Precision Granulation™

A new process for precision control of agglomeration and coating

By: Dr. Kim Walter and Bob Schurter (Niro Inc.)

Why the need for a new process?

While conventional top spray fluid bed granulation is a well-established process; there are a number of problems with this technology. Niro has developed a new granulation process, called Precision Granulation™ which offers:

• Better control of granule properties
• The ability to granulate soluble and hygroscopic materials
• The ability to granulate fine particles
• A more stable process
• Faster processing times

The Niro Precision Granulation™ process

Precision Granulation™

The Niro Precision Granulation™, process is the result of intensive study of the flow behavior of particles suspended in gas streams, and of the mechanics of contacting suspended solid particles (the feed) with atomized droplets of the wetting and bonding mixtures.

The key to the process is its ability to wet, agglomerate and dry a small quantity of particles in a fraction of a second.

How does Precision Granulation™ work?

A high velocity air stream, which is also rotating, is established in the central tube. Particles are picked up at the base of the tube and accelerated by the air stream.

While in the stream, the particles are contacted with liquid droplets produced by the spray nozzle at the base of the tube. Within the central tube the relative velocities of air, liquid droplets and particles are high so wetting is efficient and drying begins almost immediately. The agglomerates are dry by the time they leave the top of the tube.

The materials are not fluidized, they are pneumatically conveyed by the air stream. The air velocity is therefore not as critical as in a fluid bed device.

Precision Granulation™ High Volume Flow Granulation Liquid Low Volume Flow

At any given time, most of the feed material is in the outer "holding area", where the gas velocity is very low. Attrition is greatly reduced. The gas humidity is also low in the holding area so the material is dry and not tacky.

During a run, Individual particles may make repeated (typically from 10 to 1,000) cycles through the agglomeration tube, allowing very large agglomerates to be built up.

Comparison with conventional fluid bed agglomeration

Granulation Liquid Top Spray Fluid Bed Granulation Process

To understand the advantages of Precision Granulation™, it necessary to compare it with conventional fluid bed agglomeration processes.

Conventional fluid bed granulation processes work by wetting the primary particles to make them tacky, fluidizing the particles to allow them to collide, and then drying them to prevent over agglomeration and to meet final product moisture steps.

The Precision Granulation™, process uses a similar mechanism, but only a very small portion of the processor inventory is being processed at a time. This small portion is wetted, contacted in a high velocity pneumatic conveying system, and dried in a fraction of a second. The process is so rapid, surface moisture has less chance to penetrate the particles. This allows higher wetting rates to be used and faster agglomeration rates achieved. Preventing moisture from penetrating the particles eliminates or reduces the post-drying step and so reduces processing time. It also allows many soluble and hygroscopic materials to be granulated and facilitates the production of high density agglomerates.

Over Wetting

Conventional processes all involve wetting a large amount of material (essentially the entire inventory in the processor), and maintaining the material in a wet state for the entire granulation process (typically 15 to 30 minutes). During this time, moisture penetrates into the particles. Soluble materials can dissolve. Hygroscopic materials can become quite sticky and no longer fluidizable. The process requires a fine balance between underwetting (where the agglomeration rate is too slow) and overwetting, where the process can become impossible to operate.

Over wetting is much less of a concern with Precision Granulation™, because most of the inventory in the processor is dry.

Fluidizing velocity

Free Fall Velocity vs Particle Size

Fine particles can be difficult to granulate in the conventional fluid bed process. With very small particle size the viscosity effect of the fluidizing gas (usually air) can be quite pronounced. The gas velocity needed to fluidize a bed is typically in the range of 20 to 50% of the particle free fall velocity. For instance, 40-micron particles have a free fall velocity of .05 meter/second while 400-micron particles have a free fall velocity of 1.75 meter/second. This broad range of required fluidizing velocities makes it difficult to fluidize a bed containing a wide distribution of particle sizes, and difficult to control fluidization in a bed where particle sizes are growing (due to agglomeration).

With Precision Granulation™, the velocity is very high in the agglomeration section and very low in the rest of the processor. Neither chamber is really a fluid bed, so fluidizing velocities are not of concern. This makes it feasible to operate the unit with relatively small particles.

With Precision Granulation™, it is also possible to process a feed which consists of large base particles, plus fine particles of another material that is to be coated on the base particles, or distributed through the agglomerates.

Attrition

With conventional fluid beds the granulation process needs to continue for a relatively long time (typically 15 to 30 minutes). As the process continues, the fluidization and collision process breaks some agglomerates. The process then becomes a race between agglomeration and attrition. Faster agglomeration rates are needed to overcome the affects of attrition, but faster agglomeration rates can lead to overwetting and a collapse of the fluid bed.

Attrition can lead to fines in the product, causing higher than desired bulk density, dusty product, and poor dispersibility.

Attrition can be especially severe if lengthy post drying is required to meet final moisture specifications.

With Precision Granulation™, there is less attrition because the process is faster and because the post-drying phase can usually be eliminated.

Control problems

With conventional granulation, the particle collision process is somewhat random, with some particles agglomerating faster than others. This makes the process difficult to control because the charge becomes non-uniform (a large variation of particle size and particle moisture content).

As the process continues, and the feed particles become larger and wetter, the optimum airflow and spray rate change. This means a sophisticated control scheme or a highly trained operator is needed to get optimum results.

With Precision Granulation™, changes in the product charge occur more uniformly so control is easier.

Bed collapse

In conventional fluid bed agglomerators, the entire bed can become so wet it no longer fluidizes. When this occurs, the process must be stopped and the feed material manually shoveled out. The run is spoiled and the feed will either need to be dried and re-processed, or will be spoiled entirely. A bed collapse can happen in just a few seconds. It can also happen if the spray process continues for even a few seconds after a stoppage of the fluidizing airflow. (For instance, due to a power outage or machine shutdown).

With Precision Granulation™, the particles are wet for only a fraction of a second, and most of the material in the processor is dry. There is less likelihood of over wetting and bed collapse.

Bulk density control

The danger of overwetting has limited conventional fluid bed agglomeration processes to producing low bulk density, high surface area agglomerates. These are desirable for some applications (particularly where re-dispersion is a goal), but are undesirable for others (for instance, where the agglomerated material will subsequently be pressed into a solid shape - such as a tableted pharmaceutical or detergent, a powder metal part, or a timed release fertilizer).

With Precision Granulation™, the process is so rapid that, surface moisture has little chance to penetrate the particles. This allows many soluble and hygroscopic materials to be granulated. We have in fact successfully granulated powdered sugar.

High bulk density granules made on the Precision Granulator™

With Precision Granulation™, over-wetting is less of a concern, so it is possible to operate with very high humidity in the granulation tube. This makes it possible to produce high bulk density agglomerates.

The process is also capable of producing the low bulk density; high surface area agglomerates of interest where re-dispersibility is an issue.

Soluble and hydrophilic materials

With conventional granulation, the wetting agent can become physically or chemically bound to the particles, causing unwanted chemical changes and making post drying difficult. If the particles are soluble, the feed charge can actually liquefy. This has made it impractical to agglomerate many water-soluble materials.

More stable process

With Precision Granulation™, most of the product charge is dry, so the process is less affected by upsets. If airflow is interrupted there is no concern because the material is maintained in a dry and flowable state. It is in fact, possible to stop the process to take samples, or to remove and replace a plugged nozzle without stopping the process.

Scale up

The process is easy to scale up from pilot data. It is required only to scale the processing conditions (Temperature, humidity, and gas velocity) inside the granulation tube. Large units can use multiple tubes.