Separation of gases containing SF6

Details Separation of gases containing SF6 Separation of gases containing SF6

2001-02-20

2002-09-24

Spitzer, Robert H. (Department: 1724)

Gas separation: processes

Selective diffusion of gases

Selective diffusion of gases through substantially solid...

C095S131000, C095S135000, C096S004000, C096S130000, C096S134000, C096S147000, C055S356000

Reexamination Certificate

active

06454837

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to a process for the separation of gas mixtures which contain sulfur hexafluoride (SF
6
).
SF
6
-containing gases are used for widely varying purposes in industry. SF
6
/N
2
mixtures or SF
6

oble gas mixtures and SF
6
/air mixtures are used, for example, in the manufacture of windows as an insulating filler gas, and in the tyre industry as a pressure-resistant, noise-damping filler gas.
Mixtures of sulfur hexafluoride and nitrogen are used as an insulating filler gas for underground cables, see German Utility Model 297 20 507.2. Usually, these mixtures contain 5 to 30% by volume sulfur hexafluoride, remainder nitrogen to make up to 100% by volume.
Mixtures of sulfur hexafluoride and air and optionally CO
2
are used as a protective gas cushion when casting magnesium. SF
6
is usually contained in these mixtures in a quantity of 0.05 to 1% by volume.
SF
6
-containing exhaust air may also occur in industry.
SUMMARY OF THE INVENTION
It is desirable to work up these mixtures once they have been used, with the objective of re-using the SF
6
. The problem is that nitrogen or air takes up a large volume percentage in the gas mixture, and requires a large transport capacity. It is an object of the present invention to devise a process for separating the above gas mixtures which, upon re-use of the SF
6
from the mixtures, requires only a small transport capacity.
A further object is to provide a suitable apparatus for performing the process according to the invention.
The process according to the invention for separating SF
6
-containing gas mixtures provides for the mixture to be contacted with hydrophobic zeolites having an SiO
2
/Al
2
O
3
ratio (“modulus”) of at least 80 and a pore diameter of 4 to 7 Å (0.4 to 0.7 nm), in order to adsorb SF
6
preferentially. The substantially SF
6
-free nitrogen or the SF
6
-free air can be let off into the environment.
This simple way of performing the process is particularly suitable when relatively small quantities of gas mixture are to be separated or if the SF
6
content is low, e.g. below 5% by volume. It is therefore particularly well suited for the mixtures of SF
6
and air and optionally CO
2
which originate from the casting of magnesium and usually contain 0.05 to 1% by volume SF
6
. If additional impurities such as SO
2
F
2
, SO
2
etc. are contained therein, purification may be effected beforehand, such as washing with water or lye or sorption by means of e.g. Al
2
O
3
. This way of performing the process is also well suited for separating filler gases from insulating panes and car tires.
For example, the protective gas from the magnesium industry containing 0.2% by volume SF
6
, remainder air and CO
2
, is drawn off and the air is separated off. The sorbed SF
6
and CO
2
is recycled. With a few kilogrammes of adsorber material, virtually a whole week's production of exhaust gas from a melting crucible can be purified. Magnesium can therefore be produced in a far more environmentally friendly manner than previously. The desorption time is not longer, or even shorter, than the sorption time.
Although the SF
6
content is frequently 10% by volume for insulating-gas window panes, the extractor units frequently draw in secondary air, so that the effective SF
6
content is lower upon sorption.
One embodiment of the invention relates to a process for the purification of SF
6
-containing gases, in particular air or waste air contaminated with SF
6
.
This aspect of the invention is based on the finding that SF
6
can generally be separated out of gases using the selected zeolites.
Simple sorption tests show whether the separation occurs to the desired extent for a particular gas from which SF
6
is to be removed. The separation effect is very successful if the molecules of the gas from which the SF
6
is to be separated are larger or smaller than the pore diameter of the zeolite used. In this manner, for example, gases from production can be purified.
The method can also be used for the purification of air or exhaust air which is contaminated with SF
6
.
The improvement according to the invention resides in effecting the purification of air or exhaust air which is contaminated with SF
6
.
If additional impurities such as SO
2
F
2
, SO
2
etc. are contained therein, purification can be effected beforehand, such as washing with water or lye or sorption by means of, for example, Al
2
O
3
.
The purification of the exhaust air, for example from plants in which SF
6
is produced or used is effected on environmental grounds.
The purification of air containing SF
6
may be desirable because SF
6
may have a disruptive effect on industrial processes. One field of application is therefore the purification of the air which is circulated in industrial plants.
The content of SF
6
in such air is frequently very low (in the range of parts per million). For this reason, it is usually possible to dispense with the provision of membrane separation beforehand. Furthermore, in view of the small quantities of SF
6
, although it is possible to regenerate the laden sorbents, they may also be disposed of in the laden state.
To improve the purifying action, it is of course also possible to arrange a plurality of adsorption stages (2, 3 or more) in series.
This embodiment according to the invention is distinguished by a high purifying action for air contaminated with SF
6
. The purified exhaust air or air can be let off harmlessly into the environment or be passed harmlessly into plants in which SF
6
would have been a disturbing contaminant and serve as ambient air therein.
The process may for example also be applied to air or exhaust air to which SF
6
is added, e.g. as a tracer substance. Even air or exhaust air in plants manufacturing or using SF
6
can be purified if this air or exhaust air is contaminated with SF
6
.
One preferred embodiment relates to the working-up of SF
6
/N
2
mixtures, with reference to which the invention will be explained further.
If relatively large quantities are to be separated, or if the SF
6
is relatively large, the SF
6
/N
2
mixture may first be subjected to low-temperature treatment. Preferably the mixture is cooled to a temperature in the range from −70°C. to −110° C., in particular to a temperature in the range from −70°C. to −100° C. SF
6
which only has a low content of N
2
then condenses out. Furthermore, a gas phase remains which predominantly consists of nitrogen with low contents of SF
6
. The gas phase obtained in this manner is then separated by absorption as described above, so that nitrogen substantially free of SF
6
is obtained and can be let off into the environment. The SF
6
can be recycled.
An alternative, preferred embodiment for relatively large quantities of gas mixture, or those gas mixtures which have a higher content of SF
6
, will be described below. This embodiment provides for the combination of membrane separation processes and adsorption. It is highly suitable for mixtures of SF
6
and N
2
, for example from underground cables, which have an SF
6
content of 5 to 30% by volume.
This embodiment of the process according to the invention provides for an SF
6
/N
2
mixture to be separated in at least one membrane separation stage into a retentate with an increased content of SF
6
and a permeate with a reduced content of SF
6
, and for the permeate to be passed into at least one adsorption stage with the hydrophobic zeolites described above for further separation. It is preferred to provide two or more membrane separation stages and two or more adsorption stages.
The pressure on the entry side of the membrane or membranes is usually greater than the ambient pressure. For example, the gas mixture to be separated may be supplied at a pressure of up to 20 bar. If a plurality of membranes are provided, a compressor is arranged in front of each membrane. The permeate will then usually have a pressure which corresponds approximately to the ambient pressure upon entry into the adsorber stage. If desired, the permeate may

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