Dispersion in open tubular reactors of various geometries

A study comparing the band dispersions observed in tubular conduits of different overall geometry is described. Straight, coiled and various types of serpentine and superserpentine geometries of open tubular reactors (OTRs) were studied in two different tube diameters (300 µm i.d., 110cm active length, residence volume 80µl and 750µm i.d., 75cm long, residence volume 330µl) as a function of flow rate. Dispersion is presented in terms of the observed bandwidth of the peak, as measured photometrically by using an injection of slightly acidic bromthymol blue into a dilute base carrier. Very small injection volumes (200nl) and an ontube detector were used to minimize extra-reactor contributions to the observed dispersion. Some results are expected: the dispersion observed in a straight tube increases steeply with increasing flow rate. For many of the more complex designs, the increase in absolute dispersion with increasing flow rate tapers off rapidly, reaching a virtual plateau value. The dispersion observed in identically made devices are reproducible. However, dimensional changes (e.g. grid spacing) within a given geometry can have a very major effect on the observed dispersion. While there is no unique "best reactor" geometry, a knotted reactor with knots of alternating flow directions and a serpentine II design with greater horizontal than vertical spacing performed well over a large range of flow rates. The caveat on the knotted design is that such a reactor is difficult to precisely reproduce manually.