GB2150039A

GB2150039A - Process and apparatus for separating a dispersed phase from a liquid

Info

Publication number: GB2150039A
Application number: GB08331370A
Authority: GB
Inventors: Anthony Ignatious John Treanor
Original assignee: Permutit Boby Ltd
Current assignee: Permutit Boby Ltd
Priority date: 1983-11-24
Filing date: 1983-11-24
Publication date: 1985-06-26
Also published as: GB2150039B; GB8331370D0

Abstract

A dispersed phase is separated from a flowing liquid stream that enters an inlet compartment 1 and passes with laminar flow substantially horizontally through a separating compartment 3 into an outlet compartment 4. The separating compartment 3 is divided into parallel, inclined, lamellar passages 11 by a stack of inclined plates 10 having ridges 13 extending upwardly. Dispersed phase separates within each lamellar passage and travels in the direction of the ridge and is discharged into collection compartment 6 substantially without crossing the flow of liquid into that lamellar passage. <IMAGE>

Description

SPECIFICATION Process and apparatus for separating a dispersed phase from a liquid Dispersed phase will separate from liquid that is static or is moving with laminar flow and it is well known to make use of this phenomenon by passing liquid with laminar flow over a large surface. Dispersed solids collect on the surface. If the surface is horizontal their removal can be inconvenient and this has led to the development of a wide range of inclined parallel plate separators. These comprise a series of parallel, inclined, lamellar passages that are defined by a stack of parallel plates that are inclined to the horizontal. The dispersion is generally fed into the bottom of these passages and caused to flow up them.If the lamellar passages are sufficiently shallow and if laminar flow prevails in them dispersed solids in the liquid stream will separate from the liquid as it passes up the lamellar passage and will settle onto the lower plate and will slide down that plate.

The plates may be corrugated so as to provide extra strength. The solids collect at the bottom of the plates and the purified liquid is removed from the top.

Although parallel plate separators are normally designed to operate with upflow (or downflow) there have been a few proposals to operate them with the liquid stream flowing substantially horizontally. Such proposals occur in US Patent Specifications 2868384 and 4273654. Apparatus for this purpose comprises an inlet compartment having an inlet for the supply of dispersion to it, an outlet compartment having an outlet for discharge of purified liquid from it, a collection compartment having an outlet for discharge of separated dispersed phase from it and a separating compartment divided into parallel, inclined, lamellar passages by a stack of parallel plates that are inclined to the horizontal and the compartments are arranged such that liquid may flow with laminar flow substantially horizontally from the inlet compartment, through the lamellar passages and into the outlet compartment and such that dispersed phase that has separated from the liquid stream collects on the plate surfaces and flows downwardly (or upwardly) along the surfaces and into the collection compartment.

The separated solids roll down the plate surfaces and by a "snow ball" effect can develop quite a large volume before they fall into the collection compartment. As a result there is a serious risk that the separated solids will either be re-entrained by the horizontally flowing liquid or will, at least, be deflected towards the outlet compartment by the horizontally flowing liquid. As a result the liquid that emerges from each lamellar passage into the outlet compartment will tend to carry with it solids that had separated from it.

We have established that it is possible to minimise this problem by providing ridges extending upwardly along the plate surfaces for restricting horizontal travel of separated dispersed phase but even after we provided such ridges and designed them in such a way as to maintain laminar flow through the lamellar passages we found surprisingly that optimum separation still was not being achieved.

For instance we found that very poor results were obtained when the lamellar passages discharged one above the other, as in US 4273654.

The present invention relates to a process and apparatus by which these problems are overcome and by which liquid of improved purity can be achieved.

Apparatus according to the invention for separating a dispersed phase from a flowing liquid stream comprises an inlet compartment having an inlet for supply of dispersion to it, an outlet compartment having an outlet for discharge of liquid from it, a collection compartment having an outlet for discharging separated dispersed phase from it and a separating compartment divided into parallel, inclined, lamellar passages by a stack of parallel plates that are inclined to the horizontal and in this apparatus the compartments are arranged such that liquid may flow with laminar flow substantially horizontally from the inlet compartment, through the lamellar passages, and into the outlet compartment, and such that dispersed phase that has separated from the liquid will collect on the plate surfaces and flow upwardly or downwardly along the surfaces and into the collecting compartment and in this apparatus there are ridges extending upwardly along the plate surfaces for restricting horizontal travel of the separated dispersed phase and each lamellar passage opens into the collection chamber such that the separated dispersed phase from that passage is discharged into the collection chamber substantially without crossing the flow of liquid into that lamellar passage.

The invention is based in part on the realisation that the separated solids will occupy a significant volume relative to the volume of liquid flowing through the lamellar passages and that as these solids fall or rise into the collection chamber they will displace a significant volume of the liquid that is in that collection chamber. Previously the collection chamber has been constructed as a substantially closed static chamber that is initially filled with the liquid dispersion that is passed through the lamellar passages and which gradually becomes partially filled with separated dispersed phase. This dispersed phase may be discharged continuously or intermittently.

What had not previously been appreciated, and which we now realise is the cause of inferior purification, is that the separated dis persed phase that moves into the collection chamber displaces liquid from the chamber.

As the chamber has previously been closed this has forced liquid from the chamber back into the lamellar passages. In apparatus such as is described in US 4273654 separated dispersed phase from a first lamellar passage will flow downwardly across the entrance to the next lower, second. lamellar passage.

Since liquid will be flowing up into that second passage, for the reasons given above, this upwardly flowing stream of liquid will cross the flow of separated dispersed phase and will tend to re-entrain that, with the result that the liquid that is displaced from the collection chamber into the second passage will carry with it some of the previously separated solids from the first lamellar passage.In the invention we ensure that not only are there ridges along the plate surfaces for restricting, and preferably substantially preventing, horizontal travel of the separated dispersed phase but also separated dispersed phase that has been discharged from one passage does not have to cross the flow of liquid from the collection chamber into any other lamellar passage and by these two modifications we can achieve much purer liquid in the outlet compartment than has previously been obtainable.

Each adjacent pair of plates preferably defines, where they enter the collection compartment, a parallel slot. These slots may be arranged substantially horizontally with respect to one another in which event separated dispersed phase being discharged from one slot will not cross the flow of liquid from the collection chamber into that or any other slot.

However it will flow counter-current to liquid flowing from the collection chamber into that slot and this can be undesirable because of the risk of entrainment.

Preferably the collection chamber is provided with a bleed for escape from the chamber of liquid having a volume approximately the same as the volume of separated dispersed phase. For instance if the slots are arranged one above the other (as in US 4273654) the slots may discharge into a vertical zone provided, at its top, with some bleed outlets. Liquid in the chamber is thus displaced by the separated dispersed phase through the bleed outlets rather than through the lamellar passages.

Preferably however the slots are arranged substantially horizontally with respect to one another, i.e. so that they are all either on the same level or arranged along a line that is displaced from the horizontal by a small angle, generally not more than about 20 . In addition to this, the collection chamber is provided with a bleed for the escape of liquid from the chamber having a volume approximately equivalent to the volume of dispersed phase.

The liquid displaced through the bleed may be discharged from the apparatus but since it contains some dispersed solids it would be desirable to be able to separate the solids from this liquid. Preferably the bleed from a collection chamber in which the slots are arranged substantially horizontally is provided by one or more of the lamellar passages as a result of restricting or preventing the flow of liquid stream into those passages from the inlet compartment. Preferably one or more lamellar passages connect with the collection compartment and the outlet compartment but not with the inlet compartment and thus liquid bled from the collection compartment can freely move through those bleed lamellar passages and dispersed phase in the bled off liquid will be separated in those passages in the same way as in the other lamellar passages.For instance if the flowing liquid stream contains 10% by volume dispersed phase 10% of the lamellar passages may be blanked off from the inlet compartment.

If the dispersed phase is heavier than the liquid then the collection compartment must be beneath the separating compartment whilst if the dispersed phase is lighter than the liquid it will be above the separating compartment.

Often the apparatus contains two separating compartments, one for receiving light dispersed phase such as gas or oil from the top of the separating compartment and one for receiving heavy dispersed phase such as solids from the bottom of the separating compartment.

Preferably each plate is rectangular with its top and bottom edges preferably being substantially horizontal. The plates are generally inclined at an angle of 20 to 60 , most preferably 30 to 45 , to the vertical.

Generally there is the same separation between each adjacent pair of plates.

If the dispersed phase is lighter than the flowing liquid the ridges must be on the lower surfaces of the plates since the dispersed phase will move up onto those surfaces. If the dispersed phase is heavier than the liquid then the ridges must be on the upper surfaces of the plates. Generally the ridges are provided on both surfaces of the plates.

Each ridge will generally extend in a substantially rectilinear line from the top edge of the plate down to the bottom edge of the plate. It is necessary that the ridges do not prevent laminar flow across them and so normally will not have a square profile. Prefer ably the profile is trapezoidal or, most preferably sinusoidal. The ridges can be provided on otherwise flat plates but preferably are formed by corrugating the plates and preferably the position of the corrugations in one plate exactly coincides with the position of the corrugations in each adjacent plate, the separation between the plate surfaces thus being uniform throughout the lamellar passages.

We have found that the effectiveness of the apparatus for separating dispersed phase is affected significantly by the pitch (P) or distance between adjacent corrugations, the amplitude (A) of the corrugations and the distance (D) between adjacent plate surfaces, i.e.

the width of the lamellar passage. P must be at least 2.5 A since at lower values there is increasing risk of eddy currents and nonlaminar flow occurring. Preferably P is 3 A or greater. It may be up to, for instance, 5 A or even 10 A.

D must be at least 1.5 A since otherwise there is inadequate spacing for laminar flow to occur and preferably D is 2 A or more, for instance up to 4 A. It can be higher, for instance up to 8 A, but increasing the size of D with respect to A increases the risk that, particularly with high contents of dispersed phase, solids may be carried horizontally by the horizontally flowing liquid stream.

D is generally from 20 to 60 mm, most preferably 35 to 45 mm. If D is too low the corrugations must be made very small in order to maintain laminar flow, and if D is too high the rate of flow must be reduced or the size of the apparatus must be increased in order to give time for the solids to separate across the width of the lamellar passage.

The construction of the inlet and outlet compartments and of the lamellar passages must be such that laminar flow prevails throughout the separating. compartment. The Reynolds number in the separating compartment should be below 200 and generally below 150 in order that laminar flow prevails.

If nonlaminar flow exists this will be readily apparent from visual observation, since it will be seen that particles are being carried round in eddies. In these circumstances the rate of flow should be reduced until laminar flow is restored.

The invention is now described with reference to the accompanying drawings in which: Figure 1 is a perspective view of apparatus according to the invention.

Figure 2 is a section on the plane 2-2 in Figure 1.

Figures 3, 4, 5 and 6 are cross sections through various arrangements of plates according to the invention.

The apparatus shown in Figure 1 comprises an inlet compartment 1 having an inlet 2, a separating compartment 3, an outlet compartment 4 having an outlet 5 and a collection compartment 6 having an outlet 7. There is a head space 8 at the top of the separating compartment and a base space 9 at the base of the separating compartment. between it and the collection compartment 6.

This apparatus is designed for separating dispersed solids that will settle downwardly but if it desired to separate gas, oil or light solids that will separate upwardly there may be another collection compartment at the top of the head space 8 and this may be provided with, for instance, an oil skimmer.

The separating compartment 3 is divided by a stack of parallel plates 10 into a number of parallel lamellar passages 11. The majority of these lamellar passages 11 open into the inlet compartment 1 but a few are blanked off from the inlet compartment 1, as shown at 12 in Figure 2.

Each plate has corrugations 13 extending upwardly, that is to say from its lower edge to its upper edge. These corrugations may, for example, be sinusoidal or trapezoidal and may have the shape and relative dimensions shown in any of Figures 3, 4, 5 or 6.

The pitch between adjacent corrugations is P, the amplitude is A and the separation between adjacent plates is D. Preferably P 2 3A and D 2 2A.

Liquid is fed into the inlet compartment 1 at a rate such that it achieves laminar flow and flows into and through the lamellar passages 11 that are open to the inlet chamber 1. As it travels horizontally dispersed solids will settle towards the lower plate surface in each lamellar passage and, as soon as they have settled onto the surface will roll or slide down the surface towards the collection chamber 6. The upwardly extending corrugations 13 will prevent or minimise horizontal travel of the separated solids. As the solids fall from the lamellar passages 11 into the collection chamber 6 they will displace an equal volume of liquid from the zone 9 into the passages 12 that are blanked off from the inlet chamber 1.The apparatus shown is designed for a dispersion having about 15% by volume solids and a number of blanked off passages will be selected to provide a volume of lamellar passages 12 approximately equivalent to the volume of dispersed solids that are being separated. Liquid displaced from the zone 9 into the passages 1 2 will travel towards the outlet compartment 4 and any dispersed solids in it will therefore have the opportunity of settling from it before it enters the outlet compartment.

All the lamellar passages 11 and 12 open into the outlet compartment 4 and into the zones 8 and 9 and a collection compartment 6. The dispersed solids accumulate in the collection compartment 6 and are removed continuously or occasionally through the outlet 7.

The separating compartment typically is from 1 to 3 metres, most preferably 1.5 to 2 metres long, (in the horizontal direction) and typically is from 1 to 3 metres, preferably 1.5 to 2 metres wide (between the top and bottom ages of the plates).

A flocculant is generally added to water containing dispersed solids and a de-emulsifier is generally added to oil emulsions, either in the inlet compartment or before entry to the inlet compartment.

Claims

1. Apparatus for separating a dispersed phase from a flowing liquid stream and which comprises an inlet compartment having an inlet for supply of dispersion to it, an outlet compartment having an outlet for discharge of liquid from it, a collection compartment having an outlet for discharging separated dispersed phase from it and a separating compartment divided into parallel. inclined, lamellar passages by a stack of parallel plates that are inclined to the horizontal and in which the compartments are arranged such that liquid may flow with laminar flow substantially horizontally from the inlet compartment, through the lamellar passages, and into the outlet compartment, and such that dispersed phase that has separated from the liquid will collect on the plate surfaces and flow upwardly or downwardly along the surfaces and into the collection compartment and in which there are ridges extending upwardly along the plate surfaces for restricting horizontal travel of the separated dispersed phase and each lamellar passage opens into the collection compartment such that the separated dispersed phase from that passage is discharged into the collection compartment substantially without crossing the flow of liquid into that lamellar passage.

2. Apparatus according to claim 1 in which each adjacent pair of plates defines, where they enter the collection compartment, a parallel slot.

3. Apparatus according to claim 2 in which the slots are arranged substantially horizontally with respect to one another.

4. Apparatus according to any preceding claim in which the collection chamber includes a bleed for escape from the chamber of liquid having a volume approximately the

4. Apparatus according to any preceding claim in which the collection chamber includes a bleed for escape from the chamber of liquid having a volume approximately the same as the volume of separated dispersed phase.

5. Apparatus according to claim 4 in which there are means restricting or preventing the flow of liquid stream into one or more, but not all, of the lamellar passages from the inlet compartment and the said one or more lamellar passages serve as the bleed from the collection chamber.

6. Apparatus according to claim 5 in which the said one or more lamellar passages connect between the collection compartment and the outlet compartment but not with the inlet compartment.

7. Apparatus according to any preceding claim in which each plate is substantially rectangular, inclined at an angle of 20 to 60 and has its top and bottom edges substantially horizontal.

8. Apparatus according to any preceding claim in which the said ridges are provided on both surfaces of the plates.

8. Apparatus according to any preceding claim in which each ridge extends in a substantially rectilinear line from the top edge of the plate to the bottom edge of the plate.

10. Apparatus according to any preceding claim in which each ridge has a trapezoidal or sinusoidal profile.

11. Apparatus according to any preceding claim in which the ridges are formed by corrugating flat plates.

12. Appparatus according to claim 11 in which the corrugations in one plate coincide with the position of the corrugations in each adjacent plate. the separation between the plate surfaces thus being uniform throughout a lamellar passage.

13. Apparatus according to claim 11 or 12, in which the distance between adjacent corrugations is at least three times the amplitude of the corrugations.

14. Apparatus according to any of claims 11 to 13 in which the distance between adjacent plate surfaces is at least twice the amplitude of the corrugations.

15. Apparatus according to any preceding claim in which the distance between adjacent plates is from 20 to 60 mm.

16. Apparatus according to claim 1 substantially as herein described with reference to the accompanying drawings.

17. A method in which a dispersed phase is separated from a flowing liquid stream by passing the liquid stream into the inlet compartment of apparatus according to any preceding claim, through the separating compartment and out of the outlet compartment and collecting the separated phase in the collection compartment.

18. A method according to claim 17 in which the Reynolds number in the separating compartment is below 150.

19. A method according to claim 18 substantially as herein described with reference to any of the accompanying drawings.