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Pyrolysis gasoline processing
Extracting high-value components from the C5+ stream.
Ethylene production through pyrolysis (steam cracking) produces several by-products, including a heavy C5+ stream. The C5+ cut is referred to in the industry as pyrolysis gasoline or pygas. Left in its raw form, the stream has little commercial value owing to its high reactivity and low stability. However, the stream contains many high-value components, such as isoprene, benzene, toluene and xylenes.
Extracting these components can be highly economical, and typically requires hydrotreating in a pygas hydrogenation unit (PHU). The stream can be treated in various ways, depending on the required product slate. Shell Global Solutions offers a wide range of licensing packages for pygas processing to provide a custom fit for the goals of individual customers. As well as full PHU process design, Shell Global Solutions provides services to facilitate upgrades such as capacity expansions and reliability improvements to existing units.
About the technology
The Shell Group (“Shell”) has operated PHUs in its chemical plants around the globe for over 40 years. Shell Global Solutions draws on this operational experience with a range of PHU designs that are based on commercially proven technology and that provide safe, reliable and cost-effective operations.
In addition, CRI Catalyst Company (CRI), Shell’s global catalyst company, is an established leader in pygas catalysts – a crucial aspect of the pygas hydrogenation process. This arrangement helps to provide continuous technology improvements and innovations, which are then applied to licensing unit design and optimisation.
Figure 1: Pygas processing enables operators to extract valuable components from the C5+ stream. The depentaniser can also be located downstream of the first or second hydrogenation reactor, depending on customers’ needs.
For more than 40 years, Shell has operated and advised on steam crackers and hydrotreating units, and this wealth of experience is incorporated into pygas hydrogenation system design.
At the core of any PHU, lie the reactor and the hydrotreating technology. Shell Global Solutions has long been an industry leader in hydrotreating technology, and offers high-level expertise in reactor design and operation.
Although multiple arrangements are possible, pygas hydrotreating typically involves two stages: a diolefin removal step followed by an olefin and sulphur removal step.
The first stage involves hydroprocessing the feed to remove diolefins, alkenylaromatics (styrene) and some olefins in preparation for optimal downstream processing.
The second stage involves hydroprocessing all or a specific cut of the first-stage product to remove olefins and sulphur, which prepares the feed for aromatic extraction and/or motor gasoline pool addition.
Shell Global Solutions’ pygas hydrotreating technology is complemented by CRI’s catalyst technology. Through its CRI Leuna (formerly known as KataLeuna) hydrogenation catalyst line, CRI provides a comprehensive range of high-performance catalyst products for first- and second-stage pygas reactors:
- N i and Pd first-stage catalysts;
- N iMo and CoMo second-stage catalysts; and
- specialised bed grading.
CRI Leuna (formerly known as KataLeuna) catalysts are designed to maximise performance while mitigating certain operational issues that can be experienced in pygas operations, such as large pressure drops and short cycle lengths.
A major European ethylene producer had a first-stage pygas reactor with a history of short cycle lengths compared with peer plants, and chronic liquid maldistribution leading to hot spots in the reactor. CRI and Shell Global Solutions supplied new catalysts and reactor internals that enabled the company to improve liquid distribution and catalyst utilisation, and to increase the catalyst inventory. On start-up, the company observed its lowest ever start-of-run inlet temperature. Moreover, Shell Global Solutions’ pygas kinetic model predicted that the catalyst life between regenerations would increase.
CRI Leuna (formerly known as KataLeuna) catalyst products have established strong commercial track records in some of the world’s largest pygas reaction systems. This catalyst product line features inert and active grading, which is specially designed to minimise the potential for pressure drop build-up in both first- and second-stage reactors. In commercial settings, these products have been shown to extend cycle lengths by up to 50% or more.
Over the past seven years, CRI’s share of the pygas catalyst market has grown from ~4 to ~20%, and these products are being used by some of the world’s major pygas processing companies.
Reactor internal design also plays a key role in the pygas hydrogenation process. By using Shell Global Solutions’ proprietary reactor internals, operators will be helped to maximise the effectiveness of their chosen catalyst and the available reactor volume. These internals achieve nearly 100% uniform gas and liquid dispersion, which enables complete catalyst volume utilisation and reduces the potential for hot spots. The reactor internals provide optimum liquid dispersion and fouling abatement, and have achieved excellent market penetration in pygas and other hydrotreating and hydrocracking applications.
The Shell Global Solutions pygas hydrogenation design was developed to have minimal environmental impact. There are no normal process vents to atmosphere in the design, and the hydrogen consumption is optimised.
Shell Global Solutions’ reactor internals have been used to retrofit pygas units for Shell and non-Shell customers, and, in each case, the plants have experienced improvements in cycle length, production rate, temperature uniformity and product conversion.
Shell Global Solutions has wide experience of combining the pygas hydrogenation process with ethylene cracking and of integrating the products into the refinery. Operators can also benefit from in-house tools designed to help them optimise the performance and economics of pygas units and to find the best solution for specific plant requirements.