Page 193 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.6 Evaporator design 191

Boiling point elevation is usually small and can be neglected for (i) very dilute solutions and
(ii) solutions of materials with extremely high molecular weight.
Information on boiling point elevation not only provides an estimate of the boiling point of
concentrated solutions but also gives an idea whether low pressure steam can provide a proper tem-
perature difference (DT) for concentrating solutions under atmospheric pressure. 3
Boiling point elevation in multiple effect evaporators
The effect of boiling point elevation is more pronounced in multiple-effect evaporators. This is
illustrated in Fig. 6.21 for a single-, double- and a triple-effect evaporator operating under the same
terminal conditions, that is, same steam pressure to the first effect and same saturation temperature of
vapor to condenser. Total height of each column in the figure represents DT ov , the total temperature
spread from steam temperature to saturation temperature of vapor in last effect. The shaded portions
represent loss of DT due to boiling point elevation in each effect The figure reveals that in the extreme
case of a large number of effects or very high BPE, the sum of the BPEs may be equal to or even more
than DT ov , the total temperature drop available. Operation is impossible under this condition.

(A) (B) (C)
Saturation
T (P steam ) temperature
Effect 1
s
Temperature Effect 1 ΔT eff,1 BPE T 1 BPE 1 T s (P ) steam
ΔT
of supply
eff,1
T 1
1
ΔT eff,1 1 T s (P 1 ) Effect 2 ΔT eff,2
T
2
BPE 2 T (P )
Effect 2 ΔT eff,2 s 2
Effect 3 ΔT eff,3
T T T 3
BPE 1 BPE 2 BPE Saturation
1 2 3
T (P ) T (P ) T (P ) temperature
3
s
s
1
s
2
Single Effect Double Effect Triple Effect of vapour
effect
T (P steam ) = Saturation temperature of steam
s
supplied at pressure P steam
T (P ) = Saturation temperature of vapour
3
s
rd
from last (3 ) effect at pressure P
3
BPE BPE BPE = Boiling point elevation of solution
3
2,
1,
in effect 1, 2, 3.
FIGURE 6.21
Comparison of temperature driving forces in (A) single (B) double and (C) triple effect evaporator with same
terminal conditions.
3
When the boiling point elevation is not (accurately) known, it has been a customary practice to use an apparent temperature
difference, DT apparent (¼ T satd steam T satd vap ).