Method for producing hydrogen cyanide

1. A method for producing hydrogen cyanide, comprising passing a feed mixture comprising ammonia and methane through reaction tubes, coated on the inner surface with a catalyst comprising platinum, at a reaction temperature of 1000° C. to 1400° C. to provide a product gas mixture comprising hydrogen cyanide, hydrogen, unreacted ammonia and unreacted methane, the ammonia concentration in said product gas mixture being higher than the methane concentration in said product gas mixture, wherein for a time period of at least 100 h a concentration difference between said ammonia concentration and said methane concentration is maintained in a range of from 1.05% by volume to 3.0% by volume for at least 80% of the time by adjusting the reaction temperature, adjusting the molar ratio of ammonia to carbon in the feed mixture, adjusting a feed rate of said feed mixture, or any combination of said adjustments.

2. The method of claim 1 , wherein during said time period said concentration difference is no more than 3.0% by volume for at least 95% of the time.

3. The method of claim 1 , wherein for at least 95% of the time said concentration difference is maintained at a constant value in the range of from 1.1% by volume to 2.1% by volume.

4. The method of claim 1 , wherein the combined amount of ammonia and methane in said feed mixture is at least 90% by volume.

5. The method of claim 1 , wherein said feed mixture comprises less than 4% by volume of oxygen.

6. The method of claim 1 , wherein the molar ratio of ammonia to carbon in the feed mixture is kept constant and the concentration difference is maintained by adjusting the reaction temperature.

7. The method of claim 1 , wherein the methane concentration in the product gas mixture is less than 4% by volume.

8. The method of claim 1 , wherein at time intervals of from 15 to 300 h the molar ratio of ammonia to carbon in the feed mixture is increased by 10 to 30% for a time span of no more than 30 min.

9. The method of claim 1 , wherein said method is carried out in a plant comprising one or more reactors, each reactor comprising at least two reaction furnaces operated in parallel and a reactor product conduit collecting the product gas mixture of the reaction furnaces, and each reaction furnace comprising from 10 to 130 reaction tubes operated in parallel.

10. The method of claim 9 , wherein the ammonia concentration and the methane concentration is measured by IR spectroscopy in the product gas mixture from a single reaction furnace, the product gas mixture in the reactor product conduit, or both.

11. The method of claim 9 , wherein the ammonia concentration and the methane concentration is measured by gas chromatography in the product gas mixture from a single reaction furnace, the product gas mixture in the reactor product conduit, or both.

12. The method of claim 9 , wherein for each reaction furnace said concentration difference is maintained in the product gas mixture from the reaction furnace.

13. The method of claim 9 , wherein feeding of said feed mixture to a reaction tube is stopped when the reaction tube gets blocked or broken and a rate of feeding said feed mixture to the reaction furnace containing the blocked or broken reaction tube or a rate of feeding said feed mixture to the reactor containing the blocked or broken reaction tube is increased to maintain a constant production rate for hydrogen cyanide.

14. The method of claim 9 , wherein a reaction tube or a reaction furnace is temporarily taken out of operation for removing carbon deposits by passing carbon dioxide through the reaction tube or the reaction tubes of the furnace.

15. The method of claim 9 , wherein the plant comprises from 1 to 30 reactors and each reactor comprises from 2 to 20 reaction furnaces.

16. The method of claim 1 , wherein the concentration difference is maintained for a time period of from 100 h to 100,000 h.