Evaporator Inter-Effect Vapour Desuperheating
Sandwell has developed an innovative technique for extending the capacity of pulp mill black liquor evaporators by a few percent. The system is inexpensive and simple to install. When applied in one kraft mill, it resulted in a 5% increase in evaporator capacity. A system is being installed in a second mill.
When superheated vapour enters the vapour chest on an evaporator set, the vapour must first cool to the saturation temperature before condensation begins. The heat transfer coefficient for cooling the vapour, to desuperheat it, is about one-tenth of the heat transfer coefficient for condensation. By desuperheating the vapour before it enters the vapour chest, the overall heat transfer coefficient increases. The capacity of the evaporator set increases.
Vapour desuperheating is done routinely on mechanical vapour recompression evaporators, where superheat is introduced by compression of the vapour, as well as the boiling point rise of the liquor. The superheat is high.
Vapour desuperheating is done routinely on caustic evaporators, where the boiling point rise is high.
In black liquor evaporators, the superheat comes mainly from the boiling point rise of the liquor; the pressure drop in the vapour ducting adds to the superheat. In the evaporator effects where the solids concentration is high, the boiling point rise is reasonably high, about half that experienced in caustic evaporators.
In kraft evaporators, the steam entering the 1st effect is desuperheated, routinely. But the vapour going between the effects is not desuperheated routinely. Sandwell’s application of desuperheating the vapour flowing between effects is innovative.
