FREQUENTLY ASKED QUESTIONS
1. Which droplet sizes can you investigate in your levitator L800?
It is possible to levitate droplets with a diameter between 0.7 and 4 mm in the L800.
2. How do you realize the droplet dosage and the placement in the sound pressure nodes (dosage apparatus and pump)?
In the L800, injection of the droplets is performed manually via a screw piston pump. Therefore, a capillary is positioned within one pressure node in the standing acoustic wave field. Then the sample will be injected, forming a contact-free droplet in the autoclave. Afterwards, the capillary is moved out of the standing acoustic wave field. Automatization of the droplet dosage is part of current work.
3. What kind of liquids can be investigated? Solutions and slurries?
In the L800 you can investigate solutions as well as slurries. So far fluids, liquids and solids (e. g. PVP, PEG, cacao, sugar, NaCl and CO2-hydrates) were successfully investigated.
4. How can you control/influence mass transport phenomena in your L800, e. g. in drying processes? Can it be controlled only via natural convection or can you influence mass transport processes by induced gas convections?
In general, the control of mass transport phenomena in the L800 is realized via natural convection. While the droplet is levitated, it is possible to change the pressure and/or temperature with maximum rates of about 0.2 MPa/min and 5 K/min, respectively without sample losses, inducing uncontrolled gas convection. Currently we are working on the realization of controlled induced gas convection. The gas flow around the droplet will be approximately 0.3 m/s.
5. What are the maximum temperatures and pressures in the autoclave of the L800?
The standard L800 can be operated at a maximum pressure of 20 MPa at a maximum temperature of 453 K. Modifications of the L800 for special applications requiring higher pressures and temperatures can be provided by BOROSA Acoustic Levitation.
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6. Can you use the results of your acoustic levitation experiment in the L800 for an up-scale, e. g. for spray drying processes (droplet sizes, morphology, drying time)?
The results of the acoustic levitation experiment in the L800 are specific for each investigated substance and depend on the experiment parameters (pressure, temperature, humidity, gas saturation, etc.). BOROSA Acoustic Levitation cannot provide a guarantee for any realization of up-scale based on the acoustic levitation experiments with the L800.
7. How does the energy input caused by the standing acoustic wave field influence the mass transport process within the droplet?
The standing acoustic wave field of the L800 causes an increase of the droplet surface temperature of about 2 – 3 K at the beginning of the acoustic levitation experiment. Furthermore, a good mixing of the droplet is achieved. The formation of acoustically induced turbulences in the surrounding of the levitated sample is discussed in literature:
- A. L. YARIN, G. BRENN, O. KASTNER, D. RENSINK and C. TROPEA (1999). Evaporation of acoustically levitated droplets. Journal of Fluid Mechanics, 399, pp 151-204 doi:10.1017/S0022112099006266.
- H. ISHII, K. HASEGAWA, A. KANEKO and Y. ABE (2012). Internal and External Flow Structure and Mass Transport Phenomena of an Acoustically Levitated Droplet. Transactions of the Japan Society of Mechanical Engineers Series B, 78/ 794, pp 1696-1709.
In our experiments with the L800 no negative influences due to the standing acoustic wave field were detected.
8. How do you characterize the mass transport process phenomena in your L800, e. g. during a drying process?
According to the literature (O. KASTNER, G. BRENN, O. RENSINK and C. TROPEA. Chemie Ingenieur Technik. 72, 2000, pp 862-867.) the drying process curve is measured via the droplet projection surface at the beginning of the acoustic levitation experiment. In the following, the drying process curve is determined by the droplet position within the standing acoustic wave field. More specifically, the camera attached to the L800 takes shadow images of the levitated sample, which are analyzed by the help of our included software. The horizontal and vertical droplet diameter is measured. Based on these results the droplet volume of a rotationally symmetric ellipsoid is calculated. This can be done at any time during the measurement. The end of the drying process is achieved, when the vertical position of the levitated sample does not change anymore.
9. Which fluids can be used for the acoustic levitation experiments in the L800? Is the L800 only designed for measurements with air or is it also possible to use other fluids like inert gases or supercritical fluids, especially supercritical water vapor?
The acoustic levitation experiment can be performed in nearly every fluid. The L800 allows the use of inert gases, air, supercritical CO2 and water vapor with maximum pressure and maximum temperature of 20 MPa at 453 K. For special applications it is possible to modify the design of the L800 according to your requirements. E. g., experiments with supercritical water vapor (22 MPa at 648 K) require the use of other materials due to corrosion, higher pressures and temperatures. BOROSA Acoustic Levitation offers you the solution for your specific application. Please contact us for further information: firstname.lastname@example.org