Dry Granulators are machines used for dry granulation.
Dry Granulation is a type of granulation process in which granules are formed without using liquid solution. Formation of dry granules requires dry powder densification and for agglomeration.
Blog Contents
1 Factors that affect the rate and size of granules produced by a dry granulators
2 Sub-classes of dry granulators
3. Slugging
4. Factors affecting slugging process
5. Roller Compaction
6. Advantages of Roller Compactor
7. Pharmaceutical uses of Dry granulators
8. Advantages of Dry Granulators
9. Disadvantages of Dry Granulators
Dry granulation is a way to make granules without using liquid. This method is useful when the powder is sensitive to moisture or heat.
Instead of using liquid, the powder is compressed under high pressure to make it dense and stick together. There are two main methods:
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Slugging (Tablet Press):
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The powder is pressed into large tablets (called slugs).
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Then, the slugs are broken down into granules.
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Problem: the powder may not flow evenly, which can cause uneven granule sizes.
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Roller Compaction (Preferred):
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Powder is fed into two rollers using a screw feeder.
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The rollers press the powder into a sheet (ribbon) or pellets.
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These are then milled into granules of the right size.
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This method gives denser, more uniform granules.
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Steps in Roller Compaction:
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Powder is moved to the rollers (using a screw feeder).
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Rollers press the powder into sheets or pellets.
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Sheets/pellets are milled into the desired granule size.
The final product is dense, sharp-edged granules that can be blended and compressed into tablets.
Different Stages of Wet Granulation
Granulation End-Point
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The end-point in granulation is the stage where the granules have the right properties, usually defined by:
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Particle size (average size or distribution), or
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Flow behavior (rheology, like viscosity/density).
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Once the correct end-point is reached, the granules and the final tablets will have similar quality, no matter how the process factors (like impeller speed, chopper speed, or binder rate) were adjusted.
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This idea is called the principle of equifinality → different paths, same outcome.
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The main goal of measuring during granulation is to estimate:
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Viscosity (how thick/sticky the mix is)
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Density of granules
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Particle size and distribution
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One way to track this is by measuring the load on the impeller (how hard it works).
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Mixer instrumentation (sensors on the machine) is useful because it can:
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Help find the end-point.
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Detect machine problems (like worn gears).
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Identify mixing or binder issues.
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Ensure batches are consistent.
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Help improve the process and scale it up.
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1. Factors that affect the rate and size of granules produced by a dry granulators
1. Roller speed
2. Feed rate
3. Roll design
4. Difference in shape and size of screws
5. Powder cohesiveness, density and flow characteristics
6. Particles size
7. Machine type
8. Die diameter
9. Feed hopper and feed frame
10. Tooling features
11. Compression speed.
12. Slugging pressure
2. Sub-classes of dry granulators
Dry granulator sub-classes are primarily distinguished by the densification force application mechanism and they include;
1. Slugging
2. Roller compaction
3. Slugging
In slugging, a large tablet (slug) is produced in a heavy-duty tableting press. This mechanism of dry granulation leads to variation in the weight of one slug to another.
This variation is as a result of the poor flow rate of powders with small particle size which in turn causes large fluctuation in the forces applied onto the individual slugs which bring about differences of the slug’s mechanical strength.
The mechanism of dry granulation is hardly used because the properties of the granules obtained by milling of the slugs cannot be controlled well either.
4. Factors affecting slugging process
Other disadvantages of slugging include;
Single batch processing
Frequent maintenance change over
Poor economic of scale
Low manufacturing throughput per hour
Excessive air and sound pollution
Increased use of storage containers
Increased need for manufacturing space.
Increased logistics
More energy and time is required to produce 1kg of slug than 1kg of roller compact.
Factors affecting slugging process
Powder cohesiveness, density, and flow characteristics
Powder particle size distribution
Machine type
Feed hopper and feed frame
Die diameter
Compression speed
Slugging pressure
5. Roller Compaction
Roller compactors commonly referred to as chilsonator is a dry granulating equipment that produces a sheet of powder material by squeezing powder between two rollers. The roller compactor consists of two driver rollers running in opposite directions from the inside to the outside. The rollers are made of stainless steel and are equipped with teeth to produce a powerful grinding action by forcing the material against a series of breaking combs.
Picture of the parts of a dry granulator
The roller compactor or chilsonator generally consists of three major units.
Feeding system:
This unit conveys the powder to the compaction area between the rolls.
Compaction unit:
This is where the powder is compacted between two counter-rotating rolls to a ribbon by applying a force.
Size-reduction unit:
This is for milling the ribbons to the desired particle size.
Fines produced during granulation are very low (max 25%) and can be reduced to 15% by careful adjustment of the process.
Roller compactors can be classified into;
Fixed roller compactor: Roller compactor equipped with a fixed gap.
Variable gap roller compactor: A roller compactor with a floating gap.
Classifications still possess the three major units of a roller compactor. But they differ in the way in which the gap between the rolls is realized.
6. Advantages of Roller Compactor
Simplifies processing
Facilitates powder flow
Uses minimal energy to operate
Improves drug dosage weight control.
Requires less man-hours to operate
Reproduces consistent particle density
Produces good tablet and capsule disintegration
Eliminates aqueous and solvent granulation
Uses fewer raw materials
Eliminates water-induced degradants
Improves process circle time
Prevents particle segregation
Facilitates continuous manufacturing
Improves content uniformity
Does not require explosion-proof room or equipment
Produces a dry product that is process scalable.
Pharmaceutical uses of Dry granulators
1. Dry granulators are used when the drug does not compress well after wet granulation.
2. Dry granulator is the best choice for moisture sensitive drugs like Aspirin, vitamins.
3. It is used in granulating thermolabile drugs.
Advantages of Dry Granulators
1. It requires minimal floor space
2. The machine is easy to clean after use
3. The roller compactor is suitable for hard continuous operation as well as batch production
4. Dry granulators eliminate the addition of moisture and heat
5. Mechanical strength of the product is uniform
6. Low running cost/low operating cost and highly efficient process
7. It eliminates the need for binding solution
7. Disadvantages of Dry Granulators
1. It does not permit uniform color distribution
2. Dry granulation process creates more dust
3. There is increased potential for cross-contamination