A composite adsorbent bed and its application

Abstract

A composite adsorbent bed comprises a first molecular sieve layer and a second molecular sieve layer, wherein the first molecular sieve layer comprises a first molecular sieve and the second molecular sieve layer comprises a second molecular sieve; the first molecular sieve comprises Sr and the second molecular sieve comprises Li; the first molecular sieve layer is first disposed in the composite adsorbent bed along the direction from the feed end to the product end, followed by the second molecular sieve layer. Applying this composite adsorbent bed to a pressure swing adsorption oxygen generation process, compared to using an adsorbent bed solely with Li-LSX molecular sieves, can achieve lower energy consumption, lower cost, and higher yield.

Description

Technical Field

[0001] This invention belongs to the field of gas separation technology, specifically relating to an adsorbent bed used in a pressure swing adsorption oxygen generation process. Background Technology

[0002] In modern production and daily life, oxygen of varying concentrations is widely used in various industries, such as steel production, fuel combustion for energy, wastewater and waste gas treatment, as well as aerospace and healthcare. Currently, the most widely used oxygen production methods in industry are cryogenic processes and pressure swing adsorption (PSA). The industrial application of PSA oxygen production technology originated in the 1970s. Its product gas has an adjustable oxygen purity between 50% and 95%, meeting the oxygen needs of various industries. Moreover, the process is simple, the operation is stable, and the investment and operating costs are low, making it increasingly popular among oxygen-demanding enterprises.

[0003] Adsorbents are the foundation of pressure swing adsorption (PSA) separation, and their performance determines the adsorption and separation efficiency, equipment investment, and the economics of separation. The adsorbents used in PSA oxygen generation processes are mostly zeolite molecular sieves, initially primarily 5A and 13X molecular sieves. However, their low nitrogen adsorption capacity and low nitrogen-oxygen separation coefficient hindered the development of PSA oxygen generation technology. With in-depth research into the modification of oxygen generation adsorbents, in the 1990s, lithium molecular sieves, with their larger nitrogen adsorption capacity and higher nitrogen-oxygen separation coefficient, began to be gradually applied to PSA oxygen generation processes.

[0004] Numerous studies (such as US5573745A, US5584912A, US5413625A, etc.) have shown that LSX-type molecular sieves with a Si / Al ratio less than 1.2, after being processed with Li... + The Li-LSX molecular sieve prepared after exchange is currently the most ideal pressure swing adsorption (PSA) oxygen generator, with a nitrogen-oxygen separation coefficient of up to 9.68. However, due to the high cost of lithium salts, researchers have attempted to introduce other heteroions into Li-LSX molecular sieves, such as Li-Ag-LSX molecular sieves (CN102784617B) and Li-Ca-LSX molecular sieves (CN103539150A). Some have even completely replaced lithium ions with other cations. For example, US5152813A discloses a method for preparing Sr-LSX molecular sieves, showing an adsorption capacity of 0.98 mmol / g of nitrogen at room temperature and pressure, and a nitrogen-oxygen separation coefficient of 5.85. US4481018A and US4557736A also disclose methods for preparing Sr-LSX molecular sieves, as well as nitrogen and oxygen adsorption capacities and selectivity, but the adsorption capacity and selectivity of Sr-LSX molecular sieves are still inferior to those of Li-LSX molecular sieves.

[0005] In summary, while the pressure swing adsorption (PSA) oxygen generation process using Li-LSX molecular sieve adsorbent beds achieves higher yields and lower energy consumption compared to previous processes, its cost is relatively high, and further improvements in yield and reductions in energy consumption are needed. Therefore, it is necessary to develop a new adsorbent bed that achieves high yields, low energy consumption, and low cost. Summary of the Invention

[0006] One object of the present invention is to provide a composite adsorbent bed comprising a first molecular sieve layer and a second molecular sieve layer, which improves the yield of oxygen production by pressure swing adsorption and reduces energy consumption.

[0007] The composite adsorbent bed of the present invention comprises a first molecular sieve layer and a second molecular sieve layer; wherein the first molecular sieve layer comprises a first molecular sieve, and the second molecular sieve layer comprises a second molecular sieve; the first molecular sieve comprises Sr, and the second molecular sieve comprises Li; along the direction from the feed end to the product end of the composite adsorbent bed, the first molecular sieve layer is first disposed in the composite adsorbent bed, and then the second molecular sieve layer is disposed thereafter. In the pressure swing adsorption oxygen generation process, compared with an adsorbent bed using only Li-LSX molecular sieves, the composite adsorbent bed can achieve lower energy consumption, lower cost, and higher yield.

[0008] Another object of the present invention is to provide a pressure swing adsorption system comprising the above-described composite adsorbent bed.

[0009] Another object of the present invention is to provide a method for oxygen generation by pressure swing adsorption, which employs the above-mentioned pressure swing adsorption system.

[0010] The inventors of this invention unexpectedly discovered that, although the nitrogen adsorption capacity of Sr-LSX type molecular sieves is slightly smaller than that of Li-LSX type molecular sieves ( Figure 1 However, because its bulk density is greater than that of Li-LSX molecular sieves (e.g., Sr-LSX has a bulk density 1.19 times that of Li-LSX molecular sieves), it can improve the performance of the device (e.g., lower energy consumption, higher yield) when replacing Li-LSX molecular sieves in an adsorbent bed with a similar volume. Furthermore, the oxygen adsorption capacity of Sr-LSX molecular sieves is greater than that of Li-LSX molecular sieves, and the difference in oxygen adsorption capacity between them increases with increasing pressure. Figure 2 Using Sr-LSX molecular sieves at the inlet of the adsorbent bed where the oxygen partial pressure is low will be more beneficial in reducing oxygen loss.

[0011] However, if only Sr-LSX molecular sieves are used in the adsorbent bed, that is, if Sr-LSX molecular sieves are used at the product end of the adsorbent bed with a high oxygen partial pressure, the oxygen yield of the product will decrease due to the excessively large dynamic adsorption capacity of the Sr-LSX molecular sieves for oxygen. The inventors of this invention have further discovered that by installing Li-LSX molecular sieves with a smaller dynamic adsorption capacity for oxygen at the product end, the above-mentioned drawbacks can be overcome.

[0012] Based on the above findings, the following technical solution is provided:

[0013] A composite adsorbent bed includes a first molecular sieve layer and a second molecular sieve layer, wherein the first molecular sieve layer includes a first molecular sieve and the second molecular sieve layer includes a second molecular sieve; the first molecular sieve includes Sr and the second molecular sieve includes Li; the first molecular sieve layer is disposed first and the second molecular sieve layer is disposed later along the direction from the feed end to the product end of the composite adsorbent bed.

[0014] Preferably, the first molecular sieve layer is disposed in the feed end region of the composite adsorbent bed; and / or, the second molecular sieve layer is disposed in the product end region of the composite adsorbent bed.

[0015] Preferably, the composite adsorbent bed contains only a first molecular sieve layer and a second molecular sieve layer, wherein the first molecular sieve layer is disposed in the feed end region of the composite adsorbent bed, and the second molecular sieve layer is disposed in the product end region of the composite adsorbent bed.

[0016] Preferably, the first molecular sieve layer comprises Sr-LSX molecular sieve; and / or, the second molecular sieve layer comprises Li-LSX molecular sieve.

[0017] More preferably, the first molecular sieve layer is an Sr-LSX molecular sieve layer, and / or the second molecular sieve layer is a Li-LSX molecular sieve.

[0018] Preferably, the bulk density ratio of the first molecular sieve to the second molecular sieve is greater than 1, more preferably greater than 1.15.

[0019] Preferably, the first molecular sieve is an Sr-LSX molecular sieve, and / or the second molecular sieve is a Li-LSX molecular sieve.

[0020] Preferably, the volume ratio of the first molecular sieve layer to the second molecular sieve layer is 70:30 to 20:80; more preferably, the volume ratio of the first molecular sieve layer to the second molecular sieve layer is 60:40 to 30:70.

[0021] Preferably, 100% to 60% of the available ion sites in the first molecular sieve are occupied by Sr, and 0% to 40% of the available ion sites are occupied by a second ion; preferably, the second ion is Ca and / or Ba.

[0022] Preferably, 0% to 40% of the available ion sites in the first molecular sieve are occupied by Ca, and 100% to 60% of the available ion sites are occupied by Sr; more preferably, 100% of the available ion sites in the first molecular sieve are occupied by Sr.

[0023] Preferably, the composite adsorbent bed is further provided with one or more of the following: a 13X molecular sieve layer, an activated alumina layer, a silica gel layer, and a 5A molecular sieve layer.

[0024] Preferably, the composite adsorbent bed is provided with only Sr-LSX type molecular sieve layer and Li-LSX type molecular sieve layer; the Sr-LSX type molecular sieve layer and Li-LSX type molecular sieve layer have the above-mentioned characteristics.

[0025] In the composite adsorbent bed, the first and second molecular sieve layers exhibit a good synergistic effect, resulting in lower energy consumption and cost, and higher yield. However, swapping the positions of the Sr-LSX and Li-LSX molecular sieves in the adsorbent bed leads to poorer results.

[0026] The volume ratio of the first molecular sieve layer to the second molecular sieve layer is 70:30 to 20:80, preferably 60:40 to 30:70.

[0027] In the first molecular sieve, 0% to 40% of the available ion sites are occupied by a second ion (such as Ca and / or Ba), and 100% to 60% of the available ion sites are occupied by Sr; preferably, 0% to 40% of the available ion sites in the first molecular sieve are occupied by Ca, and 100% to 60% of the available ion sites are occupied by Sr; more preferably, 100% of the available ion sites in the first molecular sieve are occupied by Sr.

[0028] The composite adsorbent bed is further provided with one or more of the following: a 13X molecular sieve layer, an activated alumina layer, and a silica gel layer; a 13X molecular sieve layer, an activated alumina layer, and a silica gel layer are provided at the feed end to remove impurities in the raw material gas, such as water and carbon dioxide.

[0029] A pressure swing adsorption system includes at least two composite adsorbent beds, control valves, compressors, and other equipment.

[0030] A method for preparing oxygen by separating air using the above-mentioned pressure swing adsorption system, wherein each composite adsorbent bed sequentially undergoes seven basic steps: adsorption oxygen production step, pressure equalization step, vacuum desorption step, product gas flushing step, pressure equalization step, oxygen regeneration step, and pressurization step.

[0031] A method for producing oxygen via pressure swing adsorption (PSA) utilizes a PSA system to extract oxygen from the air. The PSA system includes at least two composite adsorbent beds and multiple control valves for controlling the flow of air. Air is compressed by a compressor before entering the composite adsorbent beds. Each composite adsorbent bed sequentially undergoes seven basic steps: adsorption for oxygen production, pressure equalization, vacuum desorption, product gas flushing, pressure equalization and boosting, oxygen recirculation, and pressurization. Different process flows can be designed by selecting the timing of each of these seven basic steps. The timing of the two or more tower process steps can be set by controlling the opening and closing of the valves, and continuous production of product oxygen can be achieved by alternating the use of two or more towers.

[0032] An application of the aforementioned composite adsorbent bed or pressure swing adsorption system in gas separation. Preferably, the application is the separation of air to produce oxygen.

[0033] Preferably, the Li-LSX type molecular sieve can be the molecular sieve described in US5573745A, US5584912A or US5413625A; the Sr-LSX type molecular sieve can be the molecular sieve described in US5152813A or US4557736A.

[0034] The adsorption capacities of Sr-LSX and Li-LSX molecular sieves for nitrogen and oxygen at ambient temperature and pressure, with 100% of the available ion sites occupied by Sr, and their bulk density in the composite adsorbent bed are shown in Table 1.

[0035] Table 1. Comparison of adsorption capacities of Sr-LSX and Li-LSX molecular sieves at ambient temperature and pressure

[0036]

[0037] The inventors unexpectedly discovered that Sr-LSX has a slightly smaller nitrogen adsorption capacity than Li-LSX, but because its bulk density is 1.19 times that of Li-LSX, it can improve the performance of the device when replacing Li-LSX in a composite adsorbent bed with an equal volume. Sr-LSX has a larger oxygen adsorption capacity than Li-LSX, and the difference in oxygen adsorption capacity between Sr-LSX and Li-LSX increases with increasing pressure. It is more advantageous to use Sr-LSX at the lower end of the composite adsorbent bed where the oxygen partial pressure is lower. Otherwise, excessive oxygen adsorption will cause a decrease in the oxygen yield of the product.

[0038] The specific process control of the pressure swing adsorption cycle of the present invention is as follows:

[0039] 1. Oxygen production step (A): Depending on the scale of oxygen consumption, one or more composite adsorbent beds are selected to simultaneously introduce air. The adsorption pressure is selected as 1.3–2 bar (absolute pressure), with an optimal selection of 1.4–1.6 bar (absolute pressure). The adsorbent in the composite adsorbent bed uses Sr-LSX molecular sieves at the feed end and Li-LSX molecular sieves at the product end. The volume ratio of the Sr-LSX molecular sieve to the Li-LSX molecular sieve is 70:30–20:80, preferably 60:40–30:70. Air is compressed by a compressor and enters from the bottom feed end of the composite adsorbent bed. Nitrogen, as an impurity, is adsorbed by the adsorbent, and the remaining gas is drawn off from the top of the tower as product gas. A purity of 50%–95% and a yield of 200 Nm³ can be obtained as needed. 3 / h to 7500Nm 3 / h of oxygen products.

[0040] 2. Pressure equalization step (ED): After the adsorption is completed, the composite adsorbent bed stops the intake of gas and oxygen production, and connects with other composite adsorbent beds that have completed flushing, so that the oxygen in the dead space is recovered into other composite adsorbent beds.

[0041] 3. Vacuum Desorption Step (V): After the pressure drop equalization is completed, the composite adsorbent bed is stopped from connecting with other composite adsorbent beds. A vacuum pump is used to evacuate the bed, causing the adsorbed nitrogen to be desorbed. The vacuum pressure is 0.3–0.7 bar (absolute pressure), with 0.4–0.5 bar (absolute pressure) being the optimal choice.

[0042] 4. Product gas flushing step (PP): The composite adsorbent bed is flushed with product gas while being evacuated. The amount of flushing gas is 5% to 20% of the gas production, with an optimal selection of 8% to 12%.

[0043] 5. Pressure equalization step (ER): After the composite adsorbent bed has finished rinsing, stop the product gas rinsing and vacuuming, connect it to the composite adsorbent bed that has completed the adsorption step, and receive the oxygen in its dead space.

[0044] 6. Oxygen Recirculation (RO) Step: After the pressure equalization and rise of the composite adsorbent bed is completed, it is disconnected from other composite adsorbent beds. The product end is then connected to the product gas tank to optimize the concentration distribution within the tower.

[0045] 7. Pressurization step (FR): After the composite adsorbent bed has completed oxygen return, stop connecting it to the product tank and start air intake to increase the tower pressure to the adsorption pressure.

[0046] The beneficial effects of this invention are:

[0047] This invention provides a method for oxygen production via pressure swing adsorption (PSA). By using a PSA system comprising the aforementioned composite adsorbent bed to produce oxygen from the air, this method offers advantages such as improved oxygen recovery rate, reduced investment and operating costs, and simple and versatile process. Specifically, the advantages of this invention include:

[0048] (I) Improving Oxygen Recovery Rate. Since a certain volume of Sr-LSX molecular sieve is installed at the feed end of the adsorbent bed, its dynamic adsorption capacity for nitrogen is higher than that of Li-LSX molecular sieve, allowing it to process more air and obtain more product oxygen. Conversely, installing Li-LSX molecular sieve at the product end, which has a smaller dynamic adsorption capacity for oxygen, overcomes the problem of excessively high dynamic adsorption capacity of Sr-LSX molecular sieve when oxygen partial pressure is high. The synergistic effect of both methods achieves low energy consumption and high yield.

[0049] (ii) Reduce investment and operating costs. Since the cost of Sr-LSX molecular sieve installed at the feed end of the adsorbent bed is much lower than that of Li-LSX molecular sieve, investment costs can be reduced; at the same time, the increase in oxygen recovery rate also reduces the power consumption of standard pure oxygen.

[0050] (III) The process is simple and versatile, and can be applied to pressure swing adsorption oxygen generation projects with different product purity requirements and different oxygen consumption scales. Attached Figure Description

[0051] Figure 1 These are adsorption isotherms of nitrogen in Li-LSX and Sr-LSX molecular sieves at room temperature.

[0052] Figure 2 These are adsorption isotherms of oxygen in Li-LSX and Sr-LSX molecular sieves at room temperature.

[0053] Figure 3 This is a schematic diagram of a composite adsorbent bed containing 13X molecular sieve, Sr-LSX molecular sieve, and Li-LSX molecular sieve.

[0054] Figure 4 This is a schematic diagram of a composite adsorbent bed containing Sr-LSX molecular sieves and Li-LSX molecular sieves. Detailed Implementation

[0055] The present invention is further described below through embodiments, but these embodiments are not intended to limit the scope of the invention in any way.

[0056] This invention provides a method for oxygen production via pressure swing adsorption (PSA). The PSA system produces oxygen from air and includes at least two composite adsorbent beds using Sr-LSX and Li-LSX molecular sieves, and multiple control valves for controlling airflow interruption. Air is compressed by a compressor before entering the composite adsorbent beds, each containing an adsorbent capable of selectively adsorbing nitrogen. Each composite adsorbent bed sequentially undergoes seven basic steps: adsorption for oxygen production, pressure equalization, vacuum desorption, product gas flushing, pressure equalization and boosting, oxygen reabsorption, and pressurization. Different process flows can be designed by selecting the timing of these seven basic steps. The timing of the two or more tower process steps can be set by controlling the opening and closing of the valves, and continuous production of product oxygen can be achieved by alternating the use of two or more towers.

[0057] The following embodiments are implemented using the method of the present invention. The pressure swing adsorption system in these embodiments operates at room temperature and consists of two composite adsorbent beds. Each composite adsorbent bed has a column diameter of 3600 mm and an adsorbent packing height of 967 mm. The timing of one cycle of the system is shown in Table 2. During each period of the adsorption-regeneration cycle, each column of the system is in a different operating state.

[0058] Table 2. Cycle sequence of the two-tower pressure swing adsorption oxygen generation process

[0059] Composite Adsorbent Bed 1 A A ED V V V PP ER RO FR Composite Adsorbent Bed 2 V PP ER RO FR A A ED V V Time (s) 8 6 4 4 4 8 6 4 4 4

[0060] The composite adsorbent beds used in Examples 1-5 are as follows: Figure 4 As shown. Figure 4 As shown, Sr-LSX molecular sieves are used at the feed end of the composite adsorbent bed, and Li-LSX molecular sieves are used at the product end. Comparative Example 1 uses only Li-LSX molecular sieves in the adsorbent bed, Comparative Example 2 uses only Sr-LSX molecular sieves in the adsorbent bed, and Comparative Example 3 uses Li-LSX molecular sieves at the feed end of the adsorbent bed and Sr-LSX molecular sieves at the product end. The Sr-LSX molecular sieves are prepared according to the method described in Example 9 of US5152813A and have the properties shown in Table X Example 4 of US5152813A; the Li-LSX molecular sieves are prepared according to the method described in Example 3 of US5413625A and have the properties shown in Table 2 Example 1 of US5413625A.

[0061] The only difference between Examples 1-5 and Comparative Examples 1-3 is the adsorbent, as shown in Tables 3 and 4; all other process conditions are the same.

[0062] The volume ratio of Sr-LSX molecular sieve to Li-LSX molecular sieve and the operating results are shown in Tables 3 and 4.

[0063] Table 3. Volume ratios and operational results of Comparative Examples 1-2 and Examples 1-5

[0064]

[0065] Note: 1. When calculating the price of Li-LSX molecular sieves for investment, the price of lithium carbonate in March 2022 shall be used as the standard.

[0066] 2. With the yield, energy consumption, and investment of the fully-equipped Li-LSX (i.e., Comparative Example 1) as 1, the yield, energy consumption, and investment were normalized and compared.

[0067] Table 4. Volume ratio and results of Comparative Example 3

[0068]

[0069] Note: 1. When calculating the price of Li-LSX molecular sieves for investment, the price of lithium carbonate in March 2022 shall be used as the standard.

[0070] 2. With the yield, energy consumption, and investment of the fully-equipped Li-LSX (i.e., Comparative Example 1) as 1, the yield, energy consumption, and investment were normalized and compared.

[0071] As shown in Tables 3 and 4, the investment cost of the equipment decreases as the proportion of Sr-LSX molecular sieves in the composite adsorbent bed increases. As shown in Table 4, Comparative Example 3, which uses Li-LSX molecular sieves at the feed end and Sr-LSX molecular sieves at the product end (volume ratio 50:50), has the lowest yield and highest energy consumption, and its operating performance is far worse than that of Comparative Example 2, which is compared in parallel. As shown in Table 3, Comparative Example 2 has a lower yield than Examples 1-5, but higher energy consumption. As shown in Examples 1-4 of Table 3, when the volume ratio of Sr-LSX molecular sieve to Li-LSX molecular sieve in the composite adsorbent bed is 70:30 to 30:70, the yield is higher and the energy consumption is lower than that of Comparative Examples 1-2. As shown in Example 5, when the volume ratio of Sr-LSX molecular sieve to Li-LSX molecular sieve in the composite adsorbent bed is 80:20, the yield and energy consumption are slightly lower than those of Comparative Example 1, but the cost is much lower than that of Comparative Example 1.

[0072] Therefore, the composite adsorbent bed described in this invention has significantly lower investment and energy consumption, as well as comparable or higher yields. Using a composite adsorbent bed with a volume ratio of Sr-LSX molecular sieve to Li-LSX molecular sieve of 70:30 can achieve even lower costs; using a composite adsorbent bed with a volume ratio of Sr-LSX molecular sieve to Li-LSX molecular sieve of 30:70 can obtain superior device performance (e.g., high yield, low energy consumption).

[0073] It should be noted that the purpose of disclosing the embodiments is to help further understand the present invention. However, those skilled in the art will understand that various substitutions and modifications are possible without departing from the spirit and scope of the present invention and the appended claims. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the scope of protection of the present invention is defined by the scope of the claims.

Claims

1. A composite adsorbent bed, characterized in that, The composite adsorbent bed comprises a first molecular sieve layer and a second molecular sieve layer; wherein the first molecular sieve layer comprises a first molecular sieve and the second molecular sieve layer comprises a second molecular sieve; the first molecular sieve comprises Sr and the second molecular sieve comprises Li; along the direction from the feed end to the product end of the composite adsorbent bed, the first molecular sieve layer is first disposed in the composite adsorbent bed, and then the second molecular sieve layer is disposed thereafter; the volume ratio of the first molecular sieve layer to the second molecular sieve layer is 70:30 to 20:

80.

2. The composite adsorbent bed according to claim 1, characterized in that, The first molecular sieve layer is disposed in the feed end region of the composite adsorbent bed; and / or, the second molecular sieve layer is disposed in the product end region of the composite adsorbent bed.

3. The composite adsorbent bed according to claim 1, characterized in that, The composite adsorbent bed contains only the first molecular sieve layer and the second molecular sieve layer.

4. The composite adsorbent bed according to any one of claims 1-3, characterized in that, The first molecular sieve layer comprises Sr-LSX molecular sieve; and / or, the second molecular sieve layer comprises Li-LSX molecular sieve.

5. The composite adsorbent bed according to claim 4, characterized in that, In the first molecular sieve, 100% to 60% of the available ion sites are occupied by Sr, and 0% to 40% of the available ion sites are occupied by a second ion, which is Ca and / or Ba.

6. The composite adsorbent bed according to any one of claims 1-3, characterized in that, The ratio of the bulk density of the first molecular sieve to that of the second molecular sieve is greater than 1; the first molecular sieve is an Sr-LSX molecular sieve, and / or the second molecular sieve is a Li-LSX molecular sieve.

7. The composite adsorbent bed according to any one of claims 1-3, characterized in that, The volume ratio of the first molecular sieve layer to the second molecular sieve layer is 60:40 to 30:

70.

8. The composite adsorbent bed according to any one of claims 1-2, characterized in that, The composite adsorbent bed is also provided with one or more of the following: a 13X molecular sieve layer, an activated alumina layer, and a silica gel layer.

9. A pressure swing adsorption system, characterized in that, The pressure swing adsorption system includes at least two composite adsorbent beds as described in any one of claims 1-8.

10. The application of the composite adsorbent bed according to any one of claims 1-8 or the pressure swing adsorption system according to claim 9 in gas separation.

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