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Micro-scale inhomogeneities in compressed systems and their impact onto the PROCESS- functioning-chain and the PRODUCT-characteristics

Periodic Reporting for period 4 - Inhomogeneities (Micro-scale inhomogeneities in compressed systems and their impact onto the PROCESS- functioning-chain and the PRODUCT-characteristics)

Reporting period: 2019-11-01 to 2020-04-30

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.
Within the project we (my group including me) first set up the new experiments at FAu Erlangen and later moved them to TU Bergakademie Freiberg. This included five experiments,
• one for the analysis of the micro-scale inhomogeneities in the supercritical antisolvent (SAS) process
• one for the analysis of micro-scale inhomogeneities in ethanol sprays under Diesel-relevant conditions
• one for the measurement of vapor-liquid-equilibria of mixtures of ethanol and air at Diesel-relevant conditions
• one for the analysis of micro-scale inhomogeneities in surfactant-free CO2-based micro-emulsions
• and one for the analysis of micro-scale inhomogeneities in gas hydrate forming systems
Though each experiment is composed of a rather complex high pressure process part and a rather complex optical measurement technique, we have been able to reliably operate the experiments and harvest profound measurement data.
The main results are:
• With respect to the supercritical antisolvent (SAS) technology we developed a measurement technique that is capable of quantifying simultaneously the progress of mixing on the macro- and the micro-scale. The measurement technique was applied to SAS-Jets for the measurement of the lag between macro and micro-mixing.
• With respect to the ethanol sprays at diesel relevant conditions, we quantified the temperature and the composition spatially and temporally resolved within the transcritical jet and won valuable insights into the prevalent mixing paths.
• The vapor-liquid-equilibria (VLE) measurements provided the spectral database for the quantification of Raman spectra and the thermodynamic database for fluid states of ignitable mixtures.
• With respect to the surfactant-free CO2 containing nanostructured fluids we have been collaborating with national research institutes situated in Barcelona and Rome. Within this collaboration we have been able to identify a new class of nanostructured fluids. This was able by combining our Raman expertise with the small angle neutron scattering expertise of Rome and the molecular dynamics computations expertise of Barcelona.
• With respect to the gas hydrate forming systems we investigated into the influence of the presence of thermodynamic inhibitors on the development of hydrogen bonds in the liquid phase before hydrate formation. We analyzed the influence of the presence of thermodynamic inhibitors on the generation of the hydrate phase (via inhomogeneities), the dissolution/melting of the solid phase and the existence of residual micro-bubbles/inhomogeneities after the complete melting of the hydrate phase.
There are no conventional measurement techniques available for this kind of in situ measurements required for reaching the objectives of this project. Therefore each step in the further development of the in situ Raman measurement techniques in each of the sub-projects of this project are beyond the state of the art.
As a consequence of the non-availability of commercial measurement equipment for these measurement conditions, each result obtained and each new insight in the high pressure processes is also beyond the state of the art.
(See list of publications)
Schematic of the existence of micro-scale inhomogeneities in macro-scale homogeneous systems during