Gas separation: processes – Solid sorption – Including reduction of pressure
Reexamination Certificate
2001-04-20
2003-02-04
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Solid sorption
Including reduction of pressure
C095S106000, C095S115000, C095S117000, C095S130000, C095S139000, C095S140000, C095S143000, C095S902000
Reexamination Certificate
active
06514317
ABSTRACT:
BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to a method for purifying a hydrogen-based gas mixture by adsorptive separation of impurity gases from the gas mixture. More particularly, it relates to a method for purifying a hydrogen-based gas mixture such as a gas produced by steam reforming of naphtha or other petroleum hydrocarbons, a coke oven gas, or a converter gas by adsorptive separation of impurity gases such as carbon monoxide, nitrogen and hydrocarbons such as methane from the hydrogen-based gas mixture.
(2) Description of the Related Art
Hydrogen-based gas mixtures such as a gas produced by steam reforming of naphtha or other petroleum hydrocarbons, a coke oven gas, or a converter gas contain impurity gases such as carbon monoxide, nitrogen and hydrocarbons. To recover hydrogen gas having a purity of at least 99% from the hydrogen-based gas mixtures, these impurity gases must be removed by adsorption or other means.
As means for separating carbon monoxide, nitrogen, hydrocarbons, carbon dioxide and water vapor, an adsorption method using an adsorbent is generally adopted. As the adsorbent, activated carbon and zeolite are widely used.
Purification of a hydrogen-based gas mixture for obtaining hydrogen gas having a high purity is generally carried out by a pressure swing adsorption (PSA) method. One-stage PSA process enables purification of the hydrogen-based gas mixture to remove impurity gases such as carbon monoxide, nitrogen, hydrocarbons, carbon dioxide and water vapor. However, in order to obtain hydrogen gas having a higher purity, a two-stage process is adopted wherein carbon dioxide, water vapor and hydrocarbons are previously removed by adsorption using activated carbon, and then, carbon monoxide, nitrogen and residual hydrocarbons are removed by adsorption using a zeolite
Conventional zeolite adsorbents include type A zeolite, and faujasite zeolite having a SiO
2
/Al
2
O
3
molar ratio of at least 2.5. However, a zeolite adsorbent exhibiting a higher adsorbability for carbon monoxide, nitrogen and hydrocarbons is eagerly desired in a H
2
-PSA method for reducing the size of equipment and the energy consumption.
A method of recovering hydrogen by using a type X zeolite having a SiO
2
/Al
2
O
3
molar ratio of 2 to 3 and exchanged with a calcium ion at an exchange ratio of at least 50% was proposed in Japanese Unexamined Patent Publication (hereinafter abbreviated to “JP-A”) No. H4-6642. The use of a type X zeolite having an Si/Al atomic ratio of less than 1.5 and exchanged with a lithium ion at an exchange ratio of at least 80% for the adsorptive removal of carbon monoxide from a hydrogen-based gas mixture was proposed in U.S. Pat. No. 5,912,422. The use of a type X zeolite having an Si/Al atomic ratio of 1 to 3 and exchanged with a lithium ion and a calcium ion at an exchange ratio of at least 85% for the removal of carbon monoxide and/or nitrogen from a hydrogen-based gas mixture was proposed in JP-A H10-212,103. Further, the use of a low-silica type X zeolite having a SiO
2
/Al
2
O
3
molar ratio of 2 for the adsorptive removal of carbon monoxide was proposed in JP-A 2000-225311. However, the above-mentioned zeolite adsorbents include a binder exhibiting no adsorption capacity, and hence, its adsorption performance was poor, as explained below.
In general zeolite adsorbents are used in the form of a shaped product prepared by incorporating a binder in zeolite and shaping the mixture of zeolite and the binder into a desired shape. However, a binder has no adsorption performance, and hence, the zeolite adsorbent containing a binder has a relatively poor adsorption performance. Therefore, proposals of using a binder capable of being converted to zeolite have been made. For example, a method of producing a low-silica type X zeolite binderless shaped product has been proposed in JP-A H5-163,015 wherein a shaped product comprised of a type X zeolite powder having a SiO
2
/Al
2
O
3
molar ratio of smaller than 2.5, metakaolin converted from kaolin clay, sodium hydroxide and potassium hydroxide is maintained in an aqueous solution containing sodium hydroxide and potassium hydroxide at a temperature of 40 to 100° C. for several hours to several days whereby metakaolin is aged and converted to zeolite to give a binderless shaped product of a low-silica type X zeolite.
A shaped product comprised of at least 95% of a low-silica type X zeolite having a SiO
2
/Al
2
O
3
molar ratio of 2, which is prepared by using a binder capable of being converted to a zeolite, is described in JP-A H11-76810. This shaped product is prepared by a process wherein a low- silica type X zeolite is agglomerated by using a binder containing at least 80% of a clay capable of being converted to a zeolite; the thus-obtained agglomerate is shaped; the shaped product is dried and then calcined at a temperature of 500 to 700° C.; and the thus-obtained solid product is placed in contact with an aqueous alkali solution containing an alkali of at least 0.5 molar concentration comprising sodium hydroxide and potassium hydroxide, wherein the proportion of potassium hydroxide is not larger than 30% by mole based on the sum of sodium hydroxide and potassium hydroxide. The low-silica type X zeolite-containing shaped product has very low crush strength and contains a small amount of type A zeolite. Thus, the SiO
2
/Al
2
O
3
molar ratio as determined by chemical analysis is larger than the theoretical value, i.e., 2.0, and the low-silica type X zeolite in the shaped product has a low purity.
Further a shaped product comprised of at least 95% of a faujasite zeolite, which is prepared by using a binder capable of being converted to a zeolite, is described in WO 99/05063. This shaped product is prepared by a process which is similar to the above-mentioned process described in JP-A H11-76810 and in which a shaped product of an agglomerate comprised of LSX zeolite and a clay capable of being converted to zeolite is placed in contact with an aqueous alkali solution containing an alkali of at least 0.5 molar concentration. This shaped product of a low-silica type X zeolite has a low crush strength, and this literature is silent on the use thereof for adsorptive separation of carbon monoxide, nitrogen and methane for the purification of hydrogen, and suggests nothing about the cation species effective for enhancing adsorptive performance for these gases.
SUMMARY OF THE INVENTION
A primary object of the present invention is to provide an improved method for purifying a hydrogen-based gas mixture by adsorptive separation of impurities such as carbon monoxide, nitrogen and hydrocarbons, especially methane, using a special zeolite adsorbent.
In accordance with the present invention, there is provided a method for purifying a hydrogen-based gas mixture comprising the step of contacting a hydrogen-based gas mixture with a zeolite adsorbent; said zeolite adsorbent being a shaped product comprised of at least 95%, as determined on the basis of the moisture equilibrium adsorption value, of a low-silica type X zeolite having a SiO
2
/Al
2
O
3
molar ratio in the range of 1.9 to 2.1.
The shaped product of the low-silica type X zeolite preferably has an average crush strength of at least 1.0 kgf as measured on particles of the shaped product having a particle diameter in the range of 1.4 to 1.7 mm as prepared for crush strength measurement.
The cation contained in the shaped product of the low-silica type X zeolite is preferably at least one kind of cation selected from lithium, sodium, potassium, magnesium, calcium and zinc. The cation contained therein has preferably been ion-exchanged with a lithium ion at an ion exchange ratio of at least 90%, more preferably at least 95%, or with a calcium ion at an ion exchange ratio of at least 50%, more preferably at least 80% and most preferably at least 90%.
The shaped product is preferably comprised of at least 98% by weight, as determined on the basis of the moisture equilibrium adsorption value, of the low-silica type X zeolite. More preferably i
Harada Atsushi
Hirano Shigeru
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Spitzer Robert H.
Tosoh Corporation
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