Page 199 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.7 Design illustration 197

2nd to Nth Effect

= h V;i h L;i ; i ¼ 1; .; N
V i ¼ L i 1 h L;i 1 h L;i þ V i 1 h V;i 1 h con;i

xv) Check validity of V i estimates in step v
N
If max ABS V i S ðV i Þ N > 0:01,i ¼ 1, .,N, then re-estimate V i ¼ values from step
i¼1
(xiv), i ¼ 1, N 1,
N 1
X
V N ¼ V total V i
i¼1
go to step v.
xvi) Report e
Overall steam economy ¼ (F L N )/S.
st
1 Effect steam economy ¼ V 1 /S.
ih
i Effect steam economy ¼ V i /V i 1 , i ¼ 2, .,N.
Capacity: Feed processed per unit mass steam required ¼ F/S.
Product produced per unit mass steam required ¼ L N /S.

6.7 Design illustration
Design example 1
Design a forward feed triple effect evaporator to concentrate 14% w/w caustic soda solution to a

product with 40% w/w NaOH. Feed liquor is available at 6 kg/s at 75 C. Last effect of the evaporator
can be connected to an existing vacuum system and operated at a pressure of 7 kPa (abs). Steam with

negligible superheat is available at 120 C. Estimated overall heat transfer coefficients for the effects
2
are 3000, 2000 and 1250 W/(m C), defined with respect to the difference in temperature of the liquid
in the effect and the condensing temperature of steam/vapor heating it.
Process design deliverables

i) Steam consumption (S kg/s)
2

ii) Heat load (q W); Heat transfer area (A m ); Operating temperature (T i C) and pressure (P i kg/
2
cm (g)) and condensate temperature (T con,i C) in each effect.

iii) Flows rate of all streams (S,V,L kg/s); Concentration of NaOH in each liquor stream (x fraction
w/w)
iv) Enthalpy of all streams (h s ,h v ,h con , h F ,h L kJ/kg)
Data

N ¼ 3; F ¼ 6 kg/s; x F ¼ 0.14; x 3 ¼ 0.4; T F ¼ 75 C;
2
P 3 ¼ 7 kPa ¼ 7/101.3 e 1.03,323 ¼ 0.964 kg/cm (g)