Determination of effective diffusion coefficients for porous packings with an impermeable centre from peak moments
✍ Scribed by P. Schneider
- Publisher
- Elsevier Science
- Year
- 1986
- Tongue
- English
- Weight
- 455 KB
- Volume
- 41
- Category
- Article
- ISSN
- 0009-2509
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✦ Synopsis
Expressions were developed for the first two moments of a chromatogmphic curve from a GSC column packed with porous particles containing a permeable layer and an impermeable central part. A modified van Dcemter equation is given for a GSC column with porous Raschig rings fitted on an impermeable string running along the column axis (pellets-on-the-string column). INTEODUCTION Gas-solid chromatography (GSC) is now widely accepted as a suitable method for determining effective diffusion coefficients of gases in porous solids (catalysts, catalyst carriers, adsorbents, etc.; for a review see, for example, [l, 23). By utilizing some simple model of the porous structure it is then possible to evaluate parameters characterizing solely the porous structure (transport p ammeters), i.e. independent of the kind of carrier and tracer gases and of the measurement conditions (temperature, pressure) [36]. The evaluation of effective diffusion coefficients is most often based on matching moments (first absolute moment, second central moment) of experimental chromatographic curves (CCs) with the theory [7-g]. More rigorous, alas more demanding, is the matching of experimental CCs with theory either in the time domain or in the Laplace or Fourier domains [lo].
GSC measurements are usually performed on columns packed with crushed particles of the porous solid. As the results obtained are the more confident the stronger the role of intraparticle diffusion, large size packing is preferred. To obtain a suitable hydrodynamic regime, it is, however, required that the column to particle diameter ratio should be not less than about 30. Thus, very wide columns have to be used which require large amounts of porous particles. In order to work under conditions of negligible transport resistance between the bulk of gas and the outer surface of the particles (external diffusion) it is imperative to use high linear carrier-gas velocities. This demand combined with the large column diameter leads to very high carrier-gas volumetric flow rates. Such flow rates cause difficulties in chromatographic detectors if gas splitting prior to the detector is not used.
It was suggested that whole, uncrushed cylindrical or spherical pellets be used in the columns which are only slightly wider than the pellets [cf. Fig. l(a)]. In %ubscrlpts r, x, t, VT, xx denote the first and second (partial) derivatives; e.g. c_ = @c/ax", c, = a&Xx, II. = &A/&, etc.