Apr-1999

Coping with sulphur and water in naphtha isomerisation feeds

An examination of the operational and economic benefits of an isomerisation process employing a superior water and sulphur tolerant catalyst in particular, potential capital cost savings by eliminating feed and hydrogen drying and reducing feed desulphurisation

Maureen F Gilbert, Carlos O Mora, Anshumali, F Mike Floyd
Kellogg Brown & Root (Now KBR Technology)
Miguel Pérez Pasqual, CEPSA
Ernst Köhler, Süd Chemie AG

Article Summary

Gasoline and fuels production trends are increasingly being determined by health and environmental related regulations. There is a long term worldwide drive to eliminate lead additives in gasoline. The USA and Japan are lead free; this movement will be completed in Europe before 2005. Other areas of the world are expected to follow this trend. Concurrently new regulations are calling for the reduction of benzene, olefins and aromatics, which are major contributors to the octane pool. These factors have created the need for alternative octane sources. Isomerisation of light naphtha is a cost effective technology that can contribute to replacing the octane deficit.

The CKS Isom process, which utilises the proprietary Hysopar zeolite catalyst, employs new features that increase the attractiveness of isomerisation of C5s and C6s as an octane source. CKS Isom can process light naphtha feedstocks with sulphur and water levels that are unacceptable in traditional isomerisation processes. CEPSA has commercially demonstrated the CKS Isom process with more than five years’ operation of an 8000bpsd unit at its Algeciras, Spain, refinery.

Increasing worldwide environmental concerns have shaped the production of automotive gasoline and diesel fuels over the past 25 years. Lead phase-down began in the United States in 1973; lead was completely banned in January 1986. Lead phase-down has been accompanied by more restrictive specifications for RVP, sulphur, olefins and aromatics. At present, California has more stringent gasoline specifications than any other part of the world (Table 1).

Similar trends are under way in Europe where lead will be eliminated by 2000. The European community has set additional fuel standards for 2000, with plans for further reductions in sulphur and aromatics in 2005 [F L Potter, “Foundation Now Set for New Global Fuel Standards”, and G H Unzelman, “Future Fuels: Hydrogen and More Hydrogen”, both in Hart’s Fuel Technology & Management, July/Aug,1998].

 The refining industry has been transforming itself in conjunction with these changing fuel specifications. Refiners have adjusted their process schemes and will continue to modify their refineries to produce product fuels which satisfy the specifications of their target markets. One can anticipate that the production of reformulated gasoline will be accomplished through more complex schemes capable of tailoring gasoline and diesel product to satisfy the increasing complex multifaceted environmental regulations.

Figure 1 shows a simplified block flow diagram of how the different refinery streams and processes may be combined to adjust to short and long term specifications.

The most cost effective way to reduce gasoline benzene content is to eliminate benzene and benzene precursors from the naphtha reformer feed. Benzene can then be saturated either in an isomerisation unit or in a separate benzene saturation unit. Controlling sulphur in gasoline will most probably involve the reduction of sulphur content in FCC gasoline. Hydrotreating of the FCC gasoline will reduce sulphur and, depending on the hydrotreating severity, saturate olefins and benzene/aromatics. In both cases, there is an octane loss from the hydrogenation of either benzene/aromatics or olefins during the saturation process.

Isomerisation of paraffinic light straight run (LSR) naphthas and C5/C6 natural gas condensates is a cost effective process to replace lost octane. Isomerisation capital costs typically range from $500 to $2000/bpsd depending on the degree of feed pretreatment required and the octane upgrade desired. In contrast, alkylation and MTBE capital costs are in the range of $3000/bpsd and $6000/bpsd, respectively. The extent to which the isomerisation process is employed is specific to the feedstock availability and gasoline octane pool requirements of each refinery.

The CKS Isom process, from Kellogg Brown and Root, is a versatile commercially proven process for isomerising C5 and C6 normal paraffins and improving the octane contribution for this gasoline component. In a once through scheme, the process is an economic alternative for those refiners needing to increase LSR naphtha octane by 9–11 RON.

With careful planning, refiners can benefit from a stepwise investment approach by first installing a once through process scheme to meet near term product specifications. Later the refiner can add fractionation steps to recycle unconverted normal and lesser branched paraffins to further increase octane. In general, the most economical isomerisation process scheme is one that employs only the technology necessary to meet a refiner’s octane requirements.

Sensitive to feed impurities
Isomerisation increases the octane value of LSR naphtha by rearranging the low octane straight chain C5-C6 paraffins into their higher octane branched isomers. These isomerisation reactions are controlled by thermodynamic equilibrium, with the isomer balance being a function of temperature. Noble metal promoted catalysts are used to enhance the reaction rate; these catalysts have both a hydrogenation function (noble metal) and an acid component.

Traditionally, the acid component has been provided by a chlorinated alumina substrate. This type of catalyst substrate is intolerant of sulphur and water. Small quantities of sulphur and water severely reduce isomerisation performance and at moderate levels will permanently deactivate the catalyst, which cannot be regenerated. The presence of these feed contaminants is a cause for ending a catalyst cycle run, necessitating catalyst replacement.

Isomerisation processes employing these traditional catalysts require full hydrotreating of the feedstock. In the event of an upset in an upstream hydrotreating unit, the performance of a traditional catalyst can be severely affected. In one case a sulphur guard was used with a typical inlet sulphur of 0.2 ppm and outlet sulphur of 0.1ppm or less. During an upset when sulphur levels rose to 0.3ppm or above, isomerate octane decreased by 4 to 5 numbers [NPRA Q&A 1994, question 39].