C.N. Kenney

Modelling of the pressure swing air seperation process

Abstract A theoretical analysis is given of the separation of oxygen and nitrogen in air by a single column pressure swing gas separation process. The problem treated includes changes in mass flow due to adsorption. The findings are compared with data obtained using molecular sieve adsorbent.

Partial wetting in trickle bed reactors — the reduction of crotonaldehyde over a palladium catalyst

Abstract The behavior of a trickle bed in which the catalytic packing was incompletely wetted was investigated using the selective hydrogenation of crotonaldehyde to n -butyraldehyde as a model reaction. The reaction was carried out at near ambient conditions using palladium deposited on porous alumina pellets as the catalyst. It was found that even when the extent of wetting was large, reaction taking place on the dry parts of the catalyst largely dominated the overall reaction rate.

Simulation of rapid pressure swing adsorption and reaction processes

Abstract A general model for non-isothermal adsorption and reaction in a rapid pressure swing process is described. Several numerical discretisation methods for the solution of the model are compared. These include the methods of orthogonal collocation, orthogonal collocation on finite elements, double orthogonal collocation on finite elements, and cells-in-series. Computationally, orthogonal collocation on finite elements is found to be the most efficient of these. The model is applied to air separation for oxygen production. Calculations confirm the formation of a concentration shock when an adsorbent bed is pressurised with air. The form and propagation of the shock over short times is found to be in excellent agreement with the exact similarity transformation solutions derived for an infinitely long bed. For air separation, novel experimental measurements, showing an optimum particle size for maximum product oxygen purity, are accurately described by the model. Calculations indicate that a poor separation results from ineffective pressure swing for beds containing very small particles, and from intraparticle diffusional limitations for beds containing very large particles. For adsorption coupled with reaction, finite rate and reversible reactions are considered. These include both competitive and non-competitive reaction schemes. For the test case of a dilute reaction A &.rlhar2; B + 3C, with B the only adsorbing species, bed pressurisation calculations are found to be in excellent agreement with the solutions obtained by the method of characteristics.

Adsorbent particle size effects in the separation of air by rapid pressure swing adsorption

Abstract Experimental and theoretical investigations into air separation by rapid pressure swing adsorption over zeolite 5A are presented. These concentrate on the effect of adsorbent particle size on the separation performance of the unit undergoing simple cycles consisting of pressurisation and depressurisation steps. An optimum particle size for maximum cyclic equilibrium product oxygen purity is shown to exist; this is accurately predicted by model simulation. Calculations indicate that for beds containing very small particles, a poor separation results from ineffective pressure swing, and for beds containing very large particles from intraparticle diffusional limitations. For the zeolite 5A adsorbent used in this work, theoretical calculations indicate that the rate limiting intraparticle diffusion is described by a parallel combination of molecular and Knudsen diffusion within the macropores of the adsorbent particles. Axial dispersion within the bed is also shown to have a significant effect upon the cyclic equilibrium value of the cycle-average product oxygen purity. In addition to measurements at the cyclic equilibrium, temporal profiles of the product oxygen purity during the approach to cyclic equilibrium are shown. Under certain operating conditions, an overshoot of oxygen purity is found to exist. This behaviour can be attributed to ineffective pressure swing within the product end region of the bed, and thus to the poor utilisation of adsorbent in this region.

Concentration oscillations of carbon monoxide, oxygen and 1-butene over a platinum supported catalyst

Abstract The effect of a wide range of experimental parameters on the sinusoidal and multiple spike oscillations observed during the oxidation of mixture of carbon monoxide and 1-butene over platinum supported on γ-alumina has been investigated in a CSTR. An elementary step model, where the rate constants are assumed independent of surface coverages, is found to be able to account largely for the observed oscillatory phenomena. The carbon monoxide and oxygen adsorption and desorption rate constant values in the model have been evaluated by fitting experimental transients, while 1-butene parameter values were obtained from stability conditions. Rate constant values and operating conditions have a profound effect on the observed and predicted oscillations. A parameter of great importance is the ratio of the number of adsorption sites to reactor volume.

Effectiveness factors in a three-phase slurry reactor: the reduction of crotonaldehyde over a palladium catalyst

Abstract Rate measurements are described of the reduction of liquid crotonaldehyde by hydrogen in a slurry reactor using palladium deposited on the surface and a porous γ-alumina. Measurements were made at ambient pressure and temperatures between 30° and 70°C using: (a) 3 16 in. cylindrical catalyst pellets and (b) powdered pellets for which the mean particle size was 30 μ. The effectiveness factor for the commercial palladium catalyst is calculated to be ca. 0.1 with an average tortuosity factor of 1·6.

Sulphur dioxide oxidation kinetics over molten salts: The absorption of sulphur dioxide in vanadium pentoxide/potassium pyrosulphate melts

Abstract The industrial catalysts used for the oxidation of sulphur dioxide to the trioxide commonly consist of a melt of vanadium pentoxide in potassium pyrosulphate. As part of a study of this reaction system, the rates of absorption of SO2 into unsupported V2O5-K2S2O7 melts have been measured at temperatures between 360°C and 480°C. The results are correlated by a rate equation of the form where A is the surface area of the melt. Values of k2 range from 5 × 10−7 to 2 × 10−6 mol/cm2sec bar. with an activation energy of 46·2 kJ/ mol. Analysis of the results using a film model of gas absorption with reaction suggests that the reaction zone extends some 9000 A below the melt surface.

Applications of penetration and film theories to the catalysis of gas reactions by liquid catalysts: the oxidation of sulphur dioxide

Abstract A general analysis is given, using film and penetration theories, of the reaction of two gases (e.g. SO 2 and O 2 ) to give a gaseous product (SO 3 ). The reaction is assumed to take place in solution and is catalysed by an involatile solute which can exist in oxidised and reduced forms. The implications for the industrial sulphur dioxide oxidation process in which the catalyst is an ionic melt of vanadium pentoxide in potassium pyrosulphate are considered.

Sulphur dioxide oxidation kinetics: The absorption of oxygen in V2O5—potassium pyrosulphate melts

The kinetics are described of the oxidation of VIV in the sulphuric acid catalyst system containing vanadium oxides in molten potassium pyrosulphate. The rate of absorption is proportional to the square root of the oxygen partial pressure and directly proportional to the mole fraction of VIV. The activation energy is 40·6 kJ/gmol. It is suggested that the reoxidation reaction is diffusion controlled, and order of magnitude calculations indicate that it is confined to a surface layer in the melt approximately 900 A in thickness.

The stability and dynamics of a gas-liquid reactor

Abstract Experiments are reported on the reaction of methanol vapour with hydrogen chloride to give methyl chloride and water in a two phase stirred tank reactor. The reactants enter and the products leave, as vapours but the reaction takes place in a concentrated aqueous solution of zinc chloride. The system may reach a steady state, display limit-cycle behaviour or overflow depending on the conditions. A theoretical analysis indicates that one or two equilibrium points can occur which have dynamic characteristics in accord with the observed behaviour. In applying Bendixsons Theorem particular attention must be paid to the boundedness constraints.