Reaction mechanism

ov CO2

L

Ha k D

= k (4.8)

and the enhancement factor in instantaneous reaction regime, E_∞, is

[ ] [

2

]

CO^Am₂

Am CO E D

Z D

∞ = (4.9)

When Eq. (4.7) is satisfied, the enhancement factor is equal to Hatta number (discussed in details in Section 2.3.3 of Chapter 2). The specific rate of mass transfer of CO₂ is given by [37]:

[ ]

[ ] ( )

2 2

2

2

2

CO CO

0 CO ov 2

CO ov

d CO dx CO

tanh

x

i

L

N D

D k

D k k

=

⎛ ⎞

= − ⎜ ⎟

⎝ ⎠

=

(4.10)

For Ha > 3, tanh Ha is near one; thus, the specific rate of mass transfer of CO2 becomes

[ ]

²

2 2 2

2

CO

CO 2 CO ov CO ov

CO

CO _i p

N D k D k

= = H (4.11)

The k_ov were calculated using Eq. (4.11) with the help of other known parameters,

CO2

p ,

CO2

D ,

CO2

H and the measured specific rate of absorption of CO₂,

CO2

N .

[ ]

- 3

3

[OH ] 1 , 10

Am , 10

W P

W P

K K

K K

αα α α

−

−

⎛ − ⎞

= ⎜⎝ ⎟⎠ ≥

= ≤

(2.17)

The water, 2-PE and AHPD protonation constants were obtained from Perrin [39], Xu et al.

[40] and Ford et al. [41], respectively.

4.4.2 Reactions of CO

2

with sterically hindered alkanolamines (2-PE and AHPD)

Following the mechanism first proposed by Caplow [42] and reintroduced by Danckwerts [43] the general consensus for the reaction of CO2 with primary or secondary amines is the formation of zwitterion intermediate, rather than one step carbamate formation. The first step of this mechanism is the formation of zwitterions:

+ -

CO +R'NH2 R'NH COO (4.12)

where R' is C7H14O for 2-PE and C4H11O3 for AHPD. The second reaction is the base- catalyzed deprotonation of the zwitterions by any base existing in the solution. The contribution of each base to the reaction rate depends on both its concentration and its strength as base. The main contributions for the deprotonation of zwitterion (Eq. (4.18)) in an aqueous amine solution are from hydroxide (OH^-), water and amine itself although the contribution of hydroxide ion can be assumed to be negligible without a significant loss of accuracy as [OH⁻] is low in the reaction of CO₂ with the amines [44, 45]. Laddha and Danckwerts [46] considered only the amine as a base for aqueous alkanolamine solutions in Eq. (4.13) which is presented as

+ - - +

R'NH COO +b→R'NCOO +bH (4.13)

As in case of sterically hindered amines the stability of the carbamate formed is less, it undergoes carbamate reversion reaction as follows:

- -

2 3

R'NCOO +H O R'NH+HCO (4.14)

To confirm the final reaction product and to establish the right reaction mechanism in the present study, a qualitative analysis on ¹³C NMR response was carried out for CO₂ loaded 2- PE solution in D₂O using a 400 MHz NMR spectrometer (Mercury Plus 400 NMR

spectrometer, Varian, USA). The peaks to the functional groups of the molecules were assigned by using information from the literature [28, 47, 48]. From the NMR spectra (Figure 4.4), no carbamate peak could be identified whereas the carbon peak for bicarbonate ion appeared at 160.509 ppm, which confirmed the formation of bicarbonate as the final reaction product. The reaction mechanism in case of absorption in (AHPD + H₂O) was considered based on the ¹³C NMR analysis performed by Park et al. [28] which confirmed the hydrolysis of unstable carbamate to form bicarbonate.

So, according to Eq. (4.14), through the hydrolysis of carbamate, free alkanolamine is released and if only alkanolamine is considered for catalyzing the zwitterion, it can be concluded that one mole of CO₂ is absorbed per mole of amine. Thus the final reaction of CO₂ with 2-PE or AHPD is as follows:

2 + -

2 2 2 3

CO +R'NH+H O ^k R'NH +HCO (4.15)

So the forward reaction rate of CO₂ with 2-PE or AHPD can be described as:

[ ][ ]

CO -R'NH2 2 CO2 R'NH

r =k (4.16)

For the absorption of CO₂ into (2-PE + H₂O) or (AHPD + H₂O), the CO₂ overall reaction rate can be expressed as follows:

2 2 -

ov CO -R'NH CO -OH ov[CO ]2

r =r +r =k (4.17)

where, k_ov is the overall reaction rate. Substituting the reaction rates from Eqs. (2.15) and (4.16) into Eq. (4.17), one has,

[ ][ ]

^{* - -}

ov 2 CO2 R'NH OH[CO ][OH ]2 ov[CO ]2

r =k +k =k (4.18)

Thus,

-

* -

ov 2[R'NH] OH[OH ]

k =k +k (4.19)

The apparent reaction rate constant, k_app, is defined as follows:

[ ]

-

* -

app ov OH[OH ] 2 R'NH

k =k −k =k (4.20)

4.4.3 Reactions of CO

2

with PZEA

In general, zwitterionic mechanism is used to describe the kinetics of the primary and secondary alkanolamines with CO2. It is assumed that this mechanism is also applicable to

PZEA although it is not an alkanolamine. PZEA reacts with CO₂ to form zwitterion, which is consequently deprotonated by any base present in the liquid. Since, PZEA is a tri-amine containing one primary, one secondary and one tertiary amine group as shown in Figure 4.1c;

the chemistry of the system is very complex. This gives rise to a large number of possible chemical reactions and form species (e.g., primary and secondary carbamates, dicarbamate and bicarbonate) for which it is difficult to identify the most important reaction(s) and to exactly determine the effects of these reaction(s) on the overall absorption rate. But, a first estimation for these reactions can be made using the Brønsted dependency of the reactivity on the pKa [49]. This techniques has shown that for many alkanolamines, a (linear) relation between the pKa value of an (alkanol)amine and (the logarithm of) the forward rate constants exists. According to Pagano et al. [50] the difference between the first and second protonation constants for PZEA is very less. The first protonation constant represents either the primary amine or the secondary amine nitrogen (probably the former) while the second protonation constant represents the other of these two nitrogen atoms [50]. They also reported the protonation constant for the tertiary amine nitrogen to be too small to be measured.

If it is assumed that the Brønsted relation is also applicable to PZEA, it seems reasonable to disregard the reaction of CO2 with tertiary amine group, whereas the reaction of CO2 with primary and secondary amine groups to form primary and secondary carbamates should be considered. Now, the possibility of formation of dicarbamate can be determined by the concentrations of the main reactant PZEA and primary and secondary mono-carbamates.

Since in this study, the conditions for the absorption of CO2 in PZEA solutions lie in the fast pseudo first-order reaction regime, the (interfacial) concentration of PZEA is not noticeably decreased due to the reaction with CO2. Hence the concentration of the reaction product mono-carbametes will be small compared to the remaining PZEA concentration (even close to the gas-liquid interface) and, consequently, the mono-carbamates can only make a small contribution to form dicarbamate. We also carried out qualitative ¹³C NMR analysis for CO2

loaded PZEA solution in D₂O using the same spectrometer used for 2-PE to get an idea about the final reaction products. The number of peaks in the NMR spectra (Figure 4.5) confirmed the formation of primary and secondary carbamates whereas the possibilities of formation of dicarbamate and bicarbonate were ruled out.

Based on above considerations concerning the various reactions with CO₂ in aqueous PZEA solutions, it seems justified to conclude that the overall absorption rate is, in the first pseudo first order kinetic regime, influenced by the reaction of CO₂ with PZEA to form primary and secondary carbamates. Thus,

- +

2 p

CO +PZEA+b PZEACOO +bH (4.21)

- +

2 s

CO +PZEA+b PZEACOO +bH (4.22)

wherePZEACOO and^-_p PZEACOO^-_sare primary and secondary carbamates, respectively. But, it is difficult to determine the actual concentration of PZEA contributed for a particular reaction. So, it is hard to find out the relative contribution of each reaction to the overall reaction with CO2. Therefore, we assume here the reaction to be first order with respect of both CO₂ and PZEA. The validity of this assumption is verified using experimental data which is discussed later on. So the reaction rate of CO₂ with PZEA is described as:

[ ][ ]

CO -PZEA2 2 CO2 PZEA

r =k (4.23)

In the present case, the relative contribution of reaction between CO2 and OH^– ion to the overall reaction was found insignificant. Because, the differences between the reaction rate constants considering and neglecting reaction of CO2 and OH^– ion were obtained less than 0.1

% in all cases of our experiments discussed later. Therefore, reaction of CO2 with OH^– ion was also neglected here. So, the overall reaction rate with PZEA is expressed as follows:

[ ][ ] [ ]

ov 2 CO2 PZEA ov CO2

r =k =k (4.24)

4.4.4 Reactions of CO

2

with blends of (PZEA + MDEA)

The reaction mechanism for the reaction of CO2 with tertiary alkanolamines is essentially a base-catalyzed hydration of CO2 forming a protonated amine and a bicarbonate anion [51]:

2 ,Amine +

1 2 3 2 2 1 2 3 3

R R R N + H O + CO ⎯⎯⎯→^k R R R NH + HCO⁻ (4.25) In most of the literature on CO2 kinetics with tertiary amine such as MDEA in aqueous solutions, it was assumed that reaction of CO2 with MDEA is a pseudo-first order reaction [44, 52–54]. So, the rate of CO2 reaction with tertiary amine MDEA is expressed by:

[ ][ ]

CO -MDEA2 2,MDEA CO2 MDEA

r =k (4.26)

Therefore, for the absorption of CO₂ into (PZEA + MDEA + H₂O), the overall CO₂ reaction rate is expressed as follows:

[ ][ ] [ ][ ]

2 2

ov CO -PZEA CO -MDEA

2,PZEA 2 2,MDEA 2

ov 2

CO PZEA CO MDEA

[CO ]

r r r

k k

k

= +

= +

=

(4.27)

where

[ ] [ ]

ov 2,PZEA PZEA 2,MDEA MDEA

k =k +k (4.28)