Patent Description:
For use in a commercial fuel cell, the ammonia must be converted back to hydrogen according to the reaction.

This is an endothermic process, i.e., a process that requires heat, and is performed over a catalyst. This process is known as cracking. The gas produced (or "cracked gas") is a combination of hydrogen (H2) and nitrogen (N2). Since the cracking reaction is an equilibrium reaction, there is also some residual ammonia. In most applications of crackers currently, the hydrogen + nitrogen mixture is utilised as is. However, as ammonia can be a poison to fuel cells, this stream, with ammonia suitably removed such as by scrubbing with water, can be used directly in a fuel cell. However, if the hydrogen is to be used in vehicle fueling, the nitrogen present provides a penalty to the process. The fuel to a vehicle fueling system is compressed to significant pressure - up to <NUM> bar. This means that the nitrogen, which is merely a diluent in the process, is also compressed, taking power, and taking storage volume and increasing anode gas purge requirement, decreasing efficiency. It is therefore beneficial where hydrogen is to be used in vehicle fueling, for the hydrogen + nitrogen to be purified.

Small scale cracking reactors, or "crackers", typically use pressure swing adsorption ("PSA") devices to separate the cracked gas and recover the hydrogen and generate a PSA tail gas (or offgas). However, these crackers are generally heated electrically and the PSA tail gas is typically vented to atmosphere.

As is common in hydrogen production from a steam methane reforming (SMR) reactor, a PSA can be used to purify the nitrogen + hydrogen. The cracking reaction is performed in tubes packed with catalyst which are externally heated by a furnace (see <CIT>).

<CIT> discloses a process for making town gas from ammonia. The ammonia is cracked and the cracked gas scrubbed with water to remove residual ammonia. The residual ammonia is recovered using a distillation column, and recycled to the cracking process. The purified hydrogen/nitrogen mixture is then enriched with propane and/or butane vapor to produce the town gas for distribution.

<CIT> discloses an endothermic catalytic reaction apparatus for converting hydrocarbon feedstock and methanol to useful gases, such as hydrogen and carbon monoxide. The apparatus comprises a tubular endothermic catalytic reactor in combination with a radiant combustion chamber. The resultant cracked gas is used directly in a fuel cell after passing through a gas conditioning system.

<CIT> discloses a process for cracking ammonia to produce hydrogen. In this process, the hydrogen is separated from the nitrogen by passing the cracked gas through a bed of molecular sieves which adsorbs nitrogen. The nitrogen is then driven off the bed and may be stored in a holder.

<CIT> discloses an ammonia cracking process in which the ammonia is contacted with an ammonia decomposition catalyst at a pressure of <NUM>/cm2 (or about <NUM> bar) and a temperature of <NUM> to <NUM>. Hydrogen is recovered from the cracked gas using a PSA device. The reference mentions that the desorbed nitrogen may be used to boost the upstream process but no details are provided.

<CIT> discloses a process in which a mixture of ammonia and hydrocarbon feedstock is passed through a fired steam reformer at <NUM> and <NUM> MPa (or about <NUM> bar) where it is converted into a synthesis gas containing about <NUM> vol. % hydrogen. The synthesis gas is enriched in hydrogen in a shift reaction, cooled and condensate removed. The resultant gas is fed to a PSA system to generate a purified hydrogen product having <NUM> vol. % hydrogen or more. The offgas from the PSA system is fed as fuel to the fired steam reformer.

<CIT> discloses a process in which ammonia is cracked in a tubular reactor within a furnace. The cracked gas is separated by adsorption to produce hydrogen gas and a nitrogen-rich offgas. The fuel demand of the furnace appears to be satisfied using a combination of cracked gas, hydrogen product gas and/or offgas.

<CIT> discloses a process for cracking ammonia using heat generated by the non-catalytic partial oxidation of ammonia with an oxygen containing gas. The residual ammonia is separated from the process gas and recycled for use in the oxidation process.

There is a need generally for improved processes for the production of hydrogen from ammonia and specifically for processes that are more efficient in terms of energy consumption and/or that have higher levels of hydrogen recovery and/or that reduce or eliminate the need to combust fossil fuels.

In the following discussion of embodiments of the present invention, the pressures given are absolute pressures unless otherwise stated.

According to a first aspect of the present invention, there is provided a method for producing hydrogen from ammonia, comprising:.

The liquid ammonia feed is typically pressurized to a pressure that is greater than <NUM> bar, e.g. at least <NUM> bar or at least <NUM> bar. In some embodiments, the liquid ammonia is pressurized to a pressure in a range from about <NUM> bar to about <NUM> bar, or in a range from about <NUM> to about <NUM> bar, or in a range from about <NUM> bar to about <NUM> bar.

Typically the liquid ammonia feed has small quantities of water added to it to prevent stress corrosion cracking in vessels during shipping and storage. The liquid ammonia feed contains water as a trace impurity, for example in an amount in the range from about <NUM> wt. % to about <NUM> wt. %, typically in an amount of about <NUM> wt. The water should be removed from the feed ammonia to prevent damage to the ammonia cracking catalyst. Some catalysts, for example iron based catalysts are water intolerant and are less compatible with feedstock containing water.

The liquid ammonia feed is heated and vaporised to produce ammonia feed vapour. The temperature of ammonia feed vapour being fed to the distillation column system is typically in a range from about <NUM> to about <NUM> which ensures that the water was also carried through to the distillation system with the vaporised ammonia. The actual temperature is determined by the boiling point of ammonia at the pressure to which it has been pumped. The vaporized ammonia entering the column may contain a small (e.g. <NUM> to <NUM> %) water-rich liquid phase that will be carried through to the distillation system. Alternatively, the water-containing ammonia stream may be vaporized leaving a water-rich liquid stream that may be fed to the distillation system separately.

The water is removed from the ammonia feed vapour using the distillation column system. As the ammonia feed is already vapourised it can be introduced near the top of the distillation column system. The water-depleted ammonia feed vapour produced typically contains less than <NUM> ppm, preferably less <NUM> ppm, more preferably less than <NUM> ppm water.

The water-depleted ammonia feed vapour is typically heated to produce heated ammonia at a temperature greater than about <NUM>, e.g. in a range from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>.

The temperature is ultimately determined by the identity of the catalyst, the operating pressure and the desired "slip", i.e. the amount of ammonia that passes through the cracking reactor without being cracked. In this regard, the process is typically operated with no more than about <NUM>% slip which would be the amount of slip if the cracking process were operated <NUM> bar and <NUM> with a close approach to equilibrium. Problems may arise with some construction materials at any appreciable pressure at temperatures above about <NUM>.

The cracking reaction takes place in catalyst-filled reactor tubes that are heated by a furnace. However, in theory any heterogeneously catalysed gas reactor could potentially be used for the conversion.

There are a large number of catalysts known in the art as useful for the ammonia cracking reaction and any of these conventional catalysts may be used in this invention. Iron-based catalysts are commonly used in the Haber-Bosch process for the manufacture of ammonia and so, since both processes are the same equilibrium-limited reaction, it is expected that such an iron-based catalyst could be utilised for the ammonia cracking process. However, it is well-known in ammonia production processes that the catalyst is poisoned by ppm levels of water and oxygen present in the feed. Therefore, it is expected that if iron-based catalysts are used, the water in the feed ammonia would need to be removed.

The primary fuel for the furnace typically comprises hydrogen, ammonia, cracked gas and/or PSA tail gas although the primary fuel preferably comprises methane. The fuel may be pure methane but is more likely natural gas or biogas.

The PSA device may operate a PSA cycle or a vacuum swing adsorption (VSA) cycle. Suitable PSA cycles include any of the cycles disclosed in <CIT>, <CIT> and <CIT>, the disclosures of which are incorporated herein by reference.

In a normal PSA system with a single PSA device, the recovery of hydrogen is typically in the range of about <NUM>% to <NUM>%. However, there are two options for increasing recovery. First, the PSA tail gas may be recycled to the first PSA. In such embodiments, the first PSA tail gas may be compressed and the compressed PSA tail gas recycled to the first PSA device. Recycling in this way can achieve an overall hydrogen recovery of about <NUM>% to about <NUM>%.

Alternatively, two PSA devices may be used in series and the first PSA tail gas is further processed in the second PSA device. In these embodiments, the process comprises compressing the first PSA tail gas to produce a compressed PSA tail gas; and purifying the compressed PSA tail gas in a second PSA device to produce a second PSA tail gas and a second hydrogen product gas. Further processing in this way can achieve an overall hydrogen recovery of about <NUM>% to about <NUM>%. For example, if the first PSA device achieves <NUM>% recovery and the second PSA achieves <NUM>% recovery, then the overall recovery is <NUM>%.

In these embodiments, the second hydrogen gas may be combined with the first hydrogen product gas to form a combined hydrogen product gas.

Similarly to the first PSA device, the second PSA device may operate a PSA cycle or a vacuum swing adsorption (VSA) cycle. Suitable PSA cycles include any of the cycles disclosed in <CIT>, <CIT> and <CIT>.

The PSA tail gas from either the first PSA device or the second PSA device may be fed as a secondary fuel for combustion in the furnace. Preferably, the PSA tail gas is warmed by heat exchange against the one or more hot fluids and/or optionally mixed with the primary fuel before being fed to the furnace.

The PSA tail gas, or a gas derived therefrom, can be separated using a membrane separator to discharge a nitrogen-rich retentate gas and recycle a hydrogen-rich permeate gas for further processing in the PSA device and/or for mixing into the hydrogen product gas,.

Like hydrogen, ammonia is a "fast gas" that readily permeates across membranes used for gas separation. Some membranes, such as those constructed of polyamide or polysulfone polymers, are more tolerant of ammonia. However, some membranes, such as those constructed of polyimide polymers, are less tolerant of ammonia. Therefore, ammonia is typically removed, or its concentration is at least reduced, upstream of the membrane separator.

The cooled cracked gas is typically produced by heat exchange against the pressurised liquid ammonia feed and the water-depleted ammonia feed vapour, and optionally against the primary fuel source, air supply to the furnace, and/or PSA tail gas.

Ammonia is removed from the cooled cracked gas by absorption in water, e.g. by washing the gas with water in a scrubbing column. The resultant ammonia-depleted gas and aqueous ammonia solution are separated so the ammonia-depleted gas can be further processed without the ammonia causing any difficulties. Ammonia is recovered from the aqueous ammonia solution by stripping in the distillation column system. Preferably, the aqueous ammonia solution is pumped from the scrubbing column to the distillation column system. Such a process may be also be applied to the PSA tail gas prior to being supplied to the membrane separator.

Recovering the ammonia from the cracked gas not only simplifies the hydrogen purification steps but, as the recovered ammonia is recycled to the ammonia feed, it may increase the recovery of hydrogen from the ammonia. It also removes ammonia from the feed to the burners, reducing concerns over production of oxides of nitrogen (NOx) caused by combusting ammonia.

The distillation column system typically produces overhead vapour containing ammonia and an aqueous bottoms liquid. Part of bottoms liquid from the distillation column may be purged. Additionally or alternatively, all or part of the bottoms liquid from the distillation column may be reboiled. In this regard, the bottoms liquid in or from the distillation column system may be reboiled by heat exchange against the one or more hot fluids.

In other embodiments, the bottoms liquid in or from the distillation column system is reboiled using an electrically powered heater. The heater may be powered at least in part by electricity generated from at least one renewable source, such as solar, wind, or tidal energy, as this would reduce the carbon intensity of the process.

Aqueous bottoms liquid from the distillation column system can be fed to the scrubbing column. Prior to entering the scrubbing column, the aqueous bottoms liquid is preferably cooled by heat exchange with a coolant, preferably the aqueous ammonia solution being fed to the distillation system from the scrubbing column.

In some preferred embodiments, the overhead vapour in or from the distillation column system is partially condensed by heat exchange against a coolant to produce a condensed stream and the water-depleted ammonia feed vapour. The condensed stream is then used as reflux for the distillation column system.

According to a second aspect of the present invention, there is provided apparatus for producing hydrogen from ammonia comprising:.

The furnace may be separate from the catalyst-filled reactor tubes although the furnace and the catalyst-filled reactor tubes are preferably integrated within the same unit. In preferred embodiments, a steam methane reforming (SMR) type reactor is used in which the furnace comprises a radiant section through which pass the catalyst-containing reactor tubes.

A compressor may be provided downstream of the first PSA device for compressing the first PSA tail gas to produce compressed PSA tail gas. The compressor may consist of one or more stages and cooling will take place between each stage and after the final stage. Water will typically condense out of the compressed PSA tail gas at the interstages or at the aftercooler stage. The aqueous condensate is typically removed after each cooling stage of the compressor and a small amount of ammonia will come out of the first PSA tail gas with this condensate.

In some preferred embodiments, the apparatus comprises:.

In some alternative preferred embodiments, the apparatus comprises:.

In these embodiments, the first and second hydrogen gas conduits combine to form a hydrogen product gas conduit.

In some preferred embodiments, part or all of the first PSA tail gas and/or second PSA tail gas can be recycled and used as a secondary fuel for combustion in the furnace. In these embodiments, the apparatus comprises a conduit for feeding first PSA tail gas and/or second PSA tail gas to the furnace. Preferably, the conduit is in fluid communication with the first heat exchanger(s) for heating by heat exchange against the one or more hot fluids prior to combustion in the furnace.

The invention will now be described by way of example only with reference to the following drawings.

A process is described herein for producing hydrogen by cracking ammonia. The process has particular application to producing so-called "green" hydrogen which is hydrogen created using renewable energy instead of fossil fuels. In this case, the ammonia is typically produced by electrolyzing water using electricity generated from renewable energy, such as wind and/or solar energy, to produce hydrogen which is then reacted catalytically with nitrogen (Haber process) to produce the ammonia which is more easily transported than hydrogen. After reaching its destination, the ammonia is then cracked to regenerate the hydrogen.

In this inventive process, heat required for the reaction is typically provided by combustion of PSA tail gas (which usually contains some amount of residual hydrogen and ammonia) in the furnace. If the PSA tail-gas has insufficient heating value than either vaporised ammonia, a portion of the product hydrogen, or another fuel may also be used.

In practice, natural gas could be used as a fuel, together with the PSA tail gas, as is practiced in SMRs for hydrogen. However, with the desire to maintain the "green" or renewable credentials of the hydrogen so produced, there is an incentive to use a "renewable fuel". This can be the cracked "renewable" ammonia, the ammonia itself, or another renewable energy source, such as biogas, or indeed electric heating whether the electricity is itself from a renewable source, in this case local to the cracking process as opposed to the renewable electricity used to generate the hydrogen which has been transported in the form of ammonia.

A reference example of the process is shown in <FIG>. The process takes liquid ammonia from storage (not shown). The ammonia to be cracked (line <NUM>) is pumped (pump P201) as liquid to a pressure greater than the desired cracking pressure (see <CIT>). The reaction pressure is a compromise between operating pressure and conversion according to Le Chatelier's principle. There is an incentive to operate the reactor (<NUM>) at higher pressure because pumping liquid ammonia requires less power and capital than compressing the product hydrogen.

The pressurised liquid ammonia (line <NUM>) is then heated, vaporised (if it is below its critical pressure) and heated further, up to a temperature of greater than <NUM> via a heat exchanger (E101) using the heat available in the cracked gas leaving the reaction tubes and the flue gas from the furnace. In the figure, the heat exchanger (E101) is shown as one heat exchanger but, in practice, it will be a series of heat exchangers in a network.

The initial heating and vaporization of the pressurized liquid ammonia may alternatively take place against an alternative heat source, such as cooling water or ambient air. Typical reaction temperatures are greater than <NUM> (see <CIT>), palladium-based systems can run at <NUM> and <NUM> bar, whereas RenCat's metal oxide-based system runs at less than <NUM> and <NUM> bar. (See https://www. ammoniaenergy. org/articles/ammonia-cracking-to-high-purity-hydrogen-for-pem-fuel-cells-in-denmark/). The operating pressure of the cracker is typically an optimization of several factors. Cracking of ammonia into hydrogen and nitrogen s favored by low pressure but other factors favor higher pressure, such as power consumption (which is minimized by pumping the feed ammonia rather than compressing the product hydrogen), and the PSA size (which is smaller at higher pressure).

The hot ammonia (line <NUM>) enters reaction tubes of a reactor (<NUM>) at the desired pressure where additional heat is provided by the furnace (<NUM>) to crack the ammonia into nitrogen and hydrogen. The resulting mixture of residual ammonia, hydrogen and nitrogen exits (line <NUM>) the reaction tubes of the reactor (<NUM>) at the reaction temperature and pressure. The reaction products are cooled in a heat exchanger (E101) against a combination of feed ammonia (from line <NUM>), furnace fuel (in this case pumped ammonia from line <NUM>, pump P202 and line <NUM>; PSA tail gas from line <NUM>; and product hydrogen to be used as fuel in line <NUM>) and combustion air (from line <NUM>, fan K201 and line <NUM>) to reduce the temperature as close as possible to that required for the inlet of a PSA device (<NUM>). Any residual heat in the cracked gas mixture (line <NUM>) is removed in a water cooler (not shown) to achieve an inlet temperature to the PSA device (<NUM>) of in a range from about <NUM> to about <NUM>, e.g. about <NUM>.

The PSA product (line <NUM>) is pure hydrogen compliant with ISO standard <NUM> - Hydrogen Fuel Quality - with residual ammonia < <NUM> ppmv and nitrogen < <NUM> ppmv - at approximately the reaction pressure. The product hydrogen (line <NUM>) is further compressed (not shown) for filling into tube trailers (not shown) for transport or it may be liquefied in a hydrogen liquefier (not shown) after any required compression. The PSA tail gas (line <NUM>) or "purge gas" from the PSA device (<NUM>) is shown as being heated via the heat exchanger E101, using the cracked gas (line <NUM>) leaving the reaction tubes of the reactor (<NUM>) or furnace flue gas (line <NUM>), before being sent (in line <NUM>) to the furnace as a combustion fuel. However, the PSA tail gas (line <NUM>) may be fed directly to the furnace (<NUM>) without heating.

The resultant warmed ammonia fuel (line <NUM>) and warmed hydrogen (line <NUM>) are depicted as combined with the (optionally) warmed PSA tail gas (line <NUM>) in a mixer (<NUM>) to produce a combined fuel which is fed (line <NUM>) to the furnace (<NUM>) for combustion to generate the flue gas (line <NUM> and, after cooling in E101, line <NUM>). However, it should be noted that one or more of the fuels could be fed directly to the furnace without prior mixing. The warmed air (for combustion of the fuel) is fed to the furnace (<NUM>) in line <NUM>.

One of the aims of preferred embodiments of the present process is to maximise the amount of hydrogen generated by cracking the renewable ammonia. That means minimising the amount of hydrogen used as fuel, or ammonia if ammonia were to be used as a fuel directly. Therefore, heat integration is important so as to use the hot flue gas and cracked gas appropriately, for instance to preheat air (line <NUM>) and ammonia (line <NUM>) to the cracker as this reduces the amount of "fuel" to be used in the burners of the furnace (<NUM>). This leads to higher hydrogen recovery as less of the hydrogen is lost in the furnace flue gas (lines <NUM> & <NUM>) as water. Therefore, steam generation, for instance, should be minimised in favour of intra-process heat integration.

<FIG> shows ammonia provided as fuel (lines <NUM> & <NUM>) and feed (line <NUM>) and it also shows product hydrogen as fuel (lines <NUM> & <NUM>) - in practice, it is likely only one of these streams would be used as fuel. In this regard, <FIG> depicts a similar process to that of <FIG> in which ammonia is used as a fuel (line <NUM>) but not product hydrogen. All other features of the process depicted in <FIG> are the same as in <FIG> and the common features have been given the same reference numerals.

The inventors are aware that stable combustion of ammonia is facilitated if hydrogen is also used as a fuel, particularly at start-up and warm-up.

<FIG> depicts a process similar to that depicted in <FIG>. In this process, the recovery of hydrogen (line <NUM>) from the PSA may be adjusted to provide a tail gas (line <NUM>) which, when burned, will provide all the heat required by the process, thus eliminating the need for a trim fuel. All other features of the process depicted in <FIG> are the same as in <FIG> and the common features have been given the same reference numerals.

Should there be a viable alternative source of renewable energy for the cracking reactions, as discussed above, one could consider recovering hydrogen from the PSA tail gas to increase the net hydrogen production from the process in addition to the hydrogen produced from the PSA. Such a process could use membranes in series or in parallel to separate hydrogen from the nitrogen rich PSA tail gas stream.

The cracked gas (line <NUM>) contains residual amounts of ammonia which can be removed and recycled to the cracking process. This has two benefits; first, it simplifies the adsorption process and secondly, it allows recovery of uncracked ammonia back to the process by stripping the ammonia from the water in a distillation system. Ammonia may need to be removed particularly but not exclusively if membranes are being used as part of the separation process since membrane material can be intolerant of ammonia and ammonia is a fast gas and would permeate with the hydrogen so would accumulate in the process if not removed. NH3 may be removed for instance by a water wash or other well-known technology for ammonia removal, upstream of the membrane. The ammonia recovered in the ammonia removal step can be recovered to the feed to the cracking process using a distillation system to recover the ammonia from the water used to absorb the ammonia from the cracked gas. This could theoretically increase the hydrogen recovery from the process up to <NUM>%. Recovering NH3 from the cracked gas simplifies the hydrogen purification steps, may increase the recovery of hydrogen from the ammonia if the separated ammonia is recovered as feed, and also removes ammonia from the feed to the burners, at least reducing and possibly eliminating concerns over production of NOx caused by burning NH3.

<FIG> depicts a process involving a conventional means of recovering residual ammonia from the cracked gas and recycling the recovered ammonia to the catalyst-containing reactor tubes for cracking. The features of the process in <FIG> that are common to the processes of <FIG> have been given the same reference numerals. The following is a discussion of the new features in <FIG>.

A fuel (line <NUM>) is warmed in the heat exchange (E101). The resultant warmed fuel (line <NUM>) is combined with the warmed PSA tail gas (line <NUM>) in a mixer (<NUM>) to produce a combined fuel which is fed (line <NUM>) to the furnace (<NUM>) for combustion to generate the flue gas (line <NUM> and, after cooling in E101, line <NUM>). However, it should be noted that one or more of the fuels could be fed directly to the furnace without prior mixing. The warmed air is fed to the furnace (<NUM>) in line <NUM>.

The cracked gas is cooled in a heat exchanger (E101) against a combination of feed ammonia (from line <NUM>), furnace fuel (in this case fuel in line <NUM>; PSA tail gas from line <NUM>) and combustion air (from line <NUM>, fan K201 and line <NUM>) to reduce the temperature as close as possible to that required for the inlet of a scrubbing column (<NUM>). The inlet temperature to the scrubbing column (<NUM>) is preferably in a range from about <NUM> to about <NUM>, e.g. about <NUM>.

The cooled cracked gas (line <NUM>) is fed to a scrubbing column (<NUM>) where water (line <NUM>) is used to recover the residual ammonia from the cracked gas to produce ammonia-depleted cracked gas and an aqueous ammonia solution. The aqueous ammonia solution is (line <NUM>) transferred to the distillation column system (<NUM>) using a pump (P301). IN some embodiments, there may be no need to use a pump for this purpose.

The distillation column system (<NUM>) removes the water from the aqueous ammonia solution to produce water-depleted ammonia feed vapour and an aqueous ammonia-depleted bottoms liquid. The water-depleted ammonia feed vapour is recycled via line <NUM> to the catalyst-containing reactor tubes for cracking. The overhead vapour containing ammonia is partially condensed by a condenser (<NUM>) and the two-phase fluid is phase separated in separator (<NUM>) to produce condensed liquid which is fed (line <NUM>) to the column system (<NUM>) as reflux, and the water-depleted ammonia feed vapour (line <NUM>).

The aqueous bottoms liquid from the column system (<NUM>) can be purged (line <NUM>) or reboiled. In this regard, the aqueous bottoms liquid (line <NUM>) can be warmed and partially vaporized in the heat exchanger (E101) before being fed via line <NUM> to a separator (<NUM>). Alternatively, or in addition, the aqueous bottoms liquid (line <NUM>) can be warmed and partially vaporized using an electrically powered heater (not shown). The separator (<NUM>) separates the liquid and vapour portions of the reboiled aqueous bottoms liquid. The vapour portion is fed back to the distillation column system (<NUM>) via line <NUM> to provide vapour for the column system. The liquid portion of the reboiled liquid, now a water-rich liquid with the majority of the ammonia stripped out, is fed to the scrubbing column (<NUM>) via line <NUM>. Alternatively, the warmed and partially vaporized aqueous bottoms liquid (line <NUM>) can be fed directly back to the distillation system (<NUM>) (not shown).

The water-rich liquid (line <NUM>) derived from the reboiled aqueous bottoms liquid is cooled prior to entry into scrubbing column (<NUM>). Preferably, the cooling is achieved by heat exchange against aqueous ammonia solution (lines <NUM> & <NUM>) in a heat exchanger (<NUM>). The cooled water-rich liquid (line <NUM>) is cooled further by heat exchange against a coolant in heat exchanger (<NUM>) prior to being fed into the scrubbing column (<NUM>).

The ammonia depleted cracked gas is fed (line <NUM>) to a first PSA device (<NUM>). The cracked gas is separated to form a first hydrogen product gas (line <NUM>) and tail gas (line <NUM>). The tail gas can be fed (line <NUM>) back to the furnace, optionally via heat exchanger (E101) and mixer (<NUM>) (not shown).

In one alternative, as shown in <FIG> and <FIG>, the tail gas (line <NUM>) from the first PSA device (<NUM>) is compressed in a compressor (K301) to produce compressed PSA tail gas (line <NUM>) which is fed to a second PSA device (<NUM>) to produce a second hydrogen product gas (line <NUM>) and a second PSA tail gas (line <NUM>). The second hydrogen product gas (line <NUM>) is combined with the hydrogen product gas (line <NUM>) from the first PSA device (<NUM>) to produce a combined hydrogen product gas (line <NUM>). The second PSA tail gas (line <NUM>) is warmed by heat exchange against the cracked gas (line <NUM>) and the flue gas (line <NUM>) in the heat exchanger (E101) and then fed as fuel (lines <NUM> & <NUM>) to the furnace (<NUM>).

Optionally, a portion of the first PSA tail gas (from line <NUM>) can be combined with the second PSA tail gas (line <NUM>) to produce a combined PSA tail gas which can then be used as fuel in the furnace (<NUM>).

The second PSA tail gas, or the combined PSA tail gas, may optionally be fed directly to the furnace (<NUM>) without heating and/or mixing with the primary fuel (line <NUM> & <NUM>) (not shown).

In another embodiment with only a single PSA device (<NUM>), the compressed PSA tail gas (line <NUM>) can be recycled back to the first PSA device (<NUM>) for purification with the cooled ammonia-depleted cracked gas (<NUM>) (not shown).

In further embodiments, a first part of the first PSA tail gas (line <NUM>) can be compressed in a compressor (K301) to produce compressed PSA tail gas (line <NUM>) and further processed as described above. However, a second part (not shown) of the first PSA tail gas can be fed back to the furnace via a flow control valve (not shown). The valve could control the ratio of the PSA tail gas used a fuel to the primary fuel, thereby controlling the carbon intensity of the process.

Water may need to be removed from the feed ammonia to prevent damage to the ammonia cracking catalyst. Some catalysts, for example iron-based catalysts, are known to be water intolerant and are not compatible with feedstock containing water. Unfortunately, ammonia typically has small quantities of water added to it to prevent stress corrosion cracking in vessels during shipping and storage. If this water is removed, any suitable cracking catalysts may be used, including water intolerant catalysts.

Water removal can be incorporated into the distillation column system as shown in <FIG>. The features of the process in <FIG> that are common to the processes of <FIG> have been given the same reference numerals. The following is a discussion of the additional features in <FIG>.

In <FIG>, pressurized liquid ammonia feed (line <NUM>) is warmed and at least partially vaporised in the heat exchange (E101). The ammonia is evaporated at the required pressure, to ensure that the water was also carried through to the distillation system with the vaporised ammonia.

The warmed and vaporised ammonia feed (line <NUM>) is fed to the distillation system (<NUM>), usually at an intermediate location of the column. The overhead vapour produced in the column system is substantially free of water (e.g. contains about <NUM> ppm water) and contains the ammonia from the feed, together with the ammonia recovered from the cracked gas.

The recovered ammonia (line <NUM>) is returned to the heat exchange (E101) to be heated further before being fed (line <NUM>) to the reaction tubes of a reactor (<NUM>).

The invention will now be illustrated with reference to the following invention Examples and by comparison with the following Reference Examples.

The process depicted in <FIG> has been simulated by computer (Aspen Plus, ver. <NUM>, Aspen Technology, Inc. ) and the results are depicted in Table <NUM>. This example assumes an equilibrium for the cracking reaction at <NUM> bar and <NUM>.

In this Reference Example, hydrogen recovery from the ammonia is <NUM>% with the PSA recovery at <NUM>%. The total power of the ammonia feed pump (P201), the ammonia fuel pump (P202) and the air fan (K201) is about <NUM> kW. In addition, the percentage of ammonia in the PSA offgas is <NUM>%.

The process depicted in <FIG> has been simulated by computer (Aspen Plus, ver. <NUM>) and the results are depicted in Table <NUM>. This example assumes an equilibrium for the cracking reaction at <NUM> bar and <NUM>.

In this Reference Example, hydrogen recovery from the ammonia is <NUM>% with the PSA recovery at <NUM>%. The total power of the ammonia feed pump (P201) and the air fan (K201) is about <NUM> kW. In addition, the percentage of ammonia in the PSA offgas is <NUM>%.

The process depicted in <FIG> has been simulated by computer (Aspen Plus, ver. <NUM>) and the results are depicted in Table <NUM>. For the purposes of the simulations, an equilibrium for the cracking reaction at <NUM> bar and <NUM> was assumed.

In this Reference Example, hydrogen recovery from the ammonia is <NUM>%. In addition, the percentage of ammonia in the PSA offgas from the second PSA is <NUM>% One can see that the crude hydrogen has had a reduction in ammonia content from <NUM> to <NUM>%. Not all of the ammonia has been recovered from the cracked gas. There is an optimization to be carried out as more ammonia recovery requires more reboiler duty in the stripping column and there is a limit to the heat available for this duty within the process without impacting the efficiency of the process.

In this Reference Example, hydrogen recovery from the ammonia is <NUM>%. In addition, one can see that the crude hydrogen has had a reduction in ammonia content from <NUM>% to <NUM>%.

In this Invention Example, hydrogen recovery from the ammonia is <NUM>%. In addition, one can see that the crude hydrogen has had a reduction in ammonia content from <NUM>% to <NUM>%.

Claim 1:
A method for producing hydrogen from ammonia, comprising:
pressurizing liquid ammonia feed comprising water as a trace impurity to produce pressurized liquid ammonia feed;
combusting a primary fuel in a furnace to heat catalyst-containing reactor tubes and to form a flue gas;
supplying heated ammonia to the catalyst-containing reactor tubes to cause cracking of the ammonia into a cracked gas containing hydrogen gas, nitrogen gas and residual ammonia;
cooling the cracked gas by heat exchange to produce cooled cracked gas;
scrubbing ammonia from the cooled cracked gas using water in a scrubbing column to produce ammonia-depleted cracked gas and an aqueous ammonia solution;
stripping ammonia from the aqueous ammonia solution in a distillation column system to produce water-depleted ammonia feed vapour and an aqueous ammonia-depleted bottoms liquid;
heating the water-depleted ammonia feed vapour by heat exchange with one or more hot fluids to produce the heated ammonia;
purifying the ammonia-depleted cracked gas in a first PSA device to produce a hydrogen product gas and a first PSA tail gas;
characterised in that the pressurized liquid ammonia feed is heated and vaporised by heat exchange against the one or more hot fluids to produce ammonia feed vapour which is fed to the distillation column system to remove water from the ammonia feed vapour,
wherein the one or more hot fluids comprises the cracked gas and/or the flue gas.