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Chemical Technology

Process Intensification:

Three Case Studies

Three case studies using spinning disc reactors illustrate how process intensification can avoid many of the problems and scale-up difficulties associated with conventional stirred tank reactors.

By Stuart Cook at Protensive

Stuart Cook has worked for 38 years in organic process development roles, managing new product development at laboratory, pilot plant and full production scale. For the 10 years to 1999, he was Science Director of Hickson & Welch Ltd – a company whose profit base was £50 million pa of complex contract manufacturing sales to major agrochemical and pharmaceutical companies. He then joined the start-up company Protensive Ltd to manage their process development activities. He graduated in Chemistry from Leeds University (UK) in 1967.

The speed and certainty of process scale-up is becoming an increasingly important issue in highly regulated life science manufacturing. Any size change in conventional stirred tank equipment introduces validation risks from changed impurity profiles. The problems arise from the intrinsic physics of stirred tanks, where the ratio of surface area to volume changes with size. It becomes progressively more difficult to add or remove heat as reactor scale increases, which often demands lengthy semi-batch reaction times. Unless this is accurately predicted from early work, subsequent regulatory compliance can be compromised.

Apart from heat transfer changes upon ‘scale-up’ of a process, it is even more difficult to confidently predict mixing and mass transfer within the reactor. Two independent factors are involved in successful mixing during a chemical reaction. Power input per unit volume will be important and can certainly be held constant as production scale increases. Unfortunately, the shear of the agitation can be equally important, and this varies independently of the power input. Keeping both constant during scale-up is thus unlikely to be possible, introducing major risks of unplanned product changes.

The addition or removal of gases and vapours is a further area of potential uncertainty in conventional stirred tanks. The surface area of a liquid in a stirred tank decreases relative to its volume as the size of reactor increases. This can lead to extended reaction times on scale-up, with potential for more changes in process selectivity and efficiency. If gases are injected by

dip-pipe, the local conditions near the dip-pipe – and mass transfer between gas bubbles and process fluid – are equally difficult to predict and model during process development.

In summary, conventional reactors pose major – albeit familiar – validation challenges to process development professionals. Despite accumulated experience in predicting and controlling these challenges, there are still frequent problems during scale-up – potentially affecting product yield, quality and project time-scale. How nice it would be if we could make process scale- up easier, and our manufacturing equipment were inherently scale-independent.

Process intensification can offer major potential manufacturing advances. Three processes are described below where intensification has avoided scale-up problems caused within conventional reactors. The case studies all use spinning disc reactors for intensification. This is a well-explored piece of process equipment; machines have recently become commercially available on a laboratory scale, with integrated feed pumps and good strip-and-clean characteristics. The principles exemplified will apply to many pieces of process intensification equipment.

The examples below have been selected to illustrate different aspects of the benefits of intensification in appropriate manufacturing. The necessity to remove customer confidential process details means that they should only be regarded as illustrative. The intention has not been to provide reproducible experimental details.

Innovations in Pharmaceutical Technology

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