Process for the synthesis of ammonia

Abstract

Process for the synthesis of ammonia comprising the steps of reforming of a hydrocarbon feedstock into a raw product gas, purification of said raw product gas obtaining a make-up synthesis gas, conversion of said synthesis gas into ammonia; said purification includes shift conversion of carbon monoxide into carbon dioxide and the reforming process requires a heat input which is at least partially recovered from at least one of said step of shift conversion, which is carried out with a peak temperature of at least 450° C., and said step of conversion into ammonia.

Claims

1. A process for the synthesis of ammonia comprising the steps of: reforming of a hydrocarbon feedstock into a raw product gas, said reforming requiring a heat input; purification of said raw product gas obtaining a make-up synthesis gas; conversion of said synthesis gas into ammonia, wherein said purification includes shift conversion of carbon monoxide into carbon dioxide, wherein said heat input of the reforming process is at least partially recovered from said step of shift conversion, which is carried out with a peak temperature of at least 450° C., wherein heat recovered from the shift conversion is used to pre-heat a mixed feed including hydrocarbon and steam, before admission of said mixed feed to the reforming step, wherein: a mixed-feed of natural gas and steam is cooled by exchanging heat with another process stream; the so obtained cooled mixed feed is then re-heated by cooling a shift reactor or the effluent thereof.

2. The process according to claim 1, wherein heat of said shift conversion is recovered by either: direct cooling of a catalytic bed of a related shift converter, and/or cooling an effluent of shifted gas.

3. The process according to claim 1, wherein pre-heated mixed feed is subject to pre-reforming before the reforming step.

4. The process according to claim 1, wherein a mixed-feed of natural gas and steam is cooled by exchanging heat with a fresh natural gas feed before a desulphurization.

5. The process according to claim 1, wherein the mixed feed, prior to reforming, is preheated with a recovered heat and said recovered heat comes exclusively from cooling of an effluent of a high-temperature shift.

6. The process according to claim 5, wherein the mixed feed, prior to reforming, is preheated with a recovered heat and said recovered heat comes exclusively from cooling of an effluent of a high-temperature shift and effluent has a temperature after the high temperature shift of at least 450° C.

7. The process according to claim 1, wherein said reforming of the hydrocarbon feedstock comprises: a primary reforming with steam and a secondary reforming with an oxidant and optionally a gas heated reformer (GHR), or a step of auto-thermal reforming and optionally a GHR.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a scheme of the plant for the production of ammonia according to the prior art.

(2) FIG. 2 is a scheme of a plant according to a first embodiment of the invention.

(3) FIG. 3 is a scheme of a plant according to a second embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(4) FIG. 1 illustrates a block scheme of a plant according to the prior art comprising: a front-end including a reforming section 1 and a purification section 2, providing a make-up synthesis gas, and a synthesis loop 3 for the conversion of said synthesis gas into ammonia.

(5) The reforming section 1 for example comprises a primary reformer 4 and a secondary reformer 5 and a first waste heat boiler 6. The primary reformer 4 is fired by a fuel F, for example natural gas.

(6) The purification section 2 comprises a high-temperature shift reactor 7, a second waste heat boiler 8 and further equipment denoted by block 9. Said block 9 may include one or more of: low-temperature shift reactor (LTS); remover of carbon dioxide, methanator, hot recovery exchangers (e.g. to preheat water), cryogenic purification etc. according to known technique.

(7) The synthesis loop 3 comprises a synthesis reactor 10 and further equipment for processing the effluent of the loop, for example an ammonia condenser 11.

(8) A mixed feed 12 of natural gas and steam is fed to a reformer 4, wherein it is first preheated in one or more mixed feed coils to about 500° C., then reformed in the primary reformer 4 and the effluent 13 is further reformed in the secondary reformer 5 with an oxidant 14 such as air or enriched air or oxygen. The raw product gas 15 leaving the secondary reformer 5 at a temperature of around 900-1000° C. is cooled in the waste heat boiler 6 to a temperature of around 320-350° C. and the cooled gas 16 is fed to the purification section 2, namely to the shift reactor 7.

(9) The effluent 17 of said reactor 7 is cooled in the second waste heat boiler 8 and further purified in the equipment 9 (e.g. by removing CO and CO2) obtaining a make-up gas 18.

(10) Said make-up gas 18 is reacted in the reactor 10 and the ammonia contained in effluent 19 is condensed in block 11.

(11) The mixed feed 12 typically has a temperature around 350° C.

(12) FIG. 2 shows the plant of FIG. 1 revamped according to an embodiment of the invention, where the mixed feed 12 is pre-heated with heat generated by the shift converter 7.

(13) More in detail, an indirect heat exchanger 20 is installed between the shift converter 7 and the waste heat boiler 8. One side of the exchanger 20 is traversed by the hot effluent 17 of the shift converter 7, and the other side is traversed by the mixed feed 12.

(14) Optionally, the heated mixed feed 21 leaving said heat exchanger 20 is further heated in a second heat exchanger immersed in a catalytic bed of said converter 7, obtaining a further heated mixed feed 22. In this case the shift converter 7 works substantially in isothermal conditions, since the temperature of the respective catalytic bed is controlled by the heat exchange with the mixed feed 21.

(15) In some embodiments, the feed 12 may be directly fed in a heat exchanger immersed in the catalytic bed of the shift converter 7 (i.e. without the exchanger 20).

(16) FIG. 2 shows a preferred embodiment where a pre-reformer 23 is also installed upstream the reforming section 1. Accordingly, the mixed feed 22 is sent to said pre-reformer 23.

(17) The peak temperature of the gas evolving in the shift converter 7 is at least 450° C., according to the invention. The temperature of mixed feed 22 is significantly higher than the temperature of the originally available feed 12, for example at least 400° C. and preferably 450° C. or higher. A trim heater may be installed to control the pre-reformer inlet temperature.

(18) Thanks to the higher temperature of mixed feed input, the consumption of fuel F is reduced.

(19) FIG. 3 shows another embodiment where heat is recovered from the synthesis loop 3. In this embodiment, the mixed feed 12 is heated in a heat exchanger 30 and the hot source is an effluent 19 of the reactor 10 (or of one of the catalytic beds), obtaining a heated mixed feed 24.

(20) The embodiments of FIGS. 2 and 3 may be combined, for example the heated feed 24 of FIG. 3 may be further heated in the heat exchanger 20 and/or in the shift converter 7 as shown in FIG. 2.

(21) Preferably, the temperature of the stream 16 is also increased, for example from around 320-350° C. to 400° C. or more.

(22) The embodiments of FIGS. 2 and 3 may also be implemented in the realization of new plants.

(23) Some embodiments of the invention involve the revamping of a plant wherein the reforming section includes also a gas-heated reactor (GHR).

(24) For example, an existing plant may include a reforming section with a primary reformer, a secondary reformer and a GHR. Part of the mixed feed is sent to the primary reformer and a remaining part is sent directly to the GHR. According to the method of the invention, the mixed feed is pre-heated in at least one newly installed heat exchanger, using heat recovered from a shift converter and/or from ammonia reactor(s), then a part of the pre-heated mixed feed is sent to the GHR and a remaining part is sent to the primary reformer; in accordance, the original line feeding the GHR can be discontinued. The related advantages are debottlenecking of the primary reformer and/or of the GHR and possible increase of the temperature of the GHR reducing the risk of metal dusting.