[membrane science and technology] fundamentals of inorganic membrane science and technology volume 4...

Fundamentals of Inorganic Membrane Science and Technology Edited by A.J. Burggraaf and L. Cot

9 1996, Elsevier Science B.V. All rights reserved

Chapter 1

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

1.1 INTRODUCTION

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 membranes 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 reactors. 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 [1] and Charpin et al. [2] 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).

TABLE 1.1

Actual and projected sales for inorganic membrane materials. From Crull [1]

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 [3] 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 [5] 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 [6] 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 [4]. Many gas membrane applications are envisaged (Table 13.1 in Ref. [7]). 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 [9], 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 membrane production is intrinsically more expensive and complicated than polymer. 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 components 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 commercial 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

TABLE 1.2

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

pressure regimes

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

1.3.1.1 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 fabrication 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 that tensile stresses surpassing a certain critical value easily give rise to catastrophic failure (fracture). This is caused by the presence of defects in the material which act as stress concentrators. Larger defects give rise to easier fracture (at lower stress value). Because defect number and size are also statistically distributed throughout the ceramic product, the strength of the material is not a unique material property. It shows a statistical fluctuation which depends on, for example, the fabrication process and the chance of failure increases with the size of the ceramic product. This chance of failure is expressed by a reliability factor for which usually the Weibull modulus m is used. For common ceramics m < 3, for high-tech products values up to 10 can be obtained (note: for metals m >_ 20 (hard metals) to 30). Despite these drawbacks, ceramic materials are in widespread use. Even high- tech components with very precise specifications such as turbine components have been developed, although at great cost. The conclusion must be that the fabrication of ceramic products is relatively expensive. Prices rise substantially with increasing demands on such product properties as porosity, pore size (distribution), reproducibility and reliability.

1.3.2 State of the Art and Needs

In this section the requirements given in Table 1.2 will be used as a starting point for the discussion.

1.3.2.1 Availability and cost

The commercial availability of high quality support (systems) is a critical issue in the further development of membrane separation units. To meet compatibility requirements with other components during assemblage, supports must fulfil strict requirements of (a) dimensional uniformity (i.e. roundness, flatness), and (b) thermal expansion coefficient and chemical inertness in high- temperature application. To make them suitable as support for thin layers the surface roughness should not be too great. Finally the pore size distribution of the support and/or support system (including layers) should be reasonably sharp and larger defects, or relatively large pores, should be absent. Commer- cially available support systems are usually developed earlier for non-mem-

6 1 - - GENERAL OVERVIEW, TRENDS A N D PROSPECTS

brane applications and are adapted only to some extent for micro- and ultrafiltration applications, these being the largest market today. For other, more demanding, applications this coupling to microfiltration-based supports is not a favourable situation. This is especially true for the further development of microporous membranes which are not yet commercially available. For the development of zeolite membranes, porous stainless steel might be a partial alternative support.

Scaling up of the processes to large surface areas (i.e. to obtain asymmetric membrane systems with several layers) as is necessary for large-scale operations has been successfully demonstrated for micro/ultrafiltration and bioseparation processes, but not for other applications such as gas/vapour separation and membrane reactors, for which only small-scale laboratory equipment is available.

The cost of inorganic membranes per unit area is reported to be much higher than for organic membranes. As argued by, e.g., Fain [10], it is not appropriate to price organic membranes by the unit area. To be comparable with polymer membranes the module cost should be reduced by an estimated factor of about three. This factor can be lower for complete installations. Nevertheless ceramic membrane systems will always be more expensive than polymer-based ones.

1.3.2.2 Reliability

Reliability problems in the sense of avoiding fracture of components result- ing in breakdown of installations is especially important in large units such as, for example, membrane reactors. The problems cannot be solved in a satisfactory way by improving the material properties only. By appropriately designing modules and processes, satisfactory solutions might be obtained as has been shown for industrial processes with related problems, e.g., ethylene oxide production.

Reliable sealing technologies for use at temperatures up to 800~ are available for, e.g., alumina-based tubular membrane systems [11] but need further development for other shapes and materials.

1.3.2.3 Long-term stability

The reliability of separation/permeation performance is coupled with (a) fouling problems and (b) the stability of the micro(pore) structure of the membrane system. Fouling is a problem in almost all liquid separation applications. Strategies have been developed to cope with this problem, usually in a satisfactory way.

The microstructure of inorganic membranes is very stable against compres- sive forces. This means that they can withstand large pressure differences

1 ~ GENERAL OVERVIEW, TRENDS A N D PROSPECTS 7

without compaction. This is an advantage compared with polymer membranes. In liquid separations under not too harsh corrosive environments long-term stability has been proven and results in long lifetimes which are usually much longer than those obtained with polymer membranes.

Remarkably enough, statements of high chemical stability under harsh corrosive environments such as pH < 3 or pH > 9 are not substantiated in the literature by reliable measurements on membrane systems and much more work is needed here. Problems that occur with the bursting pressure of support tubes after long-term usage might indicate local corrosion at the contact points between the ceramic particles making up the microstructure.

Long-term stability at high temperature of mesoporous systems can be good when appropriate materials are used, but is unknown for the new emerging microporous membranes.

1.3.2.4 Surface area to volume ratio

In polymer membrane systems large surface area to volume ratios can be obtained. For a given module size this area can be larger by a factor of 1000 times the amount obtainable with ceramic membrane systems. This is due to a membrane architecture such as spiral-wound systems (see Chapter 2) which are not available for ceramic systems; but it is also true that for many applications inorganic membranes can be produced having much greater permeance than those of polymer membranes. This means that for many applications the size needed to produce a given volume of product is about the same for inorganic and polymer membranes [10]. Nevertheless, th6producfionof large quantities of products in industrial installations or in waste water treatments requires large membrane (reactor) volumes. One development to increase the surface area to volume ratio in inorganic membrane systems has been the use of monolithic mulfichannel and/or honeycomb structures. An interesting possibility is the transfer of the architecture of flat-stack Solid Oxygen Fuel Cells (SOFCs) to the membrane field. The architecture of these SOFCs is very similar to those required for membrane systems with a relatively large surface area to volume ratio. The production of reliable ceramic hollow-fibre systems is problematic and it is doubtful whether reliable systems with a large surface area can be developed. Carbon membranes have the best chance here.

1.3.2.5 Specific combinations of high separation factors and high permeation

Scientists and developers are usually confronted with a demand for the largest possible separation factor as a first requirement. Later, it becomes obvious that for the realisation of commercial applications a high permeance (permeation) is also needed to reduce the size of the membrane separation (reactor) installation.

8 1 - - GENERAL OVERVIEW, TRENI~ AND PROSPECTS

1000 _

100

10

! t t, I- 0.1 1 10 100

permeance (a.u.)

, L

1000

Fig. 1.1. Schematic picture of separation factor 0c versus permeance (permeation).

Here we confront an intrinsic materials problem which is schematically shown in Fig. 1.1.

Larger separation factors (z are obtained at the cost of smaller permeation values. There is some degree of freedom within the indicated band width, which is also affected by process conditions.

Focusing only on higher membrane selectivity is economically not always justified if it is at the cost of strongly decreasing permeance. In every application there is an economic optimum at a given combination of selectivity and permeance [7,12]. Membrane staging can provide a dramatic increase in separation performance in many situations compared with single-stage membrane systems due to higher product recoveries. Despite an increase in the cost of multiple-stage installations, staging will often be the most cost-effective design [7]. In addition, process conditions, e.g. sweep rates and hydrodynamic conditions, are important in avoiding e.g. concentration polarisation effects which decrease permeance and separation. The use of large amounts of sweep gases or liquids is economically unfavourable and should be minimised.

The flux with a given membrane material(s) and structure can be increased by decreasing the membrane thickness. The thinner the separation layer, however, the larger the risk of forming defects which decrease the separation factor. Mesoporous separation layers of good quality with layer thicknesses down to 5-10 ~tm on macroporous supports has been realised with reasonably large surface areas. For microporous layers this has been shown only on small plates for silica (layer thickness 0.1 ~tm) and zeolites (layer thickness 5-10 ~tm).

For dense (non-porous) membranes used for oxygen separation the flux becomes insensitive for a decrease of the layer thickness for a critical thickness which is of the order of 0.1-0.3 mm depending on the permeant-membrane

1 - - GENERAL OVERVIEW, TRENDS A N D PROSPECTS 9

combination. This is due to a kinetic limitation of the permeance by surface processes. This phenomenon probably also plays a role in hydrogen permeation through metals such as Pd alloys and in the functioning of (catalytic) membrane reactors. A combination of dense membranes with adsorptive porous coatings or metal-oxide composite membranes can relieve this problem. Moreover the synthesis of defect-free, dense (non-porous) layers with a thickness smaller than a few ~tm on a porous support has not yet been definitely solved.

Combinations of high separation factors (> 50) and reasonable permeance interesting for practical applications have been realised for gas separation on small surface areas with microporous membranes. These are discussed in Chapter 9 on transport properties and particularly concern the separation of hydrogen and CO2 from each other and from hydrocarbons as well as some hydrocarbon separations.

Nanofiltration with reasonable-to-good rejection values for small molecules are reported for small membranes at the boundary of the meso- and micropore region. These fields of microporous materials are in their infancy and much more work is necessary to delineate their potential for practical purposes (see Chapter 11).

An important barrier to rapid development of supported microporous membranes is also the lack of direct measurement methods to determine porosity and pore size (distribution) of the (supported) separation layer.

1.4 TRENDS, TECHNOLOGICAL AND SCIENTIFIC PROSPECTS

1.4.1 Infrastructure for Future Work

The commercial availability of inorganic membranes is currently limited to a few applications in the micro- and ultrafiltration and bioseparation fields. The commercial development of new inorganic (ceramic) membranes is slow and production costs are high, showing no tendency to decrease. This process is not unusual in the development of new, highly sophisticated products as has been discussed in Section 1.1. Nevertheless, it has led some experts to believe that useful inorganic membranes are not viable, as cited in Ref. [10]. It is therefore important to achieve a practical and cost-effective inorganic membrane in some new applications. This must be done by bringing together solutions to most of the aspects mentioned in Table 1.2.

This is not a task for universities only; the best option is to formulate funding for the cooperation of: (a) national laboratories, for the large amounts of necessary practical developments; (b) industries, to articulate market needs and to coop- erate in the transfer of knowledge and manufacturing methods; and (c) universities, to produce the basic knowledge needed to interest national laboratories

10 1 - - GENERAL OVERVIEW, TRENDS A N D PROSPECTS

and industries and to create new opportunities and possibilities, i.e. fundamen- tal research, research with a strategic character.

1.4.2 Some Trends

The trend for market penetration will probably follow a path as shown schematically in Fig. 1.2. The figure does not pretend to give quantitative information but merely shows the relative importance of different application fields in time and illustrates the increasing complexity. Gas separation with microporous membranes will probably only start on a commercial scale if membrane business for liquid filtration has become sufficiently profitable to bear the developments necessary to produce commercial gas separation membranes. Commercial availability should therefore be improved for applications not directly making use of liquid filtration membranes.

Prospects for commercial applications as described in different reports differ significantly in their conclusions. Fain [10] describes the potential for successful implementation of inorganic membranes for hydrogen separation from coal gas, from C H 4 / C O 2 mixtures and from catalytic reactors as excellent. Sealy [13] concludes that the hydrogen membrane separation market for existing refin- ery/petrochemical applications is small and difficult to access while high temperature (>100~ should not be an advantage. This last statement is a remarkable one, which conflicts with most of the technical reports in literature. Alderliesten et al. [4] report possibilities for high-temperature applications in the same field.

o ,? ' 5 = / _,,o~ _# o. / _#~..,oO//_# #" .

/ . ~,L +,.-//,@-- .00.'0~,, +'o / .~_,~+//. ~,,oO .,,oO'#, /

/ ~o,-/ . ,

I m G E N E R A L OVERVIEW, TRENDS A N D PROSPECTS 11

The temperature requirements of membranes sometimes conflict with each other. For gas separation with microporous membranes, some authors promote finding membranes stable at temperatures higher than 500~ while others require temperatures less than 500~ [4] for dense membranes with very high separation factors, mainly due to compatibility problems of the membrane with other components of the membrane module.

The conclusion must be that reliable prospects of commercial applications can be made only for specific applications and specifications, and strongly depend on particular assumptions relating to membrane performance, process conditions and design and on particular future prospects which easily invali- date economic comparisons made today.

The acceptance of ceramic membranes in commercial applications is more difficult in fields in which other already well-developed solutions for separation problems are accepted (e.g. PSA, polymeric membranes) and where the reliability and reproducibility is very important as in, e.g., membrane reactors and gas separation in high-temperature applications.

All reports agree that acceptance of catalytic membrane reactors on a commercial scale is at least 10 years away. Here, more experimental performance data for particular processes and process conditions are required to stimulate further development. This needs the commercial availability of a larger assort- ment of microporous as well as dense membranes with a variety of combinations of good separation factors and good performance values.

The use of membrane reactors allows process conditi0ns which cannot be obtained with more conventional processes (see Chapter 10 and overviews [13]) and which allow improved yields and selectivities, the use of two simultane- ously occurring reactions (e.g. the main reaction and a decoking reaction to eliminate carbon deposits), controlled supply of reactant, etc.

By appropriately designing the membrane reactor, the possibility of decreasing the reactor volume to a given, required capacity with respect to that of a conventional unit or conversely increasing the capacity given the reactor volume is equally important. In addition, the energy balance can be improved considerably using membrane reactors, as reported by several authors.

The best strategy for acceptance in high-temperature gas/vapour separation (T > 200~ and catalytic membrane reactors is probably its introduction in small-scale processes and/or hybrid installations which exist in two types: (a) a combination of a membrane separation unit with a conventional process [4,7,12]; and (b) interstage removal of component(s) by a membrane unit in between two reactors in series [15].

This type of solution reduces the risk of accepting membrane reactors or separators because in the event of failure of the membrane unit these can be switched off without catastrophic consequences. Furthermore, process economics can be improved in this way.

12 I m G E N E R A L OVERVIEW, TRENDS A N D PROSPECTS

In all applications there will be a drive to decrease the size of the membrane unit by increasing the surface area to volume ratio. This implies a trend from tubular to multichannel (honeycomb) structures and, further, to stacked plate designs.

1.4.3 Prospects for Interesting Membrane Applications

Several groups of application fields can be distinguished: (a) industrial (production) processes; (b) energy-related applications; (c) environmental applications; and (d) others

1.4.3.1 Industrial production processes

In gas or vapour separation and membrane reactor applications the most obvious field is related to a future hydrogen economy and/or processes in which hydrogen is important. These particularly concern re-forming of natural gas and/or syn-gas production and/or syn-gas ratio adjustment. Examples are hydrogen separation from coal gas and from CH4/CO2 [10]. As stated by Alderliesten et al. [4] the re-use of H 2 by separation from reaction products instead of using it as fuel is economically interesting due to the 8-fold higher value of hydrogen as reactant.

Processes related to (de)hydrogenation reactions and isomer separation might also be interesting. Examples are (see also Chapters 10 and 13):

- naphtha reforming to provide high value (>95%) H2. - alkane dehydrogenation to alkene(s) - examples: propane ~ propene, isobutane ~ isobutene - Isomer separation of e.g. p-xylene from o-m-xylene - Methanol synthesis or methanol conversion Air separation by membranes to produce N 2 or 0 2 is interesting especially

for small to medium capacities and medium (up to 99%) purity. The highest purities of oxygen (>99%) are to be obtained with dense (non-porous) membranes. The economics of gas separation processes is discussed in detail by Spilman [7] together with a number of potentially interesting applications.

In liquid separation applications interesting fields with micro- and ultrafiltration are:

- in the food and beverage industry: concentration of whole or skimmed milk, removal of bacteria, phospholipids and casein fines, production of fresh cream cheese, extraction of serum proteins from whey, clarification and sterilisation of fruit juices, wine and beer, etc.

- in the biotechnology and pharmaceutical industry: cell culture, separation of fermentation broths and of microorganisms, extraction of vitamins, separation and concentration of proteins, separation of metabolic products from blood, etc.


Nanofiltration. This new process involves a low rejection for salts (monova- lent ions) and ionised organics (MW < 100) in combination with a high rejection of salts (multivalent ions) and organics with MW > 300 which should be separated from the earlier ones at low operating pressures compared with reverse osmosis. Examples are given in Chapter 11 and zirconia or titania membranes seem to be especially suitable due to their relatively good chemical stability.

1.4.3.2 Energy-related applications

This group is in most cases a subgroup of industrial applications but has a main goal of enhancing the energy efficiency of processes by incorporating membrane processes in the total design. Energy savings of up to 25% of the total energy costs have been reported by, e.g., manipulation of hydrogen streams in (de)hydrogenation processes or using pervaporation membranes which can reduce energy costs in distillation operations.

High-temperature membrane separation operations which avoid cooling down/heat ing up cycles can be beneficial.

A quite different example is the use of oxygen-enriched air in diesel engines to reduce fuel consumption and waste in the exhaust gas.

1.4.3.3 Environmental applications

Two main groups of application can be distinguished: (a) non-waste applications, and (b) waste removal related applications

Non-waste applications It is expected that this group has great potential and will become more and

more important in the future. Examples of potential applications are given below and are partly discussed by Spilman [7]. Applications on a smaller scale can be found here and may promote widespread acceptance of membrane systems:

- clean/pure liquid or gases in electronic industry (e.g. clean room atmos- pheric control, ultrapure water in microelectronics)

- pretreating water prior to reverse osmosis processes - recovery of homogeneous catalysts or enzymes from production proc-

esses - aeration or de-aeration of liquids - air dehumidification - atmosphere control in buildings

Waste-related applications Here the main problem is to reduce the environmental load by removal or

recovery of unwanted products from the exhaust streams of production processes,

14 1 - - GENERAL OVERVIEW, TRENDS AND PROSPECTS

energy generating systems or engines, or to design closed production processes with closed-cycle processing. Important strategies in waste management are volume reduction of the waste and/or recycling, and membrane processes can be helpful here. Many examples can be found in petrochemical, food processing, pulp and paper and biotechnology industries. A new example is the removal or conversion of CO2 from or in process streams in relation to the "greenhouse effect". Given the massive amounts of CO2 involved it is doubtful whether membrane processes can play a role here.

As previously discussed, applications on a small scale might be of interest to promote acceptance of gas/vapour separation systems. Examples are:

- organic vapour recovery systems, e.g. in domestic applications (cook- ing/frying/fat vapours) or hydrocarbon removal in industry

- gas filtration in buildings - diesel engine filtration Large-scale applications might also be possible as indicated by Bishop et al.

[16] using a honeycomb dead-end type microfiltration membrane structure with a relatively large surface area to volume ratio of 155 ft2/ft 3. Particulates are filtered from air streams and the filter cake can be easily removed by gas back-pulsing. The system can be loaded with catalysts and is said to be suitable for NOx reductions and VOC (volatile organic compounds) oxidation with efficiencies of >95% and >99% respectively.

1.4.3.4 Others

The combination of membranes and sensors has been mentioned in the literature as a potentially interesting field. To date, however, very few examples of applications have been realised. The measurement of water content in soils and in underground rock formations is potentially interesting as has been proved by field tests [11].

The use of membranes in some consumer products has been discussed in the preceding section and can be extended with, e.g., oxygen enrichment of air for medical purposes.

Photocatalytic membranes are mentioned in the literature as potentially interesting for a number of applications. Here, the membrane acts in the first place as a transparent storage medium for reactants, i.e., as a transparent microreactor system.

1.4.4 Interesting Fields for Future R&D

In this section some important technological and scientific R&D fields will be summarised. The authors do not pretend to give a complete overview but indicate fields which in their opinion certainly deserve further attention. The list of items given below is not given in order of priority.

I m G E N E R A L OVERVIEW, TRENDS A N D PROSPECTS 15

1.4.4.1 Long-term chemical stability

Reliable data as well as reliable, standardised methods for the measurement of chemical stability reflecting operational conditions in practice do not exist, neither for separation layers, nor for support systems (see Chapter 6). A meas- uring method has been proposed by Z~iter et al. [17]. Methods to improve the chemical resistance of the pore structure against strong acids by coating techniques might also be interesting. Z~iter [17] reports improvement of y-alumina membranes against acids with pH < 3 by partial coating with zirconica, Bhave [18] indicates an increase of the resistance of alumina against phosphoric acid by coating with titania.

More work is urgently needed if membranes are to be used in conditions of very large or very small pH values or in steam. This holds even more for the thermal stability for microporous membranes and for both chemical and thermal stability of membranes for nanofiltration.

In catalytic membrane reactors the compatibility of catalysts and membrane material requires attention.

1.4.4.2 Thin-layer deposition technology

Preparation of defect-free or defect-poor supported thin separation layers both porous and dense ~ with a thickness of

16 1 w GENERAL OVERVIEW, T R E N I ~ A N D PROSPECTS

For some applications non-oxide, stainless steel supports appear interesting. This support does not suffer from the brittleness of ceramic supports and can be quite easily connected to other module components, but does not allow strongly oxidising conditions or high temperatures.

1.4.4.4 Microporous membranes for gas/vapour separation

This new field is still in its infancy and offers a variety of useful and interesting R&D opportunities. For hydrogen or carbon dioxide separations from other gases good selectivities and good permeation values are reported with silica microporous membranes. For other mixtures and membrane types, however, selectivities can be very good in most cases but permeation is too low. Furthermore, microporous membranes suffer from the problem shown in Fig. 1. Research is urgently needed to resolve this problem by making use of the materials depending band width sketched in Fig. 1.1 and by optimising other factors (small thickness, large porosity) taking into account long-term stability requirements (high temperature, steam).

For further considerations it is useful to distinguish between systems with (a) small micropores (pore diameter dp< 0.5 rim) and (b) wide micropores (dp = 1.0-2.0 nm) with a transition region in between for intermediate pore diameters.

Small pore systems are important when separation of gas mixtures with the size exclusion mechanism is necessary. This is the case for separations at high temperature under conditions when adsorption selectivity does not play an important role and where selectivity will sharply drop for other mechanisms. Wide pore systems have been little investigated but are expected to have interesting properties for separation of mixtures of larger molecules which differ strongly in their interaction energy with the pore walls. Here, large permeation values in combination with good selectivities can be expected.

The theory of transport in multicomponent gas mixtures in micropore systems should be further developed and more data on competitive adsorption of multicomponent gas mixtures on the membrane material should become available together with adequate characterisation methods of porosity and pore size of supported systems.

Three groups of materials can be distinguished: (a) non-crystalline (X-ray amorphous); (b) zeolite type; and (c) crystalline, non zeolite type.

(a) Non-crystalline (X-ray amorphous) This group is mainly formed by silica or carbon membranes. For silica in the

small pore and intermediate pore region very good combinations of selectivity and fluxes are reported. The porosity of the membrane seems to be too low however (note: good measurement methods for supported microporous membranes do not exist). Porosities of at least 20% of theoretical density should give considerable improvement in the permeance. A strategy to overcome this is,

1 ~ G E N E R A L O V E R V I E W , T R E N D S A N D P R O S P E C T S 17

e.g., the use of template molecules in a variety of ways during synthesis followed by pyrolysis (see Chapter 7). Further research to fine tune the pore size and distribution is necessary, particularly in the small pore size range where the use of template molecules might be useful in addition to "engineering" of size and packing of inorganic polymeric precursor molecules. Finally, research into thermal and chemical stability problems is important.

As has already been discussed, controlled formation of very thin high porosity microporous plugs within the pore entrance of the supporting system should be very interesting. Trials with silica, carbon and SiC deposits have been reported however without giving high selectivities and permeances.

(b) zeolite type: Defect-free zeolite membranes have so far only been produced for mem-

branes of the MFI (silicalite type) with thicknesses of about 50 ~tm on stainless steel supports and 3-10 ~tm on alumina and carbon supports. They are produced by in situ methods of zeolite crystals grown directly on the support system. There are some reports of formation of defective membranes with, e.g., zeolite A. Much more research is needed to widen the range of available zeolite membrane types especially small and wide pore systems. The permeance values of the defect-free membranes is lower than that of the amorphous membranes (see Chapter 6) and to improve this the layer thickness must be decreased together with improving the crystal quality (no impurities, no surface layers, high crystallinity, crystal orientation) and microstructure (grain boundary engineering).

(c) Crystalline, non-zeolite type: Very wide pore systems with good stability are especially difficult to make

with zeolites. Packing of very small particles (diameter 1-2 nm) is reported to give membranes with very wide pores (1.5-2.5 nm) but again with low stability. Alternative strategies might be the use of "pillared-clay systems" as investigated in catalysis and of systems which form subunits in solution and have wide pores when these subunits are packed in a film to form membranes. Micro-emulsion techniques with self organising (surfactant) molecules or the use of imogolite-related materials [20] are interesting (see also Chapter 7 on sol-gel chemistry and its application to porous membranes). The problem with these alternatives is that they are usually strongly anisotropic and so need to be oriented on the support. Research to select available possibilities seems interesting.

1.4.4.5 Nanofiltration membranes

Good rejection values are reported for substances with intermediate molecular weights in liquid filtration with mesoporous membranes. To achieve optimal

18 1 - - G E N E R A L O V E R V I E W , T R E N D S A N D P R O S P E C T S

values for low molecular weight compounds (MW < 100-300) or multivalent cations, nanofiltration membranes with pore diameter in the wide-pore range (1.0-2.0 nm) are necessary. Similar problems as discussed for wide-pore systems for gas separation also obtain here and are discussed in Chapter 7.

In addition to liquid filtration membranes, extensive research has been reported to modify the chemical nature of the internal pore surface either by coating the constituting particles before making the membrane or by grafting organic coatings with functional groups relevant for the intended separation purposes onto the internal surface. Rejection values are increased and fouling can be reduced (Chapter 11, [24]). Extension of these methods to wide-pore microporous systems probably will yield interesting results.

Finally functional groups can be introduced also in a membrane system with inorganic-organic composites. Care should be taken in this case to select combinations which add advantages of both components and avoid too many of the disadvantages.

1.4.4.6 Dense (non-porous) membranes and surface reaction limitation

These types of membranes are currently only suitable for oxygen separation mainly with oxidic layers, or hydrogen separation m mainly with Pd(Ag)

alloys. As discussed in Chapter 6, reasonable fluxes (permeation values) can be obtained at high (for oxygen) to intermediate (for hydrogen) temperatures. The production of defect-free thin layers of these materials remains problematic.

The best prospects seem to be present for deposition of Pd(Ag) plugs into the pores of a porous support material as e.g. reported by Morooka et al. [21]. Further research to confirm the reproducible synthesis of this system and of its long-term stability for hydrogen separation is worthwhile.

In the case of oxygen transport the best prospects at this moment are the use of metal-oxide composites with high electronic conductivity, or separation with perovskite-derived membranes as reported by Balachandral et al. [22]. These latter membranes are thick (0.5-1.0 mm) and have long-term stability at high temperature.

The use of thinner membranes of this type increases the permeance but to a lesser extent the thinner the membrane. With a thickness around 0.3 mm (depending on the precise system) surface reactions which transfer oxygen from the gas phase to solid material completely become rate determining. This phenomenon limits the much higher permeances which are potentially based on the very high bulk permeation (see Chapters 6 and 8).

Strategies to solve this problem involve application of adsorptive porous layers on the dense membrane and of metal-oxide composites where the ex- change reaction is catalysed by the metal. The study of these surface reactions and of ways to decrease their effect is important.


1.4.4.7 Mixed (hybrid) processes and materials

The combinat ion of an existing, accepted process wi th a special membrane process might result in both attractive prospects and acceptance. An example might be the combinat ion of pressure swing absorpt ion (PSA) for gas (e.g. air) separat ion wi th membranes in the form of a rotating vo lume filled wi th small spheres consisting of a highly adsorpt ive core and a selective (microporous) membrane coating. If the core consists of a relatively wide-pore microporous material, equi l ibr ium adsorpt ion conditions can be approached. The result might be a rapid, h ighly selective hybr id (PSA-membrane) process wi th a large surface area to vo lume ratio.

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13. C.J. Sealy and A.D. Little, Report on Membranes in Hydrogen Separation, March, 1995. 14. J. Armor, Membrane catalysis, where is it now, what needs to be done. Catal. Today, 25

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20 1 u GENERAL OVERVIEW, TRENDS A N D PROSPECTS

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20. R. Segal, J.C. Huling and C.J. Brinker, Use of silica sols in inorganic molecular sieving membranes, in: Yi Hua Ma (Ed.), Proceedings of the Third International Conference on Inorganic Membranes, 10-14 July 1994, Worcester, USA. Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA 01609, USA, pp. 85-94.

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[membrane science and technology] fundamentals of inorganic membrane science and technology volume 4...

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