Investigation of the effect of reactor size on the nett rate of formation of hydrazine from ammonia in a glow discharge

Abstract

The effect of reactor size on the nett rate of formation of hydrazine in an electrical diBcharge in ammonia gas has been studied. Three geometrically similar reactor units , the largest of which was greater in volume by a factor of 6'+: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. i.e. 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 hot due to changes in the electrode surface as was previously believed.