Page 201 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

6.7 Design illustration 199

Enthalpies
h steam (T steam , P steam ) ¼ 2705.9; h con,1 ¼ h sat steam (T con1 ) ¼ 503.9
h V,1 ¼ h steam (T 1 ,P 1 ) ¼ 2697.1; h con,2 ¼ h sat steam (T con2 ) ¼ 442.9
h V,2 ¼ h steam (T 2 ,P 2 ) ¼ 2669.5; h con,3 ¼ h sat steam (T con3 ) ¼ 349.3
h V,3 ¼ h steam (T 3 ,P 3 ) ¼ 2676.1;
Enthalpy of solution found by neglecting heat of solution and considering NaOH (Specific heat of
solid ¼ 1.4915 kJ/kg) to have enthalpy “zero” at 0 C.

h F ¼ h F (75,0.14) ¼ 285.7
h L,1 (112.11,0.1787) ¼ 416.2
h L,2 (93.8,0.2471) ¼ 330.5
h L , 3 (83.4,0.4) ¼ 200.5
S ¼ (V 1 h V,1 þ L 1 h L,1 Fh F )/(h steam h con ,1) ¼ 1.703
q 1 ¼ S.(h steam h con ,1) ¼ 1.701 (2705.9 503.9) ¼ 3735.5
q 2 ¼ V 1 .(h V,1 h con , 2 ) ¼ 2931.5
q 3 ¼ V 2 .(h V,2 h con , 3 ) ¼ 3016.4
A 1 ¼ q 1/[U 1 (T con,1 T 1 )] ¼ 0.1580
A 2 ¼ q 2/[U 2 (T con21 T 2 )] ¼ 0.1240
A 3 ¼ q 3/[U 3 (T con,3 T 3 )] ¼ 0.1276
A avg ¼ (A 1 þ A 2 þ A 3 )/3 ¼ 0.1365
% deviation from A avg ¼ 5.2433, 3.0596 and 2.1837.
Recalculating DT eff,1 , DT eff,2 and DT eff,3
X
ðq i = U i Þ¼ 5.124
i¼1;3
DT eff ;1 ¼ DT eff ;total ðq 1 = U 1 Þ=5:124 ¼ 9.3829
DT eff ;2 ¼ DT eff;total ðq 2 = U 2 Þ=5:124 ¼ 11.0451
DT eff ;3 ¼ DT eff;total ðq 3 = U 3 Þ=5:124 ¼ 18.1835

Recalculated A values with new values of DT eff,1 , DT eff,2 , DT eff,3 : 0.1315, 0.1329 and 0.1328. These
are within 5% from A avg .
Re-estimating V values

V 1 ¼ q 1 þ F $ h F L 1 $ h L;1 =h steam ðT 1 ; P 1 Þ¼ 1.3000

V 2 ¼ q 2 þL 1 $ h L;1 L 2 $ h L;2 =h V;2 ¼ 1.3269

V 3 ¼ q 3 þ L 2 $ h L;2 L 3 $ h L;3 =h V;2 ¼ 1:2731
Recalculated V i values differ from corresponding V values by more than 1%. Hence, new assumed
values for are V are replaced by V i values and calculations from Step A are repeated till the V i and V
values differ within 1% and the design calculation converges.