JP7808966B2 - Method for operating nitrogen gas production equipment - Google Patents

Description

The present invention relates to a method for operating a nitrogen gas production apparatus.

The pressure swing adsorption gas separation method separates the easily adsorbed and less easily adsorbed components in a raw mixed gas by supplying the raw mixed gas to an adsorption tower that repeatedly undergoes adsorption and regeneration processes and allowing the easily adsorbed components to be adsorbed onto various adsorbents packed into the adsorption tower. For example, a widely used system uses molecular sieve carbon as the adsorbent to produce nitrogen gas from air using the pressure swing adsorption gas separation method (nitrogen gas production system, hereafter simply referred to as a "nitrogen PSA system").

A known characteristic of nitrogen PSA units is that when the flow rate (product flow rate) of the product nitrogen (product gas) is high, the purity of the product nitrogen (product purity) decreases, and when the flow rate of the product nitrogen is low, the purity of the product nitrogen increases. Furthermore, nitrogen PSA units are designed to be able to supply product nitrogen of the required purity at the required flow rate during rated operation. Therefore, when the required flow rate of product nitrogen is low, the nitrogen PSA unit will supply product nitrogen of excessive purity.

Patent Document 1 therefore proposes an operating method for a nitrogen gas production system in which, when the actual flow rate (supply amount) of product nitrogen during rated operation (design value) becomes lower, operation of the nitrogen PSA unit is stopped during the pressure equalization process carried out between multiple adsorption towers between the adsorption process and the regeneration process, and product nitrogen is supplied at the internal pressure of the product tank.

In the operating method of a nitrogen gas production system disclosed in Patent Document 1, during the pressure equalization process, when the operation of the nitrogen PSA unit is stopped (hereinafter simply referred to as the "standby process"), the nitrogen PSA unit does not require compressed air from the compressor, and the compressor motor is placed in an unloaded state (unloaded operation), thereby reducing power consumption. In other words, the lower the flow rate (supply amount) of product nitrogen, the longer the time that product nitrogen can be supplied using only the internal pressure of the product tank, resulting in significant energy savings.

Patent No. 3781187

However, the operating method of the nitrogen gas production apparatus disclosed in Patent Document 1 has the problem that, due to the characteristics of PSA equipment, the product purity in the adsorption tower during the adsorption process increases when the product flow rate is low, and therefore there is a tendency to supply product gas of higher purity than during rated operation, which produces product gas meeting the required specifications. There is also a need for an operating method of a nitrogen gas production apparatus that can further reduce power consumption than conventional methods.

The present invention was made in consideration of the above circumstances, and its objective is to provide a method for operating a nitrogen gas production system that can produce product gas of the required purity and reduce power consumption.

In order to achieve the above object, the present invention employs the following configuration.
[1] A method for operating a nitrogen gas production apparatus having a plurality of adsorption towers filled with adsorbents, the adsorption towers repeatedly performing at least an adsorption step, a pressure equalization step, and a regeneration step to separate nitrogen from compressed air as a raw material gas and supply the nitrogen as a product gas,
extending the adsorption step in response to a decrease in the product gas extraction flow rate;
A method for operating a nitrogen gas production apparatus, comprising, after completion of the adsorption step, providing a halt step of stopping the supply of raw material gas to the adsorption tower and the withdrawal of product gas from the adsorption tower.
[2] The method for operating the nitrogen gas production apparatus according to [1], wherein the compressed air is supplied using an inverter air compressor.

The method for operating a nitrogen gas production system of the present invention enables the production of product gas of the required purity and reduces power consumption.

1 is a system diagram schematically illustrating a nitrogen gas production apparatus to which a method of operating a nitrogen gas production apparatus according to an embodiment of the present invention is applied. 3 is a diagram illustrating the open/closed state of each switching valve during rated operation in the method for operating the nitrogen gas production apparatus of the present embodiment. FIG. 3 is a diagram illustrating the open/closed state of each switching valve during a reduction operation in the method for operating the nitrogen gas production apparatus of the present embodiment. FIG. FIG. 2 is a diagram showing a time chart of each adsorption tower during rated operation in the method of operating the nitrogen gas production apparatus of the present embodiment. FIG. 2 is a diagram showing a time chart of each adsorption tower during a decrease operation in the method of operating the nitrogen gas production apparatus of the present embodiment.

The following describes in detail a method for operating a nitrogen gas production apparatus, which is one embodiment of the present invention, with reference to the drawings. Note that the drawings used in the following description may show characteristic parts enlarged for ease of understanding, and the dimensional proportions of each component may not necessarily be the same as in reality.

First, a method for operating a nitrogen gas production system, which is one embodiment of the present invention, will be described together with the nitrogen gas production system used therein, with reference to Figure 1. Figure 1 is a system diagram showing a nitrogen gas production system that can be used in the method for operating a nitrogen gas production system, which is one embodiment of the present invention.

<Nitrogen gas production equipment>
As shown in FIG. 1, a nitrogen gas production apparatus (hereinafter also simply referred to as a "nitrogen PSA apparatus") 1 applicable to the method of operating a nitrogen gas production apparatus of this embodiment is a PSA apparatus that produces nitrogen from air.
The nitrogen PSA unit 1 is generally configured to include an air compressor 2 for supplying raw air, two adsorption towers 3A and 3B, a product tank 4 for storing the product nitrogen gas extracted from the adsorption towers 3A and 3B, and a plurality of switching valves AV1A to AV5A, AV1B to AV5B, and AV6 for switching the adsorption towers 3A and 3B between the adsorption process, the pressure equalization process, and the regeneration process. Note that check valves can be used for the switching valves AV2A and AV2B.

The air compressor 2 is preferably an inverter-type air compressor in which the motor rotation speed can be controlled by an inverter according to the pressure, etc. This allows the motor rotation speed to be controlled by varying the operating frequency, and compressed air can be supplied to the adsorption towers 3A and 3B while maintaining constant pressure control so that the pressure in the adsorption towers 3A and 3B approaches the set pressure (so that it remains within the target pressure) during the adsorption process.

Adsorption towers 3A and 3B are filled with adsorbents such as molecular sieve carbon, which preferentially adsorbs oxygen from air that is primarily composed of oxygen and nitrogen.

When the operation of the nitrogen PSA system 1 includes a purge operation during the regeneration process and the purge operation overlaps with the shutdown process of the nitrogen PSA system 1 of this embodiment, the switch valve AV6 is used to prevent purge gas from leaking from the adsorption tower on the purge gas supply side, causing a drop in pressure in the adsorption tower. Closing the switch valve AV6 allows the adsorption tower to be completely isolated.

Furthermore, in the nitrogen PSA system 1, during the shutdown process described below, the product nitrogen gas in the product tank 4 continues to be continuously supplied to the user, so the pressure in the product tank 4 gradually decreases. However, when molecular sieve carbon is used as the adsorbent, during the shutdown process after the adsorption process, the gas in the tower is adsorbed by the molecular sieve carbon, and the pressure in the adsorption tower also gradually decreases. Therefore, depending on the relative sizes of the product tank 4 and the adsorption towers 3A and 3B, the pressure drop in the adsorption towers 3A and 3B may be greater than that in the product tank 4, and the pressure in the adsorption towers 3A and 3B may become lower than the pressure in the product tank 4, causing the product nitrogen gas in the product tank 4 to backflow into the adsorption towers 3A and 3B. Therefore, by using check valves as the switching valves AV2A and AV2B in the nitrogen PSA system 1, backflow of nitrogen gas from the product tank 4 to the adsorption towers 3A and 3B can be reliably prevented.

In addition, the nitrogen PSA system 1 preferably includes, as operational control equipment, an air pressure gauge (not shown) that measures the pressure in the supply path that supplies compressed feed air from the air compressor 2 to the adsorption towers 3A and 3B, a nitrogen pressure gauge (not shown) that measures the pressure in the product tank 4, a nitrogen flow meter (not shown) that measures the flow rate (product extraction flow rate) in the product gas supply path that supplies product nitrogen gas from the product tank 4 to the destination (user), and a valve control device (not shown) that performs various calculations based on the nitrogen flow rate signal measured by the nitrogen flow meter and the pressure signals from the air pressure gauge and nitrogen pressure gauge, and controls the opening and closing of each switching valve based on the results of the calculations.

<Method of operating nitrogen gas production equipment>
Next, the method of operating a nitrogen gas production apparatus of this embodiment is a method of operating a nitrogen gas production apparatus that has a plurality of adsorption towers filled with adsorbent, and the adsorption towers repeatedly perform at least an adsorption process, a pressure equalization process, and a regeneration process to separate nitrogen from compressed air, which is a raw material gas, and supply it as a product gas, and the method extends the execution time of the adsorption process in accordance with a decrease in the extraction flow rate of the product gas, and provides a pause process after completion of the adsorption process to stop the supply of raw material gas to the adsorption towers and the extraction of product gas from the adsorption towers.
In the method for operating the nitrogen gas production apparatus of this embodiment, it is preferable to supply compressed air using an inverter type air compressor.

The following describes the method for operating the nitrogen gas production apparatus of this embodiment, taking the case where the nitrogen PSA apparatus 1 described above is used as an example. Figure 2 is a diagram illustrating the open/close states of each switching valve during rated operation in the method for operating the nitrogen gas production apparatus of this embodiment. Figure 3 is a diagram illustrating the open/close states of each switching valve during turndown operation in the method for operating the nitrogen gas production apparatus of this embodiment. Figure 4 is a diagram illustrating a time chart for each adsorption tower during rated operation in the method for operating the nitrogen gas production apparatus of this embodiment. Figure 5 is a diagram illustrating a time chart for each adsorption tower during turndown operation in the method for operating the nitrogen gas production apparatus of this embodiment.

(During rated operation)
In the operating method of the nitrogen PSA system 1 of this embodiment, when 100% of the specified product nitrogen gas is supplied from the product tank 4 to the destination (user), both adsorption towers 3A, 3B repeatedly perform the conventional adsorption, regeneration, and pressure equalization steps shown in FIG. 4 in conjunction with the operation of the selector valves shown in FIG. 2.

As shown in adsorption tower 3A in Figure 2(A) and adsorption tower 3B in Figure 2(C), the adsorption process includes a pressurization operation in which the pressure in the adsorption tower is increased while supplying feed air or product gas to the adsorption tower, and an operation in which the product gas is removed from the adsorption tower while supplying feed air.

As shown in adsorption tower 3B in Figure 2(A) and adsorption tower 3A in Figure 2(C), the regeneration process involves lowering the pressure in adsorption towers 3A and 3B to desorb the highly adsorbable components from the adsorbent and regenerating the adsorption towers to prepare them for the next adsorption process. Simultaneously with or following the pressure reduction in the adsorption towers, operations such as purging the adsorption towers with product gas or other operations may be performed.

As shown in Figures 2(B) and 2(D), the pressure equalization process is a process in which pressure is recovered by connecting adsorption towers 3A and 3B and moving the gas inside the towers. The two towers can be connected either at the top (product outlet side) of the adsorption towers, at the bottom (raw material inlet side) of the adsorption towers, or both, and the pressures in both towers do not need to be completely equal.

(When operating at reduced capacity)
In the operating method of the nitrogen PSA system 1 of this embodiment, when product nitrogen gas is supplied from the product tank 4 to the user at less than 100% of the specification, both adsorption towers 3A, 3B repeat the operations shown in Figure 5, adding a pause step to the conventional adsorption step, regeneration step, and pressure equalization step.

During turndown operation, the adsorption process time is extended in accordance with the decrease in the amount of product nitrogen gas supplied from the product tank 4 to the user (the product nitrogen gas extraction flow rate). For example, if the product nitrogen gas supply rate is 70% of the specified amount, it is preferable to extend the time by 200%. However, the optimal adsorption time extension is determined by the product flow rate and product tank capacity. For example, the rated adsorption time is used when the product nitrogen gas supply rate is 100% to 90%, and the adsorption time is 1.2 times the rated time when the product nitrogen gas supply rate is 90% to 80%. The relationship between the product nitrogen gas supply rate and adsorption time is determined by the equipment specifications. Furthermore, it is preferable to use an inverter-type air compressor that can be controlled by an inverter depending on the pressure of the adsorption tower. Specifically, the motor rotation speed is controlled by varying the operating frequency, and compressed air is supplied to the adsorption towers 3A and 3B while maintaining constant pressure control so that the pressure in the adsorption towers 3A and 3B approaches the set pressure (so that it falls within the target pressure) during the adsorption process, thereby extending the adsorption process to the specified time.

As shown in Figure 5, the pause process is performed after the adsorption process is completed. During the pause process, the supply of raw gas to adsorption towers 3A and 3B and the removal of product gas from adsorption towers 3A and 3B are stopped. In other words, as shown in Figure 4, during the pause process, all of the switching valves AV1A to AV5A, AV1B to AV5B, and AV6 are closed, preventing gas from flowing into or out of both adsorption towers 3A and 3B.

During the shutdown process, the inlet valves AV1A and AV1B to the adsorption towers 3A and 3B are closed, causing the air compressor 2 to operate in unloaded mode.

Since the user is still using product gas even during turndown operation, the duration of the pause step is determined according to the degree of turndown operation so that the pressure in the product tank 4 meets the specified pressure. In other words, the pause step is terminated while the pressure in the product tank 4 remains above the specified pressure, and the pause step time is set so that product nitrogen can be supplied to the product tank 4 from the adsorption tower that has started the adsorption step.

(When operating in increasing or decreasing volume)
When the operation switches from turn-down operation to normal operation, or when the supply rate of product nitrogen gas increases, the adsorption time is varied based on the relationship between the supply rate of product nitrogen gas and the adsorption time, as explained above for turn-down operation.
Furthermore, when the nitrogen gas product supply rate is further reduced from the rate reduction operation, the adsorption time is also varied based on the relationship between the nitrogen gas product supply rate and the adsorption time.

The nitrogen PSA system 1 can smoothly switch between normal operation (rated operation) when the product withdrawal flow rate is at the specified flow rate, reduced operation to reduce the product withdrawal flow rate to a desired level, and increased operation to return the reduced product withdrawal flow rate to the specified flow rate, depending on the product withdrawal flow rate. This allows users to freely change the product withdrawal flow rate within the specified flow rate.

As explained above, according to the operating method of the nitrogen PSA unit 1 of this embodiment, the adsorption process time is extended in response to a decrease in the product gas extraction flow rate, and after the adsorption process is completed, a pause process is provided in which the supply of raw material gas to the adsorption towers 3A and 3B and the extraction of product gas from the adsorption towers 3A and 3B are stopped, thereby obtaining product gas of the required purity and reducing power consumption.

Furthermore, according to the operating method of the nitrogen PSA unit 1 of this embodiment, the number of times switching to unloaded operation per unit time can be significantly reduced compared to conventional methods. Therefore, breakdowns of the air compressor 2 can be suppressed.

The technical scope of the present invention is not limited to the above-described embodiments, but also includes designs that do not deviate from the gist of the present invention.

The effects of the present invention are explained in detail below. Note that the present invention is not limited to the following description.

<Verification test>
As an example, using the nitrogen PSA unit 1 shown in FIG. 1, the power consumption and the oxygen concentration in the product gas were investigated when the flow rate of the product gas was reduced to 70%, 50%, and 30% from the rated operation (100%).
As a comparative example, the nitrogen PSA apparatus described in the above-mentioned prior art was used, and the power consumption and oxygen concentration in the product gas were investigated when the flow rate of the product gas was reduced to 70%, 50%, and 30% from the rated operation (100%), as in the above example.
Table 1 shows the power consumption and oxygen concentration in the product for each product flow rate in the nitrogen PSA apparatus 1 of the present invention and in a conventional nitrogen PSA apparatus (when the adsorption time is constant).

As shown in Table 1, in the nitrogen PSA unit of the comparative example, the idle time increased and power consumption decreased as the product flow rate decreased from 100% to 70%, 50%, and 30%. However, the oxygen concentration in the product decreased as the product flow rate decreased, and it was confirmed that excessive purity was being supplied.
In contrast to this, it was confirmed that the nitrogen PSA apparatus 1 of the example can reduce power consumption even more than the comparative example, and the oxygen concentration in the product does not become excessive.

1. Nitrogen PSA equipment (nitrogen gas production equipment)
2 Air compressor 3A, 3B Adsorption tower 4 Product tank AV1A to AV5A, AV1B to AV5B, AV6 Switching valve

Claims (2)

A method for operating a nitrogen gas production apparatus having a plurality of adsorption towers filled with adsorbents, the adsorption towers repeatedly performing at least an adsorption step, a pressure equalization step, and a regeneration step to separate nitrogen from compressed air as a raw material gas and supply the nitrogen as a product gas, comprising:
extending the adsorption step in response to a decrease in the product gas extraction flow rate;
A method for operating a nitrogen gas production apparatus, comprising, after completion of the adsorption step, providing a halt step of stopping the supply of raw material gas to the adsorption tower and the withdrawal of product gas from the adsorption tower. The method for operating a nitrogen gas production apparatus according to claim 1, wherein the compressed air is supplied using an inverter-type air compressor.