FLUID BED GRANULATION PROCESS

Abstract

A fluid bed granulation process of a predetermined substance comprises the steps of: forming, through a fluidification, air flow of predetermined flow rate, a fluid bed of granules of said substance to be granulated fed to it in the form of seeds,feeding said fluid bed with a continuous flow of a growth substance (or liquid).

Claims

1. A fluid bed granulation process of a predetermined substance comprising the steps of: forming, through a fluidification air flow of a predetermined flow rate, a fluid bed of granules of the substance to be granulated, fed to it in form of seeds; feeding said fluid bed with a continuous flow of a growth substance; inducing, through said fluidification air flow, the formation of a substantially vortex-shaped circulatory movement of the granules of the substance to be granulated in said fluid bed; and maintaining and regulating said substantially vortex-shaped circulatory movement through said fluidification air flow, wherein said substantially vortex-shaped circulatory movement is elongated and has a substantially horizontal axis, and wherein the fluidification air flow is continuously divided throughout the fluid bed in at least three parallel fractions having respective different flow rates so that the flow rate of the fluidification air flow varies continuously between a minimum value flow rate, sufficient to support the fluid bed, fed at a first zone thereof, and a maximum value flow rate, fed in a second zone of the same fluid bed that is parallel to the first zone, so as to induce and to maintain said substantially vortex-shaped circulatory movement of the granules of said substance; said substance being urea, ammonium nitrate or ammonium chloride or a similar substance susceptible to being granulated.

2. The granulation process according to claim 1, wherein the variation in fluidification air flow rates between said first zone where the flow rate is minimum and the zone where said second flow rate is maximum is step-wise.

3. The granulation process according to claim 1, wherein said granules of the substance to be granulated are made to flow with a substantially helical movement from one end of the fluid bed where a flow of seeds of said substance is continuously fed, to an opposite end of the fluid bed where a flow of finished granulated product is continuously discharged.

4. The granulation process according to claim 1, wherein finished granulated product obtained in said fluid bed is continuously discharged from a bottom of said fluid bed by gravity.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] FIGS. 1 and 2 schematically represent a longitudinal section and a cross section, respectively, of an apparatus (granulator) for carrying out the granulation process of the present invention;

[0027] FIGS. 3 and 3a schematically represent respective plan views of variant embodiments of a detail of the granulator of FIGS. 1 and 2;

[0028] FIG. 4 schematically represents a plan view of a variant embodiment of the detail of FIGS. 3, 3a;

[0029] FIGS. 5 and 5a schematically represent a plan view and cross section, respectively, of a further variant embodiment of the detail of FIGS. 3, 3a;

[0030] FIG. 5b represents a cross section of a variant embodiment of the detail of FIGS. 5, 5a;

[0031] FIG. 6 represents the same granulator of FIG. 2 for carrying out a variant of the granulation process of the present invention;

[0032] FIG. 7 schematically represents a further variant embodiment of the detail of FIGS. 3, 3a.

DETAILED DESCRIPTION

[0033] With reference to FIGS. 1, 2, 3, 3a an apparatus for carrying out a fluid bed granulation process according to the present invention is globally indicated with 1, an apparatus that in the rest of the description shall be more simply called granulator.

[0034] In an entirely schematic way, said granulator 1 comprises a container 2, represented open at the top, that is substantially parallelepiped in shape, with a rectangular section.

[0035] Said container 2 has a bottom 3, permeable to gas, consisting of a perforated element (grid), two opposite long side walls 4, 5 and two opposite short walls, front 6 or head and rear 7.

[0036] At the upper side of the head wall 6 a device, schematized as 9, per se conventional and therefore not described in detail, is supported for supplying a continuous flow of seeds 51 of substance to be granulated into the container 2. In the rear wall 7 and at a predetermined height over the bottom or grid 3, an opening 8 is formed, for the discharge, substantially by weir, of the finished (granulated) product from said container 2, as shall become clear in the rest of the description.

[0037] In a position below the container 2, a blowing system is provided (not represented since it is totally conventional) of air A or another gaseous fluid (fluidification air), used for carrying out and maintaining a fluid bed of the substance to be granulated inside the container 2.

[0038] In accordance with a characteristic of the present invention, the grid 3, which, as stated, constitutes the bottom of said container 2, is essentially a perforated plate (FIGS. 3, 3a), in which the holes 11, provided for the injection into said container 2 of a predetermined flow rate of fluidification air, are distributed in a non-homogeneous manner.

[0039] According to a first preferred embodiment of the invention (FIG. 3), all the holes 11 have the same diameter and their distribution in the grid 3 is selected so that their density, understood as the amount of holes per square centimeter of surface, increases starting from a long wall of the container 2, for example from the wall 4, towards the opposite long wall, for example towards the wall 5. In particular (FIG. 3a), according to a further embodiment, said non-homogeneous distribution is obtained by defining parallel bands 3a, 3b, 3c, of predetermined width, in the grid 3, in each of which the respective holes 11 are regularly distributed according to a predetermined pitch that is different from band to band.

[0040] Near to the upper side of the long side wall of said container 2 (in the example the wall 5), adjacent to the zone 3c of the grid 3 of FIG. 3a, where there is the greatest density of holes 11, a distributor-supplier 10 is supported with conventional means that are not represented, for feeding continuously into said container 2 a flow L of a predetermined growth substance for granules, for example in the atomized liquid state.

[0041] Said distributor 10 substantially extends for the whole length of the wall 5 and at a height over the bottom (grid) 3 which is predetermined according to the thickness of the fluid bed that, as shall become clear in the rest of the description, one intends to carry out in said container 2. Moreover, said distributor 10 is positioned so as to supply a continuous flow of growth substance orientated, in the example described here, substantially parallely to said grid 3.

[0042] With reference to the granulator 1, schematically described above (FIGS. 1 to 3a), an exemplifying embodiment of the granulation process of the present invention shall now be illustrated.

[0043] In an initial condition, a fluid bed of seeds 51 of the predetermined substance to be granulated is carried out in the container 2, with a totally conventional technique, said seeds being fed continuously at the head wall 6 of the container itself, through the distributor 9. Such a fluid bed is obtained, supported and maintained through a suitable continuous air flow A (fluidification air), of predetermined flow rate, fed from below and continuously into the container 2, through the grid 3 thereof. When the thickness of the fluid bed, continuously fed with seeds 51, is such that its free surface reaches the level of the opening 8, a continuous discharge, substantially by weir, of the finished granules out from the container 2 begins.

[0044] The fluidification air A, crossing the bottom 3, is distributed inside the bed in a non-homogeneous manner, corresponding to the non-homogeneous distribution of the holes 11, provided on said bottom (grid) 3. Where the density of the holes 11 is greater, there is a greater passage of air; in the exemplified case (FIG. 3a), a greater flow rate of fluidification air A is obtained in the band 3c of said grid 3, near to the wall 5, and a lower flow rate of fluidification air A is obtained in the band 3a near to the opposite wall 4.

[0045] Now, the flow rate of fluidification air, its speed, the diameter of the holes 11 and their density in the different bands or zones of the grid 3 are selected so that the formation and support of the fluid bed is ensured at the band(s) or zone(s) of lower density. Consequently, due to the structure of the grid of the present invention, at the other bands of said grid and, in the exemplified case, as one approaches the wall 5 of said container 2, the increasing values of flow rate and speed of the fluidification air determine a dragging upwards, towards the free surface of the fluid bed, of the granules in growth. The degree of such dragging also increases as one approaches said wall 5, at which it reaches its maximum value.

[0046] As a first effect, this increase in the degree of dragging upwards, applied on the granules of the fluid bed, determines the formation, in the fluid bed, of a circulatory movement, substantially vortex-shaped V, of the granules around an ideal axis which, in the example of FIGS. 1 and 2, is substantially horizontal, said circulatory movement extending helically from the front wall 6 to the rear wall 7 of said container 2.

[0047] Basically, with the use of a container 2 having its bottom or grid 3 structured in the way described above, the granulation process of the present invention essentially consists of distributing the flow rate of fluidification air in the fluid bed, carried out in said container 2, dividing it into a plurality of flow rate fractions, having respective values between a minimum amount, sufficient to support said fluid bed and fed at a first zone 3a thereof, and a maximum amount, fed in a zone 3c of said bed, spaced out from said first zone 3a, so as to induce and maintain a circulatory movement of granules, substantially vortex-shaped, in the fluid bed itself. It should be noted that in the exemplified case, the variation in fluidification air flow rates between said first zone where the flow rate is minimum and the zone spaced out from it where the flow rate is maximum, is of the steps type. In the case of FIG. 3, instead, the fluidification air flow rates vary, between the zone near to the side wall 4 where the flow rate is minimum and the zone near to the side wall 5 where the flow rate is maximum, in a gradual and continuous manner.

[0048] As a second effect, the aforementioned dragging upwards determines, within said circulatory movement and, more specifically, in the ascending tract thereof, a thinning of the granules, a mutual spacing out thereof, which is more evident, indeed, near to the wall 5 of the container 2, in other words at the zone of said grid 3 where the density of the holes 11 is greater, in other words where the fluidification air flow rate injected into the fluid is greater.

[0049] At the greater thinning of the granules and where the temperature of the fluidification air is greater, the wetting of said granules by the growth liquid, fed in atomized form, takes place. Precisely because they are thinned, in other words well spaced out from each other, the wetting of the granules takes place in a very uniform and optimal manner. Consequently, the uniformity of growth of the granules themselves is improved. Moreover, where the aforementioned wetting of the individual granules takes place, the air of the flow responsible for the rotary stream of granules that has formed in the fluid bed is hot and uniformly and optimally takes care of the evaporation of the possible solvent used in the growth liquid. Consequently, the thickness of the new layer of growth substance deposited on every single granule is uniform and optimal.

[0050] After wetting, the individual granules move towards the opposite wall 4 of the container 2, together with the granules that immediately follow them in the aforementioned circulatory movement, thus running through successive zones of the fluid bed in correspondence of which the grid 3 has bands 3b, 3a, with gradually reducing density of holes 11. In these zones of the fluid bed the upward thrust applied by the fluidification air, which is maximum near to the wall 5 of said container 2, decreases down to zero. For such a reason, near to said wall 4, the stream of the granules naturally deviates towards the bottom or grid 3 of the container 2.

[0051] In the course towards the bottom 3, the individual granules of said circulatory movement cross underlying layers of the fluid bed, which are gradually cooler. During this course the solidification/consolidation step of the growth liquid is carried out on the surface of every single granule, a step that is completed during the subsequent tract extending up to the wall 5 with the obtainment of respective granules having slightly increased volume and mass. From here every single granule begins a new growth cycle that is the same as the one described above, whilst it also moves towards the discharge wall (helical movement of the vortex).

[0052] In accordance with this embodiment of the invention, the granules produced are polydispersed, in terms of particle size, in a very limited range, with respect to that which has been possible up to now with fluid bed granulation processes according to the prior art.

[0053] This is advantageously made possible thanks to the fact, quoted above, that every single granule of substance is subjected to substantially the same growth process, since the operating time of each cycle (wetting, drying, exsiccation and solidification) and the number of cycles to be carried out inside the fluid bed can be controlled, controlling the variations in flow rate of the fluidification air flow in the different zones of said fluid bed.

[0054] Moreover, thanks to the granulation process according to the present invention, there is a substantial reduction in the formation of powders, with respect to the processes according to the prior art. This means a reduction, if not even an elimination, of the apparatuses necessary for the recovery of such powders which, together with the possibility of obtaining a finished product of suitable particle size, i.e. directly sellable, allow the investment and maintenance costs, as well as the energy consumption, of the relative granulation plant to be considerably reduced.

[0055] Last but not least, the use of the fluidification air to induce and maintain the aforementioned circulatory vortex movement in the fluid bed of granules in growth, advantageously allows the use of additional external energy sources to achieve the same purpose to be avoided and consequently reflects positively on the efficiency of the process, decreasing consumption.

[0056] According to an alternative embodiment (FIG. 4) of the present invention, the distribution of the holes 11 in the grid 3 is uniform, but the holes themselves have different diameters. In particular, the diameter of the holes gradually increases as one approaches the wall 5 on which the distributor-supplier 10 is supported; i.e. there are large holes near to the wall 5 and progressively smaller holes as one approaches the wall 4. In this case the variation of fluidification air flow rates is gradual and varies between the zone where it is minimum and that where it is maximum, in the same way as the embodiment of FIG. 3.

[0057] According to a further embodiment of the invention, the formation and the maintenance of the circulatory vortex movement of the granules, described above, are obtained not by dividing the fluidification air flow into a plurality of portions of different flow rate, but by suitably varying the entry direction of such a flow into the fluid bed.

[0058] For such a purpose, for example (FIGS. 5, 5a), the holes 11 of the grid 3 are uniformly distributed, they all have the same diameter, and they are all equally inclined on the horizontal by a predetermined angle .alpha., preferably between 30.degree. and 60, for example 45.degree.

[0059] The inclination of said holes is selected in such a way that the thrust of the air on the granules has a vertical component such as to ensure the support of the fluid bed and a horizontal component that allows the creation and maintenance of the rotary motion of the granules in the fluid bed.

[0060] As an alternative to this embodiment, the holes 11 of the grid 3 are uniformly distributed, they all have the same diameter and are vertical; the grid 3 is equipped with deflectors 20 (FIG. 5b) consisting of metallic foils inclined towards the wall 5 of the container 2 by a predetermined angle .alpha. to the horizontal preferably between 30.degree. and 60.degree., for example 450, associated with, preferably welded to, said grid 3 at the holes 11 and with a predetermined distance from the holes 11.

[0061] In this way, the fluidification air coming out from the holes 11 is directed, in the same way as the embodiment represented in FIG. 5a, in such a way as to allow the formation of the aforementioned rotary stream of granules inside the fluid bed. With reference to FIG. 6, a further advantageous variant embodiment of the granulation process of the present invention provides the formation of two opposite circulatory movements of granules, V1 and V2, in the same fluid bed of the substance to be granulated.

[0062] For such a purpose, the container 2 is equipped, on both of the opposite long side walls 4, 5, with respective distributors 10a, 10b for supplying flows L, L1, of the same growth liquid and with a grid 3, with through holes 11, arranged according to symmetrically opposite and equal distributions, with respect to a middle axis M-M. In such a FIG. 6, the details of the granulator 1 that are structurally and functionally equivalent to those illustrated in the previous figures are indicated with the same reference numerals. In particular, the through holes 11 of the grid 3 are of the type described with reference to FIGS. 3-5b.

[0063] Thanks to this embodiment it is possible to double the production capacity of the granulator intended to carry out the granulation process according to the invention, keeping the operating conditions of the fluid bed constant.

[0064] In accordance with a further variant embodiment of the granulator 1 of the present invention, the seeds Si and the flow L1, L2 comprising the growth liquid are fed into said fluid bed in correspondence of at least one same side wall 4, 5 of the container 2.

[0065] Such a container 2 has a bottom or grid 3 (FIG. 7) equipped with holes 11 distributed, in two symmetrically opposite zones, in the same way as the one described with reference to the previous embodiment illustrated in FIG. 4; said bottom is also equipped with a plurality of slits 14, for discharging granules, of a suitable size and with a width correlated with (greater than) the diameter of the granules intended to be produced. It goes without saying that the present embodiment of the invention can also be used with the other embodiments of the invention, relative to FIGS. 3-3a and 5-5b.

[0066] The discharge of the finished granules from the bottom 3 of the container 2 occurs by gravity, preferably in countercurrent to a flow A of air or another suitable classification gas fed into said fluid bed through said slits 14. According to this embodiment, the rear wall 7 is, of course, without the opening 8.

[0067] The invention thus conceived is susceptible to further variants and modifications all of which are within the scope of protection of the invention itself, as defined by the following claims.