Simple ways to take the edge off the edge vial effect in freeze drying

The edge vial effect is a pain-in-the-butt phenomenon that occurs during shelf freeze drying. The main result of its occurrence is poorer product quality in vials on the perimeter of the freeze-drying run compared to the rest of the batch. Here, I show you how a ferrule and empty vials can be used to minimize the negative consequences of the edge vial effect. I also offer you some experimental data to demonstrate the phenomenon in practice and to show how a metallic ring helps mitigate its effects.

I was watching «The March of the Penguins», a fascinating documentary about emperor penguins. I was amazed at the huddling scenes where the penguins cuddled together to survive the cold Antarctic winters. I wondered how the penguins on the sides don’t freeze to death so I looked for some studies on the topic. It turns out that the giant huddle is constantly moving. The penguins in the middle overheat, so they try to move out and the penguins on the edges push in and warm up. Nature is really a wonder.

But the edge effect does not only concern me when it comes to penguins. I also feel worried about edge vials when I perform freeze drying experiments.

In lyophilization, any vial that is not surrounded by six other vials is referred to as an edge vial. Unlike edge penguins that feel too cold, edge vials get too warm as they are subjected to additional heat transfer. This extra heat comes from radiation from the acrylic chamber during primary drying. The higher energy input the vials receive results in higher product temperatures and shorter primary drying time compared to the rest of the batch.

As I’ve already written about, in a shelf-freeze dryer, most of the energy is transferred to the sample via conduction.

However, the effects of radiation and convection must also be considered as they affect product quality and process control.

Radiation coming through the acrylic chamber will cause products located on the perimeter of the shelf to dry faster than the products located in the center. This well-known phenomenon in freeze drying is called the edge vial effect. And there are ways to fight its nasty occurrence.

A great weapon in the fight against edge vial effect is a ferrule.

This metallic ring (see picture below) is placed around to sample and acts as a shield that can help reduce the edge vial effect.

ferrule, metallic ring, edge vial, edge vial effect, freeze drying, lyophilization, freeze dryer

How effective is this tool? Well, I have some experimental data on the subject that I will gladly share with you.

In an experiment where I didn’t use the shield, the samples on the perimeter of the shelf dried in 19 hours, whereas those in the center dried in 23 hours. In an experiment where I used a ferrule to shield the outer layer of the sample, the drying time of the perimeter samples increased by 2 hours. In total, the outside samples needed 21 hours to dry, whereas the rest of the batch dried in 23 hours. In the graph below, the dashed line represents the freeze-drying process without a ferrule, the full lines the freeze drying-process with a ferrule, the red lines show the drying profile of the outside row on the shelf and the grey long dash dotted line indicates shelf temperature.

edge vial effect, edge vial, ferrule, metallic ring, lyophilization, freeze drying

Interestingly, the first row of samples further shields the rows in the middle.

You could then place a vial set of blanks as a shield in addition to the ferrule.

My suggestion to all of you freeze drying fans who want to achieve better product uniformity and process control, shield your vials in your freeze dryer.

While use of the ferrule alone is not enough to achieve perfect uniformity, shielding can be additionally improved by filling the outside row with vials containing water.

Do you have any bad experiences with the edge vial effect? Have you already tried using a ferrule and other shielding techniques? Let me know in the comments section, I am always looking forward to your feedback. I am now off to give my freeze drier a cuddle and I will see you back on the blog soon.

Till next time,

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