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A Kinetic Expression for the Carbonation Reaction of K2CO3/ZrO2 Sorbent for CO2 Capture Deuk Ki Lee, Da Young Min, Hwimin Seo, Na Young Kang, Won Choon Choi, and Yong-Ki Park Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie401407j • Publication Date (Web): 06 Jun 2013 Downloaded from http://pubs.acs.org on June 12, 2013

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Industrial & Engineering Chemistry Research

Revised

A Kinetic Expression for the Carbonation Reaction of K2CO3/ZrO2 Sorbent for CO2 Capture

Deuk Ki Leea,*, Da Young Min, Hwimin Seob, Na Young Kangb, Won Choon Choib, Yong Ki Parkb,* a

Department of Fire Safety, Gwangju University, Gwangju 503-703, Korea

b

Green Chemistry Process Research Division, Korea Research Institute of Chemical

Technology, Daejeon 305-600, Korea

* Corresponding authors. Tel.: +82 62 670 2394. E-mail address: [email protected] (D. K. Lee) Tel.:+82 42 860 7672. E-mail address: [email protected] (Y. K. Park)

1

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Abstract

For the kinetic study of K2CO3/ZrO2 sorbent in the carbonation reaction to capture CO2 from the flue gas, reaction experiments were carried out at temperatures between 328 and 343 K for CO2 gas compositions not exceeding 18% at 1 bar, and a phenomenological kinetic model was proposed to fit the carbonation conversion data obtained. Time-dependent carbonation conversions of the sorbent appeared as sigmoid curves. Sigmoid characteristics of the conversion curve was more pronounced for the sorption reaction conditions at lower temperatures and lower gas phase concentrations of CO2. Such conversion behavior of freshdried K2CO3/ZrO2 sorbent could be closely described with the reaction rate equation in the form: ∙ ∙ . The reaction rate constant as a temperature dependent term could be represented by Arrhenius’ equation with the negative apparent activation energy of 17.43 kJ/mol and the pre-exponential factor of 3.83 10 min-1. The term was a function introduced to reflect the carbonation rate change with the fractional carbonation conversion of the sorbent, and its parameters were determined by correlation equations of the reaction temperature and the gas phase concentration of CO2. The reaction order with respect to , the gas phase mole fraction of CO2, was determined to be 0.49. And, the characteristics of K2CO3/ZrO2 sorbent in the carbonation reaction for CO2 capture was discussed in relation to the kinetics obtained.

Keywords Carbon Dioxide (CO2) Capture, Potassium Carbonate (K2CO3), Solid Sorbent, Kinetic Equation

2


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1. Introduction

A huge amount of CO2 emission to atmosphere as a major contributing source to the global warming has become a growing concern in the recent years. To cope with the global demand for the reduction of CO2 emission, various research efforts have been made toward the development of more efficient technologies for CO2 capture from such large point sources as coal-fired power stations. Though a variety of CO2 capture technologies have been proposed, post-combustion CO2 capture from the flue gas is regarded as one of the key technology options to reduce the amount of the atmospheric emission, because this can be potentially retrofitted to the existing fleet of coal-fired power stations.1 As one of most suitable CO2 capturing options for post-combustion power plants, CO2 capture from flue gas stream using dry solid sorbents is considered to be promising. This is because the regeneration energy requirement for CO2 capture using dry solid sorbents can be significantly reduced as compared with the conventional aqueous amine-based process using a large amount of water of relatively high heat capacity.1,2 However, for their successful process application, dry solid sorbents of low cost with high CO2 sorption capacity and selectivity, fast kinetics of sorption/desorption reactions and durability are required. As the low cost sorbents with relatively high sorption capacity, alkaline metal carbonate solids such as K2CO3 and Na2CO3 have been recently investigated.3-7 K2CO3 sorbent reacts with CO2 in presence of H2O to form KHCO3 at low temperatures between 323 and 353 K as follows:

K CO s CO g H Og ↔ 2KHCO s , ∆#$% &141.2 kJ/mol

3


(1)


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Regeneration of the sorbent can be accomplished at relatively low temperature around 393 K. Some alkaline metal carbonate-based sorbents as supported on porous carriers like γ-Al2O3 or TiO2 have been developed and reported to have enhanced kinetic rates and improved applicability to a fluidized bed operation.3, 8-11 Such carbonate sorbent-based CO2 capture processes are evaluated to be currently in the experimental stages of development and is nearing implementation in post-combustion coal- and natural-gas pilot plants.12 For the process application of the solid sorbents, a riser type fast fluidized bed would be preferable to a bubbling bed as a carbonation reactor because a reactor of very wide bed area might be required for the latter to keep bubbling fluidization against a huge volumetric flue gas flow from power plants.5 However, considering the relatively short retention time of solid sorbents, the application of a fast fluidized bed as a carbonator is confronted with a problem requiring a sorbent of very fast kinetic rate in the carbonation reaction.

Zhao et al.

13

conducted kinetic experiments for the carbonation of 20 μm-sized K2CO3 particles using thermogravimetric analyzer (TGA), reporting that the time-dependent carbonation conversion of the sorbent appeared as a sigmoid curve due to the presence of the initial rate retardation in the reaction. This suggests that the kinetic rate of the carbonation reaction of K2CO3 could not be the highest at the state of the sorbent consisted of pure K2CO3, and that there would be a certain range of carbonation conversions where the carbonation reaction of the sorbent could proceed at higher rates. If the carbonation reaction would be prepared to occur around such a high kinetic rate range of the sorbent conversions, single pass carbonation conversion of sorbents in a fast fluidized bed carbonator could be enhanced. In this sense, a kinetic model capable of precisely describing the sorbent conversion behavior including the initial rate retardation should be essential to the design or simulation of this riser type carbonation reactor. However, there have been no available kinetic models on literatures in depicting the 4


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overall carbonation conversion behavior of K2CO3-based supported sorbents in spite of a few papers8, 13 dealing with the reaction from the kinetic point of view. For instance, Zhao et al. 13 tried to use the unreacted shrinking core model on a theoretical basis to represent the kinetics of the reaction, but the model seemed not to be successful in depicting the sigmoid characteristics of the time-dependent carbonation conversion behavior of the K2CO3 sorbent. In the present study, we propose a phenomenological kinetic model capable of finely predicting the overall carbonation behavior of the K2CO3/ZrO2 sorbent using kinetic data obtained from reaction experiments conducted at different temperatures and CO2 concentrations.

2. Kinetic model development

In the CO2 sorption reaction using solid sorbents, as a noncatalytic gas-solid heterogeneous reaction, kinetic rate of the sorbent carbonation conversion is generally influenced by the three factors such as the sorption reaction temperature, the degree of carbonation conversion of the sorbent, and the gas phase concentrations of CO2. In the case of K2CO3/ZrO2 sorbent, as suggested by reaction 1, the concentration of H2O as well as that of CO2 can influence on the reaction kinetics. However, Zhao et al.13 reported for K2CO3-based sorbents that similar sorbent conversion behaviors were observed irrespective of the concentrations of H2O under the condition that the concentration of H2O was comparable to or higher than that of CO2 in the reactant gas stream. Provided that this conditional concentration of H2O to that of CO2 in the reactant gas stream is kept, the sorbent conversion rate can be expressed as follows:

∙ ∙ 5


(2)


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where, is a reaction rate constant with the Arrhenius temperature dependency, is a rate-affecting term as a function of the fractional carbonation conversion X of the sorbent, and is a rate dependency as a function of the CO2 mole fraction in the bulk gas phase. In order to describe the sigmoid characteristics of the time-dependent carbonation conversion behavior, Monazam et al.14 proposed a phenomenological kinetic expression for of the sorbent of mesoporous silica grafted amine functional group, as a function of carbonation reaction time t, as follows:

1 & exp&34 5

(3)

where, a and 6 are referred in their paper as a scale parameter and a shape parameter, respectively. In this Monazam et al.’s model for the silica-immobilized amine sorbent, ultimate fractional carbonation conversion 7 approaches unity as time passes. In contrast to this silica-immobilized amine sorbent of which carbonation conversion often reaches completion actually within a few minutes, the ultimate fractional conversion of a supported or unsupported alkali metal-based sorbent such as K2CO38,9,13 or CaO15,16 is usually restricted to a value less than unity in the CO2 sorption reaction despite of its extended exposure time to CO2 even at each best carbonation reaction condition. This is because of the diffusion limitation of gaseous reactants through the outer solid product layer formed within a sorbent particle as the reaction proceeds. Therefore, eq 3 needs to be modified for the steady-state fractional conversion to become an ultimate value 7 less than unity:

6


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7 81 & exp 9&

:; < =>

?@

(4)

Figure 1 shows the profiles of X and its time-derivative dX/dt of eq 4 in variation with the parameters a and b at Xu = 0.8. As 3 increases, as shown in Figure 1(A), the slope of the curve becomes steeper at earlier time, and accordingly, taking less time to reach steady state. As 6 increases, as shown in Figure 1(B), the curve becomes more sigmoid with a longer initial induction period. Although the time at which the maximum carbonation rate occurs become later, time taken to reach steady state decreases as b increases. Here, under the condition that the reaction proceeds at a constant CO2 concentration, the sorbent conversion rate can be obtained by differentiating eq 4 with respect to time:

A= A;

364 5B exp 9&

:; < =>

?

(5)

From eq 4, time can be expressed as

4 8&

=> :

=

ln 91 & = ?@

D

(6)

Substituting eq 6 into eq 5, after rearrangement, leads to

A= A;

E ∙

where,

7


(7)


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D

D

′ 6 3 < 7B

(8)

D

B

?

(9)

The sorbent conversion rates by eq 7 are represented by the broken lines in Figure 1. Because the sorbent conversions by eq 4 do not largely deviate from the curves typical of the firstorder exponential rise, as shown in Figure 1, it is possible to introduce the time constant G as an indication of the time taken for a sorbent to reach 63.2% of its ultimate conversion. At t = G in eq 4, the conversion corresponding to 63.2% of the ultimate conversion is attained under the condition: &3G 5 /7 &1. Therefore,

=

G 9 :> ?

D

(14)

where,

′′ ∙ ′

(15)

The parameters in eq 4, 3, 6 and 7 , can be determined throughout the least squares curve fitting for the conversion data with time, and resultantly, the value of ′ in eq 8 could be obtained. The reaction order with respect to the gas phase mole fraction of CO2 in eq 13 would be determined by the slope in the plot of logarithmic ′ values against logarithmic

in the reactions conducted at the same temperature. Finally, the reaction rate constant obtained using eq 15, could be represented as an Arrhenius equation.

9



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3. Experimental

3.1. Sorbent preparation To prepare K2CO3/ZrO2 sorbent used in this study, porous ZrO2 (MEL Chemicals) support was pore-filled in incipient wetness with water solution of K2CO3 (99.5%, Samchun). The impregnated sample was aged at room temperature for 12 h, and then oven-dried at 393 K for 12 h, and finally calcined at 573 K for 4 h. The loading amount of K2CO3 was designed to be 30wt% of the prepared sorbent, and determined by 29.6wt% using AAS. The sorbent after calcination was in fine particles with surface area, 15.6 m2/g, and pore volume, 0.1 cm3/g. For carbonation reaction experiments, a mass of prepared sorbents was pressed, broken, and finally sieved to particles of 80 ~ 120 mesh sizes.

3.2. Apparatus and procedure Experiments of the sorbent carbonation reaction were conducted using AutoChem II (Micrometrics). About 300 mg of K2CO3/ZrO2 sorbent was used for the carbonation reaction. Prior to loading onto the quartz fritted tube reactor as a fixed bed of shallow depth, the sieved sorbent particles were evenly mixed using the same amount of α-Al2O3 particles as diluents. The reactor loaded fresh sorbents were pretreated using He flow (50 cm3/min) at 473 K for 1 h for dehydration, and then the reactor bed temperature was lowered to a set value for the CO2 sorption reaction. The carbonation reaction experiments were carried out at temperatures in the range of 328 ~ 338 K with CO2 concentrations 2.5, 6.25, 10, and 18% at 1 bar using the total reactant gas (CO2, H2O and He as a balance) flow rate kept at 80 cm3/min. In order to exclude the effect of H2O on the carbonation reaction rate, H2O concentrations in the reactant gas stream were adjusted to be comparable to or more than the concentrations of CO2, i.e., 18% 10


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for the reaction conducted at 18% CO2, and 10% for the reactions at other CO2 concentrations. H2O feeding was accomplished using He gas passing through a water evaporator kept at an adjusted temperature to obtain the desired composition of H2O in the total reactant gas. At first, this H2O-loaded He gas flow was directed, bypassing the reactor, to a TCD detector. Prior to reaching the detector, the gas was dehydrated using a cold trap. Once the TCD signal was stabilized, a controlled flow of CO2 was allowed to join this reactor-bypassing flow of H2O-loaded He gas. After the TCD signal on CO2 concentration was stabilized, this total reactant gas flow was switched, instead of the He flow for dehydration, to pass though the reactor to initiate the carbonation reaction. The TCD signal was monitored for the reaction duration of 0.5 h. Along with the carbonation reaction experiment as stated above, an empty run was separately conducted in the same experimental sequence to obtain the blank signal data on CO2 concentration using the reactor filled with only diluents of the same bed size without the sorbents. The amount of CO2 consumed by sorbent carbonation reaction was obtained by subtracting the blank run signal data from the reaction run signal data. The fractional conversion of the sorbent was calculated as

TUV

T

W UVX

(16)

where, YZ[ and Y\ Z[X are the amount of CO2 reacted and that of K2CO3 supported in mmoles per gram of sorbents used for the reaction experiments, respectively.

4. Results and discussion

4.1. Determination of model parameters and kinetic constants 11



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Figure 2 shows the results of the least squares regression fit to eq 4 for the carbonation conversion data experimented at 338 K and 10.0% CO2, and the rate of conversion by eq 7 with time. Overall conversion dada could be well represented by eq 4 with the parameter values determined, 7 = 0.782, 6 = 1.346, 3 = 0.379, ′ = 0.614. In addition, the sorbent conversion rate obtained by eq 7 is shown to correspond closely to the rate experimentally obtained. Table 1 lists such determined parameter values for the sorption reaction results conducted at different reaction temperatures and gas phase mole fractions of CO2. Because the parameter values are varied depending on the reaction temperature and the gas phase mole fraction of CO2, there is a need to correlate them with the two reaction variables. The results of the linear correlation analysis are listed in Table 2. And, Figure 3 shows the parity plots for the correlation results. The parameters 6 and 3 are well correlated with T and

by eqs 18 and 19, respectively, but there are some scattered data in the correlation of 7 . Figure 4(A) shows Arrhenius plots for determining the apparent activation energy, ]^,:__ , in the CO2 sorption reaction over K2CO3/ZrO2. The line slopes in Figure 4(A) were 2096 and 2249 for data experimented at the gas phase mole fractions of CO2, 0.1 and 0.0625, respectively. Because the data experimented at = 0.0625 are somewhat scattered, the values of apparent activation energy and pre-exponential factor were determined as ]^,:__ &17.43 kJ ∙ molB and A 1.24 10 minB , respectively, using the data experimented at = 0.1,. Consequently,

E 1.24 10 exp 9

Bbcd $e

?

12


(20)

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Figure 4(B) shows the kinetic rate dependence on the gas phase mole fractions of CO2 for data at 338 K, to determine the reaction order. The data are well represented by a straight line with a slope as the reaction order, = 0.49. In eq 13, the value of the constant ′′ is so determined that J K could become unity at = 0.1 and f = 338 K, resulting in EE 3.09. Therefore, eq 13 becomes

J K 3.09 d.ch

(21)

Finally, the reaction rate constant in eq 15 can be expressed as follows:

3.83 10 exp 9

Bbcd $e

?

(22)

As results of the above kinetic analysis, Figure 5 shows the comparison of the carbonation rate change with the degree of sorbent conversion between experimental and predicted by eq 14, where , 7 and 6 have been calculated using the correlation eqs 22, 17 and 18, respectively. The kinetic model predictions are shown satisfactory in depicting the experimental rate data on the whole, although some overestimations are made for data obtained at 18% CO2. Figure 6 shows the comparison between the experimental sorbent conversions and the kinetic model predictions obtained by integrating eq 14. In view of excellent agreement between experimental and calculated results at different reaction temperatures and gas phase CO2 concentrations, the kinetic model developed here is capable of providing fine predictions for the K2CO3/ZrO2 sorbent conversion within the experimental conditions employed in this study.

13



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4.2. Characteristics of the K2CO3/ZrO2 carbonation kinetics

As shown in Figure 5, starting rate of the carbonation conversion of the fresh sorbents appears to be near zero. As a result of this incipient near-zero rate of carbonation reaction, the sorbent conversion curves in Figure 6 appear more or less sigmoid. Although the conversion data were obtained by measuring the CO2 concentration in the reactant gas coming through a shallow fixed bed of the sorbents, it can be thought that this sigmoid conversion behavior was not attributed to the measurement artifacts in view of some papers reporting the same results from the experiments using TGA. Monazam et al.14 reported sigmoid conversion curves in the CO2 sorption experiments for the silica-grafted amine sorbent using TGA, by which the amount of CO2 uptake by the sorbent carbonation reaction could be directly measured. They explained that the sigmoid conversion behavior of the sorbent carbonation is attributable to the carbonation process of first nucleation and then crystalline growth, and that an induction period is required before the nucleation. For the carbonation of unsupported K2CO3 or Al2O3supported K2CO3 sorbents, Zhao et al.8,

13, 17

also reported the same sigmoid conversion

behavior. Their explanation to this conversion behavior is that H2O is required to be first adsorbed on the surface of sorbent and then CO2 reacts with the adsorbed H2O and K2CO3 to produce KHCO3. For this sequential process of carbonation reaction on K2CO3-based sorbents, they suggested that H2O adsorption could be considered as the rate-controlling step. In our experiments, due to the dehydration treatment taken prior to the carbonation reaction experiment, the reactor-loaded sorbents would be in completely dried state before contacting with the gas flow of both CO2 and H2O. Hence, a short time required for H2O adsorption first, can be considered as a cause for the very slow rate of carbonation conversion at incipient stage of sorbent carbonation. As listed in Table 1, values of the parameter 6 determined in 14


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this study are all greater than 1, indicating the existence of such incipient carbonation rate retardation. At 6 = 1, there is no retardation in the conversion rate as illustrated in Figure 1(B). Table 1 and eq 17 suggest that 6 values appeared larger for sorption reactions at lower temperatures and lower concentrations of CO2. When a riser-type fast fluidized bed is employed as a carbonation reactor, the amount of CO2 uptake on the sorbent particles would be lowered owing to the presence of initial rate retardation in the fresh K2CO3/ZrO2 sorbent if the sorbent particles are fed to the reactor after being completely decarbonated or dehydrated. Thus, a complete decarbonation/dehydration in the regenerator to a fresh K2CO3 state needs to be avoided to reduce the effect of initial rate retardation. As shown in Figure 5, in order to keep the kinetics in higher rates throughout the riser reactor, the sorbent even after regeneration needs to be somewhat moisturized and simultaneously to have a portion of KHCO3 in combination with K2CO3, optimally by the content corresponding to about 10% conversion of fresh K2CO3/ZrO2. For the sorbents intentionally regenerated like this, increased uptake amount of CO2 in the riser carbonator could be expected as compared with the sorbents completely decarbonated/dehydrated. As an example, if the solid retention time in the riser reactor is given by 20 s, and the carbonation conversion of the regenerated sorbent fed to the reactor is adjusted by 10%, because the carbonation reaction proceeds with the conversion rate, dX/dt = 0.25 min-1 ( = 0.42% s-1), as suggested in Figure 5(B), the net amount of CO2 uptake by this sorbent from a single pass of the reactor would be corresponding to the increase in the sorbent conversion by about 8.4%. Then, the sorbent leaves the reactor at the carbonation conversion of 18.4%. This simple supposition convinces us to consider the adoption of the riser reactor as a carbonator for K2CO3/ZrO2 sorbents. After this short period of initial retardation, the rate of carbonation conversion become 15



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faster with increasing conversion, reaching a maximum and then decreases with further increases of the conversion. As shown in Figure 5, the maximum rates of carbonation reaction are attained at carbonation conversions generally in the range of 10 ~ 20%. It is apparent from Figure 5(A) that the rate of carbonation reaction become faster on the whole by higher concentrations of CO2, as it is also ascertained by eq 13 with the reaction order, = 0.49. In accordance, as shown in Figure 6(C), higher ultimate conversions are achieved in the reactions at higher concentrations of CO2. However, the rates of carbonation reaction are not much discriminated by changes in the reaction temperatures between 328 and 343 K, as shown in Figure 5(B). In particular, it can be noticed at the low conversion levels before reaching the maximum rates that the carbonation reaction rates appear to be mostly same irrespective of the reaction experiments at different temperatures. In the range of higher conversion levels after the occurrence of the maximum rates, it is obvious that the carbonation conversion at higher temperatures proceeds with slower rates. As a result, the ultimate carbonation conversion reaches a lower value for the reaction at higher temperature. This can be explained by the negative apparent activation energy of relatively low value. Zhao et al.13 reported apparent activation energy for the carbonation reaction of the sorbent consisting of pure bulk K2CO3, -33.4 kJ/mol. It is typical of reactions with negative apparent activation energies that the adsorption of reactant is involved as a rate determining step in the overall reaction sequence with concurrent release of the adsorption heat in large amount as compared with any other elementary reaction steps. This reaction behavior was well explained by Lee18 for the N2 formation reaction from NO decomposition over Cu-ZSM5 catalyst, in which decreasing rate of N2 formation at temperatures higher than 773 K was attributed to the relatively large enthalpy of NO adsorption (∆#:Ai = -142.7 kJ/mol) onto the active sites as compared to that of the N2 16


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forming reaction activation energy (]^,jk = 81.6 kJ/mol), resulting in the negative activation energy (]^,:__ = -61.1 kJ/mol) by the relationship as follows:

]^,:__ ∆#:Ai ]^,jk

(23)

Due to this large heat of adsorption, the adsorption of NO onto the active sites is not thermodynamically favored at higher temperatures than 773 K, leading to the decreases in the rate of N2 formation. Likewise, it can be thought that the adsorption of CO2 onto K2CO3/ZrO2 sorbent, even though already being moisturized by H2O after the short period of initial rate retardation, is not favored at higher temperatures due to possibly high heat of adsorption of CO2 on K2CO3 as compared to the KHCO3 forming reaction activation energy, leading to decreased rates of carbonation conversion as shown in Figure 5(B), and lowered degrees of the ultimate carbonation conversion as shown in Figures 6(A) and 6(B). As for the heat of adsorption of CO2 on the material similar to that of the present study, Lee et al.19 reported the enthalpy of CO2 chemisorption on K2CO3-promoted hydrotalcite in the range between -21.0 and -29.3 kJ/mol. In the present study, ]^,:__ = -17.43 kJ/mol, and if we take ∆#:Ai = -29.3 kJ/mol, then activation energy of the KHCO3 forming reaction is obtained as ]^,jk = 11.87 kJ/mol. A facile carbonation conversion of K2CO3 at temperature as low as 328 K, as shown in Figure 5(B), could be explained by this lower KHCO3 forming reaction activation energy than the adsorption enthalpy of CO2 in their absolute magnitudes.

5. Conclusions

Carbonation conversions of the fresh-dried K2CO3/ZrO2 sorbent reacted at temperatures 17



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between 328 and 343 K for CO2 gas compositions not exceeding 18% at 1 bar appeared as sigmoid curves with time, resulting from the presence of initial retardation in the carbonation reaction rate. After this short period of initial retardation, carbonation reaction rates are maximized at carbonation conversions generally in the range of 10 ~ 20%. This kinetic behavior could be represented with good accuracy by the proposed phenomenological kinetic model:

A= A;

∙ 8& l 91 &

=

=>

B

?@

D

? ∙ . Temperature dependency of the

reaction rate constant was represented by Arrhenius’ equation with the negative apparent activation energy of -17.43 kJ/mol and the pre-exponential factor of 3.83 10 min-1. The reaction order with respect to the gas phase mole fraction of CO2 was determined to be 0.49. Other kinetic parameters were represented by correlation equations of temperature and the gas phase mole fraction of CO2 as follows: 7 &5.1 10c f 0.780 0.863; 6 &2.1 10 f & 2.142 8.659.

Nomenclature

o

pre-exponential factor (min-1)

3

parameter in kinetic model eq 3 (min-b)

6

parameter in kinetic model eq 3 (-)

]^,:__ apparent activation energy (kJ mol-1) ]^,jk surface reaction activation energy (kJ mol-1)

carbonation reaction rate constant (min-1)

′

rate constant defined by eq 8 (min-1)

E′

rate constant defined in eq 13 (min-1) 18


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reaction order with respect to (-)

p

gas constant (J mol-1 K-1)

f

temperature (K)

4

carbonation reaction time (min)

fractional carbonation conversion of sorbents (-)

7

ultimate fractional carbonation conversion of sorbents (-)

mole fraction of CO2 in the gas phase (-)

G

time constant (min)

Acknowledgements

This work is supported by Korea Carbon Capture & Sequestration R&D center. It is acknowledged that Deuk Ki Lee participates in this work during his sabbatical stay at KRICT under the financial support from this work as well as from Gwangju University.

References (1) Samanta, A.; Zhao, A.; Shimizu, G. K. H.; Sarkar, P.; Gupta, R. Post-combustion CO2 capture using solid sorbents: A review. Ind. Eng. Chem. Res. 2012, 51, 1438–1463. (2) Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D. Advances in CO2 capture technology - The U.S. Department of Energy’s carbon sequestration program. Int. J. Greenhouse Gas Control 2008, 2, 9 – 20. (3) Zhao, C.; Chen, X.; Zhao, C. Multiple-cycles behavior of K2CO3/Al2O3 for CO2 capture in a fluidized-bed reactor. Energy Fuels 2010, 24, 1009–1012. (4) Seo, Y.; Jo, S. H.; Ryu, C. K.; Yi, C. K. Effects of water vapor pretreatment time and 19



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reaction temperature on CO2 capture characteristics of a sodium-based solid sorbent in a bubbling fluidized-bed reactor. Chemosphere 2007, 69, 712–718. (5) Yi, C. K.; Jo, S. H.; Seo, Y.; Lee, J. B.; Ryu, C. K. Continuous operation of the potassium-based dry sorbent CO2 capture process with two fluidized-bed reactors. Int. J. Greenhouse Gas Control 2007, 1, 31–36. (6) Lee, S. C.; Choi, B. Y.; Ryu, C. K.; Ahn, Y. S.; Lee, T. J.; Kim, J. C. The effect of water on the activation and the CO2 capture capacities of alkali metal-based sorbents. Korean J. Chem. Eng. 2006, 23, 374–379. (7) Liang, Y.; Harrison, D. P.; Gupta, R. P.; Green, D. A.; McMichael, W. A. Carbon dioxide capture using dry sodium-based sorbents. Energy Fuels 2004, 18, 569–575. (8) Zhao, C.; Chen, X.; Zhao, C. K2CO3/Al2O3 for capturing CO2 in flue gas from power plants. Part 1: Carbonation behaviors of K2CO3/Al2O3. Energy Fuels 2012, 26, 1401-1405. (9) Zhao, C.; Chen, X.; Zhao, C. CO2 absorption using dry potassium-based sorbents with different supports. Energy Fuels 2009, 23, 4683-4687. (10) Lee, S. C.; Kwon, Y. M.; Park, Y. H.; Lee, W. S.; Park, J. J.; Ryu, C. K.; Yi, C. K.; Kim, J. C. Structure effects of potassium-based TiO2 sorbents on the CO2 capture capacity. Topics in Catal. 2010, 53, 641–647. (11) Hayashi, H.; Taniguchi, J.; Furuyashiki, N.; Sugiyama, S.; Hirano, S.; Shigemoto, N.; Nonaka, T. Efficient recovery of carbon dioxide from flue gases of coal-fired power plants by cyclic fixed-bed operations over K2CO3-on-carbon. Ind. Eng. Chem. Res. 1998, 37, 185– 191. (12) D’Alessandro, D. M.; Smit, B.; Long, J. R. Carbon dioxide capture: Prospects for new materials. Angewandte Chemie Int. Ed. 2010, 49, 6058 – 6082. (13) Zhao, C.; Chen, X.; Zhao, C. Carbonation behavior and the reaction kinetic of a new 20


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dry potassium-based sorbent for CO2 capture. Ind. Eng. Chem. Res. 2012, 51, 14361-14366. (14) Monazam, E. R.; Shadle, L. J.; Miller, D. C.; Pennline, H. W.; Fauth, D. J.; Hoffman, J. S.; Gray, M. L. Equilibrium and kinetic analysis of carbon dioxide capture using immobilized amine on a mesoporous silica. AIChE J. 2013, 59, 923-935. (15) Sun, P.; Grace, J. R.; Lim, C. J.; Anthony, E. J. A discrete-pore-size-distribution-based gas–solid model and its application. Chem. Eng. Sci. 2008, 63, 57-70. (16) Lee, D. K. An apparent kinetic model for the carbonation of calcium oxide by carbon dioxide. Chem. Eng. J. 2004, 100, 71-77. (17) Zhao, C.; Chen, X.; Zhao, C. Carbonation and active-component-distribution behaviors of several potassium-based sorbents. Ind. Eng. Chem. Res. 2011, 50, 4464-4470. (18) Lee, D. K. Thermodynamic features of the Cu-ZSM-5 catalyzed NO decomposition reaction. Korean J. Chem. Eng. 2006, 23, 547-554. (19) Lee, K. B.; Verdooren, A.; Caram, H. S.; Sircar, S. Chemisorption of carbon dioxide on potassium-carbonate-promoted hydrotalcite. J. Colloid and Interface Sci. 2007, 308, 30-39.

21



1

(A) X, - or dX/dt, min-1

0.8 0.6 0.4

a= a= a= a=

0.3 0.4 0.6 0.8

b= b= b= b=

1.0 1.4 1.7 2.0

0.2 0 1

(B) 0.8 X, - or dX/dt, min-1

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

0.6 0.4 0.2 0 0

1

2

3 4 Time, min

5

6

7

Figure 1. Plots of conversion (X by eq 4, solid line) and the rate of conversion (dX/dt by eq 7, broken line) with time at 7 = 0.8 when b = 1.4 (A) and a = 0.4 (B).

22


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0.8

0.6 X, - or dX/dt, min-1

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


X experimental X fitted to eq 4 dX/dt experimental dX/dt by eq 7

0.4

0.2

0 0

1

2

3 4 Time, min

5

6

7

Figure 2. Least squares fit to eq 4 of the sorbent conversion data experimented at 338 K and 10% CO2 to determine the parameter values: 7 = 0.782, 6 = 1.346, 3 = 0.379. The dX/dt curve was displayed using eq 7 with ′ = 0.614.

23



0.9

Xu by eq 17, -

(A) 0.8

0.7

0.6 0.6

0.7

0.8

0.9

Xu, 1.8

b by eq 18, -

(B) 1.6 1.4 1.2 1 1

1.2

1.4 b, -

1.6

1.8

0.4 a, min-b

0.6

0.8

0.8

(C) a by eq 19, min-b

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

0.6 0.4 0.2 0 0

0.2

Figure 3. Parity plots for the parameter values, 7 (A), 6 (B) and 3 (C), between calculated using the correlation equations in Table 2 and given in Table 1.

24


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0 10% CO2 6.25% CO2

Ln(k'), min-1

-0.2 -0.4 -0.6 -0.8

(A)

-1 2.85

2.9

2.95 3 1/T x 103, K-1

3.05

3.1

0 at 338 K -0.4 Ln(k'), min-1

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


-0.8

-1.2

(B) -1.6 -5

-4

-3 Ln(yCO2), -

-2

-1

Figure 4. Rate constant ′ dependency on the sorption reaction temperature (A) and the gas phase mole fraction of CO2 (B).

25



0.6 2.5% CO2 6.25% CO2

0.5

dX/dt, min-1

10% CO2 18% CO2

0.4

(A)

0.3 0.2 0.1 0 0.3 0.25

dX/dt, min-1

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

0.2

(B) 0.15 328 K 333 K 338 K 343 K

0.1 0.05 0 0

0.1

0.2

0.3

0.4 X, -

0.5

0.6

0.7

0.8

Figure 5. Comparison of the carbonation rate change with the degree of conversion between experimental and predicted (shown as lines) by eq 14 for data experimented with different CO2 concentrations at 338 K (A) and with different temperatures at 6.25% CO2 (B).

26


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Page 27 of 28

0.8

(A)

X, -

0.6 0.4 328 K, 10% CO2

0.2

343 K, 10% CO2

0 0.8

(B)

X, -

0.6 0.4 328 K, 6.25% CO2

0.2

343 K, 6.25% CO2

0 1

(C) 0.8 0.6 X, -

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


0.4

338 K, 2.5% CO2 338 K, 6.25% CO2

0.2

338 K, 10% CO2 338 K, 18% CO2

0 0

1

2

3 4 Time, min

5

6

7

Figure 6. Comparison between experimental conversions and the kinetic model predictions (shown as lines) obtained by integrating eq 14 at different sorption reaction temperature and gas phase CO2 concentrations.

27



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

Page 28 of 28

Table 1 Parameter values determined in the least squares fit of conversion data to eq 4. T, K

, -

7 , -

6, -

3, min-b

′, min-1

R2

328

0.1

0.775

1.591

0.342

0.737

0.999

333

0.1

0.780

1.472

0.359

0.677

0.999

338

0.1

0.782

1.346

0.379

0.614

0.999

343

0.1

0.725

1.253

0.394

0.558

0.996

328

0.0625

0.738

1.620

0.247

0.608

0.999

333

0.0625

0.731

1.484

0.232

0.501

0.999

338

0.0625

0.727

1.435

0.259

0.507

0.999

343

0.0625

0.764

1.301

0.260

0.434

0.998

338

0.025

0.714

1.559

0.104

0.323

0.997

338

0.18

0.834

1.180

0.710

0.859

0.986

Table 2 Linear correlation equations for the model parameters with T and . Correlation equations

R2

7 &5.1 10c f 0.780 0.863

0.743

(17)

6 &2.1 10 f & 2.142 8.659

0.964

(18)

3 2.52 10 f 3.789 & 0.842

0.995

(19)

28


ZrO2

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