Bosch–Meiser process

The Bosch–Meiser process is an industrial method used to produce urea, an important nitrogen-based chemical, on a large scale. Patented in 1922, it is named after its German discoverers, Carl Bosch and Wilhelm Meiser. The process relies on two linked equilibrium reactions that do not fully convert the starting materials. First, liquid ammonia reacts quickly with gaseous carbon dioxide under high temperature and pressure in an exothermic step to form ammonium carbamate. Second, this carbamate slowly decomposes in an endothermic reaction to yield urea and water. The overall conversion of ammonia and carbon dioxide into urea releases heat, with the energy from the first reaction driving the second. Because high temperatures favor urea formation but hinder carbamate formation, the process balances these effects by using high pressure—typically 140 to 175 bars—which promotes the first reaction, while operating at around 190 °C to support the second. Ammonia, already liquid from the production plant, can be pumped in economically, though carbon dioxide must be compressed to this pressure. To give the slow urea-forming reaction enough time to reach equilibrium, the synthesis reactor is a large, heavy pressure vessel.

Since the conversion to urea is incomplete, the product must be separated from unreacted materials, including ammonium carbamate. Different commercial processes vary in how they handle this separation and recycling. In early "straight-through" plants, the system pressure was reduced to atmospheric to decompose carbamate back into ammonia and carbon dioxide. At first, recycling these gases was not economical, so ammonia was used to make other products like ammonium nitrate or ammonium sulfate, and carbon dioxide was often discarded. Later, from the 1940s to the 1960s, the "total recycle process"—now called the "conventional recycle process"—made recycling practical. This method depressurizes the reaction solution in stages, first to 18–25 bars and then to 2–5 bars, passing it through a steam-heated carbamate decomposer at each stage. The released carbon dioxide and ammonia are then recombined in a falling-film carbamate condenser, and the resulting carbamate solution is pumped back into the urea reactor.

The conventional recycle process has largely been replaced by a stripping process, developed in the early 1960s by Stamicarbon in the Netherlands.