1. Evaporation Downstream Processing Short Course May 2007 Kevin Street Gavin Duffy

2. Learning Outcomes After this lecture you should be able to… Explain how an evaporator works Describe some film evaporators Calculate the heat transfer area of an evaporator for a given specification

3. Falling Film Evaporator Short residence time at high temperature Feed pre heated to b.p. Flows down tubes Hot vapour flows down also Vaporisation occurs Concentrate and vapours separated on exit from base Vapour condensate removed at base A: Product B: Vapor C: Concentrate D: Heating Steam E: Condensate 1: Head 2: Calandria 3: Calandria, Lower part 4: Mixing Channel 5: Vapor Separator http://www.niroinc.com/html/evaporator/falling_film_evaporators.html

4. Falling Film Evaporator Liquid must be evenly distributed to tubes A continuous wet film is needed all the way down – feed is concentrated and is removed as liquid Dry patches cause fouling or blockage

5. Thin Film Evaporator Mechanically assisted to create a thin film on inside surface of evaporator Heat transfer from jacket on evaporator Film flows under gravity and becomes concentrated Evaporated fluid leaves through top Variation is to wipe the film if gravity is not enough – i.e. fouling is possible or liquid is viscous http://www.technica.net/en/water/pollutants.htm

6. Activity – Size an evaporator What heat transfer area is needed to concentrate a mixture from 10% to 50% solids at a flowrate of 7 kg/s using steam at 205 kPa with evaporator working under a vacuum of 13.5 kPa? Specific heat capacity of 10% mixture3.76 kJ/kg K Specific heat capacity of 50% mixture3.14 kJ/kg K Temperature of steam at 205 kPa394 K Enthalpy of Steam at 205 kPa2530 kJ/kg Boiling point of water at 13.5 kPa52 ºC heat transfer coefficient 3 kW/m 2 K Taken from Chemical Engineering Vol 2, 5 th ed., Coulson and Richardson, example 14.1