Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Reexamination Certificate
2001-12-28
2003-06-03
Spitzer, Robert H. (Department: 1724)
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
C095S049000, C095S051000, C095S052000
Reexamination Certificate
active
06572678
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the treatment of natural gas that is out of specification with respect to nitrogen. 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. The gas in as many as a third or more of these reserves also contains more than 2% carbon dioxide, making it sub-quality with regard to both gases. Significant amounts of gas are also contaminated by hydrogen sulfide, for which pipeline specification is lower than 4 ppm. 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 often 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 below about 120 or 140 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.
The potential production of gas containing 10 to 30% nitrogen is about 8,500 MMscfd. At present, approximately 4,500 MMscfd of this gas is treated by cryogenic processes, leaving about 4,000 MMscfd of gas unused because it is unsuitable for cryogenic treatment. Much of this gas is in small fields and is contaminated with carbon dioxide or/and hydrogen sulfide.
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 organic components from air or other gas mixtures.
A report by SRI to the U.S. Department of Energy (‘Energy Minimization of Separation Processes using Conventional Membrane/Hybrid Systems”, D. E. Gottschlich et al., final report under contract number DE 91-004710, 1990) suggests that separation of nitrogen from methane might be achieved by a hybrid membrane/pressure swing adsorption system. The report shows and considers several designs, assuming that a hypothetical nitrogen-selective membrane, with a selectivity for nitrogen over methane of between 5 and 15 and a transmembrane pressure-normalized methane flux of 1×10
31 6
cm
3
(STP)/cm
2
·s·cmHg, were to become available, which to date it has not.
In fact, both glassy and rubbery membranes have poor selectivities for nitrogen over methane or methane over nitrogen. Table 1 lists some representative values for glassy materials.
TABLE 1
Permeability
Selectivity
(Barrer)
(−)
Polymer
N
2
CH
4
N
2
/CH
4
CH
4
/N
2
Ref.
Polyimide (6FDA-mp′ODA)
0.26
0.13
2.1
0.5
1
Polyimide (6FDA-BAHF)
3.10
1.34
2.3
0.4
1
Polyimide (6FDA-IPDA)
1.34
0.70
1.9
0.5
2
Polyimide (6FDA-MDT)
0.40
0.20
2.0
0.5
4
Polyimide (6FDA-CDM)
0.27
0.12
2.3
0.4
4
Polyimide (6FDA-HAB)
0.17
0.056
3.0
0.3
4
Polyimide (6FDA-MDA)
0.20
0.10
2.0
0.5
2
Polyimide (BPDA-MDT)
0.048
0.028
1.7
0.6
4
Polyimide (BPDA-HAB)
0.001
0.0004
2.4
0.4
4
Cellulose acetate
0.35
0.43
0.8
1.2
3
Polycarbonate
0.37
0.45
0.8
1.2
3
Polysulfone
0.14
0.23
0.6
1.7
3
Hyflon ® AD60
20
8-9
2.3
5
Hyflon ® AD80
30-
14-19
2.1
5
40
Cytop ®
5
1-2
2.7
5
1. K. Tanaka et al., “Permeability and Permselectivity of Gases in Fluorinated and Non-Fluorinated Polyimides”, Polymer 33, 585 (1992).
2. T. H. Kim et al., “Relationship Between Gas Separation Properties and Chemical Structures in a Series of Aromatic Polyimides”, J. Memb. Sci., 37, 45 (1988).
3. J. G. Wijmans, “Membrane Processes and Apparatus for Removing Vapors from Gas Streams”, U.S. Pat. No. 5,071,451 (December 1991).
4. S. A. Stern et al., “Structure Permeability Relationships in Silicone Polymers”, J. Polymer Sci: Polymer Physics Ed. 25, 1263 (1987).
5. Inventors' estimate.
The problem of separating gas mixtures containing methane and nitrogen into a methane-rich stream and a nitrogen-rich stream is, therefore, a very difficult one, owing to the low selectivity of essentially all membrane materials to these gases. In addition, many materials that are somewhat selective for one gas over the other have very low permeability.
U.S. Pat. No. 3,616,607 to Northern Natural Gas Company, discloses membrane-based separation of nitrogen from methane for natural gas treatment. 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 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 operating membranes at very low temperature operation is that, just as in conventional cryogenic plants, significant pretreatment is required to avoid system blockages and damage caused by methane hydrate formation or freezing of higher boiling point stream components individually.
A significant problem when rubbery, methane-selective membranes are used is co-permeation of carbon dioxide, hydrogen sulfide, and water that may also be present in the gas. These components permeate the first set of membrane modules and are concentrated in the natural gas product that is to be sent to the pipeline. Since more than one third of the unexp
Baker Richard W.
He Zhenjie
Pinnau Ingo
Wijmans Johannes Gerard
Farrant J.
Membrane Technology and Research, Inc.
Spitzer Robert H.
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