METHOD FOR HYDROGEN PRODUCTION COUPLED WITH CO2 CAPTURE

Abstract

A method for hydrogen production starting from a hydrocarbon feed, including a step of reacting the hydrocarbon feed with water steam to obtain a gas stream including hydrogen, carbon monoxide, and carbon dioxide (syngas), heat being provided to the step of reacting the hydrocarbon feed with water steam, the heat being obtained by electrically powered sources; and including removing carbon dioxide from the gas stream. The embodiments further relate to a plant for hydrogen production starting from a hydrocarbon feed, including an electrically powered steam reformer and at least one CO.sub.2 capture system, arranged downstream the electrically powered steam reformer.

Claims

1-11. (canceled)

12. A method for hydrogen production starting from a hydrocarbon feed, comprising the following steps: reacting said hydrocarbon feed added with a compressed recycle stream with steam to obtain a gas stream comprising hydrogen, carbon monoxide, and carbon dioxide (syngas); reacting carbon monoxide of the gas stream from the previous step with steam to obtain a gas stream enriched with hydrogen and carbon dioxide; separating hydrogen from said enriched gas stream to obtain a hydrogen product stream and a recycle stream; compressing said recycle stream to obtain a compressed recycle stream; mixing said compressed recycle stream with said hydrocarbon feed; providing heat to said step of reacting said hydrocarbon feed with water steam, said heat obtained from electrically powered sources; and a) removing CO.sub.2 from said gas stream enriched with hydrogen and carbon dioxide, before separating hydrogen; or b) removing CO.sub.2 from said compressed recycle stream, before mixing with said hydrocarbon feed; or c) removing CO.sub.2 both from said gas stream enriched with hydrogen and carbon dioxide, before separating hydrogen and from said compressed recycle stream, before mixing with said hydrocarbon feed; and removing at least part of inert components from the recycle stream.

13. The method for hydrogen production according to claim 12, further comprising a step of: removing sulfur, chlorides, and olefins from said hydrocarbon feed added with a compressed recycle stream, before reacting with steam.

14. The method for hydrogen production according to claim 12, wherein said step of removing at least part of inert components from the recycle stream is performed by intermittently or continuously purging part of the recycle stream, to remove inert components.

15. The method for hydrogen production according to claim 14, wherein CO.sub.2 is removed from the purge stream.

16. The method for hydrogen production according to claim 15, wherein 7 vol % or less of the recycle stream is purged.

17. The method for hydrogen production according to claim 12, wherein, when CO.sub.2 is removed from said gas stream enriched with hydrogen and carbon dioxide, and before separating hydrogen, the composition of the recycle stream before mixing with said hydrocarbon feed is: CH.sub.4: 14.5-15.5 vol %, CO.sub.2: 0.5-1.0 vol %, N.sub.2: 34.5-35.5 vol %, CO: 15.5-16.5 vol %, H.sub.2: 31.5-32.5 vol %, and H.sub.2O: 1.0-1.5 vol %.

18. The method for hydrogen production according to claim 12, wherein, when CO.sub.2 is removed from said compressed recycle stream, and before mixing with said hydrocarbon feed, the composition of the recycle stream before mixing with said hydrocarbon feed is: CH.sub.4: 11.5-12.5 vol %, CO.sub.2: 13.0-14.0 vol %, N.sub.2: 29.0-30.5 vol %, CO: 16.0-17.0 vol %, H.sub.2: 26.5-27.5 vol %, and H.sub.2O: 1.0-1.5 vol %.

19. The method for hydrogen production according to claim 12, wherein, when CO.sub.2 is removed both from said gas stream enriched with hydrogen and carbon dioxide, before separating hydrogen and from said compressed recycle stream, and before mixing with said hydrocarbon feed, the composition of the recycle stream before mixing with said hydrocarbon feed is: CH.sub.4: 12.0-13.0 vol %, CO.sub.2: 0.5-1.5 vol %, N.sub.2: 31.0-32.0 vol %, CO: 14.5-15.0 vol %, H.sub.2: 38.5-39.5 vol %, and H.sub.2O: 1.0-1.5 vol %.

20. The method for hydrogen production according to claim 12, wherein the steam-to-carbon ratio in said step of reacting said hydrocarbon feed added with said compressed recycle stream with steam is between 2.8 and 3.

21. The method for hydrogen production according to claim 12, wherein said electricity fed to said electrical steam reforming is derived from renewable sources.

22. A plant for hydrogen production starting from a hydrocarbon feed, comprising: an electrically powered steam reformer, a water gas shift reactor downstream said steam reformer, a pressure swing adsorber downstream said water gas shift reactor, a hydrogen product stream line and an off-gas stream line downstream said pressure swing adsorber, a recycle stream line connecting said off gas stream line to said hydrocarbon feed, a recycle stream compressor, at least one CO.sub.2 capture system arranged downstream said electrically powered steam reformer, and a split of purge gas separated from the recycle stream line.

Description

[0068] The invention will be disclosed herein below for illustrative, but non limitative purposes, according to preferred embodiments, with reference in particular to the figures of the enclosed drawing, wherein:

[0069] FIG. 1 shows a block diagram of a natural gas hydrogen production unit according to the prior art,

[0070] FIG. 2 shows a block diagram of a plant for hydrogen production coupled with CO.sub.2 capture according to a first embodiment of the method of the present invention,

[0071] FIG. 3 shows a block diagram of a plant for hydrogen production coupled with CO.sub.2 capture according to a second embodiment of the method of the present invention, and

[0072] FIG. 4 shows a block diagram of a plant for hydrogen production coupled with CO.sub.2 capture according to a third embodiment of the method of the present invention.

[0073] The method for hydrogen production coupled with CO.sub.2 capture proposed according to the present invention can be implemented according to three different configurations.

[0074] Making reference to FIG. 2, it is shown a block diagram of a plant for hydrogen production coupled with CO.sub.2 capture according to a first embodiment of the method of the present invention, based on electrical steam reforming and CO.sub.2 capture.

[0075] In particular, the method for hydrogen production coupled with CO.sub.2 capture according to this embodiment is composed of the following steps: a natural gas (NG) feed is mixed with a recycled stream coming from a pressure swing absorption (PSA) unit 14, heated up to 380? C. and conveyed to a pre-treatment unit 10, where sulfur, chloride and olefins are removed. The purified process stream is then mixed with steam, in a preferred steam-to-carbon ratio of 2.8-3. The ratio 2.8 is optimized to have zero export steam. It is possible to further lower the steam to carbon ratio depending on the possibility to have a catalyst able to operate at low steam-to-carbon ratio without deactivation.

[0076] The stream is subsequently pre-heated through a heat exchanger r (not shown) up to 550? C. The heat exchanger preferably uses the reformed stream as heat exchanging fluid, the temperature of the reformed stream being 850-900? C., to heat up the process stream. The heat of the reformed stream can also be used in a different heat exchanger to produce the steam required for the reforming reaction. Alternatively, a stand-alone heater can be used, such as an electric heater or a gas heater. In particular, the use of an electric heater would be preferable in order to eliminate the CO.sub.2 contribution coming from fuel combustion. Alternatively, in order to reduce electric power consumption, it is possible to replace the electrical heater with a gas heating step, carried out by burning a portion of natural gas with air. The flue gas generated is sent to stack. However, this solution is less compact and would be more impacting in terms of CO.sub.2 emissions, but would make the system a slightly bit more independent from availability of power produced from renewable feedstock.

[0077] The pre-heated stream is then sent to an electrical steam reformer 11, at a temperature of the pre-heated stream that allows both to protect the catalyst of the steam reformer 11 and to enter on the catalytic bed of the steam reformer 11 already over the threshold catalyst temperature.

[0078] Along the catalytic bed a temperature profile is established that is dependent on both endothermicity of the reaction and the heat provided by the electrical heat source, to achieve at the outlet of catalytic bed a temperature in the range of 850-900? C.

[0079] In some plant configurations, mainly when heavy feedstocks are used, a pre-reforming step can be installed upstream to the steam reformer 11. In this additional step a preliminary reforming occurs at lower temperature.

[0080] The reformed stream is subsequently cooled down, the heat of the reformed stream being preferably at least partly recovered to produce steam or to pre-heat the process stream upstream the reformer 11 as discussed above, the outlet stream, at a high or medium temperature, depending on available heat, preferably at 340? C., is then sent to a water gas shift reactor 12, in which a certain CO conversion to CO.sub.2 is reached.

[0081] Water gas shift conversion may be carried out at high (water gas shift inlet temperature about 320? C.-350? C.) or medium temperature (water gas shift inlet temperature about 250? C.-280? C.), depending on the heat recovery in the overall process.

[0082] The process stream from the water gas shift reactor 12, which is a stream that is rich of H.sub.2 and CO.sub.2, is cooled by a series of exchangers (not shown) to recover heat and then is sent to a CO.sub.2 capture unit 13 (namely amine, membrane separation, cryogenic, adsorption systems and combination of them) where the CO.sub.2 is separated; a pure CO.sub.2 stream is collected and, eventually, valorized.

[0083] Finally, the gas rich in H.sub.2 is sent to a PSA system 14 to be purified, meanwhile the off-gas or recycle stream is compressed in a compressor 15 and recycled to the front end of the plant. A split of purge gas (in the order of 2-3% of total) is separated from the main recycle stream to manage the amount of N.sub.2 or other inerts which are present in the natural gas to prevent accumulation thereof in the recycle stream.

[0084] With reference to FIG. 3, in an alternative plant for hydrogen production coupled with CO.sub.2 capture according to a second embodiment of the method of the present invention, the CO.sub.2 capture stage is installed on the recovered and compressed recycle stream from PSA.

[0085] According to this alternative configuration, the process stream from the water gas shift reactor 12, after heat recovery is sent to the PSA system 14 to be purified, meanwhile the off-gas or recycle stream is compressed in a compressor 15 and then is sent to a CO.sub.2 capture unit 13 where the CO.sub.2 is separated (compression and CO.sub.2 separation can also be integrated a single unit); the pure CO.sub.2 stream is collected and the remaining stream is recycled to the front end of the plant, after an optional purging of around 2-3 wt %.

[0086] Finally, in a third embodiment of the method of the present invention, a plant for hydrogen production coupled with CO.sub.2 capture is shown in FIG. 4, wherein two CO.sub.2 capture units are installed both downstream the water gas shift reactor 12 and on the recycle stream from PSA unit 14.

[0087] The method for hydrogen production coupled with CO.sub.2 capture according to the present invention represents an overcoming of disadvantages of the state of the art technology based on conventional fired heated reformer coupled with CO.sub.2 capture due to the fact that: [0088] CO.sub.2 produced is intrinsically lower, enabling for a reduction of this parameter up to 45%, [0089] CO.sub.2 capture efficiency is higher, due to the fact that CO.sub.2 capture is carried out on the process stream, where a higher concentration of CO.sub.2 is present, [0090] total CO.sub.2 emission to the atmosphere (the amount not captured) is lower due to higher CO.sub.2 capture efficiency.

[0091] The method for hydrogen production coupled with CO.sub.2 capture according to the present invention is based on the assumption that electricity needed in order to operate the electric steam reformer is produced from renewable sources, thus allowing for no CO.sub.2 production. However, in a near term scenario of energy transition, the availability of renewables may not completely satisfy the hydrogen market based on such technology. Since the method for hydrogen production coupled with CO.sub.2 capture according to the present invention enables for a reduction of CO.sub.2 emission, a breakeven point has been calculated, making the assumption that to make equal the CO.sub.2 emissions of a fired heated reformer and an electrical one, up to 30-40% of power from fossil/coal may be accepted, the remaining share coming from renewables.

EXAMPLE 1

[0092] The method for hydrogen production coupled with CO.sub.2 capture according to the present invention was used to treat a natural gas feed with the composition shown in Table 1:

TABLE-US-00001 TABLE 1 Total Molar Component Fractions % vol Pentane 0 Butane 0 i-butane 0 Propane 0.5 Ethane 2 Hexane 0 Methane 95.5 CO 0 CO.sub.2 0 H.sub.2 0 H.sub.2O 0 N.sub.2 2 O.sub.2 0 Ar 0 H.sub.2S 0 SO.sub.2 0 i-pentane 0

[0093] The main technical results achieved by the method for hydrogen production coupled with CO.sub.2 capture according to the present invention were evaluated, in comparison with a traditional fired heated steam reformer, by treating the natural gas according to the embodiment disclosed with reference to FIG. 2 and are reported in Table 2, assuming two different levels of CO.sub.2 removal efficiency.

TABLE-US-00002 TABLE 2 Fired Electrical Electrical heated Steam Steam steam Reformer - Reformer - Fired reformer CO.sub.2 CO.sub.2 heated with CO.sub.2 Capture Capture Steam capture on Efficiency Efficiency Parameter Reformer flue gas 99% 70% Plant 5000 5000 5000 5000 Capacity (Nm.sup.3/h) Molar 3.0 @ 3.0 @ 2.8 2.8 Steam - to - SMR SMR Carbon ratio T.sub.inlet SR 620 620 550 550 (? C.) T.sub.outlet SR 860 860 870 870 (? C.) T.sub.inlet WGS 340 340 340 340 (? C.) T.sub.outlet WGS 414 414 410 421 (? C.) Hydrogen 30 30 21.6 21.6 pressure, B.L. (barg) Hydrogen 42 42 40 40 Temperature, B.L. (? C.) Natural Gas 0.32 0.32.sup.(*.sup.) 0.19.sup.(*.sup.) 0.19.sup.(*.sup.) Consumption (kg/Nm.sup.3 H.sub.2) Power 0.06 0.06.sup.(*.sup.) 1.20.sup.(*.sup.) 1.28.sup.(*.sup.) Consumption (kWh/Nm.sup.3 H.sub.2) CO.sub.2 Produced 0.895 0.895 0.50 0.50 (kgCO.sub.2/Nm.sup.3 H.sub.2) CO.sub.2 0 90.0 98.6.sup.(**.sup.) .sup.97.1.sup.(**.sup.).sup.(***.sup.) captured (%) Efficiency to 3760 3760 2168 2168 hydrogen (kcal/Nm.sup.3) SR standing for Steam Reformer WGS standing for Water Gas Shift Reactor B.L. standing for Battery Limits .sup.(*.sup.)without taking into consideration optional utility consumption of CO.sub.2 removal step (in case the CO.sub.2 capture step should need electric power or natural gas consumption to be operated) .sup.(**.sup.)Residual part in purge gas .sup.(***.sup.)CO.sub.2 captured (%) being higher than indicated capture efficiency (70%) due to CO2 recycling back at the front end of the plant

[0094] Efficiency to hydrogen has been calculated as Feed (LHV)+Fuel (LHV)/Hydrogen production (Nm.sup.3), LHV standing for Lower Heating Value.

EXAMPLE 2

[0095] The technical results achieved by the method for hydrogen production coupled with CO.sub.2 capture according to the present invention were also evaluated, in comparison with a traditional fired heated steam reformer, by treating the same natural gas of the previous example according to the embodiment disclosed with reference to FIG. 3. These results are reported in Table 3, assuming two different levels of CO.sub.2 removal efficiency as for the previous example.

[0096] (Table 3 follows)

TABLE-US-00003 TABLE 3 Fired heated Electrical Electrical steam Steam Steam Fired reformer Reformer - Reformer - heated with CO.sub.2 CO.sub.2 Capture CO.sub.2 Capture Steam capture on Efficiency Efficiency Parameter Reformer flue gas 99% 80% Plant 5000 5000 5000 5000 Capacity, (Nm.sup.3/h) Molar 3.0 @ 3.0 @ 2.8 2.8 Steam - to - SMR SMR Carbon ratio T.sub.inlet SR 620 620 550 550 (? C.) T.sub.outlet SR 860 860 870 870 (? C.) T.sub.inlet WGS 340 340 340 340 (? C.) T.sub.outlet WGS 414 414 410 417 (? C.) Hydrogen 30 30 21.6 21.6 Pressure, B.L. (barg) Hydrogen 42 42 40 40 Temperature, B.L., (? C.) Natural Gas 0.32 0.32.sup.(*.sup.) 0.19.sup.(*.sup.) 0.19.sup.(*.sup.) Consumption, (kg/Nm.sup.3 H.sub.2) Power 0.06 0.06.sup.(*.sup.) 1.23.sup.(*.sup.) 1.28.sup.(*.sup.) Consumption, (kWh/Nm.sup.3 H.sub.2) CO.sub.2 Produced, 0.895 0.895 0.50 0.50 (kgCO.sub.2/Nm.sup.3 H.sub.2) CO.sub.2 0 90.0 98.6 97.7.sup.(***.sup.) captured (%) Efficiency to 3760 3760 2168 2168 hydrogen (kcal/Nm.sup.3) SR standing for Steam Reformer WGS standing for Water Gas Shift Reactor B.L. standing for Battery Limits .sup.(*.sup.)without taking into consideration optional utility consumption of CO.sub.2 removal step (in case the CO.sub.2 capture step should need electric power or natural gas consumption to be operated) .sup.(***.sup.)CO.sub.2 captured (%) being higher than indicated capture efficiency (70%) due to CO.sub.2 recycling back at the front end of the plant

[0097] Efficiency to hydrogen has been calculated as Feed (LHV)+Fuel (LHV)/Hydrogen production (Nm.sup.3), LHV standing for Lower Heating Value.

EXAMPLE 3

[0098] The technical results achieved by the method for hydrogen production coupled with CO.sub.2 capture according were also evaluated, in to the present invention comparison with a traditional fired heated steam reformer, by treating the same natural gas of the previous examples according to the embodiment disclosed with reference to FIG. 4. These results are reported in Table 4, assuming a CO.sub.2 removal efficiency of 70% on the CO.sub.2 capture unit 13 installed downstream the water gas shift reactor and a CO.sub.2 removal efficiency of 80% on CO.sub.2 capture unit 13 installed on purge gas stream.

TABLE-US-00004 TABLE 4 Fired heated Electrical Fired steam reformer Steam Reformer heated with CO.sub.2 CO.sub.2 Capture Steam capture on Efficiency Parameter Reformer flue gas 70%-80% Plant 5000 5000 5000 Capacity (Nm.sup.3/h) Molar 3.0 @ 3.0 @ 2.8 Steam - to - SMR SMR Carbon ratio T.sub.inlet SR 620 620 550 (? C.) T.sub.outlet SR 860 860 870 (? C.) T.sub.inlet WGS 340 340 340 (? C.) T.sub.outlet WGS 414 414 411 (? C.) Hydrogen 30 30 21.6 Pressure, B.L. (barg) Hydrogen 42 42 40 Temperature, B.L. (? C.) Natural Gas 0.32 0.32.sup.(*.sup.) 0.19.sup.(*.sup.) Consumption (kg/Nm.sup.3 H.sub.2) Power 0.06 0.06.sup.(*.sup.) 1.21.sup.(*.sup.) Consumption (kWh/Nm.sup.3 H.sub.2) CO.sub.2 Produced 0.895 0.895 0.50 (kgCO.sub.2/Nm.sup.3 H.sub.2) CO.sub.2 captured 0 90.0 98.5.sup.(***.sup.) (%) Efficiency 3760 3760 2168 to hydrogen (kcal/Nm.sup.3) SR standing for Steam Reformer WGS standing for Water Gas Shift Reactor B.L. standing for Battery Limit .sup.(*.sup.)without taking into consideration optional utility consumption of CO.sub.2 removal step (in case the CO.sub.2 capture step should need electric power or natural gas consumption to be operated) .sup.(***.sup.)CO.sub.2 captured (%) being higher than indicated capture efficiency (70%) due to CO.sub.2 recycling back at the front end of the plant

[0099] Efficiency to hydrogen has been calculated as Feed (LHV)+Fuel (LHV)/Hydrogen production (Nm.sup.3), LHV standing for Lower Heating Value.

[0100] Independently on the type of installation of the CO.sub.2 capture unit, the method for hydrogen production coupled with CO.sub.2 capture according to the present invention allows to reduce the CO.sub.2 emissions of up to 45%.

[0101] An important technical highlight has to be made with reference to inert components (such as nitrogen) possibly contained in natural gas.

[0102] One of the peculiar aspects of the method for hydrogen production coupled with CO.sub.2 capture according to the present invention, independent of the embodiment, is the possibility to recycle back the off-gas from PSA directly to the feed section, thereby reducing the amount of needed make up feed, provided that a compression step is applied. This solution is not applied in conventional fired heated reformer, since in the latter case, due to the presence of at least one burner, it is more convenient, from a technical point of view, to recycle back the off-gas from PSA to the fuel section, thereby reducing the amount of needed make up fuel and simultaneously avoiding the step of off-gas from PSA recompression.

[0103] The main benefits of the method for hydrogen production coupled with CO.sub.2 capture according to the present invention are highlighted herein below: [0104] reduced CO.sub.2 production compared with fired heated steam reformer; [0105] if biogas is used as feed, the overall system will be completely carbon negative, being biogas a renewable feedstock; [0106] reduced feed consumption; [0107] absence of fuel consumption; [0108] no need of stack; [0109] higher efficiency (LHV basis); [0110] reduced noise; [0111] smaller size of the steam reforming reactor; [0112] complete recovery of feed from PSA, which can be recycled and mixed with make-up feed, instead to be burned in furnace; [0113] no export of steam, provided the steam-to-carbon ratio is kept in the preferred range 2.8-3.

[0114] Moreover, since electricity becomes cheaper and is increasingly coming from renewable sources, the benefits of the method for hydrogen production coupled with CO.sub.2 capture according to the present invention will increase over time.

[0115] Additionally, according to the first embodiment disclosed above, wherein also the process stream upstream the reformer 11 is pre-heated in an electrical heater, the method for hydrogen production coupled with CO.sub.2 capture according to the present invention also involves the following advantages: [0116] no need of convective section to recover heat from flue gas, this basically means that there is no need for flue gas duct; [0117] no need for combustion air feeding system.

[0118] With reference to the embodiments shown in FIGS. 2 and 4, the CO.sub.2 capture in pre combustion enables for an increase of hydrogen partial pressure upstream of PSA. Therefore, the PSA can be of smaller size.

[0119] Still another advantage of the method for hydrogen production coupled with CO.sub.2 capture according to the present invention is the possibility to avoid to recycle back a portion of hydrogen from battery limits to carry out the desulfurization step. In fact, the amount of hydrogen required for such a step is already included in the recycled stream from PSA. However, the proposed solution can work even with a recycled stream from PSA routed directly to the electrical steam reformer. In this last case, there is the need of hydrogen from battery limits to enable the hydrodesulfurization of the hydrocarbon feedstock.

[0120] Another advantage of the method for hydrogen production coupled with CO.sub.2 capture according to the present invention is the possibility that no pollutant is released into the atmosphere. This last benefit is dependent on the end use for the split purge gas coming from the recycle stream (PSA off-gas). If it is sent to flare, it contributes to pollutants emissions. Otherwise, if it can be valorized, the environmental impact can be reduced.

[0121] Even if the purge gas is sent to flare, the overall CO.sub.2 emissions are significantly lower (about 90%) than CO.sub.2 emissions of a conventional fired heated reformer coupled with CO.sub.2 capture.

[0122] Still another advantage, depending on plant capacity and type of used shift reactor, is that all required heat to make steam production and additional services, like feed preheating can be produced by heat recovery from process stream.

[0123] The present invention was disclosed for illustrative, non-limiting purposes, according to a preferred embodiment thereof, but it has to be understood that any variations and/or modification can be made by the persons skilled in the art without for this reason escaping from the relative scope of protection, as defined in the enclosed claims.