White Paper
Solutions to Batch Mixing Issues
Download Full White Paper:
•White Paper: Solutions To Batch Mixing Issues
Mixing, like any other unit operation, should be viewed as an evolving technology. Many mixing processes in place today were designed decades ago, a time when process efficiency was not considered as important as it is in the current competitive market. Settling on a "norm" that takes time but works can put a business at risk of losing customers to competition that follow an updated and more profitable manufacturing process. The ability or failure to mix efficiently and economically can profoundly influence the growth or decline of a product line.
The good news is that improving the mixing operation does not necessarily have to involve huge costs or a complete process overhaul. Simple upgrades and practical techniques can bring better mixing performance within reach. The following discussion explores ways to improve a number of batch mixing practices. Some of these procedures involve small changes to the mixing method or minor equipment upgrades, while others entail a shift to an altogether new mixer design.
Simplify, simplify
It is quite common for batch production set-ups to consist of a large tank equipped with a slow-moving impeller serving as the main mixing vessel to which raw materials and intermediate components are added. Intermediates are batched in side tanks which may be equipped with high shear, high speed devices such as rotor/stators or saw-tooth disperser blades.
If cycle times and batch-to-batch variability are an issue, see if the overall mixing process can be simplified. In certain applications, the appropriate mixer system makes it possible to combine several operations and the eliminate transfer steps of intermediates.
For relatively flowable products (approximately = 20,000 cP), install an inline high shear rotor/stator mixer that will take product from the bottom of the main tank and recirculate it back into the vessel. By providing supplemental shear and agitation to a gently-stirred batch tank, an inline rotor/stator can significantly reduce cycle time. Depending on the formulation, the enhanced turbulent mixing may also allow some raw materials to be added directly into the main vessel instead of being prepared in separate intermediate batching tanks. Testing is recommended to confirm that the process can be modified without affecting end product quality.
Accelerate and improve powder dispersion
When large amount of powders are involved, especially those that are difficult to wet out completely, a mixer with sub-surface powder induction capabilities is extremely worth considering.
In earlier powder induction systems, a pump would propel the liquid stream into an eductor, creating a vacuum. Powders fed through an overhead tube would be drawn by this vacuum into the eductor where it joined the liquid flow. The resulting ‘pre-mix' moved on to a rotor/stator mixer which then applied shear and mixing action, breaking down agglomerates and transporting the mixture downstream.
In its day, this system offered a useful tool for powder induction. The inline system eliminated the floating solids problem of batch systems, and it offered a more precise control over the mixing process. But this set-up also presented some serious limitations. With three separate devices in series, maintenance — in terms of labor, required expertise and spare parts — was intensive. Balancing the performance of the pump, eductor and mixer was often difficult, and in many applications, downtime was quite high.
Today's inline rotor/stator mixers with integral powder induction capability are more ideal. These new generation high shear mixers no longer require the use of centrifugal pumps or eductors to create the suction for powder injection. These systems are much more tolerant to flow and viscosity changes, easier to operate and simpler to maintain.
Certain batch rotor/stator designs also offer this functionality, enabling powders to be added subsurface without having to seal the tank and pull vacuum on the batch using a vacuum pump. Combining liquids and powders right at the region where intense mixing takes place reduces the formation of agglomerates and fish eyes, and also eliminates the occurrence of floating powders.
Leave the dust behind
Another batch mixing issue is dusting, which is a nuisance from a housekeeping and operator health standpoint but can be an even more immediate danger if the powders pose an explosion-risk. Dusting can be resolved by a powder induction system.
Powders may be drawn from out of a bag or container using a flexible wand connected to the solids inlet port. This technique is particularly useful when the powders are relatively lightweight and free-flowing. Another method is to transfer powders into a hopper directly above the mix chamber. This configuration facilitates the highest rates of powder induction.
Download Full White Paper:•White Paper: Solutions To Batch Mixing Issues
