Showing papers on "Packed bed published in 1985"

hydrodynamic parameters for gas-liquid cocurrent flow in packed beds

Abstract: The hydrodynamics of cocurrent gas-liquid flow in packed beds is analyzed by extending the concept of relative permeability to the inertial regime. The relative permeabilities of the gas and liquid phases are functions of the saturation of the liquid phase. These functions are found from an analysis of experimental data. The relations obtained are used to develop empirical correlations for predicting liquid holdup and pressure drop in gas-liquid cocurrent downflow in packed beds over a wide range of operating conditions. The correlations proposed give very good results when compared to experimental data yielding, in general, mean relative deviations lower than existing correlations. In addition, a new equation is proposed for predicting static holdup in packed beds which is based on a more physically realistic characteristic length than that used in previous studies.

Autotrophic denitrification by Thiobacillus denitrificans in a packed bed reactor

Abstract: An upflow packed bed reactor with lava stones as support for the microbial growth proved to be very useful for the denitrification of industrial waste water by Thiobacillus denitrificans. The application of the plug flow principle allowed higher concentrations of nitrate to be employed than in a stirred tank reactor because inhibitory concentrations of sulfate from thiosulfate oxidation built up only in the upper part of the column — if at all. In experiments with synthetic media nitrate solutions of different strength (NO 3 − g/l: 1.8; 3.0; 4.3; 6.1) were tested, each at 5 different residence times (5; 3.3; 2.5; 2.0; 1.7 h). The combination of the two parameters which still allowed 95% denitrification was 3 g NO 3 - /l and 2.5 h residence time; this corresponded to a volumetric nitrate loading of about 25 kg/m3·d. Higher nitrate loadings led to incomplete denitrification coupled with the occurence of nitrite in the outflow. Below the “critical” loading rate nitrite accumulated only in the lower part of the column and was then gradually reduced. Experiments with simulated middle active waste from processing nuclear fuel which contained numerous heavy metals yielded similar results. — Although pure inorganic media were fed into the reactor the microflora developing as a dense layer covering the lava stones consisted not only of T. denitrificans but also of heterotrophic denitrifiers, mainly Pseudomonas aeruginosa.

Design of cyclic fixed‐bed adsorption processes. Part I: Phenol adsorption on polymeric adsorbents

Abstract: A model for the adsorption of phenol in a fixed bed of a polymeric adsorbent is developed. Model parameters (equilibrium parameters, capacity factor, axial dispersion, film mass transfer coefficient, and intraparticle effective diffusivity) are experimentally determined from independent experiments. Numerical solution of the model equations uses the method of lines with double orthogonal collocation in finite elements. The model is used for the prediction of breakthrough curves and is part of a package for the design of cyclic processes.

Longitudinal and lateral thermal dispersion in packed beds. Part I: Theory

Abstract: A new model is developed for the transient thermal response of a packed bed, using the method of spatial averaging. Equations for the average temperature of the fluid and the solid phase are derived from the point equations for thermal energy in each phase. The new model exhibits some unusual convective and dispersive coupling between the equations for the average fluid and solid temperatures. The response of the model equations to a pulse disturbance is analyzed. It is found that after a sufficiently long time has elapsed, the temperature pulses for the fluid and solid phases will be separated by a constant distance and will spread or disperse about their centroids at an equal rate. The pulse separation predicted by the new model equations is larger than that predicted using more conventional analyses of heat transfer in packed beds. Effective thermal conductivities measured under steady state conditions can differ significantly from those observed in transient experiments due to the spread in temperature pulses caused by heat exchange between phases. Estimates are made of the magnitude of the more important terms affecting longitudinal and lateral effective thermal conductivities under flow conditions, in order to make possible a direct comparison between theory and experiment in a companion paper.

Solid-liquid mass-transfer in packed-beds with cocurrent gas-liquid downflow

Abstract: Local instantaneous solid-liquid mass transfer coefficients were measured in two-phase gas-liquid downflow through packed columns for 3 X 3 mm and 6 X 6 mm cylinders. An electrochemical technique was used. Liquid flow rates from 3.0 to 26.6 kg/m/sup 2/ X s and gas flow rates from 0.07 to 1.16 kg/m/sup 2/ X s covered the gas-continuous, ripple, and pulse flow regimes. Time-averaged mass transfer coefficients in trickle flow and in pulse flow for the pulse proper and the base (outside the pulse) were found to increase with increasing gas and liquid rates. Correlations are presented in terms of liquid phase Reynolds numbers and in terms of Kolmogoroff numbers. The mass transfer coefficients in the pulse were found to correspond closely to the coefficients that would be attained in the dispersed bubble flow regime.

Longitudinal and lateral thermal dispersion in packed beds. Part II: Comparison between theory and experiment

Abstract: The thermal response of a packed bed subject to step changes in fluid temperature was measured in the Peclet number range from 10 to 103. Fluid temperature and particle temperatures were measured independently at six different axial locations in the bed as a function of time. From the temperature breakthrough curves we were able to calculate the spatial separation between the fluid and solid temperature fronts. The measured values were higher by a factor of 2 to 3 than those predicted by traditional heat transfer models. Longitudinal effective thermal conductivities were also computed and compared to theoretical predictions. The new heat transfer model developed in a previous paper is able to account for these differences between experimental results and the predictions of the dispersionconcentric and continuous solid phase models used by previous investigators. Steady-state experiments were used to measure lateral effective thermal conductivities. A temperature gradient was imposed at the top of the bed perpendicular to the flow direction and its spread was measured at different axial locations in the Peclet number range from 10 to 103. The predicted lateral effective thermal conductivities calculated using mass transfer data to estimate the hydrodynamic dispersion effect were significantly lower than the measured values. This is possibly due to the influence of viscosity and density gradients in the flow field in the packed bed.

Automatic simulated distillation of heavy petroleum fractions up to 800°C TBP by capillary gas chromatography. Part I: Possibilities and limits of the method

Abstract: The characterization of heavy petroleum fractions is essential for the design and improvement of cracking plants converting heavy feedstock into valuable “white” products. Conventional simulated distillation methods using packed columns are unsuitable for this purposes, being limited to boiling points up to about 600°C. The method presented is able to cover a boiling points interval ranging from about 150°C up to around 800°C. It employs a short, nonpolar, highly thermostable capillary column routinely operated at temperatures around 430°C. The analytical system is based on a high temperature versions of a fully automatic, capillary dedicated gas chromatograph. The experimental data demonstrate that cold on-column injection is the sole sampling system suitable for such heavy compounds. The conversion of the retention times into boiling points, based on the use of low molecular weight polyethylenes, is extremely reliable, as demonstrated by the excellent retention time reproducibilities. The lower part (up to 550–600°C TBP) of the boiling point distribution curves of heavy petroleum fractions obtained on capillary columns fits well with the corresponding distribution curves based on packed column data. For the petroleum fractions fully eluted from the column the quantitative results obtained either using internal standards or by direct processing of the elution curves are in excellent agreement (less than 0.3 weight% differences). The method has been applied to the determination of the true boiling points corresponding to short path vacuum distillation (DISTACT) cut points over 300°C.

The Growth Process of Brewing Yeast and the Biotechnological Challenge

Abstract: Immobilized brewing yeast cell technology provides the opportunity to improve productivity and volumetric efficiency compared with traditional, free cell, and batch fermentations. Successful commercial application depends on optimizing the interrelated factors of cell physiology, mass transfer, and immobilization procedures to ensure sustained cellular activity without or with very limited yeast growth. Fermentations, using wort as substrate, were undertaken with Saccharomyces uvarum cells immobilized in calcium alginate gel beads. Packed bed and fluidized bed reactor designs were used. In continuous mode, the packed bed fermentations demonstrated severe limitations at the levels of fermentative power per unit yeast, amino acid uptake, and formation of higher alcohols and esters, compared with a free cell system. These aspects were more favorable when the fluidized bed reactor was used in discontinuous mode. The results may be explained to a degree by mass transfer limitation by substrate concentration ef...

A model for pressure drop in two-phase gas-liquid downflow through packed columns

Abstract: The nature of the interaction between the flowing phases in a cocurrent gas-liquid downflow through packed beds depends on the type of the flow regime. The interaction is poor and geometric in nature in gas-continuous flow and becomes high and dynamic in pulse flow due to gas dispersion, acceleration, and mixing of the liquid in the pulses. Models to calculate pressure drop in each of the flow regimes are presented, taking into account the respective interactions. Experimental data on pressure drops and liquid holdups were measured in gas-continuous flow for 3 mm glass spheres and 6 mm Raschig rings. An air-water system is used. The literature data on pressure drops and the experimental data, covering liquid velocities from 0.001 m/s to 0.029 m/s and gas velocities from 0.097 m/s to 2 m/s, were compared with the calculated values. It was found that the pressure drop due to dynamic interaction can be as low as 10% and as much as 80% of the total pressure drop for the data examined in this work. An empirical correlation for holdup in gas-continuous flow is given for Rashig rings.

Gas-fired water heaters

Abstract: A compact, gas-fired water heater in which a reservoir of bulk water is indirectly heated by a gas burner-fired immersion tube and directly heated by mutual contact between the hot flue gases from the immersion tube outlet and feed water droplets as they pass each other in contra-flow through the apertures in a plurality of plates and through a packed bed of graded solid particles or Rashig rings. The packed bed may also include suitable chemicals for modifying the quality of the stored water. One or more calorifiers may be located in the reservoir with limited direct heat conduction contact with the immersion tube.

Preparation of formaldehyde

Abstract: Formaldehyde is prepared by (a) passing a mixture of methanol and water with a concentration of between 50 and 90% by weight, based on the total weight of the two substances, of methanol into a packed column which possesses a packing having a total layer thickness of not less than 50 cm and a total surface area of not less than 0.5 cm 2 per cm 3 of packing, a liquid circulation of from 15 to 90 g/min per g/min of methanol fed to the column, and a concentration of from 10 to 45% by weight of methanol in the recycle liquid, the temperature at the point of entry of the recycle liquid in the column being from 50° to 86° C. and the bottom temperature being 10°-25° C. lower than this, i.e. from 40° to 70° C., (b) separating off methanol and water from the column by stripping with air, an inert gas and/or exit gas, the throughput being from 0.5 to 3 tonnes of methanol and water per hour per m 2 of column cross-section, (c) converting the gaseous mixture of methanol, water, air, inert gas and/or exit gas at a space velocity of from 0.5 to 3 tonnes of methanol per m 2 of catalyst bed cross-section per hour, in the presence of a silver catalyst at from 550° to 750° C. and (d) finally, cooling and absorbing the hot reaction gases, and using the resulting heat of absorption and, if desired, the heat of reaction and/or the heat of condensation partially or completely for heating the recycle liquid, methanol containing impurities being used if desired and, after the stages (a) and (b) and before the stages (c) and (d), the measures of a stage (e) being carried out, wherein the gaseous mixture formed is passed through two layers of wire filter having a wire diameter of from 0.1 to 0.5 mm and a free layer volume of from 90 to 99.5 vol. %, based on the total volume of one wire layer, at a flow rate of from 7 to 13 m/sec in the first layer, and from 1 to 4 m/sec in the second layer. The formaldehyde obtainable by the process of the invention is a disinfectant, tanning agent, reducing agent and a useful starting material for the preparation of synthetic resins, adhesives and plastics.

Control of temperature exotherms in the conversion of methanol to gasoline hydrocarbons

Abstract: A reactor bed, particularly one designed for converting alcohols to gasoline, is provided with a section downstream of the catalyst bed filled with high heat capacity thermal absorptive material. When the circulation of recycled gas to the system is temporarily suspended automatic valves terminate the injection of alcohol feedstock and begin the injection of inert gases such as methane into the system, thereby avoiding the formation of a hot spot which would subsequently damage the catalyst and equipment downstream.

Adsorption on packed bed electrodes

Abstract: The potential-dependent sorption which occurs on a graphite surface is quite different from that on metals and permits a wide range of materials from molecules to bacteria to be sorbed. The active sites for this process are the edges of the graphite layer planes where the pK of the surface functional groups is modified by the applied potential to control the sorption in a manner analogous to that on an ion exchange resin.