Process and apparatus for cracking ammonia
The present invention concerns a process and apparatus for cracking ammonia gas at super-atmospheric pressure in catalyst-filled reactor tubes in a furnace. The tubes each have an upstream layer of a first catalyst and a downstream layer of a second catalyst, the first catalyst being more active than the second catalyst. Having the more active catalyst upstream reduces the temperature of the outer walls of the tubes in the region of the burner flames and the temperature of the inner walls of the tubes in the region with the highest mole fraction of ammonia. Nitriding of the metal of the tubes in this region is thereby reduced.
1 . A process for cracking ammonia comprising:
providing a heated ammonia gas at super-atmospheric pressure;
combusting fuel with an oxidant gas in a furnace to heat catalyst-containing reactor tubes, each tube comprising an upstream layer of a first catalyst and a downstream layer of a second catalyst, and produce a flue gas;
feeding the heated ammonia gas, or a partially cracked ammonia gas derived therefrom, to the catalyst-containing reactor tubes to cause cracking of ammonia and produce a cracked gas comprising hydrogen gas, nitrogen gas and residual ammonia gas, wherein the first catalyst is more active for cracking ammonia than the second catalyst.
2 . The process of claim 1 wherein the first catalyst is a ruthenium-based catalyst.
3 . The process of claim 1 wherein the second catalyst is a nickel-based catalyst.
4 . The process of claim 1 wherein the catalyst-containing reactor tubes comprise a layer of a third catalyst downstream of the layer of the second catalyst, the third catalyst being more active for cracking ammonia than the second catalyst.
5 . The process of claim 4 wherein the third catalyst contains the same catalytically active metal as the first catalyst.
6 . The process of claim 4 wherein the third catalyst is a ruthenium-based catalyst.
7 . The process of claim 1 comprising:
pumping liquid ammonia containing at least 0.1 mol. % water to produce pumped liquid ammonia;
pre-heating the pumped liquid ammonia to produce a pre-heated liquid ammonia;
vaporizing the pre-heated liquid ammonia to produce ammonia gas; and
heating the ammonia gas to produce the heated ammonia gas at super-atmospheric pressure,
wherein the water from the liquid ammonia is present in the heated ammonia gas.
8 . The process of claim 7 wherein at least some of the heating duty required to provide the heated ammonia gas is provided by heat exchange with the cracked gas.
9 . The process of claim 7 wherein the water is present in the heated ammonia gas in an amount of no more than 1 mol. %.
10 . The process of claim 1 wherein the catalyst-containing reactor tubes contain no iron-based catalyst.
11 . The process of claim 1 comprising partially cracking the heated ammonia gas in an adiabatic reaction unit comprising at least one catalyst bed to produce the partially cracked ammonia gas for feeding to the catalyst-filled reactor tubes.
12 . The process of claim 11 wherein the catalyst bed(s) of the adiabatic reaction unit comprise(s) at least one catalyst selected from a nickel-based catalyst and a ruthenium-based catalyst.
13 . The process of claim 11 wherein the catalyst bed(s) of the adiabatic reaction unit contain(s) no iron-based catalyst.