Fluid bed granulation process and apparatus

European).

16706 / EP / 06

EP1635938

The method of granulation of the suspension bed and the device

Field of the Invention

In the most general sense, the present invention relates to a method of granulating a suspension bed of a suitable substance such as, for example, urea, ammonium nitrate, ammonium chloride and similar substances prone to granulation.

In particular, the invention relates to a method of granulating a slurry bed comprising controlling the bed temperature by introducing a flow of hot air into it.

The invention also relates to a granulation apparatus used to carry out the above-mentioned process.

State of the art

It is known that in the process of granulation of a suspension bed, the obtaining of a predetermined substance is carried out by a constant increase (volume and mass) of embryos of granules of such substance in a continuous way supplied to the suspension bed, simultaneously by means of a flow of a suitable substance growing in a liquid state.

In general, the increment has the same properties as the substance to be granulated and is in liquid form, suitable for moisturizing, adhering and clotting on the embryos and on the incremental granules, which at the same time constitute a suspension bed.

The increment is delivered to the suspension bed at a high temperature so that the increment itself, once solidified on the embryos, can retain the adhesion properties that allow the next increment to adhere to the granules when it is inside the suspension bed.

In addition, inside the suspension bed it is necessary to maintain the temperature at predetermined and generally relatively high values ​​in order to allow evaporation of the solvent present in the increment, which is generally supplied in solution, for example aqueous in the case of urea, to the suspension bed.

The temperature of the suspension bed must be selected also taking into account that the possible cooling of the increment, before it comes into contact with the embryos and the incremental granules, could determine its premature clotting, with subsequent difficulties, if not the actual impossibility of adhering to the granules and the formation of powders, which requires subsequent restoration to normal.

To meet the abovementioned requirement, in other words to control and set the suspension bed temperatures within predetermined values, it has been proposed to supply, to the granulation suspension bed itself, an additional flow of suitably hot air, which is preferably introduced at the same level as the flow of the incoming substance.

In addition, during the initial stage, or at work with low loads, or in other words when the fluidizing air, which is used at the very high flow rates required for the formation and persistence of the suspension bed, is at a particularly low room temperature, it becomes necessary to carry out the appropriate preliminary heating this fluidizing air with suitable heat exchangers outside the slurry bed.

Despite having favorable features from some points of view, the known proposition has a serious recognized disadvantage.

Indeed, due to the very high velocity of air circulating in the suspension bed, temperature control according to a predetermined proposal necessarily entails very high energy consumption for pre-heating of fluidizing air (on demand) and additional air flow. This energy consumption has a negative impact on the operating costs of the method.

The presence of such devices for preheating the air also has a negative impact on the costs of implementation and the structural complexity of the corresponding granular installation.

Document US-A-2635684 describes a method of granulation comprising melting of a molten substance and cooling using two suspension beds. One gaseous stream is used for and drying droplets of molten granules, a single fluidization of the substance beds.

The essence of the invention

The technical problem underlying the present invention is to develop and provide a method for granulating a suspension bed of the type mentioned above, having functional properties that overcome all the cited defects in the prior art, and in particular so that the total energy consumption required to maintain the suspension bed at a predetermined temperature which ensures optimal process completion was substantially reduced.

This problem was solved in accordance with the invention by a method of granulating a suspension bed of a suitable substance, with bed temperature control, comprising the step of cooling the finished hot granules obtained in this manner in a relative cooling suspension bed, characterized in that at least a portion of the fluidizing air coming out of the cooling suspension bed from the finished granules is delivered to the granulation suspension bed.

Preferably, all fluidizing air supplied to the granulation bed comes from the cooling bed.

Preferably, substantially all of the fluidizing air exiting the cooling bed is used as the fluidizing air of the granulation bed.

Equally more preferably, the method of granulating a suspension bed of the present invention is characterized in that it uses one single flow of fluidizing air to continuously form and support, successively, cooling and granulating suspension beds, generally arranged in series relative to this single flow and in communication with each other through fluid .

Further features and advantages of the invention will become more apparent upon reading the detailed description of the embodiment of the suspension bed granulation method according to the invention, given below with reference to the accompanying drawing, for the purposes indicated and in no way limiting it.

Brief description of the figures

- figure 1 schematically shows an axonometric view of the device for carrying out the suspension bed granulation method according to the present invention;

- fig. 2 schematically shows a section of the same device in fig. 1.

Detailed description of the preferred embodiment

With reference to the drawings, the apparatus for implementing the method of granulating a suspension bed in accordance with the present invention is generally indicated by reference number 1.

Such a device comprises a self-supporting structure 2, essentially in the shape of a parallelepiped container, which defines space A, in its interior in which it is intended to realize two suspension beds F1 and F2, as will be more clearly seen in the remainder of the description.

The above-mentioned container structure 2 (which will be more simply called container 2 below), has long side walls 5, 6, short front 7 (or top) and rear 8 walls; it is closed at the top by a conventional and therefore not represented cover, and is fitted on the bottom with a double base plate 4, 4a, upper and lower respectively.

According to the characteristics of the present invention, the top wall 7 of the container 2 has a bottom side 7a, distant from the base plate 4, of the above-mentioned double base plate, with which it thus defines the passage (or through hole) 20 that binds space A in communication with the external environment of this container 2. In addition, in accordance with another feature of the present invention, the above-mentioned base plates 4, 4a extend from the rear wall 8, of this container 2, to the above-mentioned top wall 7, to a predetermined part of the length. At their free end ends, a front panel 17 is attached to the base plates 4, 4a, preferably substantially parallel to the apical wall 7, with which it forms a kind of pocket 18, in the example of the figures extending across the entire width of the wall 7 and in communication with space A, through the above passage 20.

The base plates 4, 4a of the above-mentioned double base plate, the rear wall 8 of this container 2 and the front panel 17 define a chamber 19 which is in fluid communication with the space A through the base plate 4, equipped with holes, a grate or in any case permeable for gas flow. The aforementioned chamber 19, extends below space A, has a limited height and is intended to form a chamber with uniform distribution of the flow of fluidizing air entering the space A, which will be described in more detail below.

Preferably and in accordance with further characteristic features of the present invention, the above-mentioned distribution chamber 19 has a conical profile starting from the rear wall 8 of this container 2 towards the front panel 17. To this end, the base plate 4a is made inclined to the opposite base plate 4, and coincides with it towards the above-mentioned front panel 17.

Inside the container 2, the rectangular vertical panel 15 is supported, in parallel and in a predetermined ratio of distance from the rear wall 8 of this container 2 with which it defines a slot 16.

The above-mentioned panel 15 is attached to the opposite long walls 5 and 6 and to the apical wall 3 of this container 2, while it has a horizontal bottom side 15a distant from the base plate 4 so as to define the passage (or through hole) 15a with it, suitable for binding the gap 16 in communication with space A inside the container itself. The slot 16 is in communication with the space A, also close to the top wall of this container 2, through the opening 11.

Inside the container 2 and at a predetermined distance from its base plate 4, there is a rectangular shelf 14, circumferentially attached to the long sides 5, 6, to the front wall 7, this container 2 and to the above-mentioned panel 15. The shelf 14 defines in space A the granulation zone B inside space A and is intended to support the granulation bed F1 of a predetermined substance; for this purpose, the shelf 14 is perforated, checkered or in any case permeable to the flow of fluidizing air necessary for the formation and maintenance of the aforesaid bed F1.

In Figure 1, the embryo distribution device (known as such) of the granules of the substance to be granulated, located in the container 2, at its top, is schematically depicted by reference numeral 10, while the fluid distribution and delivery devices with incremental granules (also known and therefore not shown in detail) are schematically illustrated by references 12 and 13.

In Fig. 2, the opening is schematically represented by reference number 22, associated with the rear wall 8, for introducing air into the chamber 19. The opening 22 is in fluid communication with means known per se and therefore not shown, for blowing air into the above-mentioned chamber 19.

With reference to the device of Figs. 1 and 2, an example of the granulation method according to the present invention is described below. By providing a continuous flow of embryos of the granules of a predetermined substance and at the same time a continuous flow of the substance growing into the granulation zone B, the granular suspension bed F1 is created on shelf 14. This granular bed is obtained, supported and maintained by a continuous flow of fluidizing air supplied to chamber 19 and hence, through base plate 4, to space A, below shelf 14. According to the course of granulation (growth of granules), the height of the suspension bed F1 increases, until its free surface reaches the (previously calculated) hole level 11. At this point, through the opening 11, which essentially acts as an overflow, starts pouring (or "unloading") from the bed F1 into the gap 16, granules of the substance, which are very hot (their temperature depends on the temperature of the incoming substance) and above all, which they are ready, in other words for a predetermined grain size.

From the beginning of this pouring, the height of the granulation bed F1 remains substantially constant.

The finished granules, continuously passing through the gap 16, "fall" in a substantially guided manner, or cascade, onto a suspension bed F2 containing finished granules ordered by an initial step, adjacent to the perforated base plate 4, where they are subjected to the above-mentioned flow fluidization air used for the F1 deposit. On this base plate 4, a second suspension bed F2 is thus defined, consisting exclusively of finished granules that extend in space A, on base plate 4, in slot 16 and in pocket 18, and which communicates with the above-mentioned space.

There is a lower pressure on the free surface of the suspension bed F2 in the gap 16 and pocket 18 compared to that which can be measured on the free surface of the suspension bed F2 in chamber A, between the walls 7 and 15; for this reason and because the three feeding zones are functionally comparable to connected vessels, the height of the suspension bed F2 in the gap 16 and in the pocket 18 is greater than that between the walls 7 and 15, on the base plate 4.

It should be noted that the cooling suspension bed F2 is in fluid communication with the above-lying granular bed F1 only through shelf 14, to support this bed.

It should also be noted that the above-mentioned gap 16 performs the function of a channel, the so-called downfall channel, for moving the granules from the bed F1 to the bed F2.

In the F2 suspension bed, hot finished granules exchange heat with the flow of fluidizing air delivered at room temperature. While the finished granules cool down, then the air is heated by them. And the same heated air coming out of the F2 slurry bed is used as the fluidizing air of the F1 granulation bed, to which it is supplied.

For this reason, the zone of space A between the base plate 4 and the underlying shelf 14 is known as the cooling zone of the granules and at the same time, it can be considered as the zone for pre-heating the fluidization of the air of the granulation bed F1.

Supplying preheated fluidizing air to the granulation bed means, on the one hand, providing the amount of air necessary for the formation and persistence of the slurry bed and, on the other hand, feeding into the same bed the amount of heat necessary to reduce or even prevent premature clotting of the incremental substance and at the same time to allow the solvent to evaporate, probably present in the increment supplied in solution to the incremental suspension bed.

The use of air coming out of the suspension bed of the cooling granules as preheated fluidizing air into the granulation bed also means a reduction in the total air consumption to complete the granulation process.

The height of the slurry bed F2 (the bed that cools the granules and the bed that preheats the fluidizing air) is such that its free surface in the pocket 18 reaches the upper edge of the front panel 17, ensuring that ready and cooled granules are discharged into the external environment of this container 2.

Since the suspension bed, as is well known, behaves completely in a comparable manner to a liquid, the levels of the granules in pocket 18, in slot 16 and in space A are stabilized at relative piezometric heights.

Therefore, it should be noted that the height of the front panel 17, by determining the height of the suspension bed F2, also determines the average residence time of the finished and hot granules in the cooling zone and, as a result, determines both the temperature of the finished granules unloaded from the container 2 and, above all, the temperature of the pre-heating fluidization air.

From the beginning of the above-mentioned "discharging" of the finished granules, the method of the present invention and the related device are in a working condition.

At this point, the basic property of the method of the present invention should be noted: F1 and F2 beds, respectively for granulation and cooling of the finished granules / pre-heating fluidizing air, are created and supported by the same fluidization air flow relative to the beds F1 and F2 are ordered in series.

A second feature of the above-mentioned method is that the ready and hot granules are essentially cascaded from the granulation bed to the cooling bed.

The main benefit achieved by the present invention is, as established, a large saving of energy consumption relative to what was previously required to carry out suspension bed granulation processes from the prior art, where temperature control is provided by introducing an additional flow of hot air or by preheating by means of heat exchangers, of the fluidizing air itself during predefined cycle settings of the granulation process. Taking into account the fluidizing air and the large amounts of additional hot air associated with these processes, the above-mentioned energy saving translates into an equally significant reduction in the operating costs of the process.

This advantage was made possible by using one air flow to fluidize the cooling bed of the finished granules and subsequently the granulation bed, which entails the effective preheating of the air, which is then delivered to the second operation.

According to a preferred embodiment, the front panel includes a height-adjustable movable partition 21 (able to slide vertically).

By changing the height of this front panel 17, the height of the second slurry bed F2 is thereby changed. That is, for example, if, for example, the height of this front panel 17 is increased, the height of the second slurry bed F2 increases and, as a result, the average time of granules remaining inside this bed also increases.

This entails improved heat exchange between the above-mentioned granules and fluidizing air, which may increase its inlet temperature to the F1 granulation bed.

The present invention may include further variations and modifications, e.g. the pocket 18 and the slot 16 may have a smaller width than the corresponding width of the corresponding short front wall 7, panel 15.

16706 / EP / 06

EP1635938