Mixing and Blending

Mixing and Blending

The goal of mixing or blending is to process material to a desired level of homogeneity that happens at a specific time during the processing. This processing endpoint depends on physical properties of the particles or powders including particle size and morphology, surface texture and cohesiveness. Processing time can be controlled by manipulating critical process parameters including the geometry of the vessel and impeller as well as impeller speed. Conventional means to identify the mixing end-point involve extraction of samples from the process and analyzing them offline to access the degree of homogeneity.

Offline measurements provide limited opportunity to closely track the process towards completion and rapidly detect any mechanical failure of the equipment. As a result, executing the process using offline measurements is likely to carry the unintended costs of wasted energy, compromised throughput and product quality. Inline technology that instantaneously detects the endpoint of a mixing process eliminates these issues, resulting in considerable economic return.

LIR technology is well suited to monitoring the mixing, blending or granulation processes, due to its capability of real-time measurements inline. The Force Pulse Magnitude (FPM) and Powder Consistency Factor (PCF) metrics recorded over the processing period provide unique process signatures, and, combined, the process fingerprint. The following examples of process monitoring illustrate the concept.

Detecting the point of homogeneity after liquid addition in a vertica twin-shaft mixer

A LIR-D configuration was used to monitor dispersion of a liquid component in a powdered food product being mixed in a vertical twin-shaft, with a DFF sensor installed into the mixer volume as shown in the picture.

While changes in the physical properties of the mixture were monitored in real time using the LIR,, samples were instructed at two location for off-line analysis. The liquid contained sodiul chloride (NaCl), so its distribution through the powder could be measured offline by potentiometric titration. 

Comparison of the time-dependent FPM process signature with the off-line measurements (picture below) demonstrates a close agreement between the LIR and offline measurements both indicating the blend reaches a state of homogeneity at the 25th seconds of the processing.
Liquid addition causes increase in FPM as particles coalesce and capillary bonds strengthen between particles, but FPM values rapidly decrease as the liquid addition stops and the liquid disperses through the powder. An inflection point in the FPM process signature at approximately 25 seconds separates the volatile part of the signature from a plateau of FPM values that evidently indicates the homogeneous state of the mixture. Therefore, Lenterra Inline Rheometer closely replicates an established offline technique finding the endpoint for the process with no requirement to disturb the process. Please see details of these studies in a publication of Powder and Bulk Engineering.