The inquirer does not say what type of equipment he has in the tower (trays, random or structured packing). I will assume that he has packing because he uses the term wetting rate and I will further assume that it is structured packing. The wetting rate is critical because it if it is too low per unit area, or is unevenly applied, it will cause poorer performance in all distillation or mass transfer towers. However, in this case, it is even worse because there is a second concern about coking. If an area of the packing goes dry, a hot spot will be created and the material will coke and plug up the packing. Fractional gpm/ft2 is possible. The lower the liquid rate, the harder it is to apply correctly. If sprays are being used, multi-overlap patterns should be used. I am sure that there are several equipment vendors and engineering consulting companies that are very experienced in designing equipment for this application. He can find may more references if he did an Internet search for "slop wax bed section of vacuum column."