Over the past two decades, light sheet fluorescence microscopy has developed to a powerful tool for studies of dynamics in biological systems. In a new development, we apply light sheet fluorescence microscopy as a novel experimental measurement technology within the field of multiphase process engineering and fluid dynamics. The technology enables a number of novel studies of single- and multiphase transport phenomena on a microscopic scale. In the current publication, we introduce the first implementation of light sheet fluorescence microscopy in the field and demonstrate its applicability on one exemplary measurement of diffusive oxygen mass transfer from an oxygen bubble to degassed water. The results prove that such measurements can be conducted with a high spatial resolution with a submicron pixel pitch and enable precise studies on microscopic transport phenomena. Besides measurements of mass transfer, on which we lay the focus here, light sheet fluorescence microscopy further enables studies of fluid dynamics on a microscopic scale.

Spatial oxygen limitation is a major source of physiological stress and metabolic, scale-dependent heterogeneity in aerobic bioprocesses. In gas-liquid stirred tank reactors, oxygen availability is governed by hydrodynamics and mass transfer and is therefore inherently non-uniform. Despite its importance, experimentally accessing spatial information on oxygen local supply remains challenging, as most established techniques rely on point-wise sensing or intrusive tracer-based methods. This work presents an optical method for visualizing local oxygen limitation based on the reversible redox chemistry of methylene blue in alkaline glucose solutions. After driving the reactor into a uniformly reduced, colorless state, the spatial reoxidation of the indicator during oxygen aeration is recorded and evaluated pixel-wise. The resulting two-dimensional fields of colorization time provide a spatially continuous indicator of local oxygen supply governed by hydrodynamic transport and gas-liquid mass transfer. Systematic experiments were conducted to assess the influence of glucose and sodium hydroxide concentration on the stability and reproducibility of the redox system, enabling the selection of a robust reaction medium. The method was then applied to a laboratory-scale stirred tank operated at different impeller speeds. The resulting colorization fields reveal characteristic spatial structures associated with oxygen-limiting regions, including insufficiently supplied zones under weak agitation, progressive homogenization with increasing turbulence, and residual transport limitations near reactor walls at high agitation rates. The presented approach requires no local tracer injection and operates under normal aeration conditions, providing qualitative yet physically meaningful insight into oxygen-supply and mixing related heterogeneities. It therefore offers a simple and cost-effective tool for identifying limiting operating points, supporting future scale-transfer studies, and complementing CFD-based analyses of oxygen transport in stirred tank bioreactors.

In the vast majority of gas–liquid engineering applications, the liquid phase contains a range of dissolved gaseous species. These dissolved process gases transfer from the liquid phase to the gaseous phase countercurrent to the typically desired mass transfer of gas to liquid. In process design, the resulting change in the composition of the gaseous phase is usually neglected, although a temporal change in the composition of the gaseous phase can directly influence the mass transfer performance over time. The current fundamental study quantifies the mass transfer performance of oxygen bubbles to liquid phases saturated with another gas. For this purpose, the oxygen mass transfer from a bubble to degassed, helium-, nitrogen-, argon- and carbon dioxide-saturated water is studied. Light Sheet Fluorescence Microscopy is used as imaging system for Planar Laser-induced Fluorescence measurements of dissolved oxygen concentration fields, delivering local instantaneous Sherwood numbers, diffusion coefficients and mass transfer coefficients. For the first time, the study showcases that the mass transfer performance from a gaseous dispersed to a liquid continuous phase is independent of time if mass transfer occurs in one direction only. If mass transfer occurs in both directions, the mass transfer performance of the dispersed phase is significantly lower and its time dependence higher to liquids containing gaseous species with high solubilities, such as carbon dioxide in water. Furthermore, the results suggest that applying intrinsic values, such as diffusion or mass transfer coefficients, obtained for binary systems in multicomponent systems can lead to high uncertainty.

Triply Periodic Minimal Surface (TPMS) structures have emerged as a new class of porous materials with variable geometries and favourable transport properties, making them promising for reactor internals in chemical engineering. However, experimental data on internal TPMS flow behaviour are still limited. To address this gap, the flow behaviour in additively manufactured TPMS structures is analysed using three-dimensional Magnetic Resonance Imaging (MRI) velocimetry in a large-bore vertical 3 T MRI system, in cylindrical columns of 38mm diameter and Reynolds numbers between 50 and 300. Three different TPMS geometries are investigated, and consistency between Computational Fluid Dynamics (CFD) simulations and experimentally measured MRI velocity fields is established through cross-validation. The MRI system provides fully three-dimensional velocity fields with a divergence deviation below 4%. MRI revealed distinct flow features: the Gyroid TPnS exhibited pronounced channelling, while the Schwarz-Diamond TPSf showed merge-split behaviour, achieving a 46% increase in lateral mixing compared to the Gyroid TPnS structures. Numerical simulations reproduce the flow features and show agreement with the MRI data. The combined methodology demonstrates the suitability of MRI velocimetry for the experimental validation of CFD simulations and establishes a robust foundation for future studies of heat and mass transfer, as well as reactive flow, in structured reactor systems.

This study analyzes trajectories of three particle types in an industrial-scale bioreactor, equipped with a Rushton turbine and a pitched blade turbine, to characterize hydrodynamic compartments. The trajectories obtained from measurements with Lagrangian Sensor Particles (LSP,exp) are compared to those generated by Lattice-Boltzmann large eddy simulations (LB LES). The latter method is used to reproduce analogous simulated LSPs (LSP,sim) as resolved particles. Additionally, for benchmarking purposes, massless tracer particles (tracer,sim) are incorporated to accurately represent fluid flow dynamics. Discrepancies in the axial probability of presence and velocity between LSP,exp and LSP,sim likely stem from differences in mass distribution, density, number of particles, and ratio of particle size to grid. A necessarily high LSP,sim volume fraction in LB LES leads to increased collisions and clustering, negatively impacting flow dynamics, and reducing turbulent kinetic energy by at least 3%. Circulation and residence time distributions for the three types of particles identify three hydrodynamic compartments within the bioreactor, validated by local mixing time distributions. The ratio of overall average circulation time to global mixing time is Θglob,95≈3.0⋅t¯circ for LSP,exp, which largely corresponds to literature results. A theoretical LSP size of dp,th≈1mm is estimated to be flow following on micro-scale in the bulk phase, if a Stokes number of St=0.1 is assumed. However, Stokes number estimations confirm that LSP,exp are capable to follow flow patterns on the meso-scale and macro-scale with St≈0.2 and St≈0.002, respectively. Hence, hydrodynamic structures at length scales greater than or equal to the size of the impeller can be investigated by current state-of-the-art LSPs, which proves their technological readiness for industrial bioreactors.

Utilizing pressure as a process parameter can make biotechnological processes more efficient and attractive compared to established ones. This paper presents a high-pressure reactor setup for enzymatically catalyzed gas-liquid reactions, which can be operated up to 15.0 MPa. The reactor is equipped with optical measurement technology for inline and in situ monitoring of the oxygen concentration under high-pressure conditions. The setup is characterized by assessing the influence of the process parameter pressure on the conversion of the glucose oxidation to d-glucono-delta-lactone by immobilized glucose oxidase. The study demonstrates that the increased oxygen availability due to higher solubility reduces the reaction time in a batch reactor from 270 to 90 min.

The Villermaux-Dushman protocol with UV-Vis analytics is an established tool to characterize the global micro mixing performance in process equipment. The local mixing process is described by a micro mixing model (incorporation model), originally designed for turbulent flows. The novel imaging UV-Vis spectroscopy used in this work uncovers locally resolved micro mixing phenomena in a laminar split-and-recombine (SAR) mixer unit manufactured from selective laser-induced etching (SLE). The local absorbance is recorded with a high spatial resolution camera through a telecentric lens and post-processed into local concentration fields of components. The method unveils discrepancies of the micro mixing time determined from the conventional incorporation model in laminar flow and the locally recorded mixing process. The micro mixing time rather needs to be seen as a mean micro mixing time instead. Furthermore, the spatial information obtained by the imaging UV-Vis spectroscopy gives insight into local micro mixing and selectivity, yielding new approaches to equipment optimization.

This study employs Lagrangian Sensor Particles (LSPs) with a diameter of 40 mm equipped with a pressure sensor to investigate cell lifelines in a 15,000L stirred tank reactor (STR) with three Elephant Ear impellers. The Stokes number of the LSPs is approx. 0.004 on a macro-scale. The vertical probability of presence, axial velocity profiles, circulation time distributions, and residence time distributions are quantified to analyze single-phase mixing heterogeneities, detect hydrodynamic compartments and conduct a Lagrangian regime analysis. Results reveal a similarly distributed probability of presence in the vertical reactor center but emphasize the LSP’s sensitivity to fluctuating densities. Axial velocity distributions illustrate characteristic impeller-induced flow patterns, and circulation time distributions identify three compartments with comparatively shorter times in the axial center. Residence time distributions exhibit a similar compartmentalized profile. Moreover, the study estimates a potential oxygen deprivation zone for CHO cells in the upper compartment and demonstrates the LSP’s efficacy in characterizing impeller systems. Contrary to literature, the ratio of examined global mixing times to circulation times is 1.0, highlighting macro-scale mixing. The research underscores that LSPs offer crucial insights into industrial-scale STRs, specifically for determining hydrodynamic compartments without having optical access.

The reliable transfer of bioprocesses from single-use bioreactors (SUBs) of different scales to conventional stainless steel stirred-tank bioreactors is of steadily growing interest. In this publication, a scale-up study for SUBs with volumes of 200 L and 2000 L and the transfer to an industrial-scale conventional stainless steel stirred-tank bioreactor with a volume of 15,000 L is presented. The scale-up and transfer are based on a comparison of mixing times and the modeling of volumetric mass transfer coefficients kLa, measured in all three reactors in aqueous PBS/Kolliphor solution. The mass transfer coefficients are compared with the widely used correlation of van’t Riet at constant stirrer tip speeds. It can be shown that a van’t Riet correlation enables a robust and reliable prediction of mass transfer coefficients on each scale for a wide range of stirrer tip speeds and aeration rates. The process transfer from single-use bioreactors to conventional stainless steel stirred-tank bioreactors is proven to be uncritical concerning mass transfer performance. This provides higher flexibility with respect to bioreactor equipment considered for specific processes.

Background Transjugular intrahepatic portosystemic shunt (TIPS) dysfunction in patients with liver cirrhosis and recurrent symptoms of portal hypertension is primarily assessed with US and confirmed with invasive catheter venography, which can be used to measure the portosystemic pressure gradient (PSPG) to identify TIPS-refractory portal hypertension. To avoid the risks and costs of invasive catheter venography, noninvasive PSPG evaluation strategies are needed. Purpose To demonstrate the feasibility of the combination of four-dimensional (4D) flow MRI with computational fluid dynamics (CFD) for noninvasive PSPG assessment in participants with cirrhosis and TIPS. Materials and Methods Abdominal 4D flow MRI was performed prospectively in participants with cirrhosis and TIPS between January 2019 and September 2020. Flow rates were measured within the TIPS and inferior vena cava (IVC). The portal vein (PV), TIPS, right hepatic vein, and IVC were segmented on MRI scans to create a CFD mesh. The PV and infrahepatic IVC were defined as inflows for 4D flow MRI-derived flow rates. The suprahepatic IVC was defined as the outflow. CFD simulations were used to noninvasively estimate PSPG as the difference between the simulated pressures in the PV and suprahepatic IVC. Invasive venographic measurements of the PSPG served as the reference standard, and Pearson correlation analysis was conducted to evaluate the relationship between noninvasive estimates and invasive measurements. Results In all 20 participants with cirrhosis (mean age, 58 years ± 9 [SD]; 11 men), 4D flow MRI-based CFD simulations enabled visualization of flow velocities and pressure distributions within the segmented vasculature and TIPS. Noninvasive estimates and invasive measures of PSPG were strongly correlated (r = 0.77; P < .001). The 4D flow MRI-based CFD simulations correctly classified the presence or absence of a post-TIPS PSPG greater than 12 mm Hg in 16 of 20 participants (80%). Conclusion The combination of 4D flow MRI and CFD was feasible for noninvasive PSPG assessment in participants with cirrhosis, portal hypertension, and TIPS. © RSNA, 2024 See also the editorial by Motosugi and Watanabe in this issue.

The growing demand for bio-pharmaceuticals necessitates improved methods for the characterization of stirred tank reactors (STRs) and their mixing heterogeneities. Traditional Eulerian measurement approaches fall short, culminating in the use of Lagrangian Sensor Particles (LSPs) to map large-scale STRs and track the lifelines of microorganisms such as Chinese Hamster Ovary cells. This study investigates the hydrodynamic characteristics of LSPs, specifically examining the effects that the size and position of the Center of Mass (CoM) have on their flow-following capabilities. Two Lagrangian Particle (LP) designs are evaluated, one with the CoM and a Geometric Center aligned, and another with a shifted CoM. The experimental study is conducted in a rectangular vessel filled with deionized water featuring a stationary circular flow. Off-center LPs exhibit higher velocities, an increased number of floor contacts, and moreover, a less homogeneous particle probability of presence within the vessel compared to LPs with CoM and Geometric Center aligned. Lattice Boltzmann Large Eddy Simulations provide complementary undisturbed fluid velocity data for the calculation of the Stokes number St. Building upon these findings, differences in the Stokes number St between the two LP variants of ΔSt = 0.01 (25 mm LP) and ΔSt = 0.13 (40 mm LP) are calculated, highlighting the difference in flow behavior. Furthermore, this study offers a more representative calculation of particle response time approach, as the traditional Stokes number definition does not account for non-homogeneous particles, resulting in an alternative Stokes number (ΔStalt = 0.84 (25 mm LP) and ΔStalt = 2.72 (40 mm LP)). This study contributes to the improved characterization of STRs through the use of Lagrangian Sensor Particles. Results highlight the implications the internal mass distribution has on LSP design, offering crucial considerations for researchers in the field.

Four-dimensional (4D) flow cardiovascular magnetic resonance (CMR) is feasible for portal blood flow evaluation after placement of transjugular intrahepatic portosystemic shunts (TIPS) in patients with liver cirrhosis. However, clinical acceptance of 4D flow CMR in TIPS patients is limited due to the lack of validation studies. The purpose of this study was to validate 4D flow CMR-derived measurements in TIPS stent grafts using a three-dimensional (3D)-printed flow phantom. A translucent flow phantom of the portal vasculature was 3D-printed. The phantom consisted of the superior mesenteric vein and the splenic vein draining into the portal vein, the TIPS-tract, and the hepatic vein. A TIPS stent graft (Gore® Viatorr®) was positioned within the TIPS-tract. Superior mesenteric vein and splenic vein served as inlets for blood-mimicking fluid. 4D flow CMR acquisitions were performed at 3T at preset flow rates of 0.8 to 2.8 l/min using velocity encoding of both 1.0 and 2.0 m/s. Flow rates and velocities were measured at predefined levels in the portal vasculature and within the stent graft. Accuracy of 4D flow CMR was assessed through linear regression with reference measurements obtained by flow sensors and two-dimensional (2D) phase contrast (PC) CMR. Intra- and interobserver agreement were assessed through Bland–Altman analyses. At a velocity encoding of 2.0 m/s, 4D flow CMR-derived flow rates and velocities showed an excellent correlation with preset flow rates and 2D PC CMR-derived flow velocities at all vascular levels and within the stent graft (all r ≥ 0.958, p ≤ 0.003). At a velocity encoding of 1.0 m/s, aliasing artifacts were present within the stent graft at flow rates ≥ 2.0 l/min. 4D flow CMR-derived measurements revealed high intra- and interobserver agreement. The in vitro accuracy and precision of 4D flow CMR is unaffected by the presence of TIPS stent grafts, suggesting that 4D flow CMR may be used to monitor TIPS patency in patients with liver cirrhosis.

The detection of compartments and dead zones as well as the estimation of the mixing efficiency in stirred tanks are of vital interest for a variety of biochemical and chemical processes. Here, numerically derived time-dependent 3D fluid velocity fields of a stirred tank reactor are computed using the Lattice Boltzmann Method. Mixing in the stirred tank reactor is analysed by means of Lagrangian Coherent Structures which allow to unravel the mixing states of complex flows. This Lagrangian analysis is achieved by computing Finite Time Lyapunov Exponents and applying recent trajectory-based network methods on the three-dimensional flow. The results reveal a zone of low interaction in the upper region of the stirred tank reactor and five additional transient compartments. The trajectory-based network analysis detects low cross-mixing of fluid parcels between different compartments but a very high mixing of fluid parcels inside of each compartment. This high interaction is also found in an analysis of the Finite Time Lyapunov Mixing Intensity. Time-averaging of the fluid velocity field prior to the Lagrangian analysis is considered to extract the most influential Lagrangian Coherent Structures.

The transformation of energy-intensive batch processesfor small-scalepolymer specialties into more energy-efficient continuous processesis a challenging task due to the occurrence fouling. To determinesafe operating windows, a proper reactor model is needed. Indeed,a computational fluid dynamics (CFD) can resolve potential foulingzones; however, the computational demand of a detailed CFD of entirereactors is in general uneconomical for parameter studies, transientproblems, or complex reaction mechanisms such as polymerizations.Contrary, reduced basic reactor models, such as plug flow reactors,can be used for simulation studies of entire reactors due to theirlow computational demand but cannot resolve potential fouling zones.The compartment modeling approach offers a promising method to combinerelevant fluid dynamics from CFD models with the simulation of complexreaction mechanisms. In this contribution, a compartment model ofa 3 m milli-structured reactor for polymer synthesis is derived froma reduced CFD model. The computational CFD mesh of a section of thereactor is transformed into a cascade of continuously stirred tankreactors (CSTRs) by clustering the CFD cells based on the local meanage, allowing the spatial interpretation of fouling zones. The meanage theory is used to evaluate the fluid dynamics and mixing characteristicsof the derived compartment model and compare it with the CFD. Witha scale-up, the compartment model describes the entire residence timereactor and enables transient simulations and parameter studies witha significantly decreased computational demand. The compartment modelis validated with experimental data for a catalytic polymerizationprocess.

The Mean Residence Time is a quantity which is used very often to characterise chemical reactors. This quantity can be calculated using the Residence Time Distribution of a specific, continuous reactor and assuming closed-closed boundary conditions. Many reactors in the industry though, are discontinuous ones and the direct application of these methods is in general not possible. This work presents a new distribution, the Local Resi-dence Time Distribution, which observes each position in a specific reactor. Furthermore, two new, local quantities, the Mean Distribution Time and the Mean Dead Time, are introduced, which can be applied as characteristic times to evaluate the internal behaviour of chemical reactors used for discontinuous processes such as fermentations.

Modular milli- and micro-structured systems represent a promising approach to exploit the potential of micro-process technology, including precise reaction control and scale-up. A major drawback of micro-structured devices is fouling and mixing mechanisms need to be investigated phenomenologically to better understand the processes that lead to fouling. Previous work was conducted to resolve 3D concentration fields by means of Laser-Induced Fluorescence (LIF) using a Confocal Laser Scanning Microscope (CLSM) (Frey et al., J Flow Chem, 2021, 11, 599–609). While the CLSM-LIF method yields detailed insight into concentration fields down to a few micrometers, it is limited to stationary flow structures only. Aubin et al. (Chemical Engineering Science, 2010, 65, 2065–2093) give a comprehensive review of methods to analyze mixing behavior. Most recent optical measurement methods rely on the detection of a single compound in mixtures. In case of reactive mixing, Tthe state of the art procedures to locally visualize micro mixing relies on tracking a reaction product which forms on molecular scale. In literature, only small micro-structures are manufactured from transparent materials, however larger milli-structures often lack optical accesses with sufficient quality. Selective laser-induced etching (SLE) is a new technique which enables the fabrication of larger milli-structures in transparent materials that are relevant for industry-scale applications. This work develops a method based on a concept of Kexel et al. (Chemie Ingenieur Technik, 2021, 93, 830–837) visualizing the selectivity of a competitive-consecutive gas-liquid reaction in a Taylor bubble flow. The main goal of this work is the analysis of the absorbance spectra of bromothymol blue (BTB) at different pH values in a miscible liquid-liquid system in a fused silica split-and-recombine mixer. The milli-structure of the mixer is manufactured by means of SLE. Backlight at different wavelengths is pulsed matching the recording frequency. In contrast to conventional UV/Vis setups, the absorbance is recorded locally within the mixer. The proposed method yields the 2D concentration distribution of multiple species with high spatial resolution. The spatially resolved reactant and product distribution unveils micro mixing and can yield important information about local root causes of fouling.

In this study, three-dimensional flow measurements by means of 4D-Particle Tracking Velocimetry (4D-PTV) are carried out in a laboratory-scale 3 L stirred tank reactor in order to investigate the flow-following behavior of two different inertial particle types, Polyethylene (PE) particles and alginate beads, at different impeller frequencies. Applied particles mimic Lagrangian sensor particles, which are intended to determine process parameters such as oxygen concentration at their corresponding position inside a bioreactor. Accompanying Lattice-Boltzmann Large Eddy Simulations (LB LES) provide additional information about the fluid flow and the difference in the trajectories between inertial and non-inertial particles. The data acquired from LB LES is validated with the experimental data by means of a Lagrangian and a Eulerian approach. In their tail, the probability distributions show higher Lagrangian velocities and accelerations for 4D-PTV data compared to LB LES data. Time-averaged Eulerian data is utilized to determine particle Reynolds numbers lower than 200. The Stokes number distributions show 10-fold higher values for the alginate beads than for PE particles, however, both particle types do not sufficiently meet the criterion of a flow-following Stokes number of St≤0.01. Generally, time-averaged results from LB LES are in good accordance to the 4D-PTV data. From the LB LES, a theoretical, maximum particle diameter of approx. 20 μm is determined, which meets the criterion of St≤0.01 throughout the reactor. This result implies that with current sensor particle technology it is not possible to meet the flow-following behavior and depict the lifelines of cells during a cultivation process. Therefore, further research is necessary to understand particle trajectories and to translate them into lifelines of cells.

For the purpose of the intensification of an industrial-scale gas-liquid process, the implementation in an alternative reactor concept is investigated at Hamburg University of Technology (TUHH) in cooperation with Ehrfeld Mikrotechnik GmbH. Existing process operation data from a bubble column hint at a mass transfer limitation of the gas-liquid reaction. In the project, a jet loop reactor (JLR) is chosen to increase the specific interfacial area between gas and liquid, and thus increase mass transfer, while keeping the reactor system mechanically simple and low-maintenance. For the investigation, a laboratory scale reactor has been designed on the basis of an existing industrial scale process and scaled according to a pilot scale reactor available at TUHH. For scaling, geometric similarity is desired, while specific energy dissipation rate and volumetric gas input are kept constant for the chosen scale-up strategy. Between the two different scales, the reactors are successfully characterised in a water-air system with regards to the important mass transfer, among other parameters. A pressure- and chemical-resistant twin of the laboratory-scale reactor is provided to the project partner for trials under real process conditions with the original material system. The presented work shows that the JLR concept can be transferred sufficiently well between different scales when suitable parameters are chosen, and offers a wide operating window. The investigations aim to provide a basis for a future scale-up of the chemical process in the JLR system to the industrial scale.