Droplet Breakup in High Pressure Spray Processes

H. Kroeber1), U. Teipel1, 2)

1) Fraunhofer Institute for Chemical Technology, J.-v.-Fraunhofer-Str. 7, 76327 Pfinztal, Germany 2) University of Applied Sciences Nuremberg, Germany

ABSTRACT

In the last years the special properties of supercritical fluids were used to develop new processes which can produce fine particles [1]. One of these processes is the PCA process (Precipitation with a Compressed Fluid Antisolvent) [2]. A solution of an active substance is sprayed into a supercritical fluid. After droplets are formed the solvent evaporates and simultaneous the supercritical fluid diffused into the droplets and acts as an antisolvent for the solute so that nucleation and formation of particles occur in the droplets [3]. The droplet formation process is investigated in the present contribution because of the properties of the droplet spectrum influence the particle formation and therefore the quality of the solid particles. A laser system was used to measure the droplet size in-situ and the droplet formation was visualized by a high speed camera system. Besides of different process parameters (e.g. pressure, flow rate) the influence of nozzle type and diameter on the droplet spectrum was under investigation. These investigations showed that the borders between the different spray regimes (Rayleigh breakup, sinuous wave breakup, and atomization) in the Ohnesorge-Reynolds diagram were shifted to lower Reynolds numbers.

1 INTRODUCTION The atomization of liquids in gases at atmospheric pressure is the subject of a variety of experimental investigations and also empirical and physical models. The atomization of liquids in pressurized gases has not undergone intensive investigations so far. The aim of the present investigation is in the clarification of the basic phenomena at the disintegration of liquid jets in a pressurized gas at up to 30 MPa. This is carried out by the characterization of the forces involved in the disintegration of liquid jets which are influenced by the physical properties of the phases used. With a new modified dimensionless number and the Reynolds number it was possible to determine boundaries that describe the disintegration of liquid jets in pressurized carbon dioxide.

2 PROCESS AND APPARATUS Solids which are insoluble in a compressed gas can be processed by applying the PCA process (Precipitation with a Compressed Fluid Antisolvent). For this, a solution consisting of an organic solvent which has to be complete miscible within the compressed gas and a solid material dissolved in this solvent is sprayed through a nozzle into a high-pressure vessel filled with a compressed gas. The formation of the particles is based on two mechanisms which take place simultaneously. On the one hand the solvent evaporates in the compressed gas and on the other hand the compressed gas penetrates into the droplets where it acts as an antisolvent for the dissolved material so that precipitation occurs. A distinctive characteristic of supercritical fluids is the diffusivity that can be up to two orders of magnitude higher than

those of liquids. Therefore, the diffusion of the supercritical fluid into a liquid solvent can produce a fast supersaturation of solute dissolved in the liquid and its precipitation in micronized particles [1, 2]. For this study pure solvents were sprayed into compressed carbon dioxide so that no precipitation occurs. A high-speed optical measurement system (CCD-camera with sparkflashlamp; NANOLITE, High Speed Photo System, Hamburg) and a laser light system (3-wavelength extinction measurement; WIZARD Zahoransky, Todtnau) were used to visualize the spray and to measure the droplet size, respectively. A more detailed description of the measurement systems is given in literature [3, 4]. Figure 1 illustrates the flow sheet of the apparatus and the precipitation vessel with the laser light system is shown in figure 2.

Figure 1: Flow sheet of the PCA process The experiments were carried out using capillary nozzles with different inner diameters. All nozzles were

one-flow nozzles without internal mixing. A pulsation-free high-pressure syringe pump was used to dose the solvent into the precipitation vessel. Solvents which are commonly used for the PCA process and differ in physical properties were used.

Figure 2: Precipitation vessel with the added laser light measurement system The droplet size was measured at two different positions behind the nozzle (10 mm and 40 mm below the orifice, respectively) simultaneously.

3 RESULTS 3.1 Visualization of the spray jet The different shapes of disintegrating jets in gases at elevated pressures are equal to those in gases at atmospheric pressure. With increasing jet velocity three distinct regimes of breakup can be detected: Rayleigh breakup, sinuous wave breakup, and atomization). An empirical classification of jet disintegration at atmospheric pressure was given by v. Ohnesorge [5] using dimensionless analysis. Ohnesorge determined empirical boundaries between the three different regimes on a graph of Ohnesorge number versus Reynolds number. The Ohnesorge number is given by

Nl

l

l

l

d

WeOh

⋅⋅==

σρη

Re.

These well-known boundaries are only valid at atmospheric gas pressure. Both boundaries dislocate with increasing pressure and therefore increasing density. They are shifted to regions of lower Reynolds numbers. This means that the sinuous wave breakup and the atomization in pressurized gases take place at lower outlet velocities. In figure 3 the disintegration of ethanol at atmospheric (left) and elevated pressure (right) is visualized.

0.1 MPa 10 MPa

Re 86 212 1045 8.5 23 84

0.1 MPa 10 MPa

Re 86 212 1045 8.5 23 84 Figure 3: Disintegration of ethanol through a 50 µm capillary at atmospheric (left) and elevated (right) pressure It is seen that a Reynolds number of 86 results in a Reyleigh breakup at atmospheric pressure whereas at the same Reynolds number the liquid is atomized at a pressure of 10 MPa. Figure 4 shows the Oh-Re-diagram of different liquids. The shift of the boundaries to lower Reynolds numbers can be seen very clearly.

0,01

0,1

1

10

1 10 100 1000 10000

Re [-]

Oh

[-]

I

IIIII

New borders at elevated pressure

Borders according to Ohnesorge

Ethanol

Dimethyl formamide

Acetone

Toluene

Water

Figure 4: Oh-Re diagram for the disintegration of liquids at elevated pressures (I: Rayleigh breakup, II: sinuous wave breakup, III: atomization) In pressurized gases the influence of the gas phase cannot be neglected. This influence can be considered by the Weber number of the gas phase. A modified dimensionless number Oh* was created which takes account of the gas density and viscosity.

gl

lgl

g

lgl

vWeOhOh

ηρηρ

ση

ηη

⋅

⋅⋅

⋅=⋅⋅=*

With this modified number the new boundaries between the different regimes can be described accurately. 3.2 Analysis of the droplet size The diagrams 5 and 6 show the droplet size during the atomization of ethanol in pressurized carbon dioxide versus Reynolds number and pressure, respectively. The mean droplet size was between 2 and 10 µm and it became clear that the droplets shrink from position 1 to position 2 due to the evaporation of the liquid in the gas phase.

2

3

4

5

6

7

0 1000 2000 3000 4000 5000 6000 7000 8000Reynolds number Re [-]

mea

n dr

ople

t siz

e [µ

m]

Position 1

Position 2

Figure 5: Influence of Reynolds number on the droplet size (p = 10 MPa, T = 318 K, dNozzle = 50 µm) The influence of the Reynolds number is relative small. At low Reynolds numbers a slight increase of the mean droplet size with increasing Reynolds numbers can be seen but at a Reynolds number of about 3000 no further droplet enlargement is observed.

0

1

2

3

4

5

6

7

8

9

10

0 5 10 15 20 25 30pressure [MPa]

mea

n dr

ople

t siz

e [µ

m]

Position 1

Position 2

Figure 6: Influence of pressure on the droplet size (Re = 1441, T = 318 K, dNozzle = 50 µm) A linear increase of the mean droplet size with increasing pressure is obtained. This behaviour is in opposite of what we expect. At higher pressure the surface tension of the liquid will be decrease so that droplet formation is facilitated. On the other hand the volume expansion of the liquid is much stronger at high pressure due to the better dissolution of the carbon dioxide in the solvent. The second effect seems to predominate the first one and determine the behaviour. In further experiments the influence of the nozzle diameter was under investigation. We used capillaries with inner diameters of 50 µm, 100 µm and 150 µm at a flow rate of 10 ml/min. That means that the Reynolds number differed but we did not find an effect of the orifice diameter on the mean droplet size.

4 CONCLUSIONS With respect to the particle formation using the PCA process it is of high interest to characterize the atomization of liquids at high pressure condition. It was possible to measure the droplet size in-situ during the atomization by a laser light measurement system and to visualize the jet by an optical microscope system. It

became clear that the disintegration differs from that at atmospheric pressure because the physical properties of the gas phase cannot be neglected. The atomization regime was reached at lower Reynolds numbers that means at lower velocity at the orifice so that the disintegration is easier. The droplet size is more or less independent from the Reynolds number and increased with increasing pressure. In further studies the influence of the solute in the liquid phase will be under investigation.

REFERENCES:

[1] J. Jung and M. Perrut: Particle design using supercritical fluids: Literature and patent survey, J. Supercrit. Fluids 20, (2001) p. 179-219. [2] H. Kröber and U. Teipel: Materials processing with supercritical antisolvent precipitation: Process parameters and morphology of tartaric acid, J. Supercrit. Fluids 22, (2002) p. 229-235. [3] H. Kröber and U. Teipel: Experimentelle Untersuchungen von Hochdruck-Sprühverfahren, In: R. Eggers, M. Peric, Proc. Spray 2001, Hamburg, P4. [4] K. Schaber, A. Schenkel and R.A. Zahoransky: Drei-Wellenlängen-Extinktionsverfahren zur Charakterisierung von Aerosolen unter industriellen Bedingungen, tm - Technisches Messen 61, (1994) p. 295-300. [5] W. v. Ohnesorge: Die Bildung von Tropfen an Düsen und die Auflösung flüssiger Strahlen, Z. Angew. Math. Mech. 16, (1936) p. 6.