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Cite This: Energy Fuels 2018, 32, 7978−7990
Optimization of CO2 Capture from Simulated Flue Gas Using K2CO3/ Al2O3 in a Micro Fluidized Bed Reactor Mohsen Amiri and Shahrokh Shahhosseini*
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School of Chemical, Petroleum, and Gas Engineering, Iran University of Science and Technology, P.O. Box 16765163, Tehran 16846-13114, Iran ABSTRACT: The cost-effective dry regenerable K2CO3/Al2O3 seems to be a promising sorbent for CO2 removal from the flue gas of fossil fuel power plants. In this work, the characterization of the carbonation reaction and process optimization were performed in a so-called micro fluidized bed reactor (MFBR), which has recently been applied to study gas−solid reactions. The sorbent was also characterized by BET and SEM techniques. In addition, the most important gas−solid heterogeneous models were evaluated, and the kinetic parameters were determined by the model fitting approach. Based on the kinetic study results, the homogeneous model (HM) and the shrinking core model (SCM) were selected as the reaction models. Also, the effects of the independent variables including temperature, gas flow rate, and vapor pretreatment amount on the responses (adsorption capacity and reaction rate constant) were investigated by the response surface methodology (RSM) coupled with Box−Behnken design (BBD). Regarding to the analysis of variance (ANOVA) results, the temperature and gas flow rate are the most important factors affecting the adsorption capacity and the reaction rate constant, respectively. In addition, the semiempirical polynomials were developed to find the optimum condition corresponding to the highest adsorption capacity and reaction rate. Consequently, the optimum independent variables were 60 °C, 562 CCM, and 22.2 mg of H2O condition for the temperature, gas flow rate, and vapor pretreatment amount, respectively. The best response values of 65.29 mg of CO2/g of sorbent and 0.3402 (min−1) were predicted for the adsorption capacity and reaction rate constant, respectively, at the optimum conditions which were verified experimentally. The presented results are applicable and essential for future simulation and modeling CO2 capture in the fluidized bed reactor.
1. INTRODUCTION Global warming and climate change resulting from greenhouse gas (GHG) emissions has become a widespread concern in the last two decades.1 According to the prediction of the Intergovernmental Panel on Climate Change (IPCC) the temperature increases between 1.0 to 3.7 °C through the 21 century.2,3 Among the greenhouse gases, carbon dioxide (CO2) is the most significant contributor to global warming.4,5 The main source of anthropogenic CO2 emissions is the flue gas from power plants burning fossil fuels.6,7 CO2 capture and storage (CCS) is known as an imperative option for decreasing CO2 emissions in recent years.8 Postcombustion is a key CCS technology option for the conventional power plants.9 Postcombustion processes include mainly physical and chemical absorption, adsorption, cryogenic, and membrane processes.5 Among these technologies chemical absorption and adsorption are the most appropriate ones for current power plants.1 Absorption using amine solutions is a matured technology in petroleum, natural gas, and power plants, although it has several issues including solvent losses, corrosion, hazardous byproducts, and high-energy requirement for regeneration.5,8,10 Therefore, the adsorption process using solid sorbents is an encouraging alternative in CO2 capture from flue gas and also has a good potential in the future.9 Recently, multiple research groups have been involved in the development of solid physisorbent for CO2 capture including activated carbonaceous materials, microporous/mesoporous silica or zeolites, and MOFs.9,11 However, these sorbents are not applicable under flue gas conditions (78 N2, 13% CO2, 9% H2O) due to high attraction to water vapor, low thermal and © 2018 American Chemical Society
mechanical stability, and low CO2/N2 selectivity or low CO2 adsorption capacities at moderately low CO 2 partial pressure.9,11 An especially promising opportunity is the usage of dry alkali metal-based sorbents to capture CO2 from flue gas stream.8,12,13 Dry alkali metal carbonates like K2CO3 and Na2CO3 could react with CO2 in the presence of water vapor and produce the alkali metal hydrogen carbonates salt (KHCO3 or NaHCO3) at low temperatures through the reaction R1:14,15 M 2CO2 + CO2 + H 2O ↔ 2MHCO3
(R1)
ΔH = −141 KJ mol−1 when M = K ΔH = −135 KJ mol−1 when M = Na
In the carbonation step, CO2 could react with M2CO3 in the presence of water vapor in the low temperature range of ( 50− 100 °C. MHCO3 decomposes at a moderate temperature change of 120−200 °C and produces a CO2/H2O mixture that could be converted into CO2 stream ready for transportation and storage by condensing the water vapor.4,12,16 In addition, it is reported that K2CO3 shows more CO2 capture capacities than Na2CO3.16,17 Researchers suggested several porous matrixes as support including Al2O3, TiO2, Ac, SiO2, ZrO2, CaO, and zeolites.18−23 Among them K2CO3/Al2O3 appeared to possess a high porosity, mechanical strength, and attrition Received: March 12, 2018 Revised: May 25, 2018 Published: June 8, 2018 7978
DOI: 10.1021/acs.energyfuels.8b00789 Energy Fuels 2018, 32, 7978−7990
Article
Energy & Fuels
was examined by a Philips XL30 scanning electron microscope (SEM). The impregnated amount of the K2CO3 on Al2O3 was determined by a PHILIPS PW1480 X-ray fluorescence (XRF) system. 2.3. Apparatus and Procedure. The cyclic carbonation reactions under isothermal conditions were performed in a micro fluidized bed reactor (MFBR) unit, as illustrated in Figure 1. The
resistance. It has a well-developed microstructure and seems to be an almost perfect sorbent for the fluidized bed CO2 capture process.8,17,24 According to the literature, most of the studies on potassium based sorbent have been conducted using thermogravimetric analyzer (TGA) 2 0 , 2 5 − 3 0 or in a fixed bed reactor.15,17−19,23,31−36 These studies provide a good comprehension of carbonation reaction behavior and the effect of operating factors. The carbonation reaction of K2CO3/Al2O3 in a fixed bed reactor and the effect of operating condition have been reported by Amiri et al.15 They found that the temperature and vapor pretreatment time are the most significant carbonation process variables. The so-called micro fluidized bed reactor (MFBR) has been successfully developed to study gas−solid reactions characteristics and reaction kinetics.37−49 In comparison to TGA and fixed bed, MFBR results in quicker heat and mass transfer between gas and particles, faster reaction heat removal, and less external interfacial diffusion limitations. MFBR also gives the capability of real-time measurement of the gaseous product through an online gas analyzer.40,49 Although few studies have been performed on the CO2 capture using K2CO3/Al2O3 in fluidized bed reactors in bench scale,21,50−52 there is no systematic statistical study on carbonation reaction characteristics and reaction kinetics under different operating conditions in MFBR. The final goal for this technology is to develop a CO2 capture process with dual fluidized-bed reactors under optimum conditions to achieve maximum CO2 capture capacity and reaction rate. Performance studies describing the effect of different operating variables on the adsorption capacity and reaction kinetics are essential to commercially apply CO2 capture with the fluidized bed technology.53 Recently, response surface methodology (RSM) is successfully applied to estimate the individual and interactive effects of multiple process variables and find the optimum conditions.15,54−56 Therefore, in the present study the micro fluidized bed reactor (MFBR) was employed to evaluate the carbonation characteristics of the K2CO3/Al2O3 sorbent in the simulated flue gas. RSM with Box−Behnken design (BBD) has been applied for the experimental design, proposing empirical correlation and specification of the optimum values of the process variable (Temperature, gas flow rate and vapor pretreatment time) for the desirable response variables (adsorption capacity and reaction rate constant). Kinetic parameters were investigated based on the model-fitting approach. The main objectives were to optimize the carbonation process in MFBR and evaluate the effect of the process variables on adsorption capacity and reaction rate by proposing two semiempirical correlations.
Figure 1. Schematic diagram of the micro fluidized bed reactor (MFBR) apparatus.
experimental apparatus primarily composed of five segments: (1) gas supply section, (2) steam generator system, (3) temperaturecontrolled bath, (4) micro fluidized bed reactor (MFBR), and (5) online CO2 analysis in the outlet stream. Feed gases (N2 and CO2) were provided from high-purity (vol. 99.999%) cylinders and transmitted to the MFBR with separate mass flow controllers (MFC). In addition, water feed was prepared by a high-precision liquid pump and then preheated by heat tracing of the tube line to ensure full water vaporization before mixing with N2 and CO2. Isothermal condition of the reactor was established with a circulating fluid through the MFBR jacket. The reactor temperature was measured by two thermocouples at the entrance and output of the MFBR. Dimensionally, the internal height and diameter of the bed zone were initially 20 and 12 mm, respectively. Moreover, CO2 composition in the treated outlet stream was continuously recorded by an online infrared (IR) analyzer (Vaisala, Finland, measurement limit 0−20 vol %). Furthermore, the carbonation and regeneration tests were performed at 60−80 and 300 °C, respectively. Based on the results of the previous study, H2O/CO2 mole ratio was kept as one with a balanced amount of N2 and the cyclic tests were performed by the same regenerated sorbent due to complete regeneration of the sorbent at 300 °C.15 The particle size of bed material was in the range 90−106 μm. According to the Ergun equation, the minimum fluidization velocity (Umf) and corresponding minimum fluidization flow rate of the sorbent particles were calculated as 1.91 cm/s and 129.8 mL/min, respectively. Therefore, the total gas flow rate of the inlet stream through MFBR was designed in the range of 450−650 mL/min to ensure fluidization of particles. At the beginning of each carbonation experiment about 2 g of solid adsorbent was loaded into the MFBR. Then, N2 gas was passed through the MFBR in order to reach the isothermal condition of the sorbet particles and to prevent water vapor condensation at low temperature. Thereafter, N2 stream with water vapor (10 mol %) was passed through the bed until the amount of H2O during the vapor pretreatment period reached 12−24 mg. Subsequently, CO2 was added to the gas mixture of N2 and H2O vapor, and the carbonation reaction began through the sorbent fluidized bed and CO2 was removed. Finally, the sorbent regenerated at 300 °C and prepared for the next experimental run. The reaction characteristics including
2. EXPERIMENTAL SECTION 2.1. Solid Sorbent Preparation. The solid sorbent (K2CO3/γAl2O3) was prepared by coating the conventional wet impregnation of K2CO3 (Merck, 99% purity) on the porous particles γ-Al2O3 (Merck, 99% purity). The preparation process was composed of four stages: (1) mixing and impregnation of in the aqueous mixture of deionized water and 16.2 g of K2CO3 with a magnetic stirrer at 25 °C for 14 h, (2) drying at 100 °C in an oven for 24 h, (3) calcination at 300 °C for 4 h, and (4) graining and sieving of the particles for fluidized bed tests. Additionally, the desired loading of K2CO3 on γ-Al2O3 particles was 35 wt %. 2.2. Sorbent Characterization. A PHS-1020 (PHSCHINA) system with N2 adsorption−desorption was applied to analyze surface area and pore size distributions. The microscopic shape of the sorbent 7979
DOI: 10.1021/acs.energyfuels.8b00789 Energy Fuels 2018, 32, 7978−7990
Article
Energy & Fuels Table 1. Typical Mechanism Function Using in Gas−Solid Reactions no.
F(X)
model
power law models 1 2 3 4 phase interfacial reaction 5 zero order (plate) 6 2D (cylindrical) 7 3D (spherical) diffusion models 8 1D 9 2D Valensi equation 10 3D (Jander) 11 3D (A-J) 12 3D (Ginstling−Broushtein) reaction-order models 13 first order 14 3/2 order 15 second order 16 third order nucleation models 17 n = 1.5 18 n=2 19 n=3 20 n=4
X1/4 X1/3 X1/2 X3/2
57 57 57 57
1 2(1 − X)1/2 3(1 − X)2/3
X 1 − (1 − X)1/2 1 − (1 − X)1/3
57 57 57
1/(2X) [−ln(1 − X)]−1 3/2(1 − X)2/3[1 − (1 − X)1/3]−1 3/2(1 − X)(2/3)[(1 + X)1/3-1]−1 3/2[(1 − X)−1/3 − 1]−1
X2 X + (1 − X) ln(1 − X) [1 − (1 − X)1/3]2 [(1 + X)1/3 − 1]2 (1 − 2/3X) − (1 − X)2/3
57 38,57 38,57 38,49 38,57
1−X (1 − X)3/2 (1 − X)2 (1 − X)3
−ln(1 − X) 2[(1 − X)−1/2 − 1] (1 − X)−1 − 1 0.5[(1 − X)−2 − 1]
57 57 57 57
1.5(1 − X)[−ln(1 − X)]1/3 2(1 − X)[−ln(1 − X)]1/2 3(1 − X)[−ln(1 − X)]2/3 4(1 − X)[−ln(1 − X)]3/4
[−ln(1 [−ln(1 [−ln(1 [−ln(1
∫0
ij mg CO yz 2z zz Q (1 − ψ (t ))ρ dt jjjj zz jg sorbent k {
G (X ) =
t
X=
t
∫0 f (Ci − C)Q dt
t
=
∫0 (1 − ψ (t )) dt t
∫0 f (1 − ψ (t )) dt
(2)
(3)
where F(X) is the differential reaction rate function depending on the mechanism and K(T) is the reaction rate constant determined by the Arrhenius equation in eq 4: K (T ) = K 0e−Ea / RT
38,57 38,57,58 38,57,58 38,57,58
∫0
X
dX =K (T )t F (X )
(5)
4. EXPERIMENTAL DESIGN: METHODOLOGY The response surface method (RSM) was used for experimental design and optimization of the carbonation process. RSM is a collection of statistic and arithmetical techniques beneficial for optimizing the processes, which is used widely in recent years.60 The effects of independent variables, alone or with their interactions, on the measured responses are explained by RSM. Furthermore, a multivariate empirical model based on the independent variables and their interactions is generated as a quadratic polynomial equation shown in eq 6.
where C and tf are the outlet CO2 concentration (vol %) and final time (min), respectively. 3.2. Kinetic Method. Generally, the overall kinetic rate of heterogeneous gas solid reactions under isothermal condition as a function of reaction temperature T (K) and conversion X can be defined by eq 3: dX = K (T )F (X ) dt
X)]2/3 X)]1/2 X)]1/3 X)]1/4
Accordingly, based on the conventional model-fitting method,57 the experimental conversion and time data can be used in eq 4 and K(T) is determined as the slop of linear curve G(X) ≈ t. There are 20 mechanism functions that have been commonly used for the kinetic parameters specification of heterogeneous solid-state reactions,38,49,53,57−59 as illustrated in Table 1. In the current study, the form of G(x) with the highest average correlation coefficient (R2) in all tests is selected to be the mechanism function that characterizes the carbonation reaction kinetics.
(1)
where w, Ci, Q, t, ψ, and ρ are the sorbent mass (g), inlet CO2 concentration (vol %), total gas flow rate (cm3/min), time (min), dimensionless outlet CO2 concentration (C/Ci), and CO2 density (1.55 g/cm3), respectively. Furthermore, CO2 removal conversion degree, X, was determined by eq 2:
∫0 (Ci − C)Q dt
− − − −
where K0, R, and Ea are pre-exponential factor (s−1), gas constant (8.314 J mol−1 K−1), and apparent activation energy (J mol−1), respectively. In addition, the integral reaction model G(X) can be determined by integrating eq 3 in the form of eq 5.
3. THEORETICAL BASIS 3.1. Adsorption Capacity and Conversion. CO2 capture capacity during carbonation reaction, Ac (mg of CO2/g of sorbent), was calculated using eq 1: t
refs
4X3/4 3X2/3 2X1/2 2/3X−1/2
kinetic parameters were determined by analyzing the data collected using the online gas analyzer.
1000Ci Ac = w
G(X)
3
y = α0 +
(4)
i=1
7980
3
3
∑ αixi2 + ∑ ∑ αijxixj + e i=1 j
