The Magnetic Mixer Solution
STERIDOSE®
The magnetic coupling has been used earlier in the design of pumps and mixers, but by adapting and developing this concept the range of use for this type of mixer has been widened considerably. The Sterimixer, from Steridose Systems AB, belongs to this type of mixer and has found a wide range of use in the fields of pharmaceutics and biotechnology, particularly in sterile and aseptic applications but also for special purposes, e.g., in connection with shear-sensitive products and processing under pressure and vacuum, possible applications also exist in the dairy industry.
Uncomplicated design
The mixer consists of three separate parts: An aseptically designed impeller, a weld plate which is integrated with the mixing vessel, and a drive unit.The mixer is fitted in the bottom gableof the mixing vessel, thus saving spaceand also facilitating mixing with low levels of liquid without the risk of foaming. The drive unit can also simplybe removed, which contributes to flexibility and a low centre of gravity For the vessel, i.e. a clear advantage, especially for mobile vessels.
The contact face between the impeller and weld plate consists of a bearing surface in the impeller and a corresponding bearing pin on the weld plate, both made of hard metal.
The impeller and the weld plate are made of polished AISI 316L stainless steel. The lower part of the impeller is fitted with permanent magnets. Channels enhance the action of a moving film of liquid between the impeller and weld plate during rotation for self-cleaning of the impeller. The design facilitates sterilization with steam and cleaning in place. The weld plate operates as holder and guide for the impeller inside the vessel and also as a mounting for fitting the drive unit onto the outside of the vessel. The drive unit is connected simply to the weld plate with the aid of a bayonet socket, which is rotated 1/16 of a turn and locked with two nuts. Assembling and dismounting takes very little time.
By Pontus Holmström
M. Sc (Chem. Eng.)
Advantages of the design
- No risk of contamination or leakage since there is no through-axle in the vessel.
- The mixer can be cleaned and sterilized in place.
- The impeller can easily be taken out for inspection (lifting eyelet on top of impeller).
- Excellent mixing even with low level of liquid.
- The same impeller can be used alternately in several mixing vessels.
- No wear, under normal conditions the impeller will withstand being run dry for short periods.
- No generation of particles.
- Minimal maintenance requirements, i.e. no seals, spindles or bearings to change.
Various types of drive
The drive unit consists of a motor, reduction gear and the magnetic drive plate. The motor may be an AC electric motor or a pneumatic motor the speed being regulated by a frequency converter on air control respectively, housed in a stainless steel control cabinet. An explosion safe motor can be supplied on request. The reduction gear provides a maximum impeller speed of 450 r/min. A tachometer can be utilized in fully automatic systems to control the speed.
Mixing results
Experience shows that the Sterimixer achieves effective mixing in all normal mixing vessels. Calculations concerning shear forces are dependent, for example, on the peripheral speed of the impeller and shape of the blades. Comparative trials carried out with the Sterimixer and conventional propeller mixers demonstrate that the same degree of mixing can be achieved with Sterimixer at considerably lower speeds. This implies lower shear forces, which is extremely important in the handling of sensitive or delicate products.
Ranges of use
Experience shows that the Sterimixer achieves effective mixing in all normal mixing vessels. Calculations concerning shear forces are dependent, for example, on the peripheral speed of the impeller and shape of the blades. Comparative trials carried out with the Sterimixer and conventional propeller mixers demonstrate that the same degree of mixing can be achieved with Sterimixer at considerably lower speeds. This iThis mixer is particularly suitablei n aseptic production since it does not affect the integrity of the system, which is absolutely necessary in sterile applications and in aseptic processes in pharmaceutical and dairy industry production, such as for instance starters. Sterilization in place can be made with steam or superheated water, or by sterilizing in an autoclave. Cleaning the mixer in place (CIP) ensures reproducible results and facilitates closed-circuit processing and automated cleaning procedures. Performance of the mixer results in short mixing times, solid raw materials can simply be dissolved in liquid in the tank, or, products (e.g., particulate)can be homogeneously suspended, for example, during a filling process. The efficient mixing sequence also makes it possible to control temperature, which can be a critical factor, e.g., in connection with batch sterilizing or the pasteurization of products. Location of the mixer at the bottom of the vessel closely adjacent to the outlet facilitates the mixing of very small quantities, thus ensuring that the loss of products due to variation in concentration or sedimentation can be eliminated. By adapting the speed of the bottom-mounted mixer in relation to the actual quantity being mixed it is possible to avoid mixing in air, which would otherwise cause foam.plies lower shear forces, which is extremely important in the handling of sensitive or delicate products.
Low shear forces, i.e., gentle mixing characteristics, makes it ideal for handling sensitive products such as. Proteins, cells, blood fractions and factors, etc. The mixer guarantees tightness in relation to the surroundings, which is necessary in the preparation of products under vacuum or pressure, e.g., preparation of pharmaceutical aerosols.