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  • Is there a catalytic means to increase production from our hydrogen plant without a revamp or other significant capital outlay?

    Mar-2022

Answers


  • Jumal Shah, Johnson Matthey, jumal.shah@matthey.com

    To increase production, it is important to initially understand the existing limitations of the hydrogen plant. This is typically achieved using performance evaluations of each catalyst unit against kinetic models, thermographic surveys of the steam methane reformer, and equipment specifications and design limit. Common factors that constrain production rates in a hydrogen plant include steam reformer tube temperature limits or other furnace constraints, carbon formation margins in the steam reformer pressure drop across the plant, and poor conversion in the water gas shift section.
    Steam reforming of methane is an endothermic reversible reaction. Thus, the catalyst activity and heat transfer from the burners to the catalyst are important for maximising production. The formulation and the manufacturing process determine the intrinsic activity of the catalyst. Katalco 57-6 series has a patented manufacturing technique that makes the active metal on the pellet more accessible to the gas and enables performance deeper into the operating cycle. The size and shape of the catalyst will impact the tube-side laminar film layer and, therefore, the overall heat transfer coefficient. Catalysts such as Johnson Matthey’s QuadraLobe catalysts can provide better packing characteristics, more tube wall contact points, and radial gas mixing, thereby improving the heat transfer properties. Uniformity of the catalyst loading, reliable tube wall temperature monitoring, and balancing for better utilisation of firing capacity are also important controls for easing limits caused by tube temperature limits or other furnace constraints, such as limited combustion airflow or flue gas temperature limits.

    Due to the temperatures at which steam reformers operate, carbon is constantly being formed from the hydrocarbon feed. However, carbon gasification reactions simultaneously occur that remove the carbon laid down, meaning there is no net accumulation of carbon in a well-run plant. The rate of carbon laydown depends on a number of conditions, such as the catalyst activity, degree of sulphur poisoning, and heat input to the tubes. As the steam reformer catalyst ages, the gradual deactivation of the catalyst can increase the tube wall temperatures and the potential for carbon formation. High activity, good thermal stability, and alkali promotion can ensure the carbon removal rate is faster than the carbon formation rate. Promoted catalysts, such as Katalco 25-series and Katalco 46-series, maximise protection against carbon formation and allow higher production rates to be maintained through the life of the catalyst.

    For higher production rates, the catalyst should allow for the lowest possible pressure drop, as this will enable the highest possible plant throughput before compressor limits are reached. The catalyst breakage characteristics are important for the steam reforming catalysts as all pelleted steam reforming catalysts will break due to the forces exerted on them when reformer tubes expand in operation and then contract during plant shutdowns. QuadraLobe shaped catalyst pellets are specifically designed to avoid generating small pellet fractions when breaking and hence minimise pressure drop increase over the catalyst’s lifetime. Grades range from the highest activity shape mini QuadraLobe (MQ) to the low pressure drop extra-large QuadraLobe (XQ) catalyst, allowing a tailored selection to optimise between activity and pressure drop.

    The water gas shift section of the hydrogen plant typically contributes around 15% of the hydrogen produced. High temperature shift catalyst with a higher activity, better thermal stability, and improved poisoning resistance can also enable an increase in hydrogen production. Premium catalysts such as Katalco 71-6 have enhanced activity, improved in-service strength, and more wetting resistance. The water gas shift reaction is an exothermic and equilibrium reaction. A higher activity catalyst allows operation at lower optimal temperatures and thus a more favourable equilibrium position for higher conversion. The improved strength ensures a consistently low pressure drop throughout the operating life, and simple StreamLine technology can be installed to reduce further the pressure drop across a water gas shift vessel.

    In summary, many factors limit the production rate of a hydrogen plant. To maximise the production rate, a good understanding of the plant limits is needed, and selection of the right catalyst is required to enable maximum production capacity whilst maintaining reliability and efficiency.

     

    Mar-2022



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