Article pubs.acs.org/EF
Kinetic Study of the Catalytic Partial Oxidation of Synthetic Diesel over 5 wt % Ni/Ce0.5Zr0.33Ca0.085Y0.085O2‑δ Catalyst for Hydrogen Production Md. Faysal Ahamed Khan, Ataullah Khan, Hussameldin Ibrahim, and Raphael Idem* Process Systems Engineering, Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan S4S 0A2, Canada ABSTRACT: The kinetics of the catalytic partial oxidation (CPOX) reforming of synthetic diesel (SD) has been studied for the first time for the production of hydrogen using a conventional reactor and standard liquid feed delivery system over a novel 5 wt % Ni/Ce0.5Zr0.33Ca0.085Y0.085O2‑δ (5N/CZCaY) catalyst prepared by a surfactant-assisted route. The experimental runs were conducted at atmospheric pressure, in the temperature range of 1123−1223 K, with an oxygen/synthetic diesel (O2/SD) molar ratio in the range of 6.7−9.3 and a weight of catalyst/synthetic diesel molar flow ratio (W/FSD,0) in the range of 5.28−13.21 kgcatalyst h kmolSD−1. The experimental results were used to develop a power law rate model of the form (−rSD) = 3.22 × 1015 exp(−16000/RT)NSD2NO20.5, which yielded an average absolute deviation (AAD) of 129 m2/g, average pore diameter of >87 Å (mesopore), and cumulative pore volume of 0.38 cm3/gcat. The development of a higher surface area can be attributed to the method of preparation adopted in the current work.30 Upon impregnation of a nominal 5 wt % Ni over the surface of the CZCaY support, the surface area and cumulative pore volume decreased. The observed decrease is mainly due to penetration of the dispersed nickel oxide into the pores of the support. The measurements of the pore volume per unit surface area (PV/SA) can also be found in Table 2. Idem et al. illustrated in their recent work that the supports and corresponding catalysts possessing PV/SA > 1.72 × 10−9 m exhibit exceptionally good performance.31 In this regard, it is important to mention that both CZCaY and 5N/ CZCaY exhibit a PV/SA > 2.6 × 10−9 m (Table 2), and their corresponding N2 isotherms are shown in Figure 3. The
Figure 2. (a) Extended TOS stability test over the 5 wt % Ni/ Ce0.5Zr0.33Ca0.085Y0.085O2‑δ catalyst at T = 1173 K, O2/SD = 9.3 mol/ mol, and W/FSD,0 = 10.57 kgcat h kmolSD−1, where blue ◆, XSD; red ■, SH2; and green ▲, YH2. (b) Reformate gaseous product distribution, where blue ◆, H2; red ■, CO2; green ▲, CO; and purple ×, C2H6. 2.11. Kinetic Model. The overall reaction used for the development of the kinetic model for CPOX of SD reforming is given as29
C12.87H 24.81 + 6.435O2 ⇔ 12.87CO + 12.41H 2 ° = − 1192 kJ/mol ΔHrxn
(1)
An empirical, reversible power law rate model equation can be written as
− rA = k 0 e(−E / RT )FA mFBnFCoFD p
(2)
where A = SD, B = O2, C = CO, D = H2, rA = rate of the reaction with respect to SD (kmol m−3 s−1), k0 = pre-exponential factor or collision factor, E = activation energy (J/mol), T = reaction temperature (K), R = molar gas constant (8.314 J mol−1 K−1), FA = molar flow rate of A
Figure 3. N2 isotherms of titled CZCaY support and 5N/CZCaY catalyst.
Table 2. Textural Characterization S/M = 1.25
surface area (SA) (m2/g)
pore volume (PV) (cm3/g)
average pore size (Å)
PV/SA (×10−9, m)
Ni surface area (m2/g)
Ni dispersion (%)
CZCaY 5N/CZCaY
129.8 120.3
0.38 0.32
87.3 81.9
2.9 2.6
2.2
6.6
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isotherms appear to belong to the type IV category, typical of mesoporous material, and with a type H3 hysteresis characteristic of the materials with a pillared structure (platelike), such as clays.31 The H2 chemisorption technique was employed to estimate the metallic surface area and metal dispersion of active component (Ni). As observed from Table 2, the Ni surface area of the 5N/CZCaY catalyst is reasonably high at 2.2 m2/gcat. A bigger value of Ni dispersion signifies better distribution of Ni over the surface of the support. Representative TPR patterns of the CZCaY support and 5N/CZCaY catalyst are shown as a function of the temperature in Figure 4. As observed, the TPR
Figure 5. XRD patterns of titled CZCaY support, 5N/CZCaY catalyst, and pure NiO.
3.2.1.1. Heat-Transport Effects. The internal pore heattransfer resistance was estimated using the Prater analysis, adopted from Ibrahim and Idem,35 as given by eq 6 Figure 4. TPR patterns of titled CZCaY support and 5N/CZCaY catalyst.
ΔTparticle,max =
Deff (CAs − CAc)ΔHrxn λeff
(6)
where ΔTparticle,max is the upper limit of temperature variation between the pellet center and its surface, ΔHrxn is the heat of the reaction, CAs and CAc are the concentrations at the pellet surface and center, respectively (assumed to be the same as the bulk concentration and zero, respectively, as suggested by Levenspiel32), and Deff is the effective mass diffusivity obtained from Deff = DABεp/τ,38 where DAB is the bulk diffusivity of component A in B (i.e., SD in air), which, in turn, is estimated using the Brokaw equation.39 The value for DAB was found, at the temperature of 1123 K, to be 1.521 × 10−6 m2/s. The effective diffusivity Deff was estimated to be 9.67 × 10−8 m2/s. εp is the void fraction (estimated as the ratio of the volume occupied by voids to the total bed volume = 0.5) and calculated using the formula εp = 0.38 + 0.073[1 + (((d/dp − 2)2)/((d/dp)2))],40 where d and dp are the internal diameter of the reactor and the diameter of the particle, respectively. τ is the tortuosity factor, taken as 8.8,38,41 λeff is the effective thermal conductivity obtained using the correlation λeff/λ = 5.5 + 0.05NRe,42 for PBTRs. λ is the molecular thermal conductivity calculated using the Wassiljewa correlation to be 3.4525 × 10−2 W m−1 K−1.39 The detailed calculation is shown elsewhere.29 The effective thermal conductivity, λeff, was found to be 1.908 × 10−4 kW m−1 K−1.29 A value of 0.613 K was obtained for ΔTparticle,max. The heat-transfer limitation across the gas film was determined using the following correlation adopted from Ibrahim and Idem, as shown in eq 7.8
profile of the pure support exhibits two broad H2 consumption peaks in the temperature range of 600−700 °C (670 °C) and 800−900 °C. These two peaks can be attributed to the reduction of surface and bulk oxygen anions, respectively. The TPR profiles of the NiO-impregnated support exhibit a lowtemperature H2 uptake peak at ∼400 °C (Tmax of NiO ⇒ Ni), denoting the reduction of “NiO” species to metallic “Ni” species. The shift in the TPR peaks of the support component upon impregnation of Ni from 670 to 520 °C can be attributed to the synergistic interaction between the ceria component and the nickel component, which lowers the reduction temperature. To ascertain the composition and phase purity, both the CZCaY support and 5N/CZCaY catalyst were examined by XRD, as shown in Figure 5. The support and corresponding catalyst were found to exhibit noticeably similar diffraction patterns, which could be assigned to the cubic fluorite structure of Ce1−xZrxO2 solid solution.30 Additionally, the presence of crystalline NiO in the 5N/CZCaY catalyst indicates that the impregnated NiO species are not homogeneously dispersed over the support surface, resulting in the formation of bulk crystalline structures (Figure 5). 3.2. Kinetic Modeling Study. 3.2.1. Investigation of Possible Heat- and Mass-Transport Limitations. Kinetic data collection in any experiment can only be considered intrinsic in the absence of heat- and mass-transport limitations. Because CPOX reactions at high temperatures (above 1123 K) are very rapid and tend to be mass-transfer-limited, it is very important to determine to what extent, if at all, these transport resistances affect the rate of the reaction. Several correlations are available in the literature to determine the effects that inter- and intraparticle heat- and mass-transport limitations could have on the rate of the reaction. In this work, these effects were investigated at a temperature of 1123 K.
ΔTfilm,max =
Lc( −rA,obs)ΔHrxn h
(7)
where ΔTfilm,max is the upper limit of the temperature difference between the gas bulk and the pellet surface, Lc is the characteristic length, rA,obs is the observed rate of the reaction, 5425
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3.2.2. Power Law Rate Model. An empirical, reversible power law rate model as illustrated in eq 2 was employed in the current work. Because CO and H2 were not part of the feed in the current work, the power law eq 2 can be further simplified to eq 12.
and h is the heat-transfer coefficient [estimated from the correlation JH = JD = (h/Cpuρ)NPr2/3, where JH is the heattransfer J factor, NPr = Cpμ/λ, and λ is the molecular thermal conductivity). The detailed calculation is shown elsewhere.29 The JD factor is given by the following correlations: JD = (0.4548/εp)NRe−0.4069, 40 and NRe = dpuρ/μ(1 − εp). kc is the mass-transfer coefficient obtained as 1.051 × 10−2 m/s. The detailed calculation is shown elsewhere.29 The heat-transfer coefficient, h, was determined to be 1.635 × 10−2 kJ m−2 s−1 K−1. A value of 1.344 K was obtained for ΔTfilm,max. Additionally, a more rigorous Mears,43 criterion for determining the onset of the heat-transport limitation during reaction was also employed to further ascertain the insignificance of heat-transfer resistance in the rate of the reaction, as shown in eq 8. rA,obsρb R cEΔHrxn hT 2R
< 0.15
rA = k 0 e(−E / RT )FA mFBn
(12)
The simplified power law rate equation (eq 12) was used to fit the experimental data. XSD versus W/FSD,0 graphs were drawn for all of the temperatures and O2/SD feed ratios and are shown in Figure 6.
(8)
When numerical values are substituted for the terms on the lefthand side (LHS) of eq 8, a value of 6.152 × 10−3 is obtained, which is much less than 0.15, thereby proving the absence of any heat-transport limitation. 3.2.1.2. Mass-Transport Effects. The internal pore masstransfer resistance was calculated using the Weisz−Prater criterion, adopted from Ibrahim and Idem, as given in eq 9.8 Cwp,ipd =
( −rA,obs)ρc R c 2 Deff CAs
(9) Figure 6. XSD versus W/FSD,0 at different temperatures and O2/SD molar ratios, where black ■, 1123 K and O2/SD = 8.0; red ■, 1123 K and O2/SD = 9.3; green ▲, 1173 K and O2/SD = 6.7; dark blue ▼, 1173 K and O2/SD = 8.0; light blue ◀, 1173 K and O2/SD = 9.3; pink ★, 1223 K and O2/SD = 6.7; yellow ★, 1223 K and O2/SD = 8.0; and brown ★, 1223 K and O2/SD = 9.3.
where Cwp,ipd is the Weisz−Prater criterion for internal pore diffusion, ρc is the pellet density, and Rc is the catalyst radius. The estimated value for Cwp,ipd was 0.444. This value is much less than 1. Thus, this result indicates that the concentration of the reactant on the catalyst surface is more or less the same as the concentration within its pores. The obtained result proves the absence of internal pore diffusion limitations.38 To determine whether film mass-transfer resistance has any effect on the reaction rate, the ratio of the observed rate to the rate if film resistance exists was examined, as shown in eq 10.32 ( −rA,obs) d p observed rate = rate if film resistance controls CAbkc 6
The experimental reaction rates were obtained from Figure 6 as the derivatives of the XSD versus W/FSD,0 curves, as presented in Table 3. The curves were generated using data analysis and graphing software Origin Pro 8. 3.2.3. Estimation of the Parameters of the Rate Models and Validation. Estimation of the power law model parameters was based on the minimization of the sum of the residual squares of the reaction rates by the Gauss−Newton and Levenberg−Marquardt algorithms using nonlinear regression software (NLREG). The values obtained for the parameters are presented in Table 4. The validation of the models was based on the determination of percentage average absolute deviation (AAD %) between the predicted rate using the proposed kinetic power law model and experimentally obtained rate. According to the calculation, the power law model had an AAD
