The Basics of Mixing & Blending

Biotech and food & beverage industries are constantly mixing and blending product, and mixer selections can have a huge impact on the bottom line. In fact, annual costs of poor mixing have been estimated at a 1-10 billion dollar loss in industrial chemical applications alone. Across all industries, 5% yield losses due to poor mixing are very common. In addition to avoiding high and unnecessary financial losses, using proper mixers ensures the user’s ability to seamlessly scale-up and facilitate development of new products.

There are two main types of mixers:

  • Top-mounted mixers (sometimes call top-entry mixers) are mounted on the top of the tank and have a shaft and impeller hanging down into the vessel. This type of mixer is suitable for medium and high-speed mixing operations.
  • Bottom-mount mixers have an impeller that enters from the bottom of the tank, thus enabling a continued blend during draw-down. This design is commonly used in storage tank mixing, fermentation and bioreactors, as well as food and beverage. There are two sub-types of bottom-mount mixers:

(a) Direct-drive styles (often called direct-mount) have a shaft that enters the tank through a sealing mechanism.

(b) Magnetically-driven mixers have a magnetically-coupled impeller driven by a motor that spins a magnet from outside of the tank. This design doesn’t have a shaft or seal, thus circumventing the possibility of leaks. For bioreactors, this characteristic is vital – seal failure, and either contamination or subsequent fluid loss can be catastrophic. Due to fundamental demands for hygienic and sterile environments in biotech, life sciences, and pharmaceutical industries, these mixers are commonly used in applications involving serums, vaccines, plasma fractions, bacteria, and cell cultures. Other uses for bottom entry magnetic mixers include aseptic mixing, buffer preparation mixing, and media make-up agitation.

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Every application has different needs in terms of agitation level, shear, levels of suspension and dispersion, and flow rate. As such, it’s important select a mixer that can satisfy all specifications.

Shear sensitivity – The products’ behavior and shear sensitivity helps decide the level of agitation intensity, and thus the type of mixer needed. A high shear mixer could destroy a shear-sensitive liquid and instantly kill a million-dollar batch. Other products need to have a low enough shear that the product won’t be overworked, but still kept in solution. Yet others need a high shear mixer, or they will never go into solution at all.

Applying the right kind of shear to a product is essential to avoid particle degradation, as well as heat buildup.  In general, top- or center-mounted mixers are better suited for products requiring high shear aggressive or rapid blending.  For products that need easy, gentle movement, bottom-mounted magnetic mixers are the most effective.

Tank design – Tank design – including the shape and position of the mixers – plays a crucial role in the mixing process in defining the mixture’s flow pattern. Magnetic mixers are installed in the bottom of the tank, creating a top-to-bottom vortex in which the fluid flows. On the other hand, top-mounted mixers have more flow pattern flexibility because they can be located in the center of, or offset in, the tank.

Ease of cleaning – Ingredients mixed are almost all expensive, value-adding products. Picking a design that minimizes loss and prevents down the drain flushing is vital. All mixers will leave a residue, either in the base of the mixer bowl or hanging onto sidewalls, shaft, paddles and bearings. This product is often washed away during cleaning, but can be captured with accessible design. When evaluating a design, companies should examine what areas might impede full product extraction. The mixer should be able to allow operators to safely and easily access all corners, joints, and sidewalls. In addition, use of designs with removable paddles and shafts can increase product recapture.

Clean-down time is another important factor in maintaining production efficiency. Time needed to clean a mixer can range from as little as half an hour for a dry clean and up to eight-hours for a full, wet clean and dry. Lengthy clean-down times require an intensive amount of manpower – costs that can drain company profits.

Available space – Space needs goes beyond just mixer size; companies need to be aware of space needed around the mixer for loading and unloading product, as well as for cleaning. In addition, operations must consider ceiling heights, which are necessary for access above and below. For facilities with constrained space, mixers with a low footprint and head height can be the answer.

Although it can be tempting to buy an affordable one-size-fits-all mixer, this can actually result in greater financial losses down the road, and can decrease product end-quality.  Acuity’s team can guide you in picking the best possible mixer for your purposes. While most suppliers only sell one type, Acuity sells a wide range. Acuity can help customers and companies pick the right mixer for their needs, taking all necessary details into account so you don’t have to!

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