Evaporation is an operation used to remove a liquid from a solution, suspension, or emulsion by boiling off some of the liquid. It is thus a thermal separation, or thermal concentration, process. We define the evaporation process as one that starts with a liquid product and ends up with a more concentrated liquid as the main product from the process. During the evaporation of Spent Wash we use a scheme of Falling film evaporators to concentrate Spent Wash upto 25° brix.
In falling film evaporators, liquid and vapours flow downwards in parallel flow. An even thin film enters the heating tubes via a distribution device in the head of the evaporator, flows downward at boiling temperature, and is partially evaporated. This gravity–induced downward movement is increasingly augmented by the co–current vapour flow.
In first evaporator get heat from the vapours of Analyser column. Temperature of vapour is 74ºC. Spent Wash vaporised in first effect gives energy to second effect. There is no need of any steam in this stage. Vapour condensed in first stage send back to primary condensate tank and fed to Rectifier column as feed.
There are total THREE falling film evaporators. Evaporators are in forward feed arrangement. Vapours of last evaporator get condensed in a surface condenser. Falling film evaporators can be operated at very low temperature differences between the heating media and the boiling liquid, and they also have very short product contact times, typically just a few seconds per pass. These characteristics make the falling film evaporator particularly suitable for Spent Wash evaporation.