Can a droplet of formalin evaporate in midair while falling? I am designing an evaporator for use in an enclosed environment. Due to the hazardous nature of formaldehyde, I want my evaporator to be wick-free.
   The environment in which the evaporator will be used is at a pressure of -10Pa to the outside atmosphere, has a humidity ranging from 28 - 33 %RH and a temperature that sits at around 36.5*C.
   The basic design for my evaporator is a long tube with an induction coil wrapped around its length and an opening at the bottom for the vapour to escape. As the droplet makes it way down through the heated tube I am hoping that with enough heat it will vaporise and be mixed into the enclosed atmosphere.
The solution I am using is formalin 40.
http://www.strathclydenutrition.com/index.php/Page/product_page/formalin-401
The room in which the vapour is required has a large fan that circulates the air, though I don’t know what speed this runs at or if this will effect the air in my evaporator tube.
I have yet to develop a prototype of my design and short of gaining a thermodynamics or chemical engineering degree, I thought I’d ask the friendly folks at the Physics Stack Exchange if you could help me first.
Any links to external tutorials or articles would be greatly appreciated.
 A: The theory:
The short answer is yes, a drop of formalin can evaporate while falling. The requirements are:


*

*the atmosphere around the formalin drop must have a low enough formalin vapour pressure (so that the atmosphere must be able to absorb more formalin).

*the temperature of the atmosphere around the droplet must be high enough.

*the droplets of formalin must be small enough. Smaller droplets have a higher surface area to volume ratio, meaning they can evaporate faster per given mass. Evaporation time $\tau \propto D^2$ where $D$ is the droplet size (see reference paper (1)).

*the fall must be long enough in terms of time.

*In case you want to increase the speed of gases in the opposite direction to the droplet fall, this other paper (2) shows that there is indeed a relationship between wind speed and evaporation rate.


A practical approach:
Suggested: You can design your system to allow the formalin droplets to evaporate during the fall by using the following tests to set your system specifications:


*

*Estimate your droplet size (volume and thus spherical diameter) by using a droplet dispenser such as a pipette to drop a known volume of formalin/drop it into a know volume, and counting the drops, thus being able to calculate volume and mass per drop, as well as surface area to volume ratio.

*Find out how long it takes to evaporate a given amount of formalin at an atmospheric composition and temperature similar inside your intended tube. You will also need to match surface area to volume ratio of the droplets above to see the evaporation rate under similar conditions. One can do this by placing an amount of formalin in a container such as a petri dish or beaker whose cross-sectional area can be calculated. Set the surface area to volume ratio by setting the depth (and thus amount) of formalin in the container. This will be a worst case for the droplet size selected, since, as the droplet size gets smaller, the evaporation rate will increase (as surface area per volume will increase, because volume decreases faster than surface area as a sphere gets smaller).

*Compare the evaporation rate obtained in the previous step with the predicted fall time of the formalin drop to see if you need to increase evaporation rate by, for example, increasing the temperature of the atmosphere, decrease droplet size, increasing cross wind speed.

*Achieving your goal of evaporation-in-flight might be easiest by decreasing droplet size by introducing the droplets into the pipe by a fine spray. Best to spray upwards, so that the droplets can rise before they fall down the tube, giving more evaporation time. To find the maximum required droplet size for your spray, go from step 1 again, but this time using the smaller droplet size.

*Iterate until you get a satisfactory set of conditions (small enough droplets etc) and then test in your pipe.

