Distillation tower

The boiler (or reboiler as it is typically called) heats the liquid that it is filled with, causing the liquid to be lower in density and possibly boil. In the case of a thermosyphon reboiler, the liquid rises causing circulation through the reboiler. Typically for start up, the distillation column bottom sump is filled with liquid and heat turned on the reboiler. Once the reboiler is up to temperature, additional feed is added to the column and product streams begin to be drawn off. But actually, your question is too general. More details of what you want to know are required to answer your question fully.