Process Puzzler: Deal with dryer drizzle

Our spray dryer is fed from two booms. These booms have a dozen tiny spray nozzles each. Each set of nozzles is set into a head, one to each boom. It is a new design that our management hopes will increase throughput. The result has been completely opposite. One of our engineers thinks that some of the droplets collide where the streams of the booms strike together. Conversations with the spray nozzle company and calculations confirm that this is the case. Our vendor suggests going with only one boom but this will cut our rate and the spray won’t be centered in the dryer. The droplets must fall at a specific rate to crystallize properly. Is there anything we can do that will allow us to work with the existing heads?

Use an additive
A centered stream is best for maximizing the rate and performance of the dryer. This necessitates the presence of the two opposite streams; that is also what management wants to keep. With this as a start, I would suggest considering including a chemical solution that incorporates an additive to alter the surface tension so as to promote small particle size on falling. I assume economics and product quality will not be adversely affected. The second alternative would be to increase the upstream air flow to enhance particle breakdown.
Emilio Malaguti, technical manager,
Chemtron, Hialeah, Fla.

Talk to a specialist
Without a complete redesign of the spray dryer, the best option is to keep the nozzle streams from mixing before evaporation has occurred. You may want to try to solve the problem yourself. This could give you a better understanding of the problem. If the distance between the booms cannot be adjusted because of chamber diameter restrictions, a nozzle orifice change is probably needed. Reduce the spray angle for each small nozzle in the spray head to eliminate the droplet collisions. When reducing the spray angle, the feed pressure will have to be adjusted to keep the same atomization.
After you’ve developed a better understanding of the problem, get some help. Talk with a company that specializes in spray nozzles to design the correct system for the application.
John Kelsoe, process engineer
Polyone, Peoria, Ill.

Run some tests
I assume that the spray nozzle is a simplex design involving a swirl piece producing a hollow cone or solid cone spray. This is typical for this type of application. To perform well, the spray must be atomized to an optimum droplet diameter and evenly dispersed to allow good evaporation.

Furthermore, the distribution of the droplet size must be fairly narrow. There are two components of the problem to consider: the physical conditions of the dryer and the design of the spray system.
The physical conditions consist of the slurry and the gases into which the evaporation occurs.

Considering these factors, several options exist for increasing evaporation, including: improving gas flow within the dryer; increasing the height the liquid must drop; reducing viscosity of the liquid; and increasing the liquid temperature or gas temperature. None of these tactics by themselves will eliminate the droplet collision problem. Without a solution to this problem, you may be forced to cut the dryer throughput.

One simple solution may be to change to larger nozzles for the part of the heads facing away from the booms and smaller nozzles for the nozzles pointed at each other. This might reduce the population of large drops. Another solution may be to eliminate all the interfering nozzles altogether.

Another possible approach may be to rebuild the spray heads to reduce the angles between the spray jets, giving them more time to evaporate before eventually colliding. This method combined with improvements in some of the physical conditions, such as a hotter dryer, may be enough to improve the situation.

Dirk Willard, senior process engineer
Ambitech Engineering, Hammond, Ind.