Chin. J. Chem. Eng., 15(6) 842—846 (2007)
The Enhancement of CO2 Chemical Absorption by K2CO3 Aqueous
Solution in the Presence of Activated Carbon Particles*
LU Sumin(卢素敏), MA Youguang(马友光)**, ZHU Chunying(朱春英) and SHEN Shuhua
(沈树华)
School of Chemical Engineering and Technology, State Key Laboratory of Chemical Engineering, Tianjin University, Tianjin 300072, China
Abstract The enhancement of chemical absorption of CO2 by K2CO3/H2O absorbents in the presence of activated
carbon (AC) particles was investigated. The results show that the gas absorption rates can be enhanced significantly
in the presence of AC particles, and the maximum enhancement factor 3.7 was observed at low stirring intensities.
The enhancement factor increased rapidly with the solid loading during the initial period of absorption and then became mild gradually to a maximum value. Both the liquid-solid contact area and the probability of solid particles
residing at the gas-liquid interface decreased with the increase of the particle size, leading to a negative effect on the
enhancement of mass transfer. The influence of the particles on gas absorption decreased with the reaction rate. The
stirring speed changed the interfacial coverage and mass transfer rate on the liquid side and consequently affected
the mass transfer between the gas and liquid phases; the enhancement factor decreased with the stirring intensity. A
heterogeneous two-zone model was proposed for predicting the enhancement factor and the calculated results
agreed well with the experimental data.
Keywords chemical absorption, enhancement factor, mass transfer, activated carbon particle
1
INTRODUCTION
As the main contributor to the greenhouse effect,
the recovery and rational utilization of CO2 has received considerable attention. The chemical absorption technique is usually employed in several industrial processes, in which ethanolamine[1,2] and potassium carbonate solvents[3,4] as absorbents are commonly used. Ethanolamines (mainly monoethanolamine and diethanolamine) show fast reaction rates, but
high energy consumption in regeneration. Conversely,
the reaction product KHCO3 from K2CO3 and CO2 is
easily decomposed and regenerated, whereas its rate
of reaction is slow as compared to amines; catalysts
such as hypochlorite, arsenite, and carbonic anhydrase[3] are often introduced to improve the reaction
rate. Owing to very low solubility of CO2, the
liquid-side mass transfer is also of key importance in
the CO2 absorption into aqueous K2CO3 solution.
It has been found that the gas-liquid mass transfer can be significantly enhanced in the presence of
some fine solid particles[5—11]. In this article, the
influence of activated carbon (AC) particles on CO2
absorption was investigated with K2CO3 aqueous solution as the absorbent. A two-zone heterogeneous
mass transfer model for predicting the enhancement
factors was developed based on the Higbie penetration
theory.
2
EXPERIMENTAL
The experiments were carried out in a thermostatic reactor with CO2(＞99.5% mass fraction) ab－
sorption in 0.5mol·L 1 K2CO3 aqueous solution, in
which NaClO was chosen as the catalyst (Fig.1). The
reactor consists of a stainless steel vessel with a di-
Figure 1 Experimental set-up for gas absorption
1—CO2 inlet valve; 2—junction valve; 3—balance tank;
4—pressure transmitter; 5—pressure difference transmitter(connected with the computer); 6—temperature sensor;
7—magnetic stirrer; 8—cooling coil; 9—gas outlet valve;
10—vacuum valve; 11—gas stirrer; 12—liquid stirrer;
13—baffles; 14—stainless steel top; 15—stainless steel
reactor; 16—thermostatic bath
ameter of 8cm and a total volume of 800cm3. The liquid volume added for each experiment was 400cm3.
Four verticle baffles were mounted on the vessel wall
to prevent the formation of dented liquid surface. Two
stirrers were employed to mix the gas phase and the
liquid phase, respectively. A cooling coil in the reactor
was connected to the thermostatic bath to maintain a
constant reaction temperature (298.15±0.1)K. The
absorption experiment starts with an initial pressure of
Received 2007-06-06, accepted 2007-09-24.
* Supported by the National Natural Science Foundation of China (No.20176036).
** To whom correspondence should be addressed. E-mail: [email protected]
The Enhancement of CO2 Chemical Absorption by K2CO3 Aqueous Solution in the Presence of Activated Carbon Particles
pure CO2 at 0.1MPa (gauge). Vessel 3 is a reference
cell, and a pressure difference transducer is connected
between vessel 3 and the reactor. The transducer signal was transmitted to the computer and recorded
on-line. With the value of the recorded pressure difference, the absorption rate can be calculated.
Activated carbon particles of different sizes (5μm,
250μm and 100μm) were purchased from Dalian Yixiu
Co., and the BET surface area of AC particles measured
by Gemini V 2.00 analyzer (Micromeritics Instrument
－
Corp.) were 1150,960 and 716m2·g 1, respectively.
THEORETICAL MODEL
Several researches[12—14] show that the enhancement in the gas absorption rates depends closely
on the hydrophobicity and the adsorption capacity of
the particles. As a hydrophobic adsorbent, AC particle
tends to adhere itself to the gas-liquid interfacial layer.
To facilitate theoretical analysis, the gas-liquid interface is divided into two zones: the uncovered part
(Zone I) and the covered part (Zone II); the mass
transfer rate for the uncovered part is assumed to be
equal to that measured in a particle-free liquid under
similar conditions (Fig.2). The particles are considered
to be spherical and have equal size; no direct access to
the gas phase is available[8,12], and there is a thin
stagnant layer of liquid wrapping the solid particle[15].
RA = kr CA
843
(3)
3.2
Zone II
In the continuous phase of this zone, the balance
of the solute can be written as:
∂CAD
∂C 2
(4)
= DA AD
− RAD
∂t
∂x 2
The relevant conditions of Eq.(4) are given by
3
t =0,
x≥0 ,
x=0,
t＞0 ,
CAD = 0
CAD = CA0
(5)
In the liquid film wrapping the particles,
are:
∂CAD
∂C 2
(6)
= DA AD
− RAD − kp as ( CAD − CAS )
∂t
∂x 2
The necessary initial and boundary conditions
t =0,
CAD = 0
x = L + δp ,
CAD = CAS
(7)
At the boundary between the liquid film and the
continuous phase (x＝L):
∂C
− DA AD = kp as ( CAD − CAS )
(8)
∂x
－
where as is the surface area of the particles, m 1:
6ms
(9)
as =
dp ρp
It is assuming that Langmuir type adsorption
isotherm can be employed to describe the adsorption
of the solute on the particles:
kCAS
(10)
q = qm
1 + kCAS
With these assumptions, the unsteady-state species balance for the solute can be derived respectively
in the two zones.
where q is the adsorbed amount of the solute per unit
mass of the particles. The balance for the accumulation of the solute within or on a single particle can be
written as:
∂q
ms
= kp as ( CAD − CAS )
(11)
∂t
3.1
3.3
Figure 2
Sketch of mass transfer in the slurry
Zone I
A species balance for the solute in the liquid
phase is given by
∂CA
∂C 2
= DA 2A − RA
∂t
∂x
with the following conditions:
t =0,
x≥0 ,
x=0,
t＞0 ,
x = δ ( or x → ∞ ) ,
(1)
CA = 0
CA = CA0
t＞0 ,
CA = 0
(2)
－3
where RA is the chemical reaction rate, mol·m ·s 1.
For first-order chemical reaction,
－
Absorption rate
Based on the Higbie penetration model, the absorption rate through the uncovered part is:
∂C
(12)
J 0 = − DA A
∂x x =0
If τ is the residence time of the liquid element,
the time-averaged flux follows from:
∂C
1 τ
(13)
dt
J 0 = ∫ − DA A
τ 0
∂x x =0
Similarly, the absorption rate for the covered part is:
J AD = − DA
∂CAD
∂x
(14)
x =0
Chin. J. Ch. E. 15(6) 842 (2007)
Chin. J. Ch. E. (Vol. 15, No. 6)
844
The time-averaged flux through the covered part
is:
J AD =
3.4
1
τ
−D
τ ∫0 A
∂CAD
∂x
(15)
dt
x =0
kr = kr0 + krc Ccal
Enhancement factor
The enhancement factor is defined as
⎛ gas absorption flux with solid particles ⎞
E =⎜
⎟
⎝ gas absorption flux free from solid particles ⎠in similar hydrodynamic
conditions
(16)
As assumed by Demmink et al.[8], the coverage
of the gas-liquid interface can be described by the
Langmuir-type adhesion isotherm:
km
α = α max s s
(17)
1 + ks ms
Then, the enhancement factors can be written as:
E=
(1 − α ) J 0 + α J AD
J0
(18)
The above equations are solved numerically by
gPROMs modeling software (Process System Enterprise Ltd.).
4
MODEL PARAMETERS
To evaluate the effect of the particles by the
model presented here, the parameters kr, δp, qm, k, ks,
αmax, and kp for the CO2-K2CO3/AC system must be
defined previously. The liquid-to-particle mass transfer coefficient kp is calculated from Sh＝2[16]. Although the thickness of the liquid layer δp around the
particles is very thin, it must not be neglected because
the resistance is mainly located in this stagnant liquid
film for an external diffusion controlling adsorption,
according to the penetration theory, δp＝kp/DA. The
adsorption equilibrium constant k and the maximum
adsorption amount qm of AC are determined experimentally: q is obtained by measuring the difference of
the absorption amount of the solute in the liquid with
and without particles, and then k and qm are calculated
by fitting the curves of the experimental CA and q at
different initial pressures; the experimental values for
－
－
k and qm are 95m3·mol 1 and 11.5mol·kg 1, respectively. The adhesion constants of the particles, ks and
αmax refer to the results of Shen[17] (see Table 1).
Table 1
Values for parameters of Langmuir adhesion
constants ks and αmax
Stirrer speed, s－1
ks
αmax
1
24
0.38
2
30
0.35
3
48
0.32
The concentration of K2CO3 aqueous solution
－
used in this study is 0.5mol·L 1. In such concentration,
December, 2007
the reaction between CO2 and K2CO3 can be considered as pseudo-first order chemical reaction owing to
the high concentration of K2CO3[18], and the apparent
reaction rate constant is given by[18]:
(19)
where, kr0 and krc are the uncatalytic and catalytic re－
action rate constants, respectively. kr0 is taken as 0.2s 1,
and krc is calculated using Eq.(20) in terms of the
measurement of the absorption rate at different catalyst concentrations[18]. The concentration of the
－
catalyst (NaclO) was in the range of 0—4.0kg·m 3.
J 0 = kL CA0 1 + DA ( kro + krc Ccal ) / kL2
(20)
5 RESULTS AND DISCUSSION
5.1 Effect of stirring speed and solid loading
The enhancement factors at different solid concen－
－
trations (0—1.5kg·m 3) and stirring speeds (1—3s 1)
are presented in Fig.3. The results clearly indicate that
the enhancement factors depend greatly on both the
solid loading and the stirrer speed. The highest enhancement factor 3.7 is found at the lowest stirring
intensity. The enhancement factor increases rapidly
with the solid loading during the initial period of absorption and then becomes mild gradually to a maximum value. The minimum solid loading for the
－
maximum enhancement is about 0.4kg·m 3. On the
basis of Eq.(17), there is a maximum fractional coverage by the solid particles at the interface under a
given stirring speed, which leads to a limitation of the
enhancement of mass transfer.
Figure 3 Influence of stirring speed on the
enhancement
factors (particle size: 5μm)
－
N, s 1: ● 1; ▲ 2; ◆ 3; —— cal.
Stirring speed plays a significant role in
gas-liquid interphase mass transfer. Obviously, high
stirring intensity brings about high absorption rate in
pure liquid, resulting in a decrease in the enhancement
factor referring to Eq.(18). Moreover, high stirring
speed induces low fractional coverage of the interface
by the particles[5,8], and hence most particles are
immersed in the bulk. The distance from the particle
to the gas-liquid interface has an important effect on
the mass transfer enhancement, as indicated by the
model prediction in Fig.4; the enhancement factors
calculated by Eq.(18) decrease rapidly with the increase of the distance of the particle to the gas-liquid
interface, and slight enhancement is found in high L/δ.
The Enhancement of CO2 Chemical Absorption by K2CO3 Aqueous Solution in the Presence of Activated Carbon Particles
Figure 4 Predicted influence of the distance of the particle
to the gas-liquid interface on the enhancement factors
5.2
Effect of the particle diameter
The enhancement factors at different particle
diameters (5—1000μm) are shown in Fig.5. The total
liquid-solid interfacial area decreases with the increasing particle diameter, which leads to a negative
effect on the enhancement of mass transfer; moreover,
the diameter also influences the interfacial coverage of
the particles. A correlation of parameters αmax and ks
in Eq.(17) to dimensionless mass transfer coefficient
KL was proposed by Demmink et al.[8]:
α max =
0
α max
K Lp
ks = ks0 K Lr
(21)
where KL is the dimensionless mass transfer coefficient:
kL d p
(22)
KL =
DA
0
and α max
, ks0 , p(p＞0), and r are constants in a given
system.
Figure 5
Influence of particle size on －the enhancement
factors (stirring speed: 1s 1)
dp, μm: ◆ 5; ■ 250; ▲ 1000; —— cal.
Based on Eqs.(21) and (22), the dimensionless
mass transfer coefficient KL increases with the particle
diameter, leading to a low interfacial coverage, and as
a result, the enhancement factor declines referring to
Eq.(18).
5.3
Effect of chemical reaction
The influence of chemical reaction on the enhancement factor is shown in Fig.6. It can be seen that
the chemical reaction intensified the mass transfer
between the gas and liquid phases but weakened the
enhancement action of the particles. For the reaction
of CO2 with K2CO3, the apparent reaction rate constant increases with the catalyst concentration as in
Eq.(19), and therefore, adding the concentration of
Figure 6
845
Effect of chemical reaction －rate constants
(d＝5μm, stirring speed: 1s 1)
catalyst is helpful to improve the absorption of CO2.
Nevertheless, the catalyst concentration usually has a
practical limit in industrial application on the environmental or commercial consideration, and thus, it is
still an effective and more accepted way to enhance
the absorption by adsorptive particles.
6
CONCLUSIONS
The influence of activated carbon (AC) particles
on CO2 chemical absorption under different conditions
was investigated. It can be concluded that:
(1) Fine AC particles can enhance the CO2
chemical absorption rate significantly. The highest
enhancement factor 3.7 was observed at low stirring
intensity.
(2) Stirring speed changed the interfacial coverage and the mass transfer rate on the liquid side and
consequently affected the mass transfer between the
gas and liquid phases; the enhancement factor decreased with the stirring intensity.
(3) Both the liquid-solid contact area and the
probability of solid particles residing at the gas-liquid
interface decrease with increase of the particle diameter, leading to a negative effect on the enhancement of
mass transfer.
(4) Chemical reaction improves the mass transfer
between the gas and liquid phases but weakens the
enhancement action of particles.
(5) A heterogeneous two-zone mass transfer
model was developed to predict the enhancement factor. The enhancement factor calculated by the present
model agrees well with the experimental data.
NOMENCLATURE
CA0
Ccat
DA
dp
E
JAD
surface area of the particles, m－1
solute concentration, mol·m－3
solute concentration in particle covered zone, mol·m－3
equilibrium concentration of the solute on the particle,
mol·m－3
solute concentration at the interface, mol·m－3
catalyst concentration, mol·m－3
diffusion coefficient, m2·s－1
particle diameter, m
enhancement factor
mass transfer rate in particle covered zone, mol·m－2·s－1
J AD
average mass transfer rate in particle uncovered zone,
J0
mol⋅m－2·s－1
mass transfer rate in particle uncovered zone, mol·m－2·s－1
as
CA
CAD
CAS
Chin. J. Ch. E. 15(6) 842 (2007)
Chin. J. Ch. E. (Vol. 15, No. 6)
846
J0
KL
k
kL
kp
kr
krc
kr0
ks
ks0
L
ms
N
p
q
qm
RA
r
t
x
α
αmax
0
α max
δ
δp
ρp
τ
average mass transfer rate in particle uncovered zone,
mol·m－2·s－1
dimensionless mass transfer coefficient
adsorption coefficient of the solute in Eq.(9), m3·mol－1
liquid-side mass transfer coefficient, m·s－1
liquid-solid mass transfer coefficient, m·s－1
reaction rate constant, m－1
catalytic reaction rate constant, m3·kg－1·s－1
uncatalytic reaction rate constant, m－1
particle adhesion coefficient, m3·kg－1
parameter in Eq.(21)
particle-to-interface distance, m
particle concentration, kg·m－1
rotation number of stirrer rotor per unit time, s－1
parameter in Eq.(21)
adsorbed amount of A on solid per unit mass of particle, mol·kg－1
maximum amount of adsorbed solute, mol·kg－1
reaction rate, mol·m－3·s－1
parameter in Eq.(21)
time, s
distance to the interface, m
fraction of the interface covered by particles
maximum coverage
parameter in Eq.(21)
penetration depth; m
liquid film thickness around particle, m
particle density, kg·m－3
residence time, s
6
7
8
9
10
11
12
13
14
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