DK201900370A1 - Hydrogen purification technology - Google Patents

Abstract

The present invention describes a plant and a method for purifying hydrogen by a combination of CO2 removal and cryogenic flash separation.

Description

HYDROGEN PURIFICATION TECHNOLOGY

TECHNICAL FIELD

The present invention relates to a plant and method for providing at least an H2-rich stream from a first stream of syngas. The technology can give a high yield of hydrogen (of more than 98%) at a hydrogen purity of >99.7%.

BACKGROUND

Production of hydrogen requires a purification step. In steam reforming, this is typically done by Pressure Swing Adsorption, 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 applications in a plant.

The present invention describes a method for purifying hydrogen by a combination of CO2 removal and cryogenic flash separation.

US2016/0146535 describes a process for purification of a synthesis gas containing hydrogen. The cryogenic step in this is a nitrogen wash using an external source of nitrogen, and which provides a syngas containing hydrogen and nitrogen in a 3:1 stoichiometric ratio for the production of ammonia.

A need remains for new technology which can provide a high purity H2-stream in an effective manner. Preferably, such technology should avoid the use of external gas feeds, e.g. N2. Additionally, such technology should optimise the pressures and temperatures of various gas streams in the plant/method. Ideally, the produced hydrogen should be delivered at high pressure, which allows for better process integration.

SUMMARY

It has been found by the present inventor that the combination of CO2 removal, methanation, cryogenic flash separation, feed effluent exchangers and gas expansions are used to provide efficient H2 purification.

DK 2019 00370 A1

So, in a first aspect a plant for providing at least an H2-rich stream from a first stream of syngas, is described, said plant comprising:

a. a carbon dioxide removal section arranged to receive said first stream of syngas and separate it into a CO2-rich stream and a CO2-depleted second gas stream;

b. a methanation section arranged to receive said CO2-depleted second gas stream from said carbon dioxide removal section, and methanate it to provide a third gas stream;

c. a drying section arranged to receive said third gas stream from said methanation section and to separate it into a water-containing stream and a dried fourth gas stream;

d. a first heat exchanger section, said first heat exchanger section comprising a cold side and a hot side, said hot side arranged to receive the dried fourth gas stream from said drying section and cool it to provide a cooled fifth process stream;

e. a cryogenic purification section, being a flash separation section or a distillation section, arranged to receive said cooled fifth process stream from said first heat exchanger section and separate it into an H2-rich stream and a methane-rich stream;

wherein the first heat exchanger section is configured to deliver thermal energy from the dried fourth gas stream to the H2-rich stream and the methane-rich stream; thereby providing said cooled fifth process stream.

A method for providing at least an H2-rich stream from a first stream of syngas, is also described, said method comprising:

a. providing a plant as described herein,

b. feeding said first stream of syngas to the carbon dioxide removal section and separating it into a CO2-rich stream and a CO2-depleted second gas stream;

DK 2019 00370 A1

c. feeding said C02-depleted second gas stream from said carbon dioxide removal section to the methanation section, and methanating it to provide a third gas stream;

d. feeding said third gas stream from said methanation section to the drying section and separating it into a water-containing stream and a dried fourth gas stream;

e. feeding said dried fourth gas stream from said drying section to the hot side of the first heat exchanger section and cooling it to provide a cooled fifth process stream;

f. feeding said cooled sixth gas stream from said first heat exchanger section to the cryogenic purification section, being a flash separation section or a distillation section, and separating it into an H2-rich stream and a methanerich stream;

said method comprising the additional steps of: configuring the first heat exchanger section to deliver thermal energy from the dried fourth gas stream to the H2-rich stream and the methane-rich stream; thereby providing said cooled fifth process stream. Further details of the invention are provided in the dependent claims, the detailed description and the figures. LEGENDS

Figures 1 and 2 show layouts of the plant, in schematic form.

DETAILED DISCLOSURE

The present invention describes a plant and a method for purifying hydrogen by a combination of CO2 removal and cryogenic flash separation. In particular, the plant is suitable for providing at least an H2-rich stream from a first stream of syngas.

The plant (illustrated schematically in Figures 1 and 2) comprises:

a. a carbon dioxide removal section 20

b. a methanation section 30

c. a drying section 40

d. a first heat exchanger section 50, comprising a cold side and a hot side,

e. a cryogenic purification section 60, being a flash separation section or a distillation section.

DK 2019 00370 A1

The carbon dioxide removal section is arranged to receive said first stream of syngas and separate it into a CO2-rich stream and a CO2-depleted second gas stream. Positioning of this section upstream ensures that there is no CO2 to condense or solidify in any downstream section (such as the first heat exchanger section or the cryogenic purification section).

The carbon dioxide removal section includes one or more CO2 removal units. In such units, CO2 can for instance be adsorbed in an amine or similar. A carbon dioxide unit utilizes 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 diisopropanolamine (DIPA), or tertiary amines as triethanolamine (TEA) and methyldiethanolamine (MDEA), but also ammonia and liquid alkali carbonates as K2CO3 and NaCO3 can be used. The CO2-rich stream from the CO2 removal unit is a practically pure CO2 stream at around 1 bar.

The methanation section is arranged to receive the CO2-depleted second gas stream from the carbon dioxide removal section, and methanate it to provide a third gas stream. In the methanator, all remaining CO and CO2 in the gas is converted to CH4 according to reaction (1) and (2):

CO + 3H2 CH4 + H2O (1)

CO2 + 4H2 CH4 + 2H2O (2)

A typical arrangement of the methanation section includes a feed-effluent heat exchanger, where the said CO2-depleted second gas stream is heated to ca. 300-400°C before entering an adiabatic catalytic methanation reactor. The catalyst material will often be a combination of Ni as active phase on an oxide support such as AfO3, MgAhO4, or CaAhO4, as examples. The effluent heat exchanges within the same feed-effluent reactor to be cooled. Potential further active cooling by a water cooler or air cooler can be used to provide the third gas stream.

Methanation is performed because the boiling point of CH4 is higher than that of CO, and the subsequent phase separation is easier and more efficiently done in the CH4/H2 mixture than a CH4/CO/H2 mixture.

The drying section is arranged to receive said third gas stream from the methanation section and to separate it into a water-containing stream and a dried fourth gas stream.

DK 2019 00370 A1

Depending on the precise method used for drying, the water-containing stream may be a water-rich stream, and may e.g. comprise more than 50% water, such as more than 75% water or more than 90% water. In one aspect, the drying section also removes any remaining CO2 from the third gas stream, as part of the water-containing stream.

The drying section can be a TSA (temperature swing adsorption) unit, having a molecular sieve-like compound which will retain any H2O and CO2 in the third gas stream. These components need to be removed to avoid solidification in subsequent cryogenic step(s).

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. 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. When the unit operates with changing pressures, it is called a pressure swing adsorption unit, and when the unit operates with changing temperature, it is called a temperature swing adsorption (TSA) unit. Pressure swing adsorption can generate a hydrogen purity of 99.9% or above.

In one aspect, the drying section comprises a temperature swing adsorption (TSA) unit. In another aspect, the drying section comprises one or more flash separation unit(s). In a third aspect, the drying section comprises first a flash separation and then a temperature swing adsorption (TSA) unit. In a fourth aspect, the drying section comprises a series of flash separations sequentially operating at decreasing temperatures. Notice that complete removal of water is difficult to achieve in a single flash separation step due to a typical residual partial pressure of water in the leaving gas stream, therefore it can be advantageously to include a TSA for complete water removal.

The first heat exchanger section comprises a cold side and a hot side. Heat energy in the heat exchanger moves from the hot side to the cold side of this heat exchanger. The hot side is therefore arranged to receive the dried fourth gas stream from said drying section and cool it to provide a cooled fifth process stream. The first heat exchanger may take any form commonly used in heat exchange between two fluids, typically a shell and tube heat exchanger.

DK 2019 00370 A1

The heat exchanger section cools the dried fourth gas stream to between -220 and -160°C to force a condensation of the methane, which is then separated in the subsequent cryogenic purification section.

The cryogenic purification section is arranged to receive said cooled fifth process stream from said first heat exchanger section and separate it into an H2-rich stream and a methane-rich stream. The cryogenic purification section is a flash separation section or a distillation section.

By flash separation is meant a phase separation unit, where a stream is divided into a liquid and gas phase close to or at the thermodynamic phase equilibrium at a given temperature.

Accordingly, the cryogenic purification section does not require an external gas feed, e.g. N2. Preferably, the cryogenic purification section comprises or consists of a flash separation section. By cryogenic purification is meant a process utilizing the phase change of different species in the gas to separate individual components from a gas mixture by controlling the temperature, typically taking place below -150°C.

The cold streams from the cryogenic purification section are used for cooling purposes upstream in the plant. The first heat exchanger section is thus configured to deliver thermal energy from the dried fourth gas stream to the H2-rich stream and the methane-rich stream. In this manner, the first heat exchanger section provides the cooled fifth process stream.

In one aspect, illustrated in Figure 1, the plant is arranged to feed the H2-rich stream and the methane-rich stream separately from said cryogenic purification section to the cold-side of said first heat exchanger section. By feeding both of these streams to the first heat exchanger section, maximum cooling effect can be achieved from both streams, without mixing the streams. The term separately is used here to mean that the H2-rich stream and the methane-rich stream are not mixed prior to, or after, the first heat exchanger section.

In this aspect, the first heat exchanger section may comprise two heat exchanger units, one of which uses the H2-rich stream as coolant and the other of which uses the methane-rich stream as coolant. In this way, the streams can be used for cooling without having to mix them. Alternatively, a single heat exchanger unit is configured, where said dried fourth gas can for example be fed to the shell side of a heat exchanger and the H2-rich stream and the methane-rich stream are fed to separate tube bundles placed within the same shell. This configuration still allows for using both streams separately for cooling, which potentially can be done very efficiently with the largest possible driving force.

DK 2019 00370 A1

In another aspect, which may be an alternative, or additional aspect to that above, the plant may further comprise a second heat exchange section arranged to cool the H2-rich stream using the methane-rich stream as coolant, and feed said cooled H2-rich stream to the coldside of said first heat exchanger section. This aspect is illustrated in Figure 2. This aspect has the advantage that only one stream (the H2-rich stream) need be passed through the first heat exchange section, simplifying the first heat exchange section. In addition, this will lower the temperature of the H2-rich stream and thereby increase the driving force of first heat exchanger section. When including the second heat exchange section, the resulting methanerich stream may still be used for cooling purposes and can still be included as coolant in e.g. the first heat exchange section or alternatively in between the drying section and the first heat exchange section.

Due to the Joule-Thomson effect, often a cold gas/liquid will decrease its temperature when expanded; the exact effect will be dependent on gas composition, temperature, and pressure. The plant of the invention may thus include various expansion units, which allow various streams to expand, providing streams with a lower temperature. A lower temperature of the various streams gives a greater driving force for the heat exchangers.

In one aspect, a first expansion unit may be arranged to receive the methane-rich stream from the cryogenic purification section and, at least partially, expand it, prior to said methane-rich stream being fed to the first heat exchange section or - when present - the second heat exchange section. In this way the temperature of the methane-rich stream can be decreased and the driving force for the cooling section increased. This is a preferred configuration when the resulting methane-rich gas is used as e.g. a fuel gas where high pressure has little value for process integration.

In a further aspect, a second expansion unit may be arranged to receive the H2-rich stream from the cryogenic purification section and, at least partially, expand it, prior to said H2-rich stream being fed to the first heat exchanger section. In this way the temperature of the H2rich stream can be decreased and the driving force for the cooling section increased, but at the expense of a lower partial pressure of the H2-rich stream.

In a further aspect, a third expansion unit may be arranged to receive the cooled fifth process stream from the first heat exchanger section and, at least partially, expand it, prior to said cooled fifth process stream being fed to the cryogenic purification section. In this way the temperature of the cooled fifth process stream can be decreased, improving the separation in the cryogenic purification section, and improving the driving force for the cooling section, but at the expense of a lower partial pressure of the H2-rich stream.

DK 2019 00370 A1

Optionally, the plant may include a feed/effluent heat exchanger arranged to capture heat energy from the third gas stream from the methanation section and to thereby pre-heat the CO2-depleted second gas stream fed to the methanation section.

To avoid excess heat in the drying section, a first cooler may be located between the methanation section and the drying section, and arranged to cool the methanated third gas stream.

In a similar manner, the plant may comprise a second cooler located between the drying section and the first heat exchanger section, and arranged to cool the dried fourth gas stream. Optionally, the methane-rich stream can be used as coolant in this second cooler, as illustrated in Figure 2.

The present invention also relates to a method for providing at least an H2-rich stream from a first stream of syngas. The method uses the plant defined herein, and comprises the main steps of:

a. providing a plant as described herein,

b. feeding the first stream of syngas to the carbon dioxide removal section and separating it into a CO2-rich stream and a CO2-depleted second gas stream;

c. feeding said CO2-depleted second gas stream from said carbon dioxide removal section to the methanation section, and methanating it to provide a third gas stream;

d. feeding said third gas stream from said methanation section to the drying section and separating it into a water-containing stream and a dried fourth gas stream;

e. feeding said dried fourth gas stream from said drying section to the hot side of the first heat exchanger section and cooling it to provide a cooled fifth process stream;

f. feeding said cooled sixth gas stream from said first heat exchanger section to the cryogenic purification section, being a flash separation section or a distillation section, and separating it into an H2-rich stream and a methanerich stream.

DK 2019 00370 A1

The method comprising the additional step of: configuring the first heat exchanger section to deliver thermal energy from the dried fourth gas stream to the H2-rich stream and the methane-rich stream; thereby providing said cooled fifth process stream.

All details of all sections, units and streams set out above in relation to the plant are also relevant for the method described herein.

In particular, the method may further comprise the step of feeding the H2-rich stream and the methane-rich stream separately from said cryogenic purification section to the cold-side of said first heat exchanger section.

In another aspect of the method, in which the plant further comprises a second heat exchange section, the method further comprises the step of cooling the H2-rich stream using the methane-rich stream as coolant, and feeding said cooled H2-rich stream to the cold-side of said first heat exchanger section.

Detailed Description of the Figures

Figure 1 shows a first layout of the plant, in schematic form. In this layout, the H2-rich stream and the methane-rich stream are fed separately from cryogenic purification section to the cold-side of said first heat exchanger section. References are as follows:

plant (100)

H2-rich stream (63) first stream of syngas (110) carbon dioxide removal section (20) CO2-rich stream (21)

CO2-depleted second gas stream (22) feed/effluent heat exchanger (25) (optional) first cooler (35) (optional) methanation section (30) third gas stream (33) drying section (40) water-containing stream (41) dried fourth gas stream (42) second cooler (45) (optional) first heat exchanger section (50) cooled fifth process stream (52) cryogenic purification section (60) H2-rich stream (63) methane-rich stream (64) first expansion unit (80) - optional third expansion unit (82) - optional

DK 2019 00370 A1

Figure 2 shows a second layout of the plant, in schematic form. In this layout, the H2-rich stream is cooled in second heat exchange section, using the methane-rich stream as coolant, and said cooled H2-rich stream is fed to the cold-side of said first heat exchanger section. References are as per the layout in Figure 1, with the additional inclusion of second heat exchange section (70).

EXAMPLE 1

A calculation of the various gas compositions, based on the layout of Figure 1, is provided in

Table 1:

Table 1

Composition [mole%] Stream	110	22	33	42	52	63	21	41	64
Hydrogen	75.2	91.4	89.0	91.0	91.0	99.2	0.0	28.2	1.7
Water	0.2	0.2	2.3	0.0	0.0	0.0	0.0	71.8	0.0
Nitrogen	0.2	0.2	0.2	0.2	0.2	0.2	0.0	0.0	0.6
Carbon Monoxide	1.6	1.9	0.0	0.0	0.0	0.0	0.0	0.0	0.0
Carbon Dioxide	17.7	0.0	0.0	0.0	0.0	0.0	100.0	0.0	0.0
Methane	5.1	6.2	8.5	8.7	8.7	0.6	0.0	0.0	97.7
Flow [Nm3/h]	144948	119272	114633	111016	111016	101674	25676	3617	9342
T [°C]	40	40	40	40	-185	15	40	150	15
P [barg]	23	22	21	20	19	19	0.5	0.5	0.5

The cryogenic flash separation was facilitated at -185°C in this example. The yield of H2 in the present layout was in a given example calculated to 92.5% at a hydrogen purity of 99.2%, compared to the typical 85% yield which is obtained in a PSA.

DK 2019 00370 A1

Example 2

A calculation of the various gas compositions, based on the layout of Figure 2, is provided in

Table 2:

Table 2

Composition [mole%] Stream	110	22	33	42	63	21	41	64
Hydrogen	79.2	98.6	98.0	98.6	99.7	0.0	0.0	1.8
Water	0.0	0.0	0.5	0.0	0.0	0.0	100.0	0.0
Nitrogen	0.2	0.2	0.2	0.2	0.2	0.0	0.0	3.0
Carbon Monoxide	0.4	0.5	0.0	0.0	0.0	0.0	0.0	0.0
Carbon Dioxide	19.7	0.0	0.0	0.0	0.0	100.0	0.0	0.0
Methane	0.5	1.2	1.2	1.2	0.1	0.0	0.0	95.2
Flow [Nm3/h]	128594	103323	102293	111016	101674	25271	536	9342
T [°C]	40	40	40	40	15	40	40	15
P [barg]	25.5	24.8	23.5	22.4	21.6	0.5	0.5	0.5

The cryogenic flash separation was facilitated at -200°C in this example. The yield of H2 in the present layout was in a given example calculated to 98.4% at a hydrogen purity of 99.7%, compared to the typical 85% yield which is obtained in a PSA.

The current technology therefore offers a more efficient route for hydrogen production, on an overall plant layout basis without increased energy use on e.g. compressors or similar. 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 less CO2 emissions compared to modern standards.

The present invention has been described with reference to a number of aspects and embodiments. These aspects and embodiments may be combined at will by the person skilled in the art while remaining within the scope of the patent claims. In particular, aspects of the plant of the invention may be applied to the method of the invention, where appropriate.

Claims (14)

1. A plant (100) for providing at least an H2-rich stream (63) from a first stream of syngas (110), said plant comprising;

a. a carbon dioxide removal section (20) arranged to receive said first stream of syngas (110) and separate it into a CO2-rich stream (21) and a CO2-depleted second gas stream (22);

b. a methanation section (30) arranged to receive said CO2-depleted second gas stream (22) from said carbon dioxide removal section (20), and methanate it to provide a third gas stream (33);

c. a drying section (40) arranged to receive said third gas stream (33) from said methanation section (30) and to separate it into a water-containing stream (41) and a dried fourth gas stream (42);

d. a first heat exchanger section (50), said first heat exchanger section (50) comprising a cold side and a hot side, said hot side arranged to receive the dried fourth gas stream (42) from said drying section (40) and cool it to provide a cooled fifth process stream (52);

e. a cryogenic purification section (60), being a flash separation section or a distillation section, arranged to receive said cooled fifth process stream (52) from said first heat exchanger section (50) and separate it into an H2-rich stream (63) and a methane-rich stream (64);

wherein the first heat exchanger section (50) is configured to deliver thermal energy from the dried fourth gas stream (42) to the H2-rich stream (63) and the methane-rich stream (52); thereby providing said cooled fifth process stream (52).

2. The plant (100) according to claim 1, wherein said plant (100) is arranged to feed the H2-rich stream (63) and the methane-rich stream (64) separately from said cryogenic purification section (60) to the cold-side of said first heat exchanger section (50).

3. The plant (100) according to claim 1, said plant (100) further comprising a second heat exchange section (70) arranged to cool the H2-rich stream (63) using the methane-rich

DK 2019 00370 A1 stream (64) as coolant, and feed said H2-rich stream (63) to the cold-side of said first heat exchanger section (50).

4. The plant (100) according to any one of the preceding claims, further comprising a first expansion unit (80) arranged to receive the methane-rich stream (64) from the cryogenic purification section (60) and, at least partially, expand it, prior to said methanerich stream (64) being fed to the first heat exchange section (50) or the second heat exchange section (70).

5. The plant (100) according to any one of the preceding claims, further comprising a second expansion unit arranged to receive the H2-rich stream (63) from the cryogenic purification section (60) and, at least partially, expand it, prior to said H2-rich stream (63) being fed to the first heat exchanger section (50).

6. The plant (100) according to any one of the preceding claims, further comprising a third expansion unit (83) arranged to receive the cooled fifth process stream (52) from the first heat exchanger section (50) and, at least partially, expand it, prior to said cooled fifth process stream (50) being fed to the cryogenic purification section (60).

7. The plant (100) according to any one of the preceding claims, wherein the cryogenic purification section (60) comprises a flash separation section.

8. The plant (100) according to any one of the preceding claims, wherein said drying section (40) comprises one or more flash separation unit(s).

9. The plant (100) according to any one of the preceding claims, wherein said drying section (40) comprises a temperature swing adsorption (TSA) unit.

10. The plant (100) according to any one of the preceding claims, wherein the first heat exchanger section (50) comprises two heat exchanger units, one of which uses the H2-rich stream (63) as coolant and the other of which uses the methane-rich stream (64) as coolant.

11. The plant (100) according to any one of the preceding claims, wherein the first heat exchanger section (50) is arranged to cool the fourth gas stream (42) to a temperature between -220°C and -160°C.

12. A method for providing at least an H2-rich stream (63) from a first stream of syngas (110), said method comprising:

DK 2019 00370 A1

a. providing a plant (100) according to any one of the preceding claims,

b. feeding said first stream of syngas (110) to the carbon dioxide removal section (20) and separating it into a CO2-rich stream (21) and a CO2-depleted second gas stream (22);

c. feeding said CO2-depleted second gas stream (22) from said carbon dioxide removal section (20) to the methanation section (30), and methanating it to provide a third gas stream (33);

d. feeding said third gas stream (33) from said methanation section (30) to the drying section (40) and separating it into a water-containing stream (41) and a dried fourth gas stream (42);

e. feeding said dried fourth gas stream (42) from said drying section (40) to the hot side of the first heat exchanger section (50) and cooling it to provide a cooled fifth process stream (52);

f. feeding said cooled sixth gas stream (52) from said first heat exchanger section (50) to the cryogenic purification section (60), being a flash separation section or a distillation section, and separating it into an H2-rich stream (63) and a methane-rich stream (64);

said method comprising the additional step of: configuring the first heat exchanger section (50) to deliver thermal energy from the dried fourth gas stream (42) to the H2-rich stream (63) and the methane-rich stream (52); thereby providing said cooled fifth process stream (52).

13. The method according to claim 12, wherein said method further comprises the step of feeding the H2-rich stream (63) and the methane-rich stream (64) separately from said cryogenic purification section (60) to the cold-side of said first heat exchanger section (50).

14. The method according to claim 12, wherein said plant (100) further comprises a second heat exchange section (70), said method further comprising the step of cooling the H2-rich stream (63) using the methane-rich stream (64) as coolant, and feeding said cooled H2-rich stream (63) to the cold-side of said first heat exchanger section (50).