Solvent-assisted delayed coking process

An improved process for the delayed coking of a heavy residual hydrocarbon feedstock to reduce the coking induction period and to enhance the coking process relative to the processes of the prior art is achieved by mixing a sufficient volume of a paraffinic solvent having the formula CnH2n+2, where n=3 to 8 with the heavy feedstock to disturb the equilibrium of asphaltenes in the solution of maltenes in order to flocculate substantially all of the solid asphaltenes particles to thereby increase the yield and quality of valuable liquid products and minimize undesirable cracking reactions that result in high molecular weight polymers and the formation of coke.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved process for the delayed coking of heavy residual hydrocarbons that reduces the coking induction period and thereby enhances the coking process.

2. Description of Related Art

A coking unit is an oil refinery processing unit that converts the low value residual oil, or residua, from the vacuum distillation column or the atmospheric distillation column into low molecular weight hydrocarbon gases, naphtha, light and heavy gas oils, and petroleum coke. The process thermally cracks the long chain hydrocarbon molecules in the residual oil feed into shorter chain molecules. Coking is the preferred option for processing vacuum residues containing high level of metals because metals end up in the coke by-product and are disposed of more easily and economically in this solid form. The liquid coker products are almost free of metals. The processing of heavy crude oils having high metals and sulfur content is increasing in many refineries, and as a result the coking operations are of increasing importance to refiners. The increasing concern for minimizing air pollution is another incentive for treating vacuum residues in a coker, since the coker produces gases and liquids having sulfur in a form that can be relatively easily removed from the product stream.

The most commonly used coking unit is a delayed unit, or a “delayed coker”. In a basic delayed coking process, fresh feedstock is introduced into the lower part of a fractionator. The fractionator bottoms including heavy recycle material and fresh feedstock are passed to a furnace and heated to a coking temperature. The hot feed then goes to a coke drum maintained at coking conditions where the feed is cracked to form light products while heavy free radical molecules form heavier polynuclear aromatic compounds, which are referred to as “coke.” With a short residence time in the furnace, coking of the feed is thereby “delayed” until it is discharged into a coking drum. The volatile components are recovered as coker vapor and returned to the fractionator, and coke is deposited on the interior of the drum. When the coke drum is full of coke, the feed is switched to another drum and the full drum is cooled and emptied by conventional methods, such as by hydraulic means or by mechanical means.

Typical coking unit feedstocks are vacuum residues derived from fossil fuels. Selected properties and characteristics of vacuum residue samples derived from crude oils from the various geographical regions indicated are shown in Table 1. As can be seen from Table 1, vacuum residues have low American Petroleum Institute (API) gravities in the range of from 1 to 20 degrees and a sulfur content that ranges from 0.2 to 7.7 W %. In addition, vacuum residues are rich in nitrogen and can contain metals such as nickel and vanadium in relatively high concentrations which make them difficult to process in other refinery unit operations.

Vacuum residues also contain asphaltenes in the range 0.3 to 35 W %, depending upon the source of the crude oil. Asphaltenes are defined as the particles precipitated by addition of a low-boiling paraffin solvent such as normal-pentane. It is commonly accepted that asphaltenes exist in solution in the petroleum. Asphaltenes are commonly modeled as a colloid, with asphaltenes as the dispersed phase and maltenes as the continuous phase. Petroleum residua can be modeled as ordered systems of polar asphaltenes dispersed in a lower polarity solvent phase, and held together by resins of intermediate polarity.

As schematically illustrated inFIG. 1, it is known to the prior art that asphaltenes are dispersed by resin molecules, or maltenes, while small molecules such as aromatics act as a solvent for the asphaltenes-resin dispersion and hydrocarbon saturates act as a non-solvent. If crude oil is separated into fractions and then mixed together with less resin content, asphaltenes will only be present as flocculates in solution. Addition of the maltenes or resins brings the asphaltenes back into solution until the equilibrium is disturbed by addition of hydrocarbon saturates, in which case asphaltenes will again start to flocculate.

It is well known and accepted that coke formation is delayed when the asphaltenes are in solution in the petroleum. This delay in coke formation is also referred as the “induction period” which immediately precedes the formation of coke. During this period, valuable lighter components and/or secondary products formed by coking of feedstocks are subject to continued thermal cracking and recombine to form undesirable high molecular weight polymeric compounds.

It is also known from independent studies of the thermal cracking of bitumens that the yield of gaseous products increases with the residence time in the coking unit and that liquid yields are correspondingly reduced.

It is also desirable to produce a coke having a volatile matter content of not more than about 15 W %, and preferably in the range of 6 to 12 W %.

It is therefore an object of this invention to address the problem of how to reduce the coking induction period so that the residence time of the feed in the coke drum is shortened. This will maximize the desired yield of liquids and minimize the coke yield.

As used herein, the terms “coking unit” and “coker” refer to the same apparatus, and are used interchangeably.

SUMMARY OF THE INVENTION

The present invention comprehends an improved process for the delayed coking of heavy residual hydrocarbons that reduces the coking induction period and enhances the coking process by injecting a paraffinic solvent having the formula C1H2n+2, where n=3 to 8 into the feedstock. The improved delayed coking process includes the steps of:

a. introducing a fresh heavy hydrocarbon feedstock containing asphaltenes for preheating into the lower portion of a coking product fractionator;

b. discharging a bottoms fraction that includes the preheated fresh hydrocarbon feedstock from the fractionator as a coking unit combined feedstream;

c. introducing a paraffinic solvent having the formula CnH2n+2, where n=3 to 8, into a mixing zone with the coking unit combined feedstream in a ratio of solvent-to-feedstream of from 0.1:1 to 10:1 by volume to solvent-flocculate all or substantially all of the asphaltenes present in the coking unit combined feedstream;

d. introducing the coking unit combined feedstream containing the flocculated asphaltenes into a coking unit furnace for heating to a predetermined coking temperature; and

e. passing the heated combined feedstream containing the solvent-flocculated asphaltenes and paraffinic solvent to a delayed coking drum to produce a delayed coking product stream having an increased portion of liquids and depositing a reduced amount of coke on the interior of the drum, as compared to the amount of coke deposited in the absence of the addition of the paraffinic solvent to the same heavy hydrocarbon feedstock.

In accordance with another embodiment of the invention, the improved delayed coking process comprehends the steps of:

a. introducing a fresh heavy hydrocarbon feedstock containing asphaltenes for preheating into the lower portion of a coking product fractionator;

b. discharging a bottoms fraction that includes the preheated fresh hydrocarbon feedstock from the fractionator as a coking unit combined feedstream;

c. introducing the coking unit combined feedstream into a coking unit furnace for heating to a predetermined coking temperature;

d. mixing downstream of the coking furnace a paraffinic solvent having the formula CnH2n+2, where n=3 to 8, with the furnace heated coking unit combined feedstream in a ratio of solvent-to-feedstream of from 0.1:1 to 10:1 by volume to form solvent-flocculated asphaltenes in the heated coking unit combined feedstream;

e. passing the heated coking unit combined feedstream containing the solvent-flocculated asphaltenes and paraffinic solvent to a delayed coking drum to produce a delayed coking product stream having an increased proportion of liquids and depositing a reduced amount of coke on the interior of the drum, as compared to the amount of coke deposited in the absence of the addition of the paraffinic solvent to the same heavy hydrocarbon feedstock.

The mixing in step (d) referred to in the embodiment described immediately above occurs in a mixing zone upstream of the coking unit or inside the coking drum. In the latter case, paraffinic solvent is injected directly into the coking drum to mix with the incoming feedstream. Where a separate mixing zone is established upstream of the furnace, a rotating disk contactor apparatus can advantageously be employed. Feedstock and solvent can be introduced into the top of the unit and the flocculated portion can be sent to the coking unit from the bottom. This arrangement will prevent or minimize fouling of the mixing apparatus.

The processes and systems of the invention described provide the following benefits:1. The paraffinic solvent added to the feedstream disturbs the equilibrium of the asphaltenes in the maltenes solution to flocculate the solid particles of asphaltenes. The coking induction period is therefore reduced.2. The injected paraffinic solvent facilitates the removal of reacted and/or unreacted lighter liquid compounds from the coking drum, and prevents undesirable secondary cracking reactions that form additional free radicals.3. The residence time for coking reactions is reduced. This minimizes the coking of resin molecules boiling in the vacuum gas oil range to thereby increase the yield of more valuable liquid products.

As residence time increases, the liquids in the feed are subjected to further cracking to produce gaseous products. Since the coke induction period is eliminated by the addition of solvent in accordance with the present invention, the residence time in the coke drum will be shortened and the liquids produced will not be subjected to further cracking. Accordingly, the present improved process yields more liquid and less gaseous products than the same coking process conducted without the addition of a solvent.

The process has been described above and will be described further below with reference to the use of a paraffinic solvent. However, it should be understood that an embodiment of the invention employs as the solvent a portion of the light naphtha stream recovered from the coking product stream fractionator. That product stream includes olefins that are principally C5to C8compounds. For convenience and in the interest of brevity, the term paraffinic solvent is used in describing and claiming the invention with the understanding that its source can be the light naphtha that is produced in the process which also includes olefin compounds.

Other aspects, embodiments, and advantages of the process of the present invention are discussed in detail below. Moreover, it is to be understood that both the foregoing summary 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 features and embodiments. The accompanying drawings are included to provide illustration and a further understanding of the various aspects and embodiments. The drawings, together with the remainder of the specification, serve to explain principles and operations of the described and claimed aspects and embodiments.

DETAILED DESCRIPTION OF THE INVENTION

Referring now toFIG. 2, an improved delayed coking process and apparatus10is schematically illustrated. Apparatus10includes a fractionator20, a mixing zone30, a furnace40and a coking drum50. Fractionator20includes an inlet27for receiving fresh heavy hydrocarbon feedstock, an inlet21in fluid communication with a coking drum outlet52for receiving delayed coking product stream. Fractionator20also includes an outlet22for discharging a light naphtha fraction, an outlet23for discharging a heavy naphtha fraction, an outlet24for discharging a gas oil fraction, an outlet25for discharging a heavy gas oil fraction, and an outlet26for discharging a mixture of the bottoms fraction and preheated fresh heavy hydrocarbon feedstock. Mixing zone30includes an inlet31in fluid communication with a conduit33for introducing a paraffinic solvent and fractionator outlet26for receiving the combined stream of preheated fresh hydrocarbon feedstock and the fractionator bottoms fraction. Mixing zone30also includes an outlet32for discharging a combined stream containing solvent-flocculated asphaltenes and paraffinic solvent. Furnace40includes an inlet41in fluid communication with mixing zone outlet32and an outlet42for discharging heated combined stream. Coking drum50includes an inlet51in fluid communication with furnace outlet42and an outlet52in fluid communication with fractionator inlet21for receiving the delayed coking product stream.

In the practice of the method of the invention, a fresh heavy hydrocarbon feedstock containing asphaltenes is introduced into the lower portion of the fractionator20via inlet27. The preheated feedstock is combined with the fractionator bottoms stream and passed to mixing zone30via inlet31. A paraffinic solvent is introduced into mixing zone30via conduit33in a ratio of solvent-to-feedstream of from 0.1:1 to 10:1 by volume to form solvent-flocculated asphaltenes in the combined stream. The combined stream containing solvent-flocculated asphaltenes and paraffinic solvent is discharged via outlet32and introduced into furnace40via inlet41where it is heated to a predetermined coking temperature in the range 480° C. to 530° C. The heated combined stream is discharged via outlet42and passed to coking drum50via inlet51to produce the delayed coking product stream having an increased portion of liquids and to deposit a reduced amount of coke on the interior of the drum. The delayed coking product stream is discharged via outlet52and passed to fractionator20where it is fractionated to produce a paraffinic light naphtha solvent boiling in the range 36° C. to 75° C. via outlet22, a heavy naphtha product boiling in the range 75° C. to 180° C. via outlet23, a light gas oil boiling in the range 180° C. to 370° C. via outlet24, a heavy coker gas oil boiling in the range 370° C. to 520° C. via outlet25, and a bottoms fraction boiling in the range above 520° C. via outlet26. Optionally, a portion of paraffinic light naphtha solvent is recycled back to conduit33to minimize the use of fresh paraffinic solvent.

Referring toFIG. 3, an improved delayed coking process and apparatus100is schematically illustrated. Apparatus100includes a fractionator120, a mixing zone130, a furnace140and a coking drum150. Fractionator120includes an inlet127for receiving fresh heavy hydrocarbon feedstock, an inlet121in fluid communication with a coking drum outlet152for receiving delayed coking product stream. Fractionator120also includes an outlet122for discharging a light naphtha fraction, an outlet123for discharging a heavy naphtha fraction, an outlet124for discharging a gas oil fraction, an outlet125for discharging a heavy gas oil fraction, and an outlet126for discharging a mixture of the bottoms fraction and preheated fresh heavy hydrocarbon feedstock. Furnace140includes an inlet141in fluid communication with fractionator outlet126and an outlet142for discharging heated combined stream of bottoms fraction and fresh heavy hydrocarbon feedstock. Mixing zone130includes an inlet131in fluid communication with a conduit133for receiving a paraffinic solvent and furnace outlet142for receiving heated combined stream. Mixing zone130also includes an outlet132for discharging combined stream containing solvent-flocculated asphaltenes and paraffinic solvent. Coking drum150includes an inlet151in fluid communication with mixing zone outlet132and an outlet152in fluid communication with fractionator inlet121for receiving delayed coking product stream.

A fresh heavy hydrocarbon feedstock containing asphaltenes is introduced into the lower portion of the fractionator120via inlet127. The preheated feedstock is combined with fractionator bottoms stream and passed to furnace140via inlet141where it is heated to a predetermined coking temperature in the range 480° C. to 530° C. The heated combined stream is conveyed to mixing zone130via inlet131. A paraffinic solvent is introduced into mixing zone130via conduit133in a ratio of solvent-to-feedstream of from 0.1:1 to 10:1 by volume to form solvent-flocculated asphaltenes in the combined stream. The combined stream containing solvent-flocculated asphaltenes and paraffinic solvent is discharged via outlet132and passed to coking drum150via inlet151to produce the delayed coking product stream having an increased portion of liquids and to deposit a reduced amount of coke on the interior of the drum, relative to the prior art process. The delayed coking product stream is discharged via outlet152and passed to fractionator120where it is fractionated to produce a light naphtha containing paraffinic solvent boiling in the range 36° C. to 75° C. via outlet122, a heavy naphtha boiling in the range 75° C. to 180° C. via outlet123, a light gas oil boiling in the range 180° C. to 370° C. via outlet124, a heavy coker gas oil boiling in the range 370° C. to 520° C. via outlet125, and a bottoms fraction boiling in the range above 520° C. via outlet126. Optionally, a portion of light naphtha containing paraffinic solvent is recycled back to conduit133to minimize the use of fresh paraffinic solvent.

Referring toFIG. 4, an improved delayed coking process and apparatus200is schematically illustrated. Apparatus200includes a fractionator220, a furnace240and a coking drum250. Fractionator220includes an inlet227for receiving fresh heavy hydrocarbon feedstock, an inlet221in fluid communication with a coking drum outlet252for receiving delayed coking product stream. Fractionator220also includes an outlet222for discharging light naphtha fraction, an outlet223for discharging a heavy naphtha fraction, an outlet224for discharging a gas oil fraction, an outlet225for discharging a heavy gas oil fraction, and an outlet226for discharging a mixture of the bottoms fraction and preheated fresh heavy hydrocarbon feedstock. Furnace240includes an inlet241that is in fluid communication with a conduit254for receiving a paraffinic solvent and with fractionator outlet226and an outlet242for discharging heated combined stream of bottoms fraction and fresh heavy hydrocarbon feedstock. Coking drum250includes an inlet251in fluid communication with a conduit253for receiving a paraffinic solvent and furnace outlet242for receiving heated combined stream. Coking drum250also includes an outlet252for discharging delayed coking product stream.

A fresh heavy hydrocarbon feedstock containing asphaltenes is introduced into the lower portion of the fractionator220via inlet227. The preheated feedstock is combined with fractionator bottoms stream and passed to furnace240via inlet241where it is heated to a predetermined coking temperature in the range 480° C. to 530° C. The heated combined stream is conveyed to coking drum250via inlet251. A paraffinic solvent is introduced into coking drum250via conduit253in a ratio of solvent-to-feedstream of from 0.1:1 to 10:1 by volume to form solvent-flocculated asphaltenes in the combined stream. Combined stream containing solvent-flocculated asphaltenes and paraffinic solvent is processed in coking drum250to produce the delayed coking product stream having increased portion of liquids and deposit a reduced amount of coke on the interior of the drum. The delayed coking product stream is discharged via outlet252and passed to fractionator220where it is fractionated to produce a light naphtha containing paraffinic solvent boiling in the range 36° C. to 75° C. via outlet222, a heavy naphtha boiling in the range 75° C. to 180° C. via outlet223, a light gas oil boiling in the range 180° C. to 370° C. via outlet224a heavy coker gas oil boiling in the range 370° C. to 520° C. via outlet225, and a bottoms fraction boiling in the range above 520° C. via outlet226. Optionally, a portion of light naphtha containing paraffinic solvent is recycled back to conduit253to minimize the use of fresh paraffinic solvent.

The feedstocks for the improved delayed coking process described herein are heavy hydrocarbons derived from natural resources including crude oil, bitumen, tar sands and shale oils, or from refinery processes including atmospheric or vacuum residue, products from coking, visbreaker and fluid catalytic cracking operations. The heavy hydrocarbon feedstock has a boiling point in the range of from 36° C., this being the boiling point of pentane, up to 2000° C. Some heavy hydrocarbon feedstocks such as bitumens include little light hydrocarbons. In these cases, the feedstock can have an initial boiling point (IBP) of 180° C., e.g., the IBP of gas oils, or 370° C., e.g., the IBP of vacuum gas oil.

The paraffinic solvent has the general formula of CnH2n+2, where n can be from 3 to 8. As noted above, a portion of the light naphtha stream from the fractionator can be used as the solvent that is mixed with the feedstream to the furnace or the coking drum. In accordance with the definition of light naphtha conventionally used in the art, octanes and olefin compounds, including pentenes, hexenes, heptenes and octenes, can also be present in the mixture. The presence of C3and C4compounds on the mixture will be dependent upon the prevailing pressure and temperature conditions in the coking unit and upstream. The C5to C8alkanes have boiling points in the range from about 28° C. to about 114° C., and the C5to C8olefins have initial boiling points in the range of from about 30° C. to about 121° C. The solvent is injected at a solvent battery limit temperature and a pressure of from 1 bar to 100 bars.

The coking unit is a typical delayed coking unit with two drums operating alternatively. In general, the operating conditions for the coking drum include a temperature of from 425° C. to 650° C.; in certain embodiments from 425° C. to 540° C.; in further embodiments from 450° C. to 510° C.; and in additional embodiments from 470° C. to 500° C.; and at a pressure of from 1 bar to 20 bars; in certain embodiments from 1 bar to 10 bars; and in further embodiments from 1 bar to 7 bars. The coking cycle time can be from 8 hrs to 60 hrs; in certain embodiments from 24 hrs to 48 hrs; and in further embodiments from 8 hrs to 24 hrs.

The method of the invention represents an improvement over the prior art processes by reducing the coking induction period by mixing a predetermined amount of paraffinic solvent with the heavy hydrocarbon feedstocks in order to disturb the equilibrium of the asphaltenes in the maltenes solution and to flocculate all, or substantially all of the solid asphaltenes particles. In the present process, the yield and qualities of valuable liquid products are increased while undesirable cracking and the formation of coke are minimized.

The method and system of the present invention have been described above and in the attached drawings; however, modifications will be apparent to those of ordinary skill in the art and the scope of protection for the invention is to be determined by the claims that follow.