Fluid Bed Granulation Process and Apparatus

Abstract

A fluid bed granulation process and apparatus, wherein a suitable fluid bed of a particulate material is maintained in a granulator fed by an input flow comprising a growth liquid and by a flow of seeds adapted to promote the granulation, and wherein a part of said input flow is taken upstream the feeding of the fluid bed, and used in a seeds generator, to produce the seeds for the fluid bed.

Claims

1. A fluid bed granulation process of a given substance, comprising the steps of: providing a fluid bed of a particulate material comprising granules of said substance and solid particles of a substance working as seeds for the granulation process; feeding an input flow (F) comprising a growth liquid (L) containing said substance, to said process; feeding a flow (S1) of said seeds into the fluid bed, to promote the growth of granules and to maintain the fluidized mass; and taking a flow of solid granules as output of the process carried out in the fluid bed, wherein a first portion (F1) of said input flow (F) is fed directly to the fluid bed, and a second portion (F2) of said input flow (F) is used to generate at least a part of said flow (S1) of seeds, wherein said second portion (F2) is a minor portion of said input flow, and wherein the ratio between said second portion (F2) and the input flow (F) is equal to d.sup.3/D.sup.3, where d is the mean value of a characterizing dimension of said seeds and D is the mean value of a characterizing dimension of the solid granules obtained at the output of the fluid bed.

2. The process according to claim 1, wherein said second portion (F2) of the input flow is solidified by depositing liquid drops on a cooled conveyor belt, obtaining solid pastilles.

3. The process according to claim 1, wherein said second portion (F2) of the input flow is solidified in a prilling tower.

4. The process according to claim 1, wherein said second portion (F2) of input flow is used to generate the full flow (S1) of seeds to the fluid bed.

5. The process according to claim 4, wherein the output flow of the fluid bed is directly taken as a final product of the granulation, without further screening and separation of waste granules.

6. The process according to claim 1, wherein said first portion (F1) of the input fluid flow is fed to the fluid bed along a continuous, longitudinal feeding line on one or both sides of the fluid bed.

7. The process according to claim 1, wherein said first portion (F1) of the input fluid flow is fed to the fluid bed in discrete and predetermined feeding zones (Z), aligned in a main flowing direction of the fluid bed, and alternate to non-feeding zones (Z) of the same fluid bed, said feeding zones (Z) acting substantially as wetting zones of the particulate material by the fluid flow, and said non-feeding zones (Z) acting substantially as drying and consolidation zones of the growing particles.

8. The process according to claim 1, wherein a vortex condition is induced and maintained in the fluid bed; with a transversal vortex (V) or a double transversal vortex (V1, V2) arrangement, the axis of the vortex (V; V1, V2) being substantially parallel to a main flow direction of the fluid bed.

Description

DESCRIPTION OF THE DRAWINGS

[0039] FIG. 1 is a scheme of a granulation apparatus operating according to the invention.

[0040] FIGS. 2 and 3 are schemes of a component of apparatus of FIG. 1, according to alternative embodiments.

[0041] FIGS. 4 and 5 are a view and a longitudinal section of the fluid-bed granulator of apparatus of FIG. 1, according to one embodiment of the invention.

[0042] FIG. 6 is a cross section of the granulator of FIGS. 4 and 5, showing the vortex of the fluid bed.

[0043] FIG. 7 is a cross section of a variant of the granulator of FIGS. 4 and 5.

[0044] FIGS. 8 and 9 are a view and a longitudinal section of the fluid-bed granulator of apparatus of FIG. 1, according to another embodiment of the invention.

[0045] FIG. 10 is a scheme of a granulation apparatus operating according to the known art.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0046] With reference to FIG. 1, a process according to the invention is carried out by forming a fluid-bed of a particulate material inside a granulator 1, fed with an input flow F of an appropriate growth liquid via a feeding line 30.

[0047] Feeding line 30 is divided into a first line 31 connected to granulator 1 and a second line 32 connected to a seed generator 33. Preferably, the first line 31 carries a major flow portion F1 of the input flow F, while a minor portion F2 is fed to the seeds generator 33 via line 32.

[0048] The flow portion F1 of the growth liquid is fed to the fluid bed formed inside the granulator 1 along a feeding longitudinal line 34, on one or both sides of the granulator 1 itself, continuously or in selected discrete zones.

[0049] The output of seeds generator 33 is a flow S1 of solid granules or pastilles, obtained by the solidification of the growth liquid, or of the substance to be granulated contained in said liquid. Said flow S1 of solid granules or pastilles is fed from generator 33 to the granulator 1 via line 35. The seeds produced in said generator 33 are of appropriate size to work as seeds into the fluid bed, e.g. spheres having a diameter of about 1-1.5 mm or less.

[0050] Output 36 of granulator 1 can be connected to means for screening or separating the solid granules, or can be directly taken as the final product of the granulation.

[0051] Preferably, the flow portion F2 is such that the ratio between said flow portion F2 and the total input flow F is equal to d.sup.3/D.sup.3, where d is the mean diameter of seeds produced in generator 33 and fed to the granulator 1, and D is the mean diameter of the solid granules obtained at the output 36 of the granulator 1. In other words, and referring to FIG. 1, the ratio between flow rate through line 32 and flow rate through line 30 is equal to d.sup.3/D.sup.3 as defined above.

[0052] FIG. 2 shows, in a simplified manner, an embodiment of the seeds generator 33. The flow portion F2 of growth liquid, through the line 32, is fed to a rotary former 40, depositing small droplets on the outer surface of a cooled steel belt 41. The opposite inner surface of belt 41 is cooled by an internal cooling circuit, for example by a sprayer 42 connected to a cooling water feeding 43 and return 44. The cooled surface of belt 40 promotes the solidification of the liquid, forming solid pastilles which are taken from a discharge section 45, at the opposite end of the rotary former 40, and form the output flow S1 of generator 33 directed to granulator 1.

[0053] FIG. 3 is a sketch of another embodiment of the generator 33, realized as a compact prilling tower. More in detail, the generator 33 of this another embodiment comprises a vertical tower 50 with a prilling bucket 51 installed on the top. The bucket 51 has a perforated side wall and is connected to a driving shaft for rotation around the vertical axis. The portion L2 of growth liquid is fed to the bucket 51, producing a flow 60 of small droplets, ejected by the perforated side wall of the bucket 51 and flowing down to the bottom of the tower 50.

[0054] Cooling air 61 enters the tower 50 at a bottom duct 52 and exits at a top discharge duct 53, thus flowing in counter-current relative to the droplets flow 60. The droplets are solidified by the action of said cooling air, and taken at a bottom exit 54 of the tower 50, forming the seeds flow 35.

[0055] It should be noted that the above systems are per se known, and thus they are not described in greater detail. In other (not shown) embodiments, the prilling tower of FIG. 3 can be equipped with one or more prilling showers, such as vibrating showers, which are known per se and thus not described in detail; the cooling flow inside the tower, moreover, can be co-current with the liquid droplets.

[0056] The fluid-bed granulator 1 is now described with reference to preferred embodiments.

[0057] In a first embodiment of FIGS. 4 to 7, the fluid bed granulator 1 essentially comprises a horizontal container 2 with: a bottom part 3, permeable to gas, for example made up of a perforated element; two opposite long side walls 4, 5; a head wall 6 and a discharge wall 7.

[0058] The discharge wall 7 is provided with a top opening 8 for discharging the (granulated) final product and fixing the maximum height of the fluid bed. Other appropriate discharge means may be used such as for example an automatic valve operated by the fluid bed level.

[0059] A feeder 9 is installed at the upper side of the head wall 6, receiving the flow S1 of seeds produced in said device 33, and providing uniform distribution of the seeds along head wall 6. Feeder 9 is per se conventional and therefore need not be described in detail.

[0060] A blowing system (not shown) is installed below the container 2, producing a flow of air A which creates and maintain the fluid-bed state of particulate material, comprising seeds and granules, inside the container 2, as well as a continuous vortex having a substantially horizontal axis. To this purpose, bottom part 3 of the container is perforated and is preferably provided with suitable conventional means for obtaining a non-homogeneous distribution of the air flow A into the container 2 (for example by dividing said flow A in fractions having different rates or by varying the entry direction of such a flow in the fluid bed), so as to create and maintain a vortex in the fluid bed.

[0061] In addition, the seeds S1 can be advantageously pre-heated by the air flow A.

[0062] The continuous discharge through opening 8, counterbalanced by the continuous feeding of seeds S1, determines a longitudinal main flow or fluid vein of the fluid bed, from head wall 6 towards the opposite wall 7, the free surface P being slightly tilted down in the direction of the flowing bed (FIG. 5). According to this configuration, the seeds S1 are located only near the head of the granulator 1 (wall 6) while in the remaining part of the fluid bed is formed by the progressively growing granules.

[0063] Growth liquid L atomized and mixed with air, is introduced in container 2 via a side distributor 10, slightly below the free surface P of the fluid bed. The growth liquid L may be fed in atomized form and relatively diluted in a solvent. For instance, in the case of urea granules, the atomized growth liquid can contain molten urea from 94% wt up to near 100% wt (weight percent), the balance being water (solvent).

[0064] The distributor 10 extends along the whole length of container 2, providing a continuous and distributed supply of liquid L, transversal with respect to the flow of the fluid vein. In other words, the fluid bed is fed along a continuous, horizontal feeding line (line 34 of FIG. 1) corresponding to side distributor 10.

[0065] Due to the above arrangement, a continuous vortex V is formed and maintained in the fluid bed (FIG. 6). Vortex V is transversal, i.e. with axis substantially parallel to the length of the container 2 and, hence, to the direction of the main flow (fluid vein) through the fluid bed.

[0066] The distributor can be equipped with one side distributor 10 or two distributors 10a and 10b on opposite sides (FIG. 7) with liquid inputs L and L1, obtaining feeding of the fluid bed on two opposite and parallel feeding lines, and a configuration of the fluid bed itself with a double vortex arrangement, namely a vortex V1 substantially extended in a right portion of the fluid bed and another opposite vortex V2 substantially extended in the left portion. Senses of rotation of vortex V1 and V2 are opposite.

[0067] FIGS. 6 and 7 show also upper zones Z1 of the fluid bed, where seeds are wetted by atomized liquid L and evaporation of possible solvent contained takes place, and lower zones Z2 where of solidification and consolidation of the growth liquid takes place.

[0068] Other details of the fluid bed granulator 1 can be provided according to WO 02/074427 or WO 2005/097309, which are incorporated herein by reference.

[0069] FIGS. 8 and 9 relate to another embodiment, wherein the fluid bed is fed in discrete, predetermined zones. More in detail, the granulator 1 comprises a plurality of distributors 10 spanning over the length of one or both side walls 4, 5 and at a prearranged height from the bottom 3, below the free surface P of the fluid bed. Said distributors 10 provide a feeding of the fluid bed in discrete zones Z, alternate to non-feeding zones Z.

[0070] More in detail, each distributor 10 feeds the growth liquid L to a corresponding feeding zone Z of the fluid bed substantially spanning over the entire transversal extension of the container 2, and being delimited longitudinally by portions of the long side walls 4 and 5 supporting the respective distributor 10. Said zones Z are alternated with non-feeding zones Z also substantially spanning over the entire transversal extension of the container 2 and being delimited longitudinally by portions of the long side walls 4, 5 separating two successive distributors 10.

[0071] The process starts and ends preferably in non-feeding zones Z, namely it is started in a zone Z near the head wall 6, and ended in a last zone Z close to the discharge wall 7.

[0072] A non-feeding zone Z close to the head wall 6 (i.e. in the location of seeds S1) is also preferred to establish a regular vortex for the seeds S1 before they are wetted by the growth fluid.

[0073] Other features of the granulator of FIGS. 8 and 9, including a blowing system of air A to maintain the fluid-bed state, and provision of vortex condition as seen in FIG. 6 or 7, can be provided substantially according to the above cited disclosures of WO 02/074427 and WO 2005/097309.

[0074] The granulation process which is carried out by the fluid bed is now briefly described.

[0075] In steady state conditions, seeds and growing granules inside container 2 are maintained in a fluidized (fluid-bed) condition by air flow A, crossing the bottom 3 and distributed inside the bed in a non-homogeneous manner so as to create and maintain vortex V.

[0076] The level of the fluid bed is determined by the discharge through opening 8 or an automatic discharge valve, following the main flow from head wall 6 towards the opposite wall 7.

[0077] It should further be noted that the air A carries out a thermal exchange with the growing granules that form such fluid bed heating itself progressively. Indeed, air A removes the solidification heat of a growth fluid fed onto the seeds S.sub.1 and onto the growing granules.

[0078] The fluid-bed particles (granules or seeds) located in the upper layer of the fluid bed are hit and wetted many times with the particles of atomized growth liquid of flow L, with solidification of the substance and partial evaporation of the solvent that may be inside said growth liquid. As a consequence, temperature of the granules is increased in the relative (upper) zone of the fluid bed.

[0079] Referring for example to FIG. 6, the wetted granules are pushed towards the opposite wall 4 and deflect naturally towards the bottom 3 of the container 2, under the action of vortex V. In the course towards bottom 3, the granules leave the upper hot layer of the fluid bed crossing progressively colder layers. During this course the growth liquid is solidified and consolidated on the surface of the granules. This step is completed during the course of the granules, towards the wall 5; then the granules deflect near the wall 5 and again towards the upper hot layer of the fluid bed (FIG. 6).

[0080] The course described above is substantially repeated and the steps of wetting, solidification and evaporation are repeated with progressive mass and volume increase, during the path from wall 6 to wall 7 induced by the fluid vein (FIG. 5).

[0081] The embodiment of FIG. 7 make it possible to substantially double the production yield of the granulator, while keeping the same length of container and operating conditions of the fluid bed.

[0082] With reference to granulator of FIGS. 8 and 9, non-feeding zones Z (alternate to wetting zones Z) provide a drying of the granules by the air flow A, which allows substantial evaporation of the residual solvent of the growing liquid and recover of the solidification heat, thereby obtaining a further consolidation for the growing granules that advantageously improves their mechanical properties, in particular their hardness.

[0083] It can be stated that the fluid-bed particles gain volume and mass by traversing each zone Z, where they are subjected to wetting and solidification of growth liquid; the alternate, subsequent zone Z provides a substantial drying and consolidation steps to increase hardness of the product. This embodiment of the invention with feeding and non-feeding zones Z, Z is particularly preferred as the produced granules are substantially monodispersed, thus obtaining a product directly marketable, i.e. the output line 36 (FIG. 1) can be directed to storage of the final product, without screening. A screening can be provided anyway, but waste would be very little. It should be noted that the present invention, in this case, eliminates the need to use a part of the final product for seeds production, i.e. the process as a whole is more effective.

[0084] Particularly satisfactorily results, in the field of granulation of urea, are obtained feeding the flow L comprising the growth liquid (urea solution), in the zones Z of the fluid bed, at a speed between 2 and 50 m/s, through a succession of 2 to 20 distributors 10 along a single long side wall of granulator 1. The distributor spacing between consecutive distributors may be the same or different depending on the substance to be granulated and it is preferably in the order of magnitude of the distributor length. A final product with 90% of the granules measuring from 2 to 4 mm diameter was obtained.

[0085] Together with the possibility of obtaining a final product of suitable granulometry, i.e. directly marketable, the invention allows to substantially reduce the investment and maintenance costs, as well as the energy consumption, of the corresponding granulating plant.