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A word on temperatures

  

Spray drying is an ideal way to dry materials that are heat sensitive. Despite the fact that very hot air is used in the process, the particles do not experience a significant temperature rise due to evaporative-cooling and a short residence time in the drying chamber. To illustrate this point, it is important to explain some of the basic thermodynamic principles of spray drying. 


Spray drying is an evaporative process and as such it differs from the boiling process. For example, boiling happens evenly throughout the entire mass of the liquid while evaporation occurs only at the surface of the liquid (this is why high surface area is important in spray drying). Also, boiling occurs at the boiling temperature of the liquid while evaporation occurs at all temperatures.  One of the most interesting differences between evaporation and boiling is that the rate of boiling is mostly controlled by the conditions of the water being boiled - temperature, agitation, etc. and the rate of evaporation is mostly controlled by the conditions of the air above the water being evaporated.  If the air is saturated with water then evaporation will be slow or impossible.  And if the air is very dry then evaporation will be fast.  This is part of the reason that the air in a spray dryer moves fast - the air transports evaporated water away from the droplets providing more capacity for evaporated water.  As such, the air stream is serving as both a heat transfer mechanism and a mass transfer mechanism.  


For evaporation to occur at the maximum rate the temperature of the droplet must be raised to the wet bulb temperature of the gas around the droplet. This is accomplished through a sensible heating period during which the hot drying air transfers heat to the droplet. This causes the temperature of the drying gas to decrease proportionally. Once the droplet reaches the wet bulb temperature the heat transfer process becomes a latent heat transfer process and the temperature of the droplet does not increase because the heat that is transferred to it from the drying gas is “spent” on a phase change. The temperature of the drying air continues to drop during the latent heat transfer step. Therefore, there is a temperature difference between the inlet and the outlet of the drying chamber. This difference in air temperature is commonly referred to as the “Delta T” and it is proportional to the amount of water that is being evaporated. Once evaporation is complete if the droplet (now a particle) is not removed from the hot air stream then it will enter into a second sensible heating step in which the temperature of the particle will again rise until it is removed from the hot air. Thus, it is desirable that the particle exits the drying chamber at exactly the right time, to soon then it will not be dry enough and too late then the particle can be damaged by increased temperature.   


Optimizing a spray drying process for residence time in order to produce the highest quality particle possible is a systematic process that is best done on pilot-scale equipment by experienced investigators.  At Division by Zero Labs we can guide you through this process, dramatically improving your chances of a successful scale-up.