Multi-effect Evaporators: ConcepTest and Example Problem
Try to answer this ConcepTest and solve the example problem before using this module. Studies show that trying to answer the questions before studying material improves learning and retention. We suggest that you write down the reasons for your answers. By the end of this module, you should be able to answer these on your own. Answers will be given at the end of this module.
Example Problem 1
A 5.0 wt% sugar solution at 110°C and 2.0 bar is fed to a double-effect evaporator. The first effect operates at 1.0-bar pressure and concentrates the sugar to 8.0 wt%. It is heated by saturated steam at 140°C; the condensate leaves evaporator 1 as saturated liquid. The second effect operates at 0.12-bar pressure. The overall heat transfer coefficient is 2.0 kJ/(m2 s K) for the first effect and 1.6 kJ/(m2 s K) for the second effect. The heat transfer area is 56 m2 in each effect. The results of mass and energy balances on the first evaporator: the feed rate of 5.0 wt% sugar solution is 5.61 kg/s, the steam feed rate is 2.10 kg/s, the rate that water is evaporated is 2.11 kg/s, and the 8.0 wt% stream flow rate is 3.51 kg/s.
What is the sugar concentration leaving the second effect? What is the steam economy for the system (kg water evaporated per kg of steam condensed)?
