1. Field of the Invention
This invention relates to a process for reforming a naphtha stream over a bifunctional, metal loaded aluminosilicate catalyst.
2. Description of Related Art
Naphtha streams emerging from some petrochemical refining processes generally comprise a mixture of C.sub.5 to C.sub.13 hydrocarbons which include about 15-40 wt % of C.sub.6 to C.sub.11 aromatic compounds and the balance mostly a mixture of C.sub.5 to C.sub.11 aliphatic hydrocarbons, including mixed paraffins and mixed olefins. The bulk of the naphtha stream, e.g. at least about 25 wt %, comprises C.sub.5 to C.sub.9 hydrocarbons, many of which boil near the boiling range of the benzene/toluene/xylene (BTX) fractions present in the naphtha. This makes it difficult to extract the more valuable BTX components from the naphtha by conventional distillation techniques. Therefore, a solvent extract process is required to purify the BTX component for chemical uses, which adds to the cost of recovery of BTX components. The naphtha stream from the refinery also contains sulfur contaminants such as elemental sulfur, alkyl sulfides and sulfur compounds such as benzothiophenes.
A conventional procedure for both upgrading the naphtha and removal of the sulfur is to subject the naphtha stream to hydrodesulfurization (HDS) wherein the stream is contacted at high temperatures and in the presence of hydrogen with a desulfurization catalyst such as a sulfided cobalt or nickel/molybdenum catalyst. In addition to sulfur removal, the HDS process results in some aromatization and cracking of the C.sub.5 -C.sub.9 hydrocarbons present in the naphtha, thereby facilitating the ability to separate BTX components form the hydrorefined product. However, the HDS process consumes large quantities of hydrogen, e.g. up to 10,000 SCF/B, rendering it an expensive process. Nonetheless, it is desirable to remove sulfur because it tends to poison conventional catalysts which are used to reform naphtha, e.g. noble metal loaded aluminate or aluminosilicate catalysts.
It is also known in the art that HDS treated naphtha streams, either prior to or after removal of substantial quantities of the BTX fraction, can be subjected to catalytic reforming to further enhance the aromatics content of the naphtha. In a typical reforming process, the reactions include dehydrogenation, dehydrocyclization, isomerization, and hydrocracking. The dehydrogenation reactions typically include dehydroisomerization of alkylcyclopentanes to aromatics, the dehydrogenation of paraffins to olefins, the dehydrogenation of cyclohexanes to aromatics and the dehydrocyclization of acyclic paraffins and acyclic olefins to aromatics. The aromatization of the n-paraffins to aromatics is generally considered to be the most important because of the high octane rating of the resulting aromatic product. The isomerization reactions included isomerization of n-paraffins to isoparaffins, the hydroisomerization of olefins to isoparaffins, and the isomerization of substituted aromatics. The hydrocracking reactions include the hydrocracking of paraffins and hydrodesulfurization of any sulfur compounds remaining in the feed stock.
It is well known that several catalysts are capable of reforming petroleum naphthas and hydrocarbons that boil in the gasoline boiling range. Examples of known catalysts useful for reforming include platinum (optionally with the addition of rhenium or iridium) on an alumina support, platinum on zeolites of small pore size such as type X and Y (provided the reactants and products are sufficiently small to flow through the pores of the zeolites), and platinum on zeolite KL supports as disclosed in U.S. Pat. No. 4,987,109 and WO91/06616. Catalytic reforming of essentially sulfur-free aliphatic hydrocarbons using a zinc or gallium loaded ZSM-5 catalyst is also disclosed in U.S. Pat. Nos. 3,756,942, 4,180,689, 4,490,569 and 4,933,310, as well as by Fukase et al., "Catalysts in Petroleum Refining And Petrochemical Industries 1995," 1996, pp. 456-464. Catalytic reforming of a sulfur-free naphtha using a ZMS-5 catalyst loaded with both platinum and another metal such as zinc or gallium is disclosed in WO96/03209.
However, none of these references teach reforming of a naphtha stream which has not been desulfurized and which contains high concentrations of C9+ aromatics above about 5 wt %.
Accordingly, it is a primary object of this invention to provide a catalytic process for upgrading a naphtha stream which contains BTX aromatics and also at least 5 wt % of C9+ aromatics, at least 25 wt % of C.sub.5 to C.sub.9 aliphatic hydrocarbons and greater than 10 wt. ppm sulfur.