(340c) From Liquid to Droplet: Understanding the Ligaments Break-up in Atomization Process

From
liquid to droplet: understanding the ligaments break-up in atomization process

T.
Porfirio^1,2*, F. J. Galindo-Rosales^3,4, L. Campo-Deaño⁴,
J. Vicente¹, V. Semião²

¹ Hovione Farmaciência SA, Estrada do Paço do Lumiar, 1649-038 Lisbon,
Portugal; *tporfirio@hovione.com

²LAETA, IDMEC, Mechanical Engineering Department, Instituto Superior
Técnico, Universidade de Lisboa, Av. Rovisco Pais 1, 1049-001 Lisbon, Portugal

³Instituto de Ciência e Inovação em Engenharia Mecânica e Industrial,
Rua Dr. Roberto Frias 400, 4200-465 Porto, Portugal

⁴CEFT, Departamento de Engenharia Mecânica, Faculdade de Engenharia da
Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal

The droplet formation is a key
phenomenon in spray drying process in which the particle size is mainly
dependent of atomization performance. The spray formation is described as the
breakup of a liquid jet into droplets in a gaseous atmosphere. Atomization has
been studied by many scientists from more than a century in which different
mechanism were proposed for the several applications. Currently, the
atomization process is viewed as the instability and collapse of simple
liquid-jet systems [1].
The whole process can be divided in three steps: sheet formation, sheet
destabilization into ligaments and droplet formation. Firstly, the ejection of
the liquid through the nozzle orifice forms a fine sheet of liquid in conic
shape that contacts with the surrounding gas. The cohesive and disruptive
forces in the formed sheets rise and result in perturbations and instability of
the liquid resulting in breakup into ligaments. The third phase is the further
breaking of such ligaments into droplets due to capillary force. This process
is referred as primary atomization. If the produced droplets exceed the
critical size, further disintegration can occur to form smaller droplet –
secondary atomization [2].

Figure 1 – Droplet formation
mechanism [3].

The droplet formation process is
strongly influenced liquid properties mainly viscosity, surface tension and
density [4].
Two types of viscosities must be distinguished in droplet formation process:
shear viscosity  and extensional
viscosity . The extensional
viscosity is related with the viscoelasticity and has special relevance in the
ligaments break-up and in secondary atomization. The effect is not visible in
the early stages of sheet break-up close to the nozzle discharge.  The
difference is usually observed far from the discharge in which the ligaments
are stretched and elongated [5]. 
Depending of fluid viscoelasticity, the droplet formation mechanism is
affected, and it may lead to poor atomization. The viscosity and rheological
behavior are highly dependent of the polymer (important for the formulation of
amorphous solid dispersions), mixture of solvents and concentrations. The
proposed work explores different case studies explored to link the spray drying
feed solution variables (polymer, solvents and concentration) with the
rheological behavior and atomization performance. The test fluids comprised the
most used polymer (HPMCAS, HPMC, copovidone and Eudragit) to formulate
amorphous solid dispersions and dissolved in organic solvents and water.

The rheology of the fluids was also
measured in shear and extensional flow. The extensional experiments were
conducted in a Capillary Breakup Extensional Rheometer (CaBER™ 1 Thermo Haake
GmbH, Karlsruhe, Germany) in which the sample is placed between the two plates
at an initial position and is elongated by a plate separation following the
Slow Retraction Method [6]. The sample forms a filament / ligament bridging the
two plates that thins due to surface tension until it breaks. The steady shear viscosity
curve was determined by means of a rotational rheometer with a plate-plate
geometry.

Two sample fluids are here
compared. The same polymer with a solid concentration of 6% was dissolved in
two systems: 1) acetone and water, 2) acetone. Both fluids showed a shear
thinning behavior in which the viscosity of acetone solution was lower than the
acetone/water solution. However, the extensional experiments show a contrary
result for the viscoelasticity of the fluids: the acetone-based fluid presented
a high viscoelasticity and relaxation time (Figure 2). In fact, this fluid
presents a bead-on-string effect that can lead atomization problems. Higher
deviations in the prediction of particle size distribution (mainly Dv10 and
Dv90) were observed for this case.

A set of case studies will be
explored to show the impact of feed solution variables, including polymer type,
on the atomization and spray drying process. The incorporation of these data in
atomization models to increase the accuracy of spray drying scale-up will be
discussed.

[1]       K. Masters, Spray
drying handbook, Third edit. New York: John Wiley & Sons, 1979.

[2]       V. Lefebvre,
A., McDonell, Atomization and Sprays, Second edi. CRC Press, 2017.

[3]       R. J.
Schick, “Understanding Drop Size,” Wheaton, 2008.

[4]       A. H.
Lefebvre and X. F. Wang, “Mean drop sizes from pressure-swirl nozzles,” J.
Propuls. Power, vol. 3, no. 1, pp. 11–18, 1987.

[5]       B. Keshavarz
et al., “Studying the effects of elongational properties on atomization
of weakly viscoelastic solutions using Rayleigh Ohnesorge Jetting Extensional
Rheometry (ROJER),” J. Nonnewton. Fluid Mech., vol. 222, pp. 171–189,
2015.

[6]       L. Campo
Deaño and C. Clasen, "The Slow Retraction Method (SRM) for the
determination of ultra-short relaxation times in capillary breakup extensional
rheometry experiments", J. Nonnewton. Fluid Mech., vol 165, pp
1688-1699, 2010.