Distillation Column Performance Using Mixed High and Low Energy Ring Packings

Abstract

The wide use of packed columns in industrial distillation has led to the development of numerous packings to improve column efficiencies. The shape, size and material of packings have been varied to suit different systems but the type of solid surface has never been significantly altered. The common practice is to use a high energy surface (glass, copper, ceramic, etc . . .) because of its good wetting characteristics and the liquid, flowing over the packing in a filmwise manner, offers the highest possible surface area for mass transfer. Recent investigations however show that under certain conditions, a low energy surface improves significantly the column efficiency for positive binary systems a t nearterminal concentrations. Ponter and coworkers [I] have shown that the large increase in HTU already established by a number of workers is the result of large changes in wetting properties. Since such anomaious wetting behaviour does not occur on low energy surfaces, further investigations [2] using PTFE as packing material demonstrated better column efficiencies for such systems at low compositions. The improved performance was attributed to a change in flow regime from a filmwise manner on a high energy surface to rivulets and drops on a low energy causing high liquid turbulence and mixing and consequently a n increased mass transfer coefficient.The use of plastic materials for industrial packings presents several advantages: low specific weight, high toughness and easy shaping. The usual plastics however are known to adsorb moisture [3] resulting in an apparent high energy surface defeating the purpose of the use of the material. As PTFE is a plastic which does not exhibit this property, it was employed since a permanent low energy surface WJS desired. The exorbitant cost of such a packing as well as the higher pressure drops encountered when operating it [4] suggest that a more economical alternative should be found. This investigation was therefore undertaken to determine whether mixtures containing PTFE rings would also be beneficial since intuition suggests that the low energy particles dispersed in the packings would act as trigger points where film flow would break to give revulet flow and give better mixing a t these junctions. Experimental and DiscussionThe distillation unit comprised .a 5 cm diameter glass column pa&-ed with 0.6 X 0.6 X 0.4 cm glass Raschig rings containing either 10, 30 or 50°/o (by number) PTFE rings of similar dimensions, to a height of 15 cm. The column operated a t atmospheric pressure and at total reflux at a constant boil u p rate = 1.37 moles/h. cm2. The ethanol concentrations investigated ranged from 0.14 to 36.7 mole Oio ethanol in the reboiler. The efficiencies of the column, expressed in terms of the height of a transfer unit H O G , are presented in Figures 1 and 2 both versus data since they are a consequence of changes in wetting producing low interfacial areas and so, low mass transfer coefficients.Higher efficiencies using pure PTFE Raschig rings have been reported...