ANS:B - through circulation

"Through circulation" refers to a method of air or gas drying where the drying medium is continuously circulated through the material being dried. This circulation allows for efficient heat and mass transfer, resulting in effective drying. Here's how the through circulation drying process typically works:

1. Introduction of Drying Medium: The drying medium, which can be air or a specific gas depending on the application, is introduced into the drying system.
2. Passage Through Material: The drying medium is then passed through the material that needs to be dried. This could involve the medium flowing through a bed of solid material, such as grains, pellets, or powder, or passing through a space where the material is spread out or conveyed.
3. Heat and Mass Transfer: As the drying medium comes into contact with the moist material, heat and mass transfer occur. Heat from the drying medium causes moisture within the material to evaporate, turning it into vapor. The vapor then diffuses into the drying medium.
4. Removal of Moisture-Laden Medium: Once the drying medium becomes saturated with moisture, it is typically removed from the drying chamber or space. Depending on the system design, the moisture-laden medium may be vented outside the system or passed through a moisture removal device, such as a condenser or desiccant, to extract the moisture before recirculating the medium.
5. Recirculation: In through circulation drying systems, the dried medium is often recirculated back into the drying process. This can help conserve energy by reusing the heat already present in the medium and can improve the overall efficiency of the drying operation.

Through circulation drying is commonly used in various industries for drying bulk solids, such as grains, seeds, chemicals, and pharmaceuticals. It offers advantages such as uniform drying, efficient heat transfer, and the ability to control drying parameters to meet specific requirements. Additionally, it can be adapted to different scales of operation, from small-scale batch processes to large-scale continuous operations.