The cut points in the CDU are controlled by the overhead vapor temperature
which determines how much vapor goes to the condensers to produce
light naphtha and by the flow rate of the various products straight from the
column or the side stream strippers. The atmospheric residue level control
inside the column determines its flow rate and thus its initial cut point.
The amount of light naphtha is determined by the dew point of the
naphtha at its partial pressure, which is close to the overhead temperature.
Changing the drawoff rate of any product affects the cut points of the
heavier product below it. For example, lowering the kerosene flow rate
will lower its end point (make it lighter), but will also modify the initial cut
points of the LGO and HGO and the initial cut point of the atmospheric
residue. The residue flow rate, the internal reflux rate, the drawoff temperatures
and the pumparounds are also affected.
Therefore, if the cut point of one stream is changed through a change in
its withdrawal rate, the flow rate of the heavier product next to it should be
changed in the reverse and by the same amount in order to make the
changes in the desired stream only. For example, if the end point of
kerosene is lowered by decreasing the kerosene flow rate by a certain
amount, the flow rate of LGO has to be increased by the same amount. If
this action is taken, only the cut point of kerosene is affected and the cut
points of the other products remain unchanged.
The side stream rate also affects the temperature at the withdrawal tray
and lowers the internal reflux coming out of that tray. The internal reflux
rate affects the degree of fractionation. It can be increased by increasing the
heater outlet temperature, and by lowering the pumparound duty in the
lower section of the column. When less heat is removed by the lower
pumparound, more vapours will be available up the column and more
internal reflux is produced as the vapours are condensed.