Naphtha Steam Reforming Catalyst Reduction by NH3 Cracking

GBH Enterprises, Ltd.

Naphtha Steam Reforming Catalyst
Reduction by NH3 Cracking

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Naphtha Steam Reforming Catalyst Reduction by NH3 Cracking
Scope
This procedure applies to the in situ reduction of VULCAN Series steam
reforming catalysts using ammonia cracking to form hydrogen over the catalyst in
the steam reformer. This procedure covers plants with a dry gas circulation loop
for reduction. The procedure is likely to be applied to plants using only heavier
feeds (e.g.: LPG and/or naphtha) and some combination of VULCAN Series
catalysts.
Introduction
A small number of steam reforming plants do not have an available source of the
commonly used reducing media (e.g.: hydrogen, hydrogen-rich off-gas, natural
gas). These plants will usually operate on LPG and/or naphtha feed only where
cracking of this hydrocarbon is not usually advised for reduction of the steam
reforming catalyst. In such circumstances, the plant may be designed to use the
installed steam reforming catalyst to crack ammonia to provide hydrogen for the
reformer catalyst reduction. This may be on a once through basis or with gas
recycle through a circulating loop. By control of the steam to ammonia ratio and
reformer exit temperature, oxidized catalyst cracks ammonia to generate
hydrogen which then affects a degree of catalyst reduction. Once some reduced
nickel is present, ammonia cracking becomes efficient and the period in which
ammonia is observed in the process condensate is kept to a minimum.
Procedure
1. Ensure the primary reformer catalyst is heated in a nitrogen flow to above the
dew point of the process stream. Once this temperature is exceeded by at
least 50°C (90°F), continue heating with process steam. The system pressure
should be in the usual range for the reformer start-up circulation loop
(typically 10 – 15 bara).


2. Heat the reformer to a measured inlet temperature in the range 475 to 500°C
(887-932°F) and a measured exit temperature of 780 to 800°C (14361472°F). If the plant design does not allow the inlet temperature to attain this
level, then the inlet temperature should be as high as possible within the
constraints of the plant. Temperature losses to the point of exit temperature
measurement are usual at this low load of operation and the actual tube exit
temperature will be higher than these values.
Regular (1/2 hourly)
inspections, ideally with an accurate IR pyrometer, of the reformer are
necessary to check for possible overheating.
3. At the above temperatures, control the steam flow through the primary
reformer insofar as this is possible to remain within tube skin temperature
limits and to satisfy the ratio of steam to ammonia as specified in (5)
4. Maintain a nitrogen gas circulation rate of 40-60 Nm3/hr per reformer tube.
5. Inject ammonia at an initial rate to satisfy a steam to ammonia molar ratio of
an absolute minimum of 20:1. This will be sufficient to carry-out the catalyst
reduction, but hydrogen will take 1-2 hours to be detected in the recycle
gases and ammonia in the condensate will be at high levels (>>100 ppmw)
for this period also. To minimize the time to produce hydrogen and limit the
amount of high ammonia concentration in the condensate, lower molar ratios
of steam to ammonia should be targeted in the range 14:1 to 9:1.
6. Process condensate containing ammonia will need proper attention. Initial
levels of ammonia could exceed 1000 ppmw, but will reduce quickly to <100
ppmw (typically 30-70 ppmw) once cracking occurs over the reforming
catalyst
7. Maintain continuous injection at the required rate for at least one (1) hour.
Monitor the progress of ammonia cracking by observation of process
conditions (increasing loop pressure; dry gas circulation rate) and analysis.
Adjust the rate based on the analytical results.
8. Take samples for analysis of the re-circulating gas for ammonia and hydrogen
and of the process condensate for ammonia every 30 minutes over the first
two hours of ammonia injection. Thereafter, reduce the frequency to every 60
minutes for as long as necessary.


9. If no hydrogen is detected after 2 hours and the ammonia levels in the
process condensate remain high (>>100 ppmw), then the molar ratio of
ammonia to steam is possibly incorrect and/or the reforming temperature is
low.
10. Once hydrogen is measured, the H2O/H2 molar ratio should be recorded (from
a combination of analysis and calculation of the amount injected and cracked
ammonia). The target point is when the H2O/H2 molar ratio enters the target
reduction range of 6:1 – 8:1. The molar ratio may be allowed to go as low as
4:1 without cause for concern in terms of the catalysts within the loop.
11. Once the H2O/H2 molar ratio is in the range 6:1 – 8:1, stop ammonia injection.
Continue to analyze at 60-minute intervals and calculate the H2O/H2 molar
ratio in the loop. Hydrogen will be consumed slowly as the reduction
proceeds. As the H2O/H2 ratio rises towards the top of the reducing range
(H2O/H2 molar ratio = 8:1), inject a slug of ammonia to adjust the H2O/H2
molar ratio in the loop to about 6:1.
12. Maintain reducing conditions (H2O/H2 molar ratio in the range 6:1 – 8:1) for
the following times depending on the recent shutdown history of the catalyst.
See Table 1.
13. Following this, introduce hydrocarbon feed as described in the Operating
Manual for VULCAN Series Naphtha Steam Reforming Catalysts.

Table 1 – Catalyst Reduction Times

Catalyst Steaming
Period
(Hours)
<3
3-8
>8
Fresh Catalyst Charge

Period of Reduction
(Hours)
No reduction required
6 hours of reduction


Naphtha Steam Reforming Catalyst Reduction by NH3 Cracking

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