An apparatus is provided that includes a heating zone, an overhead temperature modulation zone in fluid communication with the heating zone, and an overhead cooling zone in fluid communication with the overhead temperature modulation zone. The heating zone includes a solid reactant support and a reactant gas input, and also includes a heater. The overhead temperature modulation zone includes a temperature regulator for modifying the temperature of the overhead temperature modulation zone. The overhead temperature modulation zone also includes an overhead gas outlet and provides a reflux flow path for condensate to return to the heating zone. The overhead cooling zone has a chiller for cooling the temperature of the overhead cooling zone. The overhead cooling zone provides a barrier to separate condensate formed therein from the overhead temperature modulation zone. The apparatus further includes an apparatus wall that defines an interior chamber of the apparatus between the zones.