298 GROUP VI
There are two processes, (a) and (b), using different catalysts:(a) In the Contact process the catalyst now used is vanadium
pentoxide, V 2 O 5 , with sodium oxide on an invert carrier. Platinum
is a more efficient catalyst than vanadium pentoxide but is far more
expensive and rendered inactive or poisoned by the presence of
arsenic, which has no inhibiting effect on vanadium pentoxide, and,
consequently, platinum is no longer used.
The catalyst is carried on perforated shelves inside cylindrical
steel vessels called converters. The gas enters these at 670-720 K at
atmospheric pressure.
(b) In the older Lead Chamber process (so called because the
chamber is lined with lead, on which cold sulphuric acid has little
action), the catalyst is nitrogen oxide. This is a homogeneous catalyst.
Sulphur dioxide, oxides of nitrogen, air and steam are passed slowly
through mixing chambers and sulphuric acid of strength 60-70%
(chamber acid) is formed. As the chambers are fairly cool, it condenses.
Since the catalyst is in the gaseous state, it is being continually
removed from the mixing chambers. Its recovery, and the necessity
of continual charging of the incoming gases with it, make the lead
chamber plant complicated by comparison with that of the Contact
process.
The gases coming out of the mixing chambers pass into the Gay-
Lussac tower, packed with coke, over which concentrated sulphuric
acid trickles. The acid absorbs the nitrous fumes to form "nitrated
acid'. This nitrated acid, mixed with some of the weaker chamber
acid, is pumped to the top of the Glover tower packed with flints.
The mixture of acids passes down the tower and meets the stream of
hot gases from the sulphur burners, passing up. The nitrous fumes
are extracted from the nitrated acid and the gases now pass into
the lead-lined mixing chambers. The incoming hot gases serve also
to concentrate the chamber acid to a strength of about 80 %.
The plan of the whole process is shown in Figure 10.5.
The mechanism of the reaction in the lead chamber is complicated.
The simple representation:
NO + air =± NO 2
SO 2 + NO 2 + H 2 O -> H 2 SO 4 + NOis incomplete, for intermediate products, notably nitrosyl hydrogen
sulphate, (NO)(HSO 4 ), which are sometimes found in crystalline
form and are known as 'chamber crystals', have been identified.
The mechanism is now thought to be:
2NO + O -> 2NO