Stagnation point flow in a diffusion cell

Stagnation point flow in a diffusion cell was examined to determine how close the resulting flow approached that produced under ideal conditions, and to evaluate its application to mass transfer systems. Ideal conditions exist when the substance can flow directly into a barrier, then reflect cleanly without the distortion of surrounding walls. Numerical simulation was performed using a upwind difference method by directing the incoming flow through a tube perpendicular to the stagnation point, and diffusing the outgoing flow between two circular plates. Changes in the flow pattern were examined when varying the ratio of the gap between the two plates with the fixed radius of the inlet tube. All flow showed deviations from the ideal stagnation point flow. However, in the area near the stagnation point, a flow approximating the ideal was obtained by using a very small gap between the plates. An examination of the mass transfer near the stagnation point for three different gap sizes (d = 2.0, 1.0 and 0.2) showed that the Sherwood number was approximately a half power of the Reynolds number. The dependence of the Sherwood number on the Reynolds number increased when using a smaller gap, and the Sherwood number at Re = 500 increased by 74% when the gap was decreased from d = 2.0 to d = 0.2.     

Modeling of batch catalytic chlorination of toluene in a bubble column

The chlorination of toluene was studied in a batch bubble column in the presence of ferric chloride. Mass transfer studies (in the absence of dissolved catalyst) showed that Cl₂ absorption into toluene was controlled up to 60% by the gas side mass transfer resistance. In addition, the experimentall obtained liquid side mass transfer coefficients were found to be very low, i.e. more than 10 times lower than the predictions from common correlations. A model was developed which describes the batch chlorination process with striking agreement on the basis of the mass transfer data measured in this study and the solubility and kinetic data reported recently. It was found that the first chlorination step took place in the transition from diffusiona to fast reaction regime. However, due to the dominating gas side resistance, the overall process was only slightly enhanced by the reaction in the liqu     

The effect of homogeneous gas phase oxidations in char particle gasification

Two steady state models have been developed for the simultaneous oxidation and gasification of a single coal char particle. Because of the uncertainty in the relative importance of solid phase and gas phase reaction kinetics in a gasifier environment, the models assume that the velocities for the gas phase oxidations of hydrogen and carbon monoxide are either zero or infinitely fast. The global rates and steady state multiplicities are discussed as functions of particle size, film thickness, gas composition and ambient temperature. The restriction of infinite hydrogen diffusivity assumed by previous investigators is removed. The two models lead to considerably different predictions about the maximum temperature of char particles in coal gasifiers.     

Enhancement of absorption of a gas into a stagnant liquid in which a heterogeneously catalysed chemical reaction occurs

The CBS method[1, 2] is applied to measure the enhancement factor for the absorption of pure hydrogen from a gas bubble of constant size into a sta     

The measurement of emulsion phase voidage in gas fluidized beds of fine powders

The results of two different experimental methods, X-ray absorption and bed collapse, for the determination of the emulsion-phase voidage of gas fluidized beds are compared. It is shown that good agreement exists between them over the range of gas velocities studied, but that possible differences in voidage in an axial direction are only revealed by the X-ray technique. The bed collapse method applied to a Geldart Group A powder confirms that there is little difference between the surface settling rates of a bubbling bed and of a uniformly expanded bed of the same voidage.     

The kinetics of adsorption of surface active agents at gas-liquid surfaces

A general model of adsorption of surfactants at a gas-liquid surface has been developed, which accounts for both the diffusion in the bulk and the barrier to adsorption. In this model, the activation energy barrier to adsorption is accounted for by means of a kinetic expression. A numerical scheme for the solution of the resulting equation of the diffusion-kinetic model is illustrated for the case of the Frumkin isotherm. The model is used to estimate the extent of the adsorption barrier from the dynamic surface tension data of two aqueous surfactant systems reported in the literature. A simplification of the diffusion-kinetic model is proposed which accounts for diffusion in the bulk using diffusion-penetration theory. The simplified model compares very well with the exact model, especially for high barrier resistances.     

Experimental investigation of hydrate formation behaviour of a natural gas bubble in a simulated deep sea environment

A vertically flowing, closed circuit, high pressure water tunnel was designed and constructed for holding individual gas bubbles stationary against an opposing flow for detailed observations. Hydrate formation behavior of natural gas bubbles was studied at constant pressure as well as under conditions of controlled decompression designed to simulate buoyant rise of the bubble.A bubble of simulated natural gas suspended in 3°C salt water formed hydrates when the pressure was 4826 kPa or higher. The simulated decompression accompanying buoyant rise had very little effect on hydrate formation behavior of a bubble starting from a pressure of 5516 kPa or above. At lower starting pressures, a slight increase in the reaction rate was detected in the initial stages of a run. The conversion of the simulated natural gas to hydrates was complete in runs starting from a pressure of 4826 kPa or above.     

Effectiveness factor of a catalyst pellet in a trickle bed reactor: Limiting reactant in the gas phase

The influence of both internal and external wettings on the effectiveness factor of a partially wetted catalyst pellet in a trickle-bed reactor when the limiting reactant is in the gas phase is analyzed.A new parameter, not considered up to now, measuring the gas-liquid interfacial area inside and on the pellet surface, is introduced to study different forms of wetting.The reaction in the dry zone is also taken into account and its influence on the effectiveness factor for both the pores mouth and pores end dry is studied.An approximate analytical expression for the effectiveness factor, giving close results to the numerical solution, is proposed.     

The effect of pressure on the flow of gas in fluidized beds of fine particles

The division of gas flow between the bubble and dense phases of fluidized beds of six different types of Group A powders has been studied at pressures of up to 20 bar using surface collapse and X-ray absorption measurements. It was found that with these fine powders as pressure increases at constant volumetric gas flowrate so the size and hold-up of bubbles decrease while their frequency increases. Contrary to previous measurements the average bubb velocity appears to decrease with increasing pressure. The dominant mode of bubble break-up in all the powders was found to be division from the rear, contrast to that observed with Group B powders at atmospheric pressure. Interstitial phase voidage was found to increase with increasing pressure.The results are interpreted in terms of a model which assumes a difference between the voidages, and hence the gas flow, of powder in the wakes behind     

Particle—particle interaction force in a dilute gas—solid system

An experiment to measure pressure drop and solids velocities in a dilute pneumatic conveying pipe has been designed and constructed. A model based on momentum balance over particles has been developed to translate pressure drop and solids velocities data into an expression for particle—particle interaction. A correlation for particle—particle interaction with the relative velocity of 7 to 16 m/s has been developed. This correlation describes our experimental data within the 20% deviation.     

The role of dielectric permittivity on the stoichiometry of the charge-transfer complexes and ion-pairs of iodine with triethylamine

The charge-transfer interaction between triethylamine (Et₃N) and iodine has been studied in the solvents of different dielectric permittivity, using conductometric, refractometric and differential refractometric techniques. The K₁ value increases with the increase in the dielectric constant of the solvent, when calculated through the electrical conductance method, while the reverse was found when calculated through refractometric and differential refractometric techniques. The co-existence, in equilibrium, of intimate and solvent separated ion-pairs in a binary mixture of dioxan-THF for Et₃NI⁺, I₃^? species has been observed. The equilibrium constant (K) and solvation number (n) for the equilibrium:

have been calculated and it was found that a single THF molecule has managed to intercalate between the cation and the anion.     

Drying granular solids in a fluidised bed—II The influence of diffusion limitation on the gas—solid contacting around bubbles

When diffusion inside the solid particles which are dried in a fluidised bed can not be neglected this internal diffusion can be accounted for by two mechanisms: (a) the long term response which describes the gradual build up of a concentration profile inside the solid particles as function of the total drying time, and (b) the short term response which describes the generation of a thin concentration boundary layer inside the particles during the passage of these particles through the cloud of a rising bubble. Both mechanisms can be analyzed separately and are included in a final model which can be conceived as a further development of the model presented in Part I.     

Experimental study of ignition and extinction waves and oscillatory behavior of a tubular nonadiabatic fixed bed reactor for the oxidation of carbon monoxide

An experimental study of axial temperature profiles in a nonadiabatic tubular fixed bed reactor has been made under the transient operation. The catalytic carbon monoxide oxidation occurring on a Pt/alumina catalyst has been used. Ignition and extinction waves have been studied both in a short and a long catalyst bed. It is shown that in a short bed ignition and extinction occur at the reactor outlet while for a long bed the ignition and extinction process takes place inside the reactor and is not influenced by the outlet part of the reactor. A systematic measurement indicates that in a narrow range of inlet parameters the integral nonadiabatic nonisothermal reactor may operate in an oscillatory state. These oscillations can be periodic or aperiodic.     

Statistical study of the grid zone behavior in a shallow gas—solid fluidized bed using a mini-capacitance probe

The behavior of the grid zone in a shallow-air-sand fluidized bed was studied by means of a mini-capacitance probe coupled with the on-line correlation technique. Experiments were carried out in a bed with a diameter of 203 mm. The bed had a height of 50 mm and was operated at different air flow rates. A perforated “plexiglas” plate with a hole diameter of 1.59 mm and a thickness of 9.53 mm was used as the distributor. The technique has been found to be effective in determining locally or microscopically the jet penetration depth and the dead zone height. The phenomenon of jetting in a shallow gas—solid fluidized bed is described semi-quantitatively.     

Binary and ternary distillation in a stirred mass transfer cell

A continuous flow stirred cell operating at point conditions has been developed for the study of mass transfer in distillation. Results for the methanol-isopropanol and methanol-ethanol binary systems showed that 85–100% of the resistance to mass transfer resided in the vapour phase, and that the vapour side coefficients correlated well with those obtained in the same cell for gas phase controlled absorption. Flux rates for the ternary system methanol-ethanol-isopropanol were in fair agreement with those predicted from the binary data, using a modified form of the Krishna-Standart theory. It is concluded that the experimental technique shows considerable promise, but requires further development to overcome discrepancies in the interfacial heat balance.     

Temperature-dependent physical properties in physical gas absorption

A solution for interface temperature rise in physical gas absorption due to heat of solution was obtained by Danckwerts [1,2] with the limitation that the pertinent physical properties—diffusion coefficient, solubility of the gas and thermal conductivity of the liquid remained constant. This paper complements Danckwerts' contribution by considering the pertinent physical properties as temperature-dependent, the dependence being of the form:$\text{Q}\text{Qi}$ = exp [γ_Q(T ? T_i)]. Wagner [3] has obtained solutions for unsteady-state diffusion when the diffusion coefficient is similarly dependent on concentration. Wagner's solutions are adapted to provide a solution for the present problem. Two approximate solutions, one valid at high Lewis Number and the other when Lewis Number approaches unity are also presented. For the normal values of activation energies, Danckwerts' model is shown to be accurate when interfacial temperature rise is less than one percent of the absolute temperature of bulk liquid. The extent of departure from the Danckwerts' model is shown to depend on combinations of activation energies of the pertinent physical properties.     

A dynamic response technique for the estimation of gas—liquid mass transfer coefficients in a stirred vessel

A method is described for evaluating mass transfer coefficients, k_La, in gas sparged stirred vessels. It involves measurement of both liquid and gas concentration responses following a step change in inlet gas concentration, and differs from previously published methods in that the k_La evaluation is independent of the gasresidence time distribution. Results are presented that demonstrate the suitability and consistency of the method for a range of gas mixing levels.A modification, that involves only a knowledge of the early liquid concentration response, is suggested as worthy of further consideration.     

Catalytic hydrogen wave in presence of dimethydithiocarbamate and cobalt(II)

The presence of dimethyldithiocarbamate (Me₂DTC) in a medium containing ammonia (borax-NH₄Cl and NH₄OH-NH₄Cl buffer) and cobalt(II) gives rise to a catalytic hydrogen wave. The effects of Me₂DTC and cobalt(II) concentrations, mercury column height and addition of gelatin on the catalytic hydrogen wave are discussed. A mechanism for this wave is proposed in which the H₃O⁺ is reduced on Co(0) stabilized by the Me₂DTC at the mercury surface.