The present invention relates to a process for producing hydrogen from a hydrocarbon feedstock, such feedstock deriving from fossil sources (solid, liquid or gas) or from bacterial fermentation (biogas).

Hydrogen (¾) is an attractive alternative to fossil fuels as a non-polluting source of energy, and it is also a valuable commodity for energy transition media and for industrial uses, for example in product upgrading such as hydrocracking and hydroisomerizing liquid hydrocarbons to produce desirable products. Hydrogen is usually produced by reforming or partial oxidation of natural gas or liquid hydrocarbons, which yields synthesis gas (syngas), i.e. a mixture of hydrogen and carbon monoxide (CO) and possibly minor amounts of carbon dioxide (CO2). The relative amounts of CO/CO2 and ¾ in a synthesis gas product varies depending upon the way it is generated. For separating and purifying the hydrogen component of synthesis gas various technologies are available, e.g. pressure swing adsorption (PSA), membrane separation, or chemical reaction on solid iron oxide and calcium oxide beds, with regeneration of the solids.

It is known to produce electricity in a combined cycle power plant integrated with a reforming plant where a gas turbine is fuelled by hydrogen containing gas. For instance, WO 00/03126 relates to a process for the production of electric energy, steam, and carbon dioxide in concentrated form from a hydrocarbon feedstock, comprising formation of synthesis gas in an air driven autothermal reactor unit (ATR); heat exchanging the formed synthesis gas and thereby producing steam, treating at least a portion of the synthesis gas in a CO-shift reactor unit and carbon dioxide separator unit for formation of concentrated carbon dioxide and a hydrogen containing gas which at least partly is combusted in a combined cycle gas turbine for the production of electric energy; and where air from said turbine unit is supplied to the ATR unit, that the exhaust from the gas turbine is heat exchanged for production of steam which together with steam generated upstream is utilized in a steam turbine for production of substantially CO2 -free electric energy.

The Applicant has faced the technical problem of providing a process for producing hydrogen from a hydrocarbon feedstock (the so called "Blue Hydrogen"), especially from natural gas, by means of a reforming reactor, which does not need an external power source, which consumes reduced volumes of fresh water and which does not disperse pollutants, such as nitrogen oxides and particulate, and carbon dioxide in the atmosphere, with a carbon dioxide capture efficiency greater than 99%.

The Applicant has found that the above technical problem, and others that are apparent from the present description, can be solved by a process wherein the hydrocarbon feedstock is subjected to a reforming reaction to produce a synthesis gas stream comprising hydrogen (¾), carbon monoxide (CO) and carbon dioxide (CO2), and the produced synthesis gas stream is then improved in terms of ¾ yield by a water gas shift (WGS) step. Then, a H2-rich stream and an off-gas stream are produced by a pressure swing adsorption (PSA) step. The H2-rich stream is ready for subsequent uses, whereas the off-gas stream is subjected to an oxycombustion step, preferably a pressurized oxycombustion step, to generate power and to produce a C0 ₂ -rich stream, which is then sequestered by water condensation for further use or disposal, e.g. by injection into a geological formation (reservoir). The process can be applied in multiple configurations, including the reservoir to operate as recipient when hydrocarbons are taken from the oil&gas field under production. The process is especially suitable for small scale plants.

Therefore, according to a first aspect, the present invention relates to a process for producing hydrogen from a hydrocarbon feedstock, comprising the steps of:

(i) introducing a fuel feed stream comprising the hydrocarbon feedstock, an oxidant feed stream comprising oxygen (O2) and a steam feed stream into a reforming reactor unit to form a gas reaction mixture, which is reacted to produce a synthesis gas stream comprising hydrogen (¾), carbon monoxide (CO) and carbon dioxide (CO2);

(ii) cooling down the synthesis gas stream, withdrawn from the reforming reactor unit, to a temperature of from 200°C to 400°C;

(iii) feeding the cooled synthesis gas stream to a water gas shift (WGS) reactor unit where CO is reacted with steam to form ¾ and CO2; (iv) feeding the gas mixture produced in the WGS reactor unit to a pressure swing adsorption (PSA) unit where a hydrogen-rich stream and an off-gas stream are produced; and

(v) feeding the off-gas stream to an oxycombustion unit where the off gas stream is combusted with pure oxygen (O2) to generate power and produce a C0 ₂ -rich stream;

(vi) sequestering the C0 ₂ -rich stream and/or using the C0 ₂ -rich stream in a chemical process.

The hydrocarbon feedstock can be natural gas, refinery off-gas, pre-reformed gas, Fischer-Tropsch tail-gas, Liquefied Petroleum Gas (LPG), naphtha, or biogas. Biogas is usually obtained by anaerobic bacterial fermentation of organic substrates of vegetal or animal origin, including urban and agriculture wastes. The hydrocarbon feedstock is preferably a desulfurized hydrocarbon feedstock, so as to avoid catalyst poisoning in the subsequent steps, especially in the WGS reactor unit.

Preferably, the hydrocarbon feedstock is natural gas, which essentially consists of methane.

Preferably, the hydrocarbon feedstock is a biogas. In this case, the process according to the present invention has a negative carbon footprint, because the biogenic carbon is extracted from the atmosphere by means of direct air capture (DAC) technologies. Preferably, the biogas is a desulfurized biogas, i.e. a biogas that has been treated only to eliminate compounds, such as sulfur containing compounds, which may cause catalyst poisoning.

Preferably the oxidant feed stream is substantially pure oxygen (O2) (purity >= 95%).

The reforming reactor unit is usually a refractory-lined pressure-vessel, which operates as an auto-thermal reformer (ATR) or a partial oxidation unit (POX).

Preferably, the reforming reactor unit comprises:

(a) a combustion section, where a portion of the hydrocarbon feedstock is combusted with oxygen to heat the gas reaction mixture to a temperature of at least 1200°C; (b) a reaction section, where the heated gas reaction mixture is subjected to a reforming reaction to produce the synthesis gas stream comprising hydrogen (H2), carbon monoxide (CO) and carbon dioxide (CO2).

Preferably, the hydrocarbon feedstock is preheated, mixed with steam and fed into the combustion section of the reforming reactor unit, where the gas reaction mixture is heated to a temperature of at least 1200°C. The hot gas reaction mixture then enters the reaction section, where the endothermic reforming reaction occurs and reduces the reactor temperature. The endothermic reforming reaction may occur in the presence of a catalyst (usually a nickel-based catalyst) or in the absence of a catalyst. The reaction pressure is usually maintained at a value of from 20 to 50 barg. If a POX unit is employed, the combustion and reaction sections are usually combined in a single unit.

The synthesis gas stream exiting from the reforming reactor unit is mainly composed of ¾, CO and CO2, besides reaction steam in excess and low amounts of unreacted methane and unreacted inert impurities, such as traces of argon and/or nitrogen. The reaction steam may be recycled to the reforming reactor unit after the cooling step (ii).

After being cooled down to a temperature of from 200°C to 400°C, the synthesis gas stream is subjected to a water gas shift (WGS) reaction, in which CO reacts with steam to form additional ¾ and CO2. Generally, the WGS reaction is subdivided into two steps carried out at different temperatures to achieve high CO conversion and fast reaction rates. The high temperature step is usually carried out at a temperature of from 310°C to 450°C, whereas the low temperature step is carried out at a temperature of from 210°C to 240°C. In both steps the pressure is usually maintained at a value of from 20 and 50 barg. Preferably, the pressure in the WGS reactor unit is substantially the same maintained in the reforming reactor unit, apart a small reduction due to hydraulic loss of the plant. Preferably, the WGS reaction is carried out in the presence of a catalyst. Generally, an iron/chromium-based catalyst is used in the high temperature step, while a copper/zinc -based catalyst is used in the low temperature step. The gas mixture produced in the WGS reactor, which is enriched in ¾ and CO2 by the WGS reaction, is cooled down and fed to a pressure swing adsorption (PSA) unit, which is aimed to separate ¾ from CO2 and other gases present in the mixture. The PSA unit is based on adsorption of gases at high pressure by a plurality of adsorbent beds, and their desorption at lower pressure (regeneration). While the adsorption step is usually carried out at the same pressure of the WGS reactor unit (i.e. from 20 barg to 50 barg), the regeneration step is carried out at reduced pressure, usually from 0.2 barg to 1 barg, to maximize the hydrogen recovery yield.

According to the present invention, higher regeneration pressures, e.g. from 1 to 5 barg, may be advantageously used to obtain a pressurized off-gas stream. A high regeneration pressure increases the overall energy efficiency so as to target the PSA off-gas energy recovery, and also the pressure that can be advantageously used for the subsequent oxycombustion step and the final pressure of the captured CO2, as described below. The adsorbent beds usually contain zeolites or activated carbons.

The PSA unit generally operates at room temperature. The th-rich stream obtained from the PSA unit is of high purity (usually >= 98% and even up to 99.99%). The off-gas stream obtained from the PSA unit, which is produced by desorbing the gaseous products captured by the adsorbent beds, is mainly composed of CO2 and combustible gases, i.e. ¾, CO and CH4. The PSA unit has a yield of high purity ¾ usually from 80% to 90% of the initial amount of ¾ present in the gas mixture withdrawn from the WGS reactor.

The off-gas stream obtained from the PSA unit is fed to an oxycombustion unit. As known, in an oxycombustion process, the fuel (in the present case the fuel is the off-gas stream from the PSA unit) is burned in the presence of pure oxygen (purity >= 95%). Therefore, the exhaust of the oxycombustion process is a C0 ₂ -rich stream which mainly contains CO2 and steam, with low amounts of gaseous impurities.

To control the oxycombustion temperature, it is advantageous to recycle a portion of the CC -rich stream to the oxycombustion unit, so as to dilute the gases therein and find a proper balancing between energy efficiency and CAPEX, since uncontrolled oxycombustion temperatures are detrimental for the duration and efficiency of the oxycombustion unit. Usually, in order to achieve an effective control of the oxycombustion temperature, the oxycombustion unit is also fed with an additional fuel stream, which can be natural gas or any hydrocarbon waste or also a portion of the hydrocarbon feedstock employed for the fuel feed stream of the reforming step (i).

The oxycombustion unit may be operated at atmospheric pressure. However, it is preferred to use a pressurized oxycombustion unit, operating at a pressure of from 12 barg to 35 barg, more preferably from 15 barg to 25 barg. The pressure value is optimized depending on the intended application of the CC -rich stream and on the plant boundary conditions (e.g. the pressure of the feed gases, especially of the oxygen feed stream). A pressurized oxycombustion unit allows to minimize the additional fuel stream and to have an overall process independent from external energy sources. Moreover, the pressurized oxycombustion unit may be coupled with a PSA unity employing a higher regeneration pressure, e.g. from 1 to 5 barg, as disclosed above. This slightly reduces the hydrogen recovery factor in the PSA unit but optimizes the overall process, leading to more efficient and compact units, lower CAPEX, lighter ecological footprint and heat dispersion in the environment. This results in reducing the CO2 compression duty, which is a key parameter that allows to increase energy conversion into the final captured pressurized CO2. The CCh-rich stream is preferably subjected to cooling and compression so as to condense the steam and separate it from CO2. Advantageously, this step comprises a cascade of cooling/compression cycles, wherein condensed water in a stage is recycled back to the previous stage, to flash the dissolved CO2 so as to achieve a higher CO2 capture efficiency. This allows to gradually increase the pressure of the CC -rich stream from a relatively low value typical of the oxycombustion unit (i.e. 15-25 barg of the pressurized one) to a very high value which is useful for CO2 injection (e.g. 100 barg) as reported below.

At the outlet of the oxycombustion unit, of the reforming reactor unit and of the WGS reactor unit, energy is recovered to produce steam in closed loop and generate power in steam turbines. The oxycombustion unit allows to recover water condensation energy at high temperature, which would be otherwise lost if steam in the flue gases is sent to the atmosphere: as already pointed out, this increase the overall process energy efficiency. The combination of the pressurized oxycombustion unit with flue gas cooling and heat recovery, including a direct contact condenser, allows improved CO2 capture and energy efficiency.

The condensed steam is recycled back as steam feed and requires a very small make-up to compensate the steam that remains in the CC -rich stream or with the immiscible gases that separate from the liquid/dense phase CO2.

The CC -rich stream may be subjected to sequestering by injection into a geological formation, e.g. a hydrocarbon (oil or gas) reservoir, an aquifer or cavern, where CO2 is entrapped in a stable form so as to avoid dispersion into the atmosphere.

The CC -rich stream may also be utilized in supercritical extraction systems or in processes where a low/medium high pressure CO2 stream is demanded. The CC -rich stream may be pumped into a pipeline, or refrigerated and loaded into liquid CO2 vessels which are ready for shipping elsewhere. This last application represents indeed a carbon sequestration plant in alternative to post-combustion on flue gases or pre-combustion capture on syngas when hydrogen is used as fuel.

The CC -rich stream may be advantageously used for enhanced oil recovery (EOR) to increase production capacity from oil reservoirs. The compressed CCh-rich stream is pumped in the hydrocarbon reservoir where it displaces the hydrocarbons contained in the porous and permeable reservoir rock towards a production well for enhanced recovery of hydrocarbons therefrom.

The CCh-rich stream may also be used for the stabilization of the extracted oil, i.e. for the removal of noxious volatile components from the extracted oil for subsequent safe storage. The removal of volatile components is achieved by the stripping effect caused by CO2 injection into the oil and by direct heat exchange with water condensation which favours evaporation of the same. This minimizes or avoids the supply of heat from external sources for oil stabilization and the use of cooling media for the CCh-rich stream, thus improving energy balance of the overall process. If any CO2 is produced from the production well together with the hydrocarbons (CO2 build-up which occurs along years), CO2 will be considered at plant design stage and duly managed during operational stage with minimal constrains, having it passing through the process as inert gas and so re-injected into the hydrocarbon reservoir together with the new CO2 formed by the process. It is also envisaged that the compressed CCh-rich stream may be sequestered by injection into a depleted oil or gas reservoir where the production wells have been shut-in or into an aquifer or into a cavern for storage therein.

The CC -rich stream may be also utilized in a chemical process, e.g. in a urea production plant, according to well known techniques. Alternatively, the CC -rich stream may be sent to a carbon capture and sequestration (CCS) plant, through an export unit (e.g. a CO2 pipeline or a CO2 liquid carrier). The process according to the present invention can be easily coupled with an ammonia plant, where the nitrogen separated in the air separation unit that supplies oxygen can be used for ammonia synthesis and so obtaining Blue Ammonia. Moreover, the CC -rich stream may be used for food and beverage industry, after proper purification to food grade.

In is important to point out that the process according to the present invention has a CO2 capture efficiency of about 99% and an energy conversion efficiency from the hydrocarbon feedstock to Fh at least equal to 70%. The energy conversion efficiency is the energy percentage that is transferred from the hydrocarbon feedstock to Fh, with respect to the energy contained in the hydrocarbon feedstock. This efficiency values are remarkably higher that those that can be obtained in the production of blue-hydrogen, which are at most equal to 65%.

Cascaded utilization of low enthalpy heat can be achieved in nearby applications, such as in a biogas anaerobic digestor plant or in the oil&gas treatment plant that supplies the hydrocarbon feedstock. This heat recovery achieves an energy conversion for the process according to the present invention similar to Grey-Fh while capturing pressurized CO2 with 99% efficiency.

For the purpose of the present description and of the claims that follow, except where otherwise indicated, all numbers expressing amounts, quantities, percentages, and so forth, are to be understood as being modified in all instances by the term "about". Moreover, all ranges include any combination of the maximum and minimum points disclosed and include any intermediate ranges therein, which may or may not be specifically enumerated herein. For the purpose of the present description and of the appended claims, the words "a" or "an" should be read to include one or at least one and the singular also includes the plural unless it is obvious that it is meant otherwise.

BRIEF DESCRIPTION OF THE DRAWINGS

Further characteristics and advantages will be more apparent from the following description of some embodiments given as a way of an example with reference to the attached drawing in which:

Figure 1 shows a schematic block diagram of a process according to the present invention.

With reference to Fig. 1, the plant for carrying out the process according to the present invention comprises an air separation unit (1), which is aimed to separate pure oxygen (O2 ³ 95% v/v) from air according to known techniques, usually by fractional distillation. The pure oxygen stream is conveyed to a reforming reactor unit (2) where it is mixed with a hydrocarbon feedstock stream, preferably a methane feed stream from a natural gas network or from a reservoir (3), and a steam feed line (4a) which collects steam mainly deriving from water recovery from the subsequent steps of the process through steam feed lines (4b), especially from the cooling and compression steps of the CCh-rich stream (10) obtained from the oxycombustion unit (9). A portion of the hydrocarbon feedstock stream may be fed to the oxycombustion unit (9). The syngas stream withdrawn from the reforming reactor unit (2) is cooled down by means of a cooling unit (5) and then fed to a WGS reactor unit (6), wherein CO is reacted with steam to form ¾ and CO2 so as to increase the hydrogen yield. The cooling unit (5) allows to recover steam that can be recycled to the reforming reactor unit (2) through the steam feed line (4a). The gas mixture produced in the WGS reactor unit (6) is then conveyed into a PSA unit (7) to produce a hydrogen- rich stream and an off-gas stream. The hydrogen-rich stream may be fed to a hydrogen compression unit (8), where the ¾ gas is compressed and bottled for transportation to the final user. The off-gas stream is fed to the oxycombustion unit (9) where the off-gas is combusted with pure O2 (purity >= 95%) coming from the air separation unit (1). The CC -rich stream produced in the oxycombustion unit (9) is fed to a cooling and compression unit (10), where the steam contained in the CO2- rich stream is condensed and recycled to the cooler (5) through the condensed steam feed line (4b). A portion of the CC -rich stream is fed to the oxycombustion unit (9) to control the oxycombustion temperature. The heat obtained during condensation is recovered to increase the overall process energy efficiency. The compressed CO2- rich stream is then sequestered and/or used in a chemical process as described hereinabove.