Refinery fuel gas in steam reforming hydrogen plants
Fuel gas is an attractive feedstock for hydrogen production, but appropriate catalysts and temperature control are needed to address high olefin levels
Peter Broadhurst and Graham Hinton
Johnson Matthey Catalysts
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Operators of steam reforming-based hydrogen plants want feedstock options to minimise operating costs and
maximise operational flexibility. Consequently, new-build hydrogen plants are often designed for a number of hydrocarbon feeds. It is common to have three or four feedstocks ranging from offgases through to naphtha requiring full operational flexibility across the range.1,2 Operators of existing plants are also evaluating alternatives to the original feedstock slate and in some cases implementing changes that may necessitate modification of the plant’s operating conditions, hardware, equipment, catalyst selection and so forth.3-6 The quest for cheaper feedstock options is undoubtedly heightened by the significant increases in both natural gas and crude oil prices in the last few years, although the emergence of shale gas production has reversed this trend in certain areas. A feedstock option being considered increasingly by hydrogen plant operators associated with oil refineries is the refinery fuel gas (RFG) pool. Relative to imported natural gas and many other hydrocarbon streams and offgases in the refinery, RFG has a comparatively low value. Thus, RFG represents an attractive feedstock option where there is excess RFG available.
RFG is not widely used as a hydrogen plant feed. This is because its composition leads to a number of difficulties in processing in the feedstock purification and steam reforming sections of the hydrogen plant flowsheet. In this article, we will explore these difficulties and the strategies for hydrogen plant design and operation, which may be used to allow processing of RFG as a feedstock. This will include some recently developed catalytic and control solutions developed jointly by Johnson Matthey and Air Products & Chemicals.
RFG composition and processing difficulties
RFG is a combination of refinery unit waste or by-product gases, often referred to as offgases. The offgases that are sent to the RFG pool are those that cannot be processed elsewhere in the refinery either because of the composition or because there is an excess available. The offgases in the RFG come from various refinery unit operations (catalytic reforming, FCC, hydrotreating, coking), the availability and amount of which will depend on the refinery operation. Hydrogen-containing offgases may be partly or fully used in hydrogen-consuming units or may be treated to recover the hydrogen in a membrane or PSA unit so that a hydrogen lean gas is available to the RFG pool. Also, offgases or offgas blends with high olefin levels may be treated to recover olefins, with the olefin lean gas going to the RFG pool. Thus, RFG can differ significantly between refineries. Examples of RFGs, which have been proposed for hydrogen plant feeds, are shown in Table 1. This shows the substantial variations in: hydrogen, from 11-35 mol%; methane, from 26–65 mol%; and olefins, from 2.6–15.9 mol%.
RFG feeds often contain quite high levels of sulphur compounds. Up to 100 ppmv can be found and this can contain quite a significant proportion of mixed organic sulphur compounds. The level and speciation are necessarily dependent on the blend of gases going to the RFG pool.
Additionally, the RFG composition can fluctuate significantly in a given refinery as rates on different units change and particularly if a unit comes off line. The amount of offgas available to the RFG pool changes and so impacts on the composition of the blend, which comprises the RFG. This presents control issues in some cases.
In terms of incorporating RFG into the hydrogen plant feed slate, the aspects that may cause difficulties can be summarised:
• High olefin levels
• Variability in the RFG composition as the blend of offgases changes
• High hydrogen levels
• Significant sulphur levels
• Substantial levels of higher hydrocarbons, which may include naphthenes and/or aromatics
• Less usual trace and minor components.
Not every RFG will present each of these difficulties and each case must be considered separately. If a new-build hydrogen plant is being considered, it must be designed to include any RFG feed case(s). When considering using RFG on an existing plant, the extent to which there is a problem will be influenced by the original design basis.
Feedstock purification section
RFG feeds can cause various issues in the feedstock purification section.
High olefin levels
Olefins need to be removed from the hydrocarbon feed in a hydrogen plant to a level below 1 mol% to minimise possible olefin-derived carbon formation in the steam reformer, although higher levels may be acceptable where there is a pre-reformer in the flowsheet. The hydrodesulphurisation (HDS) catalyst is also an effective olefin hydrogenation catalyst and removes olefins almost completely as long as there is sufficient hydrogen present. Thus, for RFGs with significant olefin levels, the hydrogen available in the feed and as recycle must be sufficient for the olefin hydrogenation reaction, other hydrogen-consuming reactions and to provide the target hydrogen level specified at the HDS converter exit. Johnson Matthey recommends a different level of hydrogen be present in the HDS effluent, depending on the feed composition and how heavy it is. For the gases given as examples 5 and 6 in Table 1, insufficient hydrogen is present in the RFG on its own to hydrogenate the significant olefin content. This means that additional hydrogen must to be added, usually recycled to the purification section of the plant, of approximately 5 mol% of the RFG feed rate for example 5, and approximately 3 mol% of the RFG feed rate in example 6.
Olefins hydrogenate exothermically over the HDS catalyst and the temperature rise can be 20+°C (36°F) per mol% olefin, depending on the heat capacity of the feed gas. The inlet temperature must be adjusted to ensure that the maximum HDS bed temperature remains below 400°C (752°F). However, using standard HDS catalysts, such as Katalco 41-6T or Katalco 61-1T, the inlet temperature needs to be above 300°C (572°F) to provide sufficient activity for reactions to initiate. Given the need for some operating margin inside these restrictions, this limits the olefin that can be processed to a few mol% in a once-through reactor system.
To process higher olefin levels, a recirculation system is usually employed around the HDS reactor (see Figure 1).
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