APPARATUS AND METHOD FOR PRILLING A LIQUID, PREFERABLY UREA MELT

Abstract

Apparatus for prilling a liquid, comprising a distributor for supplying said liquid, at least one dispenser, and a pulse generator, wherein said pulse generator: is situated in either one of said supply distributor and said dispenser; is passed through by at least a portion of said liquid; and comprises at least a first surface and a second surface, which face each other and perform a relative movement and comprise respective passages for the liquid; and wherein said first surface and second surface, which are passed through by the liquid, generate in the liquid periodic pulses of pressure having a predetermined frequency dependent on the relative speed of said two surfaces.

Claims

1. An apparatus for prilling a liquid, preferably urea melt, comprising a distributor supplying said liquid, at least one dispenser and a pulse generator, wherein said pulse generator: is situated in either one of said supply distributor and said dispenser, is passed through by at least a portion of said liquid, and comprises at least a first surface and a second surface, facing one another and performing a relative movement and comprising respective passages for the liquid, and wherein said first surface and second surface, which are passed through by the liquid, generate in the liquid periodic pulses of pressure having a predetermined frequency depending on the relative speed of said two surfaces,

2. The apparatus according to claim 1, wherein said two surfaces perform a relative movement of rotation.

3. The apparatus according to claim 1, wherein said pulse generator comprises a stationary body and a rotating body, said first surface being part of said stationary body, and said second surface being part of said rotating body.

4. The apparatus according to claim 1, wherein said first surface and said second surface are cylindrical or conical, and the passage of the liquid through said surfaces occurs in a substantially radial direction.

5. The apparatus according to claim 3, wherein said stationary body has a cylindrical or conical shape and said rotating body is a drum also having a cylindrical or conical shape respectively, and rotates inside said drum, said first surface and second surface passed through by the liquid being the side surfaces of said stationary body and of said rotating drum.

6. The apparatus according to claim 5, wherein said dispenser comprises at least one rotating prilling bucket and said pulse generator is positioned inside said bucket.

7. The apparatus according to claim 1, wherein said first surface and said second surface are substantially flat and the passage of the liquid through said surfaces occurs in an axial direction perpendicular to said surfaces.

8. The apparatus according to claim 7, wherein said stationary body and said rotating body are formed respectively by a first perforated disc and by a second perforated disc, the axes of the first and second disc being parallel to each other.

9. The apparatus according to claim 8, wherein said pulse generator is positioned inside said supply distributor.

10. The apparatus according to claim 3, wherein rotation of the rotating body is imparted fluido-dynamically by the liquid passing through the pulse generator.

11. The apparatus according to claim 10, comprising a driving impeller integral with said rotating body and wherein said impeller is made to rotate by the liquid passing through the pulse generator.

12. The apparatus according to claim 10, wherein said first surface and said second surface have liquid passages with a different angle of inclination relative to the surface, such that the flow through said liquid passages is subjected to a deviation resulting in a driving torque transmitted to the rotating body.

13. The apparatus according to claim 12, wherein the stationary body and the rotating body are formed by a first and by a second disc, the first disc has holes with a first angle of inclination and the second disc has holes with a second angle of inclination opposite to the first angle, and preferably said first angle is equal to +45° and said second angle is equal to −45°.

14. The apparatus according to claim 1, wherein said surfaces have the same number and the same pattern of liquid passages.

15. The apparatus according to claim 1, comprising a pump which processes said at least one liquid portion so as to allow said at least one liquid portion to pass through said pulse generator.

16. The apparatus according to claim 15, comprising a pump impeller Integral with the rotating body of the pulse generator, wherein said pump impeller processes said at least one liquid portion so as to allow said at least one liquid portion to pass through said pulse generator.

17. The apparatus according to claim 1, wherein said at least one dispenser: is a rotating bucket with a perforated wall; or comprises one or more shower sprayers.

18. The apparatus according to claim 17, wherein the dispenser is a rotating bucket, the apparatus comprising a first motor which operates said movable body of the pulse generator, and a second motor which operates said rotating bucket.

19. The apparatus according to claim 1, wherein said distributor comprises a substantially cylindrical pipe; said pulse generator is situated inside said pipe and has a cross-section smaller than the cross-section of said pipe, thus leaving a bypass space around the pulse generator, so that a first portion of a flow entering the distributor passes through said pulse generator, while a remaining portion bypasses said pulse generator.

20. The apparatus according to claim 19, wherein the pulse generator is housed inside a coaxial tube inside said pipe, and said coaxial tube is supplied with said first portion of the liquid flow.

21. A Method for prilling a liquid, preferably urea, wherein at least a portion of said liquid passes through the passages of a first surface and a second surface of a pulse generator, said two surfaces facing one another and performing a relative movement and generating in the liquid periodic pulses of pressure having a predetermined frequency dependent on the relative speed of said two surfaces.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0055] FIG. 1 is a schematic illustration of a rotating-bucket prilling apparatus with axial-pulse generator, according to an embodiment of the invention.

[0056] FIG. 2 shows a variant of the apparatus according to FIG. 1.

[0057] FIG. 3 shows a diagram of an axial pulse generator which can be used in the apparatuses according to FIGS. 1 and 2 as well as in the apparatuses according to FIGS. 10-12.

[0058] FIG. 4 shows a detail of an axial generator of the type shown in FIG. 3, according to another embodiment.

[0059] FIG. 5 shows a variant of the axial generator according to FIG. 3, with an external supply pump.

[0060] FIG. 6 shows a further variant of an axial generator with movable body operated fluido-dynamically.

[0061] FIGS. 7 and 8 are schematic illustrations of two variants of a rotating-bucket prilling apparatuses with radial-pulse generator, according to different embodiments of the invention.

[0062] FIG. 9 is an illustration of a radial pulse generator which can be used in the prilling apparatuses according to FIG. 7 or FIG. 8.

[0063] FIGS. 10-12 are schematic illustrations of shower-head prilling apparatuses according to different embodiments of the invention.

DETAILED DESCRIPTION

[0064] FIG. 1 is a schematic illustration of a prilling apparatus denoted generally by 1, intended for prilling a stream of urea melt Q. Said apparatus 1 is located at the top of a prilling tower (not shown).

[0065] Said prilling apparatus 1 comprises essentially a urea supply distributor 2, a dispenser formed by a rotating bucket 3 and a pulse generator 4 which in this example is housed inside the dispenser 2.

[0066] The bucket 3 has a perforated wall 5 and is made to rotate by a first motor 6; more particularly the motor 6 drives rotationally a tube 7 (which is part of the distributor 2) with which the bucket 3 or at least the perforated wall 5 is integral.

[0067] The urea melt Q is introduced into the distributor 2 via an inlet 8 and travels along the tube 7 until it reaches the bucket 3. As can be noted in the figure, the pulse generator 4 is passed through by the flow Q or by a part thereof. In the example shown in FIG. 1, said generator 4 has a smaller cross-section than the tube 7 and therefore is passed through by a portion Q1 of urea, the remaining portion Q2 passing into the space around the generator 4.

[0068] Said generator 4 transfers pressure pulses with a predetermined frequency to the liquid, in particular to the portion Q1 which passes through it. In greater detail, said generator 4 comprises essentially a stationary body and a movable body moved by a second motor 9. Said stationary body and movable body comprise suitable liquid passages and the frequency of the pulses is determined substantially by the speed of the movable body. The motor 9 has a variable speed and preferably is an electric motor.

[0069] FIG. 2 shows a variant of FIG. 1 in which the portion Q1 of liquid is supplied to the said pulse generator 4 by means of a tube 10 coaxial with the tube 7. A bypass space 13 is defined between the tube 10 and the tube 7. The generator 4 in particular is housed at the end of or inside said tube 10. The two portions of urea Q1 and Q2 are supplied via two inlets 11, 12 communicating respectively with the said coaxial tube 10 and the annular portion 13 of bypass.

[0070] In this embodiment, the urea portion Q1 may have a pressure head different from that of the urea portion Q2, the former being conveyed separately inside the tube 10.

[0071] Moreover, the two portions Q1 and Q2 may have a different composition, for example owing to the varying addition of additives. For example, it is possible to improve the mechanical characteristics of the end product by adding additives, such as formaldehyde, or complex fertilizers by means of the addition of compounds, such as sulphates, nitrates or phosphates.

[0072] Further details of the pulse generator 4 are shown in FIG. 3. Said generator is essentially formed by a rotary valve comprising a casing (or stator) 20 and a rotor 21 operated by the motor 9 via a transmission shaft 22. Said stator 20 and said rotor 21 define, respectively, a first disc-like surface 23 and a second disc-like surface 24, which face each other and perform a relative movement as a result of the rotation. The surface 23 is formed by the bottom of the casing 20. Said disc-like surfaces 23 and 24 comprise respective passages for the liquid consisting of holes 25, 26. Advantageously, the holes 25, 26 have the same number and arrangement on the two surfaces 23, 24.

[0073] The liquid Q1, passing through the rotary valve or generator 4, receives a series of pressure pulses as a result of transit through the holes 25 and 26 and as a result of rotation of the body 21.

[0074] FIG. 3 also shows a pump 30 which provides the flow Q1 with the necessary pressure (head) to pass through the valve 4. Said pump 30 is also driven by the motor 9 via the shaft 22.

[0075] FIG. 4 shows a variant in which the holes 25, 26 have an inclined axis with respect to the surfaces 23 and 24. In this way, the flow is deviated, producing a driving torque which actuates the rotor 21 fluido-dynamically.

[0076] FIG. 5 shows a variant of FIG. 3 in which the pump 30 is not operated by the motor 9, but is independent. This embodiment is advantageous since the flowrate of the fluid Q1 is determined by said pump 30 and by a valve 31, independently from the speed of the motor 9 which solely drives the rotor 21 of the rotary valve 4. In other words, the frequency of the pulses transmitted to the liquid may be varied by regulating the speed of the motor 9, while the flowrate can be controlled independently by means of said pump 30 and valve 31.

[0077] FIG. 6 instead shows in schematic form another embodiment in which the rotor 21 is operated fluido-dynamically, using the energy of the liquid stream Q1. In greater detail, the rotor 21 is driven by a driving impeller 32 which is crossed by the flow Q1 and operates substantially in the manner of a turbine. In this embodiment, the frequency of the pulses is also determined by the flow Q1 and therefore ultimately by the pump 30.

[0078] Owing to the presence of said external pump 30, the body 20 of the valve 4 is under pressure. In the rotating-bucket systems, such as those shown in FIG. 1-2 or 7-8, said body rotates 20 at the same speed as the bucket 3 and therefore it is required to have a seal, for example a lip seal, on the rotating shaft integral with said pump (not shown in figure).

[0079] FIGS. 7 and 8 show an embodiment comprising a rotary valve with conical geometry, which is inserted inside the bucket 3. The details corresponding to those of FIG. 1 are indicated by the same reference numbers for the sake of simplicity. In FIG. 7, the valve 4 is positioned so as to leave a bypass space for a portion Q2 of the liquid; the valve 4 is passed through by the remaining portion Q1. In FIG. 8 instead the valve 4 has dimensions such as to process the whole flow Q.

[0080] A preferred embodiment of the conical valve is schematically shown in FIG. 9. The casing 20 and the rotor 21 in this embodiment are bodies with a substantially frustoconical shape.

[0081] It can be noted by comparing FIG. 3 and FIG. 9 that the valve in FIG. 3 may be defined as being of the axial-flow type, since the flow emerging from the passages 25 and 26 has a direction aligned with the axis of rotation and parallel to the direction of the incoming flow Q1. The valve shown in FIG. 9 may be defined instead as being of the type with a radial or substantially radial flow.

[0082] FIG. 10 shows a preferred example of a prilling system according to the invention with shower-head dispensers.

[0083] The supply distributor 2 is formed by a tank 40, inside which a rotary valve 4, preferably of the axial type shown in FIG. 3, is housed. Said tank 40 supplies a series of shower sprayers 41 at the top of a prilling tower 42, via a pipe line 43. The droplets 44 produced by the sprayers solidify, falling under gravity inside the tower 42, in a counterflow to air entering via bottom inlets 45 and exiting a top outlet 46. The solid prills are collected at the base of the tower by means of a conveyor 47.

[0084] FIG. 11 shows a variant similar to that of FIG. 2, with a coaxial tube 10 supplying the rotary valve.

[0085] FIG. 12 shows schematically an embodiment of a shower-head system comprising a plurality of pulse generators (rotary valves) 4, in the case in question each generator 4 serving a respective set of 48 shower sprayers.

[0086] In the various embodiments of the invention, the rotary valve 4 generates pressure pulses in the fluid passing through it. Said pressure pulses result in improved fragmentation of the liquid jets emitted from the perforated wall 5 of the bucket 3 (FIGS. 1 to 9) or the sprayers 41 (FIGS. 10 to 12), improving the monodispersion of the droplets and consequently the quality of the solid prills (uniformity of shape and size).