RECIRCULATING REACTOR FOR THE STUDY OF MULTIPHASE KINETICS

A new laboratory reactor was set up to measure kinetic coefficients in a solid (catalyst)-liquid-gas reacting system.

The reactor consists of two parts: an absorber, where the liquid is partially saturated by the gas reactant and a reacting zone, where the liquid alone, containing the dissolved gas, flows through a fixed bed of catalyst.

The ricircle of the liquid in the absorber maintains a high concentration of the gas reactant in the liquid also in the zone of reaction, allowing the use of a high mass of catalyst (significative from a statistical point of view) and the achievement of sufficiently high conversion.

The tested reaction is the catalysed hydrogenation of ∝-metylstyrene: in order to consider a drastic situation and to verify the results with the literature data, the experimental conditions examined corresponded to very high chemical reaction rate (instantaneous reaction) at the surface of the pellets.

The tests were carried out with the reactor working both in batchwise and in continuous operative mode (steady state); the results show the reliability of the new reactor above all when the steady state operation is considered. For the use of the reactor in batchwise condition, the accumulation of the product inside the catalyst particles must be considered for an accurate measurement of the kinetic parameters,     

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.     

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.     

PYROLYSIS AND CHLORINATION OF SMALL HYDROCARBONS

The bases of the kinetic modeling of Chlorine containing systems are addressed in this article

A kinetic scheme, involving more than 300 elementary reactions and 46 molecular and radicalic species, has been developed based on general thermochemical kinetic theories as well as the consolidated know-how in the field of hydrocarbon pyrolysis

Several comparisons with commercial and laboratory experimental data indicate a fair agreement in a wide area of lower and higher pressures (up to 30atm) also covering the high temperature range of methylchloride pyrolysis (1000°C). Mathematical model of EDC pyrolysis furnaces already allows to evaluate process performances and alternatives accounting for fouling rates and on-stream times

The kinetic scheme can be applied to study the methane chlorination reaction system and the dichloroethane (EDC) pyrolysis system. The kinetic scheme is coupled with a furnace model and appropriate mathematical techniques to solve the resulting system of material, energy and momentum conservation equations. A coking model has also been incorporated as fouling of coils determines the run length of the furnaces. Simulation of the pyrolysis of EDC to produce vinyl chloride shows that the results are in reasonable agreement with observed values and trends. Application of the model for the design of coils and evaluation of process alternatives is discussed.     

ON THE FATE OF SINUSOIDAL RADIATION TO A VOLATILE OPAQUE LIQUID

A method was developed that predicts the energy distribution of sinusoidal radiation incident on a volatile opaque liquid surface. The energy is absorbed by the liquid phase, absorbed by the gas phase and used to furnish the heat of vaporization of the volatile liquid. In an example of heptane and air, 94.8% of the energy conducted into the liquid phase 1.0% was conducted into the gas phase and 4.2% was required for the heal of vaporization

The method could be used to determine the emissivity of a material radiating to a volatile opaque liquid.

Results may also find application in reducing hydrocarbon emissions from fixed-roof tanks by as much as 30% if aluminum instead of steel tank domes were used     

STUDY OF THE PARAMETERS CONTROLLING THE ALTERNATING COPOLYMERIZATION OF METHYL METHACRYLATE AND STYRENE

The copolymerization of methyl methacrylate and styrene, in the presence of zinc chloride, was conducted in the aqueous phase. The zinc chloride acting as a complexing agent

Among the factors affecting the polymerization yield, we have; the level of complexing agents, the monomer ratio and the temperature

The polymers were characterized for their molecular weight (G.P.C.) and analyzed by N.MR, and D.T.A

A model taking into account the various parameters is proposed.     

A THEORETICAL EVALUATION OF CATALYST DISTRIBUTION ON THE PERFORMANCE OF NON-IDEAL SLURRY REACTORS

A study has been made of the influence of catalyst concentration profiles on isothermal continuous slurry reactor performance. Solid catalyst behaviour was modelled with the one dimensional sedimentation-dispersion model and that of the liquid with the one dimensional dispersion model. The Power-law kinetic model as well as Michaelis-Menten and a form of Langmuir-Hinshelwood kinetics were considered.

The results of the computations are given in terms of departures of the exit concentration obtained with a selected, non-uniform, catalyst distribution with respect to constant catalyst profile (reference case). Only for intermediate mixing conditions were differences in the exit concentration found. These differences are usually negligible for power law and Michaelis-Menten kinetics (especially for conversion not very high); on the contrary, they became meaningful for the Langmuir-Hinshelwood—at least for some ranges of the parameters.

The existence of an optimal catalyst distribution was also discussed.     

THE FLOODING CAPACITY OF PERFORATED PLATES IN ROTATING LIQUID-LIQUID-SYSTEMS†

Centrifugal extractors found wide applications in the industry, but up to now very little is known about what happens inside and how to calculate fluid-dynamic and mass transfer. For the basic research of dispersed liquid-liquid-systems in centrifugal fields a model centrifuge was built which is resistant to the high loads produced by rotation and which enabled us to observe the flow mechanism at the flooding points.

The processes inside have been measured and photographed by a high-speed-camera.

The calculation of centrifugal extractors is based mainly on the contact surfaces of the phases and the contact times. The maximum flow or flooding capacity is determined by three limits. Two of them are determined by the interface locations inside, controlled from the back pressure. The third limit is given by the maximum combined flow. This limit is dependent on the set of internals, the rotor speed and the physical properties of the liquid system. The experimental results will be shown and compared with new theories for the flooding capacities of perforated cylindrical plates in rotating liquid-liquid-systems.

The results predicted using the mathematical model for calculating the capacities are in fair agreement with the measurements.     

SIMULATION OF FORESTRY BIOMASS DRYING IN A ROTARY DRYER

Drying of forestry biomass in a rotary dryer has been performed. The raw material used was Erica Arborea belonging to the ever-green, broad leaves ecosystem which covers Central Greece and other Mediterranean countries. The study was part of a project concerning a Greek biomass pyrolysis demonstration plant where drying of biomass is very important in the contribution to the global energy balance and product yields of pyrolysis.

The study includes two parts. First, the experimental part concerns the influence of air flowrate, temperature, rotation speed and inclination of a laboratory rotary dryer to biomass residence time and biomass outlet moisture content. The second part concerns the development of a mathematical model for biomass drying in a rotary dryer. Experimental measurements in a rotary dryer were compared to the data from the model, in order to check the validity of the model.     

MEASUREMENT OF INTERFACIAL AREA AND MASS TRANSFER COEFFICIENTS BY CHEMICAL METHOD OF C02 ABSORPTION INTO AQUEOUS SOLUTIONS OF NaOH IN A MODEL COLUMN WITH EXPANDED METAL SHEET PACKING

The theory of gas absorption accompanied by fast pseudo-fast order reaction which considered dependences of diffusivity, kinetic constant and Henry's law constant on absolute temperature and ionic strength was used to obtain values of effective interfacial areas and mass transfer coefficients in gas and liquid phase.

Experimental measurement of carbon dioxide absorption from mixture with air was performed in a pilot-plant column with expanded metal sheet packing irrigated with sodium hydroxide solution.

Resulting liquid and gas-side mass transfer coefficients are compared with values obtained from physical Absorption measurement of carbon dioxide into water and with measurement of gas-side mass transfer coefficient for sulphur dioxide in the same column.

The differences between determined values are discussed.     

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.     

AN ANALYSIS OF CELLULAR FLOW OBSERVED ON THE INTERFACE OF A PARTIALLY FORMED DROPLET†

In [16], during an experiment designed to model the internal circulation of a forming droplet, secondary surface flows were observed on the droplet interface.

After summarizing the experimental results of [16], we present one possible mechanism, based on the surface surfactant mass transport equation of Levich and the surface stress-strain boundary conditions at a free surface, that provides a good qualitative explanation of the origins and the nature of the secondary motion observed in [16]. The critical hypotheses in this mechanism are that the normal component of ihe vorticity at the free surface is determined primarily by the components of the velocity field tangential to the level lines of the surface surfactant density, near the maxima and minima of that density function and that the normal component of the fluid stress does not vanish at such points.

The consequent analysis of the mass transport equation in the interface shows that the resulting surface motion may be viewed as arising from a resonance phenomenon analogous to the forced vibrations of a spring at resonance.

The effects of adsorbtion-desorbtion and surface dilational viscosity may be incorporated in this mechanism. A method for the experimental measurement of surface dilational viscosities is proposed.     

SURFACE TENSION EFFECTS IN THE SEDIMENTATION OF COAL PARTICLES IN VARIOUS LIQUID MIXTURES

The sedimentation volume, V_sed, of coal particles in mixtures of pairs of non-polar and more polar organic liquid combinations as well as in aqueous solutions was determined at 20°C.

The liquid combinations were chosen such that the surface tension, γ_SV, of the coal particles fell between the surface tensions, γ_LV, of the two liquids. A constant mass of a given coal sample was suspended in constant volumes of liquid mixtures of different concentrations.

It was found that the sedimentation volume changed with varying composition of the liquid mixtures, as did the surface tension. A maximum or a minimum occurred in V_sed when the surface tension of the suspending liquid was equal to that of the coal particles, i.e. when γ_LV = γ_SV. Maxima occurred in more polar and minima in the non-polar or less polar liquid mixtures. The position of the extrema, and hence the surface tension, γ_sv, of the particles, was found to change with particle size, in agreement with findings from other independent techniques.

It was found that the surface tension of coal, γ_sv, depends on the surrounding liquid, i.e. coal is hydrophobic in organic and hydrophilic in aqueous media, again in agreement with the results from other techniques.     

THERMODYNAMIC PROPERTIES OF BINARY LIQUID MIXTURES OF ETHANE AND ETHYLENE WITH METHANE AND THE RARE GASES

Two series of binary liquid mixtures containing either ethylene or ethane have been investigated, at one or more temperatures (usually at the triple-point temperature of the component with the higher melting-point). In the ethylene series liquid-vapour equilibrium and liquid density studies were carried out for mixtures with methane, krypton and xenon; the heats of mixing were also measured for the ethylene + krypton mixtures. In the ethane series, which comprised mixtures with methane, argon, krypton and xenon, all three properties were measured except for the ethane + methane and ethane + argon systems where the enthalpies of mixing were already known. The ethylene + ethane system was also investigated at 161.39 K.

The results have been used to estimate the thermodynamic excess functions G^E, V^E and H^E. The G^E values decrease, within each series, as one moves from the lighter to the heavier rare gas, the values being lower in the ethane series. For the ethane + xenon mixtures both G^E and H^E are negative, showing a weak attraction between the two molecules. The V^E values for the mixtures of hydrocarbons suggest the probable formation of interlocking structures between the two components.

The values of the thermodynamic excess functions have been interpreted in the light of the model of Frisch-Longuet Higgins-Widom for the liquid state.     

RECOVERY OF FATTY ACID FROM A SOLVATED HYDROCARBON MIXTURE APPLICABLE TO THE BEAVER-HERTER SOLVENT EXTRACTION PROCESS

The Herter process is a patented enhanced oil recovery process that proposes the use of a fatty acid as a solvent to recover petroleum from an oil reservoir or a tar sand. The fatty acid solvent is injected into a reservoir formation or mixed with a tar sand in order to reduce the viscosity and increase the mobility of the petroleum.

The recovery and recycle of the fatty acid solvent are fundamental parts of the process. The Herter process proposes to recover the fatty acid by saponification, migration of the soap to the aqueous phase, and desaponification of the resulting soap phase. However, the formation of a hydrocarbon-soap-water emulsion severely limits the proposed process. Addition of methanol limits emulsification to allow a meaningful separation to be accomplished.

An evaluation of two proposed revisions to the Herter process was made based on batch experiments involving the recovery of oleic acid, linolenic acid, and Georgia Pacific XTOL™304 fatty acid from solvated mixtures with n-heptane. In both of these methods, methanol was introduced to the process as a cosolvent for the saponifying base (sodium hydroxide). The two methods differed in the manner of methanol recovery. In method 1, the saponification heavy phase was distilled to recover methanol. In method 2, the desaponification heavy phase was distilled to recover methanol.

Both of these methods had fatty acid recoveries greater than 84%, n-heptane recoveries greater than 89%, and methanol recoveries greater than 67%. Method 1 recovered purer fatty acid and a slightly greater percentage of n-heptane. Method 2 recovered a greater percentage of both fatty acid and methanol. Method 2 was found to be superior when high purity of the recovered fatty acid product was not required.     

COMPARISON OF SUPERHEATED STEAM AND AIR OPERATED SPRAY DRYERS USING COMPUTATIONAL FLUID DYNAMICS.

In this paper a numerical simulation of a spray dryer using the computational fluid dynamics (CFD) code Fluent is described. This simulation is based on a discrete droplet model and solve the partial differential equations of momentum, heat and mass conservation for both gas and dispersed phase.

The model is used to simulate the behaviour of a pilot scale spray dryer operated with two drying media : superheated steam and air Considering that there is no risk of powder ignition in superheated steam, we choosed a rather high inlet temperature (973 K). For the simulation, drop size spectrum is represented by 6 discrete droplets diameters, fitting to an experimental droplets size distribution and all droplets are injected at the same velocity, equal to the calculated velocity of the liquid sheet at the nozzle orifice.

It is showed that the model can evaluate the most important features of a spray dryer : temperature distribution inside the chamber, velocity of gas, droplets trajectories as well as deposits on the walls. The model predicts a fast down flowing core jet surrounded by a large recirculation zone. Using superheated steam or air as a drying medium shows only slight differences in flow patterns. Except for the recirculation which is tighter in steam.

The general behaviour of droplets in air or steam are quite the same : smallest droplets are entrained by the central core and largest ones are taken into the recirculation zone. In superheated steam, the droplets penetrate to a greater extent in the recirculation zone. Also, they evaporate faster. The contours of gas temperature reflect these differences as these two aspects are strongly coupled. In both air and steam there is a “cool” zone which is narrower in steam than in air. Finally, the panicle deposit problem seems to be more pronounced in air than in steam.

Adding to the inherent interest in using superheated steam as a drying medium, the model predicts attractive behaviour for spray drying with superheated steam. In particular. under the conditions tested with the model, a higher volumetric drying rate is obtained in superheated steam.     

THE INFLUENCE OF COMPOSITION ON THE SWELLING OF KRAFT BLACK LIQUOR DURING PYROLYSIS

During the drying and pyrolysis phases of kraft black liquor combustion, significant swelling of individual liquor particles occurs. Swollen volumes can reach 20 to 30 times the original volume during combustion. The swelling process can affect the combustibility of black liquor and the amount of carryover in a recovery furnace.

The composition of black liquor was found to have a large influence on swelling. A combination of sugar acids and kraft lignin swelled to a larger extent than when either component was pyrolyzed separately. A 1:1 ratio of these two components resulted in maximum swelling for the various ratios tested. The molecular weight of kraft lignin had an effect on swollen volume with higher molecular weight fractions producing lower swelling chars.

Other components were found to reduce the swelling of black liquors. Extractives interfered with the swelling by appearing to change the deformable properties of the pyrolyzing material. Inorganic salts acted as a diluent.

Analysis of the surface characteristics of chars revealed that good swelling chars were composed of small bubbles 50 to 150 microns in diameter. Poor swelling liquors did not exhibit this phenomenon. The formation of bubbles was found to be initiated at 240°C, which closely corresponded to the thermal decomposition temperature of a sugar acid. Kraft lignin appeared to have a major influence on the fluid properties of the pyrolyzing particle. The composition of black liquor determines to a large extent surface forces present in black liquor; these forces are thought to be responsible for the extent to which kraft black liquors deform and swell during pyrolysis.     

APPLICATION OF TURBULENCE FUNDAMENTALS TO REACTOR MODELLING AND SCALEUP

Applications of the fundamentals of turbulent mixing become clear once those fundamentals are understood. The first article in this series presented those fundamentals, in order to show how to apply turbulent mixing fundamentals modelling and scaleup, this article covers the following topics:

1. reaction types and their interaction with mixing;

2. closure of the Reynolds equations for mixing and reactions;

3. application to complex geometries;

4. random coalescence-dispersion modelling;

5. application to complex chemistry.

The most difficult problem in applying our knowledge of turbulence to mixer modelling and scaleup is the choice of model complexity. The levels of model complexity available and how to apply them to various problems are presented following the introduction.     

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.