Decoking of a fired heater and catalyst bed.

1. DECOKING OF FIRED HEATER COIL WITH CATALYST BED
INTODUCTION
Hydrodesulphurization is a catalytic chemical process which is used for removal of sulphur
from hydrocarbon product. In this case, we are focusing on removal of sulphur from raw naphtha to
produce low content sweet naphtha. Sweet naphtha is the raw material for primary reformer and
here ‘sweet’ term implies a friendly input for reformer catalyst in terms of minimal poison
exposure.
PROCESS
The hydrodesulphurization (HDS) catalyst helps removal of sulphur present in raw naphtha
in the form of hydrogen sulphide (H2S) gas. Hydrogen rich gas is exposed to the raw naphtha and
the temperature of the mixture is increased with the help of fired heater furnace/heat exchangers.
After achieving the desired temperature, the mixture is sent over a HDS catalyst. The hydrogen
sulphide produced gets dissolved in the naphtha. H2S has to be released from naphtha via flashing
and a stripping unit. Later the sulphur free naphtha is cooled and stored as sweet naphtha.
PROBLEMS
The HDS system is quite efficient if the physical and chemical parameters of the raw
material are not tested to their limits. Due to availability of raw naphtha of higher density and
higher residue on evaporation (ROE), it was decided that we opt for the same. Initially the HDS
plant operated quite well over a recommendable period of time. The signs were there to see but
were overlooked and concluded to be an unexpected glitch.
One fine day the product requirement from the HDS unit load had dropped and finally it was
stopped. After the breakdown maintenance was successfully completed, the unit was again started
and lined up. The HDS unit was normal at lower load, but as the load started to increase the
differential pressure across the catalyst bed was increasing proportionally. On observing the
operational parameters in field it was concluded that the heat exchangers and catalyst bed were
providing more resistance. The HDS unit had to provide the product with lower sulphur content as
the sweet naphtha storage unit quality had to be maintained.
The product quality and differential pressure across catalyst bed were continuously being
monitored. In case the product was showing the tendency to fall off from the desired range,
operational parameters were stretched to their limits to avoid any incompliance. At one point, the
systems operational parameters were out of control. (I would love to discuss the problems what we
were facing and how we tackled them with a lot of viable options that were successful indeed.) It
was decided to reduce the plant load and try to maintain the feasible operational parameters to
avoid any unwanted incidents and accidents.
It was planned that we opt for decoking of fired heater coil with the catalyst bed to remove
the deposited layer of carbon. If decoking was successful, the catalyst was planned to be reused.

2. DECOKING
The basic principle of decoking is to regenerate the catalyst by burning off the deposited
carbon layer that might have occurred from cracking of heavier hydrocarbon molecules. For the
regeneration procedure the whole unit has to be purged with inert gas and ensure hydrocarbons
have been removed. Arrangements have to be done to accommodate the calculated mass flow of
inert gas, process steam and process air via fired heater coil and catalyst bed. Make sure that the
mentioned raw materials in sufficient quantity are made available during the regeneration process
and monitoring the input mass flow for them is considered. As I’m emphasizing on a unit that has a
lot of catalyst suppliers, it is not possible for me to generalize the temperature range. It would much
better to consult your catalyst supplier on rate of increasing temperature of catalyst bed and
maximum permissible temperature for regeneration.
• Establish the inert gas flow through the loop of fired heated coil- catalyst bed- water
scrubber-vent and increase the bed temperature gradually. A sample point arrangement has
to be made for analysis of reactor exit gas.
• Once the catalyst bed outlet temperature reaches to point that is well above the saturation
temperature of process steam, it is recommended to introduce process steam and maintain
the catalyst bed temperature. After successful introduction of steam, inert gas can be cut off.
• Now increase the catalyst bed temperature to the normal operation point at a steady
suggested rate. Maintain steaming continuously.
• Maintain the fired heater outlet temperature at the normal operation point. Carefully add
process air to the loop in small quantities. An increase in catalyst bed temperature will be
observed as oxidation of carbon initiates. Catalyst bed temperature should not exceed the
maximum permissible point as suggested.
• Analyze the non combustible gases at the exit of reactor for CO2 and O2.
• Increase the process air flow through the loop and monitor the O2 content at exit. If the O2
content keeps on increasing, increase the catalyst bed temperature so that O2 disappears
and regeneration is enforced. At all times ensure that the catalyst bed temperature does not
exceed the maximum permissible point as suggested.
• When the O2 content remains same at reactor exit gas even on raising the catalyst bed
temperature via fired heater, regeneration can be considered to be near completion.
• Now replace the process steam + process air with just process air. Keep a watch on catalyst
bed temperature. If the reactor outlet gas has over 15% of O2 and less than 2% of CO2,
regeneration has been completed.
The regenerated catalyst can be used and put back to service in the same way as a new
catalyst.