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Ind. Eng. Chem. Res. 2009, 48, 4022–4029
Effect of Ammonia on the Absorption Kinetics of Carbon Dioxide into Aqueous 2-Amino-2-methyl-1-propanol Solutions Won-Joon Choi,† Byoung-Moo Min,† Jong-Beom Seo,‡ Sang-Wook Park,§ and Kwang-Joong Oh*,‡ Greenhouse Gas Research Center, Korea Institute of Energy Research, 71-2 Jang-Dong, Yuseong-Gu, Daejeon 305-343, Republic of Korea, and Departments of EnVironmental Engineering and Chemical Engineering, Pusan National UniVersity, San 30 Jangjeon-Dong, Geumjeong-Gu, Busan 609-735, Republic of Korea
Reaction rate constants of aqueous 2-amino-2-methyl-1-propanol (AMP)/ammonia (NH3) solutions with carbon dioxide (CO2) were determined by measuring their absorption rates. The CO2 absorption characteristics of an NH3 solution added to AMP were investigated using a stirred-cell reactor. The CO2 absorption rates into aqueous blended amine solutions were measured at different pressures (pA ) 5, 10, and 15 kPa) and temperatures (293, 303, 313, and 323 K). Additive concentrations of 1, 3, and 5 wt % NH3 were added for each 30 wt % AMP solution. The results showed that the addition of NH3 to AMP significantly increased the CO2 absorption rates of AMP by 144%. In addition, the reaction rate constant for a blend of AMP and NH3 with CO2 as a function of temperature was 1.6- to 2.4-fold higher than that of AMP without NH3. The reaction rate constants were k2,AMP ) 1.543 × 107 exp(-3019.92/T) and k2,NH3 ) 1.847 × 1010 exp(-4954.25/T). 1. Introduction Various technologies are used to separate CO2 from the flue gas of conventional fossil fuel-fired power plants and petroleum chemical plants, such as chemical absorption, physical adsorption, cryogenic methods, membrane separation, and biological fixation. Chemical absorption is generally recognized as the most effective current technology.1 Industrially important chemical absorbents are monoethanolamine (MEA), diethanolamine (DEA), and N-methyldiethanolamine (MDEA). A different class of chemical absorbents, the sterically hindered amines such as 2-amino-2-methyl-1-propanol (AMP), has been proposed as a new, commercially attractive CO2 absorbent because of its advantages in absorption capacity, absorption rate, degradation resistance, and regeneration energy.2 Furthermore, ammonia (NH3) solution can be an alternative for the control of CO2 emitted from flue gas because of its high absorption capacity and fast absorption rate.3 Recently, the use of blended amine as a solution of two or more amines in varying concentration has been shown to produce absorbents with excellent absorption characteristics and low energy requirements. Because of the increasing importance of blended amines, an understanding of the kinetic phenomena in blended amine systems is vital.4 Determining the physical solubility and diffusivity of CO2 in the amine solutions is very important for understanding their reaction kinetics and effectively designing a process. A modified pseudo-first-order model based on the film theory is used to predict the rate of CO2 absorption into aqueous blended amine solutions.5-7 A number of research articles related to the kinetics of CO2 absorption into aqueous blended amine solutions containing AMP have been reported in the literature.8-16 These blended amines enhance the absorption capacity and absorption rate of CO2 even though they maintain the stripping characteristics of sterically hindered amines.17 * To whom correspondence should be addressed. Tel.: +82-51-5102417. Fax: +82-51-583-0559. E-mail:
[email protected]. † Korea Institute of Energy Research. ‡ Department of Environmental Engineering, Pusan National University. § Department of Chemical Engineering, Pusan National University.
In this study, NH3 was added to aqueous AMP solution to increase the CO2 absorption rates. The CO2 absorption rates of AMP and a blend of AMP and NH3 were measured at different pressures (pA ) 5, 10, and 15 kPa), temperatures (293, 303, 313, and 323 K), and aqueous concentrations by using a stirredcell reactor. In addition, the solubility and diffusivity of CO2 in aqueous solvents were estimated by using the “N2O analogy method”. Reaction rate constants were determined from the measured absorption rates. 2. Theoretical Basis 2.1. Absorption Reaction Mechanism. The reaction of CO2 with the primary amino group can produce three possible reactions: the formation of carbamate and bicarbonate, the reversion of carbamate to bicarbonate, or the formation of the carbonate ion.16 Carbamate formation: CO2(g) + 2RNH2(l) a RNH3+(aq) + RNHCOO-(aq) (1) Bicarbonate formation: RNHCOO-(aq) + H2O(l) a RNH2(l) + HCO3-(aq) (2) The equilibrium loading capacities of primary and secondary amines are limited by stoichiometry (0.5 mol of CO2/mol of amine) of eq 1. The zwitterion mechanism originally proposed by Caplow18 and reintroduced by Danckwerts19 is generally accepted as the reaction mechanism for eq 1. CO2(g) + RNH2(l) a RNH2+COO-(aq)
(3)
RNH2+COO-(aq) + B′(aq) f RNHCOO-(aq) + B′H+(aq) (4) If the carbamate ion is unstable, as in the case of a hindered amine, it undergoes the formation of bicarbonate ion as in eq 2. This reaction means that 1 mol of hindered amine allows loading of CO2 up to 1 mol. However, a certain amount of carbamate hydrolysis occurs with all amines so that even with
10.1021/ie8018438 CCC: $40.75 2009 American Chemical Society Published on Web 03/11/2009
Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 4023
MEA the CO2 loading may exceed the stoichiometry particularly at high pressures. The chemical reactions between CO2 and NH3 can be expressed by the following reactions.20,21 +
The first step in the reaction of CO2 with AMP is the formation of an intermediate zwitterion:15 k2
CO2 + RNH2 {\} RNH2+COO-
CO2(g) + 2NH3(aq) f NH4 (aq) + NH2COO (aq) (5) CO2(g) + 2NH3(1) + H2O(1) a (NH4)2CO3(s)
(6)
CO2(g) + NH3(1) + H2O(1) a NH4HCO3(s)
(7)
The wet method of CO2 scrubbing into flue gas using NH3 produces ammonium carbonate ((NH4)2CO3) and ammonium bicarbonate (NH4HCO3).3,22 2.2. Physical Properties. Determining the physical solubility and diffusivity of CO2 in the amine solutions is very important for understanding their reaction kinetics and effectively designing a process. However, the physical solubility and diffusivity are not easily classified from the chemical reaction between CO2 and amine solutions. As a result, the N2O analogy has been used to estimate the solubility and diffusivity of CO2 in amine solutions because it is necessary to use a nonreacting gas such as N2O with similarities in mass, molecular structure, and molecular interaction.23-25 The N2O analogy for measuring the physical solubility and diffusivity of CO2 in amine solutions is as follows: (HCO2)amine ) (HN2O)amine × (HCO2 /HN2O)water
(8)
(DCO2)amine ) (DN2O)amine × (DCO2 /DN2O)water
(9)
2.2.1. Physical Solubility. The physical solubility is calculated in terms of Henry’s law constant as follows: P ) HACA*
(10)
The concentration of gas in bulk of liquid at gas-liquid equilibrium (C*A) is calculated from the following mass balance: CA* )
(PfVf) - [(Pe - Pvap)Vf] RTVL
NA ) 2(DA /πtc)1/2(pA /HA)
(12)
The contact time (tc) can be derived from the wetted wall column, using the following equation: tc ) (2h/3)(πd/L)2/3(3η/Fg)1/3
(13)
The diffusivity (DA) of gas absorbed from eqs 12 and 13 is expressed as follows: DA ) (NAHA)2πtc /(2pA)2
(14)
2.3. Determination of the Reaction Rate Constant. For a chemical reaction between a gaseous constituent A and a liquid reactant B in an aqueous solution to yield a product P: kmn
A + νB 98 Products
kb
RNH2+COO- + B′ {\} RNHCOO- + B′H+
(15)
It is generally accepted that the zwitterion mechanism governs the formation of carbamate for primary and secondary amines.
(17)
k-b
On the basis of the zwitterion mechanism and the assumption of quasi-steady state for the concentration of the zwitterion, the expression for the CO2 reaction rate is as follows: rA )
k2[CO2][RNH2] 1 + k-1 /
(18)
∑ k [B] b
When the term k-1/Σkb[B] , 1, the analysis is simplified to second-order kinetics, and the following equation can be used: rA ) k2[CO2][RNH2]
(19)
The influence on absorption kinetics of all chemical reactions between dissolved CO2 and reactants in solution is usually expressed by an “enhancement factor” E over physical absorption: NA ) EkLCA*
(20)
where E is a function of the Hatta number (Ha) and the instantaneous reaction enhancement factor (Ei) is defined as follows: Ha )
( m +2 1 D k
* m-1 n 1/2 CB /kL A mn(CA)
Ei )
)
( ) ( ) DA DB
1/2
+
DB DA
1/2
CB νCA*
(21)
(22)
where CA* is the concentration of the gas at interface given by Henry’s law:
(11)
2.2.2. Diffusivity. The Higbie penetration theory gives the specific absorption rate (NA) as follows:
(16)
k-1
-
CA* ) pA /HA
(23)
The experimental conditions were selected to ensure the absorption of CO2 in amine solutions in a region of fast pseudomn-order reaction in the range of Ha between 3 and Ei. If the value of Ha is in this range, E becomes equal to Ha, and the following specific absorption rate is obtained:16 NA )
( ) pA HA
(m+1)/2
( m +2 1 D k C )
n 1/2 A mn B
(24)
The slope of the straight line fitting the data of ln NA versus ln pA will give the order of m with respect to the dissolved gas concentration CA*. The order of n with respect to the amine concentration can also be found in an analogous manner by plotting ln(NA/pA(m+1)/2)/(DA1/2/HA(m+1)/2) versus ln CB. For a fast chemical reaction between the dissolved gas and a reactant, the specific absorption rate is as follows: NA ) CA*√DAkov
(25)
Similarly, by using eq 26 the overall reaction rate (kov) can be calculated as follows: kov ) kmn ) (NAHA(m+1)/2 /pA(m+1)/2 /DA1/2)2
(26)
4024 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009
For fast pseudo-mn-order reaction conditions, when the equilibrium pressure of CO2 contributed by the CO2 in solutions is small compared to the absorption pressure and where the gasphase resistance is negligible, the absorption rate is given by eq 27.15,16 After integrating eq 28, a reaction rate constant (k2) can be achieved by eq 29.7 -
( )
( )
pA Vg dpA ) As√DAkov RT dt HA
(27)
( dndt ) ) A √D k
(28)
( )
(29)
-
s
NAHA
√DApA
2
* A ovCA
) k2CB
3. Experimental Section 3.1. Materials. Analytical grade AMP solution, with purities of 99%, was supplied by Acros Organics. A 28 wt % ammonia solution was supplied by Junsei Chemical. Aqueous solutions were prepared with distilled water. The CO2 and N2 gases were of commercial grade, with purities of 99.99%. High purity N2O (99.9%) gas was also used. 3.2. Physical Solubility Measurement. The experimental apparatus for measuring the physical solubilities is shown in Figure 1. The reactor, with a height of 160 mm and i.d. of 95 mm, was located inside a temperature-controlled vessel, with four, 5-mm-wide glass plates adhered to the inner wall of the reactor as baffles. The total volume of the reactor was about 1134 cm3 with an active interface area (As) of 70.88 cm2. A two-blade impeller (70 mm × 20 mm) was installed at a location in the middle of the liquid level. The reactor temperature was measured using a K-type thermocouple, with an accuracy of (0.1 K. A pressure transducer (MGI/MGAMP series, accuracy of (0.1 kPa) was installed in the reactor and the feeder to measure their pressure. The gas flow rates were controlled using mass flow controllers (5850E, Brooks Instruments). After the temperature of the reactor stabilized, the reactor was purged with pure N2O for one hour to remove the remaining air in the reactor. After the reactor reached an atmospheric pressure, 200 mL of an amine solution was injected into the reactor using a syringe, and the reactor was agitated. Since the reactor pressure decreased as the amine solution absorbed N2O, the N2O was continuously injected into the reactor to maintain the reactor pressure at 1 atm. When the reactor pressure stabilized without the need for extra N2O injection, the solubility was calculated by the difference of the feeder pressures before and after the absorption. 3.3. Diffusivity Measurement. A cylindrical wetted wall column absorber was constructed of 316 stainless steel, with an outside diameter of 2.54 cm and a height of 8.6 cm, to estimate the diffusivities of CO2 in water and amine solutions by the N2O analogy. The apparatus and the experimental procedure were the same as those described by Li and Lai.24 A thin solution film was maintained at the outside wall of the column by adding 0.03-0.05 vol % of Tween 80 to each amine solution. When the solution was distributed uniformly, the diffusivity was determined by the difference between the inlet and outlet gas flow rates. 3.4. Absorption Rate Measurement. An experimental apparatus was constructed to explore the absorption of CO2 into amine solutions, as shown in Figure 1. Its detailed specification was presented in section 3.2. All tests were conducted with 500 mL of solution. The stirring speed was
Figure 1. Schematic of experimental apparatus. Legend: 1, N2 cylinder; 2, CO2 and N2O cylinder; 3, mass flow controller; 4, mixing chamber; 5, feeder; 6, magnetic drive; 7, controller for temperature and agitation speed; 8, pressure transducer; 9, reactor (stirred cell); 10, computer; 11, soap bubble meter; and 12, CO2 analyzer. Table 1. Henry’s Constants of N2O and CO2 in Aqueous AMP and AMP + NH3 Solutions as a Function of Temperature HN2O wt % water
HCO2
293 K 303 K 313 K 323 K 293 K 303 K 313 K 323 K 3504
4745
5882
7035
2625
3491
4172
4875
3013 3227 3392 3569
3739 3947 4123 4298
4342 4557 4772 4883
4932 5147 5342 5571
4168 4202 4223
4876 4918 4951
5619 5648 5686
AMP 10 20 30 40
4022 4307 4528 4765
5082 5365 5604 5842
6122 6425 6728 6887
30 + 1 30 + 3 30 + 5
4545 4598 4642
5665 5711 5740
6874 6934 6980
7117 7428 7710 8040
AMP + NH3 8108 8150 8205
3405 3445 3478
limited to 50 rpm to keep the gas-liquid interface planar and smooth. The N2 gas was passed through the reactor to purge any contaminant gases in the reactor. To achieve a good gas-liquid contact, the gas was introduced into the top of the reactor. The absorption rates were calculated from the difference in the amounts of gas between the inlet and outlet. A ZRF model CO2 analyzer (Fuji Electric, 0-20 vol %) was used to measure the CO2 gas concentration at the reactor outlet. 4. Results and Discussion 4.1. Physical Properties. 4.1.1. Physical Solubility Data. The solubilities of N2O and CO2 in water were measured at 293, 303, 313, and 323 K and compared with the literature results in Figure 2.23-26 The consistency of the study results with those in literature validated the accuracy of the experimental system in determining the solubilities. The solubilities of N2O in AMP (10, 20, 30, and 40 wt %) solutions and the addition of NH3 (1, 3, and 5 wt %) to 30 wt % AMP solution were measured at 293, 303, 313, and 323 K and converted to the solubilities of CO2 by using eq 8. The results summarized in Table 1 indicate that the solubility of CO2 decreased with increasing AMP solution concentration or temperature because Henry’s constant (HA) increased with increasing concentration or temperature. In addition, the solubilities of N2O and CO2 in
Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 4025
AAD% )
Figure 2. Henry’s constants of N2O in water as a function of temperature.
1 n
∑| n
i)1
xi,exp - xi,cal xi,exp
|
(32)
4.1.2. Diffusivity Data. The diffusivities (DA) of N2O and CO2 in water were measured at 293, 303, 313, and 323 K, and the results were consistent with the literature results in Figure 3.23-25,27 The DA of N2O in AMP (10, 20, 30, and 40 wt %) solutions and the addition of NH3 (1, 3, and 5 wt %) to 30 wt % AMP solution were measured at 293, 303, 313, and 323 K and converted to the DA of CO2 by using eq 9. The results are summarized in Table 3. The DA of aqueous AMP solution decreased with increasing concentration. The addition of NH3 in 30 wt % AMP afforded a DA value higher than that of 30 wt % AMP at all measured temperatures, and the DA of N2O and CO2 in blended solutions decreased with increasing NH3 content at each temperature. The DA values of N2O and CO2 in aqueous AMP and AMP + NH3 solutions based on values presented in Table 4, for the single and blended solvent system, may be estimated as a polynomial function of the temperature and solvent concentration as shown in eqs 33 and 34, respectively. DA ) (b1 + b2M + b3M2) exp(-c2 /T) × 10-9
(33)
DA ) (b1 + b2M1 + b3M12 + b4M2 + b5M22 + b6M1M2) exp(-c2 /T) × 10-9 (34)
Figure 3. Diffusivities of N2O in water as a function of temperature.
the blended solutions decreased with increasing NH3 content at each temperature. To predict HA of N2O and CO2 in aqueous AMP and AMP + NH3 solutions for the single and blended solvent system based on the values presented in Table 1, a polynomial function of the temperature and solvent concentration may be estimated as shown in eqs 30 and 31, respectively. HA ) (a1+a2M) exp(c1 /T) × 106
(30)
HA ) (a1+a2M1 + a3M2 + a4M1M2) exp(-c1 /T) × 106 (31) The parameters a1, a2, a3, a4, and c1 were obtained from the corresponding experimental data by a regression method using OriginPro version 7.5. The results are summarized in Table 2. The average absolute percent deviation (AAD%) between the calculated values and experimental data was calculated from eq 32. The AAD% between the calculated HA of N2O and CO2 in aqueous AMP and AMP + NH3 solutions and the experimental data was
