Bosch-Meiser process

The Bosch–Meiser process is an industrial process, which was patented in 1922 and named after its discoverers, the German chemists Carl Bosch and Wilhelm Meiser for the large-scale manufacturing of urea, a valuable nitrogenous chemical.

The whole process consists of two main equilibrium reactions, with incomplete conversion of the reactants.

• The first, called carbamate formation: the fast exothermic reaction of liquid ammonia with gaseous carbon dioxide (CO₂) at high temperature and pressure to form ammonium carbamate ([NH₄]⁺[NH₂COO]⁻):

2 NH₃ + CO₂ ⇌ [NH₄]⁺[NH₂COO]⁻     (ΔH = −117 kJ/mol at 110 atm and 160 °C)

• The second, called urea conversion: the slower endothermic decomposition of ammonium carbamate into urea and water:

[NH₄]⁺[NH₂COO]⁻ ⇌ CO(NH₂)₂ + H₂O     (ΔH = +15.5 kJ/mol at 160–180 °C)

The overall conversion of NH₃ and CO₂ to urea is exothermic, with the reaction heat from the first reaction driving the second. The conditions that favor urea formation (high temperature) have an unfavorable effect on the carbamate formation equilibrium. The process conditions are a compromise: the ill-effect on the first reaction of the high temperature (around 190 °C) needed for the second is compensated for by conducting the process under high pressure (140–175 bar), which favors the first reaction. Although it is necessary to compress gaseous carbon dioxide to this pressure, the ammonia is available from the ammonia production plant in liquid form, which can be pumped into the system much more economically. To allow the slow urea formation reaction time to reach equilibrium, a large reaction space is needed, so the synthesis reactor in a large urea plant tends to be a massive pressure vessel.