Batch vs continous inline Mixing

Fig. 1: The newly developed continuous mixing and dispersing machine Ystral Coflow.

What do adhesives and toothpaste have in common? In their production, powders and liquids must be brought together in a suitable process. At first glance, this seems trivial, but it depends on the characteristics of the powders to be processed. These are of central importance when developing the right process design. Powders that need to be dispersed in a passage with a short defined dwell time are particularly suitable for continuous production. Examples of this are long-chain polymers, which would be destroyed by repeated shearing, or the processing of pyrogenic silicas for the production of polyester resins.

Continuous mixing and dispersion

Ystral's recently developed YSTRAL Coflow disperser (see Fig. 1) enables such continuous production without a mixing tank. With the YSTRAL Coflow, powdered solids are fed, mixed and dispersed into liquid streams in a controlled and quantity-proportional manner via volumetric or gravimetric solids dosing systems. The powders and liquids are brought together in a premixing zone, and fine dispersion then takes place via a rotor-stator system, whereby the dispersion tools can be designed with different slot widths depending on the application.

Fig. 3: The YSTRAL Coflow enables the production of products with a solids content of up to 80 %. The inline disperser is therefore the ideal solution for the production of toothpaste, for example.

The YSTRAL Conti-TDS is primarily used in recirculation processes. The disperser is placed outside the process container with a mixer installed in it and the powder is sucked into the liquid from a bag, hopper, big bag, silo or container. The Conti-TDS is only suitable for continuous production to a limited extent, but quasi-continuous production is possible with the use of a working tank and a buffer tank or in the form of twin-tank processes. In a twin-tank process, a Conti-TDS is connected to two process tanks. While production takes place in one tank, the finished product is pumped out of the second tank. As soon as this tank is empty, the system switches over.

Advantages of the batch mixing process

An important advantage of the batch process with modern mixing and dispersing technology is the high variability it enables in terms of the raw materials processed and the products, product variants and batch sizes produced. The YSTRAL Conti-TDS offers a wide range of powder processing options. For example, allergenic and non-allergenic powders can be aspirated in completely separate ways and processed in separate liquid circuits. A Conti-TDS can also be easily integrated into existing processing systems.

Advantages of the continuously operating disperser

The advantages of production with a continuously operating disperser, on the other hand, lie in products that are manufactured in large series or are to be produced in large volumes in the future. These include products where the end product consists of just a few components, the recipe is standardised and the quality of the components is assured. The required variability in production is low in this case: concentration adjustments are made from time to time, but product changes are rare. In the chemical industry, for example, the YSTRAL Coflow is suitable for the high-volume production of adhesives or silicone sealants. In the food sector, the continuously operating mixing and dispersing machine is the perfect solution for pre-products for dough production, milk and milk substitute products and the production of edible collagen casings, mayonnaise or dressings. Compared to a classic batch process in a container, the YSTRAL Coflow enables production with a significantly smaller system footprint and faster throughput times, while the cleaning of such a system is much easier due to the smaller product-contacting surface.

Very high solids concentrations possible

The Ystral Coflow also enables the production of products with a very high solids content. For this purpose, the Coflow is equipped with a large powder inlet port that allows solids concentrations of up to 80%, depending on the application. The large powder volume flows of the YSTRAL Coflow are particularly relevant for powders with a low bulk density. An inducer installed between the premixing zone and the rotor-stator zone serves as an axial pumping stage in order to be able to process even highly viscous products and pump them out of the dispersion stage. This makes the Coflow a suitable solution for slurry production or the manufacture of poorly flowing end products such as toothpaste, for example.

Low residual air content in the product

Whether inline or batch - in order to achieve optimum product quality, the air input during the process and the residual air content in the product must be minimised. Both the YSTRAL Coflow and the YSTRAL Conti-TDS are characterised by efficient separation and removal of the air contained in the powder compared to other process technology solutions in the inline or batch process.

“Whether inline or batch - in order to achieve optimum product quality, the air input during
 the process and the residual air content in the product must be minimised.” 
- Uwe Klaumünzner, Expert Process- and Application Engineering 

In traditional dissolver technology, for example, a large amount of air is introduced into the product by a drum, which not only reduces the dispersing effect, but also requires additives that later have a negative effect in the finished product. With Conti-TDS, on the other hand, the air contained in the powder is separated from the significantly heavier dispersion by the centrifugal effect of the high-speed rotor and coalesces into large air bubbles. These are then conveyed together with the liquid flow to the process tank, where they can easily escape. With the Ystral Coflow, the product experiences an increase in pressure via the integrated inducer. This change in pressure separates the air carried in with the powder, which is pressed upwards against the product movement and separated.

Conclusion

Both batch processes and continuous inline processes have their place in modern process engineering. Powders that are difficult to process and require intensive dispersion and a high degree of variability in production are in favour of the batch process. Powders that are easier to process and large-volume product manufacturing are in favour of an inline process. With technologies from ystral, both batch and inline processes can be realised in an efficient, time-, energy- and space-saving manner.