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A special issue of ChemEngineering (ISSN 2305-7084).

Deadline for manuscript submissions: closed (10 August 2020) | Viewed by 42676

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Prof. Dr. Heike Karbstein

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Chemical & Process Engineering Department, Chair for Food Process Engineering, Karlsruhe Institute of Technology, Karlsruhe, Germany
Interests: emulsions; emulsion processing; high-pressure homogenization; biopolymers at interfaces; spray drying; extrusion; product design; formulation technologies
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Dear Colleagues,

Emulsions have received outstanding attention from scientists and technologists for many decades because of their easy preparation, low price, and versatile applications. These extend to a broad range, including foods and feeds, creams and lotions, paints and varnishes, encapsulation and controlled delivery of actives, drugs, and other chemicals. Emulsions as intermediate products are also found in the production of, e.g., polymer-based products.
A large selection of rotor-stator machines or high-pressure homogenizers is available for the production of emulsion-based products on a large industrial scale. Ultrasonic homogenizers or membrane systems are found for special applications. Microfluidic systems are established for development and research. Although a lot of development work has already been published in the field of process design, it is still difficult today to predict the process parameters required to obtain desired product properties. Knowledge is missed on local process conditions prevailing in an emulsifying plant, how these are influenced by process parameters, and how this knowledge can be used to produce desired, specific drop size distributions. New working tools as numeric flow simulation and high-resolution inline real-time process measurement techniques offer the opportunity to broaden our knowledge and make a big step forward in emulsion process design.
We are looking for contributions which describe new process analytical tools, results, and applications they offer for emulsion processing. Given your reputed experience and the outstanding impact of your previous publications in this field, we would very much appreciate your contribution in this Special Issue of ChemEngineering. Although the issue is mainly focused on new tools for analyzing emulsion processing machines, papers reviewing and updating the state of the art, as well as novel applications are also welcome.

Prof. Dr.-Ing. Heike Karbstein
Guest Editor

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Keywords

flow simulation
emulsion process design
process analytics
stress-related droplet deformation and break-up
local flow conditions and stresses

Published Papers (11 papers)

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Research

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21 pages, 7714 KiB

Open AccessArticle

Comparison of Experimental and Numerical Transient Drop Deformation during Transition through Orifices in High-Pressure Homogenizers

by Benedikt Mutsch, Peter Walzel and Christian J. Kähler

ChemEngineering 2021, 5(3), 32; https://doi.org/10.3390/chemengineering5030032 - 22 Jun 2021

Cited by 1 | Viewed by 1988

Abstract

The droplet deformation in dispersing units of high-pressure homogenizers (HPH) is examined experimentally and numerically. Due to the small size of common homogenizer nozzles, the visual analysis of the transient droplet generation is usually not possible. Therefore, a scaled setup was used. The droplet deformation was determined quantitatively by using a shadow imaging technique. It is shown that the influence of transient stresses on the droplets caused by laminar extensional flow upstream the orifice is highly relevant for the droplet breakup behind the nozzle. Classical approaches based on an equilibrium assumption on the other side are not adequate to explain the observed droplet distributions. Based on the experimental results, a relationship from the literature with numerical simulations adopting different models are used to determine the transient droplet deformation during transition through orifices. It is shown that numerical and experimental results are in fairly good agreement at limited settings. It can be concluded that a scaled apparatus is well suited to estimate the transient droplet formation up to the outlet of the orifice. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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31 pages, 22503 KiB

Open AccessArticle

Scaling of Droplet Breakup in High-Pressure Homogenizer Orifices. Part II: Visualization of the Turbulent Droplet Breakup

by Benedikt Mutsch, Felix Johannes Preiss, Teresa Dagenbach, Heike Petra Karbstein and Christian J. Kähler

ChemEngineering 2021, 5(2), 31; https://doi.org/10.3390/chemengineering5020031 - 18 Jun 2021

Cited by 12 | Viewed by 2986

Abstract

Emulsion formation is of great interest in the chemical and food industry and droplet breakup is the key process. Droplet breakup in a quiet or laminar flow is well understood, however, actual in-dustrial processes are always in the turbulent flow regime, leading to more complex droplet breakup phenomena. Since high resolution optical measurements on microscopic scales are extremely dif-ficult to perform, many aspects of the turbulent droplet breakup are physically unclear. To over-come this problem, scaled experimental setups (with scaling factors of 5 and 50) are used in con-junction with an original scale setup for reference. In addition to the geometric scaling, other non-dimensional numbers such as the Reynolds number, the viscosity ratio and the density ratio were kept constant. The scaling allows observation of the phenomena on macroscopic scales, whereby the objective is to show that the scaling approach makes it possible to directly transfer the findings from the macro- to the micro-/original scale. In this paper, which follows Part I where the flow fields were compared and found to be similar, it is shown by breakup visualizations that the turbulent droplet breakup process is similar on all scales. This makes it possible to transfer the results of detailed parameter variations investigated on the macro scale to the micro scale. The evaluation and analysis of the results imply that the droplet breakup is triggered and strongly influenced by the intensity and scales of the turbulent flow motion. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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23 pages, 4789 KiB

Open AccessArticle

Scaling of Droplet Breakup in High-Pressure Homogenizer Orifices. Part I: Comparison of Velocity Profiles in Scaled Coaxial Orifices

by Felix Johannes Preiss, Benedikt Mutsch, Christian J. Kähler and Heike Petra Karbstein

ChemEngineering 2021, 5(1), 7; https://doi.org/10.3390/chemengineering5010007 - 7 Feb 2021

Cited by 13 | Viewed by 3601

Abstract

Properties of emulsions such as stability, viscosity or color can be influenced by the droplet size distribution. High-pressure homogenization (HPH) is the method of choice for emulsions with a low to medium viscosity with a target mean droplet diameter of less than 1 µm. During HPH, the droplets of the emulsion are exposed to shear and extensional stresses, which cause them to break up. Ongoing work is focused on better understanding the mechanisms of droplet breakup and relevant parameters. Since the gap dimensions of the disruption unit (e.g., flat valve or orifice) are small (usually below 500 µm) and the droplet breakup also takes place on small spatial and time scales, the resolution limit of current measuring systems is reached. In addition, the high velocities impede time resolved measurements. Therefore, a five-fold and fifty-fold magnified optically accessible coaxial orifice were used in this study while maintaining the dimensionless numbers characteristic for the droplet breakup (Reynolds and Weber number, viscosity and density ratio). Three matching material systems are presented. In order to verify their similarity, the local velocity profiles of the emerging free jet were measured using both a microparticle image velocimetry (µ-PIV) and a particle image velocimetry (PIV) system. Furthermore, the influence of the outlet geometry on the velocity profiles is investigated. Similar relationships were found on all investigated scales. The areas with the highest velocity fluctuations were identified where droplets are exposed to the highest turbulent forces. The Reynolds number had no influence on the normalized velocity fluctuation field. The confinement of the jet started to influence the velocity field if the outlet channel diameter is smaller than 10 times the diameter of the orifice. In conclusion, the scaling approach offers advantages to study very fast processes on very small spatial scales in detail. The presented scaling approach also offers chances in the optimization of the geometry of the disruption unit. However, the results also show challenges of each size scale, which can come from the respective production, measurement technology or experimental design. Depending on the problem to be investigated, we recommend conducting experimental studies at different scales. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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16 pages, 5936 KiB

Open AccessArticle

Influence of Cavitation and Mixing Conditions on Oil Droplet Size in Simultaneous Homogenization and Mixing (SHM)

by Vanessa Gall and Heike P. Karbstein

ChemEngineering 2020, 4(4), 64; https://doi.org/10.3390/chemengineering4040064 - 9 Dec 2020

Cited by 2 | Viewed by 3040

Abstract

High-pressure homogenizers (HPH) equipped with a Simultaneous Homogenization and Mixing (SHM) orifice allow for inducing a mixing stream directly into the disruption unit. Previous studies show that by doing so, synergies between the unit operations “emulsification” and “mixing” can be used to save energy, e.g., in homogenization of dairy products, or to extend the application range of HPH. Up to now, process design has mainly been based on the trial and error principle due to incomplete understanding of flow conditions and droplet break-up in the SHM unit. This study aims at a higher level of understanding of cavitation and mixing effects on emulsion droplet size. Experimental data were obtained using a model emulsion of low disperse phase concentration in order to avoid coalescence effects. The different flow conditions are created by varying the process and geometric parameters of an SHM unit. The results show that the oil droplet size only depends on mixing conditions when the emulsion droplets are added in the mixing stream. Furthermore, a smaller oil droplet size can be achieved by reducing cavitation, especially for droplets fed in the high-pressure stream. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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14 pages, 4113 KiB

Open AccessArticle

Development of a Pressure Stable Inline Droplet Generator with Live Droplet Size Measurement

by Felix Johannes Preiss, Teresa Dagenbach, Markus Fischer and Heike Petra Karbstein

ChemEngineering 2020, 4(4), 60; https://doi.org/10.3390/chemengineering4040060 - 10 Nov 2020

Cited by 3 | Viewed by 3013

Abstract

For the research on droplet deformation and breakup in scaled high-pressure homogenizing units, a pressure stable inline droplet generator was developed. It consists of an optically accessible flow channel with a combination of stainless steel and glass capillaries and a 3D printed orifice. The droplet size is determined online by live image analysis. The influence of the orifice diameter, the mass flow of the continuous phase and the mass flow of the disperse phase on the droplet diameter were investigated. Furthermore, the droplet detachment mechanisms were identified. Droplet diameters with a small diameter fluctuation between 175 µm and 500 µm could be realized, which allows a precise adjustment of the capillary (Ca) and Weber (We) Number in the subsequent scaled high pressure homogenizer disruption unit. The determined influence of geometry and process parameters on the resulting droplet size and droplet detachment mechanism agreed well with the literature on microfluidics. Furthermore, droplet trajectories in an exemplary scaled high-pressure homogenizer disruption unit are presented which show that the droplets can be reinjected on a trajectory close to the center axis or close to the wall, which should result in different stresses on the droplets. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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15 pages, 4748 KiB

Open AccessArticle

Rheology and Catastrophic Phase Inversion of Emulsions in the Presence of Starch Nanoparticles

by Upinder Bains and Rajinder Pal

ChemEngineering 2020, 4(4), 57; https://doi.org/10.3390/chemengineering4040057 - 19 Oct 2020

Cited by 7 | Viewed by 5915

Abstract

Emulsions stabilized by solid nanoparticles, referred to as Pickering emulsions, are becoming increasingly important in applications as they are free of surfactants. However, the bulk properties and stability of Pickering emulsions are far from being well understood. In this work, the rheological behavior and catastrophic phase inversion of emulsions in the presence of starch nanoparticles were studied using in-situ measurements of viscosity and electrical conductivity. The aqueous phase consisting of starch nanoparticles was added sequentially in increments of 5% vol. to the oil phase under agitation condition to prepare the emulsions. The emulsions were water-in-oil (W/O) type at low to moderate concentrations of aqueous phase. At a certain critical volume fraction of aqueous phase, catastrophic phase inversion of W/O emulsion to oil-in-water (O/W) emulsion took place accompanied a sharp jump in the electrical conductivity and a sharp drop in the emulsion viscosity. The W/O emulsions were nearly Newtonian at low concentrations of aqueous phase. At high concentrations of aqueous phase, prior to phase inversion, the W/O emulsions exhibited a shear-thickening behavior. The O/W emulsions produced after phase inversion were shear-thinning in nature. The comparison of the experimental viscosity data with the predictions of emulsion viscosity model revealed only partial coverage of droplet surfaces with nanoparticles. With the increase in the concentration of starch nanoparticles (SNPs) in the aqueous phase of the emulsions, the phase inversion of W/O emulsion to O/W emulsion was delayed to higher volume fraction of aqueous phase. Thus SNPs imparted some stability to W/O emulsions against coalescence and phase inversion. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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15 pages, 1446 KiB

Open AccessFeature PaperArticle

Influence of the Emulsifier System on Breakup and Coalescence of Oil Droplets during Atomization of Oil-In-Water Emulsions

by Martha L. Taboada, Nico Leister, Heike P. Karbstein and Volker Gaukel

ChemEngineering 2020, 4(3), 47; https://doi.org/10.3390/chemengineering4030047 - 3 Aug 2020

Cited by 11 | Viewed by 4015

Abstract

Spray drying of whey protein-based emulsions is a common task in food engineering. Lipophilic, low molecular weight emulsifiers including lecithin, citrem, and mono- and diglycerides, are commonly added to the formulations, as they are expected to improve the processing and shelf life stability of the products. During the atomization step of spray drying, the emulsions are subjected to high stresses, which can lead to breakup and subsequent coalescence of the oil droplets. The extent of these phenomena is expected to be greatly influenced by the emulsifiers in the system. The focus of this study was therefore set on the changes in the oil droplet size of whey protein-based emulsions during atomization, as affected by the addition of low molecular weight emulsifiers. Atomization experiments were performed with emulsions stabilized either with whey protein isolate (WPI), or with combinations of WPI and lecithin, WPI and citrem, and WPI and mono- and diglycerides. The addition of lecithin promoted oil droplet breakup during atomization and improved droplet stabilization against coalescence. The addition of citrem and of mono- and diglycerides did not affect oil droplet breakup, but greatly promoted coalescence of the oil droplets. In order to elucidate the underlying mechanisms, measurements of interfacial tensions and coalescence times in single droplets experiments were performed and correlated to the atomization experiments. The results on oil droplet breakup were in good accordance with the observed differences in the interfacial tension measurements. The results on oil droplet coalescence correlated only to a limited extent with the results of coalescence times of single droplet experiments. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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16 pages, 2598 KiB

Open AccessFeature PaperArticle

Investigation of the Applicability of Raman Spectroscopy as Online Process Control during Consumer Milk Production

by Jasmin Reiner, Kristin Protte and Jörg Hinrichs

ChemEngineering 2020, 4(3), 45; https://doi.org/10.3390/chemengineering4030045 - 19 Jul 2020

Cited by 12 | Viewed by 4465

Abstract

Online detection of product defects using fast spectroscopic measurements is beneficial for producers in the dairy industry since it allows readjustment of product characteristics or redirection of product streams during production. Raman spectroscopy has great potential for such application due to the fast and simple measurement. Its suitability as online sensor for process control was investigated at typical control points in consumer milk production being raw milk storage, standardization, and heat treatment. Additionally, the appropriateness of Raman spectroscopy to act as indicator for product application parameters was investigated using the example of barista foam. To assess the suitability of a pure online system, the merit of Raman spectra was evaluated by a principal component analysis (PCA). Thereby, proteolytic spoilage due to the presence of extracellular enzymes of Pseudomonas sp. was detected and samples based on the applied heat treatment (extended shelf life (ESL) and ultra-high temperature (UHT)) could be separated. A correlation of the content of free fatty acids and foam stability with spectra of the respective milk samples was found, allowing a prediction of the technofunctional quality criterion “Barista” suitability of a UHT milk. The results underlined the suitability of Raman spectroscopy for the detection of deviations from a defined product standard of consumer milk. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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20 pages, 5667 KiB

Open AccessArticle

Investigation the Stability of Water in Oil Biofuel Emulsions Using Sunflower Oil

by Ravin S. Sahota and Sam M. Dakka

ChemEngineering 2020, 4(2), 36; https://doi.org/10.3390/chemengineering4020036 - 5 Jun 2020

Cited by 1 | Viewed by 3877

Abstract

Targets to reduce CO₂ emissions by 75% and NO_x emissions by 90% by 2050 in aviation have been set by The Advisory Council for Aviation Research and Innovation in Europe. Sustainable fuels, e.g., emulsified biofuel, have demonstrated promise in reducing emissions and greenhouse gases. The aim of this project is to investigate the stability of a water in oil emulsion using sunflower oil. The primary objective is to achieve an emulsion which is stable for at least 4 days, and the secondary objective is to investigate how altering the emulsification parameter values of the surfactant hydrophilic-lipophilic balance (HLB), energy density and sonotrode depth in an ultrasonication procedure can impact the stability. The stability of each emulsion was measured over a period of 14 days. The main outcome is that two of the 14 emulsions made remained stable for at least 14 days using a surfactant HLB of five, which proved to be the optimum value from those tested. The results also show that, by using the sonotrode in a higher starting position, emulsions achieved a greater stability. Furthermore, over-processing of the emulsion was determined, with the point of over-processing lying between an energy density of 75 and 200 W.s/mL. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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Review

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22 pages, 5018 KiB

Open AccessReview

The Importance of Interfacial Tension in Emulsification: Connecting Scaling Relations Used in Large Scale Preparation with Microfluidic Measurement Methods

by Karin Schroën, Jolet de Ruiter and Claire Berton-Carabin

ChemEngineering 2020, 4(4), 63; https://doi.org/10.3390/chemengineering4040063 - 7 Dec 2020

Cited by 33 | Viewed by 6004

Abstract

This paper starts with short descriptions of emulsion preparation methods used at large and smaller scales. We give scaling relations as they are generally used, and focus on the central role that interfacial tension plays in these relations. The actual values of the interfacial tension are far from certain given the dynamic behavior of surface-active components, and the lack of measurement methods that can be applied to conditions as they occur during large-scale preparation. Microfluidic techniques are expected to be very instrumental in closing this gap. Reduction of interfacial tension resulting from emulsifier adsorption at the oil-water interface is a complex process that consists of various steps. We discuss them here, and present methods used to probe them. Specifically, methods based on microfluidic tools are of great interest to study short droplet formation times, and also coalescence behavior of droplets. We present the newest insights in this field, which are expected to bring interfacial tension observations to a level that is of direct relevance for the large-scale preparation of emulsions, and that of other multi-phase products. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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34 pages, 3185 KiB

Open AccessReview

Experimental Methods for Measuring the Breakup Frequency in Turbulent Emulsification: A Critical Review

by Andreas Håkansson

ChemEngineering 2020, 4(3), 52; https://doi.org/10.3390/chemengineering4030052 - 14 Sep 2020

Cited by 8 | Viewed by 2879

Abstract

The growing interest in using population balance modeling to describe emulsification processes has spurred an interest in experimentally measuring the breakup frequency. This contribution classifies, compares, and critically reviews the different methods that have been suggested for measuring the breakup frequency, applicable to emulsification devices. Two major approaches can be seen in previous studies. The first is ‘single drop breakup experiment’-based studies, which estimate the breakup frequency by observing the fate of individual drops. The second approach involves ‘emulsification experiment’-based studies, which combine measured drop-size distributions with assumptions to allow for estimations of the breakup frequency. This second approach can be further subdivided in three types: Parametric determination, inverse self-similarity-based methods, and direct back-calculation methods. Each of these methods are reviewed in terms of their implementation, reliability, and validity. Suggestions of methodological considerations for future studies are given for each class, together with more general suggestions for further investigations. The overall objective is to provide emulsification researchers with background information when choosing which method to use for measuring the breakup frequency and with support when setting up experiments and data evaluation procedures. Full article

(This article belongs to the Special Issue Emulsion Process Design)

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