37. Spray Drying, Spray Pyrolysis and Spray Freeze Drying

In conventional spray pyrolysis (CSP or simply SP), a solution is sprayed into a carrier gas forming small droplets; owing to the high temperature of the surrounding gas, the solvent is vaporized and the solute is precipitated on and within the droplets. If the air temperature is high enough, solute is decomposed to form final solid particles. A schematic diagram of the spray pyrolysis process is shown in Fig. 37.1 [1]. Spray drying (SD) is similar to spray pyrolysis, except that there is no chemical decomposition in SD and usually the process temperature is lower. SP and SD techniques may produce fully-filled or hollow particles depending on the operating conditions. In general, for most materials, hollow particles are formed if at the onset of solute precipitation on the droplet surface, the solute concentration at the droplet center is lower than the equilibrium saturation (Jayanthi et al. [2]). However, Chau et al. [3] showed that Jayanthi’s model is not applicable to the formation of NaCl particles.