Showing papers on "Packed bed published in 1991"

Oxidation kinetics of ammonia and ammonia-methanol mixtures in supercritical water in the temperature range 530-700.degree.C at 246 bar

Abstract: Oxidation of industrial chemical and metabolic wastes in a supercritical water medium is an effective method for the treatment and disposal of these materials. Partial oxidation of nitrogen-containing organics in supercritical water leads to the formation of ammonia, which is the rate-limiting step in the overall oxidation to nitrogen. In this study, the oxidation kinetics of ammonia and ammonia-methanol mixtures in supercritical water were experimentally determined in a packed and unpacked tubular plug flow reactor. The oxidation of ammonia was found to be partially catalyzed by the Inconel 625 (a nickel-chromium alloy) reactor walls. In the unpacked reactor, the activation energy was 38 kcal/mol over temperatures ranging from 640 to 700 °C. Oxidation of ammonia in the packed reactor gave an activation energy of 7.1 kcal/mol over the temperature range 530–680 °C and a reaction rate approximately 4 times larger than the tubular reactor data. A power law model and a catalytic model were tested, and the catalytic model was found to represent the data well. In experiments with ammonia-methanol mixtures, the oxidation of ammonia was unaffected by the presence of methanol for experiments conducted in the tubular reactor and retarded by the presence of methanol for packed bed experiments. These observations are consistent with the hypothesis that ammonia oxidation proceeds predominantly as a catalytic reaction in the packed bed reactor at the temperatures investigated. Competitive adsorption of methanol and ammonia reduces the oxidation rate of ammonia in the packed bed reactor. © 1991, American Chemical Society. All rights reserved.

Mechanics of Gas-Liquid Flow in Packed-Bed Contactors

Abstract: Packed beds are typically tall cylindrical vessels filled with pieces of inert solid that have been dumped or stacked to leave considerable space for fluid between them (Leva 1 95 1 ). They are used to bring flowing gas and liquid into the intimate contact needed for rapid exchange of dissolved substances and, often, for chemical reaction as well. This use began nearly two centuries ago and is common in large-scale chemical processing; there are other applications too. Flow in packed beds is justly regarded as complex, irregular, even capricious, and scaleup from laboratory and pilot­ plant models to commercial sizes is notoriously risky (Ng & Chu 1 987). Despite this, packed beds have had relatively little fluid-mechanical study (Westerterp et a1 1 984, NRC Panel Report 1 986). This article reviews what appears most significant from what has been done. The point of view we take began to emerge in the mid-1970s in the course of research into how immiscible fluids flow together in the much finer, yet no less irregular, pore space of sedimentary rocks. The key idea was H. T. Davis's-that the way to approach macroscopic two-phase flow in porous media is with the statistical physics of network processes, then blooming in applications of so-called percolation theory (Larson et al 1977, 1981, Levine et al 1977). The basic ingredient was the essence of accumulated knowledge of the fluid mechanics of microscopic behavior in the individual passages, enlargements, and junctions of the pore space (Ng

A fiber-bed bioreactor for anchorage-dependent animal cell cultures: part I. Bioreactor design and operations.

Abstract: A concentric-cylinder airlift reactor, in which the annulus is a packed bed of glass fibers, has been developed in order to facilitate the scaleup and enhance the volumetric productivity of anchorage-dependent animal cell cultures. In this bio-reactor, oxygen-containing gas is sparged through the inner draft tube, causing bubble-free medium to flow through the fiber bed in the outer cylinder and providing both oxygenation and convective nutrient transfer to the cells. Several other desirable features for reactor operation are also provided by this design. Cell cultivations in this bioreactor have been successfully carried out and provide data for the feasibility of the large-scale cell cultivation.

Adsorption and elution in hollow-fiber-packed bed for recovery of uranium from seawater

Abstract: This paper reports on a 0.9-m-high fixed bed charged with hollow fibers containing amidoxime groups placed on the coast of the Pacific Ocean for the recovery of uranium from seawater. Continuous flow of seawater at a superficial velocity of 4 cm/s provided an averaged uranium content of 0.97 g of U/kg in the amidoxime hollow fiber along the bed after 30 days contact. An elution curve having a 230 g of U/m{sup 3} peak concentration and a 45 g of U/m{sup 3} integrated concentration of uranium was obtained at a superficial velocity of 1 N HCl of 0.0125 cm/s. The required cross-sectional area of the amidoxime hollow fiber-packed bed to produce 10 kg of U per annum was calculated as 9.4 m{sup 2} with a 0.9-m bed height.

Separation of phenols by packed column supercritical fluid chromatography

Abstract: Phenols are rapidly and efficiently separated by packed column supercritical fluid chromatography (SFC). The effect of temperature, pressure, additive identity, and concentration on retention are studied. Polar modifiers do not completely suppress tailing. The addition of a very polar additive, like trifluoroacetic acid (TFA), to a modified mobile phase improves peak shapes, apparently by covering active sites on the silica packing material. Additives sometimes continue to be retained and improve peak shapes even after removal from the mobile phase

The effect of residence time on the CO/sub 2/ reduction from combustion flue gases by an AC ferroelectric packed bed reactor

Abstract: An investigation has been conducted to reduce CO/sub 2/ from combustion gases using an AC ferroelectric packed bed reactor. This ferroelectric packed bed reactor consists of two mesh electrodes packed with ferroelectric particles between them. An AC voltage is applied to the reactor to generate partial or spark discharges. The results show that: (1) The CO/sub 2/ gas reduction rate increases with increasing flue gas residential time and primary applied power; (2) The CO/sub 2/ gas reduction rated increases with decreasing gas flow rate and dielectric constant ( xi /sub s/) of packed ferroelectric particles; (3) The CO/sub 2/ concentrations are reduced by up to 18000 p.p.m., and 108 g of CO/sub 2/ are reduced by 1 kWh of primary applied energy used in the packed bed reactor. >

The effects of the column pressure drop on retention and efficiency in packed and open tubular supercritical fluid chromatography

Abstract: The effects of the pressure drop across the column on retention and efficiency in SFC have been studied. Numerical methods are described which enable the prediction of hold-up time and pressure drop in both packed and open tubular columns. Predictions of both hold-up time and pressure drop are in good agreement with experimental data. The density gradient along the column can be calculated using the numerical methods and a procedure is described which enables the calculation of the overall capacity factors of the solutes from the density profile in the column. Significant variations of the capacity factor are observed along the column. The effect of the density gradient along the column on local diffusivity and dispersion is studied. The column efficiency in systems with significant pressure drops is affected by changes in: the linear velocity of the mobile phase; the diffusion coefficients; and the capacity factors of the solutes along the column. The overall efficiency of the chromatographic system can be calculated if, as is the case for open tubular columns, adequate plate height equations are available.

Analysis of oscillating compr ible flow through a packed bed

Abstract: The forced convective flow of an ideal gas through a packed bed was investigated by finite-difference numerical means. Oscillating gas-phase temperature/pressure inlet boundary conditions were considered. The effect of oscillating boundary conditions on the transport phenomena in the packed bed was investigated and comparisons were made with the case of constant-temperature and constant-pressure boundary conditions. The average energy storage characteristics were found to be very close in both oscillating and constant inlet boundary conditions.

Pressure drop in irrigated packed columns

Abstract: In this work, an equation for determining the pressure drop of irrigated packings is introduced. This is built up from the well-known channel model. It is valid for the whole load range up to the flooding point. The validity of the formula was checked using measurements taken from about 80 different classical and modern packing elements, such as Pall rings, Raschig rings, VSP rings, Hiflow rings, ENVIPAC, DINPAC, Tellerette, Tri-Pac (Hackette), Glitsch-CMR rings, Nor-Pac, Bialecki rings, I-13 rings, Intalox saddles, Montz-Pak, Mellapak 250Y, Ralu-Pak 250YC, Gampak and other types, made of metal, ceramic and plastic (about 4000 measuring points). To apply the pressure drop formula, it is necessary to know the operating conditions, the liquid hold-up and the form factor μ specific to the packing element or group of packings. The method of calculating the pressure drop is illustrated by means of an example.

Improve pressure-drop prediction with a new correlation

Abstract: The proposed correlation for packed towers uses dry-bed packing factors. Such factors are provided for more than 90 commercial packings

A Second-Law Study on Packed Bed Energy Storage Systems Utilizing Phase-Change Materials

Abstract: Thermal modeling of packed bed, thermal energy storage systems has traditionally been limited to first-law considerations. The exceptions include a few second-law studies of sensible heat storage systems and the latent heat storage systems. The cited second-law studies treat the storage and removal processes essentially as batch heating and cooling. The approximation effectively ignores the significant temperature gradient, especially in the axial direction, in the storage medium over a substantial portion of both the storage and removal processes. The results presented in this paper are for a more comprehensive model of the packed bed storage system utilizing encapsulated phase-change materials. The fundamental equations for the system are similar to those of Schumann, except that a transient conduction equation is included for intraparticle conduction in each pellet. The equations are solved numerically, and the media temperatures obtained are used for the determination of the exergy (or availability) disposition in complete storage-removal cycles. One major conclusion of the study from both the first-law and second-law perspectives is that the principal advantage in the use of phase-change storage material is the enhanced storage capacity, compared with the same size of packed bed utilizing a sensible heat storage material. Thermodynamically, however, it does notmore » appear that the system employing phase-change storage material will always, or necessarily, be superior to that using a sensible heat-storage material. The latter conclusion is reached only on the basis of the second-law evaluation.« less

Modeling and simulation of NOX absorption in pilot-scale packed columns

Abstract: Absorption of nitrogen oxides was studied in three packed columns in series. Two were 254 mm ID and 6 m tall, the third was 800 mm ID and 3 m tall. Absorption was also studied in two packed columns of 800 mm ID, 7 and 10 m tall, operated in series. Solutions of mixed nitric and sulfuric acids were used as absorbents. Specific rates of absorption were measured using a stirred cell with a flat interface. A mathematical model is developed for an adiabatic operation. The gas phase reactions and equilibria, gas phase mass transfer, interface equilibria, and liquid phase reactions are included in the model. Heterogeneous gas-liquid equilibria are included in the model for the first time. The variation in the rates of absorption with chemical reaction (of NO2, N2O3, and N2O4) with respect to acid concentration is considered. The formation of nitric acid in the gas phase is also considered in the model. Favorable agreement is shown between the model predictions and the experimental observations.

Forced Convection in Packed Tubes and Channels with Variable Porosity and Thermal Dispersion Effects

Abstract: Previous work in chemical engineering literature on the determination of the effective transverse thermal conductivity and Nusselt number for forced convection in packed tubes and channels are reviewed. Discrepancies in existing Nusselt number correlation equations are discussed. Some of the existing experimental data are reanalyzed based on the recent thermal dispersion theory developed by Hsu and Cheng with variable porosity effects taken into consideration in an approximate manner. Numerical results are obtained for forced convection of air and water in packed tubes and channels. For forced convection in a packed column, it is found that the average Nusselt number depends not only on the Reynolds number, but also on the dimensionless particle diameter, the dimensionless length of the tube, the thermal conductivity ratio of the fluid phase to the solid phase, and the Prandtl number of the fluid.

Mass transfer in structured corrugated packing

Abstract: The performance of structured corrugated packing has been simulated by establishing mechanistic models for liquid distribution, liquid flow on the packing surface and mass transfer. The models were used to investigate the effect of packing height, liquid load, initial maldistribution as well as differing initial distribution and solid-liquid contact angle on the packed column performance. Wetted surface area is the primary value of interest and the simulated results compare very well with those predicted by Onda (1968). The results clearly demonstrate that the wetted surface area is a strong function of the solid-liquid contact angle. Other predicted values such as mass transfer coefficients and overall height of a transfer unit show reasonable agreement with published data.

Modular integrated fluidized bed bioreactor technology.

Abstract: We describe the design and demonstrate the application of a modular integrated fluidized bed bioreactor system. Basically the system is a reactor vessel equipped with an extending cylinder and a liquid distributor plate. Instead of an external recirculation loop, as used in existing fluidized bed systems, a low shear stress impeller is used as the recirculation pump. The system has several unique features, such as modular exchangeable elements, efficient oxygenation and the option of operating as a stirred tank-, a packed bed- or a fluidized bed reactor. An example of a fluidized bed run using CHO-K1 cells is shown. Under standard culture conditions a 100-fold increase in cell density (up to 1.2 x 10(8) cells/ml) was achieved.

Efficiency in packed column supercritical fluid chromatography using a modified mobile phase

Abstract: Large density differentials across small particle packed columns failed to produce losses in chromatographic efficiency. The mobile phase consisted of carbon dioxide modified with methanol. The largest density differential across the column involved an outlet density below 0.2 g/ cm3 while the density at the inlet was simultaneously above 0.9 g/cm3. Absolute efficiencies were similar to those found using the same particle size in reversed-phase liquid chromatography and were very near theoretically predicted values. In related experiments, two cases were found where low outlet pressures or densities caused degraded peak shapes. Experimental conditions were compared to vapor-liquid equilibria data. Poor peak shapes were likely caused by the mobile phase separating into two phases and/or solubility problems.

Process for separating optical isomers

Abstract: A process for separating optical isomers in a pseudo-moving bed system, which comprises introducing a solution containing an optical isomer mixture and a desorbing liquid into a packed bed containing an optical resolution packing therein and having front and rear ends thereof connected to each other endlessly via a fluid passage to circulate a fluid unidirectionally and at the same time drawing out a solution containing one of the separated isomers and another solution containing the other isomer from the packed bed, wherein a port (13) for introducing a desorbing liquid, a port (14) for drawing out a solution containing a readily adsorbable optical isomer, i.e. an extract, a port (15) for introducing a solution containing an optical isomer mixture, and a port (16) for drawing out a solution containing a difficultly adsorbable optical isomer, i.e. a raffinate, are arranged in the packed bed in this order along the direction of fluid flow and the positions of these ports are successively moved in the direction of fluid flow in the packed bed intermittently. The industrial effect of this invention is remarkably excellent, because this process allows an optical isomer mixture to be separated continuously and efficiently, works well even with a reduced amount of a desorbing liquid used, and can deal with a large amount of optical isomers.

Variable density structured packing cryogenic distillation system

Abstract: A cryogenic distillation system employing a multisection column wherein structure packing of different packing density is employed in at least two sections of the column.

Degradation of 3,4-dichloroaniline in synthetic and industrially produced wastewaters by mixed cultures freely suspended and immobilized in a packed-bed reactor.

Abstract: Degradation of 3,4-dichloroaniline (34DCA) in aqueous by undefined cultures of free and immobilized cells was examined. Batch cultures of freely suspended cells and continuous degradation in a packed-bed reactor were studied using both synthetically concocted and industrially produced waste-waters. 34DCA was found to be degraded with a concomitant evolution of chloride ions into the bulk medium. The [acked bed reactor with biomass immobilized on celite diatomaceous earth was found to be capable of degrading over 98% of the 34DCA present in a synthetically concocted inlet stream at a concentration of 250 mg l−1. Residence times of less than 4 h were employed, giving an overall volumetric degradation rate for the packed bed of 90 mg l−1 h−1. The industrially produced wastewater contained, in addition to 34DCA, aniline, 4-chloroaniline, 2,3-dichloroaniline (23DCA) and 3,4-dichloronitrobenzene. The biomass enriched on the synthetic 34DCA waste-water was found to be capable of degrading these compounds in addition to 34DCA with the exception of 23DCA. 34DCA degradation efficiencies of over 95% were obtained for the industrial waste-water at reactor residence times of 4.6 h, giving volumetric degradation rates of 24 mg l−1 h−1.