Reflux Failure Case

Reflux failure case is the major case of all tower or column. Reflux failure can be caused by the following:
- Power failure lead to reflux pump off
- Pump failure
- Control valve failure ( fail closed or stuck closed)
- Operator error etc

When reflux pump fail, there no reflux flow inlet to column. Basically the reflux flow is flown back to column (at rectifying section) to increase separation efficiency. Other that, the reflux flow is useful also for cooling the vapor flow to the top of the column. Reflux pump failure cause ‘Loss of Cooling’, as a consequence, the vapor temperature will not be cooled down and overpressure will occur.

Control valve failure has same effect as a pump failure.

Power failure case cause trip of the air cooler, cooling failure and overpressure will occur.

Based on my experience, the simplification of the calculation load capacity for reflux failure has been made. 2nd tray vapor flow is assumed as basis load capacity. In HYSYS, the column model never convergent if the reflux flow = 0, therefore in HYSYS simulation, input reflux flow with very small number. After column model convergent, we can use 2nd tray vapor flow as load capacity of the PSV.

Actually, the determining load capacity of reflux failure is very complex. Let imagine, once the pressure increase in the column, the boiling point of fluid in the reboiler increase, the number of vapor will decrease. It has many effects to separation and equilibrium condition.

I have found much discussion regarding reflux failure case, especially the method for determining load capacity. Unfortunately, until now, there is no agreement reached, no method fixed, therefore I can’t make a justification which one is the best approach for calculating load capacity for reflux failure.

Please follow the following link for more discussion about reflux failure
1.Discussion 1 from Cheresources
2.Discussion 2 from Cheresources
3.A sample dynamic method for determining load capacity reflux failure

My friend, please share to me if you have other method for calculating load capacity for reflux failure case

Thank for reading, hopefully this posting useful for you

Best regard and thank you