Multi-Pass Drum Drying Process
The dehydration process of Vandenbroek International’s drying systems takes place in a special MPS® rotary drum, the Multi-Pass System. It is characterized by:
- a turbulent rinsing of the wet product with the hot air flow
- a spiral shaped passage of air flow and wet product through the drum, which is called the Multi-Pass System
This dryer system is applied to dry various product streams.
The Multi-Pass system consists of 10 drying passes in the length of the drum. The convection drying process takes place as an adiabatic cooling process. The thermal energy that is required for the evaporation of the water comes from the hot air flow. The air flow cools down considerably during the process while the product temperature itself hardly rises.
- The spiral shaped passage, with 10 drying sections, the Multi-Pass Drum.
Flash drying, fast initial evaporation
The hot gas is co-currently mixed with the wet product at the inlet of the drum. Because of the large difference in temperature between the hot gas and the wet product, the free water is evaporated rapidly and almost 70% of the water evaporation takes place in the first section of the drum. The warm turbulent air stream flowing through the product creates a fast initial evaporation of the free surface water.
Post drying, low differences in temperature
The remaining water in the product is cellular bound moisture and needs more time to evaporate. For the final drying, up to 80% of the drying drum and drum length respectively are available. During the final drying, the remaining fringe water in the product is carefully removed at low differences in temperature.
The capillary bounded water in the product is slowly evaporated on low temperature, while product and warm air co-currently flow through the drum sections towards the drum outlet. During the post-drying the product temperature slowly rises until the adiabatic cooling line temperature which for the MPS system is between 65-85°C. Therefore, the Vadeb MPS system is tailor made for the drying of organic product streams that should not be overheated.
The residence time and temperature of the drum system can be individually adjusted for each different product.
- A typical temperature curve through the drum for the air flow and the wet product.
Drying like an adiabatic vessel
The convective dryer works in a similar way to an adiabatic vessel: no energy is lost when it is transferred from hot air to evaporated water. Therefore the physical process will follow the adiabatic cooling line represented in the Mollier diagram for energy. As long as the product temperature is lower than the air temperature and the air can absorb the water (i.e. the air is not saturated), evaporation will take place. The product never becomes warmer than the adiabatic drying temperature as long as it can evaporate its water. The adiabatic drying temperature is in the range of maximum 90°C for all drying systems.
