The Basics of Mixers and Blenders by Kathy Hunter
Monday, September 21, 2009
Mixers and blenders are widely used in the processing of food products. A mixer is a vessel that receives multiple ingredients from upstream, mixes and blends the ingredients to achieve a uniform mixture in a specified period of time, and discharges the mixture downstream. A variety of different mixer types serve solids, liquids, or liquids mixed with solids. A mixer often uses some form of active agitation to achieve the blend. The process may be low shear, offering gentle blending of friable/fragile ingredients, or may be high shear, offering homogenization or emulsification. The process mode may be either batch or continuous.
Mixer vs. Blender
The terms “mixer” and “blender” are both used in the process industry. What is the difference? According to Brian H. Kaye, author of Powder Mixing (Chapman & Hall, 1997), the meanings are similar but the two words have different origins. The Latin word ‘miscere’ means to mix. Mixture describes the product of the physical intermingling of more than one component when those components retain their physical identity. A mixer is a piece of equipment in which a mixture is achieved. “Blend” comes from an old English root word “blanden,” meaning exactly the same as the Latin root word. Blend and blender are synonymous with mixture and mixer. In the food industry, mixer is the more commonly used term; however, both are used. Some manufacturers specifically call their product a blender. The way these terms are used in the process industry is not universal. Some engineers use the word ‘blend’ to indicate the gradual addition of minor ingredients to a mixture. In this article they will be used interchangeably.
Basic Elements of a Mixer
A trough or vessel of some kind forms the largest element of the mixer. In sanitary applications this vessel is generally constructed of stainless steel, with interior angles that include a generous radius to prevent material from collecting in the corners and with all welds ground smooth. In many food applications this vessel may be jacketed to keep the ingredients at a specified temperature. The vessel may be open at the top to receive the ingredients to be mixed or may simply have an inlet to receive the ingredients. The majority of mixers use an agitator or impeller to mix the ingredients. The design of the agitator is dictated by the materials to be mixed and the type of application. Tight agitator clearance helps to ensure effective mixing and thorough clean-out.
Discharge may be controlled by a variety of valve types. Generally the process engineer is looking for a valve design that allows for fast and complete discharge and for a sanitary seal design that prevents batch-to-batch contamination and is able to hold up even with abrasive materials. The mixer drive train, support structure, and controls complete the most basic elements of a mixer or blender.
Types of Mixers: Active mixers vs. passive mixers
A variety of mixer mechanisms and designs are used to achieve the desired blend of ingredients. They may loosely be classified into two groups: active or mechanical mixers and passive or static mixers.
Passive mixers, also called static or motionless mixers, are designed so that material passes through a series of passive baffles that act as chaos-creating elements to generate the required mixture. Passive mixers are sometimes used to convey or deliver an already mixed blend in order to avoid segregation of the ingredients.
Active or mechanical mixers are more commonly used than passive mixer designs. Active mixers use internal moving parts to randomize the positions of the ingredients to form the blend, or use air jets to create convection currents or turbulence in the container. Some of the most widely used types of active mixers include:
- Tumble mixers: the tumble mixer is an enclosed rotating shell mounted on legs. The shell may be rectangular, V-shaped, or double-cone shaped. The shell’s end-over-end rotation gently tumbles the ingredients to produce a blend. It offers low shear, gentle handling. Such gentle mixing action is important if the goal is to limit particle damage with friable materials.
- Impeller driven mixers: the most common types of impeller driven mixers are ribbon mixers and paddle or plow blade mixers. The impeller driven mixer/blender generally has a U-shaped horizontal trough with an impeller shaft running the length of the trough. The shaft is mounted with a ribbon or paddle blade to aerate the mixture, forming a fluidized bed. This mixer applies more shear than a tumble blender, but still provides gentle handling with minimal particle degradation.
- Multiple-shaft mixers/blenders: these mixers have twin horizontal troughs and two parallel impeller shafts with intermeshing plow blades. The shafts can be designed to counter-rotate to fluidize and blend the material in the vessel and ease the addition of liquids. The dual shafts allow it to blend faster than a tumble blender or a single-shaft impeller driven mixer. Anchor/rotor/stator/disperser combinations also offer versatility and flexibility at a variety of speeds, offering the optimal combination of shear and flow at each stage.
Batch Mixing vs. Continuous Mixing
Batch mixtures have served the majority of food industry applications for several reasons. Batch mixing is the simplest mode of operation and it also easily allows each discrete batch of a food product to be identified for traceability and quality control. Most types of mixers can operate in a batch mode. In batch mixing the batch size is predetermined and entered into the batch controls. The mixer is filled with ingredients in the correct proportion, and when product mixing is complete the vessel’s contents are discharged for downstream processing. Processors may consider changing to continuous or “in-line” mixers when production volumes are high and they are seeking process efficiencies. Continuous mixers can time-stamp a batch to produce the same traceability and quality controls offered by batch mixing. Continuous mixing requires a different control system and a mechanical design that allows for simultaneous charging of all ingredients into the mixer and continuous mixing and discharge.
Factors to Consider When Selecting a Mixer/Blender
- Materials to be blended—the ingredients to be mixed dictate the type of mixer. Friable or fragile materials require gentle, low shear blending. If the ingredients in the mixture vary significantly in particle size, shape, and bulk density, material segregation is more likely to occur and will guide blender selection.
- Speed—if speed is critical in the manufacturing operation, a multiple shaft blender may be a good choice for producing a mixture with adequate dispersion quickly. A blender offering fast discharge means shorter blending cycles and greater production efficiency.
- Cleanability—if blend formulations are changed often or if the material easily spoils, ease of cleaning is an important factor. Cleaning times can affect the mixing cycle and slow down the process line. Thorough discharge means less cleaning, less ingredient waste, and less risk of cross-contamination between batches.
- Fast product changeover—quick changeover is an essential tool for increasing productivity. This requires a mixer with fast discharge and easy cleaning/sterilization. It also must be equipped with mixer controls to allow a fast change in recipes to accommodate another product.
- Space requirements—how much room is available on the plant floor? Keep in mind both the footprint of the mixers being considered as well as headroom available.
- Batch vs. continuous mode—is the product line best served by a batch or a continuous process?
- Flexible control system—a robust multi-axis control system allows for the storage of multiple recipes for fast changeover. The control system also needs to interface with the plant manufacturing system.
- Versatility—a versatile mixer that can handle a variety of batch sizes and perform a great variety of mixing functions, such as dispersion, emulsification, and homogenization, can be very valuable. This broad functionality may eliminate the need for a secondary piece of equipment, such as a pre-mixer, and accelerate the overall mixing cycle by performing many functions effectively.
- Drive design—identify product density, volume, and moisture content. Consider the needs of each application. Is a provision for starting at a slow speed needed? A variable speed drive, offering the ability to blend at a variety of speeds, allows fine tuning of the process and the ability to intensify blending without fear of product degradation.
- Test lab results—take advantage of the free test lab trials offered by most manufacturers to guide selection of the correct type of mixer for the application. Gather data on possible mixers under consideration, such as horsepower, start-up torque, mix time, and ingredient addition rates, to arrive at the ideal mixer selection.