KR101209159B1 - Process to make a sulphur containing steam cracker feedstock - Google Patents

Abstract

Process for producing a sulfur containing steam cracking feed from crude natural gas by performing the following steps:

(a) separating the sulfur containing liquid field condensate fraction from natural gas;

(b) separating the liquid field condensate into a high boiling fraction and a low boiling fraction;

(c) preparing a mixture of carbon monoxide and hydrogen from the gas obtained in step (a);

(d) preparing a paraffinic product by conducting a Fischer-Tropsch reaction with carbon monoxide and hydrogen from step (c), and

(e) combining the paraffinic product or a portion of the product with the low boiling fraction obtained in step (b) to obtain a steam cracking feed containing 50 to 1000 ppm sulfur.

Steam cracking feeds, natural gas, Fischer-Tropsch reactions, paraffin products

Description

PROCESS TO MAKE A SULPHUR CONTAINING STEAM CRACKER FEEDSTOCK}

1 shows a method according to the invention.

Field condensate 12 is separated in slug catcher 1 from natural gas 11 containing sulfides. The field condensate 12 is separated in the separator 2 into a low boiling fraction 13 and a high boiling fraction 14. Fraction 14 is subjected to selective condensate purification. Sulfur is removed in the sulfinol unit 4 from natural gas 15 lacking condensate. The C ₃ + hydrocarbons 17 are separated from the sulfur deficient gas 16 in the separator unit 5. The hydrocarbon 17 is divided by the distillation unit 6 into a C ₃ and C ₄ vapor 19 and a vapor 18 comprising mainly C ₅ + hydrocarbons.

Purified gas 20 is used in syngas production unit 7 to obtain syngas 21. This synthesis gas 21 is used as feed in the Fischer-Tropsch reaction step 8 in which the paraffin product 22 is obtained. This product is separated into several fractions lower than the boiling point of the naphtha fraction 24 (fraction 23) or higher (fractions 25 and 26). This naphtha fraction 24 is mixed with fractions 13 and 18 in mixing unit 10 to obtain a steam cracking feed 27 according to the invention.

The present invention provides a process for preparing a sulfur cracking feed containing sulfur from natural gas and a sulfur containing feed from another source by defecting the paraffin feed obtained by the Fischer-Tropsch process. It is about how to.

It is well known to those skilled in the art to add sulfur to the feed to reduce coke formation in the art of steam cracking processes. Coke formation in steam cracker furnace tubes is formed mainly through two mechanisms: pyrolysis and catalytic dehydrogenation. The pyrolysis reaction uses low working pressures, with high vapor-to-hydrocarbon ratios of between 30% and 60%, to avoid the possibility of introducing excess tube metal temperatures and un-evaporated raw materials into the high temperature zone. Is minimized by avoiding it. In order to maintain high tube metal temperatures up to 2100 ° F., alloys containing high contents of nickel and chromium, such as 35% nickel and 25% chromium, are used. These metals require passivated because they promote the dehydrogenation reaction. Sulfurization is a typical method for passivation. In a steam cracking device, it is a common example to inject coke inhibitors immediately after the coke removal operation. This is when the catalytic activity of the tube wall metal is at its best. Before the hydrocarbon feed is injected into the steam cracking furnace, DMS or DMDS is injected into the coil with steam to form a sulfide film. In typical liquid feeds, the sulfur content in the feed is usually sufficient to maintain the sulfide levels during the actual steam cracking process. Sulfur may be added if it is not already sufficient in the feed. In this case, approximately 20 ppm to 100 ppm sulfur is added to minimize coke and CO production.

It is also well known to use the naphtha paraffin product obtained in the Fischer-Tropsch process as a steam cracking feed. See, for example, "The Markets for Shell Middle Distillate Synthesis Products", Presentation of Peter J.A. In Tijm, Shell International Gas Ltd., Alternative Energy '95, Vancouver, Canada, May 2-4, 1995 on page 5, the steam cracking of SMDS naphtha, the Fischer-Tropsch derived naphtha fraction of the Shell MDS process It is mentioned that it is used, for example, in Singapore as a feed.

Since the Fischer-Tropsch derived product contains almost undetectable levels of sulfur, it will have to be added to this feed before it can be used as a steam cracking feed. This can be achieved by adding sulfur additives such as dimethyl disulfide (DMDS) or by using Fischer-Tropsch raw materials and 'Preliminary Survey on GTL Business Based on SMDS technology', June 2001, Japan External Trade Organization (JETRO), section 6.2.3. This can be done by combining a high sulfur content material as described in detail in.

The addition of sulfur to Fischer-Tropsch derived raw materials in the steam cracking process is also described in more recent patent document US-A-2003 / 0135077. The document describes the preparation of low sulfur Fischer-Tropsch derived naphtha at remote locations, which moves the product to a steam cracking site and transfers sulfur compounds to the naphtha before the naphtha is used as a steam cracking feedstock. It is a manufacturing method to add.

WO-A-9937736 discloses a process for reducing sulfur levels by hydrating field condensate separated from natural gas. According to this document, sulfur needs to be removed for use as fuel or in combination with other hydrocarbons and as raw material for chemical processes.

A disadvantage of the process according to WO-A-9937736 is the need for additional hydration units.

It is an object of the present invention to more efficiently use high content sulfur containing field condensate separated from natural gas, eliminating the need for an additional hydrogenation process.

This object is achieved by the following method.

The process for producing a steam cracking feed comprising sulfur from natural gas is accomplished by performing the following steps.

(a) separating sulfur-containing liquid field condensate fractions from natural gas;

(b) separating the liquid field condensate into a high boiling fraction and a low boiling fraction;

(c) preparing a mixture of carbon monoxide and hydrogen from the gas obtained in step (a);

(d) preparing a paraffinic product by conducting a Fischer-Tropsch reaction with carbon monoxide and hydrogen from step (c), and

(e) combining the paraffinic product or a portion of the product with the low boiling fraction obtained in step (b) to obtain a steam cracking feed containing 50 to 1000 ppm sulfur.

Applicants have found that in the process according to the invention a portion of the field gas condensate is used to upgrade the Fischer-Tropsch product after a simple separation step, for example a distillation step, to produce a very good steam cracking feed that meets the specifications for direct use. Formation was found. Applicants have discovered that such steam can affect the volume of the steam cracking feed and its sulfur content, thus making it possible to produce a good, directly usable steam cracking feedstock suitable for the above-mentioned specification. This saves tank equipment in the manufacturing field of the Fischer-Tropsch process and can deliver one product to an overseas customer instead of two separate products.

Step (a) can be carried out by well known processes. The crude natural gas may comprise at least 0.5 volume percent sulfur. Natural gas is usually obtained from subterrian wells. Gas hydrocarbons of 1 to 5 carbon atoms are typically separated from C ₅ ⁺ hydrocarbons after the gas has reached the surface, for example the so-called slug catcher. The liquid product thus obtained is also referred to as field condensate and has very high sulfur levels, for example sulfur levels above 1500 ppm and above 1900 ppm. Typical lower sulfur levels are 1000 ppm. Natural gas from which the field condensate has been separated will be referred to as prepurified natural gas.

The liquid condensate obtained in step (a) may have a very high final boiling point above 300 ° C. while most of the product boils simultaneously below 300 ° C. The high boiling fraction and the low boiling fraction comprising sulfur compounds, for example, by flash or distillation steps, are adapted to the boiling range suitable for use as a steam cracking feed. This fraction does not require further desulfurization treatment to be used as a blending element to obtain a mixture useful as a steam cracking feed in step (e) in the production of a steam cracking feed.

This low boiling fraction, preferably isolated from field condensate, has a final boiling point of 120 to 230 ° C., more preferably 140 to 215 ° C. The initial boiling point is consistent with the C ₅ hydrocarbons still present in the liquid field condensate. The final boiling point of the low boiling fraction may vary depending on the sulfur content of the field condensate, the low boiling fraction containing sulfur and the ratio of the Fischer-Tropsch product in step (e) and the desired sulfur level of the resulting mixture. The optimal working environment can be determined using conventional techniques. The higher boiling fraction obtained in step (b), enriched with sulfur compounds and organic acids, may be further subjected to the so-called topical condensate purification. The low boiling fraction preferably comprises 200 to 6000 ppm sulfur.

A plurality of sulfur is separated from the preliminary purified natural gas obtained in step (a) by several possible techniques before carrying out step (c). Preferably sulfur is removed from natural gas by a so-called absorbent type process. Such well known methods include contacting natural gas with a liquid mixture comprising physical and chemical absorbents. In this method the gas mixture is treated in two successive stages of two different liquid mixtures, including a physical absorbent and a chemical absorbent at superatmospheric pressure. Typical chemical absorbents include tertiary amines, especially tertiary amines comprising one or more hydroxyalkyl groups. Triethanol amine, diethyl ethanol amine (DEMEA), and methyl diethanol amine (MDEA) are very suitable. Typical physical absorbers are propylene carbonate N-methyl pyrrolidine, dimethyl formamide, dimethyl ether of polyethylene glycol, tetraethylene glycol dimethyl ether and sulfolane. Examples of such processes are described in detail in US-A-4372925 and in texts such as Gas Purification, 5th ed./Arthur Kohl and Richard Nielsen, 1997, Gulf Publishing Company, ISBN 0-88415-220-0.

Preferably any hydrocarbon compounds having at least 3 carbon atoms are separated from the natural gas prior to the conversion of the gas to the synthesis gas. Preferably the separation is carried out by cooling the gas to temperature and pressure conditions at which these hydrocarbons can liquefy. The liquid product can then be easily separated from the gas. Cooling may preferably be carried out by indirect heat exchange with liquid nitrogen. The liquid nitrogen may preferably be excess nitrogen obtained when the air is separated to obtain purified oxygen for use in step (c). A more preferable method may be to lower the pressure, or to remove the ₃ ⁺ hydrocarbons condensed, or C, optionally increasing the pressure before the production of synthesis gas using a gas. Preferably the gas pressure is lowered to a pressure below 40 bar, preferably below 30 bar, at a pressure above 50 bar. After separation of the liquid hydrocarbons, the pressure of the gas increases above 50 bar, preferably 50 to 80 bar. The C ₃ and C ₄ hydrocarbons thus obtained can be combined with the LPG product isolated from the product prepared in step (d). C ₅ ⁺ hydrocarbons are preferably combined with the fraction referred to in the transmitted (e) to step (e) step to give a good steam cracking feed.

The synthesis gas used in the Fischer-Tropsch reaction is prepared in step (c) from the gas obtained in step (a) by partial oxidation and / or vapor / methane reforming.

To control the H ₂ / CO ratio in the synthesis gas, carbon dioxide and / or vapor can be injected into the partial oxidation process. The H ₂ / CO ratio of the synthesis gas is suitably 1.3 to 2.3, preferably 1.6 to 2.1. If desired, (lower) additional amounts of hydrogen can be prepared by steam methane reorganization, preferably in combination with a water gas shift reaction. The added hydrogen can also be used for other processes, for example hydrocracking.

In another embodiment the H ₂ / CO ratio of the synthesis gas obtained in the catalytic oxidation step can be reduced by removing hydrogen from the synthesis gas. This can be done by conventional techniques such as pressure swing adsorption or low temperature methods. A preferred choice is separation based on membrane technology. Some of the hydrogen can be used in the hydrocracking step of the heaviest hydrocarbon fraction, especially in the Fischer-Tropsch reaction.

The synthesis gas obtained by the method as described above usually has a temperature of 900 to 1400 ° C., and is cooled to a temperature of 100 to 500 ° C., suitably 150 to 450 ° C., preferably 300 to 400 ° C., It is preferably cooled under simultaneous generation of power, for example in the form of steam. Also cooling to 40 to 130 ° C., preferably 50 to 100 ° C., is carried out in a conventional heat exchanger, in particular in a tubular heat exchanger. In order to remove impurities from the synthesis gas, a guard bed can be used. In particular, specific catalysts can be used to remove all traces of HCN and / or NH ₃ . Trace amounts of sulfur that may still be present in the gas used in step (c) can be removed by an adsorption process using iron and / or zinc oxide.

The purified gas mixture, which mainly comprises hydrogen, carbon monoxide and optionally nitrogen, is contacted with a suitable catalyst, usually in the catalytic conversion stage in which liquid hydrocarbons are formed.

Catalysts used for the catalytic conversion of a mixture comprising hydrogen and carbon monoxide to hydrocarbons in step (d) are known in the art and are commonly referred to as Fischer-Tropsch catalysts. Catalysts used in this process often include metals of Group VIII of the Periodic Table of Elements as catalytically active component. Particularly catalytically active metals include ruthenium, iron, cobalt and nickel. Cobalt is the preferred catalytically active metal in view of the heavy Fischer-Tropsch hydrocarbons that can be produced. As previously described, preferred hydrocarbonaceous raw materials are natural gas or associated gas. This feed usually results in synthesis gas having a H ₂ / CO ratio of about 2, and cobalt is a very good Fischer-Tropsch catalyst with a user ratio of this type of catalyst is also about 2.

The catalytically active metal is preferably supported on a porous carrier. The porous carrier can be selected from any suitable refractory metal oxide or silicate or combinations thereof known in the art. Specific examples of preferred porous carriers include silica, alumina, titania, zirconia, ceria, gaul and mixtures thereof, in particular silica, alumina and titania.

The content of catalytically active metal on the carrier is preferably in the range of 3 to 300 kWpbw, more preferably 10 to 80 kWpbw, in particular 20 to 60 kWpbw per 100 kW pbw of carrier material.

If necessary, the catalyst also includes one or more metals or metal oxides as promoters. Suitable metal oxide promoters can be selected from Groups IIA, IIIB, IVB, VB and VIB, or Actinide and Lanthanide series of the Periodic Table of Elements. In particular, oxides of magnesium, calcium, strontium, barium, scandium, yttrium, lanthanum, cerium, titanium, zirconium, hafnium, thorium, uranium, vanadium, chromium and manganese are very suitable promoters. Particularly preferred metal oxide promoters as catalysts used for wax production in the present invention are manganese and zirconium oxides. Suitable metal promoters may be selected from Group VIIB or Group VIII of the Periodic Table of Elements. Particularly suitable are the precious metals of rhenium and group VIII, with platinum and palladium particularly preferred. The content of the promoter present in the catalyst is suitably 0.01 to 100 mW pbw, preferably 0.1 to 40 MW, more preferably 1 to 20 MW pbw per 100 mW pbw of the carrier. Most preferred promoters are selected from vanadium, manganese, rhenium, zirconium and platinum.

Catalytically active metals and promoters, if present, can be dipped onto the carrier material by any suitable treatment, such as impregnation, kneading and extrusion. After immersion on the carrier material of the metal and, if appropriate, the promoter, the loaded carrier is generally calcinated. The effect of the plasticization treatment is to remove the crystallized water, to decompose the volatile decomposition products and to convert the organic and inorganic compounds into their respective oxides. After calcining, the resulting catalyst is activated by contacting the catalyst with hydrogen or a hydrogen-containing gas, usually at a temperature of about 200-350 Pa. Other processes of Fischer-Tropsch catalyst preparation include kneading / mulling, drying / baking and activation, often involving extrusion.

The catalytic conversion process can be carried out under conventional synthetic conditions known in the art. Usually, catalytic conversion can be effective in the temperature range of 150 to 300 ° C, preferably 180 to 260 ° C. Typical total pressures for the catalytic conversion process are 1 to 200 bar absolute, more preferably 10 to 70 bar absolute. In the catalytic conversion process in particular 75% by weight of C ₅ +, preferably at least 85% by weight of C ₅ + hydrocarbons are formed. Depending on the catalyst and conversion conditions, the content of heavy wax (C ₂₀ +) can be up to 60% by weight, sometimes up to 70% by weight, and sometimes up to 85% by weight. Preferably cobalt catalysts are used, low H ₂ / CO ratios (especially 1.7, or even lower) are used, low temperatures are used (190-240 ° C.), which are optionally used with high pressures. In order to avoid coke formation, it is preferable to use a H ₂ / CO ratio of 0.3 or more. The ASF-alpha value (Anderson-Schulz-Flory chain growth factor) for the obtained product having 20 or more carbon atoms is 0.925 or more, preferably 0.935 or more, more preferably 0.945 or more, particularly more preferably 0.955 or more. Preference is given to carrying out the Fischer-Tropsch reaction under conditions. Preferably a Fischer-Tropsch hydrocarbon vapor is more than 40% by weight, preferably 50% by weight, and more preferably comprises a C ₃₀ + 55% by weight, C ₆₀ + / C weight ratio of ₃₀ + 0.35 or more, Preferably it is 0.45, More preferably, it is 0.55.

Preferably, a Fischer-Tropsch catalyst is used to obtain a significant amount of paraffin, more preferably a substantial unbranched paraffin. The most suitable catalyst for this purpose is a cobalt-containing Fischer-Tropsch catalyst. Such catalysts are described in the literature. See for example AU # 698392 and WO 99/34917.

The Fischer-Tropsch process may be a slurry FT process or a fixed bed FT process, in particular a multiple associative fixed bed.

The paraffinic product can be isolated from the product obtained in step (d) by direct distillation. Preferably such fractions are first hydrogenated to remove olefins and oxygenated compounds which may negatively affect the physical properties of the product mixture used as the steam cracking feed. The paraffin product may also be isolated from the waste of the hydrocracking step carried out on all or part of the product obtained in step (d). Such paraffinic products contain higher amounts of iso-paraffins. Preferably the iso to normal ratio of paraffins in the paraffin product is less than 0.5, preferably less than 0.3.

The paraffin product preferably comprises at least 90% by weight of C _{5 relative} to the compound boiling at 370 ° C. More preferably the paraffinic product is boiled in the naphtha range, preferably starting from C ₅ Up to 90% by weight of the compound and boiling compounds at 204 ° C.

The paraffin product is combined in the liquid condensate fraction obtained in step (b) in step (e). The weight ratio of step (d) to product of step (b) is preferably from 30: 1 to 5: 1. The resulting mixture preferably has the following physical properties:

Specific gravity of 0.65 to 0.74,

Color Saybolt +20 or more,

Less than 1 vol% of unsaturated matter,

Paraffin content of 65% to 100 vol%,

Iso-normal paraffin ratio of less than 0.5,

Sulfur content of 50 to 650 ppm, preferably 100 to 650 ppm,

A final boiling point of 150 to 204 ° C., and

Reid Vapor Pressure below 13 psi at 37.8 ℃.

The invention also relates to the use of the mixture obtained by the process according to the invention as a steam cracking feed for the production of lower olefins, in particular propylene and / or ethylene.

As described above, the present invention can produce excellent steam cracking raw material, which saves the manufacturing equipment of the Fischer-Tropsch process, and has the advantage of transporting two products into one during manufacture.

Claims (9)

Process for producing a sulfur containing steam cracking feed from crude natural gas by performing the following steps: (a) separating the sulfur containing liquid field condensate fraction from natural gas; (b) separating the liquid field condensate into a high boiling fraction and a low boiling fraction; (c) preparing a mixture of carbon monoxide and hydrogen from the gas obtained in step (a); (d) preparing a paraffinic product by conducting a Fischer-Tropsch reaction with carbon monoxide and hydrogen from step (c), and (e) combining the paraffinic product or a portion of the product with the low boiling fraction obtained in step (b) to obtain a steam cracking feed containing 50 to 1000 ppm sulfur. The process of claim 1 wherein the low boiling fraction obtained in step (b) has a sulfur content of 200 to 6000 ppm. The process of claim 1 or 2, wherein the crude natural gas comprises at least 0.5% by volume sulfur. The method of claim 1 or 2, wherein the iso to normal ratio of paraffins in the paraffin product is less than 0.5. The process according to claim 1 or 2, wherein the paraffinic product comprises at least 90% by weight C _{5 relative} to the compound boiling at 370 ° C. 6. The process of claim 5 wherein the paraffinic product is a boiling fraction of the paraffinic product obtained in step (c) of boiling in the naphtha range. The process according to claim 1 or 2, wherein the sulfur content of the steam cracking feed is greater than 50 ppm. The process of claim 1 or 2, wherein the steam cracking feed has the following physical properties: Specific gravity of 0.65 to 0.74, Color Saybolt +20 or more, Less than 1 vol% of unsaturated matter, Paraffin content of 65% to 100 vol%, Iso-normal paraffin ratio of less than 0.5, Sulfur content of 50 to 650 ppm A final boiling point of 150 to 204 캜 and Reid Vapor Pressure below 13 psi at 37.8 ℃. The process according to claim 1 or 2, wherein the mixture obtained by the process is used as a steam cracking feed for producing lower olefins.

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