Process for the synthesis of ammonia

Abstract

A process for the synthesis of ammonia from a hydrocarbon feedstock, wherein the process includes reforming the hydrocarbon feedstock to produce a make-up gas and converting said make-up gas into ammonia, the process is performed in an ammonia synthesis plant requiring an electric power for operation and also requiring a start-up power (Ps) for start-up, wherein a first electric power (P1) is internally produced in the ammonia plant, and a second electric power (P2) is imported, wherein said second electric power is equal to or greater than said start-up power (Ps).

Claims

1. A process for synthesis of ammonia from a hydrocarbon feedstock, the process comprising: reforming the hydrocarbon feedstock to produce a make-up gas; converting said make-up gas into ammonia; wherein the process is performed in an ammonia synthesis plant requiring a predetermined electric power for operation and requiring a start-up power for start-up; internally producing a first amount of electric power in the ammonia synthesis plant; and importing a second amount of electric power from a source of electric power that is external to said ammonia synthesis plant, wherein said second electric power is equal to or greater than said start-up power.

2. The process according to claim 1, wherein said first electric power is internally produced by a generator operated by a steam turbine, the generator being part of said ammonia synthesis plant.

3. The process according to claim 1, wherein the importing of said second electric power, which is equal to or greater than the start-up power, takes place for at least 80% of the operation time of said ammonia plant on an annual basis.

4. The process according to claim 1, wherein reforming the hydrocarbon feedstock for the production of said make-up gas includes primary reforming of at least part of said hydrocarbon feedstock with steam obtaining a first partially reformed gas, and air-fired secondary reforming of said first partially reformed gas, thus obtaining a raw product gas, and a purification process of said raw product gas.

5. The process according to claim 4, wherein said primary reforming is performed at a temperature of at least 790 C. and absolute pressure of at least 50 bar; said secondary reforming is carried out substantially in absence of excess air relative to the stoichiometric amount, and said make-up synthesis gas has a H.sub.2 to N.sub.2 molar ratio in the range 2.5 to 3.

6. The process according to claim 5, wherein the reforming process is operated with a global steam-to-carbon ratio equal to or greater than 2.9.

7. The process according to claim 4, wherein the purification of said raw product gas comprises a step of high temperature shift (HTS) in at least one HTS converter and wherein steam produced by a step of thermal recovery from the primary reforming or from the at least one HTS converter is superheated, the heat source for steam superheating being a process gas feeding or leaving the at least one HTS converter.

8. The process according to claim 4, further comprising compression of an air feed directed to the secondary reforming within an air compressor, wherein said air feed is heated or cooled at the suction of the air compressor to maintain the inlet temperature of the air compressor within a target range.

9. The process according to claim 4, wherein said conversion of make-up synthesis gas into ammonia is carried out at a pressure which is 2.0 to 3.5 times the pressure of the primary reforming and said method comprises a step of compression of said make-up gas in a gas compressor, said gas compressor being driven by a condensing steam turbine with no steam extraction.

10. The process according to claim 4, wherein the reforming process further includes that a part of said hydrocarbon feedstock with steam is reformed in a step of gas-heated reforming in a gas-heated reformer, arranged either in series or in parallel with said air-fired secondary reforming.

Description

(1) According to a preferred embodiment, the reforming process, including the primary reforming and secondary reforming, is operated with a global steam-to-carbon ratio equal to or greater than 2.9, preferably greater than 3. The global steam-to-carbon ratio denotes the overall ratio of steam and carbon admitted to the reforming process. Such relatively high steam-to-carbon ratio is beneficial to the conversion of the feedstock. It is also synergistic with the elevated pressure of the primary reforming, namely at least 50 bar absolute.

(2) According to a preferred embodiment, the purification of said raw product gas comprises a step of high temperature shift (HTS) in at least one shift converter. Steam used to feed the steam turbines of the plant and steam used in the primary reforming (also called process steam) is recovered by thermal recovery from various process streams, mostly from the primary reforming and from the HTS.

(3) Since part of the power input of the plant is imported from an external source and less steam is expanded in the steam turbine driving the electric generator, some of the steam generated via thermal recovery is advantageously superheated. The heat source for said steam superheating is the process gas before or after the HTS, i.e. feeding or leaving the HTS converter. This embodiment reduces the steam production in the front-end section and, therefore, the steam in excess, which otherwise would be too much due to the high pressure and the relatively high steam-to-carbon ratio of the reforming process. Accordingly, steam superheating is maximized, thus minimizing steam production in the plant.

(4) In another embodiment, the heat contained in the process gas leaving the secondary reformer is conveniently used to reform part of the mixed feed in a gas heated reformer. In this way, steam production in the plant is reduced accordingly.

(5) The gas heated reforming is preferably arranged in parallel with the secondary reforming, i.e. the gas reformed in the gas heated reforming mixes with that reformed in the secondary reforming. Alternatively, the gas heated reforming may be arranged in series with the secondary reforming, i.e. the gas reformed in the gas heated reforming is fed to the secondary reforming.

(6) Advantages of embodiments featuring a GHR include: the duty of the fired primary reformer is reduced, which is an advantage to reach high capacity in terms of production of ammonia; the production of steam is reduced, which is an advantage particularly for standalone plants.

(7) The process gas used for steam superheating has a temperature preferably higher than 400 C., more preferably higher than 450 C., and even more preferably higher than 500 C.

(8) Preferably, the conversion of the make-up synthesis gas into ammonia is carried out at a loop pressure which is 2 to 3.5 times the pressure of the process gas at the exit of the primary reforming catalytic tubes. Said loop pressure is understood as the delivery pressure of a circulator of the loop. More preferably said loop pressure is in the range 100 to 200 bar, and even more preferably 120 to 160 bar.

(9) Accordingly, the make-up synthesis gas is compressed to the loop pressure in a suitable gas compressor. Preferably, the delivery of the main gas compressor is sent to the suction side of the circulator of the loop. This results in the duty of the gas compressor being reduced since part of the compression is given by the circulator. The power absorbed by the compressor, for a given capacity, is reduced accordingly.

(10) Preferably, said gas compressor is driven by a condensing steam turbine with no steam extraction and said turbine is fed with medium pressure steam. Said turbine is much simpler and cheaper. The terms medium pressure refers to a pressure which is few bars higher than the pressure of the reforming process.

(11) Preferably, said gas compressor is a single casing machine with one compression section. This is possible due to the relatively high pressure of the reforming process. Said gas compressor can run at lower speed (revolutions per minute), is more efficient and has a simplified design. This allows a significant reduction of the footprint and the cost of the plant.

(12) An air feed directed to the secondary reforming is advantageously compressed in an air compressor powered by a steam turbine. In some embodiments of the invention, the air compressor (instead of the syngas compressor) becomes the largest power user. Accordingly, the highest pressure available steam is used to drive the steam turbine coupled to said air compressor; steam discharged by, or extracted from, said turbine is preferably used for the primary reforming.

(13) This is advantageous as regards the efficiency of the process, because air compression can be achieved much more efficiently than syngas compression. This is mainly due to the possibility to use, in some embodiments, an air compressor of the integrally geared type (IG), which is unsuitable for the synthesis gas.

(14) Moreover, the steam turbine can be easily coupled to the air compressor with a dedicated pinion shaft at the desired speed: hence there is no limitation to the size of the steam turbine coupled to the air compressor.

(15) As mentioned, the air compressor is preferably an integrally geared turbomachine (IG). An integrally geared turbomachine is typically designed to operate at fixed speed and is usually controlled acting on the inlet guide vanes (IGV) installed at suction. The efficiency of said compressor is affected by fluctuations of the volumetric flowrate of the air feed, which are due to temperature fluctuations of the air feed between day and night, summer and winter.

(16) In order to work close to the point of maximum efficiency and to remain inside the range of control given by the IGV, the air feed could be heated or cooled at the suction of the air compressor, thus maintaining the inlet temperature of the air compressor within a target range.

(17) Preferably the heater and/or the cooler are integrated with an air filter at the suction of the air compressor so as to reduce the footprint and save costs.

(18) This embodiment provides an efficient way to properly control the air compressor and to keep the compressor operating within its optimal range, i.e. assuring the most efficient performance. This is particularly advantageous because even small variations of the air compressor efficiency have a significant impact on the energy consumption of the entire plant.

(19) According to a preferred embodiment, the conversion of the make-up synthesis gas into ammonia is carried out in two reactors arranged in series so that the effluent of a first reactor is further reacted in a second reactor.

(20) Preferably, the effluent of the first reactor is cooled before admission to the second reactor in a suitable heat exchanger placed between the two reactors. This is advantageous because allows to generate steam in said heat exchanger by cooling the product gas from the first reactor.

(21) This solution is also advantageous because the cold product gas can be conveniently used to flush the pressure vessel of the second converter. The gas temperature is cold enough to avoid the nitriding attack zone, assuring safe operation of the second reactor.

(22) Another aspect of the invention is a plant for the synthesis of ammonia according to the annexed claims.

(23) Another aspect of the invention is a method of operating an ammonia plant wherein the ammonia plant requires a predetermined power for operation and requires a predetermined start-up power for start-up, the method being characterized in that a first amount of electric power is internally produced in the ammonia plant by means of a generator of the plant operated by a steam turbine, and in that a second amount of electric power, is imported from a source of electric power which is external to said plant, wherein said second electric power is equal to or greater than said start-up power.