Using butane feedstock in a hydrogen plant

Case study on the use of butane feed for the Repsol YPF La Coruña hydrogen plant. The process in which a refiner and technology partner evaluated the use of lower-cost feedstock is discussed

Ashley J Winsper, Johnson Matthey Catalysts
Iñigo Crespo Irizar, Repsol YPF

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Article Summary

The evaluation and utilisation of lower-cost feedstocks within and near a European refinery was driven by increasing natural gas prices in Europe, putting ever-increasing cost pressure on European refineries, such as the Repsol La Coruña refinery to meet the lower aromatics specifications in finished gasoline. In doing so, the amount of butane available has increased, presenting itself as a lower-cost on-site feedstock for the hydrogen production unit. However, butane is much more readily prone to carbon formation in the steam methane reformer (SMR), challenging the use of this feedstock. As demand for supplemental hydrogen production in refineries continues to significantly increase worldwide, due to environmental legislation requiring production of ever-cleaner gasoline and diesel fuels, the Repsol YPF experience has lessons that are applicable to many refineries.

The hydrogen plant is an important utility for an oil refinery and is often designed to cope with a range of feedstocks. Refineries often want to consider a wider range of feeds to this unit to increase operational flexibility and, particularly, to take advantage of cheaper feedstocks available within the refinery. This allows the refinery operator to produce hydrogen by choosing the lowest-cost feedstock available at any point in time from the refinery feedstock slate, therefore enhancing overall refinery profitability.

Plant details
Repsol YPF operates a large refinery at La Coruña in the northwest of Spain. The refinery includes a hydrogen unit, the purpose of which is to provide hydrogen for the refinery. The hydrogen unit was originally designed to run on natural gas or naphtha as a feedstock, but natural gas is not available yet. Thus, the Repsol YPF La Coruña hydrogen unit had utilised a low boiling point and low sulphur content naphtha. This had a major impact on operation of the refinery crude unit, which necessitated that it run sweet crudes at reduced rates in order to produce naphtha to the specification required for the hydrogen plant. If the feedstock to the hydrogen unit could be changed from naphtha to butane, the refinery crude unit would not be limiting and could therefore be run optimally and increase the overall operating profit of the refinery.

Considering the hydrogen unit described in Figure 1, the process is made up of a feed vaporiser, hydrodesulphurising unit (HDS), feed preheat unit, a pre-reformer that had been installed for the naphtha operation, a steam reformer and a post reformer followed by a high-temperature shift vessel (HTS) and a PSA system. The post reformer is a proprietary Enhanced Heat Transfer Reformer (EHTR) designed by Technip Benelux B.v. added as part of a revamp project in 2001 to increase hydrogen production by 20 per cent.

Changing the hydrogen plant feedstock must therefore be carefully considered at all stages of the process to ensure that all issues are identified and resolved. Failure to do so can significantly impact the project’s overall success and hydrogen plant operability.

Project details

Repsol YPF commissioned a study from Johnson Matthey Catalysts to investigate the implications of changing the feedstock from naphtha to butane. The scope of the study was to:

• Determine heat and mass balance for the complete unit with current naphtha feedstock and then with operation using a butane feedstock
• Review of unit operability such as expected catalyst life, steam reformer operating conditions, flow compositions, etc.
• Identify process conditions that would change significantly so that adjustments could be made with the distributed control system (DCS)
• Gather information on existing control loops and shutdown systems to allow for:

Assessment of the process requirements
Production of design concept for new control systems
Specification of transmitters – instrumentation
Production of modifications to the block diagrams
Highlighting areas of concern with the plant emergency shutdown systems resulting from the change of feedstocks
•    Review operating procedures to change the feedstock while the plant is on-line from naphtha-to-butane and back again

Johnson Matthey chose ABB Engineering Services to complete the control part of the study.
The ability to change feedstock may be limited by hardware metallurgy, catalyst or other constraints unless suitable modifications are made. Good control systems and operating procedures are important when switching between feeds in order to maintain the steam-to-carbon levels to a steam reformer at safe levels and avoid mishaps that might cause irreparable catalyst damage, such as severe carbon formation or equipment damage. The aim of the project was to use Johnson Matthey proprietary software to model the process in its current operational mode, and then change the feedstock and investigate each unit operation for limitations.1

Project execution
In order to determine the effect of changes to the process, Johnson Matthey Catalysts’ normal procedure in these studies is to collect current plant data and build a model of the process. This model was built using a modelling tool jointly developed by Johnson Matthey Catalysts (for the catalytic unit operations) and Hyprotech (now Aspen Technology’s) based on the HYSYS simulation platform. Once the base case model was built, Repsol YPF was able to confirm that this provided a good basis for further predictive modelling.

Two options for butane feedstock were available: a commercial source of butane containing a maximum of
20 per cent olefins and a gas recovery unit source containing a maximum of
2 per cent olefins (from storage). Butane is a new feedstock option to the Repsol YPF hydrogen unit; however its use in this process should be possible as the plant is designed as a dual feedstock unit already able to process natural gas and light naphtha feeds. For example, the butane feedstocks are available in the liquid form but can be vaporised into the required gas phase using the existing naphtha vaporiser at the front end of the hydrogen unit.

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