Marsh, C.A., Furness, R.A.

Determining the correct flow conditions within a digestion flash train is crucial to achieving a safe and energy efficient process operation. Overcoming the challenges of high erosion rates associated with high velocity flashing flows combined with abrasive slurries also requires an accurate model of the process.

Computational Fluid Dynamics (CFD) models of the individual flash tank stages are presented and have been validated against detailed process calculations, which confirm that there is significant amounts of steam passing through the underflow of many flash tanks, despite a consistent and trending level indication on the vessels. The discrepancy between steam passing through the underflow and an indicated level has been resolved through additional CFD models and provides important implications with regards to the perceived versus actual process operating conditions.

The presence of steam bypass increases the steam load on the downstream flash tank and associated piping. It also increases the complexity of the hydraulic calculations. The insight gained from this work allows for a more accurate process model to be developed and this is expected to lead on to significant improvements in flash tank operation including, improved steam loads, better condensate and more predictable wear rates.