MXPA00008749A - Decarbonating gas streams using zeolite adsorbents - Google Patents

Abstract

The invention concerns a method for decarbonating gas streams polluted by CO2, which consists in contacting them with an adsorbent consisting of an agglomerate of zeolite X with a Si/Al ratio of 1 to 1.15, highly sodium-exchanged comprising not more than 20%of inert binding material, to eliminate at least the carbon dioxide.

Description

DISCHARGE OF GASEOUS FLOWS THROUGH ZEOLITHIC ADSORBENTS TECHNICAL FIELD The invention relates to the purification of gaseous flows contaminated with carbon dioxide, and in particular to the purification of the air before carrying out the separation steps of N2 / 02. PREVIOUS TECHNIQUE The production of pure gases, in particular N2 and 02 from atmospheric air is an industrial operation practiced on a large scale and can resort either to cryogenic processes or to adsorption processes based on the principle of pressure modulated adsorption. (PSA, acronym of the English "pressure swing adsorption"), that of the adsorption modulated in temperature (TSA, abbreviations of English "temperature swing adsorption") or a combination of both systems (PTSA). In addition, numerous gases produced in industrial systems contain significant amounts of carbon dioxide, which are often convenient to purify. The production of N2 or 02 from the air imposes a purification prior to the actual separation step. In fact, in the conduction of cryogenic processes, water or carbon dioxide present in the feed air can generate congestion in the equipment due to the fact that these operations are carried out at temperatures much lower than the freezing temperatures of such impurities In the processes by adsorption, water and carbon dioxide are more strongly adsorbed than nitrogen and cause in the long run a poisoning of the adsorbent, which results in a decrease in the expected life expectancy. In these procedures, a zeolite of the faujasite type (13X in which the Si / Al ratio is higher than 1.2) is generally used to ensure the elimination of carbon dioxide, with the entrapment of water usually carried out in a bed of alumina placed current above the molecular sieve bed. The regeneration is of the PTSA type, that is, a slight temperature rise is associated with a pressure drop of 150 ° C. With this system, the incoming gas, which consists of no more than N2 and 02, has about 1% by volume of argon whose behavior in terms of adsorption can be assimilated with that of oxygen. It has long been known that zeolite X is an adsorbent for carbon dioxide superior to silica gel or active carbon (US Patent No. 2,882,244). This patent also teaches that the selectivity against various adsorbates, ie adsorption materials, varies with temperature and pressure.

US Pat. No. 3,885,927 (dated May 27, 1975) shows that C02 adsorption can be carried out on an X zeolite that has been changed by more than 90% by barium: under these conditions, the C02 content of the gas does not exceed 1000 ppm and the temperature can be between -40 ° C and 50 ° C. The European application No. 88107209.4 (date May 5, 1988) shows that it is also possible to use a zeolite X with strontium exchange to effect this purification. The influence on the C02 adsorption of the number of interchangeable cations in the zeolite was studied by BARRER and colleagues in "MOLECULAR SIEVES" (Soc. Chim. Ind., LONDRES, 1968), page 233 as well as by COUGHLAN and colleagues in "JCS Faraday ", 1, 1975, 71, 1809. These studies show that the adsorption capacity of zeolite for C02 increases to the extent that the Si / Al ratio decreases to a limit of 1.2, however to date it has not yet The lower field has been explored. Zeolite X whose Si / Al ratio is close to 1.25 and which is currently used is very selective for C02 and this is all the more so as the temperature drops. At temperatures close to those of the environment, the efficiency diminishes strongly due to the fact of the competition with the nitrogen that is present in enough higher molar proportions. The ratio N2 / C02 in ambient air (with C02 ~ 300/400 vpm) is of the order of 3000. Therefore it can be said that in general terms it is indispensable to equip the floor, that is, the decarbonation phase with a cooling system for avoid thus the elevation of the temperatures to the adsorption, increase that can be considerable (several tens of degrees) because of the strong heats of adsorption involved. In U.S. Patent No. 5,531,808 (July 2, 1996), one finds the teaching that it is possible to adsorb C02 very effectively by a type X zeolite having a Si / Al ratio of less than 1.15. The advantage with respect to zeolite X (classical) lies in the fact that it is no longer necessary to lower the temperature to 1 to the decarbonation phase by means of a cold group since the zeolite's efficiency is such that the selectivity for C02 the nitrogen remains large even at 50 ° C. It is observed that the C02 adsorption capacity of a NaLSX zeolite increases with the sodium exchange rate. But it is also observed that the efficiency gain begins to reach its limit when exchange rates of the order of 90% are reached although apparently there is no interest in pushing this exchange beyond 95%. It has just been noted that this is only applicable for working conditions under relatively high C02 partial pressures: a very sensitive gain in efficiency can be obtained for decarbonation under reduced partial pressures of C02 of the order of 1000 bar with the LSX zeolites whose Sodium exchange rate (which is defined as the molar ratio between the sodium ions and the aluminum atoms in the tetrahedral position with the remainder being constituted by potassium) is at least 98% V EXPOSITION OF THE INVENTION The invention relates to thus to a process of decarbonation of gaseous flows and particularly air, which consists in putting this gaseous flow in contact with a zeolitic adsorbent of NaLSX type, the adsorbent being constituted by a zeolite X with a Si / Al ratio of 1 to 1.15, and in which the sodium exchange rate is at least 98%, the rest of the exchange capacity being occupied by the po The agglomerating agent is bonded with a binder and the inert residual binder rate of the adsorbent is less than 20% by weight and preferably at most equal to 5% by weight. In industrial installations, the use of zeolitic adsorbents in the form of agglomerates turns out to be very clearly more advantageous than the use of powders; in fact, during the handling of powders, for example during the loading and unloading phases of the adsorbent beds, it is difficult to avoid the phenomenon of the considerable losses of pulverulent material, especially due to the volatility of powders, which is not very economical for the industrialist. In contrast, agglomerates of powders, such as granules, spheres, plates, etc., do not have such drawbacks. The agglomerates of zeolite with a binder ratio greater than 5% by weight can be obtained in conventional manner by mixing a crystalline zeolite powder with water and a binder (in most cases in the form of a powder) and then by pulverization of this mixture in agglomerates of zeolite that play the role of germs of agglomeration. During spraying, the zeolite agglomerates can be subjected to a continuous rotation on themselves according to a type technique (snowball), for example in a reactor equipped with a rotating shaft. The agglomerates thus obtained are then presented in the form of spheres. Once formed, the agglomerates undergo cooking at temperatures which are generally between 500 and 700 ° C and preferably of the order of 600 ° C. As an example of a binder, mention may be made of kaolin, silica and alumina. The preferred agglomerates contain less than 5% by weight of binder. A method of obtaining these agglomerates with a low binder rate consists of converting the binder of the agglomerates described above into their zeolitic phase. For this purpose, a powder of LSX zeolite with a zeolitizable binder (for example kaolin or metakaolin) is agglomerated and then alkaline maceration is zeolitized, for example according to C, the procedure described in French application No. 97 09283 and after changes to sodium on the zeolitized granulate. In this way, granulates with a titre of at least 95% zeolite exchanged at 98% with remarkable performance characteristics can be obtained according to the invention. WAYS TO CARRY OUT THE INVENTION The decarbonation process according to the invention can be carried out by passing the gaseous flow over 1 or several beds of adsorbent associated in parallel 0 or that can be chained in a cyclic way to constitute a phase or stage of adsorption and a desorption step or stage (intended for regeneration of the adsorbent); on an industrial scale, it is preferably operated according to a process by adsorption by varying the pressure (PSA) and advantageously by adsorption by varying the pressure and temperature (PTSA). The PSA and PTSA type procedures involve the application of pressure cycles. In a first phase, the adsorbent bed ensures the separation of the contaminant by adsorption of this constituent; in a second phase of regenerating the adsorbent by low pressure. In each new cycle it is essential that the desorption of the contaminant is as complete and effective as possible, in order to find again a regenerated state of the adsorbent identical or fundamentally identical for each new cycle. The partial pressure of C02 present in the gas flow generally does not exceed 25,000 millibars and is preferably less than 10 millibars. In order to continuously purify the gaseous flow, such as air, a certain amount of adsorbent beds is generally arranged in parallel, which is alternated by a cycle of adsorption with compression and desorption with decompression. In the PSA and PTSA processes, the treatment cycle to which each bed is subjected, comprises the following steps: a / letting the gaseous flow contaminated by an adsorption zone comprising the adsorbent bed, which ensures the separation of the pollutant or of the contaminants (here C02) by adsorption; b / desorbing the C02 adsorbed by the introduction of a pressure gradient and a progressive decrease of the pressure in said adsorption zone to recover the CO2 by entering the adsorption zone; c / again raising the pressure in said adsorption zone by introducing a pure gaseous stream through the outlet of the adsorption zone. Thus each bed is subjected to a treatment cycle comprising a pure gas production phase, a second decompression phase and a third recompression phase. If the only contaminant that must be eliminated from the gas flow is C02, a single bed of adsorbent, consisting essentially of agglomerates of NaLSX zeolite as defined above, is placed in the adsorption zone. If there are several contaminants to be removed, then the adsorption zone may comprise several beds of adsorbent capable of adsorbing unwanted impurities or contaminants. Thus to remove the carbon dioxide and water contained in the air, a desiccant will be associated to adsorb water such as alumina or a silica gel and the adsorbent of the present invention. In order to optimize the PSA and PTSA procedures, the decompression and compression phases of the different adsorbent beds are synchronized; it is particularly advantageous to introduce equalization steps of the pressures between two beds of adsorbent, of which one is in the decompression phase and the other in the recompression phase. During the practical application of the process according to the invention, the adsorption pressures are generally between 0.2 and 20 bar and preferably between 1 and 10 bar while the desorption pressures are generally between 0.02 and 5 bar and preferably between 0.1 and 2 bars. As for the decarbonation processes according to the current state of the art, the temperatures prevailing in the adsorption zone are generally between 20 and 80 ° C and advantageously between 30 and 60 ° C; In the decarbonation processes according to the state of the art, the regeneration temperatures that are necessary to obtain a sufficient regeneration of the adsorbent are typically of the order of 130 to 170 ° C, which requires a heating of the adsorbent and increases the cost of industrial installation. With respect to the current state of the art, the present invention offers a substantial additional advantage at the level of the regeneration of the adsorbent to the extent necessary to obtain the same performance of the adsorbent after its regeneration. Thus the applicable regeneration temperatures are between 100 and 120 ° C and therefore are considerably lower than those practiced to date. Example In the examples presented, the zeolite is an LSX zeolite, with a Si / Al = 1 ratio, obtained according to the following experimental mode. a) Preparation of the LSX zeolite A zeolite of faujasite type LSX is synthesized, with a Si / Al = l ratio, mixing the following solutions: Solution A: 136 grams of soda, 73 grams of potash are dissolved (expressed in pure material) in 280 grams of water. The material is brought to a boil between 100 and 115 ° C and then 78 grams of alumina are dissolved. Once the dissolution is done, let it cool down and complete with water up to 570 grams to consider the evaporated water. Solution B: 300 grams of water and 235.3 grams of sodium silicate (25.5% SiO2, 7.75% Na20) are mixed under slight agitation. The silicate solution is added to the aluminate solution in about 2 minutes under strong agitation by means of a RAYNERI type deflocculating turbine rotating at 2500 revolutions per minute (peripheral speed = 3.7 m / s), and then the gel formed is left behind. 60 ° C for 24 hours without agitation. After this period an important decantation is observed, characteristic of the crystallization process. Then a filtration is carried out and then a wash with approximately 15 ml of water per gram of solid. Then it is dried at 80 ° C in the oven. The composition of the synthesis gel is: 4 Na20, 1.3 K20, 1 Al203, 2 Si02, 91 H20 The chemical analysis of the solid resulting from the synthesis provides a composition: 0.77 Na20, 0.23 K20, 2 Si02, 1 Al203 X-ray diffraction analysis confirms that the powder formed is constituted by practically pure faujasite, accompanied by traces of zeolite A whose content is estimated at less than 2%. A measure of toluene adsorption capacity is made after calcining at 550 ° C for 2 lyoras, under an inert atmosphere: an adsorbed capacity of 22.5% is found at 25 ° C and under a partial pressure of 0.5. Sodium exchange was carried out in several successive exchanges, with a liquid / solid ratio (L / S) of 10 ml / g, with a sodium chloride solution with a NaCl template per liter, at 90 ° C for 3 hours. Each exchange was followed by 1 or several intermediate washes. The C02 adsorption capacities are measured after degassing under vacuum at 300 ° C for 16 hours. Example 1: The adsorbent used is a granulate obtained as follows from the LSX powder described above. 42.5 grams (expressed in calcined equivalent), 7.5 grams of a fibrous clay (expressed in calcined equivalent), 1 gram of carboxymethylcellulose and adequate water are mixed to be able to proceed with an extrusion in the form of extrudates with a diameter of 1.6 mm and a long of the order of 4 mm. The extrudates are dried at 80 ° C and then calcined at 550 ° C for 2 hours under an inert atmosphere. Table 1 presents the results obtained regarding the adsorption capacity of C02 (in cm3 / g) at 25 ° C, under various pressures of CO2, granules of agglomerated NaLSX zeolite with 15% binder and whose exchange rate of Sodium is variable. They unequivocally reveal the interest of the NaLSX adsorbent with a high sodium exchange rate for decarbonation under reduced partial pressures.

Table 1 It is clear that relative gains in capacity are greater for low pressures than for strong pressures. Example 2: The adsorbent used is a granulate (zeolitized) obtained from the LSX zeolite powder as described above. The LSX zeolite powder of Example 1 is used, agglomerating it with a mixture of a clay of the monmorilolite type (15%), of a kaolin clay (85%), with a little carboxymethylcellulose and water. Once the extrusion is carried out, drying is carried out at 80 ° C and calcination at 500 ° C for 2 hours, under an inert atmosphere, free of water vapor. 10 grams of these agglomerates are immersed in 17 ml of a soda solution at 220 g / 1 for 3 hours at 95 ° C. The agglomerates are then washed 4 times successively by immersion in water, in a proportion of 20 ml / g. The measurements of the toluene adsorption capacity are carried out under the conditions described above and the following values are presented: agglomerated LSX (untreated) 20.2% agglomerated LSX (treated with NaOH) 22.4% This toluene adsorption value reflects the fact that that the adsorbent body is constituted by more than 95% zeolite. These are results that represent the good efficacy of the zeolitic bodies according to the invention and also reflect a better crystallinity of the LSX obtained by zeolitization with soda. The spectrum R.M.N. High resolution silicon shows that the Si / Al ratio is equal to 1.01 in the crystal lattice. Table 2 presents the results obtained regarding the adsorption capacity of C02 (in cm3 / g) under various partial pressures of C02 for NaLSX zeolite granules containing 5% zeolitized binders and whose sodium exchange rate is variable.

Table 2

Claims (9)

1. CLAIMS 1. A process for the decarbonation of a gaseous flow, preferably air, contaminated with C02, characterized in that the gaseous flow that must be purified with at least one adsorbent essentially constituted by a zeolite is brought into contact in an adsorption zone. NaLSX type with a Si / Al ratio of 1 to 1.15, is exchanged with sodium at a rate equal to greater than 98%, expressed in the exchange rate as the ratio between the number of sodium ions and the number of aluminum atoms in the tetrahedral position, the rest of the exchange capacity being occupied by potassium ions, agglomerated with a binder, the inert residual binder rate of the adsorbent being less than or equal to 20% by weight.
2. 2. The process according to claim 1, wherein the inert residual binder rate of the agglomerated zeolitic composition is at most equal to 5% by weight.
3. 3. The process according to claim 1 or 2, characterized in that it is operated by adsorption with modulated pressure (PSA) and preferably by adsorption with modulated pressure and temperature (PTSA).
4. 4. The process according to any of claims 1 to 4, wherein the zeolite X has a Si / Al ratio of 1.
5. 5. The process according to any of claims 1 to 4, wherein the pressures of the adsorption are between 1 and 10 bar and the desorption pressures are between 0.1 and 2 bar. The method according to any of claims 1 to 5, characterized in that it comprises the execution of a treatment cycle comprising the following steps: a / to pass the contaminated gaseous flow to the adsorption zone comprising the adsorbent bed which ensures the separation of the contaminant or contaminants by adsorption; b / desorb the C02 adsorbed by the introduction of a pressure gradient and progressive reduction of the pressure in said adsorption zone to recover the CO 2 by entering the adsorption zone; c / raise the pressure in the adsorption zone again by introducing a pure gaseous stream through the outlet of the adsorption zone. The process according to claim 6, wherein the adsorbent is regenerated at a temperature comprised between 100 and 120 ° C. 8. The procedure for purification of air contaminated with C02 and H20, characterized in that the gaseous flow that must be purified with at least one desiccant agent, preferably based on alumina and at least on the basis of alumina, is brought into contact in an adsorption zone. an adsorbent essentially constituted by a zeolite of NaLSX type with a Si / Al ratio of 1 to 1.15, with the sodium exchanged at a rate equal to or greater than 95%, the exchange rate being expressed as the ratio between the number of sodium ions and the number of aluminum atoms in the tetrahedral position, the remainder of the exchange capacity being occupied by potassium ions, agglomerated with a binder, the rate of the inert residual binder of the adsorbent being less than or equal to 20% by weight. The method according to claim 8, characterized in that it comprises the practical application of a treatment cycle comprising the following steps: a / to pass the contaminating gaseous flow through an adsorption zone comprising a bed of desiccant agent and a bed of adsorbent as defined in claim 1; b / desorb the C02, adsorbed, by instauration of a pressure gradient and the progressive decrease of the pressure in the adsorption zone to recover the CO2 by entering the adsorption zone; and c / raising the pressure in the sorption zone again by introducing a liqua gaseous stream through the outlet of the adsorption zone.