3rd Post Combustion Capture Conference (PCCC3, Regina, Canada on September 9th, 2015)
Analysis of the Rate-Limiting Step in Carbon Dioxide
p
to Amine-Impregnated
p g
Solid Sorbent
Absorption
Ryohei Numaguchi, Hidetaka Yamada, Kazuya Goto, and Katsunori Yogo
CO2 Capture technology
Amine solid sorbent
TEPA
PEI
CO2 emission
i i from
f
industries:
i d ti
Amine scrubbing is established.
Problems:
Large energy consumption
g stripping
pp g temperature
p
High
n
Impregnated on
porous supportt
Advantage:
Red ction of latent heat
Reduction
-- Solvent (water) not contained
Desorption at lower temperature
-- High surface area of amine to gas
Promising technology for CO2 capture
2
15
RITE’s solid sorbent project
Material development phase 2010 – 2015 (Supported by METI)
CO2
2R1R2NH + CO2
 R1R2NCOO + R1R2NH2+
：Adsorption
p
((40°C))
Am
mount of CO
O2 adsorbed
d [mol/kg]
To find novel high-performance
g p
sorbent
Amines screened by DFT calculation
 Synthesize desirable amine
5
4
：Desorption (40°C, in vacuo)
3
2
1
0
Solid sorbent with
commercial amine
Solid sorbent with
novel synthesized
amine
Gained prospect to achieve R&D target by new sorbent
(Capture energy < 1.5
1 5 GJ/t-CO2
GJ/t CO2 from coal-fired
coal fired PP)
 New phase for practical application 2015 -
3
15
Research target
Temperature dependence of CO2 absorption amount*
Absorb
bed am
mount
*A. W. Scaroni et al., Energy & Fuels, 16, 1463 (2002).
K. Yogo et al., Energy & Fuels, 29, 985 (2015).
2A + CO2 
ACOO- + AH+
Exothermic reaction
 capacity reduction
60-90℃
depending on amine
Temperature
p
Analysis for dominant factor of CO2 absorption at low temperature
1 Confirm thermodynamic equilibrium not achieved
1.
2. Estimate Rate-limiting step
4
15
Experimental methods
Solid sorbent – wet impregnation
Polyethylenimine
y y
((PEI))
(Wako, MW = 1,800)
+MeOH
Silicagel
g CARiACT Q
Q30
(Fuji Silycia Chemical)
Stirring 2h
Vacuum dry
60Ԩ 2h
Characterization
Ch
t i ti
N2 ads. isotherm(77 K) ASAP2420 (Micromeritics Corp.)
CO2 abs.
abs isotherm
ASAP2020 (Micromeritics Corp.)
Corp )
Pre-treatment: Vacuum, 80Ԩ, 6h
CO2 absorption
b
i rate
(Thermogravimetry)
JJupiter
i
STA449 F5 (Netzcsh)
(N
h)
Pre-treatment: N2 flow, 80Ԩ, 6h
5
15
Structure of amine liquid (N2 ads@77K)
N2 ads isotherm (77K)
600
450
Q30
PEI 10wt%
PEI 20wt%
PEI 30wt%
PEI 40wt%
300
150
0
0 80
0.80
0 85
0.85
0 90
0.90
0 20
0.20
Pore volu
P
ume [cm3/g]
N2 A
Adsorptio
on [cm3/g
g STP]
750
0 95
0.95
Relative Pressure [-]
1 00
1.00
0.15
0.10
0.05
0
Pore size distribution ((BJH method))
Q30
PEI 10wt%
PEI 20wt%
PEI 30wt%
PEI 40wt%
38 nm
34 nm
20-40 wt%
Pore filling
10 wt%
Surface coverage
Amine
Pore wall
10
Amine
Pore wall
20
30
40
Pore diameter [nm]
<10wt%: surface coverage by 2nm
>20wt%: surface coverage + pore filling by amine
 Used 40wt% sorbent for CO2 absorption
50
60
6
15
CO2 absorption isotherms
CO2 Absorption [mmol/g]]
3.0
2.5
PEI 40wt% on Q
Q30
Abs.
Des.
2.0
Hysteresis appears
at 40Ԩ (~1.5 mmol/g)
& 70Ԩ (~2.4 mmol/g)
100Ԩ
70Ԩ
40Ԩ
1.5
1.0
0.5
0 -2
10
10-1
100
Pressure [kPa]
101
102
Hysteresis at low temperature:
Absorption equilibrium is not achieved.
-- CO2 absorption is prohibited at a certain loading.
Threshold becomes higher by increasing temperature.
7
15
Absorption rate analysis by TG
3.0
PEI 40wt% on Q
Q30
2.5
CO2 Absorption [[mmol/g]]
CO2 Abso
orption [[mmol/g]]
3.0
100Ԩ
2.0
1.5
40Ԩ
1.0
0.5
0
0
5
10
Time [[h]]
15
20
First 10 min.
100Ԩ
95% for 20h
2.5
2.0
1.5
40Ԩ
1.0
65% for 20h
0.5
0
2
4
6
Time [min]
Absorption equilibrium not achieved < 20 hours
 Rate-limiting step?
8
10
8
15
Steps of CO2 transfer
1. Diffusion in boundary film
2. Diffusion in
remaining pore
Solid sorbent
particle
3. Diffusion in amine
4 Reaction
4.
R ti with
ith amine
i
Diffusion coeffs.
1. Boundary film
2 Remaining pore
2.
3. Inside amine
Mechanism
10-4 [m2/s]
Viscous flow
10-44 - 10-55 [m2/s]
Knudsen
<10-9 [m2/s]
Liquid phase
Rate-limiting step:
Diffusion in amine / Reaction with amine
9
15
Thiele modulus estimation
Reaction rate   L k1
Diffusion rate
D
L: diffusion length
D diffusion
D:
diff i coefficient
ffi i t
k1: 1st order reaction constant
 < 0.1  Reaction limiting
> 5  Diffusion limiting
Diffusion
ff
Diffusion coefficient: general value in liquid used
D = 1.0 × 10-9 [m2/s]
Diffusion length: assumed 1/10 of particle diameter
L = 100 [m]
Thiele modulus estimation – Reaction
PEI
Reaction model
Zwitterion formation (1
(1° 2 ° amines)
+ CO2
2°
H+
Proton elimination
 Accepted by another amino group
H+
+
10
15
R-NH3+ +
Protonated
Carbamate
Assumption: Proton transfer >> Zwitterion formation
Focusing on beginning  No reverse reaction
r = k[Amine][CO2]
3°
1°
n
Rate-limiting step estimation
11
15
*G. F. Versteeg et al., Chem. Eng. Sci., 43, 573 (1988).
r = k [[Amine]] [[CO2]
Physisorbed
Density of
Amino group concentration
Brønsted relationship model*: k = 20.2 [m3/(mol.s)]
[CO2] = 3.3 × 100 [mol/m3] (value in water used)
[Amine] = 2.39 × 104 [mol/m3]  regarded as constant
due to excess amount
CO2 reaction constant k1 = k[Amine] = 4.82 × 105 [s-1]
Thiele modulus:  ~ 22
Diffusion in amine liquid is rate-limiting step
12
15
Diffusivity evaluation by simple model
102
Diffusion from two side edges
Time dependence of abs. rate
101

Diffusion into particle
(neglect pore structure)
2



ddxx



2
 2 exp  ( 2n  1)    
d
 2  
n 1

Reduced abs. rate
e [-]


dx
 6 exp  n 2 2
d
n1
100
10-1
10-22
10-3
Diffusion rate
St i ht iin log
Straight
l scale
l
10-4
10-5
10-6
Sphere
10-7
10-8
-9
10
10-100
0
1D diffusion
5
Reduced time [-]
Based on simple models: fitting slope gives diffusion coeff.
10
13
15
CO2 Diffusion rate (exp.)
10-3
40Ԩ
CO2 Abs. rate
e [mmol/(g
g.s)]
CO2 Abs. rate
e [mmol/(g
g.s)]
10-3
10-4
10-5
10-66
0
5
10
Time [h]
15
20
100Ԩ
10-4
Abs [m
mmol/g]
Rate evaluation: average for 5 min at 0-1h, 10 min at 1-3h and 60 min at 3-20h
3.0
2.0
1.0
0
0
10-5
10-66
0
10
Time [h]
20
After 5 h
Evaluation impossible
due to noise ((~10
10-6)
5
10
15
Time [h]
40Ԩ: very slow absorption until 20 hours
 Diffusivity additionally restricted with increasing CO2 loading
20
14
15
Diffusion mechanism
40Ԩ
Abss [mmol/g]
CO2 Abs. rrate [mmo
C
ol/(g.s)]
10-3
10-4
Hypothesis:
Reduction of CO2 diffusivityy
By polyamine connecting
3.0
2.0
10
1.0
0
0
10
Ti
Time
[h]
20
10-5
Pure PEI (liq)
10-6
0
5
10
Time [h]
15
After CO2 bubbling
PEI becomes highly viscous
20  Diffusion constant should decrease
(Stokes-Einstein law for diffusion)
Controlling
C
lli diffusivity
diff i i reduction
d i by
b CO2 absorption
b
i
 Enhance CO2 capacity at low temperature
Summary
15
15
Analysis for Rate-limiting step of CO2 absorption in solid sorbent
Hysteresis analysis on CO2 absorption isotherm
Rate analysis of CO2 absorption by TG
Abso ption eq
Absorption
equilibrium
ilib i m is not achieved
achie ed at lo
low temperature.
tempe at e
Thiele modulus evaluation
Rate-limiting step: CO2 diffusion in liquid amine
CO2 diffusivityy is decreased byy p
polyamine
y
connecting
g
Controlling diffusivity reduction should improve CO2 capacity
Acknowledgement:
This project is financially supported by Ministry of Economy, Trade and Industry
(METI), Japan.