Jun-2015

Structured catalysts for steam reformers

A foil based catalyst aims to avoid the limitations of ceramic pellet substrates in steam reforming.

WILLIAM WHITTENBERGER, Johnson Matthey Process Technologies
PETER FARNELL, Johnson Matthey plc

Article Summary

Steam reforming catalyst design is a balance between many competing requirements such as strength, heat transfer, activity, pressure drop and the avoidance of carbon formation. Catacel SSR is a coated foil based alternative to metal-impregnated ceramic pellet media. A foil based structure enables SSR to avoid many of the limitations imposed by the use of ceramic pellets. It exhibits higher activity, improved heat transfer, lower pressure drop and improved carbon resistance all at the same time.

Depending on the plant design, SSR can be used to decrease tube temperatures, reduce natural gas fuel consumption or increase throughput of the reformer. New reformers can typically be designed with lower capital cost of the radiant box. This technology has been demonstrated in two commercial hydrogen plants since May 2012.

Catalyst coated foil
The principle of the technology is the ability to coat catalyst materials onto the surface of thin metal foils. The coating process ensures that the catalyst remains attached to the surface of the foil during the catalyst’s lifetime.

Alloy strip is formed into engineered foil supports called fans (see Figure 1). The fans are coated with a nickel based steam reforming catalyst. The fans are quite ‘springy’ and can easily be pulled or pushed into different diameters or shapes.

They are stacked one upon another, separated by thin metal washers (see Figure 2) in groups up to one metre long over a support structure that sits within the central space of the fans. This central structure aids in the speed and accuracy of catalyst installation by avoiding the need to install the fans individually. The stacked fans deliver superior heat transfer by impinging gas on the internal surface of the reforming tube, rather than relying on convective heat transfer mechanisms. This results in about 20-30% more heat transfer for the same (or lower) pressure drop when compared to traditional catalyst pellets. In addition, the fans offer 1.5 to 2.0 times more geometric surface area than conventional pellets.

How SSR works
The stacked fans and impingement mechanism work as follows. Gas flowing down the tube encounters the first fan structure. It cannot move through the fan as the bottom of the fan is closed and the central hole is blocked by the support structure. The gas is therefore forced out of the triangular ducts, impinging directly on the internal surface of the reformer tube, where it gathers heat (see Figure 3). Having nowhere else to go, the gas flows around the edges of the fan and back into the triangular duct on the underside side of the fan (see Figure 4). The washers that separate the fans from one another facilitate this flow back into the fan. Once inside the fan, the gas is free to move to the next fan in the stack and repeat the process.

The gas moving in and out of the fans continuously flows over all of the catalyst-coated surfaces of the fans, where the reforming reaction takes place.

The impingement heat transfer mechanism results in a significant performance benefit when compared to pellets. Results from tests are shown in Figures 5 and 6. A SSR design was selected that gave a pressure drop very similar to that of Katalco GQ size Quadralobe reforming catalyst pellets (see Figure 5). The improved heat transfer achieved by the SSR design is shown in Figure 6, which illustrates an improvement of approximately 30%.

The key performance indicators for the SSR catalyst set on a timeline of the various Johnson Matthey steam reforming catalysts is shown in Figure 7. This shows how the activity, heat transfer and pressure drop of steam reforming catalysts have developed over the past three decades.

Set against this, the performance improvements of the SSR catalyst are substantially larger than those that can be obtained by further development of ceramic based pellets and are larger than any previous improvements seen.

Any of the improvements in performance taken on their own would generate substantial benefits for operators of steam reformers. However, as the activity and heat transfer can both be markedly improved whilst at the same time reducing the pressure drop, it generates the potential for noticeable improvements in steam reformer operation. The SSR catalyst will generate reductions in tube wall temperature, increasing the heat transfer efficiency of the furnace. It will reduce the approach to equilibrium and methane slip, decrease operating pressure and the risk of carbon formation, increasing both catalyst and tube lives. In a new reformer, considerable capital can be saved. These benefits will be described in more detail in the case studies presented below.

Initial installation: Turkey
In August 2008, an early version of SSR was installed in a small can reformer in a hydrogen plant in Turkey. The plant ran well for four years in spite of numerous upsets unrelated to the catalyst. Even though it was still performing well, that catalyst was removed in January 2013 at a scheduled turnaround and replaced with the current version of SSR. The removal and reinstallation process was accomplished without major incident. The coated foils removed maintained their original integrity in spite of the process upsets and significant contamination from boiler feed water, indicating that coated foil materials can survive and thrive in a reforming environment. The new charge started up well and has operated without issue since installation.

Second installation: Mexico
In May 2012, SSR was installed in a small can reformer in a hydrogen plant in Mexico. This user sought to obtain natural gas savings by reducing fuel consumption, while having the option to increase throughput beyond the name plate capacity, and attain a longer operational lifetime for both the catalysts and reforming tubes. The reformer configuration in the plant consisted of reformer tubes of varying ages, several of which had been recharged with ceramic pellet catalyst as recently as January 2012. After thorough study and analysis, the plant’s managers decided to replace the ceramic catalyst media in all reformer tubes with SSR catalysts. The change-out was completed with minimal downtime in May 2012 by plant staff under supervision.

After installation, the hydrogen plant restarted without incident, and immediately demonstrated a 13.5% reduction in burner make-up fuel consumption. Over the following weeks, the plant’s operating conditions were optimised to take advantage of SSR. Figures 8 and 9 show furnace temperature reduction (40-60°C) and fuel savings (25-30%) realised at various plant rates with the optimised plant.

The plant continues to perform at optimum levels and the estimated payback time for the entire charge is two years based only on fuel savings alone. This does not account for savings to be realised over years to come by eliminating catalyst and tube changes.

Case studies
The development path for SSR catalyst has demonstrated its performance in two small can reformers. However, the majority of operators in the syngas industry use either top fired or side fired reformers. A series of case studies based on plant performance modelling follows, demonstrating several scenarios in which the technology can improve steam reforming operations for typical top fired and side fired reformers.