Vacuum processing system

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A vacuum processing system (also vacuum mixing system or vacuum mixer ) is a process engineering system ( process system) which is intended to process several process steps under vacuum in a closed system one after the other or in parallel. Many thousand units of such vacuum mixers are in use in the food, cosmetics, pharmaceutical and specialty chemicals industries. This machine type is characterized by great flexibility, ease of use, good scalability, simple maintenance and long service lives. The process steps can be precisely delimited and described.

Structure / functionality

First of all, the machine needs a work container . This is usually a conical base in order to ensure that products with a certain viscosity can be emptied, but it is also possible with a dished base or flat base.

"MaxxD" vacuum processing system

The container has a double jacket that can be filled with steam, hot water or cooling water.

The mixing process is carried out with a stirrer . Several types of stirrers are possible. The perforated blade stirrer has proven to be ideal, as it performs the stirring task regardless of the height of the product; that makes largely independent of the batch nSize ( batch size). This is very important because there are vacuum mixers from 10L to 10000L. For example, a maximum batch size of 2000 L can be produced on a specific vacuum mixer with a perforated blade stirrer and a batch size of 200 L can also be produced on the same machine.

At the outer edge of the stirrer are stripper plates mounted made of plastic, which have mechanical contact with the working container wall and thus significantly improve the heat transfer to the heat exchange surface. The perforated blade stirrer and the statically built-in flow breaker (or coaxial stirrer / counter-rotating device) are responsible for the necessary mixing of the contents of the working container. During the heating and cooling processes, the wiper plates ensure ideal heat transfer through frequent wiping.

The absolute vacuum cannot be achieved technically with aqueous or oily products. However, vacuum is used as a class name for this type of working machine. The generation of a negative pressure by means of a water ring pump (as a vacuum pump ) is "state of the art", as the water ring pump has proven to be very resistant, easy to clean and durable.

Many vacuum mixers have a mixing chamber underneath the work container . There are laterally welded, valve-provided nozzles that are connected to a working container or a hose.

functionality

In most cases it is about dispersing , emulsifying , suspending or deagglomerating substances. Therefore, the vacuum mixer usually also has a high-speed tool, which can be a homogenizer , rotating knife or dissolver disc. If the homogenizer is in operation, it builds up a flow through its built-in elements (mixing blades, rotor and pump impeller, all on one shaft) and the rotation through the drive. 

Homogenizers attached to the bottom of the work container have a circulation line to return the mixed material . Ideally, this has an exit at the highest possible liquid level and a second far below this highest possible liquid level.

As already mentioned, as an alternative to the homogenizer, there is in many cases the option of equipping the vacuum mixer built as a dished bottom with rotating knives with high peripheral speeds. This principle can also be used to emulsify, disperse, mix and de-agglomerate.

Another alternative to the homogenizer for some applications is the possibility of operating the vacuum processing system with one or more dissolver disks (e.g. for the production of toothpastes). The flow in the vicinity of the dissolver discs is completely inside the working container, so that no circulation line is required. The overall process is less complex, but there are fewer process control options.

Procedural steps

Mix

A slowly running scraper agitator supports the product circulation and mixing. The movable scraper plates brush over the inner surface of the container wall, thus preventing product build-up and improving the heat exchange.

Generate and maintain a vacuum

Controlled by a software routine, the negative pressure can easily be kept in a setpoint window (50..1000 mbar abs .). The incorporation of air is greatly reduced by the existing negative pressure, while air bubbles can be easily removed from the drawn in raw materials.

Collection of liquids and dry substances (from other mixing / storage and working containers) / dosing

With the intake function, liquids and dry materials, but both separately, are transported from the outside into the closed system in the vacuum mixer by means of a vacuum. In general, this is possible above and below the current liquid level in the vacuum mixer, but experience has shown that it is better to move in below. To pull it in, you now fill a template work container, which is connected to the mixing chamber by a pipe or hose, and open the valve built in between. Due to the pressure gradient, the material from the working container flows from the outside to the inside through the mixing chamber into the working container. If the flow of the perforated blade stirrer dissolves the new substance in the mixture already in the working container, the work step is completed after a short time.

Homogenize / circulate through the homogenizer

Due to the pressure gradient, the substance from the working container still flows from the outside to the inside through the mixing chamber, where it is mixed with the volume sucked in from the working container, but now follows the flow into the homogenizer. The mixture of substances passes through a first energy field in the zone around the mixing blade, then another very strong energy field in the rotor / stator zone, and a third energy field on the pump impeller that is matched to it. From there, the homogenized mix is ​​conveyed back into the working container. The shear zone itself is an annular gap volume, which is converted into a high-energy flow zone by the motor-driven mechanics. In this way, emulsification , dispersion , mixing and deagglomeration can take place in the smallest of spaces . The zone is designed in such a way that the volume elements flowing through experience the same forces and have the same dwell time regardless of the exact position of the passage. It goes without saying that the scalability is appropriate so that a product experiences almost the same amounts of forces, energy per volume and dwell time on all machine sizes.

Heating / cooling / maintaining temperature

The contents of the working container can be heated by steam or hot water in the double jacket and cooled by cooling water in the double jacket. To maintain a certain temperature, the double jacket can also be empty. It is necessary to have the scraper stirrer in operation because it ensures the necessary axial and radial mixing. Especially with parallel steps, the adjustable-speed homogenizer can also be used for mixing during the temperature control process; here you can choose between permanent and interval operation. The temperature can be regulated very well. There are systems that are designed for short temperature control times and those that have a high level of accuracy in the control loop.

Vent using vacuum

At the end of the batch, air can be vented to make the product surface more brilliant, to increase the density and to improve the shelf life of the end product by removing oxygen.

Some specific products can be quickly deflated, but not all. In many cases you have to hold the negative pressure for a few minutes and then increase the negative pressure again by one step, about 100..200 mbar. In practice, the final value is around 50..60 mbar abs. Venting too quickly is not possible. It must be noted that the density decreases due to the increasing air bubbles with more negative pressure and thus the product level increases. After a short time, the air bubbles burst and the level sinks. If the product is hot or at least still warm, the saturation vapor pressure curve of water must also be taken into account, because water evaporates at lower temperatures than under atmospheric pressure, depending on the negative pressure.

Discharge or emptying

The discharge usually takes place by means of a homogenizer, the pump impeller of which generates a pressure. The product flows along the pressure gradient into a buffer tank or container, for example. Support through overpressure is possible. A rotary lobe pump can also be installed and used for discharging products that are ultimately sensitive to shear, for conveying substances with high viscosity or for conveying over long distances.

Clean

see CIP

Sterilize

see SIP

dry

see Drying in Place

literature

  • Reinhold Buchet, Ralph Diodone: Production of liquid and semi-solid dosage forms , in: Gerd Kutz, Armin Wolff (Ed.): Pharmaceutical products and processes. Wiley-VCH, Weinheim 2007, ISBN 978-3-527-31222-1 , pp. 314-332; limited preview in Google Book search

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