Distillation Tower Design As computer technology advances, the fundamental aspects of plant design...

Distillation Tower Design

• As computer technology advances, the fundamental aspects of plant design are becoming a lost art. … N.P. Lieberman, Refinery Manager, GHR Energy Inc., La

• The following steps are taken to design and optimize a distillation tower:

R.A. Hawrelak, 22 Jan 02, CBE 497

(a) Select a Process Sequence

• Consider a five component feed as shown below. Arrange in order of descending vapor pressure.

• C2 3• C3 20• C4 37• C5 35• C6 5• Total100 lb moles/hr

Process Sequence Cont’d

• Make a split between C3 and C4• Show this as C2, C3 / C4, C5, C6• This called a depropanizer.• C3 is identified as the light key.• C4 is identified as the heavy key.

Establish Key Component Specs

• C3, light key composition in bottoms shall be 1.0 mole %. (2.0% sales spec)

• C4, heavy key composition in the overheads shall be 1.5 mole %. (3.0% sales spec).

Set Up Mass Balance for Tower

Feed Feed Ohds BtmsC2 3 3

C3 lk 20 20 - Y YC4 hk 37 X 37 - X

C5 35 35C6 5 5

100 23 + X - Y 77 + Y - X

Mass Balance Equations

Light Key In Btms = 1.00%Heavy Key In Ohds = 1.50%

Y 0.01 Eqn 177 + Y - X

X 0.015 Eqn 223 + X - Y

Mass Balance Solution

Feed mf in Feed Ohds mf in Ohds Btms mf In Btms

C2 3 0.0300 3.00 0.1330C3 lk 20 0.2000 19.23 0.8520 0.77 0.0100C4 hk 37 0.3700 0.34 0.0150 36.66 0.4734

C5 35 0.3500 35.00 0.4520C6 5 0.0500 5.00 0.0646

100 1.0000 22.56 1.0000 77.44 1.0000

Obtain Antoine Constants

• Need Antoine constants for Vapor Pressure

• Vap Press, VP = 10^(A + B / (t°C + C)) psia

Component A B CC2 5.0120015 -823.03103 328.18

C3 lk 4.3742477 -587.76681 248.90C4hk 3.8201853 -367.50819 153.30C5 4.0537542 -539.73661 169.60C6 4.0165587 -545.39181 141.15

Feed Conditions

• Temperature of feed = 225 deg F = 107.22 deg C.

• Pressure of feed = 264.7 psia

Determine Bubble Point of Feed

Assume T, deg F = 212.5714318 T, Deg C = 100.317462At Press = 264.7 psia

Component Moles/h, M VP, psia K = VP/P M*KC2 3 1233.86 4.66133 13.98400

C3 lk 20 491.08 1.85523 37.10452C4 hk 37 235.03 0.88790 32.85248

C5 35 113.27 0.42792 14.97726C6 5 57.27 0.21635 1.08173

100.00 99.999999

Determine Dew Point of Feed

Asssume T = 269.7699145 T, Deg C = 132.094397At Press = 264.7 psia

Component Moles/hr, M VP, psia K = VP/P M/KC2 3 1674.442 6.3258 0.4742

C3 lk 20 678.469 2.5632 7.8029C4 hk 37 340.773 1.2874 28.7403

C5 35 183.966 0.6950 50.3600C6 5 104.851 0.3961 12.6227

100.00 100.000001

Assess Feed Condition

• Feed Bubble Point = 100.32 deg C• Feed Temp = 107.22 deg C• Feed Dew Point = 132.09 deg C• Feed temp between Bubble Pt. and

Dew Pt.• Feed must be in a two-phase V / L

state.• Special care will have to be taken for

feed distributor design on feed tray.

Determine V / L for Feed

Assume V/L = 0.17114

1 2 3 4 5 6Component Moles/hr, M K = VP/264.7 K(V/L) 1+K(V/L) L = 1 / 4 V = 1 - 5

C2 3 5.0000 0.856 1.856 1.617 1.383C3 lk 20 2.0000 0.342 1.342 14.900 5.100C4 hk 37 0.9700 0.166 1.166 31.732 5.268

C5 35 0.4800 0.082 1.082 32.343 2.657C6 5 0.2500 0.043 1.043 4.795 0.205

100.00 85.387 14.613

Flash Fraction Vapor in Feed = 1-q = V / M =0.14613121 V/L calc'd = 0.17114V/L Ass'd = 0.17114Diff = 0.00000

Solve For Ø, The Underwood Parameter

• Example In article by J.M. Ledanois, Hydrocarbon Processing, April, 1981, P-231

• Trial and error solution with as many solutions as there are components.

• Solution is a Newton convergence method.

• Not all cases converge.

Solve For Ø, The Underwood Parameter,

Cont’dNext Ø 1.5976753 from Neqton Convergence

Assumed Ø = 1.5976753 Solution 2

1 - q = 0.1461312 enter no. manually value from cell P55 - Fraction Vapor In Feed

Temp = 225 °F 107.22 °C Ist EstimatePress, PT = 264.7 psia Eqn 13-43 Of Ø

Moles/Hr rel volatility Alpha*xFi Avg AlphaFeed Fi xF1 Ki = VPi / PT Alpha i Alpha*xFi Aplha - Ø Adj ComptsC2 3 0.0300 5.0000 5.1546 0.155 0.043 3.6082

C3 lk 20 0.2000 2.0000 2.0619 0.412 0.888 1.5309C4 hk 37 0.3700 0.9700 1.0000 0.370 -0.619 0.7474

C5 35 0.3500 0.4800 0.4948 0.173 -0.157 0.3763C6 5 0.0500 0.2500 0.2577 0.013 -0.010

100 1.0000 ∑ = 0.1461312 = 1-q

Solve For Ø, The Underwood Parameter,

Cont’d1 - q = 0.1461312

Ist Estimate Final EstEqn 13-43 Of Ø Of ØAlpha*xFi Avg Alpha Avg Alpha

Feed Alpha*xFi Aplha - Ø Adj Compts Solution No. Ø SolutionsC2 0.155 0.043 3.6082 1 4.80084859

C3 lk 0.412 0.888 1.5309 2 1.59767531C4 hk 0.370 -0.619 0.7474 3 0.64185818

C5 0.173 -0.157 0.3763 4 0.26706697C6 0.013 -0.010

∑ = 0.1461312 = 1-q

Calc Minimum Reflux Ratio by Underwood

• See Perry VI, Chem Eng HB, Page 13-36

• Solution For Minimum Reflux Ratio By Solving For ∑ [Alpha*xDi / (Alpha - Ø)] = L/D min. + 1

• Ø, The Underwood Parameter, was determined above.

Calc Minimum Reflux Ratio by Underwood, Cont’d

Underwood parameter = 1.5977Temp = 225 °F 107.22 °CPressure = 264.7 psia

Eqn 13-42Alphai Alpha*xDi

Ohds Moles/hr xD1 Ki = VPi / PT Ki/K hk Alpha*xDi Aplha - ØC2 3.00 0.1327 5.0000 5.1546 0.684 0.192

C3 lk 19.30 0.8540 2.0000 2.0619 1.761 3.793C4hk 0.30 0.0133 0.9700 1.0000 0.013 -0.022

22.6 1.0000 L/Dmin. + 1= 3.9635

L/D min. = 2.9635

Determine Minimum No. Trays by Fenske -

Underwood • Assume top and bottom pressure

equal feed pressure of 264.7 psia for now.

• Assume overhead distillate is removed as a vapor from the condenser.

Determine Minimum No. Trays by Fenske –

Underwood, Cont’dDew Point of Overhead Vapor Stream and Alpha of Keys

Temp = 118.3 °F 47.94 °CPressure = 264.7 psia

Ohds, D Moles/hr, M xD1 Ki = VPi / PT M / KC2 3.00 0.1327 2.5180 1.19

C3 lk 19.30 0.8540 0.9364 20.61C4hk 0.30 0.0133 0.3726 0.81

1.000022.6 22.61

Alpha Top = KC3 / KC4 = 2.51 For Distillate at 118 °F


Underwood, Cont’dBubble Point Of Bottoms Stream and Alpha of the Keys

Temp = 274.0 °F 134.44 °CPress = 264.7 psia

Moles/hr, M xF1 Ki = VPi / PT M(K)C3 lk 0.7 0.009044 2.6196 1.834C4hk 36.7 0.474160 1.3189 48.405C5 35 0.452196 0.7175 25.112C6 5 0.064599 0.4119 2.060

MT = 1.00000077.4 77.411

Alpha Btm = KC3 / KC4 = 1.99 For Btms at 274°F


Underwood, Cont’d• Determine geometric Average

Alpha between top and bottom of the tower.

• Geometric Avg = (Alpha Top*Alpha Btm)^0.5

• Avg Alpha = ((2.51)(1.99))^0.5 = 2.23


Underwood, Cont’d

• Min. Trays = LN((C3 lkD / C4 hkD)* (C4 hkB / C3 lkB)) / LN(Alpha Avg)

• Minimum No. Trays, Sm = 10.11

Determine Trays versus Reflux Ratio by Gilliland

Method• Use Chang equation to represent Gilliland.• Huan Yang Chang, HC Proc, Oct 1981, P-

146• A partial condenser and a reboiler

represent two theoretical trays.• No. trays = S – 2.• Assume the economic reflux ratio is 1.2

times the minimum reflux ratio,• Plot the results.

Determine Trays versus Reflux Ratio by Gilliland

Method, con’dL/D Min. = 2.9635 Sm = 10.11L/D = 3.56Chang Factor = (S - Sm) / (S + 1) = 1 - EXP(1.49+0.315*C-1.805/C 0̂.1) where C = (L/D - L/Dmin) / (L/D + 1)

Chang FactorA B C (S - Sm) By Chang

L/D L/D - L/Dmin B/(A +1) (S + 1) S N = S - 23.1 0.14 0.0333 0.65 30.30 28.303.2 0.24 0.0563 0.59 26.29 24.293.3 0.34 0.0783 0.56 24.07 22.073.56 0.59 0.1301 0.49 20.97 18.97

4 1.04 0.2073 0.43 18.39 16.394.5 1.54 0.2794 0.38 16.81 14.816 3.04 0.4338 0.29 14.53 12.53

Plot of Trays Versus Reflux RatioReflux Versus Number of Trays

0.00

5.00

10.00

15.00

20.00

25.00

30.00

35.00

40.00

2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50

Reflux Ratio, L/D

Nu

mb

er o

f Tr

ays

Example

Determine Feed Tray Location

Feed Tray Location By Kirbride Equation Oil And Gas J ournal, Oct. 20, 1980, P-138, by Henry Y. Mak

Feed Tray Location = EXP[0.206*LN(B/D*fhk/flk*((blk/B)/(dhk/D)) 2̂)]

B =77.44 fhk =37 dhk =0.34D =22.56 flk =20 blk =0.77

Kirbride Feed Tray Location = 1.25 = # trays above fd / # trays below fd

No theoretical trays = 19

Trays Above = 10.55 11 10Trays Below = 8.45 8 final selection 9Ratio A/B = 1.25 1.38 1.11

Determine Reflux Flow & Comp’n


L/D = 2.963 L = 66.868Comp of Reflux

Mole Frac Dew Pt. Liq Reflux Stream, Ohds, D Moles/hr, M y, D Ki = VPi / PT M / K x = y / K mole/hr, L

C2 3.00 0.1330 2.5246 1.19 0.05266 3.522C3 lk 19.23 0.8520 0.9392 20.47 0.90724 60.666C4hk 0.34 0.0150 0.3741 0.90 0.04009 2.681

1.0000D = 22.5641 22.5641 1.00000 66.868

Calculate Overhead Vapor Flow from Top Tray = 19

Minimum Reflux Ratio = 2.963D = 22.56 moles/hrL = (2.963)(D) = 66.868 moles/hr. V = L + D = 89.432 moles/hr

Calc Vapor Composition from Top Tray 19


Comp of Reflux Vapour VaporMole Frac Dew Pt. Liq Reflux Stream, Stream Comp'n

Ohds, D Moles/hr, M y, D Ki = VPi / PT M / K x = y / K mole/hr, L moles/hr, V mole fracC2 3.00 0.1330 2.5246 1.19 0.05266 3.522 6.52 0.0729216

C3 lk 19.23 0.8520 0.9392 20.47 0.90724 60.666 79.89 0.893316C4hk 0.34 0.0150 0.3741 0.90 0.04009 2.681 3.02 0.0337624

1.000022.5641 22.5641 1.00000 66.868 89.432 1.00000

Show Molar Balance Around Top Tray 19Top Tray n, Assume equal molar flowMolar balance = V18 = V19 + L19 - L

V19 =89.4324 V19 = L + D

Reflux LTray 19 66.868

L1966.868 Liq fr Tray 19

V18 =89.4324

Calc Dew Pt of Vapor Fr T19 and Liquid Comp’n Fr

T19Temp = 126.5378 °F 52.52 °CPressure = 264.7 psia

Comp ofMole Frac Dew Pt. Liq fr Tray n

Ohds, V Moles/hr, V y, V Ki = VPi / PT M / K x = y / KC2 6.52 0.0729 2.6753 2.44 0.02726

C3 lk 79.89 0.8933 1.0035 79.61 0.89022C4hk 3.02 0.0338 0.4091 7.38 0.08252

1.000089.4324 89.4325 1.00000

Moles/hr, V 89.4324Diff = 0.0001

Vapor Comp’n From Tray 18

Vapor, V19 Reflux, L L19 Comp L19 Flow Vap, V18 Vap, V18moles/hr moles/hr mole fr moles/hr moles/hr mole frac

C2 6.52 3.52 0.0273 1.82 4.82 0.0539C3 lk 79.89 60.67 0.8902 59.53 78.75 0.8806C4hk 3.02 2.68 0.0825 5.52 5.86 0.0655

89.43 66.87 1.0000 66.87 89.43 1.0000

Calc Dew Point of Vapor V18


V18 Mole Frac Dew Pt.Vap V18 Moles/hr, M y, V18 Ki = VPi / PT M / K

C2 4.82 0.0539 2.8044 1.72C3 lk 78.75 0.8806 1.0587 74.39C4hk 5.86 0.0655 0.4394 13.33

1.000089.4326 89.4326

Moles/hr V18 89.4326Diff 0.0001

Design Data For Top of Tower

Vap, V18 L19 Flow Liq Density MW lb/hr lb/hr lb/cf

C2 30.07 145 55 22.2C3 lk 44.10 3,473 2,625 31.6C4 hk 58.12 340 321 35.1

3,958 3,001 31.80

MW = 44.26 44.87

Dew Point of Vapor from Tray 18 = Vapour to Top Tray 19

Temp = 133.0915 Deg FPressure = 264.7 psia

Vapor Density = (MW)(Psia) / (10.73) / (Deg Rankine) = 1.841 lb/cf

Input Shortcut Tower Dia. (FWG)

Input DataVapor To Tray, V, lb/hr = 3,958 CFS Vapor = 0.60 CFS Vapor = V / DV / 3600Vapor Density, Dv, lb/cf = 1.841Liquid From Tray, L, lb/hr = 3,001 USGPM = 11.76 USGPM = L / DL / 60 * 7.481Liquid Density, DL, lb/cf = 31.80System Factor, SF = 1.00 FWG 4900 / 5, Table 1b Non Foaming SystemTray Spacing, TS, Inches = 18 Assume 18 inches or 24 inches as a First Try. 24 inch TS is preferred.Spray Ht. / Tray Spacing, = 50.00% Assume 70% as a default value. Assume Minimum Valves / AA = 8 No. Valves / sf AA A Higher value, eg 13, may lead to an unsuitable, smaller towerDowncomer Flood, % DCF = 50.00% Assume 60% as a default value.

Preliminary SizingDowncomer Area (One Side), sf = 0.14 DCA = (L*7.481 / DL /60)/(7.5*TS 0̂.5*(DL-Dv) 0̂.5*SF) / %DC FloodActive Area, AA, sf = 1.39 AA = V/3600/Dv/(((((TS*SH/TS)-4.5213)/4.3662)*DL/Dv) 0̂.5)*78.5/(No Valves / AA)Tower Area sf = 1.66 AT = AA + 2*DCATower Dia, ft. = 1.45 D = (AT*3/) 0̂.5

Select Tower Diam = 2.00 ft. (next 6" increment)

Shortcut Method by Dr Prakash

Kv = 0.04070 by Kv = -0.17*(TS) 2̂+0.27*(TS)-0.047 where TS is in meterscm/s = 0.01691 cm/s by cfs/3.2808 3̂vel = 0.16419 m/s by Vmax =Kv(DL-DV/DV) 0̂.5, the Souder EqnA = 0.103 sm cm/s / m/sD metric = 0.362 m D = (sm/.785) 0̂.5D English = 1.188 ft D = m*3.2808

Check Tower Mole Balance89.43 22.56

66.87 refluxTray 19

89.43 66.8684

14.61Feed Tray Molal Balance

100.00 Flashing feed 74.82 Feed Tray Feed In = 241.69Feed Out = 241.69

85.39

74.82 152.26

Tray No. 1

Vapour to Tray No. 1 = 74.82 152.26 Liquid From Tray No. 1In equilibrium with bottoms

Note: Tower simulation usually assume this configReboiler for the reboiler. In actual practice it is not quite like this

See tower sketch for reason

77.44 Composition From Btms Bubble Pt.

Calc Bubble Point of Bottoms


y = K(x)Mole Frac Bub Pt. Vap to Tr 1

Btms, B Moles/hr, M x, B Ki = VPi / PT M * K mole fracC3 lk 0.77 0.0100 2.6163 2.03 0.02616C4 hk 36.66 0.4734 1.3171 48.29 0.62359

C5 35.00 0.4520 0.7162 25.07 0.32371C6 5.00 0.0646 0.4110 2.06 0.02654

1.000077.4359 77.4358 1.00000

77.4359-0.0001

Vapor Rate To Tray 1

y = K(x)Vap to Tr 1 Vap to Tr 1 Molecular Vap to Tr 1

Btms, B mole frac Moles/hr, V1 Weight lb/hrC3 lk 0.02616 1.96 44.10 86C4 hk 0.62359 46.66 58.12 2,712

C5 0.32371 24.22 72.15 1,747C6 0.02654 1.99 86.18 171

1.00000 74.82 63.04 4,717

Final Vapor & Liquid Data to Tr 1

Btms, B Vap to Tr 1 Liq Fr Tray 1 Molecular Liq Fr Tray 1 Liq DensBtms, B moles/hr Moles/hr, V1 Moles/hr Weight lb/hr lb/cf

C3 lk 0.77 1.96 2.73 44.10 120 31.6C4 hk 36.66 46.66 83.32 58.12 4,842 35.1

C5 35.00 24.22 59.22 72.15 4,273 37.2C6 5.00 1.99 6.99 86.18 602 40.7

77.44 74.82 152.26 9,838 36.31

Temp = 273.76 Deg FPress = 264.70 psia

Vapor Density = (MW)(Psia) / (10.73) / (Deg Rankine) = 2.119 lb/cf

Tower Diameter For Bottom Tray 1

Vapor To Tray, V, lb/hr = 4,717 CFS Vapor = 0.62Vapor Density, Dv, lb/cf = 2.119Liquid From Tray, L, lb/hr = 9,838 USGPM = 33.78Liquid Density, DL, lb/cf = 36.31System Factor, SF = 1.00 FWG 4900 / 5, Table 1b

Tray Spacing, TS, Inches = 18 Assume 18 inches or 24 inches as a First Try. 24 inch TS is preferred.

Spray Ht. / Tray Spacing, = 50.00% Assume 70% as a default value.

Assume Minimum Valves / AA = 8 No. Valves / sf AA A Higher value, eg 13, may lead to an unsuitable, smaller tower

Downcomer Flood, % DCF = 50.00% Assume 60% as a default value.

Preliminary SizingDowncomer Area (One Side), sf = 0.36 DCA = (L*7.481 / DL /60)/(7.5*TS 0̂.5*(DL-Dv) 0̂.5*SF) / %DC FloodActive Area, AA, sf = 1.45 AA = V/3600/Dv/(((((TS*SH/TS)-4.5213)/4.3662)*DL/Dv) 0̂.5)*78.5/(No Valves / AA)Tower Area sf = 2.17 AT = AA + 2*DCA

Tower Dia, ft. = 1.66 D = (AT*3/) 0̂.5

Select Tower Diameter 2 ft. (next 6" increment)

Shortcut Method by Dr Prakash

For Bottom of Tower

Kv = 0.04070 by Kv = -0.17*(TS) 2̂+0.27*(TS)-0.047 where TS is in meterscm/s = 0.01751 cm/s by cfs/3.2808 3̂vel = 0.16348 m/s by Vmax =Kv(DL-DV/DV) 0̂.5, the Souder EqnA = 0.107 sm cm/s / m/sD metric = 0.369 m D = (sm/.785) 0̂.5D English = 1.212 ft D = m*3.2808

Tray Efficiency

O'Connell and Drickamer / Bradford Tray Efficiencies Basis, Perry VI, p 18-14, & Ludwig, Applied Process Design For Chemical Plant Design

Assume avergae column conditions at Feed temp = 225 Deg F

Liq Visc Vap Press Vap Pre RatioComp Xi, Feed cP (Xi)(cP) psia Alpha lk/hk (Alpha lk/hk)(Xi)

C2 0.03000 0.02 0.0006C3 lk 0.20000 0.06 0.0120 529.40 2.0619 0.2270C4 hk 0.37000 0.1 0.0370 256.76

C5 0.35000 0.15 0.0525C6 0.05000 0.16 0.0080

1.00000 0.1101 = X For Drickamer

O'Connell Y = 0.2270

Drickamer Y = 0.1101

Tray Efficiency cont’dBox A

O'Connell Tray Efficiency = 70.39% Perry VI, Fig 18-23a

Perry VI, Eqn 18-14 Tray Eff'y = IF Y > 4, (46.514*(Y) -̂0.2052)/100 63.06% IF Y > 1 Tray Eff'y = (48*(Y) -̂0.228)/100 67.31% IF Y > 0.45 Tray Eff'y = (48*(Y) -̂0.2797)/100 72.67% Else, Tray Eff'y = (49.83*(Y) -̂0.233)/100 Ans --> 70.39%

Drickamer Tray Eff'y = 76.43% Drickamer Tray Eff'y = -27.3*LN(Drickamer Y / 1.81) / 100Ludwig, Applied Process Design For Chemical Plant Design And Petroleum Plant, Vol Ii, Gulf Publishing, Circa 1960.

Recommend Use Average = 73.41% (O'Connell + Drickamer) / 2

Actual No. of Trays & Feed Tray Location

No. Theoretical Trays = 19

Traty Efficiency = 73.41%

Actual Trays = 26

Kirbride Feed Tray Ratio = 1.25

Trays above Feed = 15Feed Tray Location = 11

Selected Feed Tray Ratio = 1.3636 vs 1.25 by Kirbride

Tower Dimensions3 ft. top trat to top tan line

14 spcs at 1.5 ft./spc52 ft. Tan to Tan 21 ft.

3 ft. feed tray space

10 spcs at 1.5ft/sp15 ft.

10 ft. to first tray

10 ft. shirt

1

1112

26

Vessel Specs

Vessel Specs

Operating pressure = 264.7 psia Check Flange RatingsDesign Pressure = 300 psiaMax Operating temp = 273.758 deg F Flg Rating, psig 300 (150/300/400/600)Design temp = 650 deg F Flg Press, psig 541 Flgs O.K.Material = SA-516 Gr 70 Des Temp, °F = 650 deg FCorrosion allowance = 0.0625 inches.

Cost of Towers Database v1.1

Tag No., T - 100Description DepropanizerFlow Sheet No. 1000No. Eqt Items 190 Actual Cost90 Est'd CostTower Type Tray TowerTower Dia., ft 2T-T Length, ft 52Design Press, psig 300Corr Allow, in. 0.0625Yield Eff'y 0.85 10% X-RAYTower Material No. (26) 4 SA-516 Gr 70Tray Option (4) 1 ValveNo Trays 26Tray Mtl No. 1 T-410 SSPacking OptionPacking Ht, ft.Tray Cost-88

Cost Estimate for Tower with Trays

Select Tower No., T- 301 (1 to 16 in Tower dB) Time Period 2001dB Item No. 9 Fab Eqt Index 454Description Depropanizer CND$/US$ 1.54Flowsheet No. 0 Duty US>CAN 1No. Eqt Items 190 Actual Cost 0 Shell Cost = $79,204 Shop Fab90 Est'd Cost 0 Tray Cost = $9,268 Shop InstalledTower Type Tray Tower Platforms = $8,218 Shop InstalledTower Dia., ft 2 Tot Tower Cost = $96,690T-T Length, ft 52Design Press, psig 300Back Calc'd des press = 306.06996 psigCorr Allow, in. 0.0625Yield Eff'y 0.85 10% X-RAYTower Material No. 4 SA-516 Gr 70Tray Option 1 ValveNo Trays 26Tray Mtl No. 1 T-410 SSPacking Option 0 #N/APacking Ht, ft. 0Tray Cost-88 0Btm Wall Thickness = 0.53125 InchesTop Wall Thickness = 0.3125 InchesVessel Wt = 5,800 lbsSkirt Ht = 12 ft.Skirt Wt = 2,345 lbs Assuming 3/4" ThkTray Wt = 980 lbs Assuming 10 ga. Wt.Total Tower Wt = 9,125 lbs

Shortcut Method

for Packed Towers

Ekert Packing FactorsSome Ekert Wet Dumped Packing Factors, SF/Cf, for shortcut method Note:

Diam. Inches 0.625 0.75 1.00 1.50 2.00MaterialCeramic Super Intalox 60 30Plastic Super Intalox 33 21Ceramic Intalox Saddles 145 98 52 440Metal Hy-Pak Rings 42 18Plastic Pall Rings 97 52 40 25Metal Pall Rings 70 48 28 20Ceramic Berl Saddles 170 110 65 45Ceramic Raschig Ring 380 255 155 95 65Plastic Tellerettes 40 20Plastic Mapak 32User Choice - See Perry VI, P-18-23

Select Packing Factor = 155 sf/cf FP for shortcut method only

Approximate HETP• From Tray Tower design, TS = 18 inches.• For Approximated Packed Tower Design

assume one HETP = one Tray Spacing.• HETP = 18 inches.• Determine Tower Dimensions as for a

Trayed Tower.• Allow 6 ft. for feed tray and top tray for

liquid distributer.• No packing height should exceed 20 ft.• If packing height exceeds 20 ft., must

redistribute liquid which adds another 6 ft.

FRI Packed Tower V1.2

FRI Packed Tower **FRIPT** Version 1.0, 14 Nov 93

Case Study =Bottom of Tower Example for CBE 497 Dwg No. = Example from Perry III By = RAH Tag No. = CBE - 497

Input Data

Liquid Flow = 9,933 lb/hr Packing Factor = 155 sf/cf Liquid Density = 36.31 lb/cf Packing Type No. = 18 Liquid Viscosity = 0.08 Centipoise Packing Type = Ceramic Raschig Rings / Wet Packed Vapor Flow = 4,812 lb/hr Packing Size = 1 Inches Vapor Density = 2.117 lb/cf Vapor Viscosity = 0.01 Centipoise No. Theoretical Trays = 19 Tower I.D. = 2.00 ft. S = mG/L Factor = 1.01 Minimum Value = 1.01

FRI Packed Tower Results For 2 ft. Diameter Tower

From Generalized Eckert Pressure Drop Correlation Shortcut Method

X = (WL/WG)(RHOG/RHOL) 0̂.5 = 0.4984 Y = (G) 2̂(FP)/(gc*RHOG*RHOL) = 0.0113 At X above, Y Flooding = YF = 0.0444 % Flood At (X,Y) = (Y/YF) 0̂.5*100 = 50.56% Eckert Presure Drop = 0.20 Inches H2O/ft. DP = 3.87 In. H2O

FRI Packed Tower Results For 1.5 ft. Diameter Tower

From Generalized Eckert Pressure Drop Correlation Shortcut Method

X = (WL/WG)(RHOG/RHOL) 0̂.5 = 0.4984 Y = (G) 2̂(FP)/(gc*RHOG*RHOL) = 0.0358 At X above, Y Flooding = YF = 0.0444 % Flood At (X,Y) = (Y/YF) 0̂.5*100 = 89.88% Eckert Presure Drop = 1.77 Inches H2O/ft. DP = 35.14 In. H2O

FRI Detailed Method for Designing a Packed Tower• Select a Packing Factor from 18

selected packing types.• FRI have determined the design

factors which are too numerous to list here.

• FRI Packed Tower V1.2 will use this packing data and the other data in the shortcut method to design % Flood and estimate the HETP.

FRI Detailed Method For PT

From New FRI Packing Correlations, Reports 92, 94, and 95, 1984 Detailed Method

FS Factor = (V)(RHOG) 0̂.5 = 0.29 Top tower = 6 ft. USGPM = 34.11 USGPM Feed tray - 6 ft. USGPM / SF = 10.86 USGPM/sf Btm packing =9 ft. a / Epsilon 3̂ factor = 146.08 sf/cf Top packing 12 ft. Packing Height = 19.42 Ft. btm tower = 12 ft. Packing Volume = 61.01 Cu. ft. Tan - tan = 42 ft.

Skirt = 10 ft. Dry Packing Pressure Drop = 0.46 In. H2O / ft. Dry DP = 9.01 In. H2O Wet Packing Pressure Drop 0.67 In. H2O / ft. Wet DP = 13.05 In. H2O

Maximum Allowable Vapor Rate For Calculated HETP = 9,150 lb/hr % Load or % Capacity = 4812 / 9150 = 52.59%

Maximum Stable Or Flood Vapor Rate At Unknown HETP = 12,279 lb/hr skirt ht. = 12 ft. % Flood = 4812 / 12279 = 39.19%

FRI HETP Values for 2 ft. Diam.

FRI Vapor And Liquid Transfer Unit Values

Vapor Back Mixing Transfer Unit = HDUG = 3.53 Inches Liquid Phase Transfer Unit = HTUL = 4.56 Inches Vapor Phase Transfer Unit = HTUG = 4.19 Inches Overall Gas Phase Transfer Unit = HTOG = 12.33 Inches

At Above Design Vapor / Liquid Rates, HETP = 12.27 Inches

HETP Message = Within 20% to 80% Capacity Limits Total Height of packing = 19.42 ft.

HETP Calculations (Good Only Between 20% to 80% Of Capacity And For Level, FRI Tubed Drip Pan Distributor.)

Packed Tower Cost Estimate

Select Tower No., T- 302 (1 to 16 in Tower dB)Time Period 2001dB Item No. 10 Fab Eqt Index 454.00Description Depropanizer CND$/US$ 1.54Flowsheet No. 0 Duty US>CAN 1.00No. Eqt Items 190 Actual Cost $0 Shell Cost = $61,120 Shop Fab

90 Est'd Cost $0 Packing Cost = $1,406 Field Installed

Tower Type Packed Tower Platforms = $7,319 Shop Installed

Tower Dia., ft 2 Tot Tower Cost $69,845T-T Length, ft 42Design Press, psig 300Back Calcd Design Pressure, psig = 6.07Corr Allow, in. 0.0625Yield Eff'y 0.85 10% X-RAYTower Material No. 4 SA-516 Gr 70Tray Option 0 #N/ANo Trays 0Tray Mtl No. 0 #N/APacking Option 1 Ceramic Raschig Rings, 1 inPacking Ht, ft. 21Tray Cost-88 $0Btm Wall Thickness = 0.4375 InchesTop Wall Thickness = 0.3125 InchesVessel Wt = 4,482 lbsSkirt Ht = 12 ft.Skirt Wt = 2,345 lbs Assuming 3/4" ThkPacking 0 Packing Installed In FieldTotal Tower Wt = 6,827 lbs

SummaryShortcut Shortcut Detailed

Trays Pkd Twr Pkd TwrDiameter 2 2 2Shell t-t 52 53 45No. Theo Tr 19 19 19Efficiency 73.41%No. trays 26Tray Spg 18HETP, inches 18 12.27Pkg ht. 29 21

Shell Cost $79,204 $80,441 $61,120Tr/Pkg Cost $9,268 $1,942 $1,406Platforms $8,218 $8,345 $7,319Total $96,690 $90,728 $69,845

Word of Caution – Trayed Towers

• Towers with trays are huge mixing devices. Any slight restriction will cause flooding.

• Three controlling factors:• (1) % Flood by Liquid and Vapor

Load• (2) % Spray Height by number of

holes.• (3) % Downcomer flood.• Trays must be level and well

supported.

Word of Caution – Trayed Towers

• Vendors will often quote towers with many holes to reduce diameter and obtain the bid.

• Later on detailed design, they find they must reduce holes for specified diameter.

• This increases spray height beyond acceptable level and entrainment will be too high.

• Buyers must be aware of all design details.

Word of Caution – Packed Towers

• Packed towers are low pressure drop systems. Flows don’t always go where they should.

• HETPs offered by vendors are optimistic.

• Vendors claim a wide range of operation.

• In actual practice there is a narrow range.

Word of caution – Packed Towers cont’d

• Uniform liquid distribution is difficult.• If packing ht. Exceeds 20 ft.. Liquid

must be redistributed. This adds cost.• Vapor is easily misdirected to walls.• Vapor distributors are often required.

Good Luck On Your Distillation Tower Design

• Presented to CBE 497• 22 Jan 02• R.A. Hawrelak

Distillation Tower Design As computer technology advances, the fundamental aspects of plant design...

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