(186ax) Study of Transport Operations In a Novel Microstructure with Continuous Gas-Liquid-Phase Contacting for Use In Thermal Separation Processes

The successful application of micro technology in chemical engineering with gas-liquid phase contacting has already been confirmed on the basis of many examples [1], especially for absorption with instantaneous reactions. In microchannels pure physical/thermal mass transport operations between gaseous/vaporous and a liquid phase have still not sufficiently been investigated. In the present work a novel microstructure ("μTU") is developed and investigated by means of analytical and numerical methods. In this microstructure a gaseous-/vaporous- and liquid phase are guided continuously in counter-current flow without intermixing. The liquid flows down a vertical inclined semicircular microchannel driven by gravity, whereas the film thickness declines for a given evaporation rate along the flow direction. For small ratio of radius-to-channel length of about 10^-3, the three-dimensional velocity profile is partially solved with analytical methods by means of a lubrication approximation.The gaseous-/vaporous phase flows upwards in a circular channel driven by pressure. The phase contacting zone is simulated using the CFD code (6.3 FLUENT ® - 3ddp) by the approach of the VOF model. The simulations are performed with the test systems nitrogen/water (absorption) and methanol/water (distillation).

By varying the applied pressure gradient in the gaseous-/vaporous phase and the contact angle, the condition until the occurrence of instability in the liquid phase flow and flooding of the microunit “μTU” are determined. In previous studies it was shown, that the Height of a Transfer Unit (HTU) based on the gaseous phase does not reach the very low HTU based on the liquid phase despite the reduction of the characteristic length, and accordingly its higher mass transfer coefficient. This is due to the density difference between the two phases [2]. This can be improved by a special corrugation structure [3].

The mass transport in the gaseous/vaporous phase is expected to be increased due to the resulting longitudinal instability, which will be illustrated on the basis of CFD. The flooding conditions of this new configuration "μTU-II"-microunit are also investigated by means of numerical simulation (CFD), and therefore the optimum operating conditions will be identified. The results of this project are needed for the development of a demonstration microdevice to carry out a distillation or absorption without chemical reaction. In the next step, the results are verified based on the demonstration device in the laboratory.

[1] G. Tekautz,, B. Zechner, L. E. Wiesegger, D. Kirschneck, Principle and guidelines for selection of microstructured devices for mixing and reaction in Handbook of Microreactors – Volume C, System Process & Plant Engineering (Ed.: Hessel et al.), Wiley VCH Verlag GmbH & Co., KGaA, Weinheim, in print, Chapter 1.3

[2] L. E. Wiesegger, Strömungstechnische Untersuchung einer Flüssigkeits-Dampf-Kontaktierung, Diplom thesis, Microinnova Engineering GmbH, Institute of Fluid Dynamics and Heat Transfer, Graz University of Technology, April 2006

[3] P. Gschwind, Strömungs- und Transportvorgänge in gewellten Kanälen mit in einander liegender Anordnung der Wände, Dissertation, Stuttgart, 2000