Abstract: Fluidizable particulate solids are removed from a first zone operating at a relatively higher temperature and pressure to a second zone or collector operating or maintained at a lower temperature and pressure by an improved drain or particle removal system. The improved system includes moving the particles from the first zone downwardly within a substantially vertical inlet conduit or leg into the lower regions of a first chamber and upwardly through that chamber under fluidized particle transport conditions energized by a fluidizing media provided in the chamber. The particles move to a second chamber by overflowing a weir separating the first and second chambers and are mixed with cooler particles which extract heat within the second chamber under fluidized particle transport conditions energized by a second fluidizing gas source which further transports the particles to the collector or second zone. Heat is removed from the second chamber by indirect heat exchange.