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I read this answer which raises the question of what happens to the silica gel used in the ISS once it has absorbed humidity.

The ISS relies on silica gel to absorb humidity from the atmosphere to make it easier to remove excess CO2.

Is this silica gel replaced when it can't absorb any more humidity, with new silica gel being sent to the ISS from Earth along with other consumables? If so how often is it replaced?

Or is the silica gel recycled and reused? If so, how is the recycling done and how many times can it be recycled?

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The author of that answer was confused by a poorly written NASA PR document.

The majority of the humidity control on the US part of the ISS is not done using silica gel. It is done using condensing heat exchangers called Common Cabin Air Assemblies.

The temperature and humidity of the International Space Station (ISS) United States On-orbit Segment (USOS) cabin air is controlled by the Common Cabin Air Assembly (CCAA). The CCAA consists of a fan, a condensing heat exchanger (CHX), an air/water separator, temperature and liquid sensors, and electrical controlling hardware and software.

enter image description here

The silica gel referred to in the other answer is a component of the CO2 removal apparatus (CDRA). It serves only to dry the air stream going through that apparatus.

This image shows where the silica gel is located in the CDRA (labeled SG).

enter image description here

The CDRA operates in a cyclic manner so that its various absorbing beds can first collect, and then later give up, CO2 and water. Each "half cycle" of the CDRA takes 144 minutes. During each "half cycle" one of the silica gel beds is collecting water and the other one is drying out, as explained in more detail in the rest of this answer.

The half of the CDRA that is absorbing passes air from the cabin through one of the desiccant beds (including silica gel), the fan, the heat exchanger, a CO2 removing sorbent bed, and then through the other desiccant bed and back into the cabin. On the diagram this is the red / dashed green / dashed red / blue line.

When the air stream passes through the first desiccant bed, it is dehumidified so that the CO2 removing bed will work properly (silica gel is gaining water). When it passes through the second desiccant bed, it dries out the bed (silica gel is losing water).

Meanwhile, the other CO2 removal bed is connected to vacuum (orange line) to get rid of the CO2 it has absorbed.

After 144 minutes, the valves reconfigure and send the air stream to the other CO2 removal bed, the first CO2 removal bed is connected to vacuum, and the desiccant beds reverse their roles.

Sources

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    $\begingroup$ But what about the silica gel in the CO2 removal apparatus? Is it exchanged or regenerated? Regeneration may be possible by exposing it to the space vacuum. $\endgroup$
    – Uwe
    Sep 11 '20 at 14:12
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    $\begingroup$ So that second diagram show the silica gel being "recycled" in the 'desorbing" bed? $\endgroup$
    – user20636
    Sep 11 '20 at 14:13
  • $\begingroup$ So the silica gel is left in the CDRA and is neither replaced, nor recycled, right? $\endgroup$
    – usernumber
    Sep 11 '20 at 14:35
  • $\begingroup$ @usernumber that is correct, barring any failures of the apparatus (and it fails a lot). $\endgroup$ Sep 11 '20 at 14:36
  • $\begingroup$ @Uwe I corrected my first draft. The CO2 removal beds are regenerated by connecting them to vacuum. The desiccant beds are regenerated by blowing dry air over them. $\endgroup$ Sep 11 '20 at 14:50

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