Article pubs.acs.org/IECR
Kinetics Study on CO2 Absorption with Aqueous Solutions of 1,4-Butanediamine, 2‑(Diethylamino)-ethanol, and Their Mixtures Zhicheng Xu, Shujuan Wang,* and Changhe Chen Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Beijing Key Laboratory for CO2 Utilization and Reduction Technology, Department of Thermal Engineering, Tsinghua University, Beijing 100084, People’s Republic of China ABSTRACT: An aqueous solution of 2 M 1,4-butanediamine (BDA) blended with 4 M 2-(diethylamino)-ethanol (DEEA) has been proven to be a promising solvent in previous work, and BDA is a potential amine to accelerate the reaction rate of DEEA with CO2. In the present work, kinetics of CO2 absorption into aqueous solutions of BDA, DEEA, and their mixtures were studied using a wetted wall column (WWC) at 25, 40, and 60 °C with the driving force of 3−35 kPa. The BDA concentrations were 1, 2, and 3 M, with DEEA concentrations of 2, 3, 4, and 5 M, while those of the BDA/DEEA mixtures were 1 M BDA/4 M DEEA, 2 M BDA/3 M DEEA, 2 M BDA/4 M DEEA and 2.54 M BDA/2.73 M DEEA. The results show that the reaction rate constant of BDA is larger than most of the amines but lower than piperazine. BDA can largely accelerate the reaction of solvents with CO2, and the overall reaction of their mixtures can be regarded as a reaction between CO2 and DEEA in parallel with the reaction of CO2 with BDA.
1. INTRODUCTION Amine scrubbing has been proved to be a suitable technology for post-combustion CO2 capture.1 However, there are still some challenges along with the process, the most important of which being the high energy penalty during the regeneration. To mitigate this important drawback, many solvents, such as monoethanolamine (MEA), methyldiethanolamine (MDEA), diethanolamine (DEA), and piperazine (PZ) have been applied to capture CO2.2−4 Meanwhile, the biphasic solvents, such as DMX,5 2 M N-methyl-1,3-diaminopropane (MAPA) blended with 5 M DEEA,6 and lipophilic amine solvents,7,8 have also been investigated for further improvement of the energy performance. Previous works have revealed that a biphasic solvent, an aqueous solution involving the mixture of 2 M 1,4-butanediamine (BDA) and 4 M 2-(diethylamino)-ethanol (DEEA) (denoted as 2B4D) has the potential to lower the sensible heat requirement, eventually reducing the energy penalty during the regeneration.9,10 For example, the difference of CO2 concentration in a rich and lean solution of 2B4D can reach 2.278 mol CO2 per liter solution while the absorption and desorption were conducted at atmospheric pressure, 40 and 90 °C, respectively, 48.2% more than that of 5 M MEA under the same conditions.9 In order to determine the dimension of absorber in CO2 capture, it is necessary to investigate the reaction kinetics. Moreover, DEEA can be prepared from renewable resources and is regarded as a promising solvent.11 Because of its slow reaction rate with CO2, some additives, such as PZ, were used to accelerate the reaction.11 Kinetics of CO2 absorption by DEEA was originally studied by Kim and Savage.12 Littel et al. studied the kinetics of (0.2−3) M DEEA at (283−333) K and found that the secondorder reaction rate constant (k2) can be expressed as13
Li et al. investigated the kinetics of 0.2−1 M DEEA at 298− 313 K and concluded the following relationship:14 ⎛ 6238.4 ⎞ ⎟ k 2 (m 3 mol−1 s−1) = 9.95 × 107 × exp⎜ − ⎝ T ⎠
The results of Vaidya and Kenig show that, at 303 K, k2 should be 173 m3 mol−1 s−1,11 which was regressed by the data of 2−3 M DEEA. Previous researchers promoted the DEEA absorption rate with additives such as PZ, 2-(2-aminoethyl-amino)ethanol (AEEA), and hexamethylenediamine (HMDA). The results of Vaidya and Kenig show that the CO2 absorption rate was significantly enhanced when just a small amount of PZ was added into the aqueous DEEA solution.11 Konduru et al. investigated the kinetics of CO2 absorption by aqueous solution of DEEA and PZ mixtures and found that the overall reaction of CO2 with the two amines can be considered as a reaction between CO2 and DEEA in parallel with the reaction of CO2 with PZ. The second-order rate constant for the CO2 reaction with PZ was determined from the absorption rate measurements in the activated DEEA solutions, and its value at 303 K was found to be 24 450 m3 kmol−1 s−1.15 Sutar et al. compared the effects of PZ, AEEA, and 1,6-hexamethyl diamine (HMDA) and concluded that HMDA showed the best performance in increasing the specific absorption rate of DEEA. The secondorder rate constant for the CO2 reaction with HMDA was determined from the absorption rate measurements in a stirredcell reactor in the activated DEEA solutions, and its value at 303 K was found to be 4.2 × 104 m3 kmol−1 s−1.16 However, the kinetics data of DEEA with concentrations higher than 3 M are rarely reported. As for aqueous solutions of Received: Revised: Accepted: Published:
⎛ 5413 ⎞⎟ k 2 (m 3 mol−1 s−1) = 3.36 × 106 × exp⎜ − ⎝ T ⎠ © 2013 American Chemical Society
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April 24, 2013 June 15, 2013 June 24, 2013 June 24, 2013 dx.doi.org/10.1021/ie4012936 | Ind. Eng. Chem. Res. 2013, 52, 9790−9802
Industrial & Engineering Chemistry Research
Article
Figure 1. Overall experimental flow sheet of the wetted-wall column (WWC).
reservoir through a coil submerged in the oil bath, flowing through a rotameter to determine the flow rate. A saturator was added before the gas went into the column, to guarantee the water balance in the system, which has been proven to be reliable in previous work by measuring the DEEA concentration before and after the WWC experiment.10 A phosphoric acid wash and a drying pipe, as shown in Figure 1, were used after the column to absorb the possible amine vapor and water in the gas, ensuring that no vapor flows into the online IR CO2 analyzer. Other details can be found in our previous works.10,17 2.3. Mass Transfer in WWC. Based on the two-film model, the mass flux with chemical absorption can be described as follows, where the total resistance to mass transfer was divided as the sum of the resistance from the gas side and the liquid side, as eq 1 shows.
BDA and the effect of adding BDA to the aqueous DEEA solution, there is still no literature reporting the kinetics data. Also considering the good performance of the mixtures of BDA and DEEA mentioned above, in the present paper, the kinetics of 2−5 M DEEA, 1−3 M BDA, and their mixtures were investigated.
2. EXPERIMENTAL SECTION 2.1. Chemicals. The chemicals used in this work were BDA (≥98 wt %) and DEEA (≥99 wt %); each was obtained from Sigma−Aldrich and used without further purification. The BDA and DEEA structures are shown below.
1 1 1 + = kg k G′ KG
CO2 (≥99.9% pure) and N2 (≥99.99% pure) from Beijing Huayuan Gas Company were used. Deionized water was used to prepare the solutions. The amine concentrations were determined by titration against 0.2 N H2SO4, using a Metrohm 809 Titrando auto titrator. 2.2. The Wetted-Wall Column. The wetted-wall column (WWC) used in this research was constructed from a stainlesssteel tube, measuring 11.0 cm in height and 1.2 cm in diameter. The gas−liquid contact region is enclosed by a thick-walled glass tube with an outside diameter of 31.0 cm, separated by a water bath, as shown in Figure 1. Gas enters near the base of the column, contacting the liquid counter-currently as it flows up into the gas outlet. Two mass flow controllers of 10 and 1 SLPM are used to meter N2 and CO2 flow. The total gas flow for DEEA solution was ∼2 L/min, while that of BDA and BDA/DEEA mixtures was ∼5 L/min. The CO2 concentration of the gas inlet and outlet are measured by an online infrared (IR) CO2 analyzer. The oil bath, with oil circulatiing inside it, is used to control the temperature of the inlet gas, the liquid, and the reactor. The total pressure in the reactor is controlled by a needle valve on the gas outlet pipe. A Cole−Parmer micropump pushes the solution from the
(1)
The overall gas-transfer coefficient (KG) can be calculated from the absorption flux (Flux), the partial pressure of CO2 coming into the system (PCO2,in), and the partial pressure of CO2 exiting the system (PCO2,out). KG is fixed for a given temperature and amine concentration and can be calculated by the Flux−CO2 partial pressure curve. KG =
Flux * PCO2,b − PCO 2
(2)
where PCO2,b is the operational partial pressure of CO2 in the WWC, which was the logarithmic mean average, as shown by eq 3: PCO2,b =
PCO2,in − PCO2,out ⎛ PCO ,in ⎞ ln⎜ P 2 ⎟ ⎝ CO2,out ⎠
(3)
The CO2 pressure in the gas/liquid interface (PCO2,i) can be calculated by eq 4: 9791
dx.doi.org/10.1021/ie4012936 | Ind. Eng. Chem. Res. 2013, 52, 9790−9802
Industrial & Engineering Chemistry Research kg =
Flux PCO2,b − PCO2,i
Article
The reaction of CO2 with BDA is illustrated below: (4)
The gas-side mass-transfer coefficient (kg) can be given by eq 5: ⎛ Re × Sc × dh ⎞ β ⎟ Sh = α⎜ ⎝ ⎠ h
In the aqueous BDA solution, the following reactions are possible to occur:
(5)
where
Sh =
Re =
RTkgdh DCO2 ρg Vgdh μg
υ Sc = D Here, dh is the hydraulic diameter of the annulus and h is the length of the column. As Pacheco stated, the parameters α and β are fitted based on the experimental data of SO2 absorption into 0.1 M NaOH solution.18 Our previous work has fitted it as10
CO2 + 2H 2O ↔ HCO3− + H3O+
(7)
CO2 + OH− ↔ HCO3−
(8)
2H 2O ↔ H3O+ + OH−
(9)
HCO3− + H 2O ↔ CO32 − + H3O+
(10)
BDAH+ + H 2O ↔ BDA + H3O+
(11)
BDAH 2 2 + + H 2O ↔ BDAH+ + H3O+
(12)
BDA + CO2 + H 2O ↔ BDACOO− + H3O+
(13)
BDACOO− + CO2 + H 2O ↔ BDA(COO)2 2 − + H3O+ (14)
⎛ ReScdh ⎞0.5036 ⎟ Sh = 6.7097⎜ ⎝ h ⎠
−
+
BDAHCOO + H 2O ↔ BDACOO + H3O
(15)
Considering the effect of molecular structure, it has been mentioned by Albert and Serjeant that a carbon chain length of more than four carbon atoms between two diamine groups diminishes the influence of these groups on each other and this was verified by the research of 1,6-hexamethyldiamine (HMDA) and 1,6-hexamethyldiamine, N,N′-dimethyl (HMDA,N,N′).21,22 And, also, considering the fact that, in the experiment, all amines are fresh solution, the reaction of CO2 with the BDA carbamate is neglected here. The contribution of reaction 7 to the overall reaction rate is very small as the reaction has a very low rate constant (k = 0.026 s−1 at 298 K23) and usually may be neglected.24 The contribution of reaction 8 to the overall reaction rate can be described as23,25
2.4. Experimental Section. The experiment consists of three parts: CO2 absorption by DEEA, BDA, and BDA/DEEA mixture solutions. The concentrations (given by mol/L in this work) were determined at room temperature (25 °C). The reaction rates of CO2 in 2, 3, 4, and 5 M DEEA solutions at 25, 40, and 60 °C were measured under driving forces ranging from 7 kPa to 35 kPa. The experiments of CO2 with 1, 2, and 3 M BDA solutions were conducted at 25, 40, and 60 °C with the driving force of 4−20 kPa. For the BDA/DEEA mixture solutions, the concentrations studied were 1 M BDA/4 M DEEA, 2 M BDA/3 M DEEA, 2 M BDA/4 M DEEA, and 2.53 M BDA/2.73 M DEEA, with driving forces of 3−25 kPa at 25, 40, and 60 °C. The reason why 2.54 M BDA/2.73 M DEEA is studied is that it is the concentration of lower phase of 2B4D after CO2 absorption reaching equilibrium and is of great importance for the comparison of 2B4D with its lower phase alone.10 All the solvents used in this paper are fresh with no CO2 loading. In order to study the kinetics, the N2O solubilities in, densities and viscosities of BDA, DEEA, and BDA/DEEA mixed solutions at different concentrations and temperatures were also measured.
* −[CO2 ][OH−] rCO2 − OH− = k OH
(16)
* −) = 13.635 − 2895 log10(k OH T
(17)
where the k*OH− has units of m3 kmol−1 s−1 and T is given in Kelvin. The hydroxide ion concentration in the solution can be estimated from the relation given by Astarita et al.:26 [OH−] =
3. THEORY 3.1. Reaction Mechanism. For primary and secondary amines, the termolecular mechanism is generally used to describe the absorption process and to derive the reaction rate constant(s). The mechanism was proposed by Crooks and Donnellan,19 which concluded that the reaction between amine and CO2 is single-step and termolecular. This mechanism was reviewed by da Silva and Svendsen in their study of carbamate formation from CO2 and alkanolamines.20 A single-step thirdorder reaction mechanism is most likely for the formation of carbamate from CO2 and alkanolamines in solution. The observed broken-order and higher-order kinetics can be explained by this mechanism.
Kw [Am] , Ka
α < 10−3 (18)
The pKa value of BDA is estimated to be 10.68 at 298.15 K, according to ACD/Laboratories Software V11.02 (1994−2012 ACD/Laboratories). At 298.15 K, for 2 M BDA, based on eqs 17 and 18, the calculated k*OH− value is 8416 m3 kmol−1 s−1, and the calculated hydroxide ion concentration is 0.0309 M. Given that, under the same conditions, k2,BDA[BDA] ≈ 40 000 (this will be shown in detail in the following part), the contribution of OH− to the overall reaction rate was negligible (∼0.29%). So the overall reaction rate is equal to the apparent reaction rate. The overall CO2 reaction rate then can be expressed as 9792
dx.doi.org/10.1021/ie4012936 | Ind. Eng. Chem. Res. 2013, 52, 9790−9802
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T rCO2 = kov[CO2 ] = kobs[CO2 ] = k 2[BDA][CO2 ] = rBDA
The time-averaged absorption rate per unit interfacial area, integrated from eq 29, is expressed by eq 31.
(19)
Therefore, according to the termolecular mechanism, the reaction rate of BDA carbamate formation reactions, with BDA and water acting as the dominating bases, can be expressed as follows:
Flux = −
(20)
DEEA + H 2O ↔ DEEAH + OH
−
kl0 = 2
E=
(22) (23)
CO2 + OH− ↔ HCO3−
(24)
2H 2O + CO2 ↔ H3O+ + HCO3−
(25)
Flux − [CO2 ]* )
kl0([CO2 ]i
Ha =
(33)
mPCO2 RT
(34)
kobsDCO2 kl0
(35)
where the total reaction rate constant kobs is assumed to be completely being determined by the following equation: rCO2 kobs = [CO2 ] (36) According to the penetration model,28 the infinite enhancement factor for irreversible reactions is given by EA,∞ =
⎞ DCO2 ⎛ D ⎜⎜1 + Am [Am]RT ⎟⎟ DAm ⎝ DCO2 νAmPCO2mCO2 ⎠
(37)
For a constant Hatta number (Ha > 2), depending on the value of Ha and the ratio between Ha and EA,∞, there are three absorption regimes with the decreasing of infinite enhancement factor (for the experiments with constant amine concentration, this is relative to the increasing CO2 partial pressure), as stated in the following part. If EA,∞ and Ha satisfy eq 38, the absorption process can be considered to be in the PFO regime.
(27)
(28)
3.2. Mass Transfer. The following equations can be used to describe the mass-transfer reaction process of CO2 absorption by amines:
2 < Ha < EA,∞
(38)
Therefore, the enhancement factor is equal to the Ha number.29,30 Consequently, eq 34 becomes
(29)
Flux =
where rCO2 is the rate of reaction of CO2. Based on the mass balance, charge balance, and other boundary conditions, eq 6 can be integrated from t = 0 to t = θ (where θ is the total contact time for liquid and gas). For WWC, θ is calculated using eq 30.28 1/3 2/3 2h ⎛ 3μ ⎞ ⎛ πd ⎞ θ= ⎜ ⎟ ⎜ ⎟ 3 ⎝ gρ ⎠ ⎝ υ ⎠
(32)
where the chemical enhancement factor (EA) is a function of the Hatta number (Ha) and the infinite enhancement factor (EA,∞). The Hatta number is defined as
All the kinetic constants are determined by temperature, following the form of eq 28:
∂[CO2 ] ∂ 2[CO2 ] = DCO2 + rCO2 ∂t ∂x 2
D1 πθ
Flux = kl0EA
where rCO2−BDA and rCO2−DEEA are the same as the individual experiments of CO2 absorption into pure aqueous solutions of BDA and DEEA. As will be introduced below, if, in the pseudo-first-order (PFO) regime for DEEA, the reaction rate of DEEA with CO2 is just related to the temperature and DEEA concentration; however, for BDA, besides BDA itself and water, DEEA may act as a base for the reaction of BDA and CO2. Therefore, the interaction part in eq 26 could be given below.
⎛ b⎞ ki = ai exp⎜ − i ⎟ ⎝ T⎠
(31)
Considering the fresh solution ([CO2]* = 0), the liquidphase mass-transfer flux is given by
However, considering that (i) the solvent is large excess and (ii) the contribution of reactions 24 and 25 to the overall rate can be negligible, kobs is assumed to be equal to kov.27,12 In the mixtures, the total CO2 reaction rate is rCO2 = rCO2−BDA + rCO2−DEEA + rCO2−int (26)
T rCO2−int = kBDA −DEEA[BDA][DEEA][CO2 ]
∂[CO2 ] (0, t ) dt ∂x
The enhancement factor of chemical reaction to the mass transfer process (E) for the absorption of CO2 is defined from
(21)
2H 2O ↔ H3O+ + OH−
θ
Based on eq 31 and Higbie’s penetration model, the liquidphase mass-transfer coefficient (k0l ) for the physical absorption of CO2 is defined as
Since there is no carbamate formation in reaction at pH
