RELATED APPLICATIONS

Not applicable.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to hydrocracking processes, and in particular to hydrocracking processes including separation of heavy poly nuclear aromatics from recycle streams using sulfonation.

Description of Related Art

Hydrocracking processes are used commercially in a large number of petroleum refineries. They are used to process a variety of feeds boiling within the range of about 370-520° C. in conventional hydrocracking units and boiling at 520° C. and above in residue hydrocracking units. In general, hydrocracking processes split the molecules of the feed into smaller, i.e., lighter, molecules having higher average volatility and economic value. Additionally, hydrocracking processes typically improve the quality of the hydrocarbon feedstock by increasing the hydrogen-to-carbon ratio and by removing organosulfur and organonitrogen compounds. The significant economic benefit derived from hydrocracking processes has resulted in substantial development of process improvements and more active catalysts.

In addition to sulfur-containing and nitrogen-containing compounds, a typical hydrocracking feedstream, such as vacuum gas oil (VGO), contains a small amount of poly nuclear aromatic (PNA) compounds, i.e., those containing less than seven fused aromatic rings. As the feedstream is subjected to hydroprocessing at elevated temperature and pressure, heavy poly nuclear aromatic (HPNA) compounds, i.e., those containing seven or more fused benzene rings, tend to form and are present in high concentration in the unconverted hydrocracker bottoms.

Heavy feedstreams such as demetallized oil (DMO) or deasphalted oil (DAO) have much higher concentrations of nitrogen, sulfur and PNA compounds than VGO feedstreams. These impurities can lower the overall efficiency of hydrocracking units by requiring higher operating temperature, higher hydrogen partial pressure or additional reactor/catalyst volume. In addition, high concentrations of impurities can accelerate catalyst deactivation.

Three major hydrocracking process schemes include single-stage once through hydrocracking, series-flow hydrocracking with or without recycle, and two-stage recycle hydrocracking. Single-stage once through hydrocracking is the simplest of the hydrocracker configurations and typically occurs at operating conditions that are more severe than hydrotreating processes, and less severe than conventional full-pressure hydrocracking processes. It uses one or more reactors for both the treating steps and the cracking reaction, so the catalyst must be capable of both hydrotreating and hydrocracking. This configuration is cost effective, but typically results in relatively low product yields (for example, a maximum conversion rate of about 60%). Single-stage hydrocracking is often designed to maximize mid-distillate yield over single or dual catalyst systems. Dual catalyst systems can be used in a stacked-bed configuration or in two different reactors. The effluents are passed to a fractionator column to separate the H2S, NH3, light gases (C1-C4), naphtha and diesel products boiling in the temperature range of 36−370° C. The hydrocarbons boiling above 370° C. are typically unconverted bottoms that, in single stage systems, are passed to other refinery operations.

Series-flow hydrocracking with or without recycle is one of the most commonly used configurations. It uses one reactor (containing both treating and cracking catalysts) or two or more reactors for both treating and cracking reaction steps. In a series-flow configuration the entire hydrocracked product stream from the first reaction zone, including light gases (typically C1-C4, H2S, NH3) and all remaining hydrocarbons, are sent to the second reaction zone. Unconverted bottoms from the fractionator column are recycled back into the first reactor for further cracking. This configuration converts heavy crude oil fractions, i.e., vacuum gas oil, into light products and has the potential to maximize the yield of naphtha, jet fuel, or diesel, depending on the recycle cut point used in the distillation section.

Two-stage recycle hydrocracking uses two reactors and unconverted bottoms from the fractionation column are passed to the second reactor for further cracking. Since the first reactor accomplishes both hydrotreating and hydrocracking, the feed to second reactor is virtually free of ammonia and hydrogen sulfide. This permits the use of high-performance zeolite catalysts which are susceptible to poisoning by sulfur or nitrogen compounds.

A typical hydrocracking feedstock is vacuum gas oils boiling in the nominal range of 370-565° C. Heavier oil feedstreams such as DMO or DAO, alone or blended with vacuum gas oil, is processed in a hydrocracking unit. For instance, a typical hydrocracking unit processes vacuum gas oils that contain from 10V % to 25V % of DMO or DAO for optimum operation. A 100% DMO or DAO feed can also be processed, typically under more severe conditions, since the DMO or DAO stream contains significantly more nitrogen compounds (2,000 ppmw vs. 1,000 ppmw) and a higher micro carbon residue (MCR) content than the VGO stream (10 W % vs.<1 W %).

DMO or DAO content in blended feedstocks to a hydrocracking unit can lower the overall efficiency of the unit by increasing operating temperature or reactor/catalyst volume for existing units, or by increasing hydrogen partial pressure requirements or reactor/catalyst volume for grass-roots units. These impurities can also reduce the quality of the desired intermediate hydrocarbon products in the hydrocracking effluent. When DMO or DAO are processed in a hydrocracker, further processing of hydrocracking reactor effluents may be required to meet the refinery fuel specifications, depending upon the refinery configuration. When the hydrocracking unit is operating in its desired mode, that is to say, discharging a high quality effluent product stream, its effluent can be utilized in blending and to produce gasoline, kerosene and diesel fuel to meet established fuel specifications.

In addition, formation of HPNA compounds is an undesirable side reaction that occurs in recycle hydrocrackers. The HPNA molecules form by dehydrogenation of larger hydro-aromatic molecules or cyclization of side chains onto existing HPNA molecules followed by dehydrogenation, which is favored as the reaction temperature increases. HPNA formation depends on many known factors including the type of feedstock, catalyst selection, process configuration, and operating conditions. Since HPNA molecules accumulate in the recycle system and then cause equipment fouling, HPNA formation must be controlled in the hydrocracking process.

The rate of formation of the various HPNA compounds increases with higher conversion and heavier feed stocks. The fouling of equipment may not be apparent until large amounts of HPNA accumulate in the recycle liquid loop. The problem of HPNA formation is of universal concern to refiners and various removal methods have been developed by refinery operators to reduce its impact.

Conventional methods to separate or treat heavy poly-nuclear aromatics formed in the hydrocracking process include adsorption, hydrogenation, extraction, solvent deasphalting and purging, or “bleeding” a portion of the recycle stream from the system to reduce the build-up of HPNA compounds and cracking or utilizing the bleed stream elsewhere in the refinery. The hydrocracker bottoms are treated in separate units to eliminate the HPNA molecules and recycle HPNA-free bottoms back to the hydrocracking reactor.

As noted above, one alternative when operating the hydrocracking unit in the recycle mode is to purge a certain amount of the recycle liquid to reduce the concentration of HPNA that is introduced with the fresh feed, although purging reduces the conversion rate to below 100%. Another solution to the build-up problem is to eliminate the HPNAs by passing them to a special purpose vacuum column which effectively fractionates 98-99% of the recycle stream leaving most of the HPNAs at the bottom of the column for rejection from the system as fractionator bottoms. This alternative incurs the additional capital cost and operating expenses of a dedicated fractionation column.

The problem therefore exists of providing a process for removing HPNA compounds from the bottoms recycle stream of a hydrocracking unit that is more efficient and cost effective than the known processes.

SUMMARY OF THE INVENTION

Hydrocracked bottoms fractions are treated to separate HPNA compounds and/or HPNA precursor compounds and produce a reduced-HPNA hydrocracked bottoms fraction effective for recycle, in a configuration of a single-stage hydrocracking reactor, series-flow once through hydrocracking operation, or two-stage hydrocracking operation.

A process for separation of HPNA and/or HPNA precursor compounds from a hydrocracked bottoms fraction of a hydroprocessing reaction effluent comprises contacting the hydrocracked bottoms fraction with an effective quantity of a sulfonation agent. The contacting occurs under effective conditions to promote reaction with HPNA and/or HPNA precursor compounds to produce corresponding aromatic sulfonates and form a sulfonated hydrocracked bottoms fraction. The sulfonated hydrocracked bottoms fraction is separated into an HPNA-reduced hydrocracked bottoms portion and a sulfonated HPNA portion. In certain embodiments, the sulfonation agent used is liquid. In further embodiments, the sulfonation agent is gaseous.

The above methods for separation of HPNA and/or HPNA precursor compounds by sulfonation can be integrated in a hydrocracking operation using a single reactor or plural reactors in a “once-through” configuration. Accordingly, in certain embodiments a hydrocracking process for treating a heavy hydrocarbon feedstream which contains undesired nitrogen-containing compounds and poly-nuclear aromatic compounds is provided that comprises subjecting the hydrocarbon feedstream to one or more hydrocracking stages to produce a hydrocracked effluent. The hydrocracked effluent is fractioned to recover hydrocracked products and a hydrocracked bottoms fraction containing HPNA and/or HPNA precursor compounds. The hydrocracked bottoms fraction is contacted with an effective quantity of sulfonation agent promote reaction with HPNA and/or HPNA precursor compounds to produce corresponding aromatic sulfonates and form a sulfonated hydrocracked bottoms fraction. The sulfonated hydrocracked bottoms fraction is separated into an HPNA-reduced hydrocracked bottoms portion and a sulfonated HPNA portion. All or a portion of the HPNA-reduced hydrocracked bottoms portion is recycled.

In additional embodiments, the above methods for separation of HPNA and/or HPNA precursor compounds by sulfonation can be integrated in a two-stage hydrocracking configuration. Accordingly, in certain embodiments, a hydrocracking process for treating a heavy hydrocarbon feedstream which contains undesired nitrogen-containing compounds and poly-nuclear aromatic compounds is provided that comprises subjecting the hydrocarbon feedstream to one or more first hydrocracking stages to produce a first stage effluent. The first stage effluent is fractioned to recover hydrocracked products and a hydrocracked bottoms fraction containing HPNA and/or HPNA precursor compounds. The hydrocracked bottoms fraction is contacted with an effective quantity of sulfonation agent promote reaction with HPNA and/or HPNA precursor compounds to produce corresponding aromatic sulfonates and form a sulfonated hydrocracked bottoms fraction. The sulfonated hydrocracked bottoms fraction is separated into an HPNA-reduced hydrocracked bottoms portion and a sulfonated HPNA portion. All or a portion of the HPNA-reduced hydrocracked bottoms portion is recycled.

In certain embodiments, the sulfonated HPNA-containing hydrocracked bottoms fraction is separated using an aqueous separation process. In additional embodiments a solvent extraction process based on aromatic selectively can be used. In further embodiments, the sulfonated hydrocracked bottoms fraction can be separated using two or more of the separation methods described herein, for instance, an aqueous separation process followed by a solvent extraction HPNA separation process based on aromatic selectively.

In certain embodiments, a process for separation of HPNA compounds and/or HPNA precursor compounds from a hydrocracked bottoms fraction prior to recycling within a hydrocracking operation comprises: contacting the hydrocracked bottoms fraction with an effective quantity of a sulfonation agent to promote reaction with HPNA and/or HPNA precursor compounds, to produce corresponding sulfonated HPNA compounds and/or sulfonated HPNA precursor compounds, and to form a sulfonated hydrocracked bottoms fraction; separating the sulfonated hydrocracked bottoms fraction into an HPNA-reduced hydrocracked bottoms portion and a sulfonated HPNA portion; recycling all or a portion of the HPNA-reduced hydrocracked bottoms portion within the hydrocracking operation; and discharging the precipitated HPNA portion. In certain embodiments, two stage hydrocracking process comprises subjecting a hydrocarbon stream to a first hydrocracking stage to produce a first hydrocracked effluent; fractionating the first hydrocracked effluent to recover one or more hydrocracked product fractions and a bottoms fraction corresponding to the hydrocracked bottoms fraction of in the above process for separation of HPNA; wherein recycling all or a portion of the HPNA-reduced hydrocracked bottoms portion within the hydrocracking operation comprises passing all or a portion of the HPNA-reduced hydrocracked bottoms portion to a second hydrocracking stage to produce a second hydrocracked effluent; and optionally wherein the second hydrocracked effluent is fractionated with the first hydrocracked effluent. In certain embodiments, a hydrocracking process comprising subjecting a hydrocarbon stream to one or more hydrocracking stages to produce a hydrocracked effluent; fractionating the hydrocracked effluent to recover one or more hydrocracked product fractions and a hydrocracked bottoms fraction corresponding to the hydrocracked bottoms fraction of in the above process for separation of HPNA; and wherein recycling all or a portion of the HPNA-reduced hydrocracked bottoms portion within the hydrocracking operation comprises recycling all or a portion of the HPNA-reduced hydrocracked bottoms portion to at least one of the one or more hydrocracking stages. In certain embodiments, the sulfonation agent is liquid phase. A liquid phase sulfonation agent can be sulfuric acid under effective operating conditions. In additional embodiments, the sulfonation agent is gas phase. A gas phase sulfonation agent can be selected from the group consisting of SO2, SO3and mixtures thereof, under effective operating conditions. In certain embodiments the process further comprises contacting an additional feed with the sulfonation agent.

In certain embodiments, a system for separation of HPNA compounds and/or HPNA precursor compounds from a hydrocracked bottoms fraction is provided comprising a sulfonation reaction zone having one or more inlets in fluid communication with a source of sulfonation agent, and one or more inlets in fluid communication with a hydrocracked bottoms outlet of a hydrocracking fractionating zone, the sulfonation reaction zone having one or more outlets for discharging a sulfonated hydrocracked bottoms fraction; and a separation zone having one or more inlets in fluid communication with the outlet(s) discharging the sulfonated hydrocracked bottoms fraction, one or more outlets for discharging an HPNA-reduced hydrocracked bottoms portion in fluid communication with a hydrocracking operation as a bottoms recycle stream, and one or more outlets for discharging a sulfonated HPNA portion. In certain embodiments, a two stage hydrocracking system comprises a first hydrocracking reaction zone having one or more inlets in fluid communication with a source of an initial feedstock, and one or more outlets for discharging a first hydrocracked effluent stream; a fractionating zone having one or more inlets in fluid communication with the outlet(s) for discharging the first hydrocracked effluent stream, one or more outlets discharging a hydrocracked product fractions, and one or more outlets discharging a hydrocracked bottoms fraction in fluid communication with the HPNA separation zone as above; a second hydrocracking reaction zone having one or more inlets in fluid communication with the outlet(s) for discharging the HPNA-reduced hydrocracked bottoms portion of the HPNA separation zone as above, and one or more outlets discharging a second hydrocracked effluent stream; and optionally wherein the outlet(s) for discharging the second hydrocracked effluent is in fluid communication with the fractioning zone. In certain embodiments, a hydrocracking system comprises a hydrocracking reaction zone having one or more inlets in fluid communication with a source of an initial feedstock and is in fluid communication with the HPNA-reduced hydrocracked bottoms portion from the outlet(s) of the HPNA separation zone as above, and one or more outlets discharging an effluent stream; and a fractionating zone having one or more inlets in fluid communication with the outlet(s) for discharging the effluent stream, one or more outlets discharging a hydrocracked product fractions, and one or more outlets discharging a hydrocracked bottoms fraction in fluid communication with the inlet(s) of the HPNA separation zone as above. In certain embodiments, the HPNA separation zone includes a contacting and/or mixing zone upstream of the sulfonation reaction zone. In certain embodiments, the HPNA separation zone is also in fluid communication with a source of additional feed.

Still other aspects, embodiments, and advantages of these exemplary aspects and embodiments, are discussed in detail below. Moreover, it is to be understood that both the foregoing information and the following detailed description are merely illustrative examples of various aspects and embodiments, and are intended to provide an overview or framework for understanding the nature and character of the claimed aspects and embodiments. The accompanying drawings are included to provide illustration and a further understanding of the various aspects and embodiments, and are incorporated in and constitute a part of this specification. The drawings, together with the remainder of the specification, serve to explain principles and operations of the described and claimed aspects and embodiments.

