Patent Application
20220306469
The present invention relates to a plant and method for hydrogen purification, which comprise a Swing Adsorption (SA) stage and a recycle of purged gaseous impurities.
Production of hydrogen in a steam reforming process requires a purification step. In steam reforming, this is done by Pressure Swing Absorption, PSA. PSA will however also retain part of the hydrogen, which is why this technology typically gives a yield of 80-90% hydrogen. The remaining hydrogen is lost in a low pressure off-gas which is best used for heating elsewhere in the plant.
A more efficient hydrogen separation technology than PSA is desirable, which can avoid overdesign of steam reforming plants.
A plant for providing an H2-rich gas stream from a hydrocarbon feed is provided, said plant comprising:
The present technology also provides a method for providing an H2-rich gas stream from a hydrocarbon feed. The method comprises the general steps of:
Further details of the technology are presented in the following detailed description, the figures and the appended claims.
When a section, unit or stage is “arranged to receive” a particular gas from another section, unit or stage, it is typically arranged to directly receive. However, in certain circumstances, an intermediate section, unit or stage is present, via which the particular gas may be passed.
In the following the abbreviation % vol shall be used to signify volume percentage for a gas.
A hydrogen plant, i.e. a plant for providing an H2-rich gas stream from a hydrocarbon feed is provided. The term “H2-rich” should be understood to mean in the order of 95%vol or more.
The hydrocarbon feed is typically selected from natural gas, town gas, naphtha or biogas, and is preferably natural gas. The hydrocarbon feed is characterized by containing a majority (i.e. over 50%) of hydrocarbons e.g. methane, ethane, ethane, propane, butane, butane, and similar. Also, nitrogen, argon, and carbon dioxide, among others, may be present. Notice that the hydrocarbon feed will be mixed with streams containing hydrogen, steam, carbon dioxide, and or oxygen inside the reformer section to facilitate the reforming reaction.
Generally, the plant comprises:
The reformer section is arranged to receive the hydrocarbon feed and reform it in at least one reforming step to provide a synthesis gas stream. Reforming of hydrocarbons to synthesis gas is a known procedure, and need not be discussed in detail here.
Typically, and as shown in
The CO2 removal stage is arranged to receive the synthesis gas stream from said reformer section and separate CO2 from the synthesis gas stream, so as to provide a CO2-rich stream and a CO2-poor stream. The CO2 content in the CO2-poor steam will typically be below 2%, while the CO2 rich stream may comprise more than 90% CO2. By CO2 removal stage is meant a unit utilizing a process, such as chemical absorption, for removing CO2 from the process gas. In chemical absorption, the CO2 containing gas is passed over a solvent which reacts with CO2 and in this way binds it. The majority of the chemical solvents are amines, classified as primary amines as monoethanolamine (MEA) and digylcolamine (DGA), secondary amines as diethanolamine (DEA) and diiso-propanolamine (DIPA), or tertiary amines as triethanolamine (TEA) and methyldieth-anolamine (MDEA), but also ammonia and liquid alkali carbonates as K2CO3 and NaCO3 can be used.
The swing adsorption (SA) stage comprises an adsorption material and a first purge stream. The adsorption material may be selected from a zeolite, active carbon or metal organic framework, or mixtures thereof. The adsorption material is typically in the form of an adsorption bed inside the SA stage. By swing adsorption, a unit for adsorbing selected compounds is meant. In this type of equipment, a dynamic equilibrium between adsorption and desorption of gas molecules over an adsorption material is established. The adsorption of the gas molecules can be caused by steric, kinetic, or equilibrium effects. The exact mechanism will be determined by the used adsorbent and the equilibrium saturation will be dependent on temperature and pressure. Typically, the adsorbent material is treated in the mixed gas until near saturation of the heaviest compounds and will subsequently need regeneration. The regeneration can be done by changing pressure or temperature, or purging with another stream. In practice, this means that a process with at least two units is used, saturating the adsorbent at high pressure or low temperature initially in one unit, and then switching unit, now desorbing the adsorbed molecules from the same unit by decreasing the pressure or increasing the temperature or purging with another stream.
The SA stage is arranged to receive the CO2-poor stream from the CO2 removal stage. The SA stage comprises a first state (A) and a second state (B), and is interchangeable between these states. Changing between states may involve the opening or closing of streams to the SA stage. In one aspect, changing between states involves a change in temperature of the SA stage, i.e. the SA stage is a Temperature Swing Adsorption (TSA) stage. In this aspect, therefore, the temperature of the SA stage in the second state (B) is higher than in said first state (A).
Suitably, the SA stage is arranged to alternate between said first (A) and second (B) states.
To improve efficiency, and to reduce fluctuations in output, the SA stage may have several parallel adsorption reactions being in different stages (A, B) at a given time.
In the first state (A), the CO2-poor stream is arranged to contact the adsorption material so that;
The gaseous impurities are typically one or more of the following gases: CO2, CO, Ar, H2O, N2 and CH4.
The second state (B) is the purge state, in which the impurities on the adsorption material will be replaced by the purge. In the second state (B) of the SA stage, the first purge stream is arranged to contact the adsorption material so that at least a portion (and preferably all) of the adsorbed gaseous impurities and at least a portion (and preferably all) of said adsorbed hydrogen are released from said adsorption material and into the first purge stream. In this manner, a first recycle stream is provided which comprises the first purge stream, hydrogen and said gaseous impurities in admixture. As illustrated in
The SA stage may comprise a second purge stream and a third state (C). In this third state, the second purge stream is arranged to purge contact the adsorption material subsequent to purging with the first purge recycle stream so that at least a portion of the gaseous impurities are released from said adsorption material; thereby providing a second recycle stream which is recycled upstream the reforming step of said reforming section. In this way the adsorption material is flushed with a preferred gas phase before returning to state A and consequently contamination of the H2-rich stream by the first purge stream used in state B is avoided. The second purge stream may advantageously be hydrogen. In a particular embodiment, the second purge stream has a pressure equal to or higher than the first pressure.
In one preferred aspect, the first purge stream is a stream of superheated steam. Steam is a particularly attractive purge stream as it is required as co-feed to the hydrocarbon feed to the reformer section and therefore the combined stream of the first purge stream with hydrogen and gaseous impurities can be recycled collectively. As illustrated in
In an alternative aspect, the first purge stream is a fraction of the hydrocarbon feed, in the form of natural gas. This allows for the combined stream of the first purge stream with hydrogen and gaseous impurities can be recycled collectively to the reformer section.
In a further aspect, the first and/or second purge streams are stream(s) of hydrogen. In this way contamination of the H2-rich stream by the first purge stream is avoided.
A preferred configuration is to use steam as the first purge stream and no second purge stream. An alternative preferred configuration is to use natural gas as the first purge stream and hydrogen as the second purge stream.
The plant may further comprise a shift section arranged in said synthesis gas stream between said reformer section and said CO2 removal stage. The shift section is designed to adjust the content of the synthesis gas stream; particularly the H/CO ratio, depending on the desired outcome from the plant and/or the type of hydrocarbon feed.
Notice that suitable heat exchangers/temperature regulations stages and water removal stages are applied as required to facilitate the process. Details of these have not been described, as a person skilled in the art of chemical process design considers these easily adaptable.
The present technology also provides a method for providing an H2-rich gas stream from a hydrocarbon feed. The method comprises the general steps of:
Suitably, in said method, the SA stage is initially in said first state (A), and then alternates between said first (A) and second (B) states. As above, it is preferred that the temperature of the SA stage in the second state (B) is higher than in said first state (A).
All details of the plant above are relevant for the method described herein, mutatis mutandis.
The present invention is based on the recognition that it is possible to recycle part of the hydrogen produced in the swing adsorption stage and use it as feed in the reforming step with the object of increasing the overall hydrogen yield of the plant. The present invention is furthermore based on the recognition that it is feasible to provide the first purge stream of the swing adsorption stage at a pressure of equal to or higher than the pressure of the reforming reaction, and that hence the recycling of the hydrogen-rich stream from the swing adsorption stage to the reforming step may be carried out without any requirement for a compressor.
In particular, the first purge stream may be a part of the hydrocarbon feed to be fed to the reforming step or a part of the superheated steam to be fed to the reforming step and both said streams are available at pressures equal to or higher than the pressure of the reforming step. Also, the first purge stream may a hydrogen stream, which may e.g. be a high pressure stream from a separate process or a part of the hydrogen-rich first recycle stream from the SA stage, which is available at a pressure equal to or higher than the pressure of the reforming step or at a pressure slightly lower than the pressure of the reforming step, in which case the required compression is minimal.
The current technology allows for a high yield of H2, higher than the 85% of PSA and likely in the order of +95%. The current technology therefore offers a more efficient route for hydrogen production. On an overall plant layout basis, this technology will enable for construction of more contact reformers as the increased yield means less gas needs to be processed to produce a given amount of H2. This also means that the technology offers lower natural gas consumption and lower CO2 emissions compared to modern standards.
A higher yield of H2 can be achieved compared to the use of a Pressure Swing Absorption PSA stage. This will allow for building more compact steam reformers as over-production will not be an issue.
Table 1 summarizes an example of the invention. A given amount of hydrocarbon feed (101) is reformed in the reforming section (200) to produce a synthesis gas stream (201). CO2 is removed from this stream in the CO2 removal stage (300) to produce a CO2-poor stream (304) and CO2-rich stream (303). The CO2-poor stream (304) is then separated in an SA stage (400) to produce a H2-rich stream (409). The SA is purged by steam (405) and 50% of this stream is recycled back to the reformer, while the second half is condensed to the leave an off-gas. Also, steam and some hydrogen is added to the reforming section to facilitate prereforming and reforming in this section. Notice that the total feed to the reformer is the mixture of the hydrocarbon feed (101), steam, and hydrogen after being prereformed.
Table 2 summarizes a comparative example where the first recycle 408 from the SA unit is not returned to the reforming section. Similar to Example 1, a given amount of hydrocarbon feed (101) is reformed in the reforming section (200) to produce a synthesis gas stream (201). CO2 is removed from this stream in the CO2 removal stage (300) to produce a CO2-poor stream (304). This is then separated in an SA stage (400) to produce a H2-rich stream (409). The SA is in this case a more typical PSA, where the off-gas is produced directly. Also, steam and some hydrogen are added to the reforming section to facilitate prereforming and reforming in this section. Notice that the total feed to the reformer is the mixture of the hydrocarbon feed (101), steam, and hydrogen after being prereformed.
By the method of the invention presented in example 1, it is shown that the size of the H2-rich stream (409) is increased from 32103 Nm3/h in the base case of example 2 to 39752 Nm3/h in example 1. Thus, by the method of the invention, the yield of hydrogen from a given amount of hydrocarbon feed (101) is increased by 24%. By increasing the degree of purge stream (405) utilization from the 50% used in example 1, the yield can increase even further. Using 70% of the purge stream instead would result in 29% increased yield of the H2-rich stream (409).
Other references in the figure:
Preheating section 90
Flue gas 220
Hydrodesulfurisation (HDS) and sulphur adsorption unit 80
heat exchanger/waste heat boiler 209
shifted synthesis gas stream 201′
shift section 500
Although the invention has been described with reference to a number of aspects, examples and embodiments, these aspects, examples and embodiments may be combined by the person skilled in the art, while remaining within the scope of the present invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA 2019 00674 | May 2019 | DK | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/062731 | 5/7/2020 | WO |
