Title of Invention

"A METHOD FOR THE PRODUCTION OF SUBSTANTIALLY HOMOGENEOUS COMPRESSED GAS MIXTURES"

Abstract Homogeneous compressed gas mixtures can be produced from gases which are separately present, in particular gases having a great difference in density, if a mixing station is used which comprises at least the following components: a gas pre-mixing stage, in which the gases which are separately present are brought together; connected thereto, a static mixer and/or a buffer tank, a compressor which is connected to the buffer tank or the stationary mixer; if a buffer tank is present, a return line from the compressor outlet to the buffer tank. Mixtures of SF6 and N2, which are for example suitable as insulating gases for current-conducting underground cables, may, for example, be produced. High flow rates can be processed using the method. A mobile mixing station for use in this method is also disclosed.
Full Text Description
The invention relates to a method for the production of substantially homogeneous, compressed gas mixtures and a mixing station, in particular a mobile one, usable therein.
In principle, it is possible to convert gases which are separately present into a homogeneous gas mixture simply, by transferring the gases into a container and waiting for a sufficiently long time until a correspondingly homogeneous gas mixture has been produced by diffusion. Since however extremely long periods of time are required for this, such a method cannot be used industrially. Of course, mixing is also observed if gas streams are introduced into a stationary mixer and/or a common line. However, the mixing is not always so thorough that the mixtures obtained can be regarded as "homogeneous", in particular when it is desired to mix together gases having a high difference in density. Such a gas mixture is the mixture of SF6 (sulphur hexafluoride) and N2 (nitrogen). SF6 as a gas has a density of 6.18 g/1, and nitrogen as a gas has a density of 1,170 g/1, each determined at 15'C and 1 bar absolute. Such gas mixtures are used, for example, as insulating gas for current-conducting underground cables. A particular problem in this case is that the gas mixtures (which are required in very large quantities) expediently need to be produced on the spot. For if it were desired to
use gas mixtures prefabricated in a factory, these would have to be transported in gas cylinders under high pressure, in order to keep the transportation costs as low as possible; however, this is not possible, since then the content of SF6 condenses out and corresponding demixing would occur.
It is an object of the present invention to devise a method with which homogeneous compressed gas mixtures can be produced. Furthermore, it is an object of the present invention to devise a mixing station which can be used for this purpose, in particular a mobile mixing station which can be used for this purpose. Another object was to devise a mixing station which is protected from dirt and the effects of the weather. These objects are achieved by the method according to the invention or the mixing station according to the invention.
The method according to the invention for the production of substantially homogeneous compressed gas mixtures from gases which are separately present provides for the gases which are separately present to be premixed, forming a non-homogeneous gas mixture, the non-homogeneous gas mixture to be passed into a static mixer and/or buffer tank, gas mixture to be passed from the buffer tank or the static mixer into a compressor and substantially homogeneous compressed gas mixture to be delivered from the compressor, wherein, if a buffer tank is provided, a portion of the substantially homogeneous compressed gas mixture delivered from the compressor is returned into the buffer tank via a return line.
The method according to the invention makes it possible to produce homogeneously mixed gas mixtures at the place of use. It is therefore no longer necessary to
supply gas mixtures homogeneously mixed ex works. Another advantage is that high flow rates (for example above 200 standard m5 [sm3] per hour!) can be processed; in this case, the degree of mixing is independent of the cross-sections of the lines used. Metered delivery of the final homogeneous gas mixture is possible.
If a static mixer and a buffer tank are provided, it is advantageous to pass the gas first through the static mixer and then through the buffer tank.
According to a preferred embodiment, the method is performed using a buffer tank, and a control valve is installed in the return line. The return of a portion of the gas mixture is adjusted to the desired value with this control valve. This embodiment has the advantage that the compressor can be operated under gas ballast, and in addition the thorough mixing is improved still further. The control valve may for example be adjusted such that a predetermined proportion of the volume of the compressed gas delivered from the compressor is returned.
Expediently, a safety means is provided which registers the fact that the filling limit has been reached in the electric cable which is to be filled or the gas cylinder which is to be filled, and switches off the compressor. This may, for example, be an pressure-relief valve, which from a predetermined pressure onwards opens and expediently switches off the compressor. The pressure-relief line may be connected to the buffer tank. In this manner, the gas blown off remains in circulation.
Preferably the method according to the invention can be used for producing mixtures of gases which have a high difference in density, for example a difference in
density of at least 1 g/1 (determined at 15*C and 1 bar absolute). The method is particularly well suited for producing homogeneous gas mixtures which contain SF6, in particular contain SF6 and N2, or consist thereof. Such gas mixtures are used, for example, as insulating gas for current-conducting underground cables.
The compressor is adjusted such that it supplies a gas mixture having the desired pressure. Gas mixtures with SF6 and N2, which are to be used for the afore-mentioned application as insulating gas in underground cables, are advantageously delivered at a pressure of 1 to 13 bar absolute. In particular, the pressure is in the range from 7 to 13 bar absolute.
Compressors which operate without oil, in particular diaphragm compressors, but also piston compressors, are advantageously used.
The method according to the invention is particularly suitable for producing substantially homogeneous compressed gas mixtures which contain 3 to 50%, preferably 3 to 20% by volume, SF6, remainder to make up to 100% by volume N2. Preferably, the desired content of SF6 in a sample which is taken deviates by at most ±0.7% by volume from the value which is obtained with ideal thorough mixing. If applicable, the content of the already-mixed gas returned via the return line into the buffer tank is increased.
One preferred embodiment of the method according to the invention provides for a gas mixture containing or consisting of SF6 and N2 to be produced and introduced as an insulating gas into current-conducting underground cables.
The method can advantageously be performed using a mobile mixing station. A very suitable mobile mixing station is described further below. The mobile embodiment makes it possible, for example when laying underground cables, always to produce the desired gas mixture, corresponding to the progress of building, directly where it is required.
A further subject of the invention is a mixing station which can be used to perform the method according to the invention. This mixing station comprises the following components: at least two feed lines for feeding the gases to be mixed; a gas line for jointly passing on the premixed gases; a stationary mixer and/or a buffer tank into which the gas line for jointly passing on the premixed gases opens; a gas line which is connected to the buffer tank or stationary mixer and a compressor, through which gas mixture is passed from the buffer tank or the stationary mixer into the compressor; a compressor in which the gas mixture carried off from the buffer tank or stationary mixer is compressed and homogenised; a removal line for carrying the homogeneous compressed gas mixture out of the compressor; if a buffer tank is present, a return line, which is connected to the removal line from the compressor and the buffer tank; a control valve in the return line. One preferred embodiment of the mixing station has a buffer tank and a return line with control valve. Figure 1 shows such a mixing station. It comprises:
2 feed lines (1, 2); 2 valves (3, 4) for adjusting the gas flow rate; gas line (5) for passing on the premixed gases; buffer tank (6); compressor (7); gas line (8) between the buffer tank (6) and compressor (7); removal line (9); return line (10) between the buffer tank and the compressor; control valve (11) in the return line;
valve (12) for adjusting the quantity of the homogeneous gas mixture removed.
The mixing station may comprise further useful components such as one or more manometers, pressure reducers, flow meters, pressure-relief valves, automatic shut-off means for the compressor, removal points for taking samples or a removal point for the homogeneous gas mixture. The mixing station may be mobile. It then comprises the mixing station described above and an undercarriage, on which the mixing station is mounted. For example, the undercarriage may be a lorry or lorry trailer. This has the advantage that the mixing station can be moved on according to the laying of the underground cable to be insulated.
The method according to the invention will be explained in greater detail with reference to Fig. 2/2. Sulphur hexafluoride and nitrogen are introduced into a gas mixer G from the sulphur hexafluoride tank ST or the nitrogen tank NT via evaporator V, manometer M and pressure reducer D. The pressure between the manometer and the pressure reducer is 9 to 15 bar. In the gas mixer, the two gases are introduced into a common line 5 via mass flow meters and butterfly valves. The differential pressure between M and the static mixer F is at least 3 bar. The premixed gas is introduced into the buffer tank 6 via the static mixer F and is introduced into the compressor 7 from the buffer tank via line 8. A portion of the gas removed from the compressor via line 9 is returned into the buffer tank via line 10 and the control valve 11. The pressure in line 9 is up to 13 bar (i.e. 14 bar absolute). Gas samples can be taken for analysis via the sampling points 13, 13' and 13". The flow rate in line 9 is from 5 to 250 sm3/h. Homogeneous gas mixture is fed into a gas cylinder, not shown here, via line 9. The
control valve 11 is set such that the desired degree of thorough mixing is achieved - the more volume percent flows back, the more ideal the thorough mixing, but of course also the lower the quantity of compressed gas mixture delivered. The compressed gas is delivered via the shut-off valve 14 to the article to be filled (e.g. an electric cable or a gas cylinder).
The flow rates from the sulphur hexafluoride tank or the nitrogen tank were adjusted such that the volume ratio of SF6:N2 was exactly 5:95. A gas sample was taken via the sampling point 13, and according to analysis had a content of 6.7% by volume SF6 and 93.3% by volume N2. This indicates that thorough mixing is still incomplete. Samples which were taken directly after the gas buffer tank and from the gas cylinder each had a content of 5% by volume SF6 and 95% by volume N2, and prove optimum thorough mixing.
The test was repeated, with a volume ratio of SF6:N2 being set to 15:85. A sample taken directly after the gas mixer had a content of 16.7% by volume SF6 and 83.3% by volume N2. A sample taken after the buffer tank contained 15.7% by volume, and a sample taken from the gas cylinder 15.8% by volume, SF6. The deviation from the ideal value of 15% by volume was ascribed to the fact that the gas mixer had been operated in the limit region and therefore the volume ratio of SF6:N2, despite the nominal setting to 15:85, was effectively approximately 15.7:84.3, and the thorough mixing was therefore ideal.




We Claim
1. "A method for the production substantially
homogeneous compressed gas mixtures from gases which
are separately present, in which the gases which are
separately present are premixed, forming a non-
homogeneous gas mixture, the non-homogeneous gas
mixture is passed into a static mixer and/or buffer
tank, gas mixture is passed from the buffer tank or the
static mixer into a compressor and substantially
homogeneous compressed gas mixture is withdrawn from
the compressor, wherein, if a buffer tank is provided,
a portion of the substantially homogeneous compressed gas mixture withdrawn from the compressor is returned into the buffer tank via a return line.
2. A method according to Claim 1, characterised in
that mixtures of gases are produced which have a
difference in density of at least 1 g/1, preferably gas
mixtures which contain or consist of SF6 and N2.
3. A method according to Claim 1 or 2, characterised
in that a gas mixture having a pressure of up to 13 bar
is delivered from the compressor.
4. A method according to Claim 1, characterised in
that a compressor which operates without oil is used.
5. A method according to Claim 4, characterised in
that a piston or diaphragm compressor is used.
6. A method according to Claim 1, characterised in
that a buffer tank is used and a control valve is
installed in the return line, which valve adjusts the
return of gas mixture to a desired volume percentage of the compressed gas delivered.
7. A method according to Claim 1 or 2, characterised
in that substantially homogeneous compressed gas
mixtures are produced which contain 3 to 50% by volume
SF6 preferably 3 to 20% by volume SF6 remainder to
make up to 100% by volume N2, the desired content of SF6
in a sample which is taken deviating by at most ±0.7%
by volume from the value which is obtained with ideal
thorough mixing.
8. A method according to one of the preceding claims,
characterised in that a mixture containing or
consisting of SF6 and N2 is produced and is introduced
as an insulating gas into current-conducting
underground cables.
9. A method according to Claim 1, characterised in
that it is performed using a mobile mixing station.
10. A mixing station 'for use in a method according to
one of Claims 1 to 9, which has: at least two feed
lines for feeding the gases to be mixed; a gas line for
jointly passing on the premixed gases; a buffer tank or
a stationary mixer into which the gas line for jointly
passing on the premixed gases opens; a gas line which
is connected to the buffer tank or stationary mixer and
a compressor, through which gas mixture is passed from
the buffer tank or the stationary mixer into the
compressor; a compressor in which the gas mixture
carried off from the buffer tank or stationary mixture
is compressed and homogenised; a removal line for
carrying the homogeneous compressed gas mixture out of
the compressor; if a buffer tank is present, a return
line, which is connected to the removal line from the
compressor and the buffer tank; a control valve in the return line.
11. A mixing station according to Claim 10,
characterised in that it further comprises: at least
one holder for holding gas cylinders for one or more of
the unmixed gases; connections for connecting a gas
cylinder for feeding in the homogeneous compressed gas
mixture; at least one holder for such a gas cylinder.
12. A mixing station according to Claim 10 or 11,
characterised in that it comprises at least one further
component from the group comprising manometer, pressure
reducer, flow meter, removal point or removal points
for taking samples.
t
13. A mobile mixing station, comprising
a) the mixing station according to one of Claims 9 to
11;
b) an undercarriage, the mixing station being mounted
on the undercarriage.
14. A mobile mixing station according to Claim 13,
characterised in that the undercarriage is a lorry.
15. A mixing station according to one of Claims 10 to
14, characterised by means which shelter the mixing
station from external influences such as dirt and
weather.
16. A method for the production Substantially homogeneous compressed gas
mixtures substantially as herein described with reference to the foregoing
examples.
17. A mixing station substantially as herein described with reference to the
foregoing examples.
18. A mobile mixing station substantially as herein described with reference
to the foregoing examples.

Documents:

3380-del-1997-abstract.pdf

3380-del-1997-claims.pdf

3380-del-1997-correspondence-others.pdf

3380-del-1997-correspondence-po.pdf

3380-del-1997-description (complete).pdf

3380-del-1997-drawings.pdf

3380-del-1997-form-1.pdf

3380-del-1997-form-13.pdf

3380-del-1997-form-19.pdf

3380-del-1997-form-2.pdf

3380-del-1997-form-3.pdf

3380-del-1997-form-4.pdf

3380-del-1997-form-6.pdf

3380-del-1997-gpa.pdf

3380-del-1997-pct-210.pdf

3380-del-1997-petition-137.pdf

3380-del-1997-petition-138.pdf


Patent Number 213008
Indian Patent Application Number 3380/DEL/1997
PG Journal Number 01/2008
Publication Date 04-Jan-2008
Grant Date 19-Dec-2007
Date of Filing 24-Nov-1997
Name of Patentee SOLVAY FLUOR GMBH
Applicant Address HANS-BOCKLER-ALLEE 30, 30173 HANNOVER, GERMANY
Inventors:
# Inventor's Name Inventor's Address
1 MICHAEL PITTROFF BRONSARTSTR. 16, D-30161 HANNOVER, GERMANY.
2 HANS-PETER WICKEL MAGNUSSTR. 9/1, D-74206 BAD WIMPFEN, GERMANY.
3 REINER DISTEL UHLAND STR. 7, D-74206 BAD WIMPFEN, GERMANY.
4 HEINZ-JOACHIM BELT HAFERKAMP 19, D-30938 BURGWEDEL, GERMANY.
PCT International Classification Number B01F 3/02
PCT International Application Number N/A
PCT International Filing date
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 196 49 254.8 1996-11-28 Germany