Page 22 - Modeling of Chemical Kinetics and Reactor Design

The raw material source for ammonia plants is natural gas for
producing hydrogen by steam reforming. An alternative raw material
is naphtha, which also requires partial oxidation. Hydrogen streams
from catalytic reformers are another source of hydrogen. However, the
volumes available are negligible to meet the requirements of an
average size ammonia plant. Nitrogen is obtained by the liquefaction
of air or from producer gas mixed with hydrogen in the mole ratio
of 3:1.
The synthesis of ammonia is divided into four stages. In stage 1,
the natural gas undergoes catalytic reforming to produce hydrogen
from methane and steam. The nitrogen required for ammonia pro-
duction is introduced at this stage. Stage 2 involves the "synthesis gas"
(syngas) that is purified by removing both carbon monoxide and
carbon dioxide. Stage 3 is the compression of the syngas to the
required pressure. Stage 4 is the ammonia loop. A typical feed stock
for ammonia synthesis is 0.17 million standard cubic meter per day
(6 Mscfd) of natural gas at a temperature of 16°C and a pressure of
23.4 barg. Table 4 shows its composition.
Natural gas is desulfurized because sulfur has an adverse effect on
the catalysts used in the reforming and synthesis reactions. After
desulfurization and scrubbing, the natural gas is mixed with super-
heated steam at 23 barg and 510°C. Nitrogen is supplied from the air,
which is fed to the secondary reformer at 20 barg and 166°C. Table
5 shows the composition of air.

Table 4
Natural gas feed
Component Mole %
Carbon dioxide (CO ) 2.95
2
Nitrogen (N ) 3.05
2
Methane (CH ) 80.75
4
Ethane (C H ) 7.45
2 6
Propane (C H ) 3.25
3 8
Butane (C H ) 2.31
4 10
Pentane (C H ) 0.24
5 12
Used with permission from Simulation Sciences Inc.
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