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APPLICATIONS OF THREE-PHASE FLASHING FLOW IN DESIGN AND STUDY ALUMINA DIGESTION FLASH TRAINS

Tran, Q.K.

Flashing has been found to be a cost effective method for depressurising and cooling high-pressure discharge slurries from regenerative leach circuits. Thermal energy is transferred to the vapour phase during slurry flashing and this vapour can be used as a preheating medium. For efficient energy recovery, flash-cooling of the slurry must take place through a number of stages of decreasing pressure. The number of flash stages can be optimised by considering the capital cost of the installation against the cost of live steam. Incorrect application of flash train design parameters can lead to high mixture velocities, accelerated wear in piping and vessel components and poor quality condensate.

Practical and reliable methods for predicting the behaviour of three-phase flow through real piping systems are important tools for process plant designers. The objective of piping design is to generally delay the onset of three-phase flow and to limit velocities due to vapour generation by the judicious location of appropriate pressure loss devices. The characteristics of three-phase evaporative flow must be quantified to enable the hydraulic behaviour of the system to be understood and controlled.

Several mathematical models for representing three-phase flow are reviewed and discussed in this paper. The application of such models to the alumina digestion trains is given as an illustration. The example highlights the hydraulic problems in an actual three-phase system and shows how the pipe work was modified to rectify the problems that were inherent in the original design.