Jan-2012

Catalysts for maximising middle distillates

Collaboration in catalyst development and application, from laboratory scale to commercial operation, enabled a refiner to achieve production objectives

Ruben Miravalles and Tamara Galindo, Repsol

Article Summary

Increasing middle distillates consumption in Europe is a trend that began in the 1990s and is expected to continue in the future, while at the same time gasoline demand is decreasing. This has resulted in an imbalance between demand and supply for middle distillates and gasoline in Europe, meaning that refiners face an important challenge of increasing the gasoil-to-gasoline ratio in the refinery. There are many options by which refiners can raise the production of middle distillates at the expense of gasoline. For example, refineries can operate their FCC units in maximum distillates mode.

The Repsol Puertollano refinery is an inland refinery located in the centre of Spain. The refinery processes heavy crudes with a deep conversion scheme, which includes a delayed coker and FCC and mild hydrocracker (MHC) units. The complex also includes an ethyl tert-butyl ether (ETBE) and a hydrofluoric acid alkylation unit, which process the C4 fraction from the FCC unit. The FCC unit is a 
40 500 b/d Exxon Flexicracker, which was started up in 1983. In 2004, an Axens MHC was installed for FCC feedstock pretreatment, in order to adapt product quality to the more stringent regulations of maximum sulphur in fuels. Since then, 90–95% of the feedstock processed by the FCC unit is mild hydrocracker residue (RMHC), with the other 5–10% being a heavy feedstock, such as atmospheric residue, which is necessary to close the unit heat balance.

The main objective at the Puertollano FCC unit is the maximisation of middle distillates production. Other unit objectives include a reduction in light naphtha yield, maximum olefin content and a minimum motor octane number (MON) value required. To achieve these objectives, the unit operates at very low severity (low riser temperature, cat-to-oil ratio and e-cat activity), with maximum slurry recycle. Producing maximum middle distillates in the Puertollano FCC unit case is a difficult challenge. The severely hydrotreated RMHC feedstock is highly crackable with a very poor selectivity to middle distillates. In addition, the coke-making tendency of the RMHC feed is very low. Therefore, processing this feedstock at low severity requires maximum slurry recycle and the simultaneous processing of a certain amount of heavy feedstock, such as atmospheric residue, in order to close the heat balance.

Processing of atmospheric residue in this unit has several disadvantages. For example, low-sulphur atmospheric residues must be processed in the FCC unit in order not to penalise the sulphur balance. Therefore, light crudes, which are not the optimum feedstock in the refinery scheme, must be distilled to feed the FCC unit. Processing of atmospheric residue in the FCC unit also results in high metals contamination on the e-cat, with a subsequent increase in catalyst consumption.

Collaboration project
Grace Davison has been the FCC catalyst supplier for the Puertollano FCC unit for several years. Grace has continually made innovations in catalyst technology, which has allowed the Puertollano FCC unit to better adapt to the changing production scenarios. Before the start-up of the MHC unit, the Goal catalyst from Grace Davison was used. After the start-up of the FCC pretreatment and in order to adapt to the significant feed quality change and operating conditions in the FCC unit, the Goal catalyst was replaced by the Nomus-100 catalyst based on first-generation EnhanceR technology.

Later, Grace Davison developed the Nomus-Dmax catalyst, which is a second-generation EnhanceR catalyst for increased middle distillates yields, and this catalyst was also successfully applied to the Puertollano FCC unit operation. In order to fully optimise the challenging Puertollano operation, Grace Davison and Repsol decided to start a joint collaboration project to develop a new generation of catalysts for middle distillates maximisation in hydrotreated feedstock scenarios.

Two different scenarios were defined for the development of the new catalyst: a high metals scenario, corresponding to atmospheric residue processing and high metals on e-cat; and a low metals scenario, with no atmospheric residue processed and low metals on e-cat. In the first phase of the project, Grace Davison developed several catalyst formulations and tested them on a laboratory scale in an ACE (advanced catalyst evaluation) unit.

A total of 19 catalysts were evaluated in the first round to select the best zeolite, matrix and post-
treatments, and six of the highest performing catalysts were fine-tuned and tested again in a second round of trials. Upon completion, the top four catalysts were selected for the following phase of the project, which consisted of an evaluation in the Repsol DCR-II pilot plant unit. Process modelling with FCCSim was used to translate pilot plant results to the commercial unit, to check unit constraints and to optimise operating conditions for the new catalysts. The catalyst that showed the best performance in both a high and low metals scenario, the DieseliseR-Sol 16 catalyst, was manufactured by Grace Davison on an industrial scale and the results from its commercial use in Puertollano’s refinery will now be discussed.

Commercial application of DieseliseR
The DieseliseR-SOL 16 catalyst trial began in September 2009. During the catalyst turnover, the feed rate in the commercial unit was variable, with alternating operations at a minimum feed rate during the end of 2009 and early 2010 with periods of high throughput. Independently of the total feed rate, the improvement in heat balance closure achieved with the new DieseliseR-SOL 16 catalyst has allowed a desired progressive reduction in the low-sulphur atmospheric residue (AR) processing (see Figure 1).

Due to the reduction in the amount of atmospheric residue needed to close the heat balance, contaminant metals on e-cat have been significantly reduced, in particular by deactivating metals such as vanadium and sodium. The periods of low feed rate have also contributed to lower contaminant metals levels on e-cat. Consequently, the catalyst addition rate has been progressively decreased until it reached the minimum technically needed to maintain levels in the regenerator and stripper (see Figure 2).