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Mixing high viscosity products

17 January 2022

Mixing high viscosity products can be a challenging task. Food Processing looks at typical problems and finds out how to ensure the desired product consistency and texture is achieved. 

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When processing gums and thickening agents, or viscous products such as creams and gels, or sauces like ketchup or mayonnaise, certain tasks – such as incorporation of lower viscosity liquids or dispersion of powdered ingredients – become increasingly difficult as the product viscosity rises. 

While there are several types of agitators and stirrer/scrapers that are capable of handling high viscosity products, the low shear nature of agitators will limit their effectiveness when performing simple mixing tasks such as maintaining in-tank uniformity or heat transfer in jacketed vessels. 

Where the process involves more complex tasks – such as combining oils and liquids to create an emulsion, or hydrating, dissolving or disintegrating powders or solids – traditional agitators and mixing equipment can take a long time to achieve the required results. Indeed, in some applications they will not be able to achieve the desired result, even at low viscosities.

Mixing duties
High shear rotor/stator mixers are well suited for these more complex mixing duties. Traditionally, where an in-tank mixer was specified for higher viscosity mixes this would be used in conjunction with a stirrer/scraper in the vessel; the rotor/stator mixer provided the high shear required for rapid emulsifying, hydration of powders or disintegration and dissolving of solids while the stirrer/scraper distributed the output uniformly throughout the vessel. 

While this approach is still commonly used, well-suited for such larger volumes it is often preferable to mix in-line, by recirculating the product through a pipeline in which a mixer is mounted. Again, high shear rotor/stator mixers are well suited for such applications. Traditionally with higher viscosity products this was something of a challenge, because although In-Line mixers have a high volume self-pumping capacity on lower viscosity products, the pumping capacity decreases when products reach a certain viscosity. The solution has always been to add an auxiliary pump to the system, to ensure the mixer is continuously fed with product. However, in addition to the cost factor of two pieces of equipment, matching the flow rate of the mixer with that of the pump can be difficult – too low a capacity can starve the mixer of product, while too high a capacity will reduce dwell time in the mixer, reducing the amount of shear applied to the product. 

Seeking a simpler, more energy efficient solution to this challenge, a series of in-line mixers  have been developed by Silverson, specifically for high viscosity mixing. The range of production scale UHS-HV mixers has recently been broadened with the introduction of a laboratory scale high viscosity mixer, the Verso HV. 

Both the production scale models and the new laboratory scale model features an innovative pumping rotor, designed to provide increased capacity compared to standard in-line mixers. Located within the workhead the pumping rotor forces liquid and solids into the rotor/stator assembly and ensures that the flow rate of the mixer is maintained as viscosity rises. 

Material is forced out through the holes in the stator and pumped back to the vessel as fresh material is drawn into the workhead, creating a circulatory pattern of mixing in which the entire contents of the vessel passes through the workhead; bypassing is impossible. 

The mixers are aeration-free and hygienic, suitable for mixing applications in the food industry. A range of interchangeable workheads and screens are also available, allowing a mixer to be adapted to perform a wide range of mixing duties.

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