Spray performance and steadiness of an effervescent atomizer and an air-core-liquid-ring atomizer for application in spray drying processes of highly concentrated feeds

Atomization for spray drying of high viscous feed liquids is still a challenging task. For this reason, we investigated the potential of two internal mixing pneumatic atomizers, namely an effervescent atomizer (EA) and an Air-Core-Liquid-Ring (ACLR) atomizer. Both atomizers are characterized by a two-phase flow in the exit orifice. While this can be either a two-phase plug or annular flow in case of the EA geometry, the ACLR atomizer enforces annular flow conditions. In this study, spraying experiments were conducted at liquid viscosities between 0.12 and 0.69 Pa∙s. The investigations were performed at a constant liquid flow rate of 20 L/h and gas pressures from 0.3 to 0.9 MPa. Besides the commonly used correlation between Gas-to-Liquid-Ratio (GLR) and time-averaged Sauter mean diameters ((SMD) ̅), we analyzed in-depth the time dependent fluctuation of SMDs, as steady atomization is crucial for spray drying applications. We can conclude that due to strong fluctuations of the SMDs the EA is not suitable for the aimed application in spray drying of high viscous feed liquids. In contrast, the ACLR atomizer is a very promising nozzle for spray drying applications as it delivers much better performance and steadiness also at high liquid viscosities

Untersuchungen zur Zerstäubung von höherviskosen Modelllebensmitteln mittels innenmischender pneumatischer Ringströmungsdüsen

Die Sprühtrocknung ist ein weit verbreitetes Verfahren zur Überführung von Lebens-mittelflüssigkeiten in einen festen, pulverförmigen Zustand. Da Trocknungsprozesse zu den energieintensivsten Prozessen der Lebensmittelverarbeitung zählen, gibt es hier ein hohes Energieeinsparpotenzial. Eine Möglichkeit zur Reduktion des Gesamtenergiebedarfs des Sprühtrocknungsprozesses ist die Aufkonzentrierung der verwendeten Konzentrate zu höheren Trockenmassen mittels energiesparender, vorgeschalteter Prozesse. Allerdings steigt mit steigender Trockenmasse auch die Viskosität der zu zerstäubenden Flüssigkeit, was die Zerteilung in feine Tropfen grundsätzlich erschwert. Eine Möglichkeit für die Zerstäubung höherviskoser Flüssigkeiten bei relativ geringem Energieeintrag stellen innenmischende pneumatische Zerstäuber dar, bei denen sich im Düsenauslasskanal eine stabile Ringströmung einstellt. Diese Erkenntnis resultiert unter anderem aus Arbeiten mit einem optisch zugänglichen Effervescent Atomizer [1]. Auf dieser Grundlage wurde ein neues Zerstäuberdesign vorgeschlagen, welches bei geringem Gaseinsatz über einen großen Viskositätsbereich gezielt eine Ringströmung im Düsenauslasskanal erzeugen kann. Bei diesem sogenannten Air-Core-Liquid-Ring Zerstäuber (ACLR) wird das Zerstäubungsgas kurz vor dem Düsenauslasskanal mittels einer Kapillaren zentral in den Flüssigkeitsstrom eingebracht. Dadurch wird gezielt eine Gaskernbildung erzwungen. In dieser Arbeit wurde der ACLR-Zerstäuber hinsichtlich der Zerstäubungseffizienz und –stetigkeit untersucht und mit dem zuvor genannten Effervescent Atomizer verglichen. Der ACLR-Zerstäuber erzielt dabei vergleichbare mittlere Tropfengrößen bei deutlich geringeren Fluktuationen des mittleren Sprühtropfendurchmessers, insbesondere bei gesteigerten Flüssigkeitsviskositäten. Diese geringe Fluktuation ist entscheidend für den Einsatz im Sprühtrocknungsbereich, da hier prozessbedingt eine enge Tropfengrößenverteilung erforderlich ist

Air-Core–Liquid-Ring (ACLR) Atomization Part II: Influence of Process Parameters on the Stability of Internal Liquid Film Thickness and Resulting Spray Droplet Sizes

Air-core–liquid-ring (ACLR) atomization presents a specific type of internal mixing pneumatic atomization. It can be used for disintegration of high viscous feed liquids into small droplets at relatively low gas consumptions. However, the specific principle of ACLR atomization is still under research and no guidelines for process and atomizer design are available. Regarding literature on pre-filming atomizers, it can be hypothesized for ACLR atomization that the liquid film thickness inside the exit orifice of the atomizer, as well as the resulting spray droplet sizes decrease with increasing air-to-liquid ratio (ALR) and decreasing feed viscosity. In this study, the time dependent liquid film thickness inside the exit orifice of the atomizer was predicted by means of computational fluid dynamics (CFD) analysis. Results were compared to high speed video images and correlated to measured spray droplet sizes. In conclusion, the hypothesis could be validated by simulation and experimental data, however, at high viscosity and low ALR, periodic gas core breakups were detected in optical measurements. These breakups could not be predicted in CFD simulations, as the simplification of an incompressible gas phase was applied in order to reduce computational costs and time. Nevertheless, the presented methods show good potential for improvement of atomizer geometry and process design as well as for further investigation of the ACLR atomization principle

Gazette de Bayonne, de Biarritz et du Pays basque

21 juillet 19361936/07/21 (A45,N8600).Appartient à l’ensemble documentaire : Aquit

Air-Core-Liquid-Ring (ACLR) Atomization: Influences of Gas Pressure and Atomizer Scale Up on Atomization Efficiency

Air-core-liquid-ring (ACLR) atomizers present a specific type of internal mixing pneumatic atomizers, which can be used for efficient atomization of high viscous liquids. Generally, atomization efficiency is considered as a correlation between energy input and resulting droplet size. In pneumatic atomization, air-to-liquid ratio by mass (ALR) is commonly used as reference parameter of energy input. However, the pressure energy of the atomization gas is not considered in the calculation of ALR. In internal mixing ACLR atomizers, it can be assumed that this energy contributes to liquid disintegration by expansion of the gas core after exiting the atomizer. This leads to the hypothesis that droplet sizes decrease with increasing gas pressure at constant ALR. Therefore, the use of volumetric energy density (EV) as a reference parameter of energy input was investigated at different gas pressures between 0.4 and 0.8 MPa. Furthermore, scale up-related influences on the atomization efficiency of ACLR atomization were investigated by use of an atomizer with enlarged exit orifice diameter. We can conclude that EV can be applied as a reference parameter of ACLR atomization processes with different gas pressures. However, within the range investigated no clear influence of gas pressure on atomization efficiency was found. Up-scaling of ACLR atomizers allows production of similar droplet sizes, but atomization efficiency decreases with increasing exit orifice diameter