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A well-considered decision

Mixing in batch mode or continuous flow
A well-considered decision

Batch mode or continuous flow? There is no way of subsequently telling how the final product of a mixing process was manufactured. Depending on the application, one process may be superior to the other, for instance in terms of the formulation, processing times or production volumes. The decision should take account of the characteristic differences of the two processes.

The author: Dr. Thomas Meyer Development and Process Technology, Gebrüder Lödige Maschinenbau

There are essentially two options in industrial mixing: batch mode or a continuous process. There are naturally many common features. In both cases at least two heterogeneous materials are blended to form a mixture that is as homogeneous as possible. The differences – and the repercussions for the production process – lie mainly in the way the mixer is filled and emptied, namely in batches or continuously.
Batch operation in this context means that the individual, mostly dry components of a formulation are placed in the actual processing chamber. The mixing elements are then set in motion and the mixing process runs until the specified level of homogeneity has been achieved. The raw materials are weighed beforehand – usually one at a time. In addition, liquids such as aromas and fats can be added during the dry mixing process. At the end of the required processing time the product is completely emptied from the mixer. The batch process has finished when the discharge flap closes. A new batch can then be started.
Inside the batch mixer the product mainly moves from the end walls of the mixing drum towards the centre, where the discharge opening is located. This enables thorough evacuation. The correct choice and configuration of the mixing elements ensures that the product is exchanged throughout the mixing chamber – i.e. from the right end wall to the left end wall and vice versa. The mixing time can be selected more or less freely, so that adjustment to a new product is possible without any problems.
With continuous mixing all components are weighed parallel to one another and continuously. The products flow into and through the mixer together – driven by the mixing mecha-nisms – from the inlet to the outlet. They are mixed homogeneously as a result, and upon completion of the mixing process they exit the machine through the discharge opening. It is also possible to add liquids at a suitable point inside the continuous mixer. The required mixing time must be known at the design stage. It is influenced and determined by the mixer length and volume, the use of an adjustable weir and to a certain extent the speed and arrangement of the mixing elements. When the product is flung by the elements, it forms an undulating swarm reminiscent of a dancing procession – with two steps forwards and one step backwards. Changes in residence time are severely restricted or extremely complicated to realise.
Inherent differences
The main argument for using a continuous mixer is the ability to achieve high throughputs. This particularly applies if the product is always identical. In this case the mixer is smaller than a batch mixer, as no times have to be taken into account for weighing, feeding and emptying. However, this often enormous advantage simultaneously means limited flexibility in terms of product changes and mixer production times.
To be economically viable, a continuous mixer should run for as long as possible without stopping – in the best-case scenario around the clock. Starting up and shutting down a production line inevitably results in product loss because the mixer must establish stable conditions on the inside before the product can be discharged correctly. If liquids are added, these must be the first to be switched off when the machine is shut down, as local excess humidity and smearing may otherwise occur in the mixer. From this moment on, of course, the formula of the discharged material is no longer accurate.
Great differences in control
There are also differences between batch and continuous mixers with regard to the weighing and dosing method. With batch operation all raw materials (apart from very small components) can if necessary be weighed one after the other on a scale and collected in a bin. Even the mixer itself can be placed on weighing cells and used as a scale. In this case, the resolution of the scale must be taken into account. In a continuous process this resolution can seriously limit the choice of suitable dosing units for powders and liquids. Depending on the level of accuracy required, the physical reso- lution of these units may not be sufficient. In addition, the determination of a flow rate always means specifying a measurement period, which must correspond to the residence time in the mixer. Continuous mixers can compensate for fluctuations in the dosing rate – at least to a certain extent – by mixing during the residence time to ensure that the product nevertheless adheres to the formulation.
Control differences likewise exist between the two mixing processes. A batch mixer can be run in manual mode but is also capable of fully automatic operation with formulation management. By contrast, a continuous production plant has to have a certain degree of automation. Operator intervention is relatively simple with batch operation but far more complex in continuous processes.
The situation is similar when it comes to new products: batch mixers have relatively simple and readily accessible “control paths”, whereas in continuous mixers these are linked in a more complex manner. If need be, a batch mixer can even be run by “trial and error” on a production scale with ad hoc intervention in the workflow. With continuous processes this is no longer possible.
Cleaning
The method chosen to clean the production equipment is independent of the products and the composition of the process components. However, batch mixers are normally less troublesome to clean because the system components are easier to dismantle. In addition, the machine is usually more accessible. This can be difficult in a continuous process owing to the complex interaction of the individual components, which are also less straightforward to dismantle. The complete process component chain often has to be cleaned as a single unit.
Development and planning
When a new product is developed, the formulation should consider the pros and cons of batch and continuous processes. Extensive laboratory scale tests are essential. Competent manufacturers have an in-house facility for this purpose. All basic design details are specified in the test phase. The successful completion of relevant tests is an important precondition of successful mixer operation. This makes life easier both for the project engineers responsible for designing the system and for the production manager who will subsequently oversee its operation.
Continuous processes always require a more precise analysis in advance, as implementation is generally more complicated. Mixing ores and aggregates with a throughput of several dozen tons per hour is only viable in continuous operation. On the other hand, the manufacture of spice mixtures with hundreds of potential formulations and multiple changes of formulation per shift, for example, only makes sense in batch mode.
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