the Haber-Bosch process, is an artificial nitrogen fixation process and is the
main industrial procedure for the production of ammonia today.
The process converts atmospheric nitrogen (N2) to ammonia (NH3) by a reaction
with hydrogen (H2) using a metal catalyst under high temperatures and pressures.
This conversion is typically conducted at pressures above 10 MPa (100 bar; 1,450
psi) and between 400 and 500 °C (752 and 932 °F), as the gases (nitrogen and
hydrogen) are passed over four beds of catalyst, with cooling between each pass
for maintaining a reasonable equilibrium constant. On each pass only about 15%
conversion occurs, but any unreacted gases are recycled, and eventually an
overall conversion of 97% is achieved.
Ammonia is converted to nitric acid in 2 stages.
Typical conditions for the first stage, which contribute to an overall yield of
about 98%, are:
pressure is between 4-10 standard atmospheres (410-1,000 kPa; 59-150 psi) and
temperature is about 870-1,073 K (600-800 °C; 1,100-1,500 °F).
Stage 1
It is oxidized by heating with oxygen in the presence of a catalyst such as platinum
with 10% rhodium, platinum metal on fused silica wool, copper or nickel, to form
nitric oxide (nitrogen(II) oxide) and water (as steam).
This reaction is strongly exothermic, making it a useful heat source once initiated.
Stage 2
Stage two encompasses two reactions and is carried out in an absorption apparatus
containing water.
Initially nitric oxide is oxidized again to yield nitrogen dioxide (nitrogen(IV)
oxide).
This gas is then readily absorbed by the water, yielding the desired product
(nitric acid, albeit in a dilute form), while reducing a portion of it back to
nitric oxide.
The NO is recycled, and the acid is concentrated to the required strength by
distillation.
The contact process is the current method of producing sulfuric acid in the high
concentrations needed for industrial processes.
In addition to being a far more economical process for producing concentrated
sulfuric acid than the previous lead chamber process, the contact process also
produces sulfur trioxide and oleum.
The process can be divided into six stages:
Combining of sulfur and oxygen (O2) to form sulfur dioxide
Purifying the sulfur dioxide in a purification unit
Adding an excess of oxygen to sulfur dioxide in the presence of the catalyst
vanadium pentoxide at 450 °C and 1-2 atm
The sulfur trioxide formed is added to sulfuric acid which gives rise to
oleum (disulfuric acid)
The oleum is then added to water to form sulfuric acid which is very concentrated.
As this process is an exothermic reaction so the temperature should be as low
as possible.
The Solvay process or ammonia-soda process is the major industrial process for
the production of sodium carbonate (soda ash, Na2CO3).
The ingredients for this are readily available and inexpensive: salt brine (from
inland sources or from the sea) and limestone (from quarries).
In industrial practice, the reaction is carried out by passing concentrated brine
(salt water) through two towers. In the first, ammonia bubbles up through the brine
and is absorbed by it. In the second, carbon dioxide bubbles up through the
ammoniated brine, and sodium bicarbonate (baking soda) precipitates out of the
solution.
The necessary ammonia "catalyst" for reaction (I) is reclaimed in a later step,
and relatively little ammonia is consumed. The carbon dioxide required for
reaction (I) is produced by heating ("calcination") of the limestone at 950-1100 °C,
and by calcination of the sodium bicarbonate. The calcium carbonate (CaCO3) in
the limestone is partially converted to quicklime (calcium oxide (CaO)) and carbon
dioxide.
The sodium bicarbonate (NaHCO3) that precipitates out in reaction (I) is filtered
out from the hot ammonium chloride (NH4Cl) solution, and the solution is then
reacted with the quicklime (calcium oxide (CaO)) left over from heating the
limestone in step (II).
CaO makes a strong basic solution. The ammonia from reaction (III) is recycled
back to the initial brine solution of reaction (I).
The sodium bicarbonate (NaHCO3) precipitate from reaction (I) is then converted
to the final product, sodium carbonate (washing soda: Na2CO3), by calcination
(160-230 °C), producing water and carbon dioxide as byproducts.
The carbon dioxide from step (IV) is recovered for re-use in step (I). When
properly designed and operated, a Solvay plant can reclaim almost all its ammonia,
and consumes only small amounts of additional ammonia to make up for losses. The
only major inputs to the Solvay process are salt, limestone and thermal energy,
and its only major byproduct is calcium chloride, which is sometimes sold as road
salt.
In the modified Solvay process developed by Chinese chemist Hou Debang in 1930s,
the first few steps are the same as the Solvay process. However, the CaCl2 is
supplanted by ammonium chloride (NH4Cl). Instead of treating the remaining solution
with lime, carbon dioxide and ammonia are pumped into the solution, then sodium
chloride is added until the solution saturates at 40 °C. Next, the solution is
cooled to 10 °C. Ammonium chloride precipitates and is removed by filtration, and
the solution is recycled to produce more sodium carbonate. Hou's process eliminates
the production of calcium chloride. The byproduct ammonium chloride can be refined,
used as a fertilizer and may have greater commercial value than CaCl2, thus
reducing the extent of waste beds.
The most common chloralkali process involves the electrolysis of aqueous sodium
chloride (a brine) in a membrane cell.
A membrane, such as one made from Nafion (sulfonated tetrafluoroethylene based
fluoropolymer-copolymer), is used to prevent the reaction between the chlorine
and hydroxide ions. (asbestos can perform this function less efficiently)
Saturated brine is passed into the first chamber of the cell where the chloride
ions are oxidised at the anode, losing electrons to become chlorine gas:
2Cl- → Cl2 + 2e-
At the cathode, positive hydrogen ions pulled from water molecules are reduced by
the electrons provided by the electrolytic current, to hydrogen gas, releasing
hydroxide ions into the solution:
2H2O + 2e- → H2 + 2OH-
The ion-permeable ion-exchange membrane at the center of the cell allows the
sodium ions (Na+) to pass to the second chamber where they react with the
hydroxide ions to produce caustic soda (NaOH). The overall reaction for the
electrolysis of brine is thus:
2NaCl + 2H2O → Cl2 + H2 + 2NaOH
The process has a high energy consumption, for example around 2500 kWh of
electricity per tonne of sodium hydroxide produced.
Because the process yields equivalent amounts of chlorine and sodium hydroxide
(two moles of sodium hydroxide per mole of chlorine), it is necessary to find a
use for these products in the same proportion.
For every mole of chlorine produced, one mole of hydrogen is produced. Much of
this hydrogen is used to produce hydrochloric acid
The method is analogous when using calcium chloride or potassium chloride,
producing calcium hydroxide or potassium hydroxide.
With the development of industrial processes that required hydrogen, such as the
Haber-Bosch ammonia synthesis, a less expensive and more efficient method of
hydrogen production was needed.
So starting with coal and performing coal gasification:
3C (i.e., coal) + O2 + H2O → H2 + 3CO
Then using 3CO to perform the water-gas shift reaction:
CO + H2O ⇌ H2 + CO2
Low temperature shift catalysis
Catalysts for the lower temperature WGS reaction are commonly based on copper or
copper oxide loaded ceramic phases, While the most common supports include Alumina
or alumina with zinc oxide, other supports may include rare earth oxides, spinels
or perovskites.
A typical composition of a commercial LTS catalyst has been reported as 32-33%
CuO, 34-53% ZnO, 15-33% Al2O3.
The active catalytic species is CuO.
The function of ZnO is to provide structural support as well as prevent the
poisoning of copper by sulfur.
The Al2O3 prevents dispersion and pellet shrinkage.
The LTS shift reactor operates at a range of 200-250 °C.
The upper temperature limit is due to the susceptibility of copper to thermal
sintering.
These lower temperatures also reduce the occurrence of side reactions that are
observed in the case of the HTS.
High temperature shift catalysis
The typical composition of commercial HTS catalyst has been reported as 74.2%
Fe2O3, 10.0% Cr2O3, 0.2% MgO (remaining percentage attributed to volatile
components).
The chromium acts to stabilize the iron oxide and prevents sintering.
The operation of HTS catalysts occurs within the temperature range of 310 °C to
450 °C.
The temperature increases along the length of the reactor due to the exothermic
nature of the reaction.
As such, the inlet temperature is maintained at 350 °C to prevent the exit
temperature from exceeding 550 °C.
Industrial reactors operate at a range from atmospheric pressure to 8375 kPa
(82.7 atm).
The search for high performance HT WGS catalysts remains an intensive topic of
research in fields of chemistry and materials science.
Activation energy is a key criteria for the assessment of catalytic performance
in WGS reactions.
To date, some of the lowest activation energy values have been found for catalysts
consisting of copper nanoparticles on ceria support materials, with values as low
as Ea = 34 kJ/mol reported relative to hydrogen generation.
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