The "crystals" are molecular sieve and can be regenerated in an oven
but new ones here:

https://www.silicagel.com.au/molecular-sieves/4a-4x8

 

Thank you very much gentlemen for all the help and advice. The whole job took me two days to complete (I am 81 and not so nimble under cars anymore). The most difficult part was in fixing the compressor back up into the car. My X350 has the two horns right where you need your hand to hold the washers/spring/nut assy. The video Jaroslav posted (very helpful) is on a X358 and does not have the horns in the same place. Anyway, after much struggling and swearing (such as "this is unfortunate") I concluded that a special tool would be needed. First, I glued the upper most Top hat piece to the stud with just a tiny drop of super glue to hold it in place (see picture) Then with the aid of my tool was able to both hold the compressor (with the aid of a chock) up and place the spring and lower top hat washer/nut assy. Everything else was straight forward and after a test run without any faults showing I deemed it "Mission accomplished".

Chock holding up the washer assy whilst the glue sets.

Special tool.

tool closeup

 

Glad to hear you have the problem solved! My dad's Jaguar recently started throwing the Air Suspension Fault despite the car being fully leveled and no signs of struts going out. Plus, it only comes up one time before going away, so that might be a sign of a compressor job

 

Tommy, your symptoms sound exactly like mine were, so the compressor piston ring is the culprit.
I went on a lengthy drive this morning and no fault message.