METHOD AND A DRUM COATER FOR COATING SMALL ITEMS, SUCH AS TABLETS, AND A COATING SYSTEM COMPRISING SUCH DRUM COATERS

Abstract

There is described a drum coater including a substantially cylindrical drum having a peripheral drum wall and a predefined diameter, said drum having a substantially horizontal axis of rotation, a top section and a bottom section being defined above and below, respectively, the axis of rotation, said drum being adapted to contain tablets, a coating zone, one or more spray nozzles directed towards the coating zone, and driving means.

Claims

1. A drum coater comprising: a substantially cylindrical drum (11) having a peripheral drum wall (12) and a predefined diameter, said drum having a substantially horizontal axis of rotation (17), a top section (20) and a bottom section (21) being defined above and below, respectively, the axis of rotation, said drum (11) being adapted to contain tablets, a coating zone, one or more spray nozzles (26) directed towards the coating zone, and driving means, wherein the driving means is adapted to set the drum (11) to spin at a rotational speed such that the tablets are pressed towards the periphery of the drum (11) by the centrifugal force and the tablets are held against the peripheral drum wall (12) producing a substantially annular bed, and that cascade creating means are provided by loosening means, such that a cascade of tablets is created in or partly in the top section (20) of the drum.

2. A drum coater according to claim 1, wherein the loosening means comprises one or more deflecting nozzles (13) provided at the top section (20) to provide a jet or jets of air or gas.

3. A drum coater according to claim 1, wherein said loosening means comprises mechanical deflection means.

4. A drum coater according to claim 1, wherein the drum is provided with perforations.

5. A drum coater according to claim 1, wherein the drum is provided with a cut in the peripheral drum wall, creating facing first and second end edge sections (24, 25), at least one of said first and second end edge sections being deflected to create an interspace (23).

6. A drum coater according to claim 5, wherein the drum coater is provided with a mechanical blower directed towards the bottom section (21).

7. A drum coater according to claim 1, wherein the diameter of the drum lies in the interval 0.2 to 2 m and the width of the drum lies in the interval 0.04 to 1 m.

8. A coating system comprising a number of drum coaters according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0029] FIG. 1a shows a perspective view of a drum coater in an embodiment of the invention;

[0030] FIG. 1b shows a front view of the drum coater in FIG. 1a, some of the parts thereof being removed for clarity reasons;

[0031] FIG. 2 shows a front view of a prior art apparatus;

[0032] FIG. 3 shows a schematic drawing showing some principles of the method according to the invention;

[0033] FIG. 4 shows a front view showing an embodiment of a drum coater according to the invention including a schematic overview of the principles of the method according to the invention;

[0034] FIG. 5 shows a side view in cross section of the drum coater of FIG. 4;

[0035] FIG. 6 shows a partial perspective view of an embodiment of the drum coater according to the invention and

[0036] FIG. 7 shows a front view of a detail of a drum coater in one embodiment illustrating the movement of the tablets.

DETAILED DESCRIPTION OF THE INVENTION AND OF PREFERRED EMBODIMENTS

[0037] FIGS. 1a and 1b show an embodiment of a drum coater according to the invention and generally designated 1. The drum coater 1 comprises a housing 2 to which first and second air supply tubes 3 and 4a, and an air discharge tube 5a are connected. Furthermore, a supply tube 6 for coating material and atomising air is connected to the interior of the housing 2 through a front cover 8 connected to the housing 2. The cover 8 and the supply tube 6 are removed in FIG. 1b for clarity reasons. In the embodiment shown, the cover 8 further comprises a removable port 8a carrying the supply tube 6 and closure means 9, which may be formed in any suitable manner.

[0038] A substantially cylindrical drum 11 having a perforated peripheral wall and a predefined diameter is journalled in bearing 7 and is rotational around a substantially horizontal axis of rotation 17. A front cover (not shown) closes the drum and a seal acts against the drum wall, and, as will be described in further detail below, keeps the air following the correct course and prevents tablet from escaping the drum during loading, acceleration to annular bed and coating speed and discharge. As shown in FIG. 1b, the second air supply tube 4a ends in an air inlet 4, and the air discharge tube 5a ends in an air outlet 5, the function of which will be described in further detail below. The same applies to the function of the first air supply tube 3. The drum coater 1 furthermore comprises means (not shown) for loading and unloading tablets into and out of the drum 11 from outside the housing 2.

[0039] Referring now in particular to FIG. 1b, the drum coater 1 comprises a top section 20 and a bottom section 21 defined above and below, respectively, the axis of rotation 17. The top section 20 and the bottom section 21 may be located at a distance from the axis of rotation, as indicated by the two dashed lines in FIG. 1a. The air inlet 4 is placed near the bottom section 20 and the air outlet 5 is placed near the top section 21. An air inlet/outlet may be placed in other positions beneficial for drying the particles being coated, and/or beneficial to the overall working of the process, and/or to the design of the apparatus to perform the process. E.g., the inlet may be placed at the horizontal axis and may communicate directly into the drum via bearing 7.

[0040] FIG. 2 shows a prior art apparatus, in which a drum (10) rotates clockwise about a horizontal axis of rotation (11). In operation with the drum revolving clockwise, the bulk of the tablets take up a position (B) in the lower left hand quadrant of the drum. Tablets positioned adjacent the perforated cylinder wall (18) of the drum are carried round by the wall and travel in an upward direction before tumbling over past the coating spray means (S) fixedly positioned and directed downwards inside the drum adjacent its axis of rotation (11). The tumbling tablet bed rolls over the mouth of members (12) in turn in order to provide for a movement of the tablets back through the coating zone.

[0041] FIG. 3 shows a schematic drawing showing some of the principles of the method according to the invention. When the tablets or other items to be coated have been fed into the drum 11, the drum 11 is set to rotate in the counter-clockwise direction. Obviously, the drum could be designed to rotate in the clockwise direction as well. The drum 11 containing the tablets is set to spin at a rotational speed such that the tablets are held against the wall producing a substantially annular bed. The rotational speed is then reduced until tablets in the top section begin to detatch from the annular bed thereby creating a cascade of tablets into the coating zone. In the drum 11, a coating zone is provided in or near the top section 20. Supply tube 6 (cf FIG. 1a) is led through a removable part (not shown) of the cover 8 in order to provide a set of spray nozzles (not shown in FIG. 3) with a coating material such as a suspension in a number of spray nozzles 16. Depending on the rotational direction the spraying nozzles are adjusted accordingly. A set of spray nozzles 26, cf for instance FIG. 4, figuratively shown by means of arrows 16 in FIG. 3, for providing the coating material is provided in the centre of the drum 11. The spray rate of the coating material depends on the size of the drum, the tablet load, and the air rate, and may for instance be about 8 ml/min of a 15% dry matter suspension. The number of spray nozzles can be varied from one to several spray nozzles. The spray nozzles spray upwards towards the coating zone where the tablets are cascading. The spray nozzles may also spray in other directions, even at tablets which are not a part of the cascade at a given time. Different solutions or suspensions may also be used, either at the same time or successively. After the tablets are fed into the drum, the tablets form a tablet bed 18 (cf FIGS. 4 and 7) in the bottom section 21. When the drum 11 is set into rotation, the tablets are spun in the drum at a rotational speed such that a cascade of tablets into the coating zone is created as described in the above, and the tablets are sprayed in said coating zone.

[0042] In order to provide or to improve the release of the tablets from the peripheral drum wall 12 in the top section 20, a loosening means may be provided. In the embodiment shown in the drawings, the loosening means is provided by at least one deflecting nozzle directed towards the tablets in said top section, thereby forcing the tablets away from the peripheral drum wall.

[0043] As the drum 11 spins and the tablets are pushed against the inside of the peripheral wall 12 of the drum 11, the deflecting nozzles 13 contribute to creating the cascading fall of tablets. The deflecting nozzles 13 make the coating process more stable and less dependent on the peripheral speed of the drum 11. The higher the peripheral speed the higher is the centrifugal force on the tablets, making it possible to use a higher flow rate of the sprayed material. However the the higher the peripheral speed the stronger the deflecting nozzles have to be in order to deflect the tablets from their course. The deflecting nozzles 11 may be formed in any suitable manner, for instance like an air knife providing a blade-shaped stream of air or gas. Other loosening means, in the form of mechanical deflection elements, such as baffles or a plough, for dispersing the tablets may also be used. By the word deflecting nozzle is meant air or gas jet(s), an array of jets or air amplifier(s).

[0044] A number of tests of different combinations of predefined diameter of the drum and rotational speed was carried out in order to provide exemplary values. Through the air inlet 4 a stream 14 of air or gas of about 70 C. enters from below at a rate of 60 m.sup.3/hour and flows through perforations (not shown in detail in FIG. 3) in the peripheral drum wall 12, and exits in stream 15 through the air outlet 5 at a temperature at a lower temperature. In this example the drum has a diameter of 220 mm and a width of 40 mm. The tablet load is 300 g and the spray rate is 8 ml/min.

[0045] In another example the drum has a diameter of 440 mm and a width of 80 mm. The tablet load is 1250 g and the spray rate is 36 ml/min. The flow rate of the drying air is here 150 m.sup.3/hour.

[0046] Without wishing to be bound by theory, the flow rate of the drying air depends on the amount of tablets in the drum 11 and the force at which they are pushed towards the inside wall of the drum. Furthermore the drum can contain a higher load of tablets due to the coating taking place in the falling cascade of tablets and not, or not only, when the tablets are tumbling around at the bottom of the bed.

[0047] FIG. 4 shows a front view and FIG. 5 shows a side view of an embodiment of a drum coater 1 according to the invention including a schematic overview of the principles of the method according to the invention. The drum 11 itself comprises the peripheral drum wall 12 and side walls 22 (cf FIG. 5). The drum coater 1 comprises a set of three spray nozzles 26 spraying upwards towards the cascading tablets. As indicated schematically in FIG. 4, the tablets form a bed 18 in the bottom section 21 of the drum, whereas in the top section 20, the tablets form a cascade 19. The pressure from the spray nozzles should not be higher than the tablets are retained in their cascade. In this embodiment the drum 11 spins with a speed of just above =(g/r) radians per sec. The position of the cascading tablets may change depending on the peripheral speed of the drum and the amount of pressure from the deflecting nozzles 13. The spray nozzles are not necessarily positioned in the centre of the drum 11, neither are they limited to spraying into the top section 20. The spray nozzle or nozzles may be placed on the side wall of the drum, preferably in a non-rotating part of the side wall.

[0048] Here it is shown how one spray nozzle 26 is mounted on the rotational axis 17, any other nozzles being located on or near the centre of the drum 11 as well. The bearing 7 defining the rotational axis 17 may furthermore include a combined drive shaft and a concentric nozzles feed as well as a drying air inlet.

[0049] FIG. 6 shows a partial perspective view of an embodiment of the drum coater according to the invention. This embodiment shows how the drum 11 comprising a perforated drum wall 12. The peripheral drum wall 12 has been provided with a cut in order to create an interspace 23 through which the tablets can be loaded. The first end edge section 24 is deflected inwards while the second end edge section 25 is deflected outwards. Through the interspace 23, it is possible to load and unload the tablets by means of gravity. Thus by placing the interspace 23 in the top section, the drum can be gravity loaded, and by placing the interspace 23 in the bottom section, the drum can be gravity unloaded. Of course, a conventional opening with a hatch may be used instead.

[0050] Means may be provided in order for the flexure to stay in place, for instance as very schematically indicated by 27 in FIG. 6. The flexure may be permanent or temporarily while the tablets are loaded and unloaded. It may also be possible that only one of the ends are displaced, for example inwards, and when the drum begins to spin, the pressure from the tablets will push the flexure back and the drum 11 return to its cylindrical shape.

[0051] FIG. 7 illustrates the movement of the tablets in the drum 11 during rotation thereof. In the bottom section 21, the tablets form the bed 18, whereas in the top section 20, the tablets form the cascade 19.

[0052] The same numbers refer to the same features throughout the description. Features from one embodiment may be combined with features from another embodiment.

[0053] Throughout the description and claims, reference is made to tablets. When referring to tablets, what is to be understood by this term is any particle, capsule or pill or other small items, which require coating. The shape may be flat sided, oblong, round or other. The purpose of coating items may e.g. be to protect or to modify the surfaces of such items.

[0054] The speed of the drum is important. In order for the tablets to stick to the inside drum wall, the pressure on the tablets must equal or exceed 0 g at the top and 2 g at the bottom. In order for this to happen, different angular velocities are required for different sizes of drum. In practice, the speed of the drum is dependent on a number of factors. A person skilled in the art will be able to decide on the correct speed corresponding to the operational conditions. In general, a drum having a diameter of below 1 m would rotate at a speed of at least 4 rad/s, and a drum having a diameter above 1 m rotates at a speed of at least 3 rad/s.

[0055] If only one layer of tablets is present in the drum, it would be possible to define a rotational speed for each drum size. However, when considering a bed of tablets with a thickness of e.g. 5-10 tablets the most inner tablets towards the axis of rotation requires a higher speed than the outermost tablets closest to the drum wall. In this case the innermost tablets would create a cascade while the outermost tablets would require some help in order to form a cascade and not stick to the drum wall. Furthermore the flow of drying air may influence the tablets motion.

[0056] Table 1 below shows calculated approximate minimum speeds for a variety of drum sizes.

TABLE-US-00001 TABLE 1 R Speed M M rad/s Rpm 0.25 0.125 8.86 84.6 0.5 0.25 6.26 59.8 0.75 0.375 5.11 48.8 1.00 0.5 4.43 42.3 1.25 0.625 3.96 37.8 1.50 0.75 3.62 34.5 2.00 1 3.13 29.9

[0057] However in reality a higher speed is required in order to form the cascade. For example when tumbling a 30 mm tablet bed in a drum with a diameter of 433 mm the calculated speed is 66.6 rpm. However, the actual rotational speed is 83 rpm for creating a cascade.

[0058] Speeds may vary depending on the load and type of tablet as well. A thicker tablet bed requires a higher speed as the innermost tablets are subject to less centrifugal force than the outer tablets for a given angular velocity.

[0059] A rather thick tablet bed, such as 30 mm, is preferred as this will create a more stable motion.

[0060] Processing time is reduced compared to known tablet coaters. As an example a 1.25 kg batch of tablets is coated in approx. 6 minutes.

[0061] The invention should not be regarded as being limited to the embodiments shown and described in the above, but several modifications may be carried out.