Compressed fluid systems, which are handled in high pressure process technology, feature diffusivities smaller than the kinematic viscosity. This implies that during mixing the smallest hydrodynamic mixing layers are thick compared to the diffusivity mass transfer layer. In other words, the lifetime of micro-scale inhomogeneities exceeds that one of macro-scale inhomogeneities. The “zebra pattern” in Figure 1 shows a macro-scale homogeneous system still exhibiting micro-scale inhomogeneities. This means that after a certain progress of mixing the black and white fluids are mixed on a macro-scale only (they appear grey), but are still separated on a micro-scale (they appear black and white in the zebra pattern).
Chemical engineering processes such as reactions and phase-transition- or phase-separation mechanisms rely on intermolecular interactions and thus themselves also take place on scales smaller than the macro-scale (sub-macro-scale). Therefore their functioning chain is governed not by the degree of macro-scale- but by the degree of micro-scale homogeneity.
A first example; If particle precipitation starts in a system NOT homogeneously mixed on the micro-scale, particles would be formed from highly supersaturated and less supersaturated regions implying usually undesired broad particle size distributions and even particles with different morphologies and polymorphs.
As second example; If combustion starts in a system NOT homogeneously mixed on the micro-scale, there would be regions of “lean” and “fat” reaction implying an increase in undesired pollutant emissions and a decrease in the energy conversion efficiency.
In detail and in the context of the high pressure process technology relevant for this project and relevant for society, micro-scale inhomogeneities influence the functioning chain of
• the particle generation from sprays at elevated pressure, e.g. in supercritical antisolvent technology,
• the reaction, e.g. in high pressure spray/jet combustion,
• the pressure induced phase-separation mechanisms, e.g. in surfactant-free CO2-based micro-emulsions used as nano-reactors or nano-crystallizers,
• and the pressure induced phase-transition mechanisms, e.g. during the formation of gas hydrates relevant in the context of preventing natural gas pipeline blocking, deep sea methane recovery and CO2 storage.
Summarizing, micro-scale inhomogeneities govern the functioning chain of high pressure processes and the herewith produced products from which our society benefits from. Therefore, this research project aims at developing a comprehensive understanding of the impact of micro-scale inhomogeneities in compressed systems onto the respective high pressure processes and the herewith produced products.
