Isodispersion point

Figure 2.10. Dispersion of a dye, injected as a sample zone (Sy = 25 jiL) into A, straight tube By coiled tube C, knitted tube and D, a SBSR reactor. The reactor volumes (Vr = 160 iL) and pumping rates (Q = 0.75 mL/min) were identical in all experiments. The same piece of Microline tubing (L = 80 cm, 0.5 mm inside diameter) was used in experiments Ay By and C. (The injected dye was bromthymol blue, carrier stream 0.1 M borax and wavelength 620 nm, cf. Chapter 6.) The SBSR reactor was made of 0.86 mm inside diameter tube filled with 0.6-mm glass beads. Note that the isodispersion points on the peaks were recorded with microreactors made of identical length and diameter, but different geometry.

Zone penetration is an ideal tool for measuring selectivity coefficients, since the method allows readouts to be taken (1) at that vertical slice of the composite zone where the dispersion coefficients are equal (Da = Db at point M Fig. 2.26, bottom) and (2) at the peak maximum of the pure A component. The concentrations within such a composite zone and position of the point M at a time tM are readily established by an experiment where zone A is first injected alone and peak A is recorded by a detector of choice. (If chemical reactions are involved, like a reaction with a suitable reagent for colorimetric detection, a suitable manifold and colorimetric detector are used—cf. Section 4.5.2). Next, zone B is injected alone and peak B is recorded. Provided that the same solution of analyte is injected in both runs A and B, and that the detector responds linearly to the injected species, this experiment yields, (a) the isodispersion point M within the time concentration rnatrix, which is identified via time delay tM, and (b) the peak height Ha for the response of the pure species A at the time Im, because it equals the horizontal distance between point M and the baseline (Fig. 2.26). 

Figure 3.10. Residence time distribution curves in a coiled tube computed with the aid of Speberg s model [3.3]. (a) For = 0 the familiar Taylor s solution is obtained. When the function E is included, then RTD curves showing isodispersion points are obtained for both mixing cup detector b) and area average (across the flow) detector (c).

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