MULTI EFFECT FORCED RECIRCULATION EVAPORATORS AT BHP BILLITON WORSLEY ALUMINA
As part of a recent expansion at BHP Billiton Worsley Alumina a pair of multi effect forced recirculation evaporation trains were installed to evaporate Bayer liquor. These evaporators were designed to achieve higher salt concentrations than the existing units in service. The evaporators are almost exact mirror images of each other in respect to the vessels, heater and piping. This allows a high level of equipment standardization and a reduction in required equipment spares. A maintenance access road divides the two facilities and allows access to heaters for tube inspections and crane operations. The barometric condensers’ arrangement and vertical condensate return pumps have been recessed in the ground to allow for reduced steel structures as compared to standard design.
Each evaporator operates with four effects online and one spare running counter-currently, that is the feed liquor and the vapour are following in opposite direction. Four effects are equipped with two shell & tube preheaters for improved steam economy. Axial high flow pumps circulate liquor through each effect and its associated recirculation heater to limit fouling at high salt concentrations which allow increased campaign lengths. Energy efficiency is also improved by recovering heat from the live steam condensate to the process condensate via a plate heat exchanger.
High efficiency chevron type mist eliminators are installed inside each effect vessel to clean the vapour generated and produce high quality condensate (typically 20μS/cm) at design evaporation rates. This high quality water makes it a desired input for demanding applications such as condensate polishing plants (boiler feed water) or hydrate washing.
Each evaporator is equipped with motorized valves on all of the vapour and non-condensable streams controlled from the Central Control Room. Foundation Fieldbus technology allows the position of each valve to be shown in the control room. The motorized valves greatly reduce the time required to swap between effects and allow easy identification of incorrect valve line-ups. The liquor and condensate streams are manually operated. Overall, one Control Room Operator and one Field Operator are required to operate both facilities. An additional Field Operator is required when swapping effects. Cleaning is performed on a routine basis based upon heat transfer coefficient monitoring.