GROUPV 215
and the production of ammonia is an exothermic process it can
easily be deduced that high yields of ammonia will be obtained at a
high total pressure and low temperature. However, the time required
to reach equilibrium is so great at low temperatures that it is more
economical to work at a higher temperature and get nearer to a
poorer equilibrium position more quickly. In practice, a tempera-
ture of about 770 K is used and a pressure between 200 and 1000
atmospheres. Even under these conditions equilibrium is only
slowly established and a catalyst is necessary. Iron mixed with
alumina is commonly used as a catalyst, the effect of the alumina
being to reduce loss of iron surface by melting or sintering of the
iron at the high temperature used. The development of a catalyst
capable of quickly establishing an equilibrium at a lower tempera-
ture is most desirable as this would give a great yield of ammonia
and indeed much work has been done in this field.
The hydrogen required for ammonia production is largely obtained
by the steam reforming of naphtha (p. 180). Nitrogen is produced by
the fractional distillation of liquid air. The purified gases are mixed
in a 1:3 nitrogen to hydrogen ratio and passed into the catalyst
vessel (Figure 9.2). The catalyst vessel consists of a steel tower
containing relatively thin-walled tubes packed with the catalyst; the
incoming gases pass up between these tubes and down through
them, and the heat generated as the gases pass down the catalyst
tubes warms the incoming gases. The gas emerging from the catalyst
vessel contains about 10% of ammonia; on cooling, this liquefies
Outer steel
casing Catalyst-NH,Figure 9.2. The Haher process