Fermenters are an essential part of manufacturing and research facilities in the pharmaceutical industry. The starting materials for new active ingredients and drugs are produced in bioreactors. In order for the cells to grow and multiply as intended, they have to be optimally supplied with nutrients and oxygen, which are evenly distributed by agitators.
This process impacts on the level measurements in the tanks as the mixture of different media results in heterogeneous density patterns. Furthermore, the liquid in the fermenter contains protein, which causes a relatively stable foam to form on the surface. In view of the media conditions which prevail there, non-contact level measuring instruments soon come up against their limits. Hydrostatic measurement methods with submersible pressure sensors are unsuitable owing to the density differences. The radar principle, even the guided version, cannot completely rule out reactions on foam, which means the actual level in the tank cannot be determined beyond all doubt.
Always on top
Measuring instruments with a float are influenced neither by foaming nor by electrical conductivity or dielectric constants. They are equally suited for continuous and point-based limit level detection. The fundamental design of the measuring system is identical: a float with an inner tube and a magnet moves on a guide tube according to the liquid level. The float acts as a signal transmitter when its impulse is converted to a 4-20 mA, fieldbus or HART output signal.
The float can be designed so that it always stays on the actual surface of the liquid by ballasting and by computing the nominal density. Its shape depends on the pressure: a ball float is recommended at higher pressures while a cylindrical geometry is better at low pressures. The floats can thus be individually adjusted to the processes and the media conditions in the fermenter.
There are two different systems within the float-based method of continuous level measurement: magnetostriction and reed chain. In the former case, a wire made of magnetostrictive material is fixed in the guide tube of the level sensor. A current impulse produces a circular magnetic field which twists the wire. A float with a permanent magnet marks the liquid level. The interaction of both magnetic fields generates a mechanical wave in the wire, which is converted into an electrical output signal by a piezoceramic pickup in the sensor housing.
An accuracy of 0,1% is achievable with magnetostrictive level measurements. The principle simultaneously permits a resolution of 1 mm. Magnetostrictive level sensors are therefore particularly suitable for applications where high precision is required. This mainly applies to processes involving expensive or sensitive active ingredients, where every reaction or change must be detected immediately.
A practical application
The research centre of a German pharmaceutical company that specialises in cancer therapy illustrates this very well. Six fermenters are operated at the centre in a batch process; in the past, their levels were monitored using capacitive sensors. However, there came a time when these sensors no longer met the manufacturer’s requirements for precision. They were consequently replaced with WIKA’s FLM-H magnetostrictive sensors. Depending on their length (between about 400 and 1300 mm), these sensors record the level in a tank up to 10 times more accurately.
Thanks to their very high resolution, magnetostrictive measuring instruments also enable leakage losses to be reduced to an absolute minimum: any unexpected drop in the level is instantly noted and the operator can promptly take appropriate action.
Reed chains for less precise requirements
Level sensors with a reed chain exist for applications where a lesser degree of precision suffices. The guide tube contains a printed circuit board with resistors, on which the float magnet energises individual reed contacts at defined intervals. The resulting measuring chain generates a voltage that is proportional to the filling height and is converted to the desired output signal either by the sensor’s integral transmitter or by a transmitter in the control room.
The accuracy of the reed system is determined by the contact separation of the measuring chain. For example, instruments can be supplied with 5, 10, 15 or 18 mm contact separation. Between 0,2% and 1,8% measuring accuracy can be achieved with resolutions from 2,5 to 9 mm.
Point-based limit detection of filling levels with magnetic float switches is likewise based on reed contacts. With conventional instruments of this kind, up to eight switch points can be specified for monitoring defined liquid levels. Reed contacts installed at suitable points in the guide tube are energised by the approach of the float magnet. The guide tube and the float are both non-magnetic. There is no direct contact with the liquid during the switching operation, which is free from wear and requires no power supply. Once again, the reliability of the measurement derives from the ability of the float to adapt to different media – a characteristic that is not adequately ensured with alternative methods of point-based level measurement such as electrodes, tuning forks or optical sensors.
There is another key consideration for use in fermenters apart from measurement quality: the materials, surfaces and sealing material of the measuring system must meet the requirements for sanitary applications. The design of WIKA’s instruments, for instance, conforms to the 3-A Sanitary Standards, which the FDA refers to as the hygienic equipment design directive. Float-based measuring assemblies are generally made from 316L stainless steel. They have no dead spaces and are fully welded. The opening between the guide tube and the float is dimensioned so that no capillary effect occurs and no adhesive forces can act. The entire instrument can be cleaned without residues and is suitable for CIP and SIP. Two versions of the sensors can be installed in the tank: with a welded tube end or with a base support. The latter is welded separately at the bottom of the tank. Its three pins fix the guide tube and hence increase its resistance to vigorous stirring movements in the fermenter.
Although non-contact measuring instruments are increasingly commonplace in industrial processes for monitoring liquid levels, experience with level measurements in fermenters confirms that float-based methods are still a good choice. Their mechanical principle can be optimally adapted to critical media conditions and extremely precise results are guaranteed – a must in many pharmaceutical processes.
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