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• W2004391170 abstract "In the HPO process of the DSM, nitrate is reduced to hydroxylamine on Pd/C catalyst in a 5 M salt solution. Pd/C catalyst increases the interfacial area by a coalescence hindering effect. Nevertheless, mass transfer limitation is encountered and Pd/C catalyst in the liquid bulk is not just useless but even catalyses product decomposition. In this study, the more selective Pd/Al2O3 catalyst was silanised with trichloromethyl silane to reduce its wettability and enrich the catalyst at the gas–liquid interface. The silanization did not affect the catalyst activity. In a stirred tank reactor, significantly higher hydrogen absorption rates were achieved for a flat interface but the effect decreased at higher stirring speed. Dans le procédé d'oxime au phosphate d'hydroxylamine (HPO) de DSM, le nitrate est réduit en hydroxylamine dans un catalyseur Pd/C dans une solution saline de 5 M. Le catalyseur Pd/C accroît la zone interfaciale par le biais d'un effet qui entrave la coalescence. Néanmoins, on atteint la limite du transfert de masse et le catalyseur Pd/C dans la solution mère liquide n'est pas seulement inutile, mais il catalyse également la décomposition du produit. Dans cette étude, le catalyseur Pd/Al2O3 plus sélectif a été silanisé avec du trichlorure de méthylsilicium afin de réduire sa mouillabilité et d'enrichir le catalyseur à l'interface gaz-liquide. La silanisation n'a pas eu d'effet sur l'activité du catalyseur. Dans un réacteur agité, on a atteint des taux d'absorption d'hydrogène beaucoup plus élevés pour une interface plate, mais l'effet a diminué à des vitesses d'agitation accrues. Catalytic slurry reactors are used in important industrial processes like methanol and Fischer–Tropsch synthesis because of almost isothermal operation and easy exchange of the suspended catalyst. The design of such reactors requires knowledge not only of the reaction kinetics but also of the mass transfer characteristics as mass transfer often limits the conversion rate. The suspended fine particles may significantly affect gas–liquid mass transfer. Alper et al. (1980) found the absorption rate in a stirred cell with a flat gas–liquid interface to increase upon the addition of fine activated carbon particles. The authors postulated an adsorptive transport mechanism in which the particles adsorb gas at the interface and release it in the liquid bulk (“shuttle” or “grazing” effect). However, Kaya and Schumpe (2005); Kordač and Linek (2006); Rosu and Schumpe (2007); and Rosu et al. (2007) recently demonstrated the mechanism to be “surfactant-grazing.” Activated carbon (but also graphite with no adsorption capacity for the gas) can adsorb surface-active contaminants that induce surface rigidity. The surfactant removal leads to higher kL-value at a clean interface. In stirred cells with a higher degree of turbulence, hydrodynamic effects play an important role. The presence of suspended particles may decrease the effective liquid film thickness and thus increase the kL-value (Alper and Öztürk, 1985; Schumpe et al., 1987a). In the presence of gas bubbles, suspended particles can affect the coalescence rate. At high solids loadings, high viscosity of the slurry induces bubble coalescence (Öztürk and Schumpe, 1987). At low solid concentrations, Schumpe et al. (1987a) found an increase of the kLa-value by adding activated carbon to coalescence hindering liquids, specifically, electrolyte solutions. The effect was explained as coalescence hindering by particles sticking to the bubble surface, also termed “particle to bubble adhesion” (Wimmers and Fortuin, 1988a,b; Vinke et al., 1991; Ruthiya et al., 2003). It depends on the surface tension, density and viscosity of the liquid, ionic forces and the wettability of the particles. On the other hand, strictly non-wettable polymer particles can even block the interface leading to a decrease of the volumetric mass transfer coefficient (Schumpe et al., 1987b). In absorption with fast chemical reaction, the absorption rate can be influenced by suspended catalyst particles. Adhesion of the particles to the gas–liquid interface leads to a higher local concentration of the catalyst in the boundary layer where the reaction occurs. Hence, reducing the catalyst wettability can lead to higher absorption rates (Vinke, 1992; Beenackers and van Swaaij, 1993). The reaction is a step in the HPO-process (hydroxylamine–phosphate–oxime) of the DSM for ε-caprolactame production. Working in the mass transfer controlled regime leads to decomposition of hydroxylamine in the absence of hydrogen in the bulk of the liquid. In the 5 M salt concentration used, Lindner et al. (1988) reported on a triplication of the volumetric mass transfer coefficient kLa due to bubble coalescence hindering in the presence of activated carbon. This extends the regime of chemical reaction control considerably. In this study, absorption experiments are carried out under kinetic and chemical reaction control using hydrophilic and hydrophobic support materials. The influence of the particle wettability is studied in a stirred cell with either a planar gas–liquid interface or a bubble dispersion. All experiments were carried out in a steel autoclave with an inner diameter of 8.5 cm and a total volume of 414 mL. The liquid volume (250 mL) was mixed with a magnetic stirrer of 5 cm diameter. Only the centre of the stirrer rested at the bottom; the rotational speed N could be adjusted in the range of 50–1200 min−1 (±2%). The temperature was measured with a Pt-100 sensor and controlled at 333.2 K by circulating liquid through the reactor's jacket and a thermostat. The head pressure was measured with an absolute pressure sensor (Sensotec, Columbus, OH, USA, STJE, 0–1.38 MPa ± 0.05%). Hydrogen gas (99.999%) was absorbed in a buffer solution (pH 1.13) composed of 2 kmol m−3 H3PO4 (Sigma–Aldrich, Munich, Germany, 98%), 2 kmol m−3 NH4H2PO4 (Fluka, Munich, Germany, puriss. p.a., ≥99.0%) and 2 kmol m−3 NH4NO3 (Fluka, puriss. p.a., ≥99.0%). The effect of suspended particles with different surface hydrophobicity on gas–liquid mass transfer was investigated using activated carbon (Riedel–de Haen, Seelze, Germany, p.a. puriss), activated carbon loaded with 10 wt% palladium (Fluka), γ-Al2O3 (Sigma–Aldrich, Munich, Germany, 99.7%) and γ-Al2O3 loaded with 10 wt% palladium (Aldrich). The particle size of the activated carbon and the Pd/C catalyst, studied with a scanning electron microscope, was around 20 µm, whereas the γ-Al2O3 and Pd/γ-Al2O3 particle size was about 10 µm. The Pd/C-catalyst was available only in oxidic form; it was activated by reduction with 2 M sodium formate. The wettability of alumina was modified by silanization. Trichloromethyl silane (TCMS, Aldrich, 99.0%) was deposited on the particles from CHCl3 solution. Using different concentrations of TCMS, the level of hydrophobicity could be varied. Thirty-five microlitres (0.1 vol%) or 105 µL (0.3 vol%) TCMS in 35 mL chloroform (Aldrich, water-free, 99+ %) were stirred with 7 g of the alumina powder for 2 h at 323 K. Then, the particles were dried at 6 kPa and 333 K for 4 h. The silanised solids were stored at room temperature under nitrogen. The surface modification was characterised by contact angle measurements with the sessile drop method (Krüss, G-23). The mean values of receding and advancing contact angles are shown in Figure 1. Pd/Al2O3 and the pure support material Al2O3 are hydrophilic whereas activated carbon and even more the Pd/C catalyst are slightly hydrophobic. Chemical modification of the alumina surface with different amounts of TCMS decreases the wettability and leads to contact angles slightly lower (0.1 vol% TCMS) or higher (0.3 vol% TCMS) than those for the Pd/C catalyst. Liquid–solid contact angles of silanised Al2O3 and Pd/Al2O3 as compared to activated carbon and Pd/C catalyst. Neither for a planar interface nor in bubble dispersions unmodified alumina shows any effect on the volumetric mass transfer coefficient at the low concentrations studied (Figures 2 and 3). Activated carbon more than triples the volumetric mass transfer coefficient by surfactant grazing for the planar interface and almost doubles it by coalescence hindering in bubble dispersions. Note that coalescence hindering is hold responsible based on previous findings (e.g., Schumpe et al., 1987a; Lindner et al., 1988) while the present results do not allow to exclude effects on kL. Linek et al. (2008) found activated carbon to increase kL in bubble dispersions but the effects (10–60% increase) were smaller than the increase of kLa observed in this study. Solids' effects on the liquid-side mass transfer coefficient at a planar interface (N = 100 min−1). Coalescence hindering effect of silanised alumina as compared to activated carbon particles (N = 600 min−1). Silanised alumina leads to a 50% increase for a planar interface but in bubble dispersions the effect is in the order of 10%, only, although the wettability is similarly low as that of activated carbon. Obviously the wettability (contact angle) is not the only characteristics of the solid to be considered. The activity of Pd/Al2O3 catalyst is intrinsically lower than that of Pd/C catalyst, however, the silanization did not affect the catalyst activity studied in the kinetic regime (Figure 4). The Pd/C catalyst shows surfactant grazing at a flat interface and coalescence hindering in bubble dispersions. These results are not shown in order to focus on the silanised alumina-supported catalyst. Effect of the silanization on the activity of Pd/Al2O3 catalyst. In the stirred tank with flat G/L interface (N = 100 min−1), enrichment of silanised Pd/Al2O3 catalyst leads to significantly higher hydrogen absorption rate (Figure 5). As compared to the maximum physical absorption rate (), up to a threefold increase is achieved at a catalyst concentration of only 10 kg m−3. The hydrogen absorption rate is higher than at the fourfold concentration of the unmodified catalyst. This may be attributed to a locally higher concentration of the hydrophobic catalyst particles at the gas/liquid interface. Normalised absorption rate in the stirred tank with flat G/L interface (N = 100 min−1). In the stirred tank with dispersed bubbles (N = 600 min−1), the effect of silanised Pd/Al2O3 is at most 10% higher than that of the unmodified catalyst (Figure 6). Note that in the normalised absorption rate the 10% increase of kLa due to silanization is already corrected for. The overall effect on the absorption rate is thus in the range of 20%. The surprisingly low effect might indicate that the more dense Al2O3 particles show weaker bubble adherence than activated carbon at similar particle wettability. However, it should be noted that the average particle sizes used in this study, 20 µm for Pd/C and 10 µm for Pd/Al2O3, are relatively large as compared to the film thickness and the observed effects might depend on the fractions of fines. At a smaller average particle size the enhancement might be stronger. Normalised absorption rate in the stirred tank with dispersed bubbles (N = 600 min−1). Since silanised alumina shows little coalescence hindering, the effect of adding Pd-free activated carbon powder was checked. In slurries of unmodified Pd/Al2O3 catalyst, the coalescence hindering effect of activated carbon was strong, however, in slurries of the silanised Pd/Al2O3-catalyst the effect was only minor (Figure 7). This might indicate that the silanised particles suppress bubble adhesion of activated carbon (in contradiction to the explanation considered for Figure 6). The low wettability of silanised Pd/Al2O3 and activated carbon powder might also induce higher particle-to-particle adhesion. Further studies are desirable to directly observe the surface coverage of the bubbles and possible particle–particle interactions. Anyway, the silanised catalyst does not appear attractive for industrial application in the HPO process. Effect of adding Pd-free activated carbon on the normalised absorption rates in Pd/Al2O3 slurries (kLa is for the solids-free system). Silanization of alumina-supported palladium catalyst allowed reducing the wettability to that of the carbon-supported catalyst used in the industrial production of hydroxylamine (HPO process). Lower wettability should result in higher catalyst concentration in the liquid film and stronger mass transfer enhancement. This was indeed observed in the stirred cell with a flat interface thus showing the principal feasibility. Since for dispersed bubbles the effect was only weak and the silanised catalyst additionally lags the coalescence hindering effect of the carbon-supported catalyst, its industrial application in the HPO process is not attractive. However, the coalescence hindering effect of activated carbon particles added to slurries of other heterogeneous catalysts might be interesting for other industrial processes. specific interfacial area (referred to liquid volume) (m−1) equilibrium hydrogen concentration in the liquid (mol m−3) solids concentration (kg m−3) molar flux (mol m−2 s−1) liquid-side mass transfer coefficient (m s−1) stirring speed (s−1) pressure (Pa) gas constant (J mol−1 K−1) volumetric absorption rate (mol m−3 s−1) time (s) temperature (K) volume (m3) final (equilibrium) value gas initial value liquid" @default.
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• W2004391170 title "The effect of Pd-catalyst wettability on hydrogenation of nitrate to hydroxylamine" @default.
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