Apr-2001

Treating options to meet clean fuel challenges

The multiple challenges of meeting fuel specifications and upgrading low value streams, such as LCO and heavy naphtha, call for a comprehensive understanding of various hydroprocessing options

Rik B Miller, Aristides Macris & Arthur R Gentry
Kellogg Brown & Root (Now KBR Technology)

Article Summary

Regulations requiring very low levels of sulphur in gasoline and diesel have forced refiners to evaluate various sulphur removal technologies. Refineries configured with a  fluid catalytic cracking unit (FCCU) will have to deal with sulphur in the gasoline and light cycle oil (LCO) products. It is therefore important for refiners to have a comprehensive understanding of the relative benefits of various commercially proven hydroprocessing technologies for both pre-treating and post-treating options.

The following discussion describes options for reducing sulphur in FCCU products. Commercial examples illustrate the technical and financial issues that need to be included in a successful clean fuels strategy development. The approach chosen by each facility will depend on refinery configuration, crude supply, sulphur concentration in FCCU feed and types of sulphur in the products, and most of all, the available capital (and return on that capital) for the investment required. 

Regulations in Europe and North America requiring very low levels of sulphur in motor gasoline (mogas) and automotive diesel fuel products have forced refiners to consider various sulphur removal technologies to reduce emissions from internal combustion engines.  The mogas regulations address a number of fuel properties other than sulphur, including oxygen content, vapour pressure, benzene, overall aromatics content, boiling range, and olefin content.

Diesel regulations are at different implementation stages in Europe and North America. Many complex factors will impact on future diesel fuel specifications in various geographic regions, which are all geared to reduce sulphur levels and improve the burning quality of distillate fuels. Cetane improvement and reduction of the diesel sulphur, density, aromatics, and distillation 95 per cent point are proposed in Europe, while sulphur reduction is the primary focus in North America.  Although some product specifications and their implementation dates have been established, others are still uncertain, or they are expected to get tighter in the future.  This uncertainty and the minimum return on investment for such projects, emphasises the need for flexible technology solutions that can adapt with the changing regulations providing staged investment pathways to achieve the necessary product upgrades with minimum cost. Germany has mandated a 2005 sulphur specification of less than 10wppm.

While significant refinery investments will be required to meet each of the specified properties, reducing sulphur to target levels is likely to be the highest cost element over the next five years in North America.  A similar situation will be observed in Europe with the production of low sulphur mogas, but European refiners may have the additional cost of lowering aromatics and improving cetane in  diesel fuel (Table 1), depending on the final diesel specifications mandated for 2005 or 2007.

Regulatory groups have targeted sulphur reduction because sulphur in mogas or diesel significantly reduces the efficiency of catalytic converters in automobile engines. The FCCU naphtha accounts for 30–45 per cent of the gasoline pool and contributes 85–95 per cent of the total sulphur in the gasoline blend. Furthermore, the light cat naphtha (LCN) has high olefin content and the heavy cat naphtha (HCN) has high aromatics content, both of which can become a significant regulatory issue, especially in Europe.

Utilising conventional post FCCU hydrotreating technology for sulphur removal will result in olefin saturation and octane loss at significant hydrogen consumption.

The hydrogen deficient LCO (high in aromatics and poly-aromatics) is high in difficult to remove sulphur species, and also results in very poor cetane. Refiners will find it difficult to blend significant volumes of LCO into future premium distillate products without further upgrading. Disposal of LCO by using it as cutter stock or in an existing hydrocracker is the preferred solution, but not many European refiners have existing hydrocrackers. 

Including LCO in the premium diesel may become a necessity when more stringent sulphur heating oil specifications get legislated and limited amounts of untreated LCO can be blended in heating oil or off-road diesel.  

There are several commercially proven options for making FCCU products more suitable for clean fuels blending. Three broad approaches are available to refiners for lowering sulphur in the FCCU products. One approach is to hydrotreat the feed to the FCCU, while another is to treat the streams produced by the FCCU. The third approach is to operate the FCCU in ways that minimise sulphur in the products.

FCCU feed hydrotreating
Hydrotreating of FCCU feedstocks or “cat feed” hydrotreating (CFHDT) removes sulphur, nitrogen, metals and other contaminants from the FCCU feedstock. The benefits of hydrotreating FCCU feedstocks have been enumerated in many prior technical papers. Observed FCCU performance improvements include:
- Higher naphtha yield and quality
- Lower LCO production and improved LCO sulphur, density and cetane
- Lower coke make
- Reduced FCCU SOx and NOx emissions.

Most existing CFHDT configurations were designed to operate at 60 to 90 per cent desulphurisation, which will not be sufficient to meet a 50wppm S mogas pool target.   Optimising CFHDT catalyst selection and operating conditions will increase the percentage HDS and adjusting the cat naphtha end point, may make the 50wppm S target possible for FCCUs with treated feeds.

Changes to existing CFHDTs, such as addition of hydrogen purification, gas circulation improvements, or reactor revamps and catalyst volume expansion will allow the rest to of the FCCUs in the remaining cases to produce 50wppm S mogas. Post-treating the FCCU products may also be an economically preferred solution to refiners with existing low severity CFHDT, especially as the sulphur targets continue to be lowered.