**4. A particular case: Ampicillin SAS precipitation**

In our research group a study was carried out to establish a correlation between the morphologies of the particles obtained in the ampicillin precipitation assays and the estimated regimes. This correlation would be an ideal tool to establish the limiting hydrodynamic conditions for the success of the test in order to define the successful experiments; that is, the appropriate conditions to orientate the process toward the formation of uniform spherical nanoparticles instead of irregular and larger-sized particles, for the solute-solvent-SC CO2 system studied (Tenorio et al.,2009).

A series of ampicillin precipitation experiments by the SAS technique, utilizing N-methylpyrrolidone (NMP) as the solvent and CO2 as the antisolvent, under different operating conditions were carried out. Two nebulizers, with orifice diameters of 100 and 200 μm, respectively were used.

A pilot plant, developed by Thar Technologies® (model SAS 200) was used to carry out all the experiments. A schematic diagram of this plant is shown in Figure 4. The SAS 200 system comprises the following components: two high-pressure pumps, one for the CO2 (P1) and the other for the solution (P2), which incorporate a low-dead-volume head and check valves to provide efficient pumping of CO2 and many solvents; a stainless steel precipitator vessel (V1) with a 2L volume consisting of two parts, the main body and the frit, all surrounded by an electrical heating jacket (V1-HJ1); an automated back-pressure regulator (ABPR1) of high precision, attached to a motor controller with a position indicator; and a jacketed (CS1-HJ1) stainless steel cyclone separator (CS1) with 0.5L volume, to separate the solvent and CO2 once the pressure was released by the manual back-pressure regulator (MBPR1).The following auxiliary elements were also necessary: a low pressure heat exchanger (HE1), cooling lines, and a cooling bath (CWB1) to keep the CO2 inlet pump cold and to chill the pump heads; an electric high-pressure heat exchanger (HE2) to preheat the CO2 in the precipitator vessel to the required temperature quickly; safety devices (rupture discs and safety valve MV2); pressure gauges for measuring the pump outlet pressure (P1, PG1), the precipitator vessel pressure (V1, PG1), and the cyclone separator pressure (CS1, PG1); thermocouples placed inside (V1-TS2) and outside (V1-TS1) the precipitator vessel, inside the cyclone separator (CS1-TS1), and on the electric high pressure heat exchanger to obtain continuous temperature measurements; and a FlexCOR Coriolis mass flowmeter (FM1) to measure the CO2 mass flow rate and another parameters such as total mass, density, temperature, volumetric flow rate, and total volume.

Hydrodynamics Influence on Particles Formation Using SAS Process 179

Fig. 5. Effect of operating pressure on microstructure of ampicillin powder obtained by the

then, because sufficient contact area would not be generated, the liquid phase does not evaporate in the dense phase of the CO2. Instead, the liquid droplets accumulate in the filter,

In contrast, for higher pressures, the presence of a precipitate occurring as aggregates in the filter may be explained by the existence of significant mechanisms that stabilize the liquid jet. These important mechanisms of stabilization may be associated with the existence of the dynamic interfacial tension (Dukhin et al., 2003 ).Therefore, the so-called "gaseous plume" or "gas-like jet", which is characteristic of states of complete miscibility of mixtures (above

The influence of the mean velocity of the jet of liquid solution was also analyzed. The liquid solution flow rate from 1 mL/min to 5 mL/min causes the jet to disintegrate, passing through the three possible regimes: Rayleigh, sine wave break-up and atomization. The lowest flow rate tested (1 mL/min), which is equivalent to a jet velocity of 0.5 m/s (200 μm nozzle diameter), led to an unsatisfactory test result, which may be in agreement with the Rayleigh-type estimated regime; this is because the droplets that formed would not generate sufficient contact area to produce saturation while they are in motion, and, consequently, ampicillin is not precipitated. When the liquid solution flow rate is increased to 2 mL/min a dispersion of the sine wave breakup type is estimated. Considering that a polydisperse

Fig. 6. SEM images showing the microstructure of the ampicillin powder obtained by SAS

experiment with 5ml/min (at 180 bar, 328 K, and 200 μm) (Tenorio et al., 2009).

where the precipitate is obtained by the drying action of the CO2.

their MCP),would not be produced, even at 150 bar.

SAS experiments (Tenorio et al., 2009).

Fig. 4. Schematic diagram of the pilot plant

The pendant droplet method, as introduced by Andreas and Tucker, was used to determine the interfacial tension between NMP and SC CO2 (Andreas&Tucker, 1938).This method, and its application to high pressures and temperatures, are comprehensively described by Jaeger (Jaeger et al., 1996). A commercial CCD video technique allows recording of droplet shapes for subsequent video image processing.

Rayleigh breakup, sinusoidal wave break up, and atomization regimes are seen to be clearly differentiated by representing graphically the Reynolds number against Ohnesorge number Here, the forces of inertia of the liquid phase (pressure gradient), the forces of capillarity (surface tension), and those of viscosity of the liquid phase (friction) are taken into account, but the force of gravity is considered to be negligible.

Two differentiated types of morphology can be identified in the precipitated experiments: spherical nanoparticles of ampicillin that are obtained from a fine precipitate with foamy texture, and particles of ampicillin with irregular forms and larger size, which are characteristic of the precipitate formed by aggregates, compact films, and rods (Figure 5).

The aim of the work is to explain, from the estimation of the different disintegration regimes as a function of the physicochemical properties and of the velocity of the jet, the two different morphologies obtained in the ampicillin precipitation experiments for a specific range of operating conditions. Thus it should be possible to specify the hydrodynamic conditions for orientating the process toward the formation of uniform spherical nanoparticles rather than larger size irregular particles.

The morphology of the precipitate obtained at low pressure was supposed to be in accordance with the Rayleigh estimated regime, since droplets with a diameter of approximately twice the diameter of the orifice would be produced; (Badens et al., 2005)

**V1 2L**

**HE2 Automated** 

**V1-TS2**

**V1-HJ1 ABPR1**

**BPR**

**CS1**

**CS1 PG1**

**CS1-TS1**

**O.5 L CS1-HJ1**

**MBPR1**

**Manual BPR**

**P2 Solution Pump** 

**Electric High Pressure Heat Exchanger**

**V1 PG1**

**V1-TS1**

The pendant droplet method, as introduced by Andreas and Tucker, was used to determine the interfacial tension between NMP and SC CO2 (Andreas&Tucker, 1938).This method, and its application to high pressures and temperatures, are comprehensively described by Jaeger (Jaeger et al., 1996). A commercial CCD video technique allows recording of droplet shapes

Rayleigh breakup, sinusoidal wave break up, and atomization regimes are seen to be clearly differentiated by representing graphically the Reynolds number against Ohnesorge number Here, the forces of inertia of the liquid phase (pressure gradient), the forces of capillarity (surface tension), and those of viscosity of the liquid phase (friction) are taken into account,

Two differentiated types of morphology can be identified in the precipitated experiments: spherical nanoparticles of ampicillin that are obtained from a fine precipitate with foamy texture, and particles of ampicillin with irregular forms and larger size, which are characteristic of the precipitate formed by aggregates, compact films, and rods (Figure 5). The aim of the work is to explain, from the estimation of the different disintegration regimes as a function of the physicochemical properties and of the velocity of the jet, the two different morphologies obtained in the ampicillin precipitation experiments for a specific range of operating conditions. Thus it should be possible to specify the hydrodynamic conditions for orientating the process toward the formation of uniform spherical

The morphology of the precipitate obtained at low pressure was supposed to be in accordance with the Rayleigh estimated regime, since droplets with a diameter of approximately twice the diameter of the orifice would be produced; (Badens et al., 2005)

**MV2**

**Supply in**

**Low pressure heat exchanger HE1**

**FM1**

**P1 CO2 Pump**

**PG1**

**Cooling lines**

**cooling bath CWB1**

Fig. 4. Schematic diagram of the pilot plant

for subsequent video image processing.

but the force of gravity is considered to be negligible.

nanoparticles rather than larger size irregular particles.

Fig. 5. Effect of operating pressure on microstructure of ampicillin powder obtained by the SAS experiments (Tenorio et al., 2009).

then, because sufficient contact area would not be generated, the liquid phase does not evaporate in the dense phase of the CO2. Instead, the liquid droplets accumulate in the filter, where the precipitate is obtained by the drying action of the CO2.

In contrast, for higher pressures, the presence of a precipitate occurring as aggregates in the filter may be explained by the existence of significant mechanisms that stabilize the liquid jet. These important mechanisms of stabilization may be associated with the existence of the dynamic interfacial tension (Dukhin et al., 2003 ).Therefore, the so-called "gaseous plume" or "gas-like jet", which is characteristic of states of complete miscibility of mixtures (above their MCP),would not be produced, even at 150 bar.

The influence of the mean velocity of the jet of liquid solution was also analyzed. The liquid solution flow rate from 1 mL/min to 5 mL/min causes the jet to disintegrate, passing through the three possible regimes: Rayleigh, sine wave break-up and atomization. The lowest flow rate tested (1 mL/min), which is equivalent to a jet velocity of 0.5 m/s (200 μm nozzle diameter), led to an unsatisfactory test result, which may be in agreement with the Rayleigh-type estimated regime; this is because the droplets that formed would not generate sufficient contact area to produce saturation while they are in motion, and, consequently, ampicillin is not precipitated. When the liquid solution flow rate is increased to 2 mL/min a dispersion of the sine wave breakup type is estimated. Considering that a polydisperse

Fig. 6. SEM images showing the microstructure of the ampicillin powder obtained by SAS experiment with 5ml/min (at 180 bar, 328 K, and 200 μm) (Tenorio et al., 2009).

Hydrodynamics Influence on Particles Formation Using SAS Process 181

Badens, E., Boutin, O., Charbit, G. (2005). Laminar jet dispersion and jet atomization in

Balabel, A., Hegab, A.M., Nasr, M., El-Behery, S. M. (2011). Assessment of turbulence

Bałdyga, J., Kubicki, D., Shekunov, B.Y., Smithd, K.B. (2010). Mixing effects on particle formation in supercritical fluids, *Chem. Eng. Res. Des*., 88, pp. 1131–1141. Bell, P. W., Stephens, A. P.,Roberts, C. B., Duke, S. R. (2005). High-resolution imaging of the supercritical antisolvent process, *Experiments in Fluids*, 38, pp. 708–719. Benedetti, L., Bertucco, A., Pallado, P. (1997). Production of micronic particles of

Bleich, J., Cleinebudde, P., Muller, B.W. (1994). Influence of gas density and pressure on microparticles produced with the ASES process, *Int. J. Pharm.*, 106, pp. 77–84. Bouchard, A., Jovanovic, N., A. H. de Boer, Martín, A., Jiskoot, W., Crommelin, D. J.A. ,

Carretier, E., Badens, E., Guichardon, P., Boutin, O., Charbit, G. (2003). Hydrodynamics of

Czerwonatis, N., Eggers, R., Charbit, G. (2001). Disintegration of liquid jets and drop drag

Chang, S.-C., Lee, M.-J., Lin, H.-M. (2008). Role of phase behavior in micronization of lysozyme via a supercritical anti-solvent process, *Chem. Eng. J.*, 139, pp. 416–425. Chehroudi, B., Cohn, R., Talley, D. (2002). Cryogenic shear layers: experiments and

supercritical conditions, *Int. J. Heat Fluid Flow*, 23, pp. 554-563. Chigier, N. (1991). Optical imaging of sprays. *Prog. Energy Combust. Sci*., 17, pp.211–262 Dixon, D.J., Johnston, K.P., Bodmeier, R.A. (1993).Polymeric materials formed by precipitation with a compressed fluid antisolvent, AIChE J., 39 (1), pp. 127–139. Dukhin, S.S., Zhu, C., Pfeffer, R., Luo, J.J., Chavez, F., Shen, Y. (2003). Dynamic interfacial

stabilization, *Physicochem. Eng. Aspects*, 229, pp. 181-199.

*Pharm. Sci*., 90, pp. 1628-1636.

microparticles, *Pharm. Res*., 15, pp. 1233-1237.

microspheres, *Biomat.*, 19, pp.157-161.

modeling for gas flow in two-dimensional convergent–divergent rocket nozzle.

biocompatible polymer using supercritical carbon dioxide*, Biotechnol. Bioeng*., 53,

Hofland, G.W., Witkamp, G.-J. (2008). Effect of the spraying conditions and nozzle design on the shape and size distribution of particles obtained with supercritical

supercritical antisolvent precipitation: characterization and influence on particle

coefficients in pressurized nitrogen and carbon dioxide, *Chem. Eng. Tech.,* 24, pp.

phenomenological modelling of the initial growth rate under subcritical and

tension near critical point of a solvent–antisolvent mixture and laminar jet

nucleation and submicron particle production in two-phase flow of solvent-

glycol)/poly(l-lactide) (PEG/PLA) nanoparticles by gas antisolvent techniques, *J.* 

and polymer morphology in the formation of gentamycin-loaded poly (L-lactide)

characterization of ampicillin loaded methylpyrrolidinone chitosan and chitosan

Dukhin, S. S., Shen, Y., Dave, R., Pfeffer, R. (2005). Droplet mass transfer, intradroplet

Falk, R.F., Randolph, T.W. (1998). Process variable implications for residual solvent removal

Giunchedi, P., Genta, I., Conti, B., Conte, U., Muzzarelli, R.A.A. (1998) .Preparation and

supercritical antisolvent emulsion. *Colloids Surf. A*, 261, pp. 163-176. Elvassore, N., Bertucco, A., Caliceti, P. (2001). Production of insulin-loaded poly(ethylene

pressurized carbon dioxide, *J. Supercrit. Fluids* ,36, pp. 81–90.

fluid drying, *Eur. J. Pharm. Biopharm.*, 70, pp. 389–401.

morphology, *Ind. Eng. Chem. Res.*, 42, pp. 331–338.

*Appl. Math. Model.*, 35, pp. 3408–3422.

pp. 232-237.

619–624.

distribution of droplets is produced in this regime, it is very well correlated with the experimental obtained results (Tenorio et al., 2009).

When the flow rate is increased to 3 mL/min, it is estimated that the transition is complete, and the liquid is atomized. The large quantity of fine precipitate with foamy texture obtained both on the walls and accumulated in the filter (characteristic of nanoparticles) would have originated from the fully atomized and homogeneous dispersion that is occurring in the precipitation chamber. With 5 mL/min it was obtained similar results in accordance with the estimated atomization regime (Figure 6).
