Page 196 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

194 Chapter 6 Evaporators

nd
st
th
th
1 Effect 2 Effect i Effect N Effect
V 1 V 2 V i-1 V i V N
h V1 h V2 h Vi-1 h Vi h VN
Vapour 1 Vapour 2 Vapour Vapour i Vapour N
T 1 T 2 T i-1 T i T N
i-1
P 1 P 2 P i P N
P i-1
To vacuum/
Condenser
P 1 P 2 P i P N
T 1 =T s (P 1 ) T 2 =T s (P 2 ) T i =T s (P i ) T N =T s (P N )
S
+BPE(x 1 ) +BPE(x 2 ) +BPE(x i ) +BPE(x N )
h steam V N-1 Product
Steam
T steam
L N
L 1 L 2
P steam x 2 X N (=X p )
x 1
L i
h L1 h L2
F Feed T 2 L i-1 x i L N-1 h L
x F T 1 h i T = T S (P N )
x i-1
h F T i
h Li-1
T F
T i-1
S
Condensate 1 Condensate i Condensate N
h con1 V i-1 V N-1
V 1 Condensate 2
T con,1 = T s (P steam ) h con,i-1 h con,N
h con2
P steam T con,2 = T s (P 1 ) T con,i = T s (P i-1 ) T con,N = T s (P N-1 )
P i-1 P N-1
P i
FIGURE 6.23
Generalized configuration of N-effect forward feed evaporator.
The following convention is used in Fig. 6.23.
F Feed mass flow rate entering Effect 1
x F Mass fraction of solute in F
T F Temperature of feed stream
h F Enthalpy of F at T F
S Mass flow rate of heating steam entering 1st effect
P steam , Temperature and pressure of S
T steam
h con,steam Enthalpy of condensate (saturated) water at pressure P steam
h con,i Enthalpy of condensate (saturated) water at pressure P i 1 (in effect i 1)
L i ,V i Liquid and vapor mass flow rate leaving ith effect
T i ,P i Temperature and pressure in ith effect
h Li ,h Vi Enthalpy of L i and V i at T i ,P i
x i Mass fraction of solute in L i
th
x P Mass fraction of solute in product liquid (L N ) leaving N effect (last effect)
BPE i Boiling point elevation in ith effect with respect to saturation temperature of pure solvent (T s (P i )) in
the same effect