Page 211 - Applied Process Design For Chemical And Petrochemical Plants Volume II

200 Applied Process Design for Chemical and Petrochemical Plants

Design a sieve tray column to separate benzene and At 88% total tower area, = 0.12 (At), ft2; downcom-
toluene to produce 35,000 lb/hr of benzene as overhead er area
product at atmospheric pressure and use a reflux ratio of
5:l (reflux returned to net overhead). Using the top tray At = 347/[(1 - 0.12) (4.90)] = 80.47 ft2
as the first design basis, which can be followed by other
points in the column, determine by the material balance: Tower diameter, D = [ (80.47) (~)/x]O*~ = 10.12 ft for
Top tray data: 85% flooding.
Material: 95%+ benzene For fabrication convenience and practicality, select, D -
Molecular Weight: 78.1
Operating pressure: 14.7 psia 10.5 ft
Operating temperature: 176°F Actual At at 10.5 ft - 86.59 ft2
Liquid Density: 43.3 lb/ft3
Vapor Density: 0.168 lb/f$ Mechanical Features
Liquid Surface Tension: 21 dynes/cm
Maximum liquid load 5(35,000) - 175,000 lb/hr (504 4 86.59 fG
=
gpm) p41 = 0.12 (86.59) = 10.39 ft2
Maximum Vapor load: 210,000 lb/hr = (347 ft3/sec) A, = Net cross-section area for vapor flow above tray,
System: Non-foaming, non-corrosive, non-fouling (At - p41) = 86.59 - 10.39 = 76.2 ft2 (usual situation)
A, = active or bubble area of tray,
-
(4 w) [86.59 - 2 (l03)l = 65.59 ft'
=
TmerDiameter
Ah = net perforated area of tray, ft2 = (0.10) (86.59)
8.66 ft2
Flow Parameter:
Actual Flow Conditions
L'
(8 - 297A)
Vapor velocity based on net area = 347 cfs/A,
e- 175'000 (0.168/43.3)0.5 = 0.0519 U, = 347/76.2 = 4.55 fps
210,000 Approach to flooding = [Un,d&@n&,od] [lo01
Tray Spacing and Design = [4.55/4.90] (85) = 78.99%

Select as quite common; %in. dia. holes, with hole Entrainment
area/tower area = 0.10, 14 U S Std. gauge stainless steel
tray material, which is 0.078 in. thick, 2-in. weir height, Refer to Figure 8138, Fractional Entrainment, Sieve Trays
and 24in. tray spacing. [183] and for Fl\7 - FP = 0.0519, and 77.4% of flooding.
Select single cross-flow tray, segmental downcomers, and Fractional Entrainment I) = 0.06
straight weirs, with weir length equaling 77% of tower diam-
eter. The downflow segment is 12.4% of the tower area. qJ=- E
(L' + E)
Diameter E = Liquid entrainment, lb mols/hr
E = [0.06/(1 - .OS)] (175,000/78.1) - 143 mols/hr
From Figure 8-137 read, = 11,168 lb/hr
CSB = 0.36, at Flv at 0.0519 (previous calculation)
The system surface tension is approximately 20 dynes/ Pn?ssure Drop
cm, therefore no correction is necessary.
In this system, use 85% of flooding condition for design: With a hole/active area ratio = 8.66/65.59 = 0.132
Vapor flooding velocity = vf = Ui\~,n,d = csB/(h,/(Pl - With a tray thickness/hole diameter ratio = 0.078/(3/16)
= 0.416
pv) l'* Orifice coefficient, Figure 8-129, read at 0.41 tray/hole
At 85% flood
gives C, orifice coefficient = 0.75
Hole velocity = 347/8.66 = 40.06 fps
UN,flood = Vf= (0.36) (0.85)/[ (0.168)/(43.30 - 0.168]0.5 = 4.90
ft/sec, vapor velocity based on net crosssectional area for vapor Dry Tray pressure drop
flow above tray, usually, (At - Ad), ft2 hh = 0-186 (VO/CO)~
= 0.186 (0.168/43.3) [40.06/0.75]2 = 2.06 in. liquid
Weir flow = 0.77 (10.5) (12 in/ft) = 97.02 in. weir length