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No description available.

No description available.

The effect of reactor size on the nett rate of formation of hydrazine in an electrical discharge in ammonia gas has been studied. Three geometrically similar reactor units, the largest of which was greater in volume by a factor of 64:1 compared to the smallest, were used in this work. The nett rate of formation of hydrazine has been found to be inversely proportional to reactor size. Evidence has been provided which suggests that changing the reaction tube size results in a change in the concentration of atomic hydrogen in the reaction zone. As the reaction tube diameter is increased the rate of diffusion of atomic hydrogen to the reaction tube surface is decreased. This results in a decrease in the rate of recombination of atomic hydrogen at the reaction tube surface, and consequently an increase in the concentration of atomic hydrogen in the reaction zone. In turn this leads to an increase in the rate of destruction of hydrazine by atomic hydrogen attack and as a result, a decrease in the nett rate of formation of hydrazine. Attempts to minimise the concentration of atomic hydrogen in a large scale reactor, by packing the reactor with a quartz surface (wool) failed due to field distortion with subsequent discharge constriction, and also due to the catalytic nature of the quartz surface employed. In the early experimental work several of the major reactor design problems encountered in scale up were identified. One of these was the constriction of the discharge zone into a narrow beam thereby allowing some of the gas to by-pass this zone completely. Because of this, the early work was directed towards obtaining a set of reactor units, in which the discharge occupied the entire volume between the electrodes over a wide range of operating conditions. In this work it was found that under dc discharge conditions, electrodes of diameter greater than a critical size which is dependent upon a number of interdependent factors viz. a) the nature of the reactant gas, b) reactor unit geometry, c) electrode material and electrode profile, d) the flow pattern of the gaseous reactant, e) electrical and operating conditions - of which the most important are electric current density, operating pressure, and gas flow rate, give rise to non- uniform constricted discharges. In chapter 2.3 the hypothesis that physical similarity (similarity in the physics of the discharge) in two geometrically similar discharges of different size, ensures chemical similarity has been examined. This was done by testing whether or not the 'similarity principle' could be applied to electrical discharges in which chemical reactions occur. The results of this investigation show that while discharge processes depending on single electron impact activation follow the 'similarity principle' the relationship does not hold for chemical reactions where secondary processes are of primary importance. Consequently semi-theoretical methods of investigation have been used in this work and an equation of the form – [see full text for formulas] where r = nett rate of formation of hydrazine, D = reaction tube diameter, F = ammonia gas flow rate, C = ammonia gas concentration Vp = potential difference across the reaction zone (positive column) of the discharge, I = discharge current, L = length of the reaction zone (positive column), p = pressure in the reaction zone (positive column), a, b, c = constants, which was derived using the technique of dimensional analysis, has been used to correlate the experimental results satisfactorily. Finally during the course of the experimental work it was discovered that the nett rate of formation of hydrazine depended on the reactor unit ‘age’, and consequently a small amount of work was carried out to investigate this phenomenon. The results of this research indicate that the effect of reactor unit ‘age’ is primarily due to a change in either the catalytic and/or adsorption properties of the reaction tube surface, and not due to changes in the electrode surface as was previously believed.

A fully distributed mathematical model, describing the performance of a single porous compounded bifunctional catalyst pellet, promoting complex endothermic reforming reactions, demonstrates that isothermal conditions obtain within the catalyst pellet. The heat transfer resistance, therefore, resides entirely within the fluid film surrounding the catalyst particle. Application of such a lumped thermal resistance model is then extended to the examination of systems involving a physical mixture of discrete catalyst pellets. Steady state stability analyses of both bifunctional catalyst systems show that, in the case of discrete pellet mixtures contained in a reactor operated in a high temperature region, alternative stationary states may exist because one of the two types of particle supports overall exothermic reactions. A one-dimensional steady state reactor model is employed to predict the behaviour of reactors packed with the two different bifunctional catalyst preparations and which promote chemical reaction in the presence of mass and heat transfer effects. It is concluded that a compounded bifunctional catalyst is, in practice, superior to a physical mixture of discrete pellets due to the favourable mass and heat transfer characteristics of such a catalyst. It is also demonstrated that the complex reactions occurring over compounded pellets may be reasonably described by a relatively simple reaction scheme which emphasizes the role of a cooperative reaction step effecting direct conversion of methyl cyclo-pentene to benzene. Consequently, the overall complexity of the mathematical model and the high computational effort are reduced considerably. Optimisation of benzene yield with respect to the bifunctional catalyst composition indicates that it is more economic, in practice, to employ an optimum uniform catalyst composition than a spatially distributed optimum (e.g. a falling profile along the reactor) since the former policy gives a product yield which is only very slightly lower than for the latter case. A simplified unsteady state mathematical model of a reforming reactor packed with compounded pellets shows that the reactor is stable, for a wide range of operating conditions, when subjected to step perturbations in inlet fluid conditions. However, the approach to the new steady state is found to be particularly slow in the case of adiabatic operation. This is most likely due to the low overall chemical reaction rate. A well known initial value (i.e. stepwise) numerical technique is shown to be suitable for integration of the state equations, both spatially along the reactor and in the time domain, provided that an appropriate arithmetic precision is employed. Furthermore, the computational effort is reduced considerably by using a suitable integration step size distribution in the time domain.

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The application of dry-oxide absorbents to the removal of sulphur dioxide from the gas was gaining considerable interest as compared to other processes which were wet or catalytic. This topical interest of the day led to the present research work. Copper oxide on alumina was used for acceptance of sulphur dioxide and was subsequently regenerated. Apart from some patents, little has boon published on the detailed examination of the reaction mechanisms of absorption and regeneration; hence the need for the present work. Absorption studies were carried out using mixtures of sulphur dioxide, oxygen and nitrogen. Hydrogen was used as a reducing agent to regenerate copper oxide on alumina from the copper sulphate formed. Experiments have been carried out in the temperature range of 270-500 °C, the range in which the kinetics and the extent of the reaction could be measured accurately. A mass flow balance and a thermobalance fitted with a differential thermal analyser were used for single pellet work. In order to investigate the kinetics of the intermediates in the reaction, a system was designed for incremental studies. The overall reaction for regeneration was found to be complex, CuSO4.A1203 + 4H2 = CuO.A1203 + 3H20 + H2 S. Activation energies and order of reaction with respect to both fluid and solid reactant concentrations were obtained. Throughout the operating temperature range, it was observed that regeneration reached a limiting value that increased with temperature. Chemically bound water in the alumina offers an explanation of this behaviour. For absorption studies of sulphur dioxide on alumina, activation energies and dependence on fluid reactant concentrations were obtained in the range 270-500 °C. The overall reaction is CuO.Al203 + CO2 + 1/2 02 = CuSO4-A1203. One notable feature in the sulphation process not considered by other workers is that sulphation of alumina occurs and will lead to cracking and attrition of the alumina carrier. In the existing fluid-solid reaction models, a new concept of core-diffusion control was introduced in the chemically controlled conditions. Also in the particle pellet model developed, a diffuse interface was assumed to exist in the particle as well as over the pellet as a whole. Other workers using the particle-pellet model have assumed that a sharp interface exists within the particle. This can restrict the model unnecessarily. Both isothermal and non-isothermal theories were considered, but the experimental work refers only to the isothermal because of the nature of the chemical reactions involved.

No description available.

No description available.

No description available.