FR2834915A1 - Removal or nitrogen oxides and unsaturated hydrocarbons from gas stream, especially air, comprises contacting the gas stream with adsorbent containing first-series transition metal - Google Patents

Abstract

Removal or nitrogen oxides and unsaturated hydrocarbons from a gas stream comprises contacting the gas stream with an adsorbent containing vanadium, chromium, manganese, iron, cobalt, nickel and/or copper.

Description

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 The invention relates to a process for purifying a gas, such as air, into its impurities nitrogen oxides (NxOy) and unsaturated hydrocarbons (CnHm), such as C2H4, C2H2, and CsHe, and optionally HzO, CO2 and / or saturated hydrocarbons by TSA or PSA process, more particularly applied to the purification of air before cryogenic distillation.

 Before they can be used in an industrial process, certain gases need to be rid of the impurities they contain beforehand.

 Thus, air usually contains high boiling compounds, such as carbon dioxide and water. If these elements are not removed before the air enters the main exchanger of the cryogenic air separation unit, they will condense and form ice as the air is cooled to temperature. cryogenic. The exchanger will be quickly blocked.

 Many cryogenic air separation units are located near industrial sites or highways which can induce significant quantities of nitrogen oxides (NxOy) in the ambient air. These impurities can include nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (NO2). The latter can also react with each other or with oxygen to form other nitrogen oxides (N203, N204, N205). Nitrogen oxides are not very soluble in liquid oxygen (6.15 and 180 ppm in vol. For NO, NO2 and N2 respectively) and their freezing points are higher than those of nitrogen and oxygen.

 In addition, the cryogenic distillation column concentrates the compounds with a higher boiling point than the oxygen present in the air in the liquid bath of its tank.

 To prevent unwanted compounds from entering cryogenic temperature zones, cryogenic air separation units are in particular equipped with an air pre-purification unit.

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This pre-purification is usually carried out using adsorbents which operate in TSA (Temperature Swing Adsorption), PSA (Pressure
Swing Adsorption) or PTSA (Pressure and Temperature Swing Adsorption).

 A conventional PTSA cycle is a device comprising two adsorbers in which adsorption of impurities takes place, the cycles of which are in phase opposition.

 The adsorbers are conventionally filled with a first bed of adsorbent which stops water vapor, such as an activated alumina, silica gel or a zeolite A or X, and with a second adsorption bed which stops CO2. and certain other secondary air impurities. Typically, this second bed is zeolite X, such as a 13X zeolite because this adsorbent is particularly effective in stopping small amounts of CO2 and residual water, since it has a high affinity and a high selectivity for these molecules. polar.

 A strictly sized pre-purification unit for stopping carbon dioxide with a standard zeolite, typically such as zeolite 13X, 4A or 5A, only partially stops ethylene, propane, protoxide and carbon monoxide. nitrogen and almost no methane and ethane.

 This is recalled, for hydrocarbons, by Dr. J. Reyhing, Removing hydrocarbons from the process air of air-separation plants using molecular-sieve adsorbers dz Linde Reports on science and technology, 36/1983.

 Likewise, for nitrous oxide, U. Wenning of Linde AG notes the ineffectiveness of the zeolite 5A for stopping N20 compared to CO2, in the document Nitrous oxide in air separation plants, MUST'96, Munich. Meeting on Air Separation Technology, October 10-11,1996 (ISSN 0151-1637, ISBN 2- 903633-86X).

 In practice, it has been found that on an industrial air pre-purification unit equipped with activated alumina and 13X sieve, the breakthrough in N20 at each cycle is very significant although no breakthrough in CO2 is generally measured.

 In addition, document EP-A-992274 teaches the use of Na-mordenite or of a zeolite X exchanged with barium or calcium to stop N20 and

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 ethylene, but no mention is made of other nitrogen oxides and hydrocarbons. In addition, the adsorbent must be protected from COz by being positioned downstream of a bed of another adsorbent to stop the CO2.

 Likewise, document EP-A-1092465 proposes stopping nitrogen oxides and hydrocarbons by an X or LSX zeolite preferably exchanged with calcium and protected by an adsorbent stopping CO2.

 EP-A-1072300 describes a process for removing water, CO2, nitrous oxide, ethylene and propane, but not the other nitrogen oxides and hydrocarbons. It proposes, to do this, to use an exchanged adsorbent which it is preferable to place downstream of a conventional arrangement sized for stopping water and CO2.

 In EP-A-995477 and EP-A-1064978, the secondary impurities mentioned are nitrous oxide N20, ethylene, propane and possible other hydrocarbons present in the air. No mention is made of other nitrogen oxides.

 From there, the problem is to be able to purify a gas stream of its nitrogen oxide impurities (NxOy) and unsaturated hydrocarbons in an efficient, reliable and simultaneous manner, that is to say in a single purification step. , without having to protect the adsorbent C02 used during this step.

 The solution of the invention is then a process for purifying a gas stream into at least part of the nitrogen oxide impurities (NxOy) and unsaturated hydrocarbons (CnHm) which it contains, in which: a) the gas flow containing said impurities nitrogen oxides and unsaturated hydrocarbons in contact with an adsorbent containing at least one transition metal of the first series chosen from V, Cr, Mn, Fe, Co, Ni and Cu, (b) adsorbed on said adsorbent, concomitantly, at least a portion of the nitrogen oxide impurities and unsaturated hydrocarbons present in the gas flow from step (a), (c) a gas flow is recovered after step (b) freed from at least part of said impurities.

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 By "concomitant adsorption" is meant that said impurities adsorb simultaneously on the same bed of adsorbent.

- les oxydes d'azote sont choisis parmi NO, N02, N20, N203, N204, N205. - les hydrocarbures insaturés sont choisis dans le groupe formé par C2H4, C2H2 et C3Ha. Depending on the case, the method of the invention may include one or more of the following technical characteristics:

- the nitrogen oxides are chosen from NO, N02, N20, N203, N204, N205. - the unsaturated hydrocarbons are chosen from the group formed by C2H4, C2H2 and C3Ha.

 - The gas stream contains, in addition, CO2 impurities and / or one or more saturated hydrocarbons, said CO2 impurities and / or saturated hydrocarbons being adsorbed in step (b) on said adsorbent.

 - It further comprises at least one step of removing at least part of at least one impurity from among CO2, water and saturated hydrocarbons, such as propane.

 the step of eliminating at least part of the CO2 is carried out by bringing it into contact with an X or LSX zeolite, preferably said X or LSX zeolite is associated with the adsorbent containing at least one transition metal within of composite or mixed grain.

un métal de transition de la première série ou au moins un sel ou un oxyde d'un tel métal. the adsorbent comprises an inorganic solid phase containing at least

a transition metal of the first series or at least a salt or an oxide of such a metal.

 - The inorganic solid phase is chosen from the group formed by aluminas, silica gels, metal oxides, hydrotalcites, perovskites and zeolites having a Si / AI s: 1 ratio.

 - The adsorbent is a ZSM-5 zeolite exchanged with copper, cobalt, nickel or their combinations.

 - the process is a TSA process.

 - the adsorption is carried out at a temperature of -10 C to +60 C, preferably from +10 C to +40 C.

 - the gas flow is air.

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 - The gas stream freed from at least part of its impurities is subjected to a cryogenic distillation step to produce nitrogen, oxygen and / or argon.

 - The adsorbent is a ZSM-5 zeolite exchanged and / or containing from 30% to 300% with copper, cobalt, nickel or their combinations, preferably from 70% to 180%. By an exchange rate greater than 100%, it is meant that, in addition to the ion exchange, metal salt is deposited on the ZSM-5 zeolite by impregnation.

 The concept of the present invention is therefore based on a process for purifying a gas into its nitrogen oxide impurities (NxOy) preferably chosen from NO, N02 and N20 and optionally N203, N204 and N20s; unsaturated hydrocarbons such as C2H4, C2H2 and CsHe; and optionally HsO, CO2 and / or saturated hydrocarbons te) s CsHg and CzHg, comprising an adsorption step on an adsorbent containing a transition metal of the first series, that is to say belonging to the vanadium group, chromium, manganese, iron, cobalt, nickel and copper, or mixtures thereof.

 Indeed, this type of adsorbent adsorbs a lot of nitrogen oxides and hydrocarbons and possibly CO2, and also has the property of not having to be placed downstream of a bed capable of absorbing CO2 .

 The adsorbent consists of an inorganic solid phase, such as alumina, silica gel, an oxide of the genus Zr02, Ti02, hydrotalcite or perovskite, Fe203, MgO or a zeolite having a Si / AI ratio greater than or equal to 1, or alternatively an activated carbon, containing at least one transition metal from the first series or at least one compound (salt or oxide) of a transition metal from the first series.

 For example, among the adsorbents that can be used, there is the zeolite ZSM-5 containing copper, cobalt, nickel, chromium or iron cations or their mixtures.

One can also use a zeolite of type A, LSX (Si / AI from 1 to 1.15 approximately), X, Y, ferrierite, offrettite or mordenite
The transition metal or its compound may have been chemically exchanged or deposited or incorporated in or on the inorganic phase.

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 In addition to the transition metal of the first series, one can put one or more di- or trivalent cations, such as one (or more) alkaline earth or lanthanide cation.

 Note that several transition metals from the first series can also be used as a mixture.

 The preparation of the adsorbent is important, particularly the steps of bringing the transition metal compound into contact with the inorganic support, as well as the final treatment applied.

 In the case where the support is a mineral oxide, the mass area and the pore volume of the material are important; in general, it is preferable to have a mass area greater than 20 m2 / g and a pore volume greater than 0.1 cm3 / g.

 The contacting can be an ion exchange, an impregnation by imbibition or a soaking. The salt concentrations, the nature of the anion, the pH, the temperature and the contact time must be taken into account.

 In the ion exchange, for example, a zeolite is brought into contact with a solution of an ion which will replace the exchangeable cations of the zeolite.

 Consequently, it returns as many cations as its equivalent.

Thus, for example, one can use a chloride solution at a concentration of 1 M, the pH being maintained above 4 but low enough to keep the salt in solution.

 The zeolite is immersed in the solution for at least 3 hours with stirring or recirculation of the solution, at a temperature between 250C and 90 C.

 After exchange, the zeolite is rinsed with demineralized water until pH between 6 and 8, dried at 120 C, then activated between 300 C and 500 C.

 Generally, those skilled in the art will be able to adapt the exchange conditions according to the following rules: - the cation to be exchanged must be well dissolved, that is to say in the form of a hydrated cation; avoid the formation of hydroxi-cations,

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 - the zeolite must be preserved from attack by the H + ions present in the solution, - the cations must be, in the activated phase, in the form of cations without water molecule, and placed in the zeolite sites where they will be accessible to gas molecules.

 For the impregnation, in addition salt is deposited on the support, which can itself be an ion exchanger or not. In this case, the cations introduced are accompanied by their compensating anion and it is not necessary for cations present in the support to leave it.

 In the imbibition procedure, the support is brought into contact with a solution of the salt so as to always remain with a solid phase without the presence of liquid; all the salt used will then remain in the support. In the soaking process, the support is placed in contact with an excess of liquid solution which is subsequently eliminated.

précisément les conditions physico-chimiques de l'échange tel le pH et la température, ou encore d'assurer une répartition plus homogène du sel dans le solide. Soaking makes it possible to deposit sparingly soluble salts, or to control

precisely the physico-chemical conditions of the exchange such as pH and temperature, or even to ensure a more homogeneous distribution of the salt in the solid.

 For the impregnation, it may be necessary to dissolve the cation well by adjusting the pH and choosing the salt well but always respecting the integrity of the support. Indeed, some supports are acidic (silica gel), basic (MgO), amphoteric (Al203) or oxidizable (activated carbon); it will therefore be necessary to ensure that the support chosen will not react with the solution by an acid-base or redox mechanism.

 An additional treatment can be carried out in order to fix or stabilize the compound of the deposited metal. For example, the already impregnated solid can be brought into contact with another solution capable of leading to the formation of an insoluble compound with the metal (insoluble salt, hydroxide, double salt, etc.).

 After being brought into contact with the metal compound, the solid obtained can be dried as it is, that is to say without rinsing, or else rinsed with pure water.

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 Another method also consists in making the metal and its support co-precipitate in the form of a salt or a mixed oxide, as in the case of a perovskite or a hydrotalcite. In this case, the metal is inserted into the support structure.

 The final treatment can be heating to a moderate temperature at around 100 ° C. to 120 ° C. to remove the water, or else to a higher temperature between 200 ° C. and 500 ° C. to decompose the salt (for example a nitrate or an acetate) and lead to the oxide, or alternatively heating under reducing conditions, for example between 150 ° C. and 300 ° C. in the presence of a gaseous mixture of 10% H2 in N2 to obtain the metal.

 The rise in temperature may either be sudden, by direct introduction of the sample into the already hot oven, or else gradual, for example at 1 to 10 ° C./min. However, it can also be carried out in temperature steps, for example first at 120 ° C., then at 180 ° C., then at 250 ° C., finally at 4000 ° C. or 500 ° C.

pour les sels cuivre, +11 ou +111 pour les sels de fer... L'homme de l'art trouvera dans les encyclopédies de chimie les valences usuelles de différents éléments de transition. Le degré d'oxydation, lorsqu'il est positif, définit la charge de l'ion métallique et donc son pouvoir polarisant qui est important pour les propriétés d'adsorption et la localisation du cation dans la structure zéolitique. Le degré d'oxydation zéro correspond au métal'libre'capable de piéger les molécules possédant des électrons Tt par interaction avec les électrons de la structure de bande du métal. The degree of oxidation of the metal after treatment is important. By degree of oxidation is meant the state of valence of the element: 0 for the metal, +1 or +11

for copper salts, +11 or +111 for iron salts ... Those skilled in the art will find in chemical encyclopedias the usual valences of different transition elements. The degree of oxidation, when positive, defines the charge of the metal ion and therefore its polarizing power which is important for the adsorption properties and the location of the cation in the zeolitic structure. The zero degree of oxidation corresponds to the metal 'free' capable of trapping molecules having Tt electrons by interaction with the electrons of the band structure of the metal.

 The degree of division of the metal or the metal salt or the metal oxide should be as high as possible to promote contact with the gas.

 The accessibility of the metal must also be optimal, that is to say that it must be located in the open porosity of the material. It will be up to those skilled in the art to define the optimal operating conditions according to the nature of the support.

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 and metal, by playing on the conditions of synthesis of the support and of the exchange: concentrations and conditions of temperature, stirring and pH.

 The adsorption process of the invention is preferably carried out according to an TSA cycle. The adsorption is then carried out at a temperature close to ambient, that is to say from −10 ° C. to 60 ° C., preferably from 10 ° C. to 40 ° C. Desorption is moreover carried out between 70 ° C. and 350 ° C., preferably between 100 C and 250 C. During desorption, the adsorbent can optionally serve as a catalyst for the decomposition of nitrogen oxides, with the formation of O 2 and N 2.

 Among the preferably adsorbed hydrocarbons are unsaturated hydrocarbons such as ethylene, propylene and acetylene.

 It should be noted that the products desorbed may be different from those initially adsorbed, by redox reactions between the nitrogen oxides and the hydrocarbons, or else by a catalytic decomposition of the nitrogen oxides, which may occur during l adsorption, that is to say in contact with the adsorbent material.

 The step of adsorption of at least part of the nitrogen oxides and of the unsaturated hydrocarbons on the adsorbent with transition metal can be preceded or followed by at least one step of adsorption of at least one other impurity , such as water vapor, CO2 or a saturated hydrocarbon.

 To do this, the adsorbent material can also be associated with a specific COs zeolite, such as 13X or LSX, or with a specific adsorbent for water, such as an alumina. By association is meant a granular bed consisting of composite particles, or else a mixture of particles of different compositions.

 In order to demonstrate the effectiveness of the invention, the following comparative tests were carried out.

Example 1: According to the prior art
An adsorption bed 6 cm in diameter and 20 cm in height of conventional 13X zeolite is brought into contact with a flow rate of 10 Nm3 / h of air at 200C and 6 bar containing the following impurities:

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- 1 ppm en volume de NO - 1 ppm en volume d'éthylène - 1 ppm en volume d'acétylène - 1 ppm en volume de propylène. - 400 ppm by volume of C02 - 0.3 ppm by volume of N20

- 1 ppm by volume of NO - 1 ppm by volume of ethylene - 1 ppm by volume of acetylene - 1 ppm by volume of propylene.

 It is found that, when the CO 2 pierces, that is to say when the CO 2 appears downstream of the adsorbent bed, the contents downstream of the bed of nitrogen monoxide, nitrous oxide and ethylene are approximately equal to their contents upstream of the bed.

 Conversely, propylene and acetylene pierce after CO2.

 Since the prepurification installations of the air separation units are usually designed to stop CO2, this implies that the monoxide and nitrous oxide and ethylene are not completely shut down and that they enter cryogenic installations, at the risk of clogging or damaging them.

Example 2: According to the invention
The same experiment is carried out with a bed 6 cm in diameter composed of a first layer of ZSM5 zeolite exchanged (exchange rate about 80%) with copper 5 cm in height and a second layer of 13X zeolite of 15 cm in height downstream.

 In this case, when the CO2 breaks through, the monoxide and nitrous oxide, ethylene, acetylene and propylene are completely stopped.

Example 3: According to the invention
The same experiment is carried out with a bed 6 cm in diameter and 20 cm in height composed of a mixture of particles of 13X zeolite and ZSM5 zeolite exchanged (exchange rate about 140%) with copper, in the proportion of '' about 75% 13X zeolite and 25% copper ZSM5 zeolite.

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 Again, nitrous oxide and monoxide, ethylene, acetylene, and propylene all appear after the breakthrough of CO2.

 The configurations of Examples 2 and 3 according to the invention therefore make it possible to remove the monoxide and nitrous oxide, ethylene, acetylene, propylene and CO2 simultaneously, without having a protective bed located upstream. to remove CO2 before.

Claims (14)

1. Method for purifying a gas stream into at least part of the nitrogen oxide impurities (NxOy) and unsaturated hydrocarbons (CnHm) which it contains, in which: a) the gas stream containing said oxide impurities d nitrogen and unsaturated hydrocarbons in contact with an adsorbent containing at least one transition metal from the first series chosen from V, Cr, Mn, Fe, Co, Ni and Cu, (b) is adsorbed on said adsorbent, concomitantly, at least part of the nitrogen oxide and unsaturated hydrocarbon impurities present in the gas stream from step (a), (c) a gas stream is recovered after step (b), free of at least a portion said impurities.  2. Method according to claim 1, characterized in that the nitrogen oxides are chosen from NO, N02, N20, N203, N204, N2Os.  3. Method according to one of claims 1 or 2, characterized in that the unsaturated hydrocarbons are chosen from the group formed by C2H4, C2H2 and C3Hs.  4. Method according to one of claims 1 to 3, characterized in that the gas stream contains, in addition, CO2 impurities and / or one or more saturated hydrocarbons, said CO2 impurities and / or saturated hydrocarbons being adsorbed to l 'step (b) on said adsorbent.  5. Method according to one of claims 1 to 4, characterized in that it further comprises at least one step of removing at least part of at least one impurity from among CO2, water and saturated hydrocarbons, such as propane. <Desc / Clms Page number 13>  6. Method according to claim 5, characterized in that the step of eliminating at least part of the C02 is carried out by bringing into contact with an X or LSX zeolite, preferably said X or LSX zeolite is associated with l adsorbent containing at least one transition metal within composite grains or as a mixture of grains.  7. Method according to one of claims 1 to 6, characterized in that the adsorbent comprises an inorganic solid phase containing at least one transition metal from the first series or at least one salt or an oxide of such a metal.  8. Method according to one of claims 1 to 7, characterized in that the inorganic solid phase is chosen from the group formed by aluminas, silica gels, metal oxides, hydrotalcites, perovskites and zeolites having an Si / AI ratio 1.  9. Method according to one of claims 1 to 8, characterized in that the adsorbent is a ZSM-5 zeolite exchanged with copper, cobalt, nickel or their combinations.  10. Method according to one of claims 1 to 9, characterized in that the method is a TSA process.  11. Method according to one of claims 1 to 10, characterized in that the adsorption is carried out at a temperature of -10 C to +60 C, preferably from +10 C to +40 C.  12. Method according to one of claims 1 to 11, characterized in that the gas flow is air. <Desc / Clms Page number 14>  13. Method according to one of claims 1 to 12, characterized in that the gas stream freed from at least part of its impurities is subjected to a cryogenic distillation step to produce nitrogen,

 oxygen and / or argon.  14. Method according to one of claims 1 to 13, characterized in that the adsorbent is a ZSM-5 zeolite exchanged from 30% to 300% to copper, cobalt, nickel or their combinations, preferably 70% 180%.