• Steam/carbon ratio
The molar ratio of steam to carbon is one of the most critical parameters
to monitor in an SMR. To calculate the steam/carbon ratio (sometimes
abbreviated S/C):
– Use the feed detailed analysis to calculate the moles of carbon per mole of
feed. Include any CO or CO 2 in the feed as part of the calculation.
– Calculate the total moles of feed using the feed flow meter, with corrections.
Normally, this will be moles per hour of feed.
– Calculate the molar rate of carbon in the feed from the moles carbon per mole
feed times the feed rate. This will normally be moles carbon per hour.
– From the process steam flow meter, calculate the moles of steam per hour.
– Divide the moles steam per hour by the moles carbon per hour to get the
steam/carbon ratio.
For liquid feedstocks where composition is not necessarily available, the carbon
in the feed can be estimated using C/H ratio correlations from API based on
boiling range and specific or API gravity.
The S/C must be maintained within a specified range. A high ratio will make
more hydrogen, but loses some efficiency in the process. A low ratio leads to
coking and deactivation of the reforming catalyst, as well as making less
hydrogen.
Note that you do not need the whole material balance to calculate the steam/
carbon ratio. Often, the ratio is calculated online with sufficient accuracy using
average compositions to allow the direct control of the ratio by the basic process
control system.
See the Appendix for a sample calculation of the corrected steam/carbon ratio.
• Reforming catalyst approach to equilibrium
Using the reformer outlet composition from the material balance and the equilibrium K values, the equivalent reformer outlet temperature that the equilibrium
corresponds to can be calculated. This calculated temperature is subtracted from
the actual temperature to obtain the “approach to equilibrium” (ATE) temperature. This is a way to track reforming catalyst activity.
Note that an equation for calculating the equilibrium temperature from the
calculated K value is included in the overall reforming process discussion in
section “Primary Reforming” of this chapter (Eq. 13).
The ATE value is normally negative with fresh catalyst. The actual approach is
zero, but heat loss from the actual catalyst outlet to the reformer outlet
temperature measuring point results in the negative value. This is not really a
problem, because you will track catalyst activity by relative changes in
the ATE.
Over time, the reformer ATE trends upward as the furnace eventually cannot
reach equilibrium. At some point, you decide to replace the catalyst. Reformer
catalyst can typically operate over 5 years, unless it is poisoned or coked.
However, even a few minutes with very low or no steam in the feed may dictate
an immediate end of run.
608
S.A. Treese
The molar ratio of steam to carbon is one of the most critical parameters
to monitor in an SMR. To calculate the steam/carbon ratio (sometimes
abbreviated S/C):
– Use the feed detailed analysis to calculate the moles of carbon per mole of
feed. Include any CO or CO 2 in the feed as part of the calculation.
– Calculate the total moles of feed using the feed flow meter, with corrections.
Normally, this will be moles per hour of feed.
– Calculate the molar rate of carbon in the feed from the moles carbon per mole
feed times the feed rate. This will normally be moles carbon per hour.
– From the process steam flow meter, calculate the moles of steam per hour.
– Divide the moles steam per hour by the moles carbon per hour to get the
steam/carbon ratio.
For liquid feedstocks where composition is not necessarily available, the carbon
in the feed can be estimated using C/H ratio correlations from API based on
boiling range and specific or API gravity.
The S/C must be maintained within a specified range. A high ratio will make
more hydrogen, but loses some efficiency in the process. A low ratio leads to
coking and deactivation of the reforming catalyst, as well as making less
hydrogen.
Note that you do not need the whole material balance to calculate the steam/
carbon ratio. Often, the ratio is calculated online with sufficient accuracy using
average compositions to allow the direct control of the ratio by the basic process
control system.
See the Appendix for a sample calculation of the corrected steam/carbon ratio.
• Reforming catalyst approach to equilibrium
Using the reformer outlet composition from the material balance and the equilibrium K values, the equivalent reformer outlet temperature that the equilibrium
corresponds to can be calculated. This calculated temperature is subtracted from
the actual temperature to obtain the “approach to equilibrium” (ATE) temperature. This is a way to track reforming catalyst activity.
Note that an equation for calculating the equilibrium temperature from the
calculated K value is included in the overall reforming process discussion in
section “Primary Reforming” of this chapter (Eq. 13).
The ATE value is normally negative with fresh catalyst. The actual approach is
zero, but heat loss from the actual catalyst outlet to the reformer outlet
temperature measuring point results in the negative value. This is not really a
problem, because you will track catalyst activity by relative changes in
the ATE.
Over time, the reformer ATE trends upward as the furnace eventually cannot
reach equilibrium. At some point, you decide to replace the catalyst. Reformer
catalyst can typically operate over 5 years, unless it is poisoned or coked.
However, even a few minutes with very low or no steam in the feed may dictate
an immediate end of run.
608
S.A. Treese