WO1988002665A1

WO1988002665A1 - Cyclone separator device

Info

Publication number: WO1988002665A1
Application number: PCT/AU1987/000353
Authority: WO
Inventors: David Andrew Webb
Original assignee: B.W.N. Vortoil Pty. Ltd.
Priority date: 1986-10-16
Filing date: 1987-10-16
Publication date: 1988-04-21

Abstract

A cyclone (46) for separating at least two fluids having different densities, the cyclone (46) having at least two inlet means (40, 43) through one of which the fluids to be separated enter the cyclone (46) and at least two outlet means (41, 42) from one of which the fluid having the higher percentage of more dense fluid passes from the cyclone (46) and from the other of which the fluid having the higher percentage of less dense fluid passes therefrom characterized in that the or one of the other inlet means (40, 43) is adapted to permit the introduction of a fluid in the cyclone (46) to displace at least some of the existing boundary layer at the point of introduction. The invention also provides a method of separating mixtures of two or more fluids wherein said mixtures are passed through one or more cyclones (46) constructed according to the invention.

Description

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CYCLONE SEPARATOR DEVICE

This invention relates to a cyclone separator device capable of separating to a substantial extent mixtures of two or more fluids, and to a method of separating to a substantial extent mixtures of two or more fluids.

Cyclone separators (hereinafter called "cyclones") as such are well known, although mainly for separating dust from air or other fluids, steam from water, water from steam and different size classes of particles from each other.

Typicallv, cyclones used for fluid-fluid separation have included at least one first inlet means for admission of the fluid mixture to be separated in the cyclone body and at least two outlet means for expulsion of fluid after at least some separation has been achieved.

Some cyclones are known to include at least one further inlet means which may not be used significantly for the admission of the fluid mixture to be separated. Rather, some purposes of which a further inlet means is known to have been used are:-

1. to introduce chemicals to mix and react with a substantial portion of the fluid mixture within the cyclone body;

2. to introduce gas to sparge a less dense liquid in the fluid mixture in the cyclone towards the axis of the cyclone;

3. to introduce dyes or radioactive tracers or the like for experimental observation and investigation of the flow regime within the cyclone body; ^tτ. to permit access of measuring instruments into the cyclone body;

5. to introduct a fluid with substantial momentum for the purpose of imparting at least some further rotational energy to the fluid -1 -

mixture within the cyclone body.

This invention in one aspect relates to a novel second inlet means which may be used for introducing a fluid into the cyclone body for the purpose of displacing at least some of the existing boundary layer very near the wall of the cyclone, the existing boundary layer being the boundary layer upstream of this second inlet means within the cyclone body, i.e. upstream in reference to the general flow direction of the boundary layer closest the wall within the cyclone body (Note: by definition the flow velocity at the cyclone wall of the boundary is taken as zero).

Although it is often difficult to define boundary layer thickness, here the boundary layer is to be taken as that portion of fluid adjacent the cyclone wall where viscous shear forces induced by the wall are significantly greater than at some smaller radius from the longitudinal cyclone axis at the location within the cyclone along its length where one may wish to define the boundary layer thickness. In essence the purpose of the invention is to displace that portion of fluid which is substantially sourced from the first inlet means (through which the fluid mixture to be separated is admitted into the cyclone body) and which is subject to these significantly greater viscous shear forces close to the wall, with or by the fluid admitted through the second inlet means resulting in said displaced fluid sourced from the first inlet means being displaced to such an extent and in such a manner that it will be in a region of less viscous shear force induced by the cyclone wall.

An object of this invention is to displace fluids from the boundary layer without substantially disrupting the desired flow pattern in the body of the cyclone.

With this object in view, according to one aspect of the invention there is provided a cyclone for separating at least two fluids having different densities, the cyclone having at least two inlet means through one of which the fluids to be separated enter the cyclone and at least two outlet means from one of which the fluid having the higher percentage of more dense fluid passes from the cyclone and from the other of which the fluid having the higher percentage of less dense fluid passes therefrom characterised in that the or one of the other inlet means is adapted to permit the introduction of a fluid into the cyclone to displace at least some of the existing boundary layer at the point of introduction.

D. Bradley in "The Hydrocyclone" (1965) points out that the fluid in a cyclone separator can be described as moving with three velocity components i.e. an axial velocity which is parallel to the cyclone centre line longitudinal axis (hereinafter called "the cyclone axis"); a radial velocity which is normal to the cyclone axis; and a rotational velocity which is rotational about the cyclone axis. A paper presented at the 2nd International Conference on Cyclones at Bath in 1984 also refers to and discusses velocity profiles in a cyclone.

The vector sum of the rotational velocity and axial velocity can have a magnitude that is quite substantial (e.g. several metres per second or more in a cyclone used for fluid-fluid separation) at some regions of the fluid body within the cyclone. This magnitude can be very large near the wall of the cyclone but it decreases as one approaches even closer to the wall because of significant viscous shear forces in the fluid induced by the stationary wall. Figures 1A, IB and IC are sketches of some typical velocity profiles at a location along the length of a cyclone.

In these Figures, 1 represents the cyclone wall radius; 2 represents the point at zero radius from the cyclone axis and also point of zero speed on the speed axis; 3 represents a typical plot illustrating axial speed versus radius from the cyclone axis; 4 represents a typical plot illustrating rotational speed versus radius from the cyclone axis; 5 represents a typical plot illustrating the magnitude of vector sum of rotational and axial velocities; 6 represents the velocity or speed axes of the graphs; and 7 represents the radius axes of the graphs.

Figure 1 C illustrates that a large fluid speed can exist close to the cyclone wall but closer to the wall the speed reduces ^■• to become zero at the wall. "Mechanics of Fluids" by W. 3. Duncan, A. D. Thorn and A. D. Young, (2nd Edition, 1974), and "Fluid Dynamics" by J. W. Daily and D. R. F. Harleman (1973) contain discussions of fluid flow close to a wall and discussions on boundary layer theory.

The large shear stress that exists in the fluid close to the cyclone wall can break particles (or droplets) of one fluid that exist within another fluid into smaller particles. This may occur when viscous shear forces overcome the interfacial tension forces at the surface of the fluid particle effectively shearing the particle into particles of smaller size than the original particle. The shearing of fluid particles into smaller fluid particles is detrimental to cyclone separation efficiency. A smaller particle of liquid has a smaller ratio of particle mass to particle diameter (or surface area) resulting in a smaller centripetal force to drag force ratio for the smaller particle. For instance where the particle has a lower density than the fluid surrounding it, its speed of travel towards the cyclone axis may be less if the particle is smaller in size. This can be detrimental to separation efficiency.

Because of the smaller rotational velocity of fluid in the boundary layer close to the wall, the centripetal force acting on particles in this region is also smaller than where rotational velocity is larger, i.e. closer to the cyclone axis from the wall. This smaller rotational velocity resulting in smaller centripetal force can also be detrimental to separation efficiency.

It is usually desirable that the flow regime in a fluidrfluid cyclone be substantially laminar. A flow regime that is substantially turbulent leads to mixing and it may be difficult for centripetal forces to establish quasi- stable concentration gradients of the different fluids within the cyclone.

In a cyclone where the flow regime is relatively or substantially laminar, a significant proportion of the fluid in the boundary layer at the wall downstream of the fluid in the boundary layer at the wall downstream of and relatively close to the first inlet means may exit the cyclone still contained within the boundary layer, particularly if there is a greater proportion of more dense fluid than less dense fluid entering the cyclone at the first inlet means. The fluid in this boundary layer might not undergo significant separation between the more dense and less dense fluid unless, for example, two-stage cyclone treatment is used wherein the outlet means for the fluid containing the higher proportion of more dense fluid is connected by a pipe/s to a first inlet means of another cyclone. Then it is most probable that a significant proportion of the fluid in the said boundary layer in the first cyclone will not be present in said boundary layer of the second cyclone. It is of importance that the said boundary layer is displaced with a minimum of turbulence, as excessive creation of turbulence may be ^" detrimental to cyclone efficiency. Use of the invention results in at least partial displacement of the said boundary layer from the wall at the second inlet means and introduces fluid into the cyclone at the second inlet means, this introduced fluid becoming a significant proportion of the boundary layer closest the wall.

The second inlet means may be located at the wall of the cyclone at one or more positions along the length of the cyclone between the first inlet means (where a significant proportion of the mixture to undergo separation is admitted into the cyclone body) and that or those outlet means from the cyclone body where the fluid containing the higher proportion of the more dense fluid leaves the cyclone body.

The second inlet means is, or are, from a construction point of view, preferably isolated from or independent of the first inlet means. That is to say, fluid entering the cyclone through the second inlet means does not enter the cyclone body via the first inlet means. Fluid enters the cyclone body through the second inlet means at a length along the cyclone where substantial rotational and axial velocity components exist.

The fluid that enters through the second inlet means may be of a density similar to, or greater than, that of the more dense fluid in the mixture that enters through the first inlet means and is not of a significantly lower density than that of the more dense fluid in the mixture that enters through the first inlet means.

It may be advantageous if the fluid that enters the cyclone through said second inlet means be of low viscosity relative to the fluid in the boundary layer upstream of said second inlet means. If the fluid entering the cyclone through said second inlet means is of lower viscosity, the cyclone wall downstream of second inlet means may impose less drag force on the fluid witin the cyclone resulting in less rotational energy loss of the fluid as it spins about the longitudinal cyclone axis. The resulting reduction in rotational velocity loss may improve cyclone separation efficiency.

The rate of flow of the fluid that enters through the second inlet means need only be sufficient to displace fluid that is significantly sourced from the first inlet means a sufficient distance away from the wall so that a significant proportion of fluid sourced from the first inlet means is displaced into a region where viscous shear stress induced by the cyclone wall is much smaller and rotational velocity greater.

Fluid sourced from the first inlet means may be used beneficially as a fluid for the second inlet means where this fluid contains a significantly lower concentration of less dense fluid than the mixture at the first inlet means and a lower concentration of less dense fluid in the boundary layer close to the wall immediately upstream of the second inlet means.

Such a fluid may be found at that or those cyclone outlet means where the greater proportion of the more dense fluid is discharged from the cyclone after a significant proportion of less dense fluid has been separated from this more dense fluid. Some of this more dense fluid from this outlet means may be used as a fluid at the second inlet means. If this practice is to be used it is preferable that the quantity of more dense fluid to be used for the introduction via the second inlet means not be extracted from the cross-section of the exit closest the centre line axis of this exit. There ^~ S-

may exist a concentration gradient of less dense fluid within the more dense fluid as one goes from the axis of the exit to the boundary layer at the wall of the exit, there being higher concentration of less dense fluid at the axis and this concentration may reduce with an increasing radius from the axis until the boundary layer at the wall is reached.

Figure 2 depicts a typical concentration gradient at such an outlet means.

In Figure 2, 11 represents the graph axis of concentration of less dense fluid in more dense fluid; 12 represents the point on the graph where the radius from the centre line axis of the cyclone exit is zero; 13 represents the graph axis of radius from the cyclone axis; 14 represents the plot of concentration of the less dense fluid in the more dense fluid versus radius from the cyclone axis, at the outlet means from the cyclone where a greater proportion of the more dense fluid is discharged; and 15 represents _. the point of maximal concentration of less dense fluid. This graph shows that a maximal concentration of less dense fluid exists at the cyclone axis at this outlet means.

In Figure 3, 21 represents the longitudinal cyclone axis; 22 represents the region where it may be more preferable to source fluid for the second inlet means; and 23 represents the wall of the cyclone.

The invention will be further described by way of example with reference to the accompanying drawings of preferred embodiments.

Figure 4 represents a cross-section through a cyclone at a point where a second inlet means is appended according to the invention. It also shows a longitudinal section through a second inlet means. • Figure 5 represents a longitudinal section through a cyclone according to the invention.

Figure 6 represents a longitudinal section through another cyclone according to the invention.

Figures 7, 8, 9A, 9B, 9C and 10 depict representations of other cyclones according to the invention.

In Figure 4, 36 represents a second inlet means; 35 represents the cyclone axis; 39 represents the cyclone wall; 37 indicates the direction of rotational velocity in the cyclone; and 38 indicates the direction of fluid flow in said second inlet means. Fluid 31 in the second inlet means close to the wall of the second inlet means will after some period of time progress to a point 32 near the wall of the cyclone. Fluid very close to the cyclone wall at 33 will, after some period of time, progress to a point further away from the wall 34, being displaced away from the wall by the fluid entering the cyclone via second inlet means 36. - The depth of the mouth of the inlet (indicated by arrows 47) preferably need only be large enough to displace a significant proportion of the fluid that was in the zone of significantly high shear stress close to the wall upstream of the second inlet means within the cyclone.

In Figure 5, 40 represents a first inlet means; 42 represents an outlet means whereby a greater proportion of the more dense fluid is discharged from the cyclone; 41 represents an outlet means whereby a greater portion of the less dense fluid is discharged from the cyclone; 46 represents the cyclone body; 43 represents a second inlet means according to the invention. Fluid in the boundary layer close to the wall at 44 will be displaced further away from the wall to a point 45 by the action of fluid entering via second inlet means 43. In this arrangement, the width of second inlet means 43 (indicated by arrows 50) preferably is wide enough to sufficiently displace the boundary layer at the wall of the cyclone upstream of the second inlet means 43 as the fluid originally in this boundary layer follows its helix trajectory towards outlet means 42, i.e., the width shown by arrows 50 preferably should not be significantly less than the length component^' of the trajectory parallel to the longitudinal cyclone axis for one revolution of the helix at the location where fluid from this original boundary layer passes adjacent the mouth of second inlet means 43 in the cyclone.

In Figure 6, denser fluid exiting the cyclone via outlet means 52 is divided into two streams at 55, a side stream being pumped by a pump 51 to second inlet means 53.

In Figure 7 a pump 57 takes fluid from a course 56 and transfers it to a second inlet means 58.

In Figure 8 denser fluid which exits the cyclone via outlet means 62 is divided into two streams at 63. One of these streams is transferred through a tube 61 and educted into the cyclone at second inlet means 60.

Figures 9A, B and C show another arrangement. A tube 66 is oriented in a way such that its entry mouth is approximately normal to the resultant of the rotational velocity vector and axial velocity vector at the outlet of the denser fluid from the cyclone, with the objective of maximising total head recovery for eduction of the fluid captured by the tube 66 at the second inlet means 64. Figure 9A depicts restriction device 65 which may be used to impede the flow to second inlet means 64.

Figure 9B is a sectional view looking towards the end of the cyclone axis 68. The mouth of the tube 66 is located not far from the cyclone wall where rotational velocity and axial velocity are both significantly large, and not in the boundary layer existing near the wall. Captured fluid 67 flows to second inlet means 64. Solids heavier than the denser fluid will tend not to be captured by the mouth of the tube 66 because such solids will travel very close to the wall due to centripetal force action. An advantage of using tube 66 to gain a greater total head is that second inlet means 64 may be located at a length along the cyclone closer to first inlet means 70 when eduction is to be used of fluid obtained from cyclone outlet means 69 having a greater proportion of more dense fluid.

Figure 10 depicts a longitudinal section through a cyclone with two first inlet means 72 and with six second inlet means 73 located at different lengths along the cyclone. 74 is an outlet means which has a greater proportion of more dense fluid and 75 is an outlet means which carries a greater proportion of less dense fluid.

While in the foregoing, embodiments of the invention have been disclosed in considerable details for purposes of illustration, it will be understood by those skilled in the art that many of these details may be varied without departing from the spirit and scope of the invention.

The invention also provides a method of separating to a substantial extent mixtures of two or more fluids wherein said mixtures are passed through one or more cyclones constructed according to the invention.

Claims

CLAIMS:

1. A cyclone for separating at least two fluids having different densities, the cyclone having at least two inlet means through one of which the fluids to be separated enter the cyclone and at least two outlet means from one of which the fluid having the higher percentage of more dense fluid passes from the cyclone and from the other of which the fluid having the higher percentage of less dense fluid passes therefrom characterised in that the or one of the other inlet means is adapted to permit the introduction of a fluid into the cyclone to displace at least some of the existing boundary layer at the point of introduction.

2. A cyclone as claimed in claim 1 wherein the other inlet means is located along the length of the cyclone between the said one of the inlet means and the outlet means through which the fluid having the higher percentage of the more dense fluid passes.

3. A cyclone as claimed in claim 2 wherein the other inlet means is at a position along the length of the cyclone where substantial rotational and axial components of velocity exist.

4. A cyclone as claimed in claim 2 or claim 3 wherein there are a plurality of other inlet means.

5. A cyclone as claimed in claim 4 wherein the other inlet means are located at various positions along the length of the cyclone.

6. A cyclone as claimed in claim 2 wherein the depth of the other inlet, in radial section, is sufficient for fluid passing therethrough to displace only the boundary layer in the cyclone.

7. A cyclone as claimed in claim 2 wherein the axial length of the other inlet is at least of the order of the trajectory parallel -to the axis for one revolution of the helix of rotation at the other inlet.

8. A cyclone as claimed in claim 1 wherein the source of fluid for the other inlet is from the outlet means from which the fluid having the higher percentage of the more dense fluid passes therefrom.

9. A cyclone as claimed in claim 8 wherein the fluid for the other inlet is obtained by splitting the outlet means and is passed to the other inlet.

10. A cyclone as claimed in claim 8 wherein the fluid for the other inlet is pumped from the outlet means to the other inlet.

11. A cyclone as claimed in claim 8 wherein the source of luid or the other inlet is drawn from the cyclone closer to the outlet means from which the fluid having the higher percentage of the more dense fluid passes therefrom than the other inlet and is passed to the other inlet.

12. A method of operating a cyclone for separating at least two fluids having different densities wherein fluid is passed into the cyclone through a fluid inlet means other than that through which the fluids to be separated are passed into the cyclone which fluid inlet is located between the fluid inlet means through which the fluids to be separated are passed into the cyclone and the outlet from the cyclone through which the fluid having the higher percentage of the more dense fluid passes whereby the boundary layer established in the cyclone is caused to move away from the wall of the cyclone.

13. A method as claimed in claim 12 wherein the density of the fluid which enters the cyclone through the other inlet means is similar to or greater than, the density of the more dense fluid to be separated.

14. A method as claimed in claim 13 wherein the fluid which enters the cyclone through the other inlet means is less viscous than the fluids to be separated.

15. A method as claimed in claim 12 wherein the flu,.--, which enters the cyclone through the other inlet is taken from the cyclone at the outlet thereof having the higher percentage of the more dense fluid.

16. A method as claimed in claim 1.2 wherein the fluid which enters the cyclone through the other inlet is taken from the cyclone between the other inlet and the outlet thereof having the higher percentage of the more dense fluid.

17. A method as claimed in claim 12 wherein the separation of fluids ^• having different densities is effected in a multi-stage process where the fluid from at least one of the outlets of a cyclone is passed to an inlet of a second cyclone.

18. A method as claimed in claim 17 wherein the fluid passed to the other inlet of the said first cyclone is obtained from the outlet of the second cyclone through which the fluid having the higher percentage of the more dense fluid passes.