JPH05373B2 - - Google Patents

Description

[Detailed description of the invention]

Industrial Application Field The present invention uses the PSA method (pressure fluctuation type adsorption separation method) to extract raw material gas containing CH ₄ as the main component and CO ₂ .
This invention relates to an improvement in the method of removing CO ₂ to produce CH ₄ -rich gas. Conventional technology In order to produce combustible gas that can be used as city gas, liquefied petroleum gas or methanol, whose main component is C ₄ H ₁₀ , is decomposed by reacting with water vapor at high temperatures, resulting in reforming that is rich in CH ₄ . Attempts have been made to produce gas. Since this reformed gas contains a relatively large amount of CO ₂ as a result of the decomposition reaction, it lacks a calorific value and cannot be used as city gas. In order to make this reformed gas usable as city gas, it is necessary to remove as much CO ₂ from this gas as possible. It is often necessary to remove CO ₂ not only from the above-mentioned reformed gas for city gas but also from gas containing CO ₂ . Generally, as a method for removing CO ₂ from a raw material gas containing CO ₂ , 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 coke oven gas and then reacted in the presence of a cobalt catalyst to produce C ₁ to C ₄ hydrocarbons, and then the resulting gas is washed with alkali. A method for removing CO ₂ contained in the gas is shown. In addition to the alkali cleaning method described above, a PSA method is also known as a method for removing CO ₂ from a gas containing CO ₂ . Among those using the PSA method, one worth noting is Japanese Patent Publication No. 1525/1983. In other words, the publication states that when removing and recovering acidic gases such as CO ₂ from natural gas, adsorption and desorption are performed using a pressure swing method using carbon molecular sieves with an average pore diameter of about 3 Å as an adsorbent. A method is shown in which the amount of desorption gas from the beginning of desorption is recycled into raw material gas up to 70% of the total amount of desorption gas. Problems to be Solved by the Invention Among the methods of removing CO ₂ from gas containing CO 2, the method of removing _{CO 2 by} alkaline cleaning is
There are problems such as the equipment becomes larger and the thermal energy required for heating and regenerating the alkali is large. In this regard, the method of removing CO ₂ 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, a portion of CH ₄ is also adsorbed by the adsorbent along with CO ₂ , so CH ₄ is mixed into the reduced pressure gas generated in the depressurization process, and that amount of CH ₄ is absorbed by the adsorbent.
The recovery rate will inevitably decline. In experiments conducted by the present inventors, when a PSA cycle was carried out using the reformed gas of Example 1, which will be described later, as a raw material gas, the CH ₄ recovery rate was at most 89%. Furthermore, the fact that a considerable amount of CH ₄ is contained in the CO ₂ -rich gas that is the rest gas also hinders the reuse of this rest gas for other purposes. 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 force device had to be increased, and there was still room for further improvement from an industrial perspective. An object of the present invention is to provide _{an industrially advantageous method for producing a CH 4 -rich} gas by removing CO _{2 from a raw material gas containing CH 4} as a main component and containing CO ₂ by the PSA method. It is. Means for Solving the Problems The method for producing methane-rich gas of the present invention uses a raw material gas containing CH ₄ as a main component and CO ₂ by the PSA method.
When removing CO ₂ and producing CH ₄ -rich gas, an adsorbent with an average pore diameter of approximately 5 is used as an adsorbent to fill the adsorption tower.
Using a zeolite-based adsorbent with a temperature of at least 100 Å, processing while maintaining the raw material gas at a temperature of 80°C or higher, and reducing the pressure in the tower from a predetermined pressure to atmospheric pressure to vacuum after the adsorption process is completed. In the process, the reduced pressure gas that reaches a certain pressure in the range of 500 ± 200 Torr after the start of the pressure reduction operation, excluding the part used for pressure equalization, is fed back to the adsorption tower. It is. The present invention will be explained in detail below. Examples of the raw material gas containing CH ₄ as a main component and CO ₂ include the following gases. First, a reformed gas obtained by decomposing liquefied petroleum gas by reacting it with water vapor at high temperature can be used. The exit gas composition of this reformed gas is, for example, 7% H ₂ 68% CH ₄ 25% CO ₂ . If the CO ₂ content in this outlet gas can be reduced to about 5% or less, it will have the combustibility to be used as city gas. The calorific value can be adjusted by adding a small amount of liquefied petroleum gas to increase the heat. Similarly, a reformed gas obtained by decomposing methanol by reacting with water vapor at high temperature can also be used. In addition to these, various gases can be used as long as they contain CH ₄ as a main component and CO ₂ . When producing CH ₄ -rich gas by removing CO ₂ from such raw material gas using the PSA method,
As an adsorbent packed into an adsorption tower, the average pore diameter is approximately 5.
Use a zeolite-based adsorbent of Å or more. 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 about 5 Å to about 10 Å. The above zeolite-based adsorbents absorb adsorption when the degree of vacuum is increased during the depressurization process.
Shows the effect of completely desorbing CH ₄ . In other words, it is an adsorbent that has good cutting properties for CH ₄ , and this point is different from carbon molecular sieves, which have poor cutting properties. In the present invention, as described above, it is essential to use a zeolite adsorbent with an average pore diameter of about 5 Å or more as an adsorbent, and to perform the treatment while maintaining the raw material gas at a temperature of 80° C. or more. At temperatures below 80°C, it is not possible to increase the CH ₄ recovery rate to the target of 99%. Particularly preferable temperature conditions are 100~
The temperature is 130℃. 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. In the present invention, in the pressure reduction step of reducing the pressure of the tower from a predetermined pressure to atmospheric pressure to vacuum after the adsorption step is completed, 500±
The vacuum gas up to a pressure in the range of 200 Torr (in particular in the range of 500±100 Torr) is fed back to the adsorption column. However, if three to six adsorption towers are used, and the gas at the time of pressure reduction is used for pressure equalization with other towers, this pressure equalization portion is excluded from the feedback portion. In this case, the limit between the amount to be fed back and the amount to be removed from the system as rest gas is 500±.
It is important to decide on a certain pressure in the range of 200 Torr. When the pressure exceeds 700 Torr, the proportion of CH ₄ in the rest gas increases, and the recovery rate of CH ₄ decreases by that amount. On the other hand, when the pressure reaches a threshold of less than 300 Torr, a considerable amount of CO ₂ in the reduced pressure gas is returned to the adsorption tower, resulting in poor productivity, and increasing the throughput requires an increase in the size of the equipment. It turns out. Function Next, the function of the present invention will be explained based on a specific example. FIG. 1 is a curve showing the amount of reduced pressure gas of CH ₄ and CO ₂ in the pressure reduction 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 CH ₄ 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 CH ₄ and CO ₂ in the decompressed gas reverses, with CO ₂ becoming more abundant. However, the amount of CH ₄ is still large. If the degree of depressurization is further increased to below atmospheric pressure,
Although the amount of CO ₂ desorbed increases, the amount of CH ₄ in the reduced pressure gas becomes negligible. When the degree of vacuum is further increased, the desorption of CO ₂ 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. 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 at a temperature of 400°C in the presence of a catalyst. As a gas with the same composition as the reformed gas obtained by reacting with water vapor, a gas with the composition shown in the raw material gas column of Table 1 was used, and under the following conditions.
A PSA cycle was performed. Adsorption tower: 3 tower type Adsorbent: Zeolite with average pore diameter of 5 Å Space velocity: 500/hr Temperature: 100-110°C Adsorption pressure: 6 Kg/cm ² G Regeneration pressure: 40 Torr Figure 2 shows the PSA cycle. Initial decompression gas Used as pressure equalization gas Mid-decompression gas Returned to the front of the adsorption tower Late decompression gas Discarded as rest gas Boosting gas Product gas used The CH ₄ recovery rate when performing the above PSA was 99.1%
It was hot. Table 1 shows the compositions of the product gas, initial reduced pressure gas, intermediate reduced pressure gas (feedback), and later reduced pressure gas (rest gas) when carrying out the above PSA.
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 using the method of the present invention, the recovery rate of CH ₄ can be maximized.
It is extremely advantageous industrially. Furthermore, since the amount of CH ₄ 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 front of the adsorption tower 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. Further, the method of the present invention is advantageous in this respect as it does not require any special step for dehumidification even if the raw material gas contains moisture. Therefore, the present invention removes CO ₂ from a raw material gas containing CH ₄ as a main component and CO ₂ by the PSA method.
This method has great industrial significance as a method for producing gas rich in CH ₄ .

[Brief explanation of the drawing]

FIG. 1 is a curve showing the amount of reduced pressure gas of CH ₄ and CO ₂ in the pressure reduction 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 the PSA cycle in Example 1.

Claims (1)

[Claims] 1. When producing a gas rich in CH ₄ by removing CO ₂ from a raw material gas mainly composed of CH ₄ and containing CO ₂ by the PSA method, an average fine particle is used as an adsorbent to fill an adsorption tower. Pore diameter approx. 5
Using a zeolite-based adsorbent with a temperature of at least 100 Å, processing while maintaining the raw material gas at a temperature of 80°C or higher, and reducing the pressure in the tower from a predetermined pressure to atmospheric pressure to vacuum after the adsorption process is completed. In the process, the reduced pressure gas that reaches a certain pressure in the range of 500 ± 200 Torr after the start of the pressure reduction operation, excluding the part used for pressure equalization, is fed back to the adsorption tower. Method for producing methane gas. 2 In the depressurization process in which the pressure in the column is reduced from a predetermined pressure to atmospheric pressure to vacuum after the adsorption process is completed, the decompression gas used for pressure equalization that reaches a certain pressure in the range of 500 ± 100 Torr after the start of the depressurization operation is 2. The production method according to claim 1, wherein the reduced pressure gas except for the part that is absorbed is fed back to the adsorption tower. 3. The production method according to claim 1, wherein the raw material gas is a reformed gas obtained by reacting liquefied petroleum gas or methanol with steam at high temperature.