Mix It Up! Consider Continuous Blending

Whether the powder-based medicines we take, many of the processed foods we eat, or fertilizer we spread on our lawn, chances are most of the products you come across each day have been processed from a powder or bulk solid that has been aggressively mixed or gently blended.
    
Mixing or blending is the process of thoroughly combining materials to achieve a homogenous mass. In most cases, the mixture is a combination of dissimilar materials (e.g., plastic pellets and titanium dioxide to make white trash bags) using significant agitation. A mix can also be with a single component that has a wide distribution of its particles. Blending, on the other hand, usually occurs in a gentle manner (e.g., blending urea, potassium, and phosphorous to make fertilizer).
    
Manufacturers’ desire robust mixing processes that provide fast blend times, recipe flexibility, ease of equipment cleaning for minimizing product change-over time, and assurances that de-mixing (i.e., segregation) does not result in poor product uniformity with the blend or mix.
    
Blenders come in all shapes, sizes, arrangements, and modes of operation, but they fit into one of two categories -- batch or continuous. A batch blending process typically consists of three sequential steps: weighing and loading blend components; mixing/blending; and discharge of the blended product. Whereas, a continuous blending process has the weighing, loading, blending, and discharge steps occur continuously and simultaneously.  
    
In a batch blender, like a tumbling double-cone, V-shape, in-bin unit, or a ribbon/paddle/plow convective unit, solids motion is confined by the vessel and directional changes are frequent. The retention time in a batch blender is carefully controlled; not the case with a continuous blender. Blending cycles can take from a few seconds with high-intensity units to 30 minutes or more where additional processing like heating or cooling may be involved. Blender discharge may be rapid, or take substantial time, particularly if the blender is used as a surge vessel to feed a downstream process. The batch blender often creates a process bottleneck given the blender cannot do operations of storage and blending concurrently.

Batch blenders are considered when:
•    quality control requires strict batch control (critical with pharmaceuticals)
•    production quantities are small
•    ingredient properties often change
•    blender cannot be dedicated to a specific product line.

Get more information or register for the International Powder & Bulk Solids Exhibition & Conference

Advantages of batch over continuous blending include:
•    control of blending time
•    lower installed and operating costs for small to medium capacities
•    lower cleaning costs with frequent product changes

Now, let’s consider a continuous blending process where the mixing occurs during transport of the material from the in-feed point toward the mixer outlet. A ribbon/paddle/plow blender can easily be a continuous mixer whereby additives are carefully introduced to the blender along its mixing zone via use of gravimetric feeders. A process controller can readily adjust to custom recipes by varying the ratios of the main component to the minor additives. This can yield large throughputs, manufacturing flexibility, and reduce high labor costs that result with batch blender use.
    
Continuous blending should be considered when:
•    continuous, high production rate process is required
•    strict batch integrity is not essential
•    process streams require combining
•    product variations require smoothing out
    
Major benefits for a continuous blending system over batch blending include:
•    automation that improves quality, eliminates loading errors, and reduces labor costs
•    integration to an efficient continuous process
•    achievement of substantially higher throughput rates
    
Consider this: with pharmaceuticals, blend uniformity is tantamount to blend time and throughput capability given that a consumer could be injured, if not killed, from taking a poorly blended product. As a result, batch tumble blenders are used to ensure strict batch quality control, but at the consequence of limited throughput, high manufacturing costs, and requirement of specially trained operators. Continuous blending, though highly attractive, has been considered taboo by pharma. But now it exists and is working well. In 2015, Vertex Pharmaceuticals successful commissioned the world’s first commercial continuous blending process for their cystic-fibrosis drug!¹
   
So, if you are developing a new blending operation, mix it up and realize the benefits that can result from a continuous blending system.
    
Eric Maynard is the director of education and a senior consultant at Jenike & Johanson Inc., Tyngsboro, MA. The firm specializes in the storage, flow, conveying, and processing of powders and bulk solids. During his 20 years at J&J, Maynard has worked on more than 500 projects and has designed handling systems for bulk solids including chemicals, plastics, foods, pharmaceuticals, coal, cement, and other materials. He is the principal instructor for the AIChE courses, “Flow of solids in bins, hoppers, chutes, and feeders” and “Pneumatic conveying of bulk solids.” He received his BS in mechanical engineering from Villanova University and an MS in mechanical engineering from Worcester Polytechnic Institute.

¹ http://www.wsj.com/articles/drug-making-breaks-away-from-its-old-ways-1423444049

Get more information or register for the International Powder & Bulk Solids Exhibition & Conference