Russian scientists improved ammonia synthesis

Almost 200 million tons of ammonia annually produced in the world. We use it to obtain, for example, nitrogen fertilizers. However, ammonia synthesis has remained unchanged since the beginning of the 20th century. This process still consumes an enormous amount of electricity. One of the most energy-consuming stages of synthesis is the separation of ammonia from the reaction mixture.

Scientists of the Nizhny Novgorod State Technical University and the Lobachevsky State University of Nizhni Novgorod have suggested using a hybrid method for gas purification, which combines the technical capabilities of membranes and absorbents.

They created equipment divided by a membrane into two parts. The upper part receives a mixture of gases (ammonia, nitrogen and hydrogen). This mixture goes through an absorbent deposited on the membrane. This absorbent accumulates large amounts of ammonia, while nitrogen and hydrogen flow through it and come back from the absorbent into the same cavity above the membrane. Ammonia molecules are removed from the absorbent through the membrane to another part of the equipment. A large pressure drop is created between the cavities above and below the membrane, under the action of which ammonia diffuses through the membrane into the lower part of the equipment, from which the stream of purified gas is withdrawn.

As an absorbent, scientists used four different substances. All of them were deep eutectic solvents (DES). It’s a mixture of two substances, the melting point of which is lower than the melting point of individual components. DES is obtained from available reagents by simple synthesis, and since they remain liquid over a wide temperature range, they are now considered a promising class of green solvents.

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The best result was shown by an absorbent based on DES from ammonium thiocyanate mixed with glycerol. With this absorbent, the purity of ammonia reached almost 99%. However, after increasing the feed rate of the gas mixture, the efficiency of this absorbent dropped dramatically. From this point of view, solvents based on a mixture of ammonium thiocyanate and urea or ethylene glycol turned out to be more promising. The purity of ammonia, even at increased feed rates, ranged from 98.4% to 98.7%, which is comparable to industrial performance.

Not only Russian scientists are trying to apply a membrane approach to ammonia purification. However, most of the other studies in this area are still far from practical implementation. In the new study, Russian chemists not only found the most promising absorbent composition but also conducted tests in a real apparatus with a gas mixture. The degree of purification of ammonia reached 99% during the experiments, which is close to the current industrial method, which gives the purity of ammonia up to 99.8%, however, requires huge energy losses due to the use of cryogenic technologies.