Conversion of Sensible Heat to Latent Heat
When we raise the top reflux rate to our preflash tower, the tower-top temperature goes down. This is a sign that we are washing out from the upflowing vapors more of the heavier or higher-molecular-weight components in the overhead product. Of course, that is why we raised the reflux rate. So the reduction in tower-top temperature is good.
But what happened to the sensible-heat content (the heat represented by the temperature) of the vapors leaving the tower? As the vapor is cooler, the sensible-heat content decreased. Where did this heat go?
A small part of the heat was picked up by the extra liquid draining from the top tray. This extra liquid comes from the extra reflux. But the liquid flow through the tower is too small to carry away much heat. The main reason why the vapors leaving the top tray are cooler is vaporization; in other words, the sensible-heat content of the flowing vapors is converted to latent heat of vaporization.
But what is vaporizing? The reflux, of course. The sensible-heat content of the vapors, which is reduced when the reflux rate is increased, is converted to latent heat as the vapors partially vaporize the incremental reflux flow.
As the reflux rate is raised, the weight flow of vapor through the top tray, and to a lesser extent through all the trays below (except for the bottom tray), increases. This increase in the weight flow of vapor occurs even though the external heat input to the preflash tower is constant. The weight flow of vapor to the bottom tray is presumed to be solely a function of the pounds of vapor in the feed.