11.2.2. The stirred tank bioreactor
The most important type of bioreactor in use in industry today is still the stirred tank bioreactor
(Fig. 11.3). We will therefore focus on this reactor type in the rest of this chapter.
The reactor is typically cylindrical, with a ratio of reactor diameter to height - the so-called
aspect ratio - between 1:2 and 1:5. The construction material is usually Type 316L stainless
steel. The reactor has a round-bottom to facilitate cleaning and sterilization in place, and to avoid
stagnant zones during operation. The reactor is equipped with a stirrer, consisting of a shaft upon
which one or more impellers are placed. The stirrer shaft may be either top-mounted or bottom-
mounted (most common). It is essential that the shaft enters the reactor in such a way that
contamination is prevented. Typically, a double mechanical seal is used. The bioreactor may also
be equipped with a mechanical foam breaker, which could be mounted on the stirrer shaft. A
simple foam breaker could look like an extra, narrow impeller, but, located above the liquid
surface level and with a larger diameter.
A n industrial scale bioreactor for production o f enzym es. Picture courtesy o f N ovozym es.