Sample Problem #6

Sample Problem #6:

 A mixt ure of  ammonia and air is scrubbed in a plat e column wit h f resh wat er. If  t he ammonia concent rat ion is reduced f rom 5% t o 0.01% and t he wat er and air rat es are respect ively 0.65 t o 0.45 kg /m2s, how many t heoret ical plat es are required? The equilibrium relat ionship can be writ t  te   n  Y  Y =X , where X  is t he mole rat io in t he liquid phase.

GIVEN: Y1

Fresh water  0.65 kg/m2s

0.65 kg/m2s 0.01% ammonia N=?

1

YN+1 XN

0.45 kg/m2s 5% ammonia

SOLUTION: Top Coordinates: Coo rdinates: Xo = 0  Y1 = ammonia / air  = 0.0001/ 0.999 = 1.001 x 10-4 Coordinates:: Bottom Coordinates  YN+1 = YN+1  / 1-YN+1 = 0.05/1-0.05 = 0.053 NH3 in H2O = amt. in YN+1 ± am amt. t. \in \in Y 1 =0.45 =0.4 5 (0.05 (0.05)) ± {0.95 (0.45 (0.45)) / 0.999 0.999}} x0.0 x0.0001 001 = 0.022 kg/m2s XN = 0.022 / 0.65 = 0.035

Pinch

Y

pt.

YN+1 = 0.053

1

Operating line 2

 Equilibrium line Y=X 

3 Y1 = 1.001 x 10 -4

4

X

Nt = 4.3 stages heo

XN = 0.035