Microencapsulation

Electrostatic Spray Drying

In microencapsulation, each emulsion component has differing polarities. When an electrostatic charge is applied, the solvent and carrier, being the most polar of the group, will have the largest electric dipole moment. The active, being less polar, will have a smaller dipole. The solvent molecules will repel each other and the solid particles. This will force the solvent and carrier to migrate to the outer surface of the droplet, while the active will remain at the center.

Driving the solvent to the outer surface creates the ideal drying condition, leading to a near perfect encapsulation of the active without the use of high evaporation temperatures.

Traditional Spray Drying

Spray Drying Technology has been in use on an industrial scale since the late 19th Century. In that time, the technology has evolved only incrementally with the fundamentals remaining very much the same still today.

Typically, the encapsulation in a traditional spray dryer uses an emulsion made up of three components; a solvent (water or solvent), a carrier (starch), and a core/active (oil or vitamin). The object of Spray Drying is microencapsulation - forming the carrier around the active and drying off the solvent with a heated drying gas. Ideally, the carrier remains as a protective layer around the active keeping it from oxidizing.Traditionally, the emulsion is atomized using a nozzle or rotary atomizer and heated drying gas (200°C or higher) is introduced to the atomized emulsion.

Some major draw backs of traditional spray dry include intense heat that can degrade the end product, and a dried particle that has the active trapped both inside the droplet and on the surface, partially defeating the intention of microencapsulation.

Fluid Air traditional spray drying droplet temperature graph with falling-rate period and shell formation
Droplet Temperature in Traditional Spray Dryer
°C


Fluid Air electrostatic spray dryer droplet temperature constant-rate period through outlet temp graph
Droplet Temperature in Electrostatic Spray Dryer




Low Temperature Spray Drying

In both Traditional Spray Drying and in the PolarDryTM process, liquid droplets are atomized and sprayed into a stream of drying gas. Heat is transferred to the liquid from the drying gas to drive the evaporation of the solvent. Once all the solvent has been evaporated, the end product is a dry, powdered material.

In Traditional Spray Drying, there are two distinct phases of drying: a Constant-Rate drying phase, and a Falling-Rate drying phase. During the Constant-Rate phase, the majority of heat transferred to the droplet is latent, and used to drive the evaporation of the solvent. The solvent evaporation cools the surrounding drying gas, and the droplet temperature remains constant. As more and more of the solvent evaporates from the droplet, the solid content of the outer layer of the droplet increases to the point where it forms a shell. At this point, a particle with a solid shell but wet core is formed, and the drying phase switches to the Falling-Rate phase. During this phase, heat is transferred to the particle from the drying gas as sensible heat. The temperature of the particle is raised to fully evaporate the remaining solvent from the core of the particle.

In the PolarDry Process, the electrostatic effect is used to stratify the components of the droplet during atomization, based on the polarities of the materials. With a feedstock based on a polar solvent, the solid materials are driven to the inside of the droplet, and the solvent is driven to the outside. This prevents shell formation, and eliminates the need for the Falling-Rate drying period. Significantly more of the heat energy from the drying gas is used for latent heat, and less is used for sensible heat. This allows for fast, efficient drying without the need to raise the temperature of the product.