טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentDor Elad
SubjectAnalysis of Drop Transfer Mechanism through Thin
Membrane
DepartmentDepartment of Chemical Engineering
Supervisors Professor Simon Brandon
Professor Emeritus Abraham Marmur
Full Thesis textFull thesis text - English Version


Abstract

The need for economical, effective and stable separation processes is on the rise as the production rate of chemical products increases, while environmental regulations are simultaneously becoming tighter. Separation processes based on the difference in surface tension between the mixture components may address this problem in an effective way. A carefully designed surface tension based membrane will reject the mixture components while allowing one component to pass through. As a consequence, only a moderate energy investment, associated with transferring the desirable component through the membrane, is required.

An enormous amount of experimental work has been conducted in order to fabricate these kinds of membranes. According to the accepted mechanism, during the separation process a solution containing dispersed drops are in contact with an appropriate membrane. The continuous phase from the emulsion passes through the membrane while the passage of the dispersed drops is prevented. Theoretical models were established for calculating the maximum allowed pressure difference. In the case where the imposed pressure difference on the membrane exceeds that value, the dispersed drops will penetrate through the membrane as well.

Lately, experimental reports displayed an emulsion in contact with two different membranes. The continuous phase passes through one membrane while the dispersed phase passes through the second membrane. In addition, in other experimental works, the entire emulsion penetrates through a membrane. During that passage, an enlargement of the radius of the dispersed phase drop was observed. These phenomena are not explained by the accepted theoretical model for surface tension based separation. In this study, the construction of a more reliable model for surface tension based separation is attempted.

In order to do that, the focus of this thesis was placed on the transfer of a single dispersed drop through a thin membrane. First, the previously mentioned single drop penetration model was expand and used in an attempt to describe separation of the dispersed phase. The expanded model includes non-identical conditions on the membrane sides, gravity effects and non-ideal membrane surfaces.

By closely studying this expanded model, it was revealed that the regular drop penetration model fails to explain continuous separation of the dispersed phase and this model was disqualified.

Next, a new model for the surface tension separation procedure was derived. In this model, a continuous film covers the membrane surfaces and small liquid drops merge and disengage from it. Thermodynamic analysis of this model was conduct alongside calculation of the film resistance against external pressure difference. Evidence for the drop fusion was found in an experimental study. The drop coalescence and surfactant attendance phenomena are also explain by the film model.

In addition, an interesting phenomenon was observed. The center of mass of an equilibrium drop exposed to two different environments was observed to be placed in the higher pressure regime. As the pressure difference between the external fluids increases, the majority of the volume of the equilibrium drop "moves" to the higher pressure side of the membrane. A physical and mathematical explanation for this phenomenon was constructed.