First find the degree of interaction between the variables.

To properly answer your question you need, at least, 1st degree in chemical engineering and significant experience in crude distillation.

Identification of the interaction among the variables is done by changing the manipulated variables, one by one, trying to hold the rest as steady as possible and making out the influences on the whole set of the column parameters, trying to conclude it in a neat transfer function for each.

There are some companies who can support this effort.