443971 Predicting Pressure Drop of Structured Packings at Lowest Operating Pressures

Thursday, April 14, 2016: 8:30 AM
Grand Ballroom G (Hilton Americas - Houston)
Zarko Olujic1, Thomas Rietfort2 and Helmut Jansen2, (1)Process & Energy Department, Delft University of Technology, Delft, Netherlands, (2)Julius Montz GmbH, Hilden, Germany

Removal of impurities, consisting of free fatty acids (FFA) and other valuable components as well as contaminants from edible oils occurs on large scale by hot steam stripping under deep vacuum conditions, where, because of a strong deteriorating effect on separation, each Pascal of pressure drop counts. Similarly, subsequent isolation and purification of individual FFA-s and other valuable thermally sensitive components is carried out by rectification at as low as reasonable top of the column pressure. Therefore, modern continuously operating deodorization strippers and FFA distilling plants are mainly equipped with structured packings. Due to their formidable performance characteristics (good wetting and lowest pressure drop per equilibrium stage), high specific geometric area wire gauze structured packings are preferred choice, which, however, because of a rather small hydraulic diameter, may not be adequate when deep vacuum applications are considered. Another interesting aspect is that advanced geometric features of high performance corrugated sheet metal structured packings may not appear beneficial as generally believed in these applications. Due to a relatively large pressure drop (in the range of, or larger than the absolute pressure at the top of the column), operating conditions and consequently both hydraulic and mass transfer performance differ strongly at the top and the bottom of the bed. In order to arrive at an optimized design a tedious iterative procedure is required. Established, purely empirical predictive models, do not account properly for flow peculiarities of the prevailing flow regime. Namely, ascending gas moves under laminar flow conditions. Being based on common applications i.e. operating conditions where turbulent gas flow prevails, empirical models tend to underestimate the pressure drop in deep vacuum applications. One should note that a very low density gas ascends through a packed bed at a rather high effective velocity and tends to follow the shortest, vertically oriented sinusoidal flow path, which length exceeds the bed height, to the extent depending on the effective gas flow angle. The latter does not necessarily coincide with the corrugation inclination angle, and therefore needs to be determined for every type of structured packing from the pressure drop measured under operating conditions. As shown here, the effective gas flow angle may serve as single adjustable parameter in a simple phenomenological pressure drop model, which enables a theoretically founded approach to conceptual (re)design of packed columns for edible oil stripping, FFA fractionation, and similar deep vacuum distillation applications.

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