Hi pawlaw, and welcome to Cruise Chat.
The Century is scheduled to arrive back into Barcelona at 5am, but that's not the time the passengers can disembark. The ship has to clear Customs and go through other formalities before passengers are allowed off the ship. Disembarkation usually starts a couple of hours later.
Celebrity recommends 10am as the earliest flight departure time out of Barcelona. I suggest you talk to someone at the Purser's Desk when you get onboard, and let them know of your early flight departure time, so they can include you in the first group of passengers allowed off the ship.