The present invention relates to a method of operating a membrane filtration unit including plural hollow fiber membrane modules connected to each other in parallel, the method including: a filtration step; a collection step; and a recovery step, in which a relation of n.sub.1n.sub.2>n.sub.3 is satisfied, where n.sub.1 is the number of the hollow fiber membrane modules simultaneously used in executing the filtration step, n.sub.2 is the number of the hollow fiber membrane modules simultaneously used in executing the collection step, and n.sub.3 is the number of the hollow fiber membrane modules simultaneously used in executing the recovery step.

A porous hollow fiber membrane based anti-solvent crystallization (AsCr) process is disclosed. The process involves injecting an anti-solvent from a bore of a hollow fiber membrane into a shell side where a feed solution containing a solution containing a material, e.g., an API, is flowing. The shell-side feed solution flows perpendicular to the hollow fiber length; the shell side liquid is in cross flow across the hollow fiber membranes. Multiple HFM modules may be positioned in series with nanocrystal suspension product from one module fed to the next module that is independently fed with an anti-solvent. The crossflow HFM based continuous AsCr technique disclosed herein could result in continuous nanocrystal production of APIs.

The present invention relates to a hollow fiber membrane module unit including a plurality of hollow fiber membrane elements to which header pipe caps are attached at both end portions, the hollow fiber membrane elements being arranged in parallel to form a row, in which the adjacent header pipe caps attached to the hollow fiber membrane elements on a permeate side are connected by a side seal connection covering an outer periphery of a contact surface, and the header pipe caps at at least both ends of the row are fixed by a support bar.