Batch mixing versus continuous mixing
Powder mixing and accompanying processes such as humidification, deagglomeration, agglomeration, heating, drying, cooling, fumigation, bonding and ...... can take place discontinuously or continuously. This blog entry addresses the advantages and disadvantages of both mixing methods.
If we compared the number of continuous mixers and the number of batch mixers across all industries, we would quickly see that the number of batch mixers exceeds the number of continuous mixers by far. Batch mixers also dominate the array of different mixer designs.
In general, bulk goods are preferably mixed and treated continuously. However, if many components have to be mixed and if very high mixing qualities are required, then batch mixing is preferred. A batch mixer offers much more flexibility in practice. This is another reason why batch mixing plants are much more common.
Comparison
The batch mixer - on the left - works in batches. The mixing tool only starts to rotate when the individual components have been filled in. The mixer changes the position of all particles in relation to each other by three-dimensional flowing of the goods. The state changes from "unmixed" to "technically ideally mixed". Only then does the discharge process take place.
In the case of continuous mixing - shown on the right - the components involved are continuously fed into the mixer in the correct mass ratio. The material streams are homogenized in the mixer and the mixed material is continuously discharged. In this case, the concentration gradient is lower because the so-called continuous boiler flow has already produced a homogeneous mix base. Input material flows are comparatively small in relation to the already homogenized vessel contents. For example, the average residence time can span between 0.5 and 3 minutes.
At this point, it should be noted that a distinction is made between two types of continuous powder mixing: The tube flow and the boiler flow.
Question from a customer: In terms of mixing quality and reproducibility in continuous mixing, isn't the key technology the mastery of continuous dosing technology?
Yes, that is correct. Errors made during dosing lead to the wrong mixing result. Errors that occur for a short time should be corrected immediately. Continuously operating mixing systems have become established since there are accurately operating powder feeders that have high "short-term accuracies". In terms of mixing quality, gravimetric feeders should be given priority. They are also called "Loss and Weight Feeders".
Depending on the type of continuous mixing, residence time spectra may differ. Continuous mixers that have larger residence time spectra can compensate very well for dosing errors that occur for a short time.
The specifics of mixing in the form of classic tube flow and the boiler flow are described in another blog entry.
This feeder (from AZO) stands on load cells. In order not to influence the weighing signal, the inlet and outlet nozzles are coupled with folding collars. Inside the housing, a rotating raking tool serves for constant loosening of the powder. In this way, the metering screw realizes a reproducible control behavior. The filling level in the feed vessel decreases continuously. In the example shown, the feed vessel is filled pneumatically. The filling period should be as short as possible. During this time, the metering screw conveys at a constant rotational frequency.
Question from a customer: Depending on the product and the industry, every mixing plant has different demands on the mixing technology. Which criteria should be considered if you want to choose between continuous mixing and batch-by-batch mixing?
A continuous mixing process is advantageous,
- if a short defined residence time is required; e.g. defined deagglomeration, spontaneous build-up granulation during liquid addition, or when spontaneous chemical reactions take place, which may be exothermic
- if the recipe consists of only a few components
- if the formulations are standardized and the quality of the components is ensured
- if large quantities of the same or of a similar product are produced, possibly even in three shifts (bulk goods such as basic nutrients in grain mills, animal feed, basic chemical agents, washing powders, plastics, building materials)
- if the mixed goods should be packaged directly without intermediate storage
amixon® continuous mixer model AMK 400 stands on load cells. All nozzles are decoupled. The four "Loss and Weight Fiieders" are filled from above. Continuous mixers and metering systems are also available for your mixing trials in the amixon® pilot plant.
A batch mixing operation is advantageous,
- when complex preparation processes are performed, e.g. multistep mixing processes, or mixing processes with gaugepressure or vacuum application take place
- if a laboratory analysis must take place before packaging
- if quality assurance insists on batch control, and
- when cleaning must be performed after each batch
- if many components are involved. This is the case with baby food, dietary nutrients, baking agents, spice preparations, stabilizers, instant food, seasonings, ....
- when customized orders with ever changing recipes are involved (food supplements, spice preparations for meat companies, flavors for the food industry, instant meals, instant beverages,...)
This KoneSlid® mixer from amixon® is a subsequent development of our cone mixer. It mixes particularly gently with an extremely short mixing time. The rotation frequency of the mixing tool is particularly low. The discharge of free-flowing goods takes place within seconds and without residues.
The cone mixer shown above can comfortably perform both mixing processes when resting on load cells. The requirement for continuous mixing is the presence of the dosing systems.
Please feel invited to visit our test center in Paderborn with your products. We will be happy to demonstrate the mixing processes to you!
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