(120a) Droplet Formation and Reduction in Mass Transfer Equipment

This contribution presents scope and major results of the joint research project “TERESA – Droplet generation and reduction in mass transfer equipment” financed by the German government including 4 universities and 8 industrial partners and developed over 4 years.

Mass transfer and separation processes are the main unit operations in the chemical, petrochemical, pharmaceutical and food industries. Mass transfer based on evaporation and condensation is very energy intensive. It is estimated that approximately 3% of the world energy consumption is attributed to thermal separation processes. These rely on the continuous establishment of vapor/liquid equilibrium followed by a perfect phase separation. This research project centers on the separation of liquid and gaseous phases after a mass transfer step: entrained droplets must be separated from the gas phase and gas bubbles from the liquid. Entrainment and back mixing reduces separation efficiency, causes higher energy demand and can lead to corrosion and process stability problems. To avoid such troubles, a better understanding of the formation, entrainment and separation of droplets is essential.

The project is focused on four major trouble areas: the feed line of a flashing feed, the feed section inside the column, the two-phase flow of a forced circulation flash reboiler and the head vapor of a distillation column. Flow pattern maps are common to characterize the two-phase flow in the feed pipe. These maps are used and were optimized to larger pipe diameters, and typical but non-ideal industrial conditions.

To detect droplets (diameter > 2 micrometer) and to measure gas/liquid ratios existing devices (capacitance-wire-mesh-sensor, optical inline probe) were modified and adapted to the conditions and calibrated. These highly sensitive devices were used to measure droplet size, droplet concentration and gas/liquid ratios in the feed line, as well as inside an air/water column (e.g. between two trays) and the gas outlet. To adapt air/water tests to organics, investigations were made with organic mixtures in a reboiler line and after expansion in a feed line. Integral entrainment data were compared to local droplet measurements and showed excellent agreement in the quantification of critical operating parameters.

Based on these experiments improved droplet separation devices were implemented and tested. In addition, CFD calculations were used to get a better understanding of flow patterns and to improve design criteria for the development of droplet separation devices. Economic and ecologic investigations confirmed the advantages of improved droplet separation devices minimizing entrainment in mass transfer equipment.