Mixing processes

Mixing processes are particularly important in the chemical, pharmaceutical and food industries. The intermediate or end products are often dispersions or emulsions. For chemical reactions in the vessel, the reactants must be distributed as homogeneously as possible to guarantee an optimum transfer of material and heat. Mixing processes can be assigned to one of three categories:

  • Agitation (liquid-dominated media)
  • Kneading (highly viscous, paste-like media)
  • Mixing (solid substances)


Generally agitators with top entry drives, often known as DIN agitators, are used. The standard operating conditions are atmospheric pressure up to 6 bar (87 PSI) and ambient temperature up to 150 °C (302 °F). We distinguish between agitators with steel vessels and those with vessels in which the surfaces in contact with the product (including the flange of the mechanical seal) are glass-lined.

In addition to the central mixing element, some machines have another mixing element that generally turns in the opposite direction to the central element. It provides additional restructuring of the flow of mixed material and prevents sticking on the vessel wall. Again, EagleBurgmann has experience of such complex applications.

Additional units in the form of choppers or rotor/stator systems are used to accelerate mixing processes and to break up clumps. These choppers are generally arranged on the side in the bottom part of the vessel. High rotational speeds of up to 3,000 min–1 are achieved.


Kneaders are used for highly viscous and paste-like media. They need to be very robust. The same requirement applies to the mechanical seals. As a rule, several agitator blades turn slowly toward one another. The distances between the agitator blades or between the wall and blade are relatively small in order to generate the high shear forces that are required.

Extruders, which are a special type of kneader, are extrusion screws that work on the same principle as a meat grinder to press solid to thick viscous masses evenly out of an opening under high pressure and temperature. Pressures of 10 bar (145 PSI) to 300 bar (4,350 PSI), sometimes as high as 700 bar (10,153 PSI) and temperatures of 120 °C (284 °F) to 300° C (572 °F) are achieved, depending on the product.

The challenges for the seals are the small installation space and the high temperatures. The products are generally viscous and sticky and some solidify as they cool. Flushing is therefore often used to keep the product away from the seal.


Chemical reactions are the central process step in every facility in the chemical or pharmaceutical industry. The reactions take place either in the liquid phase or in a suspension with solids. The reactions that take place mean that high pressures of up to 200 bar (2,900 PSI) in individual cases and high temperatures, sometimes as much as 400 °C (752 °F), are needed in the reaction vessel. This places correspondingly high demands on the sealing technology. Mechanical seals are installed above all in tank reactors and combustion reactors and wherever a mixing element is used. This generally involves machines with top entry drive.

Mechanical and thermal separating processes

The separation of substance mixtures is one of the most important basic process operations. The majority of the products obtained by chemical reaction are mixtures of substances that have to be broken down into their components for further processing or end use.

We distinguish between mechanical, thermal and physical-chemical separating methods according to which properties are used for separation. Mechanical separating methods include centrifuging and filtering which utilize the differences in density or size between the various components. Thermal separating methods such as drying utilize different boiling points for separation. The physical-chemical separating methods include crystallization and extraction which utilize the differences in solubility of substances.

A large variety of machines are used in these processes: nutsche filters and dryers, centrifuges, separators, thin-film evaporators, paddle dryers, conical dryers, crystallizers and extractors, to name just a few. These are all special constructions that are specifically adapted to the customer’s requirements.

The sealing technology also needs to adapt flexibly to these requirements.

  • Top, side or bottom entry drives
  • High and low pressures and temperatures
  • Products with and without solids, viscous, sticky and highly corrosive media
  • Explosive media, so that the seals meet the requirements of the ATEX directive (2014/34/EU)
  • Strict hygiene requirements such as CIP and SIP compliance for the entire sealing system
  • Small shaft diameters of 20 mm (0.79") up to large diameters of 300 mm (11.81") and sometimes even larger
  • Large axial shaft deflections that need to be absorbed with metal bellows or special wiper systems.
  • Constantly changing operating conditions due to lots of start-up and shut-down operations


Centrifuging means the mechanical separation of a suspension or emulsion with the aid of centrifugal forces. Typical machines include pusher centrifuges, decanters and separators. Mechanical seals are generally only used in centrifuges that work under higher pressures or in a vacuum. In these cases, the feeder, housing and bearing all have to be sealed. Carbon segment rings are frequently used in unpressurized machines.


Filtration is the separation of a suspension into a liquid and a solid phase by means of a filter medium. Nutsche filters (pressure filters) are typically used and are sealed with mechanical seals. They consist of a pressure-tight container, generally arranged vertically with a horizontal filter base. The modern types are equipped with agitators, which are generally height-adjustable. The filtration takes place under overpressure or in a vacuum.


By drying, we mean the thermal separation of a suspension by evaporating the liquid and removing the resulting steam. Typical machines that are equipped with mechanical seals are filter dryers (nutsche dryers), paddle dryers, conical dryers and double conical dryers. Filter dryers are a variant of the nutsche filter that can be heated. They combine the filtration and cake drying steps in a single machine, generally under vacuum.

Paddle dryers are generally horizontal machines with a slowly-turning, generally heated shaft that is fitted with mixing blades. Smaller machines often have bearings on one side only; the larger machines have bearings on both sides. To improve the disaggregation of the product, choppers or blade mills can be integrated into the side of the housing.

Conical dryers are vertical machines with a mixing screw that rotates both about its own axis and along the vessel wall. Heat is supplied via the vessel wall heater and sometimes via the heated screw as well. Machines with bottom and top entry drives are used. One particular challenge for the seal technology with the bottom entry arrangement is how to seal the two coaxial agitators moving at different rotational speeds.

With double conical dryers that are operated often in a vacuum, the vessel is turned to create a tumbling action, which thoroughly mixes the product. Choppers or blade mills are optionally integrated to improve disaggregation of the product.


Evaporation uses the addition of heat to evaporate and remove the solvent from a solution. Typical machines with mechanical seals are thin-film evaporators. They generally consist of a vertical or horizontal vessel operated in a vacuum and equipped with an agitator. Rotor blades evenly distribute the liquid and a thin film of liquid forms on the vessel wall.

Size reduction processes

Reduction of the particle size of solid substances by mechanical forces is known as comminution. Machines that comminute larger particles are known as crushers; for smaller particles they are known as mills. These machines normally work under atmospheric conditions.

Bead and ball mills

Bead and ball mills are used for the fine and ultra-fine comminution and mixing of substances for manufacturing color pigments, for example. The mill feed is comminuted by special milling balls. In wet milling, the solids are mixed with a solvent.

The media to be sealed generally have a high solids content and are very abrasive. For this reason, pressurized double mechanical seals are used in these applications. Parts in contact with the product, such as the seal flange, have to be protected against excessive abrasion by a special coating, for example.

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