Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2001-07-27
2002-07-30
Capossela, Ronald (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S927000, C095S047000, C095S050000
Reexamination Certificate
active
06425267
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 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 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 membrane 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 5 and a trans membrane methane flux of 1×10
−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.
TABLE 1
Permeability
Selectivity
(Barrer)
(−)
Polymer
N
2
CH
4
N
2
/CH
4
CH
4
/N
2
Ref.
Polyimide
0.26
0.13
2.1
0.5
1
(6FDA-mp'ODA)
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-MDA)
0.20
0.10
2.0
0.5
2
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
Poly(dimethylsiloxane-
103
335
0.3
3.3
3
dimethylstyrene)
Poly(dimethylsiloxane)
230
760
0.3
3.3
3
Poly(siloctylene-siloxane)
91
360
0.25
4.0
4
Poly(silphenylene-siloxane)
3
12
0.25
4
4
Polyamide-polyether block
4.8
20
0.24
4.2
3
copolymer
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).
These separation properties are not good enough to make methane
itrogen separation by membrane practical, either by preferentially permeating the nitrogen or by preferentially permeating the methane.
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. However, 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 often 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.
Yet another problem of very low temperature operation is that equipment components must be made from comparatively expensive stainless steel or other special steels, rather than lower cost carbon steels.
Further concerns that hamper membrane process design for methane
itrogen separation are that vacuum pumps generally must not be used anywhere in the system as they may permit air to leak into lines carrying hydrocarbon mixtures, representing an unacceptable safety hazard. Indeed, for safety, reliability and cost-containment, the number of pieces of rotating or moving equipment of any kind should be kept to a minimum.
In view of these multiple difficulties, there remains an unsatisfied need for economical means of exploiting nitrogen-rich natural gas reserves, especially those contained in gas fields with smaller flow rates.
SUMMARY OF THE INVENTION
The invention is a process for treating natural gas or other methane-rich gas to remove excess nitrogen, thereby producing one, two or three product streams of value. The invention relies on membrane separation using methane-selective membranes, but does not require the membranes to be operated at very low temperatures. We have found that, by using a two-step membrane system design, and optionally controlling the operating parameters for the membrane steps within certain ranges, the capital and operating costs of the process can be kept within economically acceptable limits.
Two basic process configurations can be used, depending on the pressure of the raw gas to be treated by the process. If the ra
Baker Richard W.
Da Costa Andre R.
Lokhandwala Kaaeid A.
Wijmans Johannes G.
Capossela Ronald
Farrant J.
Membrane Technology and Research, Inc.
LandOfFree
Two-step process for nitrogen removal from natural gas does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Two-step process for nitrogen removal from natural gas, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Two-step process for nitrogen removal from natural gas will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2898117