Article pubs.acs.org/IECR
Kinetic Study of Steam Reforming of Ethanol on Ni-Based Ceria− Zirconia Catalyst Madhumita Patel, Tarun K. Jindal, and Kamal K. Pant* Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas-110016, New Delhi, India ABSTRACT: This article focuses on the hydrogen production from steam reforming of ethanol over nickel-based ceria−zirconia catalyst. A ceria−zirconia support was prepared via the coprecipitation method, followed by wet impregnation for nickel metal. The characterization of catalyst and the support was done. The experiments were conducted in a fixed-bed tubular reactor and results were reported in terms of hydrogen yield, product selectivity, and ethanol conversion. The effect of weight hourly space time, temperature, and time on stream was analyzed at a water:ethanol molar ratio of 9:1. Results revealed that the H2 yield was ∼5.3 mol (out of 6 mol), whereas ethanol conversion was almost 100% at 700 °C. Kinetics modeling was carried out on this catalyst using the Eley−Rideal mechanism and the power law model. The value of activation energy and order of reaction were found to be 64.8 kJ/mol and 1, respectively, for the power law model.
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INTRODUCTION
CH3CH 2OH → CO + CH4 + H 2
Energy is an indispensable element in our everyday lives. Most of the energy that we use comes from fossil fuelsa nonrenewable energy source. The dependence on fossil fuels as energy sources has caused serious environmental problems, such as air pollutants, greenhouse gas emissions, and natural resource depletion. The need for renewable alternatives is becoming ever more urgent. Solar, wind, and biomass are promising renewable resources but are generally site-specific, intermittent, and thus, not stable. Hydrogen has been identified an ideal future energy carrier1 with high efficiency, since it possesses the high energy content per unit of mass (∼121 MJ/kg) and burns without generating any environmental pollutants. It is also one of most important chemicals and is widely used for ammonia production, oil refineries, petrochemical industries, etc. In view of growing environmental concerns, such as global warming and the depletion of fossil fuel, major efforts are being dedicated to develop the utilization of renewable energy sources; therefore, bioethanol is seen as a good fuel alternative because the source crops can be grown renewably and in most climates around the world. In addition, the use of bioethanol is CO2 neutral.2−4 This creates an advantage over fossil fuels, which emit CO2 and poisonous gases. Lastly, bioethanol is obtained through fermentation of second-generation biomass (wasted crops and crop residue) that contains 18−23 wt % ethanol. The overall steam reforming of ethanol (SRE) process can be represented as follows:
0 ΔH298 = 49 kJ/mol
Ethanol reforming to synthesis gas: CH3CH 2OH + H 2O → 2CO + 4H 2 0 ΔH298 = 256 kJ/mol
= 174 kJ/mol
CH3CH 2OH → CH3CHO + H 2
0 ΔH298 = 68 kJ/mol
(4)
The coke formation is mainly due to the dehydration of ethanol and the Boudouard reaction: CH3CH 2OH → C2H4 + H 2O
0 ΔH298 = 45 kJ/mol
(5)
2CO → CO2 + C
0 ΔH298 = − 171.5 kJ/mol
(6)
Methane also forms due to the methanation reaction, which is reverse methane steam reforming: 3H 2 + CO → CH4 + H 2O
0 ΔH298 = −206 kJ/mol
(7)
4H 2 + CO2 → CH4 + 2H 2O
0 ΔH298
= −165 kJ/mol (8)
Water-gas shift reaction: CO + H 2O → CO2 + H 2
0 ΔH298 = − 41 kJ/mol
(9)
Reverse water-gas shift reaction:
(1)
Several other reactions may also take place simultaneously in the reactor, because of the endothermic nature of the reaction. The ethanol decomposition reaction accounts for the generation of CO and CH4: © 2013 American Chemical Society
(3)
Ethanol dehydrogenation leads to the formation of acetaldehyde:
CH3CH 2OH + 3H 2O → 6H 2 + 2CO2 0 ΔH298
(2)
Received: Revised: Accepted: Published: 15763
May 17, 2013 August 31, 2013 October 9, 2013 October 9, 2013 dx.doi.org/10.1021/ie401570s | Ind. Eng. Chem. Res. 2013, 52, 15763−15771
Industrial & Engineering Chemistry Research CO2 + H 2 → CO + H 2O
0 ΔH298 = 41 kJ/mol
Article
The effect of temperature, weight hourly space time (WHST), and time on stream (TOS) on product distribution has also been studied. An Eley−Rideal-based reaction mechanism has been proposed for SRE. The dissociative adsorption of ethanol on two active sites is determined as the rate-determining step for the catalytic process.
(10)
The SRE process has been studied over Ni, Cu, Co, and noble metals such as Pt, Ru, and Rh. Llorca et al.5 studied the ethanol steam reforming reaction over Co/ZnO catalyst and reported a considerable amount of carbon deposition during reaction. Few authors have also studied ethanol steam reforming over bimetallic-like Ni−Cu catalysts as Cu promotes the water gas shift reaction. Vizcaino et al.6 analyzed Ni−Cu for SiO2, γ-Al2O3, MCM-41, SBA-15, and ZSM-5. The highest H2 selectivity was reported for Ni−Cu/SBA-15. Ni has been extensively used as an active metal for reforming reactions, because of its high activity for C−C, O−H bond breaking, promoting H atoms to form H2 molecules via the hydrogenation reaction. Nickel is more economical, compared to noble metals; however, it is reported that nickel has high affinity toward coke formation and metal sintering, thus lowering the catalyst performance for long-term operations.7 Comas et al.8 studied ethanol reforming over Ni/γ-Al2O3 and reported high CO formation as no water-gas shift reaction occurred. The nature of support also plays an important role toward the selectivity of products and the deactivation activity of the catalyst. An acidic support such as Al2O3 leads to deactivation of the catalyst due to coke formation.7 This acidic nature can be controlled by using some basic oxides such as ZnO, MgO, CeO2, etc. The CeO2 is an effective support due to its high oxygen storage capacity (OSC) and ability to store and release oxygen during reactions (redox properties). The addition of ZrO2 further enhances its OSC, as well as its thermal and mechanical stability, and promotes the water gas shift reaction. Chirag et al.9 studied the glycerol steam reforming on Ni/CeO2/ZrO2 catalyst. To test the stability of the catalyst, they carried out the reforming reaction for duration of 15 h and concluded that there was no such degradation of the activity of the catalyst. Prakash et al.10 studied the effect of Ni loading over various composition of ceria−zirconia support for SRE. They found that the high value of Ce/Zr ratio has a lesser tendency for coke formation. Few authors have reported kinetics modeling of ethanol steam reforming over Ni- and Co-based catalysts.11−15 Akande et al.13 proposed a Eley−Rideal-based kinetics model for SRE over 15%-Ni/Al2O3. The dissociation of adsorbed crude ethanol was proposed as a rate-determining step. Mathure et al.14 also proposed a Eley−Rideal-based mechanism over a Ni/ MgO/Al2O3 catalyst for SRE. They concluded the requirement of a more-elaborate reaction scheme to describe the complex reactions occurring during the steam reforming of ethanol. Sahoo et al.15 developed a mechanistic kinetic model using Langmuir−Hinshelwood approach for SRE, water gas shift reaction, and ethanol decomposition over a Co/Al2O3 catalyst. They found that the formation of acetaldehyde from ethoxy was the rate-determining step. However, a kinetic model for Ni/ CeO2−ZrO2-based SRE has not been reported in the literature. In this study, a composition of 30% Ni, 35% CeO2, and 35% ZrO2 (by wt) was prepared in the laboratory via the wet impregnation method was analyzed for ethanol steam reforming reaction, an aqueous solution of ethanol (water:ethanol molar ratio of 9:1) was used for reforming reaction, in order to analyze the use of ethanol without distillation to separate ethanol from water. The reduction temperature of the support is ∼750 °C; therefore, it remains active in the range of experiments carried out. After that, the oxygen storage stability of ceria support is reduced as the temperature is increased.10
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CATALYST PREPARATION Most of the chemicals used for this study were 99.9% pure and procured from Merck Germany. A zirconium precursor, Zr(NO3)2·6H2O, was supplied by CDH. For the SRE process, Ni-based ceria zirconia support was chosen. The catalyst preparation was done in two steps: (i) support preparation and (ii) metal loading on the support. Support was prepared by the coprecipitation method, where Ce(NO3)2·6H2O and Zr(NO3)2·6H2O were chosen as the precursors for Ce and Zr, respectively. These precursors were added in 30 mL of deionized water, maintaining a Ce/Zr ratio of 1. With continuous stirring, ammonium hydroxide solution was added dropwise to maintain the pH of the solution at ∼9−10 at 50 °C. The solution was left unstirred for 5−6 h to get the complete precipitate. The precipitate obtained was washed separately with deionized water under vacuum filtration. The sample then was kept overnight in an oven at 110 °C and calcined at 750 °C for 6 h. The Ni metal was loaded on the support by wet impregnation method and Ni(NO3)2·6H2O was chosen as the precursor for Ni. The aqueous solution was made from the calculated amount of Ni precursor and support and kept in a rotary vacuum filter, where temperature was maintained at 90 °C for proper dispersion of metal on the support surface. The dried sample was kept inside the dryer at 110 °C for overnight, followed by calcination at 550 °C for 4 h. For the catalyst, the Ni loading was maintained at 30% (by wt).
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CATALYST CHARACTERIZATION The catalyst was characterized using a Brunauer−Emmett− Teller (BET) surface area analyzer, powder X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2 TPR), energy-dispersive X-ray diffraction (EDX), and scanning electron microscopy (SEM). The BET surface area, as well as the pore size of the catalyst and the support, were determined with N2 adsorption at −196 °C in a Micromeritics ASAP 2010 apparatus. Prior to gas adsorption measurements, the samples were degassed at 150 °C under vacuum for 3 h. The total pore volume was calculated at a relative pressure of ∼0.989. XRD characterization was performed using a Phi Psi XYZ Xray diffractometer. The X-ray diffractometer was equipped with a vertical diffractometer, a PW Bragg−Brentano (θ/2θ) goniometer. XRD patterns for the powdered samples were obtained using Cu Kα radiation that had a wavelength of 0.154 nm, with an operating voltage of 45 kV and a current of 40 mA and scanned between Braggs angles of 0°−90°. The gonimeter was maintained at a radius of 240 nm, and the step size for θ was 0.0501 per second with a scan step time of 12.15 s. The Scherrer equation was used to estimate the average metal crystallites from the obtained XRD patterns. Phase determination and lattice structure identification was done using the International Diffraction Data library (JCPDS). Micromeritics Pulse Chemisorption Chemisorb 2720 equipment was used to obtain TPR spectra of the samples. A reducing gas mixture of hydrogen and argon at a flow rate of 15764
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0.15 and 2 L h−1, respectively, was passed through the reactor. The reactor was heated to 800 °C from room temperature at a step heating rate of 10 °C/min. The surface morphology was investigated using scanning electron microscopy (SEM) (Carl Zeiss SMT EVO 50), whereas composition analysis of metal loading on support was done by energy-dispersive X-ray diffraction using the Carl Zeiss SMT EVO 50 apparatus. For SEM and for EDX analysis, pellets were used. Catalyst Testing. The experiments were performed at atmospheric pressure in a laboratory unit equipped with a massflow-controlled system, a fixed-bed reactor, and an offline gas chromatograph. The details of the experimental setup are discussed elsewhere used for glycerol reforming and are only briefly presented here.9 A peristaltic pump was used for the feeding of the liquid reactants (a mixture of ethanol and water) to the reactor (19 mm i.d.) through a preheater where temperature was maintained at 200 °C. The experiments were performed in a temperature range of 600−700 °C at atmospheric pressure for a feed mixture with a water:ethanol molar ratio of 9:1. Three grams of catalyst was taken inside the reactor bed diluted with silicon carbide (catalyst:silicon carbide ratio = 2:1 by weight) for every run to minimize channelling and hot spots. The feed flow rate was varied in the range of 0.4−1.0 mL/min. The catalyst used was in the form of pellets with a particle size of 0.5 ± 0.1 mm. Nitrogen was used as the carrier gas at a flow rate of 25 mL/min. Prior to the run, catalyst was reduced at 550 °C in 50% H2/N2 flow for 5 h before the reforming experiments. After the catalyst activation, the reaction temperature was fixed and the reaction was carried out in a nitrogen diluted atmosphere. The hot gases exiting the reactor were cooled in a coiled condenser. The liquid products and the uncondensed gases products were separated in a gas liquid separator. The gaseous product analysis was done using Nucon 5700 gas chromatograph Porapak Q column (3.2 mm OD × 1.8 m long) packed with carbosphere (80−100 mesh) and operated in TCD mode. The liquid product was analyzed using Nucon 5700 gas chromatograph SS column (3.1 mm × 2 m long) equipped for flame ionization detection (FID). Ethanol conversion (XE), hydrogen yield (yH), product selectivity (SP), and WHST were reported according to eqs 11, 12, 13, and 14, as used in another study.16 XE =
Figure 1. XRD patterns of (a) Ni/CeO2/ZrO2 and (b) CeO2/ZrO2.
was undisturbed after the addition of Ni. In Ni/CeO2/ZrO2 catalyst, NiO is the only metal oxide phase appeared on the catalyst surface and its peak intensity is very high, compared to the rest of the peaks. Results revealed that Ni particles are well dispersed on the catalyst surface in the oxide form having oxidation state of 2. The catalyst showed the characteristics peaks corresponding to (111), (200), (220), and (311) crystal planes. The crystal size of the support and the catalyst as calculated from the Scherrer equation were in the range of 17− 27 nm and 11−17.3 nm, respectively. The TPR trend of the catalyst and the support are presented in Figures 2a and 2b, respectively. A single peak at 640 °C for the support is assigned to the reduction of CeO2 to Ce, as reported by Biswas et al.10 Upon varying the ratio of Ce/Zr in the support, the peak for Ce shifted between 600 and 750 °C.
moles of ethanol in − moles of ethanol out × 100 moles of ethanol in (11)
yH =
moles of hydrogen produced moles of ethanol in feed
(12)
Sp =
moles of gaseous product × 100 moles of gaseous product
(13)
WHST (w/f 0) =
■
weight of catalyst ethanol flow rate in feed
(14)
RESULTS AND DISCUSSION Catalyst Characterization. XRD patterns of calcined Ni/ CeO2/ZrO2 and CeO2/ZrO2 are presented in Figures 1a and 1b, respectively. In the CeO2/ZrO2 support, two major phases are present: CeO2 and ZrO2. Incorporation of Ni into the CeO2/ZrO2 support did not shift the diffraction peaks of CeO2 and ZrO2, indicating that oxygen storage capacity of the catalyst
Figure 2. TPR profiles of (a) Ni/CeO2/ZrO2 and (b) CeO2/ZrO2. 15765
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There is no peak due to the ZrO2 in this wide temperature range present in the support because ZrO2 does not easily reduce at temperatures below 1000 °C.16 When Ni was loaded on the support, the H2 consumption peak appeared at two positions: one at lower temperature (220 °C) and another at higher temperature (540 °C10). Both the peaks are corresponding to the reduction of NiO to Ni. The first peak at 220 °C was ascribed to the reduction of weakly bound NiO with the support, commonly known as “free nickel”.17 These particles may be located on the surface of the support. In contrast, the reduction peak at higher temperature may correspond to smaller metallic particles, which may be present inside the pores of the support, indicating that there is strong metal−support interaction. The BET surface area, pore volume, and pore size of the support and the catalyst are reported in Table 1. The surface area and pore volume of the Ni catalyst decreased due to the particle blockage of pore surface by the metal. Table 1. BET Surface Area, Pore Size, and Pore Volume of Support and Catalyst 2
BET surface area (m /g) pore size (nm) pore volume (cm3/g)
CeO2/ZrO2
Ni/CeO2/ZrO2
31.2 ± 3.8 15.9 ± 1.5 0.109 ± 0.008
24.5 ± 1.9 11.1 ± 0.78 0.07 ± 0.001
Figures 3a and 3b show the SEM images of the CeO2/ZrO2 and Ni/CeO2/ZrO2. The SEM image of the catalyst indicates the uniform loading of Ni particles on the support and uniformity of size and shape of the catalyst. Figure 4 depicts the quantitative analysis by EDX and different peaks were indicating the final weight percentage of metal loading for fresh calcined catalyst Ni/CeO2/ZrO2. The EDX pattern confirmed the presence of Ni, Ce, and Zr in the catalyst sample and no other undesired element such as chlorine or potassium was detected in EDX diagram. In the catalyst, the initial Ni loading was 30% (by wt) and the final loading of Ni was around 28.9% (by wt). This EDX spectra indicates that the precursors were successfully handled for final loading with necessary physiochemical treatment of this sample. Thermodynamic Analysis. The thermodynamic analysis of steam reforming of ethanol was carried out using with Aspen Hysys, using the PRSV thermodynamic fluid package. This analysis involves three major reactions: eqs 3, 7, and 9. The gaseous products were H2, CO, CH4, and CO2 with nonreacted ethanol and water in the liquid phase for water:ethanol molar ratio = 9:1. Only these products were considered in product gas, because these are the primary products in SRE. Because the SRE reaction is highly endothermic in nature, the yield of H2 increases with temperature, as shown in Figure 5. These data are comparable with the published literature.18,19 The overall SRE is combination of steam reforming (eq 3) and the water gas shift reaction (eq 9). As the temperature increases, the reverse water gas shift reaction occurs; therefore, the selectivity of CO2 decreases as the CO selectivity increases. The selectivity of CH4 decreases with increasing temperature because of exothermic nature of methanation reactions 7 and 8. The thermodynamic selectivity of gaseous products is shown in Figure 5. Effect of Temperature. To study the catalytic behavior of Ni/CeO2/ZrO2 catalyst with the temperature for SRE, experiments were carried out at three different temperatures;
Figure 3. (a) SEM images of CeO2/ZrO2 support and (b) SEM image of Ni/CeO2/ZrO2.
Figure 4. EDX analysis of Ni/CeO2/ZrO2.
600, 650, and 700 °C. Figure 6 shows that ethanol conversion was significant at all range of temperatures tested. As the temperature was increased from 600 °C to 700 °C, the ethanol conversion increased from 92% to 100%, respectively. Figure 7 shows the variation of experimental and thermodynamic hydrogen yield with temperature. Experimental hydrogen yield was ∼5.3 mol at 700 °C, whereas at 600 °C, hydrogen yield was 4.5 mols. Presence of Ni in the catalyst has the tendency to break C−H bond of the ethanol and convert it in the hydrogen radicals. These hydrogen radicals are combined to form hydrogen molecules in the gaseous product. Figure 8 shows the experimental results of gaseous product selectivity 15766
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Figure 8. Effect of temperature on gaseous product selectivity, thermodynamic selectivity (···) H2, (− − −) CO2, (−·−) CO, and (−··−) CH4, experimental (◆) H2, (●) CO2, (■) CO, and (▲) CH4 (conditions: P = 1 atm, water:ethanol molar ratio = 9:1, and WHST = 99609.1 kgcat s/kmol EtOH).
Figure 5. Effect of temperature on thermodynamic product selectivity ((···) selectivity, (−−CO2, (-·-), CO and (−··−) CH4) and H2 yield (solid line, ). Conditions: P = 1 atm, water:ethanol molar ratio = 9:1.
with temperatures. At 600 °C, the selectivity of CO2 was high and was decreased as the temperature increased. This might be due to the occurrence of reverse water gas shift reaction (eq 10), which is favorable at the higher temperature. As methanation reaction is exothermic in nature, the selectivity of CH4 decreased from 3% at 600 °C to 1% at 700 °C. The SRE takes place at all temperatures to a significant extent, as proved by the increase in hydrogen selectivity. Effect of Weight Hourly Space Time (WHST). Figures 9 and10 represent the variation of ethanol conversion and hydrogen yield, respectively. As WHST increases from 39301 kgcat s/kmol EtOH to 99609 kgcat s/kmol EtOH, ethanol conversion increases from 84% to 99%, whereas the corresponding hydrogen yield increased from 3.2 mol to 5.3 mol at 700 °C. At higher residence times, the ethanol molecules have enough time to come into contact with Ni sites, which breaks the C−C and O−H bonds of ethanol. Therefore, at high residence times, the hydrogen yield was maximum. Effect of Time on Stream (TOS). To study the stability of the catalyst, experiments were carried out at three different temperatures within the range of 600−700 °C for 6 h with the inlet water:ethanol molar ratio being 9 and at a WHST value of 99609.8 kgcat s/kmol EtOH. Figures 11 and 12 show the effect of TOS on ethanol conversion and hydrogen yield, respectively, at different temperatures. At all temperatures, there is no appreciable variation in ethanol conversion and hydrogen yield with run time. At 700 °C, almost 100% ethanol conversion was obtained, whereas hydrogen yield varied within the range of 5.1−5.3 mol. Thus, the stability and activity of the catalyst remained almost constant. The reason behind this may be due to the oxygen storage capacity of ceria present in the support. This activated the oxidation−reduction cycle of the catalyst and helped in gasifying the deposited carbon on the active sites. The presence of ZrO2 in the support enhances the activity of ceria to store a greater amount of oxygen for a long time.20 It also enhances the thermal and mechanical stability of ceria and promotes water gas shift reaction. Thus ceria zirconia combination showed enhanced redox and oxygen storage properties, improved thermal resistance, and better catalytic activity for ethanol reforming. A comparison between experimental and thermody-
Figure 6. Effect of temperature on ethanol conversion at P = 1 atm, water/ethanol molar ratio = 9:1, and weight hourly space time (WHST) = 99609.1 kgcat s/kmol EtOH.
Figure 7. . Effect of temperature on H2 yield thermodynamic (solid line, ) and experimental (data points, ◆) at P = 1 atm, water/ ethanol molar ratio = 9:1, and WHST = 99609.1 kgcat s/kmol EtOH.
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Figure 9. Effect of w/f0 on ethanol conversion at temperatures of (▲) 700 °C, (■) 650 °C, and (◆) 600 °C (conditions: P = 1 atm, water:ethanol molar ratio = 9:1, and TOS = 6 h).
Figure 10. Effect of W/f 0 on H2 yield at temperatures of (▲) 700 °C, (■) 650 °C, and (◆) 600 °C (conditions of P = 1 atm, water:ethanol molar ratio = 9:1, and TOS = 6 h).
Figure 11. Effect of TOS on ethanol conversion at temperatures of (▲) 700 °C, (■) 650 °C, and (◆) 600 °C (conditions: P = 1 atm, water:ethanol molar ratio = 9:1, and WHST = 99609.1 kgcat s/kmol EtOH).
namic values of H2 yield, CO2, CH4 and CO selectivity is shown in Table 2. However, in order to achieve the thermodynamic yield and selectivity, the residence time is required. Kinetics Modeling. An Eley−Rideal type of reaction mechanism and a power law model has been proposed for SRE. It is assumed that, in the Eley−Rideal model, the overall reaction of ethanol steam reforming consists of four steps. In the first step, ethanol exhibits dissociative adsorption on two active Ni sites to give an ethoxy fraction and a hydrogen fraction. The ethoxy fraction produces hydrocarbon and oxygenate fractions on an active site in the second step. In the third step, the hydrocarbon fraction reacts with the steam and hydrogen fraction in gas phase to produce H2 and CO2. The oxygenated hydrocarbon on reaction with nonadsorbed
steam also generates H2 and CO2 in the fourth step. The reaction steps are discussed as follows: Step 1: Dissociative adsorption of ethanol on active site. CH3CH 2OH(g) + 2(S) ↔ CH3CH 2O(S) + H(S)
(11)
Step 2: Generation of hydrocarbon and oxygenated hydrocarbon fractions. CH3CH 2O(S) + (S) ↔ CH3O(S) + CH 2(S)
(12)
Step 3: Surface reaction of unadsorbed steam with adsorbed oxygenated and hydrogen fractions. CH3O(S) + H(S) + H 2O(g) ↔ CO2 + 3H 2 + 2(S) (13) 15768
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where y̆ is the mean value. The goodness of fit was evaluated by determination of correlation coefficient (R2), defined as R2 = 1 −
⎛ RSS ⎞ ⎜ ⎟ ⎝ TSS ⎠
The values of kinetic parameters for the Eley−Rideal-based model and power law are given in Table 4. The final kinetic model parameters were discriminated on the basis of their correlation coefficient value (R2) and activation energy value. Model 1 was able to explain the product distribution more closely with the experimental data with R2 = 0.92. After taking the values of kinetic parameters and equilibrium constant into consideration, Model 1 was approximated by ⎛ 66.6 kJ/mol ⎞ ⎟×E r1 = (2.6 × 105) exp⎜ − ⎝ ⎠ RT
Figure 12. Effect of TOS on H2 yield at temperatures of (▲) 700 °C, (■) 650 °C, and (◆) 600 °C at P = 1 atm, water:ethanol molar ratio: 9:1, and WHST = 99609.1 kgcat s/kmol EtOH.
Also, the activation energy values calculated for Model 1 and the power law model were comparable. The apparent order of the reaction from the power law model is 0.91, which is close to unity. Therefore, the activation energy was recalculated, considering the overall reaction as first order. The final expression was given as
Step 4: Surface reaction of unadsorbed steam with adsorbed hydrocarbon fraction. CH 2(S) + 2H 2O(g) ↔ CO2 + 3H 2 + (S)
(14)
⎛ 64.8 kJ/mol ⎞ ⎟×E r5 = (2.1 × 105) exp⎜ ⎝ ⎠ RT
An optimum catalyst pellet size 0.5 ± 0.1 mm was used based on preliminary runs to eliminate internal diffusion resistance. The catalyst:silicon carbide weight ratio was taken to be 2, to avoid any local temperature gradient. The rate law expressions were obtained assuming that each of the aforementioned steps is a rate-determining step. The remaining steps were considered to be in equilibrium. It was assumed that concentration of fractions generated during dissociative adsorption of ethanol on Ni active sites were equal during the calculation of all rate-determining steps. Table 3 summarizes the rate law expressions for each possible case. A nonlinear regression was performed based on Levenberg− Marquardt algorithm to optimize the results. The kinetic parameters were estimated using the least-squares method to minimize the sum of square of deviations of the theoretical data points from the experimental ones. The residual sum of squares (RSS) was determined:
This confirms that the assumption of dissociative adsorption of ethanol on an active site is the rate-determining step for steam reforming of ethanol for Ni/CeO2/ZrO2 catalyst. The values of activation energy for kinetic models 2 and 4 are also comparable to that of power law model; however, the R2 values for Models 2 and 4 are lower, compared to Model 1, so the generation of hydrocarbon, oxygenated fractions, and surface reaction of the hydrocarbon fraction with nonadsorbed steam cannot be assumed as rate-determining steps. Model 3 has an R2 value of 0.85, and comparing this model with the power model, there is a large variation which makes this model unfit for the rate-determining step. Table 5 shows the value of activation energy for power law, Model 1, and values reported by other authors. The comparison of different models reveals that the value of activation energy calculated from Model 1 is comparable to the values reported by other authors.11,15,21
n
RSS =
■
∑ (yi − y )̂ 2 i=0
CONCLUSION An experimental study for the steam reforming of ethanol (SRE) over 30 wt % Ni-CeO2/ZrO2 catalyst was done in a fixed-bed reactor with different temperatures, time on stream, and weight hourly space time (WHST). Ethanol conversion increased from 94% to 100% and the hydrogen yield increases from 4.5 to 5.3 mols per mole of ethanol in feed as temperature increased from 600 °C to 700 °C. The experimental hydrogen
Here, yi and ŷ are experimental and theoretical data points, respectively, and n is the number of experimental observations. The total sum of squares (TSS) about the mean is calculated as n
TSS =
∑ (yi − y )̆ 2 i=0
Table 2. Comparison between Experimental and Thermodynamic Results for the Steam Reforming of Ethanol (SRE) Reaction H2 Yield
CO2 Selectivity
CO Selectivity
CH4 Selectivity
temperature (°C)
experimental
thermodynamics
experimental
thermodynamics
experimental
thermodynamics
experimental
thermodynamics
600 650 700
4.5 4.8 5.3
5.2 5.3 5.3
23.4 18.8 15.1
20.6 19.3 18.2
5.8 7.6 10.9
5.9 7.5 8.9
0.8 0.3 0.2
1.2 0.3 0.08
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Table 3. Eley-Rideal Model Power Law Model model
step
1
dissociative adsorption of bioethanol
2
rate law expression
⎛ C H ⎞⎤ ⎛ − E ⎞ 2⎡ 2 6 ⎥ ⎟Eθ ⎢1 − ⎜ ⎟ r1 = k 0 exp⎜ 3 ⎝ RT ⎠ ⎢⎣ ⎝ KPW E ⎠⎥⎦ ⎛ C H ⎞⎤ ⎛ − E ⎞ 1/2 2⎡ ⎟E r2 = k 0 exp⎜ θ ⎢1 − ⎜ 2 36 ⎟⎥ ⎝ RT ⎠ ⎢⎣ ⎝ KPW E ⎠⎥⎦
generation of intermediate fractions
3
⎛ C H ⎞⎤ ⎛ − E ⎞⎛ EW 3 ⎞ 2⎡ ⎟⎜ r3 = k 0 exp⎜ ⎟θ ⎢1 − ⎜ 2 36 ⎟⎥ ⎝ RT ⎠⎝ CH3 ⎠ ⎢⎣ ⎝ KPW E ⎠⎥⎦
surface reaction
4
⎛ C H ⎞⎤ ⎛ − E ⎞⎛ EW 3 ⎞ 2⎡ ⎟⎜ r4 = k 0 exp⎜ ⎟θ ⎢1 − ⎜ 2 36 ⎟⎥ ⎝ RT ⎠⎝ CH3 ⎠ ⎢⎣ ⎝ KPW E ⎠⎥⎦
surface reaction
5
with
θ=
1 ⎡ I ⎢⎣1 + K1 +
CH3 K2IW 3/2
+
CH3 ⎤
K3IW ⎥ ⎦
with
θ=
1 ⎡ 1 + K1IIE1/2 + ⎣⎢
CH3
CH3
⎤
+
⎥ K3IIW ⎦
+
⎥ K3IIIW 2 ⎦
K2IIWE1/2
with
θ=
1 ⎡ 1 + K1IIIE1/2 + ⎣⎢
E1/2W 2 K2IIICH3
CH3
⎤
with
θ=
1 ⎡ K2IVEW IV 1/2 1 + K E + + 1 ⎢⎣ CH3
CH3
⎤
⎦ K3IVW 2 ⎥
⎛ −E ⎞ n ⎟(E ) r5 = k 0 exp⎜ ⎝ RT ⎠
power law
Table 4. Estimated Kinetic Parameters Value for Eley−Rideal Model and Power Law for Bioethanol Steam Reforming Reaction parameter k0 N KP Kii Ki2 Ki3 E(kJ/mol) R2
Model 1 2.6 × 105 7.07 1.06 3.90 1.09 66.6 0.92
× × × ×
1014 1015 1018 10−13
Model 2
Model 3
8.3 × 10−20
10 × 1025
4.3 × 104
13360.1 3.13 × 106 58841.4 2264.4 52.9 0.48
9.4 × 1019 1.8 × 1018 14729.7 0.09 32.5 0.85
1.13 × 1015 1.01 × 10−36 4182.5 560.6 54.5 0.72
■
E (kJ/mol) 62.2 66.6 59.7 82.7 144
■
yield was comparable with the thermodynamic yield. Methane selectivity was negligible in the experimental temperature range and carbon monoxide selectivity was insignificant. The deactivation rate of the catalyst was very low, because of the presence of CeO2, which has the tendency to store oxygen in the lattice structure. The Eley−Rideal mechanism and power law model were fitted to the experimental results for SRE. The dissociative adsorption of ethanol on the active sites of catalyst was concluded as the rate-determining step. The activation energy obtained from the optimization study was 66.6 kJ/mol.
■
power law 2.5 × 104 0.91
64.8 0.9
ACKNOWLEDGMENTS The financial support provided by the Centre for Fire, Explosive and Environment Safety (CFEES), Defense Research and Development Organization, Ministry of Defense, Government of India for the present research is gratefully acknowledged.
Table 5. Comparison of Activation Energy Reported by Different Authors Power law Model 1 Akpan et al.21 Sahoo et al.15 Mas et al.11
Model 4
AUTHOR INFORMATION
Corresponding Author
*Tel.: +91 1126596172. Fax: +91 1126581120. E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 15770
NOMENCLATURE AND UNITS ΔH0298 = standard enthalpy of reaction, kJ/mol S = active site E = activation energy, kJ/mol R = universal gas constant, J/(mol K) T = temperature, K k0 = pre-exponential factor, (kgcat s)−1 E = molar flow rate of ethanol, kmol/s W = molar flow rate of water, kmol/s w = weight of the catalyst SRE = steam reforming of ethanol TOS = time on stream H = molar flow rate of hydrogen, kmol/s C = molar flow rate of carbon dioxide, kmol/s Kp = overall equilibrium constant, (kmol s)−4 θ = active and vacant site fraction Kii = equilibrium constant, where i = I−IV Ki2 = equilibrium constant, where i = I−IV Ki3 = equilibrium constant, where i = I−IV n = order of reaction f 0 = flow rate of ethanol in feed dx.doi.org/10.1021/ie401570s | Ind. Eng. Chem. Res. 2013, 52, 15763−15771
Industrial & Engineering Chemistry Research
Article
crude ethanol (CRCCE) over a Ni-based commercial catalyst in a packed-bed tubular reactor. Chem. Eng. Sci. 2007, 62, 3112−3126.
WHST = weight hourly space time OSC = oxygen storage capacity
■
REFERENCES
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