GB2078541A - Method and apparatus for degassing a liquid - Google Patents

Abstract

In the invention a liquid feedstock containing gas is degassed by passing the feedstock down a surface (9) such as to cause the feedstock to spread sufficiently to reduce the thickness of the layer of feedstock on the surface until gas contained in the layer exits therefrom. The surface is preferably provided by a member (1) which is generally cone shaped but which, in cross-section, shows sides, constituting said surface, having a concavity such as to change the direction of liquid flow from vertical to horizontal or near horizontal. <IMAGE>

Description

SPECIFICATION Method and apparatus for degassing a liquid The present invention relates to apparatus and method for degassing a liquid.

It is an object of the present invention to provide an efficacious apparatus and method for separating out gas dispersed throughout a liquid.

The present invention provides a method of degassing a liquid feedstock containing a gas, which comprises passing feedstock down a surface such as to cause the feedstock to spread sufficiently to reduce the thickness of the layer of feedstock on the surface until gas contained in the layer exits therefrom.

In one embodiment of the present invention, the surface, which may be called the drop surface, is provided by a member which is generally cone shaped but which, in cross-section, shows sides having a concavity; and preferably the concavity is such as to change the direction of liquid flow from vertical to horizontal or near horizontal, e.g. by an angle of some 800.

In an embodiment of apparatus according to the invention, the drop surface member is used together with a liquid container providing a bottom opening in which the member is positioned to form a valve controlling the rate of outflow from the container; the position of the member axially in the bottom opening being varied to vary the rate of outflow. The apparatus may also provide a vessel to collect the degassed feedstock; and preferably the vessel is such that the run-off from the drop surface member impinges on side walls of the vessel in orderto prevent or lessen re-gassification of the effluent feedstock.

An embodiment of the present invention will now be described with reference to the accompanying drawing, in which the sole Figure is an elevational cross-section of apparatus of the embodiment.

Referring now to the sole Figure, the apparatus comprises a drop surface member 1 generally of a cone-shape providing adjacent the apex 3, an elongate cylindrical portion or neck 5 followed in the direction towards the base 7 by a concavity 9 passing from the neck to a point delimiting the maximum radius of the member adjacent the base thereof; the section between the concavity and the base being of cylindrical form.

The drop surface member receives gasified feedstock from a feed container 21 in the form of a cylindrical tube 23 having a bottom opening 25 in which the drop surface member is positioned; the member being stationed in any selected position by vanes 27 acting against the inner surface of the tube and supported on a rod 28 mounted in and extending from the apex of the member; the vanes permitting the axial i.e. vertical position, of the member in relation to the tube opening to be varied.

The tube 23 is mounted with its bottom end within a tank 31. The tank serves to collect degassed feedstock and it provides a gas outlet 33 in a top cover 35 thereof.

In one working model of the embodiment as above described. The feed tube 23 was of circular cross-section having a diameter of 51 mm, and the drop surface member had a base diameter of 140 mm and a height of 110 mm. The collection tank was a 380 mm diameter cylinder of some 750 mm high.

The gas outlet had a diameter of 22 mm; and the base of the tank provided a feedstock outlet (not shown) in the form of a 76 mm diameter tube.

In operation of the embodiment, gasified feedstock was passed down the feed tube to maintain a standing head therein and was thereby symmetrically deposited around the neck of the drop surface member to impinge on the concave surface thereof; the feedstock then flowing down that surface to run off at the lower edge thereof. The concave surface deflects the feedstock through an angle of 800 from the vertical axis. The diameter of the collection tank is so chosen that, in normal operation of the apparatus, the run-off from the drop surface member impinges on the inner surface of the side wall of the collection tank. As the feedstock runs down the concave surface of the drop surface member, it loses its contained gas; the gas appearing as bubbles which exit from the surface of the flowing feedstock.The area of the concave surface is so chosen that, with the maximum flow rate possible, before the feedstock reaches the lower edge of that surface, there is no further evolution of bubbles evident.

The expelled gas then passes out of the collection tank through outlet 33.

The flow rate over the drop surface member is determined, other things being equal, by the axial position of the drop surface member in the outlet aperture of the feed cylinder.

The effect of depth of liquid in the collection tank was found not to influence the separation process provided discharge from the drop surface member was above the water surface in the tank. In the case of separating air from water, consistent results were obtained with the water level about 100 mm below the base of the conical plug.

In separating air from water, the gap between the end of the pipe and the conical plug measured axially, varied between 1 mm for low flow and high head, and 7 mm at the maximum flow with low head. At the latter point, the slope of the side of the cone is about 450 The following hypothesis is presented as an explanation of the mechanism of the separation process.

When the water and air mixture is discharged from the lower end of the tube it spreads out over the lateral surface of the drop surface member. Neglecting friction and gravitational accelerations the velocity will remain constant, and hence the thickness of the film or layer of feedstock normal to the surface of the cone will decrease with radius.

In addition the feedstock flowing down the drop surface will be subjected to centripetal accelerations in a vertical plane. Hence the more dense water will be forced towards the drop surface member, the air bubbles, being of negligible mass, being pushed to the upper surface of the water film. An air bubble at the water surface is separated from the surrounding air by a thin water film held by the surface tension.

This surface tension force, compared with the centripetal acceleration force, is negligible, so the water film is ruptured, so connecting the air in the bubble with the free air above the water film. Hence at the outside edge of the skirt, water without any bubbles is discharged.

Thewaterfilm was observed understroboscopic light, and air bubbles could not be seen beyond a certain radius depending on the flow rate and the head at the drop surface member. Calculations showed that the thickness of the water film at the point beyond which no bubbles existed was, in most instances, greater than the diameter of the bubbles.

This confirmed the supposition that the water had in fact been forced under the air bubbles.

The same drop surface member was also used at the base of a feed tube of 101 mm diameter, satisfactorily separating air at a rate of 1.2 I/s from a water flow 6 I/s. The head loss, measured as the difference in water levels in the feed tube and collection tank, is far less than 1 m at maximum flow rate.

Efficiencies of up to 95% were achieved in separating air from water.

In view of the relationship between feed tube diameter, collection tank diameter, drop surface member diameter and the profile thereof it would appear that the arrangement could be made more compact.

It is also envisaged that the described embodiment will be useful with fluids of higher viscosity, e.g. to separate gas from oil.

Any convenient means may be used to vary the vertical position of the drop surface member. For instance, a lever system operable from the exterior of the tank 31 and acting on the bottom of the drop surface member, could be used. Or a suspension rod or cable could be used. In the latter case, the vanes 27 would then not be needed to hold the drop surface member in position; but, at least in using a cable, if the vanes were dispensed with, some device would preferably be needed to centre the drop surface member.

Claims (22)

1. A method of degassing a liquid feedstock containing a gas, which comprises passing the feedstock down a surface (hereinafter called the drop surface) such as to cause the feedstock to spread sufficiently to reduce the thickness of the layer of feedstock on the surface until gas contained in the layer exits therefrom.

2. A method according to claim 1, wherein the drop surface is such as to change the direction of liquid flow from vertical to horizontal or near horizontal.

3. A method according to claim 2, wherein the drop surface changes the direction of flow by an angle of some 80 .

4. A method according to claim 2 or 3, wherein he drop surface is provided by a member which is generally cone shaped but which, in cross-section, hows sides, constituting said surface, having a conavid in order to change the direction of flow of the eedstock.

5. A method according to any of the preceding claims, wherein feedstock is deposited against an upper portion of the drop surface from a container providing a feedstock outlet spaced from said upper portion of the drop surface; the container and said portion of the drop surface being such as to form a valve determining the rate of outflow of feedstock From the container by the spacing between said container outlet and said upper portion of the drop sur Face.

6. A method according to claims 4 and 5, wherein said container is in the form of a tube the bottom end Df which constitutes said feedstock outlet, the tube receiving in the bottom end thereof the upper por tion of said cone-shaped member so that outflow of Feedstock from the container is determined by the spacing between the bottom end of the tube and said upper portion of the cone-shaped member.

7. A method according to claim 5 or 6, wherein the spacing between the container feedstock outlet and said upper portion of the drop surface is varied to adjust the outflow of feedstock from the container.

8. A method according to any of the preceding claims, wherein degassed feedstock is permitted to run into a vessel providing an upstanding wall section; degassed feedstock being run into the vessel so that it impinges on the upstanding wall section before falling into the vessel.

9. A method according to claim 8, wherein the vessel is covered with the cover providing means to permit gas from the liquid feedstock to pass from the vessel.

10. A method of degassing a liquid, substantially as hereinbefore described with reference to the accompanying drawings.

11. Apparatus for degassing a liquid feedstock comprising a surface, hereinafter called a drop surface, such that the feedstock can be deposited on the surface to run theredown and cause the feedstock to spread sufficiently to reduce the thickness of the flowing feedstock layer on the surface until gas contained in the layer exits therefrom, means for feeding feedstock onto the surface and means for collecting the de-gassed feedstock running from said surface.

12. Apparatus according to claim 11, wherein the surface is such as to change the direction of feedstock flow from substantially vertical to horizontal or near horizontal.

13. Apparatus according to claim 12, wherein the surface changes the direction of flow bdan angle of 80'.

14. Apparatus according to claim 11, 12 or 13, wherein said feed means comprises a cbntainerto deposit feedstock against an upper portion of the drop surface and providing a feedstock outlet spaced from said upper portion; the container and the drop surface being such as to form a valve determining the rate of outflow of feedstock from the container by the spacing between said container outlet and said upper portion of the drop surface.

15. Apparatus as claimed in any of the preceding claims 12 to 14, wherein the drop surface is provided by a member, hereinafter called the drop surface member, which is generally cone shaped but which, in cross-section, shows sides, constituting said surface, having a concavity.

16. Apparatus as claimed in claims 14 and 15, wherein said container is in the form of a tube the bottom end of which constitutes said feedstock outlet; the tube receiving in the bottom end thereof the upper portion of the drop surface member so that the outflow of feedstock from the container is determined by the spacing between the bottom end of the tube and said upper portion of the drop surface member.

17. Apparatus according to claim 14 or 16, wherein the container and the drop surface or drop surface member as the case may be, are such as to permit adjustment of said spacing between the con tainerfeedstock outlet and said upper portion of the drop surface.

18. Apparatus according to claims 16 or 17, wherein the drop surface member on said upper portion thereof carries elements to bear against the internal surface of the tube to hold the drop surface member in place.

19. Apparatus according to claims 17 and 18, wherein said elements are capable of sliding on the internal surface of the tube to permit adjustment of the spacing between the upper portion of the drop surface member and the bottom end of the tube.

20. Apparatus according to any of the preceding claims 11 to 19, wherein said means for collecting degassed feedstock comprises a vessel providing an upstanding wall section to permit degassed feedstock running from the drop surface to impinge thereon before falling into the vessel.

21. Apparatus according to claim 20, wherein the vessel comprises a cover; the cover providing means to permit gas from the liquid feedstock to escape from the vessel.

22. Apparatus for degassing a liquid, substantially as hereinbefore described with reference to the accompanying drawings.