Approximately when do I have to leave Komed to get to the airport in time? I will not have any luggage to check in and I guess that a taxi is the way to go…
The Hansaring station is about 10 minutes' walk from the conference building. (The KVB planner counts 14min, which is highly pessimistic, the distance is only 700m.) From that station, line S13 (see the network map) gets you to the airport station in 17min; you can see the times on the trip planner. The S13 trains call at Hansaring at :18, :38 and :58. For a 17:50 flight inside the EU, arriving at 17:15 would be cutting it close, so I'd plan on taking the 16:38 train (arriving at the airport at 16:55), meaning you should be outside the building at about 16:25 (allow a couple of minutes to buy a ticket (€2.50), I don't think you can buy the ticket in advance).
I don't know what the traffic is at around 5pm in Cologne, but even with a clear way a taxi isn't going save a lot of time. The airport website states about 15 minutes to get to the center, to which you need to add some time for crossing the Rhine. Google's driving directions quote 23min, so in light traffic (but can you assume that? I don't know) you can leave at about 16:45 tops.