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Enrichment of Components at Interfaces and Mass Transfer in Fluid Separation Technologies

Periodic Reporting for period 4 - ENRICO (Enrichment of Components at Interfaces and Mass Transfer in Fluid Separation Technologies)

Reporting period: 2021-04-01 to 2021-09-30

Techniques for separating fluid mixtures are important in many industries like the chemical and pharmaceutical industry. The most relevant of these separation techniques, distillation and absorption, are based on mass transfer through fluid interfaces. Results from molecular thermodynamics show that for many industrially important mixtures a strong enrichment of components occurs at the fluid interface. There is a striking congruence between shortcomings of the present design methods for fluid separations and the occurrence of that enrichment. It is the central hypothesis of the research in the ENRICO project that the enrichment leads to a mass transfer resistance of the fluid interface, which has to be accounted for in fluid separation process design. The fact that it is presently neglected causes unnecessary empiricism and inconsistencies in the design. ENRICO advances the knowledge on the enrichment of components at fluid interfaces using a novel combination of two independent theoretical methods, namely molecular simulations with force fields on one side and density gradient theory coupled with equations of state on the other side. This enables reliable predictions of the occurrence of the enrichment and its magnitude. These results are used to establish a model for the mass transfer resistance of the interface due to the enrichment. On that basis, a new approach for designing fluid separation processes is developed in ENRICO, which will enable more efficient and robust designs in industry. The theoretical results are validated by experiments from laboratory to pilot plant scale, and the benefits of the new approach will be demonstrated. ENRICO will thus establish a link between molecular physics and engineering practice. The results from ENRICO will have a major impact on chemical engineering worldwide and change the way fluid separation processes are designed.
The conditions for which the interfacial enrichment at vapor-liquid interfaces occurs were elucidated. Methods for predicting the enrichment, which cannot be measured directly, were developed and successfully tested. The predictions from molecular simulations and density gradient theory agree very well. This holds both for model systems, which were studied systematically, and real systems. Experimental studies of the surface tension and the relative adsorption were carried out to validate the models. A comprehensive data base was established that contains all available data on the enrichment. Based on the results from the present work, the enrichment can now be estimated easily for basically any real system. Overall, the work in ENRICO has led to a significantly improved understanding of the nanoscopic equilibrium properties of vapor-liquid interfaces.

Furthermore, the nanoscopic mass transfer through vapor-liquid interfaces was studied using molecular simulations. Two new non-equilibrium molecular dynamics (NEMD) simulation methods (steady state and instationary) were developed for this purpose. Using these methods, the influence of the enrichment on the mass transfer on the nanoscopic level was confirmed. The molecular simulations yielded, furthermore, a wealth of insights into nanoscopic non-equilibrium processes during mass transfer across vapor-liquid interfaces, such as rebound of particles from the interface. The nanoscopic mass transfer through interfaces was also investigated using a continuum model, which was based on a new formulation of the Cahn-Hilliard equations.

Different experiments were carried out to study whether these nanoscopic findings translate into a significant mass transfer resistance on the macroscale. A novel type of laminar jet apparatus, which can be operated at pressures up to 15 bar, as well as a new method to study gas-liquid mass transfer based on magnetic resonance imaging (MRI) were developed and applied for the investigations. For comparison, diffusion in bulk liquid phases was studied by pulse-field gradient nuclear magnetic resonance spectroscopy (PFG-NMR), accompanied by molecular simulation studies. While these experimental studies have yielded an important amount of useful information on diffusion in liquid mixtures, we were not able to give a proof of the influence of the enrichment of components at the vapor-liquid interface on the macroscopic mass transfer. This is no contradiction to the findings on the nanoscale, it basically indicates that the nanoscopic resistance is small compared to macroscopic effects. While this holds for cases with simple fluid dynamics (laminar or stagnant), we cannot exclude that the enrichment influences the mass transport in turbulent situations in which the enrichment may influence the surface renewal.

The scientific results from ENRICO were disseminated in a large number of presentations at scientific conferences and papers, as well as in workshops. Awareness has been created in the chemical industry for the enrichment at vapor-liquid interfaces as well as for the insights in separation processes that can be obtained with molecular simulations.
1) A significantly improved understanding of the nanoscopic properties of vapor-liquid interfaces was obtained. This holds, in particular, for the interfacial enrichment. The reasons for the enrichment are now clear, its relations to other interfacial properties were elucidated.
2) Furthermore, a comprehensive data base was established that contains all available data on the enrichment. We can now predict if the enrichment is significant in basically any given system and quantify it.
3) Molecular simulation methods for studying the mass transfer through vapor-liquid interfaces are now available. Both stationary and instationary mass transfer can be studied now.
4) A proof for the transport resistance due to the enrichment at vapor-liquid interfaces was given for the nanoscale.
5) Particle and continuum methods for predicting properties of mixtures at interfaces are now better understood.
6) The relations between nanoscopic and macroscopic mass transfer are now better understood.
7) Two new experimental methods for studying vapor-liquid mass transfer were developed and successfully tested: magnetic resonance imaging measurements and high-pressure laminar jet measurements.
New mass transfer model
Enrichment of components at vapor-liquid interfaces
Experimental studies of mass transfer with a laminar jet apparatus