You can run the simulator with 100% tray efficiency - in fact I would recommend it. But you only use a number of trays equal to the theoretical stages. Actual trays would be simulator stages divided by expected tray efficiency.
The reason the temperatures are odd is that the 70% tray...
The freezing point of neopentane is -18°C, or about 0°F. And in dilute mixtures it would be lower than this. Therefore you probably don't have neo-pentane freezing in the exchanger. I would agree that the problem is likely on the other side of the chiller.
IF you cannot envision a scenario where the tank or vessel is being heated by an external fire (ie. there are no flammable materials nearby that could jet or pool near the tank), then I would say you don't need a fire case PSV. Of course, local codes may govern and override good judgement.
