Ultra low sulphur – low aromatic diesel

Moves to minimise sulphur emissions are causing refiners to consider more active hydrodesulphurisation catalysts in revamp projects

Alain P Lamourelle and Douglas E Nelson, Haldor Topsoe
Jerry McKnight, San Joaquin Refining

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Article Summary

The much scrutinised US Environmental Protection Agency (EPA) regulation, effective 1 June 2006, will result in almost exclusive production of 15wppm sulphur diesel in the USA. New trucks and buses will be required to meet new emission standards starting in 2007, with full compliance in 2010 (Table 1). Similar low sulphur diesel regulations taking effect in other major fuel markets, such as Europe and Asia, will affect refining facilities in these regions, to varying degrees. For example, the global onus on producing diesel at such low sulphur levels also predicates a significant reduction in aromatics and an improvement in cetane index.

As a result, revamp projects focused on existing distillate hydrotreating units, as well as plans for building new units, are compelling refiners to consider the use of more active hydrodesulfurization catalysts together with unit modifications. These include catalyst dense loading, additional reactor volume (to increase catalyst volume), replacement of reactor internals, adding an H2S scrubber and increasing the hydrogen purity of the recycle gas loop.

New catalysts have been developed recently which have a higher desulphurisation activity for such ultra low sulphur diesel (ULSD) levels. To reach a ULSD target of 15ppm or less, the revamped or new diesel unit will have to operate at low space velocities and “reasonably” high pressures. At these operating conditions, high hydrogenation activity NiMo catalysts provide a better desulphurisation performance than CoMo catalysts, most of the time.

Commercial experience exists for the production of ULSD products, as demonstrated in the first stage of existing hydrodesulphurisation/hydrodearomatisation (HDS/HAD) diesel units. New dearomatisation catalysts (noble metal) with much higher activity have been commercialised to reduce the aromatics and improve the cetane index.

The EPA acknowledged that improvements to current hydrotreaters, such as higher activity catalysts and some marginal modifications, would not generally be sufficient to meet the new 15wppm sulphur regulation for diesel fuels. However, existing diesel hydrotreaters could be incorporated into revamps if the objective is to produce ULSD. Therefore, most of the equipment added in the early 1990s to existing diesel hydrotreaters, to meet the October 1993 sulphur specification of 500wppm, could still be used to meet the new standard.

As suggested by the EPA, the primary changes to current diesel hydrotreaters would be:
• A second reactor to decrease the distillate hydrotreater unit space-velocity, or the addition of a new second-stage hydrotreater
• More active catalysts, especially new catalysts designed to desulphurise sterically hindered sulphur compounds
• Higher hydrogen purity and scrubbing of the recycle gas
• Higher reactor pressure.
The impact of reducing the diesel sulphur content from 500wppm to 15wppm for the same feedstock is shown as follows:

Product sulphur, wppm      Catalyst volume
500                                        1.0
50                                          2.0
15                                          3.0

This data was developed, using Topsoe’s kinetic model, for a typical diesel hydrotreater loaded with a CoMo-type catalyst. The left column shows the target diesel sulphur content in wppm. The right column shows how much additional catalyst, having the same activity, is required to reach the lower sulphur target while maintaining a constant cycle length.  All other operating conditions are unchanged. Therefore, about three times more catalyst will be needed to produce a 15wppm sulphur diesel versus a 500wppm sulphur diesel.

Revamp options
As indicated by the EPA, to produce ULSD,  multiple options are to be evaluated. These primarily include modifications to desulphurisation catalysts and/or operating conditions of the distillate unit (eg, space velocity, hydrogen purity, hydrogen partial pressure, catalyst cycle length). Potential changes include:
• Use of a more active catalyst system in the existing reactor
• Increasing catalyst quantity by dense loading of the existing reactor
• Increasing catalyst quantity by addition of a new reactor
• Increasing catalyst activity by increasing the hydrogen partial pressure
• Increasing catalyst activity by reducing the H2S level in the recycle gas loop
• Increasing the performance of the existing reactor by installing state-of-the-art reactor internals.

The latest CoMo catalyst, TK-574 (Table 2), has about twice the activity of the first generation catalyst, TK-550. However, for a unit currently targeting 500wppm sulphur in the product, changing the catalyst to TK-574 will not always be sufficient to achieve the new target.

Dense loading
By dense loading a reactor, the installed catalyst quantity can be increased by around 15 per cent. This results in a 7°F to 8°F reduction in the start-of-run (SOR) temperature of the unit. However, compressor pressure drop or mechanical limitations might limit the amount of dense loading that can be applied to the reactor.

Space velocity (LHSV)
Adding a reactor or replacing an existing reactor is the typical approach taken when revamping an existing unit to meet more stringent product specifications. Doubling the catalyst volume would result in a 35°F reduction in the SOR temperature. Increasing the catalyst volume has a double benefit on the unit performance:

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