I doubt the silica pack will harm the cels by 
themselves directly. However, my concern would be 
if they dessicate the atmosphere in the case too 
much. Valuable artworks and documents are not 
stored in a dry atmosphere because the material 
(paper and paint, for example) would crack over 
time. I think the optimal humidity level is 
around 40 percent. Since I have no idea how many 
packs you will put in, how well they work to 
absorb moisture, or how much air exchange occurs 
in your art portfolio case, I would think about 
doing experiments to determine how much silica to 
use to achieve a desirable and consistent 
humidity level for long periods of time (you 
would need something to measure the humidity 
inside the case). It sounds a bit extreme, but I 
am sure your cels are valuable as you are 
thinking about going to all this to protect them 
from harm, so you should make sure you are not 
doing more harm than good to them.

---Trebuchet