PROCESS FOR AMMONIA SYNTHESIS AND PLANT FOR PREPARATION OF AMMONIA

Abstract

A process for ammonia synthesis in a synthesis circuit may involve circulating a gas mixture comprising nitrogen, hydrogen, and ammonia with a conveying device (2) in the synthesis circuit, reacting nitrogen and hydrogen at least partly to ammonia in a converter, and cooling the gas mixture in a cooling device such that ammonia condenses out of the gas mixture. The disadvantages of adsorption drying and of absorption are avoided as hydrogen and nitrogen are introduced at mutually different sections into the synthesis circuit. The process may also involve introducing nitrogen in a flow direction upstream of the converter and/or directly into the converter in the synthesis circuit.

Claims

1.-12. (canceled)

13. A process for ammonia synthesis in a synthesis circuit, the process comprising: circulating a gas mixture comprising nitrogen, hydrogen, and ammonia with a conveying device in the synthesis circuit; reacting nitrogen and hydrogen at least partially to ammonia in a converter; cooling the gas mixture in a cooling device such that ammonia condenses out of the gas mixture, wherein hydrogen and nitrogen are introduced at mutually different sections into the synthesis circuit.

14. The process of claim 13 comprising introducing nitrogen in a flow direction upstream of the converter and/or directly into the converter in the synthesis circuit.

15. The process of claim 13 comprising introducing hydrogen in a flow direction upstream of the cooling device into the synthesis circuit.

16. The process of claim 13 comprising providing hydrogen by way of electrolysis of water.

17. The process of claim 16 comprising obtaining energy needed for the electrolysis from renewable energies.

18. The process of claim 13 wherein a stoichiometric ratio of introduced hydrogen to nitrogen is 3:1.

19. The process of claim 13 comprising regulating a ratio of hydrogen to nitrogen when a supply of hydrogen becomes lower.

20. The process of claim 19 wherein the ratio of hydrogen to nitrogen is 3.

21. The process of claim 19 wherein the ratio of hydrogen to nitrogen is 0.95.

22. The process of claim 13 comprising compressing hydrogen before introducing the hydrogen into the synthesis circuit.

23. A plant for preparing ammonia in a synthesis circuit, the plant comprising: a conveying device configured to circulate a gas mixture comprising nitrogen, hydrogen, and ammonia in the synthesis circuit, with the synthesis circuit being configured such that hydrogen and nitrogen are introducible at mutually different sections into the synthesis circuit; a converter configured to react nitrogen and hydrogen at least partly to ammonia in the converter; and a cooling device configured to cool the gas mixture such that ammonia condenses out of the gas mixture.

24. The plant of claim 23 comprising means for introducing nitrogen in a flow direction upstream of the converter into the synthesis circuit.

25. The plant of claim 23 comprising means for introducing nitrogen in a flow direction downstream of the converter into the synthesis circuit.

26. The plant of claim 23 comprising means for introducing hydrogen in a flow direction upstream of the cooling device into the synthesis circuit.

27. The plant of claim 23 comprising an electrolysis cell configured to provide hydrogen by electrolysis of water.

Description

[0025] In detail there are a multiplicity of possibilities for the configuration and development of the process of the invention and the plant of the invention. Reference is made in this regard both to the claims subordinate to claims 1 and 10, and to the description hereinafter of preferred exemplary embodiments in conjunction with the drawing. In the drawing

[0026] FIG. 1 shows a schematic representation of a process known from the prior art for preparing ammonia, with drying of the fresh gas;

[0027] FIG. 2 shows a further schematic representation of a process known from the prior art for preparing ammonia, with scrubbing of the fresh gas; and

[0028] FIG. 3 shows a schematic representation of a process of the invention for preparing ammonia.

[0029] FIG. 1 shows a process known from the prior art for preparing ammonia NH3 in a synthesis circuit 1. The anhydrous fresh gas introduced is mixed with the circulation gas by means of conveying device 2 in the synthesis circuit 1. For the reaction of hydrogen H2 and nitrogen N2, a converter 3 is provided. In the converter 3, hydrogen H2 and nitrogen N2 react to form ammonia NH3. After the reaction in the converter 3, the gas mixture, consisting of hydrogen H2, nitrogen N2 and ammonia NH3, is passed into a cooling device 4. In the cooling device 4, the gas mixture is cooled to an extent such that ammonia NH3 condenses and can be separated in liquid form. The reacted reactants hydrogen H2 and nitrogen N2, and also the uncondensed ammonia NH3, are run back to the conveying device 2 in the synthesis circuit 1. The conveying device may be a pump or a circulator.

[0030] The nitrogen N2 needed for the ammonia synthesis is supplied in high-purity gas form by a nitrogen provision 5. The hydrogen H2 likewise needed is generated by electrolysis 6 of water. The power needed for these purposes is obtained from fluctuating renewable energies. Accordingly, the power consumption of the electrolyzer can be reduced in this case to 20% of the nominal power.

[0031] Hydrogen H2 and nitrogen N2 are mixed and jointly compressed to the synthesis pressure in a compressor 7. The water present is removed by means of molecular sieves in an adsorption dryer 8.

[0032] Adsorption drying is costly and inconvenient, since for the adsorption dryer a plurality of adsorbers are required, which must be charged alternately with the gas mixture and alternately regenerated with a purge gas, thermally, which is costly and inconvenient.

[0033] FIG. 2 shows a further process known from the prior art for preparing ammonia NH3 in a synthesis circuit 1. On scrubbing of the gas mixture, consisting of the unprocessed nitrogen N2 and hydrogen H2, with ammonia NH3 obtained from condensation, the compressed gas mixture (also called fresh gas) still containing water is supplied to the synthesis circuit 1 ahead of the cooling device 4. The absorption drying in the liquid ammonia NH3 formed has the advantage of operating without additional apparatus for drying the fresh gas. It has the disadvantage, however, that the addition of fresh gas must be made before the condensation of the ammonia NH3. As a result, the circulation gas is diluted in terms of its ammonia content by the reactants introduced, and so, for a given condensation temperature, less ammonia is separated from the circulation gas and the ammonia content at the entrance of the converter 3 is increased relative to adsorption drying. This leads to a higher circulation quantity and hence to a higher catalyst requirement in the converter 3 and an increased driving power on the part of the conveying device 2.

[0034] FIG. 3 shows a schematic representation of the process of the invention or plant of the invention for producing ammonia NH3. The nitrogen N2 is supplied in high purity, free from oxygen and oxygen-containing compounds, by the nitrogen provision 5. The nitrogen N2 is passed in flow direction upstream of, and also directly into, the converter 3. At the entrance to the converter 3, there is then a comparatively low entry concentration of ammonia NH3 present. Accordingly more ammonia NH3 can be formed per pass through the converter 3, and so the amount of catalyst and amount of circulation gas required are lower, as compared with the simultaneous addition of hydrogen H2 and nitrogen N2.

[0035] The hydrogen H2 from the electrolysis 6 is compressed separately with a compressor 7, and so the end stages of the compressor 6 are required to compress a lower volume flow than if hydrogen H2 and nitrogen N2 are jointly compressed and supplied to the synthesis circuit 1.

[0036] The hydrogen H2, compressed to about 261 bara, is added to the circulation gas upstream of the cooling device 4. This has the advantage that the water contained in the hydrogen H2 dissolves in the condensing ammonia NH3 and is removed from the synthesis circuit 1 with the liquid ammonia NH3. Separate drying of the fresh gas, with the associated expenditure in financial and apparatus terms, and also the time-consuming and emissions-entailing regeneration of the adsorption dryers 8, are therefore no longer necessary.

[0037] The minimum hydrogen H2 to nitrogen N2 ratio for the supply to the converter is set at about 1, allowing the partial load range of the converter 3 to be reduced further, without the reaction coming to a standstill. This is particularly advantageous, as it allows autothermal behavior of the reaction, without external heating, when the hydrogen supply is low.

[0038] If hydrogen production rises again, the hydrogen H2 to nitrogen N2 ratio can be slowly brought back to normal value by increasing the water feed into the synthesis circuit 1. This technique prevents frequent starting/stopping of the plant when hydrogen production by the electrolysis 6 is fluctuating or absent, if the electrolysis 6 is driven by renewable energies and these energy sources fluctuate.

LIST OF REFERENCE SYMBOLS

[0039] (1) Synthesis circuit [0040] (2) Conveying device [0041] (3) Converter [0042] (4) Cooling device [0043] (5) Nitrogen provision [0044] (6) Electrolysis [0045] (7) Compressor [0046] (8) Adsorption dryer