Typically, evaporation is carried out within vertical tubes. However, in some applications, process fluid evaporates outside horizontal or vertical tubes. The process fluid will always evaporate downwards due to the effect of gravity as a continuous film, creating a film along the tube walls and falling downwards. A fluid distributor is created to maintain an even liquid distribution for every tube that the solution falls down.
Falling film evaporators are widely used as an evaporation technique, however, they can be inefficient. This blog will explore the drawbacks of these evaporators, comparing them with rotary evaporators.
How do Falling Film Evaporators Work?
In most applications, the heating medium is on the outside of the tubes. High heat transfer coefficients are necessary to accomplish equally balanced heat transfer resistances. As such, the most commonly used heating medium is condensing steam.
When fluids are internally evaporating, separation across the liquid phase (the solution) and the gaseous phase happens within the tubes. To uphold conservation of mass as this falls, the downward vapor velocity rises and this increases the force acting on the liquid film and thus the velocity of the solution.
On the evaporating side of the tube, the heat transfer coefficient is often subject to the hydrodynamic flow conditions of the film.
Unfortunately, the inner wall of the heat exchange tube of the falling film evaporators must be entirely wet and the membrane is not too thick. It is important to implement a precise design calculation of the material feed and the length of the heat exchange tube in order to meet the requirements.
Additionally, the cost of falling film evaporators are extremely high as there are high design and manufacturing requirements. The rotary evaporator from Ecodyst does not use expensive rotary evaporator mechanisms, meaning they are an extremely cost-effective method of evaporation.
Read More: The Hidden Cost of Falling Film Evaporators
Falling film evaporators should also not be used for materials that are prone to fouling or are made up of solid suspended matter. Conversely, the rotary evaporator from Ecodyst can efficiently and gently remove solvents a broad range of sample types, this includes both organic and inorganic analytes as well as polymeric materials.
Falling film evaporators are often used in heat-sensitive products such as milk, fruit juice and pharmaceuticals as the time inside the heated tubes is very low and the materials are unlikely to spoil. Rotary evaporators also work well for these types of materials as samples are heated whilst the boiling points are decreased using a vacuum which is induced by the lab rotary evaporator. The boiling point decreases as the pressure of the liquid is decreased, and the solvent can be vaporized at a lower temperature than the standard boiling point.
Falling film evaporators are also extremely large, requiring a high headroom and meaning that labs need to mindful of how to accommodate them. As the Ecochyll from Ecodyst combines the chiller within the equipment, it has a much smaller footprint and can be used in even small-scale laboratories.