JPH0367727B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention produces a product gas rich in useful components by adsorbing and removing impurity components from a raw material gas that is mainly composed of useful components and also contains impurity components using the PSA method (pressure fluctuation adsorption separation method). Methods, especially CH4
PSA from raw material gas containing CO2 as the main component
This invention relates to a method for producing CH4-rich gas by adsorbing and removing CO2. Conventional technology In order to produce combustible gas that can be used as city gas, liquefied petroleum gas or methanol, whose main component is C4H10, is decomposed by reacting with water vapor at high temperatures to produce reformed gas rich in CH4. That is what is being attempted. Since this reformed gas contains a relatively large amount of CO2 as a result of the decomposition reaction, it lacks heat value as it is and cannot be used as high-calorie city gas. In order to make this reformed gas usable as high-calorie city gas, it is necessary to remove as much CO2 as possible. It is often necessary to remove CO2 not only from the above-mentioned reformed gas for city gas but also from gas containing CO2. Generally, as a method for removing CO2 from a raw material gas containing CO2, an alkaline cleaning method is known in which the raw material gas is brought into contact with an aqueous solution of an alkali such as sodium carbonate or potassium carbonate. For example, Japanese Patent Application Laid-Open No. 1983-1980 filed by the present applicant
Publication No. 197793 discloses that CO-rich generator gas is added to coske furnace gas and reacted in the presence of a cobalt catalyst to produce C1 to C4 hydrocarbons, and then the resulting gas is washed with alkali to produce carbon dioxide. A method for removing CO2 contained in gas is shown. In addition to the alkali cleaning method described above, a PSA method is also known as a method for removing CO2 from a gas containing CO2. Among those using the PSA method, one worth noting is Japanese Patent Publication No. 62-1525 (Japanese Unexamined Patent Publication No.
58-159830). In other words, the publication states that when removing and recovering acidic gases such as CO2 from natural gas, adsorption and desorption are performed using the PSA method using carbon molecular sieves with an average pore diameter of approximately 3 Å as an adsorbent. The amount of desorbed gas from the beginning of time is 70% of the total amount of desorbed gas.
Methods have been shown to recycle amounts up to % to feedstock gas. In addition, Japanese Patent Publication No. 45-20082 states that in the PSA method in which pressure is increased using product gas, when performing the pressure equalization process by equalizing pressure with other columns, product gas is supplied at the same time to keep the product gas flow rate constant. is listed. Problems to be Solved by the Invention However, among the methods for removing CO2 from CO2-containing gas, the method of removing CO2 by alkaline cleaning requires large equipment and requires a large amount of thermal energy to heat and regenerate the alkali. There are disadvantages such as In this regard, the method of removing CO2 using the PSA method is
This is advantageous in that the device is compact and easy to control and maintain. However, in the PSA method, CH4 as well as CO2
Since some of CH4 is also adsorbed by the adsorbent, CH4 is mixed into the reduced pressure gas generated during the depressurization process, and the amount of CH4 is
CH4 recovery rate is inevitably reduced. In experiments conducted by the present inventors, when carrying out a PSA cycle using the reformed gas of Example 1 described below as a raw material gas,
The maximum CH4 recovery rate was 89%. Furthermore, the fact that a considerable amount of CH4 is contained in the CO2-rich rest gas has also been a hindrance to reusing this rest gas for other purposes (for example, liquefied carbon dioxide). The method described in Special Publication No. 62-1525 is PSA
This method attempts to overcome the above-mentioned problems with the method, but since approximately 70% of the total amount of reduced pressure gas (desorption gas) in the reduced pressure regeneration process is recycled, the amount of raw material gas processed is small, and the amount of processed material is reduced. In order to increase this, the scale of the momentum device had to be increased, and there was still room for further improvement from an industrial perspective. Therefore, the inventors of the present invention have developed a method to further improve the method described in Japanese Patent Publication No. 1525/1983, "CH4
From raw material gas containing CO2 as the main component
When removing CO2 and producing CH4-rich gas using the PSA method, a zeolite-based adsorbent with an average pore diameter of approximately 5 Å or more is used as the adsorbent packed in the adsorption tower, and the raw material gas is maintained at a temperature of 80°C or higher. In addition, in the depressurization process in which the pressure in the tower is reduced from a predetermined pressure to atmospheric pressure to vacuum after the adsorption process is completed, from the start of the depressurization operation,
A method for producing methane-rich gas characterized by feeding back the reduced pressure gas that reaches a pressure in the range of 500±200 Torr, excluding the part used for pressure equalization, before an adsorbent. ”,
A patent application has already been filed as Japanese Patent Application No. 185670. According to this method, the recovery rate of CH4 is increased to the maximum, and the amount of reduced pressure gas fed back to the adsorption tower is kept to the minimum necessary, so the throughput of raw material gas can be maintained at a high level. However, in this method, the product gas is used to increase the pressure of the adsorption tower after pressure equalization to the adsorption pressure, so the amount of product gas decreases during the time required for this pressure increase. However, when using this product gas as high-calorie city gas, it is necessary to adjust the calorific value by adding a small amount of liquefied petroleum gas to increase the heat. Since it is necessary to adjust the amount of heat enhancer added each time there is a change in the temperature, the operation becomes cumbersome. To level out this fluctuation, it is necessary to install an appropriate buffer tank and extract a certain amount of product gas from that tank, but such countermeasures are difficult from an industrial perspective. This was a disadvantage, and the next challenge was to find a more fundamental solution. The method disclosed in Japanese Patent Publication No. 45-20082 (method for simultaneously supplying product gas when performing the pressure equalization process by equalizing pressure with other columns in the PSA method in which pressure is increased by product gas) is to Although it seems to be effective in keeping the pressure constant, it does not take into account the waiting period after pressure equalization and pressure increase.
The stabilization of PSA operation has not been sufficiently considered.
This is because in PSA operation, the outside temperature,
Various factors such as raw material gas composition and regeneration efficiency, pressure at the time of completion of pressure equalization caused by differences in the performance of each adsorption tower, and differences in pressure increase rate when pressurizing with product gas can easily lead to insufficient pressurization. In the case of insufficient pressurization, the pressure inside the adsorption tower is lower than that of the product gas at the start of the next adsorption process, so the product gas may flow backwards.The invention of this publication specifically addresses this situation. No measures have been taken. The present invention aims to provide a suitable method for solving these various problems. Means for Solving the Problems The method for producing enriched gas by the PSA method of the present invention is as follows:
In producing a product gas enriched with useful components by adsorbing and removing impurity components using the PSA method from a raw material gas that is mainly composed of useful components and also contains impurity components,
The process from the end of the regeneration process to the start of the adsorption process in one column is a pressure increase process using pressure equalization gas, a pressure increase process using product gas as the pressure increase gas, and a waiting period until the pressure increase is completed and the next adsorption process is started. During the standby period when the pressurized gas is not used, the product gas at the same flow rate as the pressurized gas is returned to the recycle line for raw material gas or used to pressurize other columns during pressure equalization. This is characterized in that the product gas flow rate is kept constant at all times. In this case, in the depressurization step in which the pressure in the column is reduced from a predetermined pressure to atmospheric pressure to vacuum after the adsorption step is completed, the portion of the reduced pressure gas that reaches a certain degree of vacuum after the start of the depressurization operation is consumed as pressure equalization gas. It is preferable to return the removed reduced pressure gas to a recycle line for raw material gas. The present invention will be explained in detail below. As the raw material gas, a very wide range of gases can be used as long as they are mainly composed of useful components and contain impurity components. (Note that "useful components" and "impurity components" have a subjective meaning similar to "components of interest" and "components of non-concern", and do not mean that they are industrially useful or unnecessary.) , the useful component is CH4, and the impurity component is
A reformed gas obtained by reacting a raw material gas such as CO2, such as liquefied petroleum gas or methanol, with steam at high temperatures is important. Typical examples of such reformed gas include:
Reformed gas is produced when liquefied petroleum gas is decomposed by reacting with water vapor at high temperatures. The exit gas composition of this reformed gas is, for example, 7% H ₂ , 68% CH ₄ , and 25% CO ₂ . If the CO2 content in this outlet gas can be reduced to about 5% or less, it will have the combustibility to be used as high-calorie city gas. This is because the calorific value can be adjusted by adding a small amount of liquefied petroleum gas to increase the heat. Furthermore, a reformed gas obtained by decomposing methanol by reacting with water vapor at high temperature can also be used. In the present invention, in the depressurization process in which the pressure of the column is reduced from a predetermined pressure to atmospheric pressure to vacuum after the adsorption process is completed, the reduced pressure gas that reaches a certain degree of vacuum after the start of the depressurization operation is consumed as pressure equalizing gas. It is desirable to return the reduced-pressure gas excluding the remaining gas to the recycle line for raw material gas. "A certain degree of vacuum" cannot be defined unconditionally because it varies depending on the type of raw material gas, the type of adsorbent, and other PSA operating conditions, but it can be defined by reacting liquefied petroleum gas or methanol as a raw material gas with water vapor at high temperature. For example, if a zeolite adsorbent with an average pore diameter of approximately 5 Å or more is used as the adsorbent to fill the adsorption tower with the above-mentioned reformed gas obtained by Especially 500
The reduced pressure gas reaching a certain pressure in the range (±100Torr) is returned to the recycle line to feed gas. However, when the gas at the beginning of pressure reduction is used for pressure equalization with other columns, this pressure equalization portion is excluded from the feedback portion. In this case, it is important to determine the limit between the amount to be fed back and the amount to be removed from the system as rest gas at the boundary of "a certain degree of vacuum" (in the above example, a certain pressure in the range of 500±200 Torr). When the pressure is higher than this degree of vacuum, the proportion of the product gas component (CH4 in the above example) in the rest gas increases, and the recovery rate of the product gas decreases by that amount.
On the other hand, if the pressure is lower than this degree of vacuum, a considerable amount of the impurities in the reduced pressure gas (CO2 in the above example) will be returned to the adsorption tower, reducing productivity and reducing the throughput. If you try to increase the size, the device will become larger. Now, in the present invention, the steps leading to the start of the adsorption step after the end of the regeneration step in one column are a pressure increase step using pressure equalizing gas, a pressure increase step using the product gas as the pressure increase gas, and a step after pressure increase is completed to start the next adsorption step. It is divided into a waiting period until the transition, and during the waiting period when the pressurized gas is not used,
The product gas at the same flow rate as the pressurized gas is returned to the recycle line for raw material gas or used for pressurizing other columns during pressure equalization. By doing this, the product gas always has a constant flow rate, so when adding an additive to the product gas, the amount of the additive added can be kept constant, which greatly simplifies management. In addition, when using the above-mentioned reformed gas obtained by reacting liquefied petroleum gas or methanol with steam at high temperature as the raw material gas, the adsorbent and temperature conditions described below should be selected or set as the adsorbent and temperature conditions. is advantageous. That is, as an adsorbent, the average pore diameter is about 5 Å.
It is desirable to use the above zeolite-based adsorbent; however, the object of the present invention cannot be fully achieved with an adsorbent having an average pore diameter of 4 Å. A preferred average pore size range is from about 5 Å to about 10 Å. When the degree of vacuum is increased in the depressurization process, the above zeolite adsorbent
Shows the effect of completely desorbing adsorbed CH4. so to speak
It is an adsorbent that has good cutting ability for CH4, and this point is different from carbon molecular sieve, which has poor cutting ability. Further, it is desirable to use the above-mentioned adsorbent and to perform the treatment while maintaining the raw material gas at a temperature of 80°C or higher, particularly 100 to 130°C. This is because at temperatures below 80°C, it is difficult to increase the CH4 recovery rate to the targeted 99%. Performing the PSA operation using a specific zeolite adsorbent and at a temperature of 80°C or higher is also advantageous in terms of dehumidification. In other words, even if the raw material gas contains water, a balance is achieved between the ability to adsorb water and the ease of desorption, and dehumidification is performed simultaneously during the PSA operation. Therefore,
There is no need to provide a special process for dehumidification. Effects Next, the effects of the present invention will be explained in that the useful ingredient is CH4,
An example will be explained in which a raw material gas whose impurity component is CO2 is used. FIG. 1 is a curve showing the amount of CH4 and CO2 depressurized gas in the depressurization process. The horizontal axis is the pressure, and the vertical axis is the amount of pressure reduction. The solid line is the integral value, and the dotted line is the differential value. The composition of the decompressed gas is rich in CH4 at the beginning of decompression, so
This amount is used to boost pressure in other columns if necessary. As the degree of depressurization approaches atmospheric pressure, the ratio of CH4 and CO2 in the decompressed gas reverses, with CO2 becoming more abundant. However, CH4 is still included in an amount that cannot be ignored. If the degree of depressurization is further increased to below atmospheric pressure,
Although the amount of CO2 desorbed increases, the amount of CH4 in the reduced pressure gas becomes negligible. When the degree of vacuum is further increased, the desorption of CO2 becomes minimal, and the regeneration of the adsorbent is completed. In the example shown in Figure 1, where the pressure is reduced from the pressure immediately after the adsorption process to atmospheric pressure to 40 Torr,
After pressure equalization, the gas desorbed during the pressure reduction period until it reaches 500 Torr is fed back to the adsorption tower.
It can be seen that it is most efficient to remove the gas desorbed during the depressurization period from 500 Torr to 40 Torr as rest gas. FIG. 2 shows an example of a PSA cycle and the flow rate of product gas corresponding to the cycle. FIG. 3 is a diagram showing the flow of gas when the PSA cycle of FIG. 2 is carried out. 1 is an adsorption tower, which consists of three towers 1a, 1b, and 1c. 2 is a vacuum pump, 3 is a recycle gas holder, 4 is a recycle gas compressor, and V1 and V2 are flow rate control valves. f is raw material gas, p is product gas,
p' is part of the product gas, r is the rest gas, and c is the recycling flow. i and ii are channels. In FIG. 3A, gas f+c is being sent to the column 1a while the recycle gas c is being recycled into the raw material gas f, and adsorption is being performed in the column 1a. The pressure equalization gas is sent from the column 1c to the column 1b, and a part of the product gas p' is also sent to the column 1b, and pressure increase during pressure equalization has begun. In FIG. 3B, gas f+c is being sent to the tower 1a while the recycle gas c is being recycled into the raw material gas f, and adsorption is being performed in the tower 1a. In the column 1b, pressure is subsequently increased using a part of the product gas p' as a pressurizing gas. Reduced pressure gas is led out from the column 1c where pressure equalization has been completed, and the reduced pressure gas flows through channels i,
Gas up to a certain degree of vacuum below atmospheric pressure is sent to the recycle gas line through channel ii, and the vacuum-desorbed portion exceeding that degree of vacuum is discharged outside the system as rest gas r. In FIG. 3C, gas f+c is being sent to the tower 1a while the recycle gas c is being recycled into the raw material gas f, and adsorption is being performed in the tower 1a. The column 1b has completed pressurization and is in a standby period. Vacuum depressurization continues in the column 1c, and the depressurized gas is discharged outside the system as rest gas r. At this stage, the product suspension
is not used for pressure boosting, so a part of the product gas
p′ (product gas corresponding to the pressure increase) is returned to the recycle gas line. In other words, if we focus on column 1b, column 1b is in the pressure equalization process using pressure equalization gas in Fig. It is in a waiting period before moving on to the process. Using three towers as shown in Figures 2 and 3
When the PSA cycle is carried out, the flow rate of the product gas changes as shown in Figure 2 (b) if the product gas is only used as a pressurizing gas. However, in the present invention, the product gas is used as the pressurizing gas, and during the standby period when the pressurizing gas is not used, the product gas at the same flow rate as the pressurizing gas is returned to the recycle line for raw material gas, or during pressure equalization. Since the product gas is used to raise the pressure of other columns, the flow rate of the product gas is as shown in Figure 2 (c), and the flow rate is always constant. According to the present invention, even if product gas is used for pressurization and recycling in other columns, the product gas used for pressurization and recycling is only circulated within the system and is not lost, reducing the recovery rate of product gas. There is no risk of it happening. Examples Next, the present invention will be further explained with reference to Examples. Example 1 A reformed gas obtained by reacting liquefied petroleum gas containing C4H10 as a main component with steam at a temperature of 400°C in the presence of a catalyst, or a reformed gas obtained by reacting methanol with steam at a temperature of 400°C in the presence of a catalyst. As a gas with the same composition as the obtained reformed gas, a gas with the composition shown in the raw material gas column of Table 1 below was used. Adsorption tower: 3-column type Adsorbent: Zeolite with an average pore diameter of 5 Å Space velocity: 500/hr A PSA cycle was carried out according to FIGS. 2 and 3 at a temperature of 100-110°C. The initial reduced pressure gas was used as equalization gas, the middle reduced pressure gas was returned to the raw material gas recycle line via a recycle gas holder, and the latter reduced pressure gas was disposed of as rest gas. The product gas is used as the pressurizing gas, and during the standby period when the pressurizing gas is not used, the same flow rate of the product gas as the pressurizing gas is returned to the raw material gas recycling line via the recycling gas holder, or during pressure equalization. It was designed to be used for pressurizing other columns. CH4 recovery rate when performing the above PSA was 99.1%
It was hot. Product gas, initial reduced pressure gas, intermediate reduced pressure gas (feedback gas) when carrying out the above PSA,
Table 1 shows the composition of the late decompression gas (rest gas). Note that the composition of the raw material gas is also shown in Table 1.

[Table] Effects of the Invention If the PSA cycle is carried out according to the method of the present invention, the recovery rate of useful components in the raw material gas can be increased to the maximum, which is extremely advantageous industrially. Furthermore, since the amount of useful components transferred into the rest gas is extremely small, there is no problem when reusing this rest gas for other purposes. In addition, since the amount of reduced pressure gas fed back to the raw material gas recycling line is kept to the necessary minimum, the throughput of raw material gas can be maintained at a high level, or the apparatus can be made more compact. Furthermore, in the present invention, since the flow rate of the product gas is always constant, when additives are added to the product gas to produce the final product gas (for example, the product gas is methane-rich gas, liquefied gas is added as a heat enhancer). (When adjusting the calorific value by adding petroleum gas), the amount of the additive added can be kept constant, which greatly simplifies management. Therefore, the present invention has great industrial significance as a method for producing enriched gas by the PSA method.

[Brief explanation of drawings]

FIG. 1 is a curve showing the amount of CH4 and CO2 depressurized gas in the depressurization process. The horizontal axis is the pressure, and the vertical axis is the amount of pressure reduction. The solid line is the integral value, and the dotted line is the differential value. FIG. 2 shows an example of a PSA cycle and the flow rate of product gas corresponding to the cycle. FIG. 3 is a diagram showing the flow of gas when the PSA cycle of FIG. 2 is carried out. 1, 1a, 1b, 1c...adsorption tower, 2...vacuum pump, 3...recycle gas holder, 4...
Recycle gas compressor, V1, V2...
Flow rate control valve, f...source gas, p...product gas,
p'... Part of product gas, r... Rest gas, c...
...recycled gas.

Claims (1)

[Scope of Claims] 1. In producing a product gas rich in useful components by adsorbing and removing impurity components from a raw material gas containing useful components as a main component and containing impurity components by the PSA method, after the regeneration step in one column is completed. The process leading to the start of the adsorption process is divided into a pressure increase process using pressure equalization gas, a pressure increase process using product gas as the pressure increase gas, and a waiting period until the pressure increase is completed and the next adsorption process is started. During the standby period when it is not in use, the product gas at the same flow rate as the pressurized gas is returned to the recycle line for raw material gas or used to boost pressure in other columns during pressure equalization, thereby keeping the product gas flow rate constant at all times. A method for producing enriched gas using the PSA method, which is characterized in that the enriched gas is maintained. 2. The production method according to claim 1, which uses a raw material gas in which the useful component is CH4 and the impurity component is CO2. 3. The production method according to claim 2, wherein the raw material gas in which the useful component is CH4 and the impurity component is CO2 is a reformed gas obtained by reacting liquefied petroleum gas or methanol with steam at high temperature. 4. When producing a product gas rich in useful components by adsorbing and removing impurity components using the PSA method from a raw material gas that is mainly composed of useful components and also contains impurity components, after the adsorption process is completed, the pressure of the tower is reduced from a predetermined pressure to atmospheric pressure. In the depressurization process in which the pressure is reduced to vacuum after the decompression operation starts, the reduced pressure gas excluding the part consumed as pressure equalization gas from the reduced pressure gas that reaches a certain degree of vacuum after the start of the depressurization operation is returned to the recycling line for raw material gas, and (1) The process from the end of the regeneration process to the start of the adsorption process in two columns is divided into a pressure increase process using pressure equalization gas, a pressure increase process using product gas as the pressure increase gas, and a waiting period until the pressure increase is completed and the next adsorption process is started. During the standby period when the pressurized gas is not used, the product gas at the same flow rate as the pressurized gas is returned to the recycle line for raw material gas or used for pressurizing other columns during pressure equalization. A method for producing enriched gas using the PSA method, characterized in that the flow rate of the product gas is kept constant at all times. 5. The manufacturing method according to claim 4, characterized in that a raw material gas in which the useful component is CH4 and the impurity component is CO2 is used. 6. The production method according to claim 5, wherein the raw material gas in which the useful component is CH4 and the impurity component is CO2 is a reformed gas obtained by reacting liquefied petroleum gas or methanol with steam at high temperature. 7 Claims characterized in that the reduced pressure gas that reaches a certain pressure in the range of 500±200 Torr after the start of the pressure reduction operation, except for the part consumed as pressure equalization gas, is returned to the recycle line for raw material gas. Manufacturing method of Section 5. 8 The average pore diameter of the adsorbent packed in the adsorption tower is approximately
6. The production method according to claim 5, characterized in that the treatment is carried out using a zeolite adsorbent of 5A or more and maintaining the raw material gas at a temperature of 80°C or more.