Gases - Fundamentals of Industrial Chemistry: Pharmaceuticals, Polymers, and Business - page 27

rate of ammonia production and added costs associated with cooling the
exothermic reaction. Higher pressure also necessitates higher costs in reactor
design and compression pumps. For example, higher pressures require
reciprocating pumps, which are like bicycle floor pumps, versus centrifugal
pumps, which can be used at lower pressures and operate on a fan-blade
type principle. A variety of catalysts can be used to improve the rate and
the introduction of copper catalysts has enabled somewhat lower process
temperatures [20]. However, an ammonia process is a balance between
conversion and cost. A typical process might run at temperatures from
about 350 to 550 ∘ C and pressures of 120 to 250 atmospheres. Under these
conditions, the conversion rate may be 30% or less. You may wonder how it
can be economical to produce ammonia with such a low conversion rate. It is
because the product ammonia is readily separated from the unreacted hydro-
gen and nitrogen. This can be done by cooling. Ammonia has a boiling point
of 33 ∘ C but nitrogen and hydrogen have boiling points of 196 ∘ Cand − −

253 ∘ C respectively. Therefore the ammonia can be selectively condensed
and unreacted nitrogen and hydrogen gas returned to the reactor as a recycle
stream. Because of the recycling of unreacted starting material, the overall
reaction yield is very high, despite the low conversion (amount of product
per pass).
The reaction is shown with an exotherm of 92 kJ. Expressed as the
exotherm per mole of ammonia, it is 46 kJ. This is at standard temperature
and pressure. Although this is fine for most academic discussions, one
should realize that enthalpy changes are influenced by both temperature and
pressure. Ammonia is not commercially made at standard temperature and
pressure (a temperature of 273.15 ∘ K, 0 ∘ C and a pressure of 1 atmosphere;
1 atmosphere

−

= =

0.101 MPa) so heats need to be either calculated
or experimentally determined at the conditions of the synthesis. For example,
at atmospheric pressure with an increase in temperature from 300 ∘ Kto
600 ∘ K to 900 ∘ K, the enthalpy change per mole of ammonia is increased
from 46.35 kJ to 52.04 kJ to 55.06 kJ [21]. The enthalpy change increases
with increasing pressure. For example, at 400 ∘ C, it is 53.09 kJ/mol NH 3 at
10 MPa. At the same temperature and 40 MPa, it increases to 57.18 kJ. At
100 MPa, it is 65.37 kJ [22].
For a plant operating at 400 ∘ C and 10 MPa, somewhat typical commercial
conditions, we can calculate the enthalpy change per metric ton of ammonia
produced. A ton is 2,000 pounds and a metric ton, sometimes referred to as
“tonne” is 1,000 kg which is 2,200 pounds.

1.01 bar

( 53 . 09 kJ ∕ mol NH 3 )( ∕ )( ∕ )( )

1mole 17g 1000 g kg 1000 kg

10 9 J = 3 . 12 million kJ ( 3 . 12 × ) .