EP0220788B1

EP0220788B1 - Liquid flow mixer

Info

Publication number: EP0220788B1
Application number: EP86202094A
Authority: EP
Inventors: Garry Hoyland
Original assignee: Water Research Centre
Current assignee: Water Research Centre
Priority date: 1985-07-15
Filing date: 1985-07-15
Publication date: 1992-09-30
Anticipated expiration: 2005-07-15
Also published as: EP0220788A3; DE3586721D1; ATE81031T1; EP0220788A2

Abstract

A mixer for two liquid flows comprises a generally cylindrical chamber (51) having one open end (52) providing an axial flow outlet for the mixed liquids, and one or more radial inlets (80,81,82) for the liquid flows to be mixed. Within the chamber are one or more radially extending impeller plates (55,56,57) rotatable to cause thorough mixing of the liquids as they flow through the chamber. The impeller plates are preferably mounted on an axial drive shaft (54) and sweep substantially the whole volume of the chamber upon rotation of the shaft.

Description

This invention relates to a liquid flow mixer.
There are many instances in which it is desirable to mix two or more flowing liquid streams. One such instance occurs in sewage sludge dewatering processes which require the sludge to be flocculated by dosing with a flocculant such as polyelectrolyte.
US-A-4341476, FR-A-1488593 and DE-B-1093335 all disclose apparatus comprising a generally cylindrical chamber, having an inlet and an outlet, the chamber having one or more rotatable, plate-like impellers, rotatable about the axis of the chamber and means for driving the impellers in rotation. In the French and German specifications the impellers extend generally axially of the chamber. FR-A-1488593 describes apparatus for mixing two liquid flows which comprises a cylindrical chamber having at one end an outlet for the mixture, said chamber including at least one plate-like impeller mounted on a drive shaft and rotatable about the axis of chamber and means for driving the impeller(s) in rotation. However, none of these documents describes an apparatus for mixing two liquid flows whereby the liquids are subjected to immediate and thorough mixing as they pass quickly through the apparatus.
We have now devised apparatus for mixing two (or more) liquid flows which apparatus is useful particularly but not exclusively in filter plate pressing of sludges as described in EP-A-172649.
According to the present invention there is provided apparatus for mixing two or more liquid flows which comprises a cylindrical chamber having at one end an outlet for the mixture, said chamber including at least one plate-like impeller mounted on a drive shaft and rotatable about the axis of chamber and means for driving the impeller(s) in rotation, said impeller(s) extending axially in the chamber (51) and radially outwardly of the drive shaft (54); characterised in that said chamber has one open end forming an outlet coaxial with said cylindrical chamber and has no axial inlet but at least two radial inlets in the cylindrical wall of the chamber, whereby at least two separate incoming liquids can enter the chamber simultaneously to impinge directly on said rotating impeller(s).
In use of the apparatus of the invention, the liquid flows to be mixed enter the chamber substantially radially, and exit axially. When the apparatus is used for dewatering a solids suspension as described in EP-A-172649, the suspension is mixed with flocculant by passing both radially into the cylindrical chamber with the impellers rotating therein, the mixture exiting from the open end of the chamber.
Preferably, the drive shaft is mounted substantially axially in the cylindrical chamber, and the or each impeller preferably extends longitudinally of the chamber and radially outwardly of the shaft to terminate close to the walls of the chamber.
In order that the invention may be more fully understood, reference is made to the accompanying drawings, wherein:

FIGURE 1 is an axial section of one embodiment of in-line mixer of the present invention; and
FIGURE 2 is an end elevation of the mixer of Figure 1.

Referring to the drawings, the mixer comprises a cylindrical body 50 constituting a mixing chamber 51. The chamber is open at one end 52 (the right-hand end as shown in Figure 1) to provide an axial exit for the mixed liquids. The other end of chamber 51 is closed by a low clearance impeller back-plate 53. Extending through back-plate 53 and axially into chamber 51 is a drive-shaft 54 on which is mounted a circular disc 90 and three plate- like impellers 55,56,57 each extending longitudinally of the chamber at 120° with respect to the other two (see Figure 2). As can be seen, each impeller extends radially outwardly from the shaft 54, almost to the inner wall 58 of body 50. Disc 90, at one end of the impellers (the left-hand end as viewed in Figure 1), protects the seal 63 from any debris that may be present in the suspension.
The drive shaft 54 is connected to a drive motor/gear box combination 60, passing through bearing assemblies 61,62 and a mechanical seal 63 on the back-plate 53. To cope with any small fluid leakage through seal 63, a disc 64 is mounted coaxially on shaft 54, to throw leak fluid centrifugally outwardly through drainage holes 65. Bearing assembly 61 includes a support member 70 conveniently mounted between two coaxially arranged cylindrical bodies 71,72 containing the bearing assembly 62, fling plate 64 and seal 63, as shown schematically in Figure 1.
Communicating radially with chamber 51 is a first liquid inlet pipe 80 (eg. for sewage sludge). It is to be understood that the term liquid includes not only liquids per se but mixtures, eg. of liquids and solids. Also two further liquid supply pipes 81,82 are shown (in practice, only one will normally be used). These could for example be used for flocculant or other dosant. Pipe 81 passes through the wall of pipe 80 and both terminate level with inner wall 58 of body 50. Pipe 82 is downstream of pipe 80 and is disposed radially of body 50, again terminating at inner wall 58 of body 50.
In one preferred arrangement, the appropriate diameter of inlet 80 is 200 mm and the maximum speed of drive shaft 54 is about 500 rpm.
In operation of the mixer for dosing flocculant into aqueous suspensions, suspension and flocculant (the latter in controlled quantity) are fed into chamber 51 whilst the impellers are rotated to cause mixing of the suspension and flocculant. The mixture passes out of chamber 51 via end 52, for example to the filter plate press. It is preferred to add the flocculant to the suspension either as the latter enters the mixing chamber 51 or immediately thereafter.
The motor used in the mixer of Figures 1 and 2 can be an induction motor, a variable speed reluctance motor or a DC motor. Best results are obtained from the flocculant when the motor speed is changed during a pressing cycle in accordance with the flowrate and viscosity of the suspension, i.e. when the motor speed is turned to suit the particular flocculant/suspension mixture being treated. The or each impeller is so designed as to create maximum mixing without the problem of fouling with debris such as fibres that may be present in the suspension.

Claims

Apparatus for mixing two or more liquid flows which comprises a cylindrical chamber (51) having at one end (52) an outlet for the mixture, said chamber (51) including at least one plate-like impeller (55,56,57) mounted on a drive shaft (54) and rotatable about the axis of chamber (51) and means (60) for driving the impeller(s) (55,56,57) in rotation, said impeller(s) (55,56,57) extending axially in the chamber (51) and radially outwardly of the drive shaft (54); characterised in that said chamber (51) has one open end (52) forming an outlet coaxial with said cylindrical chamber and has no axial inlet but at least two radial inlets (80,81,82) in the cylindrical wall of the chamber (51), whereby at least two separate incoming liquids can enter the chamber (51) simultaneously to impinge directly on said rotating impeller(s) (55,56,57).
Apparatus according to claim 1, wherein the or each impeller (55,56,57) extends radially outwardly of the shaft (54) to terminate close to the cylindrical wall (58) of the chamber (51).
Apparatus according to claim 2, wherein upon rotation of the shaft (54), the or each impeller (55,56,57) sweeps substantially the whole of the volume of the chamber (51).
Apparatus according to claim 2 or 3, wherein, at the other end of the chamber from the outlet (52), the chamber (51) is substantially closed by a low clearance impeller back plate (53) through which the drive shaft (54) enters the chamber.
Apparatus according to claim 4, wherein said back plate (53) includes a seal (63) around the drive shaft.
Apparatus according to claim 4 or 5, wherein a circular disc (90) is mounted radially on the drive shaft (54) in the cylinder (51) adjacent the back plate (53).
Apparatus according to any preceding claim, wherein one radial inlet (81) lies within another (80).