Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2000-03-08
2002-05-21
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S918000, C062S922000
Reexamination Certificate
active
06389845
ABSTRACT:
FIELD OF THE INVENTION
The invention generally relates to the separation and recovery of SF
6
from a gas stream comprising CF
4
and at least one of air and nitrogen. The invention specifically relates to a method and apparatus for the separation and recovery of SF
6
from a gas stream comprising CF
4
and at least one of air and nitrogen using a combination of membrane and liquefaction separation techniques.
BACKGROUND OF THE INVENTION
Sulfur hexafluoride (SF
6
) is widely used in the electric power distribution industry. In particular, SF
6
is used as an insulator or dielectric gas in power distribution equipment such as transformers, switch boxes, gas insulated lines, and substations. Under high voltage conditions, SF
6
decomposes into various polar components including HF, F
+
, SO
2
, and the like. These by-products degrade the insulating qualities of the gas. As a result, the gas has to be replaced or refined periodically, In addition to the formation of these polar by-products, carbon tetrafluoride (CF
4
) is also generated during arcing in the presence of carbon containing insulators such as the materials known under the trade designations Teflon® and Megelit®.
Currently, there are several known methods for purifying and recycling SF
6
used as an insulator in electrical equipment. These methods are based upon adsorption and liquefaction. The SF
6
polar decomposition by-products are removed by soda lime, activated alumina, or molecular sieves. The refined SF
6
is then refilled into the circuit breakers, substations, or transformers.
Due to both technical and environmental concerns, the use of SF
6
/N
2
(or SF
6
/air) mixtures has been suggested to replace pure SF
6
as a gaseous dielectric in the electric power distribution industry. However, there is no known method or apparatus that can economically and efficiently recover and refined SF
6
from a gas mixture containing N
2
or air and CF
4
. Thus, there is a need in the industry for an economical and efficient method and apparatus for the capture and recycle of SF
6
/N
2
mixture containing CF
4
and the polar by-products. The present invention is intended to address this need in the art.
SUMMARY OF THE INVENTION
Briefly, in one aspect, the present invention relates to a method for the paration of SF
6
from a gas mixture consisting essentially of SF
6
, CF
4
, and N
2
.
The method comprises the steps of:
(a) contacting a gas mixture consisting essentially of SF
6
, CF
4
, and N
2
with a membrane at conditions effective to obtain a permeate stream rich in N
2
and a retentate stream rich in SF
6
and CF
4
; and
(b) liquefying the retentate stream at conditions effective to obtain liquid SF
6
and gaseous CF
4
.
In another aspect, the present invention relates to an apparatus for the separation of SF
6
from a gas mixture consisting essentially of SF
6
, CF
4
, and N
2
.
The apparatus comprises:
(a) at least one membrane separation unit which permeates N
2
faster than SF
6
and CF
4
; and
(b) means for liquefying a retentate stream comprising SF
6
and CF
4
from the at least one membrane separation unit to form liquid SF
6
and gaseous CF
4
.
REFERENCES:
patent: 4274851 (1981-06-01), Stokes
patent: 4687498 (1987-08-01), Maclean et al.
patent: 5053067 (1991-10-01), Chretien
patent: 5779763 (1998-07-01), Pinnau et al.
patent: 5843208 (1998-12-01), Anumakonda et al.
patent: 5919285 (1999-07-01), Li et al.
patent: 5976222 (1999-11-01), Yang et al.
patent: 98/32521 (1998-07-01), None
American Air Liquide Inc.
Doerrler William C.
Russell Linda K.
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