Analysis of Dissolved-Gas Atomization: Supercritical CO2 Dissolved in

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Ind. Eng. Chem. Res. 2010, 49, 9454–9461

Analysis of Dissolved-Gas Atomization: Supercritical CO2 Dissolved in Water Giuseppe Caputo,* Renata Adami, and Ernesto Reverchon UniVersita` di Salerno, Dipartimento di Ingegneria Chimica e Alimentare, Via Ponte Don Melillo, 1 I-84084 Fisciano (SA), Italy

Supercritical dissolved-gas atomization is an atomization process in which carbon dioxide at temperature and pressure above its critical point is used as the atomizing gas. The spray characteristics in terms of droplets size and distribution have been experimentally studied using a laser diffraction method based on a Malvern apparatus. The main parameter that influences the droplets size is the gas-to-liquid mass ratio (GLR); the injection pressure in the range of 7.4-13 MPa has a minor effect. Upon variation of the GLR from 0.5 to 3, the droplet mean diameter changes from about 8.0 to 2.0 µm; very narrow droplet size distributions are also produced. From the point of view of the atomization mechanism, the mean droplet diameter is mainly influenced by the sudden release of the gas dissolved in the liquid. The overall analysis of the experimental data confirms that dissolved-gas atomization allows for the formation of micrometric droplets that can produce precipitates with controlled sizes and distributions that are useful in several fine-particles production processes. 1. Introduction Sprays are important in many practical applications. An indicative list includes combustion of liquid fuels, liquid metal processes, powder production, spray drying, coating and painting, and production of health-care products. The selection of a particular atomization method involves the consideration of several factors that might include economic considerations, production scale, physical and chemical properties of the droplets/particles to be produced, and dimensions and morphologies of the droplets/particles desired. The atomization mode is also strictly related to the application of the spray process and to the range of droplet diameters of interest for the given application. For example, a spraying process aimed at the production of drugs for aerosol delivery formulations must produce particles in the range from 1 to 5 µm because only these particles are effectively delivered into the deep lung. The energy that is required for liquid-jet atomization can be imparted to the liquid in a variety of ways. Depending on the mode in which the energy is supplied, atomization processes can be classified into the following major categories: (a) pressure atomization, (b) centrifugal atomization, and (c) twin-fluid atomization.1 More specifically, in twin-fluid atomization mode, a secondary fluid (atomization fluid) is used to break up the liquid jet into droplets. If the fluid is a gas, the process is called gas atomization. The twin-fluid atomization method can be further classified into external and internal mixing. Most airblast atomizers are of the external type, in which the bulk liquid is first transformed into a jet or sheet before being exposed to the atomization gas flowing at high velocity. When internal mixing is employed, the contact between the gas and the liquid takes place within the atomizer body. Three types of internal-mixing atomization mode have been reported in the literature: effervescent atomization, flash atomization, and dissolved-gas atomization.1 The basic principle of effervescent atomization is to inject a two-phase bubbling flow through a discharge orifice. The basic atomization mechanism has not yet been studied in detail; however, because both liquid and atomizing gas exit through * To whom correspondence should be addressed. Tel.: +39 089964091. Fax: +39 089964057. E-mail: [email protected].

the same orifice, the area available for the liquid flow is reduced, causing the liquid to be discharged at a higher velocity. At the same time, the liquid is “squeezed” by the gas bubbles into thin shreds and ligaments, which are further shattered into small drops by the rapid expansion of gas bubbles that occurs immediately downstream of the discharge orifice as a result of the sudden pressure drop.2 In flashing atomization, a liquidized propellant is dissolved into the liquid to be dispersed.3 Because of the high vapor pressure of the propellant, the saturation pressure of the solution will be greater than the ambient pressure; therefore, the solution is discharged into the atmosphere, and the propellant is transformed into a gas by an “explosion”, causing the formation of liquid droplets. Dissolved-gas atomization is widely used in commercial spray cans (aerosols, perfumes, etc.). It relies on a dissolved gas emerging from a liquid to form bubbles. Therefore, this technique applies only to a limited range of liquids that can hold significant quantities of dissolved gas. Both effervescent and flashing atomization can contribute to dissolved-gas atomization. Indeed, the dissolved gas at the exit orifice nucleates to form gas bubbles as in effervescent atomization and can also rapidly evaporate by flashing.4 The mentioned limitations of dissolved-gas atomization can be overcome by using carbon dioxide at supercritical conditions as the atomization gas, because of its large miscibility in most organic liquids comprising solvents and fuels, producing supercritical dissolved-gas atomization (SDGA). Indeed, the solubility of supercritical CO2 (scCO2) in organic liquid solvents can vary from 0.1 to 0.7 mole fraction and more, as demonstrated by the large compilation of binary equilibrium data for CO2-organic solvent mixtures collected by Ohe.5 To take advantage of this characteristic, a solubilization device can be used to put CO2 and the liquid into contact prior to atomization. Therefore, the atomization apparatus is based on the use of a packed saturator characterized by a high specific surface area and large residence times. In the saturator, supercritical CO2 dissolves in the liquid solution (an organic solvent or water) before the atomization in a thin-wall injector. The atomization is particularly efficient because CO2 is released from the insides of the droplets and enhances their fragmentation.

10.1021/ie100925w  2010 American Chemical Society Published on Web 09/08/2010

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Figure 1. SDGA apparatus: C, CO2 vessel; P1, CO2 pump; M, mixer; H, electric heating; L, liquid vessel; SL, laser source; R, receiver.

The concept of SDGA has been applied in studies of the supercritical-assisted atomization (SAA) process, developed for the production of fine particles of pharmaceuticals and chemical products.6,7 The SAA process can be considered as a modification of a classical atomization process in which supercritical carbon dioxide is used as a cosolute. It is based on the solubilitazion of controlled quantities of scCO2 in a liquid solution containing a solid solute and on the subsequent atomization of the ternary solution through a nozzle. Atomization forms small droplets that, after drying, usually produce a spherical micrometric powder. A peculiarity of SAA is that it can use either an organic solvent or water; thus, it is possible to process water-soluble as well as non-water-soluble compounds. Moreover, this technique has demonstrated good control over the particle size and distribution of the produced powders.6-12 To date, the SAA process has been successfully tested on some pharmaceutical compounds, salts, dyes, and catalyst and superconductor precursors, yielding micrometric and submicrometric particles of controlled size and distribution ranging between 0.5 and 5 µm. The main advantage of the SAA process is the capability to produce particles with very small dimensions and narrow size distributions that are difficult to obtain using traditional methods. Thus, the process is very promising and has also been used on the pilot scale,11 but until now, the mechanisms of atomization have not been investigated. Therefore, in this work, SDGA using water as the liquid to be atomized, has been explored. Indeed, water is an important medium in a large variety of pharmaceutical and consumerproduct processes aimed at the production of micrometric particles. Despite the practical importance of this solvent, water is not easily atomized by supercritical atomization because the solubility of scCO2 in water is very small compared to that in most organic liquids. For this reason, because of the low solubility of scCO2 in water, the mechanisms of droplet formation should comprise both bubbling flow typical of effervescent atomization and rapid expansion of bubbles from the liquid phase typical of dissolved-gas atomization. The effects of the gas-to-liquid mass ratio (GLR), nozzle diameter, gas velocity, and atomization pressure on the droplet mean size and size distribution have been studied. 2. Experimental Section 2.1. Experimental Setup. The SDGA apparatus is illustrated in Figure 1. It mainly consists of two feed lines, used to deliver the CO2 and the liquid to a mixing vessel. Carbon dioxide, stored in a vessel (C), is preheated in a water bath and delivered to the mixer (M) by a volumetric pump (P1) (type LDB1, Lewa, Leonberg, Germany). The liquid is delivered to the mixer by a

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membrane pump (P2) (type LDB, Lewa, Leonberg, Germany) from a 500 mL graduated cylinder (L). The two streams are fed to the mixer through a three-way connection. The mixer is a high-pressure vessel with an internal volume of 0.15 dm3 heated by thin band heaters (model 125CH47a4X, Watlow, St. Louis, MO) and is packed with stainless steel perforated saddles with a high specific surface area. It provides a large contacting surface and an adequate residence time (3-5 min depending on the flow rates) for the mixing of the liquid and supercritical CO2. Therefore, an efficient, continuous solubilization of supercritical CO2 in the liquid solution is allowed. As a result, CO2 dissolves in the liquid and tends to form a fluid phase near the saturation limit in terms of the operating conditions of temperature and pressure. The pressure in the mixer is measured by a pressure gauge mounted on the top of the mixer. The liquid-gas solution at the exit of the mixer is sprayed into the atmosphere using a single plain orifice. Two different orifices with diameters of 100 and 120 µm and length-to-diameter (l/d) ratios of 8 and 6.67, respectively, were used in the present study. The orifice dimensions were selected to obtain pressure values above the critical pressure of CO2 (7.38 MPa) at the given flow rates of gas. 2.2. Spray Characterization. The characteristics of the spray were investigated using a laser diffraction technique. A Mastersizer S (Malvern Instruments, Malvern, U.K.) particle size analyzer fitted with a 300-mm-focal-length lens was used to measure the spray droplet size distribution. This provides a lower droplet size boundary of about 0.5 µm. The technique is based on measuring the scattered light intensity caused by the drops as they pass through the analyzer sampling area using a series of semicircular photodiodes housed in the receiver unit. The principle of operation and limitations of this instrument are well established.1,13,14 The effect of CO2 on the refractive index of the liquid droplets was not considered because the gaseous CO2 is transparent to laser light. The instrument-reported obscuration was used to indicate the presence of multiple scattering due to dense spray. Instrument software compensates for multiple scattering, and according to manufacturer, it works satisfactorily up to 95% obscuration. Droplet size measurements were carried out at distances of 7.5, 13.5, 19.5, and 25 cm downstream of the nozzle exit with the laser beam passing through the centerline of the spray. The recirculation of fine droplets was kept to a minimum by absorbing the spray on an adsorbent located about 40 cm downstream of the nozzle exit. The temperature along the spray axis was measured at different operating conditions using a thermocouple. We found that the temperature was 20 °C at the exit of the nozzle and that it was about 3 °C lower at the base of the spray. The effect of this slight change of temperature on the refractive index was assumed to be negligible. Each experiment consisted of a set of five consecutive analyses, each composed of 10000 data points acquired by the analyzer. The droplet sizes and distributions reported in the Results and Discussion are the average values over the five analyses. A curve fitting program was used to convert the light intensity distribution into droplet size distribution curves. To measure the jet cone angle, the spray was illuminated and then photographed using a digital camera. Spray cone half-angles were calculated measuring the width of the spray 50 mm downstream of the injector orifice on the digital photo. In the atomization literature, the Sauter mean diameter (SMD) is frequently used to represent the mean droplet diameter of a spray because it expresses the fineness of a spray in terms of the surface area produced by the spray. The SMD is defined as

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Table 1. Ranges of Parameters Explored controlled parameter

range of values investigated

GLR injection pressure exit orifice diameter downstream distance CO2 flow rate atomized liquid

0.7-40 7.4-13 100-120 7.5-25 12.0-18.0 water

units g of CO2/g of liquid MPa µm cm g min-1

the volume-to-surface mean diameter and is calculated from the equation ∞

∑ (∆N )d i

SMD ≡ d32 )

3 i

i)1 ∞

(1)

∑ (∆N )d i

2

Figure 2. Picture of a typical spray during droplet analysis by laser diffraction. In this figure, the laser beam intersects the spray 7.5 cm downstream of the exit orifice.

i

i)1

where ∆Ni is the fraction of droplets counted in the size interval ∆di. The amplitude of the droplet size distribution was evaluated using the SPAN parameter, defined as SPAN )

D0.9 - D0.1 D0.5

(2)

where D0.9, D0.1, and D0.5 are the droplet diameters of 90%, 10% and 50% by volume of the drops in the population. Very narrow droplet size distributions have SPAN values less than 1. 3. Results and Discussion All the experiments were performed at steady-state conditions with the injector spraying into the atmosphere. The spray characteristics reported here include the mean droplet size (SMD), the amplitude of the droplet distribution (SPAN), the droplet size distribution curves, and the cone half-angles of the spray. Their variations with the GLR, CO2 flow rate, injection pressure, exit orifice diameter, and downstream distance were studied. Table 1 provides the ranges over which these parameters were varied. Experimental observations are presented in this work as function of GLR, because it is the most relevant parameter controlling this atomization process. 3.1. Spray Shape and Properties. The plain-orifice atomizer produces only a solid jet of water, when pure water is injected. With the addition of scCO2, the water jet disintegrates into a fine spray in which it is not possible to visually observe any phase separation between gas and liquid. A picture of a typical supercritical dissolved-gas spray is reported in Figure 2; the spray appears as a homogeneous jet with a well-defined conical shape and in the fully developed atomization regime. The common fluid flows encountered in gas-assisted spray atomization are classified in three main regimes: bubbling flow, slug flow (also called transition regime), and annular flow. In bubbling flow, which occurs at very low GLRs, single bubbles form a “train” through the exit orifice. As the jet discharges from the orifice, the bubbles expand rapidly and shatter the jet into ligaments and drops. With increasing GLR, the number of bubbles increases until a GLR is reached above which the bubbles start to coalesce and form voids in the liquid flow. This marks the onset of the slug-flow regime characterized by unsteadiness due to voids. A further increase in GLR suppresses instabilities, and a new steady flow is formed in which a central round jet of gas is surrounded by a thin annular sheet of liquid. At very high values of GLR, as in this work, the two-phase

Figure 3. Spray cone half-angle as a function of GLR. QCO2 ) 18 g min-1, exit orifice ) 120 µm.

flow inside the nozzle is fully dispersed and discharges in the form of drops suspended in the atomizing gas.2 Under these conditions, the jet appears as a gaseous plume in which no phase separation can be visually observed, but it has a white color as a result of visible light scattering produced by the droplets. In addition, in the case of supercritical atomization, liquid drops contain carbon dioxide that, after phase separation, can contribute to the formation of smaller droplets. Because of the large quantity of gas used for atomization, the spray is highly dilute, as confirmed by the low obscuration of the laser light, calculated by the instrument used to take multiple scattering into account. During experiments, obscuration was always below 20%. As a comparison, it can be mentioned that effervescent atomization produces a significantly denser spray, with obscuration as high as 95%.15 The cone-angle is practically constant along the axial distance. This fact indicates that gas expansion occurs in the near nozzle region when the gas-liquid mixture emerges from the injector. In Figure 3, the cone half-angle is reported for a wide range of GLR values. The cone of the spray is very narrow, and the cone angle is a function of GLR. In particular, the cone halfangle is about 13° at low GLR and decreases to 4° when the GLR is increased. 3.2. Water-CO2 Phase Equilibria. Knowledge of the phase equilibria is fundamental in dissolved-gas atomization because it provides information on the maximum quantity of gas that can be dissolved in the liquid. The compositions of the liquid and gas phases can be determined from thermodynamic phase diagrams representing, on a pressure-mole fraction (p-x) plane, for example, the miscibility curve of the binary system CO2-solvent. Extensive solubility data of the system water-CO2 can be found in a recent article16 in which phase diagrams obtained by a model validated through various experimental studies drawn from the literature are reported. An

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Figure 4. Phase equilibrium diagram of the water-CO2 binary system at 80 °C. The equilibrium composition data were obtained from Diamond and Akinfiev.16

Figure 5. Liquid-phase equilibrium curve and operating points of the experiments carried out at 16 g min-1 of CO2. (Data at other flow rates have been omitted to avoid overlapping.)

equilibrium diagram of water and CO2 at 80 °C drawn on the basis of these data is shown in Figure 4, where both the liquidand vapor-phase equilibrium compositions are reported. In this case, the miscibility gap is very large, and the liquid and vapor lines are very close to the vertical axis. The solubility of CO2 in water at relatively low pressures and temperatures is relatively low: for example, at 80 °C, the solubility is 1.25 mol % at 8.0 MPa and 1.6 mol % at 10.0 MPa. To understand the role played by dissolved CO2 in the atomization process, it is useful to report the solubility data along with the operating points of the system water-CO2 during spraying process on the p-x diagram. An enlargement of part of the diagram is shown in Figure 5. An operating point is characterized by the mole fraction of the mixture inside the mixer and the total pressure. Mole fraction (xCO2) and GLR are directly correlated by the relationship xCO2 )

GLR × MWCO2 /MWH2O GLR × MWCO2 /MWH2O + 1

(3)

where MW represents the molecular weights of CO2 and water. The pressure decrease is due to the fact that, operating at constant CO2 flow rate, the amount of liquid with respect to gas decreases when the GLR increases. As a consequence of the large miscibility gap of the system water-CO2, all of the operating points employed in this work fall into the two- phase gas-liquid region. Moreover, the effect of the GLR on the composition of the phases formed in the saturator is very small. For example, when the GLR in terms of molar fraction is increased from xCO2 ) 0.3 to xCO2 ) 0.6, the total pressure decreases from 10.6 to 8.0 MPa, and correspondingly, the liquidrich phase equilibrium composition changes only from 1.4 to 1.1 mol %. Because the amount of CO2 that is solubilized in water is small throughout the range of GLRs employed, a large fraction of the CO2 remains in the vapor phase.

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Figure 6. Influence of the GLR on the SMD of the droplets at various CO2 flow rates. Exit orifice diameter ) 120 µm; axial distance ) (a) 13.5 and (b) 7.5 cm.

On the basis of the phase behavior of the CO2-water mixture, it can be preliminarily hypothesized that the atomization mechanism is influenced both by the gas portion (undissolved CO2) and by the dissolved portion of CO2. 3.3. Influence of Atomizing Gas-to-Liquid Ratio and Gas Flow Rate. It is common practice to present droplet size distribution data as a function of the gas-to-liquid mass flow rate ratio (GLR).1 The GLR is an important operating parameter in dissolved-gas atomization that has a strong effect on the droplet size and distribution. It also influences the economic viability of the atomization process, because an elevated gas flow rate translates into a high cost per unit amount of product. SMD data were acquired for GLR values from 0.7 to 20 (and in some cases up to 40) to explore a large range of flow conditions. Experiments were performed at a temperature of the mixing vessel of 80 °C. Because the experiments were performed at constant gas flow rate, an increase of in the GLR was necessarily accompanied by a decrease in the liquid flow rate. Representative examples of the effect of the GLR on the SMD are reported in Figure 6, where SMDs obtained at different CO2 flow rates are reported. The results show that the SMD is a nonlinear function of the GLR: Droplet size decreases rapidly as the GLR is increased to around 2.5-3; then, it decreases at a lower rate with further increase in GLR, eventually reaching an asymptotic value. Experiments at a distance of 7.5 cm were stopped at GLR ) 7.5 when the asymptotic value was reached. Very small droplets were obtained with mean diameters lower than 2 µm. These results demonstrate that SDGA provides good control over droplet size. As mentioned in the Introduction, when SDGA is applied to the SAA process for the production of microparticles, it allows microparticles with very narrow size distributions to easily be obtained.6-11 The results reported in Figure 6 show that the droplet size decreases with GLR at the same rate for all gas flow rates investigated; however, at GLRs higher than about 3, the SMD becomes relatively insensitive to changes in the GLR. The lower limit of GLR is obtained when the spray falls out of the atomization regime and the liquid is no longer atomized. The limiting GLR beyond which the water spray deteriorates is about 0.7, independent of CO2 flow rate. In summary, from the data reported in Figure 6, it can be deduced that the effect of the CO2 flow rate on the SMD is negligible throughout the GLR range investigated. This finding is somewhat surprising because an increase in CO2 flow rate should increase the available atomization energy, which should produce a refinement of droplet size. To explain this behavior, the gas velocity (Vg) and gas flow area (Ag) were calculated as functions of GLR at different gas flow rates. The gas velocity through the exit orifice can be

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Figure 7. Variations of (a) gas flow area and (b) gas velocity as functions of GLR obtained with a 120-µm exit orifice.

calculated assuming one-dimensional steady flow and applying mass continuity to the total flow and to the gas flow as m ˙ ) FATV

(4)

m ˙ g ) FgAgVg

(5)

where m ˙ g is the CO2 mass flow rate, Fg is the CO2 density, and Ag is the average flow area occupied by the gas. The relationship between the total area, AT, and Ag is given by the void fraction, defined as17 R)

Ag AT

(6)

with AT ) Ag + Al, where Al is the average flow area occupied by the liquid. Substituting AT into eq 6 and utilizing the expression of continuity (eq 4) yields

R)

m ˙g FgVg m ˙l m ˙g + FgVg FlVl

(7)

Because the velocities of gas and liquid into the orifice can be assumed to be equal, eq 7 can be simplified as

R)

m ˙g Fg m ˙l m ˙g + Fg Fl

(8)

Considering that the GLR is defined as m ˙ g/m ˙ l, Ag is thus given by m ˙g Fg d2 Ag ) π m ˙ g /GLR 4 m ˙g + Fg Fl

(9)

where d is the exit orifice diameter. Equation 9 gives the value of the gas flow area as a function of GLR when Fg and Fl are known. Values of Fg were calculated using the Bender equation of state18 at the given pressure inside the orifice (taken to be equal to that in the mixer). Values of Fl were calculated considering the solubility of CO2 in water at the given pressure and temperature. Finally, Vg was calculated from eq 5. Values of Ag and Vg are plotted in Figure 7a,b against GLR at two different CO2 flow rates. Ag increases with GLR up to about GLR ) 6 and then reaches a plateau. Ag does not change

with m ˙ g (the two curves in Figure 7a are perfectly superimposed) because the change in gas flow rate is balanced by the increase in gas density due to the increase in pressure. The increase in gas flow area is beneficial to atomization because it reduces the area available for the liquid flow; as a consequence, it squeezes the liquid into thinner films and ligaments where it flows through the injector orifice. On the contrary, gas velocity decreases with the GLR and increases with the gas flow rate. A decrease of gas velocity (and thus of liquid velocity) is not beneficial to atomization because the flow of liquid is decelerated through the exit orifice, causing it to be discharged at a lower velocity. Based on these calculations, it can be concluded that the mean droplet diameter shown in Figure 6, which is insensitive to gas flow rate and gas velocity, is mainly influenced by the sudden release of dissolved gas from the inside of the liquid, which is able to overcome the kinetic effect of the atomizing gas associated with the decrease of velocities. It is now possible to make a hypothesis about the role played by carbon dioxide in this atomization mechanism. It can be assumed that supercritical CO2 acts on the liquid breakup through two different mechanisms: (1) in analogy with effervescent atomization, it squeezes the liquid into ligaments as the liquid flows through the injector orifice, and (2) in analogy with flashing atomization, the gas explodes downstream of the nozzle exit, shattering the ligaments and producing droplets. The former effect is attributable to undissolved CO2, which reduces the area available for the liquid flow by squeezing it toward the orifice walls. The latter is related to dissolved CO2, which, flashing from the liquid ligaments downstream of the orifice, produces liquid droplets. A comparison of the SMD data reported in Figure 6 with those obtained using twin-fluid atomization, and particularly effervescent atomization, is difficult because of the differences between the experimental conditions usually employed. However, the nonlinear dependence of the SMD on the GLR shown in Figure 6 is consistent with the results of a number of investigations on twin-fluid atomization. For example, in the case of effervescent atomization, Ramamurthi et al.19 observed that the SMD decreases rather rapidly with an increase in GLR in the bubbling flow regime and the rate of change of the SMD reduces in the annular flow regime. Sovani et al.2 reviewed various works that indicated a decrease of the SMD with GLR when the GLR was below about 0.03 and a slow decrease above this value. Supercritical atomization differs from conventional effervescent atomization in the GLR values employed, which are more than 1 order of magnitude higher, but the trends of the curves are similar. The change in the slope of the SMD vs GLR curve in the case of effervescent atomization is observed at around GLR ) 0.03, whereas in supercritical atomization, it is observed at a GLR value of about 3. Thus, even if the shapes of the curves are similar, the mechanisms that produce droplets should be different, and in particular, the roles played by the gas and the natures of the two-phase flow are different. Indeed, effervescent atomization is usually operated in the bubbling flow regime, whereas in SDGA, the liquid droplets are dispersed in a gaseous plume. For example, according to data reported by Whitlow et al.17 for the effervescent atomization of water with air, the minimum SMD is 20 µm for low-pressure operation (6.89 MPa). Highpressure atomization of fuels has been studied by Sovani et al.15 and Satapathy et al.20 up to 27.6 MPa. A comparison can be established with the data of Sovani et al.15 obtained at pressures

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Figure 8. Droplet size distributions of a supercritical spray at high and low GLR, measured at an axial distance of 7.5 cm, a CO2 flow rate of 18 g min-1, and an exit orifice of 100 µm.

closer to those of this work. Atomization of a diesel oil carried out at 12.3 MPa and GLR of 0.0125 produced an SMD of 6.25 µm. In the present work, atomization of water, which is denser and more viscous than diesel oil, performed with a GLR of 3 at about 10.0 MPa produced an SMD of 2 µm. In addition to mean droplet size, another parameter of importance in the definition of a spray is the droplet size distribution. In Figure 6, the SPANs of the droplet distributions are reported, along with SMD data. The trend of the SPAN with GLR follows that of the SMD. Thus, an increase in GLR produces a decrease in the mean droplet size and a narrower distribution. The trends of SMD and SPAN with GLR are the same at all of the CO2 flow rates investigated (data not reported). Another important factor in the definition of the droplet population is the droplet size distribution curve. A typical representation of the droplet size distribution obtained in this work is reported in Figure 8, where experimental data obtained from the laser diffraction analyzer have been fitted with distribution functions. The general trend observed in this work is that, at high GLRs, the distribution is unimodal and, at low GLRs, an enlargement of the distribution is observed until a bimodal shape is found. In Figure 8, two representative examples of these trends are reported in the case of a high GLR (6.43) and a low GLR (1.0). When the GLR was further reduced below the value at which the onset of the bimodal distribution was observed (generally about GLR ) 1), a transition of the spray flow was also observed from fine atomization to the onset of instabilities and ejection of liquid ligaments, as a result of liquid slugs passing through the orifice. 3.4. Influence of Atomization Pressure. The atomization pressure is a variable correlated with the GLR: When the liquid flow rate is increased at constant CO2 flow rate, a moderate increase of the pressure within the mixing vessel is observed. The trends of pressure with GLR at three CO2 flow rates are reported in Figure 9. The pressure is higher on the left (low GLR, high liquid flow rate) and lower on the right (high GLR, low liquid flow rate) of the diagram. The overall result is that the SMD of the droplets decreases despite the decrease of the atomization pressure. However, the variation in pressure is relatively small (about 2.0 MPa over the whole GLR range) because it depends on the CO2 flow rate more than on the liquid flow rate; moreover, it is concentrated in the GLR range between about 0.8 and 2. Most studies on twin-fluid atomization indicate that the SMD decreases with increasing injection pressure and that, usually, the effect of pressure is more pronounced at low injection pressures than at high injection pressures.15 As shown in Figures 5 and 9, in the case of SDGA, an increase in the GLR produces decreases in both the SMD and the pressure; this implies that

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Figure 9. Pressure variation in the mixer as a function of GLR at CO2 flow rates of 12, 16, and 18 g min-1. Orifice ) 120 µm; axial distance ) 7.5 cm.

Figure 10. Effect of GLR on SMD at various injection pressures.

Figure 11. (a) SMD as a function of GLR measured at various axial distances. (b) Droplet size distributions measured at GLR ) 1.5 and various axial distances from the exit orifice.

the injection pressure has a minor role in the breakup mechanism. This observation is fully supported by the data reported in Figure 10, in which SMD vs GLR curves in different pressure ranges are reported. The trend of these curves demonstrates that, in the range investigated, the SMD is virtually independent of pressure. The minor role of pressure confirms that the decrease of the droplet size is mainly due to the action of dissolved gas. 3.5. Influence of the Axial Distance from the Atomizer Orifice. The mean droplet size and distribution were measured at four different locations downstream of the atomizer orifice, namely, 7.5, 13.5, 19.5, and 25 cm. As shown in Figure 11, very similar values of SMD were measured at 7.5 and 13.5 cm throughout the range of GLR investigated. At higher distances, the SMD increased with increasing distance from the orifice and with GLR. The mean droplet size increased by up to 40% with an increase of the downstream distance from 7.5 to 25 cm. this change can be attributed to the combined effect of the evaporation of the smallest droplets and droplet coalescence. Moreover, it indicates that the hydrodynamic breakup of the droplets due to the relative velocity between the droplets and the surrounding air does not occur or is negligible. However, the influence of the axial distance on the SMD decreases when the value of the GLR is decreased (Figure 11a).

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Table 2. Characteristic Values of Volume-Based Size Distribution Parameters of Water Droplets and Cromolyn Sodium Particles Precipitated from Water by SAA

size range (µm) D10 (µm) D50 (µm) D90 (µm) SPAN

particles

droplets

0.2-12 0.49 1.52 4.2 2.44

1-12 2.5 4.09 7.81 1.3

The effect of the axial distance on the droplet size distribution was also studied. Representative results are reported in Figure 11b in the case of atomization at GLR ) 1.5. Similarly to the SMD, the width of the distribution is a function of the axial distance. Droplets measured at 7.5 cm had a very narrow distribution ranging between 0.1 and 20 µm. At higher distances, the distributions enlarged; in particular, at 19.5 and 25 cm, the presence of a second population of droplets with dimensions up to hundreds of micrometers was found. This result confirms the tendency of droplets to coalesce at longer distances from the injector. At this point, it is useful to compare the droplet sizes and distributions with those of particles obtained by SAA under the same conditions. The particle diameters and distributions were measured generally by scanning electron microscopy but also, in some cases, by light scattering (i.e., the same technique as used in this work). For example, a comparison can be established between particles of cromolyn sodium precipitated from water10 and droplets obtained in this work under the same operating conditions. Droplet data at an axial distance from the orifice of 7.5 cm were considered here. In Table 2, a comparison between the main size parameters of droplets and particles obtained at GLR ) 1.8 is reported. The maximum sizes of droplets and particles are very similar (about 12 µm), and also the shapes of the distribution curves are similar (not reported). The main difference is in the low size range of the population. Indeed, particles have a larger size distribution (see the SPAN value and size range), but the distribution curve is shifted toward lower sizes (see the D10 and D90 values). This comparison is useful for two reasons: (1) It indirectly validates the reliability of the droplet size measurements obtained in this investigation, and (2) it allows for the conclusion that the particles are the result of droplet evaporation that follows a one-droplet, one-particle drying step. Two considerations can be made on the basis of these data. First, the formation of particles seems not to be influenced by droplet coalescence, probably because the very fast evaporation of water in SAA processing (in which hot nitrogen is added to the precipitator to hasten the evaporation) occurs at small distances downstream of the orifice. This is beneficial for the process, because coalescence works on the opposite side of the process, producing an increase of the final particle size. Second, in the particle population, very small sizes down to 0.2 µm are present. On the contrary, in the droplet population, the lowest size is about 1 µm. We can hypothesize that small droplets were not recorded by the analyzer because they evaporated very soon after the orifice. Instead, when the solute was present, smaller particles were in every case measured by the instrument. However, it can be also hypothesized that droplet measurements were made too far from the orifice and, hence, that the results presented cannot be exploited for an SAA interpretation. To solve this problem, droplet measurements in the presence of the solute will be performed in the future development of this work.

Figure 12. Effect of injector diameter on the droplet SMD. QCO2 ) 18 g min-1; axial distance ) 13.5 cm.

3.6. Influence of the Orifice Diameter. The atomizer size and geometry control the gas-liquid flow structure inside the atomizer and influence the droplet size. Several different atomizer designs can be adopted, but simple orifices with a circular hole having diameters of 100 and 120 µm and length/ diameter ratios of 8 and 6.67, respectively, were used in this work to simplify the process of parameter analysis. SMD as a function of GLR was measured using the two orifices; the results are compared in Figure 12. The SMD slightly decreased with an increase in nozzle diameter from 100 to 120 µm. The size difference was found to be larger at higher GLR; below a GLR of about 2, the curves tended to a common value. If we consider a fixed value of GLR, for example, GLR ) 3, the SMD decreased from 3.4 to 2.8 µm when the exit orifice changed from 100 to 120 µm. The effect of the orifice diameter on the SMD is limited to some extent by the fact that the injection pressure slightly decreased when the orifice diameter was increased, because, as previously discussed, the experiments were performed at constant flow rates. To explain the reduction of the droplet mean size with the larger nozzle, a comparison can be made with effervescent atomization studies performed under similar conditions. In the case of effervescent atomization performed at low pressure (kilopascal range), most of the authors reported that droplet size was largely independent of the final discharge orifice diameter. On the contrary, Wade et al.,21 who measured the droplet sizes produced by an effervescent atomizer at a very high injection pressure (33 MPa) for relatively small orifice diameters (180-340 µm), noted that the mean droplet size decreased slightly as the diameter of the discharge orifice was increased. No arguments were proposed to explain this observation, but Chin et al.22 hypothesized that it could be due to the reduction in the length/ diameter ratio of the orifice. Indeed, they found that the lengthto-diameter ratio (l/d) of the discharge orifice has a significant effect on atomization performance, with the mean droplet size decreasing as the l/d ratio is reduced as in our case, where l/d changed from 8 to 6.67 when nozzle diameter was increased from 100 to 120 µm. They attributed this result to lower frictional losses associated with a reduction in the orifice l/d ratio. 4. Conclusions It has been shown that water-based supercritical dissolvedgas atomization has the potentiality to produce very fine spays with droplet dimensions down to about 2 µm and with very narrow droplet size distributions. Droplet diameters typical of SDGA are usually not attainable with other twin-fluid atomization processes at comparable pressures. For example, effervescent atomization usually produces droplets that are 1 order

Ind. Eng. Chem. Res., Vol. 49, No. 19, 2010

of magnitude larger. The performance of SDGA can be exploited in particle formation processes, as shown by the SAA process for the production of microparticles of a large variety of compounds such as drugs and polymers. Regarding the atomization mechanism, the SDGA of water is a complex process in which gaseous CO2 acts on the liquid breakup through two different mechanisms: (1) by squeezing the liquid into ligaments as it flows through the injector orifice and (2) by exploding downstream of the nozzle exit to shatter these ligaments and produce small droplets. The results of this work indicate that the second mechanism is the one that mainly controls the efficiency of water atomization. Indeed, the mean droplet diameter is mainly controlled by the sudden release of the dissolved gas. Because SDGA operates with very dilute sprays, the tendency of droplets to coalesce is limited. This fact allows for the production of very fine droplets with narrow size distributions. However, a moderate tendency to coalescence was observed in a series of measurements performed at long axial distances downstream of the orifice. A comparison of the previous results on particle analysis obtained by SAA and those proposed in this work for droplets demonstrate that fast evaporation in SAA processing probably hinders the problem of coalescence. Literature Cited (1) Lefebvre, A. H. Atomization and Sprays; Taylor & Francis: Boca Raton, FL, 1989. (2) Sovani, S. D.; Sojka, P. E.; Lefebvre, A. H. Effervescent atomization. Prog. Energy Combust. Sci. 2001, 27, 483. (3) Sher, E.; Elata, C. Spray Formation from Pressure Cans by Flashing. Ind. Eng. Chem. Process Des. DeV. 1977, 16, 237. (4) Rashkovan, A.; Sher, E. Flow pattern observations of gasoline dissolved CO2 inside an injector. Atomization and sprays 2006, 16, 615. (5) Ohe, S. Vapor-Liquid Equilibrium Data at High Pressure; Elsevier: Tokyo, 1990. (6) Reverchon, E. Supercritical assisted atomization to produce microand/or nanoparticles of controlled size and distribution. Ind. Eng. Chem. Res. 2002, 41, 2405. (7) Reverchon, E.; Della Porta, G. Particle design using supercritical fluids. Chem. Eng. Technol. 2003, 8, 840.

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(8) Reverchon, E.; Della Porta, G. Micronization of some antibiotics by supercritical assisted atomization. J. Supercrit. Fluids 2003, 26, 243. (9) Reverchon, E.; Della Porta, G. Terbutaline microparticles suitable foraerosol delivery produced by supercritical assisted atomization. Int. J. Pharm. 2003, 258, 1. (10) Reverchon, E.; Adami, R.; Caputo, G. Production of cromolyn sodium microparticles for aerosol delivery by supercritical assisted atomization. AAPS PharmSciTech 2007, 8 (4), Article 114. (11) Reverchon, E.; Adami, R.; Caputo, G. Supercritical assisted atomization: Performance comparison between laboratory and pilot scale. J. Supercrit. Fluids 2006, 37, 298. (12) Wang, Q.; Guan, Y. X.; Yao, S. J.; Zhu, Z. Q. Microparticle formation of sodium cellulose sulfate using supercritical fluid assisted atomization introduced by hydrodynamic cavitation mixer. Chem. Eng. J. 2010, 159, 200. (13) Swithenbank, J.; Beer, J. M.; Taylor, D. S. Experimental diagnostics in gas phase combustion engines, Experimental diagnostics in gas phase combustion systems. Prog. Astron. Aeron, AIAA 1977, 53, 421. (14) Dodge, L. D. Change of calibration of diffraction based particle sizers in dense sprays. Opt. Eng. 1984, 23 (5), 626–630. (15) Sovani, S. D.; Crofts, J. D.; Soika, P. E.; Gore, J. P.; Eckeerle, W. A. Structure and steady-state spray performance of an effervescent diesel injector. Fuel 2005, 84, 1503. (16) Diamond, L. W.; Akinfiev, N. N. Solubility of CO2 in water from1.5 to 100 °C and from 0.1 to 100 MPa: Evaluation of literature data and thermodynamic modelling. Fluid Phase Equilib. 2003, 208, 265. (17) Whitlow, J. D.; Lefebvre, A. H. Effervescent atomizer operation and spray characteristics. Atomization Sprays 1993, 3, 137. (18) Brunner, G. Gas Extraction; Springer: New York, 1994. (19) Ramamurthi, K.; Sarkar, U. K.; Raghunandan, B. N. Performance characteristics of effervescent atomizer in different flow regimes. Atomization Sprays 2009, 19 (1), 41. (20) Satapathy, M. R.; Sovani, S. D.; Sojka, P. E.; Gore, J. P.; Eckerle, W. A.; Crofts, J. D. The effect of ambient density on the performance of an effervescent atomizer operating in the MPa injection pressure range. In Proceedings of the Spring Technical Meeting of the Central States Section of the Combustion Institute (CSS/CI) 1998 The Combustion Institute: Pittsburgh, PA, 1998, pp 76-80. (21) Wade, R. A.; Weerts, J. M.; Sojka, P. E.; Gore, J. P.; Eckerle, W. A. Effervescent atomization at atomization pressure in MPa range. Atomization Sprays 1999, 9 (6), 651. (22) Chin, J. S.; Lefebvre, A. H. A design procedure for effervescent atomizers. ASME J. Eng. Gas Turbines Power 1995, 117, 227.

ReceiVed for reView April 20, 2010 ReVised manuscript receiVed August 15, 2010 Accepted August 26, 2010 IE100925W