GB2531804B - Separator and separation apparatus - Google Patents

Description

Separator and Separation Apparatus

Field of the invention

The present invention relates to a separator and separation apparatus. More particularly, embodiments of the present invention relate to significant improvements to an existing spiral separator.

Background to the invention

Gravity separation of solids from a liquid or two liquids is possibly where the factions have sufficiently different specific gravities for them to separate if the mixture is held stationary (if for example a sample of the mixture separates satisfactorily if stood in a glass laboratory beaker on a bench).

Gravity separation of solids from a liquid or of two liquids is carried out in a number of industries and from bench scale to liquid flows of many cubic meters per second. Some common applications for gravity separation include river water clarification at water supply treatment works, so-called “final separation” at sewage treatment works or separation of oil and water in the oil industry.

The most commonly used devices used for gravity separation are settlement or sedimentation tanks or lagoons and these can be used in either a batch or a continuous process. These work well and are usually the solution chosen where there is plenty of space available.

However where space is limited, a more compact form of treatment process is required. One way to perform gravity separation in a more compact process is to use a lamella separator. In a lamella separator a number of inclined plates are arranged in parallel in the process tank. Usually the plates are rectangular but other shapes are possible and the plates are usually evenly spaced apart. A well designed lamella separator tank will have arrangements to spread the mixed liquid flow evenly across all of the lamella plates and have take-offs for the separated factions normally with the lower specific gravity faction take off at the top of the process tank and the higher specific gravity take off at the bottom of the process tank. A typical lamella separator process used for primary treatment of domestic sewage may have an overall process footprint of 10% of the footprint required for equivalent treatment provided in conventional settlement tanks.

The Spiral Separator is a development of the lamella separator. In the Spiral Separator, there is an arrangement of plates within a circular process tank. Each plate is in the shape of a conical helix and there may be several evenly spaced interleaved plates.

The advantages of the Spiral Separator over the lamella separator include: (A) The shape of the plates gives them increased inherent stiffness (compared with the normally flat lamella plates) so plates can be made of thinner and lighter materials and require less support to hold them in shape. (B) The shape of the plates facilitates an improved design to provide more even distribution of mixed liquid flow across all of the plates, for removal of the lower specific gravity faction from the top of the process tank and for concentration and removal of the higher specific gravity faction from the bottom of the process tank. (C) Rotating the arrangement of plates within the process tank increases separation efficiency. A typical Spiral Separator process used for primary treatment of domestic sewage may have an overall process footprint of one third of that of a lamella separator or 3% of the footprint required for equivalent treatment provided in conventional settlement tanks.

Whilst the lamella separator and the spiral separator reduce the plan area required for the treatment process when compared with settlement tanks there is usually an increase in process tank depth required.

Examples of prior spiral separators are described in WO01/21273 and WO95/01215.

Embodiments of the present invention seek to provide improvements over the existing spiral separators described above.

Summary of the invention

According to the present invention, there is provided a separator for separating solid materials from a liquid or for separating two liquids, comprising at least one plate following a helical path around a central axis, the slope of the plate at any given position having a circumferential component and a radial component, wherein the radial component of slope varies with distance from the central axis, and wherein the radial component of slope increases with distance from the central axis and further wherein a portion of the plate nearest to the central axis has a radial component of substantially zero and an outer portion of the plate having a non-zero radial component has a constant slope.

The resulting spiral separator, by using different plate shapes, can be made even more compact than existing spiral separators, and the range of applications of the technology can be extended into areas where current technology would not be either practical or commercially worthwhile. The complex shape of plate used in this case can be optimised using calculations. In particular, the shape can be optimised to give the maximum inherent plate stiffness consistent with the minimum requirements of the separation process to be performed in the spiral separator. Moreover, a reduction in treatment tank height can be achieved when compared with previous spiral separator designs.

The new plate shape provides further inherent stiffness allowing for even thinner or less stiff materials to be used in construction (substantially reducing costs). The overall height of the plate pack can be reduced allowing a reduction in the depth of the required process tank.

The slope of the outer portion may be between approximately 55° and approximately 60°.

The pitch of the helical path may be set such that the circumferential component is greater than or equal to a desired slope for the plate.

The plate extends radially from an inside diameter to an outside diameter. A hollow core may be defined within the inside diameter of the plate. In some examples, the outside diameter is approximately twice the inside diameter.

The helical pitch of the plate may be at least approximately three times the inside diameter of the plate. Preferably, the helical pitch is at least approximately four times the inside diameter of the plate, more preferably at least approximately five and a quarter times the inside diameter of the plate, and still more preferably at least approximately six and a half times the inside diameter of the plate.

The separator may comprise a plurality of plates following interleaved helical paths, each plate defining an approximately 180° of a full helix, wherein the outside diameter is approximately twice the inside diameter, and wherein the helical pitch is approximately six times the inside diameter.

Detailed description

The invention will now be described by way of example with reference to the following Figures in which:

Figure 1 schematically illustrates a conventional (Mark 1) spiral separator plate pack;

Figure 2 schematically illustrates a large Mark 1 spiral separator in a concrete tank;

Figure 3 schematically illustrates the effect of increasing the plate pack pitch on the radial profile of the plate;

Figure 4 schematically illustrates an improved (Mark 2) spiral separator plate pack;

Figure 5 schematically illustrates a large Mark 2 spiral separator in a concrete tank;

Figures 6 schematically illustrates a small Mark 2 Spiral Separator plate pack, in an inverted position within an alternative tank; and

Figure 7 schematically illustrates the derivation of an expression for plate slope.

From here on, the original spiral separator as described for example in the earlier patent documents WO01/21273 and WO95/01215 is referred to as the Mark 1 spiral separator and the embodiment of the invention described herein is referred to as the Mark 2 spiral separator. A Mark 1 spiral separator comprises a process tank with an inlet and two outlets - an overflow (or high level) outlet for the faction with the lower specific gravity and an underflow (or low level or bottom) outlet for the faction with the lower specific gravity. The Mark 1 spiral separator also comprises an arrangement of interleaved plates, each having the shape of a conical helix. The Mark 1 spiral separator also comprises hydraulic arrangements inside the process tank which ensure that the in-flowing mixture of liquid and solid or two liquids passes evenly through the arrangement of plates and also to divert the separated factions to the different outlets. Referring to Figure 1, there is shown a half section of a Mark 1 spiral separator plate pack 100. The left hand side shows a section through the plate pack and the right hand side is an elevation. In Figure 1, the outside diameter of the plate pack is twice the inside diameter. The helical pitch is 3 times the inside diameter. There are six plates 101 in total, and each plate does a full turn (360°) of the helix.

Figure 2 shows a large Mark 1 spiral separator in a concrete tank 200. The outline of the plates 201 is shown with dotted lines and the plates 201 are mounted to and spiral around a hollow central core 202 of the plate pack. A mixed flow of liquid and solid 203 enters the spiral separator from beneath, and the central core 202 of the plate pack acts as a baffle to slow down the velocity of flow and direct it to the bottom of the plate pack. More particularly, it can be seen from Figure 2 that the incoming liquid and solid 203 enters the apparatus via a pipe coming from one side and then turning to extend up into the hollow central core 202. The direction of flow upwards within the pipe is indicated by arrows. This pipe extends up to near the liquid surface near the top of the tank. The liquid exiting the pipe then drops down (again, following the direction shown by the arrows) within the central core and then passes outwardly into a region beneath the plates 201. The flow then passes upwards through the plate pack and as it does, the solids 204 are deposited on the plates, where they coalesce to form a sludge. This sludge slides off the plates and is concentrated in the annular gap between the plate pack and the tank wall and from there to the floor of the tank, where it exits from an outlet in the floor of the tank. The cleaned liquid 205 passes out of the top of the plate pack and out of the process tank. A drive mechanism 206 is provided that drives both the plate pack and a rake 207 to rotate. Rotating the plate pack improves the efficiency with which solids are removed from the liquid, while rotating the rake pushes the sludge accumulating on the floor of the tank to the outlet. A man access hatch 208 is provided for maintenance purposes.

The text book “Wastewater Engineering, Treatment and Reuse”, Metcalf and Eddy, Fourth Edition, International Edition 2004, published by McGraw Hill and in particular page 375 and page 377 explains that in a lamella separator the slope on the plate has to be at least 45 to 60 degrees to the horizontal for the settled solids to slide from the plate by gravity. This angle range is for all applications, not just primary treatment of sewage for which the slope on the plate is more preferably at least 55 to 60 degrees. In practice, most (if not all) lamella separators, commercially available for primary treatment of sewage, have plates inclined at 60 degrees to the horizontal.

All Mark 1 Spiral Separators, used for primary treatment of sewage built to date have a radial slope on the conical helix shaped plates of 60° to the horizontal. Because a conical helical plate has both a radial and a circumferential component of slope, this means that the actual slope on the plate is slightly more than 60° to the horizontal everywhere. For a conical helix, the circumferential component of slope is greater nearer the axis of the helix. This means that the actual slope on a conical helical plate in a Mark 1 spiral separator is slightly steeper near the centre of the process tank than at the periphery of the tank. It is possible to calculate the actual slope on the plate at every point on the plate surface. When combining radial and circumferential angles to give a combined angle, the equation used to determine slope is as follows: cotan2(c) = cotan2(a) + cotan2(b) (A)

Where c = slope angle, a = circumferential angle, b = radial angle

The derivation of Equation A and the equation for the direction of the angle of steepest slope (in plan relative to the radius) is described later with reference to Figure 7.

Furthermore, for any given helical pitch, it is possible to calculate the required radial slope at every point on the plate surface so that the actual slope on the plate is at exactly the required angle (60° to the horizontal for primary treatment of sewage). If this required radial shape is calculated and plotted out it gives a curve to the radial profile. The resulting shape is no longer a conical helix, but is a new shape with improved properties. It is this shape which forms the basis of a Mark 2 spiral separator. In other words, one way in which the Mark 2 Spiral Separator differs from a Mark 1 Spiral Separator is that the interleaved plates are not in the shape of a conical helix but are in a more complex three dimensional shape calculated to give the required slope at every point on the plate surface.

The curve in the radial profile becomes more pronounced as the helical pitch is increased. So it is possible to select a helical pitch to give a definite curve in the radial direction. In all Mark 1 spiral separators built to date, the helical pitch has not been high enough to have made much of a difference. Part of the improvement going from Mark 1 to Mark 2 spiral separators is to deliberately increase the helical pitch so that the calculation above can be used to put a definite curve in the radial profile of the plates. For the Mark 2 spiral separator it is possible to further increase the helical pitch so that the circumferential component of slope on the plate (nearest the axis of the helix) is more than the required slope. In this case, a section of the radial profile can be horizontal and the effect of this is to put a marked bend in the radial profile making the overall shape much stiffen

As a result, the Mark 2 spiral separator has an inherently stiffer plate pack than the Mark 1 spiral separators, allowing the use of thinner or cheaper materials for fabrication. Moreover, when compared with the Mark 1 spiral separator, the Mark 2 spiral separator requires a shallower process tank, which is cheaper and gives more flexibility of layout for the overall treatment process stream on any given treatment works site.

In other words, the new plate shape is achieved by (a) increasing the pitch of the helix to the point where it adds significantly to the overall slope on the plate surface, then (b) calculating the slope in the radial direction so that the combination of circumferential and radial gives the slope required for the actual application (which would for example be 55° to 60° to the horizontal for primary treatment of sewage with self-cleansing plates). The nature of the calculation is such that it is possible to choose a helical pitch on the inside edge of the plate that will allow a definite and significant curve in the radial direction. It is this curve in the radial direction that gives the increased plate stiffness and reduces the depth of the process tank required in the improved spiral separator.

From here on, illustrative calculations and examples are based on providing a combined slope angle on the plates of 60°, but exactly the same principles apply to designing any Mark 2 spiral separator for any other required plate angle.

Figure 3 shows the effect of using Equation A for a variety of different helical pitches. A line 301 represents the radial profile of a Mark 1 spiral separator as described above. It can be seen that this radial profile is straight. Until the plate pack pitch is increased to at least three times the inside diameter of the plate pack, the effect of using Equation A is very limited. A line 302 represents the radial profile, using Equation A, where the plate pack pitch is four times the inside diameter of the plate pack. A noticeable curve in the radial profile is starting to appear. A line 303 represents the radial profile where the plate pack pitch is five and a quarter times the inside diameter of the plate pack. At about this point the circumferential angle at the inside edge is about 60°, with the result that the joint between the plate and the plate pack core is substantially perpendicular. A line 304 shows the pitch increased to six and a half times the inside diameter of the plate pack. At the plate pack core the circumferential angle is greater than 60° and it does not reduce to 60° until the diameter has increased to about 1.2 times the inside diameter of the plate pack.

This means that there can be a horizontal section in the radial profile, in particular at its inner portion. Then as the diameter increases further, the radial profile curves to ensure that at every point on the plate surface, the slope is 60° or more.

Where the radial profile is horizontal, there is still a movement of settled solids away from the centre towards the periphery, as the path of a particle moving along the line of greatest slope will be tangential to the circumference in plan when the radial profile is horizontal and will turn more steeply towards the periphery as the radial profile curves downwards. In other words, at the inner portion of the radial profile where the only component of slope is the circumferential component, a particle will follow a line tangential to that circumferential component, which will guide the particle slightly further away from the central axis of the separator. This will continue until the particle reaches the curved part of the radial profile, at which time the particle will tend to follow a line of greatest slope, which will point increasingly outwardly with proximity to the outer diameter of the plate.

Figure 4 shows a Mark 2 spiral separator plate pack 400. This is a half section. The left hand side shows a section through the plate pack and the right hand side is an elevation. In this Figure, the outside diameter of the plate pack is twice the inside diameter. The helical pitch is 6 times the inside diameter and there are twelve plates 401 and each plate does a 180° of turn of the helix. It can be seen from Figure 4 that each plate comprises a substantially horizontal portion (in a radial direction) adjacent to the central core, and that at a predetermined distance from the central core (or more specifically at a predetermined radial position) the plate curves downwards radially. This Mark 2 plate pack would be suitable to replace the Mark 1 plate pack in Figure 2 to turn the spiral separator into a Mark 2 spiral separator. It should be understood that the total plate area and treatment capacity for the plate packs in Figures 1 and 4 are substantially the same. The advantages of the Mark 2 over the Mark 1 is that the plate pack is inherently stiffer so can be made with cheaper/lighter materials and it is shorter so there is a saving in tank depth.

Figure 5 shows a Mark 2 spiral separator in a concrete tank 500. Other than the spiral separator itself, and the reduced tank depth, the features of Figure 5 are substantially identical, with the same function. These features are therefore not described again here. Comparing Figures 2 and 5 gives an indication of the scope for reducing treatment tank depth by moving from a Mark 1 to a Mark 2 Spiral Separator. Although the difference does not look very dramatic in these diagrams, it can have a significant effect on costs and the ability to rearrange plant on a treatment works. For a 5m diameter Mark 1 spiral separator as used for primary treatment of sewage at Newhaven and Lowestoft in the UK, the treatment tank depth is some 10m. Changing to a Mark 2 would have allowed treatment tank height to be reduced to about 7.5m. This would have allowed much greater headroom above the Spiral Separators within the building at Lowestoft or, at Newhaven, would have reduced the extent to which the bottom of the Spiral Separators needed to be sunk into the ground.

Of equal or greater financial significance is the increased inherent stiffness of the Mark 2 spiral separator plate packs. The plate packs at Lowestoft and Newhaven were manufactured of resin transfer moulded glass reinforced plastic platelets for the Mark 1 design. Using a Mark 2 design, the increased inherent stiffness of the shape could allow the plates to be made of vacuum formed unreinforced plastic.

Figure 6 shows a very small spiral separator 600 with an inverted Mark 2 plate pack 601. However, the Figure 6 spiral separator design could also be used with an inverted Mark 1 plate pack. It is possible to build a wide range of spiral separator sizes using the standard design as shown in Figure 2 with variations as appropriate for the specific duty and site conditions for the particular project.

However as the size is reduced, it is not possible to reduce all dimensions to scale. There are minimum dimensions for some parts of the spiral separator. For example, the annular gap between the plate pack and tank wall cannot be reduced below a practical minimum as the risk of blockage increases. So, for very small spiral separators of the standard design, the annular sludge gap would become too large a proportion of overall tank area, increasing the risk of flow by-passing the plate pack.

Also, for very small spiral separators, it is worth sacrificing some enhancing features in favour of simplicity of construction or operation. In Figure 6, the flat floor and rake has been replaced with a conical hopper bottom in the tank 602, which by shape alone channels the heavier separated material down to an outlet at the bottom of the tank. This is only practical for small diameter tanks as depth of construction would limit its application as diameter increases. The plate pack has been inverted so that it fits into the hopper bottomed tank shape better. As a result, the overall depth of the tank (including hopper part) can be reduced. The benefit of plate pack rotation is foregone for the simplicity of attaching the plates to the tank wall 603 and the design plate loading rate is reduced to maintain required solids removal performance without rotating the plates.

With the inverted plate pack, sludge 604 now moves to the centre of the tank and the sludge gap area is only a small proportion of tank cross section area. In the design of the spiral separators shown in Figures 2 and 5, the suspension of solids in a liquid (or two liquids) enters at the centre of the tank, the sludge (or heavier liquid) leaves the treatment tank at the bottom and the liquid (or lighter liquid) leaves the tank at the top. In the inverted design shown in Figure 6, the incoming mixture enters at the bottom through the side wall (shown in Fig 6 as a tangential entry pipe 605). The mixture progresses upwardly through the plate pack, and as it does, the solids 604 are deposited on the plates, where they coalesce to form a sludge. This sludge slides off the plates and is concentrated in the annular gap between the plate pack and a central column (which is in fact the outside of a conduit used to remove the lighter liquid from the tank), and from there drops into the hopper part at the base of the tank, where it exits from an outlet in the floor of the tank. The lighter liquid 606 spills into the hollow central column, which guides it to an outlet. It will therefore be appreciated that the sludge (or the heavier liquid) leaves from the bottom of the hopper 604 and the liquid (or lighter liquid) leaves the treatment tank over a central outlet pipe 606, which leads down through the core of the separator and out to the side.

In a variant (not shown) of Figure 6, the outlet may be on the outside of the tank at the top, in the manner shown in Figure 2 (element 205). In this case, the whole of the hollow central area of the spiral separator would then be available for sludge to flow to the bottom of the tank.

It will be understood from the above that a Spiral Separator using a complex shape of plate for separation of a solid from a liquid or the separation of two liquids by gravity can be provided. The improved shape of plate pack reduces the size of the treatment tank required and improved inherent stiffness allowing plate packs to be manufactured of thinner or less stiff material. A new design for an “inverted” spiral separator 600 is also described, which can optionally (but beneficially) be combined with the new plate pack. This inverted design allows the spiral separator process to be used effectively for much smaller flows than would be economic in the normal spiral separator design.

Referring to Figure 7, a derivation is provided for equation A, and for an equation for the direction of the angle of steepest slope (in plan relative to the radius). Figure 7 is an isometric projection of an inclined plane corresponding to the surface of the helically disposed plate described above. Figure 7 also indicates (top right) the directions of the x, y and z axes. In effect, the z axis corresponds to the longitudinal axis of a spiral separator. The helical pitch of a spiral separator is measured in the direction of the z axis. The circumferential position of the plate can be represented as a circle in the plane of the x, y axes. In Figure 7, PRT is an inclined plane (which may correspond to a surface of a plate, in the context of the separator described above), PQR and PQT are two orthogonal vertical planes in the x and y directions, a is the inclination of the inclined plane in the x direction, and β is the inclination of the plane in the y direction.

If a and β are known; calculate the angle of greatest slope on the inclined plane χ and its direction in plan given by δ.

By Trigonometry

(1)

From Similar Triangles in the xy plane

(2)

By Pythagoras in the XY plane

(3)

Substitution using (2) in (3) to replace x and y

(4)

Multiply (4) by a2b2

(5)

Substitute using (1) in (5) to get back to trig functions

(6)

To calculate steepest slope from two orthogonal angles By Trigonometry in xy plane

(7)

Substitute using (1) in (7) to get back to trig functions

(8)

To calculate direction of steepest slope

Claims (9)

1. A separator for separating solid materials from a liquid or for separating two liquids, comprising at least one plate following a helical path around a central axis, the slope of the plate at any given position having a circumferential component and a radial component, wherein the radial component of slope varies with distance from the central axis, and wherein the radial component of slope increases with distance from the central axis and further wherein a portion of the plate nearest to the central axis has a radial component of substantially zero and an outer portion of the plate having a non-zero radial component has a constant slope.

2. A separator according to claim 1, wherein the slope of the outer portion is between approximately 55° and approximately 60°.

3. A separator according to claim 1, wherein the pitch of the helical path is set such that the circumferential component is greater than or equal to a desired slope for the plate.

4. A separator according to any preceding claim, wherein the plate extends radially from an inside diameter to an outside diameter.

5. A separator according to claim 4, wherein a hollow core is defined inside the inside diameter of the plate.

6. A separator according to claim 4 or claim 5, wherein the outside diameter is approximately twice the inside diameter.

7. A separator according to claim 6, wherein the helical pitch of the plate is at least approximately three times the inside diameter of the plate.

8. A separator according to claim 7, wherein the helical pitch is at least approximately four times the inside diameter of the plate, is more preferably at least approximately five and a quarter times the inside diameter of the plate, and still more preferably at least approximately six and a half times the inside diameter of the plate.

9. A separator according to any preceding claim, comprising a plurality of plates following interleaved helical paths, each plate defining an approximately 180° of a full helix, wherein the outside diameter is approximately twice the inside diameter, and wherein the helical pitch is approximately six times the inside diameter.