A MONTE-CARLO APPROACH TO THE INTERPRETATION OF BUBBLE PROBE SIGNALS IN FREELY BUBBLING FLUID BEDS†

The interpretation of the signals generated by a double probe may be done by the three characteristic times method:

t₁ the time duration of the pulses;

t₂ the time shift between the signals of the two probes;

t₃ the time interval between two pulses on one channel.

Each of these times is largely dispersed and the corresponding histograms may be constructed.

The present work is a trial to go over from the time histograms to physical properties of the bubbles combining a Monte-Carlo simulation and a flexible simplex optimisation procedure.

As a result, the percentage of oblique bubbles cutting just one level, the bubble size distribution, the average velocity-size relation, and the individual dispersion around it may be defined.

The procedure is finally applied to experimental results obtained with a light probe in a fluid bed of glass beads.     

ON THE PERMEATION OF SOME GASES THROUGH THE LAMELLAE OF STATIC FOAMS. A PRELIMINARY APPROACH

Basing on the capillary method for the determination of foam bubble size, a method was proposed for establishing values of permeability coefficients of several gases. The permeability coefficients of He, Ar, H₂, N₂, CH₄ through the lamellae of static foam bubbles were preliminarily determined using this method.

A rough dependence of the N₂ permeability coefficient through the lamellae of static foam bubbles on the foam wetness and surfactant concentration was also found.

The results obtained lead to the conclusion that the Princen-Mason diffusion model of gas permeation through an isolated soap film was inadequate in the case of permeation through lamellae of bubbles forming a static foam.

A hypothesis that the gas permeation through the lamellae of a static foam is primarily caused by liquid convection in the lamellae core was put forward.     

VISCOSITY OF BIOMATERIALS

Viscosity data for honey, corn oil, mayonnaise, yogourt, blood and banana puree have been analyzed using two Theological models: the Herschel-Bulkley model and a proposed model. The proposed model contains three parameters: a yield stress, a parameter having the units of time and a parameter having the units of viscosity.

The model parameters were obtained by non-linear regression and the proposed model was shown to compare favorably with the Herschel-Bulkley equation.

An Arrhenius-type of correlation could be verified between the viscosity of banana puree and the inverse of the temperature. Also, the time parameter (t₁) of the proposed model could be correlated with the temperature and the parameter η₁.

It is asserted that the proposed model should replace advantageously the commonly used Casson expression.     

Drying Sawdust in a Pulsed Fluidized Bed

The objective of this work was the experimental and theoretical study of sawdust drying, in batch and continuous experiences, using a pulsed fluidized bed dryer.

In the batch experiences, a 2³ factorial design was used to determine the kinetics of drying, the critical moisture content, and the effective coefficients of both diffusivity and heat transfer, all of them as a function of the velocity and temperature of the air, the speed of turning of the slotted plate that generates the air pulses in the dryer, using sawdust with 65% moisture in each run.

In the continuous operation, a 2³ factorial design was used to study the effect of the solid flow and the velocity and temperature of the air on both the product moisture and the distribution of residence times. In order to determine these last ones, digital image processing was used, utilizing sawdust colored by a solution of methylene blue as tracer.

The statistically significant factors were the velocity and the temperature of the heating air, for both the continuous and batch operations. Although the speed of turn of the slotted plate was not significant, it was observed that the air pulses increased the movement of particles, facilitating its fluidization, especially at the beginning of drying.

The heat transfer coefficients were adjusted according to the equation Nu = 0.0014 Re_p^1.52, whose standard deviation of fit is 0.145.

The period of decreasing rate was adjusted to several diffusivity models, giving the best fit the simplified variable diffusivity model (SVDM). The curve of distribution of residence times was adjusted using the model of tanks in series, with values between 2.6 and 5 tanks.     

EVALUATION OF CONDUCTIVE HEAT TRANSFER MECHANISMS BETWEEN AN IMMERSED SURFACE AND THE ADJACENT LAYER OF PARTICLES IN BUBBLING FLUIDIZED BEDS

The evaluation of the heat transfer coefficient h_wp between a heat exchanging surface immersed in a gas fluidized bed and the adjacent layer of dense phase particles is analyzed in this contribution. Gas convective and radiant effects are not included in the present analysis.

The inclusion of h_wp, or an equivalent formation, in mechanistic models describing heat transfer has been necessary because the sudden voidage variation close to the immersed wall restrains significantly the heat transfer rate. However, there is not at present a widely accepted expression to evaluate h_wp.

A precise formulation for h_wp accounting for transient conduction inside spherical particles, the Smoluchowski effect, the concentration of particles in the adjacent layer (N_p) and an effective separation gap (l₀) is developed here.

Although N_p can be estimated, in principle, from experimental evidence in packed beds, and it is reasonably expected that l₀ = 0, the analysis of experimental heat transfer rates in moving beds, packed beds, and bubbling fluidized beds indicate that values of h_wp are, in general, smaller than expected from these assumptions. Appropriate values of l₀ and N_p are then stimated by fitting the experimental data.

The probable effect of surface asperities is also discussed by analyzing a simplified geometrical model. It is concluded that the parameter l₀ can be also effective to account for particle roughness, independently of thermal properties.     

EFFECTS OF GEOMETRY ON HYDRODYNAMICS IN EXTERNAL-LOOP AIRLIFT REACTORS

Experimental investigations were carried out in model external-loop airlift reactors. Two reactors of laboratory scale (riser liquid height ranged between 1.16-1.56 m, riser diameter 0.03 m, A_D/A_R ratio between 0.111-1,000, total liquid volume V_T = (1.189-2.446).10^-3m³) and pilot-plant scale (riser liquid height of 4.4 and 4.7 m, respectively, riser diameter 0.200 m, A_D/A_R ratio of 0.1225 and 0.040 m, total liquid volume, V_T = (0.144-0.170) m³) were used.

The influences of reactor geometry characterized by some parameter as: A_D/A_R ratio, liquid height in riser and downcomer and liquid height in gas separator, together with the amount of introduced air, on the basic hydrodynamic design parameters: gas holdup and liquid circulation velocity were analysed.

The influence of gas sparger design on gas holdup and liquid velocity was found to be negligible.

The experimental liquid circulation velocity was correlated using a simplified form of the energy balance in airlift reactors, valid for external-loop airlift reactors with almost complete phase separation at the top.

An original dimensionless correlation for gas holdup prediction involving superficial velocities of gas and liquid, cross sectional areas, dispersion height, riser diameter, as well as Froude number, was obtained.     

Reversible pyroelectric effect in Pt(Sc1/2Ta1/2)O3 ceramics under dc bias

—It has been shown that quenched Pb(Sc_1/2Ta_1/2)O₃ (PST) disordered ceramics and crystals show diffuse dispersive dielectric properties, while well annealed ordered materials exhibit normal sharp first order transition. The pyroelectric depolarization measurements taken using a Hewlett Packard Model 4140B picoammeter/DC Source under computer controlled heating cycle also have shown different behaviors between disordered and ordered materials.

In this work pyroelectric measurements by Chynoweth method under DC bias up to 1.8 KV/mm within a temperature range of 70°C around the temperature of maximum dielectric constant have been studied. A very significant enhancement of the pyroelectric signal under DC bias is observed in thermally quenched disordered samples. The largest enhancement of the signal appears at temperatures some degrees below the temperature of maximum dielectric constant.

The existence of microdomains in disordered materials is believed to be responsible for this new extrinsic component of reversible pyroelectricity.

The large reversible pyroelectric effect is a promising phenomenon for developing new pyroelectric devices.     

Hg REMOVAL FROM HYDROCARBON LIQUID USING POLYSULFIDE

Some crudes and condensates are known to contain a small quantity of Hg. A process has been devised to remove up to 80-90% of this Hg from the oil. In the process, the oil is washed with an aqueous solution containing 50 to 1000ppm of Na₂S_x.

The efficacy and the feasibility of the process concept have been demonstrated in the laboratory using both batch and flow systems. When the Na₂S_x concentration in the treating solution was higher than 100ppm, the Hg removal reaction was mass transfer limited. By increasing mixing intensity and the number of contacting stages, Hg was removed from 200 to 10ppb using a solution of 500 ppm Na₂S_x. The mixing can be accomplished using one on-line centrifugal pump and preferably two in series.

Phase separation between the aqueous solution and the oil product was rapid. In the process, the sulfur contamination of the oil product, due to Na₂S_x, was low.     

HYDRODYNAMIC CHROMATOGRAPHY: THREE DIMENSIONAL LAMINAR DISPERSION IN RECTANGULAR CONDUITS WITH TRANSVERSE FLOW

Experimental observations^1,9 indicate much poorer separations than are predicted by two dimensional theory. The purpose of this work is to explain these differences and suggest ways in which system performance can be improved.

The large effect of span-wise variation in axial velocity caused by side walls on hydrodynamic separations carried out in rectangular conduits with transverse flow is studied theoretically. As the aspect ratio increases, the steady stale retentivity (convection coefficient) approaches an asymptotic value obtained by neglecting side wall effects. However, the dispersion coefficient does not reduce to that for a flow with no side walls. Indeed, the asymptotic steady state dispersion coefficient is at least six times larger than that obtained by two dimensional theory which neglects side wall effects. As the transverse Peclet number increases, the effect of side walls on the dispersion coefficient becomes much larger.

The present three dimensional theoretical predictions, in contrast to two dimensional ones, are in good agreement with the experimental data of Caldwell, et al.⁹ and Kesner, et al.¹ on electrical field flow fractionation. The results indicate that side wall effects may be of major importance in hydrodynamic chromatography even when the aspect ratio is 70 or more.

The adverse effect of side walls may be avoided by having the membranes enclose thin annular regions rather than rectangular conduits. This should improve performance significantly.     

CHARACTERIZATION OF FLUIDIZED BED REACTORS WITH GAS TRACER MEASUREMENTS

In a steady state bench scale fluidized bed the decomposition reaction of NaHCO₃ was carried out. The residence times distributions, DRT, of carbon dioxide (the gaseous product) and non adsorbing argon (the reference tracer) were mass spectroscopically measured as a function of the bed temperature. By means of single-, two- and three-phase dispersion models as well as by a cell model, the DRT's were evaluated on line by a computer.

The steady state transverse and longitudinal concentration profiles of these tracers upstream from the plane source were also measured and evaluated by a dispersion model as well as by a counter current back mixing model. Comparison of the steady state and nonsteady state dispersion coefficient measurements indicate that the longitudinal gas mixing is only partially due to backmixing. The experimentally determined wake fractions agree well with those published in the literature. Since the adsorption rate of CO₂ on the pore surface area of the particles in the dense phase is high no interphase transfer from the interstitial gas of the dense phase into the bubble phase takes place.

The desorption of CO₂ and its return into the interstitial gas and than into the gas phase occurs only slowly and with an initial time lag. The on-line DRT can be used as a diagnostical technique for investigation of the reactor during its operation, if operation disturbances or breakdowns occur.     

GAS MIXING IN SLUGGING AND TURBULENT FLUIDIZED BEDS

The gas phase mixing in a fluidized bed of glass beads (d_p = 0.362 mm) in the slugging and turbulent flow regimes has been studied in a 0.1 m-ID × 3.0 m high Plexiglas column.

The gas dispersion in the downstream of the bed has been described by a diffusion process with the axial and radial dispersion coefficients. The radial dispersion coefficient of the gas phase is nearly constant with the variation of gas velocity in the slugging flow regime, but it increases with an increase in gas velocity in the turbulent flow regime.

Appreciable backmixing of the gas phase is pronounced in the slugging flow regime whereas the lower gas backmixing is produced in the turbulent flow regime. The gas backmixing coefficient increases with an increase in gas velocity in the slugging flow regime, but it decreases slightly with an increase in gas velocity in the turbulent flow regime.

The radial mixing and backmixing coefficients of the gas in terms of Peclet numbers have been correlated with the relevant dimensionless parameters (U_g/U_mf, p_s/p_g, d_p/D_t).

The gas flow pattern in the bed has been well represented by a simplified model based on the two gas phases in the dilute and dense phases which are percolating through the bed in plug flow. The present model can predict the gas exchange coefficient between the phases, the fractions of the dilute phase, the interstitial gas in the dense phase, and the interstitial gas velocity in the bed.     

Particle Adhesion in Model Systems: 16. Barium Sulfate Particles on Glass and Protein Surfaces

The adhesion phenomena of monodispersed barium sulfate (BaSO₄) particles on gelatin-coated glass beads were evaluated using the packed column technique and compared with the same system in the absence of the protein.

Multilayer deposition was observed with the uncoated glass beads at pH 4, 5 and 6, while at pH 9, which is above the isoelectric point (pH ∼ 6) of BaSO₄ particles, monolayer deposition took place, even though the BaSO₄ particles and glass beads bore the same sign of charge. At pH = 10, no uptake was observed on the glass beads, but the addition of 10^-4 mol dm^-3 BaCl₂ induced multilayer deposition due to the adsorption of the Ba²⁺ cation on BaSO₄ particles, which causes a reversal of their charge to positive.

The formation of multilayers was found to occur over a much wider pH range on the gelatin coated glass beads.

BaSO₄ particles deposited in multilayers could not be removed from either glass beads or gelatin-coated glass beads by rinsing the loaded column with solutions of pH 11.5, but could be detached from monolayers on glass beads only.     

CHEMICAL AND TECHNICAL ASPECTS OF HYDROFORMYLATION OF OLEFINS WITH SUPPORTED LIQUID-PHASE RHODIUM CATALYSTS

Supported liquid-phase catalysts containing RhH(CO)(PPH₃)₃ dissolved in PPH₃ are used in the gas-phase hydroformylation of several olefins. Their activity in the hydroformylation of ethylene, propylene and the butenes is reported.

Various physicochemical aspects of the rhodium SLP catalysts, such as the adsorptive withdrawal of the rhodium complexes by the supports, the activity of the rhodium complexes at the gas-liquid interface and the solubility of the reactants and products in the liquid part of the catalysts, are discussed.

The results are presented of a bench-scale process study of the hydroformylation of propylene, from which the design parameters of a future plant are calculated.     

IN SITU LEACHING OF URANIUM USING DILUTE SULFURIC ACID AND MOLECULAR OXYGEN

The technical feasibility of in situ uranium leaching using dilute sulfuric acid and molecular oxygen has been assessed and the important process parameters examined by use of laboratory high pressure leaching columns.

The dilute H₂SO₄/O₂ lixiviant was effective in leaching uranium from the ore samples tested. The leaching process was chemical reaction rate limited and can be represented using pseudo first-order kinetics. The leaching rate constant is proportional to the proton concentration of the lixiviant.

Much of the uranium was leached from the ore before decomposition of carbonate minerals by the acid was complete. Acid consumption per pound of U₃O₈ increased sharply as the uranium recovery level exceeded 70%. There appears to be a minimum oxygen pressure for effective uranium leaching. A pressure of 2758 KPa was adequate for the ore samples tested     

TRANSIENT CONVECTIVE MASS TRANSFER IN RECTANGULAR FIELD-FLOW FRACTIONATION CHANNELS

The work of Takahashi and Gill (1980) on the transient convective mass transfer in three dimensional rectangular channels in the presence of a transverse field is extended to include the time dependence of the convection and dispersion coefficients. The methodology of generalized dispersion theory is used to predict the breakthrough curves. The functions, f₀ and f₁, are determined by introducing a two-dimensional Sturm—Liouville operator.

The results suggest that the side wall effects on the axial dispersion cannot be neglected and a dispersion equation with time dependent coefficients is appropriately describable of the practical field-flow fractionation system.     

THE CHARACTERIZATION OF KINETICS BY SINGULAR POINTS: COMPARISON OF SUPPORTED VS. UNSUPPORTED AND ISOTHERMAL VS. NONISOTHERMAL Pt CATALYSTS IN AMMONIA OXIDATION

Similar multiplicity features were observed during ammonia oxidation on supported or unsupported Pt catalysts, whether in the isothermal or nonisothermal modes of operation. Two types of isothermal multiplicity patterns were observed upon varying ammonia concentration. The simplest model predicting the transition between the two must have a pitchfork singularity. The same singularity accounts also for the diagrams observed with varying oxygen concentration. The singularities of the isothermal kinetics are inferred also from nonisothermal experiments

These similarities suggest that identical kinetic models are responsible for multiplicity in supported and unsupported catalysts. The dependences on either reactant is used to characterize the kinetics by defining the highest order singularity in the temperature-concentrations space. This information is utilized to develop the simplest rate expression which accounts for the observations

The two simultaneous reactions, producing N₂ and N₂O, were found to ignite and extinguish simultaneously.     

Surface Characterization of Vulcanized Rubber Treated with Sulfuric Acid and its Adhesion to Polyurethane Adhesive

Modifications produced on a vulcanized styrene -butadiene rubber surface by treatment with sulfuric acid were studied and several experimental variables were considered.

The treatment of R1 rubber with sulfuric acid produced a noticeable decrease in contact angle which was mainly ascribed to an increase in surface energy due to the formation of sulfonic acid moieties and C=O bonds, and the removal of zinc stearate. The rubber surface swelled and became brittle as a result of the treatment, and when flexed microcracks were created. A rubber surface layer modification was produced with a consequent decrease in tensile strength and elongation-at-break values. The treatment enhanced the T-peel strength of R1 rubber/polyurethane adhesive joints and the locus of failure was cohesive in the rubber.

The optimum immersion time in H₂SO₄ solution was less than 1 min., and the reaction time in air was not found to be critical; the neutralization with ammonium hydroxide and the high concentration of the sulfuric acid (95 wt%) were essential to produce adequate effectiveness of the treatment.     

STEAM CONDENSATION IN A VERTICAL CORRUGATED DUCT

An experimental investigation was made on the condensation of water steam in a vertical corrugated duct. The data have been correlated as follows

Co=5.11Re^-0.431 150 ≤ Re ≤ 350 Co = 0.034311Re^-0.425350 ≤Re ≤l000

The vertical corrugated duct is constructed of two corrugated plates with corrugation inclination angles of β = 0 and β = 45° respectively (relative to the overall flow direction).

The condensation heat transfer coefficient in the corrugated duct is more than two times higher than that of bulk condensation on a vertical plate. A physical model was proposed to explain the heat transfer enhancement. Attention was also paid to the effect of exit steam velocity on the heat transfer during partial condensation. It was demonstrated that the heat transfer in the corrugated duct was strongly affected even at a low exit velocity, which was different from the case of bulk condensation on a vertical plate. Experimental apparatuses and the method for examining their reliability are described in detail.     

Preadhesion Laser Treatment of Aluminum Surfaces

An excimer laser may be used for preadhesion treatment of aluminum alloys. This method presents an alternative to the use of ecologically unfriendly chemicals involved in conventional anodizing pretreatments.

Experimental results indicate that preadhesion laser surface treatment significantly improved the shear strength of modified-epoxy bonded aluminum specimens compared with untreated and anodized substrates. The best results were obtained with laser energy of about 0.2 J/Pulse/cm² where single lap shear strength was improved by 600-700% compared with that of untreated Al alloy, and by 40% compared with chromic acid anodizing pretreatment.

The mode of failure changed from adhesive to cohesive as the number of laser pulses increased during treatment. The latter phenomenon has been correlated with morphology changes as revealed by electron microscopy, and chemical modification as indicated by Auger and infrared spectroscopy.

It can be concluded that the excimer laser has potential as a precise, clean and simple preadhesion treatment of Al alloys.     

EXPERIMENTAL STUDY OF THE SELECTIVE HYDROGENATION OF STEAM CRACKING C2CUT. FRONT END AND TAIL END VARIANTS

Selective hydrogenation is the habitual industrial process to eliminate the most unsaturated hydrocarbons, which are harmful for later applications. In this paper, the kinetics of the selective hydrogenation of a C₂ mixture over two palladium/alumina catalysts with both front end and tail end variants, have been studied. Experiments have been carried out to analyse the influence of temperature, hydrogen/acetylene molar ratio, carbon monoxide content in the feed and hydrocarbon volumetric flow rate on the corresponding conversion and selectivity.

The experiments were performed in an integral plug flow reactor and the integral method was used for the kinetic analysis. The minimization of the objective function was made by the Marquardt algorithm for multiple response and the continuity equation set integrated by fourth order Runge-Kutta technique.

The most adequate models were the power law type for the experimental range. The comparison between experimental and observed values of the acetylene and ethane molar fraction in the hydrocarbon mixture, which are used for minimization, confirm the suitability of the fit.