As I have seen for the four years of my Ph.D., membranes manufacturers always send spiral-wounded membrane modules for RO and NF, but no hollow fiber membrane modules.

I guess some of the possible reasons are the fact that Re and mass Pe numbers are quite low when the feed is circulating through the lumen of hollow fibers, so there are huge fouling and concentration polarization phenomena.

Besides, as in some cases (seawater filtration) the solutes in the feed are very concentrated, the pressure required to carry out the process is relatively high, and can get even bigger when the concentration polarization phenomenon is strong.

Do think there are some other reasons?