FR2640159A1 - Process and device for purifying a neutral gas containing oxygen and/or hydrogen as impurities - Google Patents

Abstract

Purification of a neutral gas containing oxygen and/or hydrogen by catalytic conversion of these impurities into water vapour in the catalytic reactor 1 after addition of hydrogen or oxygen via 23 and optionally 25. The excess hydrogen is removed in the catalytic reactor 7 containing a reducible oxide and a cycle for regenerating this oxide is performed at regular intervals by modifying the quantity of hydrogen and/or oxygen added in 1 in order that the gas entering the reactor 7 during the cycle should have an excess of oxygen. This cycle is governed by the detector 29 which detects the presence of hydrogen or oxygen in the gas and orders the cycle to begin when hydrogen is detected and to end when oxygen is detected.

Description

 Process and device for purifying a neutral gas containing oxvqene and / or hodrogen as impurities.

 The subject of the present invention is a method and a device for purifying an inert gas containing as impurities Oxygen and / or Hydrogen.

 It applies in particular to the purification of rare gases such as argon, krypton and xenon, used as shielding gas in industrial installations.

A known method for removing traces of oxygen and / or hydrogen present in an inert gas is to eliminate them in the form of water vapor by catalytic conversion, after having added to the gas a sufficient quantity of hydrogen- or oxygen to convert these traces of oxygen and / or hydrogen to water vapor. Such a purification method is described for example in the French patent
FR-A-2 270 662 which relates to the elimination of isotopes of hydrogen, in particular tritium, from a gas stream.

 In a process of this type, in order to completely remove the impurities in the form of water vapor, it is necessary to add to the gas to be purified an excess of oxygen or an excess of hydrogen and this is therefore obtained after the conversion catalytic a purified gas still containing this excess oxygen or hydrogen, which is undesirable for many applications where it is desired to obtain an inert gas of high purity.

 The present invention relates specifically to a process for purifying an inert gas containing oxygen and / or hydrogen, which makes it possible to obtain purified gases containing less than 10 vpm volume per million) of hydrogen or oxygen.

 According to the invention, the process for purifying a neutral gas containing oxygen and / or hydrogen as impurities by catalytic conversion of these impurities into water vapor in a first catalytic reactor consists of: to add to the gas to be purified introduced into the first catalytic reactor an amount ql of hydrogen (or an amount q'1 of oxygen) such that the gas introduced into the first reactor contains oxygen and hydrogen with a excess of hydrogen relative to the stoichiometric quantity necessary to convert all of the oxygen into water vapor, - to pass the gas leaving the first reactor into a second reactor containing a reducible oxide with hydrogen to convert the excess of hydrogen into water vapor by obtaining a reduced form of the oxide, - collecting the purified gas leaving the second reactor ~ which contains practically no more than water vapor, and - periodically performing a cycl e regeneration of the reducible oxide of the second reactor by modifying, during the duration of this cycle, the quantity of hydrogen (or oxygen) added to the gas to be purified in such a way that, during the duration of this cycle, the gas entering the first reactor contains oxygen and hydrogen with an excess of oxygen relative to the stoichiometric amount necessary to convert all the hydrogen into water vapor and that the gas entering the second reactor and containing this excess oxygen reacts with the reduced form of the oxide to regenerate this reducible oxide, and it is characterized in that the presence of hydrogen or oxygen is continuously detected in the gas circulating in the second reactor , before it leaves the second reactor A regeneration cycle is started as soon as hydrogen is detected and this regeneration cycle is interrupted as soon as oxygen is detected.

 According to a first embodiment of the invention, the impurities consist only of oxygen.

 In this case, a quantity q1 of hydrogen is added to the gas to be purified introduced into the first reactor greater than the stoichiometric quantity necessary to transform all the oxygen into water vapor, and during the regeneration cycle, we add to the gas introduced into the first reactor a quantity q2 of hydrogen less than the stoichiometric quantity necessary to transform all the oxygen into water vapor.

 According to a second embodiment, the impurities consist only of hydrogen. In this case, a quantity q'1 of oxygen is added to the gas to be purified introduced into the first reactor which is less than the stoichiometric quantity necessary to transform all of the hydrogen into water vapor, and for the duration of the regeneration cycle, adding to the gas introduced into the first reactor a quantity q12 of oxygen greater than the stbechiometric quantity necessary to transform all the hydrogen into water vapor.

 When the gas to be purified contains hydrogen and oxygen, the procedure is as in the first embodiment if the molar ratio between the oxygen and the hydrogen present as impurities is greater than 0.5. In the case where this molar ratio is less than 0.5, the procedure is as in the second embodiment.

 By carrying out according to the invention, the regeneration of the combustible oxide of the second reactor under such conditions, it is possible to obtain purified gas containing less than 10 vpm of hydrogen or oxygen, by operating continuously with a single reactor containing the reducible oxide and without having to reject a possible gas for regeneration of this combustible oxide.

 In the process of the invention, the catalysts used in the first catalytic reactor to convert hydrogen and oxygen to water vapor are conventional catalysts, for example precious metal catalysts, optionally comprising suitable adjuvants. The catalyst can advantageously be supported on a substrate with a large specific surface, to ensure the catalytic reaction.

 Examples of catalysts which can be used include platinum, palladium, rhodium catalysts and their mixtures.

 Preferably, a palladium-based catalyst is used.

 To carry out this reaction with a catalyst of this type, it is possible to operate at atmospheric pressure or at a higher pressure and at room temperature or at a higher temperature. The pressure and the temperature are chosen according to the neutral gas to be purified.

 This reducible oxide used in the second reactor is an easily reducible and easily regenerable oxide, that is to say the reduction or oxidation reaction of which is not too exothermic.

 According to the invention, it is preferable to use cupric oxide CuO because it is easy to use. In addition, it can be reduced or regenerated at temperatures compatible with those used in the first catalytic reactor.

 Also, according to an advantageous characteristic of the process of the invention, only the gas to be purified is heated at the inlet of the first reactor to bring it to a temperature of 180 to 2000 ° C., which makes it possible to obtain an appropriate temperature in the second reactor containing cupric oxide.

 Advantageously, the temperature of the gas at the inlet of the second reactor is continuously centralized and action is taken on the means for heating the gas entering the first reactor so that the temperature of the gas at the inlet of the second reactor does not exceed a value. given. When the reducible oxide is Cupric oxide this value is approximately 2000 C.

The invention also relates to a device for the implementation of this method,
Like the devices of the prior art, this device comprises a first catalytic reactor and means for circulating in the first catalytic reactor The gas to be purified to which a metered amount of hydrogen (or oxygen) has been added, and it is characterized in that it further comprises - a second reactor containing an oxide reducible by Hydrogen, - means for circulating in the second reactor the gas leaving the first reactor, -means for detecting the presence hydrogen
or oxygen in the gas flowing in the oxide
reducible from the second reactor near the
output from the second reactor, and -means controlled by said detection means
to adjust the amount of hydrogen (or oxygen)
added to the gas to be purified.

According to a preferred embodiment
of the invention, the means for adjusting the quantity
hydrogen (or oxygen) added to the gas to be purified
include a first line to introduce
continuously in the gas to purify hydrogen
(or oxygen) at a flow rate dl, a second line
equipped with a valve to introduce into the gas
to purify an additional amount of hydrogen
(or oxygen) at a flow rate d2, and means for
order the opening and closing of said
valve.

Advantageously, the device comprises
Furthermore
- two gas drying columns filled with adsorber
butter, mounted in parallel,
- means to put in alternative circulation
in each of these columns the purified gas
leaving the second reactor,
- means for heating the gas entering each
column,
- means to collect the dried and purified gas
coming out of each column,
- means to put in alternative circulation
part of the dried gas in each column
collected at the exit of the other column in view
regenerate the adsorber,
- means for condensing the water vapor present
in this part of the dried gas which was used for the regeneration of the adsorber, - means for separating the water thus condensed, and - means for recycling in the first reactor this part of the dried gas after condensation.

 Other characteristics and advantages of the invention will appear better on reading the description which follows, of course given by way of illustration and not limitation, with reference to the appended drawing which schematically represents a device for implementing the method of 'invention.

 In this figure, it can be seen that the device comprises a first catalytic reactor (1) filled with catalyst 2, comprising means (3) for heating the gas entering this first reactor, a pipe (5) for collecting the gas leaving the reactor (1) and introduce it into a second reactor (7) containing a reducible oxide (9). The gas leaving the second reactor (9) via line (11) is then directed to drying columns which will be described later.

 The heating means (3) of the first catalytic reactor (1) consist of an immersion heater which is controlled by a detector (13) for measuring the temperature of the gas in the reactor (7) at the entrance of the oxide bed. reducible (9).

 The gas to be purified is introduced through a line (15) provided with a circulation pump (17), a flow control valve (19) and a mixer (21) in the first reactor (1). In the mixer (21), the metered quantity of hydrogen or oxygen is added to this gas via the lines (23 and 25) provided respectively with valves (24 and 26) and connected to a source of hydrogen (or d oxygen) (27). The opening and closing of the valve (26) is controlled by a device (29) for detecting the presence of oxygen or hydrogen in the second reactor (9) before the exit from this second reactor.

 This detection device (29) comprises a pipe (30) making it possible to take a sample of the gas circulating in the reducible oxide bed (9) at a level higher than the base (9a) of this bed. The sample taken is introduced into an analyzer (29) which is sensitive to the presence of hydrogen, then to the presence of oxygen. It includes a permeator 29a for adding oxygen to the withdrawn gas and a cell 29b for measuring the partial pressure of oxygen in a gas.

 This cell can be a cell whose measuring element is a stabilized zirconia sheath placed in an electric oven at around 850 C.

This sheath behaves like the solid electrolyte of a concentration cell whose electromotive force with open circuit is given by the law of Nernst.

in which :
R = ideal gas constant
T = temperature of the measurement area
F = Faraday charge
P1 = partial pressure of oxygen in the gas analyzed
P2 = partial pressure of oxygen in the reference gas
C = cell constant
E = electromotive force in mV.

 In the invention, the gas analyzed is constituted by the sampled gas to which oxygen permeate 29a is added at a given partial pressure P.

 Also, as soon as there are traces of oxygen in the sampled gas, the electromotive force of the cell increases since the partial pressure of oxygen increases.

On the other hand, when there are traces of hydrogen in the sampled gas, the electromotive force of the cell decreases because the partial pressure of oxygen decreases due to the combination of these traces of hydrogen with oxygen added to
The sample taken, at the temperature of the measuring cell (--8500C).

This detector controls the opening of
The valve (26) when the presence of oxygen is detected and it controls the closing of the valve (26) when the presence of hydrogen is detected.

 The device of the invention can be completed by drying columns filled with adsorber making it possible to eliminate the water vapor present in the gas leaving the second reactor (7) via line 11. These drying columns (31, 33) are mounted in parallel so that one column can be used for the adsorption of water vapor and the other column can be regenerated simultaneously.

 These drying columns (31 and 33) are associated with means for heating the gas entering the column (35 and 37) such as immersion heaters, and filled with adsorber (39), constituted for example by a molecular sieve.

 The pipe (11) is provided with a steam condenser (41) and a condensed water separator (43), and it can be connected to each of the drying columns (31 and 33) by means of the pipes (45 and 47) provided with valves (46 and 48) respectively. The dried gas leaves the columns via the pipes (49 and 51) provided respectively with the valves (50 and 52). These pipes (49 and 51) are connected to a single outlet pipe (53).

 For the regeneration of the columns, these can be connected to outlet pipes (55 and 57) provided respectively with valves t56 and 58), which open into a single pipe (59) provided with a condenser (61) and a water separator (63). The pipe (59) is then connected to the pipe (15) for supplying the gas to be purified.

 The operation of the installation is as follows for the treatment of argon containing approximately 5000vpm of oxygen.

 Argon is introduced via line (15) at a flow rate regulated by valve 19 of 10Nm3 / h and is added to the mixer (21) 8000vpm of hydrogen via line (23) and 3000vpm of hydrogen through line (25), the valve (26) being open and the valves (24 and 26) being adjusted to obtain the addition of these quantities of hydrogen.

 The temperature of the gas introduced into the reactor (1) is brought to a temperature of about 2000C by means of the immersion heater (3), then the gas circulates in the direction of the arrow over the palladium catalyst (2) where the hydrogen and oxygen combine into water vapor. This exothermic reaction increases the temperature of the gas up to 220-2400C and one recovers in the pipe of Argon not containing more oxygen but the excess of hydrogen.

 This gas is then introduced into the second reactor (7) where the excess hydrogen is trapped on the cupric oxide (9) which is transformed into copper.

 When all the cupric oxide located above the pipe (30) has been transformed into copper, the analyzer (29) detects the presence of hydrogen, and it acts on the valve (26) to close it and start a regeneration cycle.

 Argon is then introduced into the reactor (1) to which only 8000vpm of hydrogen is added, that is to say an amount insufficient to transform the 5000vpm of oxygen into water vapor. Also, the gas leaving the catalytic reactor (1) contains an excess of oxygen which reacts with the copper formed in the previous phase to regenerate the cupric oxide. When all the copper located above the pipe (30) has been transformed into CuO, the analyzer (29) again detects the presence of oxygen, which controls the opening of the valve (26) to operate. again the catalytic reactor (1) with an excess of hydrogen.

 The positioning of the pipe (30) in the cupric oxide bed (9) makes it possible to keep a height protective layer h of unmodified cupric oxide to guarantee the purity of the gas leaving the reactor (3). This height h is chosen according to the apparatus and the catalysts used.

During the two operating phases described above, the temperature detector (13) acts on the heating means (3) so that the temperature of the gas at the inlet of the reactor (9) does not exceed 2000C, temperature at which
Cupric oxide becomes brittle.

 During these two operating phases, the gas leaving the second reactor (7) via the pipe (11) which contains water vapor, is first of all brought back to room temperature, for example 300C, in the condenser ( 41) where part of the water vapor it contains is condensed. It then passes through the separator (43) where the condensed water is separated, then it is introduced at room temperature into one of the drying columns (31) through the line (45), the valve (46) being open while the valve (48) is closed.

 In the column (31>, the gas circulates in the direction of the arrows F1 and the water vapor is adsorbed on the adsorber (39). 6 The outlet of this column is thus collected via the pipe (49) of the purified gas rid of water vapor and it is evacuated via the line (53>, the valve (50) being open.

During this time, the column (33) can be regenerated by introducing into it by
line (51) a fraction of the purified and dried gas leaving the heated column (31) has
a temperature of 250 to 3000 C. In this case,
the valve (52) is open but does not allow passage
that a fraction of the gas flow leaving the first
re column by line (49) and heating
the sieve, for example at a temperature of 250 to
2700C by the heating means (37), while
the heating means (35) do not work
in the first column (31). The heated gas passes
then in the adsorber (39) charged with steam
of water by following - The path of arrows F2. At this
temperature, the water vapor is desorbed and
collects via line (55) hot gas containing
the desorbed water vapor, the valve (56) being open while the valves (48 and 58) are closed. This gas is evacuated via line (59) then cooled in the condenser (61) to room temperature, and freed from the water condensed in the separator t63); It is then recycled through line (59) into Line (15) for introducing argon to be purified.

 When the adsorber (39) of the first column t31) is saturated, the operation of the columns (31) and (33) is reversed by actuating the valves (50, 52, 46, 48, 58 and 56), stopping the heating the column (33) and turning on the heating of the column (31>.

 The fraction of gas used for the regeneration of columns (33) and (31) can correspond to a flow of 10 to 30% of the nominal flow.

 The fact of regenerating the drying columns in this way makes it possible to avoid obtaining another effluent since the gas used for regeneration is recycled at the entrance to the installation.

 The method of the invention thus has many advantages.

 Indeed, it makes it possible to obtain a purified gas containing less than 10vpm, and generally less Ivpm, of oxygen without using regeneration gas to be rejected not only for the catalytic conversion stage but also for the stage which is very interesting When it comes to expensive or toxic gases.

 Although in this example the operation of the device for purifying an oxygen-containing gas has been described, it goes without saying that the process of the invention can be used to purify a gas containing oxygen. hydrogen and / or a mixture of oxygen and hydrogen, and that other catalysts than palladium, other reducible oxides than cupric oxide and other adsorbers other than molecular sieves can be used.

 The process of the invention can also be used to purify a gas containing oxygen and / or hydrogen in the form of isotopes.

Claims (16)

 1. Process for purifying a neutral gas containing as impurities oxygen and / or hydrogen by catalytic conversion of these impurities into water vapor in a first catalytic reactor (1), consisting : - to add to the gas to be purified introduced into the first catalytic reactor an amount q1 of hydrogen Cou an amount q'1 of oxygen), such that the gas introduced into the first reactor contains oxygen and hydrogen with an excess of hydrogen compared to the stoichiometric quantity necessary to convert all of the oxygen into water vapor, - to pass the gas leaving the first reactor into a second reactor (7) containing an oxide reducible by hydrogen to convert the excess hydrogen into water vapor by obtaining a reduced form of the oxide, - to collect the purified gas leaving the second reactor which contains practically no more than water vapor, and - to carry out periodically a cycle of re generation of the reducible oxide of the second reactor by modifying, during the duration of this cycle, the quantity of hydrogen (or oxygen) added to the gas to be purified so that, during the duration of this cycle, the incoming gas in the first reactor contains oxygen and hydrogen with an excess of oxygen compared to the stoichiometric quantity necessary to convert all the hydrogen into water vapor and that the gas entering the second reactor and containing this excess oxygen reacts with the reduced form of the oxide to regenerate this reducible oxide, characterized in that the presence of hydrogen or oxygen in the gas circulating in the second reactor is continuously detected (at 30) before leaving the second reactor, in that a regeneration cycle is started as soon as hydrogen is detected and that this regeneration cycle is interrupted as soon as oxygen is detected .  2. Method according to claim 1, characterized in that the impurities are oxygen.  3. Method according to claim 2, characterized in that one adds to the gas to be purified introduced into the first reactor a quantity ql of hydrogen greater than the stoichiometric quantity necessary to transform all the oxygen into water vapor, and in that, during the duration of the regeneration cycle, a quantity q2 of hydrogen is added to the gas to be purified introduced into the first reactor, less than the stoichiometric quantity necessary to transform all the oxygen into water vapor.  4. Method according to any one of claims 1 to 3, characterized in that the neutral gas is a rare gas chosen from argon, krypton and xenon.  5. Method according to any one of claims 1 to 4, characterized in that the first catalytic reactor comprises a catalyst based on palladium.  6. Method according to any one of claims 1 to 5, characterized in that the oxide reducible by hydrogen is cupric oxide CuO.  7. Method according to claim 6, characterized in that the gas to be purified is heated at the inlet of the first reactor to bring it to a temperature of 180 to 200 OC.  8. Method according to claim 7, characterized in that one continuously controls (at 13) the temperature of the gas at the inlet of the second reactor and in that one acts on the heating means (3) of the gas at the inlet of the first reactor so that the temperature of the gas at the inlet of the second reactor does not exceed a given value.  9. Process according to any one of claims 1 to 8, characterized in that the purified gas collected at the outlet of the second reactor is subjected to drying by putting it into circulation. Lation at room temperature alternately in a first and a second drying column (31, 33) filled with adsorber, and in that each of the drying columns is periodically regenerated by passing a fraction of the gas through it purified and dried leaving the other drying column after having heated it.  10. Method according to claim 9, characterized in that the water vapor is condensed in the fraction of purified and dried gas used for the regeneration of each of the drying columns and in that this is then recycled fraction at the entrance to the first reactor.  11. Method according to any one of claims 4 and 5, characterized in that the fraction of purified and dried gas used for regeneration is heated to a temperature of 250 to 3000 C.  12. Device for purifying a neutral gas containing oxygen and / or hydrogen as impurities comprising a first catalytic reactor (1) and means (17, 19, 21) for circulating in The first catalytic reactor the gas to be purified to which we have added  a metered amount of hydrogen (or oxygen),  characterized in that it further comprises  - a second reactor (7) containing a reduced oxide  ble by hydrogen, ~ means to circulate in the second  reactor the gas leaving the first reactor, means for detecting (29, 30) the presence  hydrogen or oxygen in the flowing gas  in the reducible oxide of the second reactor above  from the outlet of the second reactor, and  means (24, 26) controlled by said means  sensor to adjust the amount of hydrogen  (or oxygen) added to the gas to be purified.  13. Device according to claim  12, characterized in that the means for adjusting  the amount of hydrogen (or oxygen) added  gas to be purified include a first line  (23) to continuously introduce into the gas to be purified  hydrogen (or oxygen) at a flow rate dl,  a second pipe (25) provided with a valve (26)  to introduce a quantity into the gas to be purified  additional hydrogen (or oxygen) to a  flow d2, and means for controlling the opening  and closing said valve (26).  14. Device according to any one  claims 12 and 13, characterized in  that the detection means (29, 30) are made up  by an analyzer comprising sampling means  of a sample of the gas flowing in the second  reactor, means for adding to the sample  sampled a metered amount of oxygen and means  for hot measurement of partial oxygen pressure  of the sample thus treated.  15. Device according to any one  claims 12 to 14, characterized in that it further comprises means for measuring the temperature (13) of the gas flowing in the second reactor in the vicinity of the inlet of the second reactor, and heating means (3) gas entering the first reactor, controlled by said measuring means.  16. Device according to any one of claims 12 to 15, characterized in that it further comprises: - two drying columns (31, 33) of gas filled with adsorber, mounted in parallel, - means (45 , 46, 47, 48) to circulate alternately in each of these columns the purified gas leaving the second reactor, - heating means (35, 37) of the gas entering each column, - means (49, 50 , 51, 52) to collect the dried and purified gas leaving each column, - means (49, 50, 51, 52) for alternately circulating in each column part of the dried gas collected at the outlet of the other column for regenerating the adsorber, - means (61) for condensing the water vapor present in this part of the dried gas which was used for the regeneration of the adsorber, - means (63) for separating the water thus condensed, and - means (59) for recycling in the first reactor this by tie of the dried gas after condensation.