My memory is foggy on that one now as well but I seem to recall something about the piping around that reactor not being p0roperly cleared.
Found this on Wiki:
https://en.wikipedia.org/wiki/Phillips_disaster_of_1989
I read that Wikipedia entry.
I remember some of the workers in the area saying "eh, not so fast..." but those kind of conversations tend to happen when things like this occur.
Some plants, the culture of safety, process, and procedures are higher than other, as we all know.
I wonder if the valves now have been made in such a way that they cannot be reversed?
In my recip compressor world, API 618 was amended for that very thing.
No longer allowed to have reversible suction and discharge valves on a compressor, which used to be quite common.