Mass transfer in ideal and geometrically deformed open tubes : III. Deformed metal and plastic tubes

Abstract

Dispersions ( h values) in deformed tubes are much better than those in coiled ideal tubes,even at lower velocities ( u < 50 cm/sec). Such tubes are of interest as connecting pipes, heat exchangers, reaction detectors, liquid chromatographic (LC) separation columns and possibly in catalysis. No theory exists for deformed tubes and hence improvements must be made by trial and error. Squeezed, twisted and coiled tubes are much better than ideal tubes, but they are complicated to make, their geometry is difficult to optimize and is dependent on the kinematic viscosity of the eluent. Coiled wavy tubes, having a circular cross-section, are easy to prepare, and have an efficiency that is relatively insensitive to the diamemter of the tube, its exact geometry, the kinematic viscosity of the eluent and the ability of the eluent to wet the tube. Wavy tubes can be prepared from glass, metal or plastic. In all deformed tubes the laminar region is extended to unusually high flow velocities, because of secondary flow. The transition from laminar to turbulent flow is gradual and continuous. These effects are strongest in wavy tubes. For example, in a 2-mm I.D. tube h decreases almost linearly from 2.6 to 1 cm as the linear velocity is increased from 3 to 130 cm/sec, i.e., comparable h values are obtained at low velocities and in the turbulent region. Up to 70 theoretical plates per second can be generated for an inert sample. If this column were 100 m long, then with n-heptane as the eluent more than 7000 theoretical plates would be generated for an inert sample at u = 100 cm/sec at the cost of a pressure drop of only 7 bar. At high velocities the loss in permeability is only about 50% of that of an ideal tube. Efficient wavy tubes of large diameter (1–4 mm) can be produced, which would have advantages as LC columns and as reaction detectors, because their loadability would be high. Because of the low pressure drop and large peak volume, simple equipment could be used.