Question #248778

An evaporator is used to concentrate 12 000 kg/h of a 15% solution of NaOH entering at 60oC to a product of 50% solid. The pressure of the saturated steam used is 300 kPa and the pressure in the vapour space of the evaporator is 11.7 kPa. The overall heat transfer coefficient is 1 560 W/m2K. Calculate the steam used and the steam economy in kg vaporised/kg steam.

Expert's answer

Given:

*F = 4536 kg/h*

*TF = 60 ºC*

*xF = 0.2*

*S = ?*

Solution

* For the total balance,*

* *"F = 4536 = L + V"

* For the balance on the solute alone,*

* *"F \\times F = L \\times L"

* *"4536 (0.2) = L (0.5)"

* **L = 1814 kg/h of liquid*

* Substituting into total balance and solving,*

* **V = 2722 kg/h of vapor*

*To determine T1 = Tsat + BPR of the 50 % concentrate product, first we obtain Tsat of pure water from steam table. At 11.7 kPa, *

**Tsat = 48.9 ºC. **

** **

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