Spray Drying Basics 4

Spray Drying Basics Diagram 3

Spray Drying Extraction Cyclone

The high efficiency cyclone (figure 9) is the most commonly used item of plant for separating  the dried powder from the exhaust air and is often preferred where multiple or difficult products are produced due to the relative ease of cleaning down and low cost. Although over 99% collection efficiency is possible, back up after the cyclone in most locations is usually necessary to meet emission regulations. Back up air cleaning is usually provided by either a wet scrubber or a “policeman” bag filter.

Where only a single material is being dried or products are manufactured in campaigns, the ‘bag filter’ or ‘baghouse’ may be used without the need for a cyclones. The bag filter has a much higher efficiency rating than cyclones and meets all emissions regulations. The most common type of bag filter employed would be the reverse jet bag filter. This type ensures that the bags operate at maximum efficiency at all times as the compressed air jet timings can be adjusted to suit the application.

Where the investment capital is plentiful a CIP capable bag filter can be employed giving all the collection benefits of the bag filter with the wash capabilities of a cyclone.

Cyclone Dead Zone

Powders enter a cyclone tangentially, air and entrained powder spin down the length of the cyclone wall, while air is pulled tangentially in the opposite direction creating an inner vortex. Where the two vortex meet at the bottom of the cyclone a dead zone is created, when the powder entrained in the air hits the dead zone it is spun out free of the vortex while the air is pulled from the system. Cyclones must be sealed so powder exits through a rotary valve or similar device.

Bag Filters

Bag filters work very simply, air is pulled through the filter drawn by the exhaust fan powder collects on the bags while air is allowed to pass through the filter medium. A compressed air jet dislodges the powder from the bag and drops into a hopper and out of the system.      

Cyclone
Bag Filter

Exhaust

On small basic spray drying plant an exhaust fan may generate all the air passing trough the system. On larger plant two fans are usually the preference reducing the overall noise level and allows a more balanced system. The exhaust fan draws the air from the chamber and through the separation system discharging spent air to atmosphere.

To produce powder from spray dryers you don’t need a lot of instrumentation. Temperature is critical to the process, air must be hot enough to evaporate liquids but not too hot to damage the material being dried. The exhaust air temperature is the controlling temperature and must remain hot enough to keep the moisture from the evaporation process absorbed. The inlet temperature must be high enough to maintain the exhaust temperature after the evaporation process. The greater the difference between inlet and exhaust temperature then greater is the rate of drying. The final relative humidity of the exhaust air should not be greater than 60%. For this reason even basic plant are supplied with instrumentation to read both inlet and exhaust temperatures.

What is the best temperature for your product would normally be determined during product testing and varies widely between products dependent upon their sensitivity to heat. It is important to note that however hot the inlet temperature, the temperature of the powder in the air stream will not rise above the exhaust air temperature unless all internal moisture has been removed.

It’s also important to know the amount of feed material entering the system. On pressure nozzle systems a pressure indicator is employed, this indicates the amount of material the nozzle will allow to pass.

There are many other instruments that can be added to a spray dryer that will give you good information on the health of your system.