Testing a Mixer

Many mixers can be operated in either batch or continuous mode. It is advised that the selection of batch versus continuous operation be made before any tests are conducted. The mixing tests should be conducted in appropriate mode. For instance, performance of a paddle mixer in continuous mode of operation cannot be reliably estimated from batch experiments. 

In either case, we need to address the following questions regarding sampling and analysis  

1 Sample Size. An ideal sample size is equal to the scale of scrutiny or the scale at which the product specification has been developed. If the scale of scrutiny is much smaller than the minimum amount of sample that can reliably be obtained from a sampler, suitable sample reduction techniques must be utilized. The spinning riffler has been found to be the most reliable method of reducing the sample size (Allen, 1981). The sample size must be greater than the minimum amount required for the analytical technique. 

For continuous mixers, full-stream samplers at the mixer outlet are recommended. In-hne or in situ probes to measure the physicaFchemical property of interest, if available, can give useful insight into the mixing process. Before sampling, make sure that the system has reached a steady state and that at least three residence times have passed. 

4 Number of Samples. Determination of the number of samples depends largely on the confidence level in the estimate of population standard deviation from sample standard deviation. The precision of the estimate of standard deviation increases with the number of samples however, so does the analytical cost. The analytical cost is generally a small price to pay to avoid selection of an improper mixer. It is a common practice to take 15 to 30 samples during testing. As a rule of thumb, the total amount of sample removed from a batch mixer should not exceed 5% of the charge. This will prevent sample bias. Repeat tests must be conducted if the total number of samples required for analysis exceeds 5% of the charge. 

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