Adsorbents for separating nitrogen from a feed gas

Details Adsorbents for separating nitrogen from a feed gas Adsorbents for separating nitrogen from a feed gas

1998-04-09

1999-11-30

Spitzer, Robert

Gas separation: processes

Solid sorption

Including reduction of pressure

95130, 95902, 96108, 502 60, B01D 53047

Patent

active

059935162

DESCRIPTION:

BRIEF SUMMARY
This invention is concerned with clinoptilolite adsorbents suitable for the separation of nitrogen from mixtures of gases of larger molecular dimensions, particularly methane, but also ethylene, ethane, propylene, butane, butene, etc. These adsorbents are designed for use in cyclic adsorption processes such as pressure swing (psa) and thermal swing (tsa) adsorption separation (see for example, D M Ruthven, ADSORPTION AND ADSORPTION PROCESSES, Academic Press, 1984, and R T Yang, GAS SEPARATION BY ADSORPTION PROCESSES, Butterworths, 1987).
Within the natural gas industry, it has been recognised that the reduction of nitrogen levels in natural gas (whether that nitrogen is present initially, or introduced by injection into a depleted well) represents a significant technical challenge. The current favoured solution to this challenge, by cryogenic separation, suffers from high capital and running costs. Separation by cyclic adsorption offers a potentially cheaper solution.
The chief problem hindering the development of a process for natural gas denitrogenation by cyclic adsorption lies in the lack of an adsorbent suitably selective for nitrogen over methane. Conventional cyclic adsorption processes such as for example the production of oxygen-enriched air (see D W Breck, ZEOLITE MOLECULAR SIEVES, John Wiley and Sons, 1974) rely on adsorbents with an energetic, or equilibrium, selectivity for one or more components of the gaseous mixture to be separated. Common microporous adsorbents for separation processes, such as microporous carbons and zeolites, possess equilibrium selectivities for methane (a principal component of natural gases) over nitrogen. Although these afford a potential means of natural gas denitrogenation, energy costs associated with the repressurisation of adsorbed methane would be economically prohibitive.
An alternative means of achieving selectivity in adsorbents is to attempt to differentiate molecules on the grounds of their molecular dimensions. Nitrogen is smaller than methane in terms of both its kinetic diameter, and in its critical van der Waals' dimension (see D W Breck, ZEOLITE MOLECULAR SIEVES, John Wiley and Sons, 1974, p636). What is required, therefore, is an adsorbent with a pore size such that it adsorbs the smaller nitrogen molecule at a much faster rate than the larger methane. Such a material would possess a kinetic selectivity for nitrogen adsorption over methane, and be suitable for use in natural gas denitrogenation by psa.
T C Frankiewicz and R G Donelly (Chapter 11, INDUSTRIAL GAS SEPARATIONS, American Chemical Society, 1983) disclosed that both mineral clinoptilolite (a naturally occurring and abundant zeolite), and clinoptilolite directly exchanged with calcium ions, display a kinetic selectivity for nitrogen over methane. This selectivity was used on a pilot plant scale to demonstrate that nitrogen separation from methane by psa was technically feasible if an appropriate, short, cycle time was employed. If too long a cycle time was used, however, more methane than nitrogen was adsorbed on the clinoptilolite. This demonstrated that the zeolite possessed only rate selectivity for nitrogen, having an equilibrium selectivity for the hydrocarbon component.
K Igawa et al. (ADSORPTIVE SEPARATING AGENT, Japanese Patent 62-132,542) disclosed that calcium exchanged clinoptilolites with CaO/Al.sub.2 O.sub.3 ratios of 0.40 to 0.75 were useful for separating molecules with kinetic diameters of less than 3.70 .ANG. from mixtures with molecules of greater dimensions. Separation of nitrogen from methane on these grounds was demonstrated.
C C Chao (SELECTIVE ADSORPTION OF NITROGEN ON MAGNESIUM-CONTAINING CLINOPTILOLITES, European Published Patent Application No. 437,027) disclosed further that clinoptilolites containing at least five percent of their exchangeable cations (as defined within the above patent) as magnesium ions were suitable for the separation of molecules of dimensions equal to or smaller than nitrogen from those larger than nitrogen. Disclosed also in this patent were

REFERENCES:
patent: 4935580 (1990-06-01), Chao et al.
patent: 4964888 (1990-10-01), Miller
patent: 4964889 (1990-10-01), Chao
patent: 5587003 (1996-12-01), Bulow et al.
patent: 5616170 (1997-04-01), Ojo et al.
M.W. Ackley et al., "Clinoptilolite: Untapped potential for Kinetic gas separations", Zeolites, 1992, vol. 12, Sep./Oct., pp. 780-788.
M.W. Ackley et al., "Adsorption Characteristics of High-Exchange Clinoptilolites", Ind. Eng. Chem. Res., vol. 30, No. 12, 1991, pp. 2523-2530.

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