Membrane quality

After each CVI experiment nitrogen was used to check the change in permeance of the membrane. For several membranes also permeance measurements were performed with He, H2 or CO2. In these cases the corresponding permselectivities could be calculated, which are a better indication of the membrane quality than the change in nitrogen permeance only. Unfortunately this type of measurements was not possible with our silane set-up (see Figure 2) because of safety regulations involved. 

In addition to the traditional deep bed filtration, other interesting examples of different processes and techniques can be described by the same basic principle (i) the tangential micro-filtration and ultra-filtration where a slow deep filtration produces the clogging of the membrane surface (ii) some processes of impregnation of porous supports with a sol in order to form a gel which, after precipitation, will form a membrane layer. Here the sol penetration inside the support is fundamental for the membrane quality. 

Control membrane quality, detection of microdefects, and influence on membrane performance. 

The lack of methods for a fast and reliable assessment of membrane quality is stiU one of the outstanding issues in zeolite membrane development. The usual meaning of the term quality relates to the ability of the membrane to carry out a given separation, therefore, is a system-specific property and the universal membrane quality test does not exist. In general, specific permeation measurements at different temperatures, either of single gases (or vapors) or of multicomponent mixtures in the gas or liquid (pervaporation) phase, provide extremely useful information on the effective pore structure of the membrane, on the... 

Alternatively, selective blocking of membrane pores by condensing vapors combined with permeation measurements (permporometry) could be used to evaluate defects [18]. However, this method gives only pore sizes and does not consider other specific interactions between the membrane and the permeating molecules. Bemal et al. [19] have developed a fast and simple technique to assess membrane quality taking into account the nature (organophilic vs. hydrophUic) of the zeolite... 

A survey of recent literature on zeolite membrane preparation reveals that synthesis processes, even for well-known zeolite structures (i.e., MFl, LTA), are still carried out batchwise, using a hydrothermal route to produce a thin layer from hydrogels or sols containing the corresponding nutrients. As a general rule, the reactant mixture in contact with the support changes in composition with time provoking a reduction of the membrane quality. 

Separation due to molecular sieving alone can only take place if the membrane is defect free and there is no adsorption of the permeating species on the zeolite crystals (i.e., at high temperatures) or the components in the mixture adsorb with similar heats of adsorption. For those mixtures, the shape and size of the components compared to the zeolite-pore sizes direct the separation toward molecular sieving. For example, the separation of H2/SF6 or N2/SF6 on MFl zeolite membranes at high temperatures where SFe does not adsorb significantly. In fact, the latter separation is commonly used to evaluate the membrane quality. Methane/f-octane represents another example of this mechanism since f-octane cannot fit into the MFl stmcture and the heat of adsorption of methane is very low in the zeolite [2]. 

With the development of more complex and sophisticated inorganic membranes there is a need for a better understanding of membrane structures and their influence on the mechanisms of separation processes. This requirement for a better insight into the relationships between (a) the membrane synthesis route, (b) the membrane microstructure or morphological properties and (c) the permeation properties, has been widely emphasised in the literature. Information on membrane characteristics is essential for membrane users, manufacturers and scientists to choose an appropriate membrane for a specific application, controlling membrane quality and preparation process parameters or understanding transport mechanisms. 

From gas permeation experiments (see Chapter 9) it was found that before template removal even very thin layers (1-2 pm) could be produced in a gastight state. After template removal a good membrane quality could be obtained with somewhat thicker layers. To obtain high quality, defect-free MFI membranes, two subsequent hydrothermal treatments resulting in a total layer thickness of about 3-4 pm give best results with excellent separation properties. 

Discussion of permeation and separation requires some characteristic parameter for the membrane quality. As shown below the apparent activation energy for H2 permeation gives a good correlation with the separation factor and is used as a measiure of quality. Furthermore, the total measured permeation has to be corrected for influence of the support to obtain permeation and activation energy values characteristic for the silica layer (see also Section 9.5). 

Typical values of b and D for a range of membranes are given in Table 9.10 together with some other parameter values. The range in D values reflects differences in membrane quality, the smallest D values being formed in high quality membranes. The diffusion coefficients become smaller in the same order as the kinetic molecular diameter (see Table 9.6) increase. The large differences in the D values indicate that the pore diameter is of the order of the molecular diameters (0.4-0.5 nm). 

Separation results with supported MFI membranes of good quality are reported by the Delft group (Kapteyn, Geus, Bakker [69-71,89]) with membranes on porous steel supports and by the Twente group (Burggraaf, Vroon et al. [72,73]) with membranes on porous a-alumina support and of medium (defect quality) by, e.g., the groups of Noble and coworkers [27,77,97] on y-alumina supports, of Ma and coworkers [76,96]. Some other groups report results without sufficient indication of membrane quality [64,75,98]. 

In order to obtain a good membrane quality and a high degree of utilization, a careful operation is necessary. This requires skilled people who know their job. 

If the starting sol becomes below supersaturation before the membrane is fully developed, growth must be repeated with a fresh sol. Two or three hydrothermal syntheses are often required to gel good quality membranes. In many cases, large quantity of non connected zeolite crystals can be observed on the surface of infiltrated composite membranes, after several synthesis steps. The methods for evaluating the membrane quality will be detailed in the Characterization section. In the case of MFI membranes, a very simple method can be used at this step the membrane is washed, dried at about 150-200 °C and tested by single gas permeation (lie or N2). As far as the TP A ions are blocking the zeolite channels, no gas permeance should be detected if there are no defects. 

The zeolite membranes quality and the development of methods able to rapidly identify the defects are key-factors for large scale applications. A large number of static and dynamic methods can be used to evaluate the quality of membranes [125J. Static methods allow to study the physico-chemical characteristics of the membrane material. Dynamic methods allow the detection of defects affecting the transport properties and consequently the membrane performance. For zeolite membranes the quality of the adhesion between the zeolite layer and the support, the orientation of zeolite crystals, the layer thickness, and the number and quality of... 

The single gas permeance ratios N2/SF6 or FF/SFf, are often considered as representative of the zeolite membrane quality. However these values can be quite low even for high quality membranes [131]. Indeed, these ratios are affected by the defect distribution but can also depend on other factors such as the applied feed pressure, the film thickness and the support type. For example the N2/SF6 permeance ratio increases with pressure due to non-linear adsorption of SF6 and also increases with the film thickness due to the mass transfer resistance of the support. In the same way, the ideal or mixture selectivity /-butane/n-butane is often considered as a good indication of the membrane quality, but varies largely (from 10 up to 200) even for good quality membranes. It is then difficult to use only one of these criteria to compare zeolite membranes. 

Two membranes of the same type (TFC-S), but different batches were tested for the effect of their significantly different pure water flux on rejection. Rejection of organics decreased from 80 to 65% for a flux increase from 33 to 74 Lm Tr bar. This is an expected trend due to the increase in wall concentration with an increased fltrx. Therefore a large error can be expected due to variations in membrane quality. However, these problems can be minimised by measuring the water flux of each membrane prior to experiments. 

This chapter deals with CS or its derivatives as a membrane material in the field of membrane technology. This covers the brief history of membranes, qualities of CS as a good membrane material, methods to prepare CS-based membranes, cross-linking agents and its effects, and modification of CS and its applications in the various fields like wastewater purification, pervaporation, fuel cells, and hemodialysis. 

For A. craccivora, specific binding of [ H]thiamethoxam was similarly sensitive to temperature and membrane quality as in M. persicae IT). The data for affinity and capacity at 2 C were about 90 nM and 1000 finol/mg protein. Higher values for A. craccivora compared to M. persicae were also obtained with imidacloprid 8, IS). 

Gu et al studied the effects of the seeding procedures and the conditions of hydrothermal treatment on the FAU membranes quality. For the seeding stage, they used rubbing and dip-coating. They found that dip-coating combined with... 

Samples Membrane Quality SIDA Mn (g/mol) Polymer Mn ( mol) Tg (°C) d-spacing (A) Permeate Flux (103 kg/[m2h]) Separation Factor... 

The exponent n for the partial pressure is 0.5 provided that the bulk phase diffusion of atomic hydrogen is the only rate-limiting step. In this case Equation 14.10 is named Sievert s law. The hydrogen separation factor 5 is sometimes used to specify membrane quality. It is defined as... 

The major area of application for plastics in bioscience is in the two areas indicated. The plastics make interesting materials to be used for mechanical implants into all living systems, including animals and plants where they can serve as repair parts or as modifications of the system. Figure 18-4 shows a variety of plastics used in hospitals. The other applications are based on the membrane qualities of plastics which can control such things as the chemical constituents that pass from one part of a system to another, the electrical surface potential in a system, the surface catalytic effect on a system, and in some cases the reaction to specific influences such as toxins or strong radiation. A Dacron artery is shown in Fig. 18-5. 

The hydrogen separation factor is sometimes used to specify membrane quality. It is defined as ... 

There are applications based on the membrane qualities of plastics. They can control such things as the chemical constituents that pass from one part of a system to another, the electrical surface potential in a system, the surfiice catalytic effect on a system, and in some cases the reaction to specific influences such as toxins or strong radiation. 

Characterization of zeolite membrane quality by using permporosimtrey... 

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