How does it work? I do not get it my brain won’t comprehend how you can freeze something and also remove moisture without heat
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I use freeze driers (aka lyophilizers) in my work from time to time. Water moves from warm areas to cold areas, like what happens when your glasses fog up if you come into a warm house after being outside when it’s cold outside. In the cold glasses case the glasses are considered a “cold sink” and the hot area is the room. The water goes from the hot area (air) to the cold sink (your glasses).
The process of freeze drying includes the following: The sample (thing to be freeze dried) is first frozen. Once it’s frozen everything is locked in place–sort of. In this case let’s say the sample is at -20 Celsius. The sample is placed into a vacuum chamber which is pumped down to remove the gaseous nitrogen, oxygen, CO2, and some *water vapor molecules but not all*. The vacuum chamber is now connected to a cold sink, usually it’s at -80 C. So now the system consists of a frozen sample (at -20C) in a vacuum with a cold sink (at -80). Since the frozen sample is not at absolute zero the water molecules are actually still moving! The water molecules will tend to fly off of the hot thing (in this case the “hot thing” is merely the hottest thing in the system which is at -20C) and bounce around until they encounter something to make them settle down, a cold sink. If the vacuum is low enough the water molecules will bounce around a few times and find their way to the cold sink (at -80C). The water molecules will tend accumulate on the cold sink and literally evaporate from the “warm” -20C frozen sample without going through a liquid phase. The vacuum is essential for avoiding the liquid phase*. So in essence freeze drying is no different than water condensing on a pair of cold glasses after you come in from the outside, water is merely going from a warm place to a cold place. What’s odd about freeze drying is that ALL the temperatures involved are below 0C and it messes with our everyday experience. In the end it’s water going from something hot to something cold. I hope this helps a little bit.
*Technically the role of the vacuum is to lower the number of air molecules in the system so that the mean free path from the “hot” sample to the cold sink is lower. If there is too much air in the system then the water molecules bounce off of the air molecules and can’t find the cold sink quick enough; this causes the sample to melt before the water sublimes. Most freeze driers will use a single rotary vacuum pump and a cold trap. Ultra high vacuum systems will often use multiple types of vacuum pumps coupled together and always use a cold trap called a “cryo pump” to remove residual water molecules.
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