Showing papers on "Packed bed published in 1997"

Ultrahigh-pressure reversed-phase liquid chromatography in packed capillary columns.

Abstract: The use of extremely high pressures in liquid chromatography can improve the efficiency and reduce analysis time for columns packed with small particles. In this work, fused-silica capillaries with inner diameters of 30 μm are slurry packed with 1.5 μm nonporous octadecylsilane-modified silica particles. These columns are prepared in lengths up to 66 cm with packing pressures as high as 4100 bar (60 000 psi). Near the optimum flow rate, columns generate as many as 300 000 theoretical plates for lightly retained compounds (k‘ < 0.5) and over 200 000 plates for more retained compounds (k‘ ≈ 2). These translate to plate heights (Hmin) as low as 2.1 μm. The pressures required to run at optimum flow rates are on the order of 1400 bar (20 000 psi). Analysis times at these pressures are on the order of 30 min (k‘ ≈ 2) and can be reduced to less than 10 min at higher than optimum flow rates. Capacity factors are observed to increase linearly with applied pressure.

Fluid flow through catalyst filled tubes

Abstract: Network models are an effective means of incorporating pore-scale heterogeneity into flow models of porous materials. The drawback to these models used to be the inability to obtain quantitative macroscopic parameters representing larger (experimental-scale) media. However, recently developed modeling techniques, combined with more widely available computational resources, make the simulation of macroscopic parameters from a network approach viable. A network model for the slow flow of an incompressible fluid in disordered packed beds is presented. Fundamental fluid mechanics equations are solved at the pore scale and then translated to macroscopic behaviour using a network approach. The results reproduce experimental permeabilities and show excellent quantitative fits to residence time distributions for mechanical dispersion in real beds. Simulations of the RTD are of special interest, because they are definitive links between pore-scale flow behavior and macroscopic responses.

Modeling flow in disordered packed beds from pore-scale fluid mechanics

Abstract: Network models are an effective means of incorporating pore-scale heterogeneity into flow models of porous materials. The drawback to these models used to be the inability to obtain quantitative macroscopic parameters representing larger (experimental-scale) media. However, recently developed modeling techniques, combined with more widely available computational resources, make the simulation of macroscopic parameters from a network approach viable. A network model for the slow flow of an incompressible fluid in disordered packed beds is presented. Fundamental fluid mechanics equations are solved at the pore scale and then translated to macroscopic behaviour using a network approach. The results reproduce experimental permeabilities and show excellent quantitative fits to residence time distributions for mechanical dispersion in real beds. Simulations of the RTD are of special interest, because they are definitive links between pore-scale flow behavior and macroscopic responses.

Nanofiber packed beds having enhanced fluid flow characteristics

Abstract: The general area of this invention relates to porous materials made from nanofiber packed beds. More particularly, the invention relates to altering the porosity or packing structure of a nanofiber packed bed structure by blending nanofibers with scaffold particulates having larger dimensions. For example, adding large diameter fibers to a nanotube packed bed to serve as a scaffolding to hold the smaller nanofibers apart and prevent the nanofiber bed structure from collapsing. This increases the average pore size of the mass by changing the pore size distribution and alters the packing structure of the packed bed. The increase in average pore size is caused by the creation of larger channels which improves the flow of liquids or gasses through these materials.

Development of a complete simulation model for predicting the hydraulic and separation performance of distillation columns equipped with structured packings

Abstract: This paper describes the work oriented toward the development of an advanced expert system capable of predicting the hydraulic and separation performance of corrugated sheet structured packings. The simulation programme consists of a number of modules covering different types of performance calculations. Traditional, overall approach relying on the assumption of plug flow of both phases is represented by a newly developed method, which unlike the known methods accounts explicitly for the effects imposed by micro and macro characteristics of packing geometry. A special feature of this method is that the effective areas are determined using the author's small scale liquid distribution method. This liquid flow distribution model represents the hart of a detailed calculation approach. The gas flow profile is assumed to be uniform and the established liquid distribution pattern enables the calculation of local L/V values which are used together with other relevant information's in a separate module to calculate the concentration changes along the bed and consequently the corresponding HETP value. A separate module is used to visualise the liquid flow pattern in the bed. Any kind of initial liquid distribution profile can be arranged, however, a separate module can be used to evaluate the performance of commercially available high performance liquid distributors. Generally the detailed model is designed to predict the mass transfer efficiency of a column operating in the preloading range, and is particularly suitable for the prediction of the effects of varying degrees of small scale liquid maldistribution. This in turn enables the determination of the optimum quality of initial liquid distribution for a given packing and creates a basis for optimisation of packing geometry. Present developments of the model and related software are discussed as well as the future refinements and improvements.

Mechanistic pressure drop model for columns containing structured packings

Abstract: A mechanistic model was developed to predict pressure drop and flooding in packed columns equipped with corrugated packing of the regular type. It was developed after considerating the interaction of falling liquid film with the gas phase, based on mass-and momentum-conservation equations. Among the most common structured packings, the behavior of the Mellapak and BX types was analyzed. The aim of this work is to demonstrate how mechanistic models, developed for simple geometry, can also be used to compute pressure drops in cases where the geometry is more complex, as with a structured packing. This approach, based on the geometric characteristics of the packing and measurable parameters such as liquid holdup, enables the development of a basic model by limiting the number of adjustable parameters, which are numerous in all the available models. Because of its nature, this model is extremely easy to extend to different types of structured packings.

Capillary electrochromatography with 1.5 μm ODS-modified non-porous silica spheres

Abstract: The particle realization of electrochromatography on capillaries packed with 1.5 μm ODS-modified non-porous silica spheres is demonstrated. In order to realize stable separation conditions it is crucial to add sodium dodecylsulphate (SDS) to the mobile phase and to pressurize both buffer vials at 10 bar. The presence of SDS stabilizes the current and makes the electro-osmotic flow in the packing more uniform so that no air bubbles are generated at high field strengths. The capillary columns are extremely efficient and on a 24 cm long column about 120,000 plates can be generated (a reduced plate height of about 1.3). The columns are very stable and no loss in efficiency was found after using a column continuously for two months.

Contribution of the radial distribution of the flow velocity to band broadening in HPLC columns

Abstract: The radial distributions of the local linear velocity of the mobile phase, the local efficiency, and the local analyte concentration were determined at the outlet of 0.3−0.4 in. i.d. chromatographic columns by simultaneously recording the elution bands at different radial locations over the column exit cross section. The flow profiles exhibited by these columns are parabolic in shape, as was found previously for analytical and preparative size columns. The linear velocity measured in the wall region of the packing is about 2−5% lower than that in the center of the column, depending on the quality of the packed bed. The local efficiency is highest in the core region of the packed bed, where local, on-column detection would yield better separations, exhibiting a higher degree of resolution. The radial distribution of the analyte concentration is not uniform but shows important fluctuations along the radius of the column. The differences in local linear velocity, local efficiency, and local analyte concentra...

A new method for sorptive enrichment of gaseous samples: Application in air analysis and natural gas characterization

Abstract: A new method for the preconcentration of trace components from gaseous samples is described. The system is based on enrichment of the solutes on a packed bed of 100% polydimethylsiloxane (PDMS) particles followed by thermal desorption and GC analysis. It was demonstrated that breakthrough volumes for packed PDMS traps can be calculated from theoretical equations. The PDMS material exhibits excellent thermal stability even after 200 consecutive runs. Additional advantages of packed sorbent beds over conventional adsorbent materials include high inertness, the absence of displacement effects and a reduced affinity for water. The practicality of the system was demonstrated by the analysis of air and natural gas using the system described here, components in the range of octane to benso[a] pyrene could be quantitatively trapped and thermally desorbed at mild temperatures. With the present set-up the detection limit is in the order of 0.1 ng/m3 (MSD detector).

Two‐flux radiation‐field model for an annular packed‐bed photocatalytic oxidation reactor

Abstract: One of the remaining challenges in application of heterogeneous photocatalysis for treatment of air streams containing dilute VOCs is to design a cost-effective photocatalytic reactor that simultaneously allows efficient contact of the contaminated air and solid catalyst while uniformly irradiating the solid catalyst with light. A pseudohomogeneous model was developed to study effects of system parameters on process performance for a gas–solid lamp-in-tube annular-photocatalytic-oxidation (PCO) reactor in which the annular space is filled with photocatalyst-coated packing. In this model the flow field is assumed to be uniform and radial diffusion negligible. Homogeneous reactions are neglected. Heterogeneous reaction rates follow Langmuir–Hinshelwood–Houghen–Watson kinetics with rate parameters extracted from independent experiments. A 1:D “two-flux” incidence submodel is used to account for the radial UV light distribution throughout the reactor annular space. This submodel requires knowledge of the UV lamp radiant emittance, the optical characterstics of the catalytic thin-film coating, and the UV irradiance at the outer wall of the reactor and contains only a single adjustable parameter—the mean free path between photon–catalyst interactions. The model was validated with experimental performance measurements for destruction of acetone and isopropyl alcohol in a bench-scale photoreactor. The validated model can be used to predict the optimum catalyst film thickness for given reactor dimensions, packing shape and size, and VOC abatement problem.

Mass Transfer in Beds of Modern, High-Efficiency Random Packings

Abstract: A new model has been developed for the prediction and correlation of mass-transfer rates in distillation columns containing random packings. Emphasis is placed on the characteristics of the newer “high-efficiency” random packings: IMTP, CMR, Fleximax, and Nutter. These packings are of the high void fraction, through-flow type and have become quite popular for new designs as well as for retrofits. In building the model use has been made of a large bank of experimental data from the laboratories of Fractionation Research, Inc., and the Separations Research Program (SRP) at The University of Texas at Austin. The model is based on an earlier SRP study of liquid holdup and gas pressure drop in beds of random or structured packing (Stichlmair et al. Gas Sep. Purif. 1989, 3, 19]. The only packing parameter needed is a “packing characteristic” which has a value of about 0.030 for a 2-in. Pall and Raschig rings and about 0.050 for the 2-in. nominal size of the high-efficiency packings listed above. The model was ...

Dehydrogenation of propane using a packed-bed catalytic membrane reactor

Abstract: In a high-temperature catalytic membrane reactor, a plug-flow reactor is combined with a gas-separative membrane. By selectively removing one of the reaction products, the reaction mixture is prevented from reaching equilibrium, and a higher conversion can be obtained. This concept is only valid for reactions that are limited by the thermodynamic equilibrium, such as the direct dehydrogenation of propane to propene. A tubular H2-selective silica membrane was characterized [αH2/C3H8 = 70–90 at 500°C] and used as the gas-separative membrane. The membrane reactor was filled with a chromia/alumina catalyst. The kinetics of the catalyst was studied. At 500°C the deactivation of the catalyst is slow, and the propene yield is almost constant for at least 10 h of operation. Under well-chosen process conditions, the propene yield is at least twice as high as the value obtained at thermodynamic equilibrium in a conventional reactor.

Fundamental Liquid Flow Correlations for the Computation of Design Parameters for Ordered Packings

Abstract: The foundations have been laid out for estimating design parameters in ordered packing materials based on fluid mechanics and mass transfer without any adjustable parameters. Design parameters for heat- and mass-transfer processes over ordered packing materials can be estimated using the knowledge developed on the study of flow patterns and velocity parameters for thin viscous films over complex surfaces. Velocity profiles can be integrated to calculate liquid holdup and interfacial areas. The effect of the size of the structured packing as well as liquid properties on these parameters was explored. Flow visualization experiments were performed to determine the path followed by tracer particles in the liquid film flowing over the inclined corrugations of a packing element. Contact times for tracer particles to travel one wave of the packing element can be evaluated using the packing geometry and the path followed by the particles. Mass-transfer coefficients for the liquid phase were estimated using a pene...

Capacity and efficiency of reactive distillation bale packing : Modeling and experimental validation

Abstract: The geometry of catalyst-containing bale packing is characterized in this paper. The calculated packing parameters (specific surface area and void fraction) are employed in conjunction with a model to predict two-phase pressure drop, maximum capacity, and height equivalent to a theoretical plate (HETP). Experimental data obtained in a 5.3-cm (2.1-in.) column, operated at total reflux, are presented for two systems (cyclohexane/n-heptane and acetone/methyl ethyl ketone) at pressures of 138 and 241 kPa (20 and 35 psia). Model predictions for pressure drop and HETP are validated with experimental data obtained under nonreactive conditions. An appropriate procedure for scaleup of HETP and pressure drop, with associated limitations, is also discussed.

Particle capture and plugging in packed-bed reactors

Abstract: Fine particles in the liquid feed to packed-bed reactors can be trapped in the catalyst bed, which eventually leads to excessive pressure drop. The fine particles can include coke, corrosion products, clays, and other minerals. The catalyst bed functions as a granular filter to remove particles much smaller than the size of the pores between the catalyst pellets. The efficiency for trapping the particles in the packed bed depends on the flow fields and the attractive forces between the packing and the fine particles. In order to understand the capture of fine particles from nonaqueous media, we studied a model system of carbon black in kerosene. Columns packed with glass beads and a catalyst were operated over a range of flow velocities to Reynolds numbers from 0.1 to 2.3, on the basis of the diameter of the packing in the bed. Flow was in the upward and in the downward direction. The filter coefficient and efficiency were sensitive to liquid velocity. Trapping was slightly more efficient with downward fl...

Mist Deposition onto Hairy Root Cultures: Aerosol Modeling and Experiments

Abstract: We analyzed the applicability of the standard models for aerosol deposition in randomly packed fibrous filter beds to mist deposition across a bed of hairy roots in the nutrient mist bioreactor. Although the assumptions inherent in the models are met on a local level, the overall structure of the root bed introduces some uncertainty into the correct choice of root packing fraction and gas velocity required by the model. For reasonable parameter values, the minimum in the deposition efficiency curves is close to the peak in the mist number and mass distributions, and good penetration of the root bed is possible. We then measured the deposition of mist across a packed bed of Artemisia annua transformed roots as a function of droplet size, bed length, and gas flow rate at a root packing fraction R) 0.5. We compared the experimental measurements with the predictions of the aerosol deposition model and found good agreement between the measured and predicted values for the diameter where the deposition efficiency across the bed is 50%,D0.5. Agreement between the model and the experiments broke down when the flow rate was increased to the point where the creeping flow assumptions were no longer valid.

Effect of contact resistance between particles on the current distribution in a packed bed electrode

Abstract: A microscopic, modelistic approach was carried out to elucidate the electrochemical reduction of a nuclear waste solution in a packed bed electrode. The interfacial surface reactions within the packed bed were taken into account and the particle–particle contact resistance through oxide films was found to be big enough to effect the potential distribution throughout the bed. On the basis of equations developed here, the contribution of the resistance of the oxide film to the potential and current distribution throughout the bed was compared with the macroscopic homogeneous approach.

Charge dynamics of a methane adsorption storage system: Intraparticle diffusional effects

Abstract: The charge of natural gas adsorption storage systems is studied numerically, With emphasis given to the impact on its dynamics of intraparticle diffusional resistances to mass transport. Besides adsorption kinetics and thermal effects, the simulation model takes into account both mass transport inside the adsorbent and hydrodynamics of flow through the packed bed. Numerical results are presented for change with methane of a 50 liter cylindrical reservoir, filled with hypothetical adsorbents with diffusional time constants in the range 10−3 s1D/Rp2 ≤ ∞. and with the adsorption equilibrium curve of a commercially available activated carbon with a good adsorptive storage capacity. An attempt is made to assemble the charge histories for different values ofD/Rp2 , in a single cure by using a modilied time scale.