英语翻译The second stage is the key to the process.The direct re

英语翻译
The second stage is the key to the process.The direct reaction between sulphur dioxide and oxygen to give sulphur trioxide is slw and requires a catalyst.In the ole lead chamber process nitrogen dioxide (NO2)was used for the oxidation though in practice mixture of nitrogen oxides were used.This process has now been completely superseded by the contact process,which speeds the direct reaction via a solid-state catalyst of vanadium pentoxide(v2o5).The catalyst is normally absorbed on an inert silicate support and lasts for about 20 years.
The reaction between oxygen and sulphur dioxide is exothermic so the equilibrium favors the product at lower temperatures.So much heat is produced by the reaction that it is difficult to achieve good yields in a single-bed reactor:the reactor warms up so much that the reaction goes into reverse.Most plants therefore used several reactor stages(with coolers between the reactors) so that the heat produces does not drive the reaction backwards.The first reactor chamber converts the mixture of oxygen and sulohur dioxide into a stream about 60%-65% converted to products at about 600度.it is then cooled to about 400度 and passed into the next layer of catalyst,and so on.After three layers 95%-96% of the starting materials have been converted to products(this is near the maximum possible conversion unless sulphue trioxide is removed).