I would use "preferably", not "preferentially" with "will". Alternatively, I would delete "prefertially" and use "should", not "shall/will".
I don't pretend to understand the meaning but, for me, it should read:
The temperature is defined by the process and, preferably, will not exceed 90 degrees C.
The temperature is defined by the process and should not exceed 90 degrees C.
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