Abstract: A permselective membrance suitable for obtaining fresh water by the desalination of sea water and salt water is provided herein, said permselective membrane being prepared by the reaction of an aromatic polycarboxylic acid component with a mixed diamine component comprising a diaminodiphenylsulfone compound of the formula (I), ##STR1## wherein R.sup.1 is hydrogen atom or a C.sub.1 -C.sub.12 hydrocarbon group, R.sup.2 s may be same or different, each representing a monovalent organic group, and n is an integer of 0 or 1 to 3, and an alicyclic diamine compound containing two secondary amino groups in the same or different ring(s).

Abstract: A fluid separation element comprising a casing and a bundle of selectively permeable hollow fibers accommodated therein, the bundle being potted with a resinous potting agent at the end portions and the gaps among the hollow fibers being blocked with the resinous potting agent, characterized in that (1) the ratio of the porosity of the membrane of the hollow fiber at the effective permeation portion (HA) and the porosity of the membrance of the hollow fiber at the potting portion (HB) satisfies the following condition: 0.9.gtoreq.HB/HA.gtoreq.0.4 and (2) the outer diameter of the hollow fiber at the effective permeation portion (OD.sub.A) and the outer diameter of the follow fiber at the potting portion (OD.sub.B) satisfies the following condition: 0.95.gtoreq.OD.sub.B /OD.sub.A .gtoreq.0.7, whereby the firm potting between the hollow fibers and the resinous potting agent is achieved, and a good separation efficiency without the increase of a pressure loss can be maintained for a long period of time.

Abstract: A hollow fiber package body for separation of fluid comprising a core and layers of hollow fibers having a selective permeability to fluid arranged around said core, said hollow fiber layers being formed by tape-like bundles of hollow fibers and composed of the arrangement of hollow fibers in nearly parallel relations one another and in a flat state, and said hollow fiber bundles being arranged in multi-layers by intersecting at least at a spot between one end and the other end of said package body.