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Feeding And Conveying In Tablet Press Applications

Tue, 10/23/2007 - 9:40am
Tips to achieve an efficient and affective operation
By Sharon Nowak,
K-Tron Process Group

The introduction of both granulate as well as lubricant directly to tablet presses is of key interest to a wide variety of pharmaceutical, nutraceutical, and food manufacturers. The method of feed for both of these applications is of key importance in providing an efficient and cost effective press operation. Due to the tight space limitations which exist in many tabletting suites, dense phase vacuum conveying as a method of introducing granulate to the press is often utilized. Solutions that are easy to clean and versatile are ideal for this application. In addition, the possibility of introducing a lubricant directly into the tablet press via a specially designed feeder and eductor is gaining increased popularity in tabletting operations.
Tablet Press Lubricant Feeding

K-Tron Pharma Feeder with eductor and controls, mounted on cart for easy use by tablet press.
Lubricant addition
Lubricants (such as magnesium stearate) are used in tablet press applications to reduce the compression force during tableting, to avoid product buildup on the tablet press tools, and to make the tablet surface smooth. Control of the precise amount of lubricant to the process is critical to the overall product quality. If too much lubricant is added in the mixing stages, the tablet can often become softer than required. This becomes critical in the formulation of effervescent tablets, where excessive lubricant (i.e. magnesium stearate) can actually cause the tablet to repel water, thus affecting the dissolution properties.

The addition of the lubricant in the mixing process is an important variable in the tablet formulation. Not only is the actual required amount of lubricant often in question, but also the stage in the process at which it is introduced. Typical formulations which include this lubricant can be sticky, cohesive, and difficult to press. The introduction of this lubricant directly to the tablet press tooling is currently being utilized as a method of overall lubricant consumption reduction, as well as a method of improving the overall quality of the tablets produced by the press.

When mixing the lubricating agent directly in the mixer, a higher percentage of lubricant is often utilized than is required for the end product quality. As an alternative, the lubricant can be sprayed directly onto the press tooling through the use of a specialized feeder design. The introduction of the lubricant directly to the press tooling ensures that not only is there less sticking of the product to the tablet press , but also that significantly less lubricant is used. This consumption of the lubricant can be reduced dramatically, sometimes by as much as 97%!

Process Details
Lubricants are fed continuously by means of a twin screw feeder directly into a gas injector/eductor. Twin screws are used not only due to their suitability to convey cohesive materials, but also their consistency of feed without buildup in the screw trough. After exiting the screws, the material is entrained in a gas stream as provided by the eductor. This same gas stream then aspirates and conveys the lubricant to a nozzle fixed to the tablet press. The nozzle is mounted on the press in such a way that all surfaces of the tablet press tools that are in contact with the incoming tablet granulate (the dies as well as the punches) are uniformly coated with the lubricant. Any dust created in the tablet press is then eliminated by an internal vent or exhaust system.

Lubricant Delivery - Volumetric or Gravimetric?
The choice between a volumetric versus a loss-in-weight feeder for delivery of the lubricant is based upon the requirement to know the exact mass flow of the lubricant delivered. A loss-in-weight feeder is a type of gravimetric feeder that is mounted on a scale to detect the loss in weight as material is fed. In the case of a pharmaceutical installation a gravimetric feeder design enables validation of the product mass flow and ensures higher accuracy.

By definition, gravimetric feeders measure the flow's weight, and then adjust feeder output to achieve and maintain the desired setpoint. Volumetric feeders, again by definition, don't weigh the flow. Volumetric feeders operate by delivering a certain volume of material per unit time which is then translated into an inferred weight-based flow rate by the process of sampled calibration. Flow rate changes are accomplished by altering screw speed. A range of screw designs, sizes and geometries as well as agitation systems are available to optimize feeding the given material.

As such, volumetric feeders, while simple and relatively inexpensive, are open-loop devices in the sense that they cannot detect or adjust to variations in the material's density. For materials whose density does not vary significantly, volumetric feeders may perform to the required accuracy.

The difficult flow properties of a lubricant like magnesium stearate may cause buildup or bridging problems. Since, as mentioned above, volumetric feeders are open-loop devices from the viewpoint of discharge rate, headload variations and material buildup on the flights of a feed screw change the volume-per-revolution relationship, throwing off calibration without any outward sign. Gravimetric feeders automatically detect and adjust to these conditions.

For most volumetric feeders partial or complete material blockage upstream of discharge is likely to remain undetected for some time unless the feeder is outfitted with a no-flow detector. Most gravimetric feeders can automatically detect and alarm to these conditions.


Side view of K-Tron Pharma feeder with eductor mounted for feeding magnesium stearate into gas injector for lubricating tablet press.
In using loss-in-weight technology, it is imperative that a highly accurate load cell or weighing mechanism is utilized. The gravimetric mode of a feeding system should be continuously monitored and measured to ensure that the weight of the lubricant is accurate and consistent. Integrated controls keep the weight reduction per time constant by adjusting the screw speed of the twin screw feeder. By adjusting speed to maintain a consistent mass flow (or weight per time) the unit can be validated to prove a steady and uniform feed of lubricant to the press. In addition, due to the extremely short time in which the press tools can be lubricated (e.g. milliseconds), high short term accuracy is an essential benefit. The increased investment cost of the gravimetric design is easily balanced by the savings in lubricant since there is far less lubricant consumption.

Delivery of the Granulate to the Press
The relatively tight space in many processing suites often presents a challenge in delivery of the granulate to feed the tablet press. The use of pneumatic vacuum conveying, via specialized receivers and pickup methods is often utilized. In addition, for most sensitive granulate conveying applications, the principle of dense phase vacuum conveying is used. In most applications the gas is air, however, in the pharmaceutical industry, nitrogen is also widely used due to its inerting capabilities as well as the natural purity of the gas.Dilute phase conveying should be used with materials where segregation or attrition in the conveying line is not a concern. Comparative gas/air velocities in a 3 inches pipe for dilute phase can range from 3000 ft./min. (15 m/s) up to 7000 ft./min. (36 m/s). In dense phase operations, a reduced gas velocity range of 80 ft./min. (.4 m/s) to only 1700 ft./min. (8.6 m/s) is utilized.

Due to the lower gas velocity used with dense phase conveying, a much gentler action is delivered to the conveyed powder or granulate. This gentle action also reduces the segregation issues often experienced with the more aggressive dilute phase operation. It should be noted, however, that there are limitations to dense phase conveying. These limitations include conveying distances in excess of 12 ft. vertical and 15 ft. horizontal, and conveying materials which are cohesive, hygroscopic, or so coarse in particle size that they will not readily form slugs.

Dense Phase Principle of Operation

K-Tron/ Premier Pneumatics P-Series Vacuum Receiver offers the sanitary design and ease of cleaning needed by the pharmaceutical industry.
By definition, dense phase means a higher product to gas ratio, or a smaller amount of gas is used to move a large quantity of product. The less the gas requirement, the less the power consumed by the exhauster or vacuum pump. Typically material is picked up from the outlet of a specialty hopper, which minimizes the amount of air entrained in the material, and allows the slugs of product to form. In addition, the hopper also includes a type of makeup air inlet, which aids in the forming of the slugs as they travel and pulse through the conveying line. The combination of the relatively low air velocity and an expanded line size result in a type of "siphon-like" effect for conveying to the vacuum receiver, with less resultant attrition and segregation.

Regardless of the type of conveying technique used (dilute or dense), the material is delivered directly to a vacuum receiver which allows for the break of the vacuum and the release of the material from the gas stream. This receiver includes a reverse jet filter, which is pulsed when the vacuum breaks to allow for cleaning of the filter media. The outlet of the receiver includes a valve, either flap type or butterfly. This valve is sequenced to open after the vacuum breaks and the filter has been pulsed, thus delivering product to the process below.
Summary
The material handling, cost savings and process advantages of using both dense phase pneumatic conveying and direct lubricant feeding to the tablet press have been recognized by a wide variety of manufacturers. When selecting equipment vendors to provide this equipment, it is imperative that the vendor possess the experience not only in the often difficult material handling characteristics which are present in many of these pharmaceutical, food and nutraceutical applications, but also in providing equipment which is easy to handle, maintain and clean.

About the author: Sharon Nowak serves as Global Business Development Manager for the Food and Pharmaceutical Industries for the K-Tron Process Group. Nowak works closely with the Research & Development and Engineering departments to identify new applications and focus on the very specific needs of these industries. Nowak has a 20-year background in the process equipment industry for food and pharmaceuticals as well as a degree in chemical engineering from Rutgers University. She has extensive experience integrating feeders into food and pharmaceutical systems for mills and micronizers, blenders/mixers and contained processes for potent compounds.

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