JP7789306B2 - Gas Separation Method - Google Patents

Description

The present invention relates to a gas separation method using a porous polymer complex that exhibits a gate phenomenon (hereinafter simply referred to as a "gate-type porous polymer complex").

Pressure Swing Adsorption (PSA) systems using solid adsorbents are widely used as a method for producing industrially important gases such as oxygen and nitrogen. For example, when separating two gases from a mixture, an adsorption tower is filled with solid adsorbents that have different adsorption capacities for the two gases. When the mixed gas is introduced, one gas is selectively adsorbed. As a result, a large amount of unadsorbed gas remains in the gas phase, and the adsorbed gas is recovered by depressurizing this. For this purpose, so-called porous solid adsorbents such as zeolite and activated carbon are widely used, and PSA for oxygen generation, PSA for nitrogen generation, and other applications are widely used (Patent Document 1).

Generally, existing porous materials such as zeolites and activated carbon can be classified into the six IUPAC adsorption isotherm categories. In other words, existing porous materials have the property that their gas adsorption capacity increases with increasing pressure, and existing PSA systems are optimized for this property. However, materials that exhibit unique adsorption isotherms that cannot be classified into the six IUPAC categories have been announced. These are types of porous coordination polymers (PCPs) formed from metal ions and ligands, and are known as gate-type porous polymer complexes or flexible porous polymer complexes. The unique gas adsorption behavior they exhibit is known as the gate phenomenon or gate adsorption.

The gate phenomenon occurs when a gate-type porous polymer complex used as an adsorbent undergoes a sudden change in gas adsorption, primarily due to a change in its structure. At low gas pressures, the gate-type porous polymer complex barely adsorbs gas. However, once the gas pressure reaches a certain value (called the adsorption initiation gate pressure), the structure of the gate-type porous polymer complex changes (e.g., the stacking structure shifts, the interlayer spaces widen, etc.), and gas molecules are incorporated into the structure. This results in a sudden increase in gas adsorption above the adsorption initiation gate pressure. This phenomenon is thought to occur because, below the adsorption initiation gate pressure, it is energetically more stable for the gate-type porous polymer complex and gas molecules to exist separately. However, above the adsorption initiation gate pressure, it is more energetically advantageous for the gas molecules to be incorporated into the gate-type porous polymer complex, forming a more stable inclusion complex than if the gate-type porous polymer complex and gas molecules existed separately.

The opposite phenomenon occurs with gas release. That is, when the gas pressure drops below a certain pressure (desorption initiation gate pressure), the gas molecules trapped in the gate-type porous polymer complex are released and attempt to return to the original gate-type porous polymer complex structure, resulting in a sudden release of gas. In other words, this gating phenomenon is based on the flexibility of the gate-type porous polymer complex structure; the gating phenomenon does not occur with existing porous materials such as zeolite and activated carbon, which do not possess this flexibility. The gating phenomenon is unique to gate-type porous polymer complexes.

When this gate phenomenon is applied to gas separation, there are two major advantages. The first advantage is highly efficient gas separation due to the rapid adsorption and desorption of gas. Existing porous materials exhibit adsorption isotherms in which the gas pressure is roughly proportional to the amount of adsorption. In order to recover the entire amount of gas adsorbed by such adsorbents, large pressure fluctuations are required. In contrast, with gate-type porous polymer complexes that exhibit the gate phenomenon, gas can be recovered by fluctuating the pressure across the gate pressure, so adsorbed gas can be recovered with very small pressure fluctuations (Figure 1).
Pressure fluctuations during adsorption gas recovery are directly linked to the cost of electricity used. In other words, the gate-type porous polymer complex, which can recover adsorbed gases with small pressure fluctuations, is a material that can separate gases at low cost.

The second benefit is its high gas separation efficiency. As mentioned above, gas adsorption by gate-type porous polymer complexes is not based on the simple mechanism of gas molecules being trapped in the pores of the adsorbent, as in conventional methods. Rather, it is based on a mechanism in which the gate-type porous polymer complex and the adsorbed gas molecules form stable inclusion complexes with the gate-type porous polymer complex when the gas pressure (adsorption onset gate pressure) is above a certain level. With existing porous adsorbents, even if two gases have different affinities, the presence of pores makes it easy for co-adsorption, in which the gas with lower affinity is also trapped within the pores. In contrast, gate-type porous polymer complexes have the property of selectively trapping gases with higher affinity, thereby forming more stable inclusion complexes, because the inclusion complex's stability decreases when gas molecules with lower affinity are also trapped due to co-adsorption. As a result, gate-type porous polymer complexes have high gas selectivity, and when applied to PSA systems, they can produce high-purity separated gases with high recovery rates and high efficiency.

However, when this gate-type porous polymer complex is actually applied to a PSA system, a problem known as slipping-off can occur, resulting in a decrease in the recovery rate of the adsorbed gas. Slipping-off refers to the phenomenon in which, when gas is injected into a PSA unit, the components to be adsorbed are not adsorbed by the gate-type porous polymer complex and instead flow out of the PSA unit. This phenomenon is due to the gate-type porous polymer complex's characteristic of barely adsorbing a gas when the partial pressure of the adsorbed gas is low (below the adsorption initiation gate pressure). Furthermore, even when the partial pressure of the adsorbed gas is higher than the adsorption initiation gate pressure, the recovery rate of the adsorbed gas may be low (approximately 0.4). This occurs because the partial pressure of the adsorbed gas is high (above the adsorption initiation gate pressure) near the gas inlet of the PSA unit, causing the gate adsorption phenomenon, but the adsorbed gas that is not completely adsorbed near the inlet flows toward the gas outlet. This unadsorbed gas flows out of the PSA unit without being adsorbed near the outlet because the partial pressure of the gas to be adsorbed is lowered by the amount of gas adsorbed near the inlet, falling below the adsorption start gate pressure. When this slipping-off phenomenon occurs, the gas that should have been adsorbed ends up leaking out, resulting in a lower recovery rate.

To solve this slipping-off phenomenon, Patent Document 2 proposes a PSA system in which two adsorption columns are connected in series. One of the two columns is filled with a gate-type porous polymer complex, and the other with an existing porous material. In this system, gas to be adsorbed leaks from the column filled with the gate-type porous polymer complex due to slipping-off, but is adsorbed by the column filled with the existing porous material, thereby preventing gas leakage.

This system has the following problems. The first is the high equipment cost. Because two adsorption columns are required, the equipment cost is higher than that of a typical PSA system that uses a single column for adsorption.

The second issue is the shortened useful life of equipment. The two types of adsorbents have different useful lives, and equipment replacement must be carried out according to the material with the shorter useful life, which requires more frequent replacement than PSA systems that use a single column for adsorption. Equipment replacement is costly because it requires the entire production system to be shut down, and frequent replacement directly leads to reduced productivity.

The third issue is the increased energy required to desorb gases adsorbed in existing porous materials. Slipping-off is a phenomenon in which low-concentration gases below the gate pressure at which adsorption begins leak out. For this reason, existing porous materials that capture gases must have strong adsorption power. Thus, a large amount of energy is consumed to desorb gases adsorbed from adsorbents with such strong adsorption power. As a result, the energy-saving feature of PSA systems using gate-type porous polymer complexes may be lost.
For this reason, a PSA system utilizing a gate-type porous polymer complex has not yet been put to practical use, and this problem remains unsolved.

Japanese Patent Application Laid-Open No. 2000-061244 Japanese Patent Application Laid-Open No. 2020-018995

The present invention aims to provide a gas separation method that reduces the slipping-off phenomenon in PSA systems using gated porous polymer complexes, improves gas recovery rates, and produces highly efficient, high-purity gas.

In order to solve the problems described above, the inventors investigated the following three methods to reduce the slipping-off phenomenon and improve gas recovery rates in PSA systems using gate-type porous polymer complexes.

The first method involves recovering the gas flowing out from the gas outlet of the adsorption column of the PSA system, returning it to the inlet side, and re-pressurizing it to adsorb it, thereby increasing the gas recovery rate.

A gas separation device using this method has an outlet gas recycling system, as shown in Figure 4. This stores the outlet gas discharged from the adsorption column in a tank, re-pressurizes it with a compressor, and feeds it back to the adsorption column inlet, thereby re-adsorbing the target gas. This prevents the target gas from leaking from the adsorption column outlet, making it possible to improve the recovery rate of the target gas.

The gas separation system is operated by repeating steps (1) to (4) shown in Figure 5. The dotted line indicates the magnitude of the gas partial pressure of the component to be adsorbed in the adsorption column. First, in step (1), the low-concentration gas stored in the tank in step (3) of the previous cycle is compressed into the adsorption column by a compressor. During this process, gas adsorption occurs in a portion of the column (near the inlet). Note that step (1) is not performed in the first cycle. Next, in step (2), the raw gas is compressed into the column. At this time, the system switches to step (3) just as gas begins to leak from the outlet due to slipping off. In step (3), this leaked gas is stored in the tank. The tank is replenished with the low-concentration gas used in step (1). When the amount of gas adsorbed in the tank reaches saturation, the system transitions to step (4). Here, the column is first depressurized to atmospheric pressure, and then a vacuum pump is used to reduce the gas pressure in the column to below the gate pressure, allowing the gas to be desorbed. Once gas desorption is complete, the system returns to step (1).
This is a method of circulating the effluent gas, but circulating it makes the entire system more complicated.

The second method consists of a PSA system consisting of two adsorption columns, the first and second, each containing a gate-type porous polymer complex. The first adsorption column is kept at room temperature or heated, and the second adsorption column is cooled, thereby appropriately controlling the gate pressure. This aims to increase adsorption capacity by cooling the gate material of the second column, thereby reducing the amount of gas that leaks out.

Gas separation equipment using this method has a system that reduces slipping-off through heating and cooling. Figure 6 shows an apparatus that connects a room-temperature adsorption column A to a low-temperature, high-pressure adsorption column B. The dotted lines indicate the magnitude of the partial pressure of the gas to be adsorbed within the columns, with the gate pressures for columns A and B being [Pgate, A] and [Pgate, B], respectively. In this case, the gate pressure of the gate-type porous polymer complex decreases at low temperatures, resulting in [Pgate, A] > [Pgate, B]. Gas above the adsorption initiation gate pressure within the column is adsorbed, so that gas to be adsorbed leaking from column A at a partial pressure lower than the gate pressure [Pgate, A] can be adsorbed in column B until the partial pressure of the gas to be adsorbed reaches [Pgate, B]. This reduces the amount of gas that slips off compared to PSA using only column A.

This method also includes a system that reduces gas leakage due to slipping-off through pressure control. In Figure 6, a compressor is installed between columns A and B, and the gas at the outlet of column A is repressurized and allowed to flow into column B, further reducing the amount of gas that slips off.

As explained in the aforementioned Patent Document 2, this method involves preparing two adsorption columns and filling them with adsorbents with different lifespans, which increases equipment costs and reduces the useful life of the entire equipment due to the different lifespans of the materials.

The third method is to increase the partial pressure of the gas to be adsorbed at the inlet of the PSA system column above the adsorption start gate pressure, thereby reducing the amount of gas that flows out relative to the total amount of gas that flows in. Using this method, the gas recovery rate unexpectedly increased.

The present invention is as follows.
(1) A gas separation method using a gas separation apparatus including at least an adsorption tower packed with a gas adsorbent containing a gate-type porous polymer complex and a gas supply means for supplying a mixed gas containing a plurality of gases to the adsorption tower,
A gas separation method characterized in that the ratio of the partial pressure of the gas to be adsorbed among the plurality of gases at the inlet of the adsorption tower to the adsorption start gate pressure of the gas to be adsorbed is greater than 1.3 and less than 400.
(2) The gas separation method according to (1), wherein the pressure difference between the adsorption completion gate pressure and the desorption completion gate pressure of the gate-type porous polymer complex is 2 kPa to 8000 kPa.
(3) The gas separation method according to (1) or (2), wherein the gate-type porous polymer complex is an ELM, a Kagome, an MIL, or a CID.

The gas separation method of the present invention solves the problem of reduced recovery rates of the adsorbed gas due to the slipping-off phenomenon that has traditionally been encountered in PSA systems using gated porous polymer complexes, achieving high efficiency, high purity, and high recovery rates of the adsorbed gas. Examples of gas separation devices according to the present invention include pressure swing adsorption (PSA) gas separation devices. Specifically, either vacuum swing adsorption (VSA) or pressure swing adsorption (PSA) can be suitably used.

1 is a graph showing the relationship between gas pressure and gas adsorption amount for an existing solid adsorbent and an adsorbent made of a gate-type porous polymer complex. FIG. 1 is a diagram showing the gate pressures of an adsorbent made of a gate-type porous polymer complex used in the present invention. FIG. 1 is a diagram illustrating gate coefficients using ELM-11 as an example. Diagram of a slip-off mitigation system using outlet gas reuse. Operation method of the system shown in Figure 4 Diagram of a slip-off mitigation system using heating and cooling. 1 is a conceptual diagram of a process operation simulating the gas separation method of the present invention. Adsorption isotherm equation for ELM-11 used in the examples. Adsorption and desorption isotherm model (1) for ELM-11 used in the simulation. The adsorption and desorption isothermal model of ELM-11 used in the simulation (2).

The gate-type porous polymer complex of the present invention is a material that exhibits a gate-type isotherm. A gate-type isotherm refers to an adsorption isotherm and desorption isotherm that show inflection points as shown in Figure 2, primarily due to structural changes in the porous polymer complex. As shown in Figure 2, the isotherm of a gate-type porous polymer complex is defined by the adsorption start gate pressure, adsorption completion gate pressure, gate adsorption amount, desorption start gate pressure, desorption completion gate pressure, and gate desorption amount.

The adsorption start gate pressure is the pressure at which the proportional relationship between the adsorption amount and pressure in the initial stage of adsorption in the adsorption process suddenly increases (the adsorption amount increment suddenly increases relative to the gas pressure increment). The adsorption completion gate pressure is the pressure at which the proportional relationship between the adsorption amount and pressure after the adsorption start gate pressure suddenly decreases (the desorption amount increment suddenly decreases relative to the gas pressure increment). The gate adsorption amount is the adsorption amount between the adsorption start gate pressure and the adsorption completion gate pressure. The desorption start gate pressure is the pressure at which the proportional relationship between the adsorption amount and pressure in the initial stage of desorption in the desorption process suddenly increases (the desorption amount increment suddenly increases relative to the gas pressure increment). The desorption completion gate pressure is the pressure at which the proportional relationship between the adsorption amount and pressure after the desorption start gate pressure suddenly decreases (the desorption amount increment suddenly decreases relative to the gas pressure increment). The gate desorption amount is the desorption amount from the desorption start gate pressure to the desorption completion gate pressure. Both the adsorption start gate pressure and desorption start gate pressure vary depending on the affinity between the adsorption material and the gas to be adsorbed; that is, the higher the affinity, the lower the pressure, and the lower the affinity, the higher the gate pressure. Also, at lower temperatures, affinity increases, so the gate pressure decreases. Therefore, the pressure cannot be determined uniquely as it varies depending on the adsorption material, gas type, and temperature.

Furthermore, the adsorption and desorption characteristics of the gate-type porous polymer complex can be defined by the following two equations:

Equation 1: Adsorption gate coefficient = (adsorption amount at adsorption completion gate pressure - adsorption amount at adsorption start gate pressure) / (adsorption completion gate pressure - adsorption start gate pressure)
Equation 2: Desorption gate coefficient = (adsorption amount at desorption start gate pressure - adsorption amount at desorption completion gate pressure) / (desorption start gate pressure - desorption completion gate pressure)
Note: The adsorption amount is the amount of adsorption of the gas to be measured at the measurement temperature, and is expressed in m(STP)L/g.
Note: Pressure is in kPa.

Currently known gate-type porous polymer complexes include ELMs (Elastic Layer-structured metal organic frameworks), Kagome complexes, MILs, CIDs, and others. The literature on these gate-type porous polymer complexes is as follows: ELMs are described in Ueshiro et al., Int. J. Mol. Sci. 2010, 11, 3803; Kagome complexes are described in Sato et al., SCIENCE (2014) 343, 167, and Zaworotko et al., Chem. Commun., 2004, 2534; MILs are described in Ferey et al., Chem. Soc. Rev., 2009, 38, 1380. CIDs are described in Inubushi et al., Chem. Commun., 2010, 46, 9229, and Nakagawa et al., Chem. Commun., 2010, 46, 4258. Other examples are described in Kitaura et al., Angew. Chem. Int. Ed. 2003, 42, 428. Of these, ELMs, Kagome compounds, MILs, and CIDs are preferred because they exhibit a clear gate phenomenon and are highly effective in the present invention.

The gate coefficient will be explained using Figure 3, using ELM-11 as an example. The carbon dioxide adsorption/desorption isotherm for ELM-11 at 273 K is shown in Figure 3. The x-axis is pressure (kPa) and the y-axis is adsorption amount (mL (STP)/g). In this case, each isotherm passes through the following points, and therefore these isotherms can be approximated by the following formula:

Adsorption isotherm before the adsorption gate:
Representative waypoint 1: x = 5.23, y = 0.188; waypoint 2: x = 27.4, y = 0.665, and the approximate equation is y = 0.022x + 0.0753.
Adsorption isotherm after adsorption gate:
Representative waypoint 3 : x = 34.2, y = 23.3; waypoint 4 : x = 35.2, y = 50.7, and the approximate equation is y = 27.4x - 913.

By solving the above simultaneous equations, x ("adsorption start gate pressure") = 33.4 kPa (the amount of adsorption before the gate is 1.1 mL (STP)/g).
Similarly, the "adsorption completion gate pressure" calculated from the isotherm was 36.0 kPa, and the amount of adsorption at that time was 74.4 mL.
From the above, the adsorption gate coefficient is calculated as follows: (77.4-1.1)/(36.0-33.4)=29.3.
Similarly, the desorption start gate pressure is 28.41 kPa (the adsorption amount is 75.2 mL (STP)/g), and the desorption completion gate pressure is 26.20 kPa (the adsorption amount is 2.71 mL (STP)/g).
From the above, the desorption gate coefficient is calculated as (75.2-2.71)/(28.41-26.20)=32.8.

The gate-type porous polymer complex that can be used in the gas separation method of the present invention preferably has an adsorption gate coefficient of 0.7 to 75 and a desorption gate coefficient of 0.4 to 80.
When the adsorption gate coefficient is less than 0.7, the variation in adsorption amount with respect to pressure fluctuation is too small, and no clear difference is obtained from general adsorbents, making it difficult to say that the behavior is gate-type. Furthermore, when the adsorption gate coefficient exceeds 75, the variation in adsorption amount with respect to pressure fluctuation is too steep, resulting in the rapid generation of adsorption heat and a sudden change in the concentration ratio of the mixed gas due to the rapid adsorption of a specific gas, making it difficult to practically use (control) as a gas separation material. When the desorption gate coefficient is less than 0.4, the variation in adsorption amount with respect to pressure fluctuation is too small, and no clear difference is obtained from general adsorbents, making it difficult to practically use (control) as a gas separation material. Furthermore, when the desorption gate coefficient exceeds 80, the variation in desorption amount with respect to pressure fluctuation is too steep, resulting in a sudden drop in temperature due to the rapid heat of desorption and a sudden change in the concentration ratio of the mixed gas due to the rapid release of a specific gas, making it difficult to practically use (control) as a gas separation material.

In the gate-type porous polymer complex that can be used in the gas separation method of the present invention, the pressure difference between the adsorption completion gate pressure and the desorption completion gate pressure is preferably 2 kPa to 8,000 kPa. If the pressure difference between the adsorption completion gate pressure and the desorption completion gate pressure is less than 2 kPa, the adsorption pressure and the desorption pressure are too close to each other and cannot be fully controlled by the PSA device. On the other hand, if the pressure difference exceeds 8,000 kPa, the adsorption pressure and the desorption pressure are too far apart, and recovering the gas adsorbed from such a material requires very large pressure fluctuations, which increases the power cost and operation time, eliminating the benefit of using the gating phenomenon for gas separation.
Particularly preferred gate-type porous polymer complexes that can be used in the gas separation method of the present invention include ELMs, Kagomes, MILs, and CIDs.

These various gate-type porous polymer complexes each have their own specific adsorption initiation gate pressure, which can vary with temperature. However, since slipping-off occurs when the partial pressure of the gas to be adsorbed within the adsorption tower drops below the adsorption initiation gate pressure, slipping-off can be prevented by setting the partial pressure of the gas to be adsorbed at a certain level higher than the adsorption initiation gate pressure. In other words, even for materials with different adsorption initiation gate pressures, excellent recovery rates for the gas to be adsorbed can be achieved as long as the ratio of the partial pressure of the gas to be adsorbed at the inlet of the adsorption tower to the adsorption initiation gate pressure is greater than 1.3 and less than 400.

Devices packed with the gate-type porous polymer complex of the present invention can be used to separate a variety of gases. Examples of gas species include oxygen, nitrogen, argon, carbon dioxide, carbon monoxide, hydrogen, alkanes, alkenes, and alkynes. When a device packed with the gate-type porous polymer complex of the present invention is used for gas separation, it is possible to easily separate high-purity gases with high recovery rates and high efficiency by utilizing the gate phenomenon.

In a gas separation device filled with the gate-type porous polymer complex of the present invention, the container shape, container material, and type of gas valve do not need to be particularly special equipment; those used in gas separation devices can be used. However, this does not exclude improvements to various devices, and any device used is within the technical scope of the present invention as long as it uses the gas polymer metal complex of the present invention.

The gas separation method using a gas separation apparatus packed with a gate-type porous polymer complex according to the present invention is a separation method in which the ratio of the partial pressure of the gas to be adsorbed at the inlet of the adsorption tower to the adsorption initiation gate pressure of the gas to be adsorbed among a plurality of gases is greater than 1.3 and not greater than 400. If the partial pressure ratio of the gas to be adsorbed at the inlet to the adsorption initiation gate pressure is less than 1.3, even if gate adsorption occurs, gas separation cannot be performed due to slipping-off in the adsorption tower. On the other hand, if it exceeds 400, the mixed gas must be pressurized very high, resulting in high power and equipment costs.
In the gas separation method of the present invention, the specific means for making the ratio of the partial pressure of the gas to be adsorbed at the inlet of the adsorption tower to the adsorption start gate pressure of the gas to be adsorbed greater than 1.3 can be any means commonly used for controlling the gas pressure in an adsorption tower.

Table 1 below shows the ratio of adsorbed gas partial pressure to the adsorption initiation gate pressure, and the change in gas recovery rate. This is the result of simulating a PSA system using an ELM-11, supplying a gas mixture of 20% carbon dioxide and 80% nitrogen at a temperature of 25°C. The adsorption isotherm model used was that proposed by Hefti et al. (Faraday Discuss (2016), pp. 153-179, Adsorption 20, 359-371 (2014)), and the dynamic model used was that used by Ko et al. (Ind. Eng. Chem. Res. 2005, 44, 21, 8084-8094).

In the case of a mixture of carbon dioxide and other gases, it has been reported in many publications that ELM-11 does not adsorb these gases, even if the gases other than carbon dioxide are one or more of oxygen, argon, helium, hydrogen, and methane. Therefore, when considering the separation of these gases from carbon dioxide, the same results can be obtained as when separating a mixture of carbon dioxide and nitrogen.

The ratios of adsorbed _CO2 partial pressure to adsorption initiation gate pressure for Examples A to G in Table 1 were 4.87, 2.43, 1.22, 5.73, 1.72, 4.65, and 1.55, respectively, and the corresponding recovery rates were 0.863, 0.710, 0.337, 0.890, 0.574, 0.853, and 0.498, respectively. These results confirm that the recovery rate increases as the ratio of adsorbed gas partial pressure to adsorption initiation gate pressure increases. Example C was selected as a comparative example because its recovery rate was below 0.4.

(Note)
Capture rate: The ratio of the amount of _CO2 captured to the amount of _CO2 input, expressed by the following formula:
Note that u [m/s] is the flow velocity at the inlet, Pi is the partial pressure of component i at the inlet [Pa],
R is the gas constant [J/(kg·K)], T is the temperature at the inlet [K], the feed step is the process of introducing the raw gas into the column, and the exhaust step is the process of extracting _CO2 from the column.
Purity: The purity of the recovered _CO2 gas, expressed by the following formula:
BSF: An index showing the reciprocal of the amount of _CO2 that can be treated per unit amount of adsorbent, and is expressed by the following formula:
where m _PCP is the amount of adsorbent used [kg], and t _cycle is the time [days] per PSA cycle.
Power Consumption: The amount of energy used to capture a unit of _CO2 , expressed as follows:
Here, P _in is the total pressure at the column inlet [Pa], P _atm is the atmospheric pressure [Pa], γ is the specific heat ratio [-], and R _bed is the column radius [m].

Table 2 shows the results of simulating a PSA system supplying a mixture of 40% carbon dioxide and 60% nitrogen at a temperature of 25°C.

As can be seen from Table 1, in Example A (invention example), under the condition of an inlet adsorption CO2 _partial pressure of 2.0 [ ¹⁰⁵ Pa] (inlet adsorption CO2 _partial pressure/adsorption start gate pressure = 4.87), it was found that the recovery rate was improved by 2.56 times compared to Example C (comparison example) without a significant decrease in the purity of the recovered gas. In addition, the BSF (the amount of material required to obtain a certain amount of _CO2 recovery) was also reduced by nearly 80%. In other words, it was confirmed that a large amount of gas could be recovered and separation efficiency was significantly improved. This indicates that slipping-off was reduced.

Gas separation, particularly _CO2 separation, is often expected to be carried out on a scale of several million tons per year. This means that separation costs are high, the equipment is large, and the site area required is large. For example, the separation cost for chemical absorption using an amine solution is said to be 5,000 yen/ton- _CO2 , but the cost of separating 1 million tons would be as much as 5 billion yen. Therefore, if the separation efficiency of _CO2 separation can be improved, significant cost reductions are expected, and if the equipment is made smaller, significant site area reductions are expected. This technology dramatically improves recovery rates and BSF, as shown in Table 1 above, and can be said to contribute to cost reductions and site area reductions.

The simulation results shown in Table 1 were calculated as follows.
Example A (invention example) Inlet adsorption CO ₂ partial pressure 4.00 [10 ⁵ Pa]
In this example, a lab-scale process using a single column apparatus is assumed. The column length is 1 m, and the column tube radius is 1.1 cm. The pellet diameter is 3 mm, and the pellets are packed to give a porosity of 34.8%. The pellets are ELM-11 supported on cellulose, with a weight ratio of 3:1, and the weight of ELM-11 is 0.293 kg. The temperature outside the tube is 298.15 K.

The process procedure is shown in Figure 7. First, in the (a) pressurization process, the column outlet is closed and exhaust gas is injected through the inlet. The inlet pressure is gradually increased from 0.25 [10 ⁵ Pa] to 20 [10 ⁵ Pa] over 40 seconds. The temperature of the inflowing gas is 298.15 K, and the molar fraction is 20% CO ₂ and 80% N ₂ . This process continues for 40 seconds. Next, in the (b) adsorption process, the column outlet is opened and gas continues to be injected through the inlet. The gas temperature and molar fraction are the same as in (a), and the gas flow rate is 0.302 m/s. This process continues for 84.4 seconds. In the (c) depressurization process, the column outlet is closed, and the column is depressurized and CO ₂ gas is recovered through the inlet. The inlet pressure is gradually reduced to 0.25 [10 ⁵ Pa] over 120 seconds. This process continues for 120 seconds. (d) During the desorption process, the pressure is maintained at 0.25 [10 ⁵ Pa] and CO ₂ gas is collected from the inlet. This process continues for 250 seconds. The simulation results are obtained by repeating the above operations (a) to (d) for 10 cycles.

Example B (invention example) Inlet adsorption CO ₂ partial pressure 2.00 [10 ⁵ Pa]
In this example, the process was the same as in Example A, but with the following two changes to the operating conditions: The first change was the CO2 _partial pressure during the pressurization and adsorption process, which was changed from 4.00 [ ¹⁰⁵ Pa] to 2.00 [ ¹⁰⁵ Pa], and the second change was the time required for the adsorption process, which was changed from 84.4 seconds to 169 seconds.

Example C (Comparative Example) Inlet adsorption CO ₂ partial pressure 1.00 [10 ⁵ Pa]
Like Example B, this example is the same process as Example A, but with modified operating conditions. The inlet adsorption CO ₂ partial pressure during the pressurization and adsorption process was changed from 4.00 [10 ⁵ Pa] to 1.00 [10 ⁵ Pa].

In Examples D to M, similar to Examples A, B, and C, simulations were performed by changing the inlet adsorption CO ₂ partial pressure and the temperature of the raw material gas.
The parameters used in the example are summarized in Table 3.

The adsorption isotherm model used in the above simulation was that proposed by Joss et al. (Energy Procedia (2017), 114, 2271-2278). This method is described below. _CO2 adsorption by ELM-11 consists of two processes: adsorption around the framework and adsorption within the framework. Here, we assume two hypothetical adsorption isotherms that represent these adsorption behaviors. Specifically, _nL denotes the adsorption isotherm when all gates are closed regardless of pressure, and _nU denotes the adsorption isotherm when all gates are open regardless of pressure. If we assume that the number of open gates increases depending on the _CO2 partial pressure when ELM-11 performs gate adsorption, the actual adsorption/desorption isotherm should lie between _nU and _nL . Under this assumption, the adsorption/desorption isotherm for ELM-11 is expressed as Equation 1 (Figure 8). Note that _wj is a weight that ranges from 0 to 1 and indicates the proportion of open gates relative to the total number of gates.

Here, j indicates whether the process is adsorption or desorption. _nU and _nL are expressed by the Langmuir equations shown in Equations 2 and 3.

The weighting function w _j is expressed by Equation 4.

X _j and P _step,j in Equation 4 are expressed by Equations 5 and 6, respectively.

Here, P _step,j indicates the gate pressure in adsorption/desorption, and X _j is a parameter that determines the pressure width when the weight function w _j rises.

Furthermore, modeling of changes in adsorption/desorption isotherms due to hysteresis caused by pressure operation conditions was performed with reference to the following paper (Hefti et al. Adsorption 20.2-3 (2014): 359-371). In this model, as shown in Figure 9, the isotherm after switching is a scaled-down version of the isotherm shown in Equation 1, calculated as the ratio of the adsorption amount at operation switching to the saturated adsorption amount n _sat (n _switch) . Specifically, if the adsorption amount when switching from desorption to adsorption is defined as n _ads→des and the adsorption amount when switching from adsorption to desorption is defined as n _des→ads , these can be expressed by Equations 7 and 8, respectively.

The dynamic model used in the simulation was the same as that used by Ko et al. for the _CO2 capture process using zeolite 13X (Ind. Eng. Chem. Res. 2005, 44, 21, 8084-8094), except that the driving force q ^* in the linear driving force model, which determines the direction and rate of adsorption and desorption, is expressed as Equation 9 in this study.

The mode in Equation 9 is a binary variable that is 1 during the pressurization/adsorption process and 0 during the depressurization/desorption process. This is expressed in Equation 10.

The process used to carry out the above simulation is shown below.
1. The gas is an ideal gas.
2. Temperature, pressure, and concentration changes in the pipe diameter direction are not taken into consideration.
3. The adsorption rate is calculated using the linear driving force (LDF) model.
4. The physical properties of the adsorbent, such as its density, do not change with temperature.
5. Pressure loss follows the Elgin formula.
6. The change in flow rate can be calculated using the total mass balance equation.

Claims (4)

A gas separation method using a gas separation apparatus including at least an adsorption tower packed with a gas adsorbent containing a gate-type porous polymer complex and a gas supply means for supplying a mixed gas containing a plurality of gases to the adsorption tower, the method comprising the steps of a pressurization step, an adsorption step, a depressurization step, and a desorption step in this order :
In the pressurization process, a mixed gas containing the plurality of gases is injected into the adsorption tower through an inlet of the adsorption tower with the outlet of the adsorption tower closed, and the pressure at the inlet of the adsorption tower is increased until a partial pressure of the gas to be adsorbed among the plurality of gases at the inlet of the adsorption tower reaches a predetermined pressure that is higher than an adsorption start gate pressure of the gas to be adsorbed among the plurality of gases;
In the adsorption process, the outlet of the adsorption tower is opened, and the mixed gas containing the plurality of gases is continuously injected into the inlet of the adsorption tower at the predetermined pressure, thereby adsorbing the adsorption target gas among the plurality of gases onto the gas adsorbent;
In the depressurization step, the pressure in the adsorption tower is reduced, and in the desorption step, the reduced pressure state is maintained, and the adsorption target gas adsorbed by the gas adsorbent is recovered;
A gas separation method characterized in that the ratio of the partial pressure of the gas to be adsorbed among the plurality of gases at the inlet of the adsorption tower to the adsorption start gate pressure of the gas to be adsorbed is greater than 1.3 and less than 400. The gas separation method described in claim 1, characterized in that the pressure difference between the adsorption completion gate pressure and the desorption completion gate pressure of the gate-type porous polymer complex is 2 kPa to 8000 kPa. The gas separation method according to claim 1 or 2, characterized in that the gate-type porous polymer complex is an ELM, Kagome, MIL, or CID. 4. The gas separation method according to claim 1, wherein the pressurization step, the adsorption step, the depressurization step, and the desorption step are repeated.