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A Description of the Spheronization Process

Spheronization

example of spheroids

Example of Spheroids

1mm mean diameter spheres produced as part of a process development investigation using Caleva spheronization equipment.

Spheronization has many advantages (link). The process is useful in several ways: it can improve the product; simplify manufacturing processes and help to reduce costs. The process is well known and widely used in the pharmaceutical industry but spheronization is becoming increasing recognized in other areas of industrial materials handling.

The extrusion of the product is a required step prior to spheronization (link). The size of the spheres are determined by the diameter of the extrudate used for the spheronization process. For example, in order to obtain spheres with a diameter of 1 mm, a 1 mm screen is used on the extruder, although spheres with a slighter bigger diameter will sometimes be obtained. In a spheronizer, it is possible to obtain spheres with a diameter ranging from about 0.5 mm to about 10 m but in practical terms the range 0.7 to 3 mm is considered normal. Larger sizes would have a poor product appearance (not round) and a low yield of product and smaller sizes would be difficult to extrude.

 

 

The Spheronization Process:

These are the basic steps in converting a pharmaceutical formulation into a spheronized product:

Basic Configuration

diagram of spheronizer

In principle the basic machine consists of a rotating friction disk, designed to increase friction with the product, which spins at high speed at the bottom of a cylindrical bowl. The spinning friction disc has a carefully designed groove pattern on the processing surface. This is most often crosshatched, but several sizes and other types are available.

 

 


diagram of spheronizer

Extrudates are charged to the spheronizer and fall on the spinning disc. At first, the cylindrical extrudate segments are cut into segments with a length ranging from 1 to 1.2 times the diameter. These segments then collide with the bowl wall and they are thrown back to the inside of the friction plate. Centrifugal force sends the material to the outside of the disc. The action of the material being moved causes the extrudate to be broken down into pieces of approximately equal length relative to the diameter of the extrudate. These cylindrical segments are gradually rounded by the collisions with the bowl wall, the plate and each other.

 


diagram of spheronizer

The ongoing action of particles colliding with the wall and being thrown back to the inside of the plate creates a "rope-like" movement of product along the bowl wall.

The continuous collision of the particles with the wall and with the friction plate gradually turn the cylindrical segments into spheres, provided that the granules are plastic enough to allow the deformation without being destroyed. It is essential that this rope movement is present for an optimal spheronisation

When the particles have obtained the desired spherical shape, the discharge valve of the chamber is opened and the granules are discharged by the centrifugal force.

The design principle of the spheronizer is relatively simple but additions and adaptations are available have widened the range of applications and greatly improved the flexibility of the machines. For example, the Caleva "Twin" incorporates two spheronizers which allow continuous productions of spheroids.

Key Spheronization Factors:

Disc Speed

There is an optimum disc speed and load for each disc diameter

Momentum too low:

Momentum too high (from under loading or disc speed too high):


The typical rotation speeds for disks < 500 mm in diameter is about 200 to 1000 RPM. The higher the speed, the more energy is imparted to the particle which results in more force during a collision. The optimum speed depends on the characteristics of the product being used and the particle size. In general, smaller particles require higher speed while bigger particles with (higher mass which results in more force during a collision) require lower speeds. In practice the optimum speed is determined by experimentation. Generally, it is best to start at a high speed and then to lower the speed in the final stage of the process. But again this can be determined by simple practical tests. The spheronization process allows considerable flexibility with most materials.

 

The Spheronizer Drum Charge Volume:

The optimum charging volume depends upon the machine size and the product characteristics; there is an optimum batch size introduced into the spheronizer chamber that will produce the most narrow particle distribution and the best spheres. A typical charge volume for a machine with a 380 mm diameter disc is about disc is 4 liters, depending on the density of the material. Increasing the batch size increases the hardness of the spheres and smoothens the granule surface.

 

Disc Groove Geometry:

Spheronizer disk with a radial pattern

The most common groove pattern used for spheronizer discs is the square cross-hatched square design, where the processing surface is covered with a grid of truncated pyramids. Generally, extrudates up to 0.8 mm in diameter are normally processed on a 2 mm pitch plate with a 3 mm pitch plate is used for extrudates up to 3 mm in diameter. Discs with a radial design are used for genteler action on the material being spheronized.

Disc Diameter:

Peripheral speed (related to disk diameter) was a major influence on the spheronization process

With constant peripheral speed:

Retention time:

Graph of residence time in the spheronizer versus time to make a sphere

Typical spheronization retention time necessary to obtain spheres is from 3 to 8 minutes. Again, this value is best determined by performing simple trials on specific products. Generally, speaking, the longer the spheronization time results in more perfect spheres at the expense of creating more dust. Typically, the edges of cylindrical granules are the most likely to break apart and generate dust during handling. Spheronisation with a short retention time can help to reduce the amount of dust significantly. If the objective is to reduce dust and not necessarily obtain perfect spheres, then a shorter processing time is sufficient to break the long extrudates into small segments and round the edges. The "roundness" of the spheres can be expressed as the One Plane Critical Stability.

Definition: One Plane Critical Stability (OPCS)- The measure of deviation from an ideal sphere

For an idealized sphere:

The angle through which the center of gravity of a particle outline has to be raised for the outline to roll

Product Parameters:

The particles must be plastic enough to allow deformation during collisions, but also must be strong enough to withstand collisions with the disc, other particles and the spheronizer wall without breaking up.

In some cases, the strong cohesive force in the extrudate prevent it from breaking into smaller pieces and therefore, more processing time is required.

The result obtained in the spheronizer depends on the rheology of the product, which can can be easily investigated using a Caleva Mixer Torque Rheometer - MTR (link).

Other Factors:

Table Summarizing the Different Types of Caleva Spheronizers
For Pharmaceutical Productin and Development

Types of Caleva Spheronizerx

Equipment
Description
Main Uses
Micro Spheronizer xxxxxxxxxxxxx Laboratory: small quantity (???g) development use
Spheronizer 120 Bench top Bench Top Spheronizer Laboratory
experimental / small scale production
(30 ?????)
Spheronizer 250 Lab scale bench top Spheronizer Lab/Production
Low cost-high output
(????)xx
Spheronizer 380 xxxxxxxxxxxxx Quality Spheroids
Output
?????
Spheronizer 500 xxxxxxxxxxxxx Quality Spheroids
Output ?????
     

 

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