WO2009101669A1 - Method for recovering and purifying methane from fermentation biogas using adsorbent - Google Patents

Abstract

A method for recovering and purifying methane, which is a method for recovering and purifying methane gas from biogas generated by anaerobic fermentation of organic waste by a pressure swing method comprising the steps of: A: feeding biogas to an adsorption tower by increasing pressure; and B: in the adsorption tower, (1) removing water by passing the biogas through a water adsorbent under relatively high pressure conditions; (2) removing siloxane by adsorbing it through an acid-treated silica gel adsorbent; (3) removing hydrogen sulfide and methylmercaptan by adsorbing them through a silicalite or acid-treated silicalite adsorbent; and (4) removing CO2 by adsorbing it through at least one type of zeolite adsorbent selected from the group consisting of Ca-A and Na-A type zeolites whose crystal surface has been coated with silica.

Description

Method for recovery and purification of methane from biofermentation gas using adsorbent

The present invention relates to a method for recovering and purifying methane from biofermentation gas using an adsorbent.

In order to effectively use resources and preserve the environment, various studies have been made to effectively use biomass, which is an organic resource derived from living organisms, instead of simply incineration. These biomass include food waste such as raw garbage, livestock manure, organic waste water, organic waste such as sewage sludge generated in a sewage treatment plant, resource crops or waste thereof. Development of technologies for generating biogas by anaerobic fermentation of these biomass and using this biogas as energy is being promoted. Biogas (digestion gas) produced by anaerobic fermentation of sewage sludge generated in a sedimentation basin at a sewage treatment plant is mainly composed of methane and CO ₂ , and sulfur such as hydrogen sulfide and methyl mercaptan as trace impurities. It is a gas containing. It is known that shampoo-derived siloxane compounds are contained in urban digestion gas.

Water, CO _2, siloxane, methane from bio-fermentation gas mainly composed of methane containing at least one more of the hydrogen sulfide and methyl mercaptan, water, CO ₂ and siloxane, hydrogen sulfide, to remove the methyl mercaptan The most frequently used method for recovering and purifying lysine is performed as follows. In this method, hydrogen sulfide and methyl mercaptan are absorbed and removed by a chemical absorption method, siloxane is adsorbed and removed by an activated carbon adsorbent, and CO ₂ is absorbed and removed by another chemical absorption method.

Although this method is suitable for large-capacity treatment because the chemical absorption method has a large scale-up factor, it is not suitable for the purification of small and medium-sized biofermentation gas of about 2,000 m ³ N / h at the maximum. Further, the deterioration of the absorbent due to the coexisting impurities is unavoidable, and problems such as continuous replenishment of the absorbent and secondary contamination associated with the absorbent treatment occur. Similarly, in the activated carbon adsorption method, it is necessary to discard the activated carbon after removing the adsorbed siloxane, and the amount of consumption over time is large, and the disposal of activated carbon becomes a burden.

In order to overcome these problems, an adsorbent is used to adsorb moisture, siloxane, hydrogen sulfide, and methyl mercaptan at a relatively high pressure, and then adsorb CO ₂ to recover, purify, and adsorb methane. After completion, a pressure swing method (PSA) in which siloxane, hydrogen sulfide, methyl mercaptan, and CO ₂ are removed at a relatively low pressure has been proposed.

However, in the currently proposed PSA, activated carbon and X-type zeolite are used as the CO ₂ adsorbent. However, co-adsorption of methane accompanying CO ₂ adsorption cannot be ignored, and siloxane and hydrogen sulfide used for pretreatment The adsorption performance of high-silica zeolite for adsorbing methyl mercaptan is gradually deteriorated by long-term use due to precipitation of solid sulfur caused by hydrogen sulfide and methyl mercaptan, and precipitation of silica caused by siloxane.

An object of the present invention is to provide a method for recovering and purifying methane from biofermented gas using an adsorbent that overcomes the problems of the prior art described above.

The method includes the following steps:
(1) Pressurize the biogas and supply it to the adsorption tower.
(2) Silicalite or hydrogen sulfide, silica gel treated with acid as a moisture adsorbent, siloxane adsorbent, methane gas containing at least one of moisture, CO ₂ and siloxane, hydrogen sulfide, methyl mercaptan or silicalite as a methyl mercaptan adsorbent Silicalite with acid treatment, Ca-A or Na-A type zeolite with silica coating on the crystal surface as CO ₂ adsorbent, moisture, siloxane, hydrogen sulfide, methyl mercaptan, moisture under relative high pressure conditions By first adsorbing, then adsorbing CO ₂ and separating it from methane, and then introducing the adsorbent adsorbing moisture, siloxane, hydrogen sulfide, methyl mercaptan, CO ₂ to a relatively low pressure condition and releasing it. A combination of methane and moisture, at least one of siloxane, hydrogen sulfide, and methyl mercaptan. Methane is separated by the pressure swing method with existing gas.

Detailed Description of the Invention
General description The following steps:
According to the present invention, the following steps:
A. Pressurize the biogas and supply it to the adsorption tower.
B. In the adsorption tower,
(1) Under relative high pressure conditions, the biogas is removed from the moisture through the moisture adsorbent,
(2) The siloxane is adsorbed and removed through an acid-treated silica gel adsorbent,
(3) Hydrogen sulfide and methyl mercaptan are adsorbed and removed through silicalite or acid-treated silicalite adsorbent,
(4) Provided is a method for recovering and purifying methane by adsorbing and removing CO ₂ through at least one zeolite adsorbent selected from the group consisting of Ca-A and Na-A type zeolites coated with silica on the crystal surface.

In a preferred embodiment, step C. The adsorbent adsorbing moisture, siloxane, hydrogen sulfide, methyl mercaptan, and CO ₂ is guided to a relatively low pressure condition, and the respective components are desorbed from the adsorbent to regenerate the adsorbent. A method for recovering and purifying methane by a pressure swing method that repeats the steps B and B is provided.

In a preferred embodiment, in step A, moisture, siloxane, hydrogen sulfide, methyl mercaptan, and CO ₂ are adsorbed near atmospheric pressure, and desorbed under vacuum and reduced pressure conditions.

In a more preferred embodiment, one column filled with the adsorbent at a relatively high pressure after completion of the adsorption step and another column filled with the adsorbent at a relatively low pressure after completion of the desorption step are connected at the rear of the column. Provided is a method for recovering and purifying methane by a pressure swing method with a coexisting gas that improves methane recovery by transferring methane remaining from one tower after the adsorption process to the other tower after the desorption process.

In a preferred embodiment, a gas mainly composed of desorbed CO ₂ is supplied to the adsorbent adsorption tower after completion of the adsorption process from the front of the tower, and methane remaining in the tower is purged from the rear of the tower to purge gas. To improve the methane recovery rate.

The waste is removed from the exhaust gas containing biofermented gas, and the environment is not polluted. In addition, methane can be effectively recovered and used for fuel and the like, and resources can be recovered effectively.

Embodiments of the present invention will be described in detail below with reference to the drawings.

As used herein, the term “biofermentation gas” is generated as biogas by anaerobic fermentation of biomass, and includes moisture, CO ₂ , siloxane, hydrogen sulfide, methyl mercaptan and methane and other impurities. Containing.

In the present invention, the acid-treated silica gel adsorbent used for adsorbing moisture and siloxane can be obtained by treating commercially available silica gel with an acid aqueous solution. Silica gel reacts with hydroxyl groups and siloxanes, hydrolyzing siloxanes and precipitating silicic acid at the adsorption active sites, leading to a decrease in adsorption performance. Here, when the silica gel is subjected to an acid treatment, the hydroxyl group is protonated and the hydrolysis of the siloxane is remarkably suppressed. For this reason, the adsorption of siloxane becomes reversible, and smooth adsorption-desorption proceeds. The acid used to treat the silica gel is not particularly limited, and any acid may be used, but it is common to use sulfuric acid, nitric acid, etc., which are easily available. Further, the acid concentration of the acid aqueous solution is not particularly limited, and may be any concentration. The temperature and time for treating the silica gel with the aqueous acid solution are not particularly limited, and any temperature may be used, but the higher the temperature, the shorter the treatment time.

In the present invention, the adsorbent used for adsorbing hydrogen sulfide and methyl mercaptan is silicalite or an adsorbed silicalite adsorbent. A commercially available silicalite may be used. Silicalite that has been subjected to acid treatment uses hydrochloric acid, phosphoric acid, boric acid, etc. as the acid, and the acid is added to pure or deionized water to prepare an aqueous solution with a pH of about 0.5-6. Suspends silicalite in an aqueous solution and stirs for about 10 minutes to 3 hours to allow the silicalite to carry an acid.

Hydrolysis proceeds by the reaction of the hydroxyl group with H ₂ S and methyl mercaptan, and elemental sulfur is deposited at the adsorption active site, leading to a decrease in adsorption performance. When silicalite is used here, precipitation of elemental sulfur is suppressed because the hydroxyl group concentration on the adsorbent surface is low. Further, when the silicalite is subjected to an acid treatment, the remaining hydroxyl group is protonated and the precipitation of elemental sulfur is remarkably suppressed. For this reason, the adsorption of H ₂ S and methyl mercaptan becomes reversible, and smooth adsorption / desorption proceeds. You may mix and use the silicalite which performed the silicalite and the acid treatment.

In the present invention, a Ca-A or Na-A type zeolite having a silica coating on the crystal surface used for adsorbing CO ₂ is obtained by suspending zeolite powder in a slurry form in a solvent, for example, methyl alcohol, to which a template, For example, when tetraethoxyorthosilicate (TEOS) is added to the crystal surface in an amount corresponding to the required thickness, and water is added thereto at a H ₂ O / TEOS ratio of about 5 to 20, silica is precipitated.

After the coating is completed, silica sol is added to prepare a slurry of about zeolite: silica sol: deionized water = 5 to 30: 1 to 10: 100, and this is immersed and supported on the honeycomb substrate, and the temperature is about 90 to 150. The surface moisture was removed at 0.5 ° C for about 0.5 to 3 hours, heated at about 30 to 80 ° C / h and maintained at about 250 to 450 ° C for about 0.5 to 3 hours to complete the dehydration of silicic acid. Thus, the Si—O—Si network on the zeolite crystal surface is completed and the activation by dehydration is completed. Under this coating condition, a silica thin film of 0.05 to 0.1 μm is purified on the crystal surface.

The amount of CO ₂ adsorption is larger for Ca-A, and Na-A is larger for the CO ₂ / CH ₄ separation factor. Under this coating condition, a silica thin film of 0.05 to 0.1 μm is purified on the crystal surface.
Since Na-A has a window diameter of 4 mm, Ca-A has a window diameter of 5 mm, CO ₂ molecular diameter of 3.2 mm, and CH ₄ molecular diameter of 4.2 mm, both Na-A and Ca-A have CO ₂ adsorption rates. CH ₄ greater than the adsorption rate. The silica-coated Ca-A and Na-A type zeolites may be mixed and used.

Hereinafter, the sequence of one embodiment of the present invention is shown in Table 1, and will be described with reference to FIG.

First step (equalization process between tower A, tower B and tower)
In FIG. 1, when the tower A after the adsorption process and the tower B after the regeneration process are opened, the valves 8a and 8b behind the tower are opened, and the methane remaining behind the tower A is transferred to the tower B for high efficiency. Since the pressure is recovered and the pressure in the towers A and B is equalized, smooth pressure increase for the adsorption process and smooth pressure decrease for the pressure reduction process proceed.

Second step (A tower-pressurization process, B tower-depressurization process)
When the tower A and the product tank 12 that have been pressurized to equal pressure are connected by the valve 8b, the product methane is supplied from the rear of the tower A and the pressure is increased to a point close to an adsorption pressure of 80 to 150 kPA. When the B tower decompressed to equal pressure is connected to a vacuum pump through the valve 9b, the pressure inside the tower is reduced and adsorbed siloxane, hydrogen sulfide, methyl mercaptan, moisture, and CO ₂ are desorbed. Here CO ₂ to desorb from the most downstream CO ₂ adsorbent, siloxane is upstream of strongly adsorbed component, hydrogen sulfide, methyl mercaptan, to act as a purge gas at the time of moisture desorption significantly the partial pressure of these gases in the tower Desorption progresses with high efficiency because of lowering.

Third step (A tower-adsorption process, B tower-regeneration process)
Biolite gas containing methane, CO ₂ , moisture, hydrogen sulfide, methyl mercaptan, siloxane is treated with silicalite or acidite treated with hydrogen sulfide, methyl mercaptan as an adsorbent through flow blower 2 and valve 3a, It is supplied to a honeycomb of acid-treated silica gel as a moisture adsorbent, a siloxane adsorbent, and a Ca-A or Na-A type zeolite granular product having a silica coat on the crystal surface as a CO ₂ adsorbent or a packed tower 4a of the honeycomb. In the packed tower 4a, a honeycomb adsorbent filled with silicalite-based hydrogen sulfide, methyl mercaptan adsorbent, silica gel-based siloxane, moisture selective adsorbent, and CO ₂ adsorbent of A-type zeolite silica coated product in this order from the upstream. 5 is filled. (The biofermentation gas supply is stopped immediately before CO ₂ flows from the back of the packed tower 4a. The packed tower 4b is in a state in which the CO ₂ adsorption zone has moved to some extent by decompression to the rear of the tower, and is supplied from the flow path 7. The product methane is supplied through the pressure reducing valve 18 and the valve 8b and brought into countercurrent contact with the adsorbent honeycomb 5, whereby siloxane, hydrogen sulfide, methyl mercaptan, moisture, and CO ₂ are desorbed.

Since the desorbed siloxane and CO ₂ are harmless, they are discharged out of the system. Hydrogen sulfide and methyl mercaptan are generally absorbed and fixed in an alkaline solution. Thus, according to the present invention, no harmful substances are released into the environment.

Here, the same operation as the first to third steps is performed in the fourth to sixth steps by changing the A tower and the B tower.

Hereinafter, the present invention will be described more specifically with reference to examples.

The method of the present invention was carried out using the apparatus shown in FIGS. 1 and 2 under the following conditions.

First step (equalization process between tower A, tower B and tower)
In FIG. 1, when the tower A having an adsorption pressure of 120 kPa after completion of the adsorption process and the tower B having a regeneration pressure of 5 kPa after completion of the regeneration process are opened, the valves 8a and 8b at the rear of the tower are opened. Since the pressure in the towers A and B is equalized to about 60 kPa, smooth pressure increase for the adsorption process and smooth pressure reduction for the pressure reduction process proceed.

Second step (A tower-pressurization process, B tower-depressurization process)
When the A tower boosted to about 60 kPa and the product tank 12 are connected by the valve 8b, the product methane is supplied from the rear of the A tower and the pressure is increased to a position close to the adsorption pressure of 120 kPa. When the column B reduced in pressure to about 60 kPa is connected to a vacuum pump through the valve 9b, the adsorbed siloxane, hydrogen sulfide, methyl mercaptan, moisture, and CO ₂ are desorbed by reducing the pressure in the column to 10 kPa or less. Here CO ₂ to desorb from the most downstream CO ₂ adsorbent, siloxane is upstream of strongly adsorbed component, hydrogen sulfide, methyl mercaptan, to act as a purge gas at the time of moisture desorption significantly the partial pressure of these gases in the tower Desorption progresses with high efficiency because of lowering.

Third step (A tower-adsorption process, B tower-regeneration process)
100m ³ N / h of biofermented gas containing methane 60vol%, CO ₂ 35vol%, moisture 5vol%, hydrogen sulfide 100ppm, methyl mercaptan 10ppm, siloxane 100ppm is passed through the blower 2 and valve 3a through the blower 2 and valve 3a. silicalite or acid-treated silicalite subjected as mercaptan adsorbent, moisture adsorbents, honeycomb silica gel acid treated as siloxane adsorbent, Ca-a was subjected to silica coated on the crystal surface as a CO ₂ adsorbent or Na-a Is supplied to a granular zeolite product or honeycomb packed tower 4a. Packed column 4a diameter 30 cm, height 120cm of a size where the silicalite-based hydrogen sulfide 90l, methyl mercaptan adsorbents, silica gel-based siloxane, moisture-selective adsorbent, A-type zeolite silica-coated article of CO ₂ adsorption The honeycomb 5 filled with the agent from the upstream in this order is filled. (The superficial velocity is 0.5 m / sec and the adsorption load is 650 m ³ N / h / ton.) The biofermentation gas supply is stopped immediately before CO ₂ flows from the back of the packed tower 4a. The packed tower 4b is in a state in which the CO ₂ adsorption zone has moved to some extent by the decompression to the rear of the tower, and 4 m ³ N / h of product methane supplied from the flow path 17 is supplied through the pressure reducing valve 18 and the valve 8b. By countercurrent contact with 5, siloxane, hydrogen sulfide, methyl mercaptan, moisture, and CO ₂ are desorbed.

Here, the same operation as the first to third steps is performed in the fourth to sixth steps by changing the A tower and the B tower.

Example 1: Ca-A was subjected to silica-coated as CO ₂ selective adsorbent, Preparation Examples and Evaluation CO ₂ selective adsorbent honeycomb 5 of Na-A type zeolite, Na-A, Ca-A , Comparative evaluation of Na-A (10 nm), Ca-A (10 nm), Na-A (50 nm), Ca-A (50 nm), Na-A (100 nm), and Ca-A (100 nm) was performed.

Here, the values in () of Na—A and Ca—A are the thickness of the silica coat thin film. Here, for the thin film growth on the zeolite crystal by the silica coat of Na-A and Ca-A, the zeolite powder is suspended in a slurry form in methyl alcohol, and tetraethoxyorthosilicate (TEOS) is added to the crystal surface at the required thickness. When an amount corresponding to the above is added and moisture is added to this at an H ₂ O / TEOS ratio of about 10, silica is precipitated. (This time, adjustment was made so that 10 to 20 nm of silica was precipitated by one coating, and this time, adjustment was made so that 50 nm was obtained 3 times and 100 nm was obtained 5 times.
After coating is completed, silica sol is added and a slurry is prepared with zeolite: silica sol: deionized water = 15: 3: 100, and this is immersed in a honeycomb base material to carry a bulk specific gravity of about 0.4. Remove surface moisture over time, heat up at 50 ° C / h and hold at 350 ° C for 1 hour to complete silicic acid dehydration to complete the Si-O-Si network on the zeolite crystal surface and dehydration The activation by ends.

The results are shown in Table 2.

In both cases, the methane concentration in the raw material gas exceeds 60 vol%, and the methane recovery rate of Li—X (SiO ₂ / Al ₂ O ₃ ratio 2) currently used as a CO ₂ adsorbent is exceeded. The effectiveness of is shown.

In particular, Na-A, Ca-A and these silica-coated products showed high CO ₂ adsorption performance showing a molecular sieve effect on methane. Ca-A (50 nm) showed the highest CO ₂ removal performance. This seems to be due to the relatively large CO ₂ adsorption rate and the window diameter (crystal gas passage) that has a molecular sieving effect on methane.

Example 2: Preparation Example and Performance Evaluation of Silicalite Treated with Acid as Sulfur Compound-Selective Adsorbent Silicalite Silicalite (1), Silicalite (4), acid-treated as sulfur compound-selective adsorbent honeycomb 5 Siliconite (PO4-0.5), Siliconite (PO4-0.5), Siliconite (PO4-1), Siliconite (PO4-2), Siliconite (PO4-4), Siliconite (BO3-0.5), Siliconite ( BO3-0.5), Siliconite (BO3-1), Siliconite (BO3-2), Siliconite (BO3-4) and untreated silicalite were comparatively evaluated.

Here, the values in () of Silicalite are the pH of the treatment solution when it is acid-treated with HCl, the carrying amount (mass%) when phosphoric acid treatment is carried, and the carrying amount (mass%) when boric acid is carried. . First, for the acid treatment of silicalite by solution in HCl solution treatment, HCl was added to pure water to prepare HCl solutions of pH 1 and pH 4, and silicalite was suspended in these aqueous solutions and stirred for 30 minutes.

After completion of stirring, filtration is performed, silica sol is added, and a slurry is prepared with zeolite: silica sol: deionized water = 15: 3: 100, and this is immersed in a honeycomb base material to carry a bulk specific gravity of about 0.4, The surface moisture is removed at 110 ° C. for 1 hour, the temperature is raised at 50 ° C./h, and the temperature is maintained at 350 ° C. for 1 hour to complete the acid treatment of silicalite.

Next, for phosphoric acid and boric acid, 15 g of silicalite and A gram (g) of phosphoric acid or boric acid are added to 100 g of deionized water, where A gram (g) is Cw% (1) Calculate with the formula.

A = 15 × C × 1/100 (1)
The results are shown in Table 3.

HCl treatment, phosphoric acid dope, and boric acid dope all show durability against removal of hydrogen sulfide and methyl mercaptan during long-time operation compared to untreated silicalite. This is also confirmed from the fact that the amount of sulfur deposited on the adsorbent after operation for 720 hours is suppressed compared to untreated silicalite. Here, with regard to HCl treatment, those with pH 1 showed more excellent durability, and it was evaluated that the addition of about 2 g was optimal for phosphoric acid dope and boric acid dope.

This is because when the acid point strength near the adsorption active point increases, the amount of chemical adsorption of hydrogen sulfide and methyl mercaptan decreases, and the Claus reaction (Equation (2.1 to 2.3)) at the adsorption active point decreases. I think that the.

_{H 2 S + 1 / 2O 2} → S + H 2 O (2.1)
_{H 2 S + 3 / 2O 2} → SO 2 + H 2 O (2.2)
2H ₂ S + SO ₂ → 3S + 2H ₂ O (2.3)
Example 3: Preparation Example and Performance Evaluation of Silica Gel Treated with Acid as Siloxane-Selective Adsorbent As Silica-Selective Adsorbent Honeycomb 5, acid-treated silicalite silica gel (1), silica gel (4), silica gel (PO4) -0.5), silica gel (PO4-0.5), silica gel (PO4-1), silica gel (PO4-2), silica gel (PO4-4), silica gel (BO3-0.5), silica gel (BO3-0) .5), silica gel (BO3-1), silica gel (BO3-2), silica gel (BO3-4) and untreated silica gel were comparatively evaluated.

Here, the values in () of silica gel are the pH of the treatment solution when it is acid-treated with HCl, the loading amount (mass%) when phosphoric acid treatment is performed, and the loading amount (mass%) when boric acid is supported. . First, for acid treatment of silica gel with a solution in HCl solution treatment, HCl was added to pure water to prepare HCl solutions of pH 1 and pH 4, and the silica gel was suspended in these aqueous solutions and stirred for 30 minutes.

After completion of stirring, filtration is performed, silica sol is added, and a slurry is prepared with silica gel: silica sol: deionized water = 15: 3: 100, and this is immersed in a honeycomb base material to carry a bulk specific gravity of about 0.4, The surface moisture is removed at 110 ° C. for 1 hour, the temperature is raised at 50 ° C./h, and the temperature is maintained at 350 ° C. for 1 hour to complete the acid treatment of the silica gel.

Next, for phosphoric acid and boric acid, 15 g of silica gel and phosphoric acid or boric acid A gram (g) are added to 100 g of deionized water, where A gram (g) is the formula (1) where the target loading is Cw%. Calculate with

A = 15 × C × 1/100 (1)
The results are shown in Table 4.

HCl-treated silica gel shows durability against siloxane removal during long-time operation compared to untreated silica gel. This is also confirmed from the fact that the amount of silica deposited on the adsorbent after operation for 720 hours is suppressed compared to untreated silica gel. Here, with regard to the HCl treatment, those having a pH of 1 showed more excellent durability, and it was evaluated that the addition of about 2 g was the most optimal for the phosphoric acid dope and the boric acid dope.

This seems to be because when the acid point strength near the adsorption active site increases, the amount of siloxane chemisorption decreases, and the hydrolysis reaction of siloxane at the adsorption active site (formula (3)) decreases.

Rl · Sim · On + kH ₂ O → mSiO ₂ + lROH (4)
Next, silica gel acid-treated with a pH 1 HCl solution as a siloxane adsorbent, hydrogen sulfide, silicalite acid-treated with a pH 1 HCl solution as a methyl mercaptan adsorbent, a silica gel honeycomb as a water adsorbent, a CO ₂ adsorbent Using a granular cast of Ca-A zeolite (1.6 mm diameter pellets) with silica coating (coat thickness 50 nm) three times using TEOS on the crystal surface, the raw material flow rate, product methane concentration, product The relationship of methane recovery was investigated. The results are shown in Table 5.

As the raw material flow rate decreases, the methane concentration at the outlet of the adsorption tower increases, but the methane recovery rate decreases. Therefore, when the flow rate is reduced to about 80 m ³ N / h, the methane concentration increases to 96 vol%, while when the raw material flow rate is increased to 150 m ³ N / h, the methane recovery rate increases to 95%.

Next, the relationship between the cycle time, the product methane concentration, and the product methane recovery rate at a raw material gas flow rate of 100 m ³ N / h under the same adsorbent filling conditions was examined. The results are shown in Table 6.

As the cycle time decreases, the product concentration increases and the product methane recovery rate also increases. With a cycle time of 1 minute, the product methane concentration reached 95 vol% and the product methane recovery rate reached 94%. This is presumably because CO ₂ having a small molecular diameter is adsorbed even in a short cycle time due to the Ca-A molecular sieve effect, but methane having a large molecular diameter is difficult to be adsorbed. Therefore, it is possible to simultaneously reduce the amount of adsorbent used by shortening the cycle time, improve the product methane concentration, and improve the product methane recovery rate.

Next, the relationship between the amount of product methane purge gas, product methane concentration, and product methane recovery rate was examined. The results are shown in Table 7.

As the product methane purge amount increases, the product methane concentration increases and reaches 98 vol% at 8 m ³ N / h. However, since the methane once recovered is used as the purge gas, the methane recovery rate is reduced to 86%. On the other hand, when the product methane purge amount is reduced to 2 m ³ N / h, the product methane concentration decreases to 84%. Since the recovery rate increases to 94%, this condition may be adopted when a high methane concentration is not required.

All the product methane recovery so far is between the towers by connecting the adsorption tower after the adsorption process (adsorption pressure 120 kPa) and the adsorption tower after the regeneration (regeneration pressure 5 kPa) at the rear so that the pressure in the tower is about 62.5 kPa. A pressure equalization was adopted. Here, the relationship between the product methane concentration and the product methane recovery rate with the case of shifting to the depressurization step after the end of the adsorption step and shifting to the pressurization step after the end of the regeneration step was examined without adopting the uniform pressure between the columns. The results are shown in Table 8.

When there is no uniform pressure between the towers, the product methane concentration increases to 94 vol%, but the product methane recovery rate remains at 72%. On the other hand, if the uniform pressure between the columns introduced so far is adopted, the product methane concentration is slightly reduced to 92 vol%, but the recovery rate is very high at 92%. It can be understood that the uniform pressure between the columns can be easily transferred from the pressure change of adsorption-desorption-adsorption, and is very effective from the viewpoint of improving the recovery rate and smooth PSA operation.

Example 4
In the first embodiment, the product adsorption of methane was performed in the “adsorption process” by means of pressure equalization / pressure increase / adsorption between the columns and in the “regeneration process” by pressure equalization / pressure reduction / counterflow purge between the columns. Since product methane is used as the purge gas in the countercurrent purge, the loss of product methane cannot be ignored. As a method of avoiding the loss of methane, in the “regeneration process”, when the adsorption tower after the adsorption process is purged with a desorption gas mainly composed of CO ₂ from the front of the tower, the methane remaining in the adsorption tower is replaced with CO _2. Methane flows from the rear of the tower, and the loss of methane in the desorption process is significantly reduced.

Fig. 2 shows the flow sheet of the equipment at this time, and Table 9 shows the equipment flow sheet.

In the figure, the same numbers as those in FIG. 1 indicate the same parts. In FIG. 2, when the vacuum pump 11 is used as a blower from the desorption gas tank 14 and the valves 15, 16, 19, 9b, and 20b are opened, the product methane remaining in the tower flows through the tower B after completion of the adsorption process. It is refluxed from the channel 21 to the channel 1 and collected.

This operation is called a cocurrent purge, but when the desorption gas amount is G2 (m ³ N / h) and the cocurrent purge gas flow rate is G4 (m ³ N / h), the cocurrent purge rate K is
K = G4 / G2
Define in. The desorption gas amount G3 is G3 = G2-G4.

Here, the relationship between the cocurrent purge rate, the product methane concentration, and the product methane recovery rate was examined. The results are shown in Table 10.

As the co-current purge rate increases, the product methane recovery rate increases, reaching a co-current purge rate of 70% and a recovery rate of 98%. However, since the product methane concentration decreases as the cocurrent purge rate increases, the maximum value of the cocurrent purge rate should be limited to about 70%.

It is a figure which shows the 1st embodiment of this invention. It is a figure which shows the 2nd embodiment of this invention.

Explanation of symbols

4a, 4b packed tower 5 adsorbent honeycomb 12 product tank 14 desorption gas tank 18 pressure reducing valve

Claims (6)

[Correction based on Rule 91 17.03.2008]
A method for separating methane gas from a biogas generated by anaerobic fermentation of organic waste by a pressure swing method, comprising the following steps:
A. Pressurize the biogas and supply it to the adsorption tower.
B. In the adsorption tower,
(1) Under relative high pressure conditions, the biogas is removed from the moisture through the moisture adsorbent,
(2) The siloxane is adsorbed and removed through an acid-treated silica gel adsorbent,
(3) Hydrogen sulfide and methyl mercaptan are adsorbed and removed through silicalite or acid-treated silicalite adsorbent,
(4) A method for recovering and purifying methane by adsorbing and removing CO ₂ through at least one zeolite adsorbent selected from the group consisting of Ca-A and Na-A type zeolites coated with silica on the crystal surface. Further, Step C. Regenerating said adsorbent by introducing said adsorbent adsorbed with moisture, siloxane, hydrogen sulfide, methyl mercaptan, CO ₂ to a relatively low pressure condition and desorbing each component from said adsorbent; The method for recovering and purifying methane according to claim 1, wherein the process according to claim 1 is repeated by a pressure swing method. [Correction based on Rule 91 17.03.2008]
The methane according to claim 1, which is obtained by a pressure swing method of methane and a coexisting gas that adsorbs moisture, siloxane, hydrogen sulfide, methyl mercaptan, CO ₂ in the vicinity of atmospheric pressure in the process according to claim 1 and desorbs under vacuum decompression conditions. Recovery and purification method. [Correction based on Rule 91 17.03.2008]
Adsorbing one tower packed with the relatively high-pressure adsorbent after completion of the adsorption process according to claim 1 and another tower packed with the relatively low-pressure adsorbent after the desorption process at the rear of the tower A method for recovering and purifying methane by a pressure swing method with a coexisting gas that improves the methane recovery rate by transferring methane remaining from one tower after the process to the other tower after the desorption process is completed. [Correction based on Rule 91 17.03.2008]
A gas mainly composed of desorbed CO ₂ is supplied to the adsorbent adsorption tower after the adsorption step from the front of the tower, methane remaining in the tower is purged from the rear of the tower, and a purge gas is refluxed to the raw material line to methane. The method for recovering and purifying methane according to claim 1 or 2, based on a pressure swing method with a coexisting gas that improves the recovery rate. The method for recovering and purifying methane according to claim 1, wherein the biofermentation gas contains water, CO ₂ , siloxane, hydrogen sulfide, methyl mercaptan, methane, and other impurities.

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