Page 108 - Chemical and process design handbook

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AMMONIA

Ammonia (NH , melting point –77.7°C, boiling point –33.4°C, and density
3
0.817 at –79°C and 0.617 at 15°C) is a colorless gas with a penetrating, pun-
gent-sharp odor in small concentrations that, in heavy concentrations, pro-
duces a smothering sensation when inhaled. Ammonia is soluble in water
and a saturated solution contains approximately 45% ammonia by weight
at the freezing temperature of the solution and about 30% ammonia by
weight at standard conditions. Ammonia dissolved in water forms a
strongly alkaline solution of ammonium hydroxide (NH OH) and the
4
aqueous solution is called ammonia water, aqua ammonia, or sometimes
ammonia (although this is misleading). Ammonia burns with a greenish-
yellow flame.
The first breakthrough in the large-scale synthesis of ammonia resulted
from the development of the Haber process in 1913 in which ammonia was
produced by the direct combination of two elements, nitrogen and hydro-
gen, in the presence of a catalyst (iron oxide with small quantities of
cerium and chromium) at a relatively high temperature (550°C) and under
a pressure of about 2940 psi (20.3 MPa).

N + 3H → 2NH
2 2 3
In the Haber process (Fig. 1), the reaction of nitrogen and hydrogen
o
gases is accomplished by feeding the gases to the reactor at 400 to 600 C.
The reactor contains an iron oxide catalyst that reduces to a porous iron
metal in the nitrogen/hydrogen mixture. Exit gases are cooled to –0 to
o
–20 C, and part of the ammonia liquefies; the remaining gases are recy-
cled.
The process for ammonia manufacture will vary somewhat with the
source of hydrogen, but the majority of ammonia plants generate the
hydrogen by steam reforming natural gas or hydrocarbons such as naphtha
(Fig. 2).
If the hydrogen is made by steam reforming, air is introduced at the sec-
ondary reformer stage to provide nitrogen for the ammonia reaction. The

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