1. Quick and Easy Troubleshooting of a
Packed Tower:
Thermal Imaging as a Novel Method
Hendri Barnard and Elzette du Toit
Sasol Ltd, Secunda Synfuels Operations

2. Problem Definition
An Acid Gas Scrubber had an unstable top temperature.
The top temperature of the distillate was swinging 10 °C higher and 10 °C lower
than the set point and this posed a risk contamination of the overheads product to
the downstream units.
The reflux line entering the top of the tower was frozen on the outside of the line
which also raised suspicion.
ACID GAS SCRUBBER CASE STUDY

3. Process Description
The purposes of the acid gas scrubber are to:
• remove the steam and CO2 from NH3,
• concentrate the NH3.
An ammonium carbonation reaction is used to ensure that
all the CO2 is removed from the NH3 before sending the NH3
for further purification. The NH3 would be in excess and this
causes the CO2 to react until extinction. The excess NH3 is
removed at the top of the tower.
Two sections of packing are used to enlarge the contact area
For the ammonium carbonate ((NH4)2CO3) formation. The
reaction is exothermic and excess heat is removed in the
middle and top pump around systems.
The feed enters the tower below the first tray. The steam
present in the feed needs to be condensed which is done by a
bottom pump around system.

4. Process Description
Temperature is controlled to favour the (NH4)2CO3 formation
reaction.
A stream from the Trap Out Tray 2 in the middle section is
pumped through the middle pump around cooler to attain
the required temperature after which it is then reintroduced
above Packing Section 1.
Vapour leaving the middle pump around passes through a
chimney into Packing Section 2.
The top pump around system also aids in the formation of
(NH4)2CO3.
A stream from Trap Out Tray 3 is drawn off and pumped
through the top pump around cooler and reintroduced above
the second section of random packing.
A stream with a high purity of NH3 is used as a reflux and is
introduced right at the top of the tower.
This stream also controls the temperature at the top of the
tower and washes down any traces of (NH4)2CO3 present.

5. Conventional Troubleshooting
The first steps that were done to try and alleviate the problem were:
Check Instrumentation - The temperature transmitter used for the control of the
top temperature was found to be operating outside its specified range. This was
corrected but this didn’t lead to an improvement in the temperature control,
Top temperature control loop was investigated - The control loop was found to be
functioning as intended.
All pump around temperatures and tower pressures were within normal
operating limits,
The temperature above packing section 2 was seen to be much higher than on
the other phases; possibly indicating that the problem might be in the middle
section of the tower,

6. Conventional Troubleshooting
The top reflux was investigated - It was found to be different when compared
to the other phases. This was also detected by visual inspection as this was the
only reflux line that indicated ice formation on the outside of the line.
The reflux was adjusted to allow for similar compositions of the reflux when
compared to the other phases. This resolved the ice formation on the line
and did influence the temperature, however the temperature above packing
section 2 remained higher than normal,
Finally we decided to take thermal scans of the tower to help determining
the possible root cause for the instabilities seen.

7. Tower 1 (showing the instabilities)
Unedited picture displayed on the camera taken from adjacent column.
Thermal Imaging

8. Tower 1 (showing the instabilities)
Unedited picture displayed on the camera taken from adjacent column.
Thermal Imaging

9. Tower 1 (showing the instabilities)
Unedited picture displayed on the camera.
Thermal Imaging

10. Tower 2 (stable operation)
Unedited picture displayed on the camera taken from adjacent column.
Thermal Imaging

11. Tower 2 (stable operation)
Unedited picture displayed on the camera taken from adjacent column.
Thermal Imaging

12. Tower 2 (stable operation)
Unedited picture displayed on the camera taken ground angle.
Thermal Imaging

13. Top Section
Tower internals schematic drawing.
Thermal Imaging Comparison
Trays
Distillate Outlet
Top Pump
Around ReturnLiquid
Distributor
Man Hole
Vent
Reflux
Distributor

14. Tower 1 (edited) Tower 2 (edited)
Thermal Imaging Comparison
Cold Spot

15. Top Pump Around Section
Tower internals schematic drawing.
Thermal Imaging Comparison
Top Pump
Around ReturnLiquid
Distributor
Packing Section 2
Man Hole
Man Hole

16. Tower 1 (edited) Tower 2 (edited)
Thermal Imaging Comparison

17. Top Section and
Top Pump Around Section
Tower internals
schematic drawing.
Thermal Imaging Comparison
Trays
Distillate Outlet
Top Pump
Around
Return
Liquid
Distributor
Man
Hole
Vent
Reflux Distributor
Packing Section 2
Packing Section 1
Man
Hole
Chimney Stand
Pipe
Top Pump
Around Outlet
Trap Out
Tray 3

18. Tower 1 (edited) Tower 2 (edited)
Thermal Imaging Comparison

19. Suspected Deviations
• Uneven or damaged liquid distributor.
• Blocked or damaged random packing.
• Blocked or damaged bed limiter.

20. Internal Inspections
Damaged random packing

21. Internal Inspections
A support grid installed on top of the packing section.
Damaged random packing and fouling causing blockages.

22. Repairs
New bed limiter before it was installed.

23. Conclusion
• Thermal imaging proves to be another viable option
in packed tower troubleshooting.
• Able to visually inspect the flow pattern inside a
packed tower while it is in operation.
• Pro active planning and procurement of spares to
ensure that downtime is minimized.

24. Conclusion
• Very little expertise is required for capturing the
images.
• Deviations can easily be identified.

25. Drawback
• Lagging/insulation would need to be removed before
the thermal image can be taken.
• External fouling also acts as insulation and need to
be cleaned.