Distillation Tower Design

1
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
2
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
• Total 100 lb moles/hr

3
Process Sequence Contd

• 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.

4
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).

5
Set Up Mass Balance for Tower
6
Mass Balance Equations
7
Mass Balance Solution
8
Obtain Antoine Constants

• Need Antoine constants for Vapor Pressure
• Vap Press, VP 10(A B / (tC C)) psia

9
Feed Conditions

• Temperature of feed 225 deg F 107.22 deg C.
• Pressure of feed 264.7 psia

10
Determine Bubble Point of Feed
11
Determine Dew Point of Feed
12
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.

13
Determine V / L for Feed
14
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.

15
Solve For Ø, The Underwood Parameter, Contd
16
Solve For Ø, The Underwood Parameter, Contd
17
Calc Minimum Reflux Ratio by Underwood

• See Perry VI, Chem Eng HB, Page 13-36
• Solution For Minimum Reflux Ratio By Solving For
  ? AlphaxDi / (Alpha - Ø) L/D min. 1
• Ø, The Underwood Parameter, was determined above.

18
Calc Minimum Reflux Ratio by Underwood, Contd
19
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.

20
Determine Minimum No. Trays by Fenske
Underwood, Contd
21
Determine Minimum No. Trays by Fenske
Underwood, Contd
22
Determine Minimum No. Trays by Fenske
Underwood, Contd

• Determine geometric Average Alpha between top and
  bottom of the tower.
• Geometric Avg (Alpha TopAlpha Btm)0.5
• Avg Alpha ((2.51)(1.99))0.5 2.23

23
Determine Minimum No. Trays by Fenske
Underwood, Contd

• Min. Trays LN((C3 lkD / C4 hkD) (C4 hkB /
  C3 lkB)) / LN(Alpha Avg)
• Minimum No. Trays, Sm 10.11

24
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.

25
Determine Trays versus Reflux Ratio by Gilliland
Method, cond
26
Plot of Trays Versus Reflux Ratio
27
Determine Feed Tray Location
28
Determine Reflux Flow Compn
29
Calculate Overhead Vapor Flow from Top Tray 19
30
Calc Vapor Composition from Top Tray 19
31
Show Molar Balance Around Top Tray 19
32
Calc Dew Pt of Vapor Fr T19 and Liquid Compn Fr
T19
33
Vapor Compn From Tray 18
34
Calc Dew Point of Vapor V18
35
Design Data For Top of Tower
36
Input Shortcut Tower Dia. (FWG)
37
Shortcut Method by Dr Prakash
38
Check Tower Mole Balance
39
Calc Bubble Point of Bottoms
40
Vapor Rate To Tray 1
41
Final Vapor Liquid Data to Tr 1
42
Tower Diameter For Bottom Tray 1
43
Shortcut Method by Dr PrakashFor Bottom of Tower
44
Tray Efficiency
45
Tray Efficiency contd
46
Actual No. of Trays Feed Tray Location
47
Tower Dimensions
48
Vessel Specs
49
Cost of Towers Database v1.1
50
Cost Estimate for Tower with Trays
51
Shortcut Method for Packed Towers
52
Ekert Packing Factors
53
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.

54
FRI Packed Tower V1.2
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FRI Packed Tower Results For 2 ft. Diameter Tower
56
FRI Packed Tower Results For 1.5 ft. Diameter
Tower
57
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.

58
FRI Detailed Method For PT
59
FRI HETP Values for 2 ft. Diam.
60
Packed Tower Cost Estimate
61
Summary
62
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.

63
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.

64
Word of Caution Packed Towers

• Packed towers are low pressure drop systems.
  Flows dont 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.

65
Word of caution Packed Towers contd

• 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.

66
Good Luck On Your Distillation Tower Design

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