FIELD OF THE INVENTION

This invention relates to a zeolite-X adsorbents exchanged with barium and potassium and a method of preparing zeolite-X adsorbents exchanged with barium and potassium and their use in removing carbon dioxide by adsorptive separation from a gaseous mixture containing carbon dioxide in the range of 4 to 15 % by volume as in power plant flue gas and gases less polar than carbon dioxide such as nitrogen, oxygen and hydrogen.

BACKGROUND AND PRIOR ART

Atmospheric CO ₂ concentration has been increasing steadily since the industrial revolution. It has been widely accepted that the CO ₂ concentration was about 280 parts per million by volume (ppmv) before the industrial revolution, while it has increased from 315 ppmv in 1959 to 370 ppmv in 2001 [Keeling, C. D. and T. P. Whorf. 2002, Atmospheric CO ₂ records from sites in the SIO air sampling network. In Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, Term., U.S.A. This data is also available from http://cdiac.esd.ornl.gov/ftp/maunaloa- co2/maunaloa.co2]. Rising CO ₂ concentrations has been reported to account for half of the greenhouse effect that causes global warming [IPCC Working Group I. IPCC Climate Change 1995 — The Science of Climate Change: The Second Assessment Report of the Intergovernmental Panel on Climate Change; Houghton, J. T., Meira Filho, L. G., Callander, B. A., Harris, N., Kattenberg, A., Maskell K, Eds.; Cambridge University Press: Cambridge, U.K., 1996]. Although the anthropogenic CO, emissions are small compared to the amount of CO ₂ exchanged in the natural cycles, the discrepancy between the long life of CO ₂ in the atmosphere (50-200 years) and the slow rate of natural CO ₂ sequestration processes leads to CO ₂ build up in the atmosphere. The IPCC (Intergovernmental Panel on Climate Change) opines that "the balance of evidence suggests a discernible human influence on the global climate." Therefore, it is necessary to develop cost effective CO ₂ management schemes to curb its emission.

Carbon dioxide is contained in natural gas, exhaust gas from combustion, as well as atmospheric air although in a minor amount. Carbon dioxide is produced as by-product in industrial processes, for example, at a step of steam-reforming natural gas, naphtha, coke or methanol to produce hydrogen. In recent years carbon dioxide is industrially produced as by-product including that contained in combustion gas. Further, removal of carbon dioxide by cryogenic separation of air has a problem such that a trace amount, i.e., about 300 to 400 ppm, of carbon dioxide in air is solidified upon cooling, leading to clogging an equipment such as a heat exchanger.

For separating and removing carbon dioxide contained in a gas mixture, there can be mentioned two methods. The first method is involving chemically absorbing carbon dioxide in a solution of an alkali or amine, and the second method of physically adsorbing carbon dioxide by an adsorbent such as activated carbon or a zeolite. The methods of physical adsorption of carbon dioxide using a zeolite adsorbent include a pressure- temperature swing adsorption (PTSA) method or a pressure swing adsorption (PSA) method. In these methods, adsorption of carbon dioxide by a zeolite adsorbent is affected at a low temperature and a high pressure and desorption thereof from the zeolite adsorbent for the reactivation of the zeolite adsorbent is affecced at a temperature higher and a pressure lower than those for adsorption. Upon desorption, the zeolite adsorbent may be purged with a gas which contains no carbon dioxide and is less adsorbed than carbon dioxide.

It is known that a zeolite adsorbent adsorbs a gas molecule by the interaction between the cation present in the zeolite adsorbent and the gas molecule. The interaction is enhanced and the amount of the molecule adsorbed increases with an increase of polarity of the gas molecule.

The zeolite adsorbent hitherto used for removing carbon dioxide from a gaseous mixture such as air includes zeolite-A and zeolite-X having SiO ₂ /Al ₂ O ₃ molar ratio of at least 2.5. The removal of carbon dioxide from air involves use of large size equipments and/ or higher energy consumption for the activation of large amount of zeolite adsorbents. In view of reducing the size of equipment and/or the energy consumption, an adsorbent exhibiting a high adsorption capacity for carbon dioxide even in the co-presence of a large amount of nitrogen is eagerly desired.

A newer process for gas dehydration and carbon dioxide recovery technology uses molecular sieves, natural and synthetic zeolites. It is known that synthetic zeolite A and X types are effective adsorbent Of CO ₂ and water. For instance, U. S. PaL 3,981,698, (Leppard et al., September 21, 1976), U. S. Pat. 4,039,620 (Netteland, et al. August 2, 1977), U. S.

Pat. 4,711,645, (Kumar Ravi, December 8, 1987), U. S. Pat. 4,986,835, (Uno et al., January

22, 1991) and U. S. Pat 5,156,657, (Jain Ravi et al. October 20, 1992), suggest the use of standard molecular sieves 5 A, 1OA and 13X as carbon dioxide adsorbents. These molecular sieves adsorb CO ₂ by physical adsorption and are regenerable at ambient temperatures.

However, they do not possess sufficient adsorption capacity for carbon dioxide. Thus, such adsorbents cannot provide extensive gas purification, demand an increased loading volume and often require the use of supplemental adsorbent beds to decrease the water and carbon dioxide concentration prior to introduction into the zeolite bed.

A number of patents disclose molecular sieve adsorbents having improved adsorption capacities, especially for the removal of carbon dioxide from gas mixtures. For example. U. S. Pat. 2,882.244, (Milton et al., April 14, 1959) discloses a variety of crystalline aluminosilicate useful for CO ₂ adsorption. U. S. Pat 3,078,639, (Milton et al., February 26, 1963) discloses a zeolite-X useful for the adsorption of carbon dioxide from a gas stream. British Patent Nos. 1,508,928, Mobil Oil, and 1,051,621, (Furtig et al., January 28, 1913) disclose faujasite-type zeolites having silica to alumina ratio from 1.8 to 2.2. However, the adsorption capacity for carbon dioxide of these adsorbents are limiting factor.

To increase carbon dioxide adsorption capacity, several adsorbents have been proposed based on various cation exchanged forms of molecular sieve X and other crystalline structures. U. S. Pat. 3,885,927, (Sherman et al., May 27, 1975) discloses a barium cation form of zeolite X in which 90-95% of the Na ⁺ ions are replaced by Ba ²⁺ ions. For carbon dioxide removal, U. S. Pat. 4,775,396, (Rastelli et al., October 4, 1988) describes the use of zinc, rare earth metals, a proton and ammonium cation exchanged forms of synthetic faujasite having silica to alumina ratio in bread range of 2-100.

All of these molecular sieve adsorbents are characterized by carbon dioxide adsorption capacity extended at moderate and high partial pressures of the admixture to be adsorbed. However, their capacity to adsorb at low partial pressure of CO ₂ (< 5 Torr) and at ambient temperatures is not sufficient to provide the purity of the gas required. In addition, due to the relatively short time before CO ₂ breakthrough, the water capacity of these adsorbents appears to be only 10-15 percent of potential. This decreases adsorbent performance in such applications as TSA and PSA air pre-purification units where carbon dioxide inlet adsorption is very low. For employing the above mentioned adsorbents in such applications demands gas chilling to a temperature below about 5° C. In turn, this results in a substantial increase in operation and capital costs.

A method of removing carbon dioxide from a gas stream by u?ing a zeolite adsorbent has been proposed in U.S. Pat. 5,531,808 (Ojo et. al., July 2, 1996) wherein the gas stream is contacted with a type X zeolite having a silicon / aluminum atomic ratio of about 1.0 to 1.15 and having been ion-exchanged with a cation selected from the ions of groups IA, 2A, 3A, 3B, the lanthanide series of the periodic table and mixtures thereof at a temperature of about -50° C to about +80° C. It is noted that the change of the uptake of carbon dioxide depending upon the pressure of carbon dioxide is examined, but, the selective adsorption of carbon dioxide in a gaseous mixture containing carbon dioxide and nitrogen is not examined therein. Further, as preferable exchangeable cations, sodium and lithium are mentioned in U. S. Pat. 5,531,808. It is shown that Li-LSX, Ca-LSX is superior to Na-LSX in the uptake of carbon dioxide, but, the adsorption selectivity between carbon dioxide and nitrogen is not examined.

A method of removing water vapor and carbon dioxide from a gas wherein water vapor is first removed and then carbon dioxide is removed by using sodium LSX zeolite as adsorbent has been proposed in U.S. Pat. 5,914,455 Gain Ravi and Tseng J. K., June 22, 1999). However, this patent is silent on the removal of carbon dioxide from a gaseous mixture containing carbon dioxide and nitrogen.

A zeolite adsorbent for gas purification comprising a sodium-type low-silica faujasite having a SiO ₂ /Al ₂ O ₃ of about 1.8 to 2.2 with a residual content of potassium ions less than about 8.0 equiv. % and a binder has been proposed in VVO 00/01478 (Rode et al., January, 2000). The zeolite adsorbent used therein has low zeolite crystal purity and contains a binder, and hence, the adsorption performance inherently possessed by zeolite crystal is not sufficiently manifested.

U.S.Pat. 6,616,732 (Grandmougin, et al. September 9, 2003) disclosed a novel family of zeolite adsorbents, suited to the decarbonatation of gas flows contaminated by CO ₂ , comprising a mixture of zeolite X and zeolite LSX, these adsorbents being predominantly exchanged with sodium or with strontium. The process for producing an agglomerated adsorbent as per this patent comprises many steps. Further, the use of a treatment cycle involve the steps of a) passing the contaminated gas flow into an adsorption region comprising the adsorbent bed, the adsorbent bed providing for the separation of the contaminant or contaminants by adsorption, b) desorbing the adsorbed CO ₂ by establishing a pressure gradient and gradually reducing the pressure in the adsorption region to recover the CO ₂ via the inlet into the adsorption region, c) increasing the pressure in the adsorption region by introducing a pure gas stream via the outlet of the adsorption region. Moreover, the adsorption is carried out at pressures of between 1 and 10 bar and desorption is carried out at pressures of between 0.1 and 2 bar. It does not disclose the adsorption selectivity between carbon dioxide and nitrogen and also does not provide the breakthrough data of carbon dioxide and nitrogen ad/desorption.

U.S.Pat. 6,530,975 (Rode, et al. March 11, 2003) discloses the preparation of a molecular sieve adsorbent for the purification of gas streams containing water vapor and carbon dioxide. The adsorbent is a combination of sodium form of a low-silica faujasite, having a residual content of potassium ions less than about 8.0 percent (equiv.), a low content of crystalline and amorphous admixtures and crystal sizes generally within the range of 1- 4 μm, and a binder. The process for the adsorbent preparation comprises specific parameters of low silica faujasite synthesis, sodium-potassium ion exchange, blending and granulation. It provides the carbon dioxide adsorption isotherms for low-silica faujasite having differing percentages of residual potassium cations and compares the carbon dioxide adsorption of various adsorbents including the molecular sieve 5A (CaA-94.5 % Ca ²⁺ ), molecular sieve 1OA (CaX), molecular sieve 13X (NaX) and calcium low-silica faujasite with a potassium ion content of 0.16 percent. However, it does not disclose adsorption data for nitrogen.

U. S. Par. 6,537,348 (Hirano, et al., March 25, 2003) discloses ε method of adsorptive separation of carbon dioxide from a gaseous mixture comprising carbon dioxide and gases less polar than carbon dioxide comprising contacting the gaseous mixture with a zeolite adsorbent is effected wherein carbon dioxide present in the gaseous mixture as contacted with the zeolite has a partial pressure of 0.1 to 50 mmHg, and the zeolite adsorbent is a 5 shaped product comprised of at least 95%, as determined in the basis of the moisture equilibrium adsorption value, of a low-silica type X zeolite having an SiO ₂ /Al ₂ O ₃ molar ratio of 1.9 to 2.1. Preferably, the zeolite adsorbent is preferably ion-exchanged with lithium and/or sodium, and is prepared by a process including a step of contacting with a caustic solution a calcined product of a mixture of a low-silica type X zeolite and kaolin 10 clay whereby the kaolin clay is converted to a low-silica type X zeolite. The method of this invention is claimed to be employed for purification of air when cryogenic separation of air is conducted, or for purification of natural gas. It does not disclose adsorption data for adsorptive separation of carbon dioxide from a gaseous mixture wherein carbon dioxide content is higher; about 4 -15 %, as in flue gas from power plant.

15 U. S. Pat. 7,011,695 (Moreau, et al., March 14, 2006) discloses zeolite adsorbent, and method of production, exchanged with calcium and barium cations, for purifying or separating a gas or gas mixture, in particular air, so as to remove there from the impurities found therein, such as hydrocarbons and nitrogen oxides. The adsorbent is preferably an X or LSX zeolite and the gas purification process is of the TSA type. It

20. does not disclose sorption data for carbon dioxide and nitrogen.

Historically, CO ₂ separation was motivated by enhanced oil recovery [Kaplan, L. J. Cost- Saving Processes Recovers CO ₂ from Power-Plant Flue gas. Chem. Eng. 1982, 89 (24), 30 31; Pauley, C. P.; Smiskey, P. L.; Haigh, S. N, ReN Recovers CO ₂ from Flue Gas 25 Economically. Oil Gas J. 1984, 82(20), 87 92]. Currently, industrial processes such as limestone calcination, synthesis of ammonia and hydrogen production require CO ₂ separation. Absorption processes employ either physical or chemical solvents such as a mixture of dimethylethers of polyetheleneglycol (Union Carbide' s 'SelexoD or chilled methanol (Linde' s 'RectisoD, methyl-ethanolamine (MEA) or methyl-diethanolamine (MDEA) and hindered amine solvents (KS-2, developed by Mitsubishi Heavy Industries and Kansai Electric Company) [Reimer, P.; Audus, H.; Smith, A. Carbon Dioxide Capture from Power Stations. IEA Greenhouse R&D Programme, www.ieagreen.org.uk, 2001. ISBN 1 898373 15 9; Blauwhoff, P. M. M.; Versteeg, G. F.; van Swaaij, VV. P. M. A study on the reaction between CO ₂ and alkanolamines in aqueous solution, Chem. Eng. Sci. 1984, 39(2), 207 225; Mimura, T.; Simayoshi, H.; Suda, T.; lijima, ¹ SA.; Mitsuake, S. Development of Energy Saving Technology for Flue Gas Carbon Dioxide Recovery by Chemical Absorption Method and Steam System in Power Plant, Energy Conversion Management 1997, 38, Suppl. P. S57 S62]. Adsorption systems capture CO ₂ on a bed of adsorbent materials such as molecular sieves and activated carbon [Kikkinides, E. S.; Yang, R. T.; Cho, S. H. Concentration and Recovery of CO ₂ from flue gas by pressure swing adsorption. Ind. Eng. Chem. Res. 1993, 32, 2714-2720]. CO ₂ can also be separated from the other gases by condensing it out at cryogenic temperatures. Polymers, metals such as palladium, and molecular sieves are being evaluated for membrane based separation processes [Reimer, P.; Audus, H.; Smith, A. Carbon Dioxide Capture from Power Stations. IEA Greenhouse R&D Programme, www.ieagreen.org.uk, 2001, ISBN 1 898373 15 9].

OBJECTS OF THE INVENTION

Accordingly, the object of the present invention is to eliminate the above-mentioned disadvantages of the conventional zeolite based adsorbents and to provide an adsorbent in pellet form, which is selective to carbon dioxide over nitrogen and can be used commercially for the separation of CO ₂ from a gas mixture, such as power plant flue gas, containing carbon dioxide in the range of 4 to 15 % by volume and gases less polar than carbon dioxide such as nitrogen, oxygen and hydrogen. Another object of the present invention is to provide a method of preparing zeolite based adsorbents for their potential use in removing carbon dioxide by adsorptive separation from a gaseous mixture, such as power plant flue gases, containing carbon dioxide in the range of 4 to 15-% by volume and gases less polar than carbon dioxide such as nitrogen, oxygen and hydrogen.

Yet- another object of the present invention is to provide a carbon dioxide selective adsorbent based on zeolite-X.

Yet another object of the present invention is to provide a method of preparing cation exchanged zeolite-X adsorbent for the selective uptake of CO ₂ from its gaseous mixture with N ₂ .

Yet another object of the present invention is to provide a method of preparing mixed cation, particularly, barium and potassium cations, exchanged zeolite-X based adsorbent for the selective uptake of CO ₂ from its gaseous mixture with other less polar gases like nitrogen and oxygen.

Other objects and advantages of the present invention will be apparent from the following descriptions.

SUMMARY OF THE INVENTION

Accordingly, the present invention provides a barium and potassium exchanged zeolite Na-X adsorbent composition useful for removal of CO ₂ from a gaseous mixture, comprising zeolite X powder wherein exchangeable potassium and barium cations are in the range of 5-95% and 35-95% respectively; wherein the sum of said potassium and barium cations represent at least 95% of the total exchangeable cations available in the said zeolite NaX powder and at least one clay binder is in the range of 10-20% by weight of the total weight of said adsorbent, wherein the adsorption capacity of the adsorbents is in the range 122 to 133 cc of CO,/g at 30 ⁰ C, selectivity is in the range 161 to 287 at 25 mmHg and 9.6 to 18.6 at 840 mmHg and break through capacity is in the range 53.3 to 62.9 cc of CO ₂ /g at 303K and 26.2 cc /gram to 33.3 cc /gram at 348K.

In an embodiment of the present invention wherein the adsorbent is prepared by the process comprising the steps of: a)subjecting Zeolite NaX powder to a first ion exchange by bringing said zeolite - NaX powder in contact with a solution containing potassium/barium cations. b)subjecting said potassium/barium exchanged zeolite X as obtained in step (a) to a second ion exchange by bringing said zeolite X in contact with a solution containing barium/potassium cations. clashing the zeolite as obtained in step (b) till free from chloride ions, followed by drying at 60 to 80 ⁰ C, and recovering zeolite - X exchanged with potassium and barium cations, d)incorporating and mixing dried zeolite powder as obtained in step (c) with clay binder (rich in Montmorillonite content, having cation exchange capacity >75 meq/100 gram clay, swelling in water 22 cc) at the ratio in the. range of 4:1 to 9:1 for about 15 to 30 minutes and adding water to the mixture followed by kneading for about 20 to 40 minutes; e)extruding the kneaded product of step (d) in desired shapes of pellets and drying at a temperature ranging between 60 to 80° C and calcined at a temperature ranging between 500 to 600° C for about 2 to 4 hours under air atmosphere to give a shaped adsorbent product. f)activating the dried extrudes of zeolite adsorbent obtained as in step (e) at a temperature in the range of 300 to 45O ⁰ C in fixed bed column. g)contacting the activated zeolite Na-X exchanged with barium and potassium cations as obtained in step (f) with a gaseous mixture containing CO ₂ , N ₂ and gases less polar than

CO, _.

In another embodiment of the present invention, wherein the gaseous mixture comprises carbon dioxide in the range of 10 to 20 % by volume and nitrogen in the range of 80 to 90 % by volume.

In still another embodiment of the present invention, wherein carbon dioxide is selectively removed from said gas mixture.

In yet another embodiment of the present invention, wherein the gaseous mixture is having carbon dioxide and nitrogen in a proportion similar to those found in flue gas from power plants.

In a further embodiment of the present invention, wherein the gaseous mixture is treated - by a process selected from the group consisting of pressure swing adsorption, temperature swing adsorption and optionally in combination of the two.

In an embodiment of the present invention, wherein the adsorbent comprises clay binder in the range of 10-20% by weight of the total weight of said adsorbent.

In an embodiment of the present invention, wherein the clay binder used is Bentonite clay.

In a further embodiment of the present invention a process for purifying or separating carbon dioxide from the gaseous mixture using said adsorbent, wherein the process steps comprises bringing the gaseous mixture containing carbon dioxide in contact with the adsorbent.

The zeolite used as starting material in the present invention possesses mole composition 0.9 ± 0.2Na_O:Λi_ O,:2.5 ± 0.5 SiO,: nH ₂ O; where i: = 6 to 8 and having sodium as the major extra framework cation. An ion-exchange solution containing the desired cation from barium or potassium is contacted with a predetermined quantity of zeolite so that all of the zeolite adsorbent is exposed to the exchanging cations, which rapidly attaches to the zeolite. An extended period of time, during which the zeolite soaks in a Ba ² VK ⁺ -rich solution, allows the ion concentrations to equilibrate and become uniform throughout the zeolite. For maximum cation loading the cation exchange cycle is performed repeatedly. The cation exchange is carried out in a batch process. The maximum potassium loaded (94%) zeolite is designated as KX and maximum barium loaded (93%) zeolite-X is designated as BaX. Mixed cation zeolite-X adsorbents, KBaX-I & KBaX-II, were prepared by treating the KX with 0.2M BaCl ₂ solution once and two times respectively. Whereas, KBaX-III and KBaX-IV were prepared by treating the BaX with 0.2M solution of KCl, once and two times respectively. The contents of K ⁺ and Ba ²⁺ were determined in solids using ICP-AES (Inductive Coupled Plasma-Optical Emission Spectrophotomer) method.

BRIEF DESCRIPTION OF THE DRAWINGS

For further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, wherein:

FIG.l is X-ray diffraction patterns for the commercial zeolite NaX powder, KX powder obtained in Example-2 and BaX powder obtained in Example- 5.

FIG.2 is X-ray diffraction patterns for the KBaX-I, KBaX-II, KBaX-III and KBaX-IV powders obtained in Example- 3, Example- 4, Example-6 and Example- 7, respectively.

FIG. 3 shows adsorption isotherms for nitrogen and carbon dioxide on zeolite NaX powder as mentioned in Example- 1.

FIG.4. shows adsorption isotherms for nitrogen and carbon dioxide on KX obtained in

Example- 2.

FIG.5 shows adsorption isotherms for nitrogen and carbon dioxide on KBaX-II obtained in Example-4.

FIG. 6 shows adsorption isotherms for nitrogen and carbon dioxide on BaX obtained in Example-5.

FIG. 7 is a CO ₂ breakthrough curve on NaX adsorbent bed at 30 ⁰ C. FIG. 8 is a CO ₂ breakthrough curve on NaX adsorbent bed at 75 ⁰ C. FIG. 9 is a CO ₂ breakthrough curve on KX adsorbent bed at 30 ⁰ C. FIG. 10 is a CO ₂ Breakthrough curve on KX adsorbent bed at 75 ⁰ C. FIG. 11 is a CO ₂ breakthrough curve on BaX adsorbent bed at 30 ⁰ C FIG. 12 is a CO ₂ breakthrough curve on BaX adsorbent bed at 75 ⁰ C FIG. 13 is a CO ₂ breakthrough curve on KBaX-II adsorbent bed at 30 ° C. FIG. 14 is a CO ₂ breakthrough curve on KBaX-II adsorbent bed at 75 ° C.

DESCRIPTION OF PREFERRED EMBODIMENTS

Within the context of the invention, the expression "exchanged with cations" is understood to mean that the cations in question are those associated with AlO ₂ ^" tetrahedral units of the zeolite (zeolitic phase), which exchanged cations play a role in the mechanism of adsorbing the gaseous compounds to be removed. Likewise, the term "exchangeable cations" is understood to mean cations that can be substituted or replaced with other cations by performing an ion exchange process.

The expression "percent of exchange of a cation 'M' " is understood to mean the number of charges carried by the cations 'iVT present in the zeolite with respect to the total number of charges of all of the cations. The degree of exchange varies between 0 and 100%. The total positive charge carried by the cations is equal to the total negative charge carried by the AlO ₂ ^" groups. The stoichiometric amount corresponds to this total charge.

The invention also relates to a process for manufacturing a zeolite adsorbent exchanged with potassium and barium cations, in which: (a) a zeolite containing sodium cations is subjected to a first ion exchange by bringing the zeolite into contact with a solution containing potassium/barium cations; (b) the zeolite from step (a) is subjected to a second ion exchange by bringing the zeolite into contact with a solution containing barium/potassium cations; (c) if necessary, steps (a) and/or (b) are repeated until the desired degree of exchange for each of the barium and potassium cations has been reached; and (d) a zeolite exchanged with potassium and/or barium cations is recovered. Depending on the case, the manufacturing process of the invention may include one or more of the following technical features: after each step (a) and after each step (b), the exchange solution is drained and, optionally, the zeolite obtained is rinsed; preferably with demineralized water; the zeolite in powder form or in agglomerated form; the solutions used in step (a) and/or in step (b) are recovered and reused during another subsequent step (a) and/or (b), in particular the barium salt solution; the zeolite is in powder form or in agglomerated form; the solutions used in step (a) and/or step (b) are recovered and reused during step (c), in particular the barium salt solution; an amount of barium ranging from 110% to 200% of the amount introduced into the zeolite is consumed; steps (a) and (b) are successive, in any order, or, depending on the case, carried out at the same time and in a single step using a solution of potassium or barium salts; steps (a) and (b) are carried out in a stirred batch reactor; steps (a) and (b) are carried out simultaneously and in a single step using a solution containing potassium and barium cations; the starting zeolite is a commercially available zeolite - X in sodium form (i.e. not exchanged) which; after step (d), the zeolite exchanged with potassium and/or barium cations is mixed with a binder, such as bentonite clay in order to obtain agglomerated zeolite particles. The invention will now become more clearly understood through the explanations and comparative examples given below as illustration and with reference to the appended figures.

A primary object of the present invention is to suggest / provide a method of adsorptive separation of carbon dioxide from a gaseous mixture comprising carbon dioxide in the concentration range of 4 to 15 % by volume and gases less polar than carbon dioxide, for example, nitrogen which is a cumbersome ingredient in flue gases. More particularly, it is intended by the present invention to effect the adsorptive separation of carbon dioxide by using a mixed cation zeolite-X adsorbent, which exhibits an enhanced adsorption capacity. When a zeolite ion-exchanged with a specific alkali metal cation such as barium / potassium ion to a high degree of exchange level is used, the adsorption selectivity of carbon dioxide and the efficiency of adsorptive separation thereof can be mere enhanced. In accordance with the present invention, there is provided a method of adsorptive separation of carbon dioxide from a gaseous mixture comprising carbon dioxide and gases less polar than carbon dioxide such as nitrogen comprising contacting the gaseous mixture with a zeolite adsorbent whereby carbon dioxide is adsorbed by the zeolite to be thereby separated, characterized in that carbon dioxide present in the gaseous mixture as contacted with the zeolite adsorbent has a partial pressure in the range of 25 - 150 mmHg, and the zeolite adsorbent is comprised of at least 80%, as determined on the basis of the moisture equilibrium adsorption value, of zeolite-X having an Si /Al ratio of 1.25.

The zeolite adsorbent preferably has been ion-exchanged with at least one kind of cation selected from barium and potassium, at an ion exchange ratio of at least 90%, more preferably at least 93%.The ion exchange is carried out on the starting zeolite, which is zeolite-X, initially containing sodium and/or potassium/barium which are easily exchangeable cations, the zeolite possibly being in non-agglomerated powder form or else formed into extrudate. To carry out the ion exchanges, a solution of potassium or barium salts, such as a chloride solution, with a pH of about 6 is preferably used.

The zeolite powder is placed into a stirred suspension in water and then the solution of potassium or barium salts is slowly added, with stirring which is sufficient to distribute the solution throughout the entire volume in suspension.

In all cases, the contact must be carried out under conditions in which the potassium or barium salt is distributed throughout the entire volume of zeolite, before the exchange has had time to take place, thereby ensuring that the potassium or barium is distributed homogeneously throughout the mass of the zeolite.

The salt molarities are between 0.01M and 0.25M; the temperature is between 30° C and 100° C and the contact time is between 30 minutes and 5 hours.

After exchange, the zeolite is washed with pure water for several times, dried at 80° C and then activated between about 300 and 450° C in a stream of dry gas or vacuum, under conditions which minimize contact between the moisture released and the zeolite.

The zeolite adsorbent pellet is preferably comprised of at least 80%, as determined on the basis of the moisture equilibrium adsorption value, of zeolite. More preferably the zeolite adsorbent consists essentially of the zeolite-X.

The zeolite adsorbent particles preferably have an average particle diameter in the range of 1 ^~ 5 mm, more preferably of 1.6-3.2 mm.

The adsorptive separation' of carbon dioxide may be effected by pressure swing adsorption

(PSA) and/or temperature swing adsorption (TSA). Preferably the adsorption is carried out at a temperature of 30 or 75° C, and then desorption is carried out at a temperature of 30 to 200° C.

The invention will now be described more specifically by the following working examples that by no means limit the scope of the invention. Evaluation of zeolite and its shaped product was conducted by the following methods.

(1) Chemical Composition:

A zeolite specimen is dissolved in a mixed solvent of nitric acid and hydrofluoric acid, and concentrations of metal ions are measured by an ICP (inductively coupled plasma) emission analyzer ("Optima 2000D V" available from Perkin Elmer Instruments). The concentration of a cation is expressed by an ion equivalent ratio. For example, the concentration of Ba/K ion in a solution containing Ba/K ion, Na ion and K ion is expressed by the ion equivalent ratio of Ba ion represented by the formula Ba/(Ba + Na + K).

(2) Crystal Structure:

Determination is conducted by an X- ray diffraction apparatus ("X' Pert' available from Phillips, The Netherlands).

(3) Adsorption Capacity:

Adsorption capacity of carbon dioxide and nitrogen are measured by using adsorption measurement apparatus "ASAP-2010" available from Micromeritics Instruments, USA at a temperature of 30° C or 75° C. The specimen used is kept in vacuum (under a pressure of not higher than lxlθ ^~3 mmHg) at a temperature of 350 ° C for 16 hours before measurement of the adsorption capacity. Using the thus-obtained adsorption isotherm, (i) adsorption capacity of carbon dioxide in a single-component system of carbon oxide under a desired pressure is calculated, and further, (ii) adsorption selectivity of carbon dioxide in a two-component system comprising carbon dioxide and nitrogen are calculated according to the following equation:

V ₁ CO. a AlB = T.

Where, V _co2 and V _χ2 are the volumes of gases CO ₂ and N ₂ respectively adsorbed at any given pressure P and temperature T. (4) CO ₂ Breakthrough Measurements:

CO ₂ breakthrough data were collected on a breakthrough measurement system comprising of an adsorbent column having a length of 35 cm and inside diameter of 1.9 cm at two temperatures 303K and 348K respectively. All breakthrough measurements were conducted using the same column mentioned above. The feed and outlet concentrations of the gases to and from the adsorbent column were measured using a gas chromatograph (GC-7610 supplied by Chemito Technologies FVt-. Ltd., Nashik, India) equipped with a TCD detector (TCD 866). The GC was equipped with a Porapaq column and the H ₂ gas at a flow rate of 40ml/min was used as the carrier gas. Around 1.5 ml of the gas samples were taken in a gas tight syringe and analyzed in the GC. NaX used is extrudes with 3 mm diameter, BaX, KX and KBaX-II is 3 mm extrudes prepared using 20% bentonite clay as binder. The feed gas concentration is having CO ₂ = 15.0 ± 0.5 % and N ₂ = 85.0 ± 0.5 %, Feed flow rate = 120 ± 2 ml/min. Desorption is carried out counter currently to feed flow by purging with N ₂ only. Prior to the CO ₂ breakthrough measurements, around 100 ml of the particular adsorbents were activated in situ in the adsorbent column at a temperature of around 623K under N ₂ flow overnight and then cooled to the breakthrough measurement temperatures. The feed gas consist of around 15% CO ₂ and 85% N ₂ , in which N ₂ acts as a carrier gas for the breakthrough measurements, is passed through the adsorbent column at a flow rate of around 120 ml/min.

Prior art teaches the preparation and use of Barium and potassium exchanged zeolite X for adsorption of N ₂ and CO ₂ . It is mainly used for removing CO ₂ for air stream. In the prior art wherein mixture of barium and potassium ions in zeolite X is used for the preferential adsorption of both para-and meta-cresol from their mixture with ortho cresol. None of prior art teaches the separation of CO ₂ form the mixture of CO ₂ and N ₂ using mixed cation (Ba ÷ K) zeolite X. and this is net obvious to the persons skilled in the art at the time of filing the application. The advantages can be clearly seen from the CO ₂ adsorption capacity (129.6 cc/g at 30 ⁰ C), selectivity (200.3 at 25mmHg and 7.5 at 840mmHg) and break through capacity 62.9 cc/g at 30 ^Q C)

The mixture of Barium and potassium exchanged zeolite which is the subject matter of the present invention gave better results when compared to individual K and Ba exchanged zeolite X with respect to adsorption capacity, Selectivity of CO ₂ /N ₂ and breakthrough capacity for CO ₂ at 3O ⁰ C. When zeolite containing (Ba + K) exchanged zeolite X was used in the present invention, the heat generated was very low; hence CO ₂ adsorption was on higher side. This is also advantageous during desorption process where in the heat requirement is less.

Comparative data given in Table 1 show how the adsorbent of the present invention is better than the closest adsorbent exchanged individually with Ba and K ions and their combination.

Table 1 Comparative data for Breakthrough capacity, Heat of adsorption, Adsorption capacity and selectivity of adsorbents of the present invention.

The novelty of the present invention lies in developing a zeolite based adsorbent comprising exchangeable cations in the amount of from about 5 to 95% potassium cations with from about 35 to 95% barium cations, and wherein the sum of said barium and said potassium cations represents at least about 95% of the total exchangeable cations available in said adsorbent for selective adsorption of carbon dioxide from flue gas like compositions. The variation of adsorption properties of mixed (barium and potassium) cation exchanged zeolite based adsorbents on account of varying cation contents resulting in difference in adsorption sites are reported for the first time and determined both by equilibrium and dynamic adsorption studies. The dynamic adsorption data as reported here are not reported in any prior art to the best of our knowledge.

The following examples are given by way of illustration and therefore should not construe to limit the scope of the present invention. EXAMPLE- 1

Adsorption capacities of carbon dioxide and nitrogen gas from a single component gas were measured in zeolite NaX powder having mole composition 0.9 ± 0.2Na ₂ OiAl ₂ O ₃ :2.5 ± 0.5 SiO ₂ : nH ₂ O; where n = 6 to 8 and having sodium as the major extra framework cation (considered as 100%). Heat of adsorption for carbon dioxide and nitrogen pure gases were calculated and the results found to be 55 and 19 kj/mol, respectively. The adsorption capacity was 129 cc /gram for CO2 and 9 cc / gram for N2 at 303K and 1 atmospheric pressure. The adsorption selectivity ( α CO2/N2) was 166 at 25 mmHg and 13 at S40mmHe. To make NaX adsorbent pellets, 100 grams of zeolite NaX powder having mole composition 0.9 ± 0.2Na ₂ ChAl ₂ O ₃ :2.5 ± 0.5 SiO ₂ : nH ₂ O; where n = 6 to 8 and having sodium as the major extra framework cation, 20 grams of Bentonite clay (rich in Montmorillonite content having cation exchange capacity >75 meq/100 gram clay, ■ swelling in water 22. cc) were incorporated and mixed together for 15 minutes and then a required amount of water was added, followed by kneading for 0.5 hours. The kneaded product was extruded by using a hand-operated, kitchen type noodle making machine to give a shaped product in the form of extrudes having an average diameter of 3 mm. Extrudes were dried at 80° C overnight and broken in to pieces of about 8 mm to 12 mm length manually. The dried extrudes were calcined at 600° C for 3 hours under air atmosphere in a muffle furnace to give a shaped product containing zeolite NaX. X-ray diffraction analysis of the zeolite NaX shaped product confirmed that it was comprised of faujasite zeolite and dehydroxylated bentonite clay.

EXAMPLE- 2

100 grams of zeolite NaX powder was placed in contact with an aqueous potassium chloride solution (75 grams potassium chloride in 5 liters of water) whereby an ion exchange was affected to give a zeolite KX powder product. To obtain maximum potassium exchanged product the treatment cycle is repeated three times. The aqueous potassium chloride solution used for ion-exchange in each cycle was prepared freshly by dissolving potassium chloride in water. Finally, the product is separated, washed and dried at 80° C overnight. Chemical analysis of the zeolite KX powder product revealed that the K ion exchange ratio was 94%, the Na ion exchange ratio was 6%, and the Si/ Al ratio was ~1.25. The pure component adsorption isotherms of CO ₂ and N ₂ were measured in KX powder at 303K. Heat of adsorption for carbon dioxide and nitrogen pure gases were calculated and the results found to be 49 and 16 kj/mol, respectively. The adsorption capacity was 122 cc /gram for CO _n and 6 cc /gram for Nj at 303K and 1 atmospheric pressure. The adsorption selectivity ( α CO ₂ /N ₂ ) was 287 at 25 mmHg and 19 at 840 mmHg. By the same procedures as described in Example-1, zeolite KX shaped product was prepared by incorporating and mixing together 100 grams of zeolite KX powder and 20 grams of bentonite clay.

EXAMPLE- 3

100 grams of zeolite KX powder as prepared in example- 2 was placed in contact with an aqueous barium chloride solution (245 grams barium chloride dihydrate in 5 liter of water), whereby an ion exchange was affected to give a zeolite KBaX-I powder product. Chemical analysis of the zeolite KBaX-I powder product revealed that the Ba ion exchange ratio was 37% and the K ion exchange ratio was 58%, and the Na ion exchange ratio 5% and the Si/Al ratio was ~1.25. The pure component adsorption isotherms of CO ₂ and N ₂ were measured in KBaX-I powder at 303K. Heat of adsorption for carbon dioxide and nitrogen pure gases were calculated and the results found to be 47 and 24 kj/mol, respectively. The adsorption capacity was 128 cc /gram for CO ₂ and 7 cc /gram for N2 at 303K and 1 atmospheric pressure. The adsorption selectivity ( α CO ₂ /N ₂ ) was 256 at 25 mmHg and 19 at 840mmHg. By the same procedures as . described in Example-1, zeolite KBaX-I shaped product was prepared by incorporating and mixing together 100 grams of zeolite KBaX-I powder and 20 grams of bentonite clay.

EXAMPLE-4

100 grams of zeolite KBaX-I powder as prepared in example-3 was placed in contact with an aqueous barium chloride solution (245 grams barium chloride dihydrate in 5 liters of water), whereby an ion exchange was affected to give zeolite KBaX-II powder product. Chemical analysis of the zeolite KBaX-II powder product revealed that the Ba ion exchange ratio was 53%, the K ion exchange ratio was 43%, Na ion exchange ratio was 4%, and the Si/Al ratio was ^" 1.25. The pure component adsorption isotherms of CO ₂ and N ₂ were measured in KBaX-II powder at 303K. Heai of adsorption for carbon dioxide and nitrogen pure gases were calculated and the results found to be 47 and 24 kj/mol,

">? respectively. The adsorption capacity was 129.6 cc /gram for CO ₂ and 7 cc /gram for N2 at 303K and 1 atmospheric pressure. The adsorption selectivity ( α CO ₂ /N ₂ ) was 200.3 at 25 mmHg and 18 at 840 mmHg. By the same procedures as described in Example- 1, zeolite KBaX-II shaped product was prepared by incorporating and mixing together 100 grams of zeolite KBaX-II powder and 20 grams of bentonite clay.

EXAMPLE-5

100 grams of zeolite NaX powder having mole composition 0.9 ± 0.2Na ₂ O:Al ₂ O ₃ :2.5 ± 0.5 SiO ₂ : nH ₂ O; where n = 6 to 8 and having sodium as the major extra framework cation was placed in contact with an aqueous barium chloride solution (245 grams barium chloride dihydrate in 5 liter of water), whereby an ion exchange was affected to give a zeolite BaX powder product. To obtain maximum barium exchanged product the treatment cycle is repeated three times. The aqueous barium chloride solution used for ion-exchange in each cycle was prepared freshly by dissolving barium chloride in water. Finally, the product is separated, washed and dried at 80° C overnight. Chemical analysis of the zeolite BaX powder product revealed that the Ba ion exchange ratio was 93%, the Na ion exchange ratio was 7%, and the Si/ Al ratio was ~1.25. Heat of adsorption for carbon dioxide and nitrogen pure gases were calculated and the results found to be 43 and 26 kj/mol, respectively. The adsorption capacity was 127 cc /gram for CO ₂ and 14 cc /gram for N2 at 303K and 1 atmospheric pressure. The adsorption selectivity ( α CO ₂ /N ₂ ) was 161 at 25 mmHg and 9 at 840 mmHg. By the same procedures as described in example- 1, zeolite- BaX shaped product was prepared by incorporating and mixing together 100 grams of zeolite BaX powder and 20 grams of bentonite clay.

EXAMPLE-6

The zeolite-BaX powder (100 grams) product as prepared in example-5 was placed in contact witn an aqueous potassium chloride solution (75 grams potassium chloride in 5 liters of water), whereby an ion exchange was affected to give zeolite KBaX-III powder product. Chemical analysis of the zeolite KBaX-III powder product revealed that the Ba ion exchange ratio was 68%, the K ion exchange ratio was 31%, Na ion exchange ratio was 1%, and the Si/ Al ratio was ^" 1.25. Heat of adsorption for carbon dioxide and nitrogen pure gases were calculated and the results found to be 47 and 25 kj/mol, respectively. The adsorption capacity was 132 cc /gram for CO ₂ and 10 cc /gram for N2 at 303K and 1 atmospheric pressure. The adsorption selectivity ( α CO ₂ /N ₂ ) was 179 at 25 mmHg and 12 at 840 mmHg. By the same procedures as described in example- 1, zeolite KBaX-III shaped product was prepared by incorporating and mixing together 100 grams of zeolite KBaX-III powder and 20 grams of bentonite clay.

EXAMPLE-7

100 gram of zeolite KBaX-III powder as prepared in example-6 was placed in contact with an aqueous potassium chloride solution (75 grams potassium chloride in 5 liter of water) .whereby an ion exchange was affected to give zeolite KBaX-FV powder product. Chemical analysis of the zeolite KBaX-IV powder product revealed that the Ba ion. exchange ratio was 93%, the K ion exchange ratio was 6%, Na ion exchange ratio was 1%, ^• and the Si/ Al ratio was ~1.25. Heat of adsorption for carbon dioxide and nitrogen pure gases were calculated and the results found to be 46 and 25 kj/mol, respectively. The adsorption capacity was 133 cc /gram for CO ₂ and 12 cc /gram for N2 at 303K and 1 atmospheric pressure. The adsorption selectivity ( α CO ₂ /N ₂ ) was 169 at 25 mmHg and 10 at 840 mmHg. By the same procedures as described in example- 1, zeolite KBaX-FV shaped product was prepared by incorporating and mixing together 100 grams of zeolite KBaX-FV powder and 20 grams of bentonite clay.

EXAMPLE-8

The CO ₂ breakthrough curve in NaX pellets, prepared as described in example- 1, was measured by taking around 38 grams of the adsorbent pellets and activating in situ in the adsorbent column at ^" 623K under N ₂ flow overnight and then cooled to the breakthrough measurement temperature. It showed a sharp increase in CO ₂ concentration at the outlet after the breakthrough, which means that the adsorption process inside the adsorbent column is of equilibrium controlled in nature. The CO ₂ breakthrough in NaX pellets is shown in FIG. 5 & 6. The breakthrough capacity of CO ₂ in NaX adsorbent pellets was found to be 56.3 cc /gram and 28.6 cc /gram at 303 K and 348K respectively.

EXAMPLE-9

The CO ₂ breakthrough curve in KX adsorbent pellets, prepared as per example-2, was measured by taking around 39 grams of the adsorbent pellets and activating in situ in the adsorbent column at a 623K under N ₂ flow overnight and then cooled to the breakthrough measurement temperatures. The feed gas consist of around 15% CO ₂ and 85% N ₂ , in which N ₂ acts as a carrier gas for the breakthrough measurements, is passed through the adsorbent column at a flow rate of around 120 ml/min. The CO ₂ breakthrough in KX pellets is shown in FIG. 7 & 8. The breakthrough capacity of CO ₂ in KX adsorbent pellets was found to be 53.3 cc /gram and 26.2 cc /gram at temperatures 303 K and 348K respectively.

EXAMPLE-10

The CO ₂ breakthrough curve in BaX adsorbent pellets was measured by taking around 55 grams of the adsorbent pellets prepared as described in example-5 is activated in situ in the adsorbent column at a temperature of around 623K under N ₂ flow overnight and then cooled to the breakthrough measurement temperatures. The feed gas consist . of around

15% CO ₂ and 85% N ₂ , in which N ₂ acts as a carrier gas for the breakthrough measurements, is passed through the adsorbent column at a flow rate of around 120 ml/min. The CO ₂ breakthrough in BaX pellets is shown in FIG. 9 & 10. The breakthrough capacity of CO ₂ in BaX adsorbent pellets was found to be 54.3 cc /gram and 29.6 cc

/gram at temperatures 303 K and 343K respectively. EXAMPLE-Il

The CO ₂ breakthrough in KBaX-II adsorbent pellets was measured by taking around 49 grams of the adsorbent pellets prepared as described in example-4 is activated in situ in the adsorbent column at a temperature of around 623K under N ₂ flow overnight and then cooled to the breakthrough measurement temperatures. The feed gas consist of around

15% CO ₂ and 85% N ₂ , in which N ₂ acts as a carrier gas for the breakthrough measurements, is passed through the adsorbent column at a flow rate of around 120 ml/min. The CO _j breakthrough in KBaX-II pellets is shown in FIG.ll & 12. The breakthrough capacity of CO ₂ in KBaX-II adsorbent pellets was found to be 62.9 cc /gram and 33.3 cc /gram at temperatures 303 K and 348K.

The adsorbent KBaX-II showed the highest breakthrough capacity for CO ₂ among the other adsorbents used in this study.

MAIN ADVANTAGES OF THE INVENTION

1. Commercially available zeolite NaX is used as starting material to prepare adsorbent of present invention. This will have advantage while scaling up the process to pilot and/or commercial plant. 2. Potassium chloride and barium chloride used to modify zeolite NaX are also available easily and commercially.

3. The method used for the modification of zeolite NaX is simple and require very common unit operations like mild stirring, heating, decantation, washing, pelletization and drying. 4.The advantages can be clearly seen from the high adsorption capacity (129.6 cc/g at 30 ⁰ C), selectivity (200.3 at 25mmHg) and break through capacity (62.9 cc/g at 30 ⁰ C). 5.When zeolite containing (Ba + K) exchanged zeolite X was used for the CO ₂ adsorption in the present invention, the heat generated was comparatively low; hence CO ₂ adsorption was on higher side. This is also advantageous during de≤orption process where in the energy requirement either in form of heat or vacuum is less. 6. The adsorbent of the present invention can function in temperature range from 30 to 75° C.

7. The adsorbent of the present invention has potential use in CO ₂ capture from post- combustion flue gas from thermal power plants. 8. The present invention can contribute to prevent global warming by CO ₂ capture from flue gas of thermal power plant.