Mar-2013

Catalysts for hydrogen management

Use and optimisation of high-performance catalysts are an important tool to increase hydrogen supply in a refinery

ROSS BRUNSON, UWE FLESSNER and PAIGE MORSE
Clariant

Article Summary

Hydrogen management is a critical requirement for refineries these days. This simple molecule provides the means to meet environmental regulations and process more challenging feedstocks, both of which are keys to sustainable and profitable operations. The Catalysts business unit of Clariant (formerly the Catalyst Division of Süd-Chemie) is a leading supplier of catalysts and technical service to the refining industry, and a specialist in hydrogen operations. Our high-
performance catalysts and technical service experts enable optimal performance for operations throughout the hydrogen plant.

The demand for hydrogen at modern refineries is an increasingly challenging topic for operators. Available crudes are often lower in quality – heavier and more sour – than in recent years, thus requiring more hydrogen to process. Additionally, sulphur and aromatics specifications for transportation fuels continue to be lowered worldwide, which again demands more hydrogen to reach these new targets. For refiners, these demands often require an increase in hydrotreating capacity and a boost to hydrogen purity levels in operations.

There are four basic operations for hydrogen plants: feedstock treatment; steam reforming; water gas shift; and raw hydrogen purification, typically with the use of a pressure swing adsorption (PSA) unit. This article discusses the high-performance catalysts for each of these operations that enable the best overall performance, and can boost production levels and purity of existing units. Optimal operation of hydrogen plants is best achieved by a thorough understanding of catalyst-related topics, including reaction chemistry, design information, operations optimisation, equilibrium considerations, startup/shutdown procedures and troubleshooting.

A simplified schematic of hydrogen operations is shown in Figure 1. Catalysts commonly used in these processes are shown in Table 1.

Feedstock treatment
Feedstock purification is a fundamental first step in hydrogen operations. It removes sulphur, chlorides, olefins and other impurities that can poison downstream catalyst systems and foul or corrode equipment. Hydrogenation is the first step, with three key processes to consider: saturation of olefins to paraffins, hydrodesulphurisation of sulphur-containing materials to hydrogen sulphide, and hydrodechlorination of 
alkylchlorides to hydrogen chloride.

Products used in these applications include HDMax 200 and 300 series for hydrogenation, ActiSorb Cl2 for chloride removal, ActiSorb S2 for desulphurisation, and ActiSorb S6 for deep desulphurisation. The HDMax series will hydrogenate olefins and alkylchlorides, and will convert most sulphur-containing species. HDMax catalysts are typically used with a downstream zinc oxide-based sulphur trap and are resistant to heavy hydrocarbons. The HDMax 200 series are cobalt-molybdenum catalysts, and the HDMax 300 series are nickel-molybdenum catalysts with the advantage of remaining in the sulphided state more easily than the CoMo option.

The ActiSorb series products are absorbents used to purify the hydrogenated feedstocks. ActiSorb S2 and S6 are particularly effective for sulphur removal in hydrogen operations, and are typically installed in beds following HDMax series catalysts. The ActiSorb S2 is a zinc oxide material with a very high surface area that enables high pick-up of sulphur-containing species; that is, greater than 32 wt% pick-up. For deep desulphurisation, ActiSorb S6 is a copper-zinc-based product that is typically applied to protect a pre-reforming catalyst from premature poisoning by sulphur. ActiSorb Cl2 is used to remove hydrogen chloride.

Pre-reforming and reforming
Steam reforming is the conversion of hydrocarbon feeds to synthesis gas (a mixture of CO and H2) using steam. The simplified reaction scheme is as follows:

CxHy + H2O → x CO + (x + ½ y) H2

The preferred configuration for reforming depends on the quality of the feedstock 
available and the amount of heavy hydrocarbons that a plant will process into hydrogen. The most common process uses methane and is referred to as steam methane reforming (SMR). Hydrogen production utilising SMR is increasingly being considered a utility gas within the refining and petrochemical industry.

For units with heavier feeds, including naphtha, a pre-reforming unit can be installed upstream of the SMR and offers several advantages. A pre-reformer will allow the processing of heavy feed including naphtha at low steam-to-carbon ratios (S/C) without issues from coking of the catalysts used in the production of hydrogen. The pre-reformer also allows the SMR to operate at 8-10% higher rates (at the same heat load) by reducing the required reforming duty. It can contribute to a longer operating lifetime for the SMR equipment at increased hydrogen production capacity by reducing the heat loads and avoiding problems due to carbon formation and the associated pressure drop increase. The overall S/C can be reduced by utilisation of a pre-reformer, and it will lower energy consumption.

ReforMax 100 is a catalyst for pre-reforming, with higher activity and better poison resistance when compared to benchmark catalysts (see Figure 2).

The reforming reaction is endothermic and the equilibrium is favoured by a high exit temperature, high steam-to-gas ratio and low exit pressure. Catalysts used for SMR need 
to combine the highest 
performance under extreme operating conditions with high structural integrity and minimum pressure drop.

Nickel-based reforming catalysts have been produced by Clariant (and predecessor company Süd-Chemie) for more than 50 years. The ReforMax series are the preferred choices for this reaction. Each of these catalysts has several decades of commercial application and more than 100 users. ReforMax 330 is preferably used in methane-rich feeds, while ReforMax 210 is potassium promoted for use with heavier feeds such as heavy natural gas or LPG, and ReforMax 250 is the best choice for naphtha feeds. These catalysts are often used in layers in a reformer tube, geared to the quality of the feedstock.