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Catalytic Partial Oxidation of iso-octane over Rh/#-Al2O3 in adiabatic reactor: an experimental and modeling study Andrea Carrera, Alessandra Beretta, and Gianpiero Groppi Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b00255 • Publication Date (Web): 15 Apr 2017 Downloaded from http://pubs.acs.org on April 20, 2017

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Industrial & Engineering Chemistry Research

Catalytic Partial Oxidation of iso-octane over Rh/αAl2O3 in adiabatic reactor: an experimental and modeling study Andrea Carrera, Alessandra Beretta, Gianpiero Groppi*

Laboratory of Catalysis and Catalytic Processes, Dipartimento di Energia, Politecnico di Milano, Via La Masa 34, 20156 Milano (Italy)

*Corresponding author: [email protected]

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ABSTRACT

Catalytic Partial Oxidation (CPO) of hydrocarbons represents an interesting technology for hydrogen production on mobile systems. We investigated the CPO of 2,2,4-trimethyl pentane (iso-octane), chosen as surrogate for gasoline. CPO experiments were carried out in a lab scale auto-thermal reformer with honeycomb monolith catalysts (2% Rh/α-Al2O3), equipped with probes for spatially resolved measurements of temperature and concentration. The iso-octane CPO process follows a reaction pathway which mainly consists of the exothermic combustion reaction and the endothermic steam reforming. The chemical reaction is very fast and sharp gradients of temperature and concentration establish at the catalyst inlet. Similarly to the CPO of light hydrocarbons, the consecutive reaction mechanism results in the formation of a hot spot of temperature at the catalyst inlet. However, compared to light hydrocarbons, this phenomenology is specifically emphasized in the case of isooctane, because of the higher overall exothermicity and the lower diffusion rate, which limits the steam reforming reaction rate. The reactor design strategy previously suggested in the CPO of methane, based on the enlargement of the channel opening to selectively limit the rate of oxygen consumption, does not work for the CPO of iso-octane where the consumption of the fuel is also considerably limited by mass transfer.

KEYWORDS Catalytic Partial Oxidation, iso-octane, rhodium catalyst, mass transfer limitations, spatially resolved measurements, deactivation


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Introduction Hydrogen is currently used in refining and chemical industries. In the next years, its role as energy carrier may become relevant, for its exploitation in efficient energy systems, in particular in the transportation field1,2. In this view, on-board hydrogen usage has gained attention, for several purposes such as: cold start of catalytic mufflers, regeneration of Lean NOx Traps during their rich phase, H2 assisted combustion

and conversion into electric energy for Auxiliary Power Units3,4. Hydrogen

storage and distribution infrastructures are not available yet, and their construction may be inconvenient, compared to those of logistic fuels4. Moreover, on-board hydrogen storage is not competitive with liquid fuels, because of higher cost and lower energy density. Hydrogen production from liquid fuels is a viable solution to overcome this problem. Hydrocarbon fuels have higher energy densities than those of alcohols and other oxygenate fuels, moreover, the former have massive storage and capillary distribution facilities. Different processes may convert diesel, gasoline, LPG and methane into hydrogen rich streams: catalytic reforming, partial oxidation, plasma reforming and supercritical water reforming1. The last two processes have the drawback of high electric energy consumption and the need of operation at high pressure. Among catalytic processes, the most common are: Steam Reforming (SR), Catalytic Partial Oxidation (CPO) and Auto-Thermal Reforming (ATR). SR, which consists of the reaction between hydrocarbons and water, is a strongly endothermic process, and requires high heat load at high temperature, thus requiring large reactors, not suitable for on-board application. CPO represents a valid alternative, in which the fuel reacts with oxygen (contained in air), leading to the production of hydrogen, carbon monoxide and some by-products, mainly deep oxidation products (water and carbon dioxide) and methane. The process is globally exothermic, and the usage of catalyst allows to reach thermodynamic equilibrium within small reactor volumes, making this process feasible for the development of a compact fuel reformer. ATR is a combination of SR and CPO, in fact, it consists of 3 ACS Paragon Plus Environment


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the reaction between fuel, oxygen and water, resulting in a thermally neutral process. ATR has the advantage of higher hydrogen yields, better temperature management and higher coking resistance, compared to CPO, but it also requires higher reactor volumes than CPO. Recently, the Reforming of Exhaust Gas Recirculation (REGR) process has been developed: it consists of the reforming of fuel with exhaust gases, thus producing a hydrogen rich stream, which can be recirculated to the engine, lowering pollutant emissions and fuel consumption, thanks to an increase in combustion efficiency5,6. Catalytic Partial Oxidation represents an effective process to convert hydrocarbon fuels into synthesis gas7. Methane CPO has been extensively studied8–11, and the focus moved recently towards LPG12–16. A less extensive literature is available on heavier hydrocarbons as CPO feedstock, compared to methane CPO, even though they offer the big advantage of higher energy densities than gaseous fuels, and so they are ideal for on-board reformers17–19. Both transition and noble metals allow carrying out CPO. Noble metals, such as Rh and Pt, present the advantage of high activity and resistance to deactivation, compared to traditional Ni catalysts20. Moreover, the higher activity of rhodium in the C-C bond cleavage is a helpful feature for the CPO of hydrocarbons with a large C skeleton, which enhances hydrogen selectivity20. In previous works, we have studied the kinetics of the CPO of methane, propane, iso-octane and noctane using an isothermal annular microreactor, at high space velocity and high dilution of the reacting feed. For all the fuels, we found that the overall process over Rh/α-Al2O3 catalysts can be well explained by an indirect consecutive reaction scheme, consisting of the hydrocarbon deep oxidation to CO2 and H2O and the hydrocarbon steam reforming to CO and H2; H2 and CO post combustions, WGS (Water Gas Shift), RWGS (Reverse Water Gas Shift) and CO methanation can then affect the syngas composition. When comparing the reactivity of different fuels, we found that the rate of the oxidation reaction increases with increasing hydrocarbon chain length (CH4 < C3H8 < iso-C8H18 < n-C8H18), while steam reforming reaction rate increases in a different order (n-C8H18 < iso-C8H18 < CH4 < C3H8) 4 ACS Paragon Plus Environment

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because higher hydrocarbons exert a stronger interaction with the catalyst surface compared to light hydrocarbons, which hinders the reforming route. Under operating conditions close to the real application (adiabatic reactor, absence of feed dilution, washcoated structured catalysts, millisecond contact times) the combustion-reforming scheme results in the onset of a hot spot of temperature at the very entrance of the reactor, which can cause an autocatalytic deactivation process. High temperatures can in fact cause catalyst sintering, with loss of activity 21. The development of axially resolved sampling techniques has largely contributed to finely characterize the evolution of temperature and composition in short contact time CPO reactors

9,13,16,22

.

In our laboratory these techniques have been applied to study the CPO of light hydrocarbons (methane and propane) over Rh and validate a 1D model of the adiabatic reactor implementing the reaction schemes derived from kinetic investigations. Interestingly, in the case of propane CPO we found evidence of the onset of gas phase chemistry, responsible for the formation of small hydrocarbons (CH4, C2H6, C2H4, C3H6, C2H2) which react at the surface and contribute to the overall production of synthesis gas 16. In this paper, we extend our investigations and address the analysis of the CPO of iso-octane, a model compound for branched transportation fuel, in adiabatic reactor. In his pioneering studies, Schmidt and his group investigated the CPO of many liquid hydrocarbons with different moieties 23,24. O’Connor et al. demonstrated that the CPO of n-hexane, cyclohexane, isooctane, iso-octane/toluene mixture, may be successfully performed over rhodium coated foams, with high yields towards synthesis gas (>90% for pure fuels)25. The ratio between the total amount of C atoms and the total amount of O atoms in the feed was shown to affect the product distribution, with an optimum for hydrogen production at a unitary C/O ratio. Fuel rich conditions (C/O>1) result in lower reactant conversion, higher olefins selectivity with respect to the optimal C/O = 1 for syngas production. Vice versa, while lowering the C/O ratio, fuel conversion and deep oxidation product 5 ACS Paragon Plus Environment


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selectivities increased. The authors investigated the CPO of a commercial gasoline, finding catalyst deactivation during the experiment, due to sulfur and metal poisoning. Panuccio et al. investigated the CPO of octane isomers, as pure fuels and in a 1:1 mixture, over a broad range of C/O ratios (from 0.8 to 2), with focus on the effect pore size of the ceramic foams

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.

Fuel conversion decreased with increasing C/O ratio. Gas phase reactions, responsible for the formation of olefins, were found more important in the catalyst with larger pore sizes. A first attempt to measure spatially resolved species and temperature profiles for n-octane CPO in a packed bed reactor, has been carried out by Panuccio and Schmidt

27

. They found that Rh catalysts

exhibited higher hot spot temperatures and higher oxidation rate than Pt catalysts. The rate of hydrogen formation was also higher on rhodium catalysts with lower exit temperatures than on Pt. Importantly, ethylene, methane, propylene and butylene formation was also detected inside the reactor. Hartmann studied iso-octane CPO, finding that C/O ratio determines conversion and selectivities28. While oxygen was completely converted at all C/O ratios, iso-octane was entirely converted only at fuel lean conditions (C/O ratio lower than 1.1). At C/O 1.1 hydrogen yield was the highest. Methane was found to be the main by-product; minor amounts of iso-butylene, propylene and ethylene were detected formed. The authors employed a 2D reactor model, accounting for both homogeneous (only in the second part of the monolith) and heterogeneous reaction mechanisms. Model results revealed that catalytic reaction rates are very high at the reactor entrance, and the process is mass transfer controlled. Almost all data found in the literature for iso-octane CPO were collected in integral fashion17,26,28–30. In this work we apply the combination of spatially resolved sampling techniques and mathematical modeling of the adiabatic reactor in order to gain a better insight on the specific behavior of iso-octane in the CPO process. In particular, goal of this work is the comparison between methane CPO and isooctane CPO on Rh coated monoliths, focusing on the role of mass transfer limitations, the thermal management and catalyst deactivation. 6 ACS Paragon Plus Environment

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Methods Experimental Setup The catalysts used in this study consist of samples of 400/7 cordierite honeycomb monolith, coated with a 2% wt. Rh/α-Al2O3 active phase. The honeycomb samples are 2.5 cm diameter, 3.1 cm to 4 cm length. The catalytic powders are prepared via the incipient wetness impregnation of α-Al2O3 with a Rh(NO3)3 solution, dried in an oven over night at 120°C, mixed with water and nitric acid before undergoing to a 24 h long ball milling. The cordierite honeycomb is then coated through the dipcoating technique. Details about the catalyst preparation can be found elsewhere31. The monolith may be partly uncoated, in the rear part, thus forming a Continuous Back Heat Shield (CBHS); the measurement of the temperature profile along the length of the inert CBHS allows to verify the presence of heat losses across the reactor. Instead, the catalysts were fully coated at the entrance thus avoiding a Continuous Front Heat Shield, in order to reduce the hot spot temperature10. The catalysts were placed in a quartz pipe between a FeCrAlloy 15 ppi foam (located 1 cm upstream) and an inert cordierite monolith (Back Heat Shield, BHS, placed downstream). Overall, the adiabaticity coefficient of the reactor, evaluated as reported in previous studies12, was maintained above 90%. The whole quartz pipe is inserted in a 27 mm ID stainless steel reactor, insulated with quartz wool. Table 1 lists the geometric properties of the samples used in this study. The reactor is equipped with the spatially resolved sampling apparatus. One capillary, inserted in a central channel of the catalyst, may slide along the axial coordinate of the monolith 22. The capillary is fixed to a Vici Valco union fitting, joined together to a Zaber TLA-60 linear stage, which can move it along the reactor axial coordinate. The capillary sealed at the end can host an optical fiber or a K-type thermocouple. The optical fiber, whose tip is 45° ground, measures catalyst temperature, while the thermocouple gives measurement of gas-phase temperature. The capillary opened at its tip can withdraw gas samples, which are analyzed with an Agilent MicroGC 3000A, in order to determine the 7 ACS Paragon Plus Environment


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gas phase-concentration of different species. The MicroGC is equipped with two columns: a Plot U and a Molecular Sieve (with a Plot U pre-column) which use He and Ar, respectively, as carrier gases. The first column is operated at 60°C, in order to get better resolution for light hydrocarbons (CH4, C2H6, C2H4, C3H6, C3H4, C4H8), and then at 160°C, in order to get peaks with small tailing effect for isooctane. Nitrogen has been chosen as internal standard. Since the column Plot U is not able to separate nitrogen from methane, oxygen, carbon monoxide and hydrogen, the deconvolution of the sum peak is based on the response of the Molecular Sieve column. Mass balances are evaluated on atom basis for carbon, oxygen and hydrogen, as the ratio between the total amount of ith element in each sampling point and the total amount of ith element in the inlet. All balances are between 0.95 and 1.05 in case of methane CPO. Balances are between 0.9 and 1.05 at the catalyst outlet for every octane experiment. Near the catalyst inlet, carbon and hydrogen balances have higher errors. Based on our experience, this phenomenon has been associated with the incorrect measurement (systematically underestimated) of the of iso-C8H18 concentration where the capillary tip is close to the entrance, where the axial gradient is extremely high, possibly due to condensation in the sampling circuit. This is the reason why in the case of iso-C8H18 CPO tests, the local iso-C8H18 concentration is recalculated from the C-balance, that is by considering the consumption of input C that is needed to explain the measured product distribution. Hydrogen balance is then used to compute water mole fraction. All the experimental tests were performed with a standard flow rate of 10 Nl/min, corresponding to GHSV values between 36059 and 43038 (1/h) based on reactor volume basis, at the C/O ratio of 0.9. A Gilson Minipuls 3 peristaltic pump delivers the octane liquid feed into an evaporator, which consists in a 40 cm long ½ inch. pipe, filled with random packings, located inside an oven, whose set point temperature is 170 °C. Dilution nitrogen is fed to the evaporator, in order to ensure complete iso-octane evaporation. Before every test, the catalyst is preheated with a 6% v/v hydrogen and 3% v/v oxygen in 8 ACS Paragon Plus Environment

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nitrogen mixture, with a flow rate of 5 Nl/min. The catalytic combustion of hydrogen preheats the catalyst. As soon as the temperature measured by a rear thermocouple (placed 2 cm downstream of the back heat shield) reaches 200 °C in case of iso-octane CPO or 240 °C in case of methane CPO, the inlet mixture of the reactor is switched to the effective CPO feed mixtures. The procedure lasts approximately 20 to 30 min. Results are collected in steady-state conditions, 30 min after the feeding of the reactant mixture. High purity (4.5) nitrogen, air and methane were used. Liquid fuel was iso-octane (2,2,4trimethylpentane, 99.5% purity, analysis grade) from Carlo Erba Reactants.

Reactor Model and Kinetic Scheme A one dimensional, dynamic, fixed-bed, heterogeneous, single-channel reactor model was used for the analysis of the experimental results and for reactor design. The model has been developed in previous works

13,32

, validated against CH4 and C3H8 CPO measurements and applied to analyze the

impact of mass transfer, heterogeneous and homogeneous chemistry in the thermal behavior of the CPO reactor. The model consists of mass and energy balances for both solid phase and gas phase. It accounts for axial convection and diffusion, gas-solid transport term and solid conduction32. In this work, the model has been modified by the incorporation of the lumped reaction scheme of iso-C8H18 oxidation and steam reforming derived from previous studies8,14,33–35; the reactor model has been applied in a fully predictive way. The complete heterogeneous kinetic scheme is reported in the Supporting Information. Homogeneous reactions were instead neglected due to their limited impact under diluted test conditions.



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Results and Discussion Thermodynamic Equilibrium of iso-octane CPO Previous CH4 and C3H8 CPO experiments performed on Rh in the adiabatic reactor revealed that the thermodynamic equilibrium is approached within very short contact times12,13. An equilibrium analysis of the iso-octane/oxygen/nitrogen system has then been therefore performed by using the software Chemical Equilibrium with Applications (CEA) NASA

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. The analysis was

performed at constant enthalpy and pressure (adiabatic reactor), by assigning an inlet temperature of 85°C and a feed C/O ratio of 0.9, including the possible formation of solid graphitic carbon. Figure 1 presents the temperature and the selectivity to graphitic carbon (Panel a) and the selectivity to syngas, deep oxidation products and CH4 (panel b) as a function of nitrogen dilution (or iso-octane inlet concentration). Equilibrium temperature increases with decreasing N2 dilution (and increasing isooctane inlet mole fraction), from 520°C at 1% iso-octane feed mole fraction to 940°C at 4%, because of the global exothermicity of the CPO process. The amount of solid carbon decreases with decreasing nitrogen dilution and increasing temperature, and its formation becomes negligible below 35% N2 dilution, that corresponds to 0.03 iso-octane feed mole fraction. Selectivity to syngas (CO and H2) increases with decreasing N2 dilution. Equilibrium selectivities to CO and H2 equal to 95% and 92% respectively are obtained in undiluted conditions. The selectivity to deep oxidation products and methane consistently decreases with iso-octane mole fraction in the feed. In principle, the operation of the CPO reactor with undiluted feed mixture is desirable, since syngas productivity is maximized and C formation is avoided. However, the high equilibrium temperatures (in the order of 900°C), due to the overall exothermicity of the process, which increases with the number of C atoms of the hydrocarbon molecules, increase the risk for catalyst sintering and rhodium particles volatilization, with a reduction of surface area and loss of activity. Additionally, it is expected that a hot spot will establish at the reactor entrance with surface temperatures well above the equilibrium 10 ACS Paragon Plus Environment

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temperature. This is the reason why, in this work, we mostly operated the reactor at about 50% dilution, such that the catalyst stability was not affected and reproducible experiments could be performed for a detailed analysis of the reacting system. Less diluted experiments were then performed to verify the catalyst temperature profiles under more concentrated feeds, as well as the catalyst stability.

Comparison between methane CPO and iso-octane CPO Tests of methane CPO and iso-octane CPO were carried out on samples A and C respectively, with an inlet flow rate of 10 Nl/min and a C/O ratio of 0.9. Inlet temperature of the feed mixture was 25°C for methane CPO and 85°C for iso-octane CPO to avoid iso-octane condensation. Methane CPO tests were performed with an undiluted reacting gas mixture, i.e. by co-feeding only methane and air, while iso-octane CPO tests were carried out in diluted conditions, co-feeding 56% N2 with the reacting gas mixture, consisting of alkane and air (Table ). Results are shown in Figure 2, for methane CPO (panels a, c, e) and iso-octane CPO (b, d, f). Horizontal dotted lines, plotted at the right-end side of the graphs, represent thermodynamic equilibrium, while vertical dotted lines show the position of the catalytic washcoat. Both iso-octane and methane CPO exhibit similar features. Rhodium is a very active catalyst and equilibrium mole fractions were reached within 1.5 cm from the monolith entrance. The reactants were consumed (panels c, d) by total oxidation in the first 5 mm ÷ 6 mm of the catalyst, mainly producing water and carbon dioxide. Due to mass transfer limitations, which deplete oxygen at the catalyst interface starting from the catalyst inlet13, the excess fuel reacted with water, via steam reforming reaction, producing hydrogen and carbon monoxide (e, f). The stoichiometry of steam reforming reactions for the two fuels are responsible for the different H2/CO ratio, much higher in the case of methane CPO with respect to iso-octane CPO. At the catalyst outlet, a residual part of methane was



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unconverted, since its mole fraction at equilibrium is close to 0.024, while iso-octane was almost completely converted, consistently with equilibrium predictions. Two distinct reaction zones were present: •

the Oxy-Reforming Zone, located at the catalyst inlet, where oxygen is still present in the gas phase; here an overlap of oxidation and reforming reactions on the catalyst surface is realized;

•

the Reforming Zone, downstream in the catalytic bed, where oxygen is totally converted, and reforming reactions contribute to lower the temperature, and increase syngas productivity. The co-presence of exothermic and endothermic reactions in the Oxy-Reforming Zone results in the

onset of a hot spot of temperature on the monolith wall, near the catalyst inlet (panels a, b). The net release of reaction heat at the solid is progressively transferred to the flowing gas, where the temperature rises passing through a maximum located downstream from the catalyst hot spot. Further downstream, the steam reforming reaction prevails driving the catalyst temperature at values lower than the gas-phase temperature. Heat transfer then reduces the temperature radial gradient and favors the equilibration between solid and gas phase, which is reached in the ending portion of the catalyst. At the catalyst outlet, the pyrometer signal has a sudden drop since the tip of the optical fiber collects the radiation from the surrounding surfaces of the reactor, which are at lower temperature. The temperature measured by the thermocouple decreases as well, but at a lower extent, because of cupmixing phenomena, i.e. the mixing of gases of the central and outer channels which might be colder due to some residual heat dispersion of the system. Noteworthy, even though the iso-octane experiment at 2% was carried out at 56% N2 dilution, the hot spot temperature was higher than in the undiluted methane CPO test, despite of higher equilibrium temperature in the case of methane CPO test than iso-octane one (Table 2). In a previous study, Livio et al. have shown that propane CPO is characterized by higher hot spot temperatures than CH4 CPO

13

,

due to the impact of propane diffusivity (slower than that of methane) which reduces the rate of steam 12 ACS Paragon Plus Environment

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reforming and thus the rate of heat sinking in the oxy-reforming zone. It may be expected that a similar factor is also involved in iso-octane CPO and a dedicated analysis is addressed in a following paragraph. Homogeneous cracking reactions had a limited role in the diluted iso-octane CPO test; small concentrations of light alkanes (methane, ethane) and olefins (mainly propylene, iso-butene) were detected in correspondence with the gas-phase hot spot, as shown in Figure 3. Since gas temperatures were lower than 800 °C, a modest mole fraction of cracking products (< 1.1·10-3) was observed, which correspond to an overall C-based selectivity below 5%. Despite the small amount, the shape of the concentration curve was characteristic of the same formation-consumption sequence previously detected in propane CPO tests. In fact, light alkanes and olefins were produced at the catalyst inlet, via gas phase reactions, then consumed via heterogeneous steam reforming, leading to the production of synthesis gas. Concerning methane, this species was likely produced by cracking reactions in the gas phase within the first cm of reactor length, while it was further produced by a heterogeneous route of CO methanation along the rest of the monolith.

Modeling analysis of the CPO tests The CPO tests above illustrated were simulated and the predicted trends of temperature and concentration are reported in Figure 2 as solid lines. These are in good agreement with the axially resolved data, confirming, for both methane and iso-octane CPO, that the observed evolution of temperature and product distribution along the monolith are coherent with the indirect consecutive reaction scheme. In more detail, we observe that in the case of i-C8H18 CPO, some discrepancy between calculated and measured values is evident in the distribution between CO and CO2 at the very reactor inlet, the former being underestimated and the latter being overestimated. The oxygen mole fraction approaches zero at the coordinate 6 mm ÷ 7 mm, after which steam reforming reactions lower 13 ACS Paragon Plus Environment


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the temperatures and produce synthesis gas. The hot spot of temperature predicted by the model is slightly higher than that measured in the experiment, because of the assumption, in the reactor model, of the absence of any radiative heat loss from the catalyst inlet. This assumption well suits the case of monoliths with an inert continuous Front Heat Shield (FHS), i.e. an inert part of 5 mm length. However, in the present experimental setup, the continuous FHS was avoided on purpose. The relation between the reactor geometry and the heat dispersion has been studied by Livio et al.13. The difference of temperature between the experiment and the model, in correspondence of the hot spot, is consistent with our findings on methane CPO (30 °C). Even though methane and iso-octane have different reaction stoichiometry, the reaction schemes are qualitatively similar to each other. However, some of the physical properties of these molecules, such as gas diffusivity, are very different, and this has a significant impact on the reactor behavior. Model simulations allow a better understanding of the impact of transport phenomena in the CPO of methane and iso-octane. In analogy with the analysis performed in a previous work

13

, on the impact of mass transfer and

surface reaction in propane CPO, we evaluated the Carberry number of reactants. The Carberry dimensionless number for the generic ith reactant, consists in the ratio between the observed reaction rate and the maximum external mass transfer rate, as defined in equation (1) 38. ‫ܽܥ‬௜ = ௬

௬೔,್ೠ೗ೖ ି௬೔,ೢೌ೗೗

(1)

೔,್ೠ೗ೖ ି௬೔,೐೜ೠ೔೗೔್ೝ೔ೠ೘

Accordingly, the Carberry number is a measure of the degree of control of mass transfer limitations. When the Carberry number approaches 0, the consumption of the generic ith reactant is under chemical control, while when the Carberry number reaches 1, the global reaction rate is solely controlled by the external mass transfer. Figure 4 reports the evolution of the calculated concentration of reactants in the bulk and at the wall (a, b), Carberry numbers (c, d), reactant conversions (e, f) for methane CPO (a, c, e) and iso-octane 14 ACS Paragon Plus Environment

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CPO (b, d, f). As noted above, in the gas phase oxygen is completely converted across 5 mm, for the CPO of both fuels. Oxygen wall mole fraction drops down to zero immediately at the catalyst inlet, meaning that the surface oxidation chemistry is much faster than the gas-solid diffusion. As a result, the Carberry number of oxygen is equal to one in both methane CPO and iso-octane CPO (c, d). Oxygen consumption is controlled by its diffusion rate, and this explains why its conversion rate is the same in the CPO of the two alkanes. The Carberry number of methane is lower than 0.5 in all the positions along the reactor. This means that methane consumption is under a mixed chemical-diffusive control. This is in perfect accordance with past studies, for methane CPO on 400 CPSI monolith, coated with Rh /α-Al2O3 catalyst9. The Carberry number of iso-octane is close to one at the catalyst entrance, and it decreases at about 0.25 at 1 cm, being however always higher than that of methane. This implies that octane consumption is more sensitive to diffusional limitations than methane consumption. The reason for this is that iso-octane diffusivity is three times lower than methane diffusivity, hence its gas-solid transport is slower and kinetically limiting. Accordingly, the iso-octane conversion approaches more gradually the equilibrium value. The slower reforming rate associated with the higher diffusion control determines the higher hot spot temperature despite of N2 dilution.

Sensitivity analysis on molecular diffusion in iso-octane CPO A parametric analysis has been carried out using the reactor model, in order to highlight the impact of fuel diffusivity on transport phenomena and so on temperature profiles along the reactor. The simulation of the iso-octane CPO tests has been compared with two additional simulations wherein we assumed that the diffusivity of the fuel was equal to methane and to n-hexadecane diffusivity in N2, which are 3.22 and 0.69 times the diffusivity of iso-octane, respectively39.



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The other simulations parameters, such as catalyst geometry, reaction kinetics and operating conditions, are the same for all cases. Figure 5 displays conversions for oxygen (a) and iso-octane (b), and temperature profiles (panel c), along the reactor axial coordinate for the three cases. Expectedly, oxygen conversion profiles are perfectly overlapped in the three cases, since the oxidation rate is always completely controlled by the oxygen diffusion rate, which was not changed in the simulations. On the other hand, the fuel conversion rate decreases with the fuel diffusivity, because of the increasing impact of mass transfer limitations on the rate of reforming reactions. The reduction of the rate of endothermic reactions has a strong effect on temperature profiles. At the catalyst entrance, it is clearly observable that the hot spot of temperature is strongly affected by the fuel diffusion coefficient that is assumed in the simulation. The hot spot of temperature approaches 800°C when the diffusivity of methane is assumed, and increases up to 900°C when the diffusivity of n-hexadecane is taken. These results confirm that fuel diffusivity is very important in the CPO process, because it affects strongly the global efficiency of steam reforming and consequently the temperature profiles.

Effect of channel opening The effect of the channel hydraulic diameter was simulated, for both methane and iso-octane CPO, by keeping constant the void fraction (0.7) and the catalyst mass (0.7 g). The reactor diameter was set to 1 inch and the catalyst total length was set to 3 cm, with 0.5 cm inert continuous FHS, hence the assumed catalytic bed was 2.5 cm long. A Rhodium loading of 2% and a dispersion of 20% were also assumed. Simulations were performed at 10 Nl/min, atmospheric pressure, inlet temperatures of 60°C for iso-octane CPO and 25°C in the case of methane CPO. The fuel mole fraction was 0.27 and 0.02 for methane and iso-octane, respectively, with a C/O ratio of 0.9.


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Three cell densities, 200, 400, 900 CPSI, were analyzed corresponding the channel openings of 1.50, 1.06, 0.71 mm, respectively. The predicted effect of channel opening on the thermal behavior of the CH4-CPO reformer is reported in Figure 6, left panels. Results are fully in line with a previous study from Beretta et. al.9 and confirm that the channel diameter is a sensitive parameter in order to control the hot spot of temperature in methane CPO. In particular, large channel diameters are beneficial in lowering the hot spot. The decrease of cell density decreases the oxygen consumption rate by lowering the rate of gassolid mass transfer, while steam reforming (more chemically controlled) is affected to a lesser extent, thus reducing the hot spot of temperature. Figure 6 also reports the calculated gas and solid temperature profiles of the iso-octane CPOreformer at varying channel opening (right panels). A different trend was found, since, at increasing the cell density the calculated hot spot temperatures keep almost unchanged. In fact, given the important control of gas-solid diffusion on the rate of consumption for both oxygen and iso-octane, an increase of cell density, or a reduction of channel opening, results in faster oxidation and steam reforming reaction rates, higher reactant conversions and higher synthesis gas yields.

Effect of iso-octane feed concentration on reactor performance and catalyst stability Iso-octane CPO tests were performed on sample B at decreasing N2 dilution, which was reduced from 56% to 10.6%, while keeping constant the C/O ratio at 0.9. The fuel concentration was accordingly increased from 2% to 4%. Figure 7 reports the measured temperature profiles. Consistently with thermodynamic predictions, catalyst and gas-phase temperatures increased with decreasing nitrogen dilution. This was due to a greater heat release, while replacing inert gas (nitrogen) with reactive gases (iso-octane and oxygen).



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In parallel with the increase in the outlet temperature, the hot spot temperature also increased. At 2.5% iso-octane in the feed, a peak temperature higher than 900 °C was reached. At the highest reactant concentration (i.e. 4% iso-C8H18 in the feed), the surface hot spot was as high as 1050°C and it further increased to 1100°C, after 2 h time on stream. This was probably due to catalyst deactivation by sintering. The integral reactor performance was measured by sampling the product mixture at some distance from the monolith outlet; Figure 8 reports it in terms of reactant conversions (panel a) and yields to product species (b) as a function of iso-C8H18 feed mole fraction. Symbols represent the experimental results, while dotted lines represent the thermodynamic equilibrium. Oxygen conversion was complete in all the experiments. The octane conversion increased with the increase of iso-octane in the feed, an effect of the progressive increase of temperature. Hence the process became more and more selective; the productivity of H2 and CO increased and the product distribution closely approached the equilibrium. It is worth noticing that at the lowest temperature, corresponding to 0.02 and 0.025 iso-octane mole fractions in the feed, significant methane formation is predicted at equilibrium; however, methane was produced in much smaller amounts than the equilibrium values, which suggests a peculiar inhibition of the methanation route during the CPO of iso-octane. Reference methane CPO tests were repeated on the fresh catalyst and after each iso-octane test, in order to verify the catalyst stability. During the tests, temperature profiles and integral performance were monitored. Temperature measurements are shown in Figure 9; conversion and syngas selectivity were constant. Recalling that the hot spot of temperature is a sensitive function of catalyst steam reforming activity9, an increase of the hot spot temperature is an index of a deactivation process. We found that the temperature profiles were unaffected after the iso-octane CPO tests at 2% and 2.5% iso-C8H18 in the 18 ACS Paragon Plus Environment

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feed, while a progressive growth of temperatures in the initial zone of the monolith was observed after the experiment with 3%, 3.5% and 4% iso-C8H18 in the feed and the measured hot spot increased from 840°C to 870°C and 910°C. The probable cause of deactivation was Rh particles sintering, that, as suggested from the present and previous CPO experiments with light hydrocarbons, initiates when surface temperatures approach 900°C 13,40. This certainly represents the main critical issue related to the application of iso-octane CPO; the present results seem to exclude that stable operation of the reactor can be guaranteed with concentrated feed mixture and high space velocity as herein tested. While the N2-dilutionadopted in this study seems not a viable solution for vehicle applications, Rijo Gomes suggested that EGR dilution could represent a practical solution for maintaining the reactor temperatures below a safe threshold 6.

Conclusions The CPO of iso-octane was investigated on a rhodium catalyst, exhibiting high yields towards synthesis gas. The monolith reactor operated in nearly auto-thermal conditions, and it ensured a high throughput of synthesis gas. Due to the combustion-reforming reaction pathway, which causes heat release by oxidation and heat consumption by steam reforming, a hot spot of temperature was present in all experiments. Thermal management emerges as the most critical issue of the process: too harsh conditions, in terms of high reactant concentration and high flow rates, result in high front catalyst temperatures. The lower diffusion coefficient of iso-octane, compared to methane, is responsible for the important impact of mass transfer limitations on the global rate of steam reforming reaction. This results in a hot spot of temperature close to 1000°C under stoichiometric feed mixtures which is detrimental for the catalytic activity. In fact, the experience gained so far suggests that temperatures above 900°C cause rhodium particles sintering, hence loss of catalytic activity. In highly diluted feed mixtures, however, the catalyst does not reach complete iso-octane conversion, because of the slower kinetics. 19 ACS Paragon Plus Environment


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Modeling predictions revealed that the enhancement of reactant gas-solid mass transfer allows reducing the hot spot of temperature. Nevertheless, the employment of catalyst with higher cell densities may not be enough in order to avoid thermal sintering, and alternative strategies, such as water and carbon dioxide co-feeding, may be exploited.

Supporting Information Table S1: Catalytic kinetic scheme and rate equations for iso-octane CPO. This information is available free of charge via the Internet at http: //pubs.acs.org

Acknowledgements The financial support from the I2015 Microgen30 project is gratefully acknowledged. Experimental work performed by Marco Gerla, Luca Guzzetti, Simone Crespi and Alfredo Colaprete is gratefully acknowledged.


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Figure Captions Figure 1. Equilibrium calculations for iso-octane CPO at various nitrogen dilution; C/O=0.9, Tin=85°C. Figure 2. Experiments (symbols) and modeling results (lines) for methane CPO (catalyst C, left panels) and iso-octane CPO (catalyst A, right panels). Panels a and b show the temperature profiles, c and d indicate the reactant mole fractions and e and f represent the product mole fractions, along the axial coordinate of the reactor; flow rate = 10 Nl/min, C/O = 0.9; yCH4=0.273; yiso-C8H18=0.02; Tin,CH4=25°C, Tin,iso-C8H18=85°C. Figure 3. Axial molar fraction profiles of methane (panel a) and cracking species (panel b) in 2% isooctane CPO; flow rate = 10 Nl/min, C/O = 0.9; yiso-C8H18=0.02; Tin =85°C. Figure 4. Molar fractions, Carberry numbers and reactant conversions for CH4 CPO (left panels) and for iso-C8H18 CPO (right panels); flow rate = 10 Nl/min, C/O = 0.9; yCH4=0.273; yiso-C8H18=0.02; Tin,CH4=25°C, Tin,iso-C8H18=85°C. Figure 5. Sensitivity analysis of fuel diffusivity on iso-octane CPO: calculated reactant conversions and temperature profiles; flow rate = 10 Nl/min, C/O = 0.9; yiso-C8H18=0.02; Tin=85°C. Figure 6. Effect of the channel opening on the temperature profiles in the CPO of methane (a, solid temperature; c, gas temperature) and iso-octane (b, solid temperature; d, gas temperature); flow rate = 10 Nl/min, C/O = 0.9; yCH4=0.273; yiso-C8H18=0.02; Tin,CH4=25°C, Tin,iso-C8H18=85°C. Figure 7. Measured temperature profiles in CPO tests at increasing iso-C8H18 mole fractions in the reactant mixture; flow rate=10 Nl/min, Tin=85°C, C/O=0.9, catalyst B.


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Figure 8. Reactant conversions and product yields for iso-C8H18 CPO experimental tests; flow rate=10 Nl/min, Tin=85°C, C/O=0.9, catalyst B. In the legend, Yi, j is the yield of the i-species (i = H2, CO, CO2, H2O, CH4), referred to the j-atom (j = H, C). Figure 9. CH4 CPO tests performed on the fresh catalyst and after every iso-C8H18 CPO test; yCH4=0.273; flow rate=10 Nl/min, Tin=25°C, C/O=0.9, catalyst B.



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Table 1. Geometric properties of the catalysts. Catalyst

Cell density

LTOT

LCBHS

D

Washcoat

Void fraction

[CPSI]

[cm]

[cm]

[cm]

[mg]

[-]

Sample A

400

3.1

0

2.5

507.6

0.76

Sample B

400

4.02

0.42

2.5

615.5

0.75

Sample C

400

4.06

0.36

2.5

604.8

0.75

Table 2. Operating conditions for methane and iso-octane CPO. Alkane mole

O2 mole

Reacting

Reactant

C/O

Equilibrium

fraction [-]

fraction [-]

gases [%]

dilution [%]

[-]

Temperature [°C]

CH4

0.273

0.153

100 %

0%

0.9

688

i-C8H18

0.020

0.089

44 %

56 %

0.9

641

Alkane


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Figure 1.



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Figure 2.


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Figure 3.



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Figure 4.


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Figure 5.



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Figure 6.


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Figure 7.



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Figure 8.


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Figure 9.



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in an Adiabatic Reactor - ACS Publications - American Chemical Society

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