Showing papers in "Chemical Engineering & Technology in 2002"
TL;DR: In this paper, the size of the precipitated primary particles is strongly dependent on the mixing intensity, and it was possible to generate particles of approximately 50 nanometers in diameter by increasing mixing intensity.
Abstract: Precipitation is a promising method for the economic production of commercial amounts of nanoparticles because it is fast, and operable at ambient temperature. However, process control – due to the rapidity of the involved processes of mixing, nucleation, growth, and agglomeration – and stabilization against agglomeration represent challenges. This paper shows how these challenges can be successfully handled. The focus of this work is therefore set on how to tailor the particle-size distribution in continuous precipitation. Precipitation experiments with barium sulfate in a T-mixer are presented. It was found that the size of the precipitated primary particles is strongly dependent on the mixing intensity. On increasing the mixing intensity, it was possible to generate particles of approximately 50 nanometers in diameter. The second challenge, to stabilize the particles against agglomeration, was successfully met by adsorbing potential-determining ions on the particle surfaces, i.e., by increasing repulsive particle interactions. Thus, stable suspensions of barium sulfate nanoparticles were obtained.
109 citations
TL;DR: In this article, a review of the synthesis, properties, and emerging applications of dendritic polymers is presented, as well as the impact of polymer branching on the phase behavior of DPMs within a comprehensive thermodynamic context.
Abstract: Dendritic polymers, i.e., dendrimers and hyperbranched polymers, attract increasing attention due to their unique structures and properties. The step-wise methodologies for the synthesis of dendrimers allow the tailoring of physical and chemical properties and thus provide a powerful tool to design dendrimers for a wide variety of applications. The complex syntheses of dendrimers often result in expensive products with limited use for large-scale industrial applications, an area where hyperbranched polymers appear to be promising alternatives.
The large body of interdisciplinary research on dendritic polymers is a guarantor for emerging applications. However, the understanding of essential fundamentals such as the phase behavior of dendritic polymer solutions is still in its infancy. Therefore, this review intends to cover the syntheses, properties, and emerging applications of dendritic polymers as well as to discuss the impact of polymer branching on the phase behavior of dendritic polymer solutions within a comprehensive thermodynamic context.
89 citations
TL;DR: In this paper, a reproducible preparation of large area tubular zeolite membranes together with ways for their regeneration and repair are shown, as well as potential application fields are discussed.
Abstract: Molecular sieve membranes are characterized by their high thermal and chemical resistance and by their monodisperse micropore system. Mixtures can be separated by their molecular size as well as by adsorptive interactions and differences in the diffusion coefficients. In this paper, techniques for a reproducible preparation of large area tubular zeolite membranes together with ways for their regeneration and repair are shown. The separation figures demonstrate the efficiency of the molecular sieve membranes developed. Potential application fields are discussed.
78 citations
TL;DR: In this article, the authors describe the application of microscopic techniques to investigate structure-mobility-relations in zeolites and in mesoporous materials of type MCM-41.
Abstract: Mass transfer in micro- and mesoporous materials is of crucial importance in their practical application for separation and catalysis, since the mobility of adsorbed molecules ultimately limits the rate of the overall processes. Diffusion, i.e. the irregular thermal motion of the molecules, is the dominating process. Diffusion measurements are therefore indispensable for the evaluation of the quality parameters of porous materials. Due to their ability to directly follow the diffusion path of the molecules, microscopic techniques are to the forefront, amongst the various methods of diffusion measurement. Besides describing their fundamentals, this contribution describes the application of these techniques to investigate structure-mobility-relations in zeolites and in mesoporous materials of type MCM-41. Some features of particular technological relevance, e.g., the phenomena of correlated diffusion anisotropy, of single-file diffusion and of molecular traffic control, are discussed in detail.
68 citations
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TL;DR: In this article, heat data for pure gas adsorption and binary gas mixtures are available, but heat data is not available for mixtures containing three or more components. But, the available heat data are limited.
Abstract: Separation of gas mixtures by pressure swing and thermal swing adsorption processes is an established unit operation in the chemical industry. Mathematical simulations of these processes require precise knowledge of multicomponent gas adsorption equilibria, kinetics, and heats for the system of interest over all conditions of pressure, temperature, gas composition and adsorbate loading encountered by the adsorber during the separation process. Unfortunately, the published data on heats of adsorption are often not adequate. Limited heat data are generally available for pure gas adsorption, heat data for binary gas mixtures are rare, and heat data for mixtures containing three or more components are nonexistent.
66 citations
TL;DR: Particle Image Velocimetry (PIV) is a measurement technique that has received a lot of attention for this purpose in the last decade as mentioned in this paper, and it is an optical and thus non-intrusive measurement technique for a whole plane in a 3D flow field.
Abstract: Particle Image Velocimetry (PIV) is a measurement technique that has received a lot of attention for this purpose in the last decade. PIV is an optical and thus non-intrusive measurement technique that gives instantaneous 2D velocity data for a whole plane in a 3D flow field. In this paper we will focus on recent developments in PIV for dispersed two-phase flows in particular for bubbly flows.
66 citations
TL;DR: In this article, a phenomenological analysis of the friction factor (Reynolds number relation and the longitudinal dispersivity -Peclet number dependence for random sphere packings) was performed to derive characteristic lengths (i.e., equivalent particle dimensions) for a monolith with regard to its hydraulic permeability and dispersion originating in stagnant zones.
Abstract: Due to the complex, often sponge-like structure of monolithic adsorbents it is difficult to define appropriate constituent units that characterize the hydrodynamics of the material, or to determine relevant shape and size distribution factors comparable to those for spherical particles in (particulate) fixed beds. Based on a phenomenological analysis of the friction factor (Reynolds number relation and the longitudinal dispersivity – Peclet number dependence for random sphere packings) we derive characteristic lengths (i.e., equivalent particle dimensions) for a monolith with regard to its hydraulic permeability and dispersion originating in stagnant zones. Equivalence to the hydrodynamic behavior in “reference” sphere packings is established by dimensionless scaling of the respective data for the monolithic structure. This phenomenological approach, which is simply based on liquid flow and stagnation in a porous medium, can successfully relate hydrodynamic properties of the monolith to that of particulate beds.
59 citations
TL;DR: In this paper, a new mode of operation for simulated moving bed (SMB) chromatographic processes is presented, where a modulation of the feed concentration during the switching cycles is introduced.
Abstract: A new mode of operation for simulated moving bed (SMB) chromatographic processes is presented. While conventional SMB processes are operated with constant feed concentration, a modulation of the feed concentration during the switching cycles is introduced in the new mode of operation. Thereby the performance compared to the conventional SMB process is improved significantly. The productivity and the product concentration is increased. Simultaneously, the solvent consumption is decreased.
57 citations
TL;DR: In this paper, the effect of chemical modification on the sorption capacity of natural Turkish zeolite, clinoptilolite for its potential application as a sorbent was determined, based on XRD, 27 Al MAS NMR and BET methods.
Abstract: This work determines the effect of chemical modification on the sorption capacity of natural Turkish zeolite, clinoptilolite, for its potential application as a sorbent. Pore size distribution and surface area are critical for assessing the suitability of the zeolite for sorbent application. Because natural clinoptilolite has small pore sizes and low surface area compared to synthetic zeolites, modification studies have been performed to improve the sorption capacity. The conversion of natural clinoptilolite to the hydrogen form has been carried out by two different ion exchange procedures, namely ammonium exchange followed by calcination and direct treatment with HCl. The natural and modified clinoptilolite samples were characterized by XRD, 27 Al MAS NMR and BET methods.
52 citations
TL;DR: In this paper, a solution-diffusion mechanism for species permeation through nonporous membranes was developed for direct use in process simulators, without the need for external, custom programming.
Abstract: Mathematical model equations and their corresponding solution methods for membrane gas permeation and pervaporation were developed for direct use in process simulators, without the need for external, custom programming. The models are based on the solution-diffusion mechanisms for species permeation through nonporous membranes. Cross-flow and counter flow models using the logarithmic-mean trans-membrane partial pressure were used to simulate spiral wound and hollow fiber membrane configurations. The Chen approximation of the log-mean partial is recommended for avoiding divergence in the iterative solution methods required to solve the non-linear model equations. The models incorporate temperature, pressure, composition, mass flow, membrane area, and species permeance effects into the simulations. The models were designed for implementation in process simulators using intrinsic capabilities for calculating material and energy balances and predicting physical and thermodynamic properties. Examples are given using the process simulator HYSYS.
48 citations
TL;DR: In this article, an alternative concept for the recycling of homogeneous catalysts has been developed, which combines the aspects of the two-phase technique with the principles of thermo-regulated phase-transfer catalysis.
Abstract: An alternative concept for the recycling of homogeneous catalysts has been developed, which combines the aspects of the two-phase technique with the principles of thermo-regulated phase-transfer catalysis. Special temperature-dependent multicomponent solvent systems make it possible to carry out a single-phase reaction by repression of the miscibility gap at reaction temperature and guarantee an easy catalyst/product separation by the two-phase technique at room temperature.
TL;DR: In this paper, the authors extended distillation's use of side-stream withdrawal to moving-bed chromatography in order to investigate the suitability of a sidestream SMB for separating multicomponent systems.
Abstract: Simulated countercurrent moving-bed chromatography is a thermal separation process which due to its high selectivity, is often used to separate very similar molecules. As with a rectification column, a conventional chromatographic separator unit is designed with two product outlets for each component of the binary mixture. However, since multicomponent mixtures are more likely to be encountered in reality, this work extends distillation's use of side-stream withdrawal to moving-bed chromatography in order to investigate the suitability of a side-stream SMB for separating multicomponent systems.
TL;DR: In this paper, a special and unconventional cryogenic grinding system was established using liquid nitrogen, where a jet-vortex mill was used as the grinding mill and the major feature of this grinding process is that heat generation during the grinding period was eliminated.
Abstract: In order to produce fine polymer powders, a special and unconventional cryogenic grinding system was established using liquid nitrogen, where a jet-vortex mill was used as the grinding mill. The major feature of this grinding process is that heat generation during the grinding period was eliminated. The results suggest that this cryogenic grinding system may be suitable for studying the grinding properties of polymeric materials. It may also be helpful in understanding mechanochemistry, e.g., the t-P-T conditions for different mechanochemical processes under cryogenic conditions (where T is the temperature, and P the pressure of the gas mixture in the grinding chamber). In addition, an Elbow-jet classifier was attached to the jet-vortex mill so that fine, medium and coarse products of polymeric powders could be obtained simultaneously. Chitin, a type of renewable natural polymer, was ground in the system and XRD analysis of ground powders showed they displayed highly activated properties. Unlike a high-energy mechanical milling process, such as a vibratory (bead) mill which requires more milling time t, the final properties of the ground polymer in the cryogenic grinding system were highly dependent on the temperature in the chamber of the jet-vortex mill. The grinding results of chitin also showed that the minimum diameters of the ground polymer products are larger than several tens of micrometers (e.g., 75 μm). The developed method offers a new choice for the production of materials, polymer modification (e.g., degradation), and recycling of wasted rubber and plastic.
TL;DR: In this article, experiments of multicomponent desorption of fixed-bed-adsorbers are presented and an overview is given about the different mechanisms appearing during the process.
Abstract: Desorption by microwaves is an alternative to standard industrial processes for the regeneration of fixed-bed-adsorbers. There are several advantages to using microwaves as the heat supply, but the mechanisms of microwave desorption, especially if several components are present, still have not been analyzed. In this article experiments of multicomponent desorption of fixed-bed-adsorbers are presented. An overview is given about the different mechanisms appearing during the process.
TL;DR: In this paper, an explicit equation for the enhancement factor of a fast irreversible second-order reaction is presented, which makes it possible to determine the effective interfacial area and the liquid-phase mass-transfer coefficient of this reaction regime.
Abstract: The paper presents an explicit equation for the enhancement factor of a fast irreversible second-order reaction. The equation makes it possible to determine the effective interfacial area and the liquid-phase mass-transfer coefficient of this reaction regime. With the help of a new plot as described in this paper, the Danckwerts plot, and the method for determining the interfacial area by means of a pseudo-first-order reaction the paper discusses a novel method for characterizing the reaction regime of experimental data.
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TL;DR: In this article, the authors describe a way to reach the maximum possible contact angle, namely 180°, by texturing the liquid surface instead of the solid one, as it is done for super-hydrophobic solids.
Abstract: Recently, many ways (sometimes inspired by nature) for achieving super-hydrophobic surfaces have been proposed in the literature. On such surfaces, water makes a contact angle close to 180°, which produces spectacular properties: droplets do not stick and the surfaces repel water, which bounces when thrown on them. In this paper, we describe a way to reach the maximum possible contact angle, namely 180°, by texturing the liquid surface instead of the solid one, as it is done for super-hydrophobic solids. It is shown that the contact between such a marble and the solid on which it is deposited is very small, which dramatically reduces the friction when these marbles move. High speeds are thus observed. Together with the fact that the marbles roll as they move, this produces spectacular changes in shape. But the marbles resist to these changes, which can be of interest for practical applications in microfluidics.
TL;DR: In this paper, the photocatalytic degradation of benzoic acid using titanium dioxide (Degussa P-25) as a photocatalyst has been studied.
Abstract: The photocatalytic degradation of benzoic acid using titanium dioxide (Degussa P-25) as a photocatalyst has been studied. The effects of various anions, which are common in industrial wastewater, and of the initial pH were also studied. The effect of the presence of low concentration of cations was also studied. From the intermediates detected a photocatalytic degradation pathway for benzoic acid has been proposed.
TL;DR: In this paper, the breakup of a Newtonian liquid jet into droplets injected horizontally into another flowing immiscible Newtonian fluid was studied experimentally under creeping flow conditions, and the laminar breakup length of the filament was found to correlate with the flow rates of both phases and their viscosity ratio.
Abstract: The breakup of a Newtonian liquid jet into droplets injected horizontally into another flowing immiscible Newtonian fluid was studied experimentally under creeping flow conditions. Different breakup mechanisms take place in different flow regions. No filament is generated at very low velocities of the continuous phase when the droplets peel off directly at the nozzle tip. As soon as the flow rate of the continuous phase exceeds a critical value, a filament of a characteristic length begins to grow. The filament breaks up due to instabilities in terms of developing interfacial waves. The laminar breakup length of the filament is found to correlate with the flow rates of both phases and their viscosity ratio. The impact of the capillary diameter, through which the disperse phase is injected, on the filament length was investigated and the maximum droplet size was estimated.
TL;DR: In this article, the authors advocate the use of non-steady state operation of trickle-bed reactors and develop several operation modes that involve the artificial induction of natural pulses and control of the catalyst wetting efficiency over longer times.
Abstract: Trickle-bed reactors are usually operated in the steady state trickle flow regime. Uneven liquid distribution and the formation of hot spots are the most serious problems experienced during trickle flow operation. In this paper, we advocate the use of non-steady state operation of trickle-bed reactors. Based on a square-wave cycled liquid feed, several operation modes are developed that involve the artificial induction of natural pulses and control of the catalyst wetting efficiency over longer times. The operation modes aim at increasing the mass transfer rate of the limiting reactant and simultaneous prevention of flow maldistribution and hot spot formation. The operation modes are distinguished by a relatively fast and slow cycling of the liquid feed. The potential advantages of the developed feed strategies on reactor performance are evaluated.
TL;DR: In this paper, the authors applied the classic Wilhelmy balance method and the dynamic contact angle analysis (DCA) on initial interfacial reactions of surface-modified biomaterials.
Abstract: Recently, the author successfully applied the classic Wilhelmy balance method and the dynamic contact angle analysis (DCA) on initial interfacial reactions of surface-modified biomaterials. In this study, the authors present further results which underline the potential of these methods to yield time-resolved data of ionic and protein interfacial reactions. In contrast to many spectroscopic methods, an on-line method, which works time-resolved and without disturbing the interface, would be important in the process-engineered quality control. This is underlined by the fact, that many biomedical material surfaces currently are pre-biofunctionalized before their application in order to increase their biocompatibility and bifunctionality. The above-outlined statement of the problems involves many disciplines. In this approach, it is highlighted and discussed on the background of current research on biomaterials.
TL;DR: In this paper, the recovery of organic acids from aqueous solutions using ion-exchanger-impregnated resins has been investigated, and the key parameters for process design are equilibria, mass transfer kinetics and loss of the liquid ion exchanger (bleeding).
Abstract: The recovery of organic acids from aqueous solutions using ion-exchanger-impregnated resins has been investigated. The key parameters for process design are equilibria, mass transfer kinetics and loss of the liquid ion exchanger (bleeding). The maximum bleeding was observed when the resin was used for the first time. The ion exchanger concentration in the aqueous phase decreases to a final concentration lower than 1 mg/L after the first usage. The equilibria of citric and tartaric acid have been investigated for the different macroporous resins XAD4 and XAD16 impregnated with tri-n-octylamine (TOA). A modified Langmuir isotherm taking the loading of ion exchanger TOA on the resin into account is able to describe the equilibrium in both test systems in dependence of the temperature. Experiments in a stirred vessel have been carried out to characterize the intraparticular mass transfer. The results have been used to predict the effective pore diffusion coefficient using the tortuosity.
TL;DR: In this paper, a technique of positron emission particle tracking (PEPT) was used to obtain information on the flow behavior of coarse particles suspended in pseudoplastic liquids agitated by axial-hydrofoil Lightnin impellers A320 and A410.
Abstract: A technique of Positron Emission Particle Tracking (PEPT) was used to obtain information on the flow behavior of coarse particles suspended in pseudoplastic liquids agitated by axial-hydrofoil Lightnin impellers A320 and A410. PEPT enables the position of a 600 μm radioactive particle tracer inserted inside one of the suspended particles to be detected many times per second and its full trajectory followed inside the vessel. Particle trajectory analysis yielded information on particle circulation, velocity distribution, and spatial occupancy. The minimum speed for complete particle suspension, N js , was also determined. The well-known Zwietering correlation failed to predict the measurements by a substantial margin, suggesting that it is inadequate for viscous non-Newtonian liquids.
TL;DR: Hyper-branched polyesters with different functionalities and different mechanical properties can be synthesized in a conventional melt polycondensation from various AB 2 monomers as mentioned in this paper, by suitable modification it is possible to optimize these materials for application as additives, in blends, for coatings or as sensor material.
Abstract: Hyper-branched polyesters with different functionalities and different mechanical properties can be synthesized in a conventional melt polycondensation from various AB 2 monomers. By suitable modification it is possible to optimize these materials for application as additives, in blends, for coatings or as sensor material. The high functionality in a constrained molecular geometry, high solubility, low solution viscosity, interesting melt rheology as well as the branched and thus globular structure allow an improved property profile compared to linear polymers.
TL;DR: In this article, the authors compared the behavior of the simulated moving bed (or ring reactor) with a network of two or three reactors and showed that the network has a narrower stability range than the reverse-flow reactor.
Abstract: Catalytic reactors in forced nonstationary operation enable autothermal VOC (volatile organic compounds) oxidation even when the adiabatic temperature rise of the combustible mixtures is extremely low. The simulated moving bed (or ring reactor), realized with a network of two or three reactors, has been suggested as an alternative to the well-investigated reverse-flow reactor. The behavior of these configurations has been compared, showing that the reactor network has a narrower stability range than the reverse-flow reactor; the stability range is decreased if we increase the number of reactors. The maximum temperature of the catalyst is higher in the network than in the reverse-flow reactor and in both configurations it is increased if part of the catalyst is substituted by inert material.
TL;DR: In this paper, the authors show that low-frequency vibrations in the 50-200 Hz range of an air-water bubble column can significantly reduce the number of bubbles generated at the distributor plate.
Abstract: We show that application of low-frequency vibrations, in the 50-200 Hz range, to the liquid phase of an air-water bubble column causes significantly smaller bubbles to be generated at the distributor plate. For bubble column operation in the homogeneous flow regime, measurements of the volumetric mass transfer coefficient using the oxygen absorption technique show that the increase in the k L a values ranges from 50-100 % depending on the flow rate. It is concluded that application of low-frequency vibration has the potential of improving the performance of bubble columns.
TL;DR: In this paper, the microchannel walls are provided with aluminum oxide coatings, which have a large surface area and can be applied as catalyst carriers for heteregeneously catalyzed reactions.
Abstract: The methods presented allow vacuum-tight microstructured components to be produced, in which the microchannel walls are provided with aluminum oxide coatings. These coatings have a large surface area and may be applied as catalyst carriers for heteregeneously catalyzed reactions, as was demonstrated by the catalytic conversion of hydrogen with oxygen.
TL;DR: In this paper, the authors present a cost optimization method for a wet limestone flue gas desulfurization system, where the most important stage of SO 2 removal runs in the absorber and in the holding tank.
Abstract: In this article the method of cost optimization of the Wet Limestone Flue Gas Desulfurization System is presented. The optimization calculations include process and cost models. The process model describes the most important stage of SO 2 removal that runs in the absorber and in the holding tank. It includes absorption of sulfur dioxide, oxidation of SO 3 2- , dissolution of limestone, and crystallization of gypsum. The model was applied to calculate indispensable parameters for estimating costs and then to minimize capital and operating costs. Costs of all important equipment were estimated, such as SO 2 removal systems with the absorber and the holding tank, reagent feed system with the ball mill and dewatering gypsum slurry system. Optimum values of the process parameters for different conditions of running flue gas desulfurization system were found. The process and cost model can be useful when designing the wet limestone FGD systems and carrying out economic analysis of the flue gas desulfurization plants.
TL;DR: In this paper, an optical sensor was used to measure the deoxygen desorption from organic liquids (ethanol, 1-butanol, toluene, and water) into nitrogen gas.
Abstract: Oxygen desorption from organic liquids (ethanol (96%), 1-butanol, toluene) and water into nitrogen gas has been studied with an optical sensor. Gas hold-ups and volumetric mass transfer coefficients have been determined in the pressure range of 1-10 bar. In this range both quantities are found to increase with the gas density to the power of 0.24. However, by comparison with literature data and on theoretical grounds, the gas density effect can be shown to depend on the gas velocity and on gas density itself. The effect becomes negligible at gas velocities below 0.01 m/s and at gas densities below about 0.1 kg/m 3 .
TL;DR: In this paper, the effects of solid loading on the performance of a cyclone with pneumatic extraction of solids were investigated, and a new method for detecting cyclone fouling was proposed based on the evolution of the pressure drop resistance coefficient.
Abstract: This paper presents the effects of solid loading on the performance of a cyclone with pneumatic extraction of solids. The cyclone is a non-conventional design, especially used for hot-gas cleaning applications such as pressurized fluidized bed combustors (PFBC). A scaled-down cold-flow model was employed for the research. Experiments were conducted at 9–14 m/s inlet gas velocities, inlet solid loadings ranging from 30 to 230 g/kg gas, and bottom gas extraction percentages from 0.3 to 1.5%. Experimental results of pressure drop resistance coefficients and collection efficiency were compared with literature predictions. At PFBC operating conditions, cyclone geometry and solid concentration are the main parameters influencing cyclone pressure drop and collection efficiency. The vortex penetration in dipleg causes lower pressure drop values and higher collection efficiencies than predicted. These parameters can be suitably predicted for PFBC cyclones by introducing a modified penetration length in Muschelknautz's model [1]. For the present cyclone design, a new correlation of pressure drop, including the influence of solid loading, is proposed. A new method for detecting cyclone fouling, not previously addressed, is also presented, based on the evolution of the pressure drop resistance coefficient. An enhanced separation efficiency has been found, related to collection efficiency, which is especially important for particle sizes below 10 μm revealing agglomeration effects.