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

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

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.

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.

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,

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.

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

Thermal Imaging

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

Thermal Imaging

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

Thermal Imaging

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

Thermal Imaging

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

Thermal Imaging

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

Thermal Imaging

Top Section

Tower internals schematic drawing.

Thermal Imaging Comparison

Trays

Distillate Outlet

Top Pump Around ReturnLiquid

Distributor

Man Hole

Vent

Reflux Distributor

Tower 1 (edited) Tower 2 (edited)

Thermal Imaging Comparison

Cold Spot

Top Pump Around Section

Tower internals schematic drawing.

Thermal Imaging Comparison

Top Pump Around ReturnLiquid

Distributor

Packing Section 2

Man Hole

Man Hole

Tower 1 (edited) Tower 2 (edited)

Thermal Imaging Comparison

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

Tower 1 (edited) Tower 2 (edited)

Thermal Imaging Comparison

Suspected Deviations

• Uneven or damaged liquid distributor.

• Blocked or damaged random packing.

• Blocked or damaged bed limiter.

Internal Inspections

Damaged random packing

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

Repairs

New bed limiter before it was installed.

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.

Conclusion

• Very little expertise is required for capturing the images.

• Deviations can easily be identified.

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.