Where We Go Wrong In Distillation Towers

1
Where We Go Wrong In Distillation Towers

• Dick Hawrelak
• Presented to ES317Y in 1999 at UWO

2
Introduction

• 3 of large property damage losses are caused by
  failures in process towers.
• Average Trended Loss was 53.8MM, the largest of
  all unit operations.

3
Distillation Tower Problems

• The following list of problems have been drawn
  from my personal experience as a design engineer
  with Dow Chemical for 33 years (1960 - 1993).

4
Poor Mass Balances

• Normal mass balance.
• Start-up, shut down or upset.
• Re-run of off-spec material.

5
Tower Variables

• Poor understanding of tower variables, reflux,
  downcomers, reboilers, condensers.
• VLE data can be confusing and misleading during
  design.
• Tower configuration not the best - Seadrift tower
  explosion kills one operator.

6
Tower Controls

• Process controls not adequate for upset
  conditions.
• Often dual level controls are required on bottoms
  of towers.

7
Reboilers

• Poor reboiler selection.
• High rate of fouling due to high steam
  temperatures.
• Reboiler stall due to loss of level in tower -
  Seadrift tower explosion.
• Reboiler lines too small and choke recirculation.

8
Condensers

• Pressure drops higher than design due to fouling.
• Hydrates form and plug condenser.
• Entrainment from tower not considered in design.
• Inerts blanket condenser, failure to remove heat
  cause PSVs to blow.

9
VCM Upset
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VCM Process
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VCM mb
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HCL Dispersion
13
VCM Dispersion
14
Mechanical Design of Tall Towers

• Tower shell thickness not suitable to prevent
  sway in high winds, tower topples.

15
Impurities In Tower Feed

• Impurities not considered in design, exothermic
  reaction destroys tower - Seadrift

16
Flammable Inventory

• Designers specify excessive inventory in reflux
  drum when tower logistics not well understood.
• Large bottoms inventory may promote formation of
  unwanted polymerization products.

17
Tower Operation at Low Rates

• Ballast tray units experience high rate of wear
  due to low unit reference. Broken valves plug
  downcomers or end up in pump suction causing
  further upsets.

18
Vacuum Operation

• Designers too optimistic about air leakage.
  Vacuum pump or steam jets too small to pull
  required vacuum.
• Poor management of pressure drop.

19
Recessed Sumps On Trays

• Vendors often specify recessed sumps to save 6
  inches on tower diameter.
• Recessed sumps are perfect traps to collect
  solids and plug downcomers.

20
Packed Towers

• Poor vapor and liquid distribution affects HETP.
  Dow wins 3MM lawsuit.
• Vendors predict high HETP over wide range of
  flow. Customers beware.
• Poor rangeability in packed tower operation.

21
Data Collection

• Grab samples not suitable to check tower
  performance.
• Require sample range over 24 hour period.
• Troubleshooting difficult because of poor
  instrumentation.

22
Tower Internals

• Flashing tower feeds not considered in feed tray
  design.
• Feed distributors do not work.
• Demisters often required but are ignored.
• Vortex breakers omitted in tower bottoms.

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Tower Optimization

• Select a process sequence.
• Mass balances to match product specs.
• Thermal condition of the feed.
• Minimum reflux ratio.
• Minimum no. of trays.
• No. trays versus reflux ratio.

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Optimization Contd

• Feed tray location.
• Tray efficiency.
• Valve tray tower design.
• Select tower internals.
• Repeat optimization for a packed tower.

25
UC Configuration
26
Dow Configuration
27
Previous Exam Problem

• The students were presented a paper on Union
  Carbides Seadrift, Texas, EO tower explosion.
  They were asked to comment on the explosion with
  respect to what they had learned in this safety
  course.

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Other Possible Exam Questions

• Why do designers often specify abnormally large
  flammable inventory in the reflux drum?
• Why should the designer minimize the reboiler
  steam temperature in a flammable distillation
  tower?
• What are the main problems with packed towers?

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Summary

• This short list is indicative of some of the
  problems caused by poor engineering discipline in
  distillation tower design.
• Recommend you obtain a copy of the Chemical Plant
  Design programs and follow the procedures built
  into the spreadsheets.