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Reverse Osmosis
P. HOORNAERT
European Patent Office, The Hague, The Netherlands
Pergamon International Information Corporation
a member of the Pergamon Group
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Copyright © 1984 Commission of the European Communities
All Rights Reserved, No part of this publication may be
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First edition 1984
Library of Congress Cataloging in Publication Data
Hoornaert, P.
Reverse osmosis.
(EPO applied technology series ; v. 4)
1. Reverse osmosis. I. Title. II. Series.
TP156.07H66 1984 660.2'8424 83-27550
British Library Cataloguing in Publication Data
Hoornaert, P.
Reverse osmosis.—(EPO applied technology series
no. 4)
1. Cells 2. Membranes (Biology)
3. Osmosis
I. Title II. Series
574.87'5 QH615
ISBN 0-08-031144-X
In order to make this volume available as economically and
as rapidly as possible the authors1 typescript has been
reproduced in its original form. This method
unfortunately has its typographical limitations but it is hoped
that they in no way distract the reader.
Printed in Great Britain by A. Wheaton & Co. Ltd., Exeter
This study has been made for the Commission of the
European Communities, DC XIII - Information Market and
Innovation, Division Technological Information and Patents.
Introduction
1. Definitions; limitations
Reverse osmosis is a separation process using semi-
permeable membranes in order to achieve a separation
between a solvent (e.g. water), which is allowed to pass
through the membranes, and the low molecular weight
solutes (e.g. salts), which are rejected,by the application
of an elevated working pressure in excess of the osmotic
pressure of the solution which has to be separated.
The study is complete regarding reverse osmosis, but
related separation processes such as ultrafiltration
and dialysis will not be discussed. In ultrafiltration,
high molecular weight solutes (e.g. polymers and proteins)
and colloidal substances (e.g. clays, latex particles)
are separated from their solvents. The osmotic pressure
of these solutions is lower, and the working pressures
are much lower than those necessary for reverse osmosis.
In dialysis, a difference in concentration is responsible
for the separation; its principal application is the dia
lysis of blood in artificial kidneys.
Documents only relating to ultrafiltration or dialysis
have not been taken into account; but documents relating
to separation membranes or membrane separation devices in
general, which can be used as effectively for reverse osmosis
as for dialysis or ultrafiltration have been discussed.
However, especially concerning the membranes, the dis
tinction between reverse osmosis and ultrafiltration or
dialysis membranes is not always very clear (see general
reference (h) ). Because of this, documents dealing with
"separation membranes" in general will be taken into
account, but documents which are clearly indicating that
1
2 Introduction
their use as membranes is strictly limited to ultra-
filtration or dialysis will be omitted.
Also the field of artificial kidneys (based on hemo-
dialysis and/or hemofi1 trat ion) will be excluded from
this study, as will some other membrane separation
processes such as electrodialysis, permeation,
pervaporation, gas diffusion and microfi1 trat ion.
2. Purpose; Extent; Presentation
The purpose of this study is to give a review of the
recent developments and more particularly of the
Japanese activities in the field of reverse osmosis.
It is based on the documents (mainly patent documents)
in our systematic documentation belonging to the
fields B01D 13/00, B01D 13/04 and C02F 1/44 of the
international patent classification system (third
edition). Only the documents published since 1st
January 197 5 have been considered.
In view of the principal purpose of the present study
to find the directions towards which the Japanese
industry is developing, we have extended our search
to the English abstracts of the Japanese patent
applications published before examination. A large
number of these Japanese documents in a specific fields
can be an indication for future Japanese activities
in the western market in this field.
These abstracts have been published by the Japanese
Patent Office since March 1977. The documents from which
an English abstract is made by the Japanese Office are
those applications which are filed by a Japanese
applicant and which belong to a fast developing technical
field. For our study we have considered the abstracts
classified in the fields BC1D 13/00, B01D 13/04 and
C02F 1/44 of the third edition of the international
patent classification, system and those classified in
the field of C02B 1/82 of the second edition.
Introduction 3
In order to realize our purpose, the field of reverse osmosis
has been divided into five parts. The first part concerns the
membranes constituting the vital component of each reverse
osmosis plant; in a second part we treat the different kinds
of reverse osmosis modules (array of the semipermeable
membranes); the third part deals with process and plant
design; the fourth describes the reverse osmosis applications
and the fifth part looks at accessories and auxiliary
operations (e.g. membrane cleaning). These parts are further
subdivided into specific subjects (see table of contents). Each
relevant document has been classified in the most appropriate
subdivision. For each subdivision we have listed the documents
in question : first the non-patent literature (when available),
then the patent documents in our systematic documentation
- later referred to as "documents filed in western countries" -
identified by their publication number, name of the firm
and publication year; and finally the Japanese patent
applications published before examination - later referred
to as "Japanese documents" - also identified by their publication
number, name of the firm and publication year, in addition, the
documents filed in western countries, and which are of
Japanese origin (having a Japanese priority or being filed
by a Japanese applicant) have been underlined to allow a
quick tracing of these documents. The ratio between these
documents of Japanese origin and the total number of
documents filed in western countries is called the Japanese
penetration.
As a matter of course, only one document has been taken
into account, when corresponding patents were available.
Furthermore, for each important subdivision a brief description
and a review concerning the prior art has been given
(e.g. illustrated with examples).
4 Introduction
Special documents have been briefly discussed where
possible.
Relating to the purpose of this study, a striking
Japanese penetration has indeed become evident in some
fields. This will be expounded in the conclusions of
the study.
Finally we draw your attention to the general references
at the end, giving general information concerning reverse
osmosis.
-o-o-o-o-o-o-o-
7
Part I : Membranes
Introduction
The efficiency of the reverse osmosis method is greatly
affected by the properties of the semipermeable membrane
used. The characteristics required of semipermeable mem
branes are basically a high permeability for the solvent
(water) and a high rejection of the low molecular weight
solutes (salts). In addition, they should have high re
sistance to compaction caused by the high pressures used
in the R.O. process, superior resistance to chemical and
biological attack (workability in wide pH-range, resis
tance to hydrolysis, chlorine, etc.) and resistance to
high temperatures.
The first membrane for practical use, invented by Loeb
and Sowaragan was an asymétrie cellulose acetate membrane
(US 3133132 and US 3133137). It consisted of a very
thin skin layer supported by a thicker, more porous layer
(sponge layer). This asymétrie structure made it possible
to combine a good salt rejection with a high water flux.
The active skin layer was dense and responsible for the
salt rejection, but also extremely thin so that a reason
able water permeability could be obtained. The support
layer provided the necessary mechanical strength. The
fabrication process consisted of the following steps :
1) preparing the casting solution : polymer
(cellulose acetate), solvent and swelling agent.
2) casting on a support.
3) partial evaporation.
4) coagulation in non-solvent.
5) washing and shrinkage in hot water.
By chosing the appropriate composition of the casting
solution, this homogenous system is changed into two
separate phases by steps 3) and 4). A solvent-rich phase
becomes distributed in a polymer-rich phase; the first
phase is ultimately responsible for the pores, the second
for the membrane network (see ref (i) ). This process is
called the "phase-inversion process" and the thus produced
Membranes
membranes "integral asymétrie membranes" (see ref (d)).
These known membranes have the disadvantage of poor
pressure compaction resistance and low resistance to
chemical and biological degradation (hydrolysis of the
cellulose acetate).
In an attempt to overcome these defects, the casting
variables were changed and other polymers instead of
cellulose acetate were introduced.
A second successful achievement were the membranes based
on synthetic organic nitrogen-linked aromatic condens
ation polymers - especially aromatic polyamides -
invented by Du Pont (US -3567632). They have an extended
pH operation range, are inherently less water-permeable
but more salt-retentive than the cellulose acetate mem
branes. The material is usually spun from solution into
very fine hollow fibers. The water-permeability through
this fibers is well below that exhibited by cellulose
acetate membranes, but the high surface volume ratio of
the hollow fiber configuration results in an acceptable
water flux.
Many other polymers have been proposed (see under 1.2.B.1).
the difficulties are perhaps not to find appropriate
polymers, but in transferring their properties
reproducibly into thin, defect-fine and resistant
membranes under technical fabrication conditions.
The asymétrie structure can also be obtained by applying
a thin active layer in a separate step onto an existing
porous (not salt-rejecting) membrane. This process allows
the optimization of both active layer and support layer
(contrary to the integral asymétrie membranes). These
membranes are called "composite membranes"; they are dis
cussed under I.2.A.1.
For further information about the reverse osmosis membranes
we refer to the general references at the end.
