min; ~~, 24 L/min; 5!, 30 L/min; ^^, 33.3 L/min and liquid flow rates (open symbols, 80 mL/min; solid symbols, 100 mL/min)).
fixed liquid flow rate of 80 mL/min and for the gas flow rates higher than 24 L/min at a fixed liquid flow rate of 100 mL/min. Based on the different amounts of CO2 present in gas at different temperatures and nearly the same loading at different temperatures, a less capture efficiency therefore was observed at lower temperatures. Fig. 4 also shows that the CO2 loadings at high gas flow rates for a liquid flow rate of 80 mL/min approached to the equilibrium capacity (0.5 mole per mole of MEA) and were higher than those for 100 mL/min at which the CO2 loading was dropped to about 90% of its equilibrium capacity. More apart from the equilibrium capacity for a liquid flow rate of 100 mL/min was due to the presence of more MEA in liquid. It has been pointed out by Ma’mun et al. (2007b) that absorption rate of an alkanolamine with CO2 becomes reduced when absorption approaches to its equilibrium loading. The decreased absorption rate might be the other reason to cause the capture efficiency at liquid flow rate of 100 mL/min higher than that at liquid flow rate of 80 mL/min. It is seen from Fig. 4 that CO2 loading increased significantly with increasing gas flow rate when the gas flow rate varied from 12 to 24 L/min. This was due to a reduction of gas-side mass transfer resistances and the presence of more CO2 in gas allowed to be
Fig. 4. Loadings at different gas flow rates, liquid flow rates, and temperatures for the 30 wt% MEA solution and a rotating speed of 1000 rpm.
Fig. 5. Temperature dependence of CO2 capture efficiency and loading for the 30 wt% AEEA solution at different gas flow rates for a rotating speed of 1000 rpm and a fixed liquid flow rate of 80 mL/min (&&, 12 L/min; **, 18 L/min; ~~, 24 L/ min; 5!, 30 L/min; ^^, 33.3 L/min; open symbols represent capture efficiency and solid symbols represent loading).
absorbed by the 30 wt% MEA solution when gas flow rate increased (Lin et al., 2003). However, when the equilibrium loading was approached, the variation of loading with gas flow rate became small and the loading even dropped, though slightly at a gas flow rate over 30 L/min for a fixed liquid flow rate of 100 mL/min, shown in Fig. 4. The reason might be due to that the effect by the decrease in contact time between gas and liquid overweighed that by the decrease in mass transfer resistances at such a high gas flow rate. Because the loadings were sufficiently close to the equilibrium capacity at high gas flow rates (very short contact time in a RPB), the conclusion that MEA is an effective absorbent for capturing CO2 from a gas stream containing high CO2 concentration in a RPB operation can then be drawn. When a 30 wt% AEEA solution instead of a 30 wt% MEA solution was used to capture CO2, Fig. 5 shows a less dependence of the CO2 capture efficiency on temperature, indicating that the absorption was not controlled by reaction rate with CO2. It is known that the reaction rate of AEEA with CO2 is higher than that of MEA with CO2 (Ma’mun et al., 2007a). In a consequence, the effect of enhancement of reaction rate with temperature was not so pronounced for the use of AEEA as the absorbent. The increase in reaction rate with increasing temperature was then compensated by the decrease in CO2 loading. It is therefore seen from Fig. 5 that the loadings at gas flow rates higher than 18 L/min were different at different temperatures, not as those for the use of MEA as the absorbent as shown in Fig. 4. It is also seen from Figs. 4 and 5 that the capture efficiency for the 30 wt% AEEA solution were higher than those for the 30 wt% MEA solution, indicating the advantage of using an absorbent with higher reaction rate with CO2 in a RPB, just as suggested by Tan and Chen (2006) for treatment of a gas with 10 vol% of CO2 using a relatively low concentration of alkanolamines. To achieve a desired capture efficiency for the 30 wt% MEA solution, Fig. 3 shows that the highest gas flow rates allowed to treat depended on liquid flow rate and operation temperature. For examples, the hot stove gas with a flow rate of 24 L/min could be treated to achieve a capture efficiency of 83% for a liquid flow rate of at least 100 mL/min and a temperature of at least 323 K, but only 18 L/min gas could be treated to achieve a capture efficiency of 95% at the same liquid flow rate and operation temperature. It is also seen from Fig. 6 that a higher capture efficiency could be achieved using a higher MEA concentration solution. This was an expected result because more MEA was allowed to absorb CO2. It likes to point out here that the loadings for the liquid solutions containing
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Fig. 6. Capture efficiencies for different alkanolamine solutions at different gas flow rates for a rotating speed of 1000 rpm, a fixed liquid flow rate of 100 mL/min, and a temperature of 333 K (&, 30 wt% MEA; *, 5 wt% PZ + 25 wt% MEA; ~, 10 wt% PZ + 20 wt% MEA; &, 30 wt% AEEA; *, 5 wt% PZ + 25 wt% AEEA; ~, 10 wt% PZ + 20 wt% AEEA; !, 20 wt% MEA).
20 and 30 wt% MEA were nearly the same and close to the equilibrium capacity at high gas flow rates (the CO2 loadings for the 20 wt% MEA solution are not shown here), indicating that MEA is an effective absorbent used in RPB even though the reaction rate of MEA with CO2 is smaller than that of AEEA and PZ. It has been recognized that PZ is an effective promoter for chemical absorption of CO2 in a packed bed absorber and a RPB for gases with CO2 content less than 10 vol% (Bishnoi and Rochelle, 2000; Tan and Chen, 2006). The capture efficiencies for different alkanolamines and their mixtures for a total alkanolamine concentration in liquid of 30 wt%, a liquid flow rate of 100 mL/ min, and a temperature of 333 K, verify this recognition in a RPB for treatment of a gas with 30 vol% CO2 as well, shown in Fig. 6. It is seen that the mixtures containing PZ exhibited better CO2 capture efficiency over MEA and AEEA itself, and the more PZ was present in the mixture, the more CO2 could be captured from the gas stream. The enhancement of capture efficiency by PZ was mainly attributed to the faster reaction rate of PZ with CO2. Besides the better capture efficiency, PZ is easier to regenerate compared to MEA and AEEA based on the fact that the heat of reaction of PZ as 72 kJ/mol of CO2 is lower than that of MEA and AEEA, both about 80 kJ/mol of CO2. Though PZ possesses these advantages, a very high content of PZ in water is not possible because the solubility of PZ in water is limited to 13 wt% at 298 K. With this concern, the maximum content of PZ in water was at 10 wt% in the study. When a 90% capture efficiency was desired in the treatment of the hot stove gas with a flow rate of 24 L/min, only the alkanolamine mixtures containing 10 wt% of PZ and 20 wt% of AEEA or MEA could achieve this goal, shown in Fig. 6. To use these two mixtures in the RPB, Fig. 7 shows that the HTU were less than 2 cm, much smaller than that using a packed bed absorber. This verifies the conclusion by Lin et al. (2003) and Jassim et al. (2007) that the size of an absorber can be significantly reduced if a RPB instead of a packed bed absorber is used to capture CO2 from a gas stream. The significant volume reduction would be beneficial for CSC to treat its hot oven gas because of limited land on the plant site. The capture efficiency and HTU for the mixed alkanolamine solutions 10 wt% PZ + 20 wt% MEA or AEEA at a liquid flow rate of 80 mL/min (not shown here) were found to be nearly the same as those for the 30 wt% single MEA or AEEA solution at a liquid flow rate of 100 mL/min, indicating less absorbent is required to achieve the same capture purpose when a properly mixed alkanolamine
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Fig. 7. HTU for different alkanolamine solutions at different gas flow rates for a fixed liquid flow rate of 100 mL/min and a temperature of 333 K (&, 30 wt% MEA; *, 5 wt% PZ + 25 wt% MEA; ~, 10 wt% PZ + 20 wt% MEA; &, 30 wt% AEEA; *, 5 wt% PZ + 25 wt% AEEA; ~, 10 wt% PZ + 20 wt% AEEA; !, 20 wt% MEA).
solution is used. Obviously, this would be beneficial for CO2 capture because less energy is required in the subsequent regeneration of absorbent after the absorption. Fig. 8 shows the calculated total energies for regeneration of the alkanolamine solutions using the equation suggested by Chakma (1997) as total energy ¼ heat of reaction þ sensible heat þ latent heat of vaporization of water þ latent heat of vaporization of absorbent
(3)
and the reported heat of reaction and heat capacities of MEA, AEEA, and PZ (Steele et al., 1997; Chiu and Li., 1999; Cullinane and Rochelle, 2005, 2006; Kim and Svendsen, 2007; Mundhwa and Henni, 2007) and vapor pressures and latent heats of vaporization of water and alkanolamines (Smith et al., 2001; Chickos and Acree, 2003; Ma’mun et al., 2006; VonNiederhausern et al., 2006; Kim et al., 2008).
Fig. 8. Regeneration energy for different alkanolamine solutions at different gas flow rates for a rotating speed of 1000 rpm, a liquid flow rate of 100 mL/min, and a temperature of 333 K (&, 30 wt% MEA; *, 5 wt% PZ + 25 wt% MEA; ~, 10 wt% PZ + 20 wt% MEA; &, 30 wt% AEEA; *, 5 wt% PZ + 25 wt% AEEA; ~, 10 wt% PZ + 20 wt% AEEA; !, 20 wt% MEA).
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When the alkanolamine and gas flow rates were fixed at 100 mL/min and 24 L/min, respectively, the total regeneration energy calculated using Eq. (3) for the 30 wt% MEA and 20 wt% MEA solutions were 191.9 and 210.2 kJ, respectively. Because the CO2 absorbed by the 30 wt% MEA and 20 wt% MEA solutions were 0.23 and 0.15 mole/min, respectively, the regeneration energies for 30 wt% MEA and 20 wt% MEA solutions were then as 834 and 1401 kJ/(mol CO2), respectively, shown in Fig. 8. The main reason for more energy required to regenerate the 20 wt% MEA solution is due to more energy required to vaporize water. Obviously higher alkanolamine concentration in solution is desired if the corrosion problem caused by alkanolamine can be overcome. It is seen from Fig. 8 that regeneration energy decreased with increasing gas flow rate. This was because that the total energies at different gas flow rates calculated by Eq. (3) were not in significant difference, but a large difference existed for the CO2 absorbed at different gas flow rates, as illustrated in Fig. 4. It can also be seen from Fig. 8 that the required regeneration energies for the solutions 10 wt% PZ + 20 wt% MEA and 10 wt% PZ + 20 wt% AEEA were the least as compared to the other alkanolamine solutions, indicating the superiority of these two solutions for CO2 capture from a gas stream containing 30 vol% CO2. Figs. 7 and 8 show that the HTU for other 30 wt% single and mixed alkanolamine solutions were all less than 5.0 cm and the energies required for regeneration were comparable to those for the solutions 10 wt% PZ + 20 wt% MEA and 10 wt% PZ + 20 wt% AEEA, indicating the applicability of chemical absorption using the absorbents with fast reaction rate with CO2 in a RPB. 4. Conclusions The capture of CO2 from the hot stove gas of China Steel Corporation containing 30 vol% CO2 by chemical absorption using 30 wt% single alkanolamine such as MEA and AEEA and mixed alkanolamine solutions containing PZ, MEA, and AEEA over different rotating speed, temperature, gas flow rate, and liquid flow rate in a rotating packed bed was studied. From the measured CO2 capture efficiency, height of transfer unit, CO2 loading, and regeneration energy, the alkanolamine aqueous solutions containing 10 wt% PZ and 20 wt% MEA or AEEA were found to be the most effective absorbents to capture CO2, resulting in a HTU as low as 2.0 cm for a 90% CO2 capture efficiency. This was attributed to the highest reaction rate of PZ with CO2. A higher portion of PZ in solution was found to be more favorable to CO2 capture. However, PZ concentration in solution higher than 15 wt% is not possible due to its limited solubility in water at room temperature. The CO2 capture efficiency was found to increase with increasing temperature in a range of 303–333 K. It was also found to be strongly dependent on gas and liquid flow rates but less dependent on rotating speed when the speed was higher than 700 rpm. The highest gas flow rates allowed to achieve a desired CO2 capture efficiency varied with aqueous solution flow rate. Because all the 30 wt% single and the mixed alkanolamine solutions besides 10 wt% PZ and 20 wt% MEA or AEEA could resulted in a HTU less than 5.0 cm as well for treating a gas with a flow rate higher than 30 L/min in a RPB with a height of only 2 cm and a volume of 311.4 cm3 at proper operation conditions, chemical absorption in a RPB is therefore suggested to use for capturing CO2 from the flue gases in steel production processes. This operation is especially helpful for the company such as China Steel Corporation with limited land on the plant site. Acknowledgement Financial support from the China Steel Corporation, the grant number 95T6D0031E, is gratefully acknowledged.
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Contents lists available at ScienceDirect
International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc
CO2 capture from hot stove gas in steel making process Hsu-Hsiang Cheng a, Jui-Fu Shen b, Chung-Sung Tan a,* a b
Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC New Material Research & Development Department, China Steel Corp., Kaohsiung 81233, Taiwan, ROC
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 December 2008 Received in revised form 15 July 2009 Accepted 12 December 2009 Available online 7 January 2010
The capture of CO2 from a hot stove gas in steel making process containing 30 vol% CO2 by chemical absorption in a rotating packed bed (RPB) was studied. The RPB had an inner diameter of 7.6 cm, an outer diameter of 16 cm, and a height of 2 cm. The aqueous solutions containing 30 wt% of single and mixed monoethanolamine (MEA), 2-(2-aminoethylamino)ethanol (AEEA), and piperazine (PZ) were used. The CO2 capture efficiency was found to increase with increasing temperature in a range of 303–333 K. It was also found to be more dependent on gas and liquid flow rates but less dependent on rotating speed when the speed was higher than 700 rpm. The obtained results indicated that the mixed alkanolamine solutions containing PZ were more effective than the single alkanolamine solutions. This was attributed to the highest reaction rate of PZ with CO2. A higher portion of PZ in the mixture was more favorable to CO2 capture. The highest gas flow rates allowed to achieve a desired CO2 capture efficiency and the correspondent height of transfer unit (HTU) were determined at different aqueous solution flow rates. Because all the 30 wt% single and mixed alkanolamine solutions could result in a HTU less than 5.0 cm at a liquid flow rate of 100 mL/min, chemical absorption in a RPB instead of a packed bed adsorber is therefore suggested to capture CO2 from the flue gases in steel making processes. ß 2009 Elsevier Ltd. All rights reserved.
Keywords: Carbon dioxide Hot stove gas Chemical absorption Alkanolamine Rotating packed bed Height of transfer unit
1. Introduction CO2 is a major greenhouse gas. It contributes to the global warming more than 60% (Halmann and Steinberg, 1999). Though CO2 capture from power generation plants has received a lot of attention in the past years, little attention has paid to CO2 capture from the exhausted gases in iron and steel industry so far. Nearly 27 Gt of CO2 was emitted to atmosphere in 2007 and about 9% of the total emission came from iron and steel industry (International Energy Agency, 2007). It is therefore required to capture CO2 from the exhausted gases in steel making processes to cope with the worldwide CO2 reduction demanding. China Steel Corporation (CSC), established in 1971, is the biggest steel-making corporation in Taiwan. It produces 9.55 million tonnes of crude steel annually. Because steel production is a highly energy intensive process (Farla et al., 1995), about 20 million tonnes of CO2 is emitted to atmosphere annually from CSC. One of the emitted gases containing CO2 in CSC is the hot stove gas. Its temperature and pressure are at about 400 K and 0.1 MPa, respectively, and its flow rate is around 2.4 105 m3/h comparable to that emitted form power plants. This gas stream contains about 30 vol% of CO2, 68 vol% of N2, 2.0 vol% of O2, less than 30 ppm of
* Corresponding author. Tel.: +886 3572 1189; fax: +886 3572 1684. E-mail address: [email protected] (C.-S. Tan). 1750-5836/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2009.12.006
SOx, and 13 ppm of NOx, all in dry basis. The CO2 content is quite different from that in the flue gases of power plants varying from 3 to 15 vol% depending on fossil fuel used. In this situation, a CO2 capture process specifically suitable for treatment of the hot stove gas is needed to develop. There are several existing technologies such as chemical absorption (Mariz, 1998; Chakma, 1997; Singh et al., 2003), physical absorption (Kohl and Nielsen, 1997; Aaron and Tsouris, 2005; Yokoyama, 2006), physical adsorption (Kikkinides et al., 1993; Siriwardane et al., 2005; Ruthven, 1984; Yang, 1987), and membrane (Hyun et al., 1999; Kuraoka et al., 2000; Chen et al., 2000; Feron and Jansen, 2002; Zhang et al., 2002) have been proposed to capture CO2 from gases. In this study, chemical absorption was chosen to capture CO2 from the hot stove gas of CSC. For chemical absorption in a packed bed absorber, significant mass transfer limitations usually exist. A huge volume is consequently required. To reduce absorber volume and to enhance mass transfer rate between gas and liquid, a rotating packed bed (RPB) in which liquid and gas come into contact in the presence of a high centrifugal field is suggested (Ramshaw and Mallinson, 1981; Tung and Mah, 1985; Munjal et al., 1989a,b; Lin et al., 2003; Rao et al., 2004; Cheng and Tan, 2006). This technology has been denoted as Higee and was originally proposed by Ramshaw and Mallinson (1981). To capture CO2 from a gas containing 10% of CO2, the overall mass transfer coefficients in a RPB had been proved to be larger than those in a conventional packed bed absorber,
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implying that RPB is an effective gas-liquid contactor (Lin et al., 2003; Tan and Chen, 2006). Jassim et al. (2007) calculated the height of transfer unit (HTU) for chemical absorption of CO2 from a gas stream containing 3.5–4.5% CO2 in a RPB. The calculated HTU was found to lie in a range of 14–21 cm that is much lower than 340 cm in a packed bed absorber, indicating a significant volume reduction using RPB. This advantage is especially important for CSC, because there is limited land on the plant site to install CO2 capture equipments. The absorbents commonly used for capturing CO2 from a gas stream containing high CO2 concentrations such as 10 vol% in a packed bed absorber are alkanolamines including monoethanolamine (MEA), 2-(2-aminoethylamino)ethanol (AEEA), diethanolamine (DEA), methyldiethanolamine (MDEA), 2-amino-2-methyl1-propanol (AMP), and piperazine (PZ) (Tontiwachwuthikul et al., 1992; Aroonwilas and Veawab, 2004; Yeh and Pennline, 2001; Mandal et al., 2001; Ma’mun et al., 2007b; Aroonwilas et al., 1999; Lu et al., 2005; Akanksha et al., 2007; Paul et al., 2007). The reaction rate with CO2 of these alkanolamines follows the order of PZ > AEEA > MEA > DEA > MDEA > AMP (Bishnoi and Rochelle, 2000; Ma’mun et al., 2007a; Ramachandran et al., 2006; Saha and Bandyopadhyay, 1995; Hikita et al., 1979; Alper, 1990). Though PZ is the most effective absorbent, it can only be used as the promoter because of its solubility limitation in water, about 150 g/L at room temperature. Due to a short contact time in a RPB, alkanolamines with higher reaction rate with CO2 such as PZ, MEA, and AEEA were found to be more effective than those with lower reaction rate with CO2 for the alkanolamine concentrations less than 15 wt% (Lin et al., 2003; Tan and Chen, 2006). To capture CO2 from the flue gases of power plants, SOx and NOx scrubbers prior to a packed bed absorber and the addition of oxygen inhibitor into alkanolamine solutions are required in order to overcome the problems as the loss of alkanolamine by the reaction between alkanolamine and SOx/NOx and degradation of alkanolamine by oxygen. Because SOx, and NOx, and O2 contents in the hot stove gas are much lower than those in the flue gases of coal-fired power plant as 300–3000 ppm for SOx, 100–1000 ppm for NOx, and 5–15% for O2 (Chi and Rochelle, 2000; Chakravarti et al., 2001; Gielen, 2003), the problems caused by SOx and NOx may not be serious and less oxygen inhibitor is expected to add into alkanolamine solution in the treatment of the hot stove gas of CSC. The objective of this study is to verify the applicability of chemical absorption in a RPB for capturing CO2 from the CSC hot stove gas containing a CO2 concentration as high as of 30 vol%. The problems caused by SOx, NOx, and O2 are therefore not concerned in the study. Because of such a high CO2 concentration, 30 wt% of alkanolamine solutions containing the alkanolamines with high reaction rate with CO2 including MEA, AEEA, and PZ and their mixtures were chosen as the absorbents in this study. To assess the performance of the single and mixed alkanolamine aqueous solutions, CO2 capture efficiency, CO2 loading, and HTU were calculated and compared. The effects of rotating speed, temperature, liquid flow rate, and gas flow rate on CO2 capture efficiency, CO2 loading, and HTU were studied. From the obtained experimental data and the calculated regeneration energy after absorption, the appropriate alkanolamine solutions could then be chosen and the highest gas flow rate allowed to enter the RPB to achieve a desired CO2 capture efficiency and the correspondent HTU could also be determined. 2. Experimental MEA, AEEA, and PZ with a purity of at least 99% were purchased from Tedia, Sigma–Aldrich, and Acros Organics, respectively. They were used as received. The physical and chemical properties of
these absorbents can be found elsewhere (Cheng and Tan, 2006). CO2 with a purity of 99.5% and nitrogen with a purity of 99.99% were purchased from Boclh Industrial Gases (Taiwan). The experimental apparatus used in this study was the same as that used by Tan and Chen (2006) except that the larger tubing was used to allow higher gas flow rates. The RPB packed with stainless wire mesh with an opening of 5 mm 2 mm possessed an inner and outer diameter of 7.6 and 16 cm, respectively, and a height of 2 cm. The total volume of the RPB was of 311.4 cm3. The specific surface area and void fraction of the stainless wire mesh were 803 m2/m3 and 0.96, respectively. The inlet liquid and gas streams were heated prior to entering the RPB. The flow rates of CO2 and N2 gases were controlled to result in a CO2 gas stream containing 30 vol% of CO2. The total gas flow rate was varied from 12 to 33.3 L/ min. The fed alkanolamine aqueous solution was prepared by adding a predetermined amount of single or mixed alkanolamines into deionized water. In the operation, the gas stream flowed inward from the outer edge of the RPB and the liquid solution flowed outward from the inner edge of the RPB and left from the outer edge via a centrifugal force. The flow rates of the liquid solution were observed to be high enough to avoid entrainment in the effluent gas stream. All the ratios of gas flow rate to liquid flow rate were less than 1000 to satisfy the suggestion by Lin et al. (2003) to avoid flooding. The CO2 concentrations in the inlet and effluent gas streams were measured by a NDIR CO2 analyzer (Drager, Polytron Transmitter IR CO2). The measurement range is from 0% to 30% with a resolution of 0.01%.
3. Results and discussion The CO2 capture efficiency and HTU used for assessing the performance at different operating conditions were calculated from the experimental data using the following equations (Jassim et al., 2007), capture efficiency ¼
HTU ¼
ro ri lnðY i =Y o Þ
Yi Yo 100% Yi
(1)
(2)
In Eqs. (1) and (2), Yi and Yo are CO2 content in vol% in the inlet and effluent gas streams, respectively, and ro and ri are the outer and inner radii in cm, respectively. Obviously, a higher capture efficiency represents the better CO2 capture from gas feed and a smaller HTU represents the smaller volume of absorber needed to achieve a desired capture efficiency. The CO2 loading, representing the number of CO2 moles absorbed per mole of alkanolamine absorbent fed to the RPB, was also calculated in this study. The equilibrium CO2 loading of each alkanolamine depends on its thermodynamic limitation, for example, the CO2 loading is 0.5 mole of CO2 for one mole of MEA and 1.0 mole of CO2 for one mole of AEEA. A higher CO2 loading for an absorbent therefore represents the closer to the equilibrium loading. The reproducibility tests at almost all of the operating conditions were performed in this study. The CO2 concentration in the effluent gas stream, capture efficiency, and HTU were observed to reproduce with an average deviation of less than 2%, indicating the reliability of the measurement. To assure the superiority of chemical absorption to physical absorption, the latter using pure methanol as the absorbent (Rectisol process) to capture CO2 was also performed in this study. However, due to the pressure limitation of the present RPB, the operations could only be carried out at a maximum pressure of 0.31 MPa. Fig. 1 shows that the capture efficiencies for using pure methanol as the absorbent at
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Fig. 1. Comparison of capture efficiency between physical absorption using pure methanol and chemical absorption using the mixtures 6.1 wt% MEA + 10.4 wt% AEEA and 6.1 wt% MEA + 20.8 wt% AEEA in a rotating packed bed with a rotating speed of 1000 rpm.
283 K and 0.31 MPa were about 15% and nearly invariant with liquid flow rate. They were much lower than those for using the solutions 6.1 wt% MEA + 10.4 wt% AEEA and 6.1 wt% MEA + 20.8 wt% AEEA as the absorbents in the same RPB at 333 K and 0.1 MPa. Obviously, higher capture efficiency could be achieved if the total alkanolamine concentration in liquid solution was increased to 30 wt%. In this situation, CO2 capture efficiency by physical absorption cannot be comparable with that by chemical absorption using alkanolamine mixtures, possibly due to the lower absorption rate of CO2 in methanol. Because a 30 wt% MEA aqueous solution is commonly used in commercial chemical absorption processes such as Fluor Daniel Econamine FGSM, this solution was first used to examine its CO2 capture efficiency for a gas stream containing 30 vol% CO2 in the RPB operation. Fig. 2 shows the dependence of capture efficiency on rotating speed at different gas flow rates for a fixed liquid flow rate of 60 mL/min and at different liquid flow rates for a fixed gas flow rate of 33.3 L/min using the 30 wt% MEA aqueous solution as the absorbent at 333 K. When the rotating speed varied from 100 to 1600 rpm, the correspondent centrifugal acceleration varied from 7 to 1726 m s2. It is seen from Fig. 2 that the capture efficiency was very low at a rotating speed of 100 rpm, because the centrifugal force is not high enough to generate small liquid droplets and thin films over the packing. When the rotating speeds were equal to and higher than 700 rpm, a similar trend as that reported by Guo et al. (1997), capture efficiency was nearly invariant, was observed. These capture efficiency results indicate that small liquid droplets and thin films, favorable to mass transfer, could be formed at relatively higher rotating speeds. At a very high rotating speed, an opposite effect resulting from a reduction of contact time between liquid and gas was supposed to happen. However, this opposite effect was not observed in the present study, probably due to high gas flow rates used in this study compared with that in our previous studies (Lin et al., 2003; Tan and Chen, 2006). Because a rotating speed of 1000 rpm exhibits a higher capture efficiency, though not so significant, compared to others, this rotating speed was therefore chosen for the further study. Because higher temperature results in higher amine evaporation, thermal degradation of alkanolamine, and lower CO2 equilibrium absorption capacity, the temperature generally lies in a range of 313–333 K in a packed bed absorber for capture of CO2 from the flue gas of power plant (Chakravarti et al., 2001). With the
Fig. 2. Dependence of capture efficiency on rotating speed for the 30 wt% MEA solution at 333 K and different (a) gas flow rates, and (b) liquid flow rates.
same concern, the experiments in this study were performed at 303–333 K. Fig. 3 shows a notable decrease in capture efficiency with gas flow rate in the studied temperature range at a fixed liquid flow rate using 30 wt% MEA solution as the absorbent. The reduction in capture efficiency was mainly resulted from the more CO2 needed to remove and the less contact time at higher gas flow rates. It is therefore not surprised to see from Fig. 3 that the capture efficiency increased with increasing liquid flow rate, resulting from more MEA allowed to absorb CO2 and a reduction of liquid-side mass transfer resistance. Fig. 3 also shows an increase in capture efficiency with increasing temperature at a fixed liquid and gas flow rate. It is known that equilibrium absorption capacity decreases with increasing temperature due to exothermic absorption nature and an increase in reaction rate with increasing temperature according to the Arrhenius equation. An increase in capture efficiency with temperature, at a first glance, seemed to be caused more by an increase in reaction rate than a decrease in equilibrium capacity at higher temperatures. However, it should be noted that more CO2 was present in the gas stream with a fixed volumetric flow rate at lower temperatures because of the higher density at a lower temperature. In this situation, a decrease in capture efficiency with decreasing temperature does not simply imply that less CO2 was captured at lower temperatures. This can be seen from Fig. 4 that the CO2 loadings at different temperatures were nearly identical for the gas flow rates higher than 18 L/min at a
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Fig. 3. Temperature dependence of CO2 capture efficiency for the 30 wt% MEA solution at different gas flow rates (&&, 12 L/min; **, 18 L/min; ~~, 24 L/min; 5!, 30 L/min; ^^, 33.3 L/min and liquid flow rates (open symbols, 80 mL/min; solid symbols, 100 mL/min)).
fixed liquid flow rate of 80 mL/min and for the gas flow rates higher than 24 L/min at a fixed liquid flow rate of 100 mL/min. Based on the different amounts of CO2 present in gas at different temperatures and nearly the same loading at different temperatures, a less capture efficiency therefore was observed at lower temperatures. Fig. 4 also shows that the CO2 loadings at high gas flow rates for a liquid flow rate of 80 mL/min approached to the equilibrium capacity (0.5 mole per mole of MEA) and were higher than those for 100 mL/min at which the CO2 loading was dropped to about 90% of its equilibrium capacity. More apart from the equilibrium capacity for a liquid flow rate of 100 mL/min was due to the presence of more MEA in liquid. It has been pointed out by Ma’mun et al. (2007b) that absorption rate of an alkanolamine with CO2 becomes reduced when absorption approaches to its equilibrium loading. The decreased absorption rate might be the other reason to cause the capture efficiency at liquid flow rate of 100 mL/min higher than that at liquid flow rate of 80 mL/min. It is seen from Fig. 4 that CO2 loading increased significantly with increasing gas flow rate when the gas flow rate varied from 12 to 24 L/min. This was due to a reduction of gas-side mass transfer resistances and the presence of more CO2 in gas allowed to be
Fig. 4. Loadings at different gas flow rates, liquid flow rates, and temperatures for the 30 wt% MEA solution and a rotating speed of 1000 rpm.
Fig. 5. Temperature dependence of CO2 capture efficiency and loading for the 30 wt% AEEA solution at different gas flow rates for a rotating speed of 1000 rpm and a fixed liquid flow rate of 80 mL/min (&&, 12 L/min; **, 18 L/min; ~~, 24 L/ min; 5!, 30 L/min; ^^, 33.3 L/min; open symbols represent capture efficiency and solid symbols represent loading).
absorbed by the 30 wt% MEA solution when gas flow rate increased (Lin et al., 2003). However, when the equilibrium loading was approached, the variation of loading with gas flow rate became small and the loading even dropped, though slightly at a gas flow rate over 30 L/min for a fixed liquid flow rate of 100 mL/min, shown in Fig. 4. The reason might be due to that the effect by the decrease in contact time between gas and liquid overweighed that by the decrease in mass transfer resistances at such a high gas flow rate. Because the loadings were sufficiently close to the equilibrium capacity at high gas flow rates (very short contact time in a RPB), the conclusion that MEA is an effective absorbent for capturing CO2 from a gas stream containing high CO2 concentration in a RPB operation can then be drawn. When a 30 wt% AEEA solution instead of a 30 wt% MEA solution was used to capture CO2, Fig. 5 shows a less dependence of the CO2 capture efficiency on temperature, indicating that the absorption was not controlled by reaction rate with CO2. It is known that the reaction rate of AEEA with CO2 is higher than that of MEA with CO2 (Ma’mun et al., 2007a). In a consequence, the effect of enhancement of reaction rate with temperature was not so pronounced for the use of AEEA as the absorbent. The increase in reaction rate with increasing temperature was then compensated by the decrease in CO2 loading. It is therefore seen from Fig. 5 that the loadings at gas flow rates higher than 18 L/min were different at different temperatures, not as those for the use of MEA as the absorbent as shown in Fig. 4. It is also seen from Figs. 4 and 5 that the capture efficiency for the 30 wt% AEEA solution were higher than those for the 30 wt% MEA solution, indicating the advantage of using an absorbent with higher reaction rate with CO2 in a RPB, just as suggested by Tan and Chen (2006) for treatment of a gas with 10 vol% of CO2 using a relatively low concentration of alkanolamines. To achieve a desired capture efficiency for the 30 wt% MEA solution, Fig. 3 shows that the highest gas flow rates allowed to treat depended on liquid flow rate and operation temperature. For examples, the hot stove gas with a flow rate of 24 L/min could be treated to achieve a capture efficiency of 83% for a liquid flow rate of at least 100 mL/min and a temperature of at least 323 K, but only 18 L/min gas could be treated to achieve a capture efficiency of 95% at the same liquid flow rate and operation temperature. It is also seen from Fig. 6 that a higher capture efficiency could be achieved using a higher MEA concentration solution. This was an expected result because more MEA was allowed to absorb CO2. It likes to point out here that the loadings for the liquid solutions containing
H.-H. Cheng et al. / International Journal of Greenhouse Gas Control 4 (2010) 525–531
Fig. 6. Capture efficiencies for different alkanolamine solutions at different gas flow rates for a rotating speed of 1000 rpm, a fixed liquid flow rate of 100 mL/min, and a temperature of 333 K (&, 30 wt% MEA; *, 5 wt% PZ + 25 wt% MEA; ~, 10 wt% PZ + 20 wt% MEA; &, 30 wt% AEEA; *, 5 wt% PZ + 25 wt% AEEA; ~, 10 wt% PZ + 20 wt% AEEA; !, 20 wt% MEA).
20 and 30 wt% MEA were nearly the same and close to the equilibrium capacity at high gas flow rates (the CO2 loadings for the 20 wt% MEA solution are not shown here), indicating that MEA is an effective absorbent used in RPB even though the reaction rate of MEA with CO2 is smaller than that of AEEA and PZ. It has been recognized that PZ is an effective promoter for chemical absorption of CO2 in a packed bed absorber and a RPB for gases with CO2 content less than 10 vol% (Bishnoi and Rochelle, 2000; Tan and Chen, 2006). The capture efficiencies for different alkanolamines and their mixtures for a total alkanolamine concentration in liquid of 30 wt%, a liquid flow rate of 100 mL/ min, and a temperature of 333 K, verify this recognition in a RPB for treatment of a gas with 30 vol% CO2 as well, shown in Fig. 6. It is seen that the mixtures containing PZ exhibited better CO2 capture efficiency over MEA and AEEA itself, and the more PZ was present in the mixture, the more CO2 could be captured from the gas stream. The enhancement of capture efficiency by PZ was mainly attributed to the faster reaction rate of PZ with CO2. Besides the better capture efficiency, PZ is easier to regenerate compared to MEA and AEEA based on the fact that the heat of reaction of PZ as 72 kJ/mol of CO2 is lower than that of MEA and AEEA, both about 80 kJ/mol of CO2. Though PZ possesses these advantages, a very high content of PZ in water is not possible because the solubility of PZ in water is limited to 13 wt% at 298 K. With this concern, the maximum content of PZ in water was at 10 wt% in the study. When a 90% capture efficiency was desired in the treatment of the hot stove gas with a flow rate of 24 L/min, only the alkanolamine mixtures containing 10 wt% of PZ and 20 wt% of AEEA or MEA could achieve this goal, shown in Fig. 6. To use these two mixtures in the RPB, Fig. 7 shows that the HTU were less than 2 cm, much smaller than that using a packed bed absorber. This verifies the conclusion by Lin et al. (2003) and Jassim et al. (2007) that the size of an absorber can be significantly reduced if a RPB instead of a packed bed absorber is used to capture CO2 from a gas stream. The significant volume reduction would be beneficial for CSC to treat its hot oven gas because of limited land on the plant site. The capture efficiency and HTU for the mixed alkanolamine solutions 10 wt% PZ + 20 wt% MEA or AEEA at a liquid flow rate of 80 mL/min (not shown here) were found to be nearly the same as those for the 30 wt% single MEA or AEEA solution at a liquid flow rate of 100 mL/min, indicating less absorbent is required to achieve the same capture purpose when a properly mixed alkanolamine
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Fig. 7. HTU for different alkanolamine solutions at different gas flow rates for a fixed liquid flow rate of 100 mL/min and a temperature of 333 K (&, 30 wt% MEA; *, 5 wt% PZ + 25 wt% MEA; ~, 10 wt% PZ + 20 wt% MEA; &, 30 wt% AEEA; *, 5 wt% PZ + 25 wt% AEEA; ~, 10 wt% PZ + 20 wt% AEEA; !, 20 wt% MEA).
solution is used. Obviously, this would be beneficial for CO2 capture because less energy is required in the subsequent regeneration of absorbent after the absorption. Fig. 8 shows the calculated total energies for regeneration of the alkanolamine solutions using the equation suggested by Chakma (1997) as total energy ¼ heat of reaction þ sensible heat þ latent heat of vaporization of water þ latent heat of vaporization of absorbent
(3)
and the reported heat of reaction and heat capacities of MEA, AEEA, and PZ (Steele et al., 1997; Chiu and Li., 1999; Cullinane and Rochelle, 2005, 2006; Kim and Svendsen, 2007; Mundhwa and Henni, 2007) and vapor pressures and latent heats of vaporization of water and alkanolamines (Smith et al., 2001; Chickos and Acree, 2003; Ma’mun et al., 2006; VonNiederhausern et al., 2006; Kim et al., 2008).
Fig. 8. Regeneration energy for different alkanolamine solutions at different gas flow rates for a rotating speed of 1000 rpm, a liquid flow rate of 100 mL/min, and a temperature of 333 K (&, 30 wt% MEA; *, 5 wt% PZ + 25 wt% MEA; ~, 10 wt% PZ + 20 wt% MEA; &, 30 wt% AEEA; *, 5 wt% PZ + 25 wt% AEEA; ~, 10 wt% PZ + 20 wt% AEEA; !, 20 wt% MEA).
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When the alkanolamine and gas flow rates were fixed at 100 mL/min and 24 L/min, respectively, the total regeneration energy calculated using Eq. (3) for the 30 wt% MEA and 20 wt% MEA solutions were 191.9 and 210.2 kJ, respectively. Because the CO2 absorbed by the 30 wt% MEA and 20 wt% MEA solutions were 0.23 and 0.15 mole/min, respectively, the regeneration energies for 30 wt% MEA and 20 wt% MEA solutions were then as 834 and 1401 kJ/(mol CO2), respectively, shown in Fig. 8. The main reason for more energy required to regenerate the 20 wt% MEA solution is due to more energy required to vaporize water. Obviously higher alkanolamine concentration in solution is desired if the corrosion problem caused by alkanolamine can be overcome. It is seen from Fig. 8 that regeneration energy decreased with increasing gas flow rate. This was because that the total energies at different gas flow rates calculated by Eq. (3) were not in significant difference, but a large difference existed for the CO2 absorbed at different gas flow rates, as illustrated in Fig. 4. It can also be seen from Fig. 8 that the required regeneration energies for the solutions 10 wt% PZ + 20 wt% MEA and 10 wt% PZ + 20 wt% AEEA were the least as compared to the other alkanolamine solutions, indicating the superiority of these two solutions for CO2 capture from a gas stream containing 30 vol% CO2. Figs. 7 and 8 show that the HTU for other 30 wt% single and mixed alkanolamine solutions were all less than 5.0 cm and the energies required for regeneration were comparable to those for the solutions 10 wt% PZ + 20 wt% MEA and 10 wt% PZ + 20 wt% AEEA, indicating the applicability of chemical absorption using the absorbents with fast reaction rate with CO2 in a RPB. 4. Conclusions The capture of CO2 from the hot stove gas of China Steel Corporation containing 30 vol% CO2 by chemical absorption using 30 wt% single alkanolamine such as MEA and AEEA and mixed alkanolamine solutions containing PZ, MEA, and AEEA over different rotating speed, temperature, gas flow rate, and liquid flow rate in a rotating packed bed was studied. From the measured CO2 capture efficiency, height of transfer unit, CO2 loading, and regeneration energy, the alkanolamine aqueous solutions containing 10 wt% PZ and 20 wt% MEA or AEEA were found to be the most effective absorbents to capture CO2, resulting in a HTU as low as 2.0 cm for a 90% CO2 capture efficiency. This was attributed to the highest reaction rate of PZ with CO2. A higher portion of PZ in solution was found to be more favorable to CO2 capture. However, PZ concentration in solution higher than 15 wt% is not possible due to its limited solubility in water at room temperature. The CO2 capture efficiency was found to increase with increasing temperature in a range of 303–333 K. It was also found to be strongly dependent on gas and liquid flow rates but less dependent on rotating speed when the speed was higher than 700 rpm. The highest gas flow rates allowed to achieve a desired CO2 capture efficiency varied with aqueous solution flow rate. Because all the 30 wt% single and the mixed alkanolamine solutions besides 10 wt% PZ and 20 wt% MEA or AEEA could resulted in a HTU less than 5.0 cm as well for treating a gas with a flow rate higher than 30 L/min in a RPB with a height of only 2 cm and a volume of 311.4 cm3 at proper operation conditions, chemical absorption in a RPB is therefore suggested to use for capturing CO2 from the flue gases in steel production processes. This operation is especially helpful for the company such as China Steel Corporation with limited land on the plant site. Acknowledgement Financial support from the China Steel Corporation, the grant number 95T6D0031E, is gratefully acknowledged.
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