C. Royo

Improved explicit equations for estimation of the friction factor in rough and smooth pipes

Abstract The most common correlations for calculating the friction factor in rough and smooth pipes are reviewed in this paper. From these correlations, a series of more general equations has been developed making possible a very accurate estimation of the friction factor without carrying out iterative calculus. The calculation of the parameters of the new equations has been done through non-linear multivariable regression. The better predictions are achieved with those equations obtained from two or three internal iterations of the Colebrook–White equation. Of these, the best results are obtained with the following equation: 1 f =−2.0 log e/D 3.7065 − 5.0272 Re log e/D 3.827 − 4.567 Re log e/D 7.7918 0.9924 + 5.3326 208.815+Re 0.9345 .

Acetylene hydrogenation on Ni–Al–Cr oxide catalysts: the role of added Zn

Abstract Ni-containing catalysts for selective acetylene hydrogenation to ethylene have been prepared by controlled calcination of hydrotalcite-like precursors. In addition to Ni and Al, Cr and Zn were added to improve the catalytic performance. The hydrotalcite-like precursors and the calcined catalysts have been characterized by powder X-ray diffraction, FT-IR and Vis–UV/Diffuse Reflectance spectroscopies, temperature-programmed reduction, and specific surface area assessment by nitrogen adsorption at 77 K. Despite addition of Zn hinders coke formation, the activity to gaseous products decreases as the Ni content is increased. An increase in coke concentration increases activity and selectivity to ethylene, specially in those samples with not too high Ni contents. The highest selectivity to ethylene is achieved for Zn/Ni≈4 (molar ratio).

Fourier transform infrared spectroscopic study of coke deposits on a Cr2O3Al2O3 catalyst

Abstract The process of coke deposition during the dehydrogenation of butene to butadiene over a chromia-alumina catalyst has been studied by Fourier transform infrared spectroscopy. The study was carried out on samples of coked catalyst dispersed on KBr pellets. The aromatic CC stretching (1630-1500 cm −1 ) and aliphatic CH bending (1450-1300 cm −1 ) regions of the spectra were examined by curve-fitting analysis in order to improve the information provided by Fourier transform infrared spectroscopy. Different steps in the process of coke deposition were observed. At low temperature the coke is formed by polyolefinic chains rather than by aromatic structures. Aromatization of coke takes place with increasing temperature and can be followed by observing the peak at 1590 cm −1 . The aging of the coke in nitrogen does not produce modifications in its aromaticity and the observed dehydrogenation is due to the loss of aliphatic structures as is revealed by the 1385 cm −1 band.

Unraveling the growth of vertically aligned multi-walled carbon nanotubes by chemical vapor deposition

The interaction between the main operational variables during the growth of vertically aligned multiwalled carbon nanotubes (VA-MWCNTs) by catalytic chemical vapor deposition is studied. In this contribution, we report the influence of the carbon source (i.e. acetylene, ethylene and propylene), the reaction/activation temperature, the rate of heating, the reaction time, the metal loading, and the metallic nanoparticle size and distribution on the growth and alignment of carbon nanotubes. Fe/Al thin films deposited onto silicon samples by electron-beam evaporation are used as catalyst. A phenomenological growth mechanism is proposed to explain the interaction between these multiple factors. Three different outcomes of the synthesis process are found: i) formation of forests of non-aligned, randomly oriented multi-walled carbon nanotubes, ii) growth of vertically aligned tubes with a thin and homogeneous carbonaceous layer on the top, and iii) formation of vertically aligned carbon nanotubes. This carbonaceous layer (ii) has not been reported before. The main requirements to promote vertically aligned carbon nanotube growth are determined.

Effect of thermal aging upon the regeneration kinetics of a coked Cr2O3Al2O3 catalyst

Abstract The effect of thermal aging on the combustion kinetics of coke deposited on a commercial Cr 2 O 3 Al 2 O 3 catalyst has been studied. Catalyst aging was produced in several operation and regeneration cycles involving high temperatures and the alternate use of reducing or steam-containing oxidizing atmospheres. As a consequence of aging, the intrinsic regeneration constant decreased, while the activation energy remained unchanged compared with that of the fresh catalyst. This indicates that the regeneration mechanism is not modified, and the effect of catalyst aging is to reduce the number of active regeneration sites on the surface through a partial sintering process.

Kinetics of catalyst regeneration by coke combustion. II. Influence of temperature rise in the catalyst particles

A grain-pellet model has been used to study the effect of high reaction rates upon the temperature profiles developed during regeneration of coked catalyst particles. The possibility of falsification of kinetic data is discussed in view of the results obtained.AbstractИспользовали модель типа таблетки-зерна в рассмотрении влияния высоких скоростей реакций на температурные профили, образующиеся в ходе регенерирования зауглероженных катализаторов. На основе полученных данных обсуждают возможность фальсификации кинетических данных.

Effect of the Operating Conditions on the Growth of Carbonaceous Nanomaterials over Stainless Steel Foams. Kinetic and Characterization Studies

Abstract This work is an advance on the development of structured catalytic reactors. Here, we present the results of the effect of the main operational variables (reaction temperature, % H2 and % C2H6) on the kinetics of carbonaceous nanomaterials (CNMs) formation by catalytic decomposition of ethane over stainless steel foams. Some of the main drawback problems that occur during the operation of chemical structured reactors are related to the preparation of long term stable coatings. The washcoating is the most used technique to deposit the catalytic layer over the substrate. The application of this procedure is quite complex in the case of geometries such as foams or cloths. In the case of the deposition of layers of carbonaceous nanomaterials, an alternative route, avoiding the washcoating, is their direct growth by catalytic decomposition of light hydrocarbons over the surface of the metallic substrate. In the case of structured steel foams, the substrate already contains the catalytic active phases for this reaction, like Fe and Ni, among of the minor components (Cr, Mn, Mo) that can act as promotors/stabilizers. The nanomaterials obtained after reaction were characterized by Raman spectroscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The characterization results indicate that there is a maximum, obtained at ca. 900 °C, 33.3 % of C2H6 and 1.7 % of H2, in the quality of the carbonaceous nanomaterials grown. Under these conditions, the CNMs consist mainly of few layer graphene (FLG) and graphite nanolayers (GNL) encapsulating the metallic nanoparticles. In addition, the kinetic results indicate the existence of another optimum, at ca. 800 °C, 33.3 % of C2H6 and 1.7 % of H2, in the productivity to the carbonaceous nanomaterials. The existence of these optimums is due to the driving force for the diffusion of the carbon atoms through the Fe-Ni nanoparticles (NPs) obtained at high temperatures (e. g. above 800 °C) caused by the competence between two opposite phenomena: the increase of the rate of carbon diffusion through the metallic nanoparticles of Fe-Ni and the deactivation of these nanoparticles. The deactivation is the consequence of the encapsulation and reconstruction of the nanoparticles during the formation of the several types of CNMs. The evolution of the carbon mass during the reaction time was analyzed using a phenomenological kinetic model that takes into account the main stages involved during the formation of carbonaceous nanomaterials: hydrocarbon decomposition, carburization, diffusion, precipitation and deactivation. The results obtained from the kinetic model, along with the characterization results, enable quantify the influence of the operating variables on each stage of the carbonaceous nanomaterial formation and therefore open the way to optimize the process.