JPS6259041B2 - - Google Patents

Description

Detailed Description of the Invention "Industrial Application Field" The present invention is directed to carbon dioxide, carbon monoxide,
Impure gases such as methane and moisture are selectively adsorbed and removed using the pressure swing adsorption method (hereinafter referred to as PSA method).
This invention relates to a hydrogen purification method that obtains high purity hydrogen of 99.99% or more in high yield.

"Conventional technology" Conventionally, impurity gases are removed from a mixed gas containing hydrogen using an adsorption tower filled with adsorbents such as silica gel, activated carbon, and synthetic zeolite under pressure.
The method for purifying hydrogen is well known as the PSA method.

The PSA method is characterized by the ability to obtain high purity hydrogen of 99.99% or more in one pass using a relatively small adsorption tower, but in order to obtain such high purity hydrogen, sufficient regeneration of the adsorbent is required. This requires vacuum desorption to the lowest possible pressure, backwashing (countercurrent scavenging) with product hydrogen, and countercurrent pressurization.

For this reason, with depressurization and scavenging purge, a considerable amount of hydrogen content escapes to the purge side, so normal PSA
Hydrogen purification unit produces 75% hydrogen (dry basis)
When processing a certain amount of crude hydrogen gas, the hydrogen yield (recovery rate) remains at about 75%. Therefore, in order to obtain the desired amount of hydrogen product, approximately 30% of the crude hydrogen generation equipment must be used.
You have to take it big.

This is the only point in which the PSA method is inferior to other hydrogen purification methods, such as absorption methods and cryogenic separation methods. Therefore, various proposals have been made to improve the hydrogen yield of the PSA method.

``Problem to be solved by the invention'' In the purification of hydrogen gas where multi-component impurity gases other than hydrogen are present at high and low concentrations, conventionally, three
One to three kinds of adsorbents that selectively adsorb impurities were mixed or packed hierarchically in four adsorption towers, and the treatment was carried out at a single rate in one tower. Therefore, the adsorption tower design or switching cycle time program such as gas linear velocity and tower size cannot necessarily be set to the optimum conditions for each impurity component gas and adsorbent, resulting in wasted adsorption capacity and cycle time, and oversized towers. Since the capacity increases the voids and increases the hydrogen loss associated with the purge gas, it was not possible to improve the hydrogen yield.

There is also a proposal for a two-stage PSA method in which the PSA tower is treated with multiple towers, one in the front stage and the other in the latter stage, but this also simply treats impure gas selectively in the front and rear stages, and then uses the purge gas in the latter stage to further treat the former stage. Only purging is performed, no consideration is given to the column size, and the adsorption-regeneration cycle time between each stage is the same. In this case, it is not possible to improve the hydrogen yield for reasons described later.

In general, to improve the hydrogen yield of the PSA method, (1) pretreatment of impurities, (2) recovery of depressurization, desorption gas, and scavenging gas (recycling to the inlet), and (3) extension of the regeneration cycle are considered effective. On the other hand, to improve purity (1) pretreatment (2) sufficient regeneration (highly reduced pressure, vacuuming, etc.) (3)
Sufficient scavenging (backwashing with product hydrogen) and countercurrent pressurization with product hydrogen are effective.

The conventional methods for improving hydrogen yield are 3.
~ Four adsorption towers are used for alternate adsorption, and the tower pressure after the completion of adsorption in one tower is pressure balanced (pressure equalized) to the rising tower, a part of the vacuum desorption gas is recovered, and the scavenging gas is These include efforts to reduce waste, adsorption, and regeneration cycles.

However, simply recycling and recovering the purge gas to the inlet results in a large amount of circulating gas, which is not practical, and extending the adsorption cycle increases the capacity of the adsorption tower and increases the hydrogen loss during depressurization due to the increase in voids. It does not improve yield. Further, simply increasing the number of columns to five or more increases the degree of freedom of the cycle, but it also causes increased loss of hydrogen in the reduced pressure gas, and does not bring about any merit in terms of the complexity of the equipment. Furthermore, although sufficient scavenging is effective, as described above, the hydrogen loss associated with the purge gas increases proportionally.

As mentioned above, yield improvement and purity improvement are contradictory to each other, and it is difficult to satisfy both.
This has been considered difficult with the PSA method.

"Means for Solving the Problem" Therefore, the present inventors completed the present invention for the purpose of obtaining high purity hydrogen in the conventional two-stage PSA method and further improving the hydrogen yield.

In the two-stage PSA method, this invention uses an adsorption tower in which the diameter, size, or number of the first and second stages is changed, and the capacity of the second stage is smaller than that of the first stage, and the first stage adsorption tower adsorbs high-concentration impure gas components. Then, the remaining low concentration impure gas components are adsorbed and removed in the adsorption tower in the latter stage, and then the desorption gas released in the depressurization process after the adsorption process in the latter stage is directly transferred to the depressurized gas component after the adsorption in the former stage tower is completed. It can be used for scavenging of the tower during the scavenging process, or it can be collected in a recovery tank and used for scavenging. A portion of the purge gas is also collected in a recovery tank. Furthermore, the scavenging gas from the column outlet by the product hydrogen of the latter column is directly used for column scavenging after scavenging by the depressurized gas in the first column, or the scavenging gas is collected in a temporary recovery tank and then used during the scavenging period of the first column. Use as. The scavenging purge gas from the front tower is returned to the recovery tank and recycled by the compressor. Furthermore, the gas in the recovery tank is used to pressurize the column in the repressurization process after the completion of regeneration of the former column using a compressor.

The time schedule for switching the adsorption towers is such that the cycle time programs for the first stage tower and the second stage tower are shifted within about 1/2 of the adsorption time.

If high-concentration impurities are removed in the first column and low-concentration impurities are removed in the second column for final purification, the capacity of the second column may be smaller than that of the first column, and the cost of the second column will be reduced proportionally. Not only
Since the voids in the column become smaller, the amount of reduced pressure desorption gas is small in areas where the hydrogen concentration is high, and the amount of countercurrent scavenging gas and countercurrent pressurized hydrogen amount due to product hydrogen may also be small. Moreover, since most of the pure volume of impure gas to be desorbed is adsorbed and removed in the first stage column, only a small amount is absorbed in the second stage, and the hydrogen concentration in the purge gas does not decrease much. This hydrogen-rich vacuum desorption gas and scavenging gas are recovered and recycled to the previous stage.
Used for pre-stage playback. Furthermore, most of the purge gas is recovered into a recovery tank, boosted in pressure by a compressor, and used to pressurize the column after the first-stage regeneration is completed.
Since the purge gas is thoroughly recovered and recycled in this way, hydrogen loss is small and sufficient regeneration is possible.

Furthermore, if the adsorption removal of high-concentration impurity components and constant-concentration impurity components is performed in two stages: a large-capacity first-stage adsorption tower and a small-capacity second-stage adsorption tower, the optimal cycle time for adsorption and regeneration of impure components can be set for each stage. I can do it. For example, if the cycle times of the first and second columns are shifted by about one-third of the adsorption time (corresponding to the pressure equilibration time), the vacuum desorption gas from the second column can be used for scavenging of the first column. The scavenging gas in the front tower can be saved.

Further, the adsorbent is filled in the first stage with one that easily adsorbs high concentration impurity components, and the second stage with one that easily adsorbs low concentration impurity components. For example, gases containing a large amount of CO ₂ and methane are removed in a front stage filled with activated carbon, while CO and N ₂ , which are difficult to adsorb to activated carbon and need to be sufficiently removed even though their concentration is relatively low, are filled with synthetic zeolite. Make sure to remove it completely in the later stage.

High purity hydrogen of 99.99% or more can be produced using the above method.
It can be obtained with a high yield of more than 1%.

"Example" The method of the present invention will be explained in detail based on the drawings. FIG. 1 is a flow sheet showing an example of the present invention, in which the first stage has three columns and the second stage has two columns.
An example of the raw material crude hydrogen gas composition is as follows, the adsorption pressure is 15 Kg/cm ² G, and the temperature is 35°C.

Inlet gas composition (mol%) (a) Natural gas reformed gas (b) Methanol reformed gas H ₂ 77.36 H ₂ 74.54 CO ₂ 18.66 CO ₂ 24.47 CH ₄ 1.48 CO 0.51 CO 2.00 CH ₃ OH 0.12 N ₂ 0.12 H ₂ O 0.36 H ₂ O 0.36 100.00 100.00 A ₁ , A ₂ , A ₃ are the first stage adsorption towers, B ₁ , B ₂ are the second stage adsorption towers. Filled with activated carbon that selectively adsorbs ₂ or CH ₄ , the remaining water and
Filled with synthetic zeolite that adsorbs CO, CO ₂ and CH ₃ OH. The column capacity (and thus the amount of adsorbent packed) of the two columns in the B column is smaller than that in the A column with the three columns.

The raw material crude hydrogen gas 1 enters the A ₁ adsorption tower from below, where it adsorbs most of the water, CO ₂ and CH ₄ ,
From the outlet pipe 2, primary purified hydrogen gas with a purity of approximately 98% is obtained. This gas then enters the B ₁ adsorption column, where small amounts of impurities such as remaining CO, CO ₂ , CH ₄ or CH ₃ OH, H ₂ O are almost completely adsorbed and removed, and the B ₁
High purity hydrogen 4 of 99.999% is obtained from the outlet pipe.

FIG. 2 shows an example of the adsorption-regeneration step cycle of the two-stage PSA method. The horizontal direction indicates the cycle time, the dotted line indicates the internal pressure level, and the diagonal line indicates the adsorption process. In this example, row A (3 towers) row B (2 towers)
For both columns, the adsorption time is about 6 minutes, the depressurization, desorption, and scavenging times are about 6 minutes, and the pressure equilibration and repressurization times are about 6 minutes, and one cycle ends in about 18 minutes. However, the time schedule for column A and column B is such that the start timing is slightly shifted so that column _B1 starts the adsorption process about 2 minutes after column _A1 starts adsorption, as shown in Figure 2. This is because the reduced pressure desorption gas and scavenging gas in the B row are used as the scavenging gas in the A row.

In Fig. 1, when the first stage _A1 column is adsorbing, _A2 ,
The inlet and outlet valves of the _A3 tower are closed, and the _A3 tower is undergoing vacuum desorption. After completion of regeneration, the vacuum desorbed gas from the _A3 tower is sent to the _A2 tower, where it is recovered in about 1 minute until the pressure reaches equilibrium. Approximately 1/2 residual pressure in _A3 tower is
Purge within 3 minutes from lines 5 and 7 (line 6 closed). 14 is a purge gas, which is discharged into the atmosphere or sent to a fuel line. The latter stage _B1 tower is under adsorption, and the _B2 tower is under reduced pressure desorption and regeneration.
0 (closed at 11), and after being depressurized to near the atmospheric pressure, it is sent from the outlet side of the first stage _A3 tower and used as scavenging gas. Most of this _A3 column scavenging purge gas is recovered to the recovery tank 8 through lines 5 and 6 (7 is closed). However, the purge gas after scavenging from the _A3 tower, which contains a relatively large amount of impurities due to the reduced pressure gas at the end of the _B2 tower, is purged from the 7th line. When the pressure in the _B2 tower is reduced to near atmospheric pressure (desorption and regeneration), a part of the product gas 4 is then diverted from 3 and sent from the outlet direction for backwashing and scavenging. The required amount of product gas for scavenging is B
Since the capacity of the tower is small, only a small amount is required, and the scavenging time can be shortened. The scavenging purge gas continues to pass through lines 9 and 10, and after being used for scavenging of the first stage _A3 tower,
Collect into recovery tank 8 or directly 9, 11, (10
Collected from the line (closed line) to the recovery tank 8.
The recovered gas in the recovery tank 8 is compressed and recovered by a compressor to the _A2 tower (standby tower after regeneration and pressure equalization). Therefore, there is no loss of product gas to the outside.

Finally, the _A2 column is pressurized in countercurrent by the primary purified hydrogen gas ( _A1 column outlet gas) and pressurized with the raw material gas from the inlet, and the inlet and outlet valves are switched to the _A1 column to enter the adsorption process.

Similarly for column B, once the scavenging of the _B2 tower is finished, it is pressurized with product hydrogen gas and primary purified hydrogen gas.
By switching the inlet and outlet valves, the _B2 tower enters the adsorption process and the _B1 tower enters the regeneration process.

The above steps are repeated according to the program sequence to continuously obtain 99.999% high purity hydrogen.

In the above method, as shown in Figs. 2 and 4,
Although the cycle times of the A column and the B column are staggered, it is of course possible to set them to the same period. However, in this case, the depressurized gas in the B row cannot be used as is for scavenging in the A row, and must be depressurized once to a recovery tank, and further pressurized by a compressor and recycled to the A row.

As described above, according to the method of the present invention, the amount of regenerated purge gas discharged outside the system is only the residual pressure after equilibrium of about half of the adsorption pressure in column A and column B, and the pressure in column A is reduced. Since the desorption gas has a low hydrogen concentration, the loss of released hydrogen is relatively small.Also, since the inlet gas amount in column B is small and the adsorption tower capacity is small, the amount of vacuum desorption gas is also small, compared to the normal PSA method. Hydrogen loss is kept low, and overall hydrogen yield is significantly improved.

For example, in a 200Nm ³ /hr hydrogen purification PSA unit,
When processing the gas with the above composition (b) at 15Kg/cm ² G at 35°C (regeneration cycle time 6 minutes) A tower, void volume is 0.49m ³ , 7.5Kg/cm ² G to 0.1
The amount of purge gas hydrogen when depressurized to Kg/cm ² G is (hydrogen concentration 70%) (0.49) (60/6) (7.4) (273/308) (0.7) = 22.5
(Nm ³ /hr) A tower, void volume is 0.15m ³ , 7.5Kg/cm ² G to 0.1
The amount of purge gas hydrogen when depressurized to Kg/cm ² G is (hydrogen concentration 90%) (0.15) (60/6) (7.4) (273/308) (0.9) = 8.85
(Nm ³ /hr) Total purge hydrogen amount 22.5 + 8.85 = 31.35 (Nm ³ /
hr) Therefore, the hydrogen yield is 200/231.35=0.864, or approximately 86% hydrogen yield.

Next, an example of the three first-stage columns and the second-stage three columns will be explained with reference to a flow sheet in FIG. 3 and an adsorption cycle program diagram in FIG. 4. As in the previous example, the rear tower is smaller in size than the front tower.

In the case of three columns in the latter stage, the regeneration time can be longer than in the case of the two columns, so that the flexibility of operation is increased. Figure 4 shows an example in which the adsorption times of columns A and B are shifted by the pressure equilibration time, and a part of the purge gas is recovered by using the reduced pressure desorption gas of column B for scavenging of column A. The scavenging gas is collected in a tank, and some relatively impure scavenging gas is purged out of the system. Further, the countercurrent scavenging gas generated by the product hydrogen in the B row is further utilized for the A row scavenging gas and recovered in the recovery tank.

The purge gas is also recycled using a compressor. Further, these gases are pressurized by a compressor and recovered under pressure to the column before the A column adsorption.

The method shown in FIG. 3 has the advantage that the regeneration time can be extended by using three towers in the latter stage, but the overall equipment cost increases.

In the above explanation, in order to use the reduced pressure desorption gas in row B for scavenging in row A, the adsorption times in rows A and B are shifted by the pressure equalization time or 1/2 time of one cycle. It is also possible to once collect the remaining pressure in a recovery tank, increase the pressure with a compressor, and use it for scavenging (scavenging gas recycling). In this case, the cycle time of the A and B rows can be set freely.

In addition to the above two examples, the front stage A row has two towers, and the rear stage B
It is also possible to have two columns or three columns in the column. In this case, a pressure accumulator tank is provided in column A, and the reduced pressure desorption gas (hydrogen-rich gas) after adsorption in column A is collected in the pressure accumulator tank to an intermediate pressure. The recovered intermediate pressure gas is used to pressurize the tower after regeneration.

The use of the B-row depressurizing gas and scavenging gas for A-row scavenging and for pressurization after A-row regeneration is the same as in the above two examples.

[Brief explanation of the drawing]

FIG. 1 is a flow sheet for the case of the present invention with three columns in the front stage and two columns in the latter stage. FIG. 2 is an explanatory diagram of the adsorption cycle program same as above. Figure 3 is a flow sheet for the case of three towers in the front stage and three towers in the latter stage. FIG. 4 is an explanatory diagram of the adsorption cycle program same as above. A ₁ , A ₂ , A ₃ , B ₁ , B ₂ , B ₃ ...Adsorption tower, 1...
Raw material gas inlet, 2... Row A exit gas, 3... Row B exit gas, 4... Purified hydrogen, 8... Recovery gas tank, 12... Compressor.

Claims (1)

[Claims] 1 In the two-stage pressure swing adsorption method in which crude hydrogen gas is passed through a plurality of adsorption towers in the front and rear stages to purify hydrogen, the diameter, size, or number of columns in the front and rear stages are changed, so that the former stage column is replaced with the latter stage tower. Using an adsorption tower with a small capacity, the first-stage adsorption tower adsorbs and removes high-concentration impure gas components, and the second-stage adsorption tower adsorbs and removes the remaining low-concentration impure gas components. The desorption gas released in the depressurization process after the adsorption process in the previous column is completed is used for scavenging of the column in the scavenging process, or it is collected in a recovery tank and used for scavenging, and part of the purge gas is Furthermore, the scavenging gas from the column outlet by the product hydrogen of the latter column is directly used for column scavenging after scavenging by the depressurized gas in the former column, or after being temporarily collected in a recovery tank, the scavenging gas from the column outlet is It is used as a scavenging gas during the tower scavenging period, and
The scavenging purge gas from the front column is returned to the recovery tank and recycled by the compressor, and the gas in the recovery tank is used to pressurize the column during the repressurization process after the completion of regeneration of the first column, and the adsorption column is switched. The time schedule is a hydrogen purification method using a two-stage pressure swing adsorption method, characterized in that the cycle time programs in the first column and second column are shifted within about 2 minutes of the adsorption time.