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Abstract: The external and internal mass transfer for a rotating foam stirrer reactor has been studied using the hydrogenation of 3-methyl-1-pentyn-3-ol as a model reaction. Catalytic foam stirrers with different number of pores per linear inch (ppi) and washcoating thicknesses are used and their performance are described in terms of activity and selectivity. The liquid–solid mass transfer does not limit the overall reaction rate when a 20ppi foam is used. For this catalytic foam stirrer, a k LS a LS value of 0.2s−1 is estimated at 300rpm. The use of finer pore foam structures is not convenient due to a drastic decrease of catalyst surface refreshment. As a result, the lower k LS value negates the benefit of a larger area (a LS) resulting in a lower liquid–solid mass transfer. The catalyst accessibility is a controlling factor in this fast reaction. The optimal washcoat thickness to avoid diffusion limitations is 5μm. Gas–liquid mass transfer rate is not a limiting factor. A k GL a GL of 0.6s−1 is measured at 300rpm by the physical absorption method. When compared to the conventional slurry reactor, the liquid–solid and the gas–liquid mass transfer rates are 17 and 6 times higher, respectively. However, the intrinsic activity of the foam catalyst is 4.6 times less than the slurry catalyst prepared with the same palladium loading. This is explained by a difference in palladium dispersion caused by a non-homogenous concentration of active sites along the washcoat layer. On the other hand, the less active foam catalyst leads to higher selectivity. The production rate is then increased by mounting more active blades on the stirrer shaft, thus creating more surface area for catalyst deposition while still keeping the high accessibility of a 4μm washcoat layer. Hence, rotating foam reactors have promising applications for multiphase processes. High mass transfer rates can be easily achieved, catalyst filtering is not longer needed and reusability of the foam catalyst is possible. [Copyright &y& Elsevier]

Abstract: Three-phase reactor designs based on rotating solid foams for the application in the fine chemical industry are developed. The aim is to use solid foams both as a catalyst support and stirrer in order to mix the gas and liquid phases and create fine gas bubbles. Gas–liquid mass transfer data are presented for different solid foam stirrer configurations and compared to an optimized Rushton stirrer. Solid foam stirrers were developed in a blade and a block design. Both foam reactor designs work at stirring rates below 600rpm. Using the foam blade design, gas bubbles are mainly created by the turbulence at the gas–liquid interface. Large bubbles are broken up by the foam blades. Using a foam block design, rotation leads to the structurization of the reactor volume into sections strongly differing in gas holdup, flow behavior and bubble size distribution. This results in a gas–liquid mass transfer, which is 50% higher than the Rushton stirrer used as comparison. The foam stirrer designs can be easily used in ordinary three-phase reactors and show a high potential for further optimization of the gas–liquid flow pattern and therefore for further increase of the rate of mass transfer. [Copyright &y& Elsevier]

Abstract: The novel monolithic stirrer reactor is a promising replacement for a conventional slurry reactor in multi-phase reactions. In this reactor monolithic structures are used as stirrer blades, creating a catalytic stirrer. The most important advantage of the monolithic stirrer reactor is the easy catalyst handling. The influences of different parameters on the performance of the monolithic stirrer reactor in the selective hydrogenation of 3-methyl-1-pentyn-3-ol were investigated. The decreasing catalyst layer thickness with increasing cell density proved to be beneficial for the performance of the monolithic stirrer reactor. An increasing stirrer speed caused an increase in the activity because of the higher flow through the monolith channels and the improved gas–liquid mass transfer. If the length of the monolithic stirrer blades was smaller than 0.03m the performance of the monolithic stirrer was enhanced. [Copyright &y& Elsevier]

Temporal analysis of products (TAP) is a transient pulse-response technique that allows to extract kinetic information from reacting and adsorbing systems. In a previous work (Chem Eng Sci 57(2002) 1835), a detailed-transport model for the Multitrack set-up, a TAP-like system, was developed, which allows studying systems with a low bed resistance. The use of structured beds, having both a low bed resistance and small sorbent particles, is required to determine adsorption and diffusion parameters when strong adsorption and slow diffusion occurs. A method is presented to extend the range of measurable adsorption and diffusion parameters in zeolitic sorbents in the TAP technique by a structured approach. Small zeolite crystals are coated on larger non-porous glass beads. Adsorption and diffusion parameters for n-butane, SF6 and 3-methylpentane have been determined in MFI-type zeolites. Absolute diffusivity values in the zeolite coating are estimated by using a well-defined silicalite sample as a reference to determine the effective diffusion length in the coating.Two criteria have been derived, one for the characteristic time for transport through the bed, 0.05 s, and one for the ratio of the latter and the characteristic diffusion time in the zeolite crystal, 0.01<α<200tbed/(1+tbed), which should be satisfied both to be able to determine values of the zeolite diffusivity. [Copyright &y& Elsevier]

A kinetic study into the styrene hydrogenation over a palladium on alumina catalyst has been made. Styrene was used as a model component for pyrolysis gasoline. A kinetic rate expression has been derived and the inhibiting effect of sulfur components has been included. Using this kinetics and mass-transfer models compiled from literature, the performance of two types of reactors for the styrene (pyrolysis gasoline) hydrogenation has been evaluated. A structured reactor such as a monolith has large advantages over a conventional trickle-bed reactor. For the monolithic reactor a more than 3 times higher volumetric productivity is obtained with much less catalyst. The modeling results indicate that deactivation by gum formation should be significantly less due to much better hydrogen mass transfer in the reactor. [Copyright &y& Elsevier]

A detailed transport model for the Multitrack setup, a TAP-like system, has been developed, which allows further analysis of adsorption, diffusion and catalysis phenomena. This includes the transport in the void part between the pulse valve and the reactor inlet. The effects of viscous flow and thermal transpiration, aspects that have not been studied in detail before for this type of setup, have been analyzed. A new expression for the modeling of the output signal is proposed depending on the positioning of the MS detector used in the study. The transport parameters of the model have been estimated by the analysis of the experimental pulse responses of the empty reactor system and the reactor charged with an inert packed bed. The proposed model reproduces the experimental pulse responses very well, and therefore can be extended to study systems with reacting or adsorbing beds by including the corresponding rate equations for the processes occurring in the bed of particles. [Copyright &y& Elsevier]

Solid-acid catalysts are attractive replacements for processes using conventional homogeneous catalysts. In the esterification of an alcohol with a carboxylic acid, however, the side product water strongly inhibits the activity of a solid-acid catalyst. Since an esterification is an equilibrium limited reaction, full conversion is not possible unless one of the products is removed. A novel reactor type, utilizing a solid-acid catalyst coated monolith, is presented in which water can be removed from the liquid reaction mixture by means of reactive stripping. In this manner the inhibition is eliminated and complete conversion can be reached. The advantages of this reactor concept are demonstrated by both experiments and reactor modeling. [Copyright &y& Elsevier]

The decomposition of hydrogen peroxide (H 2 O 2 ) on Pt, PtO and Au catalysts has been investigated in the presence of nitrogen, propylene and hydrogen. H 2 O 2 formation on the catalyst is known to be a key intermediate step for the direct synthesis of propylene oxide (PO) from hydrogen, propylene and oxygen. Therefore, during this reaction, H 2 O 2 is in contact with the catalyst on which it is produced, propylene and hydrogen. In this work we investigate the effect of the simultaneous presence of a metal catalyst (Pt, Au) and these gases on the H 2 O 2 decomposition. The presence of hydrogen favors the decomposition of H 2 O 2 over all the studied catalysts. This is attributed to the combination of direct decomposition and hydrogenation reactions. Furthermore, hydrogen changes the catalyst from an oxidized to a more metallic state accelerating the H 2 O 2 decomposition. We also observed the positive effect of propylene on decreasing the decomposition activity of Pt and Au. The experimental results were used to estimate reaction rate constants. The obtained rate constants hinted to changes of the catalyst during the decomposition as the main reason for reaction rate changes. This was confirmed to happen for Pt but not for the Au catalyst. The decrease of H 2 O 2 decomposition on Au in the presence of propylene is due to their interaction where propylene blocks the Au active sites responsible for the H 2 O 2 decomposition. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents the use of ultraviolet–visible spectroscopy (UV–Vis) spectroscopy in a slurry of particles, a packed bubble column, and a trickle bed to assess the changes in the state of an active component on the surface of the solid support. As a model system, insoluble pH indicators deposited on the particles and on a solid foam packing (used as the packing material in the packed bubble column and the trickle bed) are used which fluoresce different UV–Vis spectra according to the liquid pH. The experimental results indicate that for the slurry the UV–Vis spectra obtained from the moving particles can be used to characterize the state of the pH indicator and to determine the transition point. The UV–Vis spectra can also be used to characterize the concentration of particles. Bubbles in the packed bubble column result in disturbances in the UV–Vis spectra collected from the pH indicator adsorbed to the solid surface and this interference is removed successfully with a newly developed tolerance-and-averaging method. In the trickle bed the liquid film flowing over the solid surface does not disturb the UV–Vis spectra. An abrupt change in the state of the pH indicator is therefore observed successfully. [Copyright &y& Elsevier]

In the present work, the direct synthesis of propylene oxide (PO) in the liquid phase is successfully performed in a packed bed membrane reactor. We show that this engineering device can be implemented under mild reaction conditions (low temperature and pressure) and outside the explosive regime (low hydrogen concentration), making it appealing for industrial applications. The ceramic membrane allows for a separate feed of the reactants, therefore addressing the PO selectivity issue related to the propylene hydrogenation. The reaction is operated in a continuous methanol flow fed with propylene inside the tubular membrane; hydrogen and oxygen are fed through the porous material. We observe that the feeding strategy of the reactants has an important effect on key parameters such as PO selectivity and productivity. By separating propylene from hydrogen, the propane formation was reduced with respect to conventional packed bed reactors. Moreover, the addition of small amounts of NaBr to the reaction medium increases the catalytic activity to PO, but also to propane formation. Therefore, this study provides a good starting point in the design of a membrane reactor device for the direct synthesis of PO where the main limitations such as propylene hydrogenation and water formation can be addressed. [ABSTRACT FROM AUTHOR]

In the present work, the epoxidation of propylene with hydrogen peroxide in the presence of acids and halides is studied. The presence of acids and halides is indispensable for increasing the selectivity of the direct synthesis of hydrogen peroxide, the first step of the direct propylene oxide production from hydrogen, oxygen and propylene. Therefore, we focus on the systematic study of the effect of these compounds on the epoxidation of propylene with hydrogen peroxide in prospective of its future integration with hydrogen peroxide in-situ formed. Indeed, the epoxidation of propylene with hydrogen peroxide is the second step of the direct PO synthesis from hydrogen, oxygen and propylene. The acid concentration and type were shown to have an effect on the PO selectivity. Sulphuric and hydrochloric acids, typical compounds used to stabilize the hydrogen peroxide, are not suitable for the direct propylene oxide due to their fast dissociation in the solution. This leads to a fast ring opening reaction of the epoxide. Phosphoric acid was shown to have a much milder effect on the propylene oxide selectivity, therefore being an option for the direct propylene oxide synthesis. Also, the amount of halides (NaBr) needed to stabilize the hydrogen peroxide, does not have an effect on the epoxidation step. Therefore phosphoric acid and NaBr can be implemented in the direct propylene oxide synthesis from hydrogen, oxygen and propylene. [ABSTRACT FROM AUTHOR]

A method for the incorporation of zeolite coatings in glass chip-based microreactors has been developed. The use of a fluoride-based hydrothermal pretreatment method provides a microchannel surface with high roughness, good wettability, and a silica nutrient source in a single pretreatment step. This pretreated glass surface is ideal for the subsequent in-situ zeolite hydrothermal synthesis which enables the formation of zeolite coatings within the chip-based microreactor. The optimization of the zeolite precursor suspension in terms of aluminium and silica source and amount, allows for the formation of zeolite coatings which are uniform, have high surface area and high Brønsted acidity. [ABSTRACT FROM AUTHOR]

Using numerical simulations, a new membrane reactor is proposed for the direct synthesis of propylene oxide (PO) in liquid phase. The reactor is a combination of two consecutive catalytic reactor units, one for the hydrogen peroxide (H 2 O 2 ) synthesis on a Pd/SiO 2 catalytic membrane layer, and the second for the conversion of hydrogen peroxide with propylene (C 3 H 6 ) to PO on a titanium silicalite-1 (TS-1) catalytic layer. The membrane reactor is described numerically by a set of kinetic-diffusion mass balance equations. The optimization of the reactor design is achieved by determining membrane pore size, thickness and gas pressures which provide conversion and selectivity performance comparable to the industrial requirements. An optimal pore size of 0.2–0.4 μm was found for the Pd/SiO 2 membrane layer. The results show that a Pd/SiO 2 membrane thickness of 250 μm and a TS-1 layer of 100 μm are necessary to ensure conversion and selectivity performance of the catalytic membrane reactor comparable to the industrial ones. Calculated these optimized dimensions of the membrane reactor, a total membrane area of 84,000 m 2 is required for the production of 300 kton/year of propylene oxide. [ABSTRACT FROM AUTHOR]

Aqueous phase reforming of sorbitol for hydrogen production was conducted in a Pt–Ru-washcoated microchannel reactor with continuous hydrogen stripping. This study is an extension of our earlier investigation using the same reactor configuration with a monometallic Pt catalyst. The excellent mass transfer of hydrogen in the microchannel reactor operated under the Taylor flow regime by co-feeding nitrogen gas as stripping agent is highly beneficial for the selectivity to hydrogen. Nevertheless, the overall reaction rate is kinetically limited. Thus, more reactive conditions are needed to increase the productivity of hydrogen. The Pt–Ru bimetallic catalyst increases the catalytic activity by a factor of 2–3. However, in a reference case (with no hydrogen stripping), the Pt–Ru bimetallic catalyst resulted in a decrease of the hydrogen selectivity by a factor of 3 with respect to the reference monometallic Pt, leading to a nearly neutral effect in the productivity of hydrogen. By operating the Pt–Ru-washcoated microchannel reactor under the Taylor flow regime with simultaneous hydrogen production and hydrogen stripping we were able to maintain the benefits in catalyst activity while the selectivity to hydrogen was greatly increased with respect to the reference case with no stripping. The combined effect of a more active catalyst with the benefits of this reactor configuration in terms of selectivity led to an overall increase of the hydrogen production rate from 0.2 to 6.6 min −1 . Increasing the reaction temperature from 220 to 240 °C in this reactor configuration resulted in a further increase in the conversion of sorbitol and a mild increase in the selectivity to hydrogen (from 30 to 70% and from 35 to 39% respectively). The use of hydrogen stripping was essential to combine high activities and high selectivities. Increasing the nitrogen flow ratio with constant liquid flow rate was beneficial for the sorbitol conversion and the selectivity to hydrogen. The increase in sorbitol conversion reaches a saturation point at a gas to liquid ratio of ca. 1 m N 2 3 / m liq 3 , while the selectivity to hydrogen continuously increases in the range of 0–2 m N 2 3 / m liq 3 . This reactor configuration allows operating under more reactive conditions ( i.e. more reactive catalyst and higher reaction temperatures) without significant loss in hydrogen selectivity. [ABSTRACT FROM AUTHOR]

Abstract: Rotating solid foam reactors have already proven to show high mass transfer rates and to be a potential alternative to slurry reactors. The rotation of a foam block stirrer results in a high mass transfer and in the development of different reactor sections showing specific hydrodynamics and gas holdup distributions. In order to optimize the reactor system the hydrodynamics in a lab scale reactor are studied using tomography, a powerful method to measure the gas holdup in three-phase reactors. The influence of liquid properties, such as viscosity and surface tension, and the rotational speed on the gas/liquid distribution in the different reactor sections is investigated. Especially the viscosity has a strong effect on the entrapment of gas bubbles in the foam block structure, while the surface tension is the dominant parameter in the outer reactor section. The influence of these parameters on the inset of foaming and the collapse of the gas/liquid dispersion is investigated. Conclusions on the mass transfer performance are drawn and recommendations for further optimizations of the reactor design and the operational conditions depending on the liquid properties are developed. [Copyright &y& Elsevier]

Abstract: In this paper, the hydrodynamics and the pressure drop of liquid–liquid slug flow in round microcapillaries are presented. Two liquid–liquid flow systems are considered, viz. water-toluene and ethylene glycol/water-toluene. The slug lengths of the alternating continuous and dispersed phases were measured as a function of the slug velocity (0.03–0.5m/s), the organic-to-aqueous flow ratio (0.1–4.0), and the microcapillary internal diameter (248 and 498μm). The pressure drop is modeled as the sum of two contributions: the frictional and the interface pressure drop. Two models are presented, viz. the stagnant film model and the moving film model. Both models account for the presence of a thin liquid film between the dispersed phase slug and the capillary wall. It is found that the film velocity is of negligible influence on the pressure drop. Therefore, the stagnant film model is adequate to accurately predict the liquid–liquid slug flow pressure drop. The influence of inertia and the consequent change of the slug cap curvature are accounted for by modifying Bretherton’s curvature parameter in the interface pressure drop equation. The stagnant film model is in good agreement with experimental data with a mean relative error of less than 7%. [Copyright &y& Elsevier]