Structured catalyst reactor system for steam methane reforming
Results from pilot plant test programme confirm significant advantages and value creation. Steam methane reforming is the predominant and most widely used process for syngas and hydrogen production.
Sanjiv Ratan and Bruce Boisture, ZoneFlow Reactor Technologies LLC
William Blasko and Wolfgang Spieker, Honeywell UOP
Florent Minette and Juray De Wilde, Université Catholique de Louvain
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The catalyst-induced reforming reaction is highly endothermic, thus calling for multiple tubes filled with catalyst and suspended in a furnace known in the industry as a steam methane reformer (SMR). The design, efficiency, tube life, and operational reliability of a SMR are governed to a large extent by its catalyst.
Although a matured technology, it has seen only incremental improvements in its catalyst design in conjunction with superior tube metallurgy over the years. The size and shape of current state-of-the-art pellet catalysts reflect the various developments aimed at improving heat transfer, pressure drop, and catalyst effectiveness. However, SMR performance with pellets catalyst remains inherently limited by heat transfer between the inner tube wall and the process gas, pore diffusion limitations, and heat transfer improvement at the cost of a higher pressure drop.
ZoneFlow Reactor Technologies LLC (ZFRT) developed the proprietary ZoneFlow structured catalytic reactor system (hereafter called ‘ZF reactor’) for steam methane reforming as a technological advancement (see Figure 1). It has been shown to provide numerous benefits over conventional pellet catalyst in terms of heat transfer, pressure drop, and catalyst effectiveness properties.1,3
ZF reactor modules have an annular and structured flexible casing, designed to double heat transfer compared to pellets without increasing pressure drop.³ The development journey of ZF reactors included catalyst selection for coating based on kinetic testing and modelling,² optimisation of the reactor design, and cost-effectiveness based on heat transfer-pressure drop testing,³ as well as computational fluid dynamics (CFD) modelling.4,5
Past work and referred publications reported the results of detailed CFD modelling and comparative measurements of the pressure drop and heat transfer of commercial pellets vs ZF reactors. ZF reactors showed a heat transfer enhancement to the extent of 2.2-2.4 times that of the conventional pellet catalyst without any pressure drop increase at commercial flow regimes (mass flux of 9-12 kg/sec-m²).³,4.5
Subsequently, a rigorous pilot plant test programme for performance validation of the ZF reactors on near-commercial conditions was successfully conducted and completed by November 2022 in a joint effort based on the Joint Development Agreement between Honeywell UOP and ZFRT (‘Pilot Plant Test Program’). Details and results of the pilot plant test programme are described further in this article.
Table 1 summarises the main differentiators of ZF reactors over state-of-the-art pellets based on the results of the pilot plant test programme. Apart from the main advantages of enhanced heat transfer (2.2-2.4 times that of pellets without pressure drop penalty) and lower pressure drop (40-50% lower vs pellets at the same flow rate), ZF reactors are stacked as modules in uniform loading in all the tubes. This results in uniform flow distribution over each tube compared to the randomly packed pellets causing inherent non-uniformity of flow even with the best pellet loading methods.
Based on industry best practices and modern methods for loading pellets, at best, the target can achieve as low as +/-3-4% variation in pressure drop over the multiple tubes in the SMR (measured using a preset air flow in each tube during loading), leading to inherent flow variation. In contrast, since ZF reactors are in a uniformly stacked identical metallic structure assembly in all the tubes, the non-uniformity of pressure drop and related flow rate per tube is deemed to be negligible (as confirmed by 3-D CFD modelling4,5). The uniformity of feed flow per tube minimises the variations in heat pick-up across the multiple tubes (based on a homogeneous stirred firebox). It thus minimises temperature spread and maldistribution in terms of tube skin temperatures as well as the outlet gas temperature from each tube, thereby requiring lower design margins for the outlet system.
The results and findings of the pilot plant test programme alidated the uniformity and robustness of the metallic foil substrate and stacked module design assembly. It also thereby prevents settling, crushing, and breakage. These are typical problems with pellets, causing increasing pressure drop over their operating life and thus worsening the maldistribution of flow and related tube temperatures.
The reactor’s design capabilities were demonstrated and verified in all the test campaigns based on the measured methane slip being very close to that simulated from the operating conditions (and expected approach-to-methane equilibrium). If there were any feed bypassing along the tube wall due to gaps between the ZF assembly and tube wall under hot/operating conditions, the methane slip in the reformed gas (and the approach to methane equilibrium) would have been far higher than observed.
Pilot plant description
The ZFRT pilot plant has been built for extensive testing and performance evaluation of different steam reforming catalysts. It is a world-class unit in terms of capacity, reformer geometry, boundary conditions, instrumented provisions, and operational safety.
The driving force and underlying objective to realise such an ambitious venture was to have an unconstrained and dedicated capability for testing and demonstrating the performance of ZF reactors compared to conventional state-of-the-art pellet catalysts under the same operating conditions covering (near-) commercial levels in terms of operating conditions, heat flux, and feed conversion.
Referring to the diagram of the ZFRT pilot plant in Figure 2, the main individual functioning units (IFUs) are desulphurisation and compression of the supplied natural gas, feed section for other gases (N2, Ar, H2), water pretreatment, an electric steam boiler, an electrical SMR furnace for steam reforming, an outlet expansion interconnecting piece, a syngas steam generator, a steam superheater and flow and pressure control system, and back pressure regulation for flaring the generated syngas.
The feed natural gas is drawn from the local low-pressure supply net. It is almost fully desulphurised in a two-layer catalytic adsorption reactor before compression in a compact multi-stage compressor. Other gases like N₂, Ar, and H₂ are fed from gas cylinder batteries. Boiler feed water is pretreated (softener, RO unit, degassing, dosing) and fed to an electrical start-up/make-up boiler and to the syngas steam generator, which is used to cool the syngas exiting the reactor. The steam is superheated before being mixed under the imposed S/C-ratio control with natural gas and other gases. The reactor pressure is controlled through a back pressure regulator, and the pressure in the steam supply system through an independent back pressure regulator via which excess steam is vented. Mass flow controllers are used to control all flow rates. A syngas sample line for gas analysis is installed in the syngas exit line to the flare.
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