Chemistry of inorganic compounds – Zeolite
Reexamination Certificate
2002-03-25
2003-06-03
Sample, David (Department: 1755)
Chemistry of inorganic compounds
Zeolite
C423S713000, C423SDIG002, C502S085000, C095S902000
Reexamination Certificate
active
06572838
ABSTRACT:
FIELD OF INVENTION
The present invention relates to a process for the preparation of a molecular sieve adsorbent for selectively adsorbing nitrogen and argon from a gaseous mixture with oxygen.
BACKGROUND AND PRIOR ART REFERENCES
Adsorption processes for the separation of oxygen and nitrogen from air are being increasingly used for commercial purposes for the last three decades. Oxygen requirements in sewage treatment, fermentation, cutting and welding, fish breeding, electric furnaces, pulp bleaching, glass blowing, medical purposes and in the steel industries particularly when the required oxygen purity is 90 to 95% is being largely met by adsorption based pressure swing or vacuum swing processes. It is estimated that at present, 4-5% of the world's oxygen demand is met by adsorptive separation of air. However, the maximum attainable purity by adsorption processes is around 95% with separation of 0.934 mole percent argon present in the air being a limiting factor to achieve 100% oxygen purity. Furthermore, the adsorption-based production of oxygen from air is economically not competitive to cryogenic fractionation of air for production levels more than 100 tonne oxygen per day. Of the total cost of the oxygen production by adsorption processes, it is estimated that capital cost of equipment and power consumption are the two major factors influencing the overall cost with their share being 50% and 40% respectively. Along with the factors like process and system design, the adsorbent is the key component, which can bring down the cost of oxygen production by adsorption. The adsorbent selectivity and capacity are important parameters for determining the size of the adsorption vessels, compressors or vacuum pumps. It is desirable to have an adsorbent, which shows a high adsorption capacity as well as selectivity for nitrogen compared to oxygen. The improvement in these properties of the adsorbent directly results in lowering the adsorbent inventory of a system and hence the size and power consumption of the air compressor or vacuum pump. Furthermore, adsorbent having a high. nitrogen capacity and selectivity can also be used to produce reasonably pure nitrogen along with oxygen by evacuating nitrogen adsorbed on the adsorbent. Furthermore, adsorbents having both nitrogen and argon selectivity over oxygen can be used for producing high purity (>96%) oxygen from air.
It is, therefore, highly desirable, for an adsorbent to have good adsorption capacity and adsorption selectivity for a particular component sought to be separated.
Adsorption capacity of the adsorbent is defined as the amount in terms of volume or weight of the desired component adsorbed per unit volume or weight of the adsorbent. The higher the adsorbent's capacity for the desired components the better is the adsorbent as the increased adsorption capacity of a particular adsorbent helps to reduce the amount of adsorbent required to separate a specific amount of a component from a mixture of particular concentration. Such a reduction in adsorbent quantity in a specific adsorption process brings down the cost of a separation process.
The adsorption selectivity of a component results from steric factors such as the differences in the size and shape of the adsorbate molecules; equilibrium effect, i.e. when the adsorption isotherms of components of a gas mixture differ appreciably; kinetic effect, when the components have substantially different adsorption rates.
It is generally observed that for a process to be commercially economical, the minimum acceptable adsorption selectivity for the desired component is about 3 and when the adsorption selectivity is less than 2, it is difficult to design an efficient adsorption process.
In the prior art, adsorbent which are selective for nitrogen from its mixture with oxygen and argon have been reported wherein the zeolites of type A, X and mordenite have been used after ion exchanging alkali and/or alkaline earth metal ions. However, the adsorption selectivity reported for the commercially used zeolite A based adsorbents for this purpose varies from 3 to 5 and adsorption capacity from 12-15 cc/g at 765 mmHg and 30° C. The efforts to enhance the adsorption capacity and selectivity have been reported by increasing the number of exchangeable cations into the zeolite structure by modifying the chemical composition of the zeolite. The adsorption selectivity for nitrogen has also been substantially enhanced by exchanging the zeolite with cations like lithium and/or calcium in some zeolite types.
Zeolite A having a specific amount of calcium has been commercially used for oxygen production from air by selectively adsorbing nitrogen. However, presently used adsorbent has the following limitations:
Low adsorption capacity compared to other commercially used adsorbents.
Low adsorption selectivity.
It gives oxygen with only 95% maximum purity.
Sensitivity to moisture.
It needs multiple exchange with calcium salt.
The activation of the adsorbent requires much care, in order to prevent the hydroxylation.
R. V. Jasra et al. reviewed the recent status of pressure swing adsorption as a process for separating multi component gas mixture in “Separation of gases by pressure swing adsorption”; Separation Science and Technology, 26(7), pp. 885-930, 1991, The application of a new generation of adsorbents were described in detail. In “Adsorption of a Nitrogen-Oxygen mixture in NaCaA zeolites by elution Chromatography”, Ind. Eng. Chem. Res. 1993, 32, 548-552, N. V. Choudary et al. describes the influence of calcium content on adsorption of nitrogen and oxygen is studied on various NaCaA zeolite samples. N. V. Choudary et al. describes the adsorption and desorption of nitrogen, oxygen and argon in mordenite type zeolite having different Si/Al ratios in ‘Sorption of nitrogen, oxygen and argon in mordenite type zeolites’, Indian Journal of Chemistry Vol. 38A January 1999, pp.34-39. The heat of adsorption of nitrogen and argon in mordenite, NaA and NaX were compared to revels the sorbate interactions with extra-frame work sodium ions as well as lattice oxygen atoms.
Reference may be made to J. J. Collins et al in U.S. Pat. No. 3,973,931(1976) entitled “Air separation by adsorption”, wherein an adiabatic pressure swing process for air separation by selective adsorption in atleast two zeolitic molecular sieve beds in which air is introduced at below 90° F., the coldest gas temperature in the inlet end is 35° F., delta T atleast 15° F., the inlet end is heated to maintain the gas at maximum of at least 20° F. warner than without heating, but below 175° F. The main drawback is it require heating and temperature control in the air separation process.
C. G. Coe et al. in U.S. Pat. No. 4,481,018 (1984) entitled “Polyvalent ion exchanged adsorbent for air separation”, describes the use of a thermally activated polyvalent ion exchanged faujasite-containing compositions with selectivity 3.4 to 6.7 at 30° C. for the separation of air into oxygen and nitrogen. The drawbacks are the thermal activation process requires very slow heating to prevent hydroxylation and the selectivity of the adsorbent is only 3.4 to 6.7 at 30° C.
S. Sircar et al in U.S. Pat. No. 4,557,736 (1985) entitled “Binary ion exchanged type X zeolite adsorbent”, describes the use of an adsorbent comprises a binary ion exchanged type X zeolite, in which 5%-40% of the available ion sites are occupied by calcium and 60%-95% of the available ion sites are occupied by strontium is used for the adsorption of nitrogen from an air stream at superambient pressure to produce an oxygen rich product streem. The main drawback is the preparation of the adsorbent requires multistage cation exchange process.
S. Sircar in U.S. Pat. No. 4,756,723 (1988) entitled “Preparation of high purity oxygen”, describes the use of a single stage pressure swing adsorption method for the production of approximately 95% pure oxygen. The main drawback is the maximum attainable oxygen purity is only 95%.
C. C. Chao in U.S. Pat. No. 4,859,217 entitled (1989) “Process for se
Jasra Raksh Vir
Sebastian Jince
Council of Scientific and Industrial Research
Dority & Manning P.A.
Sample David
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