The correlation for the static holdup was taken from [1] (equation 2), the correlation for the dynamic holdup was taken from [2] (equation 5).
References:
[1] Engel, V.: Fluiddynamik in Füllkörper- und Packungskolonnen für Gas/Flüssig-Systeme, Chemie Ingenieur Technik, Vol. 72, pp. 700-703
[2] Wagner, I et al.: Mass Transfer in Beds of Modern, High-Efficiency Random Packing, Ind. Eng. Chem. Res, Vol 36 (1997), pp. 227-237