[Membrane Science and Technology] Fundamentals of Inorganic Membrane Science and Technology Volume 4 || Chapter 1 General overview, trends and prospects
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Fundamentals of Inorganic Membrane Science and Technology Edited by A.J. Burggraaf and L. Cot
9 1996, Elsevier Science B.V. All rights reserved
General overview, trends and prospects
A.J. Burggraaf I a n d L. CoF
1Laboratory of Inorganic Materials Science, Faculty of Chemical Technology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands 2Laboratory des Materiaux et Proc6des Membranes, UMR 9987 CNRS-ENSCM-4411, Ecole Nationale Sup6rieure de Chimie, 8, rue de I'Ecole de Chimie, 34053 Montpellier, France
The aim of this chapter is threefold: (i) to account for some important focal points in the book, (ii) to identify main barriers to technological development of membranes and their commercialisation in potentially important areas and (iii) to summarise some interesting trends, developments and R&D areas.
In recent years the volume of research and development of inorganic mem- branes has grown considerably and a large diversity of new ideas, development directions and potential applications have emerged. Today, research funding is more and more coupled with concepts and developments which give promise for at least medium-term commercialisation. As will be shown below, most of the interesting long-term concepts and developments depend on progress in the field of ceramic membranes and on successful commercialisation in liquid filtration as a basis for R&D in other directions. Focus is therefore given to macro-, meso-, and microporous ceramic based (composite) systems. A separate chapter treats dense (non-porous) oxide membranes because of their importance for oxygen or hydrogen permeation properties in gas (air) separation and chemical reac- tors. The planned chapter on dense metal membranes and bioseparations could not be produced due to problems with the scheduled authors. Properties of metal membranes are treated in two chapters (Chapters 8 and 10).
2 1 -- GENERAL OVERVIEW, TRENDS AND PROSPECTS
1.2 MARKET SITUATION A N D PROSPECTS
The market situation and prospects have been described in several documents. Although the reported figures are not always consistent, the trend is clear.
Crull  and Charpin et al.  predict total sales of inorganic membranes in 1999 of US$ 432 million, of which ceramic membranes make up 80%. According to Crull, carbon membranes and metal membranes will make up 11.5 and 5.5% respectively of this market (see Table 1.1).
Actual and projected sales for inorganic membrane materials. From Crull 
Material 1986 1989 1994 1999 Growth
in US$ million (%)
Ceramics 6 18 75 345 34
Carbon 0 3 9 50 32
Metals and other 5 8 13 25 12
Glass 0 0 1 3 >100
Other 1 2 4 9 16
Total 12 31 102 432 30
In a more recent study by the Business Communication Company  the total sales of inorganic membranes is estimated to be US$ 228 in the year 2003 (about 40 million in 1993), of which 70% are ceramic membranes. These reports predict growth rates of about 30% or more. The total market for membrane sales including polymer membranes is much larger and it is stated that 15% of it will be inorganic in 2003. In a recent study [3,4] these total sales are estimated to be US$1000 million in the year 2000. Larger figures for this total market are given by the Freedonia Group  which estimates the total membrane sales in the USA at US$1300 million in 1998 with an annual growth rate of 5%. Studies by Frost and Sullivan  indicate growth rates of about 10% for liquid filtration which make up 70% of the total market to 16.5% for use in industrial production.
The market for gas separation applications is considered to be potentially very important. In 1993 the total membrane (polymeric) sales for gas separation are about US$ 75 million, which is expected to grow by a factor of three in the year 2000 . Many gas membrane applications are envisaged (Table 13.1 in Ref. ). The market for N 2 o r 0 2 production especially is expected to grow for non-cryogenic and membrane applications (in 1993 this was 3-5%). According to Thorogood , membrane applications will particularly be found in medium
1 - - G E N E R A L OVERVIEW, TRENDS A N D PROSPECTS 3
and small capacity processes. So far, commercialised membrane applications have been strongly (liquid
separation) or exclusively (gas separation) dominated by polymer membranes. Inorganic membranes will have their share of the future growth if use can be made of their following strong points:
(a) a relatively high thermal stability; (b) a relatively high chemical stability and biocompatability of specific mate-
rials; and (c) good erosion resistance and non-compactability under high pressure. This holds especially for ceramic membranes and to some extent for carbon
membranes and ceramic-metal composite membranes. The weak points of inorganic membranes should be minimised (see next section). Ceramic mem- brane production is intrinsically more expensive and complicated than poly- mer. Inorganic membrane applications should therefore preferably be found in fields where polymer membranes cannot or do not perform well.
1.3 M A I N BARRIERS TO TECHNOLOGICAL DEVELOPMENT A N D ACCEPTANCE
1.3.1 Requirements and Issues
A membrane system is built from components and assembled into modules which, in turn, form the complete system (see Chapter 2)vThe-single compo- nents consist of a (usually ceramic) supporting system and the final (usually ceramic) separation layer. The supporting system can be a single plate, tube, hollow fibre or monolithic multichannel or honeycomb structure. The final separation layer can be porous or dense and single phase or composite. A hierarchic system can be built, as discussed in Chapter 2, from a sandwich of macro-, meso-, microporous layers, which can be tailor-made by changing the chemical or physical nature of the pore system. Each step (product) in the manufacturing process can be used for specific applications. The quality of the underlying support (system) determines, to a high degree, the properties and quality of the final top layer and the number of steps necessary in a multi-step coating process to obtain a defect-free final separation layer. The support system must also fulfil strict quality standards and requirements and must be compatible with other components of the membrane module and system.
Before a membrane system is accepted by users in applications on a commer- cial scale, many requirements must be fulfilled. The main requirements are related to a large number of technological problems to be solved and/or a variety of possibilities for realisation. A brief overview of important aspects is given in Table 1.2.
4 1 -- GENERAL OVERVIEW, TRENDS AND PROSPECTS
Requirements for commercial application of membrane systems
1. Low cost production of separation units (modules and/or installations) Relation with: -easy scaling up from laboratory to production installations - reproducible fabrication processes - availability of not too expensive, high quality supports
2. Reliability of components in: 2.1. ambient conditions 2.2. high-temperature applications (T >_'200~
Relation with: - reliability of ceramic components (2.1 and 2.2) - availability of reliable sealing technologies (2.2)
3. Long-term stability of pore (material) structure Relation with: - thermal or chemical properties - separation and/or permeation properties - mechanical stability of support and separation layer under cyclic temperature and/or
4. Reasonable to good surface area to volume ratio Relation with: - module architecture
Specific conditions of (high) separation and (high) permeation Relation with: - intrinsic membrane properties and limitations - process conditions and membrane architecture - fabrication technology of thin separation layers on large surface areas of asymmetric graded
support systems - knowledge of permeation limiting surface processes
126.96.36.199 Characteristics of ceramic fabrication
Prepara to ry to discuss ing Table 1.2, it is useful to summar i s e briefly the ma in characteristics of ceramic materials and their product ion technology. The fabrica- tion process of ceramic materials always involves stages where particle assemblies (powder suspensions) are formed which, dur ing processing, form in a statistical w a y secondary particles called agglomera tes and aggregates. In the next stage po rous compacts are fo rmed by the packing of the above-men t ioned particle assemblies into "green" compacts wi th a certain shape (plate, tube, honeycomb, etc.). The particle organisa t ion in the compact is again a statistical process and results in a d is t r ibut ion of pore size and shape which is related to the dis t r ibu- t ion of the particle size and shape and of local statistical f luctuat ions of the poros i ty and of the pore size (so a certain inhomogene i ty occurs). In the last stage this "green" compact is consol idated by heat t rea tment at high temperature
1 - - GENERAL OVERVIEW, TRENDS AND PROSPECTS 5
(sintering process) during which considerable shrinkage occurs. In this process the porosity and pore size distribution as well as the product dimensions change again to their final values. Due to the statistical nature of the initial powder and the subsequent processing, reproducibility of the final product properties is not easy to obtain and requires many precautions. The last step in the production process is machining to obtain final dimensions and surface quality.
Ceramic materials are intrinsically brittle. This means