Abstract: A heat exchanger is provided which is suitable for use as a condenser/reboiler in a double distillation column. The heat exchanger has alternating first and second passages and a reservoir located above the first and second passages for receiving a first fluid and for containing a froth. A series of orifices can be provided by a distributor tray to provide flow communication between the reservoir and the first passages to allow at least a liquid phase of the froth to weep into the first passages and to be partly vaporized through indirect heat exchange with a second fluid flowing through the second passages. The partial vaporization produces a vapor phase of the froth which can at least in part escape from the orifices and interact with the first fluid to thereby form the froth. A remaining part of the liquid phase, not vaporized through the indirect heat exchange, is discharged from the bottom of the first passages into a sump.