EVAPORATION

Example 1:
a. What is the boiling of the 5% NaCl – Water solution if the separation is operated at 80 kPa?
b. What is the Boiling Point Rise of the solution?

Solution:
p = 80kPa = 0.080 MPa
 

a:
Boiling Point of Water at 80kPa:

Thermodynamic Properties of Water: 8th Perry’s Handbook, T 2 – 30, p. 2-413
p(MPa)	T(K)
0.062194	360
0.090535	370
0.08	–

Evaluate the temperature at p=0.080 MPa by interpolating. Refer to the example of interpolation.
T_sat = 366.28 K = 93.13^oC
 

Evaluate the boiling point of 25% NaCl – Water solution by referring to the graph or by using any available correlation.

Tb 100 ^oC
 

b:
BPR = = Tb – T^sat
BPR = 100 – 93.13
BPR = 6.87^oC
 

Example 2:
What is the enthalpy of the 10% NaOH solution at 130^oF?
 

Solution:

From the plot: h 200 kJ/kg 85 Btu/lbm

 

B. Evaporator Set-up

Single-Effect Evaporator

Evaporation is carried out in a single stage. It is usually utilized in a small-scale operation.
 

F – the amount of liquid solution fed S – the amount of Steam introduced into the evaporator V – the amount of vapor/steam produced L – the amount of concentrated liquid product xf – concentration of solute in the feed x – concentration of solute in the liquid product y – the concentration of solute in the Vapor

 
The evaporation involves straightforward material and energy balances. Unless conditions are specified, these simplifying assumptions almost always apply:

1. The vapor is taken to be pure so that y = 0.
2. Steam and condensate are both saturated.
3. The tank is well-mixed so that the temperatures of the Vapor and the Liquid Product are the same.

 

a. Steam Economy

Steam Economy refers to the total amount of Vapor, V_T, evaporated per amount of Steam, S, supplied.

 

b. Heat Supplied, q

The heat supplied to the evaporator comes from the phase change of steam. For saturated steam, the heat supplied is:

 

c. Surface Area of Heat Exchange, A

The Total Heat Transfer Surface Area can be derived from the Over-all Heat Transfer Equation.

q – over-all heat transfer U – over-all heat transfer coefficient A – total heat transfer surface area Ts – temperature of the condensing Steam Tb – boiling temperature