RCTU, NSTU and NNSU: Ammonia synthesis using hybrid gas purification technology

2020: Announcement of ammonia synthesis using hybrid gas purification technology

On December 11, 2020, it became known that Russian chemists optimized the synthesis of ammonia.

As explained, the world produces almost 200 tons of ammonia per year, from which nitrogen fertilizers, various plastics, explosives and much more are obtained. At the same time, the process of synthesis of ammonia has remained almost unchanged since the beginning of the 20th century and consumes enormous volumes of electricity, and one of its most energy-consuming stages is the separation of ammonia from the reaction mixture. Russian scientists from RSTU, NSTU and NNSU proposed to conduct it using hybrid technology that combines the capabilities of membrane purification and modern absorbers and showed that it is possible to obtain ammonia with a purity of up to 99%, spending much less energy. The results of the work are published in the journal ChemicalEngineering Journal. The study is supported by the Russian Scientific Foundation.

Nitrogen is necessary for all living organisms because it is part of many proteins and amino acids. And although the Earth's atmosphere is almost 80% composed of molecular nitrogen N2 in this form, it is practically not absorbed by organisms. Therefore, humanity produces so many fertilizers that contain nitrogen already in bound form, that is, in the form of different chemical nitrogen compounds with other atoms, from which it is easier for organisms to pull nitrogen for their needs. And most nitrogen fertilizers, in turn, are obtained from ammonia (NH3), which is also used to synthesize various cooling agents (refractories), plastics, polymers, explosives, and even to microelectronics. in. In total, almost 200 million tons of ammonia are produced each year for these needs in the world - about 25 kilograms for each resident of the planet and this is the largest chemical production.

Almost all ammonia is obtained in the Gaber-Bosch process, proposed at the beginning of the 20th century and a few years later brought one of its developers (Fritz Gaber) the Nobel Prize in chemistry. In it, hydrogen (H2) and nitrogen (N2) are passed at a certain pressure through the catalyst, and a gaseous mixture of initial reagents and ammonia is obtained at the outlet, from which pure NH3 must then be isolated. As of December 2020, for this, the mixture is cooled using a large amount of refrigerants and because of this, the purification process consumes a lot of energy - more than 3 MW-h for each ton of ammonia, this is the average electricity consumption in the apartment in about two years. Therefore, although the synthesis of ammonia has been worked out by centuries of experience and scaling, scientists are constantly looking for other more economical ways to separate the products of the Gaber-Bosch process.

So, researchers from RSTU named after D.I. Mendeleev, as well as Nizhny Novgorod universities NSTU named after R.E. Alekseev and NNSU named after N.I. Lobachevsky have already proposed using a hybrid gas purification method for this, which combines the technical capabilities of membranes and absorbers. Now they have published a work in which, using absorbents, researchers received ammonia with a purity comparable to industrial.

One day I made a plenary presentation at a scientific conference in Portugal, and then a scientist in the field of membranes, Professor Edward Cussler, approached me, he took my hands and showed, folding his hands, the principle of the membrane, and then told me that in the science of membranes you need only three directions. One of them was ammonia. told Ilya Vorotyntsev, one of the authors of the work, head of the laboratory 'SMART Polymer Materials and Technologies' RKTU named after D.I. Mendeleev, professor

Hybrid gas purification technology has been developed by Russian scientists and flows in the apparatus of their own design. It is divided by a membrane into two parts - a mixture of gases enters the top - in this case, NH3, N2, H2 - which passes through the absorbent deposited on the membrane. It accumulates large amounts of ammonia and vice versa almost does not absorb nitrogen and hydrogen, which exit back from the absorbent into the same cavity above the membrane. NH3 molecules are removed from the absorbent through the membrane to another part of the apparatus: a large pressure drop is created between the cavities above and under the membrane, under the influence of which ammonia diffuses through the membrane to the lower part of the apparatus, from which a stream of purified gas is already removed.

As an absorbent, scientists used four different substances, which, according to preliminary data, looked promising for the purification of ammonia. All of them were deep eutectic solvents (GER) - the so-called mixture of two substances, the melting point of which is lower than the melting point of individual components. GER is obtained from available reagents by simple synthesis, and since they remain liquid in a wide range of temperatures for December 2020, they are considered a promising class of green solvents. The researchers worked with three already tested ERTs based on ammonium thiocyanate, which according to the literature can absorb large amounts of ammonia, and one composition based on 1-butyl-3-methylimidazolium methanesulfonate, which was already promising according to preliminary experiments by Russian scientists themselves.

The original gaseous three-component mixture consisted of NH3, N2, H2 in volumetric ratios of 15.5/62.8/21.7. The best result was shown by an absorbent based on GER from ammonium thiocyanate mixed with glycerol: with it, the purity of ammonia at the outlet was almost 99%. However, after increasing the feed rate of the gas mixture, the efficiency of this absorbent greatly decreased, which can be critical when scaling the technology to real production. From this point of view, solvents based on a mixture of ammonium thiocyanate and urea or ethylene glycol were more promising: with them, the purity of ammonia, even at increased feed rates, ranged from 98.4 to 98.7, which is comparable to industrial indicators.

Not only Russian scientists are trying to apply a membrane approach to the purification of ammonia. However, most other studies in this area, according to the authors of the work, are material science in nature and are still far from practical implementation. The study of Russian chemists not only selected the most promising composition of absorbent, but also conducted flow experiments in a real apparatus with a mixture of gases extremely close to the composition of the products of the Gaber-Bosch process. At the same time, the degree of purification of NH3 was up to 99%, which is close to the modern industrial method, which gives ammonia purity up to 99.8%, but interfaced with large energy losses due to the use of cryogenic technologies.