TABLE C
LOW SURFACE TENSION(SEE NOTE 1) Foaming
Factor
Molecular Weight
(Note 3) Foaming Factor MW Lean Oil Foaming Factor Multiplier
Surface Tension (dyn/cm (mN/m)
1 <150 1.05 <100 1.05 2-2.5
1.05 150-250 1.1 100-150 1.1 1.5-2
1.1 250-300 1.2 150-300 1.2 1-1.5
1.2 >300 1.3 >300 1.3 <1
Notes 1: Apply "low surface tension foaming factor multiplier" only if a foam factor not already available from Table 2.
2: The foam factors for ethylene recovery towers are being reevaluated. If applying these foam factors results in exceeding the allowable tray design criteria please consult with a FRACTIONATION SPECIALIST.
3: The information on the foam factor dependency on molecular weight is general guidance and may be conservative at times and it may be overridden if a particular engineer has experience that shows otherwise.
Table 3
Equations For Determining Liquid And Vapor Splits
The equations for liquid, vapor, and pressure drop distribution are presented here for three and four pass trays. The appropriate equations in each set are modified for the special case of vapor crossover. Each set of equations must be solved by trial-and-error. EMoTIP contains a convergence procedure that solves the flow rates and pressure drops for multipass trays. This program should be used for all multipass tray designs. Note that feed trays, drawoff trays, and reboiler return areas of three and four pass trays can get complicated, and may not be thoroughly discussed in Section III-H. Therefore, consultation with a FRACTIONATION SPECIALIST is recommended.
Three Pass Trays
To determine the vapor and liquid flow rates for each pass of a three pass tray, the following equations must be solved (refer to Figure 8). This is a trial and error procedure for rating purposes only.
NO VAPOR CROSSOVER WITH VAPOR CROSSOVER
Replace Eqs. (4) and (5) (1) QLa = QLC
(2) hdb = hdc
(3) QLa + QLb + QLc = QLtotal (4) If hta > htb, then hta = htb + hvt (4) wva = wvc If htb > hta, then htb = hta + hvt (5) hta + htc = 2 htb (5) htb = htc
(6) wva + wvb + wvc = wvtotal where:
hta = heda + hca htb = hedb + hcb htc = hedc + hcc
Note: Sub-subscripts a, b, and c refer to the pass of the tray.
Four Pass Trays
To determine the vapor and liquid flow rates for each pass of a four pass tray, the following equations must be solved (refer to Figure 9). This is a trial and error procedure.
NO VAPOR CROSSOVER WITH VAPOR CROSSOVER
REPLACE EQS. (5), (6), (7), AND (8) (1) QLa = QLc
(2) QLb = QLd (3) hdc = hdd
(4) QLa + QLb + QLc + QLd = QLtotal (5) If hta > htb, then hta = htb + hvt (5) wva = wvc If htb > hta, then htb = hta + hvt (6) wvb = wvd (6) htc = htd
(7) hta + htc = htb + htd (7) 2wva + 2wvb = wvtotal (8) wva + wvb + wvc +wvd = wvtotal (8) 2wvc + 2wvd = wvtotal where:
hta = heda + hca htb = hedb + hcb htc = hedc + hcc htd = hedd+ hcd
Note: Sub-subscripts a,b,c, and d refer to the pass of the tray
Table 4
Default Design Algorithm Values Table 4A (Customary Units)
Default Values (1)
Min Max Step Target
Fouling and service-specific geometries
Hole diameter (do), in. Determined from fouling factor
Tray thickness (t), in. Determined from service
Geometry limits
Downcomer clearance (C), in. (2) 3.5 0.125
Downcomer top width (r), in. 6 H (3) 0.25
Downcomer bottom width (rud), in. (4) 6 r 0.25
1-pass: rud fracmin = rud / Diameter (5) 0.1097
2-pass: rud fracmin = rud / Diameter (6) 0.0993
4-pass: rud fracmin = rud / Diameter (7) 0.0764
Tray spacing (H), in. (2) 36 3
Flow path length (lfp), in. 16 180
Number of passes (Np) 1 4 (Allows 1,2,4)
Tower diameter (Dt), ft.: 1-pass 2.5 50 0.5
2-pass 5 50 0.5
4-pass 10 50 0.5
Hole area to bubble area ratio (Ao/Ab): sieve 0.035 0.15 0.0025
valve 0.05 0.18 0.0025
Effective weir length [for cases w/ picket fence], in. 0.25
Glitsch valve pitch, in. 3.0
Operating limits
Liquid flowrate per outlet weir length, gpm/in. 1.5 17.5
Universal ultimate capacity, % 80
Tray ultimate capacity, % OFmax(8)
Downcomer flood, % OFmax(8)
Jet flood, % OFmax+6(8)
Dry tray pressure drop, in. of hot liquid 1.25 5.5 (9)
Overall flood, % 80
Downcomer choke due to vapor bubble velocity, % 95
Froth to spray transition (% of PEGASYS limit) 110
Entrainment [both Exxon and Mobil models], % 10
Geometric mean of % DC Froth Backup and % DC Choke 70
Downcomer seal, in. -0.5
Weeping, % 20
Velocity under the downcomer, (Vud) ft/s 1.3 1.1
Notes: Table 4A
(1) Allows user override of all geometry limits and operating limits (min, max, step, target).
(2) Determined from fouling factor and foaming factor.
(3) Large diameter towers are designed at the minimum rud / diameter given in table, and limited to a maximum of (r / H) = 1.20.
(4) Only straight downcomers are allowed when foaming factor ≥ 1.3.
(5) Based on 62.5% DC outlet length / diameter.
(6) Based on 60% side DC outlet length / nearest center DC chord length.
(7) Based on 60% side DC outlet length / nearest off-center DC chord length.
(8) OF designates overall flood.
(9) 2.25 if foaming factor ≥ 1.1.
Default Design Algorithm Values Table 4B (Metric Units)
Default Values (1)
Min Max Step Target
Fouling and service-specific geometries
Hole diameter (do), mm Determined from fouling factor
Tray thickness (t), mm Determined from service
Geometry limits
Downcomer clearance (c), mm (2) 90 3.125
Downcomer top width (r), mm 150 H (3) 6.25
Downcomer bottom width (rud), mm. (4) 150 r 6.25
1-pass: rud fracmin = rud / Diameter (5) 0.1097
2-pass: rud fracmin = rud / Diameter (6) 0.0993
4-pass: rud fracmin = rud / Diameter (7) 0.0764
Tray spacing (H), mm (2) 900 75
Flow path length (lfp), mm 400 4500
Number of passes, (NP) 1 4 (Allows 1,2,4)
Tower diameter (Dt), mm: 1-pass 750 15,000 150
2-pass 1500 15,000 150
4-pass 3000 15,000 150
Hole area to bubble area ratio (Ao/Ab): sieve 0.035 0.15 0.0025
valve 0.05 0.18 0.0025
Effective weir length [for cases w/ picket fence], mm 6.25
Glitsch valve pitch, mm 75
Operating limits
Liquid flowrate per outlet weir length, dm3/s/m 3.73 43.47
Universal ultimate capacity, % 80
Tray ultimate capacity, % OFmax(8)
Downcomer flood, % OFmax
Jet flood, % OFmax+6
Dry tray pressure drop, mm of hot liquid 31.75 139.7 (9)
Overall flood, % 80
Downcomer choke due to vapor bubble velocity, % 95
Froth to spray transition (% of PEGASYS limit) 110
Entrainment [both Exxon and Mobil models], % 10
Geometric mean of % DC Froth Backup and % DC Choke 70
Downcomer seal, mm -12.7
Weeping, % 20
Velocity under the downcomer, m/s 0.396 0.335
Notes: Table 4B
(1) Allows user override of all geometry limits and operating limits (min, max, step, target).
(2) Determined from fouling factor and foaming factor.
(3) Large diameter towers are designed at the minimum rud / diameter given in table, and limited to a maximum of (rud / H) = 1.20.
(4) Only straight downcomers are allowed when foaming factor ≥ 1.3.
(5) Based on 62.5% DC outlet length / diameter.
(6) Based on 60% side DC outlet length / nearest center DC chord length.
(7) Based on 60% side DC outlet length / nearest off-center DC chord length.
(8) OF designates overall flood.
(9) 57.15 if foaming factor ≥ 1.1.
Figure 1
Weeping And Dumping Regions
1.0
0.2
Weeping Region
Vapor Rate Dump
Point Dumping
Region 0
Fractional Weepage, fw
Unacceptable Efficiency
Fair To Poor
Efficiency Good
Efficiency
Good Efficiency
DP3BF10
Weep Point
Figure 2
EMoTIP Tray Performance Diagrams (Customary Units)
Figure 2A
EMoTip Performance Diagram For C6/C7, 24 PSIA Physical properties:
ρV = 0.321298 lb/ft3 ρL = 40.7 lb/ft3 µV = 0.0082 cP µL = 0.239 cP σ = 13.91 dyn/cm ff = 1.00
0 0.1 0.2 0.3 0.4 0.5 0.6
0 2 4 6 8 10 12 14 16
Liquid rate (gpm / inch of weir) Cb (ft/s)
Figure 2B
EMoTIP Performance Diagram For iC4/nC4, 165 Psia
Physical properties:
ρV = 1.78 lb/ft3 ρL = 30.7 lb/ft3 µV = 0.0096 cP µL = 0.089 cP σ = 5.15 dyn/cm ff = 1.00