| Pre-Reforming, Primary Reforming, Secondary and Auto-Thermal Reforming CatalystsHydrocarbon Reforming with steam in the presence of catalyst has been an essential means for hydrogen production in
Chemical Industry since the 1930's. While the catalysts for operating plants have improved, the underlying principles of
operation have remained steadfast throughout time. In Primary and Pre-Reforming equipment, hydrocarbon is mixed with
steam at elevated temperatures and passed across catalysts supported within tubes in a fired heater or within a pressure
vessel. Competing simultaneous hydrocarbon reforming and water-gas-shift reactions occur on the active sites of the
catalyst, as follows:Methane Reforming:
CH4 + H2O = CO + 3H2 +49.2 kcal/mole (Endothermic)
CH4 + 2H2O = CO2 + 4H2 +39.4 kcal/mole (Endothermic)Water-Gas-Shift:
CO + H2O = CO2 + H2 -9.84 kcal/mole (Exothermic)Higher hydrocarbons are first cracked catalytically to Olefines and Methane and then these primary products react further
with Steam yielding Hydrogen and oxides of Carbon. Thus, a mixture of H2, CO, CO2, CH4 and Water vapors is obtained,
the composition of which depends upon reaction chemical equilibria and catalyst activity, operating temperature and
pressure conditions and the ratio of steam to hydrocarbon feeding into the reformer. Over decades of design, operating
conditions for hydrocarbon reforming have become much more severe, intensifying the design requirements of the catalyst.
Earliest equipment operated at near atmospheric pressure, whereas modern reforming equipment can operate up to 40
atmospheres pressure. (588 psia, 4034 kPa, 41.3 kg/cm2 abs)
| Carbon Forming ReactionsThe following carbon forming reactions are
possible, but are largely prevented by proper
maintenance of Steam/Carbon ratio in the mixed
feed gas to the Reformer:
2CO = C + CO2
CO + H2 = C + H2O
CH4 = C + 2H2
The potentiality for carbon formation in Primary
Reforming is quite complex and depends on
pressure, temperature and composition of the
Reformer mixed feed gas, as well as catalyst
composition. Generally thermodynamic carbon
formation reactions will not occur for Natural Gas
feedstock as long as S/C ratio is maintained
substantially above 1.0. Such risks generally
occur only during plant upset conditions and
properly designed plant trip instrumentation
should largely prevent it. |
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| Catalyst FormulationsNickel has been found to be the most cost-effective active species for Reforming catalysts, although other elements can
work and are more active, including Cobalt, Lanthanum, Platinum, Palladium, Iridium, Rhodium and Ruthenium.
Nickel is combined with various support materials including alpha-Alumina, Calcium-Aluminate, Magnesia-Alumina,
sometimes including Alkali-promoters through proprietary means within an efficient open pore-structure catalyst shape to
overcome gas diffusion limitations, while supporting high catalyst activity.Catalyst PoisoningReforming catalysts are poisoned primarily by Sulfur, but other compounds can also cause loss of catalyst activity,
including Arsenic, Chlorine and other halogens, Copper, Lead, Silver, Vanadium and Cadmium. Under normal
circumstances only Sulfur and Chlorine present significant threats to loss of Reforming catalyst activity and these effects
on activity are completely reversible when the poisoning contaminant has been removed from the feedstock. Sulfur and
Chlorine concentrations of greater than 0.2 ppmv in the mixed feed (wet basis) or 0.7 ppmv in the hydrocarbon feedstock
can cause significant loss of catalyst activity over the period of a few minutes to a few hours, depending on the
contaminant concentration. Increased levels of Methane and reduced Hydrogen production will take
place until the poisoning is eliminated.
Catalyst PerformanceCatalyst performance is frequently measured by a variety of ways, including the extent of conversion of hydrocarbon
(particularly Methane) into Hydrogen, or the Methane content of the exit gas (Methane Leakage) at given temperature,
pressure and gas throughput. Increased temperature reduces the amount of Methane for otherwise fixed operating
conditions. In actual practice, the Methane concentration in the exit gas from Reforming catalyst is greater than the theoretical
minimum at a given temperature so that there is a lower equilibrium temperature where that higher Methane concentration
would exist at equilibrium. This difference in temperature is referred to as the Methane Approach. Most generally, conversion
is equated to this approach to the Methane-Steam reaction equilibrium, or "Methane Approach." The Methane Approach is
dependent on gas throughput, Steam/Carbon ratio, operating temperature and pressure and catalyst activity.
Catalyst activity, (which decreases over time for all catalysts), can be promoted by several means, including constituents
combined with manufacturing techniques, catalyst size and shape. Catalyst size and shape also establish Reformer gas
pressure drop and have direct impact on catalyst strength, which has a major influence on practical useful catalyst life. For
tubular Hydrocarbon Reforming equipment, catalyst activity is a direct influence of catalyst tube metal temperature during the
life of a catalyst charge, apart from the separate and distinct influence of plant throughput and inter-related Reformer
operating conditions. In normal service as Reforming catalyst ages, tube metal temperature increases for otherwise fixed
operating conditions, chiefly from the net loss of active Nickel surface, primarily from sintering of active metal crystallite size
and from the gradual loss of Alkali promoters. Thus tubular Reforming catalyst performance can chiefly be measured by three
variables:
Exit Gas Methane Leakage (and resulting Methane Approach)
Tube Metal Temperature (gradually increasing to the equipment limitations)
Gas Pressure Drop (increasing from catalyst attrition, primarily due to plant cycles)
For other Pre-Reformers, Auto-Thermal and Secondary Reformers, Methane Leakage and Gas Pressure Drop over catalyst
life would fundamentally define catalyst performance.
Greater performance for all Reformer types infers longer life due to strong catalyst with low gas pressure loss and close
Methane Approach to equilibrium and the smallest possible changes over time.Pre-ReformingPre-Reformers have gained some recognition, during the past 5-10 years due to the expansion of older plants. Primary
Reformers have been expanded with re-tubing projects as equipment has aged and worn out. Primary Reformer tubes must
be periodically replaced about every 15 years for a typical plant, whether enlarged or replaced in-kind. Pre-Reformers offer
the option of spending capital on an up-stream Reforming catalyst vessel, which unloads Primary Reforming catalyst duty.
The economics are rather complex, but make good sense for some facilities. Unfortunately, true process debottlenecking of
Primary Reforming doesn't occur when Primary Reforming tubes are not enlarged, since gas pressure losses will remain the
same, or increase with higher throughput. |
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