GB2083381A

GB2083381A - Uniflow decanter centrifuge

Info

Publication number: GB2083381A
Application number: GB8126730A
Authority: GB
Original assignee: Alfa Laval Separation AS
Current assignee: Alfa Laval Copenhagen AS
Priority date: 1980-09-09
Filing date: 1981-09-03
Publication date: 1982-03-24
Also published as: GB2083381B; IT8168177A0; DE3134935A1; IT1144763B; FR2489716B1; FR2489716A1

Abstract

A decanter centrifuge for separating a raw material into solids and two liquid phases of different densities has inlet apertures (11) for the raw material and outlet apertures (12) for solids. Each liquid phase is withdrawn intermediate the ends of the separating space (10) through a separate set of outlet tubes (13, 14) extending through a conical section (20) of the drum wall which section diverges towards the solids outlet end at a steeper angle than the preceding wall section (18), counted from the inlet end. The solids-conveyor screw (3, 4) has cut-outs in the region of the tubes (13, 14). The inwardly oriented openings of one set of tubes e.g. (13), are closer to the drum axis than the openings of the other set of tubes (14). Downstream of the tubes (13) a radial partition wall (15) is secured to the conveyor screw and the periphery of that wall is farther away from the drum axis than the outermost tube openings. The partition wall efficiently retains the lighter liquid phase on its upstream side and permits the heavier liquid phase to pass together with the solids so that each liquid phase is withdrawn with practically content of the other liquid phase. <IMAGE>

Description

SPECIFICATION Uniflow decanter centrifuge The present invention relates to a decanter centrifuge comprising a rotatable drum and a conveyor screw rotatable within the drum for transporting solids separated from a raw material introduced through inlet means at one end of the separating space within the drum, towards a solids outlet at the opposite end of the separating space, said drum comprising a first section extending from the inlet end and having a substantially cylindric or slightly tapering inner wall, a second section with an inner wall, the diameter of which increases more steeply towards the solids outlet end, and a third section with an inner wall, the diameter of which decreases towards said solids outlet end, the transporting flight or flights of said conveyor screw being interrupted in a region of the separating space defined by said second drum section, and outlet tubes for liquid separated from the raw material extending through the wall of said second drum section.

An advantage of such centrifuges, which are known from Danish Patent Specification No. 61,399, is that during the separating or clarifying process the liquid phase moves in the same direction as the raw material and the successively separated solids, so that turbulence phenomena, which would result in a reduced separating efficiency, are substantially eliminated. The rather steeply tapering section of the drum wall around the outlet tubes contributes to ensuring a continuous transportation of the separated solids along the drum wall through that region in which the screw flight or flights have been omitted in order to accommodate the outlet tubes which rotate relative to the screw.

According to the present invention a centrifuge of the kind referred to above is characterized in that two axially staggered sets of outlet tubes extend through the wall of said second drum section, that the inner openings of one set of tubes are located closer to the drum axis than the openings of the other set of tubes, and a circular partition wall having an outer radius larger than the radius to the outermost located tube openings is secured to the conveyor screw downstream of the first mentioned set of outlet tubes.

The invention utilizes the fact that the steeply tapering wall section permits a rather considerable axial length of that region in which the conveyor screw is interrupted so that this region may accommodate two sets of axially staggered liquid outlet tubes. Consequently, the invention permits an efficient separation of a raw material into a solids phase and two liquid phases of different density or specific weight. The lighter liquid phase will leave the separating space through the first mentioned set of tubes, the openings of which are located nearest to the drum axis, since the partition wall located downstream of these tubes retains that phase whereas it permits the passage, along the tapering drum wall, of the solids and the heavier liquid phase which latter is withdrawn through the other set of outlet tubes.

In this way it is ensured that each liquid phase can be withdrawn in a practically pure state without being mixed with the other liquid phase. This is especially advantageous when the lighter liquid phase has a high economic value, such as in the production of olive oil wherein it is attempted to keep the oil content in the separated pulp and the heavier liquid phase, in this case water, as low as possible. A centrifuge according to the invention may, however, also be employed in processes in which the heavier liquid phase is the most valuable product and in which it is therefore important that as little as possible of that phase is discharged together with the lighter liquid phase.In such cases it may be desirable to subject the solids to a subsequent operation in order to recover a possible residual content of the heavier liquid phase, but this operation will have been substantially simplified by the preceding efficient removal of the contaminating lighter liquid phase in the centrifuge.

According to a feature of the invention the first mentioned set of tubes may be located nearest to the inlet end of the separating space. This embodiment is particularly simple because it requires only one partition wall arranged between the two sets of outlet tubes, and thus the necessary interruption of the flight or flights of the conveyor screw is relatively short.

The invention may, however, alternatively be embodied in a centrifuge wherein the last mentioned set of tubes are located nearest to the inlet end of the separating space while upstream of those tubes and between the two sets of tubes, respectively, there are provided two additional radial partition walls, the chambers located upstream and downstream, rs spectively, of said additional partition walls communicating through tubes secured in said walls and extending substantially parallel to the drum axis at the radial level of the inner openings of the first mentioned set of tubes.

Through the tubes secured in the two additional partition walls the lighter liquid phase flows to the chamber located between the two walls remote from the inlet end and from that chamber it is withdrawn through the outlet tubes extending into the chamber whereas it is prevented from flowing into the intermediate chamber in which the openings of the other set of outlet tubes for withdrawal of the heavier liquid phase are located.

At least one set of liquid outlet tubes may be mounted in the drum wall in such a way that the radial level of their inner openings is adjustable. This feature permits, without dismantling of the centrifuge, an adjustment of the relevant liquid level or levels within the drum with due regard to the composition of the raw material being processed.

The optimum value of the apex angle of the tapering wall of the second drum section will depend inter alia on the frictional characteristics of the solids, but in most cases it should be at least 30 , and it may be considerably larger, e.g. between 60 and 80 .

The invention will now be described in more detail with reference to the somewhat schematical drawings in which Figure 1 is an axial or longitudinal section through a first embodiment of a decanter centrifuge according to the invention, and Figure 2 is a corresponding fractional view, on a larger scale, of a modified embodiment, showing that region of the drum length in which the outlet tubes are provided.

The centrifuge illustrated in Fig. 1 comprises a drum generally designated by 1, and a coaxial inner rotor 2 to which is secured a double-start conveyor screw, the flights or turns of which are designated by 3 and 4, respectively. At its ends drum 1 is rotatably supported in schematically shown bearings 5 and 6. Through the stub shaft 7 and 8, respectively, the drum and the rotor are coupled to a drive mechanism (not shown) adapted to cause the two component parts to rotate at different rpm's. Due to the differential rotation of the drum and rotor, which corresponds to a relative rotation of rotor 2 in the direction shown by arrow 9, screw 3, 4 effects a transportation of solids, which during the centrifuging operation are separated in the separating space 10 defined between the rotor and the inner wall of the drum, towards the left-hand end of Fig. 1.

A raw material to be separated into a solids phase and two liquid phases of mutually different densities, is supplied through the hollow rotor 2 from which it flows into the separating space through apertures 11 located in the vicinity of one end of the separating space. From there the material flows along the inner side of the drum wall towards the opposite end of the separating space, and during its movement the material is separated into a solids phase transported by screw 3, 4 and finally discharged through apertures 1 2 at the drum end, and said two liquid phases.

For withdrawing the liquid phases two sets of axially staggered or offset outlet tubes 1 3 and 14, respectively, are mounted in the drum wall. Each set comprises a plurality of tubes distributed along the circumference of the drum. Between the sets of tubes 1 3 and 14 a circular ring-shaped partition wall 1 5 is secured to rotor 2. In view of the previously mentioned relative rotation of conveyor screw 3, 4 and drum 1 each screw flight is, as shown, interrupted on a part of its axial length so that the outlet tubes 1 3 and 14 can rotate freely between the terminal edges of the flight portions.

Drum 1 is composed of three sections, viz.

first-counted from the raw material inlet end-a cylindric section 1 7b having a cylindric inner wall 18, then a conical section 1 9 with an inner wall 20, the diameter of which increases rather steeply towards outlet apertures 12, and finally a drum section 21 with a conical inner wall 22 of opposed taper.

The two sets of outlet tubes 1 3 and 14 are mounted in the conical drum section 19, the axial length of which is the same as or slightly larger than the length of that region of the separating space in which screw flights 3 and 4 are interrupted. With a view to the desired transporting action of the screw the contour the flights is adapted in such a way to the sections 18, 20, and 22 of the drum wall that the gap between the screw and the drum wall is rather narrow, normally between 0.5 and 2 mm dependent on the drum diameter.

The rather steep inclination of the conical wall portion 20 towards outlet apertures 1 2 ensures that within this region the solids slide outwardly along the wall section towards the lowermost point thereof, i.e. the transition between section 20 and the following section 22, solely under the influence of the centrifugal force. When said lowermost point is loca texas shown-shortly downstream of the point where the screw flights commence after the interruption, the screw, can readily get hold of the solids and transport them further on to apertures 1 2 without any risk of jamming or clogging due to accumulated solids.

As shown in Fig. 1 the outlet tubes 1 3 nearest to inlet apertures 11 are mounted in the drum wall in such a way that their inwardly oriented openings, which determine the liquid level 23 upstream of partition wall 15, are closer to the rotational axis of the drum and rotor than the openings of outlet tubes 14 which latter determine the liquid level 24 downstream of wall 1 5. The outlet tubes are preferably mounted in the drum wall for replacement and/or radial adjustment so that levels 23 and 24 can be altered accord- s ing to desire or need.

Since the outer periphery of partition wall 1 5 is located radially outside the inwardly oriented openings of outlet tubes 14 and hence also outside the openings of tubes 13, the partition wall ensures that only the relatively heavier liquid phase and the solids can move past the partition wall to the final part of separating space 10 from which the heavier liquid phase is withdrawn through tubes 14 while the lighter liquid phase is withdrawn through tubes 1 3.

In Fig. 2 those component parts of the centrifuge, which correspond to, and serve the same functions as, component parts in the embodiment described above, have been designated by the same reference numerals. The axial positions of the two sets of outlet tubes 1 3 and 1 4 have been reversed so that tubes 1 3 serving for the withdrawal of the lighter liquid phase are farthest away from the inlet end (not shown) of separating space 10. Partition wall 15, which is secured to rotor 2 and which has to retain the lighter liquid phase on its upstream side in order to permit the withdrawal thereof through tubes 13, is-as in Fig. 1-located downstream of tubes 1 3.

Between the two sets of tubes 1 3 and 14 an additional partition wall 25 is secured to the rotor and a third partition wall 26 is secured to the rotor upstream of tubes 14. At their ends a suitable number of connecting tubes 27, which extend in parallel to the drum axis, are sealingly secured in walls 25 and 26 at the level 23 of the lighter liquid phase. Tubes 27 thus provide an unimpeded communication for the flow of the lighter liquid phase from that part of separating space 10 which is nearest to the inlet end, i.e.

to the right of partition wall 26, to chamber 28 defined between walls 1 5 and 25. Partition wall 26 extends so far outwardly towards the conical inner wall 20 of drum section 1 9 that the lighter liquid phase is at the same time prevented from flowing into the second chamber 29 defined between walls 25 and 26. The heavier liquid phase can, however, flow freely into that chamber through the gap between partition wall 26 and the drum wall so that it can be withdrawn from the chamber through outlet tubes 14, the inwardly oriented openings of which determine the level 24 of that liquid phase, similar to the embodiment of Fig. 1.

While in both embodiments the first section 1 7 of the drum has been shown with a cylindric inner wall it should be understood that if desired the wall could be slightly conical, but in any event its generatrix will include a smaller angle with the drum axis than the generatrix of the subsequent wall section through which the liquid outlet tubes extend.

Claims

1. A decanter centrifuge comprising a rotatable drum and a conveyor screw rotatable within the drum for transporting solids separated from a raw material introduced through inlet means at one end of the separating space within the drum, towards a solids outlet at the opposite end of the separating space, said drum comprising a first section extending from the inlet end and having a substantially cylindric or slightly tapering inner wall, a second section with an inner wall the diameter of which increases more steeply towards the solids outlet end, and a third section with an inner wall, the diameter of which decreases towards said solids outlet end, the transporting flight or flights of said conveyor screw being interrupted in a region of the separating space defined by said second drum section, and outlet tubes for liquid separated from the raw material extending through the wall of said second drum section, characterized in that two axially staggered sets of outlet tubes extend through the wall of said second drum section, that the inner openings of one set of tubes are located closer to the drum axis than the openings of the other set of tubes, and that a circular partition wall having an outer radius larger than the radius to the outermost located tube openings is secured to the conveyor screw downstream of the first mentioned sets of outlet tubes.

2. A decanter centrifuge as claimed in claim 1, characterized in that the first mentioned set of tubes are located nearer to the inlet end of the separating space than the other set of tubes.

3. A decanter centrifuge as claimed in claim 1, characterized in that the last mentioned set of tubes are located nearer to the inlet end of the separating space, that upstream of those tubes and between the two sets of tubes, respectively, there are provided two additional radial partition walls and that the chambers located upstream and downstream, respectively, of said additional partition walls communicate through tubes secured in said walls and extending substantially parallel to the drum axis at the radial level of the inner openings of the first mentioned set of tubes.

4. A decanter centrifuge as claimed in any of claims 1-3, characterized in that at least one set of outlet tubes is mounted in the drum wall in such a way that the radial level of their inner openings is adjustable.

5. A decanter centrifuge as claimed in any of claims 1-4, characterized in that the apex angle of the tapering wall of the second drum section is at least 30".

6. A decanter centrifuge as claimed in claim 5, characterized in that said apex angle is between 60 and 80 .

7. A decanter centrifuge substantially as described herein with reference to and as illustrated in Fig. 1 or Fig. 2 of the accompanying drawings.