Tailor-made membrane

With the recent implementation of high-throughput experimental techniques in this type of membrane separation, the development of new membranes and novel applications will surely be accelerated [49]. Probably, tailor-made membranes will be needed to solve specific separation problems in industrial chemical processes. 

Regarding the retention of pollutants membranes can be manufactured more or less tailor made, thus increasing potential applications... 

Another method of producing composite hollow fibers, described by Kusuki etal. at Ube [108] and Kopp et al. at Memtec [109], is to spin double-layered fibers with a double spinneret of the type shown in Figure 3.37. This system allows different spinning solutions to be used for the outer and inner surface of the fibers and gives more precise control of the final structure. Often, two different polymers are incorporated into the same fiber. The result is a hollow fiber composite membrane equivalent to the flat sheet membrane shown in Figure 3.26. A reason for the popularity of composite hollow fiber membranes is that different polymers can be used to form the mechanically strong support and the selective layer. This can reduce the amount of selective polymer required. The tailor-made polymers developed for gas separation applications can cost as much as... 

The discussion in Section 4.4.1.3 on transport mechanisms in SLM has manifestly demonstrated another facet of tuning analyte-selective extraction. For example, Figure 4.5 clearly demonstrates the selective extraction of a basic compound—all that is required here is a simple adjustment of the pH on either side of the membrane. Also, Figure 4.6 neatly illustrates the possibility of performing such selective extraction of anionic and cationic species in another transport mechanism that employs selective carriers. Thus, by fine-tuning the chemistry/composition of the sample, membrane liquid, and acceptor phases, analyte-selective extraction can be tailor-made. 

As one can see from Table 1, a spin-off result of this work is a list of recipes for the preparation of membranes with different amounts of doping, covering a complete range of pore-sizes with a resolution of 1-2 nm. This shows that we are now able to produce membranes with a tailor-made pore-size, which may be important for retaining certain large molecules by high-flux nanofiltration. 

Completely hydrothermally stable intermediate layers were developed (chapter 5). Synthesis routes were developed to realise mesoporous membranes with tailor-made pore-radii ranging from 2 to 10 nm. 

Because the currently used y-alumina is not stable in all acid and basic environments used in industry [2], the development of mesoporous layers other than y-alumina deserves attention as well. Most common materials that can be used for the mesoporous layer are zirconia and ti-tania [3,4], but recently also the preparation of mesoporous hafnia is described [5], Hafnia seems to be a very interesting membrane material, because it can, unlike zirconia and titania, be fired up to 1850°C without a phase transformation of its monoclinic form. Hafnia also has a high chemical resistance toward acid and basic media. Another interesting material, currently under investigation by the group of Brinker is mesoporous silica [6,7], This material is especially interesting because a tailor made morphology and pore-size is possible. 

There are different possibilities for performing whole cell biotransformations with several enzymes. Each enzyme can be produced from another strain. To carry out the desired reaction, these strains are combined in different amounts which depend on the particular enzyme activities. Disadvantages of this method are diffusion problems and the membrane barrier [126]. Alternatively, all enzymes can be expressed in one single host strain. This can be reached by means of recombinant DNA techniques in several ways The genes can be expressed under one single promoter or under different promoters. Furthermore, it is possible to express all genes from the same promoter but with different copy numbers. Using these methods it is possible to create tailor-made catalysts for manifold purposes. 

Activated charcoal was originally regarded as a relatively inexpensive adsorbent with an assortment of pores of ill-defined size and shape. However, in recent years considerable progress has been made in the development of tailor-made porous carbons such as molecular sieves, activated carbon fibres and carbon composites (Marsh et al., 1997). Superactive carbons are now made on a commercial scale with BET areas of around 3000 m2g-1. Activated carbons can be manufactured as fine particles or granules or in the form of a cloth, felt or consolidated membrane. The properties of some of these special types of activated carbon are discussed in Chapter 12. 

If a membrane has a graded pore structure but is made in one processing step, frequently from the same material across its thickness, it is called an asymmetric membrane. If, on the other hand, the membrane has two or more distinctively different layers made at different steps, the resulting structure is called a composite membrane. Almost invariably in the case of a composite membrane, a predominantly thick layer provides the necessary mechanical strength to other layers and the flow paths for the permeate and is called the support layer or bulk support. Composite membranes have the advantage that the separating layer and the support layer(s) can be tailored made with different materials. Permselective and permeation properties of the membrane material are critically important while the material for the support layer(s) is chosen for mechanical strength and other consideration such as chemical inertness. The composite membranes can have... 

The use of zeolitic membranes in separation or combined reaction and separation processes is very appealing. Advantages of using this type of membrane include not only their ability to discriminate between molecules based on molecular size but also their thermal stability. The large variety of zeolite types could provide a tailor-made separation medium for specific processes. Moreover, the properties of zeolites are often easily adjustable (ion exchange, Si/Al ratio, etc.). This makes zeolitic membranes also very promising for use as catalytic membranes. 

Membrane technology covers various chemical technology disciplines, such as material science and technology, mass transport and process design. By manipulating material properties, membranes can be tailor-made for particular separation tasks... 

Among the proton-conducting membranes Nation or Nafion-like sulfonated perfluorinated polymers should also be mentioned. These materials are used for polymer electrolyte membrane (PEM) fuel cells, and in addition to being chemically very stable, they exhibit high proton conductivity at temperatures lower than 100°C. It is believed that permeability and thermal stability may be increased if tailor-made lamellar nanoparticles are added to a proton conducting polymer. 

Separation of CO2 from gas streams is required in four areas (1) purification of natural gas (gas sweetening), (2) separation of CO2 from enhanced oil recovery (FOR) gas streams, (3) removal of CO2 from flue gas, and (4) removal of CO2 from biogas. A fifth area vital for the space age should be mentioned removal of CO2 from life support systems onboard space ships, and also in submarines. All these applications have different specifications for the purified gas or for the recovered CO2, and future membrane applications will most likely be based on tailor-made materials. 

Membrane science and technology is expected to play an increasingly important role in the future for various industrial sectors. The availability of new membrane with tailor-made properties and new membrane processes offers important tools for the design of alternative production systems appropriate for a sustainable growth. 

Fine chemicals, ranging from some hundred to some thousand tons per year, are usually cost sensitive in many directions. Here electrochemistry will find strong competition but also really good opportunities. If commercial cells and standard electrodes and membranes are available, this will be ideal. If the capacity is distinctly higher than 1000 metric tons/year, opportunities increase for suppliers of electrochemical equipment to develop a tailor-made system. 

Pore structures of membranes formed in a primary process can be modified by subsequent modification processes to change properties and to obtain tailor-made systems for specific applications. Figure 2.4 shows schematically some typical microstructures. 

Fig. 7.4. Formation of tailor-made porous membrane materials by insertion and thermal decomposition of template agents in a gel layer.

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