Slow reaction in boric acid coated vessels

2 SLOW REACTION IN BORIC ACID COATED VESSELS 

The slow reaction in aged boric acid coated vessels has been extensively studied by Baldwin and Mayor [45], While studying the effect of withdrawal rate on the second limit, Baldwin and Mayor observed that in a freshly coated vessel at 500 °C and 500 torr pressure, the rate of 

The reaction is autocatalytic, resembling in this respect the situation already encountered with uncoated quartz or glass vessels. However, in contrast with the results of Lewis and von Elbe [23] for a quartz vessel, Baldwin and Mayor [45] found little or no effect of addition of up to 22 torr added water on either the induction period or the maximum rate, irrespective of whether the water was added a short time before, or together with, the reactants themselves. They concluded that the autocatalytic effect cannot be due to poisoning (by absorption of water vapour produced in the reaction) of the ability of the surface to destroy chain centres, as had previously been suggested. 

The reaction in aged boric acid vessels shows no significant effect of vessel diameter, either on the maximum rate or the induction period, over the range 15, 24, 36 and 51 mm. For a constant total pressure of 500 torr. Fig. 19 shows the variation of the maximum rate R with (a) oxygen mole fraction y over the range 0.07—0.72, the H2 mole fraction x being 

The variation of R with total pressure for the standard mixture at 500 °C is given approximately by f ocP . Addition of nitrogen causes an increase in rate, but the increase is substantially less than with salt coated vessels, e.g. 200 % addition of Ni only increases the rate by about 50 %. The effect is least marked at low x. Over the temperature range 470—540 °C, the log R versus 1/T plot is closely linear, and gives an activation energy of 55.8 0.7 kcal. mole . All these properties contrast sharply with the behaviour in porcelain and salt coated vessels described earlier, with which, for example, the activation energy is 100 kcal. mole or greater. 

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Attention has already been drawn to the presence of hydrogen peroxide in the products from the oxidation in Pyrex tubes and its absence for KCl coated tubes. The build up of hydrogen peroxide concentration during the slow reaction in boric acid coated vessels has been investigated by Baldwin et al. [45, 64], and is shown for one set of conditions in F. 20. The hydrogen peroxide concentration reaches a maximum at the same time as the reaction rate. 

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