Process for reduction of sulfur content in a hydrocarbon mixture

Abstract

A process for reduction of sulfur content in a hydrocarbon mixture, without considerable increase in gas production, comprising the introduction of a fraction of products obtained by the FCC process in a reaction zone of said unit. The cut must be introduced into the unit riser, in an area below the feed line of the heavy hydrocarbon streams to be cracked. Preferably, through one or more injection lines (5) placed near the point of gas injection line (3) entraining the catalyst in the riser (1).

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon, claims the benefit of, priority of, and incorporates by reference, the contents of Brazilian Patent Application No. PI 0404048-1 filed Sep. 22, 2004.

SCOPE OF THE INVENTION

The field of application of the present invention belongs amongst the processes for the reduction of the sulfur content in a hydrocarbon mixture. Preferably, within the processes for the reduction of sulfur content through modifications in the reaction zone in a fluid catalytic cracking (FCC) unit. More specifically, through recycling a fraction of the product obtained from said process at the vapor injection point that entrains the catalyst in the base of riser.

DESCRIPTION OF THE STATE OF THE ART

About 30% of anthropogenic NO_x released into the atmosphere is produced by combustion engines. One way that was found to attenuate the problem was to use catalysts in automobile exhaust system that convert the CO, the unburned hydrocarbons, and the NO to CO₂ and N₂. Sulfur present in the fuel acts as a poison, reducing the efficiency of the catalyst. This led to the adoption of more and more restrictive measures regarding the reduction of sulfur content in fuels, initially adopted by European and North American countries. Brazilian specifications were gradually adjusted in accordance with the standard requirements set by European and North American countries.

A good portion of gasoline is produced using the fluid catalytic cracking (FCC) process. In this way, many patents were filed for the purpose of introducing improvements into the FCC process. The majority of them to produce quality fuels in an amount capable of meeting the demand.

North American U.S. Pat. No. 5,616,237 protects a process to attain high octane fuel, where the stream to be cracked is divided in stream fractions and recycled to the reaction zone by injecting them at points throughout the riser at different temperatures. It does not concern itself with limiting the production of light fractions resulting from the increase in octane.

In the same way, North American U.S. Pat. No. 5,154,818 protects a hydrocarbon catalytic cracking process in two sections. In this patent, a stream of light hydrocarbons is placed in contact with the spent catalyst in the first section of riser, while a stream containing heavy hydrocarbons is placed in contact with the regenerated catalyst in the second section of riser. The temperature of the first section is lower than the temperature in the second section.

International patent WO 87/02695 protects a process to increase the octane of gasoline by injecting the stream to be cracked at several different points and by using vapor or inert gases to suspend the catalyst in the riser.

It is important to point out that in the patents above, recracking of hydrocarbon streams for the purpose of improving the octane of gasoline, involves significant modifications in the reaction conditions that changes the productivity profile of an FCC unit.

Using the FCC's spent catalyst to convert hydrocarbons is also presented in North American U.S. Pat. No. 6,162,402. In this patent, the spent catalyst is partially recycled in the reaction zone to promote conversion of the stream. This solution presents the same disadvantages as the previous ones, as it also reduces the productivity of the unit. Moreover, the introduction of considerable modifications in the FCC unit implies a greater implementation investment.

North American U.S. Pat. No. 6,238,548 requires recycling the 100-180° C. naphtha cut in the dense stream bed area, situated on top of the stripper, in order to increase octane. Injection in this area exposes the hydrocarbons to conditions that favor thermal cracking (long residence times in the reactor vessel), which may compromise the stability of the gasoline.

An alternative solution is proposed in the European patent EP 0 940 464. Said patent proposes the use of a process to reduce sulfur where the gasoline produced in the FCC process is fractioned in three parts, followed by hydrotreatment of the heaviest fraction, mixing of the obtained effluent as an intermediate fraction and hydrotreatment of the mixed stream, so as to assure sulfur removal.

The use of catalysts, intended for use in an FCC, as an additive to reduce sulfur content is also widely used. Brazilian patent PI 0014102-0, proposes using a different molecular sieve support containing a high level of vanadium, used together with a catalytically active cracking catalyst.

In the same way, Brazilian patent PI 0015509-8 protects an absorbent composition made up of a bimetallic booster of transition metals and a particulate support. European patent EP 0 609 971, proposes using Lewis acid supported in oxidized alumina. The Lewis acids used are some transition metals.

British patent GB 2 341 191, suggests that an additive may be used either together with the FCC catalyst or not. The additive comprises a metal with an oxidation number greater than zero in the structure of the molecular sieve. The zeolite used has large pores, such as USY or ZSM-5.

British patent GB 2 345 293 protects an additive for sulfur reduction in gasoline made up of a transition metal like vanadium and a rare earth like cerium, both supported in a large pored zeolite.

North American U.S. Pat. No. 4,464,252, in addition to raising the resulting expense of producing an additive, introduces other stages into the process, thus making it a less agile process. Said patent proposes regenerating the spent catalytic by coke burning, whether it is followed or not by placing it in contact with hydrogen to reduce metals.

These solutions increase production costs of gasoline, once a new additive must be used. In addition, the use of an additive containing a high level of vanadium encounters limitations due to an excessive production of coke, hydrogen and possible deactivation of the catalytic base that may harm the operation of the FCC units.

Thus, the state of the art seeks a solution in which: it is not necessary to recycle the spent catalyst and neither the residence time of the stream in the riser is reduced, nor is the productivity of the FCC unit. Also, it is interesting that it has a uniform temperature profile so that a quality fuel is produced with a low sulfur level.

SUMMARY OF THE INVENTION

The process for reducing the sulfur content in a hydrocarbon mixture, which is the object of this invention, attempts to eliminate wholly or in part the above-mentioned disadvantages, by introducing a heavy naphtha cut which comes from the FCC process in the reaction section of said unit. In this way, it is possible to perform a selective recracking, for the purpose of reducing the sulfur content in the fuel, without increasing gas production considerably. The cut must be introduced into the unit riser, in an area below the feed line of the heavy hydrocarbon streams to be cracked. Preferably, at the point of injection of the gases entraining the catalyst in the riser. The heavy naphtha cut must be distributed in a homogeneous manner, with compatible dispersion. Alternately, the stream to be recracked may be made up of a mixture of heavy naphtha with an initial cut of light cycle oil (LCO). In this way, the FCC unit operates in conditions of low severity, so as to increase the production of medium distillates to incorporate in the diesel oil pool.

BRIEF DESCRIPTION OF THE DRAWINGS

The process to reduce the sulfur content in hydrocarbon mixtures, the object of the present invention, will be better understood through the detailed description given below, with example heading, together with the referenced diagram below, which makes up an integral part of the present report.

FIG. 1 shows a schematic of a section of the riser in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The process for reducing the sulfur content in hydrocarbon mixtures, which is the object of this invention, shall be made according to the identification of the respective components, based on the figure described above.

The field of application of the present invention belongs amongst the processes for the reduction of the sulfur content in a hydrocarbon mixture. Preferably, within the processes for the reduction of sulfur content through modifications in the reaction zone in a fluid catalytic cracking (FCC) unit. More specifically, through recycling a fraction of the product obtained from said process at the vapor injection point that entrains the catalyst in the base of riser.

The invention is based on the catalytic recracking concept with a greater residence time for a portion of the products generated by the FCC process, in order to reduce the sulfur content in the final product. The stream selected to be injected, must have a boiling point within a range of 130° C. and 280° C. Preferably, within a range between 160° C. and 220° C.

Alternately, the stream to be recracked may be made up of a mixture of heavy naphtha with an initial cut of light cycle oil (LCO). In this way, the FCC unit operates in conditions of low severity, so as to increase the production of medium distillates to incorporate in the diesel oil pool.

The stream to be recracked must be injected at one or more points of injection, near the point of injection of the gases entraining the catalyst in the riser. Also, residence time of said stream will increase in the reaction zone and will increase the efficiency of reducing the sulfur content.

One possible prototype of the invention is described in FIG. 1.

In this figure, a section of the riser (1), containing two injection points (2) for hydrocarbons to be cracked, has undergone a modification in the gas injection line (3) area to the regeneration section (4), so as to accommodate a second injection line (5), where the hydrocarbons to be recracked are injected.

The process of the present invention permits sulfur to be reduced in the final gasoline produced without considerably increasing the production of liquefied petroleum gas (GLP) and gas fuel in the FCC process. In this way, this invention may be adapted in FCC units limited by gas compressors.

EXAMPLE

In an initial study, cracked naphtha produced in a pilot unit by cracking gas oil with a balancing catalyst was reprocessed in a 9 gram catalyst bench unit at a reaction temperature of 535° C., with a catalyst/oil ratio between 7 and 11.

A little less than a third of the gas oil was converted into other products; specifically, 17% GLP, 4.5% coke, 2.5% gas fuel, and 6% LCO. The sulfur content in the gasoline was reduced from 824 ppm to 420 ppm and the level of olefins from 28% by weight to 16% by weight. The diene content of the reprocessed naphtha was below the detection limit, which indicates excellent stability.

A second study was performed using commercial cracked naphtha and a balancing catalyst. This time, the gasoline was reprocessed in a pilot unit using reaction temperatures between 565° C. and 610° C.

The range of gasoline conversion and distribution of products were similar to the previous study, and quality improvements were as follows: 36% reduction of sulfur content, 44% reduction in olefin resin content, and 90% reduction of diene content.

The description made here of the process for reducing sulfur content in hydrocarbon mixture, the object of the present invention, should be considered only as a possibility or possible models, and any particular characteristics introduced herein should be understood only as something that was described to facilitate understanding. In this way, they should not in any way be considered as limitations of the invention, which is limited to the scope of the claims that follow.

Claims

1. Process for the reduction of sulfur content in a hydrocarbon mixture, through catalytic recracking of a fraction of the products generated in the fluid catalytic cracking process, characterized by a product stream to be recracked, injected through one or more injection lines (5), placed close to the point of gas injection line (3) that entrains the catalyst in the riser (1).

2. Process to reduce sulfur content in a mixture of hydrocarbons, in accordance with claim 1, characterized by a stream of products to be cracked having a boiling point within the range of 130° C. and 280° C., preferably within a range between 160° C. and 220° C.

3. Process to reduce sulfur content in a mixture of hydrocarbons, in accordance with claim 1, characterized by a stream of products to be cracked, that is, a mixture of a heavy naphtha cut and a light stream of light cycle oil (LCO).