Concentration is mainly done for removing water (generally coming from nitric acid or water creation during reaction) inside the solution and get the required concentration for the next step (granulation, prilling, direct selling, etc.)
A falling film evaporator is a special type of heat exchanger used to concentrate solutions. The evaporation takes place inside vertical tubes through which the process fluid to be evaporated flows downwards by gravity as a continuous film. Steam or process steam can be used as the heating medium is placed on the outside of the tubes.