JPH0461684B2 - - Google Patents

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention uses a mixed gas such as air whose main components are nitrogen and oxygen as a raw material, and uses an adsorption bed made of molecular sieve charcoal (hereinafter referred to as MSC). It relates to a method for inexpensively separating nitrogen with a purity of 99.99% or higher (nitrogen purity is the volume % of the total of nitrogen and argon, the same applies hereinafter) using a pressure fluctuation adsorption method (hereinafter referred to as the PSA method).

(Prior art) The method of separating nitrogen from a mixed gas such as air using the PSA method using MSC as an adsorbent is widely known. It has been put into practical use as a means of supply.

The equilibrium adsorption amount of oxygen and nitrogen on MSC is
If the temperature and adsorption pressure are specified, the values are almost the same, but there is a noticeable difference in the adsorption rate in the initial stage, and this characteristic is used when separating nitrogen gas using the PSA method using MSC. In other words, oxygen, moisture, etc. are adsorbed by MSCs to a region close to equilibrium,
However, the adsorption operation is normally completed within a time of around 60 seconds, when almost no components such as nitrogen and argon are adsorbed.

The nitrogen purity of the resulting product is generally considered to be 99.0% when the adsorption bed is regenerated under normal pressure, and 99.9% when the adsorption bed is regenerated under vacuum. There is no PSA type nitrogen gas separation tank with a floor on the market.

Therefore, if higher purity nitrogen is required, there is a method in which the remaining 0.1 to 1.0% by volume of oxygen is reacted with hydrogen or ammonia using a catalyst, and the resulting water is removed, or the method is to react with the metal to be oxidized. It is customary to combine it with a purification device that utilizes a contact removal method. However, when adopting such purification equipment, in addition to using highly dangerous gases for reaction or regeneration as described above, there are many other problems such as the contamination of unreacted gas and a significant increase in equipment and operating costs. issues remain.

(Problems to be Solved by the Invention) Existing high-purity nitrogen production methods, such as the cryogenic separation method, generally require large-scale equipment, which requires a large amount of investment, and requires close proximity to the place of consumption. It is unlikely that the product can be installed in a similar manner, and the price of the product becomes extremely high when the cost of transportation is taken into account. In addition, the PSA nitrogen gas separation method, which uses synthetic zeolite as an adsorption bed that takes advantage of the difference in equilibrium adsorption capacity between nitrogen and oxygen, requires an extremely complex mechanism, which significantly increases the proportion of fixed costs in the product price. do.

On the other hand, MSC, which has a relatively simple mechanism and can easily obtain nitrogen at low cost, was used as an adsorption bed.
The PSA nitrogen gas separation method has application limitations in terms of nitrogen purity, and as mentioned above, there are many problems when a purification device is added to the nitrogen gas separation device, making it difficult to supply inexpensive high-purity nitrogen. Development of a method was awaited.

(Means for solving the problem) The inventors of the present invention have developed a PSA method using MSC as an adsorption bed to absorb nitrogen from a mixed gas using air etc. as a raw material.
As a result of intensive research into the possibility of inexpensive separation with a purity of 99.99% or higher, we came up with the following method.

In other words, the nitrogen adsorption method is a pressure fluctuation adsorption method that uses a pressurized mixed gas consisting of 78% by volume or more of nitrogen and a residual component mainly composed of oxygen as a raw material, and consists of two adsorption tanks filled with molecular sieve coal. In the gas separation equipment, a. The raw material gas inlets and product nitrogen outlets of both adsorption tank A, which has completed adsorption operation, and adsorption tank B, which has completed vacuum regeneration operation, are communicated with each other, and adsorption under pressure is carried out. A process of moving the gas in the tank from tank A to adsorption tank B placed in a reduced pressure state in a short time, and stopping the gas movement before both tanks reach equal pressure; The nitrogen outlet and the inlet of the product storage tank are connected to allow the product nitrogen to flow back into the adsorption tank B, and when the gauge pressure in the adsorption tank B reaches 50% of the maximum gauge pressure during adsorption operation, the raw material mixed gas is removed. The step of introducing the nitrogen gas into the adsorption tank B and starting the adsorption operation, c. However, the gauge pressure in the product storage tank is always maintained, including the period when the product nitrogen gas is flowing back into the adsorption tank B in the step b) above. Maintain at least 70% of the maximum gauge pressure in the adsorption tank during adsorption operation, dMeanwhile, in adsorption tank A, from the time when gas transfer between the adsorption tanks in step a) is stopped, the residual gas in the tank is brought to near atmospheric pressure. The regeneration operation is performed by releasing the gas into the outside air until the temperature reaches 150 Torr, and then reducing the pressure to below 150 Torr using a vacuum pump. From then on, adsorption tanks A and B alternately repeat both adsorption and regeneration operations.
The time required for the regeneration operation and the adsorption operation including the product nitrogen gas backflow process is 90 seconds to 180 seconds each.
Within seconds, the nitrogen purity is 99.99% by volume
The above is a nitrogen gas separation method for obtaining the product.
The above-mentioned items are essential conditions for achieving the purpose, and the present invention can only be achieved by satisfying all of them.

Hereinafter, the present invention will be explained in more detail using FIG. 1.

Using air or a mixed gas mainly composed of nitrogen and oxygen, such as recovered nitrogen gas mixed with air,
In order to continuously obtain nitrogen with a purity of 99.99% or higher using the PSA method using MSC as an adsorption bed, a minimum of two adsorption tanks are required.

The pressurized mixed gas enters the adsorption tank (A) 1 via valve 5, and components to be removed such as oxygen are adsorbed by the MSC filled in the tank, and the separated product nitrogen is passed through valve 9. The nitrogen is temporarily stored in the product storage tank 3 and then consumed, but the pressure within this adsorption tank affects the purity of the product nitrogen.

Although it varies depending on the characteristics of the MSC, the experimental results of the present inventors show that an ultimate pressure of at least 5 kg/cm ² G is required for practical use, and when the pressure reaches a region of 10 kg/cm ² G or more, the pressure decreases. The results will be almost unrecognizable. Therefore, the range of 6 to 10 kg/cm ² G is more preferable. However, pressure fluctuations in the supplied mixed gas must be avoided as much as possible.

Especially when separating high purity nitrogen of 99.99% or more,
Desorption of oxygen etc. adsorbed by a small amount of MSC, incomplete adsorption separation due to pressure difference within the adsorption tank, contamination of remaining low-purity nitrogen, etc. become serious inhibiting factors.

Valves 5 and 9 are closed, and adsorption tank A1, in which the adsorption operation has been completed, and adsorption tank B2, in which the vacuum regeneration operation has been completed, are connected to each other by opening valve 11 to communicate the product nitrogen outlet sections of both tanks (hereinafter referred to as upper communication). At the same time, by opening the valve 12 and communicating the raw material gas inlets with each other (hereinafter referred to as lower communication), the adsorption tank A1 under pressure is transferred from the adsorption tank A1 under pressure to the adsorption tank B2 under reduced pressure. The remaining gas with a high nitrogen content in the tank is moved to improve the raw material gas consumption rate.

In this operation, the amount of gas moving through the upper communication section must exceed the amount of gas moving through the lower communication section, and the flow rate of gas moving through the lower communication section is 3 to 30% of the flow rate of gas moving through the upper communication section. These flow rates are preferably adjusted by orifices 15 and 16.

Moreover, this gas movement must be stopped before both adsorption tanks become equal pressure. The reason is not clear, but it seems to be because the amount of components adsorbed to MSCs that are desorbed increases rapidly. When valves 11 and 12 are closed to stop gas movement, the internal pressure of adsorption tank B2 on the receiving side is 0.10 to 0.70 in absolute pressure ratio, more preferably 0.15 with respect to adsorption tank A1 on the side supplying stored gas. This is when the range of ~0.40 is reached. Note that the radius of the orifices 15 and 16 is adjusted so that the time required for the gas movement is stopped within 2 seconds.

Next, the valve 10 is opened to allow the product nitrogen to flow back from the product storage tank 3 to the adsorption tank B2. This backflow operation has traditionally been used to wash out low-purity nitrogen that has accumulated in piping connected to the upper part of the adsorption tank due to pressure equalization between the adsorption tanks, but it causes pressure fluctuations in the product tank. For this reason, the amount of backflow is usually not so large, and it is common to carry out backflow operation and supply of raw material mixed gas at the same time.

The purpose of the backflow operation in the present invention is to prevent the generation of insufficiently separated nitrogen gas due to the large pressure difference between the internal pressure of the adsorption tank and the supplied raw material mixed gas at the start of the adsorption operation, which is equivalent to or more effective than the above-mentioned washing effect. have. Therefore, due to the backflow operation, the gauge pressure in the adsorption tank B2 becomes lower than the maximum gauge pressure reached during the adsorption operation.
Only when the concentration reaches 50% or more, preferably 70% or more, valve 7 is opened and the raw material mixed gas is transferred to adsorption tank B.
2 and start the adsorption operation.

However, while a certain amount of product nitrogen gas flows out from the product storage tank 3 due to consumption, even during the period when the backflow operation is being performed to the adsorption tank B2, the product storage tank 3
The volume of the product storage tank 3 must be selected so that the gauge pressure within the tank is always maintained at 70% or more, more preferably 80% or more, of the maximum gauge pressure reached by the adsorption tank during adsorption operation.

On the other hand, in the adsorption tank A1, valves 6 and 13 are opened when the gas movement between the adsorption tanks is stopped, and the adsorbed and remaining gas in the tank is released to the outside air to near atmospheric pressure. Close the valve 14 and use the vacuum pump 4 to reduce the pressure to 150 Torr or less, preferably 50 to
The pressure is reduced to a range of 100 Torr, and a vacuum regeneration operation is performed until the adsorption operation in adsorption tank B2 is completed.
After that, adsorption tank A1 and adsorption tank B2 alternately repeat adsorption and regeneration operations with a phase difference of 180°.
The optimal time to perform the regeneration operation or the adsorption operation including the product nitrogen gas backflow process, that is, the half cycle time, is 60 to 90 seconds when using the same MSC and using the conventional method with a product nitrogen gas purity of 99.0 to 99.9%. However, in the method of the present invention, it is necessary to shift to a range of 90 to 180 seconds.

Space velocity is an important dynamic index that represents the efficiency of a PSA nitrogen gas separation system. Space velocity refers to the capacity of the MSC that is used for constant adsorption operation, i.e., the capacity of the MSC that is used for constant adsorption operation, i.e., the capacity of the MSC filled in one adsorption tank in the present invention.
This is the value divided by the MSC capacity, and the performance of the MSC itself also affects this characteristic.

Therefore, when the space velocity takes a small value, the necessary internal volume of the adsorption tank becomes large, and the amount of gas released to the outside air increases accordingly, so that the basic unit of raw material mixed gas becomes high. This phenomenon means that in addition to the adsorption tank, the entire equipment such as the compressor for raw material mixed gas and the vacuum pump becomes larger. This increases not only the equipment cost but also the operating cost, mainly electricity cost, which makes it difficult to put it into practical use. It becomes a fatal inhibiting factor.

The space velocity in the method of the present invention is assumed to be 0.6 min ^-1 or more, more preferably 1.2 min ^-1 or more, and equipment with a capacity range of 1 to 1000 Nm ³ /Hr is installed in the necessary location to achieve a high speed of 99.99% or more. It has become possible to provide pure nitrogen at low cost.

Example Equilibrium adsorption amount of oxygen 7.7cc/g at 25℃ and 1 atm.
Oxygen/nitrogen adsorption capacity ratio at 60 seconds adsorption time: 8.44
An adsorption tank was filled with MSC having the following performance, and a PSA nitrogen gas separation device as shown in the drawing was used.

Air pressurized to 8.5 kg/cm ² G was used as a raw material, and adsorption and regeneration operations were performed in a half cycle time of 120 seconds.
The ultimate vacuum level during regeneration operation is 80 Torr. For gas movement between the adsorption tank that has completed the adsorption operation and the adsorption tank that has completed the vacuum regeneration operation, adjust the lower communication flow rate to 6% of the upper communication flow rate, and transfer the gas to the adsorption tank on the gas supply side. Gas movement was stopped when the absolute pressure ratio of the tank reached 0.25. The time required for this gas movement was 1.2 seconds.

Next, product nitrogen was flowed back from the product storage tank to the receiving adsorption tank for 0.2 seconds, and when the pressure reached 85% of the maximum gauge pressure reached during the adsorption operation, supply of pressurized raw material air was started.

While the product storage tank continued to supply 10Nm ³ /Hr to external equipment, it was possible to always maintain 85% or more of the maximum gauge pressure reached in the adsorption tank during adsorption operation, even during the backflow process. The product storage tank used has a capacity four times that of the adsorption tank.

The quality of the product nitrogen obtained in this example was such that a residual oxygen concentration of 73 ppm and an atmospheric pressure dew point of −70° C. or less were stably obtained, and the space velocity at this time was 1.5 min ⁻¹ .

Furthermore, when the supply amount from the product storage tank to external equipment was reduced to 5Nm ³ /Hr using this device, the residual oxygen concentration in the product nitrogen decreased to 5.7ppm.

[Brief explanation of drawings]

FIG. 1 is a flow sheet showing an example of the present invention. In the figure, 1 and 2 are adsorption tanks A and B, respectively, 3 is a product storage tank, 4 is a vacuum pump, 5, 6, 7, 8,
9, 10, 11, 12, 13, 14 are valves, 15,
16 indicates an orifice.

Claims (1)

[Scope of Claims] 1. Pressure fluctuation consisting of two adsorption tanks filled with molecular sieve charcoal and using a pressurized mixed gas as a raw material consisting of 78% by volume or more of nitrogen and residual components mainly consisting of oxygen. In an adsorption type nitrogen gas separation device, a. The raw material gas inlets and product nitrogen outlets of both adsorption tank A, which has completed adsorption operation, and adsorption tank B, which has completed vacuum regeneration operation, are connected to each other, a step of moving gas in the tank from adsorption tank A under pressure to adsorption tank B under reduced pressure in a short time, and stopping the gas movement before both tanks reach equal pressure; The product nitrogen outlet of tank B and the product storage tank inlet are communicated to cause the product nitrogen to flow back into adsorption tank B, and when the gauge pressure in adsorption tank B reaches 50% or more of the maximum gauge pressure during adsorption operation. The process of introducing the raw material mixed gas into the adsorption tank B to start the adsorption operation; c. However, in the step b) above, the product storage tank The gauge pressure in the tank is always maintained at 70% or more of the maximum gauge pressure in the adsorption tank during adsorption operation. The residual gas is discharged into the outside air to near atmospheric pressure, and then a vacuum pump is used to reduce the pressure to below 150 Torr to perform a regeneration operation.
and B repeat both adsorption and regeneration operations alternately, but the time required for the regeneration operation and adsorption operation including the product nitrogen gas backflow step is 90 seconds or more, respectively.
Within 180 seconds, the nitrogen purity is 99.99% by volume
Nitrogen gas separation method to obtain the above products. 2. The nitrogen gas separation method according to claim 1, wherein the maximum pressure achieved when adsorbing and separating the raw material mixed gas is 5 kg/cm ² G or more. 3. The nitrogen gas separation method according to claim 1 or 2, wherein the space velocity is 0.6 min ^-1 or more.