A pretty hot way to speed up the freeze drying process

If you use a shelf freeze dryer and want to perform your lyophilization faster, then this post might pique your interest. Here I describe a common accessory for quickening the lyophilization process, mainly heated shelves. I also point out important considerations for using heatable shelves and give advice on how to avoid destroying your product while aiming for maximal speed.

 

I recently attended a wedding at a venue that was decorated with thousands of balloons. It reminded me of my nephew’s birthday party where my panicked sister decided to put me in charge of balloon decorations. You might find this hard to believe, but until then I had always used my mouth to blow balloons. Well my sister showed me this fantastic balloon pump and I went crazy. I had something like 20 balloons done in under a minute.

It’s funny how a little energy boost can help you increase your throughput in the laboratory as well. Let us consider heatable shelves in freeze drying.

During freeze-drying, energy in the form of heat needs to be added to the product for phase change from solid to vapor to occur. In fact, roughly 2800 Joules are needed to convert 1 g of ice into vapor. As demonstrated in the figure below, heat can be transferred by convection (b) and radiation (c) from the surrounding environment in both shelf freeze-drying (left image) and manifold freeze-drying (right image). For a shelf freeze dryer, heat can also be transferred by conduction from heated shelves (a).

convection, radiation, conduction, heated shelves, heatable shelves, freeze drying, lyophilization

The more energy transferred to the sample, the faster the process.

Heatable shelves are the most common accessory for providing extra heat to the samples. Depending on the freeze-drying system, the temperature of the shelves can be automatically controlled and adjusted according to programmable methods. But how efficient are heated shelves at improving the freeze-drying process?

Well, an increase in sample temperature of 1°C is reported to shorten the freeze-drying process by 13%, while an increase of 5°C could shorten the process by 50%.

Experimental data shows that for 400 g of frozen water in a plate, only 32 % of the ice is sublimated in 7.5 hours with non-heated shelves. When the shelves are heated up to 20 °C, 61 % of the ice is sublimated within the same amount of time. These results are confirmed by the temperature profiles of the samples during the freeze-drying cycle in the figure below. The graph depicts sample temperature profiles during lyophilization using non heatable shelves (blue line) and heatable shelves (orange line) compared to the set shelf temperature (grey line).

heatable shelves, heated shelves, experimental data, temperature profiles, lyophilization, freeze drying

To go back to my balloon blowing experience, I did overestimate my pumping power several times and ended up popping some of the balloons whenever I applied more pressure than the balloon could handle. The same could happen with your heated shelves if you go out of control.

A good balance between heat energy and sample critical temperature needs to be found to maintain product integrity.

Therefore, heat input and output needs to be carefully controlled. A good way to do this is to use heatable shelves with precise heat input control, combined with thermocouples in the product to measure the product temperature and ensure the collapse temperature is not exceeded. This combination helps achieve fast freeze-drying rates while maintaining product quality.

If you are interested in speeding up manifold freeze-drying instead, check out my previous blog post on the topic.

And my nephew’s birthday party was a great success. He not only liked the balloons, he also liked my present. Read what I bought him and how the selection process was rather similar to picking flash chromatography cartridges in a previous blog post.

Till next time,

The Signature of Bart Denoulet at Bart's Blog
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