Page 303 - Separation process engineering

Top Operating Line:

where x is water mole frac in distillate = 1—x benz in w = 1—0.00039 = 0.99961
D
y intercept (x = 0) = 0.132

y = x = x = 0.99961
D
Plot top operating line (Figure 8-5A).
Feed Line: Saturated liquid.

Bottom Operating Line:
goes through y = x = x = 0.001 and intersection of top operating line and feed line.
B
Reflux is the benzene phase from the liquid-liquid separator (see Figure 8-3A); thus, x reflux = x w in benz

= 0.015. Use this to start stepping off stages on Figure 8-5B. Optimum feed stage is top stage of
column. We need 2 stages plus a partial reboiler.
Settler mass balance requires distillate flow rate. From Eq. (3-3),

E. Check. All of the internal consistency checks work. The value of α w–benz agrees with the
calculation of Robinson and Gilliland (1950). The best check on α w–benz would be comparison

with data.
A check on the settler flow rates can be obtained from a water mass balance. The vapor leaving
Stage 1, y 1,w , is a passing stream to the reflux,

y = (0.868)(0.015) + (0.132)(0.99961) = 0.145
1,w
This stream is condensed and separated into the water layer (distillate) and the organic layer (reflux).

9.94 = 9.90, which is within the accuracy of the graph.

F. Generalize. Since the solubility of organics in water is often very low, this type of heterogeneous
azeotrope system requires only one distillation column.

Even though water has a higher boiling point than benzene, the relative volatility of water dissolved in
benzene is extremely high. This occurs because water dissolved in an organic cannot hydrogen bond as it
does in an aqueous phase, and thus, it acts as a very small molecule that is quite volatile. The practical
consequence of this is that small amounts of water can easily be removed from organics if the liquids are
partially immiscible. There are alternative methods for drying organics such as adsorption that may be