Page 371 - Applied Process Design For Chemical And Petrochemical Plants Volume II

360 Applied Process Design for Chemical and Petrochemical Plants

Average liquid rate = (695) (1.553) = 1,080 lb/hr Correcting Height of Transfer Unit for Inert Gases in System
Outlet liquid rate = (724) (1.553) = 1,124 Ib/hr
1080 ( Diffusivity : D, = 0.0069 T3I2 MA + ~/MB
L [ pG)o" WA 113 + vB1/3 12
=- - =0.0744
G PL 1047 62.3
Because : T (for 90°F) = 90 + 460 = 53O"R at liquid inlet

T (for 90 + 72") = 162 + 460 = 622"R max. at liquid outlet

MA = 17 for NH3
where Vg = superficial gas velocity, ft/sec MB = 29 for air

Solve for Vp' at abscissa = 0.0744
P = 164.7/14.7 = 11.2 atm
Read Figure 9-21D at flooding, ordinate = 0.150
VA = 26.7 for NH3 from Table 9-44 (molecular volume) Note:
The value of VA is from Ref. 63. Molecular volume values vary
2 (0.130) (32.2) (62.3) = 7.83 between references.
vg =
(0.324) (0.949) (1) (125)
VB = 29.9 for air
Vg = 2.8 ft/sec flooding velocity
Diffusion coefficient of NH3 through air:
Read Figure 921D at upper loading, ordinate = 0.084

Vg = 2.1 ft/sec upper loading velocity D, = 0.0069 (550)~/~ = 0.065 ft2 / hr
11.2 [(26.7)'13 + (29.9)'"f
Base diameter on 50% of flooding
Diffusion coefficient of NH3 through 3:l H2 - N2 mix-
Operating velocity = 0.5 (2.8) = 1.4 ft/sec ture gas:
MA = 17 for NH3
Gas flow rate = (1047) = 0.896ft3/sec
(0.324) (3600)
MB = 11.2 for inert gas mixtures, less NH3
Required tower cross-sectional area at 50% of flooding:
VA = 26.7 for NH3

VB = (0.75) (14.3) + (0.25) (31.2) = 18.5 for H2 - N2 mixture
Tower dia. = ,/- = 0.902 ft = 10.8 in.
Diffusion of ammonia through hydrogen-nitrogen
inert gas:
If a standard 10-in. pipe is used (10.02-in. I.D.) the
superficial velocity = 0.896/0.546 ft2 = 1.64 ft/sec.
(550)~~~
D, = 0.0069
% of flooding = 1.64 (100)/2.8 = 38.6% 11.2 [(26.7)'13 + (18.5)1/3f

% of upper loading = 1.64 (100)/2.1 = 78.3% D, = 0.096 ft2 / hr

Pressure drop is approximately 0.5 in./ft packing For an ammonia-air system using mass rates in the tower
height, Figure 9-21D. the same as the H2 - N2 system: G' = 1047/0.546 = 1920
This should be an acceptable operating condition, lb/hr (ft2) and L' = 1080/0.546 = 1980 lb/hr (ft2)
therefore, use tower diameter of 10-in. nominal Sch. 40 From Figure 9-73 Extrapolated HOG = 0.95 (this should
pipe. This pipe is satisfactory for 150 psig operating pres- be maximum to expect)
sure (see Figures 9-7411 and B) . Then substituting, for ammonia-air mixture: