Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2002-09-17
2003-10-07
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S050000
Reexamination Certificate
active
06630011
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the treatment of nitrogen-laden natural gas. More particularly, the invention relates to the removal of nitrogen from such natural gas by means of gas-separation membranes.
BACKGROUND OF THE INVENTION
Fourteen percent of known U.S. natural gas reserves contain more than 4% nitrogen. Many of these reserves cannot be exploited because no economical technology for removing the nitrogen exists.
Cryogenic distillation is the only process that has been used to date on any scale to remove nitrogen from natural gas. The gas streams that have been treated by cryogenic distillation, for example streams from enhanced oil recovery, have large flow rates and high nitrogen concentration, such as more than 10 vol %. Cryogenic plants can be cost-effective in these applications because all the separated products have value. The propane, butane and heavier hydrocarbons can be recovered as natural gas liquids (NGL), the methane/ethane stream can be delivered to the gas pipeline and the nitrogen can be reinjected into the formation.
Cryogenic plants are not used more widely because they are costly and complicated. A particular complication is the need for significant pretreatment to remove water vapor, carbon dioxide and C
3+
hydrocarbons and aromatics to avoid freezing of these components in the cryogenic section of the plant, which typically operates at temperatures down to −150° C. The degree of pretreatment is of ten far more elaborate and the demands placed upon it are far more stringent than would be required to render the gas acceptable in the pipeline absent the excess nitrogen content. For example, pipeline specification for water vapor is generally about 120 ppm; to be fit to enter a cryogenic plant, the gas must contain no more than 1-2 ppm of water vapor at most. Similarly, 2% carbon dioxide content may pass muster in the pipeline, whereas carbon dioxide must be present at levels no higher than about 100 ppm for cryogenic separation. For streams of flow rates less than about 50-100 MMscfd, therefore, cryogenic technology is simply too expensive and impractical for use.
Other processes that have been considered for performing this separation include pressure swing adsorption and lean oil absorption; none is believed to be in regular industrial use.
Gas separation by means of membranes is known. For example, numerous patents describe membranes and membrane processes for separating oxygen or nitrogen from air, hydrogen from various gas streams and carbon dioxide from natural gas. Such processes are in industrial use, using glassy polymeric membranes. Rubbery polymeric membranes are used to separate volatile organic compounds from air or other gas mixtures.
An application that is very difficult for membranes is the separation of nitrogen from methane. Both glassy and rubbery membranes have very poor selectivities, typically of 3 or less, for nitrogen over methane or methane over nitrogen.
U.S. Pat. No. 3,616,607 to Northern Natural Gas Company, discloses membrane-based separation of nitrogen from methane for natural gas treatment, using nitrogen-selective membranes. The patent reports extraordinarily high nitrogen/methane selectivities up to 15 and 16. These numbers are believed to be erroneous and have not been confirmed elsewhere in the literature. Also, the membranes with these alleged selectivities were made from polyacrylonitrile, a material with extremely low gas permeability of the order 10
−4
Barrer (ten thousandths of a Barrer) that would be impossible to use for a real process.
It was discovered a few years ago that operating silicone rubber membranes at low temperatures can increase the methane
itrogen selectivity to as high as 5, 6 or above. U.S. Pat. Nos. 5,669,958 and 5,647,227 make use of this discovery and disclose low-temperature methane
itrogen separation processes using silicone rubber or similar membranes to preferentially permeate methane and reject nitrogen. However, such a selectivity is obtained only at very low temperatures, typically −60° C., for example. Temperatures this low generally cannot be reached by relying on the Joule-Thomson effect to cool the membrane permeate and residue streams, but necessitate additional chilling by means of external refrigeration. While such processes may be workable in industrial facilities with ready access to refrigeration plants, they are impractical in many gas fields, where equipment must be simple, robust and able to function for long periods without operator attention.
Another problem of very low temperature operation is that, even though the membranes themselves may withstand the presence of liquid water and hydrocarbons, considerable pretreatment is of ten necessary to avoid damage to ancillary equipment by condensed liquids. Streams must also be dried to a very low water content to prevent the formation of methane or other hydrocarbon hydrates that can clog the system.
U.S. Pat. Nos. 6,361,582 and 6,361,583, co-owned with the present application, describe processes for separating gases such as nitrogen from C
3+
hydrocarbons. The processes make use of membranes made from certain fluorinated polymers that are resistant to plasticization by hydrocarbons. U.S. patent application Ser. No. 10/100,459, entitled “Nitrogen Gas Separation Using Organic-Vapor-Resistant Membranes” and co-owned and copending with the present application, describes processes for separating nitrogen from gas mixtures such as natural gas using the same type of fluorinated membranes.
U.S. patent application Ser. No. 10/105,861, entitled “Gas Separation Using Organic-Vapor-Resistant Membranes in Conjunction with Organic-Vapor-Selective Membranes” and co-owned and copending with the present application, describes processes for separating gas mixtures, such as nitrogen-containing natural gas, by means of flow schemes that use combinations of rubbery and glassy membranes.
U.S. Pat. No. 6,425,267, co-owned with the present application, describes processes for removing nitrogen from natural gas using a two-step arrangement of nitrogen-rejecting membranes.
U.S. patent application Ser. No. 10/035,404, entitled “Natural Gas Separation using Nitrogen-Selective Membranes” and Ser. No. 10/033,680, entitled “Natural Gas Separation using Nitrogen-Selective Membranes of Modest Selectivity”, both co-owned and copending with the present application, describe processes for separating nitrogen from natural gas using only glassy, nitrogen-selective membranes.
There remains a need for improvements to the above-described processes, especially for treating gas streams containing relatively high concentrations of nitrogen, or those where the composition varies over time.
SUMMARY OF THE INVENTION
The invention is a process for treating natural gas or other methane-containing gas to remove excess nitrogen.
It is envisaged that the process will be particularly useful as part of a natural gas processing train. Pipeline specification for natural gas is usually no more than about 4% nitrogen, but raw gas frequently contains more than 4% nitrogen and not infrequently contains 10% nitrogen, 20% nitrogen or more. The process of the invention frequently enables gas that is out of specification with respect to nitrogen to be brought to pipeline specification. Gas streams associated with oil wells, landfill gas, coal-seam gas and the like fall within this general type of treatable gas stream.
Other application areas where the process is expected to be useful include, but are not limited to, treatment of off-gases from petrochemical manufacturing and other industrial processes.
The invention relies on membrane separation using a combination of methane-selective membranes and nitrogen-selective membranes. Specifically, the process in preferred form uses three membrane separation units, configured as a two-step, two-stage operation, as defined below. The first and second steps of the first stage use methane-selective membranes, and the second stage uses nitrogen-selective membranes.
In a basic embodiment, the
Baker Richard W.
Da Costa Andre R.
Lokhandwala Kaaeid A.
Wijmans Johannes G.
J. Farrant
Membrane Technology and Research, Inc.
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