Reforming Catalyst Technology

Reforming Catalyst Technology Hydrocarbon reforming is a basic chemical process used to develop synthesis gas used in the manufacture of hydrogen, methanol and ammonia.  The article includes such topics as hydrocarbon reforming, carbon forming reactions, catalyst poisons, catalyst formulations and advanced catalyst shapes.  Primary Reforming, Auto-Thermal Reforming, and Pre-Reforming reactors are explained and limitations and problems are discussed.
	Reforming Catalyst Technology
	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. 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.