Institute of Thermal Separation Processes

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Molecular Methods for Separation Processes

Dr.-Ing. S. Müller

 

Novel separation processes require non-volatile, stable and cheap solvents with high selectivities and capacities. To meet these objectives, our group develops innovative surfactant-based separation processes for the separation of complex mixtures. Furthermore, modern molecular methods are applied and extended for the investigation and design of these separation processes.

 

Surfactant based separation processes

Above the critical micellar concentration (cmc), surfactants form aggregates of different sizes and shapes (micelles). Due to their hydrophobic core and hydrophilic outer shell, micelles can dissolve hydrophobic components in aqueous media. The micellar phase can be separated by cloud point extraction (temperature induced phase separation of nonionic surfactants) or by membrane processing.

Based on these features, micellar systems are applied in reaction engineering, environmental technology and in the area of pharmaceuticals. In particular, they are applied to solubilize hydrophobic catalysts in aqueous solutions, to separate pollutants and to estimate the partition behavior of active substances in physiological systems. The environmental compatibility of many surfactants, easy availability as well as their high selectivities and capacities illustrates the potential of surfactants for separation processes.

 

 

Besides the selection of appropriate surfactants for a given system, parameters like temperature, pressure, pH and additives like alcohols and salts have a significant influence on the phase behavior of micellar solutions. A fundamental prerequisite for the establishment of surfactant based separation processes is the precise knowledge of the phase behavior of these systems. Our group also determines partition equilibria as well as liquid-liquid equilibria by means of ultrafiltration and micellar chromatography.

For example, in our lab we are working on the design of new in situ extraction of microalgae cultures by using mild extracting agents such as nonionic surfactants. Several of these surfactants (e.g., Triton X-114) are biocompatible with the green microalgae. This permits the design of in situ extraction process, directly from the cultivation broth. The possibility of simultaneous cultivation and extraction allow for a continuous counter current extraction of microalgae products using a stirred column. To increase the productivity, this laboratory process was scaled-up to a pilot application. An extraction plant for production of micellar microalgae product is currently operating at the BIQ algae house in Hamburg.

 

 

The good compatibility of nonionic surfactants with enzymes motivates the integration of enzymatic catalysis in surfactant-based separation processes. The application of extractive biocatalysts in aqueous micellar two-phase systems leads to a reduction of the unit operations. Moreover, the reaction yield can be increased due to the simultaneous separation of hydrophobic products. This type of process intensification is also applied as a continuous set-up.

 

 

Development of molecular methods

In addition to our experimental work, we apply and extend molecular methods to predict and understand the underlying process of separation processes. With these methods, the behavior and interaction of individual molecules can be studied. In engineering sciences they are just starting to be become an established tool (molecular engineering). With experiments, one obtains data which are often difficult to explain and lack a deep physical understanding. With molecular methods it is possible to gain a better understanding and furthermore, it is possible to predict macroscopic properties with these techniques. Nowadays, they can be applied to a wide variety of systems and problems. In our group we are mainly using molecular simulations (molecular dynamics and Monte Carlo) and COSMO-RS. These techniques are used to understand and design new separation processes.

Molecular dynamics (MD) simulations are computer simulations in which the motion of atoms followed over time. Whereby, the interactions of the atoms are modeled by molecular mechanics force fields. These force fields provide a set of functional forms and the corresponding parameters to describe inter- and intramolecular interactions. Due to the large number of interacting particles, these simulations are computationally demanding and have to be carried out on supercomputers. We are using MD simulations to study various systems, such as partitioning in biological membranes, self-assembly of surfactants, and complexation of cyclodextrins.

 

 

COSMO-RS is an established model to predict thermodynamic data. Based on the structure of participating molecules, partition equilibria in complex mixtures can be predicted a priori. In collaboration with Prof. A. Klamt, our group evaluates and extends the applicability of COSMO-RS for complex mixtures containing electrolytes. COSMOmic is an extension of COSMO-RS to anisotropic systems such as micelles and membranes. We are evaluating and extending COSMOmic to study solute partitioning in micellar systems and biological membranes. Based on the combination of MD simulations and COSMOmic, we are providing input data for COSMOmic on our web page.

 

 

Ongoing projects: