Drying Process Characterization of a Novel Continuous Vacuum Drying Technology

In the last years continuous technologies have emerged in the pharmaceutical industry, but still some steps are not sufficiently developed. One of those technologies is the last step in the active pharmaceutical ingredient (API) production, drying. Suitable, continuous equipment is not available, especially for the case of cohesive, poorly flowable and thermosensitive materials, as many APIs are. Such a technology would be promising for a truly continuous primary manufacturing route and with attractive particle properties. Additionally, the time to market can be reduced significantly, and scale-up and scale-down considerations can be neglected.

Drying is an especially challenging process, as it involves simultaneous mass and heat transfer and can have challenging consequences for the particle structure and morphology which eventually control powder behavior. In addition, agglomeration and attrition compete. This is an undesired effect, as during crystallization the particle properties are usually fixed in an optimal way for the intended use of the API. Often, the drying process changes the tailored particle properties, as particle size distribution (PSD) and morphology.

This presentation will show the concept of a continuous drying process, using a newly developed dryer design which intends to overcome these challenges by a unique approach of balancing forced feed, powder bed motion and residence time. The results indicate that the particle properties are maintained to a large extent. The influence of several process parameters (mass flow, air flow, rotational speed, temperature, inlet moisture) is initially evaluated, and the product morphology and PSD is investigated.

As further improvement to the dryer, a vacuum-tight design was implemented, to enable the dryer to dry off organic solvents and thermosensitive materials. This enables the dryer to continuously operate at the intended scale, in an industrially relevant way. The governing drying kinetics are investigated under vacuum and the drying efficiency and limitations of the systems are discussed.

As main test substance Ibuprofen was used, with varying inlet moisture levels (10 to 50 wt. %). It was assured that the technology is able to dry cohesive powders with a small particle size (<100 µm) continuously, in the range of 0.5 to 2.0 kg/h, dry basis. In this range, the PSD was maintained and the residual moistures of the product for some process configurations could be reduced to below 1 wt. %. Additionally, the residence time distribution (RTD) was investigated throughout the process and scanning electron microscopy (SEM) was used to classify the morphology prior and after the drying step.

In summary, the presented continuous technology gives a robust drying process, suitable to dry temperature-sensitive cohesive particles at low mass flows under vacuum.