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Scaling up the mixing process

02 January 2023

Matt Smith explains the key things that should be considered when expanding from pilot scale to full production.

As batch sizes increase it is necessary to decide whether to use in-tank or in-line mixing. Over a certain batch size, it becomes more efficient and economical to switch to in-line mixing, because an in-line mixer’s energy is concentrated on the small volume within the mixing chamber at any given moment, rather than expended on circulating the entire contents of the vessel, as well as the mixing task itself. Consequently, a relatively small in-line mixer will be able to process a larger batch than an in-tank mixer of similar power. 

When batch sizes and vessels increase in size, the processing time will do the same, so it is necessary to take into consideration the tank turnover rate – the amount of time it takes to recirculate the product – and be prepared to mix for longer because mixer flow rates will decrease as batches become larger. This may mean that additional equipment, such as feed pumps or scraper/stirrer units in the vessel are needed to maintain flow rates when mixing more viscous products.

When mixing at pilot scale the process is often manually assisted. For example, using a spatula to agitate the product in the vessel. When manual intervention is necessary at smaller scale, then it may be necessary to invest in additional agitating equipment when scaling up, in order to replicate the process. 
 
Other types of ancillary equipment may also be needed when scaling up – such as vessels, hoses/pipes and pumps so it is important to consider these requirements early in the upscaling process. Usually, of course it will be necessary to produce the new product using existing equipment, so this also needs to be considered. 

Mixing parameters
The usual high shear mixing parameters – such as tip speed, stator design, mixing time – need to match. It is necessary to understand prior to the start of any pilot scale trials that anything extreme among these parameters can become extremely difficult to scale up. As such, these parameters should stay within sensible limits. For example, a laboratory mixer running at 10,000 rpm for over an hour in a one-litre beaker to get the desired product quality may be impossible to scale up. 

Another issue to remember, when it comes to mixer speed, is that lab mixers need to be run at a faster rpm than their production counterparts in order to ensure the peripheral tip speed – and therefore the shear rate – is comparable between laboratory and production scale equipment. It is a common error to run a lab mixer at 3,000 rpm because this is the speed most production models run at, but this will not give an accurate idea of full-scale results. Indeed, a lab mixer running at 8,000 rpm has a tip speed that more closely matches the standard 3,000 rpm of most larger in-tank batch mixers.

Matt Smith is sales director at Silverson Machines.


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