The choice of blender type dramatically can affect end results. Yet, often we put a product in whatever blender is available. That’s a mistake. Each blender is unique; they may vary in typical blend time and quality, blender capacity, scaleup, maintenance and attrition. So, let’s compare blender types.
The number of options can seem overwhelming. One manufacturer’s selection guide I recently looked at showed several hundred blender types. I don’t doubt that their differences are real in the eyes of the designer — but I’ve usually found that the particulate solids characteristics of the material to be blended are more important than some subtle variation in mechanical design. For simplicity, I suggest concentrating on three overall types of blenders: mechanical, gravity and fluid-assist devices.
Most such units are small capacity. Let’s look at four options.
• The double-cone blender has been the workhorse of the industry, especially for batch operations. Blend times typically run one to two hours. Maintenance is low. Attrition is moderate but particles may segregate on discharge. Internals, which come in many variations (ribbons, baffles, etc.), can improve blend quality and minimize segregation.
• Paddle or ribbon blenders provide much lower blend times and often serve in continuous processes. Low-speed units may reach a good blend in one to 15 minutes. In contrast, high-speed devices may achieve the same blend in five to 30 seconds but the number of fine particles generated may override blend quality. Maintenance is moderate but attrition is high for high-speed devices.
• Riffle blenders can match the blend quality of a high-speed blender and create less attrition. However, their maintenance (cleaning) is high and they don’t scale up very well.
• Screw blenders aren’t known for blending applications but can reach a good batch blend in three to five minutes with moderate attrition. Maintenance is high but scaleup is easy.
For continuous operation and the best blend quality, I suggest a loss-in-weight feeder. It scales up easily and creates little to no attrition. The device demands only low to moderate maintenance but often is an expensive alternative.
Storage of large quantities of bulk material frequently requires some blending to compensate for segregation or process variations. Achieving the best quality blend may call for recirculation via pneumatic conveying. The traditional static flow tube smooths out small variations. However, mass-flow or multi-cone blender designs largely have replaced it because they can reach a high quality blend in a couple of turnovers. Moreover, they are easier to scale up than flow tubes and are lower in maintenance. Other than attrition from the pneumatic conveyor, particle breakage is minimal in these devices.
These devices often are costlier than the other two categories but compensate by providing better blend quality. Let’s examine four alternatives.
• The lift-tube blender could be considered a gravity blender — but recirculation is internal to the device and blending can take place during tank filling, which smooths out production variations over a longer time frame. Scaleup is moderately easy and maintenance is low. Attrition is negligible because the recirculation loop has no feeders or elbows. The design of the internal cone controls blend quality.
• The fluidized bed probably is the most effective fluid-assist blender. It works well in batch or continuous operation. Blend time is on the order of minutes versus hours and scaleup is easy. Most maintenance is external to the device; proper grid design minimizes attrition.
• The spouted bed, a cousin to the fluidized bed, is a favorite for coating particles as well as giving a good blend in only one turnover. Attrition is very low, especially with big and light particles, because most attrition occurs as the particles fall back onto the bed. Scaleup is moderately easy; units require little maintenance and cleanup is easy.
• Blenders using a jet or venturi work well with fine materials and can give an excellent blend. Scaleup is easy but maintenance can be high. These devices better suit liquid/particle blending.
Throughout this column I haven’t talked much about blend quality because this mostly is a function of the particles in the system. Experimental data are a must to define blend quality.