An analytical simple algebraic expression for isothermal effectiveness factor (q) in a porous pel... more An analytical simple algebraic expression for isothermal effectiveness factor (q) in a porous pellet is presented. Arbitrary kinetic expressions are investigated and the external mass transfer effect is also considered. The resulting analytical expression is found after matching asymptotic expressions of r) valid for large and small values of Thiele modulus. The agreement between approximated and numerical results is surprisingly good. For most cases analyzed maximum deviation are below 3% for power law type kinetic expressions provided the reaction order is greater than 0.5. More severe limitation arises for Langmuir-Hinshelwood kinetic expressions. In actual facts the proposed analytical expressions is unable to predict effectiveness factor greater than one. Nevertheless, as will be shown in future works, this very simple procedure can be safely used to predict the effectiveness in more complex situations such as those where activity distribution must be considered or where the kinetic parameters will be function of composition and/or spatial coordinates.
A previously presented method[l], to predict isothermal effectiveness factor with a single comple... more A previously presented method[l], to predict isothermal effectiveness factor with a single complex reaction in isothermal slab pellets is extended to encompass much more complex and general situations. In this work the geometry of the pellet can be arbitrary and a non uniform distribution of the catalyst is considered. Though the previously presented method[l] has had to be slightly modified to predict with great accuracy the effectiveness factor, (with less than 3% deviation from the exact values), an almost general and very simple algebraic expression is deduced to predict effectiveness factor values within 10% of their respective exact values. Thus for many applications in engineerin desi8n and catalytic reactor simulation, this simple general expression can be extremely useful since only one easily generated parameter is needed, as shown throughout the present contriiution.
−− In this contribution, effectiveness factor (η) calculations are performed by a perturbation an... more −− In this contribution, effectiveness factor (η) calculations are performed by a perturbation and matching technique developed by the authors which takes into account both the intrinsic kinetic expression and external heat and mass transfer resistances. The simplified method of Papadias et al. (2000) was used to consider the non-uniform wash-coat thickness usually present in monolith channels. As a result a global effectiveness factor (η o) is calculated at each point on the grid to simulate monolith reactor performance. The procedure was tested to predict experimental findings taking into account the actual kinetic expression to describe CO oxidation on Pt catalyst. Agreement among theoretical predictions and Ullah et al. (1992) and Holmgren and An-dersson (1998) experimental results are fairly good.
The hydrogenation of cyclohexene was studied in both gas and liquid phases on six different palla... more The hydrogenation of cyclohexene was studied in both gas and liquid phases on six different palladium catalysts supported on silica gel and alumina between 264 and 308 K at hydrogen pressures from 4 to 89 kPa, cyclohexene partial pressures from 0.46 to 1.5 kPa, and cyclohexene concentrations from 0.15 to 3.0 M. Under these conditions the turnover frequency is independent of particle size (1.5 to 10 nm) in both liquid and gas phases, of the nature of the support, and of the nature of nonaromatic solvents (methanol, n-heptane, ethyl acetate, and cyclo-hexene). The order of reaction was one-half with respect to hydrogen for the reaction in both liquid and gas phases and zero with respect to cyclohexene partial pressure (gas phase) or with respect to cyclohexene molar concent'ration (liquid phase). At the same temperature and hydrogen pressure, the turnover frequency was about twice as high in the gas phase as in the liquid phase.
In this contribution it is shown that the influence of the intra-phase (washcoat) diffusion - rea... more In this contribution it is shown that the influence of the intra-phase (washcoat) diffusion - reaction process is of crucial importance in the simulation and design of monolith reactors. Applying a simple but rigorous method for estimating global and intrinsic effectiveness factors, the bulk and washcoat surface concentration and temperature profiles, along a monolith channel is presented for the case of carbon monoxide oxidation. It is also shown how the global and intrinsic effectiveness factors varies, as a function of temperature and concentration, along the reactor. The simple 1D model was used. Comparison of simulated results with experimental data reported in the literature shows a very good agreement.
Cr/SiO 2 catalysts (4-8% Cr, w/w), obtained from Cr(III) salts and tetraethylortosilicate (TEOS),... more Cr/SiO 2 catalysts (4-8% Cr, w/w), obtained from Cr(III) salts and tetraethylortosilicate (TEOS), were prepared by the sol-gel method. After TEOS hydrolysis in ammoniacal solution (pH: 9.5), the gel was treated under hydrothermal conditions at 120-220 • C. The solids were characterized by N 2 sorptometry, TG-DTA, DRUV-vis, XRD, and FTIR. In vacuum or N 2 atmosphere, Cr 3+ ions were stable up to 450 • C. However, calcination in air promotes the partial oxidation of Cr 3+ to Cr 6+ as the temperature increases from 200 to 450 • C. The amount of Cr 6+ formed depends on the temperature of the hydrothermal treatment. 2,6-Lutidine adsorption allows to determine the evolution, with respect to calcination temperature, of both Lewis and Brönsted acid centers. Cr 3+ ions were related to the presence of Lewis sites, and Cr 6+ ions to the development of weak Brönsted acid sites. Accordingly, the 2-propanol dehydration reaction rate shows an increase when the catalysts are treated in N 2 at 450 • C, while catalysts calcined in air develop a lower acidity. The dehydration reaction is attributed to the presence of Lewis acid centers.
Latin American applied research Pesquisa aplicada latino americana = Investigación aplicada latinoamericana
A simple, precise and fast procedure to simulate monolith reactors is presented. The method allow... more A simple, precise and fast procedure to simulate monolith reactors is presented. The method allows the estimation of effectiveness factors (η) in monolith with washcoat of irregular geometries and arbitrary catalytic activity distribution. Catalytic washcoat with the same quantity of active material, deposited in different manners, are compared in their influence on monolith reactor performance. Intrinsic effectiveness factor estimations, with the approximate method, for first order reaction gave results very close to the rigorous 2D calculation. It is shown that differences between η values can be as much as 54% when non uniform catalytic activity distribution is considered. It is also shown the influence of different catalyst distribution on the behavior of a monolith reactor where the isothermal NO decomposition on Cu/ZSM-5 washcoat with complex Langmuir-Hinshelwood kinetic expression, is carried out. Estimated results are in close agreement with experimental findings. The influe...
Sol-gel followed by hydrothermal treatment let to obtain MCM-41 Cr/SiO 2 materials. The Cr +3 ion... more Sol-gel followed by hydrothermal treatment let to obtain MCM-41 Cr/SiO 2 materials. The Cr +3 ions are stable and resistant to segregation in air up to 300 1C. The Cr +3 ions produce Lewis acid sites that catalyze the 2-propanol dehydration. XPS determine the dispersion and the Cr +3 /Cr +6 ratio on the synthesized materials.
Nickel borides doped with transition metal (Cr, Mn, Fe, Co, Cu, and Zn) MNiB, 4% w/w respect to N... more Nickel borides doped with transition metal (Cr, Mn, Fe, Co, Cu, and Zn) MNiB, 4% w/w respect to Ni were prepared by chemical reduction of nickel and metallic salts in methanol solution with borane-tetrahydrofurane (BH 3 -THF) complex obtained in aprotic solvent (THF anhydrous). Different techniques were used to characterize these materials. The MNiB amorphous structure was verified by XRD. Thermal treatment in N 2 shows Ni˚, Ni 2 Band NiO phases. Spongy-morphology was evident by SEM studies in all cases. XPS and TEM show that doped and non-doped nickel boride was present as Ni˚ in metallic state as principal phase and nickel alloying with boron was observed in minor quantities. The oxidation stability, magnetic susceptibility and catalytic effect are correlated with parameters of each dopant metal.
Based on the well-known solution of Maxwell [1] for the thermal conductivity of dilute dispersion... more Based on the well-known solution of Maxwell [1] for the thermal conductivity of dilute dispersions of spheres and the improved form of Maxwell's equation developed by Chiew and Glandt [2], two general correlation for k eff estimation are proposed. One of them apply to medium dense dispersions (0.15 ≤ φ ≥ 0.85) and the other one for dense porous materials (φ > 0.9). Both correlating equations encompass a wide range of phase conductivity ratio values. The comparison with experimental measurements shows very good agreement.
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