high, abnormal pressure drops occurring across the tower. Separation by fractionation is not possible under these conditions. Heat input (and feed rate) to the tower
must be reduced to bring the unit back to a normal pressure drop.
Low Tower Loading
Most towers have been designed with at least a 50 % turndown ratio for the trays.
This means that the trays should operate satisfactorily at 50 % of their loading.
Nevertheless tray performance does fall off at these low loadings. Below this
turndown ratio, performance, particularly in sieve trays, is drastically reduced.
This is almost certain to be due to “weeping” where liquid falls from tray to tray.
If the low loads are to be for only a short time due to temporary reduced throughput,
tray loading can be increased by increasing reflux. If the low throughput is to
continue for an extended period of time, a tray blanking schedule should be
considered to reduce the active tray area.
Operating Close to Critical Conditions
Deethanizers in particular operate close to critical pressure at the bottom of the
tower. Careful attention should be paid to avoid any pressure surges in this unit.
Feed to the unit and reflux streams should be on flow control.
No separation by fractionation can occur at pressures in excess of critical. Very
often chilled water is used for overhead condensing to reduce reflux drum pressure
but maintaining minimum C 3 loss in the case of deethanizers.
Distillation of the ‘‘Light Ends´´ from Crude Oil in Petroleum Processing
227
must be reduced to bring the unit back to a normal pressure drop.
Low Tower Loading
Most towers have been designed with at least a 50 % turndown ratio for the trays.
This means that the trays should operate satisfactorily at 50 % of their loading.
Nevertheless tray performance does fall off at these low loadings. Below this
turndown ratio, performance, particularly in sieve trays, is drastically reduced.
This is almost certain to be due to “weeping” where liquid falls from tray to tray.
If the low loads are to be for only a short time due to temporary reduced throughput,
tray loading can be increased by increasing reflux. If the low throughput is to
continue for an extended period of time, a tray blanking schedule should be
considered to reduce the active tray area.
Operating Close to Critical Conditions
Deethanizers in particular operate close to critical pressure at the bottom of the
tower. Careful attention should be paid to avoid any pressure surges in this unit.
Feed to the unit and reflux streams should be on flow control.
No separation by fractionation can occur at pressures in excess of critical. Very
often chilled water is used for overhead condensing to reduce reflux drum pressure
but maintaining minimum C 3 loss in the case of deethanizers.
Distillation of the ‘‘Light Ends´´ from Crude Oil in Petroleum Processing
227