C₄/C₅ hydrogenation

In a steam cracker, the olefin yield and the furnace run length are strongly dependent on the diolefin content and, to some extent, on the olefin content in the feed. As the raw C₄ and C₅ cuts are high in olefins and diolefins, further upgrading is typically required before blending the streams back into the cracker feed.

Ethylene production through pyrolysis (steam cracking) typically produces several by-product streams. The type and the quantity of these streams are functions of the hydrocarbon source used to produce the ethylene. with heavier cracker feeds, the by-products will typically include C₄ and C₅ fractions. Both these fractions contain valuable petrochemical components, but extracting them in a usable form requires additional processing. Valuable components such as butadiene and butenes are typically extracted from the raw C₄ cut. The C₅ cut upgrading is less common, but valuable components, including isoprene, piperylenes and dicyclopentadiene, can be extracted from it. If the petrochemical facility is integrated with a refinery, the streams from these extractions can be further processed and sent to an alkylation unit. Another option for the C₄ and C₅ fractions is to blend them into the motor gasoline pool, but this is becoming less common owing to the increased use of ethanol in gasoline blending and the concerns about volatility that come with blending in the lighter components.

Even with these options, there are times when such outlets are unavailable and the ethylene producer needs to find another outlet for one or both of these product fractions. One attractive option is to recycle them back to the steam cracker.

Upgrading the C₄ and C₅ fractions can be highly economical, and typically requires hydrotreating in a C₄, C₅ or C₄/C₅ hydrogenation unit. The streams can be treated in various ways, depending on customers’ ultimate objectives. Shell Global Solutions offers various licensing packages for C₄/C₅ processing to provide a custom fit for the goals of individual customers. As well as grassroots C₄/C₅ hydrogenation unit process designs, the organisation provides services to facilitate upgrades such as capacity expansions and reliability improvement of existing units.

About the technology

The Shell Group (“Shell”) has operated C₄/C₅ hydrogenation units in its chemical plants around the globe for over 40 years. Shell Global Solutions draws on this operational experience with a range of C₄/C₅ hydrogenation unit designs that are based on commercially proven technology and provide safe, reliable and cost-effective operations.

Reactor internal design also plays a key role in the C₄/C₅ hydrogenation process. Here, Shell Global Solutions offers its proprietary reactor internals. These internals achieve nearly 100% uniform gas and liquid dispersion, which enables complete catalyst volume utilisation to minimise the required reactor volume and reduce the potential for hot spots. The reactor internals provide optimum liquid dispersion and fouling abatement, and have achieved excellent market penetration in many hydrotreating and hydrocracking applications.

In addition, CRI Catalyst Company (CRI), Shell’s global catalyst company, is a leading provider of C₄/C₅ hydrogenation catalysts – a crucial aspect of the hydrogenation process. The combination of in-house operational, process and catalyst expertise helps to provide continuous technology improvements and innovations, which are then applied to licensed unit design and optimisation.

Shell Global Solutions, through its support of Shell’s in-house steam crackers, has developed a detailed steam cracker model that can predict product composition based on cracker feeds. The organisation uses this model to optimise the level of hydrogenation of the C₄/C₅ stream and strike the balance between the costs of hydrogenation and the yield benefit in the steam cracking furnaces.

In the main reactor, the feed is hydroprocessed to remove diolefins and most of the olefins. In some cases, an optional trim reactor is installed for hydroprocessing the main reactor product to remove more of the trace olefins.

Steam cracking makes use of CRI’s hydrogenation catalyst products. CRI offers several catalyst products for C₄/C₅ processing: the choice is determined by the characteristics of the feed and the final product specifications. These products include:

• nickel catalysts;
• palladium catalysts; and
• active and inert grading materials.

Performance data

CRI’s catalysts are designed to maximise performance while mitigating the operational issues, such as pressure drop build-up, low conversion and short cycle lengths, that can be experienced in C₄/C₅ operations. This catalyst product line features inert and active grading, which are specially designed to minimise the potential for pressure drop build-up in C₄/C₅ reactors. In commercial settings, these products have been shown to extend cycle lengths by 50% or more.

The Shell Global Solutions C₄/C₅ hydrogenation design was developed to have minimal environmental impact. There are no normal process vents to atmosphere and the hydrogen consumption is optimised.

Business value

By drawing on its wide experience of combining the C₄/C₅ hydrogenation process with ethylene cracking technology, Shell Global Solutions can help operators to optimise the performance and economics of their C₄/C₅ units and to find the best solution for specific plant requirements.