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|1. Key stages of
spray drying process
The drying effect, in the drying chamber, is based on the intimate contact in between the drying air and the product to be dried. Over the course of the history of development of spray drying researchers and industrials have developed different ways of putting in contact the drying air and the material leading to the commercialization of 3 types of drying chambers :
- Co-Current flow
- Counter current flow
- Mixed flow
This page is explaining each concepts and details the pros and cons of each solutions.
In co-current flow, the product and the drying air are introduced in the drying chamber at the same place, typically at the top of the drying chamber for vertical ones. This means that the hotter air is contacting the product to be dried 1st.
This creates a very quick drying in the 1st part of the spray drying tower but, as the material is saturated in water just after atomization, it must be noted that its temperature remains more or less constant equal to the wet bulb temperature of the drying air. This is a very interesting property as it allows the material to be protected against overheating. This can be very useful for thermal sensitive materials.
The air temperature drops after 1st contacting the material and the air carries away the particles while drying them. As the temperature of the air has dropped and its humidity increased the drying efficiency is lower when the particles and the air exits the chamber. As the air is pneumatically conveying the material in the chamber the residence time is usually very short.
|The material to be dried is protected against overheating||Higher residual moisture that may requiring a 2nd drying stage|
|Short residence time|
|The material is pneumatically conveyed by the drying air|
In counter-current spray drying, the material to dry and the air are injected at opposite location in the drying chamber - usually the material at the top of the chamber and the air at the bottom - which means that the dry air is contacting 1st the particles which have been dried. In principle this way of operation allows to dry more as the driving force in between the dry air and the humid particle is always maximal.
As the dried particles contact the hotter air, their temperature increases and reaches a level close to the dry air inlet temperature. They are not protected by the free water evaporating like it is the case in counter current drying which may have as a consequence to overheat the material and degrade it. Thus this kind of drying can only be chosen for heat resistant products.
|The material to be dried is protected against overheating
||Possible overheating and degradation of the product
|Longer residence time than co-current spray drying
||Larger amount of fines carried away by the air to be
Mixed flow spray drying is combining the 2 techniques presented above. The hot air is introduced at the top of the drying tower but the liquid is injected on the bottom half of the chamber, with the atomization facing upwards. This means that the particles travel counter-current at 1st then, when carried by the gas, travel co-current.
The atomization happens in air already cooled down which results
in lower drying rate and in a denser product. Care must be taken
not to damage the product at the particles partially dried get
into contact with the hotter air at the top of the chamber.