You have opened a couple of completely different topics within a single thread. Perhaps you should start a new one.
In case you have water in the tower feed, there are several possibilities - it all depends on the amount of water, composition of feed, and operating conditions of the tower:
1) Water may go overhead and condense in the overhead receiver;
2) Water could condense on the tray(s);
3) Water could freeze inside the tower;
4) Water may end up with the bottom product.
Obviously, with so many unknown factors, it is not possible to provide you with a simple, straightforward answer.
As for the second question (operating in the retrograde region), the answers are again not simple - this depends on the amount of condensation as pressure gets lower along the pipeline, abilities of the plant to handle varieties in gas/liquid loads, frequency and intensity of slugging, etc. A detailed analysis of operating scenarios and variation in fluid composition is normally required before one can come up with definite conclusions.
Dejan IVANOVIC
Process Engineer, MSChE