Falling Film Evaporator

This type of evaporator, sometimes referred to as a FFE, finds use in a wide range of industries when either doing bulk solvent removal to concentrate a product or to recover a solvent. Falling film evaporators are similar to wiped film evaporators since they both are able to remove solvent without too large of a heat impact and do so with one HETP.

Falling Film Distillation

Both the falling film evaporator and wiped film evaporator are vertically oriented and cylindrical and receive feed liquid pumped in steadily to the top portion. Also, in both cases, feed liquid flows downward while being heated and under controlled vacuum, during which the more volatile components condense on a condenser, discharging as one stream, (condensate or distillate), while the less volatile, non-evaporated components discharge as a separate stream, (concentrate or residue). The differences between these systems are a WFE is a single vertical cylinder with a set of wipers actively spreading the liquid evenly around the inner wall while being partially evaporated, whereas an FFE is a bundle of many tubes without wipers in which the liquid falls downward while being partially evaporated.

Because a falling film evaporator has no moving parts except for the feed and discharge pumps, it is typically less costly than an equivalent WFE. These are often chosen for applications where there is a high percentage of volatile components such as solvent or water in the feed, and where heat sensitivity or viscosity are not extreme. Examples include the concentration of orange and other citrus juices and the elimination of solvents and water to decrease the volume of waste streams which are heavy and/or contain solids. If heat degradation or tube fouling becomes an issue with falling film evaporator units in certain applications, wiped-film evaporator units, with their active, evenly distributed wiping mechanisms, are a better choice.

Careful evaluation of a process leads to optimized system design involving multiple stages. For example, if the first stage is not called upon to concentrate to very low solvent levels, an FFE may be selected. If the product from the overall processing requires very low solvent, e.g. ppm levels, a WFE or WFMS may be the best choice for a second stage, fed from the residue of the first stage. Such a system is shown in the photo where stage 1 is a FFE and stage 2 is a WFE. In this example, because the evaporated fraction is a flammable solvent, the entire skid system and controls are designed and rated as explosion-proof.