stallone,
when the heat load of the vacuum column is high the circulating reflux at times is inadequate for the rectification section.also the Cr is always a cold reflux,while the pump back reflux or pump around reflux is only partially cooled.This stream of PA is used for heat integrity as well as adjust the IBP-FBP of the side cut.The PA is as u must be knowing sent back to the column few trays above the side draw.this serves as additional reflux over and above the internal refux and helps in adjusting the side cut temperatures.
