Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-10-20
2001-06-12
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
Reexamination Certificate
active
06244071
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for purifying a cryogenic fluid, such a nitrogen, oxygen, helium, hydrogen or argon, or in particular its NO
2
O, C
n
H
m
and/or NO
x
impurities.
BACKGROUND OF THE INVENTION
Cryogenic fluids such as oxygen, nitrogen, helium, hydrogen or argon are of great industrial importance, especially in the electronics field.
Thus, it is common practice to use nitrogen for inerting or cooling printed circuits during their manufacturing process and helium is often employed for cooling hot optical fibres.
At the present time, the various cryogenic fluids are obtained by cryogenic distillation from ambient air or from gas mixtures containing them or by non-cryogenic separation techniques, for example by pressure swing adsorption processes, usually called PSA processes, or by membrane permeation.
However, for some applications, especially in the electronics field, the cryogenic fluid must be of high purity, that is to say it must contain minimal amounts of impurities and other undesirable contaminants, in order to prevent these impurities from causing undesirable physico-chemical reactions or reactions incompatible with the intended aim.
Thus, it is usual to purify the cryogenic fluids very thoroughly, that is to say down to impurity levels of less than a few tens of ppb (parts per billion) or even to 1 ppb.
Up to the present time, many methods of purifying cryogenic fluids have already been proposed.
Thus, mention may be made of document EP A-662,595 which describes a process for preparing high-purity liquid nitrogen, in which process the carbon monoxide, oxygen and hydrogen impurities present in the liquid nitrogen are removed by adsorption on a zeolitic material or a material of the porous metal oxide type.
Furthermore, document U.S. Pat. No. 4,746,332 describes the removal of carbon monoxide present in liquid nitrogen by adsorption on a type 5A zeolite.
Moreover, EP-A-590,946 teaches prepurification, using a TSA (Temperature Swing Adsorption) type process, of carbon monoxide present in gaseous nitrogen at a temperature of 90 K. to 150 K., followed by ultrapurification by subsequent distillation of the nitrogen thus prepurified.
Document EP-A-750,933, for its part, relates to removal of carbon monoxide and oxygen which are present in liquid or gaseous nitrogen or argon by adsorption on a transition metal oxide or oxides, for example a hopcalite.
Moreover, document U.S. Pat. No. 4,717,406 teaches the purification of a stream by mechanical filtration and adsorption on an adsorbent, especially of the molecular-sieve, activated-charcoal or silica type.
Document WO-A-98/28226 recommends purifying a cryogenic fluid, such as helium, hydrogen or argon, of its impurities by mechanical filtration of the impurities in the form of crystals and adsorption of the impurities in dissolved or gaseous form, so as to obtain a purified fluid containing less than 1 ppb of impurities.
Furthermore document U.S. Pat. No. 4,425,143 describes a zeolite having enhanced adsorption performance and being characterized by a high Si/Al ratio in order to withstand acids, this zeolite being free of Fe
2
O
3
.
Document EP-A-747,118 teaches the removal of oxygen impurities from an inert gas by means of a support impregnated with an alkali or alkaline-earth metal oxide.
Finally, document U.S. Pat. No. 3,597,169 relates to a process for removing methane impurities contained in liquid oxygen by employing a zeolite X highly exchanged with calcium or silver cations.
In practice, the adsorbent particles used in these various processes usually have an average size of about 2 mm to 5 mm.
However, although these processes allow certain impurities contained in the cryogenic fluids, especially such as carbon monoxide, carbon dioxide or oxygen impurities, to be removed relatively well, it appears that the problem of effective removal of certain other impurities likely to be present in relatively large amounts in cryogenic fluids have not been solved hitherto, or have only been solved incompletely.
SUMMARY OF THE INVENTION
The object of the present invention is therefore to improve the existing processes by proposing a cryogenic-fluid purification process allowing effective removal of, in particular, N
2
O, C
n
H
m
and/or NO
x
impurities liable to be present in a cryogenic fluid, such as nitrogen, oxygen, helium, hydrogen or argon.
Within the context of the present invention, NO
x
should be understood to mean NO and/or NO
2
and C
n
H
m
should be understood to mean one or more hydrocarbons, such as molecules with a saturated or unsaturated carbon backbone.
The present invention therefore relates to a process for purifying a cryogenic fluid containing impurities chosen from N
2
O, C
n
H
m
(hydrocarbons) and NO
x
, in which process:
(a) at least some of the impurities contained in the cryogenic fluid to be purified are removed by bringing the cryogenic fluid to be purified into contact with particles of at least one adsorbent, the particles having an average size of less than or equal to 1.5 mm; and
(b) a purified cryogenic fluid is recovered which contains less than 100 ppb of the impurities, preferably less than 10 ppb of the impurities.
The reason for this is that the inventors of the present invention have demonstrated that the size of the adsorbent particles has a great influence on the performance, that is to say the effectiveness, of the purification process.
As shown in the examples below, it appears that a beneficial effect on the performance of the process may be obtained by reducing the size of the adsorbent particles used in the purification process, this probably being due to the improvement in the kinetics of impurity adsorption on the adsorbent.
Depending on the case, the process according to the invention comprises one or more of the following characteristics:
the particles have an average size of less than or equal to 1.4 mm, preferably less than or equal to 1.3 mm, preferably less than or equal to 1.2 mm and advantageously approximately 0.8 mm to 1.1 mm;
the cryogenic fluid is chosen from among oxygen, argon, nitrogen, hydrogen and helium;
the cryogenic fluid is in the liquid and/or gaseous state;
at least some of the impurities contained in the cryogenic fluid to be purified are removed by adsorption or by chemisorption on the adsorbent particles;
the removal of the impurities is carried out at a temperature below approximately −120° C., preferably below −150° C.;
the removal of the impurities is carried out at a pressure of 10
5
Pa to 3×10
6
Pa;
the contact time between cryogenic fluid and adsorbent particles is less than or equal to 300 s, preferably from 70 to 200 s;
the adsorbent particles have a pore diameter of 0.5 to 2.5 mm and/or a pore volume of 0.2 to 0.8 ml/g and/or a specific surface area of 100 to 1500 m
2
/g, preferably from 600 to 850 m
2
/g;
the adsorbent particles are in bead, extruded, crushed or elliptical form;
the adsorbent particles are made of a material chosen from among silica gel, zeolites, metal oxides, such as hopcalite, and carbon materials;
a purified cryogenic fluid is recovered which contains at most 1 ppb of impurities;
the process is of the PSA (Pressure Swing Adsorption) of TSA (Temperature Swing Adsorption), preferably TSA, type;
the cryogenic fluid possibly contains one or more impurities chosen from the group formed by H
2
, CO, CO
2
and H
2
O, which are also removed;
the adsorbent is of the zeolite type, especially an X, LSX (for Low Silica X) or A zeolite, which is unexchanged or exchanged with metal cations, particularly calcium, silver, copper, cobalt, nickel, palladium or platinum cations.
REFERENCES:
patent: 3011589 (1961-12-01), Meyer
patent: 5106399 (1992-04-01), Fisher
patent: 5425240 (1995-06-01), Jain et al.
patent: 5706674 (1998-01-01), Hsiung et al.
Fournis Dominique
Gary Daniel
Moreau Serge
Doerrler William
Drake Malik N.
L'Air Liquide Societe Anonyme pour l'Etude et l'E
Young & Thompson
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