Subscriber access provided by University of Newcastle, Australia
Article
Optimization of CO2 Capture Process from Simulated Flue Gas by Dry Regenerable Alkali Metal Carbonate-Based Adsorbent Using Response Surface Methodology (RSM) Mohsen Amiri, Shahrokh Shahhosseini, and Ahad Ghaemi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b03303 • Publication Date (Web): 07 Mar 2017 Downloaded from http://pubs.acs.org on March 7, 2017
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
1
Optimization of CO2 Capture Process from Simulated
2
Flue Gas by Dry Regenerable Alkali Metal Carbonate-
3
Based Adsorbent Using Response Surface
4
Methodology (RSM)
5
Mohsen Amiri1, Shahrokh Shahhosseini1
6
1
School of Chemical Engineering, Iran University of Science and Technology, P.O. Box 16765163,
7
Tehran, Iran.
8
Abstract
9
The low cost of K2CO3/Al2O3 adsorbent is encouraging to use it for CO2 capture from the flue gas of
10
fossil-fuel power plants. In this study, optimization of CO2 capture process using K-based adsorbent
11
in a fixed-bed reactor has been investigated. The sorbent was also characterized by different
12
techniques such as SEM, BET, and XRD analysis before and after the reactions. Response surface
13
methodology (RSM) combined with Box–Behnken design (BBD) was employed to evaluate the
14
effects of the process variables (temperature, mole ratio of H2O/CO2 and vapor pretreatment time)
15
and their interaction on the responses (CO2 capture capacity and deactivation rate constant) to
1 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 2 of 48
16
achieve the optimal conditions. In addition to the experiments, the deactivation model in the non-
17
catalytic heterogeneous reaction system was employed to evaluate the kinetic parameters (sorption
18
rate and deactivation rate constants) using nonlinear least square technique. According to the analysis
19
of variance (ANOVA), the vapor pretreatment time and temperature were found to be the most
20
important process variable, which affect CO2 adsorption capacity. Moreover, two quadratic semi-
21
empirical correlations were established to calculate the optimum operating conditions of CO2 capture
22
process. The predicted values of the correlations showed very good agreement with the experimental
23
data. The optimum process variables obtained from the numerical optimization corresponded to 61.3
24
°C, 1 and 9 min for adsorption temperature, mole ratio of H2O/CO2 and min vapor pretreatment time,
25
respectively. Based on the optimal condition, the highest adsorption capacity of 87.71 (mg CO2/g
26
sorbent) in 100% CO2 removal zone (corresponding to 97.82% of theoretical adsorption capacity in
27
the total zone) and the lowest deactivation rate constant of 0.1872 (min-1) were obtained.
28
Furthermore, additional experiments performed in the optimal conditions resulted in 86.97 (mg
29
CO2/g sorbent) adsorption capacity and deactivation rate constant of 0.1874 (min-1). The results
30
indicate that the presented models could adequately predict the responses and provide suitable
31
information for the process scale-up.
2 ACS Paragon Plus Environment
Page 3 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
32
Keywords: CO2 capture; Optimization; Response surface methodology (RSM); Adsorption; Alkali
33
metal carbonate; Kinetic.
34
3 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
35
Page 4 of 48
1. Introduction
36
Environmental concerns such as global warming and climate change have inspired researchers to
37
develop more effective and improved processes for carbon dioxide (CO2) capture 1. The climate of
38
the earth is changing continuously due to numerous factors, particularly growth in greenhouse gas
39
(GHG) concentrations 2. Among the greenhouse gases, CO2 contributes more than 60% to the global
40
warming
41
atmospheric concentration of CO2 has increased from 280 ppmv to 404 ppmv and as a result, the
42
average global temperature has increased between 0.6 °C to 1 °C 5. International Panel on Climate
43
Change (IPCC) predicts that, by the year 2100, the atmosphere may contain up to 570 ppmv CO2,
44
causing a rise of mean global temperature of around 1.9 °C and an increase in mean sea level of 38 m
45
2, 4, 6, 7
46
Extreme atmospheric greenhouse gases are responsible for several environmental problems such as
47
melting of the snow cover and ice caps, rising the sea levels, increasing number of the ocean storms
48
and floods 2, 9.
3, 4
. From the origin of the industrial revolution in about 1850 till now, the mean
. Man-made sources of CO2 include power plants, refineries, and cement industries
1, 8
.
49
CO2 Capture and sequestration (CCS) from the huge CO2 sources is recognized as the main option
50
to address the problem of global warming and climate change. CCS consist of four principal stages:
51
CO2 capture, compression, transport, and storage 9. In CO2 capture, CO2 emissions from thermal
4 ACS Paragon Plus Environment
Page 5 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
52
power plant flue gas can be diminished by any of the following methods: Pre-combustion CO2
53
capture, post-combustion CO2 capture and oxy-fuel combustion 2. In pre-combustion CO2 capture,
54
the fossil fuel is gasified and reacted in a water gas shift reactor to generate CO2 and H2. The CO2 is
55
captured, while the H2 is used for energy generation. In post-combustion technology, the fossil fuels
56
are combusted as in regular energy generation, and then the CO2 is captured from the discharge gas 9.
57
Current power plants use air for combustion and produce a flue gas at atmospheric pressure and
58
typically have a CO2 concentration of less than 15% 2. Oxy-fuel combustion consumes pure or nearly
59
pure O2 for combustion, such that mainly CO2 and H2O are generated 9. Among these technologies,
60
the post-combustion process is more appropriate for CO2 capture of the conventional power plants 5.
61
Multiple post-combustion technologies are available for CO2 capture from fossil fuel power plant.
62
These include mainly physical and chemical absorption, adsorption, cryogenic and membrane
63
processes 2. Among these processes, chemical or physical absorption processes using amine solutions
64
,such as monoethanol amine (MEA), are regularly used in the petroleum, natural gas and power
65
plants as well as chemical industries for separation of CO2
66
conventional amines are simply degraded and tend to lose its capacity because of the presence of
67
classic flue gas pollutants (e.g., O2, SO2, HCl, and particulates) and need high capital and operating
68
costs associated with CO2 separation from large volumes of flue gas at low CO2 concentrations 6. In
1, 2, 4, 9
5 ACS Paragon Plus Environment
. However, in this process
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 6 of 48
69
addition, the sorption of water into the gas stream involves an extra drying process, the evaporation
70
of the water rises the cost of the process 5.
71
Reported alternative process for liquid absorption is adsorption using solid adsorbents such as
72
zeolites, activated carbon and MOFs are not appropriate for working under flue gas condition due to
73
the high affinity to the moisture, low selectivity or low CO2 adsorption capacities at relatively low
74
CO2 partial pressure
75
sorbents to capture CO2 from flue gas 6, 11, 12. Alkali metal carbonate, such as K2CO3 and Na2CO3 can
76
react with CO2 in the existence of H2O and transform to alkali metal hydrogen carbonates salt
77
(KHCO3 or NaHCO3) at low temperatures. Water vapor is consistently essential as shown in the
78
reaction (1) 5:
1, 10
. A particularly promising option includes the use of dry alkali metal-based
M2CO3+CO2+H2O⟺2MHCO3
(1)
∆H= -141 kJmol-1, M=K ∴ -135 kJmol-1, M=Na 79
In the carbonation step, CO2 could react with M2CO3 in the presence of water vapor at low
80
temperature range of (50-100 °C). Thermal regeneration basically happens at a temperature lower
81
than 300 °C. After steam condensation, high-concentrated CO2 can be obtained and compressed,
82
ready for transportation and storage
83
capacities than Na2CO3 13, 14. It has been suggested that to use porous matrix as support to place solid
84
chemisorbent on it in order to improve the carbonation rate of K2CO3. Therefore several studies have
11, 13
. K2CO3 has been discovered to give more CO2 capture
6 ACS Paragon Plus Environment
Page 7 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
85
been performed by researches on various supports, such as Al2O3, TiO2, Ac, SiO2, ZrO2, CaO and
86
zeolites
87
and high attrition resistance, well-developed microstructure due to the Al2O3 support, and appears to
88
be a nearly ideal sorbent for CO2 capture 6, 14, 21.
15-20
. K2CO3/Al2O3 in particular, appeared to have a high porosity, high mechanical strength
89
The process of CO2 capture using dry alkali metal carbonate-based adsorbents has been
90
extensively studied in recent years, and was believed to have potential to be practical in fossil-fuel
91
power plants
92
amount of CO2 capture capacity are temperature, H2O concentration, CO2 concentration and water
93
pretreatment time 18, 22, 25. However, by the fact that these variables have interactions with each other,
94
up to the present time no optimal operational condition has been proposed by the researchers in order
95
to maximize the CO2 capture capacity. It is worth noting that the best condition is the one, which has
96
the highest possible removal capacity and reaction rate together. No systematic statistical study has
97
been done on this subject
11, 18, 22-27
. As, reported by the researchers, most important variables that affect the
98
Response surface methodology (RSM), initially described by Box and Wilson (1951), is useful for
99
design of the experiments, estimation of the effects of multiple process variables with their
100
interactions on response variables and finding the optimum conditions 28, 29. Latterly, RSM has been
101
applied to decrease the required experimental data in order to attain the best operating conditions for
7 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 8 of 48
102
a desired response in numerous chemical processes 30-34. Therefore, in the present work, RSM based
103
on Box-Behnken design (BBD) has been used for experimental design, proposing empirical models
104
and determination of the optimum values of the variables (carbonation temperature, mole ratio of
105
H2O to CO2 and water pretreatment time) for the desirable response variables (CO2 capture capacity
106
and reaction rate). Besides, the deactivation model was applied to determine the kinetic rate and
107
deactivation rate constants using the breakthrough curve data with non-linear least square technique.
108
The main objectives were to optimize the CO2 capture process and investigate the influence of
109
process variables on adsorption capacity and reaction rate. In addition, two semi-empirical
110
correlations were developed for each response based on experimental data.
111 112
2. Experimental Section
113
2.1.
Solid Sorbent Preparation
114
The potassium based sorbent used in this study was prepared by dry impregnation of K2CO3 (99%
115
purity from Merck) on porous γ-Al2O3 (from Merck) as a support. The preparation process consisted
116
of three steps: (1) mixing and impregnation in deionized water, (2) drying at 100 °C for dehydration
117
and calcination at 300°C. Then, the sorbent was grained and sieved for collecting the solid sorbent
8 ACS Paragon Plus Environment
Page 9 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
118
particle size in the range of 150-300 µm. In addition, the designated loading of K2CO3 on γ-Al2O3
119
was 35 %wt.
120
2.2.
Adsorbent Characterization
121
Phase composition of the sorbent before and after the carbonation reaction was examined with a
122
PHILIPS PW 3830 X-ray diffraction (XRD) system. A PHS-1020 (PHSCHINA) system with N2
123
adsorption-desorption was used to determine surface area and pore size distributions. The
124
microscopic shapes of the sorbent was observed by a Philips XL30 scanning electron microscopy
125
(SEM). The impregnated amount of the alkali metal was determined by a PHILIPS PW1480 X-ray
126
fluorescence (XRF).
127
2.3.
Apparatus and Procedure
128
The cyclic carbonation reactions were conducted in lab-scale fixed bed reactor (FBR) unit, as
129
shown in Figure 1. The experimental apparatus mainly consisted of four sections: (1) simulated flue
130
gas system and injection, (2) temperature controlled bath, (3) fixed-bed reactor (FBR), (4) CO2
131
analysis in discharge stream. N2 and CO2 were supplied from high-purity cylinders and transported
132
to the experimental apparatus with individual mass flow controllers. Besides, water was supplied by
133
a high-precision liquid pump and then heated to guarantee complete vaporization of the water before
134
mixing with other gases. Additionally, heat tracing was applied to keep the temperature of the 9 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 10 of 48
135
pipeline high enough to avoid condensation of the steam. The temperature of the reactor was
136
controlled with a circulating fluid through the FBR jacket. The reactor temperature was measured by
137
the thermocouples at the inlet and outlet of the reactor. The diameter and height of the reaction zone
138
were 12 mm and 35 mm, respectively. The mass of the loaded solid adsorbent was about 3.5 g for
139
each experimental run. The CO2 concentration in the treated discharge stream was continuously
140
measured by an online infrared (IR) analyzer (Vaisala, Finland, measurement limit 0-20 vol%). The
141
total flow rate of the feed gas was 80 ml/min. In addition, temperature ranges for carbonation and
142
decarbonation stages were 55-80 °C and 300 °C, respectively. The simulated flue gas consisted of
143
different amount of CO2 and H2O with a balanced amount of N2. The adsorbent was pretreated with
144
the vapor for (3-9) min to achieve higher adsorbent capacity.
10 ACS Paragon Plus Environment
Page 11 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
145 146
Figure 1. Schematic diagram of the fixed-bed reactor apparatus
147
The sorption operation is as follows. Firstly, N2 gas stream flows through the FBR in each
148
experimental run in order to achieve isothermal condition of the sorbet particles and also to prevent
149
water condensation at low temperature. Secondly, water vapor is added to the N2 stream for about 3-
150
9 min. This time is considered as the vapor pretreatment stage. Then, CO2 is added to the mixture
151
stream of N2 and water vapor. After that, carbonation reaction starts through the bed and CO2 is
152
adsorbed.
153 154
3. Theoretical Basis 11 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
155
156 157
3.1.
Page 12 of 48
CO2 capture capacity calculation
CO2 capture capacity during carbonation reaction, Ac (mg CO2/g sorbent), was determined through Eq (1):
Ac =
1000C i w
t
∫ Q (1 −ψ (t) ) ρ .dt 0
mg CO 2 g sorbent
(1)
158
Where w, Ci, Q, t, and are sorbent mass (g), inlet CO2 concentration (%vol), gas flow rate
159
(cm3/min), time (min), dimensionless outlet CO2 concentration (C/Ci) and CO2 density (g/cm3),
160
respectively. In addition, the fractional CO2 removal, Fr, was expressed as Eq (2):
Fr =
161
CO2 capture capacity during carbonation reaction (Aci ) Theoretical CO2 capture capacity
3.2.
(2)
Mathematical Modelling
162
Along with the progress of CO2 reaction, a dense product layer is formed on the solid surface of
163
the adsorbent particles. Moreover, the amounts of pore volume and surface area as well as the
164
activity of the adsorbent change over the reaction time. In the deactivation model (DM), all of these
165
effects on reduction of CO2 reaction rate are combined into a deactivation rate term 35.
166 167
The mass conservation equation of CO2 in the fixed bed with the assumption of the pseudo-steady state and isothermal condition is given as Eq (3):
12 ACS Paragon Plus Environment
Page 13 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
(1 − εb )τ K ψ a dψ =− εb dζ ψ=
C z L , ζ = ,τ = Ci L ug
(3)
B .C . ζ = 0 ⇒ ψ = 1
168
Where ψ, ζ, εb, τ, z, L, ug, Co, C and K are dimensionless outlet CO2 concentration (C/Ci),
169
dimensionless length of bed, bed porosity, time scale of flow through the bed (min), axial coordinate
170
in fixed bed (m), total length of bed (m), superficial velocity of gas flow (m/min), inlet and outlet
171
CO2 concentration (%vol) and reaction rate constant (min-1), respectively. The activity (a) in the
172
above equation is the ratio of the reaction rate for the adsorbent at a time, to the reaction rate for the
173
fresh adsorbent. According to the DM, the variation of the activity over time is expressed as Eq (4):
da = − k d a nC mψ m dt I .C . t = 0 ⇒ a = 1
(4)
174
Where a, t, kd, n and m are sorbent activity, reaction time (min), deactivation rate constant (min-1)
175
and corresponding exponents. The outlet concentration profile of CO2 was obtained from numerical
176
solution of Eqs (3, 4) (n=1 and m=0) with corresponding boundary and initial conditions using the
177
fourth-order Runge-Kutta method. In addition, the two unknown parameters in the equations (K and
178
kd) were determined by using nonlinear least-squares technique using experimental data.
179
13 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
180
4. Experimental Design
181
4.1.
Page 14 of 48
Methodology
182
Experimental design of the process for optimization of CO2 adsorption from the simulated flue gas
183
was carried out using the response surface method (RSM). RSM is a collection of statistical and
184
mathematical techniques advantageous for developing, improving and optimizing the processes36. It
185
clarifies the effects of independent variables, alone or in combination, on the processes. In addition
186
to analyzing the effects of independent variables, this experimental methodology generates an
187
empirical model, which describes the corresponding quantity of the process37. Response surface
188
methodology (RSM) is the most prevalent optimization method used in recent years. The
189
experimental design and statistical analysis were performed using Stat-Ease software (Design-Expert
190
7.0 trial). A three-level three-factor Box-Behnken design (BBD) consisting of 17 experimental runs
191
was employed including five replicates at the center paint. The effects of unexplained inconsistency
192
in the observed response due to inessential factors were minimized by randomizing the order of the
193
experiments.
194
Three variables considered in this study were (X1) bed temperature (°C), (X2) H2O/CO2 mole ratio
195
in feed and (X3) vapor pretreatment time, which are presented in Table (1). Each variable was varied
14 ACS Paragon Plus Environment
Page 15 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
196
over three levels -1,0,+1 at the determined ranges based on some preliminary experiments. The
197
generalized quadratic polynomial model used in the RSM is given as Eq (5): 3
3
3
Y = β 0 + ∑ βi X i2 + ∑∑ βij X i X j + Ε i =1
(5)
i =1 j < i
198
Where Y is the predicted response (i.e. Initial capacity and Rate constant), 0, i, ii and ij are
199
regression coefficients for intercept, linear, quadratic and interaction terms, respectively. Xi and Xj
200
are independent variables, and Ε is the unanticipated error.
201
Table 1. Independent variables and their coded and actual values for optimization Independent variable
Unit
Symbol
Coded Levels -1
0
+1
Temperature
°C
X1
50
65
80
H2O/CO2
-
X2
0.5
1
1.5
Vapor pretreatment Time
min
X3
3
6
9
202
The analysis of variance (ANOVA) table was generated and the effect and regression coefficients
203
of the individual linear, quadratic and interaction terms were determined. The significances of all
204
terms in the polynomial model were judged statistically by calculating F-value at a probability (P-
205
value) of (0.001, 0.01, and 0.05).
206
5. Results and Discussion
15 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
207
5.1.
Characterization of K2CO3/Al2O3 sorbent
208
5.1.1.
Porous structure performances
Page 16 of 48
209
It is generally verified that the microstructure of the sorbents play an important role in their CO2
210
capture processes. Therefore, the porous structure of the K2CO3/Al2O3 (KAL) sorbents were
211
characterized using N2 adsorption-desorption analysis. The specific surface areas and pore volumes
212
of the KAL sorbents (before and after the carbonation reaction) and fresh Al2O3 were calculated
213
using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively.
214
The results are presented in Table (2).
215
Table 2. Structural characteristic of γ-Al2O3 and K2CO3/Al2O3 (before and after carbonation reaction) Sorbent
Surface area (m2/g)
Pore volume (cm3/g)
γ-Al2O3
173.9
0.49
K2CO3/Al2O3 (fresh)
73.4
0.24
K2CO3/Al2O3 (used)
52.1
0.13
216
Based on the results of Table (2), both surface area and pore volume of fresh KAl sorbent are
217
much less than those of the pure support (γ-Al2O3). In addition, pore size distributions (PSDs) of γ-
218
Al2O3 and KAL before and after reaction are illustrated in Figure 2. The results confirm that support
219
γ-Al2O3 is a mesoporous material (with pore diameter of in the range of 2-50 nm) and about 80% of
220
pore diameters are in the range of 2-10 nm with a peak point at 4.3 nm. After loading of K2CO3 on 16 ACS Paragon Plus Environment
Page 17 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
221
γ-Al2O3 (fresh KAL) the amount of pore volume decreased and also the curve peak shifted towards
222
larger pore diameter (5 nm), based on Figure 2 and Table (2). These variations could be explained by
223
K2CO3 partial filling or blocking of the smaller γ-Al2O3 pores and by hydrothermal treatment of
224
support during preparation of the sorbent. Similarly, both surface area and pore volume of the used
225
KAl are less than those of the fresh KAL, which shows that CO2 could penetrate well through the
226
pores of the sorbent and react with K2CO3 to produce KHCO3.
227
228 229
Figure 2. BJH pore size distributions for γ-Al2O3 and K2CO3/Al2O3 (before and after carbonation
230
reaction)
231
5.1.2.
SEM Analysis
17 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 18 of 48
232
Another key factor for the CO2 capture process is the distribution type of K2CO3 on the AL2O3
233
support before and after the reaction. The effect of carbonation reaction (1) on the morphology of
234
sorbent and the characteristics of the active-sorbent distributions over the support particles were
235
evaluated using SEM analysis. The SEM images were taken at a magnification of 5000 which is
236
shown in Figure 3.
237 238 239 (a)
(b)
240
Figure 3. SEM images of K2CO3/Al2O3 sorbent, (a) before carbonation reaction, (b) after carbonation
241
reaction
242
As exhibited in Figure 3a, the porous surface of Al2O3 where well covered by rod-shaped
243
crystallites of K2CO3 in the range of 1-5 µm before the carbonation reaction, similar result is 18 ACS Paragon Plus Environment
Page 19 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
244
reported by Qin et al 2014 11; whereas after carbonation reaction some aggregate is created over the
245
surface of support (shown in Figure 3b). This aggregates imply the creation of KHCO3 crystals,
246
which block the porous sorbent and results in a reduction in active sites for CO2 adsorption.
247
5.1.3.
XRD Analysis
248
In order to find out the reaction paths of K2CO3/Al2O3, the phase composition change of sorbent
249
before and after the reaction was studied by XRD analysis. XRD patterns of γ-Al2O3 support and
250
KAl sorbent samples before and after the carbonation reaction are shown in Figure 4.
251 252
Figure 4. XRD patterns for: (a) fresh γ-Al2O3 support, (b) K2CO3/Al2O3 before carbonation reaction,
253
(c) K2CO3/Al2O3 after carbonation reaction in 8% CO2 + 8% H2O +84% N2 at 65 °C. Crystalline
254
phases: (■) γ-Al2O3, (▲) K2CO3, (♦) KHCO3, (✯) K4H2(CO3)3.1.5H2O
19 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 20 of 48
255
As shown in Figure 4a, there are mainly six peaks attributed to γ-Al2O3 [ICDD PDF-2 #10-0425],
256
which selected as support material. The diffraction peaks present in Figure 4b are assigned to the
257
monoclinic crystalline phase of potassium carbonate [K2CO3, ICDD PDF-2 #71-1466] and γ-Al2O3,
258
which are consistent with the K2CO3/Al2O3 design samples before reaction. Based on the peak
259
intensity of XRD pattern (Figure 4c) the main product of carbonation reaction at the condition of 8%
260
CO2 + 8% H2O +84% N2 at 65 °C was KHCO3 and implies the nearly complete carbonation
261
conversion of K2CO3. However, the XRD pattern (Figure 4c) not only shows two phases of γ-Al2O3
262
and the monoclinic crystalline phase of potassium bicarbonate [KHCO3, ICDD PDF-2 # 1-0976] but
263
also smaller values of the mixed potassium hydrogen carbonate hydrate phase [K4H2(CO3)3.1.5H2O,
264
ICDD PDF-2 # 20-0886]. This indicates that K2CO3 could be converted to KHCO3 and
265
K4H2(CO3)3.1.5H2O in the presence of CO2 and H2O. Similar results already have been declared by
266
other researchers
267
concentration of 5-9% at 60 °C
268
reaction temperature and high H2O concentration as reported by Zhao et al. 25. In addition, Luo et al.
269
presented the following reaction mechanism instead of reaction (1) based on the XRD pattern tests.
16, 24, 25, 38, 39
. It was stated that K4H2(CO3)3.1.5H2O was produced in H2O 16
. More K4H2(CO3)3.1.5H2O is formed in the condition of low
K2CO3(s) + 1.5H2O(g) K2CO3·1.5H2O(s)
(2)
2K2CO3·1.5H2O(s) + CO2(g) K4H2(CO3)3·1.5H2O(s) + 0.5H2O(g)
(3)
K4H2(CO3)3·1.5H2O(s) + CO2(g) 4KHCO3(s) + 0.5H2O(g)
(4)
20 ACS Paragon Plus Environment
Page 21 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
270
They claimed that the carbonation reaction process of K2CO3 could be separated into three
271
reactions, including the formation of K2CO3·1.5H2O from K2CO3, the subsequent production of
272
K4H2(CO3)3·1.5H2O from K2CO3·1.5H2O, and the production of KHCO3 from K4H2(CO3)3·1.5H2O.
273
Moreover, they reported that the production of KHCO3 from K4H2(CO3)3·1.5H2O was
274
thermodynamically promising upon the increase of the CO2 concentration. As a result, the
275
concentration of H2O and CO2 (or the mole ratio of H2O/CO2) and temperature could be the key
276
variables that affect the reaction path, which is discussed more in the next section.
277
Moreover, based on the XRD results in Figure (4), neither in the fresh sorbent nor in the reaction 17, 26
278
product the phase of KAl(CO3)2(OH)2 is found. Similar results were obtained by Zhao
279
According to the literature, this phenomenon could be related to two reasons. Firstly,
280
KAl(CO3)2(OH)2 could be generated by the reaction of the γ-Al2O3 support with K2CO3 during
281
calcination in the preparation of sorbent
282
completely converted into the K2CO3 phase during calcination at the temperature higher than 290
283
°C.15, 40 The second reason for the absence of KAl(CO3)2(OH)2 phase in reaction product could be
284
due to the water pretreatment effect. Lee et al. found that after water pretreatment the amount of
285
KAl(CO3)2(OH)2 phase id significantly reduced 38.
17, 38
.
. In addition, the KAl(CO3)2(OH)2 phase was
21 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
286
287 288
5.2.
Page 22 of 48
CO2 Breakthrough Curve of K2CO3/Al2O3
The result of a typical test during carbonation reaction for K2CO3/Al2O3 (under the reaction condition of H2O/CO2=1.5, 65 °C with 9min vapor pretreatment) is shown in Figure 5.
289
Figure 5. Breakthrough curve of CO2 outlet concentration (reaction condition: of H2O/CO2=1.5, 65
290
°C with 9min vapor pretreatment), (a) comparison of the experimental and modelling results (R2
291
=0.99), (b) definition of adsorption zone (full and partial adsorption zones)
292
As is illustrated in Figure 5a, CO2 concentration of the outlet gas varies over the reaction time,
293
which is also called as “Breakthrough Curve (BC)”. Furthermore, the modelling result is compared
294
with the real experimental one and shows high accuracy (R2=0.99). Calculated kinetic parameters for
295
this test, K and kd were equal to 210452.817 (min-1) and 0.142 (min-1), respectively. Besides, the
296
CO2 concentration at the outlet is definitely zero for about 50 min, indicating 100% CO2 removal
297
(also called “Breakthrough Time (BT)”), then CO2 concentration at the outlet rapidly increases to
298
reach as same as the inlet concentration ( equals one). Based on this definition, the longer BT is, the 22 ACS Paragon Plus Environment
Page 23 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
299
more efficient CO2 removal is. CO2 removal capacity could be evaluated by integrating the area
300
under the curve of (1-(t)), as displayed in Figure 5b. The adsorption zone is divided into full and
301
partial adsorption zones, FAZ and PAZ respectively, where FAZ corresponds to 100% removal. For
302
this run test, adsorption capacities are 82.55 and 27.61 (mg CO2/g sorbent) for FAZ and PAZ,
303
respectively. This result indicates that about 75% of total CO2 removal capacity (110.16 (mg CO2/g
304
sorbent)) belongs to FAZ, which is more important in terms of industrial consideration than PAZ.
305
Moreover, Fr value for this test is evaluated as 96.1%, (real K2CO3 ratio in sorbent was 35.5%
306
through XRF result), which shows the reaction practically reaches to the complete conversion.
307
Therefore, it is so useful to discover the optimum reaction condition in which the adsorption capacity
308
in the FAZ (Aci) reaches the highest possible value.
309
5.3.
Analysis of variance (ANOVA)
310
The designed experiments (17 runs) using BBD method and the responses obtained from the
311
experiments are shown in Table (3). The observed Aci and Kd vary between (35.15-82.55 (mg CO2/g
312
sorbent)) and (0.18-0.68 (min-1)), respectively.
313
Table 3. Corresponding experimental design and response values STD X1 X2
X3 Aci (mg CO2/g sorbent) Kd (min-1)
1
50 0.5 6
35.152
0.68
2
80 0.5 6
48.290
0.543
23 ACS Paragon Plus Environment
Energy & Fuels
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Page 24 of 48
3
50 1.5 6
37.909
0.27
4
80 1.5 6
53.150
0.22
5
50 1
3
44.048
0.352
6
80 1
3
63.243
0.25
7
50 1
9
68.673
0.255
8
80 1
9
81.642
0.21
9
65 0.5 3
58.754
0.51
10
65 1.5 3
64.139
0.18
11
65 0.5 9
80.566
0.39
12
65 1.5 9
82.548
0.142
13
65 1
6
66.865
0.253
14
65 1
6
67.325
0.25
15
65 1
6
65.864
0.263
16
65 1
6
67.270
0.2535
17
65 1
6
68.420
0.2453
314
An acceptable fit of the model is needed to avoid poor or uncertain optimized results. This is
315
essential to confirm the accuracy of the model. Table (4) shows the analysis of variance (ANOVA)
316
of regression parameters of the predicted response surface quadratic model for Aci and kd using the
317
results of the experiments.
318 319
Table 4. ANOVA results for the RSM-BBD model of each responses Analysis of variance (Aci)
24 ACS Paragon Plus Environment
Page 25 of 48
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Source
D.f.a
Sum of squares Mean square F-value
Pr>F
Model
9
3233.41
359.27
407.45
4), which clarify
330
that noise to ratio of the model is placed in the desirable range
331
well the model fits the data. Strong lack of fit (p