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Expert representation chemical looping reforming: A comparative study of Fe, Mn, Co and Cu as oxygen carriers supported on Al2O3
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JIEC 2021 1–12 Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6
Expert representation chemical looping reforming: A comparative study of Fe, Mn, Co and Cu as oxygen carriers supported on Al2O3 Q1 H.R. a b
Forutan a, E. Karimi a, A. Hafizi a, M.R. Rahimpour a,b,*, P. Kashavarz a
Chemical Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran Department of Chemical Engineering and Material Sciences, University of California, Davis, CA 95616, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 January 2014 Received in revised form 24 April 2014 Accepted 25 April 2014 Available online xxx
In this study, a comparison between the performance of alumina supported Fe, Mn, Co and Cu oxygen carriers has been performed in chemical looping reforming (CLR) of methane. This process is consisting of two stages: ‘air reactor’ and ‘fuel reactor’, in which the oxygen carriers were placed in a fixed bed tubular reactor. Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) were applied to check the carrier specifications before and after the process. Also, response surface method based on central composite design was applied to investigate the operating conditions including reaction temperature, time and oxygen carrier type. The results showed that the effective retention time of oxygen carriers varies for different metals in the air and fuel reactors, which depends on the amount of adsorption and desorption of oxygen in each stage. Results of these experiments revealed that iron based oxygen carrier had the highest resistance against sintering and the maximum capacity for oxygen adsorption among the mentioned carriers. It was also found that copper had a significant oxygen transport capacity; however its resistance against sintering and agglomeration is relatively low. Finally, Design expert software suggested several optimized solutions; among them the best choice was obtained to be Fe-based oxygen carrier with reaction temperature and time of 1014.13 8C and 50.5 min in the second cycle respectively. ß 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Chemical looping reforming (CLR) Oxygen carriers Experimental design Hydrogen production Methane reforming
7 8
Introduction
9 10 11 12 13 14 15 16 17 18 19 20 21
Hydrogen (H2) is also a significant feedstock in the production of ammonia, methanol and fertilizers, as well as for upgrading of fuels in the refining industry [1–4]. Various technologies are employed to obtain hydrogen from other hydrogen-containing compounds such as fossil fuels, biomass, or water. Currently, commercial hydrogen is mostly produced from the fuel processing by natural gas reforming, partial oxidation of heavy crude, naphtha as well as coal gasification. Steam methane reforming (SMR) is the most common hydrogen production method for large scale; however, the appreciable amount of CO2 releases during its operation units [3,5–7]. Thus, the development of concepts for H2 production via SMR is extremely pleasant due to reduced capital costs and CO2 emissions. So the chemical looping concept might be
* Corresponding author at: Chemical Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran. Tel.: +98 711 2303071; fax: +98 711 6287294. E-mail address: [email protected] (M.R. Rahimpour).
able to suitable alternative for H2 production [1,4,8]. Chemicallooping reforming (CLR) which is a technology for synthesis gas (H2 and CO) production from natural gas and light hydrocarbons. This new process was offered by Mattisson and Lyngfelt [9]. The most important advantage of CLR is production of synthesis gas, and also synthesis gas could be used for production of pure H2 accompanies without any change in the composition of product gas. Since the heat transfer occurs directly between gas and oxygen carrier, it is more effective to reduce the size of reformer, CLR is economic than conventional technology [10,11]. As shown in Fig. 1 the CLR system consists of two interconnected reactors which are designated as air and fuel reactors. In methane CLR, the fuel and some steam are burnt with the oxygen-carrier e.g. metal oxide (MenOm) to form a mixture of CO, H2, CO2, and H2O in the fuel reactor while oxygen carrier particles are reduced to MenOms. Then oxygen carriers are transferred to the air reactor in order to react with air which cause oxidizes original oxidation state [12]. If CO2 and steam reforming occurred, with two network reactions of CLR are as described below; they can effective on coke removing. Fuel Reactor :
sCH4 þ Men Om $ sCO þ 2H2 þ Men Oms
http://dx.doi.org/10.1016/j.jiec.2014.04.031 1226-086X/ß 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
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(1)
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Fig. 1. A general scheme of the CLR process.
42 41 Air Reactor : 45 44 43 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87
2Men Oms þ sO2 $ 2Men Om
(2)
It is essential to select an oxygen-carrier that tends to partially oxidize methane to syngas rather than CO2 and H2O. The main advantage of this process is that heat needed for converting CH4 to H2 is obtained without costly oxygen production and mixing the air with carbon-containing fuel gases [2,12,13] Selection of an oxygen-carrier is vital for the CLR technology development. An oxygen carrier in a CLR power plant must show high mechanical and chemical stability (high melting point), sufficient durability and reactivity in successive cycles reactions. It should be low in the tendency to agglomerate, fragmentation and particles attrition [12,14,15]. The most researchers have been considered Cobalt- based catalysts for the reforming of methane processes due to high catalytic activity and relatively low cost [16,17]. Besides, the main disadvantage of Cu-based oxygen carriers is related to the agglomeration problems caused through the low melting point of Cu (1085 8C) [10,11,18–20]. Chuang et al. [19] focused on the development and performance of Cu-based oxygen-carriers and Al2O3 (as a support) in three different methods: mechanical mixing, wet-impregnation, and co-precipitation in chemical looping process. It was found that particles produced by coprecipitation did not agglomerate and showed a high carrying capacity after 18 cycles of operation. In comparison with other metal oxides, most of the oxygen-carriers supported on Al2O3 compounds showed very high reactivity with all fuel gases. Adanez et al. [21] studied Cu-based oxygen carriers on SiO2 or TiO2 and Febased oxygen carriers on Al2O3 and ZrO2. A number of studies have also concentrated on employment different oxygen carriers in CLC and CLR processes. Zafar and co-workers [22] performed an experimental study in a fluidized-bed reactor with NiO, CuO, Fe2O3 and Mn2O3 particles. Several materials using ZrO2 doped with Ca, Mg, and Ce were tested by Johansson et al. [23]. They found calcination temperature affects the agglomeration tendency. One of these materials supported on MgO-ZrO2 was successfully used in a CLC reactor for 70 h without defluidization [24]. A continuously operating laboratory reactor consisting of two interconnected fluidized beds using the oxygen carrier made of NiO and MgAl2O4 was considered by Ryde´n et al. [25] in CLR process. Abad et al. reported the range of operational conditions for Cu, Fe and Nibased oxygen carriers in CLC process [26]. Mattisson et al. [18] prepared Mn3O4/Al2O3 oxygen carrier in order to investigate its reactivity. It was shown that Mn3O4/Al2O3 reactivity is low due to the formation of MnAl2O4 during sintering, which does not react
with oxygen and fuel. Iron oxides have been considered as the most preferred materials among many oxygen carriers employed in CLC and CLR processes due to high availability, low price and high environmental safety. It should be noticed that reduction from hematite to magnetite (Fe2O3 ! Fe3O4) is the fast step while the subsequent steps of magnetite to ferrous oxide (Fe3O4 ! FeO) and ferrous oxide to iron (FeO ! Fe) are much slower. The secondary steps were known to be appropriate for CLR process. However manganese oxides are rarely reported as oxygen carriers in CLC and CLR processes [27], some experimental investigations in a batch fluidized bed [2] and fixed bed [27,28] reactors for hydrogen production with inherent CO2 separation using iron oxides supported on magnesium, silica, chromium, titania and aluminium have been conducted by several research groups. Chiesa et al. [30] investigated a process which consist three-reactor chemicallooping using iron oxides as oxygen carriers for hydrogen production from natural gas. Investigation and determination of the optimum conditions for evaluating the catalytic performance of different oxygen carriers is a tedious and complex process involving many steps, variables and complex interactions among them. The most of previous studies includes using the one-factorat-a-time experimental approach. This approach has some disadvantages such as time consuming and costly procedures, and of course does not consider the interaction of various operating parameters. Consequently, it results in inexactness of the process optimization and tends to functionalize only for a single varied factor process or systems with linear interactions of operating parameters [31]. The application of design of experiments (DOE) procedure seems to be one of the best methodologies for studying and improving the complicated processes [32]. One of the appropriate multivariate techniques of experimental design approaches which can deal with statistical modeling and process optimization is the response surface method (RSM) [33]. It is used to examine the relationship between one or more response variables and a set of quantitative experimental variables [34]. The main objective of this study is to investigate the catalytic activity and feasibility of four different oxygen carriers in chemical-looping reforming process in terms of their oxygen release and uptake capabilities and their activity toward produce hydrogen in a fixed- bed micro- reactor. Furthermore, RSM is taken up to investigate and optimize the CLR process operating conditions while the previous researches did not use experimental design for the optimization of the reaction parameters.
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Experimental method
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Oxygen carrier preparation
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Oxygen carriers supported on alumina support were prepared using co-precipitation method. This method is based on the precipitation by changing the pH level of a desired solution. By using this method good distribution of high active metal content could occurs on the support internal surface. Four oxygen carriers, including Fe, Mn, Co and Cu were prepared for the experiments according to the following procedure: A desired amount of Al (NO3)39H2O was added slowly in an aqueous solution of the metal nitrate. When the salts were completely dissolved, the desired amount of NH4OH was added slowly until the solution pH reached 8–9. Then the solution was stirred and heated to 70–80 8C. The homogeneous solution was maintained in a water bath for 10 h. Consequently, the precipitates were filtered and washed with distilled water. After that, in order to evaporate the surface water, the paste mixture allowed to dry for 6 h at 100 8C in an oven. In the first, the synthesized materials were calcined in an electrical furnace under air atmosphere increasing the temperature from ambient conditions up to 700 8C,
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kept at this temperature for 2 h, and followed by a second calcination step under air at 1250 8C (for Fe, Mn, Co) and 1000 8C (for Cu) with a ramp of 5 8C/min and remained 6 h at the final temperature.
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Oxygen carriers characterization
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X-ray diffraction (XRD) analysis allowed investigating the new phases formation in the oxygen carrier after reaction and comparing them with the ones present in the initial oxygen carrier. XRD patterns were recorded by a powder diffractometer (Bruker D8 Advance, Germany) using Cu Ka radiation, operated at 40 kV and 40 mA. The samples were scanned in a step-scan mode, with a step size rating of 0.058/s over the angular 2U range of 10– 908. Scanning electron microscopy (SEM) was performed using a Vega 2 Tescan (Czech Republic) microscope instrument in order to study the morphology of the oxygen carriers. The same instrument was also used to perform energy dispersive X-ray analysis (EDX) of each oxygen carriers. In addition to the specific surface area (BET) and porosity of the fresh oxygen carriers was measured, using an ASAP 2010 instrument (Micrometrics). The crushing strength a real density was measured using a digital force gauge machine (Shimpo FGN-5)
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Reactor system
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Fig. 2 shows the experimental setup consisting of a fixed bed micro-reactor (made of stainless steel, i.d. 12 mm), an electrical furnace, electronic low pressure mass flow controller (LP–MFC) and a gas chromatography (GC). In the present work, multicycle reduction–oxidation reactions (redox) were conducted in a cylindrical fixed bed reactor. The oxidation and reduction reactions occurred alternatively at atmospheric pressure in the fixed bed reformer tube. The oxygen carriers were located inside the reactor in a crucible (for allowing rapid cycling of a small amount of carrier in a repeatable way with acceptable manual work). The initial weight of metal sample in the reactor was 2.1 g for each test. The oxygen carriers were then sieved to achieve a particle size of 20–40 mesh. Reactor design of a CLR reformer must be done carefully, because direct contact between the oxygen carrier and the gas phase is important. The reformer reactor was located in an
3
electrical furnace for heating the system to desired reaction temperature (a very high furnace temperature is required). After reaching the desired temperature, the reducing gas (CH4) controlled by an electronic LP–MFC was introduced into the reactor. CLR process consists of three consecutive steps. In the oxidation step (first step), oxygen reacts with the oxygen carriers in the air reactor. The oxidation stream designed 25% O2 and 75% Ar. Then in the transition step (second step), the residual oxygen from previous step is pushed out using argon and finally in reduction step (third step), the fuel is burnt with the oxygencarrier to form a mixture of H2 and CO along with a small amount of CH4 and CO2 in the fuel reactor. The fuel composition was 40% CH4, 10% CO2, 3% water (steam) and 47% Ar as a carrier inert gas. The argon was used as an inert carrier gas for dilution of methane and oxygen. GC analyzed continuously the outlet gas composition at each time.
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Results and discussion
205
Characterization of oxygen carrier
206
The distribution of the metal oxide on the support depends on the reactivity, structural strength and stability during the consecutive cycles. The characteristics of oxygen carriers used in this paper are given in Table 1. In this work, crushing strength was measured with the force required to fracture the desired particle. The crushing strength has highly dependent on the type of active metal oxides. The particles with low porosity have high crushing strength. Correspondingly, the specific surface area (SBET) of alumina supported Fe, Cu, Co and Mn oxygen carriers are shown in Table 1. It is clear that Fe-based oxygen carrier exhibits the maximum surface area (123 m2/g) while Cu-based oxygen carrier has the lowest surface area (79.8 m2/g). The structure of the catalysts was determined by X-ray powder diffraction (XRD). The X-ray diffractions of fresh and reduced oxygen carriers are presented in Fig. 3. As mentioned in this figure, Fe, Mn, Co and Cu atoms are visible in the carriers supported on Al2O3. This figure indicates different oxides of oxygen carriers, and almost all of them can be converted to stable oxide in high temperature. For example, with increasing the reaction temperature, Co3O4 is reduced to CoO and small metallic cobalt particles [35]. On the other hand, limited formation of copper aluminate compounds was found in both XRDs of Cu based oxygen carrier. The copper aluminate compounds are formed due to the high reactivity between CuO and Al2O3, and diffusion of copper ions into the alumina even at temperatures as low as 500 8C [36]. Fig. 4 shows the elemental analysis and SEM images of different catalyst before and after CLR experiments. It is clear from this figure that each oxygen carrier has its own metal clusters on the support with different sizes. The pore blocking (impurities) and sintering are referred as the main reasons of Fe and Mn based catalysts deactivation (shown in Fig. 4a and b). Also, sintering degrades catalyst performance due to the decrease of active surface area. Fig. 4c and d reveals the effect of sintering on Co and Cu based catalyst at high temperature which is the main cause of
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Table 1 Characterization of the fresh oxygen carrier particles.
Fig. 2. Reactor system for chemical looping reforming of natural gas with hydrogen production.
Oxygen carrier
Fe/Al2O3
Co/Al2O3
Mn/Al2O3
Cu/Al2O3
Theoretical weight ratio [wt%] Porosity (%) Real density [g/ml] BET surface area [m2/g] Crushing strength [N/mm]
40–60 53.2 2.7 123 2.1
40–60 31.3 4.2 87.8 13
40–60 44.8 3.8 110.0 3.8
40–60 39.3 5.1 79.8 10.3
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Fig. 3. XRD patterns of oxygen carriers in the best operating temperatures during cycles: (a) before oxidation of Fe/Al2O3, (b) after reduction of Fe/Al2O3, (c) before oxidation of Mn/Al2O3, (d) after reduction of Mn/Al2O3, (e) before oxidation of Co/Al2O3, (f) after reduction of Co/Al2O3, (g) before oxidation of Cu/Al2O3, (h) after reduction of Cu/Al2O3.
Fig. 4. SEM images of the oxygen carrier particles both fresh (left) and reacted in the CLR unit (right); (a) Fe/Al2O3 (reacted at 1050 8C), (b) Mn/Al2O3 (reacted at 950 8C), (c) Co/ Al2O3(reacted at 950 8C), (d) Cu/Al2O3 (850 8C).
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Oxygen carrier
Estimated (wt%)
Component
Fresh (wt%)
Deactivate (wt%)
Fe/Al2O3
40/60
Fe Al O Impurities
40.66 30.67 28.7 0
40.46 30.55 28.77 0.75
Cu/Al2O3
40/60
Cu Al O Impurities
38.95 32.23 28.82 0
38.75 27.82 30.45 2.63
Co/Al2O3
40/60
Co Al O Impurities
39.41 30.93 29.66 0
39.01 28.62 29.82 2.54
Mn/Al2O3
40/60
Mn Al O Impurities
39.13 27.97 39.13 0
39.08 27.4 35.26 0.97
the catalyst deactivation. So an increase in deposited coke was observed in these oxygen carriers. EDX analysis of fresh and reacted oxygen carriers are presented in Table 2. This table confirms the precipitation of 40% metals on alumina support. Also, it shows the percentages of the elements and their suitability when the carriers are fresh. However, after the last cycles the existence of small amount of impurities due to carriers deactivation changes these percentages that seem to be due of carbon deposition and impurities of catalyst synthesis on catalyst surface. Nevertheless, EDX analysis reveals that the
5
catalyst leaching is avoided and the catalysts loading percent do not considerably change after the four cycles.
252 253
Oxygen carrier activity
254
In this study Fe/Al2O3, Mn/Al2O3, Cu/Al2O3 and Co/Al2O3 at the stoichiometric ratio of 4:6 are used as oxygen carriers in 4 cycles of oxidation and reduction in CLR process. The approximated required time for each carrier in the reduction step is suggested to be 100 min while this time for the oxidation stage is 35 min. In order to investigate the influence of cycle number on activity and stability, all the oxygen carriers were tested at 850 8C and 950 8C in four cycles. Results are indicated in Fig. 5 for all oxygen carriers. As mentioned in Fig. 5a the maximum hydrogen percent for Fe2O3 as oxygen carrier was about 72 and 75% occurring in the second cycle at 850 and 950 8C, respectively. As mentioned in this figure, the second cycle has the higher hydrogen production performance than other ones for all of oxygen carriers. It can be concluded that the particles need 1 cycle to reach a more favorable structure. So, in the following, the second cycle is applied to investigate the effect of other parameters. As indicated above, the second cycle revealed the maximum hydrogen production yield. Therefore, the second cycle was selected for further catalytic investigations. An experimental design was performed to investigate the effect of reaction temperature, time and catalyst type on hydrogen production yield, methane conversion and CO2 selectivity in the second cycle.
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Experimental design
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Center composite design (CCD) which is known as the most popular response surface method was applied for further investigation of the oxygen carriers in CLR process. The CCD is
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Fig. 5. Hydrogen concentration profiles versus time at each cycle at 850 and 950 8C; (a) reduction of Fe2O3, (b) reduction of Mn3O4, (c) reduction of CoO, (d) reduction of CuO.
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Table 3 Experimental levels of independent variables.
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Variables
Unit
Symbol
Temperature Time Catalyst type
8C min
X1 X2 X3
Xi X0 DX i
1
0
+1
+a
656.25 2.5 Fe Mn Cu Co
700 10
875 40
1050 70
1093.75 77.5
(3)
xi is independent variable coded value, Xi is independent variable real value, X 0 is independent variable real value in the center point and DX i is step change value. Thus, the coded variables range from 1 to +1. In order to formulate a simple model appropriate for optimization, the experimental results obtained from the CCD are related to the variables by linear and quadratic terms, as in Eq. (4): Y ¼ b0 þ
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a
well suited for sequential experiments to obtain appropriate information for testing lack of fit with a low number of design points. In CCD the total number of experiments is calculated with 2k + 2k + n0, where, k is the number of independent variables and n0 is the number of repetitions of the experiments at the center point. Four star points (1), four axial points (a = 1.25) and five replicates at the center point (0) were chosen as experimental points for each catalyst type (X3) and consequently a total of 52 experiments were performed. Central points were used to check the reproducibility and stability of results. The runs were conducted in randomized manner to guard against systematic bias [37]. The process parameters and their values involved in the present study are shown in Table 3. In this study, experimental design was developed to maximize hydrogen yield and methane conversion and minimizing carbon dioxide selectivity by the optimization of operating variables including reaction temperature, time and catalyst type. The catalyst type (X3) which is a categoric factor studied in this paper includes alumina supported Fe, Mn, Cu and Co catalysts. The numerical test factors (X1 and X2) were coded based on the following equation: xi ¼
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Variable levels
k X
k X
X
i¼1
i¼1
i< j
bi xi þ
bii x2i þ
bi j xi x j þ e
(4)
where b0 is offset or constant term, bi the linear coefficient, bii the quadratic effect, bij the cross product coefficient and e is the statistical error [31]. Our CCD design includes 52 sets of test conditions in a single block while the responses of this design are hydrogen yield, methane conversion and CO2 selectivity.
Data analysis
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The objective of the present research is to study the combined effects of reaction temperature, time and oxygen carrier type on its performance in CLR process. The responses including hydrogen yield, methane conversion and CO2 selectivity are complex under various combinations of those independent variables. Fit summary output of Design Expert software revealed that the quadratic model is statistically significant for all three responses. Therefore, quadratic models were utilized to represent the responses for further analysis. The importance of each term of the quadratic model (Eq. (3)) and the validity of the obtained model was valuated using analysis of variance (ANOVA). Table 4 summarizes the ANOVA results for the three responses of the quadratic models and their statistical parameters. To estimate the accuracy of the acquired models, F-value and Pvalue tests were conducted. The F-values of the reduced quadratic models are 20.54, 34.02 and 38.95, for hydrogen yield, methane conversion and carbon dioxide selectivity respectively which show the significance of the models. Furthermore, the significance of each term in the quadratic models are also tested and the terms with P-value greater than 0.05 are eliminated from the models to enhance the accuracy. From ANOVA table it is apparent that catalyst type (X3) has largest effect on all responses due to their high F-values. On the other hand, reaction temperature revealed the lowest impact on CO2 selectivity. The coefficient of determination (R2) was calculated as 0.8754, 0.8924 and 0.9048 for hydrogen yield, methane conversion and CO2 selectivity, respectively (Table 4), indicating that the statistical model can explain a wide range of variability in each response. The R2 value is always between 0 and 1 and the closer R2 to 1, the better predictability of the responses with the models. As indicated in the ANOVA table the adjusted R-squared values are 0.8328, 0.8662 and 0.8815 for the three responses and the proximity of R2 and adjusted R2 indicates that non-significant terms are not involved in the models. An additional way to evaluate the adequacy check of obtained models is analyzing the distribution of residuals. Residuals are the deviation between model predicted and experimental values and are supposed to follow a normal distribution if the experimental
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Table 4 ANOVA table for the response surface reduced quadratic models. Source
Hydrogen yield (Y1)
Methane conversion (Y2)
Carbon dioxide selectivity (Y3)
F-value
P value Prob > F
F-value
P value Prob > F
F-value
P value Prob > F
Model X1-temperature X2-time X3-catalyst type X1 X3 X2 X3 X12 X22 Model statistics
20.54 5.18 7.86 62.00 4.06 12.48 6.83 11.20
<0.0001 0.0285 0.0079 <0.0001 0.0135 <0.0001 0.0128 0.0019
34.02 0.56 24.60 69.62 – 29.17 5.03 13.28
<0.0001 0.4575 <0.0001 <0.0001 – <0.0001 0.0304 0.0007
38.95 0.055 46.99 74.86 – 34.68 3.06 10.53
<0.0001 0.8163 <0.0001 <0.0001 – <0.0001 0.0877 0.0023
R2 R2-adj R2-predict Adeq. precision
0.8754 0.8328 0.7523 16.924
0.8924 0.8662 0.8088 23.418
0.9048 0.8815 0.8224 27.900
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Fig. 6. The studentized residuals versus predicted response plot: (a) hydrogen yield; (b) methane conversion; (c) CO2 selectivity.
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errors were random [38]. The studentized residual plot is attained by normalizing the residuals with respect to their standard deviations and afterwards a normal distribution function was fitted to the plot. Internally studentized residuals versus predicted for hydrogen yield, methane conversion and carbon dioxide selectivity are demonstrated in Fig. 6 a–c. The random scatter of experimental points indicates that there is no need to assume any violation of the independence. Also, this figure illustrates that the obtained models are in an appropriate description of the process and do not have any obvious pattern and unusual structure. By applying multiple linear regressions on the experimental results, the data of the central composite design were fitted with a quadratic full polynomial equation (Eq. (4)). The empirical relationships between three responses for each of four catalysts with independent variables in actual values are obtained by the application of RSM and are given in Eqs. (5)– (16) as follows: Mn catalyst pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi H2 yield þ 0:88 ¼ 15:0786 þ 0:041347 temp 2
þ 0:097147 time 0:00002053 temp 0:00895 time2
1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ þ2:81397 0:0044005 temp SCO2 þ 0:18 0:016207 time þ 0:0000024868 temp2 0:0004191 time2
378 377 376
þ 0:009793 time 0:00002053 temp2 0:0008949 time2
(8) 380 379
lnðCH4 conversion þ 0:99Þ ¼ 2:48135 þ 0:01364 temp 0:021626 time 0:000007582 temp2 0:0004191 time2
(9)
1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ þ2:56888 0:0044005 temp SCO2 þ 0:18
382 381
0:013539 time þ 0:0000024868 temp2 0:00015696 time2
373 372
(10)
Cu catalyst
lnðCH4 conversion þ 0:99Þ ¼ 2:93417 þ 0:01364 temp
385 384 383
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 yield þ 0:88 ¼ 8:08479 þ 0:03466 temp
þ 0:044528 time
þ 0:01397 time 0:00002053 temp2
0:00000758 temp2 2
(7)
Co catalyst pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi H2 yield þ 0:88 ¼ 10:2248 þ 0:03456 temp
(5)
0:0004191 time
375 374
(6)
0:0008949 time2
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1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ þ2:92198 0:0044005 temp SCO2 þ 0:18
lnðCH4 conversion þ 0:99Þ ¼ 2:10506 þ 0:013640 temp þ 0:01148 time
0:015387 time þ 0:0000024868 temp2
0:000007582 temp2 0:0004191 time2
(12)
389 388 1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ þ2:06542 0:0044005 temp SCO2 þ 0:18 2
0:00107282 time þ 0:0000024868 temp 0:00015696 time2 392 391 390
(13)
Fe catalyst pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi H2 yield þ 0:88 ¼ 16:1955 þ 0:0436 temp þ 0:09028 time 0:00002053 temp2 0:00089494 time2
(14)
394 393 lnðCH4 conversion þ 0:99Þ ¼ 2:93417 þ 0:01364 temp þ 0:044528 time 0:00000758 temp2 0:0004191 time2
396 395
(15)
0:00015696 time2
(16)
Effect of process variables on oxygen carriers efficiency
397 398 399
In order to evaluate the effects of reaction temperature and time on hydrogen production yield, methane conversion and CO2 selectivity for each oxygen carrier, graphical representations have been made in Figs. 7–10. So, contour plots have been created by means of obtained models. Fig. 7a shows the interacting effect of reaction temperature and time and their mutual interaction on hydrogen production yield. This figure indicates both reaction temperature and time have positive effect on hydrogen yield in the presence of Mn/Al2O3 oxygen carrier. On the other hand, Fig. 7b revealed that the maximum methane conversion of this carrier occurs around the temperature of 900 8C and 55 h. However, more increasing of the temperature results in the agglomeration of active sites which reduces the catalytic activity of Mn carrier. The performance of Cu/Al2O3 oxygen carrier is illustrated in Fig. 8. The maximum hydrogen yield and methane conversion was obtained at about 850 8C while further increasing of reaction temperature shows decreasing effect on these responses. Like Mn
400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417
Fig. 7. Contour plots of Mn/Al2O3 oxygen carrier performance: (a) hydrogen production yield, (b) methane conversion, (c) CO2 selectivity.
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Fig. 8. Contour plots of Cu/Al2O3 oxygen carrier performance: (a) hydrogen production yield, (b) methane conversion, (c) CO2 selectivity.
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based oxygen carrier, the reduction of Cu carrier activity in higher temperatures is due to its low melting point (1085 8C) which tend to agglomeration and sintering. On the other hand, comparison of fresh and reacted Cu oxygen carrier SEM (Fig. 4) shows that active sites reduce due to coke formation on the surface. Besides, the carbon dioxide selectivity decreases by reaction time because of side reactions which consumes CO2. The investigation of cobalt oxygen carrier reveals the same results of Cu carrier. Nevertheless, it revealed lower activity and hydrogen production yield in comparison with Cu carrier. The results obtained for Co40Al2O360are demonstrated in Fig. 9a–c. Also, Fig. 10a–c shows the responses of alumina supported iron based oxygen carrier. As indicated in this figure increasing the reaction temperature tends to improve the hydrogen production yield. The maximum hydrogen production yield occurs around 1050 8C while the maximum methane conversion comes about around 950 8C.
435
Oxidation
436 437 438 439 440 441 442 443
In the oxidation process of each carrier and cycle, the flow of input gas was 166 ml/min and the oxidation gas mole fraction was 25% oxygen and 75% argon. The amount of oxygen adsorbed on the oxygen carriers is the deference of oxygen in the initiation and the amount in each time. In the presence of carbon that is the pollutant of oxygen carriers, oxygen bonds break and some CO2 would be observed in the output gases. This process continued till the oxygen content run into a constant amount in the down-stream. As
the time passes and different cycles, in the same reaction conditions, hydrogen production rate has been decreased, this trend will continue even after re-oxidizing carrier which shows the deactivation of carriers. Fig. 11 shows the first to the fourth cycles of oxidation processes for manganese as carrier. As it is illustrated, the needed time for carriers to reach the maximum amount of oxygen adsorption is the same for all cycles, as well as of oxygen adsorption decreases after each cycle. Since most of active sites are reduced in the first reduction reaction, active sites regeneration is more difficult in the next oxidation reactions. After the fourth cycle, the maximum oxygen adsorption has been decreased significantly due to catalyst deactivation. Fig. 12 shows the oxidation (oxygen consumption) in the second cycle for all oxygen carriers at 850 8C. It is clear the iron has the maximum value in 12 min and the oxygen consumption slope is low; thus, the adsorption would be the highest. Iron had the maximum oxygen consumption and manganese, cobalt and copper are next respectively.
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Process optimization
463
It is clearly obvious that RSM is a powerful method for the examination and optimization of multi-variable procedures. In the Design Expert software numerical optimization could be applied for finding the combination of independent parameters accomplishing the desired requirements instantaneously. In this manner, the higher and lower levels of each independent variable and the
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Fig. 9. Contour plots of Co/Al2O3 oxygen carrier performance: (a) hydrogen production yield, (b) methane conversion, (c) CO2 selectivity.
Fig. 10. Contour plots of Fe/Al2O3 oxygen carrier performance: (a) hydrogen production yield, (b) methane conversion, (c) CO2 selectivity.
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Table 5 Optimal operating conditions and validation tests for the maximizing of hydrogen production yield, methane conversion and minimizing carbon dioxide selectivity.
RSM prediction Validation test
470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491
Temperature (8C)
Reaction time (min)
Catalyst type
H2 yield
CH4 conversion
CO2 selectivity
1014.13 1014.13
50.50 50.50
Fe Fe
83.45 80.91
79.09 77.61
2.26 2.30
responses were provided according to the contour plots. The optimization was performed and the optimum variables that satisfied all of the specified conditions for the maximization of the response were achieved as reported in Table 5. The optimal operating variables including reaction temperature, time and catalyst type were achieved to be 1014.13 8C, 50.5 min and Fe catalyst, respectively. As indicated in Table 5, the maximum hydrogen yield and methane conversion were predicted to be 83.45% and 79.09% and the minimum carbon dioxide selectivity of 2.26%, applying the acquired optimal operating variables. Also, validation test was carried out at these obtained conditions in order to evaluate the exactness of obtained optimums. As shown in Table 5, the experimental validation test at optimal operating conditions (80.91) was in a good agreement with RSM predicted (83.45) for the hydrogen yield. Similarly, the predicted methane conversion (79.09) revealed good fitness with the validation test result (77.61). Also, carbon dioxide selectivity validation test showed good fitness with predictions as mentioned in Table 5. The difference of RSM predicted and experimental values are justifiable due to the coefficients of determination and model F-values obtained for each response.
Conclusion
492
This work represents the suitability of different oxygen carriers and operating conditions in a CLR process. Four different oxygen carriers including alumina supported Fe, Mn, Co and Cu were synthesized with co-precipitation method. In order to investigate the influence of reaction temperature (700–1050 8C), time (10– 70 min) and carrier type (alumina supported Fe, Mn, Cu and Co), a CCD based response surface method was applied and experiments were implemented corresponding. By applying central composite design, the quadratic model was selected for all responses including hydrogen production yield, methane conversion and CO2 selectivity. According to F-values indicated in ANOVA table, the oxygen carrier type showed the largest effect on all responses. According to the results of the experiments, the iron has the highest resistance against sintering among the four carriers. Also the alumina supported iron carrier has the highest capacity for oxygen adsorption. Capacity of oxygen consumption and hydrogen production is increased as the temperature increases. This trend is continued until the process is reached to the optimum temperature, while hydrogen production will be reduced in higher temperature due to sintering phenomena. The second cycle is the best since the adsorption and desorption of oxygen in the process for these cycles are the maximum at the operating temperature. The optimization was performed and the best optimized answer was temperature of 1014.13 8C and the reaction time of 55.5 min for iron oxygen carrier, aimed to obtaining 83.45, 79.09 and 2.26 of hydrogen production yield, methane conversion and carbon dioxide selectivity, respectively.
493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520
Uncited reference
Fig. 11. Absorbed oxygen percentage during 4 oxidation reaction cycles for Mn/ Al2O3 at 950 8C.
Fig. 12. Oxygen consumption in the second cycle for all carriers at 850 8C.
Q2521
[29].
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Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec 1 2 3 4 5 6
Expert representation chemical looping reforming: A comparative study of Fe, Mn, Co and Cu as oxygen carriers supported on Al2O3 Q1 H.R. a b
Forutan a, E. Karimi a, A. Hafizi a, M.R. Rahimpour a,b,*, P. Kashavarz a
Chemical Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran Department of Chemical Engineering and Material Sciences, University of California, Davis, CA 95616, USA
A R T I C L E I N F O
A B S T R A C T
Article history: Received 31 January 2014 Received in revised form 24 April 2014 Accepted 25 April 2014 Available online xxx
In this study, a comparison between the performance of alumina supported Fe, Mn, Co and Cu oxygen carriers has been performed in chemical looping reforming (CLR) of methane. This process is consisting of two stages: ‘air reactor’ and ‘fuel reactor’, in which the oxygen carriers were placed in a fixed bed tubular reactor. Scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and X-ray diffraction (XRD) were applied to check the carrier specifications before and after the process. Also, response surface method based on central composite design was applied to investigate the operating conditions including reaction temperature, time and oxygen carrier type. The results showed that the effective retention time of oxygen carriers varies for different metals in the air and fuel reactors, which depends on the amount of adsorption and desorption of oxygen in each stage. Results of these experiments revealed that iron based oxygen carrier had the highest resistance against sintering and the maximum capacity for oxygen adsorption among the mentioned carriers. It was also found that copper had a significant oxygen transport capacity; however its resistance against sintering and agglomeration is relatively low. Finally, Design expert software suggested several optimized solutions; among them the best choice was obtained to be Fe-based oxygen carrier with reaction temperature and time of 1014.13 8C and 50.5 min in the second cycle respectively. ß 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Keywords: Chemical looping reforming (CLR) Oxygen carriers Experimental design Hydrogen production Methane reforming
7 8
Introduction
9 10 11 12 13 14 15 16 17 18 19 20 21
Hydrogen (H2) is also a significant feedstock in the production of ammonia, methanol and fertilizers, as well as for upgrading of fuels in the refining industry [1–4]. Various technologies are employed to obtain hydrogen from other hydrogen-containing compounds such as fossil fuels, biomass, or water. Currently, commercial hydrogen is mostly produced from the fuel processing by natural gas reforming, partial oxidation of heavy crude, naphtha as well as coal gasification. Steam methane reforming (SMR) is the most common hydrogen production method for large scale; however, the appreciable amount of CO2 releases during its operation units [3,5–7]. Thus, the development of concepts for H2 production via SMR is extremely pleasant due to reduced capital costs and CO2 emissions. So the chemical looping concept might be
* Corresponding author at: Chemical Engineering Department, School of Chemical and Petroleum Engineering, Shiraz University, Shiraz 71345, Iran. Tel.: +98 711 2303071; fax: +98 711 6287294. E-mail address: [email protected] (M.R. Rahimpour).
able to suitable alternative for H2 production [1,4,8]. Chemicallooping reforming (CLR) which is a technology for synthesis gas (H2 and CO) production from natural gas and light hydrocarbons. This new process was offered by Mattisson and Lyngfelt [9]. The most important advantage of CLR is production of synthesis gas, and also synthesis gas could be used for production of pure H2 accompanies without any change in the composition of product gas. Since the heat transfer occurs directly between gas and oxygen carrier, it is more effective to reduce the size of reformer, CLR is economic than conventional technology [10,11]. As shown in Fig. 1 the CLR system consists of two interconnected reactors which are designated as air and fuel reactors. In methane CLR, the fuel and some steam are burnt with the oxygen-carrier e.g. metal oxide (MenOm) to form a mixture of CO, H2, CO2, and H2O in the fuel reactor while oxygen carrier particles are reduced to MenOms. Then oxygen carriers are transferred to the air reactor in order to react with air which cause oxidizes original oxidation state [12]. If CO2 and steam reforming occurred, with two network reactions of CLR are as described below; they can effective on coke removing. Fuel Reactor :
sCH4 þ Men Om $ sCO þ 2H2 þ Men Oms
http://dx.doi.org/10.1016/j.jiec.2014.04.031 1226-086X/ß 2014 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
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(1)
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2
Fig. 1. A general scheme of the CLR process.
42 41 Air Reactor : 45 44 43 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87
2Men Oms þ sO2 $ 2Men Om
(2)
It is essential to select an oxygen-carrier that tends to partially oxidize methane to syngas rather than CO2 and H2O. The main advantage of this process is that heat needed for converting CH4 to H2 is obtained without costly oxygen production and mixing the air with carbon-containing fuel gases [2,12,13] Selection of an oxygen-carrier is vital for the CLR technology development. An oxygen carrier in a CLR power plant must show high mechanical and chemical stability (high melting point), sufficient durability and reactivity in successive cycles reactions. It should be low in the tendency to agglomerate, fragmentation and particles attrition [12,14,15]. The most researchers have been considered Cobalt- based catalysts for the reforming of methane processes due to high catalytic activity and relatively low cost [16,17]. Besides, the main disadvantage of Cu-based oxygen carriers is related to the agglomeration problems caused through the low melting point of Cu (1085 8C) [10,11,18–20]. Chuang et al. [19] focused on the development and performance of Cu-based oxygen-carriers and Al2O3 (as a support) in three different methods: mechanical mixing, wet-impregnation, and co-precipitation in chemical looping process. It was found that particles produced by coprecipitation did not agglomerate and showed a high carrying capacity after 18 cycles of operation. In comparison with other metal oxides, most of the oxygen-carriers supported on Al2O3 compounds showed very high reactivity with all fuel gases. Adanez et al. [21] studied Cu-based oxygen carriers on SiO2 or TiO2 and Febased oxygen carriers on Al2O3 and ZrO2. A number of studies have also concentrated on employment different oxygen carriers in CLC and CLR processes. Zafar and co-workers [22] performed an experimental study in a fluidized-bed reactor with NiO, CuO, Fe2O3 and Mn2O3 particles. Several materials using ZrO2 doped with Ca, Mg, and Ce were tested by Johansson et al. [23]. They found calcination temperature affects the agglomeration tendency. One of these materials supported on MgO-ZrO2 was successfully used in a CLC reactor for 70 h without defluidization [24]. A continuously operating laboratory reactor consisting of two interconnected fluidized beds using the oxygen carrier made of NiO and MgAl2O4 was considered by Ryde´n et al. [25] in CLR process. Abad et al. reported the range of operational conditions for Cu, Fe and Nibased oxygen carriers in CLC process [26]. Mattisson et al. [18] prepared Mn3O4/Al2O3 oxygen carrier in order to investigate its reactivity. It was shown that Mn3O4/Al2O3 reactivity is low due to the formation of MnAl2O4 during sintering, which does not react
with oxygen and fuel. Iron oxides have been considered as the most preferred materials among many oxygen carriers employed in CLC and CLR processes due to high availability, low price and high environmental safety. It should be noticed that reduction from hematite to magnetite (Fe2O3 ! Fe3O4) is the fast step while the subsequent steps of magnetite to ferrous oxide (Fe3O4 ! FeO) and ferrous oxide to iron (FeO ! Fe) are much slower. The secondary steps were known to be appropriate for CLR process. However manganese oxides are rarely reported as oxygen carriers in CLC and CLR processes [27], some experimental investigations in a batch fluidized bed [2] and fixed bed [27,28] reactors for hydrogen production with inherent CO2 separation using iron oxides supported on magnesium, silica, chromium, titania and aluminium have been conducted by several research groups. Chiesa et al. [30] investigated a process which consist three-reactor chemicallooping using iron oxides as oxygen carriers for hydrogen production from natural gas. Investigation and determination of the optimum conditions for evaluating the catalytic performance of different oxygen carriers is a tedious and complex process involving many steps, variables and complex interactions among them. The most of previous studies includes using the one-factorat-a-time experimental approach. This approach has some disadvantages such as time consuming and costly procedures, and of course does not consider the interaction of various operating parameters. Consequently, it results in inexactness of the process optimization and tends to functionalize only for a single varied factor process or systems with linear interactions of operating parameters [31]. The application of design of experiments (DOE) procedure seems to be one of the best methodologies for studying and improving the complicated processes [32]. One of the appropriate multivariate techniques of experimental design approaches which can deal with statistical modeling and process optimization is the response surface method (RSM) [33]. It is used to examine the relationship between one or more response variables and a set of quantitative experimental variables [34]. The main objective of this study is to investigate the catalytic activity and feasibility of four different oxygen carriers in chemical-looping reforming process in terms of their oxygen release and uptake capabilities and their activity toward produce hydrogen in a fixed- bed micro- reactor. Furthermore, RSM is taken up to investigate and optimize the CLR process operating conditions while the previous researches did not use experimental design for the optimization of the reaction parameters.
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Experimental method
131
Oxygen carrier preparation
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Oxygen carriers supported on alumina support were prepared using co-precipitation method. This method is based on the precipitation by changing the pH level of a desired solution. By using this method good distribution of high active metal content could occurs on the support internal surface. Four oxygen carriers, including Fe, Mn, Co and Cu were prepared for the experiments according to the following procedure: A desired amount of Al (NO3)39H2O was added slowly in an aqueous solution of the metal nitrate. When the salts were completely dissolved, the desired amount of NH4OH was added slowly until the solution pH reached 8–9. Then the solution was stirred and heated to 70–80 8C. The homogeneous solution was maintained in a water bath for 10 h. Consequently, the precipitates were filtered and washed with distilled water. After that, in order to evaporate the surface water, the paste mixture allowed to dry for 6 h at 100 8C in an oven. In the first, the synthesized materials were calcined in an electrical furnace under air atmosphere increasing the temperature from ambient conditions up to 700 8C,
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kept at this temperature for 2 h, and followed by a second calcination step under air at 1250 8C (for Fe, Mn, Co) and 1000 8C (for Cu) with a ramp of 5 8C/min and remained 6 h at the final temperature.
155
Oxygen carriers characterization
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X-ray diffraction (XRD) analysis allowed investigating the new phases formation in the oxygen carrier after reaction and comparing them with the ones present in the initial oxygen carrier. XRD patterns were recorded by a powder diffractometer (Bruker D8 Advance, Germany) using Cu Ka radiation, operated at 40 kV and 40 mA. The samples were scanned in a step-scan mode, with a step size rating of 0.058/s over the angular 2U range of 10– 908. Scanning electron microscopy (SEM) was performed using a Vega 2 Tescan (Czech Republic) microscope instrument in order to study the morphology of the oxygen carriers. The same instrument was also used to perform energy dispersive X-ray analysis (EDX) of each oxygen carriers. In addition to the specific surface area (BET) and porosity of the fresh oxygen carriers was measured, using an ASAP 2010 instrument (Micrometrics). The crushing strength a real density was measured using a digital force gauge machine (Shimpo FGN-5)
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Reactor system
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Fig. 2 shows the experimental setup consisting of a fixed bed micro-reactor (made of stainless steel, i.d. 12 mm), an electrical furnace, electronic low pressure mass flow controller (LP–MFC) and a gas chromatography (GC). In the present work, multicycle reduction–oxidation reactions (redox) were conducted in a cylindrical fixed bed reactor. The oxidation and reduction reactions occurred alternatively at atmospheric pressure in the fixed bed reformer tube. The oxygen carriers were located inside the reactor in a crucible (for allowing rapid cycling of a small amount of carrier in a repeatable way with acceptable manual work). The initial weight of metal sample in the reactor was 2.1 g for each test. The oxygen carriers were then sieved to achieve a particle size of 20–40 mesh. Reactor design of a CLR reformer must be done carefully, because direct contact between the oxygen carrier and the gas phase is important. The reformer reactor was located in an
3
electrical furnace for heating the system to desired reaction temperature (a very high furnace temperature is required). After reaching the desired temperature, the reducing gas (CH4) controlled by an electronic LP–MFC was introduced into the reactor. CLR process consists of three consecutive steps. In the oxidation step (first step), oxygen reacts with the oxygen carriers in the air reactor. The oxidation stream designed 25% O2 and 75% Ar. Then in the transition step (second step), the residual oxygen from previous step is pushed out using argon and finally in reduction step (third step), the fuel is burnt with the oxygencarrier to form a mixture of H2 and CO along with a small amount of CH4 and CO2 in the fuel reactor. The fuel composition was 40% CH4, 10% CO2, 3% water (steam) and 47% Ar as a carrier inert gas. The argon was used as an inert carrier gas for dilution of methane and oxygen. GC analyzed continuously the outlet gas composition at each time.
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Results and discussion
205
Characterization of oxygen carrier
206
The distribution of the metal oxide on the support depends on the reactivity, structural strength and stability during the consecutive cycles. The characteristics of oxygen carriers used in this paper are given in Table 1. In this work, crushing strength was measured with the force required to fracture the desired particle. The crushing strength has highly dependent on the type of active metal oxides. The particles with low porosity have high crushing strength. Correspondingly, the specific surface area (SBET) of alumina supported Fe, Cu, Co and Mn oxygen carriers are shown in Table 1. It is clear that Fe-based oxygen carrier exhibits the maximum surface area (123 m2/g) while Cu-based oxygen carrier has the lowest surface area (79.8 m2/g). The structure of the catalysts was determined by X-ray powder diffraction (XRD). The X-ray diffractions of fresh and reduced oxygen carriers are presented in Fig. 3. As mentioned in this figure, Fe, Mn, Co and Cu atoms are visible in the carriers supported on Al2O3. This figure indicates different oxides of oxygen carriers, and almost all of them can be converted to stable oxide in high temperature. For example, with increasing the reaction temperature, Co3O4 is reduced to CoO and small metallic cobalt particles [35]. On the other hand, limited formation of copper aluminate compounds was found in both XRDs of Cu based oxygen carrier. The copper aluminate compounds are formed due to the high reactivity between CuO and Al2O3, and diffusion of copper ions into the alumina even at temperatures as low as 500 8C [36]. Fig. 4 shows the elemental analysis and SEM images of different catalyst before and after CLR experiments. It is clear from this figure that each oxygen carrier has its own metal clusters on the support with different sizes. The pore blocking (impurities) and sintering are referred as the main reasons of Fe and Mn based catalysts deactivation (shown in Fig. 4a and b). Also, sintering degrades catalyst performance due to the decrease of active surface area. Fig. 4c and d reveals the effect of sintering on Co and Cu based catalyst at high temperature which is the main cause of
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Table 1 Characterization of the fresh oxygen carrier particles.
Fig. 2. Reactor system for chemical looping reforming of natural gas with hydrogen production.
Oxygen carrier
Fe/Al2O3
Co/Al2O3
Mn/Al2O3
Cu/Al2O3
Theoretical weight ratio [wt%] Porosity (%) Real density [g/ml] BET surface area [m2/g] Crushing strength [N/mm]
40–60 53.2 2.7 123 2.1
40–60 31.3 4.2 87.8 13
40–60 44.8 3.8 110.0 3.8
40–60 39.3 5.1 79.8 10.3
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Fig. 3. XRD patterns of oxygen carriers in the best operating temperatures during cycles: (a) before oxidation of Fe/Al2O3, (b) after reduction of Fe/Al2O3, (c) before oxidation of Mn/Al2O3, (d) after reduction of Mn/Al2O3, (e) before oxidation of Co/Al2O3, (f) after reduction of Co/Al2O3, (g) before oxidation of Cu/Al2O3, (h) after reduction of Cu/Al2O3.
Fig. 4. SEM images of the oxygen carrier particles both fresh (left) and reacted in the CLR unit (right); (a) Fe/Al2O3 (reacted at 1050 8C), (b) Mn/Al2O3 (reacted at 950 8C), (c) Co/ Al2O3(reacted at 950 8C), (d) Cu/Al2O3 (850 8C).
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Oxygen carrier
Estimated (wt%)
Component
Fresh (wt%)
Deactivate (wt%)
Fe/Al2O3
40/60
Fe Al O Impurities
40.66 30.67 28.7 0
40.46 30.55 28.77 0.75
Cu/Al2O3
40/60
Cu Al O Impurities
38.95 32.23 28.82 0
38.75 27.82 30.45 2.63
Co/Al2O3
40/60
Co Al O Impurities
39.41 30.93 29.66 0
39.01 28.62 29.82 2.54
Mn/Al2O3
40/60
Mn Al O Impurities
39.13 27.97 39.13 0
39.08 27.4 35.26 0.97
the catalyst deactivation. So an increase in deposited coke was observed in these oxygen carriers. EDX analysis of fresh and reacted oxygen carriers are presented in Table 2. This table confirms the precipitation of 40% metals on alumina support. Also, it shows the percentages of the elements and their suitability when the carriers are fresh. However, after the last cycles the existence of small amount of impurities due to carriers deactivation changes these percentages that seem to be due of carbon deposition and impurities of catalyst synthesis on catalyst surface. Nevertheless, EDX analysis reveals that the
5
catalyst leaching is avoided and the catalysts loading percent do not considerably change after the four cycles.
252 253
Oxygen carrier activity
254
In this study Fe/Al2O3, Mn/Al2O3, Cu/Al2O3 and Co/Al2O3 at the stoichiometric ratio of 4:6 are used as oxygen carriers in 4 cycles of oxidation and reduction in CLR process. The approximated required time for each carrier in the reduction step is suggested to be 100 min while this time for the oxidation stage is 35 min. In order to investigate the influence of cycle number on activity and stability, all the oxygen carriers were tested at 850 8C and 950 8C in four cycles. Results are indicated in Fig. 5 for all oxygen carriers. As mentioned in Fig. 5a the maximum hydrogen percent for Fe2O3 as oxygen carrier was about 72 and 75% occurring in the second cycle at 850 and 950 8C, respectively. As mentioned in this figure, the second cycle has the higher hydrogen production performance than other ones for all of oxygen carriers. It can be concluded that the particles need 1 cycle to reach a more favorable structure. So, in the following, the second cycle is applied to investigate the effect of other parameters. As indicated above, the second cycle revealed the maximum hydrogen production yield. Therefore, the second cycle was selected for further catalytic investigations. An experimental design was performed to investigate the effect of reaction temperature, time and catalyst type on hydrogen production yield, methane conversion and CO2 selectivity in the second cycle.
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Experimental design
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Center composite design (CCD) which is known as the most popular response surface method was applied for further investigation of the oxygen carriers in CLR process. The CCD is
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Fig. 5. Hydrogen concentration profiles versus time at each cycle at 850 and 950 8C; (a) reduction of Fe2O3, (b) reduction of Mn3O4, (c) reduction of CoO, (d) reduction of CuO.
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Table 3 Experimental levels of independent variables.
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Variables
Unit
Symbol
Temperature Time Catalyst type
8C min
X1 X2 X3
Xi X0 DX i
1
0
+1
+a
656.25 2.5 Fe Mn Cu Co
700 10
875 40
1050 70
1093.75 77.5
(3)
xi is independent variable coded value, Xi is independent variable real value, X 0 is independent variable real value in the center point and DX i is step change value. Thus, the coded variables range from 1 to +1. In order to formulate a simple model appropriate for optimization, the experimental results obtained from the CCD are related to the variables by linear and quadratic terms, as in Eq. (4): Y ¼ b0 þ
310 309 311 312 313 314
a
well suited for sequential experiments to obtain appropriate information for testing lack of fit with a low number of design points. In CCD the total number of experiments is calculated with 2k + 2k + n0, where, k is the number of independent variables and n0 is the number of repetitions of the experiments at the center point. Four star points (1), four axial points (a = 1.25) and five replicates at the center point (0) were chosen as experimental points for each catalyst type (X3) and consequently a total of 52 experiments were performed. Central points were used to check the reproducibility and stability of results. The runs were conducted in randomized manner to guard against systematic bias [37]. The process parameters and their values involved in the present study are shown in Table 3. In this study, experimental design was developed to maximize hydrogen yield and methane conversion and minimizing carbon dioxide selectivity by the optimization of operating variables including reaction temperature, time and catalyst type. The catalyst type (X3) which is a categoric factor studied in this paper includes alumina supported Fe, Mn, Cu and Co catalysts. The numerical test factors (X1 and X2) were coded based on the following equation: xi ¼
302 301 303 304 305 306 307 308
Variable levels
k X
k X
X
i¼1
i¼1
i< j
bi xi þ
bii x2i þ
bi j xi x j þ e
(4)
where b0 is offset or constant term, bi the linear coefficient, bii the quadratic effect, bij the cross product coefficient and e is the statistical error [31]. Our CCD design includes 52 sets of test conditions in a single block while the responses of this design are hydrogen yield, methane conversion and CO2 selectivity.
Data analysis
315
The objective of the present research is to study the combined effects of reaction temperature, time and oxygen carrier type on its performance in CLR process. The responses including hydrogen yield, methane conversion and CO2 selectivity are complex under various combinations of those independent variables. Fit summary output of Design Expert software revealed that the quadratic model is statistically significant for all three responses. Therefore, quadratic models were utilized to represent the responses for further analysis. The importance of each term of the quadratic model (Eq. (3)) and the validity of the obtained model was valuated using analysis of variance (ANOVA). Table 4 summarizes the ANOVA results for the three responses of the quadratic models and their statistical parameters. To estimate the accuracy of the acquired models, F-value and Pvalue tests were conducted. The F-values of the reduced quadratic models are 20.54, 34.02 and 38.95, for hydrogen yield, methane conversion and carbon dioxide selectivity respectively which show the significance of the models. Furthermore, the significance of each term in the quadratic models are also tested and the terms with P-value greater than 0.05 are eliminated from the models to enhance the accuracy. From ANOVA table it is apparent that catalyst type (X3) has largest effect on all responses due to their high F-values. On the other hand, reaction temperature revealed the lowest impact on CO2 selectivity. The coefficient of determination (R2) was calculated as 0.8754, 0.8924 and 0.9048 for hydrogen yield, methane conversion and CO2 selectivity, respectively (Table 4), indicating that the statistical model can explain a wide range of variability in each response. The R2 value is always between 0 and 1 and the closer R2 to 1, the better predictability of the responses with the models. As indicated in the ANOVA table the adjusted R-squared values are 0.8328, 0.8662 and 0.8815 for the three responses and the proximity of R2 and adjusted R2 indicates that non-significant terms are not involved in the models. An additional way to evaluate the adequacy check of obtained models is analyzing the distribution of residuals. Residuals are the deviation between model predicted and experimental values and are supposed to follow a normal distribution if the experimental
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Table 4 ANOVA table for the response surface reduced quadratic models. Source
Hydrogen yield (Y1)
Methane conversion (Y2)
Carbon dioxide selectivity (Y3)
F-value
P value Prob > F
F-value
P value Prob > F
F-value
P value Prob > F
Model X1-temperature X2-time X3-catalyst type X1 X3 X2 X3 X12 X22 Model statistics
20.54 5.18 7.86 62.00 4.06 12.48 6.83 11.20
<0.0001 0.0285 0.0079 <0.0001 0.0135 <0.0001 0.0128 0.0019
34.02 0.56 24.60 69.62 – 29.17 5.03 13.28
<0.0001 0.4575 <0.0001 <0.0001 – <0.0001 0.0304 0.0007
38.95 0.055 46.99 74.86 – 34.68 3.06 10.53
<0.0001 0.8163 <0.0001 <0.0001 – <0.0001 0.0877 0.0023
R2 R2-adj R2-predict Adeq. precision
0.8754 0.8328 0.7523 16.924
0.8924 0.8662 0.8088 23.418
0.9048 0.8815 0.8224 27.900
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Fig. 6. The studentized residuals versus predicted response plot: (a) hydrogen yield; (b) methane conversion; (c) CO2 selectivity.
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errors were random [38]. The studentized residual plot is attained by normalizing the residuals with respect to their standard deviations and afterwards a normal distribution function was fitted to the plot. Internally studentized residuals versus predicted for hydrogen yield, methane conversion and carbon dioxide selectivity are demonstrated in Fig. 6 a–c. The random scatter of experimental points indicates that there is no need to assume any violation of the independence. Also, this figure illustrates that the obtained models are in an appropriate description of the process and do not have any obvious pattern and unusual structure. By applying multiple linear regressions on the experimental results, the data of the central composite design were fitted with a quadratic full polynomial equation (Eq. (4)). The empirical relationships between three responses for each of four catalysts with independent variables in actual values are obtained by the application of RSM and are given in Eqs. (5)– (16) as follows: Mn catalyst pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi H2 yield þ 0:88 ¼ 15:0786 þ 0:041347 temp 2
þ 0:097147 time 0:00002053 temp 0:00895 time2
1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ þ2:81397 0:0044005 temp SCO2 þ 0:18 0:016207 time þ 0:0000024868 temp2 0:0004191 time2
378 377 376
þ 0:009793 time 0:00002053 temp2 0:0008949 time2
(8) 380 379
lnðCH4 conversion þ 0:99Þ ¼ 2:48135 þ 0:01364 temp 0:021626 time 0:000007582 temp2 0:0004191 time2
(9)
1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ þ2:56888 0:0044005 temp SCO2 þ 0:18
382 381
0:013539 time þ 0:0000024868 temp2 0:00015696 time2
373 372
(10)
Cu catalyst
lnðCH4 conversion þ 0:99Þ ¼ 2:93417 þ 0:01364 temp
385 384 383
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 yield þ 0:88 ¼ 8:08479 þ 0:03466 temp
þ 0:044528 time
þ 0:01397 time 0:00002053 temp2
0:00000758 temp2 2
(7)
Co catalyst pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi H2 yield þ 0:88 ¼ 10:2248 þ 0:03456 temp
(5)
0:0004191 time
375 374
(6)
0:0008949 time2
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(11)
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1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ þ2:92198 0:0044005 temp SCO2 þ 0:18
lnðCH4 conversion þ 0:99Þ ¼ 2:10506 þ 0:013640 temp þ 0:01148 time
0:015387 time þ 0:0000024868 temp2
0:000007582 temp2 0:0004191 time2
(12)
389 388 1 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ¼ þ2:06542 0:0044005 temp SCO2 þ 0:18 2
0:00107282 time þ 0:0000024868 temp 0:00015696 time2 392 391 390
(13)
Fe catalyst pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi H2 yield þ 0:88 ¼ 16:1955 þ 0:0436 temp þ 0:09028 time 0:00002053 temp2 0:00089494 time2
(14)
394 393 lnðCH4 conversion þ 0:99Þ ¼ 2:93417 þ 0:01364 temp þ 0:044528 time 0:00000758 temp2 0:0004191 time2
396 395
(15)
0:00015696 time2
(16)
Effect of process variables on oxygen carriers efficiency
397 398 399
In order to evaluate the effects of reaction temperature and time on hydrogen production yield, methane conversion and CO2 selectivity for each oxygen carrier, graphical representations have been made in Figs. 7–10. So, contour plots have been created by means of obtained models. Fig. 7a shows the interacting effect of reaction temperature and time and their mutual interaction on hydrogen production yield. This figure indicates both reaction temperature and time have positive effect on hydrogen yield in the presence of Mn/Al2O3 oxygen carrier. On the other hand, Fig. 7b revealed that the maximum methane conversion of this carrier occurs around the temperature of 900 8C and 55 h. However, more increasing of the temperature results in the agglomeration of active sites which reduces the catalytic activity of Mn carrier. The performance of Cu/Al2O3 oxygen carrier is illustrated in Fig. 8. The maximum hydrogen yield and methane conversion was obtained at about 850 8C while further increasing of reaction temperature shows decreasing effect on these responses. Like Mn
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Fig. 7. Contour plots of Mn/Al2O3 oxygen carrier performance: (a) hydrogen production yield, (b) methane conversion, (c) CO2 selectivity.
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Fig. 8. Contour plots of Cu/Al2O3 oxygen carrier performance: (a) hydrogen production yield, (b) methane conversion, (c) CO2 selectivity.
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based oxygen carrier, the reduction of Cu carrier activity in higher temperatures is due to its low melting point (1085 8C) which tend to agglomeration and sintering. On the other hand, comparison of fresh and reacted Cu oxygen carrier SEM (Fig. 4) shows that active sites reduce due to coke formation on the surface. Besides, the carbon dioxide selectivity decreases by reaction time because of side reactions which consumes CO2. The investigation of cobalt oxygen carrier reveals the same results of Cu carrier. Nevertheless, it revealed lower activity and hydrogen production yield in comparison with Cu carrier. The results obtained for Co40Al2O360are demonstrated in Fig. 9a–c. Also, Fig. 10a–c shows the responses of alumina supported iron based oxygen carrier. As indicated in this figure increasing the reaction temperature tends to improve the hydrogen production yield. The maximum hydrogen production yield occurs around 1050 8C while the maximum methane conversion comes about around 950 8C.
435
Oxidation
436 437 438 439 440 441 442 443
In the oxidation process of each carrier and cycle, the flow of input gas was 166 ml/min and the oxidation gas mole fraction was 25% oxygen and 75% argon. The amount of oxygen adsorbed on the oxygen carriers is the deference of oxygen in the initiation and the amount in each time. In the presence of carbon that is the pollutant of oxygen carriers, oxygen bonds break and some CO2 would be observed in the output gases. This process continued till the oxygen content run into a constant amount in the down-stream. As
the time passes and different cycles, in the same reaction conditions, hydrogen production rate has been decreased, this trend will continue even after re-oxidizing carrier which shows the deactivation of carriers. Fig. 11 shows the first to the fourth cycles of oxidation processes for manganese as carrier. As it is illustrated, the needed time for carriers to reach the maximum amount of oxygen adsorption is the same for all cycles, as well as of oxygen adsorption decreases after each cycle. Since most of active sites are reduced in the first reduction reaction, active sites regeneration is more difficult in the next oxidation reactions. After the fourth cycle, the maximum oxygen adsorption has been decreased significantly due to catalyst deactivation. Fig. 12 shows the oxidation (oxygen consumption) in the second cycle for all oxygen carriers at 850 8C. It is clear the iron has the maximum value in 12 min and the oxygen consumption slope is low; thus, the adsorption would be the highest. Iron had the maximum oxygen consumption and manganese, cobalt and copper are next respectively.
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Process optimization
463
It is clearly obvious that RSM is a powerful method for the examination and optimization of multi-variable procedures. In the Design Expert software numerical optimization could be applied for finding the combination of independent parameters accomplishing the desired requirements instantaneously. In this manner, the higher and lower levels of each independent variable and the
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Fig. 9. Contour plots of Co/Al2O3 oxygen carrier performance: (a) hydrogen production yield, (b) methane conversion, (c) CO2 selectivity.
Fig. 10. Contour plots of Fe/Al2O3 oxygen carrier performance: (a) hydrogen production yield, (b) methane conversion, (c) CO2 selectivity.
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Table 5 Optimal operating conditions and validation tests for the maximizing of hydrogen production yield, methane conversion and minimizing carbon dioxide selectivity.
RSM prediction Validation test
470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491
Temperature (8C)
Reaction time (min)
Catalyst type
H2 yield
CH4 conversion
CO2 selectivity
1014.13 1014.13
50.50 50.50
Fe Fe
83.45 80.91
79.09 77.61
2.26 2.30
responses were provided according to the contour plots. The optimization was performed and the optimum variables that satisfied all of the specified conditions for the maximization of the response were achieved as reported in Table 5. The optimal operating variables including reaction temperature, time and catalyst type were achieved to be 1014.13 8C, 50.5 min and Fe catalyst, respectively. As indicated in Table 5, the maximum hydrogen yield and methane conversion were predicted to be 83.45% and 79.09% and the minimum carbon dioxide selectivity of 2.26%, applying the acquired optimal operating variables. Also, validation test was carried out at these obtained conditions in order to evaluate the exactness of obtained optimums. As shown in Table 5, the experimental validation test at optimal operating conditions (80.91) was in a good agreement with RSM predicted (83.45) for the hydrogen yield. Similarly, the predicted methane conversion (79.09) revealed good fitness with the validation test result (77.61). Also, carbon dioxide selectivity validation test showed good fitness with predictions as mentioned in Table 5. The difference of RSM predicted and experimental values are justifiable due to the coefficients of determination and model F-values obtained for each response.
Conclusion
492
This work represents the suitability of different oxygen carriers and operating conditions in a CLR process. Four different oxygen carriers including alumina supported Fe, Mn, Co and Cu were synthesized with co-precipitation method. In order to investigate the influence of reaction temperature (700–1050 8C), time (10– 70 min) and carrier type (alumina supported Fe, Mn, Cu and Co), a CCD based response surface method was applied and experiments were implemented corresponding. By applying central composite design, the quadratic model was selected for all responses including hydrogen production yield, methane conversion and CO2 selectivity. According to F-values indicated in ANOVA table, the oxygen carrier type showed the largest effect on all responses. According to the results of the experiments, the iron has the highest resistance against sintering among the four carriers. Also the alumina supported iron carrier has the highest capacity for oxygen adsorption. Capacity of oxygen consumption and hydrogen production is increased as the temperature increases. This trend is continued until the process is reached to the optimum temperature, while hydrogen production will be reduced in higher temperature due to sintering phenomena. The second cycle is the best since the adsorption and desorption of oxygen in the process for these cycles are the maximum at the operating temperature. The optimization was performed and the best optimized answer was temperature of 1014.13 8C and the reaction time of 55.5 min for iron oxygen carrier, aimed to obtaining 83.45, 79.09 and 2.26 of hydrogen production yield, methane conversion and carbon dioxide selectivity, respectively.
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Uncited reference
Fig. 11. Absorbed oxygen percentage during 4 oxidation reaction cycles for Mn/ Al2O3 at 950 8C.
Fig. 12. Oxygen consumption in the second cycle for all carriers at 850 8C.
Q2521
[29].
522
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523
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