Membrane processes can be employed for the separation of liquid, vapourous or gaseous mixtures. Comparing to the conventional competitors, membrane processes are often advantageous due to their low energy consumption, simple process design and small footprint. Often membrane processes can be advantageously combined with other unit operations to hybrid processes. Applications of these processes can be found in the chemical and petrochemical industries, in renewable and conventional energy technology, in biotechnology, in environmental technology, in the food industry and in the pharmaceutical industry.
The department “Process Engineering” concentrates on the membrane processes gas permeation, vapour permeation, pervaporation, and solvent nanofiltration.
The polymeric membranes employed in these processes work according to the solution-diffusion principle. This principle necessitates the application of a driving force across the thin, dense polymer, i.e. the selective layer of the membrane. The driving force causes an absorption of the components present in the feed mixture into the polymer matrix and a subsequent diffusion of these components in their dissolved state to the backside of the membrane where they are desorbed. The separation is caused by the different interactions of the various components of the feed mixture with the membrane polymer in the solution and diffusion steps.
Essential for successful implementation of research results into industrial applications is that the entire research and development chain is processed holistically. In the department of “Process Engineering” the following aspects are considered:
The research focus of our department:
- Membrane production
- Development of membrane modules
- Modeling and simulation
- Investigation of permeation properties
- Pilot plant investigations
- Optimisation of membrane materials
- Examples of investigated applications
Flat membranes are manufactured in 100 m2 scale. These membranes are prepared by the phase inversion principle. This results in porous sponge-like structures, which may form a dense film on the surface. Most of membranes developed by the Helmholtz-Zentrum Geesthacht are composite membranes. These are produced by coating porous membranes prepared according to the principle of phase inversion with one or more dense polymer films in a second step. The layer thickness can be reduced to 70 nm, which allows the production of high-flux membranes, as is required for many applications discussed today.
For converting the intrinsic separation properties of a membrane material into the technical process with only minimal losses, process equipment has to be designed accordingly. The design of these membrane modules aims at installing as much membrane area as possible per unit module volume. Additionally the minimisation of effects influencing the transport of substances through the membrane negatively, for example pressure losses and concentration boundary layers, are a prime concern.
For the design of membrane processes simulation models are employed. The simulation models describe the properties of the membrane modules as flow patterns, pressure losses, velocity, concentration and temperature profiles and mass transfer through the membrane as a function of pressure, temperature and composition.
Mathematically this leads to a system of ordinary differential and partial differential equations as well as algebraic equations. These equations are transferred to appropriate software and implemented in commercial process simulators. Thus it becomes possible to display the behaviour of membrane modules in the context of the overall process. Here, both the interconnection of individual membrane modules, including the necessary units for temperature and pressure changes as well as the combination of membrane processes with other unit operations to hybrid processes are examined.
The fundamental understanding of the effects contributing to the transport through membrane materials is essential to evaluate their operating performance. Automated apparatuses are employed for measuring the single gas permeation behavior of membrane materials as a function of temperature and pressure. The experimental data is described by means of permeation models the parameters of which are determined from the data. Gas mixtures are investigated using especially designed experimental set-ups. One target is the description of the multicomponent permeation behavior by means of single gas permeation data and to validate this description by experiments. Analogous approaches are employed for the investigation of liquid phase processes as pervaporation and solvent nanofiltration.
The pilot plant infrastructure of the department is used to investigate the operating performance of newly developed membrane materials installed in membrane modules under conditions closely resembling those of industrial application. The feed mixtures are synthetically produced or supplied by project partners. Pilot plants are also operated on site at research partners, the feed streams are taken directly from the industrial process.The investigations aim at the selection of the optimum process conditions with respect to pressure, temperature and composition ranges, the proof of long term stability of the membrane material for a selected application, the evaluation of different membrane module concepts and the validation of simulation models.
The targeted modification of membrane materials, the selection of appropriate solvent systems and additives for membrane production as well as the development of processing routes for membrane production are in the focus of this activity. The aim is to achieve better processing and improved separation characteristics of the considered polymers and polymer blends. Furthermore, mixed matrix membranes are considered where inorganic compounds are added to the membrane polymer in order to achieve a separation performance and stability superior to pure polymer membranes. The activities are conducted in close collaboration with the other departments of the institute.