3rd Post Combustion Capture Conference (PCCC3)
Analysis of the Rate-Limiting Step in Carbon Dioxide Absorption to
Amine-Impregnated Solid Sorbent
Ryohei NUMAGUCHI*, Hidetaka YAMADA, Kazuya GOTO, and Katsunori YOGO
Research Institute of Innovative Technology for the Earth (RITE), 9-2 Kizugawadai, Kizugawa, Kyoto 619-0292, Japan
Rate-limiting process in CO2 absorption to amine-impregnated solid sorbent has been discussed. The solid sorbent, consisting of polyethyleneimine and silica gel
particle support, is prepared by wet impregnation method. A hysteresis in CO2 absorption-desorption isotherm and time dependence of absorption rate are
measured. The hysteresis analysis indicates that a certain kinetic barrier constraints the CO2 absorption at low temperature, whereas the solid sorbent seems to
accomplish the absorption equilibrium at high temperature. Slow absorption remains after CO2 flows on the sorbent for more than 4 hours, and over-time change
in absorption rate is non-exponential, suggesting that the kinetic barrier becomes stronger with loading CO2 more. By estimating the Thiele modulus, CO2
diffusion in supported amine liquid is proposed as the rate-limiting step at low CO2 loading. CO2 diffusivity should decrease with its loading by taking account of
increment of amine viscosity, and this results in the slow absorption at high loading region.
Keywords: Amine solid sorbent; Rate-limiting step; Gas diffusion
1.
Introduction
There has been growing interest in amine-impregnated solid sorbent, which consists of hardly-volatile amine liquid and
mesoporous support, as a new-generation material for CO2 separation with low energy consumption. Through the vast
examination of amine and support in the last decade, it is generally known that CO2 absorption capacity increases with
temperature rising at low temperature.[1,2] This trend is against the theory of thermodynamic equilibrium, which indicates that the
capacity for CO2 absorption decrease due to exothermic process. This fact suggests that a certain kinetic process dominates the
performance of the solid sorbent; however, what kind of the process restricts the absorption rate is still unclear. In this study, we
investigate dependences of the rate-limiting process on temperature and CO2 loading through hysteresis analysis for CO2
absorption-desorption isotherms, long-term absorption rate measurement, and estimation of the Thiele modulus.
2.
Materials and methods
The solid sorbent was prepared by wet impregnation method. Polyethyleneimine (molecular weight = 1,800, Wako Pure
Chemical Industries, Ltd.) was dissolved to methanol in flask, and then silica gel particles (CARiACT Q30, 1.18-2.36 mm, Fuji
Silysia Chemical Ltd.) was added. After stirring for two hours, the mixture was evaporated at 60 °C, so that the 40wt% amine
was impregnated in the solid support. CO2 absorption-desorption isotherm was measured with ASAP2020 (Micromeritics
Instrument Corp.), after pre-treatment under vacuum condition at 80 °C for 6 hours. Time dependence of CO2 absorption amount
was evaluated by thermogravimetry (ThermoPlus, Rigaku Corp.). Pure CO2 gas was injected at 40 °C, after N2 gas injection at
80 °C for 6 hours for pre-treatment.
2
3.
Results and discussion
Figure 1 shows CO2 absorption-desorption isotherms of the prepared solid sorbent at two different temperatures. At 40 °C (Fig.
1a), CO2 absorbs at less than 0.1 kPa, and hysteresis is appeared at a CO2 loading higher than 1.4 mmol/g. Maximum amount of
CO2 absorption is recorded on desorption branch, indicating that the system does not reach to absorption equilibrium and
continue to absorb CO2 even at depressurization process. On the other hand, at 100 °C, CO2 absorbs from 1 kPa, and no large
hysteresis like that in 40 °C was detected (Fig. 1b). This difference of hysteresis in temperature supports the general
understanding that the CO2 absorption is inhibited by a certain kinetic barrier at low temperature although the CO2 absorption
can be saturated at high temperature.
3.0
(a) 40 °
C
2.5
2.0
CO2 Absorption [mmol/g]
CO2 Absorption [mmol/g]
3.0
absorption
desorption
1.5
1.0
0.5
0 -2
10
10-1
100
101
P ressure [kP a]
102
2.5
(b) 100 °
C
2.0
absorption
desorption
1.5
1.0
0.5
0 -2
10
10-1
100
101
P ressure [kP a]
102
Fig. 1 Temperature dependence of CO2 absorption-desorption isotherm
To investigate the CO2 absorption kinetics at the low temperature, the time dependence of the absorption amount was measured
at 40 °C (Fig. 2a). At first 4 hours, the solid sorbent absorbs 2.20 mmol/g of CO2, and the system seems to reach to the
absorption equilibrium; however, CO2 absorption continues slowly, and the CO2 uptake reaches 2.26 mmol/g after 35 hours. By
calculating a slope of this curve, over-time change in absorption rate was obtained (Fig. 2b, in scale of logarithm). The
absorption rate decreases exponentially in the first 4 hours, but the rate of 10-6 to 10-7 mmol·g-1·s-1 remains after 4 hours. In the
solid sorbent system, CO2 diffusion in amine liquid accommodated by support or carbamate formation reaction is considered as
the rate-limiting process for CO2 absorption, but the observed un-exponential curve cannot be explained by a first-order chemical
reaction model or simple model for intra-particle gas diffusion. We divide the term of CO2 absorption to two part: primary term
(0-4 hours in Fig. 2b) and secondary term (> 4 hours), and discuss the rate-limiting step of each term.
For evaluation of the rate-limiting step in the primary term, the Thiele modulus, which is ratio of diffusion and chemical
reaction rate, was calculated. The reaction rate for carbamation can be described with first-order model when conversion of CO2
to carbamate is enough low, and a kinetic constant was estimated by Brønsted correlation model
[2]
as 4.82×105 s-1. By setting
the diffusion coefficient as 1×10-9 m2/s, which is general value in liquid phase, and diffusion distance as 100 µm, the Thiele
modulus was estimated as 21.9, indicating that the gas diffusion in amine liquid is the rate-limiting step. For the secondary term,
CO2 diffusivity can be reduced, by considering the increment of amine viscosity with absorption of CO2. This will explain the
slow absorption after 4 hours, and diffusion should be also the rate-limiting process in the secondary term.
3
10-3
(a)
CO2 Abs. rate [mmol/(g.s)]
CO2 Absorption [mmol/g]
3.0
2.5
2.0
1.5
1.0
0.5
0
0
5 10 15 20 25 30 35
Time [h]
(b)
10-4
10-5
10-6
10-7
0
5 10 15 20 25 30 35
Time [h]
Fig. 2 Time dependence of (a) absorption amount and (b) absorption rate of CO2 at 40 °C
4.
Conclusion
We investigated the rate-limiting process for CO2 absorption to amine solid sorbent. The temperature dependence of CO2
absorption isotherm indicates that a certain kinetic barrier inhibits the CO2 absorption at low temperature. The time dependence
of absorption rate was measured, and exponential curve was observed at first 4 hours, whereas slow absorption continues after 4
hours. To understand the rate-limiting process, the Thiele modulus for primary term of absorption (0-4 hours) was estimated as
21.9, suggesting that the gas diffusion in amine liquid dominates the absorption rate. CO2 diffusion should slow down with
increasing the loading of CO2, which is implied by increment of amine viscosity, and it causes the drastic decrease in absorption
rate after 4 hours. Our analysis suggests that the accelerating the CO2 diffusion should contribute to performance improvement of
the amine solid sorbent.
References
[1] X. Xu, C. Song, J. M. Andresen, B. G. Miller, and A. W. Scaroni, Energy and Fuels, 16, 1463 (2002)
[2] D. S. Dao, H. Yamada, and K. Yogo, Energy and Fuels, 29, 985 (2015)
[3] G. F. Versteeg and W. P. M. Van Swaaij., Chem. Eng. Sci., 43, 573 (1988)
Acknowledgement
This work was financially supported by the Ministry of Economy, Trade and Industry (METI), Japan.
*Corresponding author. Tel.: +81-774-75-2360; fax: +84-774-75-2355.
E-mail address: [email protected].