1 Introduction and Aim of Work
A journey of a thousand miles begins with a single step.
Lao Tzu In daily life as well as in technical and industrial applications, interfaces play an important role in many processes. It is worth noting that dealing with the interface between two phases is unavoidable and in general, the properties of an interface will be affected by physical or chemical changes in either or both of the phases involved. An adequate knowledge and understanding of the interfacial phenomenon provide an advantageous basis in improving and optimizing a process.
In this thesis, several core disciplines such as thermodynamics, interfacial phenomena, surface physical chemistry, wetting, intermolecular long-range interaction and fluid dynamics are combined and observed from an engineering point of view in order to serve as useful tools in creating a link between the natural and the engineering sciences. The main interest of this thesis is to provide the raw, basic principles derived from the natural sciences to be used in the high pressure engineering field.
High pressure engineering, mainly those processes which use carbon dioxide as a solvent, has become more popular [26,67,104,111,116,131,153,155,177]. However, due to the non-inert property of carbon dioxide and the combination of both the interfacial phenomena and high pressure engineering such as supercritical extraction using carbon dioxide, some unanswered questions occur. These questions are closely related to the optimisation of the processes for which precise answers are necessary in order to have an optimally designed column. Some typical columns used in process engineering are tray columns, spray columns, random and structured packing columns. In spray columns, the liquid drop size in the continuous phase (either in a gaseous or a liquid phase) decides the ratio of volume to exchange area [114] and thus, the effectiveness of the heat and mass transfer between the coexisting phases. The liquid drop size is in turn, controlled by its interfacial tension [35,46,51,64,82,131,165,194,197].
Therefore liquid interfacial tensions against carbon dioxide, both gaseous and supercritical, are measured and reported in this work.
Packing columns are developed with the objective to provide a large exchange area between the coexisting phases coming into touch with each other. This is achieved by forming a film phase which flows down the solid material. However, the existence of the solid material to be wetted does not guarantee the formation of a wide covering film. The latter is controlled solely by the wetting characteristics of the sytems at the given operating conditions [5,95,132,150,160,178].
In order to provide an adequate answer to this issue, the wetting characteristics of fluid-solid-liquid systems are investigated intensively for both static (single sessile drop) as well as dynamic systems (falling film).
In this thesis, the wetting behaviour of various systems which are relevant for industrial applications is studied. The investigation of the wetting ability comprises the study of a single,
1 Introduction and Aim of Work
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sessile drop which rests on a horizontal surface and, macroscopically seen, is stationary with respect to the continuous phase, and the falling film which, compared to a single liquid drop, contains a larger amount of liquid and conducts a relative motion towards the embedding continuous phase.
The interfacial phenomena study consisting of one single drop is started in Chapter 2 where the interfacial tension of liquid-vapour is explained thermodynamically. The measurement method and the results of the measurements containing pressurized carbon dioxide are also given there.
The wetting behaviour of various systems containing a horizontal solid surface, a drop phase and an embedding fluid is reported in Chapter 3. Here, the wettability of a system is given in terms of contact angle. Data from the interfacial tension of liquid-vapour and the contact angle are correlated in a single equation known as Young equation. In Chapter 4 the physical chemistry of the surface and the long-range intermolecular interaction come into play. A second, required correlation consisting of the vapour, the liquid-vapour and the solid-liquid interfacial tension is developed and combined with the initial Young equation in order to estimate the magnitude of the solid-vapour interfacial tension. This second correlation is initially not for high pressure use. Some cautious adaptations, especially in the choice of the molecular properties required, are conducted here with the hope of obtaining the right value of the solid-vapour interfacial tension and so that this quantity can provide the right prediction of the wetting behaviour of a given system.
From Chapter 5 to 9, attention is given to the falling film where the relative motion between the film and the continuous phase is not negligible. To begin with, in Chapter 5 the dimension of the film is measured and both, the thickness and the width of the falling film, are consolidated in one single measure, called the wetting angle [106].
Due to the relative motion of the falling film and the continuous phase, it is necessary to discuss the fluid dynamics. In Chapter 6 two extreme models, the Nusselt and the wall models are introduced. The Nusselt model is known in the literature and applied for the design of falling film apparatus. Here, the shear stress-free state at the film surface is assumed [13,22,108,135,138]. However, working with supercritical carbon dioxide, it is interesting to find out whether the assumption is still justifiable. In case the assumption does not meet the reality, it should be found out how this supercritical phase affects the fluid dynamics of the liquid phase which moves relatively to it.
The wall model describes a state where the shear stress at the film surface is maximum and thus, is the other extreme case. Hence, a third, more universally applicable model is required since the given operating conditions are not always under extreme conditions. The tau model which can be employed on both a wide covering film and a narrow rivulet, is explained and derived in Chapter 6. The mean velocity of the falling film according to these three different calculation models is given and compared with the experimental measured value in Chapter 7.
The tau model postulates the existence of a velocity boundary layer between the film and the continuous phase, and a finite shear stress exerted by the continuous phase on the film phase
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due to the relative motion and the large density of the fluid used as the continuous phase (larger friction at the film surface). The thickness and the fluid dynamics in the velocity boundary layer are explained in Chapter 8. In both Chapter 6 and 8, the colour-coded simulation results show the velocity profile in the film and the boundary layer respectively. In the last chapter, the magnitude of the over-the-surface averaged shear stress is shown.
For further investigation of the wetting behaviour and the fluid dynamics under high pressure conditions, experiments with systems containing Teflon, glass, steel (solid material), water, ethanol (liquid phase) and carbon dioxide (continuous phase) are conducted at temperatures up to 373 K and pressures up to 27 MPa.
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