Feb-2006

Amino acid-based refinery gas treatment

Use of gasoline sulphur reduction catalyst technology in refinery applications is discussed. Marathon Petroleum Company LLC (MPC) operates an FCC (fluidised catalyst cracking) unit in its Robinson, Illinois, USA, refinery.

Jeffrey Sexton, Kenneth M Barnes and Ron L Cordle, Marathon Petroleum Company
Joe B McLean, C J Farley and James C Fu, BASF Catalysts

Article Summary

In late 2003, MPC initiated a programme to evaluate options to meet its corporate gasoline sulphur pool requirements. The Robinson FCC engineers planned to undercut gasoline to minimise gasoline sulphur prior to the installation of a gasoline post-treater in 2005. This interim period of operation sought a catalytic technology solution to improve unit profitability. The result of this programme was the use of Engelhard’s NaphthaClean catalyst technology. The development and commercialisation of this FCC catalyst is based on a proprietary matrix platform technology called DMS (Distributed Matrix Structures). It provides gasoline sulphur reduction while maintaining excellent catalyst properties for the conversion of gas oils to gasoline and lighter products.

Optimising diffusion and zeolite-based cracking
The DMS technology platform was first introduced in 2000 with the commercial launch of the NaphthaMax FCC catalyst and has since been demonstrated in over 100 commercial FCC unit operations, including all seven of MPC’s FCC units.

The structure imparted by the novel DMS matrix is designed to provide enhanced diffusion of the feed molecules to pre-cracking sites located on the external, exposed surface of highly dispersed zeolite crystals. The feed initially cracks on the zeolite surface itself rather than on an active amorphous matrix material, as is the case with other FCC catalyst technologies. This provides better selectivities, with reduced coke formation, characteristic of zeolite cracking. The secondary diffusion of the cracked products to the internal crystalline zeolite surface is also minimised, resulting in less overcracking. The net result is high bottoms conversion with low coke, and higher yields of valued gasoline and light olefins products.1,2

Reducing FCC gasoline sulphur
The majority of gasoline sulphur species are formed in the FCC riser and are not found in the gas oil feed. Some compounds can be formed through the cracking of heavier alkyl thiophenes or through the cyclisation and dehydrogenation of alkyl benzothiophenes. However, the high partial pressure of H2S in the FCC riser makes recombinate reactions with olefins and diolefins a likely mechanism. The use of other FCC materials, such as SOx additives, ZSM-5 and NOx additives, can have an impact on gasoline sulphur. Additional operating variables also affect FCC gasoline sulphur, including riser outlet temperature, riser residence time, catalyst rare earth content and contaminant metals levels. All of these operational changes serve to increase the rate of hydride ion transfer, which reduces FCC gasoline sulphur levels. The appropriate catalytic technology to achieve the desired gasoline sulphur level needs to consider all of these factors and interactions.

The distillation range of the gasoline also heavily influences FCC gasoline sulphur levels. As the gasoline endpoint increases, gasoline sulphur will rise dramatically, particularly when fractionation results in benzothiophenes being included in the “as-cut” gasoline product. It is often possible to impact gasoline sulphur by ~50% with a 10% reduction in as-yielded gasoline, but this may also further constrain FCC operations.

NaphthaMax-LSG (low-sulphur gasoline) is specifically formulated to combine the benefits of NaphthaMax with gasoline sulphur reduction. This technology provides refiners additional flexibility to reduce gasoline sulphur content by up to 50% via cut-point control while preserving or even increasing total gasoline yield due to the improvement in gasoline selectivity achieved with the catalyst. This option offers the additional benefits of low coke, excellent dry gas selectivity and improved bottoms cracking.3,4 The previously mentioned NaphthaClean catalyst preserves all of these features and benefits while adding chemical desulphurisation functions.5 NaphthaClean extends the sulphur reduction performance to the full-range naphtha.

Robinson refinery FCC overview
The MPC Robinson refinery’s FCC unit is designed to process 48 000 bpd of gas oil. Design elements include a UOP side-by-side geometry, Stone & Webster feed nozzles, and Stone & Webster’s proprietary Linear Disengaging Riser Termination Device (RTD). The regenerator operates at near-complete combustion, with a typical flue gas composition containing 2 vol% or less carbon monoxide (CO) upstream of the CO boiler. Typical feed temperatures are 500–550°F. Riser outlet temperature is controlled between 985°F and 1000°F.

The typical feedstock to the unit is a mixture of atmospheric gas oils, vacuum gas oils and coker gas oils. None of the feed streams are hydrotreated. At high feed rates, the unit is constrained by the wet gas compressor and regenerator maximum allowable pressure.
Catalyst addition to the unit is typically 0.14–0.15lb of fresh catalyst per barrel of feed. This will sustain 75 wt% conversion on equilibrium catalyst, as measured by Engelhard’s refinery returns laboratory.

The FCC main fractionator at Robinson is limited during routine operation by one of the highest loadings in the industry. At high throughput, there is often poor fractionation efficiency, with high boiling point sulphur containing molecules present at routine gasoline endpoints. Gasoline sulphur can be controlled strictly by undercutting, but this represents a substantial penalty in the feed rate. In order to meet the clean fuel specifications for MPC, the Robinson unit had to reduce its FCC gasoline sulphur level by nearly 40% on a daily basis.

Laboratory screening results
MPC has conducted pilot unit testing in its circulating riser unit (CRU) FCC pilot unit facility in the refining analytical and development facility in Catlettsburg, Kentucky. It has incorporated pilot unit testing for all FCC catalyst evaluations and has a history of validating improved catalyst selections through an extensive post-auditing process.

MPC has also conducted internal evaluations of available competitive technologies. It concluded that many different levels of FCC gasoline sulphur reduction were available, but often the yield penalties associated with non-selective cracking reactions far outweighed the benefits of reducing gasoline sulphur. Balancing yield selectivity with FCC gasoline sulphur reduction was therefore a priority and resulted in the selection of NaphthaClean for a unit trial.