3.1. CO2-Amine-H2O System
The reaction kinetic data between CO
2 and aqueous tertiary amine (TEA and DEEA) solution with temperatures ranging from 293 K to 313 K was listed in
Table A1. In the CO
2-tertiary amine-H
2O system, amine itself is the major contributor to the reaction and the contributions of OH
− and water can be negligible [
15,
16]. Then, the base-catalyzed hydration mechanism (Equations (1)–(3)) was used to interpret the possible reaction mechanism of CO
2 when reacting with aqueous TEA and DEEA solutions. The collected kinetic data shown in
Figure 3 (
Table A1) was fitted by Equation (
5) to verify the possible reaction order of CO
2 absorption in amine solution at the temperatures of 293, 298, 303, 308, 313 K, in which the reaction order is approximately equal to one (1.30–1.36 for TEA and 0.83–1.0 for DEEA) with respect to amine concentration over temperature ranging from 293 to 313 K, respectively.
As can be observed from
Figure 3, the pseudo first-order rate constants show a growing trend with increasing concentration and temperature of amine solution. Under experimental temperature, the reaction order with respect to amine concentration was approximately equal to 1 by fitting
values using the power law Equation (
9).
where
n is the reaction order.
The above obtained
values listed in
Table A1 were fitted with Equation (
10) in order to get
values of these amines. All the fitted results of
for TEA and DEEA are given in
Table A2. It can be easily found that the
value is just a function of temperature. It is widely accepted in published studies that
can be fitted according to the Arrhenius expression:
where
A is Arrhenius constant or pre-exponential constant (m
/kmol·s
), and
represents the activation energy (kJ/mol) and
R represents the universal gas constant (0.008315 kJ/mol·K
).
The corresponding Arrhenius equation for
with TEA was correlated according to Equation (
10) and the results displayed in
Figure 4. The calculated activation energy was 45.29 kJ/mol. The Ea values of these amines in the Arrhenius were replaced with calculated values in Equations (11)–(12), respectively.
Figure 5 showed a comparison between the
values obtained with base-catalyzed mechanism and the experimental data. The absolute average deviations (AADs) is calculated as shown in Equation (
13). The AADs for CO
2–TEA–H
2O and CO
2–DEEA–H
2O systems were 10.8% and 4.5%, respectively.
3.2. CO2-Amine-H2O Containing CA
The collection of kinetics data was operated almost the same as
Section 3.1, except that a quantitative enzyme is added to the amine solution. The concentration of CA was over a range of 0–50 g/m
in 0.2 kmol/m
aqueous TEA solutions. The results of collected kinetic data was listed in
Table A3 and fitted by Equation (
14) shown in
Figure 6. Compared with the obtained results of
Figure 3, it found that the
value was obviously accelerated by the existing of CA in the aqueous amine solution. There is a non-linear relationship between
and CA concentration, which is inconsistent with the test results provided by Alper and Deckwer [
19]. The rate constants were well fitted by a linear regression at low amine concentration and reached a maximum value and not changed at higher enzyme concentration. The similar phenomenon was observed in the work of Van Elk et al. [
20] on the reaction of CO
2 with DMMEA catalyzed by a thermostable variant of the human carbonic anhydrase (5X CA) provided by CO
2 Solutions Inc. However, the increase of overall reaction rate constants in the DEEA soulution were not significant. This is because the enzyme shows better activity in a near neutral environment and the pKa value of DEEA has a key impact on the catalytic activity of enzyme. Similar conclusions were reached by Wilk et al. [
21] when using N-methyldiethanolamine (MDEA) and the activator in the form of piperazine.
Subsequently, the overall catalyst enhancement (CE) was defined in enzyme catalyzed CO
2 absorption system, which represented the ratio of reaction rate constant as listed in Model 1 (
) and another correlation one marked as Model 2 (
), which is used to correct the experimentally determined
values in CO
2-MDEA system using human carbonic anhydrase (HCA) as additive agent introduced by Penders-van Elk et al. [
8] However in order to further study the effect of CA on the
, the influence of amine itself needs to be removed. The termed Model 3 listed in Equation (
14) was selected in this work, in which the reaction rate constant of CO
2 reacting with hydroxyl ion in present of enzyme was considered to exclude the effect of amines on CA activity, which is different from Model 1 and Model 2.
It is worth noting that both above Model 1 and 2 were not selected in this work but Equation (
14) was used for data fitting as listed in our previous work on MDEA and DMEA containing carbonic anhydrase [
6].
Table A4 listed the kinetic data of CO
2 absorption in various amine solutions at temperatures from 293 to 313 K and the obtained results as shown in
Figure 7 show that there is a linear trend in both aqueous solutions of TEA and DEEA. In CO
2-DEEA-H
2O system, the CA shows partial activity at lower concentrations and no obvious regular catalytic activity in this amine system, which means the model of Equation (
14) is not suitable for this system. This phenomenon is likely because the overall absorption rate is not limited by enzymatic turnover once the enzyme is sufficiently concentrated.
where
is reaction rate constant of CA-CO
2-amines-H
2O system,
is the reaction rate constant of CO
2-amine-H
2O system, and
represents the reaction rate constant of CO
2 reacting with OH
− in the solution which can be found in the work of Pinsent et al. [
22].
and
are parameters of Equation (
14) as a function of temperature following Arrhenius relationship. The catalyst enhancement value reflects the extent to which CA promotes the CO
2 absorption into the amine solution compared to that uncatalyzed reaction.
Equations (15) and (16) is the results of the intercept values in
Figure 8 fitted in an Arrhenius relationship. In this work, the reaction rate constant is defined as
. The reaction kinetics fitting of CO
2 reacting with aqueous solution of different amines including TEA were performed. Equation (
20) can be obtained combing Equations (4) and (14). The
can be replaced by
multiplying amine concentration. The values of the calculated
shown in Equations (19) and (20) are plotted versus the experimental values in
Figure 9. It showed a good performance in predicting
values of CO
-TEA-H
O solution with the AADs of 23.38%.