Why ports and catheters (or fistula or grafts)
Plasmapheresis cannot be done through a port designed for infusions. It is best for the patient to use the smallest diameter tubing practical for the task at hand. In an infusion, the flow rates are relatively low (less than 1000 ml/hr) and red blood cells do not pass through the port. In plasmapheresis, the flow rate is usually around 5000 ml/hr and red blood cells pass through the access. Because red blood cells are damaged by high velocity, the high flow rate used in plasmapheresis means that big tubing must be used, tubing that is much bigger than what is used for infusions.
It is theoretically possibly to use the plasmapheresis venous access for infusions. I think, however, the infusion does not have a high enough flow rate to keep blood out of the tubing. I suspect that once blood enters the tubing, it tends to clot. It might be possible to administer an anti-coagulant, but I suspect a high dose would be needed.