METHOD OF PRODUCING PRILLS

Abstract

Method of producing prills includes providing a hollow body rotatable about a first axis, the body having a wall rotationally symmetrical around the first axis forming an interior space, the wall including nozzles; providing a second body disposed in the hollow body such that a gap exists between the hollow body and the second body; supplying liquid to the gap; generating jets of liquid from the nozzles radially outward by driving the rotational motion of the hollow body and/or second body around the first axis of rotation using a rotary drive unit; applying a reciprocal pressure excitation on the jets of liquid by moving the hollow body and/or second body along the first axis; and decoupling the rotations of the one of the hollow body and second body and a reciprocating drive-unit.

Claims

1. A method of producing prills, comprising the steps of: providing a hollow body arranged to rotate about a first axis of rotation, the hollow body comprising a wall that is arranged rotation symmetrically around the first axis, thereby enclosing an interior space, the wall being provided with a plurality of through-holes forming nozzles; providing a second body being shaped to fit into the interior space of the hollow body, nesting the second body inside the hollow body, such that a gap is obtained between an inner surface of the wall of the hollow body and an outer surface of the second body; supplying a flow of liquid to the gap through a liquid inlet that is in liquid connection with the gap; generating jets of liquid from the nozzles in at least a radially outward direction with respect to the first axis by driving the rotational motion of at least one of the hollow body and second body around the first axis of rotation using a rotary drive unit; applying a reciprocal pressure excitation on the jets of liquid by moving, using a reciprocating drive unit, one of the hollow body and second body with respect to the other of the hollow body and second body along the first axis of rotation; and decoupling the rotations of the one of the hollow body and second body and the reciprocating drive-unit.

2. The method of producing prills according to claim 1, wherein the step of decoupling the rotations comprises: providing a coupling mechanism between the reciprocating drive-unit and the one of the hollow body and second body; and enabling, by means of the coupling mechanism, relative rotations between the one of the hollow body and second body and the reciprocating drive-unit.

3. The method of producing prills according to claim 2, wherein the coupling mechanism comprises a first rotational bearing unit and a second axis of rotation, wherein the step of providing the coupling mechanism comprises coupling a lower end of the reciprocating drive unit to a first part of the first rotational bearing unit and coupling the one of the hollow body and second body to a second part of the first rotational bearing unit, and; wherein, during the step of applying a reciprocal pressure excitation, a lower end of the reciprocating-drive unit moves in a direction substantially parallel to the second axis between a first and second position; and wherein the step of enabling relative rotations comprises rotating the second part of the rotational bearing unit with respect to a first part of the first rotational bearing unit around the second axis of rotation.

4. The method of producing prills according to claim 1, wherein the provided hollow body is at least partially substantially cylindrically and/or conically shaped, wherein the interior space is at least partially substantially cylindrically or conically shaped, and wherein the provided second body is shaped substantially similar to the interior space of the hollow body, such that a width of the obtained gap is substantially constant along an entire circumference of the second body.

5. The method of producing prills according to claim 4, comprising the step of coaxially arranging the hollow body, second body and the first rotational bearing unit, such that the first and second axes of rotation coincide and the hollow body, second body and the first rotational bearing unit rotate around the same axis of rotation.

6. The method of producing prills according to claim 1, further comprising the step of controlling the reciprocating drive-unit, using a controller, to move the one of the hollow body and second body with respect to the other of the hollow body and second body with a predefined frequency and amplitude of motion.

7. The method of producing prills according to claim 1, further comprising: coupling the reciprocating drive-unit to a frame assembly and suspending the coupling mechanism in the axial direction from the reciprocating drive-unit; and arranging a second rotational bearing unit, which is arranged to rotate around a third axis of rotation, between the reciprocating drive-unit and the frame, wherein the third axis of rotation is substantially parallel with the second axes of rotation.

8. (canceled)

9. The method of producing prills according to claim 1, further comprising: blocking, by means of a rotational blocking mechanism, substantially any rotation of reciprocating drive-unit around the second axis of rotation; and providing the blocking mechanism with a blocking pin for blocking the rotation.

10. (canceled)

11. The method of producing prills according to claim 1, further providing a stacked piezo-electric element in the reciprocating drive-unit, and contracting and/or expanding the stacked piezo-electric element in a direction substantially parallel to the first axis for moving the one of the hollow body and second body with respect to the other of the hollow body and second body

12. The method of producing prills according to claim 1, further comprising preloading the reciprocating drive-unit using a biasing mechanism; wherein the step of preloading the reciprocating drive-unit using a biasing mechanism comprises suspending the coupling mechanism and the one of the hollow body and second body from the reciprocating drive-unit for applying a tensile preload to the reciprocating drive-unit.

13. (canceled)

14. The method of producing prills according to claim 1, further comprising providing a shaft-assembly comprising a first and second shaft, and arranging the second shaft between the coupling mechanism and the one of the hollow body and second body and arranging the first shaft between the rotary drive unit and the other of the hollow body and second body.

15. The method of producing prills according to claim 14, further comprising the step of arranging the first and second shafts coaxially.

16. The method of producing prills according to claim 14, comprising the step of arranging the first shaft to at least partially enclose the second shaft in the radial direction, or arranging the second shaft to at least partially enclose the first shaft in the radial direction.

17. The method of producing prills according to claim 14, wherein the step of providing a shaft-assembly further comprises providing a third bearing system, arranging the third bearing system in between the first and second shafts, wherein the third bearing system comprises at least a linear bearing member; wherein the method further comprises the step of moving the second shaft with respect to the first shaft in the axial direction while moving, using a reciprocating drive unit, one of the hollow body and second body with respect to the other of the hollow body and second body along the first axis of rotation.

18. The method of producing prills according to claim 1, further comprising the steps of: arranging a rotational transfer mechanism between the hollow body and second body; and coupling the rotational motion of the hollow body and second body around the first axis by means of a rotational transfer mechanism.

19. The method of producing prills according to claim 1, wherein the step of supplying a flow of liquid to the gap comprises supplying the liquid through the liquid inlet that debouches in the second body and subsequently through at least one through hole of the second body that forms a liquid connection between the gap and the liquid inlet; the method further comprising: arranging a primary through hole that runs substantially parallel to the first axis and debouches in a lower section of the hollow body; and supplying the liquid through the primary through hole to the lower section of the hollow body.

20. (canceled)

21. The method of producing prills according to claim 19, further comprising: arranging, in the circumference of the second body, secondary through holes that run at least outwardly in the radial direction as seen from the first axis, and supplying the liquid through the secondary through holes to the gap wherein the step of arranging the secondary through holes comprises arranging the secondary through holes of the second body, as seen along the radial direction, at a nonzero distance from the nozzles of the hollow body.

22. (canceled)

23. The method of producing prills according to claim 19, comprising the steps of: arranging substantially fin shaped members in the through-hole of the second body; and urging, by means of the fin shaped members, the flow of liquid to rotate with the rotation of the second body.

24. The method of producing prills according to claim 1, wherein the hollow body and second body are hollow conical frustums and wherein the step of applying a reciprocal pressure excitation comprises reciprocally varying the width of the gap between the hollow body and second body.

25. The method of producing prills according to claim 1, wherein the jets of liquid break up into droplets, the method further comprising the steps of: generating a flow of a cooling fluid; and at least partially solidifying the dispensed droplets by cooling as they move through the generated flow of cooling fluid.

Description

[0033] The present invention is further illustrated by the following figures, which show preferred embodiments of the method, wherein a droplet dispensing apparatus is used for generating prills from a flow of liquid. The figures are not intended to limit the scope of the invention in any way, wherein:

[0034] FIG. 1 schematically shows a 3D perspective view of a droplet dispensing apparatus for producing prills that is used for performing an embodiment of the method according to invention.

[0035] FIG. 2 schematically shows a cross-sectional view of the droplet dispensing apparatus of FIG. 1 in a first plane.

[0036] FIG. 3 schematically shows the cross-sectional view of the droplet dispensing apparatus in the first plane zoomed in on a top section of the apparatus.

[0037] FIG. 4 schematically shows the cross-sectional view of the droplet dispensing apparatus in the first plane zoomed in on a bottom section of the apparatus.

[0038] FIG. 5 schematically shows a cross-sectional view of the droplet dispensing apparatus of FIG. 1 in a second plane.

[0039] FIG. 6 schematically shows the cross-sectional view of the droplet dispensing apparatus in the second plane zoomed in on a top section of the apparatus.

[0040] FIG. 7 schematically shows the cross-sectional view of the droplet dispensing apparatus in the second plane zoomed in on a bottom section of the apparatus.

[0041] FIG. 8 schematically shows a preferred embodiment of a reciprocal driving-unit and a coupling mechanism for use in the droplet dispensing apparatus.

[0042] FIG. 9 shows a picture of experimental results of traditional droplet dispensing method.

[0043] FIG. 10 shows a picture of experimental results of droplet dispensing method according to an embodiment of the invention.

[0044] FIG. 11 schematically shows two different types of nozzles arranged in the circumferential wall of the hollow body.

[0045] FIG. 1 schematically shows a 3D perspective view of a droplet dispensing apparatus that is used for performing an embodiment of the method according to invention. The droplet dispensing apparatus 1 comprises a lower rotating assembly 2 that comprises the hollow and second bodies 21, 22. A rotary drive unit 3 is arranged for driving the rotation of the lower rotating assembly 2, a reciprocating drive unit 4 for reciprocally driving the second body 22 along the axis of rotation I and a coupling mechanism 8 for decoupling rotations from the reciprocating drive unit 4. The apparatus can further comprise a stationary frame assembly 5 that comprise, for instance, a mounting bracket 51 for mounting the apparatus in a suitable cooling tower, i.e. prilling tower (not shown). Also, cylinder 52 resembles the size of the opening through which the apparatus typically needs to be inserted for mounting it in a prilling tower. An inlet piping system 6 is provided for supplying a liquid to the lower rotating assembly 2 of the apparatus 1, as is described in more detail below. With reference to FIGS. 1-8 and 11 the workings of the embodiment of the apparatus 1 will be described below in more detail.

[0046] FIGS. 2-4 schematically show the cross-sectional view of the droplet dispensing apparatus 1 for producing prills of FIG. 1 in a first plane. FIGS. 5-7 schematically show the cross-sectional view of the droplet dispensing apparatus of FIG. 1 in a second plane which is substantially perpendicular to the first plane. Lower rotating assembly 2 comprises a rotating hollow body 21, wherein a rotating second body 22 is arranged. Hollow body 21 and second body 22 are shaped such the second body 22 is shaped (at least on its outside) to fit into, and substantially match in shape with, the inner space 211 of the hollow body 21, thereby forming a gap 23 between the outer surface of the second body 22 and the inner surface of the circumferential wall of the hollow body 21. The hollow body 21 and second body 22 are preferably substantially bucket-shaped (i.e. are formed as hollow conical frustums) and are arranged such that, once installed in a prilling tower, the top sections of the bodies 21, 22 have a larger dimension (e.g. diameter) than the bottom sections of the bodies 21, 22. This aids in a more evenly distribution of the droplets throughout the prilling tower.

[0047] The second body 22 can comprise an opening 221 at its bottom in addition to a plurality of smaller through holes 222 that can be arranged over a number of rows of through holes 222 that can be arranged at different (angular) locations in the circumferential wall of the second body 22, as seen around the axis of rotation I. These rows of through holes 222 run can run over substantially the full height of the second body 22.

[0048] A central inlet 24 directs, in use, a flow of liquid to an interior space of the second body 22. The central inlet 24 can be fitted with a plurality of flow directing elements 241, 242 that aid in directing the flow towards the interior space of the second member 22 and/or in the direction of rotation. After this, it flows through opening 221 and/or the plurality of through holes 222 to the gap 23. A plurality of through holes, also referred to as nozzles 91, 92, is arranged in the circumferential wall 212 of the hollow body 21. In use, the hollow body 21 spins around the rotational axis I, such that any liquid held in the gap 23 is exposed to the centrifugal forces originating from this spinning, thereby creating a pressure in the liquid and which is forced out the plurality of nozzles 91, 92, thus forming jets of liquid 901, 911 (see FIGS. 9 and 10) that are directed at least partially in the radial outward direction with respect to the rotational axis I. As the width of gap 23 can be reciprocally varied by driving the reciprocating drive unit, as is described in more detail below, pressure pulsations can be introduced to the liquid that is present in the gap 23. These pulsations will propagate to the jets shooting from the nozzles 91, 92. By tuning the frequency and the amplitude with which the width of the gap 23 is varied, pressure pulsations can be obtained that lead to a fast break up of the jets into droplets, wherein substantially equally sized droplets are obtained, such that a spread in droplet-size is significantly reduced.

[0049] Nozzles 91, 92 (see FIG. 11) can be arranged in different manners in the circumferential wall 212. For instance, first nozzles 91, second nozzles 92 or any combination of these and other types of nozzles can be arranged in the circumferential wall 212. The first nozzle 91 is arranged as a through hole that is substantially perpendicular to the outer and/or inner surface of circumferential wall 212. The second nozzle 92 is arranged as a through hole that, once the droplet dispensing apparatus is installed in a prilling tower, runs substantially horizontal, i.e. substantially perpendicular to the axis of rotation I. Alternatively, a recession 93 can be arranged in the outer surface of circumferential wall 212 of the hollow body 21, such that the though hole of the second nozzle 92 debouches in the recession 93, wherein the surface of the recession 93 is substantially perpendicular to the through hole of the second nozzle 92.

[0050] The second body 22 can further comprise a number of fin-shaped members 223 that extend from a centre-axis connecting body 224 in a substantially radial direction towards the circumferential wall 225 of the second body 22. These fin-shaped members 223 force the liquid that enters the interior space of the second body 22 to rotate with the second body 22. Furthermore, additional flow direction elements 226 can be arranged at a top section of the fin-shaped members 223 to help distribute the liquid throughout the second body 22. Hereby, a stable and substantially constant vortex of the rotating liquid can be obtained in the second and hollow bodies, thereby generating more constant process conditions at the nozzles 91, 92 and thus a better control of the process. In this embodiment, the second body 22 is connected at its top section 226 to the top section 213 of the hollow body 21. The hollow body 21 itself is driven by the rotary drive unit 3. An outer shaft 71 is provided that is connected to the rotary drive unit on a first end 711 and to the top second 213 of the hollow body at the second end 712. The outer shaft 71 can be connected to a stationary frame assembly 5 by means of rotary bearings 74. Stationary frame assembly comprises a frame mounting bracket 51 for installing, or arranging, the droplet generating apparatus 1 in a prilling tower.

[0051] The second body 22 is connected to an inner shaft 72 using the centre-axis connecting body 224 that is arranged to receive and couple to a lower portion 721 of the inner shaft 72. Inner shaft 71 is for the most part enclosed by the outer shaft 72 and supported by a number of sliding bearings 73, such that the inner shaft 71 is, preferably only, movable with respect to the outer shaft 72 in a direction along the axis of rotation I and in the rotational direction around axis of rotation I. To shield the sliding bearings 73 and the space between the inner and outer shafts 71, 72 from dust and/or liquid contamination, a flexible shaft cover 75 is arranged between the bottom section 712 of the outer shaft 71 and the centre-axis connecting body 224.

[0052] FIG. 3 schematically shows the cross-sectional view of the droplet dispensing apparatus in the first plane zoomed in on a top section of the apparatus 1. Rotary drive unit 3 is provided for driving the outer shaft 71 using, in the current embodiment, a second pulley 34 that can be directly connected to the first end 711 of the outer shaft 71. The rotary drive unit 3 can comprise (electro-) motor 31 that drives a first pulley 32 and wherein the first and second pulleys 32, 34 are coupled by means of a drive belt 33 that transfers the rotary motion from the motor 31 to the outer shaft 71. Note however, that any other suitable rotational transfer, or gearing, mechanism can be used for this. The outer shaft 71 is coupled, through rotational bearings 74 to a shaft holding frame 54, which is formed by a tubular member connecting and holding the rotational bearings 74, and wherein the shaft holding frame 54 is in turn connect to a frame base member 53 that also comprises the mounting bracket 51.

[0053] The inner shaft 72, which for the largest part enclose by the outer shaft 71, extends at its upper end 721 from the first end 711 of the outer shaft. The upper end 721 is received by an output shaft 81 of the coupling mechanism 8. The coupling mechanism 8, which comprises a rotational bearing 82, takes up the rotational motion of the inner shaft 72, thereby shielding the reciprocating drive-unit 4 from any torsional forces that could potentially damage the vibrating element 41, which is preferably a stacked-piezo element. Stacked-piezo elements are able to generate vibrations in a large band of frequencies, with a sufficiently large force-amplitude and be controlled precisely, such that small amplitudes of vibration can be obtained.

[0054] To secure the vibrating element 41 accordingly, the element 41 is held between a lower 42 and upper connecting member 43. The vibrating element 41 is directly connected and held to the upper connecting member 43. The lower connecting member 42 is directly coupled to an upper section 83 of the coupling mechanism 8. The upper connecting member 43 is held by a secondary coupling mechanism 84 that also comprises a rotational bearing 85. Hereby, the reciprocating drive-unit 4 is unconstrained in its rotation around axis of rotation I, such that even if minor torsional forces are transferred through the coupling mechanism 8, the vibration element 41 is virtually isolated from any potentially damaging torsional forces that could potentially transfer from the inner shaft 72. To further aid in this, coupling mechanism 8 comprises a blocking pin 86 that transfers resulting torsional forces to a frame suspension member 55.

[0055] The secondary coupling mechanism 84 is directly connected through its upper, stationary, section 86 to the suspension member 55. Hereby, the reciprocating drive unit 4, coupling mechanism 8, inner shaft 72 and second body 22 are all suspended from the suspension member 55. The axial forces from these parts are thus transferred through the vibrating element 41, which thereby has a preload applied to it. These suspended parts 8, 72, 22 thus effectively form a biasing mechanism for the vibrating element 41. Suspension member 55 is part of the stationary frame assembly 5.

[0056] FIG. 5, which shows a cross-sectional view taken in a plane substantially perpendicular to the plane of FIGS. 2-4, shows the liquid inlet section 6 that comprises an assembly of tubular members and is arranged to connect on its first end 61 to a liquid feed system and on its second end 62 it debouches in the central inlet 24. Through a stationary first section 244 of the central inlet 24, the liquid is arranged to flow to a second section 244 of the central inlet 24, wherein the second section 242 rotates with the hollow body 21.

[0057] In use, the hollow body 21 is driven by the rotary drive unit 3 to rotate along the axis of rotation I, as described above. Liquid is fed to the second body 22 through the liquid inlet section 6, such that the liquid reaches the gap 23 through the second body 21 that comprises a number of openings 221, 222. The reciprocating drive unit 4 is in turn used to vary the width of gap 23. In the current embodiment this is achieved by driving the vibrating element 41 that, through the coupling mechanism 8 and inner shaft 72, transfers the reciprocating motion along the axis of rotation I to the second body 22. By controlling the vibrating element 41 according to a predefined frequency and amplitude, pressure pulsations are introduced to liquid held in the hollow body 21. The combination of the centrifugal forces due to the rotation and the pressure pulsations introduced in the liquid, allows jets to form through nozzles 91, 92 that break up in individual droplets, wherein the individual droplets have only a small variation in size (when compared to a traditional rotational droplet generating apparatus), such that they can be considered to be substantially evenly sized.

[0058] Results from an experimental setup using such a device are show in in FIG. 10, whereas results from an experimental setup of a traditional rotational droplet generating apparatus are shown in FIG. 9. The photo's show a hollow body 121 comprising both first and second nozzles 91, 92. The second nozzles 92 are arranged to debouche in a recession 93 arranged in the outer surface of the hollow body 121. In FIG. 9 an actual jet of liquid 901 can be seen leaving the nozzles 91, 92, which only after a certain distance break up into a series of differently sized droplets 902. The droplets 902 have a large distribution in size, as the jets breaks up in larger primary droplets and smaller secondary, or satellite, droplets. As further downstream a number of different droplets might recombine to form even larger droplets, a large spread in droplet size is obtained.

[0059] In FIG. 10, the liquid in the hollow body 22 is excited by pressure pulsations with a predefined frequency and amplitude. It is clear that the jets 911 start to break up practically immediately after it exits from the nozzles 91, 92 and that the resulting droplets 912 are much more similar in size when compared to droplets 911. In addition, the individual droplets 912 can be seen to spread out in more regular intervals, thereby leading to less merging of droplets. In the experimental setup water/glycerine mixtures having different viscosities have been tested. In the first test, water with a viscosity of 1 mPa*s was used, wherein it was found that excellent results could be obtained (in terms of a narrow distribution in droplet size) by rotating the bucket such that the water velocity in the resulting jets is 1.5 m/s and by introducing a pressure pulsations caused by introducing vibrations at approximately 280 Hz and with an amplitude 20 μm. When using a somewhat more viscous mixture of 4 mPa*s, a set of substantially ideal conditions was, for instance, found when velocities of the jet of liquid are 1.3 m/s and by introducing a pressure pulsations caused by introducing vibrations at approximately 240 Hz and with an amplitude 35 μm. When using a more extreme mixture having a viscosity of 35 mPa*s, excellent results were, for instance, obtained for velocities of 1.15 m/s and by introducing the vibrations at approximately 190 Hz and with an amplitude 35 μm.

[0060] FIG. 8 furthermore shows an alternative embodiment of the coupling mechanism 108 and an alternative embodiment of the secondary coupling mechanism 1084, wherein all the other components are equal to the embodiment shown in FIGS. 1-7. Coupling mechanism 108 comprises two rotational bearings 1082, in particular spherical roller thrust bearings, which are preferably substantially equal. The bearings 1082 are arranged such that the first ends 1086 are arranged adjacent to one and another, such that the bearings only allow for the rotational motion of the output shaft 82 with respect to housing unit 187 of the coupling mechanism 108, such that a reliable coupling mechanism 108 with minimal play in the axial direction is obtained. Play in the axial direction of the coupling mechanism 108 influences the transfer of vibrations from the vibrating element 41 to the second body 22 and would therefore negatively affect the performance of the droplet generating apparatus 1. Also, spherical roller thrust bearings are highly suitable for transferring high axial (i.e. trust) loads, such that a reliable coupling mechanism 108 is obtained for transferring axial forces from the reciprocating driving unit 4 to the second body. Also the secondary coupling mechanism 1084 comprises a similar arrangement of two rotational bearings 1085, in particular spherical roller thrust bearings.

[0061] The present invention is not limited to the embodiment shown, but extends also to other embodiments falling within the scope of the appended claims.