The separation of light gases has become increasingly important over the years particularly in areas relating to natural gas recovery and enhanced oil recovery where there can be large amounts of light gases present. Often times, there are appreciable amounts of nitrogen present, as well as other smaller molecules such as carbon dioxide, oxygen and water, which need to be separated from other larger molecules such as methane, ethane, propane, butane and corresponding olefins.
In the natural gas industry, the separation of nitrogen from hydrocarbons, and particularly methane, is of primary interest in order to avoid nitrogen contamination from gas wells into natural gas pipelines. Natural gases which contain significant amounts of nitrogen may not meet minimum heating value specifications, reduce pipeline capacities and require additional compression horsepower and fuel consumption. Furthermore, as high quality gas reserves are gradually being depleted, the natural gas industry is becoming more dependent on lower qualtiy gas which contains larger amounts of nitrogen and other undesired gases. Accordingly, nitrogen removal from natural gases has attained increased importance.
The depletion of oil reserves has also influenced the petroleum industry where successful recovery of petroleum often requires the use of an enhanced recovery technique. One such often used technique involves the injection into the reservoir of a fluid which will not support combustion; an often used fluid for this technique is nitrogen or a nitrogen-containing gas. However, the use of this technique increases the level of nitrogen contaminant in the gas fraction recovered from the reservoir, i.e., the petroleum gases, above their naturally-occurring nitrogen concentration.
Nitrogen injection for enhanced oil recovery introduces a further problem because the nitrogen concentration in the petroleum gases, also known as case-head or well-head gases, does not remain constant over the life of the recovery operation. Although the nitrogen concentration variation will strongly depend upon particular reservoir characteristics, a general pattern is predictable. Typically during the first few years that enhanced recovery with nitrogen injection is employed, the nitrogen concentration in the gases may remain at about the naturally-occurring level, increasing thereafter as more nitrogen injection is required to recover the oil.
Hence, in order to render the use of nitrogen effective for enhanced oil and gas recovery, processes which are both cost effective and tolerant to compositional variations are required to separate nitrogen from methane and other gases.
The nitrogen contaminant can be removed from the nitrogen-containing gas by distillation. For example, U.S. Pat. No. 4,415,345 discloses a solution to this problem by providing processes that utilize distillation to remove nitrogen from natural gas streams, wherein a nitrogen heat pump is employed with both single and double distillation column arrangements to process streams of varying nitrogen content. In general, however, distillation processes such as described in the above-mentioned patent are well suited only for larger-scale operations due to their relatively high equipment costs and complex operating procedures.
In smaller-scale natural gas operations as well as in other areas such as synthesis gas and coke oven gas processing, adsorption processes can be especially well suited. The adsorption capacities of adsorption units can, in many cases, be readily adapted to process gas mixtures of varying nitrogen content without equipment modifications, i.e., by adjusting adsorption cycle times. Moreover, adsorption units can be conveniently skid-mounted, thus providing easy mobility between gas processing locations. Further, adsorption processes are often desirable because more than one component can be removed from the gas. As noted above, nitrogen-containing gases often contain other gases that contain molecules having smaller molecular dimensions than nitrogen, e.g., for natural gas, carbon dioxide, oxygen and water, and for coke oven gas, carbon monoxide.
However, despite the advantageous aspects of adsorption processes, the adsorptive separation of nitrogen from methane has been found to be particularly difficult. The primary problem is in finding an adsorbent that has sufficient selectivity for nitrogen over methane in order to provide a commercialy viable process. In general, selectivity is related to polarizability, and methane is more polarizable than nitrogen, i.e. The polarizability volumes for nitrogen and methane are respectively, N.sub.2 =17.6.times.10.sup.-25 cm.sup.-3 ; CH.sub.4 =26.0.times.10.sup.-25 cm.sup.-3. See P. W. Atkins, PHYSICAL CHEMISTRY, p. 773, Freedman (1982). Therefore, the equilibrium adsorption selectivity of essentially all known adsorbents such as large pore zeolites, carbon, silica gel, alumina etc. all favor methane over nitrogen. However, since nitrogen is a smaller molecule than methane, it is possible to have a small pore zeolite which adsorbs nitrogen faster than methane. Clinoptilolite is one of the zeolites which has been disclosed in literature as rate selective adsorbent for the separation of nitrogen and methane.
The separation of gaseous mixtures of methane and nitrogen using both raw clinoptilolite and clinoptilolite which had been ion-exchanged with calcium cations is described in the following publication; T. C. Frankiewicz and R. G. Donnelly, METHANE/NITROGEN SEPARATION OVER THE ZEOLITE CLINOPTILOLITE BY SELECTIVE ADSORPTION OF NITROGEN, Chapter 11, INDUSTRIAL GAS SEPARATION, American Chemical Society, 1983. They disclose that at long adsorption times, adsorption approaches thermodynamic equilibrium and there is a tendency for adsorbed nitrogen to be replaced by methane. However, since methane diffusion is slower than nitrogen diffusion into clinoptilolite, the separation can be made on a rate basis.
Japanese Patent Application (Kokai) No. 61-255,994 discloses a process for producing a high-caloric gas comprising two adsorption zones wherein nitrogen and other non-combustible low-caloric components are removed from a feed gas, e.g., coke oven gas or methane reaction gas, which also contains hydrogen, methane and other hydrocarbons. This Japanese patent application discloses that the nitrogen is adsorbed on a clinoptilolite adsorbent that may be naturally produced clinoptilolite, natural clinoptilolite that has been crushed as required either in its original form or after ion exchange or other chemical treatment, natural clinoptilolite that has been combined with a suitable binder, then compacted and sintered, natural clinoptilolite that has merely been heat-treated, or from clinoptilolite obtained by a synthetic process. This Japanese patent application does not, however, disclose any specific cations that would be suitable as ion-exchange agents in clinoptilolite for adsorbing nitrogen or smaller molecules.
Japanese Patent Application (Kokai) No. 62-132,542 discloses an adsorbing and separating composition composed of a clinoptilolite type zeolite containing calcium cations in a mole ratio of CaO/Al.sub.2 O.sub.3 of 0.4 to 0.75. The application discloses that the composition is useful for separating molecules with a kinetic diameter of less than 3.7 angstroms from molecules with that of 3.7 angstroms or greater, e.g., removal of small quantity of nitrogen from methane gas, or bulk separation of nitrogen from a methane-containing coke oven gas or coal mine draught gas, etc.
Although the above-described references propose various adsorbents and processes for separating nitrogen from methane other gases, more efficient and effective adsorbents and commerically viable processes are sought. In particular, it is desired to provide adsorbents and processes that are particularly useful for separating nitrogen or smaller molecules from methane or larger molecules.