“Alkylation” generally refers to the reaction of a hydrocarbon, such as an aromatic or a saturated hydrocarbon, with an olefin. For example, in one type of reaction of particular interest, a branched saturated hydrocarbon, such as isobutane, may undergo alkylation with an olefin containing 2-6 carbon atoms, such as 2-butene, to produce an alkylate that has a higher octane number and which boils in the gasoline range. Processes directed to the alkylation of paraffins with olefins produce branched hydrocarbon molecules for gasoline components, such as isomers of octane, e.g. trimethylpentanes (“TMPs”), which have high octane numbers. A gasoline with a high octane number, often expressed as research octane number (“RON”), can reduce engine knock, which lessens the need to add environmentally harmful anti-knock compounds such as tetraethyllead. A second octane measurement, motor octane number (“MON”), also describes the anti-knock properties of gasoline. MON is measured when the test engine is run under heavy load (higher rpm), and the RON is measured at lower load (lower rpm).
Gasoline produced by the alkylation process is essentially free of contaminants, such as sulfur and nitrogen that may be present in gasoline obtained by other processes, such as cracking heavier petroleum fractions, e.g. vacuum gas oil and atmospheric residue. Sulfur oxides (“SOx”), a combustion product, are the primary cause of pollutants. In addition to direct SOx emissions, SOx can significantly lower the effectiveness of catalytic converters, thereby adversely impacting SOx, NOx, and CO emissions. SOx also form indirect particulates—a combination of water and SOx to form sulfurous and sulfuric acids. These indirect particulates normally exist in the 1-10 micron range, which are “inhalable particulates” that cause health problems, especially to people who suffer from asthma or emphysema. Also, unlike gasoline obtained by reforming naphtha or by cracking heavier petroleum fractions, alkylate contains few if any aromatics or olefins. Aromatics, especially benzene, are toxic, and olefins are reactive in photochemical reactions which cause ozone and smog.
The alkylation reaction is acid-catalyzed. Liquid acid catalysts, such as sulfuric acid or hydrofluoric acid, have been commonly used in alkylation processes. The use of liquid acid catalysts has several disadvantages. The liquid acids used are highly corrosive, requiring special quality, more expensive equipment. Because the presence of these acids in the resulting fuel is undesirable, any acid remaining in the alkylate must be removed. This process is complicated and expensive. In addition, the liquid acids, especially hydrofluoric acid, are dangerous if released into the environment.
To address these and other deficiencies of liquid acid catalysts, solid acid catalysts have been developed for use in alkylation processes. The solid catalysts typically employ a solid acid catalyst and a metal that provides a hydrogenation function. For example, U.S. Pat. No. 6,855,856 describes a catalyst comprising a solid acid, such as a zeolite, and a hydrogenation function. The solid acid described has a defined range for the ratio of the volume of catalyst pores to the specific length of the catalyst particles.
A disadvantage of the prior solid acid catalysts is that the catalyst can become rapidly deactivated due to the formation of polyalkylates (e.g. C12+ product) which inhibits the alkylation reactions—somewhat like very soft coke. As soon as the catalyst forms a certain level of polyalkylates, the catalyst essentially stops the alkylation reactions. In a fixed bed reactor, an often preferred configuration, one can view the deactivation as occurring as a band-wise aging, with the deactivation zone moving as a band throughout the bed until most of the bed is inactive. This catalyst deactivation requires that the catalyst be periodically regenerated to ensure that the process produces a sufficient yield of the desired product. Regeneration of the catalyst typically requires that the alkylation process be stopped for a period of time. This reduces production and increases the cost of the alkylation process, especially by lowering the “onstream” factor of the process.
A preferred method of regeneration of the catalyst is hydrogenation. The hydrogenation function is typically provided by a metal of Group VIII of the Periodic Table of the Elements, in particular the noble metals such as platinum (Pt) or palladium (Pd). Unlike the classical bifunctional (metal/acid) catalyst, the hydrogenation function plays little or no direct role in the alkylation reactions itself. Instead, it plays a critical role in the effective H2 reactivation (also called “regeneration” here) of the deactivated catalyst. The hydrogenation function is important in both the so-called low temperature (“low T”) and high temperature (“high T”) regenerations, described below.
Various attempts have been made to develop improved solid acid catalysts. For example, U.S. Publication No. 2004/0162454 describes an alkylation catalyst comprising nanocrystalline zeolite Y and a hydrogenation metal. The pore size of the nanocrystalline zeolite Y provides an alkylate with a higher RON/MON, as well as a longer run time for the catalyst. The nanocrystalline zeolite Y catalyst also includes a metal of Group VIII of the Periodic Table of the Elements, such as Pt or Pd, to provide a hydrogenation function.
To increase the efficiency and productivity of the alkylation process using solid acid catalysts, various methods have been developed to improve the process of regenerating solid acid catalysts. For example, U.S. Pat. No. 7,176,340 describes a continuous process for alkylation using a total of at least four catalyst containing reactors. However, use of multiple reactors increases the cost of the process; this cost increase may be offset, at least in part, by the efficiency increase of the overall process U.S. Pat. No. 5,986,158 describes an alkylation process in which the catalyst is subjected intermittently to a regeneration step by being contacted with a feed containing a saturated hydrocarbon and hydrogen, with the regeneration carried out at 90% or less of the active cycle of the catalyst. While these regeneration methods improve the overall efficiency of the alkylation process, the relatively large amounts of solid acid catalysts and associated noble metals required could be a problem that affects the commercial viability of the alkylation process.
It would be desirable to have a solid acid catalyst for the alkylation process that provided longer run times prior to deactivation. It would also be desirable to have a solid acid catalyst that that utilizes a metal for the hydrogenating function that provides equal or improved performance as compared to Pt or Pd, and that may be available at a lower cost. The present invention overcomes one or more of these and other drawbacks or disadvantages of prior solid acid catalysts used in alkylation processes.