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

Reporting period: 2018-01-01 to 2019-10-31

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 first scientific reporting period between May 2015 and October 2017 we (my group including me) first set up the new experiments. 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 after the first scientific reporting period 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. The respective results were published in the Journal “Chemical Engineering Sciences”. Since we also established a collaboration with a French research group that is expert in the numerical modelling of jets at high pressure conditions and try to further understand the experimentally measured phenomena by comparing them with numerical computations (not submitted for publication yet). In the meantime we also measured the lag between macro and micro-mixing at various pressures and temperatures and developed an understanding of the results (not submitted for publication yet).
• With respect to the ethanol sprays at diesel relevant conditions, we have already carried out two measurement campaigns at a diesel injector test bench. Based on the experiences we made in the first (not very successful) measurement campaign, we were able to significantly improve our Raman optical measurement technique. The quality of the results we were able to achieved in the second measurement campaign are first class and promise to be of outmost relevance for mixture generation at elevated pressures but especially for engine combustion. With the technique developed we measure spatially and temporally resolved the mixture composition in the spray/jet region of diesel injectors at diesel relevant conditions (10 MPa & 900 K) in specially developed chamber. Simultaneously we measured the shape of the Raman spectrum of the hydroxyl group of the ethanol molecules. From the shape we can extract information on whether the ethanol is still liquid, already gaseous or features supercritical properties. As the second measurement campaign is still in progress, the thorough evaluation of the results will follow (not submitted for publication yet).
• The vapor-liquid-equilibria (VLE) measurements are required for the correct interpretation of the Raman results that we have received during the second Diesel-measurement-campaign. Measuring VLE between a fuel (ethanol) and an oxidizer (air) is challenging because of the permanent risk of ignition. Therefore we have developed a special VLE-cell that enables the safe measurement of VLE of these fluid combinations, even at diesel relevant conditions. We accessed the performance of our experiment by comparing for well understood and well known fluid combinations our results with results published in literature. Due to deviations of our results from the literature results we have been modifying the experiment in various steps and improved the quality of the achieved results step by step(not submitted for publication yet).
• With respect to the surfactant-free CO2 containing nanostructured fluids we collaborate with national research
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.
The various articles the group has already published in renown journals, such as Physical Chemistry B, ACS Nano and Chemical Engineering Journal underline this.
Schematic of the existence of micro-scale inhomogeneities in macro-scale homogeneous systems during