Heat recovery in milk powder drying by using a liquid sorption process

The last step in the production of milk powder is drying, an energy intensive process that demands 30% to 40% of the total energy input of a typical plant. It takes place in Spray Dryers (SD), where concentrated milk is sprayed and placed in direct contact with hot and dry air that cools down and gains humidity as water is evaporated from milk. The warm and humid air leaving the SD contains a small portion of potentially recoverable sensible heat and a large portion of latent heat that is impractical to recover by direct condensation due to the low dew point of this stream and due to the presence of milk powder particles that become sticky at high relative humidity values. In this research, the thermodynamic feasibility of a liquid sorption system for the recovery of heat from the exhaust of SD’s was investigated. The system proposed has two main advantages: the dehumidification of air in the absorber for reuse in the SD, and the production of medium pressure steam in the regenerator for integration to the steam network of the plant. A mathematical model was implemented in Matlab, and two system configurations were evaluated. The calculations showed that a SD equipped with this system can achieve energy savings between 58% and 99% when using aqueous solutions of phosphoric acid as liquid desiccant depending on the system configuration. The challenge with this liquid desiccant remains on the construction materials.