Multichannel elements

Different membrane shapes are used, such as plates, foils, spirals, hollow fibers, tubes, and even monilithic multichannel elements have been mentioned in the context of membrane reactors. In the following section, a general survey will be given indicating the main characteristics of the different types of inorganic membranes used in CMRs. More details can be found elsewhere [13-15]. 

Although some ceramic membrane elements are proposed with a flat geometry, most of them exhibit a cylindrical shape for a multichannel element (Figure 6.1). The reason for that is the much better mechanical properties obtained for cylindrical-shaped ceramics and the easier sealing of the elements compared to flat shapes. 

Cross-sectional schematic of a monolithic multichannel element. (After Hsieh. -)... 

The front opening of such a microchannel element has a diameter of only a few microns, but it is only one element of a whole multichannel array (Figure 31.2). Whereas the orifice to one micro-channel element covers an area of only a few square microns, an array of several thousand parallel elements covers a much larger area. In particular, the area covered by the array must be larger than... 

An alternative type of spectrometer is the energy dispersive spectrometer which dispenses with a crystal dispersion element. Instead, a type of detector is used which receives the undispersed X-ray fluorescence and outputs a series of pulses of different voltages that correspond to the different wavelengths (energies) that it has received. These energies are then separated with a multichannel analyser. 

Multichannel instruments are capable of measuring the intensities of the emission lines of up to 60 elements simultaneously. To overcome the effects of possible non-specific background radiation, one or more additional wavelengths may be measured and background correction (see Section 21.12) can be achieved. 

Walter et al. studied the flow distribution in simple multichannel geometries by means of the finite-element method [112]. In order to reduce the computational effort, a 2-D model was set up to mimic the 3-D multichannel geometry. Even at a comparatively small Reynolds number of 30 they found recirculation zones in the flow distribution chamber and corresponding deviations from the mean flow rate inside the channels of about 20%. They also investigated the influence of contact time variation on a simple two-step reaction. 

The main detectors used in AES today are photomultiplier tubes (PMTs), photodiode arrays (PDAs), charge-coupled devices (CCDs), and vidicons, image dissectors, and charge-injection detectors (CIDs). An innovative CCD detector for AES has been described [147]. New developments are the array detector AES. With modem multichannel echelle spectral analysers it is possible to analyse any luminous event (flash, spark, laser-induced plasma, discharge) instantly. Considering the complexity of emission spectra, the importance of spectral resolution cannot be overemphasised. Table 8.25 shows some typical spectral emission lines of some common elements. Atomic plasma emission sources can act as chromatographic detectors, e.g. GC-AED (see Chapter 4). 

Simultaneous spectrometers consist of various combinations of analyser crystals and detectors, arranged around the sample at fixed angle settings. Use of a multichannel X-ray spectrometer with simultaneous determination of up to 24 elements can considerably increase the analysis speed (a few seconds to a few minutes). 

Table 8.40 compares the main characteristics of WDXRF and EDXRF. Multidispersive XRF combines the benefits of the WDXRF technique for routine elemental analysis with the complete flexibility offered by EDXRF for nonroutine analysis. Clearly, modem XRF instrumentation is rather varied, ranging from simple benchtop EDXRF equipped with a low-power X-ray tube and high-resolution proportional counter for some key elements, to 4 kW simultaneous multichannel spectrometers with 28 fixed element channels for... 

The multichannel coulometric detection system serves as a highly sensitive tool for the characterization of antioxidant phenolic compounds because they are electroactive substances that usually oxidize at low potential. The coulometric efficiency of each element of the array allows a complete voltammetric resolution of analytes as a function of their oxidation potential. Some of the peaks may be resolved by the detector even if they coelute (Floridi and others 2003). 

The use of a multichannel support made of a sintered oxide carrying a separation layer deposited on the surface of the channels was not a new concept. This was described in the patent literature as far back as the 1960s (Manjikian 1966). The multichannel geometry is particularly attractive in terms of its sturdiness, lower production cost compared to the single tube or tube-bundle geometry and lower energy requirement in the cross-flow recirculation loop. However, Ceraver was the first company to industrially produce multichannel membranes. Since 1984 these membranes, which have 19 channels per element with a 4 mm channel diameter are sold under the trademark Membralox. ... 

A few other players in the nuclear membranes activity also developed inorganic membranes for the filtration of liquids. This was the case with Norton-USA who with the know-how of Euroceral developed MF membranes made of an 0-AI2O3 tubular support with an a-Al203 layer. The inner tube diameter was 3 mm and the outer diameter 5 mm. In 1988-1989, Norton also produced the multichannel membrane elements. These membranes produced by Norton are now sold by Millipore under the trademark Ceraflo . 

In the second half of the 1980s, an increasing number of companies entered the field of inorganic membranes, the most significant ones being ceramic companies such as NGK of Japan which also developed a multichannel membrane element (19 channels, 3 mm diameter), Nippon Cement and Toto also from Japan and very recently Coming who also developed a multichannel membrane structure. 

The structural elements of commercial inorganic membranes exist in three major geometries disk, tube or tube bundle, and multichannel or honeycomb monolith. The disks are primarily used in laboratories where small-scale separation or purification needs arise and the membrane filtration is often performed in the flow-through mode. The majority of industrial applications require large filtration areas (20 to over 200m ) and, therefore, the tube/tube bundle and the multichannel monolithic forms, particularly the latter, predominate. They are almost exclusively operated in the cross-flow mode. 

Figure 3.15. Schematic of permeate flow path through a porous multichannel monolithic membrane element (Hsieh 1988, Hsieh, Bhave and Fleming 1988).

Conventional wisdom leans towards a simple measurement system featuring one or two optical elements and with a simple constraction. In practice there are many forms of filter assemblies that can be used and these are not limited to the use of a single filter and implementations featuring multiple filters on a wheel are often implemented in multichannel analyzers. Such assemblies (Figure 6.6C) can be quite small (25mm diameter as illustrated) and can accommodate a relatively large number of filters. Other forms of filter assemblies include clusters or arrays (multiple wavelengths), correlation filters (customized to the analyte), continuously variable filters, such as the circular variable filter (CVF) and the linear variable filter (LVF), and tunable filters (both broad and narrow band). 

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