WO2012080931A2

WO2012080931A2 - Feed mechanism

Info

Publication number: WO2012080931A2
Application number: PCT/IB2011/055605
Authority: WO
Inventors: Alexei Egmar Krass De Krassnokutski
Original assignee: Roymec Technologies (Proprietary) Limited
Priority date: 2010-12-15
Filing date: 2011-12-12
Publication date: 2012-06-21
Also published as: WO2012080931A3

Abstract

The invention relates to a feed mechanism suitable for use in introducing a feed flow into a process vessel. More particularly, but not exclusively, the invention relates to a feed mechanism suitable for use as a feed well for a thickener. The feed mechanism includes a feed well chamber for receiving the feed, the feed well chamber being of cylindrical configuration and having an operatively top end and operatively bottom end; and a flow guiding arrangement comprising a plurality of blades located at least partially below the bottom end of the feed well chamber.

Description

FEED MECHANISM

BACKGROUND OF THE INVENTION

This invention relates to a feed mechanism suitable for use in introducing a feed flow into a process vessel. More particularly, but not exclusively, the invention relates to a feed mechanism suitable for use as a feed well for a thickener.

In this specification the feed mechanism will generally be described with reference to the use thereof as a feed well for a thickener. However, it should be appreciated that the invention is not limited to this particular application, and that it will be useful in many different applications, especially those where a high velocity feed is introduced into a relatively small process vessel.

Thickeners are separation devices for separating solids from suspensions and are used in the mining, metallurgical, pollution control, water treatment and chemical industries.

A thickener typically comprises a circular tank into which a liquid containing suspended solids is pumped, with a holding area large enough to allow the solids time to settle to the bottom. Once the solids have settled to form a sludge, scraper blades, diagonally attached to rotating rake arms push the sludge to an exit point at the bottom centre of the thickener. Usually, thickeners include a feed well that receives the liquid containing suspended solids which it distributes into the thickener body or annulus. Conventional feed wells may be open at the bottom (open feed well) or may include a deflector mounted underneath or in the feed well to direct the flow horizontally and radially outwards and with the intention to achieve uniform radial flow towards the overflow collection system (closed feed well). Turbulence, rotation of the fluid body or recirculation cells forming within the body of the thickener are undesirable as they impede settling particles from falling towards the floor of the thickener. The exiting flow from conventional feed wells possesses a relatively large component of kinetic energy in the form of (i) a tangential or swirling velocity and (ii) a radially directed recirculation velocity, in which flow adjacent to the floor of the thickener is flowing radially outwards, whereas the flow at the surface flows inwards. Both these adverse flow components also result in short-circuiting whereby a measurable portion of the incoming stream reports directly to the discharge without being mixed with supernatant or flocculent in the feed well. Some zones of the thickener will be quiescent. Flow exiting the feed well with surplus energy results in high outward radial velocities at the level of the floor of the thickener, observable components of inward velocity towards the top of the thickener and observable tangential circulation components in the body of the thickener. In some thickeners, flow exiting the feed well will scour the floor of the feed well, entraining settled solids and driving solids away from the centre of the thickener towards the overflow collection system. The flow exits a conventional feed well at high velocity in the order of 5 - 10 m/min.

The supernatatant level is the normal operating level of the thickener outside the feed well defined by the level of the overflow collection system.

It is an object of the invention to provide an improved feed mechanism introducing a process feed into a process vessel. It is also an object of this invention to provide an improved feed mechanism for a thickener.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a feed mechanism for use in introducing a process feed into a process vessel, the feed mechanism including:

a feed well chamber for receiving the feed, the feed well chamber being of cylindrical configuration and having an operatively top end and operatively bottom end; and

a flow guiding arrangement comprising a plurality of blades located at least partially below the bottom end of the feed well chamber.

There is provided for the blades to extend from the bottom end of the feed well chamber.

The blades may also extend beyond an outer perimeter of the feed well chamber.

Preferably, the blades extend downwardly from the bottom end of the feed well chamber.

More preferably, the blades are at least partially inclined relative to a longitudinal axis of the feed well chamber.

There is provided for the blades to be inclined radially inwardly when viewed from bottom to top. There is furthermore provided for each blade to be orientated in an at least partially radial direction when viewed in cross-section.

A further feature provided for the blades to be arcuate when viewed in cross-section.

The feed well chamber may have a top shelf, preferably an annular top shelf, at the top end of the side wall, which defines a flow restricting orifice, preferably a circular orifice, to contain the inlet stream flow profile and to control the incoming supernatant flow rate overflowing into the feed well chamber. The level of the shelf may be above, at the level of the supernatant in the thickener or may be lower than the supernatant level.

The feed well chamber may have a bottom shelf, preferably an annular bottom shelf, at the bottom end of the side wall, which defines an outlet orifice, preferably a circular outlet orifice, out of the feed well chamber and from which the flow guiding arrangement extends.

Preferably, the feed well chamber comprises at least one feed inlet which is arranged to feed the feed into the feed well chamber tangentially relative to the side wall.

According to a second aspect of the invention, there is provided a feed mechanism for a thickener, whereby splashing and air entrapment of the feed stream in the feed well is controlled and reduced by a feed well top and bottom shelf wherein:

the orifice of the top shelf of the feed well is smaller or equal in diameter to the office in the bottom shelf of the feed well; and the top shelf of the feed well chamber is suspended 0 to 100 cm below a supernatant liquid level in the thickener during normal operation of the thickener. Supernatant will overflow the top shelf and enter into the annulus to mix with the incoming feed stream containing solids. The mixed stream will leave the bottom annulus.

According to a third aspect of the invention, there is provided a method of removing solids from a stream containing suspended solids in a thickener using the feed mechanism described above.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting embodiments of the invention are described by way of example only, and with reference to the accompanying figures in which: is a cross sectional side view of a feed mechanism for a thickener in accordance with first embodiment of the invention;

Figure 2 is a bottom view of the feed mechanism illustrated in

Figure 1 ;

Figure 3 is a perspective view of a feed mechanism for a thickener in accordance a further embodiment of the invention;

Figures 4 (a) to (c) illustrate a number of adverse flow conditions that may be encountered in feed wells;

Figure 5 (a) to (i) show CFD comparisons for various feed well designs; Figure 6 shows CFD time lapse series of flow trajectories for various feed well designs;

Figure 7 shows CFD and scale model experimental normalized cumulative residence-time distributions; and

Figures 8(a) and (b) show vertical velocities in thickener bodies with throttled (a) and non-throttled (b) feed pipes just before the new feed well.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to Figures 1 to 3, in which like reference numerals indicate like features, a feed mechanism according to the invention is shown generally by the numeral 10. In use, the feed mechanism 10 is located centrally within a thickener tank (not shown), with the drive shaft 12 of a rake mechanism (not shown), extending through the centre of thereof.

As pointed out above, the feed mechanism will generally be described with reference to the use thereof as a feed well for a thickener. However, it should be appreciated that the invention is not limited to this particular application, and that it will be useful in many different applications, especially those where a high velocity feed is introduced into a relatively small process vessel, such as for example filter vessels and clarifiers.

The feed mechanism 10 comprises a feed well chamber A, and a flow guiding arrangement B depending from the lower end of the feed well chamber A. The feed well chamber A is defined by a cylindrical sidewall 14 having an operatively top end supporting a top annular shelf 16, an operatively bottom end supporting a bottom annular shelf 18, and has at least one tangentially orientated inlet pipe 20.

The top annular shelf 16 defines a circular orifice 17 into the feed well chamber A. The bottom annular shelf 18 defines a circular orifice 19 out of the feed well chamber A. The top circular orifice 17 is smaller than or equal in diameter to the circular orifice 19 of the feed well chamber A. The top and bottom annular shelves 16 and 18 respectively, may be flat or may be conically sloped and may be perforated with a succession of holes about the circumference.

The flow guiding arrangement B comprises an arrangement of between six and sixty blades 22 suspended and extending downwardly and at least partially radially from the bottom end of the feed well chamber A. The blades 22 may protrude beyond the sidewall 14 of the feed well chamber A, and may hang vertically or be radially sloped inwards or outwards from the centre annulus. In the preferred embodiment shown in Figure 3 the blades are radially inwardly sloping towards a longitudinal axis of the feed well chamber when viewed from top to bottom. It should be noted that the salient aspect it for the plates to be positioned in a zone below the feed well chamber, and even blades extending from the side of the feed well chamber will therefore be able to meet this criteria if suitably configured.

The blades 22 may be flat plates (as seen in Figure 1 and 2) or curved plates (as seen in Figure3), and will preferably depend downwardly from the bottom end of the feed well chamber. It is however also foreseen that it may be possible for the blades to be at least partially helically shaped in order to resemble a large screw. The base of the flow guiding arrangement B may include a floor 24 which may be a plate, an annular ring or an arrangement of flat plates around a circumference of the blades 22. The floor 24 may be suspended from the flow shaping device B, or may be mounted on the drive shaft 12 passing through the centre of the feed mechanism 10. Alternatively, as is seen in the embodiment of Figure 3, the bottom end of the flow guiding arrangement may be open.

In one embodiment of the invention, the feed well chamber A has an internal diameter of 12-25% of the thickener diameter, and the side wall 14 has a height of 10-100% the feed well diameter. The circular orifice 17 in the top shelf 16 has a diameter of 30-60% the feed well diameter. The circular orifice 19 in the bottom shelf 18 has a diameter of 50-80% the feed well diameter. The blades 22 of the flow shaping device B have a length of 80-120% the sidewall height, and a width of 10-30% feed well diameter.

In use, the feed mechanism 10 is installed in a thickener with the top shelf 16 of the feed well chamber A suspended 0 to 100 cm below the supernatant liquid level 26 during normal operation of the thickener. A partially flocculated feed containing suspended solids 28 enters the feed well chamber A through the at least one feed inlet pipe 20. The feed inlet pipe 20 is arranged to feed the flocculated feed 28 into the feed well chamber 20 tangentially relative to the side wall 14. The feed well chamber A geometry assists the incoming feed to mix thoroughly with the injected flocculent. By limiting the orifice 17 in the top shelf 16 to smaller than or equal to the diameter of the orifice 19 in the lower shelf 18, splashing and air entrainment are significantly reduced. The effect of the top shelf 16 and bottom shelf 18 is to constrain the incoming feed stream 28 and to allow mixing to take place. A mixed stream exits axially downwards from the feed well chamber A through the circular orifice 19 of the bottom shelf 18 (which defines the floor of the feed well chamber A), and into the flow guiding arrangement B. The radially mounted arrangement of blades 22 that depend from the feed well chamber A convert the entering feed into a stable, uniform and radial flow 30 which exits the flow shaping device B. The arrangement of radially-mounted blades 22 suspended from the feed well chamber A slow the incoming stream and dissipate excess kinetic energy. The radial blades 22 also direct the flow 30 horizontally outwards with a low vertical velocity component. Seen from above, the exiting flow 30 resembles a source flow with little or no tangential velocity component, while seen from the side, the flow 30 is uniformly distributed from top to bottom of the radial plates, resembling a radial plug flow with little or no vertical velocity.

In the case where the flow guiding arrangement B includes a floor 24, the floor 24 corrects any remaining vertical velocity component in a centre hollow chamber of the flow shaping device B. It is evident that the flow guiding arrangement B, which does not have a side wall, is located outside the feed well chamber A, and makes a large discharge area available for reduced radial velocity.

The feed mechanism of this invention is a combination of a thickener feed well, and a flow guiding device suspended from the bottom of the feed well. The combined assembly stimulates the recirculation of thickener supernatant liquor back into the feed well chamber to dilute the feed and directs the exiting flow radially.

EXPERIMENTAL RESULTS

In order to validate the above design, various industrial standard feed wells of various configurations were constructed and tested in a scale model thickener. In particular the performance of the new design was compared to that of an open feed well and a closed feed well, both of which are known in the art. It should be noted that operating a scale model thickener with only water is equivalent to operating an industrial thickener without an established pulp-bed or hindered settling zone, i.e. a thickener at initial start-up. Although this view may be limited in that factors such as hindered settling zone feed injection are not considered, the inventor believes that this technique establishes the fundamental fluid dynamics within a thickener. Justifying this statement is the observation that the fluid dynamics remained largely invariant when the scale thickener was retrofitted with a rotating rake arms and resin was used as a flocc and pulp- bed emulator.

Initially, flow patterns were established in the scale model with dye tracer tests. Three major adverse conditions were observed when testing an open feed well, namely tangential swirl (Figure 4a), radial recirculation (Figure 4b) and flow asymmetry (Figure 4c).

Tangential swirl can be seen on the top of almost all operational thickeners. The disadvantage of this flow regime is that if the tangential swirl is too great, then floccs may be scoured off the pulp bed back into solution. Furthermore, due to centripetal forces, floccs are flung further radially outwards in the thickener than for a non-rotating fluid body, and rake arms then need to do more work in drawing the pulp back to the centre. Unlike tangential swirl, radial recirculation and flow asymmetry cannot be seen on the surface of an operational thickener. However, similar to tangential swirl, radial recirculation also transports floccs to the outer extremities of the thickener at which point some floccs settle and others are washed upwards along the thickener wall and short circuit directly into the launder (see Figure 4b). Moreover, again like tangential swirl, high shear stresses on the pulp bed can scour already settled floccs back into solution which may then also short-circuit.

Flow asymmetry is an observation of the variation in velocities, or energy, at similar radii around the thickener body and is expressed mainly in the form of radial recirculation intensity. For example, when testing the open feed well it was observed that there was far greater radial recirculation in the thickener body opposite the feed pipe entry point than adjacent to it. When testing the scale model with an emulated pulp bed flow asymmetry was observed as flocc souring and short circuiting in the high energy zone and a relatively dead zone at the low energy zone.

When testing the closed feed well, it was observed that the tangential swirl was marginally reduced compared to the open feed well. It was also found that the energy asymmetry was eliminated, i.e. there was a uniform radial recirculation around the thickener.

All these undesirable conditions were eliminated or at least substantially reduced using the new feed mechanism. This was seen in the scale model experiment, but is also apparent from a CFD analysis described in more detail below. A Computational Fluid Dynamics (CFD) analysis of a 0 21m thickener operating at a rise rate of 4 m/h was run in Flo.EFD™. Similar to the early scale model experimentation, this model used water as a working fluid with no underflow and in which rake arms were omitted.

Velocities and shear stresses

Various feed well were tested in the above thickener under identical conditions. These feed wells included the open, the closed feed well and the new feed well, the results of which are shown in Figure 5. When comparing the velocity magnitudes (colour contours) of Figure 5 a), b) and c), it can be see that the velocities in thickener body using the open feed well is greatest, while that of the new feed well is significantly less than both the open and closed feed wells. This indicates that the new feed well dissipates the greatest amount of the incoming feed's kinetic energy and gently introduces the feed/flocculent mixture into the thickener body. Fluid velocity in the thickener body impedes flocc settling and therefore the quiescent zone surrounding the new feed well allows for floccs to settle out of solution more quickly. More importantly, this regime allows for a greater number of floccs to settle per unit time, i.e. achieve greater flux rate. An alternative way to view the velocity magnitudes is to view the velocity iso-surfaces in the thickener body as shown in Figure 5 d), e) and f). Again, it is apparent the new feed well dissipates almost all the feed's kinetic energy within the feed well as compared with the other feed wells. Figure 5 d), shows a large velocity iso-surface "tail" extending along the thickener floor opposite to the feed pipe entry point. This asymmetric velocity distribution confirms the scale model observations of a flow asymmetry with greatest velocities opposite to the feed pipe entry). According to the CFD calculations, the new feed well dissipates twice the energy of a closed feed well and up to 5 times the energy of the open feed well. Finally Figure 5 g), h) and i) show the shear stress on thickener floor. It can be seen that the open feed well induces a patch of high shear on the floor that induced by the high velocity tail. During scale model experimentation, it was observed that floccs (resin) were sheared from the pulp-bed in this area which then were carried by the high velocity (Figure 5d) radially outwards where many floccs short-circuited directly to the launder.

Flow trajectories

A time-lapse series of CFD images of flow trajectories is shown for various feed wells in Figure 6. The flow trajectories are coloured with velocity magnitude limited to 0.3 m/s (faster velocities will also only appear as red). Again, it is apparent how effectively the new feed well dissipates the feed's kinetic energy (reduces its velocity) when compared to the other feed well designs. It can also be seen that the flow from the open feed well is the first to reach the launder at approximately 150 seconds. On the other hand, the flow trajectories from the closed feed well all seem to reach the launder simultaneously at about 200 seconds (not shown) at which point most trajectories flow into the launder. The flow trajectories from the new feed well only appear to reach the launder at approximately 500 seconds. This radical increase in residence time maximizes the probability of floccs settling out of solution.

It is also interesting to note in Figure 6 that the flow trajectories emanating from the new feed well begin to tangentially swirl in a clockwise direction which opposes the counter clockwise direction in which it was injected into the feed well. The magnitude of this tangential swirl is small and opposite to both that induced by open and closed feed wells and should easily be counteracted by the swirl induced by the rake arms. Hence, the rake arms would be set to rotate in the same direction as the feed injection into the feed well.

Residence-times

Another technique used to show the agreeable correlation between scale model experimentation and CFD simulations is through cumulative residence-time distribution curves derived from time-concentration curves. As the name suggests, time-concentration curves are derived by adding a tracer into a thickener's feed pipe at zero time and measuring the concentration of the flow exiting the thickener with respect to time.

For scale model experimentation this was achieved by introducing 2I of saturated salt solution into the feed pipe just before the feed well, starting a stop watch and measuring the conductivity as the fluid left the thickener. On the other hand, for CFD analysis, 500 particles were introduced into the feed pipe and the residence time of these particles was recorded as they left the thickener.

A CFD normalized cumulative residence-time distribution curve is constructed by dividing the number of particles that passed through the thickener up to a given time, through by the total number of particles. Hence at a normalized cumulative residence time distribution of 1 means all the particles have passed through the thickener. In a similar manner, an experimental normalized cumulative residence time distribution is constructed by measuring the cumulative concentration up to a given time by the total cumulative concentration.

The normalized cumulative residence-time distribution curves are plotted with respect to normalized time, or t_n = tQ/V where V is the thickener volume. Hence, at t_n = 1, all particles composing a plug flow will have left the thickener. The normalized cumulative residence-time distribution times for both CFD and scale model results, for the open, closed and new feed wells are shown in Figure 7. From Figure 7 it can be seen that experimental (scale model) and CFD results for all feed well designs correlate relatively well. For both CFD and experimental results, the open feed well concentration curve develops the fastest (t_n = 0.04 for Exp and t_n = 0.06 for CFD), but is soon overshot by that of the closed feed well. This is indicative that the particles from the open feed well reach the launder first as can be seen in Figure 7, whereas the particles from the closed feed well reach the launder afterwards, but at a higher concentration (more exiting flow trajectories).

Possibly the most informative variable that can be drawn from Figure 7 is the feed well half time or the time taken for half the particles to exit the thickener. For the open feed well, this is between t_n = 0.22 and t_n = 0.32 (for CFD and experimentation respectively), the closed feed well is worse at t_n = 0.1 and t_n = 0.2 (for CFD and experimentation respectively), and the new feed well is best at t_n = 0.6 and t_n = 0.7 (for CFD and experimentation respectively). Hence the new feed well maximizes the residence time for the majority of particles introduced into the thickener which hence maximizes the probability of a flocc settling minimizes the possibility of short circuiting. 1 055605

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Bed injection

It may be argued that while the new feed well performs impressively in the above CFD analysis and scale model testing whilst using a uniform liquid (water) as a working fluid, it may not perform well in industrial thickeners with established pulp-beds and hindered settling zones. This is a valid argument as the floccs may skim along the hindered settling zone interface and short circuit to the launder. In fact, the success of closed feed well design is largely attributable to its uniform feed injection into the hindered settling zone which effectively acts like a porous filter for floccs.

In order to inject into the hindered settling zone a radial recirculation needs to be established. However, optimizing the degree of radial recirculation is a fine balancing act; if the injection pressure is too weak, feed will not penetrate the hindered settling zone and the feed well will overflow, and if too strong, floccs composing the hindered settling zone will be scoured back into solution. Considering that the injection pressure (radial recirculation magnitude) for both the open and closed feedwells is directly proportional to the operating flow rates, there is little to no practical control over this parameter. On the other hand, it has been found that by simply varying the feed velocity into the new feed well, via a throttling valve in the feed pipe just before the feed well, the degree of radial recirculation in the thickener can be accurately controlled. This is shown in Figure 8 a) and b) which both show a thickener operating with a rise rate of 4 m/h, but in which b) has been throttled to reach a feed well injection velocity of 3 m/s as opposed to the non-throttled velocity of 1.5m/s in a). The magnitude of the radial recirculation in Figure 8 b) is approximately equivalent to that of the closed feed well, which has shown to be adequate for hindered settling zone injection by the closed feed well. The prospect of controlling the by simply controlling a throttling valve suggests the potential to use this valve as a dynamic control parameter which may be linked to pulp-bed measuring devices. T IB2011/055605

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With promising CFD and scale model results, the applicant commissioned a pilot plant. This plant used a 02.4 m plastic tank for the thickener body which incorporated transparent plastic windows and a submerged orifice launder feed system. Both an open feed well and new feed well were constructed to be used in this pilot plant. Field trials were carried out at Gold One™ in Springs, Gauteng, South Africa. Although tests are still ongoing, however, preliminary results indicate that when using the new feed well a rise rate of 8 m/h is possible at a fiocculent dosing ratio of 20g/ton. In contrast, in order to achieve satisfactory operation at the same rise rate when using the open feed well, the fiocculent dosing ration would need to be multiplied by 4 times to 80g/ton. Furthermore, through the windows it was noted that the velocities within the thickener body were greatly reduced when operating with the new as opposed to the open feed well. Moreover, flocc concentrations viewed through the windows were lower when using the new as opposed to the open feed well; this indicates that the majority of floccs settled to the thickener floor before reaching the circumferential windows.

Claims

CLAIMS:

1. A feed mechanism for use in introducing a process feed into a process vessel, the feed mechanism including:

a feed well chamber for receiving the feed, the feed well chamber being of cylindrical configuration and having an operatively top end and operatively bottom end; and a flow guiding arrangement comprising a plurality of blades at least partially below the bottom end of the feed well chamber.

2. The feed mechanism of claim 1 in which the blades extend from the bottom end of the feed well chamber.

3. The feed mechanism of claim 1 or claim 2 in which the blades extend beyond an outer perimeter of the feed well chamber.

4. The feed mechanism of any one of claims 1 to 3 in which the blades extend downwardly from the bottom end of the feed well chamber.

5. The feed mechanism of claim 4 in which the blades are at least partially inclined relative to a longitudinal axis of the feed well chamber.

6. The feed mechanism of claim 5 in which the blades are inclined radially inwardly when viewed from bottom to top.

7. The feed mechanism of any one of claims 1 to 6 in which each blade is orientated in an at least partially radial direction when viewed in cross-section.

8. The feed mechanism of any one of claims 1 to 7 in which the blades are arcuate when viewed in cross-section.

9 The feed mechanism of any one of the preceding claims in which the feed well chamber includes an annular top shelf at the top end thereof, with an opening in the top shelf defining a flow restricting orifice to contain the inlet stream flow profile and to control the incoming supernatant flow rate overflowing into the feed well chamber.

10. The feed mechanism of any one of the preceding claims in which the feed well chamber includes an annular bottom shelf at the bottom end thereof, with an opening in the bottom shelf forming an outlet orifice, and with the flow guiding arrangement extending from the bottom shelf.

11. The feed mechanism of any one of the preceding claims in which the feed well chamber includes at least one feed inlet which is arranged to feed the feed into the feed well chamber tangentially relative to the side wall.

12. The feed mechanism of any one of the preceding claims in which the process vessel is a thickener.

13. A feed mechanism for a thickener in which splashing and air entrainment of the feed stream in the feed well is controlled and reduced by a feed well top and bottom shelf characterised in that the orifice of the top shelf of the feed well is smaller or equal in diameter to the office in the bottom shelf of the feed well.

14. The feed mechanism of claim 13 in which the top shelf of the feed well chamber is suspended 0 to 100 cm below a supernatant liquid level in the thickener during normal operation of the thickener.

15. A method of removing solids from a stream containing suspended solids in a thickener using the feed mechanism as claimed in any one of claims 1 to 14.

16. A thickener including a feed mechanism as claimed in any one of claims 1 to 14.