Wet dielectric separation

We have studied the possibility of using electrical phenomena in order to selectively separate minerals with various densities and dielectric constants in non-homogeneous electric fields. This technique is based on levitation forces acting on a solid particle immersed in dielectric fluid.When the gradient of the electric field is directed downwards, the levitation force in the dielectric fluid (pure liquid or electrical colloids) will be in an upwards direction to float sinking minerals by counterbalancing their density mismatch with the liquid medium.The separation process can be made by using two possible modes: either by direct application of electrical forces or by change in the density and dielectric constant of a liquid medium in which the mineral particles are placed. We have investigated several electrode configurations in order to produce electrical fields which are strongly non-homogeneous. Further investigations on the dielectric properties of liquids have been performed.     

Composite separation units and their application in dialysis for the isolation of intermediate-sized molecules

Composite separation units (i.e. assemblages of similar elementary separation units) are studied. Dialysis is used as an example of a basic unit. The overall input-output response of a composite group made up of a system of interconnected dialyzers is developed in relation to the system configuration. In certain flow configurations, a composite dialyzer has the permeability characteristics of a band-pass filter. Since the selectivity of composite dialyzer systems can be amplified by repeating the units in a cascading manner, a specific solute of intermediate permeability can, in principle, be extracted from a complex multicomponent solution.     

Computer-aided analysis and design of robust multivariable control systems for chemical processes

A conceptual framework to design robust process control systems is develope d and its realization through an interactive computer-aided design software is presented. The overall design methodology is based on a unified treatment of recent theoretical results in modern control and new computational techniques in symbolic logic manipulation, singular value decomposition and optimization. Several physical examples are given to demonstrate the application of the design approach and the utility of its computer software.     

Analytical optimization and sensitivity analysis of forced periodic chemical processes

Intentional periodic manipulation of the inputs to a chemical reactor can in some cases improve the amount of product and its composition. A variational method for analyzing the effects of single and multivariable periodic forcing functions on process performance is presented in this work and illustrated in several examples. While the method discussed here is not theoretically justified for large-amplitude input variations, comparisons of analytical results obtained with the method and simulation results for large-amplitude reactor cycling show good qualitative agreement. The approximate analytical method provides a reasonable initial estimate of the structure of an advantageous periodic control.     

Simulation of packed-bed separation processes using orthogonal collocation

The two-point boundary value problem resulting from the heat and material balance equations of a packed separation column are solved using polynomial approximation techniques. The model equations are based on the two-film theory of mass transfer. The resulting partial differential equations are first reduced to ordinary differential equations and then integrated using semi-implicit Runge-Kutta method of integration. Application of orthogonal collocation simplifies the solution of the two-point boundary value problem. For the examples studied, the algorithm is found to converge rapidly with respect to the number of collocation points used in the polynomial approximation.     

Simultaneous diffusion and chemical reaction in two-phase systems

An approximation is given for the case of simultaneous steady state diffusion and chemical reaction in a liquid—liquid system under such conditions that diffusion and chemical reaction are of the same order of magnitude in the two phases. The two-film model was adopted to represent the system, and results were plotted as variations of the enhancement factor with the diverse parameters affecting it. Comparison was made with an exact numerical solution, and the results show that the proposed approximation leads to small errors for low reaction rates, and gives a first approximation for the mass transfer rate for very rapid chemical reactions.     

Heat and mass transfer in honeycomb catalysts—II

Heat and mass gas-to-solid coefficients associated with the vaporisation of water and some hydrocarbons from the surface of a porous, monolithic structure were experimentally established. The mass transfer results are correlated using Reynolds and Schmidt number and the length-to-diameter ratio. The data for heat transfer are correlated on the Nu-Re(d/L) plot. The experimental results can be used for design of afterburner reactors utilizing monolithic structures.     

“Inverted” peaks and current oscillations in cyclic voltammetry

Inverted peaks and strongly increased peaks have been reported and different explanations of these phenomena have been suggested[1,2]. The present communication discusses the peak shape distortions (“inverted” peaks, current oscillations) in terms of tangential movements of the mercury electrode surface. These distortions are explained similarly to the maxima in classical polarography: inhomogeneous polarization and inhomogeneous adsorption are considered as the two major forces causing the mercury surface movement and as a result, leading to the peak shape distortions. Experimental examples of the phenomena in aprotic and protic media and for organic and inorganic redox couples are given and explained in accordance with the proposed theory.     

Reaktive extraction of salicylic acid and D, L-phenylalanine in a bench-scale pulsed sieve plate column

The reactive (ion pair) extraction of salicylic acid with a secondary amine (Amberlite LA-2) in xylene and, for comparison, its physical extraction with xylene alone were investigated in a bench-scale pulsed sieve tray column. Also, the reactive (ion exchange) extraction of an amino acid (D,L-phenylalanine) with a quaternary ammonium salt (Adogen 464) in xylene was investigated in the same column. To characterize the two-phase system, the holdup and the droplet size distribution of the dispersed phase as well as the residence time distributions of both phases were measured.

The influence of the volumetric flow rate of the dispersed phase, the stroke frequency and the carrier concentration on the holdup, Sauter diameter, specific interfacial area, mean residence time and numbers of equivalent stages in the cascade, as well as on the degree of extraction and the longitudinal concentration profile of the solute in the column, were investigated. The degree of extraction and the longitudinal concentration profiles of the solute were calculated by means of a countercurrent cascade model and the measured kinetics for the extraction of salicylic acid. For physical extraction, satisfactory agreement between measured and calculated values was found. For reactive extraction, the measured and calculated degrees of extraction agree well. However, the longitudinal concentration profiles of the solute are not in agreement. The cause of this has not been evaluated yet.     

Kinetics,poisoning and mass transfer effects in liquid-phase hydrogenations of phenolic compounds over a palladium catalyst

The influence of poisoning and mass transport on the performance of a suspended palladium catalyst for the liquid-phase hydrogenation of o-cresol h for the various transport and reaction steps occuring in the three-phase slurry system and these were combined into an overall rate expression. From th analysed either by an experimental approach or by calculation.The hydrogenation in the absence of poison is described by a Langmuir—Hinshelwood adsorption model with adsorption of hydrogen and substrate on diffe present in the feed, the activity loss can be described by an extended model of the same type; experimental results show that the poison is very strong between poison and substrate on the remaining catalyst surface. The selectivity is not affected by increasing poison concentration.     

Walden product, ion-solvent interactions and solvent structure in water-rich mixtures ionic conductances in water-sulfolane, water-acetonitrile and water-dimethylsulfoxide

Conductance measurements are reported for several salts in binary aqueous mixtures containing up to 60 mole % sulfolane, 20 mole % acetonitrile and 20 mole % dimethylsulfoxide. The variations of R = (λ^±₀η₀)_s/(λ^±₀η₀)_w with solvent composition have been compared with those observed in other water-rich mixtures. Alkali cations show R values greater than one with maxima in all the solvent mixtures. This behaviour has been discussed in terms of “sorting”, “averaging” and “steric” effects. Contrary to what happens to alkali cations, halide ions show R values greater or lesser than one according to whether the organic solvent respectively increases or decreases water structure. On these bases we suggest that conductometric behaviour of the halide ions may be indicative of the effect of the cosolvent on the water structure in water-rich mixtures and that DMSO is a water structure breaker.     

Electrophoretic studies of mixed ligand complexes in solution—copper(II), nickel(II), cobalt(II), uranyl(II) and thorium(IV)—tartarate—nitrilotriacetate

Paper electrophoresis has been used for the study of the equilibria in mixed ligand complex systems in solution. The method is based on the migration of a spot of a metal ion, with the complexants added in the background electrolyte (0.1 M perchloric acid), at a fixed pH. The concentration of one of the complexants [A] is kept constant, while that of second ligand [L] is varied. A graph of—log [L] against mobility is used to obtain information on the formation of the mixed ligand complex, and to calculate the stability constant. Using this technique, the values of overall stability constant of the complex metal—tartarate—nitrilotriacetate have been found to be 10^12.25, 10^5.98, 10^?3.56, and 10^3.74 for Cu(II), Ni(II), Co(II), UO₂(II) and Th(IV) complexes, respectively at μ = 0.1 and temp. = 40°C.     

Acidization III—The kinetics of the dissolution of sodium and potassium feldspar in HF/HCl acid mixtures

The dissolution of a potassium and a sodium feldspar in hydrochloric-hydrofluoric acid mixtures was studied in a rotating disk system at 25 and 100°C under a pressure of 40 psig. Under these conditions, the dissolution rate is limited by the rate of reaction of the acids with the solid surface.The rate of dissolution may be described by the following type of rate law:

The parameters a and b for potassium feldspar have the values 0·4 and 1·2, respectively, whereas for sodium feldspar both have a value of 1·0. A mechanism for the dissolution reaction is proposed which is consistent with experimental observation. The analysis suggests that the dissolution is limited by the rate at which the hydrofluoric acid reacts with the solid surface and that the strong influence of the hydrochloric acid on the dissolution rate is due to the adsorption of hydrogen ions on the mineral surface.     

Homogeneous models for porous catalysts and tubular reactors with heterogeneous reactions

One-dimensional models for porous catalysts and laminar flow tubular reactors with first-order reactions at the walls are developed. The method which is used replaces the transport equations containing the local concentrations with equations, similar to those used in the dispersion theory, for the concentrations averaged over the cross section. The main assumption is the approximation of the coefficients in the dispersion equations by those valid for a pulse of concentration introduced at time zero at the mouth of the pore or at the inlet of the tube.The effectiveness factor depends not only on the Thiele modulus but also on the length-to-radius ratio of the catalyst pore. The one-dimensional model for the catalytic tubular reactor provides a good approximation to the area-mean concentration for β (k_sR/D) up to 1·0. The comparison is made with an exact orthogonal expansion solution developed also in the paper. For larger values of β, say β ≈ 100, the discrepancies between the exact solution and the one-dimensional model may be as high as 50 per cent. Therefore, a generalized dispersion solution is presented to obtain more accurate predictions for β > 1·0.     

Electrochemistry of ferrocene in anionic,cationic and nonionic micellar solutions. Effet of the micelle solubilization of the half-wave potentials

The redox properties of the ferrocene/ferricinium Fc/Fc⁺ couple were studied in aqueous 0.1 M NaCl solution of sodium dodecylsulphate (SDS), N-cetyltrimethylammonium bromide (CTAB) and polyoxyethylene-23 lauryl ether (Brij 35). Solubilities of ferrocene were measured by spectrophotometry and by limiting current measurements in direct current voltammetry. The reversible half-wave potentials were determined by cyclic and differential pulse voltammetry. The results are discussed on the basis of the micelle solubilization equilibrium of both the reductant Fc and the oxidant Fc⁺. Indeed, diffusion coefficient ratios of the oxidant to the reductant, measured by coulometry, show that the micelle solubilization of the electrogenerated ferricinium cation Fc⁺ must also be considered in the nonionic and the anionic micellar solutions. Thus in every solution studied, a standard potential of Fc/Fc⁺ in water may be obtained from the experimental values of the half-wave potentials, the partition coefficients of Fc and Fc⁺ and the ratio of their diffusion coefficients. The behaviour of ferrocene in the SDS solutions and the effect produced by the addition of pentanol seem to confirm that SDS forms micelles with a less ordered structure than the other surfactants studied. The invariance of the half-wave potentials of ferrocene in pure SDS solutions whatever the surfactant and ferrocene concentrations suggests the use of this solute-solvent couple as a reference potential system.     

Acidization—V: The prediction of the movement of acid and permeability fronts in sandstone

A new model has been developed which describes the acidization of sandstone cores. The results show that the reaction of hydrofloric acid with the dissolvable minerals occurs in a narrow zone which moves as a reaction front through the sandstone core. The width of the reaction front is related to the Damkohler Number and the velocity of the reaction front is related to the acid capacity number. The model has virtually no adjustable parameters and agrees extremely well with the experimental results.     

Oscillations in crystallite shape during heating in hydrogen of model iron/alumina catalysts

Transmission electron microscopy is used to bring evidence for an alternation in the shape of iron crystallites supported on planar, nonporous, alumina substrates, when the specimens were heated, at 400 °C, in hydrogen contaminated with traces of oxygen (less than 1 ppm) and/or moisture. The crystallites alternated between a torus which encloses a cavity and a shape in which an annular gap separates the torus from a core in the cavity. Electron diffraction indicated that this is accompanied by a corresponding alternation in the chemical state of the catalyst. The torus with an enclosed cavity corresponds to an oxidized state, in which iron oxides, resulting on oxidation by the contaminant oxygen, form solid solutions in alumina, with the tendency to approach the aluminates FeAl₂O₄ and/or Al₂Fe₂O₆. The torus containing a core in the cavity corresponds to a less-oxidized state, in which a part of the previously dissolved oxide diffuses out to be (at least partially) reduced by hydrogen to the metal and/or to a lower oxide (including possibly nonstoichiometric oxides). After prolonged heating, the two alternating shapes become less distinct, probably because of mechanical fatigue which eventually leads to a breakup of the crystallites. Similar behavior is observed at temperatures up to 600 °C, though with diminished sharpness of the two alternating shapes above 500 °C. An attempt is made to explain the shape alternation on the basis of surface phenomena resulting from the physical and chemical interactions between gas, crystallite, and substrate.