Process and apparatus for cracking ammonia
The invention concerns a process and apparatus for cracking ammonia in which heated ammonia gas at super-atmospheric pressure is partially cracked catalytically in an adiabatic reaction unit to produce partially cracked ammonia gas which is heated and fed to catalyst-containing reactor tubes in a furnace to cause cracking of further ammonia and produce a cracked gas comprising hydrogen gas, nitrogen gas and residual ammonia gas. At least some, preferably all, of the duty required to heat the partially cracked ammonia gas is provided by heat exchange with the cracked gas, enabling more efficient heat integration within the process.
1 . A process for cracking ammonia comprising:
providing a heated ammonia gas at super-atmospheric pressure;
partially cracking the heated ammonia gas in an adiabatic reaction unit comprising at least one catalyst bed to produce a partially cracked ammonia gas;
heating the partially cracked ammonia gas to produce a heated partially cracked ammonia gas;
combusting a fuel with an oxidant gas in a furnace to heat catalyst-containing reactor tubes and produce a flue gas; and
feeding the heated partially cracked ammonia gas to the catalyst-containing reactor tubes to cause cracking of further ammonia to produce a cracked gas comprising hydrogen gas, nitrogen gas and residual ammonia gas,
wherein at least some of the duty required to heat the partially cracked ammonia gas is provided by heat exchange with the cracked gas.
2 . The process of claim 1 wherein the adiabatic reaction unit comprises a first adiabatic reactor and a second adiabatic reactor in series, wherein the first adiabatic reactor comprises a catalyst bed comprising a ruthenium-based catalyst and the second adiabatic reactor comprises a catalyst bed comprising an upstream layer comprising a nickel-based catalyst and a downstream layer comprising a ruthenium-based catalyst, the process comprising:
passing the heated ammonia gas through the catalyst bed of the first adiabatic reactor to produce an intermediate partially cracked ammonia gas;
heating the intermediate gas to produce a heated intermediate gas; and
passing the heated intermediate gas through the catalyst bed of the second adiabatic reactor to produce the partially cracked ammonia gas.
3 . The process of claim 2 wherein at least some, preferably all, of the duty required to heat the intermediate partially cracked gas is provided by heat exchange with the flue gas.
4 . The process of claim 3 wherein a part of the heating duty required to provide the heated ammonia gas is provided by heat exchange with the flue gas downstream of the heat exchange with the intermediate partially cracked ammonia gas relative to the flow of flue gas.
5 . The process of claim 1 comprising:
after cooling, recovering hydrogen from the cracked gas in a hydrogen recovery unit to produce a hydrogen gas product and an offgas comprising nitrogen gas, residual hydrogen gas and residual ammonia gas;
heating at least a portion of the offgas to produce a heated offgas; and
feeding the heated offgas as at least part of the fuel to the furnace.
6 . The process of claim 5 wherein all of the offgas is fed as fuel to the furnace.
7 . The process of claim 5 comprising:
dividing the offgas into a first portion and a second portion, wherein the first portion is heated and fed to the furnace; and
compressing the second portion of the offgas to produce a compressed offgas and recycling the compressed offgas to the hydrogen recovery unit for further hydrogen recovery.
8 . The process of claim 1 wherein all of the duty required to heat the partially cracked ammonia gas is provided by heat exchange with the cracked gas.
9 . The process of claim 1 wherein the heat exchange between the partially cracked ammonia gas and the cracked gas takes place in a shell-and-tube style heat exchanger with the partially cracked ammonia gas passing through the tubes of the heat exchanger and the cracked gas passing through the shell side of the heat exchanger.
10 . The process of claim 1 wherein the cracked gas is not cooled by heat exchange against another process fluid prior to heat exchange with the partially cracked ammonia gas.
11 . The process of claim 1 wherein at least some, preferably a majority, of the heating duty required to provide the heated ammonia gas is provided by heat exchange with the cracked gas downstream of the heat exchange with the partially cracked ammonia gas relative to the flow of cracked gas.
12 . The process of claim 1 wherein the catalyst-filled tubes of the furnace and the or each catalyst bed of the adiabatic reaction unit contains no iron-based catalyst.
13 . The process of claim 1 wherein the or each catalyst bed of the adiabatic reaction unit comprises a nickel-based catalyst and/or a ruthenium-based catalyst.
14 . The process of claim 1 comprising pumping liquid ammonia containing at least 0 . 1 mol % water to produce a pumped liquid ammonia;
pre-heating the pumped liquid ammonia to produce a pre-heated liquid ammonia;
vaporizing the pre-heated liquid ammonia to produce an ammonia gas; and
heating the pressurized 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.
15 . The process of claim 1 wherein the water is present in the heated ammonia in an amount of no more than 1 mol %.