JP2572616B2 - How to remove carbon dioxide - Google Patents

Description

The present invention relates to a method for removing carbon dioxide, and more particularly, to a method for removing carbon dioxide using zinc oxide as an adsorbent.

Gases such as hydrogen, nitrogen, helium, argon and oxygen are commercially available in packed or liquefied state and are used for various industrial and academic purposes.

In recent years, with the advancement of technologies in fields such as a semiconductor manufacturing process, an optical fiber manufacturing process, a metal heat treatment process, and analysis, these gases are also required to have high purity.

[Conventional technology]

Various purification methods are known according to the type of impurities contained in the gas in order to obtain a high-purity gas. For impurities such as hydrogen, hydrocarbons, and carbon monoxide, these are burned. A method has been used in which carbon dioxide and water are converted to water, and the carbon dioxide and water, which are originally mixed, are brought into contact with an adsorbent to remove these.

As these adsorbents, synthetic zeolites are generally and widely used, for example, molecular sieves (Union Carbide, USA) are best known. The adsorbent having adsorbed impurities is repeatedly used by regenerating at high temperature.

[Problems to be solved by the invention]

Although the degree of integration of semiconductors has increased, the purity of gas has also increased, and the concentration of impurities has been decreasing.
Further, it is required to remove impurities. On the other hand, in a semiconductor process or the like, there is a strong demand for downsizing of the device due to the installation space and the like.

When the equilibrium adsorption amount of low-concentration carbon dioxide is measured for molecular sieve 5A that is currently widely used, when the concentration of carbon dioxide is as low as 1 ppm, it is about 1 / Drops to 7. That is,
In order to purify the same amount of gas, the volume of the adsorption column is reduced to only 7/10 even if the concentration is reduced to 1/10. To make the adsorption cylinder even smaller, shorten the cycle of heating and regeneration, or
Alternatively, it is necessary to increase the amount of adsorption by lowering the adsorption temperature to a low temperature such as 0 ° C. or lower.

However, shortening the heating cycle also has a design upper limit, and lowering the adsorption temperature requires installation of a chiller, a refrigerator, and the like, and thus has a drawback that it cannot meet the demand for downsizing.

It has also been experimentally known to use calcium oxide, magnesium oxide, barium oxide, caustic potash, soda lime, etc. to fix and remove carbon dioxide as an alternative to synthetic zeolite. There is a problem that the removal capacity is small, the purity of the outlet gas must be monitored and the chemical must be replaced frequently, and the regeneration after adsorption is difficult and cannot be repeated.

[Means and actions for solving the problems]

The present inventors did not particularly require cooling, and as a result of intensive research to efficiently remove carbon dioxide contained in the gas together with accompanying moisture and the like, as a result, zinc oxide formed into a granular form as a main component The present inventors have found that impurities can be efficiently removed by using an adsorbent that can be used, and that the adsorbent can be used repeatedly by heating and regenerating.

That is, in the present invention, a gas containing carbon dioxide as an impurity is brought into contact with an adsorbent, and in a carbon dioxide removal method for removing carbon dioxide from the gas, a molded body mainly composed of zinc oxide is used as an adsorbent. A method for removing carbon dioxide, characterized in that:

The present invention is applied to the removal of carbon dioxide contained as an impurity in gases such as hydrogen, nitrogen, helium, argon and oxygen. When moisture is contained in these gases, the moisture is also removed at the same time.

The adsorbent used in the present invention is a molded product containing zinc oxide as a main component.

As the zinc oxide, those manufactured by various methods such as a wet method using an aqueous solution of a zinc salt as a raw material, a French method using metal zinc as a raw material, and an American method using zinc ore as a raw material can be used. The wet method is generally preferred in that a material having a large specific surface area can be obtained. As a wet method, for example, an aqueous solution of zinc nitrate, zinc chloride or the like is added with an aqueous solution of soda ash to precipitate basic zinc carbonate, which is filtered, washed with water, dried, and further calcined at 200 to 700 ° C., thereby oxidizing. Should be zinc. Alternatively, an appropriate zinc oxide may be selected from commercially available zinc oxide.

The adsorbent used in the present invention is obtained by molding zinc oxide into pellets or the like, or by crushing a molded product to an appropriate size. As a molding method, a dry method or a wet method can be used. For example, as the former, there is a method in which a small amount of a lubricant such as graphite or talc is mixed with powder of zinc oxide to form a tablet, and as the latter, a binder such as alumina cement and water are added to zinc oxide and kneaded. There is a method of extrusion molding. These are refired as necessary. The mixing ratio of alumina cement to zinc oxide is usually 50 g or less, preferably 10 to 30 g, per 100 g of zinc oxide. Also, for example, a molded product of a zinc oxide by adding a lubricant, a binder and the like at the stage of the basic zinc carbonate may be fired at 300 to 700 ° C. to obtain a molded zinc oxide. Generally, a wet method is preferred from the viewpoint of ease of processing.

The size and shape of the molded pair are not particularly limited, but spherical, cylindrical, cylindrical and the like are representative examples,
In some cases, these are used after being crushed to an appropriate size. The size of the molded body is 1 to 10 mm in diameter if it is spherical, 1 to 10 mm in diameter if it is cylindrical, and about 2 to 20 mm in height,
0.84 to 5.66 with a sieve opening
A range of mm is used.

The density of the molded body used in the present invention is usually 0.6 to 3.5 g / ml,
Preferably it is in the range of 0.8 to 2.5 g / ml. In the present invention, the density refers to a value obtained by dividing the weight of a molded article (particle) by the geometric volume of the molded article.

The filling density when the molded body is filled in an adsorption column is usually 0.5 to 2.0 g / ml, preferably 0.6 to 1.5 g / ml.

In the present invention, the adsorbent is usually filled in an adsorption column, and the carbon dioxide in the gas is adsorbed and removed by passing a gas containing carbon dioxide through the gas and bringing them into contact with each other. At this time, when water is contained in the gas, the water is also removed at the same time, and a purified gas having a low dew point is obtained. The adsorption temperature is generally 80 ° C. or lower, and a lower one is preferable. However, it usually has a sufficient adsorption performance at room temperature, and does not particularly require cooling. The velocity of the gas at the time of contact may be the same as that in the case of using synthetic zeolite, and is usually 150 cm / sec or less, preferably 3 to 70 cm / sec, as the linear velocity of the cylinder. The pressure at the time of contact is not particularly limited, but is usually in the range of 1 to 10 kg / cm ² G for practical use.

In the present invention, usually, two adsorption columns are used, and the adsorption and purification of the gas and the heat regeneration of the adsorbent are alternately switched. The adsorbent that has adsorbed carbon dioxide, moisture, etc. is preferably at 200 to 500 ° C. while flowing a purified gas through the adsorption column.
By heating at 300 to 400 ° C., impurities such as carbon dioxide are desorbed, removed and regenerated, and used repeatedly for gas adsorption purification.

〔Example〕

Example 1 While a 20 wt% aqueous solution of zinc nitrate was stirred in a stirring tank, a 20 wt% aqueous solution of sodium carbonate was added dropwise thereto to form a precipitate of basic zinc carbonate. The precipitate was filtered, washed and dried at 120 ° C. for 10 hours.
Calcination for a period of time gave zinc oxide. A mixture of 41 g of zinc oxide thus obtained and 9 g of alumina cement was mixed with a small amount of water, kneaded with a kneader, and extruded and dried at 120 ° C. for 2 hours. Its density was 1.9 g / ml.

This molded product was crushed, and 1 g of a molded product of 20 to 35 mesh was obtained with an inner diameter of 7.
Filled into a 53mm, 100mm long stainless steel adsorption cylinder (filling density 1.3g / ml), and purified nitrogen at 350 ℃ 1.8Nl / mi
Reflowing was performed for 2 hours while flowing with n. Raw material nitrogen containing 10 ppm of carbon dioxide and having a dew point of -61 ° C (9 ppm as moisture) was introduced into this adsorption column at a pressure of 5 kg / cm ² G and a temperature of 35 kg.
C. and a flow rate of 1.8 Nl / i min for 6 hours to measure the amount of adsorbed carbon dioxide and the dew point of the outlet gas.

 The adsorption amount of carbon dioxide was measured as follows.

The gas flowing out of the outlet of the adsorption column was analyzed by FID-gas chromatography. The carbon dioxide discharged from the separation tube of the gas chromatograph is brought into contact with a nickel catalyst at 600 ° C. in the presence of hydrogen to convert it to methane, and then guided to an FID (hydrogen flame ionization detector) to detect the methane concentration. The concentration of carbon dioxide at the outlet is plotted against the time axis, the amount of carbon dioxide coming out of the outlet of the adsorption column is obtained from this curve, and the amount obtained by subtracting this amount from the amount of carbon dioxide in the raw material nitrogen is calculated as the carbon dioxide concentration. The amount of adsorption was used. The dew point was measured by a Hamada dew point meter.

As a result, the amount of carbon dioxide adsorbed was 2.55 mg / g adsorbent, and the dew point of the outlet gas was -76 ° C (1 ppm as moisture).

Next, a heating regeneration test of the adsorbent was performed. Purified nitrogen gas was flowed at a pressure of 5 kg / cm ² G and a flow rate of 1.8 Nl / min for 1 hour while heating the above-mentioned adsorption column adsorbing carbon dioxide at 350 ° C., and carbon dioxide in the outlet gas desorbed from the adsorbent was removed. The amount was 2.3 mg / g adsorbent as determined by the above measurement method.

Comparative Example 1 Example 1 was performed using an adsorbent obtained by crushing molecular sieve 5A (manufactured by Union Carbide, USA) into 20 to 30 mesh.
When the test was conducted in the same manner as above, the amount of carbon dioxide adsorbed was 0.65 mg / g adsorbent, and the dew point of the purified gas was -76 ° C.
Met. The carbon dioxide desorbed in the regeneration test
It was 0.59 g / g adsorbent.

Comparative Example 2 As a result of measuring the amount of adsorbed carbon dioxide under the same conditions as in Example 1 using a mixture of 79 g of calcium oxide and 17 g of magnesium oxide as an adsorbent under the same conditions as in Example 1, 0.59 mg / g
It was an adsorbent. Subsequently, the amount of desorbed carbon dioxide from the adsorbent was measured while heating at 350 ° C., but no peak of the methane curve by FID was observed.

Example 2 As a result of measuring the amount of adsorption by changing the concentration of carbon dioxide in the raw material nitrogen gas to 30.2 ppm in Example 1, the amount of adsorption was 4.0.
mg / g adsorbent. Subsequently, purified nitrogen was flowed at 350 ° C. for 1 hour, and the amount of desorption was measured. As a result, it was 3.5 mg / g adsorbent.

Examples 3 and 4 The nitrogen content in Example 1 was changed to hydrogen, and the content of carbon dioxide was
As a result of measuring the adsorption amount of two kinds, 10.2 ppm and 2.7 ppm, it was 3.05 mg / g adsorbent and 3.7 mg / g adsorbent.

Example 5 The content of carbon dioxide in Example 1 was changed by replacing nitrogen with oxygen.
As a result of measuring the amount of adsorption for 32 ppm, it was 2.17 mg / g adsorbent.

Example 6 The amount of adsorption was measured by changing the adsorption temperature to 55 ° C. in Example 1, and the result was 2.31 mg / g adsorbent.

〔The invention's effect〕

In the present invention, by using a zinc oxide molded body as an adsorbent, the adsorption performance is extremely high as compared with a method using a synthetic zeolite which has been conventionally used, and carbon dioxide can be efficiently removed. In addition, the adsorbent is regenerated at a relatively low temperature, like the synthetic zeolite, and can be used repeatedly. Therefore, the size of the device can be reduced as compared with the conventional dimensions, and the device can be easily installed in a limited space in semiconductor manufacturing or the like.

Claims (1)

(57) [Claims] 1. A method of removing carbon dioxide from a gas containing carbon dioxide as an impurity by contacting the gas with an adsorbent, wherein a molded body mainly composed of zinc oxide is used as the adsorbent. A method for removing carbon dioxide.