Showing papers on "Packed bed published in 2002"

Ammonium removal from aqueous solution by ion-exchange using packed bed natural zeolite

Abstract: Ammonium removal from aqueous solution by a natural ion-exchange resin was investigated by considering the factors affecting the ammonium-exchange capacity including the zeolites’ particle size, the loading flow rates and the impact of a number of regenerations upon the ion-exchange capacity. The resin column was exhausted by downflow at 10, 25, 50 and 75 bed volume (BV)/h, until effluent ammonium concentration of more than 10 mg/l NH 4 + was achieved. The results indicate that conditioning of the zeolite improves the ion-exchange capacity and that the smaller particle size also causes a higher ion-exchange capacity due to greater available surface area. The actual ion-exchange capacity of the conditioned fine (-1.00+0.125) and coarse (-2.00+1.00) mm clinoptilolite was found to be 0.57 and 0.38 meq/g zeolite, respectively.

Absorption of VOCs in a Rotating Packed Bed

Abstract: A Higee absorption process was developed to remove VOCs (volatile organic compounds) from air into an aqueous phase under a centrifugal field. The experimental results showed that the overall volumetric gas-phase mass-transfer coefficient (KGa) increased as a function of the gas Grashof number (GrG) to the power of 0.18. In analyzing kG and a individually, it was found that the enhancement of mass transfer by the centrifugal force can mainly be attributed to an increase in the effective gas−liquid interfacial area. The values of kG lie in a range similar to that for conventional packed beds.

n-Butyl Acetate Synthesis via Reactive Distillation: Thermodynamic Aspects, Reaction Kinetics, Pilot-Plant Experiments, and Simulation Studies

Abstract: The development of a heterogeneously catalyzed reactive distillation process for the production of n-butyl acetate by the esterification of n-butanol with acetic acid is presented. Thermodynamic aspects of the considered system are discussed, and UNIQUAC interaction parameters are given. The reaction was catalyzed heterogeneously by a strongly acidic ion-exchange resin (Amberlyst-15). The reaction kinetics was investigated, and the kinetic constants for a pseudohomogeneous kinetic model are presented. Reactive distillation experiments were performed using the structured catalytic packing Katapak-S. Several operation conditions were varied (reboiler duty, reactant ratio, total feed rate), and two different setups were realized experimentally. The experimental results are presented in comparison with simulation results. An equilibrium stage model is capable of describing the experiments quantitatively. n-Butanol conversions of 98.5% accompanied with n-butyl acetate purities of 96.9% were achieved using an e...

CFD of multiphase flow in packed‐bed reactors: I. k‐Fluid modeling issues

Abstract: The Eulerian k-fluid CFD model was used to simulate the macroscale multiphase flow in packed beds. The geometric complexity of the bed structure is resolved by statistically describing the porosity distribution. The complicated multiphase interactions are computed using the Ergun type of formula developed based on bench-scale hydrodynamic experiments. The work is presented in two sequential articles. Part I discusses implementation issues of the k-fluid CFD model for packed beds. The drag exchange coefficients are obtained from the model of Holub et al. for the particle-fluid interfaces Xks and from the model of Attou et al. (1999) for the gas–liquid interface, Xgl. The effect of particle external wetting on flow distribution was incorporated into the model through the capillary pressure evaluated by either the J-function of Leverett (1941) for air–water or by the expression of Attou and Ferschneider (1999) for other fluids. In the framework of CFDLIB, the choice of the grid size and boundary conditions are discussed. An appropriate relationship between the section size and variance of the sectional porosity distribution was used for flow simulation. Part II discusses the extensive numerical results, and the CFD model is compared with experimental data in the literature.

Distillation in a Rotating Packed Bed

Abstract: This work presents a novel rotating packed bed owing to its applicability in distillation. Distillation experiments were conducted using the methanol-ethanol system at atmospheric pressure and total reflux conditions with two packings. Rotor speeds ranged from 600 to 1600 rpm, which provided 42–298 equivalent gravitational force. Experimental results indicated that 1–3 theoretical plates were achieved with a packing thickness of 8.6 cm. Consequently, the height equivalent of a theoretical plate (HETP) of about 3–9 cm can be obtained in a rotating packed bed, i.e., much lower than that in a conventional packed bed. An empirical correlation was also proposed, implying that the HETP value varied with centrifugal force to the 0.23–0.26 power.

Compact solid source of hydrogen gas

Abstract: A compact solid source of hydrogen gas, where the gas is generated by contacting water with micro-disperse particles of sodium borohydride in the presence of a catalyst, such as cobalt or ruthenium. The micro-disperse particles can have a substantially uniform diameter of 1-10 microns, and preferably about 3-5 microns. Ruthenium or cobalt catalytic nanoparticles can be incorporated in the micro-disperse particles of sodium borohydride, which allows a rapid and complete reaction to occur without the problems associated with caking and scaling of the surface by the reactant product sodium metaborate. A closed loop water management system can be used to recycle wastewater from a PEM fuel cell to supply water for reacting with the micro-disperse particles of sodium borohydride in a compact hydrogen gas generator. Capillary forces can wick water from a water reservoir into a packed bed of micro-disperse fuel particles, eliminating the need for using an active pump.

Effect of coexisting components on aromatic decomposition in a packed-bed plasma reactor

Abstract: Decomposition of the aromatics benzene (C6H6), toluene (C6H5CH3), and o-xylene (C6H4(CH3)2) was carried out in a plasma reactor packed with BaTiO3 pellets in the presence of various background gases. The order of decomposition efficiency was C6H6 C6H5CH3>C6H4(CH3)2. When the reactant gas consisted of a mixture of C6H6, C6H5CH3, and C6H4(CH3)2, decomposition of C6H6 was suppressed, whereas the decompositions of C6H5CH3 and C6H4(CH3)2 were enhanced compared with the single-component decompositions of these aromatics. It seems that the migration of reactant molecules to the solid surface and the molecular sizes play important roles in these behaviors.

Monitoring of temperature effects on animal cell metabolism in a packed bed process.

Abstract: Animal cell (Chinese Hamster Ovary) concn. was detd. online in a packed bed process using dielec. spectroscopy. This enabled the evaluation of the effect of temp. on specific metabolic rates during 3 mo of continuous culture. The effect of low cultivation temp. on cell growth and metab. was monitored, and the data were used for process development. At 37 DegC cells grew exponentially with a specific growth rate of 0.038 d-1 and specific glucose uptake and lactate prodn. rates increased continually. Redn. of the temp. to 33.5 DegC resulted in a lowering of these metabolic rates while having no effect on cell proliferation. Subsequent redn. of the temp. to 32 DegC resulted in stabilization of the cell concn. at a high d. (3.6 * 107 cell per mL of packed bed). In addn., the specific prodn. rate of the protein of interest increased by a factor of 6 compared to the value at 37 DegC. During the stationary phase at 32 DegC, all other specific metabolic rates could be controlled to low and const. levels. [on SciFinder (R)]

Dynamics of protein uptake within the adsorbent particle during packed bed chromatography.

Abstract: A new experimental set-up for on-line visualization of the intra-particle uptake kinetics during packed bed chromatography has been designed and tested. Confocal laser scanning microscopy was used to analyze the dynamics of protein adsorption to porous stationary phases. In combination with this, a flow cell was developed that could be packed with chromatography media and operated as a fully functional mini-scale chromatography column. Adsorption profiles of single- and two-component mixtures containing BSA and IgG 2a during packed bed cation-exchange chromatography were investigated. The two proteins appear to exhibit different transport characteristics. For BSA a classical "shrinking core" behavior could be detected. The profiles obtained during IgG 2a adsorption point toward a different transport mode, which deviates from the classical pore-diffusion picture. For the two-component system, a superposition of the single-component profiles combined with a classical displacement of the weaker bound protein species was found. The results indicate that depending on the adsorbed protein the uptake can vary tremendously, even for adsorption to the same chromatographic support. It is clearly shown that the new microcolumn allows in situ quantitative investigations of protein adsorption dynamics within a single particle, which adds a new tool to the available methods for characterizing and optimizing protein adsorption chromatography.

CFD of multiphase flow in packed-bed reactors: II. Results and applications

Abstract: Numerical simulations of multiphase flow using the k-fluid CFD model described in Part I of this issue are presented for packed beds at various operating conditions. Both steady-state and unsteady-state (e.g., periodic operation) feed conditions were studied numerically. Predictions of the k-fluid CFD model are comparable with the experimental data in the literature for liquid upflow in a cylindrical packed bed. In addition to the mean porosity and the longitudinally averaged radial porosity profile, the variance of the porosity distribution is needed for predicting the probability density function of the sectional flow velocity. In the trickling flow regime, the k-fluid CFD model provides reasonable predictions of the global liquid saturation and the pressure gradient. Relevant applications of the k-fluid CFD model are identified in quantifying the relationship between bed structure and flow distribution in various-scale packed beds. The combined flow-reaction modeling scheme is proposed through the “mixing-cell” network concept, in which the k-fluid CFD simulation can provide the information on sectional flow distribution.

Operational Limits of Ignition Front Propagation against Airflow in Packed Beds of Different Wood Fuels

Abstract: Propagation of the ignition front against airflow in packed beds of different wood fuels has been studied. The results of experiments carried out with pellets and mixtures of wood chips and sawdust are presented and compared with earlier experiments with different wood fuels. Increase in particle density and size was found out to widen the range of possible airflow rates, and transfer the maximum rate of ignition front propagation toward fuel lean conditions. Increase in the average sphericity of particles decreases the porosity of the bed. Mixing of small and large particles seems to be advantageous for combustion so that small particles change the optimum airflow rate to fuel rich conditions and large particles widen the usable range of airflow rates. A correlation was found for the maximum rate of ignition front propagation in beds of wood fuels.

Absorption of NO2 in a packed tower with Na2SO3 aqueous solution

Abstract: This paper looks at the development of a two-stage chemical scrubber for NOx control. In the first stage, the most practical oxidizing agents for NO oxidation are sodium chlorite and sodium hypochlorite. Although a considerable amount of work has been done on the reaction kinetics of NO2 with Na2SO3, there are major gaps in developing an Na2SO3 aqueous solution for NO2 absorption in the second stage. Of particular importance is the rate of chemical absorption. A pilot-scale research program was initiated to evaluate the absorption rate of NO2 with Na2SO3 aqueous solution in a packed tower. The research is directed at obtaining height of a transfer unit (HTU) for NO2 absorption, and to determine reasonable operation conditions for the packed bed scrubber. A sulfite concentration of 0.25 M is essential to have a reasonable HTU in 2 to 5 feet for a gas rate between 1,050 to 2,350 lb/ft2 hr. The results indicate that the scrubbing effectiveness of NO2 increased with the L/G ratio, and an L/G of more than 3 is required for high NO2 absorption.

A comparison of steady-state equilibrium and rate-based models for packed reactive distillation columns

Abstract: A steady-state equilibrium model and a rate-based model were developed and compared for packed reactive distillation columns for the production of tert-amyl methyl ether (TAME) and methyl acetate. For the methyl acetate system, both models yield good agreement with experimental data. The results predicted by the equilibrium and rate-based models are similar with very few differences found under all simulation conditions. However, the rate-based model is much more complicated than the equilibrium model and also more difficult to converge. The influence of the reflux ratio, the operating pressure, the catalyst amount, and the heat loss was studied. It was found that reactive distillation behaves very differently from ordinary distillation. The existence of an optimal reflux ratio and an optimal pressure is predicted by both models.

Enhanced dispersion in cylindrical packed beds.

Abstract: The effective longitudinal dispersion constant, D(L)(eff), in cylindrical packed beds is larger than in the bulk due to the existence of radial inhomogeneities induced by the cylinder walls. For dense random packed beds, D(L)(eff) can be several times larger than the bulk value, even for arbitrarily large cylinder radius, R. The time-scale for attaining asymptotic dispersion rates in a cylindrical geometry is neither the convective nor the diffusive time-scale, but rather D(T)/R(2), where D(T) is the bulk transverse dispersion rate. Similar effects are predicted for packed beds confined in ducts of any cross-sectional geometry. The case of a rectangular duct, compared with an infinite slit, provides an intuitive model for the influence of walls in the limit as R goes to infinity.

Production of structured lipids in a packed-bed reactor with Thermomyces lanuginosa lipase

Abstract: Lipase-catalyzed interesterification between fish oil and medium-chain TAG has been investigated in a packedbed reactor with a commercially immobilized enzyme. The enzyme, a Thermomyces lanuginosa lipase immobilized on silica by granulation (lipozyme TL IM; Novozymes A/S, Bagsvaerd, Denmark), has recently been developed for fat modification. This study focuses on the new characteristics of the lipase in a packed-bed reactor when applied to interesterification of TAG. The degree of reaction was strongly related to the flow rate (residence time) and temperature, whereas formation of hydrolysis by-products (DAG and FFA) were only slightly affected by reaction conditions. The degree of reaction reached equilibrium at 30–40 min residence time, and the most suitable temperature was 60°C or higher with respect to the maximal degree of reaction. The lipase was stable in a 2-wk continuous operation without adjustment of water content or activity of the column and the substrate mixture.

Simulation of packed-bed chromatography utilizing high-resolution flow fields: comparison with models.

Abstract: A computer simulation of a section of the interior region of a liquid chromatographic column is performed. The detailed fluid flow profile is provided from a microscopic calculation of low Reynolds number flow through a random packed bed of nonporous spherical particles. The fluid mechanical calculations are performed on a parallel processor computer utilizing the lattice Boltzmann technique. Convection, diffusion, and retention in this flow field are calculated using a stochastic-based algorithm. This computational scheme provides for the ability to reproduce the essential dynamics of the chromatographic process from the fundamental considerations of particle geometry, particle size, flow velocity, solute diffusion coefficient, and solute retention parameters when retention is utilized. The simulation data are fit to semiempirical models. The best agreement is found for the “coupling” model of Giddings and the four-parameter Knox model. These models are verified over a wide range of particle sizes and fl...