Dispersion coiled tubes

The reactor can consist of a short packed tube or a length of coiled tube. Open tubes can give very serious band dispersion as already discussed and, if a tube is used for the reactor, it should be constructed of low-dispersion tubing. Low dispersion tubing will not only reduce band dispersion but will also produce highly efficient mixing and thus accelerate the reaction. 

Figure 5 Schematic diagrams of reactors used in FIA in order of increasing dispersion (a) single bed string reactor, (b) knitted tube, (c) coiled tube, (d) straight tube, and (e) external mixing chamber with stirring.

For the sake of simplicity, the equations that Tijssen derived for radial dispersion in coiled tube are given in terms of conventional chromatographic terminology. At relatively low linear velocities (but not low relative to the optimum velocity for the tube) Tijssen derived the equation,... 

The low dispersion serpentine tube developed by Katz et e> OO) was an alternative approach to the coiled tube and was designed to increase secondary flow by actually reversing the direction of flow at each serpentine bend. A diagram of a serpentine tube is shown in figure 3. In fact, the serpentine tubing shown in figure 3 was designed to be an interface... 

The flow rate is employed as the independent variable, an alternative to the more usual linear velocity, as the flow rate is defined by the column with which the low dispersion tubing is to be used. It will be shown in due course that the column flow rate is independently defined by the nature of the separation that is to be achieved by the column, It is seen that a similar curve is obtained for the serpentine tube, as that for the coiled tube, but the the maximum value of (H) is reached at a much lower flow rate than that with the coiled tube. Furthermore, the variance remains more or less constant over a wide range of flow rates that encompass those usually employed in normal LC separations. 

The objectives of liquid mixing in stirred tanks are to (i) make the liquid concentration as uniform as possible (ii) suspend the particles or cells in the liquid (iii) disperse the liquid droplets in another immiscible liquid, as in the case of a liquid-liquid extractor (iv) disperse gas as bubbles in a liquid in the case of aerated (gassed) stirred tanks and (v) transfer heat from or to a liquid in the tank, through the tank wall, or to the wall of coiled tube installed in the tank. 

Our main concern here is to present the mass transfer enhancement in several rate-controlled separation processes and how they are affected by the flow instabilities. These processes include membrane processes of reverse osmosis, ultra/microfiltration, gas permeation, and chromatography. In the following section, the different types of flow instabilities are classified and discussed. The axial dispersion in curved tubes is also discussed to understand the dispersion in the biological systems and radial mass transport in the chromatographic columns. Several experimental and theoretical studies have been reported on dispersion of solute in curved and coiled tubes under various laminar Newtonian and non-Newtonian flow conditions. The prior literature on dispersion in the laminar flow of Newtonian and non-Newtonian fluids through... 

Further details of the mass transfer enhancement techniques in membrane separation processes are reported by Belfort and Al-Bastaki and Abbas. In this entry, the focus is on the flow instabilities produced by Dean vortices in curved and coiled tubes because of their advantages over the other techniques, viz., lower axial dispersion, better radial mixing, residence time distribution closer to plug flow, higher mass... 

Fig. 7 Comparison of various axial dispersion correlations (k = Dc/Dg, ratio of dispersion in coiled tube to the straight tube).

Janssen, L.A.M. Axial dispersion in laminar flow through coiled tubes. Int. J. Heat Mass Transfer 1976, 31, 215-218. 

Saxena, A.K. Nigam, K.D.P. Axial dispersion in laminar flow of polymer solution through coiled tubes. J. Appl. Polym. Sci. 1981, 26, 3475-3478. 

Saxena, A.K. Laminar Dispersion in Helically Coiled Tubes. Ph.D. Thesis, Indian Institute of Technology, New Delhi, India, 1982. 

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.

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