Vertical drying

The author: Matthias Böning Authorized Officer, Sales Director, Amixon

Amixon vacuum mixer-dryers and reactors have a vertical helical-band mixing gear. This rotates in a thick-walled, round cylinder with a flat base, passing extremely close to the walls and floor. The very good equidistance between the mixing gear and the fully heated walls guarantees ideal thermal transfer, reliably avoiding wall adhesions. The special shape of the helical spring creates mixing and flow without compacting or pressing the materials. It is of particular significance that all the surfaces of the mixing gear are also temperature controlled: the shaft of the mixing gear, the shovel-like arms and the helical spring.

So far, Amixon has replaced 30 horizontal paddle-through dryers with vertical models. The batch sizes vary from 50 l up to 18,000 l. According to feedback from operators, the use of vacuum mixer-dryers can help cut the normal drying times of paddle dryers by more than 40 %. The reason for this is that the flow of the material to be dried extends throughout the entire chamber – thanks to the patented mixing gear, there are no dead spaces.

On the other hand, as the mechanical energy input (drive power at the mixing tool) is substantially lower, the exchange of heat and material is obviously more effective. The products to be dried are conveyed vertically by the helical spring at the periphery of the mixing chamber and slide downwards at the centre, following the laws of gravity. Static mixing tools prevent them from rotating by combing with the rotating mixing tool.

The alloy and Hastelloy drying equipment in the Amixon laboratory is already part of the apparatus manufacturer’s standard equipment. This offers a major advantage for all manufacturers and future customers interested in experimental trials: all products can be tested in their true form and it is no longer necessary to resort to a dummy product. Amixon mixer-dryers and reactors have hygienically designed CleverCut inspection doors, making access for inspection and cleaning purposes much easier. The internal filters allow valuable height to be saved, as does the compact structure.

Amixon has profound practical experience with a wide variety of drying processes and materials to be dried. For example, Amixon engi-neers have developed an upscale programme founded on knowledge from thermodynamic experience, to enable large-scale dryer mixing results to be reliably correlated from experience with mixing tests. Amixon is therefore able to predict processing times and provide process guarantees on the basis of test results.

During the drying process, very diverse consistencies occur at various stages. Beginning with the pumpable suspension, the product characteristics change as drying progresses. The products become increasingly incapable of flowing and are compact and possibly sticky, viscous or pasty. In the next phase they become crumbly and form granules, which at a later stage are further reduced with the loss of moisture until a finely dispersed powder is obtained.

Aqueous suspensions also develop a viscous, pasty consistency. The mass can then be compared with the viscosity and stickiness of chewing gum. Depending on the product, classic rheological phenomena such as shear thinning or thickening occur. These states are problematic to process and place high demands on the dryer’s stability. If it adheres to the mixing tool, the mass tends to follow its rotation. This, however, would interrupt the material and heat exchange.

Typical kneading properties of the dryer are applied here, with static and dynamic mixing tools combing together. The mass is pulled apart at a reduced tool speed, new surfaces are opened and dried and the strands that were pulled apart then join together again, only to be pulled apart again afterwards. This way, the plastic mass is kneaded and effectively dried with no dead spaces.

Crystalline materials, which are often found as initial pharmaceutical products, represent a special case. Due to their crystalline structure they are not easily mixed and it is difficult to generate a flow in the mixer. On the other hand, this structure must not be damaged by the mechanical or thermal input of energy involved in the drying process.

The vacuum mixer-dryer rises to this task magnificently. The process materials are screwed upwards by the mixing gear at the periphery of the container, then flow down along the shaft again under the influence of gravity. Moreover, this kind of mixer can be operated with variable circumferential speeds from >4 ms-1 to <1 ms-1, as this has no effect on the dead space-free flow.

The ability to choose circumferential speeds of <1 ms-1 if required has another crucial advantage for many companies: if dust or solvent can give rise to a risk of explosion in the equipment when it is started up or shut down under atmospheric conditions, it is possible to reduce the circumferential velocity in order to satisfy the need for safety (refer also to Atex 94/2/EC).

The drive, the bearings and the seals for the mixing gear shaft are all above the mixing chamber. Neither thermal expansion nor mechanical stress impact the face seal. Both high excess pressures and high-quality vac-uums are sealed with virtually no leakage. The environment of the mixer-dryer is thus reliably protected from the materials reacting within the equipment; at the same time, the operating costs are lower and the purity of the materials preserved.

Amixon equipment is designed and built in accordance with the latest EHEDG guidelines and conforms to GMP and FDA requirements. It can be used in all areas of the chemical and pharmaceutical industries. If the customer’s appa-ratus needs to be operated under sterile conditions, Amixon has techniques and special solutions expressly for this purpose.

The “WaterDragon” system can be used for automatic wet cleaning. The washing lance moves into the mixing chamber automatically. Driven by the water pressure, the washing head rotates and the nozzles spray the complete process apparatus. After this apparatus has been cleaned, the rotating lance returns to the parking position and seals the processing chamber gas- and liquid-tight.

If it is important to have very high residual emptying rates, Amixon conical mixers and mixer-dryers are particularly appropriate. Together with the patented “sliding blades”, the conical base ensures a rate of almost 100 %. The sliding blades tilt the mixing gear to a horizontal position, improving the removal of the product residues. The requirements related to, and governing, the technical aspects of the process are otherwise the same as described above for the vertical vacuum mixer-dryers with flat bases.

In addition to the equipment itself, Amixon supports the qualification of the supplied equipment as a service. All the necessary documentation is drawn up and the installation qualified in cooperation with the people who will later operate the plant.

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