Pan Speed: Hit a Moving Target
Optimizing pan speed is a critical, but often overlooked, way to improve aqueous tablet coating. There’s no magic formula, but simple visual checks can improve results.
By Fred A. Rowley, Watson Pharmaceuticals, Inc.
Researchers have thoroughly studied the aqueous coating of pharmaceuticals and nutritional supplements, and documented the impacts of film build and operating parameters on stability and on product potency at release [1,2]. Equipment vendors have also examined the impact of nozzle placement and humidity on tablet quality and general process optimization [3,4,5].
However, most of these studies have failed to consider one key operating parameter: pan speed, or the speed at which the coating drum is rotated. Most studies assume that this rotation is fixed during the coating run, when, in fact, it can vary significantly depending on the operation, product and equipment vendor involved. There is no magic equation for optimizing pan speed, but this article examines the connection between pan speed and coating quality and summarizes techniques that operators can use to optimize pan speed during coating operations.
Any discussion of the key operating parameters involved in aqueous coating must include both fixed and variable settings. Generally, there are eight critical operating parameters to consider. Not all of them can be seen or adjusted via computer. At least three require an experienced operator to set, and two of those three also require monitoring and continued observation. These parameters are:
One Number Does Not Fit All Circumstances
- Gun geometry, or how the spray system is set up inside the pan. Gun-to-bed, gun-to-gun and gun-to-side (of the pan) are all equally critical to overall success. Most published work, with one exception , focuses on gun-to-bed geometry, neglecting other important factors.
- Spray rate, or the quantity of suspension sprayed per unit time. This is commonly published as a gross number (kg/min. total) or, more correctly, as an individual number (ml/min. per gun).
- Atomizing/pattern air -- Atomizing air is combined with the suspension to form a mist. Pattern air is the air used to construct the concavity of the spray cone. Both settings are commonly given in bar, psi, or standard liters per minute (slpm).
- Inlet air temperature, or the temperature of the processing air used to dry the tablets in the coating pan.
- Total air volume, or the quantity of air, usually expressed in cubic feet per minute (cfm), used to dry the tablets, and the quantity of air pulled through the bed of tablets from the coating pan. Ideally, the air volume is adjusted so that there is slightly more air pulled from the pan than enters the pan. This results in a slightly negative pressure in the pan and pulls the coating spray into the tablet bed.
- Pan pressure, or the differential pressure between the pan and the room it is in, commonly expressed in negative inches of water. Insufficient pan pressure prevents suspension intake through the tablet bed and results in suspension adhering to the pan door, pan surfaces and the spray system. Excessive pan pressure pulls excessive quantities of suspension through the bed of tablets.
- Adhesion of particles on the spraying system surfaces -- a situation in which dried material adheres to the spray gun surfaces near the exit ports of the atomizing/pattern air and the suspension. The pressure changes are very slight and cannot be seen by a computer even though they may direct the suspension spray in a radically different direction. Figure 1 illustrates how particulates can quickly adhere to critical areas of the gun.
- Pan speed -- the speed at which the pan drum rotates, usually in rpm. In most published work [4-7], regardless of pan type and size, pan speed is held constant.
Optimized pan speed may vary from one size pan to another, between equipment from different vendors, and even from one size pan charge to another. There is therefore no magic set point for research or commercial production, unless the processing conditions used are exactly the same, batch to batch, with the same size and shape tablet and pan charge.
For a process yielding varying quantities of tablets batch to batch, pan charge may have to be altered to achieve the same quality of film build. Unfortunately, pan charge is frequently fixed during academic study and analyzed as a constant.
Complicating matters further is the fact that different pan speeds may be used in two separate and distinct operations: protecting the tablet during the tablet formation stage and optimizing final coating appearance in the tablet elegance phase .
Although a number of excellent software programs are available to help control coating operations, they cannot distinguish between an optimized pan speed and an inadequate speed setting because they cannot link visual observation of film build on the tablet surface with pan drum rotation. It is up to operators to analyze and then recommend initial settings, or changes for pan speed.
As with other variables, pan speed may vary. Too low a setting assures batch failure with numerous tablets stuck together, or will result in heavily defective batches containing partially-coated tablets, known as "twins" and "picked" tablets. Too high a setting, however, will result in excessive abrasion, and "eroded" or even broken tablets.
Given the variety in pan sizes, vendors, pan charges and product requirements, and the fact that there is no magic number covering all situations, how does one optimize pan speed? One can apply meaningful guidelines to establish a speed range that can still be challenged during coating process validation.
Simple visual inspection will provide an indicator. Any bed of tablets properly rotated in a moving drum will take the shape of a modified kidney bean.Evaluating Pan Speed
The goal is to maintain the tablet bed speed to assure vigorous movement while keeping tablets from jumping forward and out of the bed. The minimum speed selected should not allow the tablet bed to stall, even for a short time. This initial setting may be quantified in very broad strokes of between two and six rpm, depending upon the size of the coating pan, the product being coated and the size and shape of the tablet.