Process for the synthesis of ammonia with low emissions of CO2IN atmosphere

Abstract

Process for the synthesis of ammonia from natural gas comprising conversion of a charge of desulphurized natural gas and steam, with oxygen-enriched air or oxygen, into a synthesis gas, and treatment of the synthesis gas with shift reaction and decarbonation, wherein a part of the CO2-depleted synthesis gas, obtained after decarbonation, is separated and used as fuel fraction for one or more furnaces of the conversion section, and the remaining part of the gas is used to produce ammonia.

Claims

1. A process for synthesis of ammonia from natural gas, the process comprising: conversion of a charge of desulphurized natural gas and steam, with oxygen-enriched air or with oxygen, into a synthesis gas containing hydrogen, CO and CO2, in a conversion section; treatment of said synthesis gas including at least a shift reaction of the carbon monoxide into CO2, and subsequent separation of CO2 from the gas, thus obtaining a CO2-depleted synthesis gas and a CO2-rich gaseous flow containing the CO2 separated from the gas; separation of a part of said CO2-depleted synthesis gas as fuel fraction, wherein said fuel fraction is fed as fuel to at least one furnace and wherein said separation of the fuel fraction includes the split of said CO2-depleted synthesis gas into at least a first stream and a second stream, said first and second streams having the same composition, wherein the first stream forms the fuel fraction and the second stream is process gas intended for the synthesis of ammonia.

2. The process of claim 1, wherein the first stream forms the fuel fraction and the second stream is the process gas intended for the synthesis of ammonia after further purification.

3. The process of claim 1, wherein said fuel fraction is between 1% and 40% of the total amount of said CO2-depleted syngas.

4. The process of claim 1, wherein said fuel fraction is between 10% and 30% of the total amount of said CO2-depleted syngas.

5. The process of claim 1, further comprising a step for methanation of the synthesis gas and/or addition of nitrogen to the synthesis gas.

6. The process of claim 5, wherein said fuel fraction is separated upstream of the methanation step or upstream of the nitrogen addition step.

7. The process of claim 1, wherein said CO2-depleted syngas contains no more than 1000 ppm by volume of CO2.

8. The process of claim 1, wherein said enriched air contains at least 50% oxygen.

9. The process of claim 1, wherein said enriched air contains at least 90% oxygen.

10. The process of claim 1, wherein said fuel fraction of the synthesis gas feeds one or more of the following apparatuses: a steam production and/or superheating furnace; one or more burners of a primary steam reformer; or one or more furnaces for pre-heating the charge of an autothermal reformer or partial oxidation reactor or natural-gas desulphurization section.

11. The process of claim 1, wherein: a purge gas of the ammonia synthesis process, containing hydrogen and methane, is treated so as to recover the hydrogen contained in the purge gas, obtaining a hydrogen-rich gas and a tail gas containing methane; at least part of said tail gas is recycled as process gas and subjected to said step of conversion of the natural gas into synthesis gas.

12. The process of claim 1, wherein: a part of said CO2-depleted syngas is treated in a unit for separation of nitrogen, obtaining a first stream of nitrogen-depleted syngas and a second nitrogen-enriched stream; at least one part of said first stream, is used as fuel; any remaining part of said first stream is fed to the synthesis of ammonia together with the second stream.

13. The process of claim 1, further comprising a step of expansion, with production of mechanical work, of the CO2-depleted syngas, after heating the CO2-depleted syngas.

14. The process of claim 1, wherein the overall steam/carbon ratio in the conversion step is greater than 2.0.

15. The process of claim 1, wherein the overall steam/carbon ratio in the conversion step is greater than 2.4.

16. The process of claim 1, wherein the overall steam/carbon ratio in the conversion step is greater than 2.7.

17. The process of claim 1, wherein said synthesis gas fuel fraction forms all of the fuel of the furnaces used in the conversion section, or an amount of natural gas is provided as trim fuel, said trim fuel accounting for not more than 15% of the total combustion heat.

18. The process of claim 1, wherein: the fuel fraction is produced in excess with respect to the fuel necessary for the conversion section, the quantity of said fuel fraction exceeding that required being exported from the process; and regulation of the conversion section is performed by modulating the quantity of said fuel fraction directed towards the conversion section and the quantity exported from the process.

19. The process of claim 1, wherein: the conversion of the natural gas charge into synthesis gas includes a step of autothermal reforming, or a step of partial oxidation; said step of autothermal reforming or partial oxidation is performed with a steam/carbon ratio not greater than 2.

20. The process of claim 1, wherein the conversion of the natural gas into synthesis gas includes primary reforming of the natural gas in the presence of steam and secondary reforming performed with said oxygen flow or enriched air.

21. A method for revamping a plant for synthesis of ammonia from natural gas or ammonia/urea plant, wherein the plant includes a reforming section that includes a primary steam reformer and an air-fired secondary reformer, and a section for treatment of the synthesis gas by a shift reaction of carbon monoxide into carbon dioxide, and decarbonation; the method comprising: providing a flow of enriched air or oxygen and using said flow as oxidant, instead of air, for the secondary reformer; separating a part of the synthesis gas, downstream of decarbonation, for use as fuel fraction, and allocating a remaining process fraction of the synthesis gas for ammonia conversion; feeding said fuel fraction of the gas, which has a reduced CO2 content, as fuel for at least one furnace of the plant.

22. The method of claim 21, further comprising installing an air separation unit so as to provide said oxygen or enriched air flow, wherein said separation unit also provides a nitrogen flow which is added to the process fraction of the synthesis gas in order to obtain the desired H2/N2 ratio.

23. The method of claim 21, wherein the section for treatment of the synthesis gas by the shift reaction of carbon monoxide includes methanation.

Description

DESCRIPTION OF THE FIGURES

(1) FIG. 1 shows a first embodiment of the invention.

(2) FIG. 2 shows a second embodiment.

(3) FIG. 3 shows a third embodiment.

(4) FIG. 4 shows a fourth embodiment.

DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(5) FIG. 1 shows a process according to a mode of implementing the invention based on an oxygen autothermal reformer ATR and pre-reformer.

(6) The blocks in FIG. 1 denote the following systems or apparatus:

(7) AUX charge pre-heater furnace and auxiliary boiler

(8) PRE pre-reformer

(9) ATR autothermal reformer

(10) POX partial oxidation reactor

(11) SHF shift section

(12) CDR CO2 removal section (decarbonation)

(13) MET methanator

(14) SYN synthesis section

(15) HRU hydrogen recovery section

(16) ASU air separation (fractioning) section

(17) A charge 10 comprising natural gas and steam is preheated in AUX and converted into a synthesis gas 11 via the reformers PRE and ATR. The autothermal reformer ATR operates with a flow 22 of enriched air or oxygen.

(18) The synthesis gas, which contains H2, CO and CO2, undergoes a shift reaction in SHF and then removal of CO2 in CDR. The section SHF may comprise shift treatments at different temperatures. The section CDR operates for example with chemical/physical washing of the gas using an aqueous amine solution.

(19) The section CDR produces a CO2-depleted syngas 12 and a flow 13 of CO2 which may be sequestered or used for other process purposes, for example in order to produce urea.

(20) Preferably the synthesis gas 12 has the following composition: 98% H2, 1% CO+CH4, 0.1% CO2, Ar+N2 0.9%.

(21) A part 14 of said CO2-depleted syngas 12 is used as fuel in a furnace in the front end, for example in a charge heater (AUX block). The remaining part 18 of synthesis gas is further treated in the methanator MET.

(22) In the example, the fuel fraction 14 of synthesis gas, together with a tail gas 15 of the HRU section, feeds one or more users of the block AUX via the fuel line 16. Optionally a suitable quantity of natural gas 17 (trim gas) is added to the gas 16.

(23) Nitrogen 18 from the air separation section ASU is added to the process fraction 18 of the synthesis gas, after it has passed inside the methanator (optional).

(24) The gas thus obtained, together with a flow of hydrogen 20 recovered from the HRU section, feeds the synthesis section SYN. Said synthesis section produces ammonia 23 and a flow 21 of purging gas treated in the section HRU for recovery of the hydrogen contained inside it.

(25) The separation section ASU produces, in addition to the nitrogen 19, also the flow 22 of enriched air or oxygen for the autothermal reformer ATR.

(26) FIG. 2 shows a process according to a preferred mode of implementing the invention based on primary reforming and ATR.

(27) The block SMR in FIG. 1 denotes the primary reformer (or steam reformer) plus auxiliary apparatus, for example steam generators. The remaining symbols in FIG. 2 correspond to the symbols in FIG. 1 and therefore the description is not repeated.

(28) FIG. 2 also shows an optional line 15a for recycling part of the tail gas 15 from the unit HRU for feeding the primary reformer SMR.

(29) FIG. 3 shows an ATR-based process, similar to FIG. 1, but comprising a purification of the process fraction of the synthesis gas in a section PUR, which in this example comprises a PSA unit.

(30) The fuel fraction 14 separated from the CO2-depleted synthesis gas 12 is divided into a first portion 14a used as fuel in a furnace in the front end, for example in the block AUX, and a second portion 14b exported as excess fuel. The remaining part 18 of synthesis gas is further treated in the PSA unit.

(31) A portion 23 of the effluent of the PSA unit is (optionally) exported as excess fuel. Said portion 23 has a greater hydrogen purity than the gas portion 14b.

(32) In the example shown in FIG. 3, the portion 14a of synthesis gas separated as fuel fraction, together with a tail gas 25 of the PSA unit, feeds one or more users of the block AUX via the fuel line 26.

(33) FIG. 4 shows a process based on POX, comprising purification of the process fraction of the synthesis gas in a section PUR comprising dryer and LNW (liquid nitrogen wash).

(34) In a similar manner to the embodiment shown in FIG. 3, also in this case the fuel fraction 14 separated from the CO2-depleted synthesis gas 12 is split into a first portion 14a used as fuel for example in the block AUX, and a second portion 14b exported as excess fuel. Said portion 14a, together with a tail gas 35 of the section PUR, feeds one or more users of the block AUX via the fuel line 36. Said tail gas 35 results from the combination of the tail gas of the LNW and of the dryer.

(35) The process according to the invention allows to produce ammonia with emissions of CO2 of about 0.1 t/t compared to about 0.7 t/t in the prior art.

(36) All the configurations illustrated may be the result of revamping of an ammonia plant with air-fed secondary reformer.

(37) In one embodiment, the revamping procedure comprises essentially the following steps:

(38) installing the ASU unit in order to generate the flow 22 of oxygen or enriched air;

(39) modifying the secondary reformer in order to accept said flow 22 instead of air;

(40) increasing the fraction of natural gas intended for the process and reducing the fraction of natural gas intended for fuel;

(41) revamping the existing section for desulphurization of the natural gas so as to increase the capacity thereof owing to the greater process gas flowrate;

(42) revamping the existing section CDR so as to increase the capacity thereof owing to the greater quantity of CO2 to be separated.

(43) It can be understood that the revamping does not require costly modifications to the reformer, the exchangers and piping of the front end and the shift section. This is due to the fact that the volumetric flowrate of the gas through the front end does not increase, despite the generation of the additional gas fraction (fuel fraction).

EXAMPLE

(44) The following Table 1 refers to a plant with a capacity of 3000 metric tonnes per day (MTD) of ammonia produced.

(45) Table 1 compares the following options:

(46) (1) Plant of the prior art with primary reformer and air-fed secondary reformer.

(47) (2) Plant of the prior art, as per option (1), with capturing of the CO2 from the fumes of the primary reformer.

(48) (3) Plant according to the invention, as per FIG. 1.

(49) (4) Plant according to the invention, as per FIG. 2.

(50) TABLE-US-00001 TABLE 1 Option (1) (2) (3) (4) Prior art Prior art Inv. Inv. NH3 production MTD 3000 3000 3000 3000 Net Natural Gas consumption based on gas LHV Gcal/t NH3 6.7 7.4 7.0 6.9 (FEED + FUEL-Steam exported e.g. to urea) Gcal/t NH3 +0.7 +0.3 +0.2 CO2 captured in process MTD 3600 3600 5200 5100 CO2 to atmosphere (contained in flue gas) MTD 2100 200 200 200 CO2 to atmosphere (contained in flue gas) t/t NH3 0.7 0.1 0.1 0.1 Main Equipment size Total fired heater duty % 100 120 75 105 (reformer/fired heater + auxiliary boiler) Process CO2 removal capacity % 100 100 145 140 ASU O2 capacity % n/a n/a 100 60 Flowrate of front end (secondary output/ATR) % 100 100 120 97

(51) The table shows that the process according to the invention (option 3) requires a front end suitable for producing also the gas fraction to be used as fuel in the furnaces; in fact the quantity of CO2 captured in the process amounts to 5200 MTD compared to 3600 for options (1) and (2), and the synthesis gas flowrate in the front end is about 20% more than FIGS. 1 and 2. Despite this, the consumption of the plant is much less than that of option (2) (7.0 instead of 7.4 Gcal/MT) and is competitive with the plant without capturing according to option (1). The total duty of the furnaces is only 75% of option (1).

(52) In option (4) the energy consumption obtained is less than in option (3), i.e. equal to 6.9 Gcal/MT, and also the cost of the plant is less. This because, in option (4), the quantity of CO2 separated in the process is less than in option (3); the duty of the furnaces is greater than in (3), but is comparable with that of options (1) and (2); the oxygen consumption is markedly less than (3); the flowrate in the front end is markedly less than (3) and comparable with (1) and (2).

(53) The examples show how the novel process is more efficient and competitive than the prior art.

(54) The example also shows the advantage of modifying a plant operating as per option (1), preferably implementing option (4). In fact, in this case the duty of the furnaces and the flowrate of the gases remain substantially unvaried, while the CO2 emissions are reduced. The invention thus achieves the object of minimizing the CO2 emissions of a conventional plant without losing ammonia production.

(55) The following Table 2 refers to a plant with a capacity of 3000 metric tonnes per day (MTD) of ammonia produced, which is self-sufficient from the point of view of the steam and electric power. The process comprises the consumption relating to the compression of the CO2 sequestered up to the pressure of 100 bar. For each process, the natural gas consumption and the CO2 emissions indicated also consider the superheated steam production in AUX for driving all the turbines, and the production of the electric power necessary for the process.

(56) Table 2 compares the following options:

(57) (1) Plant according to the invention, with SMR+ATR as per FIG. 2, in the following variants:

(58) a. with the overall S/C ratio=2.7, shift composed of HTS and LTS, methanator, trim fuel with flowrate corresponding to 10% of the duty required by the furnace;

(59) b. as for 1 a, but without trim fuel.

(60) (2) Plant according to the invention, with pre-reformer+ATR as per FIG. 3, in the following variants:

(61) a. with the overall S/C ratio=2, shift composed of MTS and LTS, PSA, trim fuel with flowrate corresponding to 10% of the duty required by the furnace;

(62) b. as for 2a, without trim fuel, with the Selectoxo™ process downstream of SHF and upstream of CDR for converting the residual CO.

(63) (3) Plant according to the invention, with POX, as per FIG. 4, in the variants:

(64) a. with the overall S/C ratio=2, shift composed of MTS and LTS, LNW dryer, trim fuel with flowrate corresponding to 10% of required duty of the furnace;

(65) b. as for 2a, without trim fuel, with selective catalytic oxidation of CO (Selectoxo™ process) downstream of the shift reaction and upstream of the CO2 removal.

(66) TABLE-US-00002 TABLE 2 1a: SMR + ATR 1b SMR + ATR 2a ATR 2b ATR 3a POX 3b POX Daily production 3000 3000 3000 3000 3000 3000 of NH3, MTD Daily throughput 5600 5800 5100 5300 5350 5600 of sequestered CO2, MTD Net 100%  100%  93% 93% 95% 95% consumption of natural gas (for Example 1a), dimensionless CO2 emissions 0.15 0.075 0.17 0.10 0.15 0.05 into atmosphere per unit of ammonia produced, t CO2/t NH3 Distribution of the CO2 emissions trim fuel 50% zero 20% zero 20% zero CH4 slip in the 15% 29% 40% 79% 30% 63% process gas CO slip in the 34% 69% 39% 19% 49% 34% process gas CO2 slip in  1%  2%  1%  2%  1%  3% the process gas

(67) In Table 2:

(68) “Trim fuel” as defined in the patent;

(69) “CH4 slip in the process gas” is the residual methane content of the synthesis gas produced in the front end (i.e. in SMR+ATR in option 1, in the pre-reformer+ATR in option 2, POX in option 3), quantified upstream of optional methanation.

(70) “CO slip in the process gas” is the residual carbon monoxide content of the synthesis gas downstream of the shift section;

(71) “CO2 slip in the process gas” is the residual carbon dioxide content of the synthesis gas downstream of the CO2 removal section;

(72) Several comments may be made in view of Table 2: In the case of SMR+ATR in option 1a, the trim fuel provides the greatest percentage of the CO2 emissions; avoiding the trim fuel by exporting surplus fuel enables the emissions to be significantly reduced to less than 0.1 t/t, as shown in the Table for option 1b; at the same time, the quantity of sequestered CO2 increases from 5600 to 5800 MTD. In the case of SMR+ATR, the CO slip in the process gas provides the second greatest percentage of the CO2 emissions in option 1a, and provides the greatest percentage in option 1b; application of the Selectoxo process to option 1a or 1b would significantly reduce the CO2 emissions; the CH4 slip in the process gas and the CO2 slip in the process gas provides a smaller percentage of the CO2 emissions. The overall energy consumption of the options based on ATR and POX+purification (2 and 3) is decidedly lower than that of the options based on SMR+ATR (1a and 1b); this is mainly due to the synthesis gas purification (which improves the efficiency of the synthesis section and avoids the purge); the quantity of CO2 sequestered in these options (2a, 3a; or 2b, 3b) is consequently less than in options 1a or 1b. The emissions due to the use of the trim fuel in options 2a (ATR) and 3a are of about 20% of the total emissions, much less than in option 1a, by way of confirmation of the fact that the heat required by the furnace in these two options is much less than in option 1a. In the embodiments with ATR or POX, the residual CO and CH4 are of higher relevance owing to the lower S/C ratio at which ATR and POX operate compared to SMR+ATR; POX operates at a higher temperature than ATR, which results in a smaller residual amount of CH4, but in a greater residual amount of CO. In both the options 2b and 3b, the improvement achieved by selective oxidation of CO, in order to reduce the CO slip, and by avoiding trim fuel, is evident. It should be noted that both POX and ATR achieve very low emissions—as low as 0.05 for POX and 0.1 for ATR—comparable with those achieved by SMR+ATR, but with lower energy consumption, and therefore preferable.

(73) The above example shows how the invention is able to achieve extremely low CO2 emission levels, corresponding to almost total capture (>98%), while maintaining competitive energy consumption levels.