Liquid, electrostatic atomization

Electroslag remelting, 23 255 Electroslurry process, 23 576 Electrospinning, 11 186 Electrospray ionization, liquid chromatography, 4 625 Electrospray ionization source, 15 654-658 Electrostatic atomization, in spray coating, 7 72-73... 

Electrostatic Atomization 0.1-1000[88] 300-600[88] 100-250[126] Paint spraying, Printing, Oil burner Fine and uniform droplets Very low flow rates, Strongly dependent on liquid electrical properties... 

In electrostatic atomization, an electrical potential is applied between a liquid to be atomized and an electrode placed in the spray at a certain distance from liquid discharge nozzle. As a result of the mutual repulsion of like charges accumulated on the liquid surface, the surface becomes unstable and disrupts when the pressure due to the electrostatic forces exceeds the surface tension forces of the liquid. Droplets will be generated continuously if the electrical potential is maintained above a critical value consistent with liquid flow rate. Both DC and AC systems have been employed to provide high electrical potentials for generating fine droplets. Many configurations of electrode have been developed, such as hypodermic needles, sintered bronze filters, and cones. 

A number of investigations have been made to explain the droplet formation mechanisms associated with electrostatic atomization. 119][1201 It has been hypothesized that the dispersion of a liquid by electrostatic atomization occurs via the detachment of a single droplet from the capillary tip of the liquid. However, this mechanism has not been proven experimentally. Due to the complex physics involved, a generic theoretical model has not yet been established. 

Generally, the droplet size generated in electrostatic atomization is a function of applied electrical potential, electrode size and configuration, liquid flow rate, liquid nozzle diameter, and liquid properties such as surface tension, dielectric constant and electrical conductivity.[121] [124] When a low electrical potential is applied to a liquid, a stream of relatively uniform droplets will form below the liquid discharge nozzle. As the applied electrical potential is increased, the droplets produced become smaller, and the liquid velocity and droplet production rate both increase, with concomitant... 

It should be noted that, to produce a reasonably monodis-perse spray, the liquid flow rate should be maintained at an extremely low level, and thus the scaling up of such devices may pose some difficulties. It is also rather difficult to assess the liquid flow rate that can be achieved due to few quantitative studies and lack of comprehensive understanding of the underlying principles. Another drawback of the electrostatic atomization technique is that both the production and properties of droplets are significantly dependent on the electrical properties of the liquid, limiting the type of liquids that can be successfully atomized. 

Electrostatic forces can be utilized for the direct dissemination of materials, both liquid and solid. The dissemination of liquids by this means (electrostatic atomization) has been studied extensively. The corresponding dissemination of powders is referred to in the literature but has not received much attention in depth. Dissemination of bulk solids (i.e., consolidated solids as distinguished from powders) by electrostatic means has not been studied and, although it may be technically possible, is probably not feasible. 

Peskin and Raco (P3) have given a theoretical analysis of both ultrasonic and electrostatic atomization from the point of view of liquid instability. They conclude that atomization with low frequency ac will require about twice the field strength as dc but that, by going to high frequency, lower fields are possible with conducting liquids. The value for the critical field for atomization given by these authors for a dc field is, however, smaller than that which would be calculated from Eq. (39) by a factor of (1/32)1/2. This presumably reflects the simplified one-dimensional model used in their derivation. 

Shaffer (S5) also made some exploratory evaluations of the electrostatic atomization of dibutyl phthalate using a camel s hair brush for the atomizing nozzle. On a count basis 73% of the particles were smaller than 10 microns and the largest particle obtained was 40 microns. The energy input corresponded to 0.5 cal/g liquid atomized (0.00026 kWh/lb) and the charge level on the particles as atomized corresponded to a value of Sps of the order of 3-5 V/micron. Current and flow rate measurements reported by Vonnegut and Neubauer (V4) would correspond to an energy input of 0.1 kWh/lb. 

Electrostatic Atomization. If the following modifications are made in the apparatus of Figure 2, then electrical forces can be utilized to stress a jet of low-conductivity liquid sufficiently to atomize (and charge) the liquid with no additional energy input (12) (a) remove point P (b) reduce the length Z of the... 

A somewhat more recent approach to electrostatic atomization and charging has been developed for hydrocarbon fuels in various combustion systems (13). This method employs direct charge injection into the continuous stream of hydrocarbon liquid by means of... 

All liquids will spray the same, that is they will have the same mean droplet size independent of fluid properties, if they are charged to the same level. For instance, for oils of interest having viscosities less than approximately 200 centlpolse, uniform spray charging to 4 C/m will result in the electrostatic atomization of a 42 2 ym diameter spray."... 

A basic requirement of burner combusting liquid fuels is a high-quality fuel atomization [9], necessary for complete evaporation and burnout in the area of the flame. If some fuel drops are not evaporated and combusted in the area of flame, concentrations of carbon monoxide and unburned hydrocarbons (UHCs) in flue gas increase rapidly. For the above mentioned reason most liquid fuel burners are designed as diffusion burners with fuel atomized in the combustion chamber. The fuel atomization system itself is rather dependent on physical and chemical properties of fuel and availability of auxiliary atomizing medium. Thus there are three basic types of atomization [10] (i.e., pressure, pneumatic, and rotary atomization). Besides these, there are other, less frequent types of atomization using vibrational, acoustic, ultrasonic, and electrostatic atomizers or flash liquid atomization. 

Electrostatic atomization and electrostatically assisted atomization have been employed in a variety of areas, including paint spraying [1], electrostatic printing [2], and cell immobilization [3]. The basic concept behind these applications involves electrostatic forces, which work to disrupt the liquid surface to form a charged stream of fine droplets. The effect of electrostatic forces on mechanically atomized liquid droplets was first studied in detail by Lord Rayleigh [4,5], who investigated hydrodynamic stability of a jet of liquid with and without applied electric field. 

Electroencapsulation is an application of electrospraying in which liquid is atomized into micro- or even nanosized droplets using electrostatic forces alone. The method allows better controllability of the capsulation process than, e.g., the most commonly used spray drying. Due to the applied electrostatic forces, it also enables production of complex capsule structures, like solid shell covered liquid core particles. In this review, we will focus on electroencapsulation processes used to improve the handling, processing, and administration of porous silicon-based drug delivery systems. 

The range of systems that have been studied by force field methods is extremely varied. Some force fields liave been developed to study just one atomic or molecular sp>ecies under a wider range of conditions. For example, the chlorine model of Rodger, Stone and TUdesley [Rodger et al 1988] can be used to study the solid, liquid and gaseous phases. This is an anisotropic site model, in which the interaction between a pair of sites on two molecules dep>ends not only upon the separation between the sites (as in an isotropic model such as the Lennard-Jones model) but also upon the orientation of the site-site vector with resp>ect to the bond vectors of the two molecules. The model includes an electrostatic component which contciins dipwle-dipole, dipole-quadrupole and quadrupole-quadrupole terms, and the van der Waals contribution is modelled using a Buckingham-like function. 

M. P. Allen, D. J. Tildesley, Computer Simulation of Liquids Oxford, Oxford (1987). Chemical Applications of Atomic and Molecular Electrostatic Potentials P. Politzer, D. G. Truhlar, Eds., Plenum, New York (1981). 

Semidry Scrubbers The advantage of semidry scrubbers is in that they remove contaminants by way of a solid waste that is easier to dispose of (less expensive). Initially, the scrubbing medium is wet (such as a lime or soda ash slurry). Then a spray dryer is used to atomize the slurry into the gas which evaporates the water in the droplets. As this takes place, the acid in the gas neutralizes the alkali material and forms a fine white solid. Most of the white solids are removed at the bottom of the scrubber while some are carried into the gas stream and have to be removed by a filter or electrostatic precipitator (discussed later). Although semidry systems cost 5-15% more than wet systems, when combined with a fabric filter, they can achieve 90-95% efficiencies. Dry scrubbers are sometimes used in a very similar fashion, but without the help of gas-liquid-solid mass transfer, these systems use much higher amounts of the solid alkali materials. 

---