Drying

Overview

Drying removes water or solvent from a pharmaceutical material. Drying is performed after wet granulation to remove excess liquid, but can also be performed to improve stability, bioavailability, or processing compatibility. There are several approaches to drying including freeze drying (or lyophilisation), spray drying, or fluidized bed drying. These varied approaches allow for drying of materials with different thermal stabilities, result in different dried material properties, and have different material and energy requirements. For example, biologics are thermally labile, and freeze drying is used because it is a low temperature drying process even though the process requires more energy than spray drying. Spray drying involves the use of a hot gas to evaporate moisture from the atomized droplets, and often results in an amorphous material. Fluidized bed drying involves suspending granules in a hot air bed, and is often used after wet granulation.

For any drying operation, it is important to identify any process-related changes in product crystal form whether they are intended or not intended. Drying endpoint is a critical process parameter to maintain efficient plant operations and ensure product quality. Inline PAT is commonly used to measure moisture as an indication of process endpoint. This can be done either inside the dryer, or at the dryer discharge.

At the dryer discharge, the resulting dried powder is disperse in volume and often below the limits of detection for spectroscopic PAT if the powder were to simply flow past a spectroscopy probe window. An approach to enable spectroscopic PAT of these disperse solid streams is a specialized “process-to-PAT” interface that allows solids to accumulate locally in front of the probe window. With enough volume accumulated in front of the probe window, the presented sample is now at an amount measurable by NIR or Raman spectroscopy. The Taso™ interface uses a “spoon” that is immersed in the stream, allowing a powder bed to accumulate. Once the accumulated powder bed is measured, the material is mechanically wiped from the window.

Inside the dryer, dried granules or powders can stick to process equipment. Mechanical wiping of a spectroscopic window ensures a representative measurement and prevents probe fouling. The Rado™ interface has an integrated heating element and mechanical wiping mechanism to ensure that sticky granulates or powders do not interfere with spectroscopic PAT during granulation or drying unit operations.