Drying: moving from batch to continuous processes

The requirements of a granule drying process haven't really changed much in recent years: pharmaceutical companies still need a process/technology that is capable of drying the produced granules as fast as possible, without affecting critical quality attributes (CQAs), such as particle size and homogeneity. However, as with all other pharmaceutical processes, the pharma industry has started to focus on quality by design (QbD) for the development of granule drying processes. As such, companies are now working on controlling and analysing CQAs inline and during the process, as well as understanding the process better and correcting it when necessary, rather than analysing the product afterwards without a thorough understanding of what might have gone wrong during the process in case of an outofspecification result.

Griet Van Vaerenbergh

Two major breakthroughs in granule drying technologies have taken place in recent years: the development of better and more robust and integrated PAT tools and probes, with the ability to clean and calibrate without interrupting the process, and the introduction of continuous processing for granulation and drying. The use of PAT tools for inline/inprocess measurement of CQAs has helped the pharma industry obtain a better understanding of processes, which makes it possible to implement QbD in the development of new products, as well as to improve current processes. This helps to improve production efficiency, product quality and safety.

However, with current batch drying technologies, it is often not possible to correct a detected out-of-specification situation during the process, which is why the introduction of continuous granulation and drying technologies is so important. A continuous system is ideally suited for the implementation of online PAT measurements because it is very easy to detect a deviation from steady state and to modify the process parameters via a feedback (or even feed forward) loop to prevent the product from becoming out-of-specification. This development will lead to better process control, high production efficiency, less wasted product, and better product quality and safety.

Tabletting product showcase

Drawbacks of existing systems and solutions

One of the main drawbacks of current drying systems is that they are all batch systems. Even with the implementation of on-line measurement tools, batch processes are more difficult to understand and control because they are in a continuous state of change. As mentioned above, when an outofspecification result is detected in batch systems, it is often not possible to correct it by changing process parameters and a complete batch is wasted.

Continuous systems can overcome this problem. As a continuous system reaches a steady state, it is much easier to understand and control. An advanced control system continuously monitors the process and when it detects a trend to deviate from steady state, it will automatically take action by changing process parameters to avoid out-of-spec results. In newly developed continuous systems, steady state is reached very fast and there is always only a small amount of product in process; in the unlikely event that out-of-spec product is produced, only a small amount needs to be discarded.

Because of the advantages, we will see a move from batch systems to continuous systems, and a widespread implementation of online measurement tools and advanced control systems, such as those already seen in the chemical industry, only on a smaller scale. I also believe that continuous systems will be more flexible; not continuous systems producing only a single product for years, but systems that can produce several products in campaigns of varying length.

Additionally, given the fact that a tablet press is already a continuous process, we will see the implementation of truly continuous production systems, going from powders to tablets without products in quarantine waiting to be released after drying and before tabletting. With advanced control systems containing feed forward and feedback loops, the online measurements of CQAs will be sufficient for realtime release.