PL196032B1 - Process for sulfur reduction in naphtha streams - Google Patents

Abstract

A process for fractionating and treating of a full range naphtha stream. The full boiling range naphtha stream is first split into a light boiling range naphta, an intermediate boiling range naphtha and a heavy boiling range naphtha. The bottoms are subjected to hydrodesulfurization and the effluent combined with the intermediate boiling range naphtha containing thiophene and thiophene and thiophene mercaptans and subjected to a second hydrodesulfurization. The effluent from the polishing reactor may be combined with the light boiling range naphtha to produce a new full boiling range naphtha containing substantially less total sulfur than the original feed. The mercaptans in the light naphtha may be removed by thioetherification prior to splitting or by wet caustic wash afterwards. The object being to meet higher standards for sulfur removal, by treating the components of the naphta feed with the process that preserves the olefinic while most expediently removing the sulfur compounds.

Description

Description of the invention

The invention relates to a process for the simultaneous fractionation and treatment of a full boiling gasoline fraction with hydrogen. More specifically, the full boiling range gasoline fraction is split into light (low boiling) gasoline, mid boiling point gasoline and naphtha (high boiling point). Each boiling point of gasoline is processed separately to achieve the total required sulfur content.

The fractions from the crude oil distillation contain a variety of organic chemicals. Generally, fractions are determined by their boiling point ranges which define the composition of the ingredients. The processing of fractions also affects their composition. For example, products from catalytic or thermal cracking processes contain both high concentrations of olefinic materials and saturated (alkanes) and polyunsaturated (diolefins) materials. Additionally, these compounds can be some of the different isomers of the compounds. The composition of untreated gasoline, such as coming from a feedstock distillation reactor or straight run gasoline, is primarily influenced by the feed source. Gasolines from paraffinic raw material sources have more saturated straight-chain or cyclic compounds. In general, most "sweet" (low sulfur) feedstocks and gasolines are paraffinic. Naphthenic raw materials contain more unsaturated, cyclic and polycyclic compounds. Raw materials with higher sulfur content tend to be naphthenic more often. The treatment of different raw gasoline streams may vary slightly depending on their composition due to the source of the feed.

Reformed naphtha or reformate generally requires no further treatment except perhaps distillation or solvent extraction to isolate a valuable aromatic product. Reform gasolines are essentially sulfur-free due to the severity of their pre-process and process pretreatment. Cracked gasoline, such as comes from catalytic cracking, has a relatively high octane number due to the olefinic and aromatic compounds contained therein. In some cases, this fraction along with a significant portion of the octane can make up as much as half of the gasoline in the refinery tank.

The material with the gasoline boiling range in the catalytically cracked product currently makes up a significant fraction (= 1/3) of the amount of gasoline produced in the United States and provides the largest portion of sulfur.

Sulfur contamination may need to be removed, usually by exposure to hydrogen, to meet product requirements or environmental regulations.

The most popular method of removing sulfur compounds is hydrodesulfurization (HDS), in which the petroleum distillate is passed over a special solid catalyst containing a hydrogenation metal deposited on an alumina substrate. Additionally, significant amounts of hydrogen are found in the feed. The following equations illustrate the reactions in a typical HDS unit.

(1) RSH + H2 RH + H2S (2) RCl + H2 RH + HCl (3) RN + 2H2 RH + NH3 (4) RCOOH + 2H2 RH + 2H2O

Typical operating conditions for HDS reactions are as follows:

Temperature, ° C 315-415 (600 - 780 ° F)

Pressure, kPas 4140-20700 (600-3000 psig)

H2 recycling rate, SCF units / barrel 1500 -3000

Addition of fresh H2, SCF units / barrel 700 -1000

The reaction with hydrogen of the organic sulfur compounds in a refined stream over the catalyst to form H 2 S is typically called hydrodesulfurization. Hydrogen interaction is a broader concept which also includes saturation of olefins and aromatics and the reaction of organic nitrogen compounds to form ammonia. However, hydrodesulfurization is included and is sometimes simply referred to as hydrogen treatment.

After the hydrogen interaction is over, the product can be fractionated or simply flashed to liberate hydrogen sulfide and collect the desulfurized gasoline.

In addition to providing a high octane blend of ingredients, cracked gasolines are often used as olefin sources and in other processes such as etherification. Conditions of influence

Hydrogen into the gasoline fraction to remove sulfur also saturates certain olefinic compounds in the gasoline fraction, lowering the octane number and causing a loss of the olefin source.

Various proposals have been made to remove sulfur leaving the more desirable olefins. Since the olefins in cracked gasoline are mainly in its lower boiling fraction and the sulfur-containing impurities tend to accumulate in the higher boiling fraction, the most popular solution has been to pre-fractionate prior to exposure to hydrogen. Pre-fractionation produces a gasoline with a lower boiling range which boils in the range of C5 to about 120 ° C (250 ° F) and a gasoline with a higher boiling range which boils in the range of about 120 to 246 ° C (250 - 475 ° F) .

The predominant light or lower boiling sulfur compounds are mercaptans, while the heavier or higher boiling compounds are thiophenes and other heterocycles. Separation by fractionation alone will not remove mercaptans. However, in the past, mercaptans have often been removed by the oxidative processes associated with lye washing. The combination of oxidative removal of mercaptans with subsequent fractionation and treatment with hydrogen on the heavier fraction is disclosed in US Patent No. 5,320,742. In the oxidative removal of mercaptans they are converted to the corresponding sulfides.

In addition to treating the lighter gasoline portion to remove the mercaptans, the lighter fraction has traditionally been used as a feed for catalytic reformer plants to increase the octane rating if necessary. The lighter fraction can also be further separated to separate valuable C5 olefins (amylenes) which are used in the preparation of the ethers. The simultaneous treatment and fractionation of petroleum products, including gasoline, particularly liquid catalytically cracked gasoline (FCC gasoline) is disclosed in US Pat. Nos. 5,110,568; 5,597,476; 5,779,883; 5 807 477 and 6 083 378.

For example, US Patent No. 5,510,568 discloses a full-boiling FCC gasoline treated with hydrogen in a split reactor, which contains a catalyst for the thioetherification of the lighter fraction. The light fraction mercaptans react with the lower diolefins (thioether) to form higher boiling sulfides which are removed as a residue with heavy (higher boiling) FCC gasoline.

It was found that the FCC light gasoline fraction split in the split reactor contains just mercaptans and a significant amount of thiophenes in addition to the light fraction. The mercaptans in this fraction can be removed by thioetherification. The total sulfur content of the thiophene fraction is relatively low and, more importantly, does not require the same severe treatment as heavy fraction sulfur compounds to convert thiophene to H2S, therefore it is less likely that the olefins in the thiophene fraction will be hydrogenated.

An advantage of the present invention is that sulfur can be removed from the light stream fraction to the heavier stream fraction without significant loss of olefins. Essentially all of the sulfur in the heavier fraction is converted to H 2 S by hydrodesulfurization and easily distilled from the hydrocarbons. Also, the sulfur in the middle fraction will also decrease.

Summary of the invention

Briefly, the present invention provides a method of removing sulfur from a full boiling range cracked liquid gasoline stream to achieve higher sulfur removal standards, by cleaving the light fraction of the stream and treating gasoline feed components in a manner that preserves olefins during the most favorable sulfur removal.

Preferably, the method comprises the steps of:

(a) separating the full boiling cracked gasoline stream into three fractions comprising a light cracked gasoline cut, preferably boiling in the range of C5 to about 65 ° C (150 ° F), an intermediate fraction of cracked gasoline preferably boiling in the range of about 65 ° C up to about 120 ° C (150-250 ° F), naphtha preferably boiling from about 120 ° C to about 230 ° C (250-450 ° F);

(b) subjecting the naphtha cracker to hydrodesulfurization in a first desulfurization reactor containing a hydrodesulfurization catalyst;

(c) combining the effluent from the first hydrodesulfurization reactor with intermediate cracked gasoline and subjecting the combined stream to hydrodesulfurization in a second hydrodesulfurization reactor.

The advantage of this system is the reduced size and capital costs of the hydrodesulfurization distillation column reactor. The level of recombinant mercaptans entering the distillation column reactor is lowered. Ultimately, an octane saving is possible due to a gentler treatment of the thiophene-rich olefin fraction.

PL 196 032 B1

Brief description of the drawings

Figure 1 is a simplified flow chart in one embodiment of the invention.

Figure 2 is a simplified flow chart of an alternative embodiment of the invention having pre-treatment by thioether verification.

Detailed Description of the Invention

The feed to the process comprises a sulfur-containing petroleum fraction which boils in the gasoline boiling range. These feedstocks include light gasoline having a boiling range of about C5 to 165 ° C (330 ° F) and full boiling range gasoline having a boiling range of C5 to 215 ° C (420 ° F). Generally, the method is useful for gasoline with the boiling range of catalytic cracking material as it contains the desired olefins and unwanted sulfur compounds. The crude gasolines have very little olefin material and very little sulfur unless the source of the feed is "acidic".

The sulfur content of the catalytically cracked fractions will depend on both the sulfur content of the cracking feed and the boiling range of the selected fraction used as process feed. Lighter fractions will have a lower sulfur content than higher boiling fractions. The starting gasoline fraction contains most of the high-octane olefins but relatively little sulfur. The sulfur constituents in the initial fraction are mainly mercaptans and typical compounds are: methyl mercaptan (methanethiol) (boiling point 6 ° C) (43 ° F), ethyl mercaptan (ethanethiol) (boiling point 37 ° C) (99 ° F) , n-propyl mercaptan (propanethiol-1) (boiling point 68 ° C) (154 ° F), iso-propyl mercaptan (propanethiol-2) (boiling point 57-60 ° C) (35 - 140 ° F), mercaptan isobutyl (2-methylpropanethiol-1) (boiling point 88 ° C) (190 ° F), tert-butyl mercaptan (boiling point 64 ° C) (147 ° F), n-butyl mercaptan (butanethiol-1) (boiling point 98 ° C) (208 ° F), sec-butyl mercaptan (boiling point 95 ° C) (203 ° F) and 3-mercaptan hexane (boiling point 57 ° C) (135 ° F). Typical sulfur compounds found in the heavier boiling fraction include heavier mercaptans, thiophene sulfides and disulfides.

The reaction of these mercaptans with the olefins contained in gasoline is called thioetherification and its products are higher boiling sulfides. A suitable catalyst for the reaction of diolefins with mercaptans is 0.4 wt% Pd on 7 to 14 mesh Al2O3 (Alumina) beads, supplied by Sϋd-Chemie designated G-68C-1. Typical physical and chemical properties of the catalyst as supplied by the manufacturer are as follows:

Tabe l a I.

Mark G-68C-1 Character balls Nominal Size 7x12 mesh % Pd by weight 0.4 ± 0.03 Carrier high purity alumina (99.0-99.5)

Another catalyst used for the mercaptan-diolefin reaction is Ni on silica / alumina in extrudate form, supplied by Sϋd-Chemie, denoted as C46-7-03RS. Typical physical and chemical properties of the catalyst as stated by the manufacturer are as follows:

Tabe l a II

Mark C46-7-03RS Character presswork Nominal Size 1/16 " wt% Ni 52 ± 4 Carrier silicon oxide / alumina

PL 196 032 B1

The rate of hydrogen feed to the reactor must be sufficient to sustain the reaction, but is considered an "effective amount of hydrogen" as this term is used here. The molar ratio of hydrogen to diolefins and acetylenes in the feed is at least 1.0 to 1.0 and preferably 2.0 to 1.0.

Another used method of removing mercaptans from light gasoline is the lye wet scrubbing process. In such a process, light gasoline is brought into contact with caustic soda. Mercaptans solubilize to the aqueous phase of the liquor. The mercaptans then react to form disulfides. The amount of mercaptan extracted is limited by the solubility of mercaptan in the caustic soda solution. The catalyst which is useful for the hydrodesulfurization reaction comprises Group VIII metals such as cobalt, nickel, palladium, alone or in combination with other metals such as molybdenum or tungsten, on a suitable support which may be alumina, silica-alumina, titanium-zirconium oxide or the like. Normally metals are provided as metal oxides deposited on extrudates or spheres and as such are generally not useful as distillation structures.

The catalysts contain components made of metals from groups V, VIB, VIII of the periodic table or their mixtures. The use of a distillation system reduces deactivation and provides longer operation than prior art fixed bed hydrogenation plants. The Group VIII metal provides increased overall average activity. Catalysts containing a Group VIB metal such as molybdenum and a Group VIII metal such as cobalt or nickel are preferred. Catalysts suitable for the hydrodesulfurization reaction include cobalt-molybdenum, nickel-molybdenum and nickel-tungsten. The metals generally exist as oxides deposited on a support such as alumina, silica-alumina, or the like. Metals are reduced to sulfides both during use and before use by exposure to streams containing sulfur compounds. The catalyst can also catalyze the hydrogenation of olefins and polyolefins contained in light cracked gasoline and, to a lesser extent, the isomerization of some olefins. Hydrogenation, especially of the mono-olefins in the lighter cut, may not be desirable.

Typical preferred conditions for the thioetherification reaction in a standard fixed bed reactor include temperatures ranging from 75 to 205 ° C (170 - 400 ° F), pressures from 1000 to 2000 kPas (145 - 290 psia), and an hourly volumetric velocity of 1 to 10 volumes of liquid. gasoline / catalyst volume / hour.

The properties of a typical hydrodesulfurization catalyst are shown in Table III below.

Tabe la III

Manufacturer Criterion Catalyst Co. Mark C-448 Character three-plane pressings Nominal Size diameter 1.2 mm Metal, weight percentages Cobalt 2 - 5% Molybdenum 5 - 20%

Typically the catalyst is in the form of extruded particles with a diameter of 1/8, 1/16 or 1/32 inch and an L / D ratio of 1.5 to 10. The catalyst may also be in the form of spheres having the same diameters. They can be directly loaded into standard fixed bed reactors that have support structures and reagent separation. The sulfur removal reaction conditions in a standard fixed bed reactor only are in the range 260-370 ° C (500-700 ° F) at pressures between 2760-6900 kPas (400-1000 psig).

The residence times defined as liquid space velocity per hour are typically between 1.0 and 10.0. The gasoline in simple fixed bed reaction can be in the gaseous or liquid phase depending on temperature and pressure with the total pressure and the amount of hydrogen gas supplied adjusted to a hydrogen partial pressure in the range 690-4830 kPas (100-700 psia.). The operation of hydrodesulfurization by simple passage through a solid bed is well known in the art.

Referring now to Figure 1, a simplified diagram of one embodiment of the invention is shown in schematic form. The gasoline feed enters the splitting reactor 10 via line 101. Light gasoline containing mainly the C5 fraction is withdrawn at the top.

Via line 102. Light gasoline also contains most mercaptans and some other organic sulfur compounds. The intermediate gasoline fraction, boiling in the range of C6 to about 150 ° C (300 ° F), is withdrawn through line 104 as a sidestream. The intermediate gasoline fraction mostly contains thiophenes along with a few mercaptans. Heavy gasoline boiling in the range of 150-230 ° C (300-450 ° F) is received as a distillation residue through line 106. Heavy gasoline may contain some thiophene but essentially contains less boiling organic sulfur compounds, which for better definition are other organic sulfur compounds .

The light gasoline in line 102 is scrubbed with aqueous lye to remove mercaptans in reactor 20 and withdrawn as a product through line 103 for use primarily as a feedstock for the tert-amyl methyl ether process. The distillation residue in line 106 is hydrodesulfurized in reactor 40 with hydrogen added to the process through line 107. In reactor 40, substantially all of the thiophene and most of the other organic sulfur compounds are converted to hydrogen sulfide, which can be easily removed by flashing or distillation. The effluent from reactor 40 is combined with the gasoline intermediate in line 104 and sent to a second hydrodesulfurization reactor 30 where hydrogen is added through line 105 to complete the reaction. Basically the thiophene in the gasoline intermediate and the remaining organic sulfur compounds in the naphtha are converted to hydrogen sulfide. The combined gasoline products are withdrawn from reactor 30 via line 109.

Referring to Figure 2, a second embodiment of the invention is shown therein. All of the gasoline feed is fed through line 101 to the reactor 20, where the diolefins in the gasoline react with the mercaptans to form sulfides. The effluent from reactor 20 is fed via line 102 to gasoline splitting reactor 10 where it is split into three fractions. Light gasoline containing mainly C5 hydrocarbons is withdrawn at the top through line 103. Since the mercaptans were removed in the thioetherification reactor, the light gasoline contains very little organic sulfur. The intermediate gasoline fraction boiling in the range of C6 to about 150 ° C (300 ° F) is withdrawn through line 104 as a side stream. The intermediate gasoline fraction mostly contains thiophenes along with some mercaptans. Heavy gasoline boiling in the range of 150 to 230 ° C (300-450 ° F) is received as a distillation residue through line 106. Heavy gasoline may contain some thiophene, but essentially contains higher boiling organic sulfur compounds, which are better termed other organic sulfur compounds.

The distillation residue from line 106 is hydrodesulfurized in reactor 40 with hydrogen added to the process through line 107. In the reactor, substantially all of the thiophene and most of the other organic sulfur compounds are converted to hydrogen sulfide, which can be easily removed by flashing or distillation. The effluent from reactor 40 is combined with the gasoline intermediate in line 104 and fed to the second hydrosulfurization reactor 30, where hydrogen is added through line 105 to complete the reaction. Basically the thiophene in the gasoline intermediate and the remaining organic sulfur compounds in the naphtha are converted to hydrogen sulfide. The combined gasoline products are withdrawn from reactor 30 via line 109.

Claims (2)

A method for reducing the content of organic sulfur compounds in the full boiling range cracked gasoline fraction containing olefins, diolefins, mercaptans, thiophenes and other organic sulfur compounds, characterized by the following steps: (a) separating the full boiling cracked naphtha fraction into three fractions comprising the light cracked naphtha fraction, the intermediate cracked naphtha and naphtha; (b) subjecting the naphtha cracker to hydrodesulfurization in a first hydrodesulfurization reactor containing a hydrodesulfurization catalyst; and (c) combining the effluent from the first hydrodesulfurization reactor with intermediate cracked gasoline and subjecting the combined fraction to hydrodesulfurization in a second hydrodesulfurization reactor. 2. The method according to p. The process of claim 1, wherein the light cracked naphtha contains substantially all of the mercaptans and is subjected to a wet lye washing process while more mercaptans are converted to sulfides, and then the sulfides are removed.