GB2255140A - Method of hermetic sealing - Google Patents

Abstract

A method for the hermetic sealing of chemical reaction vessels comprises providing from the reaction vessel a vertical conduit 2 with an outlet to the ambient atmosphere to enable product to be removed without touching the conduit wall and the conduit is hermetically sealed by a column of liquid eg. mercury 9 within the conduit, the hermetic seal being maintained against gravitational effects on the liquid by the application of gaseous pressure at the liquid/atmospheric interface eg. pressurized hydrogen gas via tube 5. The method is of use in the production of silicon carbide fibres. A reservoir of mercury is used to fill and empty the column. Also, by raising and lowering the reservoir it is possible to change the mercury level during operations in response to pressure fluctuations in the reaction vessel. The conduit 2 may have PTFE baffles 3 which stop the fibre touching the sides of the conduit to which it is attracted by filament attraction or surface tension. <IMAGE>

Description

METHOD OF HERMETIC SEALING The present invention relates to a method of hermetic sealing chemical reaction vessels specifically, but not exclusively, for reaction vessels used in the production of silicon carbide fibres.

Hermetic gas seals are used to enable the maintainence of environmental conditions within a vessel with respect to the ambient atmosphere. This may be of importance for various reasons, such as, the necessity to contain dangerous substances or, to prevent gaseous contamination of a reaction vessels environment.

Hermetic sealing, with complete air exclusion is used in chemical vapour deposition reactions (CVD) at pressures below atmospheric. An example of this is the production of silicon carbide fibres by (CVD). The reaction chamber in this process typically contains the gases CH3SiC13 and H2 at reduced pressure. It is conventional to use mercury for forming hermetic seals in this type of process with the product being removed from the production apparatus as it is made. Because reaction vessels like this will during the course of reaction undergo significant pressure changes due to factors including thermal expansion and contraction or volume changes resulting from product and by product formation there is difficulty in maintaining a mercury hermetic seal.This and other reasons have necessitated the use of complicated computer controlled pressure systems to ensure that the mercury seal is maintained. Even with this type of control only extremely small apertures have been able to be used otherwise mercury drop outs and leaks tend to occur. Accordingly, there is provided a method for the hermetic sealing of a reaction vessel used for the production of fibres which comprises providing from the reaction vessel a vertical conduit with an outlet to the ambient atmosphere to enable product to be removed without touching the conduit wall, the conduit being hermetically sealed by a column of liquid within the conduit, the hermetic seal being maintained against gravitational effects on the liquid by the application of gaseous pressure at the liquid/atmospheric interface.

This simple method relies on underlying physical principles and requires little in the way of extraneous control unlike the prior known techniques.

A hermetic seal produced in this way is of special benefit in the production of fibres such as silicon carbide where it is important that frictional effects on the fibre are as small as possible to minimise damage or breakage to the fibres. The vertical orientation of the conduit helps in keeping the fibre away from the conduit wall and minimising friction to the fibre.

The introduction of annular baffles within the conduit also helps in preventing filament attraction to the conduit wall. This attraction may also occur through the effects of surface tension. The baffles are preferably made from polytetrafluoroethylene (P.T.F.E.) in order to minimise the frictional effects of fibre contact with the baffles. Mercury is the preferred liquid for the hermetic seal in the production of silicon carbide fibres. Its high density means that inertia is high and pressure changes in the reaction vessel have a limited effect on mercury movement. Furthermore mercury does not interfere in the reaction for formation of silicon carbide.

Preferably, the lower end of the conduit is of narrow bore and ideally of capillary tube diameter to help prevent mercury drop-out from the system.

Ideally, the narrow bore tube should be carefully aligned with the main conduit and reaction vessel above so that fibres can pass freely through the system.

To ensure that mercury does not fall out of the narrow bore tube a gas stream is applied at the mercury/ambient atmosphere interface. The pressure applied by the gas stream is sufficient to prevent the mercury falling out of the tube. In practice this translates to a fairly low flow rate.

Preferably, the gas used is an inert gas however in the case of silicon carbide production the gas is preferably the same as one in the reaction chamber, such as, hydrogen.

The gas stream is conveniently introduced part way along the narrow bore tube via a side arm tube into the vertical narrow bore tube. The bore of this side arm tube is not critical but an internal diameter of capillary dimensions will be appropriate.

In a preferred arrangement of the present invention there is also included a reservoir of mercury to fill and empty the column etc. Where silicon carbide fibre is being made this eases the threading of the fibre through the apparatus. Conveniently, the mercury reservoir is connected via a flexible plastics tube into the conduit. By raising and lowering the reservoir it is possible to change the mercury level in the conduit.

This may also be used to change the mercury level during operation in response to pressure fluctuations which may occur as a result of reaction. Preferably, a tap is present along the plastics tube to provide added control to the system.

It will be appreciated that as certain reactions occur in environments below atmospheric pressure that the seal of the present invention will in this case to some extent be supported by atmosheric pressure acting upon it. It is therefore possible that a seal, such as mercury, can be supported solely by atmospheric pressure, providing the pressure in the reaction vessel is sufficiently low. In this respect the length of a column of a liquid forming a seal will determine the pressure differential required for maintainance of the seal under the influence of atmospheric pressure. It is possible for column length to be less than the barometric height with the limit of height reduction being limited by the onset of bubbling of atmospheric air upwards through the column.

The invention will now be described by way of example only and with reference to the accompanying drawing which shows a hermetic mercury seal apparatus for use in the production of silicon carbide fibres according to the present invention.

A mercury sealed apparatus (1) for the production of silicon carbide fibres comprises a glass tube (2) about 90cm long and lem in diameter and held in vertical orientation is connected to a reaction vessel (not shown) which is positioned axially above the top of the tube (2). The joint between the tube (2) and the reaction vessel is airtight. Within tube (2) there are a plurality of P.T.F.E. annular baffles (3) which fit against and are held in place by the tube (2) wall.

At the base of the tube a glass capillary tube (4) continues vertically downwards from the tube base. The joint between the tube (2) and capillary tube (4) is an airtight one. Branching from, and hermetically sealed to, the capillary tube is a side arm tube (5). The side arm tube (5) projects in a perpendicular direction from the capillary tube and is connected to a pressurised supply hydrogen gas (not shown).

A side arm tube (6) formed of resilient plastics material projects laterally from the base of the tube (2). The tube (6) is connected to the base of an open topped container (7) which is in parallel alignment with tube (2). The container (7) is however capable of vertical movement when not clamped in position. A (8) tap is positioned along tube (6) close to tube (2).

In operation tap (8) is initially opened and the system is filled with mercury (9) via container (7). The mercury travels along tube (6) and into tube (2). Hydrogen gas is introduced at a low flow rate through side arm tube (5) which is just enough to prevent mercury from entering capillary tube (4). This prevents mercury drop-out through capillary tube (4).

Silicon carbide is produced as a continuous fibre (10) in the reaction vessel. As the fibre is produced it passes down the centre of tube (2) and then through capillary tube (4) to exit the system (1). The fibre is prevented from touching the sides of tube (2) by filament attraction or surface tension by the baffles (3). The mercury present in tube (2) provides a hermetic seal which ensures that the reaction vessel is sealed against ambient influences of pressure and atmospheric gases.

Because the reaction takes place under reduced pressure the level of mercury in tube (2) will necessarily be higher than the level in container (7) when tap (8) is open. If the level of mercury in tube (2) changes such that there is a danger of the mercury hermetic seal being broken then tap (8) can be opened to allow more mercury into tube (2).

If there is still difficulty in achieving the desired level of mercury in tube (2) further change can be effected by either raising or lowering the height of container (7) to respectively increase or decrease the potential energy of the mercury and cause a consequent change in the level of mercury in tube (2). In some cases there is no need to replenish the mercury in tube (2) during operation and tap (8) may be closed once the tube (2) if filled to the desired level. However, it may be advantageous to leave tap (8) open all the time.

Claims (15)

1. A method for the hermetic sealing of a reaction vessel used for the production of fibres which comprises providing from the reaction vessel a vertical conduit with an outlet to the ambient atmosphere to enable product to be removed without touching the conduit wall, the conduit being hermetically sealed by a column of liquid within the conduit, the hermetic seal being maintained against gravitational effects on the liquid by the application of gaseous pressure at the liquid/atmospheric interface.

2. A method for the hermetic sealing of a reaction vessel as claimed in claim 1 wherein there are provided annular baffles within the conduit.

3. A method for the hermetic sealing of a reaction vessel as claimed in claim 2 wherein the baffles are made from P.T.F.E.

4. A method for the hermetic sealing of a reaction vessel as claimed in any one of the previous claims wherein mercury is used for the hermetic seal when silicon carbide fibres are being produced.

5. A method for the hermetic sealing of a reaction vessel as claimed in claim 4 wherein the lower end of the conduit is of narrow bore and ideally of capillary tube diameter to help prevent mercury drop-out from the system.

6. A method for the hermetic sealing of a reaction vessel as claimed in claim 5 wherein the narrow bore tube should be carefully aligned with the main conduit and reaction vessel above so that fibres can pass freely through the system.

7. A method for the hermetic sealing of a reaction vessel as claimed in any one of claims 4 to 6 wherein to ensure that mercury does not fall out of the narrow bore tube a gas stream is applied at the mercury/ambient atmosphere interface.

8. A method for the hermetic sealing of a reaction vessel as claimed in claim 7 wherein the gas used is an inert gas.

9. A method for the hermetic sealing of a reaction vessel as claimed in claim 7 wherein when the reaction vessel is used for the production for silicon carbide the gas is the hydrogen or some other gas present within the reaction vessel.

10. A method for the hermetic sealing of a reaction vessel as claimed in one of claims 7 to 9 wherein the gas stream is introduced part way along the narrow bore tube via a side arm tube into the vertical narrow bore tube.

11. A method for the hermetic sealing of a reaction vessel as claimed in any one of claims 4 to 10 wherein a reservoir of mercury is used to fill and empty the column.

12. A method for the hermetic sealing of a reaction vessel as claimed in claim 11 wherein the mercury reservoir is connected via a flexible plastics tube into the conduit.

13. A method for the hermetic sealing of a reaction vessel as claimed in claim 12 wherein the mercury level in the conduit is changed by raising and lowering the reservoir.

14. A method for the hermetic sealing of a reaction vessel as claimed in claim 12 or 13 wherein a tap is present along the plastics tube to provide added control to the system.

15. A method for the hermetic sealing of a reaction vessel specifically as herein described and with reference to the accompanying drawing.