Jul-2020

Steam reforming catalysts raise production efficiency

The primary reformer is the most energy-intensive element in syngas production. New steam reforming catalysts reduce its energy costs and raise productivity.

CHRISTIAN LIBRERA
Clariant Catalysts

Article Summary

The production of synthesis gas (also known as syngas) involves the process of steam reforming, in which steam is used to convert natural gas containing hydrocarbons such as methane and naphtha into hydrogen and carbon based components (CO, CO2) in the presence of a nickel catalyst. The process takes place in steam methane reformers which are furnaces where catalysts are loaded in vertical tubes (each tube is considered to be a separate reactor), and externally fired to provide the required heat for the desired endothermic reforming reaction. Burners are positioned on the top, bottom or the sides of the reformer, depending on the plant’s process design.

The steam reformer is the heart of a syngas plant — but also the element that consumes the most energy and costs. Its complex structure includes costly materials needed for the furnace and tubes, placing a considerable burden on the capital expenses of a project. Due to the high energy costs of the extreme temperatures required for the reaction the reformer also makes a major contribution to the operating expenses of running a plant.

Steam reforming catalysts
One critical factor that greatly affects process costs and efficiency is the performance of the reforming catalyst used to boost the chemical reaction in the tubes. High operating temperatures, close to the limits of the reformer tube’s material, combined with the endothermic steam reforming reaction, require highly active and stable catalysts to prevent the reformer tubes from overheating. In addition, the steam methane reformer typically has the highest pressure drop of installed catalyst beds, which can increase costs due to additional natural gas compression, and therefore can limit the throughput of a plant in some cases.

Catalyst properties
As the catalyst is loaded in a multitude of tubes (up to several hundred), each tube needs to maintain the same pressure drop level during the entire lifetime of the catalyst charge in order to guarantee a satisfactory run for the whole reformer. Otherwise the gas would preferentially flow through tubes with lower pressure drop which can affect the performance of the reformer in terms of higher methane slip or increased tube wall temperatures, causing a restriction in the plant’s throughput. Consequently, a catalyst which lowers pressure drop is preferred to reduce syngas compression energy.

The catalysts used for steam reforming are usually supplied in the oxide state (as nickel oxide on a suitable carrier), and then reduced during the plant’s start-up since nickel is the desired active phase for the reforming reaction. During every shutdown and start-up process, the tubes undergo thermal cycling, meaning that they may potentially contract and expand to a larger extent than the loaded catalyst particles, applying very high localised forces on the catalysts. Hence the catalyst needs to be physically strong in both the oxidised and reduced state.

Optimal reforming catalysts must also enable a lower methane (CH4) concentration at the tube outlets to maximise production and energy savings. This would not only ensure high hydrogen (H2) yield, but also reduce the inert level and purge in the ammonia synthesis loop, thereby reducing energy consumption. Furthermore, catalyst designs that promote lower tube wall temperatures are favoured as they extend tube lifetimes, thus reducing expenses.

Performance criteria
There are several basic requirements that steam reforming catalysts must satisfy in order to ensure efficient syngas production. One essential feature is high catalytic activity — both intrinsic and apparent. A reforming catalyst’s intrinsic activity is influenced by the interaction between its nickel content and carrier, on the nickel dispersion, as well as on the formulation of the carrier material. Apparent activity is determined by the catalyst’s diffusion capacity, the volume and size distribution of its pores, and its geometric surface area. Other critical catalyst performance parameters include low pressure drop, efficient heat transfer, high physical strength and stability, as well as resistance to carbon formation.

Importance of shape
A steam reforming catalyst’s form is as important as its formulation. Every shape parameter — such as particle size, aspect ratio of catalyst height to diameter, void spaces in holes and external channels, or packing property between catalyst particles — influences both the geometric surface area per loaded volume, and the pressure drop across the catalyst bed. However, it is important that any optimisation during shape development must consider high crush strength as the guiding boundary condition.
 
ReforMax LDP series
Nickel-based steam reforming catalysts have been available for several decades. One of the leading examples, focused on ensuring low differential pressure drop, is Clariant’s ReforMax 330/210 LDP series. These catalysts are designed with a special 10-hole shape that ensures uniform radial crush strength, and an optimised geometric surface area. Those characteristics directly result in high activity, long catalyst life, and low tube wall temperatures, as well as a stable and minimum methane slip.
Each particle consists of a calcium aluminate based carrier on which the active metal nickel oxide is applied. The carrier chemistry is based on a significant amount of irreversibly formed hibonite (CaAl12O19), which plays a vital role in the mechanical strength of the catalyst particle and allows easy reduction of the impregnated nickel oxide to the active metal. The catalyst’s robustness also allows its fast regeneration through steaming in case of catalyst poisoning (carbon formation). The catalysts are commercially proven to be highly efficient under various process feed or design conditions and are currently running in more than 130 ammonia, methanol, and hydrogen reformers worldwide.

LDP Plus generation
The performance benefits of the 10-hole ReforMax LDP series have now been further enhanced in a new generation of eight-hole, flower- shaped steam reforming catalysts (see Figure 1). The most striking feature of the new ReforMax LDP Plus series is that it combines high activity with a very low pressure drop, thus helping to further improve plant operations and efficiency. The series consists of ReforMax 330 LDP Plus (non-promoted) and ReforMax 210 LDP Plus (lightly alkalised). The size, aspect ratio and inner channels of these catalysts have been designed to optimise geometric surface area, pressure drop, and crush strength.

Benefits of Plus series
Thanks to its high geometric surface area, the LDP Plus catalyst series maintains the high activity and longevity of its predecessor. The main difference, however, is that with the new eight-hole floral shape, the void fraction inside and between particles has been significantly increased (see Figure 2), thus reducing pressure drop in the reactor tubes by approximately 20% (see Figure 3). This gives plant operators the possibility of increasing gas throughput by up to 11%, provided there are no other limitations present. Another option for producers is to maintain current gas throughput levels and benefit from energy savings due to the reduced compressor load. Furthermore, the catalysts’ large holes support highly efficient heat transfer in the reformer.