Matthew Walczer and Albin Friedrich | Alexanderwerk
This article discusses how dry granulation can help ensure a homogenous powder blend and flow for a tablet press. The article also discusses supplier-conducted tests on how recycling screened-out granules back into the raw powder affects that blend.
Making tablets from powders is a mature technology, and some of the same powder handling problems that impacted the industry in its early days continue to challenge scientists and process engineers today. For instance, efforts to get a homogeneously blended powder mix to flow well and resist segregation is a concern that can be managed but not eliminated. That’s because during handling, the physics of powder movement pushes particles to segregate based on size and mass. A segregated powder going to a tablet press, for instance, will produce tablets that vary in weight and content. This is unacceptable.
Dry granulation by use of a roller compactor can help avoid this less-than-desirable state and shepherd a homogenous powder blend through the manufacturing process to a tablet press. Basically, dry granulation involves using compaction and comminution to agglomerate and then break these poorly flowing powders into granules. (In wet granulation, a liquid or alcohol binder is added to the process along with a downstream drying step.)
During roller compaction, powders are compressed and densified between two counter-rotating rollers and then compression-milled to a granule of a certain size. This yields a powder that’s physically different but chemically unaltered and has flow characteristics that are more predictable and reliable than the original raw material feed.
In an ideal world, the process begins with a perfectly blended powder. This material is delivered to a feed hopper and conveyed by a feed screw to the set of counter-rotating rollers. These come in various configurations and sizes depending on the equipment manufacturer.
The powder then reaches what are known as the “slip” and “nip” regions of the process where particles react to the compression force exerted on them. The “slip” region is the first phase where the powder particles move with respect to each other. The “nip” region is the second phase in which the particles cease to move with the same speed and position as phase one and move relative to the rollers.
There are four process phases that compress and transform powder into a cohesive, densified ribbon or “flake.” These phases are rearrangement, deformation, fragmentation, and bonding.
Particle rearrangement. Particle rearrangement occurs as the powder begins to move, with particles filling the interparticle spaces. Air also begins to move out of the spaces between the particles. Particle shape and size are key factors in the effectiveness of this step.
Particle deformation. Particle deformation occurs as the pressure forces exerted by compaction between the rollers increases the points of contact between particles. This causes plastic deformation, which is a permanent change to the shape of a solid body without fracture.
Particle fragmentation. Particle fragmentation happens as the compression forces increase and cause particles to fracture into smaller particles. This creates multiple new contact points and surface and bonding sites.
Particle bonding. Particle bonding occurs at the compression pressure setpoint when plastic deformation and fragmentation occur, allowing Van der Waals forces to create a stable bond between particles.
The powder, having been compressed and densified in this way, exits the rollers as a ribbon or “flake.” One or more size-reduction techniques are then employed to change that densified flake into granules. This is typically accomplished by using a rotating bar to press the densified material through one or more screens or wire screen mesh of certain sizes. When done successfully, the resulting granulated material will have a particle size distribution that meets the final material specification and can be reliably and predictably handled and delivered to a tablet press for transformation into a stable oral solid dosage form.
The significance of this process is that it helps in maintaining powder blend homogeneity. For example, a powder comprised of three ingredients that differ in size and mass will separate based on those parameters if no action is taken to prevent the segregation. By compressing the homogeneously blended powder in this process, the powder’s homogeneous makeup is locked using Van der Waals forces to create granules that are a blend of all three ingredients. Depending on the particular characteristics of the powder blend and depending on the final product’s particle size distribution requirements, additional mechanical separation techniques can be employed to separate and capture fines that have escaped the rollers or been sieved out of the final material. This yields a granulated powder with a tight particle size distribution of the homogeneous granules.
Reintroducing (recycling) off-size granules into the raw material feed
What happens to the segregated material that was screened out of the final material? Does it become waste? The answer to that question varies from product to product and from company to company. In general, however, a reasonable assumption would be that the desired objective would be to capture the material and find a way to reintroduce it back into the process to eliminate waste. Taking this step, however, raises the question of whether the captured material can be reintroduced to the raw material process feed in a way that continues to maintain the desired content homogeneity. Would this reintroduction produce a process feed stream that has significantly different physical properties than a stream without the recycled material?
An experiment was conducted to answer these questions and demonstrate if strategic reintroduction of the captured material would produce a compacted flake that maintains the original blend homogeneity without significantly changing the final product.
The process efficiency of blend uniformity was studied by simulating recycle feeds and observing the resulting output. This was achieved using a dual-chamber feed hopper design available on some production-scale roller compactors. The dual-chamber feed allows recycled material to be directed to the feed stream using a segmented compartment in the main feed hopper where recycled material is combined with raw material process feed.
The experiment used lactose powder as the raw material and red extrafine sugar as the simulated recycled material. The intent was to produce a qualitative, visual representation of how well the captured and redirected material could be reintroduced to the feed stream.
During normal dry granulation operation, the fines generated are commonly recycled into the feed hopper. Utilizing a feed system that allows for controlled, even delivery of the recycled material is critical to maintaining the efficient mixing of a recycle stream. The functionality of this type of feed system eliminates the potential of nonhomogeneous product mix from affecting the compaction process. In other words, the system shouldn’t process raw material and recycled material differently. Rather, the recycled material should be evenly mixed into the raw material feed to create a homogeneous process feed stream that can be maintained for the process run.
A stream of red, extrafine granulated sugar representing the segregated and captured fines was dosed into the recycle section of the feed hopper at three different rates: 5.0 kg/h, 10.0 kg/h, and 20.0 kg/h. The roller compactor was configured to run at approximately 100 kg/h of total throughput. The objective criteria for the testing was to generate a uniform red granule (sugar) distribution in the flake exiting the compaction rollers of the machine. Over a period of several minutes, the sugar distribution pattern was observed and evaluated for consistency and homogenous distribution. The process was repeated at the different speed setpoints and the results were compared. A consistent uniform distribution indicated efficient mixing action of raw and recycled material from the feed assembly.
Upon reaching a steady process state, several samples of the flake at the three different doses were obtained and evaluated for visual confirmation of the red sugar dispersion. The sugar appeared to be uniformly dispersed throughout the flake at all three dosing rates, as shown in Figure 1. This observation remained constant over the entire test period.
Visual comparison of the three different feedrates indicates that reintroducing off-size recycled granules using a dual-chamber feed hopper design delivers efficient mixing, and segregation concerns can be nearly eliminated by granulation of the powder. When considering dry granulation utilizing a screening operation for recirculation of fines (at whatever cut point is effective for the process), a well-designed recycle loop with an efficient feed mechanism will produce a batch that’s optimized for downstream operations. This helps to minimize concerns in final product quality that are traced back to inconsistencies in raw material content homogeneity.
For further reading
Matthew Walczer is national sales manager for Alexanderwerk Inc. He has a BA in biology.
Albin “Al” Friedrich also is national sales manager at the company. He has a BA in mechanical engineering. Both men have been with the company for nearly 3 years.
Alexanderwerk • Montgomeryville, PA
215-442-0270 • www.alexanderwerkinc.com
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