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Coupled Transport Processes in Zero-Gravity Distillation Columns

Preußer, Niklas (2020):
Coupled Transport Processes in Zero-Gravity Distillation Columns.
Darmstadt, Technische Universität Darmstadt,
DOI: 10.25534/tuprints-00011447,
[Ph.D. Thesis]

Abstract

Distillation is an essential process in the chemical industry. To cope with small production quantities of specialized chemicals, modular production plants have gained increasing attention in the past years. Zero-gravity distillation is a small-scale distillation process, which offers high separation efficiencies and can be used as a part of modular production plants. Instead of gravity, capillary forces are utilized to sustain the fluid circulation. In order to apply the process in industry, the influence of the operating parameters and the capillary structure on the process variables has to be understood. In this thesis, the results of experiments and simulations of zero-gravity distillation are presented and discussed.

The experimental setup, which is similar to a flat heat pipe, allows measuring the wall and vapor temperatures, the fluid composition at the condenser, the pressure, and the liquid-vapor interface shape. Two different capillary structures were investigated at infinite reflux with mixtures of water and ethanol. For the extensively studied triangular groove structure, the overall average ethanol fraction, the heat flux, and the inclination of the system were varied. Negative inclination angles indicate that the condenser lay higher than the evaporator. The separation efficiency defined as the difference between the ethanol mole fraction at the condenser and the system average ethanol mole fraction increased with decreasing overall average ethanol fraction, decreasing heat flux, and decreasing inclination angle. For the advanced rectangular groove structure, three fluid compositions were tested. By comparing the two groove structures, no clear conclusion could be drawn on which capillary structure promotes separation.

In the theoretical model, heat transfer, mass transfer, and hydrodynamics are coupled for zero-gravity distillation in channel-shaped capillary structures. In contrast to existing models for porous media as capillary structures, the influence of the variation of the area occupied by the liquid along the flow direction is accounted for. Additionally, heat conduction in the channel wall is included.

The theoretical model of zero-gravity distillation was applied to a re-entrant channel structure at different values of the heat flux, the inclination, and the radius of the circular part of the cross section. As in the experiments, the separation improved with decreasing heat flux, which means the difference between the ethanol mole fractions between evaporator and condenser increased. Changing the inclination of the system did not lead to changes in the distribution of the component mole fractions in the system. With decreasing the radius of the circular part of the capillary structure cross section, an improved separation was observed. Thus, an influence of the capillary structure design on the separation was shown.

Item Type: Ph.D. Thesis
Erschienen: 2020
Creators: Preußer, Niklas
Title: Coupled Transport Processes in Zero-Gravity Distillation Columns
Language: English
Abstract:

Distillation is an essential process in the chemical industry. To cope with small production quantities of specialized chemicals, modular production plants have gained increasing attention in the past years. Zero-gravity distillation is a small-scale distillation process, which offers high separation efficiencies and can be used as a part of modular production plants. Instead of gravity, capillary forces are utilized to sustain the fluid circulation. In order to apply the process in industry, the influence of the operating parameters and the capillary structure on the process variables has to be understood. In this thesis, the results of experiments and simulations of zero-gravity distillation are presented and discussed.

The experimental setup, which is similar to a flat heat pipe, allows measuring the wall and vapor temperatures, the fluid composition at the condenser, the pressure, and the liquid-vapor interface shape. Two different capillary structures were investigated at infinite reflux with mixtures of water and ethanol. For the extensively studied triangular groove structure, the overall average ethanol fraction, the heat flux, and the inclination of the system were varied. Negative inclination angles indicate that the condenser lay higher than the evaporator. The separation efficiency defined as the difference between the ethanol mole fraction at the condenser and the system average ethanol mole fraction increased with decreasing overall average ethanol fraction, decreasing heat flux, and decreasing inclination angle. For the advanced rectangular groove structure, three fluid compositions were tested. By comparing the two groove structures, no clear conclusion could be drawn on which capillary structure promotes separation.

In the theoretical model, heat transfer, mass transfer, and hydrodynamics are coupled for zero-gravity distillation in channel-shaped capillary structures. In contrast to existing models for porous media as capillary structures, the influence of the variation of the area occupied by the liquid along the flow direction is accounted for. Additionally, heat conduction in the channel wall is included.

The theoretical model of zero-gravity distillation was applied to a re-entrant channel structure at different values of the heat flux, the inclination, and the radius of the circular part of the cross section. As in the experiments, the separation improved with decreasing heat flux, which means the difference between the ethanol mole fractions between evaporator and condenser increased. Changing the inclination of the system did not lead to changes in the distribution of the component mole fractions in the system. With decreasing the radius of the circular part of the capillary structure cross section, an improved separation was observed. Thus, an influence of the capillary structure design on the separation was shown.

Place of Publication: Darmstadt
Divisions: 16 Department of Mechanical Engineering
16 Department of Mechanical Engineering > Institute for Technical Thermodynamics (TTD)
16 Department of Mechanical Engineering > Institute for Technical Thermodynamics (TTD) > Interfacial Transport & Complex Wetting
Date Deposited: 12 Apr 2020 19:55
DOI: 10.25534/tuprints-00011447
Official URL: https://tuprints.ulb.tu-darmstadt.de/11447
URN: urn:nbn:de:tuda-tuprints-114476
Referees: Gambaryan-Roisman, Apl. Prof. Tatiana and Stephan, Prof. Dr. Peter and Hardt, Prof. Dr. Steffen
Refereed / Verteidigung / mdl. Prüfung: 28 January 2020
Alternative Abstract:
Alternative abstract Language
Destillationen sind essenziell in der chemischen Industrie. Um die Produktion kleiner Mengen von Spezialchemikalien leisten zu können, sind modulare Produktionsanlagen in den Fokus gerückt. Die Gravidestillation ist ein kleinskaliger Destillationsprozess, der mit hohen Trennraten modular eingesetzt werden kann. Anstatt der Gravitation werden Kapillarkräfte genutzt, um die Fluidströmung aufrechtzuerhalten. Um den Prozess industriell nutzen zu können, muss der Einfluss der Betriebsparameter und der Kapillarstruktur auf die Prozessvariablen verstanden werden. In dieser Arbeit werden Ergebnisse von Experimenten und Simulationen zur Gravidestillation vorgestellt und diskutiert. Mit dem Versuchsaufbau, der mit einer Flat Heat Pipe vergleichbar ist, können die Wand- und Dampftemperaturen, die Zusammensetzung am Kondensator, der Druck und die Form der Phasengrenzfläche gemessen werden. Zwei Kapillarstrukturen wurden mit Wasser-Ethanol-Gemischen bei unendlichem Rückfluss untersucht. Für die detailliert untersuchte dreieckige Rillenstruktur wurden der mittlere Ethanolanteil, die Wärmestromdichte und die Neigung des Systems variiert. Hierbei stehen negative Winkel dafür, dass der Kondensator höher liegt als der Verdampfer. Die Trenneffizienz, definiert als die Differenz zwischen dem Ethanolanteil am Kondensator und dem mittleren Ethanolanteil, nahm mit abnehmendem mittleren Ethanolanteil, mit abnehmender Wärmestromdichte und abnehmendem Neigungswinkel zu. Für die fortgeschrittene rechteckige Rillenstruktur wurden drei Fluidzusammensetzungen untersucht. Durch den Vergleich der zwei Rillenstrukturen konnte nicht eindeutig identifiziert werden, welche Struktur eine Trennung begünstigt. In dem theoretischen Modell werden Wärmetransport, Stofftransport und Hydrodynamik für kanalförmige Kapillarstrukturen gekoppelt. Im Gegensatz zu existierenden Modellen für poröse Medien wird der Einfluss der Veränderung des von der Flüssigkeit eingenommenen Querschnitts entlang der Strömungsrichtung berücksichtigt. Zusätzlich wird die Wärmeleitung in der Kanalwand betrachtet. Mit dem theoretischen Modell der Gravidestillation wurde eine Re-Entrant-Kanalstruktur für variierte Wärmestromdichten, Neigungen und Radien des kreisförmigen Teils des Kanalquerschnitts untersucht. Wie im Experiment verbesserte sich die Trennung für eine abnehmende Wärmestromdichte, was bedeutet, dass die Differenz zwischen den Ethanolstoffmengenanteilen im Verdampfer und denen im Kondensator zunahm. Für variierte Neigungswinkel konnte keine Veränderung in der Trennung beobachtet werden. Eine Abnahme des Radiusses des kreisförmigen Teils des Kanalquerschnitts führte zu einer verbesserten Trennung. Dadurch konnte ein Einfluss der Kapillarstruktur belegt werden.German
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