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Networks of microdevices can be used to form, sort, split and recombine droplets by varying the fluidic resistance of the channels. To facilitate design of such systems we propose two models to calculate pressure drop of liquid-liquid slug flow in square microchannels. By approximating a droplet surrounded by liquid film as annular flow, the moving film model includes the velocity profile in the lubricating layer. In the no-film model, the presence of liquid film is neglected. Pressure drop measurements of water-toluene and water-silicone oil slug flow generated in glass/silicon devices of 200 and 400 μ m width serve to validate these two models. We found that the droplet viscosity influences the applicability of the models. Good agreement is obtained for water-toluene slug flow, with the average error of 15–20% between the measured and calculated values for both models, proving that the presence of the lubricating film may be neglected for flows with similar viscosities of both phases. On the contrary, both predictions agree poorly with experiments for silicon oil-water flow which we attribute to the lower than expected liquid film thickness around the silicon oil droplets. Therefore we use the moving film model to estimate the thickness of the lubricating layer which we find to lie between 0.5% and 0.75% of the channel width, lower than the assumed 2%. Finally we compare the performance of our models against two correlations adapted from literature. We thus demonstrate that our approach allows reliable pressure drop prediction for liquids with similar viscosities and delivers important information on flow hydrodynamics.

This work presents a study on the continuous operation of a structured reactor for alkyne hydrogenation in the field of Process Intensification. The reactor consists of a laser sintered metal structure characterized by a regular geometry, coated with a layer of ZnO/Al2O3 and impregnated with palladium nanoparticles. The partial hydrogenation of 2-methyl-3-butyn-2-ol with co-current gas-liquid upward flow was used as the test reaction system. A plug flow reactor model was applied to study the mass transfer phenomena under the reacting conditions. The reaction kinetics with the Pd/ZnO-based catalyst were simplified using a power rate law expression. The results in terms of the overall mass transfer coefficient Kov were modelled with a predictive Sherwood number correlation whose parameters were estimated by means of an optimization procedure. The structured reactor shows an overall mass transfer coefficient ranging between 0.2 and 1.2 s−1 depending on the operating conditions. The model is able to predict the impact of temperature (333–363 K), pressure (3.0–7.0 bar), gas velocity (0.005–0.024 m s−1) and liquid velocity (0.025–0.085 m s−1) on the overall mass transfer coefficient with a maximum deviation of 15%.

This work presents a study of the hydrogenation of 2-methyl-3-butyn-2-ol in flow over a structured catalyst. A porous tubular metal structure is coated with a layer of ZnO/Al 2 O 3 and impregnated with palladium. This reactor is operated with recirculation of the process liquid and continuous supply of hydrogen with co-current upward flow. A Langmuir-Hinshelwood mechanism based on noncompetitive adsorption of hydrogen and organics is used to model the experimental results. The kinetic and adsorption parameters governing the process are estimated in the kinetic regime by means of a numerical optimization procedure. The extension of the model to the mass transfer limited regime, at lower hydrogen flowrates, allows the estimation of an overall mass transfer coefficient K ov . The reaction appears to be strongly dependent on the hydrogen flowrate with estimated K ov lower than 0.25 s −1 in the range of flowrates 0.1–0.5 nL min −1 . The mathematical model, successfully validated, is able to accurately predict the concentration profiles of the species involved in the system at varying temperatures, in both kinetic and mass transfer limited regimes.

The axial dispersion in tubular reactors is frequently modeled by the dispersion model, which is an analytical solution under the assumption that the vessel dispersion number is sufficiently small. In this study, limits until which the analytical solution is valid are investigated by comparison with the numerical method. The results show that it is applied only if the vessel dispersion number is small enough and the entrance effect of the tracer is avoided. Therefore, an algorithm based on the numerical solution of the control equation of dispersion is proposed, which allows for higher vessel dispersion numbers.

Isophytol is a key intermediate in the industrial synthesis of vitamins, produced by selective hydrogenation of dehydroisophytol. This work presents a study of the kinetics of this process in the absence of any solvent. The experimental data were modeled using power-law kinetic expressions and mechanistic models based on Langmuir–Hinshelwood kinetics. The hypothesis of noncompetitive adsorption between hydrogen and organics was chosen in order to describe the changes in the reactivity of the system at varying temperatures (313–353 K) and pressures (5.0–9.0 bar). A simplified approach was used to describe the kinetics of the overhydrogenation reaction to the corresponding alkane. A list of the relevant kinetic and adsorption parameters, the result of an optimization procedure, is reported. The mathematical model, successfully validated, allows the prediction of the influence of the operating conditions and of the addition of quinoline, as reaction modifier, on the selectivity of the process.

In the frame of process development for the generation of chemical energy carriers, we studied the synthesis of methanol and formic acid from methyl formate hydrolysis in a continuous-flow millireactor. The aim was to establish the link between kinetics and phase behavior in the biphasic liquid regime. Reaction performance using the acidic ion-exchange resin Amberlyst 15 as catalyst was examined at various operating conditions such as space velocity, catalyst particle size, temperature, and initial reactant ratio. Results revealed substantially higher yields without mass transfer impediment with feed compositions exceeding methyl formate saturation in water. The simultaneous decrease in methyl formate and increase in polar product concentrations sufficed to bring the initially biphasic mixture to a homogeneous system as confirmed by thermodynamic UNIFAC equilibrium calculations. The separate analysis of effluent liquid phases unveiled a quasi-homogeneous catalytic process rooted in an aqueous layer at the...

Electrochemical micro-probes are used to study the flow regime and liquid–solid mass transfer inside a designed porous structure. In order to place the electrodes inside the porous structure avoiding gluing and polishing, the electrodes are glued flush with pins. These pins are then simply plugged into the structure. In a first step, the influence of the pin on the local flow behavior is studied. It is shown, that the departure from the laminar flow regime can be determined within 10% accuracy in empty pipes. Additionally, the influence of the pin length on the point of transition is studied inside the porous structure showing errors below 10% for pin lengths with an accuracy of ±0.5 mm. In a second step, the flow regime is determined with a simple algorithm, allowing a reproducible detection of the laminar plateau and the onset of fluctuations. The departure from the laminar regime is found at Re lam , end = 162 ± 3 . In the third and last step, the liquid–solid mass transfer is studied for a single-phase liquid flow and gas–liquid two-phase flow. It is shown, that the second phase is enhancing the mass transfer at low volumetric transport fractions but degrading the mass transfer at volumetric transport fractions higher than 6%. The Sherwood number is compared to the literature and correlations are presented to describe the Sherwood number in the laminar and turbulent flow regime.

Highly porous structures (porosity ≥75%), as metal foams or designed foam-like structures, are often used in industry to augment heat and mass transfer. In the present study, we focus on the mixing in highly porous structures in continuous millireactor characterized with a relatively low Reynolds number (ReD=240 – based on the ligament diameter and bulk velocity) and a high Schmidt number fluid (Sc=2400). We apply a combined numerical and experimental approach. The numerical simulations employ a Large Eddy Simulation (LES) method with dynamic Lagrangian approach for subgrid-scale turbulent stress and turbulent mass flux. The results of the numerical simulation are compared with our own combined Particle Imaging Velocimetry (PIV) and Laser Induced Fluorescence (LIF) measurements. The application of combined PIV/LIF makes it possible to simultaneously measure velocity components, turbulent stresses, concentration and concentration fluxes. The mechanism of the mixing is analyzed in detail with specific focus on validity of a simple gradient diffusion hypothesis (SGDH) in modeling of the turbulent mass transfer in complex porous structure.

“Designed Porous Structured Reactor” (DPSR) is porous stream-wise periodic structure that can serve both as static mixers and catalyst carriers. Two DPSRs with different inner diameters (A: 14 mm, B: 7 mm) were compared. Residence time distributions were determined in an aqueous system at different Reynolds numbers using wire-mesh electrodes. Similar dispersion was observed in both structures, which was attributed to fully developed flow in DPSR A and B. Axial dispersion coefficients increased monotone with pore Peclet numbers and were in good agreement with literature. DPSR A showed higher pressure drops due to lower porosity values than DPSR B. Pressure drops followed the Forchheimer equation using an Ergun model. The coating reduced the surface roughness and the porosity of the DPSRs’ metal fabric. Chemical mass transfer was evaluated for the hydrogenation of 2-methyl-3-butyn-2-ol (MBY) at various gas/liquid combinations. While gas–liquid mass transfer was determined as limiting process step, liquid–solid mass transfer was ruled out. Superior gas–liquid mass transfer coefficients were determined for DPSR B. Differences in mass transfer rates DPSR A and B were attributed to the ratios of inner diameter to the pore size and the porosities. A Stanton correlation was suggested to estimate gas–liquid mass transfer coefficients.

This study compares a two-phase flow through a foam-like porous structure and a Sulzer SMX TM static mixing element. Refractive index matching between two immiscible fluids and the internal structures enables optical measurements to be performed. The droplet size and position within the internal structures were observed by laser-induced fluorescence. The results show that at low flowrates, droplets follow preferred paths, whereas at higher flowrates, they are more homogeneously distributed within the structures. The droplet size distribution was found to be well represented by the Sauter mean diameter. Measuring along the axis of the two static mixers, we found that droplets disintegrate more quickly in the foam-like porous structure. As both geometries have the same porosity and hydraulic diameter, we conclude that the change in the free cross section is also an important parameter. We observed that in the geometry with large changes in the free cross section, the droplets are smaller.

A general kinetic model has been developed to simulate the three-phase solvent-free hydrogenation of 2-methyl-3-butyn-2-ol (MBY) over a commercial palladium-based catalyst. A Langmuir–Hinshelwood mechanism with noncompetitive adsorption between hydrogen and the organic species is assumed. Gas–liquid mass-transfer resistance is included in the model. Experiments were carried out in a stirred slurry reactor to estimate the kinetic parameters. The proposed model is able to predict the concentration profiles of the species involved during MBY hydrogenation, at varying temperatures (313–353 K), pressures (3.0–10.0 bar), and catalyst loadings (0.075–0.175 wt %). The model predictions were successfully validated using additional experimental runs conducted under different operating conditions and at a lower initial concentration of MBY.

Methyl formate synthesis by hydrogenation of carbon dioxide in the presence of methanol offers a promising path to valorize carbon dioxide. In this work, silica-supported silver nanoparticles are shown to be a significantly more active catalyst for the continuous methyl formate synthesis than the known gold and copper counterparts, and the origin of the unique reactivity of Ag is clarified. Transient in situ and operando vibrational spectroscopy and DFT calculations shed light on the reactive intermediates and reaction mechanisms: a key feature is the rapid formation of surface chemical species in equilibrium with adsorbed carbon dioxide. Such species is assigned to carbonic acid interacting with water/hydroxyls on silica and promoting the esterification of formic acid with adsorbed methanol at the perimeter sites of Ag on SiO2 to yield methyl formate. This study highlights the importance of employing combined methodologies to verify the location and nature of active sites and to uncover fundamental catalytic reaction steps taking place at metal–support interfaces., Journal of the American Chemical Society, 140 (24), ISSN:0002-7863, ISSN:1520-5126

We characterize shapes and volumes of droplets generated in PDMS T-junctions and assess the use of this type of microfluidic device to generate droplets suitable for the study of nucleation. Water droplets were formed in oil in a PDMS T-junction and subsequently stored. Droplet volume reproducibility and stability were investigated from acquired micrographs. By theoretically analyzing the influence of the mean volume of a population of droplets on the estimation of nucleation rates, we have shown that deviations in mean volumes can seriously affect the estimates, unless such deviation is smaller than 10%. This condition is fulfilled if experiments are repeated using the same microdevice. Measured droplet polydispersity remained low enough to treat the droplets as monodisperse. Immersing the microdevice in a water bath mitigates solvent evaporation, and allows for very accurate temperature control. Finally, a screening procedure was used to select the ideal operating conditions to obtain droplets with the desired sizes. Applying this method in devices with increasing T-junction cross sectional area, we have demonstrated a scaling-up of droplet volumes close to an order of magnitude while tuning the droplet shape, i.e., the average length to width ratio, at values between 1 and 1.2., Chemical Engineering Research and Design, 138, ISSN:0263-8762, ISSN:1744-3563

A novel combination of catalyst carrier and reactor design was developed for intensified production of vitamin intermediates. The so called Design Porous Structured Reactor ( DPSR ) is a laser sintered porous 3D-structure that can be tailored to the desired reaction properties such as fluid conditions or heat removal and can also act simultaneously as catalyst support. The selective hydrogenation of 2-methyl-3-butyn-2-ol ( MBY ) to 2-methyl-3-buten-2-ol ( MBE ) under solvent-free conditions was chosen as the reaction to evaluate the potential of DPSR s in comparison to conventional batch reactors. DPSR experiments were performed at varying temperatures and liquid flow rates. DPSR s exceeded batch performance in terms of selectivity, yield and turnover frequency in the analyzed process parameter range. However, DPSR s showed some mass transfer effects. Selectivities and yields increased with higher liquid flow rate due to reduced system pressures and sharper residence time distributions. Overall mass transfer coefficients for DPSR s were determined based on an isothermal non-ideal plug flow model applying heterogeneous Langmuir–Hinshelwood kinetics to account for the chemical conversion. The model showed sufficient accuracy to describe the occurring mass transfer processes. DPSR s were found to be viable alternative for batch reactors, demonstrating the potential for process intensification with an inherent potential for further improvement.

Highly porous structures (porosity ≥ 75 % ) used as inserts in continuous chemical reactors with characteristic length scale in the millimeter range exhibit at industrial relevant throughputs the transport phenomenon vortex shedding. The dependency of the ligament shape and thickness of the porous media on vortex shedding is investigated in two geometries: a highly porous structure consisting of round ligaments and a highly porous structure whose ligaments are thicker and have a square cross section. The contribution of vortex shedding to the total turbulent kinetic energy is determined from proper orthogonal decompositions (POD) of fluctuating velocity fields measured with particle image velocimetry (PIV). The spatial structure of the vortex shedding phenomenon is addressed by POD eigenfunctions and by the wake length extracted by the position of zero mean streamwise velocity along the wake axis. Shedding frequencies are evaluated for a large Reynolds number range ( 6 ≤ Re D ≤ 1500 ) by a dye injection technique. The vortex shedding phenomenon is more intense in case of the highly porous structure with square and thicker ligaments. This means that larger vortices are created and detached, Strouhal numbers are larger and the contribution of vortex shedding to the total turbulent kinetic energy is more substantial in this case compared with the structure with round and thinner ligaments.

a Supercritical water jets are of interest for the hydrothermal spallation drilling technology to thermally fragment rock. The important thermal field of such jets was investigated via axial temperature measurements inside a high-pressure vessel simulating the harsh pressure and temperature conditions found downhole. Deionized water was electrically preheated to supercritical temperatures and afterwards discharged into a slowly co-flowing cooling water ring. Furthermore, a numerical model was developed and applied. Difficulties in simulating the thermal field due to the strongly varying fluid properties around the pseudo-critical point of water were highlighted and critically discussed. An approach of a locally varying turbulent Prandtl number is introduced to improve the predictions compared to the experiment. Finally, all numerical results were validated with experimental data.

Highly porous media (with a porosity ≥85%), like metal foams, fins or designed porous inserts are widely used in industrial flow applications. Motivated by the use of highly porous inserts in continuous chemical reactors, the time dependent behavior of flows through a highly porous structure is investigated experimentally and numerically for Reynolds numbers ReD=33 and 330 (based on the ligament diameter and the bulk velocity). The experimental studies are based on the Particle Image Velocimetry (PIV), whereas the numerical studies are conducted by using the Large Eddy Simulation (LES) approach with the dynamic Lagrangian subgrid closure. The vortex shedding phenomena is observed for both values of Reynolds number. It is demonstrated that the length of the wake and the orientation of the shedded vortices are dependent on the position within the highly porous structure.

Lignin represents one of the most prospective renewable resources for the production of chemicals and fuels in the future. However, current research on the valorization of lignin is often limited by conventionally used batch autoclaves which feature long heating/cooling times, laborious effort in running single data points and limited experimental freedom due to safety issues. In the current article, we present a continuous microreactor setup that is able to overcome these drawbacks. With this setup, it was possible to study the acidic oxidation of kraft lignin in detail. The reaction network of the main monomeric products, their concentrations with residence time in the minute range, and the influence of temperature (150–250 °C), pressure (32–96 bar) and lignin concentration (2.5–10 g L−1) are presented. While the influence of pressure and lignin concentration turned out to be rather negligible, temperature was found to be the most important parameter. At elevated temperatures, products were obtained in substantially higher amounts and concentration peaks appeared within less than 1 min of residence time. Kinetic simulations on the vanillin concentration confirmed the experimental results and underline that high temperatures and short residence times are essential for obtaining high vanillin yields from lignin.

Lignin represents a promising renewable source of chemicals. Valuable aromatic monomers, such as vanillin and methyl vanillate, can be obtained through its acidic oxidation and subsequently have to be separated and purified for further usage. Extraction directly from the aqueous reaction mixture using supercritical carbon dioxide as solvent was evaluated as a possible separation step. Fast screening of different extraction conditions up to pressures of 121 bar at temperatures from 39.8 to 59.3 °C was enabled using a continuous microfluidic device. Distribution ratios of the main five monomeric products were calculated from concentration measurements. With increasing pressure and decreasing temperature higher quantities of monomers were extracted, thereby decreasing selectivity. Overall, selectivity towards specific monomers was high, especially at conditions close to the critical point of supercritical carbon dioxide, confirming the potential of this green and cheap purification method.

The CO2 hydrogenation to methanol is favored by high pressure from the thermodynamic point of view. Mostly experimental work on this reaction is limited at 400 bar due to technical and safety reasons. In this work we present a high pressure plant able to conduct CO2 hydrogenation reactions at pressures up to 950 bar in a capillary microreactor; we focus on the influence of pressure concerning process intensification. To validate the plant functionality the reverse water–gas shift (RWGS) reaction was conducted over a 1 wt% Pt/CeO2 catalyst at 450 °C and between 200 and 950 bar. A mass flow controller for hydrogen was developed due to lack of commercial available hydrogen mass flow controller able to work in the micro liter per minute range and up to 1000 bar. Additional to the RWGS reaction two more reactions take place. The first is the CO disproportionation reaction which results in deposited carbon on the catalyst. The second is the subsequent hydrogenation of carbon to methane. The experimentally determined CO2 conversion is clearly below the equilibrium of the entire reaction network, hence the reaction is kinetically limited. The reaction performance increases with pressure showing process intensification.

Many industrial products and intermediates in powder form suffer from low flowability and poor dissolution behavior. Current techniques to overcome such problems often include time-consuming admixture of additives or wet-chemical processes that are harmful to the environment. Therefore, the objective of this work was to develop a continuous non-equilibrium plasma process to modify the flowability and dissolution behavior of fine-grained, cohesive and temperature sensitive powders. As test substance salicylic acid was used due to its low melting point, cohesive nature and similarity to many pharmaceutical products. In the proposed process the powders were passed through a low-pressure discharge to modify the single particle surfaces. In a first process step SiO x nanostructures and thin films were formed in the plasma and deposited on the salicylic acid particles. The created nanostructures acted as spacers and reduced the prevailing interparticle van der Waals forces. Hence, the flowability of the dry powder was improved from the cohesive to the easy-flowing regime. The flowability was higher the more nanoparticles were deposited per surface area. On the other hand, the deposition of nanostructures retarded the dissolution, which was attributed to the partial coverage of the particle surfaces. Therefore, these already coated powders were surface activated in a second plasma-assisted process step in the same reactor. The particle surfaces were enriched with polar groups, which increased the surface free energy. This improved the powder wettability and the dissolution time was reduced even below the measured value of uncoated powder. Hence, with the introduced process both the flowability and dissolution time of the cohesive, temperature-sensitive and fine-grained test substance salicylic acid could be adjusted within a very short process time in the order of 1 s.

The influence of an inert gas phase on liquid extraction using a microstructured device is analyzed. The gas phase establishes a modified flow pattern. The performance of the gas–liquid–liquid flow is compared to that of a segmented two phase flow, as regards mass transport as well as separation. The extraction of vanillin dissolved in water with toluene was chosen as an example and experiments at different residence times were conducted by varying the total volumetric flow rate. μ-PIV measurements were performed to reveal the influence of the inert gas phase on recirculation within the liquid slugs. Addition of the gas leads to an increase in mass transfer at flow velocities above 0.08 m/s. However, no difference can be noted at lower flow velocities and longer residence times, respectively. The two liquid phases were separated within the microstructured device by using a capillary separator. Purity was always higher than 96%. For two phase segmented flow, the toluene phase was pure, whereas the water phase was free of toluene rests when applying the inert gas phase. Thus, the inert gas phase can be used to enhance mass transfer under certain circumstances and to tune the separation behavior of a capillary separator.

The surfaces of lactose particles are coated with SiOx nanostructures in a plasma downstream reactor to increase the surface roughness and thus, to reduce the prevailing van der Waals forces between the substrate particles. As a consequence of the smaller interparticle friction, both the bulk density and the flowability of plasma modified powders are increased. In this study different powder fractions with mean particle sizes between 5 and 40 μm are processed separately to investigate the influence of the particle size on the process. The measured bulk densities and flow factors become larger with increasing mean particle size for both untreated and plasma treated powders. The bulk density of all powder fractions is increased by 40 to 50% and all originally cohesive powders reach an easy flowing regime through the short low-temperature plasma treatment. Scanning electron microscope images are presented to appraise the particle size and distribution of the deposited spacer structures on the lactose surface. This estimation leads to the conclusion that the spacer structures have typical diameters between 15 and 60 nm and cover the surface with approximately 5 to 15%. Comparative studies investigating the influence of admixed glidants on the powder flowability show that this range is desirable for a maximal reduction of attractive van der Waals forces.

We present a dynamic large eddy simulation of flow in a periodic open-cell structure in the turbulent regime. The flow domain is represented by a tube with inner diameter of 7 mm with integrated highly porous media. Simulations were performed for three different Reynolds numbers ranging from 1200 to 4500 based on the empty tube diameter. The obtained results were compared to experimental data for which measurements were performed downstream of the same periodic geometry manufactured from a transparent material by stereolithography. The measured pressure drop and turbulence statistics compare well to the results of the large eddy simulations. The numerical results give an insight into the forced convection mechanism within the porous media and enhance the understanding of flow through foamlike structures. Furthermore specific geometric details like the influence of the ligament shape of the porous material can be investigated applying this technique.

We present an investigation on species mass transfer in a milli-scale plug flow reactor using metal foams to enhance the mixing process. In the current design the reactor consists of a pipe with an inner diameter of 7 mm and metal foam inserts with different pore sizes of 20, 30 and 45 ppi. Simultaneous PIV and LIF measurements were performed in orthogonal planes normal to the radial and axial direction downstream of a foam element of 50 mm length. The investigated Reynolds numbers range from 600 to 7600 defined with the empty tube diameter and the bulk velocity. We discuss the influence of the pore sizes on the scalar mixing efficiency and compare the results to the reference empty tube case. A strongly intermittent flow caused by the metal foam was observed over the whole range of Reynolds numbers. The increased radial velocities lead to an enhanced mixing performance. Coefficients of Variation in the order of 0.1 were achieved.

In this work, a process for producing vanillin and methyl vanillate from an industrial kraft lignin by oxidation in aqueous methanol at acidic pH is proposed. A series of consecutive runs, comprising the steps of kraft lignin oxidation, monomer extraction, and filtration of residual lignin, were conducted. The depolymerization of kraft lignin (1 g per run) was carried out in a stirred batch reactor with 80 vol % methanol/water as the solvent, with H3PMo12O40 as a homogeneous catalyst, and with 10 bar of oxygen initially present. The monomeric products were recovered from the process by extraction with chloroform and quantified by gas chromatography/mass spectrometry (GC/MS). Residual kraft lignin was recovered from the reaction medium by filtration. After extraction and filtration, the reaction medium, containing the homogeneous catalyst, was reused in the next run. The sum of all quantified products (gas phase, extract, and solids), as well as the amount of consumed oxygen, reached constant levels after ...

The influence of the channel radius on the mass transfer in rectangular meandering microchannels (width 200 – 400 μ m and height of 150 μ m ) has been investigated for gas–liquid flow. Laser induced velocimetry measurements have been compared with theoretical results. The symmetrical velocity profile, known from the straight channel, was found to change to an asymmetrical one for the meandering channel configuration. The changes in the secondary velocity profile lead to an enhanced radial mass transfer inside the liquid slug, resulting in a reduced mixing length. In the investigated experimental range (superficial gas velocity 0.08 m / s and superficial liquid velocity 0.01 – 0.07 m / s ) the mixing time was reduced eightfold solely due to changes in channel geometry. An experimental study on the liquid slug lengths, the pressure drop and their relation to the mass transfer have also been performed. Experimental results were validated by a simulation done in Comsol Multiphysics ® . To obtain information for higher velocity rates, simulations were performed up to 0.64 m / s . These velocity variations in the simulation indicate the occurrence of a different flow pattern for high velocities, leading to further mass transfer intensification.

The surface of lactose particles was modified by a plasma-enhanced chemical vapor deposition process to improve the flow behavior of the powder. For this, the particulates were treated in a plasma down stream reactor which provides a short (50 ms) and homogeneous exposure to the capacitively coupled RF discharge. The organosilicon monomer hexamethyldisiloxane (HMDSO) was used as a precursor for the formation of SiO x which is deposited on the substrate particle surface. For varying process gas mixtures (O 2 /Ar/HMDSO) and RF power applied, the amount of the deposited material was determined gravimetrically after dissolution of the lactose substrate particles and the chemical composition of the accumulated deposition material was investigated by means of attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy. The concentration of the deposited SiO x relating to the substrate material was found to be in the range of 0.1 wt.%. Based on the ATR-FTIR analysis, the inorganic, i.e. oxidic SiO x fraction of the obtained deposits was shown to be controllable by varying the process parameters, whilst a relatively large amount of organic structures must be considered.

To increase mass transfer in solid catalyzed gas–liquid hydrogenations, the reactions are often operated at high pressures. Silicon/glass microreactors present a possibility to safely handle high-pressure and provide optical access into the reaction channel for flow investigations. In this study we present a Si/glass microreactor with soldered microfluidic connections for high-pressure and high-temperature applications. Mechanical testing of the device by tensile and pressure tests showed no failure for continuous operation at 140 bar and 80 °C. The improved microreactor design is applied for a well-described solid catalyzed exothermic hydrogenation at operating conditions up to 51 bar and 71 °C. Additionally the two-phase gas–liquid flow is investigated in the microreactor. Results clearly indicated the improvement of the reaction rate from 0.004 mol g cat − 1 mi n − 1 at ambient conditions to 0.046 mol g cat − 1 mi n − 1 at 51 bar and 71 °C. The reaction was observed to be mass transfer limited.

Due to the laminar flow in microfluidic devices, geometrical adaptations are required to enhance the transport across the center line of a microreactor. In gas–liquid segmented flow, the liquid slug between two gas bubbles creates two counter rotating vortices. Using μ-PIV measurements for microchannels with hydraulic diameters of DH = 150 μm and 200 μm, we show that this recirculating movement is very sensitive to the channel geometry. Our results show that the vorticity and swirling strength have the same characteristics before and directly after channel curve. We present the influence of the superficial velocities, channel diameter and curve radius on the recirculating motion. We demonstrate that for segmented gas–liquid flow, an increase of mass transfer over the complete channel diameter is possible by using meandering channels. The flow pattern for straight channels, characterized by two counter rotating vortices symmetrical to the channel center line, changes to two different sized vortices providing strong mass transfer over the center line. Considering these cognitions, we finally propose an optimized mixing design for liquid mixing in a gas–liquid two-phase flow.

A novel down flow reactor is being developed and tested for the functionalization, here the change of contact angle, of temperature sensitive polymer particles (high density polyethylene (HDPE) and co-polyamide (Co–PA) powders) in a low-pressure plasma treatment with O2/Ar glow discharge. The effects on wettability and chemical composition of processed material at exposure times to the plasma of less than 0.13 s have been determined. X-ray photoelectron spectroscopy (XPS) was used to characterize the powder surfaces. HDPE and Co–PA powder reached a water contact angle reduction down to 658 and 768. Ageing effects are being observed with HDPE. Contact angles on copolyamide do not change over time. XPS analysis of the treated HDPE powders indicates the formation of oxygen containing groups, e.g. diethyl ether (C–O–C), carbonyl (C=O) and carboxyl (O–C=O) groups even after short-time treatment. A correlation between the O2-content in the plasma process gas and the oxygen to carbon (O/C) ratio is found. The O/C-ratio increased from 0.0 (no oxygen on the non-treated powder) up to 0.15 in the short-time plasma treated HDPE powder surface. D 2005 Elsevier B.V. All rights reserved.

This paper introduces a plasma reactor for short-time plasma surface modification of polymer powders. This reactor type, the so-called plasma downer reactor (PDR), provides a high reaction rate, which is forced by the high gas velocity and high particle dispersion in the plasma. It combines the advantageous characteristics of the downer principle for the fluid dynamics of gas-solid phases with plasma surface modification. This work demonstrates that a very short plasma exposure time of 0.1 s effectively improves the wettability of HDPE and Co-PA powders. The water contact angles of HDPE and Co-PA powder can be reduced to 72° and 76°, respectively. The surface tensions of both polymer powders could be increased by a factor of 2 up to 43 mN/m. Water/polymer suspensions and pastes can be made without tensides.

A design procedure for tubular reactors is developed for liquid-phase reactions with gas release in horizontal tubes of small diameters. It is based on a one-dimensional calculation approach, which assumes a stationary turbulent flow. For each stream-wise position on the tube axis, the model delivers the actual temperature, void fraction, chemical composition and slip of the mixture. The possible retardation of the gas release from the liquid proves to be a decisive factor. Approaches to handle this phenomenon are described. The calculation of the tubular flow reactive system requires the knowledge of the reaction macro kinetics. It can be determined separately in suitable small scale batch experiments as shown. Experiments with a homogeneous model reaction in a tubular pilot plant validate the theoretical method.

AmorphousTiO2 films were deposited on glass microbeads using a specially designed circulating fluidized bed plasma-CVD reactor. The film thickness was varied between 7 and 120 nm. While only little carbon impurity was found, XPS analysis revealed the presence of silicon, sodium and alkaline earth elements in the titania coating. Reduced amounts of these substrate-originating impurities were observed in the thicker films. By ToF-SIMS imaging, cross-sectional TEM andtime-resolved dissolution, the titania coatings were proven to be uniform, both per particle and in terms of the film thickness distribution.The photocatalytic performance of the composite particles was evaluated in a fully irradiated fluidized-bed photoreactor. The thinnest films had some photocatalytic activity in the as-deposited state, possibly induced by the high specific power of the microwave plasma or silicon doping. The thicker films needed a post-deposition calcination at 723K to achieve catalytic activity. Both the degree of anatase crystallization and the activity were improved by applying thicker films and after UV irradiation-plus-calcining. All films showed good adhesion and abrasion resistance during the photocatalytic tests. The best plasma-CVD films were about 70% as efficient (per unit reactor volume) as the reference material, P-25 immobilized on quartz sand. Fil: Karches, Martin. Eidgenossische Technische Hochschule Zurich; Fil: Morstein, Marcus. Eidgenossische Technische Hochschule Zurich; Fil: Rudolf von Rohr, Philipp. Eidgenossische Technische Hochschule Zurich; Fil: Pozzo, Roberto Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Giombi, Jose Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina Fil: Baltanas, Miguel Angel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Desarrollo Tecnológico para la Industria Química. Universidad Nacional del Litoral. Instituto de Desarrollo Tecnológico para la Industria Química; Argentina

We report on the microwave plasma-enhanced chemical vapor deposition (PECVD) of silicon dioxide from oxygen/tetraethoxysilane and oxygen/hexamethyldisiloxane under continuous wave and pulse modes. The influence of process parameters such as oxygen/organosilicon flow rate ratio, pressure and microwave power on the film characteristics has been investigated. We were interested in the film deposition rate, density, refractive index and atomic concentrations. Mass spectrometry and Langmuir probe measurements provided useful information about the gas phase and the plasma discharge. A comparison was made between the properties of films deposited with continuous wave and with equivalent powered pulsed plasma. The effect of the pulse frequency and of the pulse duty cycle was also studied.

Coating of powders by Plasma-CVD is an adequate way to protect particles from environmental attack or to provide a large catalytic surface on carrier particles. The μ-SLAN® microwave plasma source is placed into the riser of a circulating fluidized bed, and its performance as a particle-coating reactor is evaluated for silicon oxide deposition on glass beads. Fluctuations of the particle concentration reduce the efficiency of MW coupling. The minimum MW input power for stable plasma generation is 4 times higher in the presence of particles and increases with oxygen partial pressure. The temperature of the entrained particles can be adjusted to very low values, but higher temperatures are achieved near the MW-coupling slots. The conversion ratio of HMDSO only depends on the specific power ( P / F ). It is slightly lower with particles and 5 W/sccm are required for 80% conversion.

Wet oxidation (WO) reactions of the azo dye Orange II were carried out under moderate conditions (T = 130−190 °C, pH = 2, poxygen = 1.0 MPa) with addition of a promoter and FeIISO4. The promoted wet oxidation (PWO) leads to a faster decay of the dye compared with the unpromoted WO experiments. The decomposition of the dye can be described by first-order kinetics over the temperature range. The temperature dependency can be described by an Arrhenius relationship. A reduction in TOC of 70% was achieved at 160 and 190 °C. The influence of the promoter was established at 160 °C. The results indicate a strong dependency on the dye decomposition rates and show a changing reaction order. Whereas the nonpromoted decay can be described with zero-order kinetics, the addition of the promoter changes the decay to first-order kinetics. A combined rate law was adapted for the experimental results to describe the influence of the promoter on the dye decomposition. The influence of the initial promoter concentration was ...

Cf Article en Anglais dans Chemical Engineering & Technology, 1998, Vol 21, No 5, pp. 427-430.

Chemical Engineering and Processing, 69, ISSN:0255-2701, ISSN:1873-3204

International audience; In this paper, we present the synthesis of organic nanocrystals in a 3D hydrodynamic focusing device through a non-solvent crystallization process. This microreactor was designed in order to control the supersaturation level, while manipulating mixing conditions, and to avoid fouling and clogging within the main channel. Rubrene, the molecule of interest, was dissolved in a THF–ethanol mixture and was injected through a silica capillary. Two side flows containing the non-solvent and the surfactant, water and CTACl respectively, surround and interact with the capillary flow and mixing occurs through diffu- sion. First, we quantify the water diffusion process within the focused beam by confocal fluorescence microscopy in the presence of fluorescein. It is shown here that the efficiency and the quality of the mix- ing between the aqueous side flow and the focused organic solution are improved by increasing the side flow rate to the capillary flow rate ratio, i.e. the focusing ratio. Second, the synthesis of rubrene crystals were performed under a variety of flow rate conditions. We show that, by increasing the focusing ratio (from 5 to 40), the mean size of nanocrystals decreases (from 110 to 50 nm). The characterization of the mixing process allows for a better understanding of rubrene nanocrystal synthesis: a fast supersaturation induces the generation of numerous nuclei, which limits the crystal growth. Herein, we show that the 3D hydrodynamic focusing microreactor efficiently produces organic nanoparticles of controlled size, without the deposition of large, unwanted crystals on the microchannel walls.