Steam Reforming Catalyst Reduction with LPG Feed

GBH Enterprises, Ltd.

Steam Reforming Catalyst Reduction with
LPG Feed

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Steam Reforming Catalyst Reduction with LPG Feed
Scope
This procedure may be used for the reduction of VULCAN Series Catalysts for
the general steam reforming of LPG.
It is strongly advised that this procedure is adopted only where there is no
other option available to use hydrogen, a hydrogen-rich gas or natural gas
for the reduction stage.
Reduction using the cracking of heavier
hydrocarbons carries an extreme risk of catastrophic carbon formation in
the event of any error in execution of the procedure.
Introduction
LPG is not normally utilized for steam reforming catalyst reduction although it can
be used successfully. Caution is required if heavier hydrocarbons are used for
catalyst reduction. Although operators have been able to reduce catalysts by
using heavier hydrocarbon cracking, this has only been adopted where no other
reductant option is available. The risk of carbon formation greatly increases as
the carbon number of the feed increases when the catalyst is in the unreduced
state. For the purposes of this procedure, LPG may range from a hydrocarbon
mixture which is predominantly propane to one which is predominantly butane.
Reduction Using LPG
The probability of success is greatly enhanced by increasing the care taken
during the reduction, over and above that used for hydrogen or natural gas
reduction. The operator must be confident that both steam and hydrocarbon flow
metering are properly calibrated and accurate during the reduction procedure as
one of the prevalent causes of inadvertent carbon formation during this
procedure arises from metering errors. It is therefore important that the
flowmeters be span checked and if possible calibrated for the conditions that will
be utilized during the catalyst reduction. All changes to feed rates and
temperatures should be carried out with extreme care. When increasing feed
rate, ensure that the steam rate is increased before the feed rate is increased.
Conversely when reducing feed rate, ensure that the feed rate is reduced before
the steam rate is reduced. Steam reformer operation must be extremely closely
monitored including very frequent visual inspection of the furnace including tube
appearance, flame stability and so forth.


1.

Purge the plant free of oxygen using nitrogen, and heat the reformer
above the condensation temperature while still circulating nitrogen.
Steam may be added as soon as possible after the temperature of the
exit header is at least 50°C above the condensation temperature of
steam.

2.

The steam flow should be increased to approximately 50% of the design
rate (commensurate with plant design constraints) as soon as possible
to allow even firing of the furnace. During reduction of catalysts with LPG
operation at the correct steam ratio is critical. With too high a steam to
carbon ratio, the catalyst will not reduce whereas too low a steam to
carbon ratio will lead to carbon formation. During reduction, it is usual to
stay below design pressure to allow early introduction of steam without
condensation and to protect the tubes by providing an additional margin
against over temperature and hence failure. Therefore, the accuracy of
the steam flow meter must be verified for operation at approximately
50% of design rate and this lower pressure operation. The use of a
separate DP cell on the steam orifice calibrated for start-up conditions
may be considered.

3.

Nitrogen circulation may be stopped at any convenient time after steam
has been added and before LPG is introduced.

4.

Increase the reformer exit temperature to 750°C at an appropriate rate
dependent upon the allowable furnace heating rate. At all critical stages
during reduction it must be emphasized that the temperatures referred to
are those at the actual tube exit. Temperatures indicated by control room
instruments are inevitably lower than the true value because heat
losses. Allowance must be made for the discrepancy. Depending on
the location of the thermocouples, indicated values may be 25 -100°C
(45 – 180oF) lower than actual temperatures.

5.

Introduce the LPG feedstock at about 5% design rate. This will result in
a steam:carbon ratio of approximately 30:1. Increase the LPG feed rate
to 10% of design rate over a period of 1 hour. This will reduce the steam
to carbon ratio to approximately 15:1. At the same time, increase the
reformer exit temperature to the design level or 800oC (1472oF)
whichever is achieved first. The heat requirement in the furnace will
increase as the endothermic steam reforming reaction begins. As
catalyst reduction proceeds, the furnace firing should be trimmed to
maintain the necessary exit temperature.


The recycle hydrogen rate should be adjusted to give the recommended
hydrogen concentration for acceptable hydrodesulfurization as soon as
possible after feed has been introduced.
6.

Since at start up the LPG flow is very low (5% - 10% of design) the
accuracy of the flow meter should be verified at this low flow. The use of
a low range flow transmitter would decrease the risk of operating with
too high an LPG flow and low steam to carbon ratio. It should be noted
that when the LPG feedstock pumps are lined up and the discharge
pressure raised there may be a small leakage of LPG past the isolation
valves due to the high differential pressure. This may result in a
reduction in the reformer exit temperature as reforming reactions take
place. Small leaks are not likely to create carbon in the reformer due to
the very small leakage rates experienced but more serious leaks
introduce the possibility of severe carbon formation.

7.

Increase the LPG feed rate to give a steam to carbon ratio of 7:1. At a
steam flow of ~50% of design and a steam to carbon ratio of 7:1 this
corresponds to an LPG feed rate of approximately 20% of design.

8.

During catalyst reduction, the tube inlet temperature should be as high
as possible to promote maximum reduction at the inlet of the tubes.

9.

Hold the steam to carbon ratio at 7:1 for a period of approximately 12
hours, by which time the catalyst will be reduced. As the catalyst
reduces more LPG will be reformed. During this stage, the exit methane
and heavier hydrocarbons concentrations should be checked at hourly
intervals. Reduction should be complete when the exit methane
concentration reaches a low steady value and the presence of heavier
hydrocarbon cannot be detected.
At this point the reformer exit
temperature can be decreased to the design exit temperature, if a higher
temperature has been used to promote desulphurisation.

10. When the catalyst has been reduced, the LPG feedstock rate should be
increased slowly to the design steam ratio and flow rate. This should
take about 2-3 hours. Check the methane concentration in the reformer
exit gas after each change to ensure that it stays at a low steady value.
If the methane or heavier hydrocarbon concentration increases or the
tubes show hot zones, continue reduction for a further period at a steam
to carbon ratio of 7:1, before once again increasing the LPG flow to the
design steam to carbon ratio. When flow rates are being increased, it is
always important to increase the steam flow before the feed flow in order
to maintain the steam ratio at or above the design value.

11. If the catalyst has not been fully reduced, the tubes may appear to be
hot. However, the catalyst should reach its fully reduced state after
approximately 24 hours normal operation. If this is not the case, it may
be necessary to stop the feed and restore reducing conditions for a few
hours.


Steam Reforming Catalyst Reduction with LPG Feed

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