PROCESS TO MAKE A SULPHUR CONTAINING HYDROCARBON PRODUCT

The invention is directed to a process to make a sulphur containing hydrocarbon product from crude natural gas .

It is well known to the skilled person in the technical field of steam cracker processes to add sulphur to the feed in order to reduce coke formation. The coke formation in steam cracker furnace tubes is principally formed through two mechanisms, pyrolysis reactions and catalytic dehydrogenation reactions. The pyrolysis reactions are minimized by using low operating pressure, having high steam-to-hydrocarbon ratios, on the order of 30% to 60%, and avoiding excessive tube metal temperatures and the possibility of allowing unvaporized feed into the high temperature zones. In order to withstand high tube metal temperatures, up to 2100 ⁰ F, alloys containing high nickel and chromium are used, for instance 35% nickel and 25% chrome. These metals catalyze dehydrogenation reactions and therefore need to be passivated. Sulphiding is the typical solution for passivation. It is a common practice for steam cracker operators to inject coke inhibitors right after the decoking operation. That is when the tube wall metal catalyzing activities are highest. Before the hydrocarbon feedstock is fed into the furnace, DMS or DMDS is injected into the coil with steam to form a sulphide film. For a typical liquid feedstock, the sulphur content in the feedstock usually is sufficient to maintain the sulphide level during the actual steam cracking process. Sulphur may be added to the feed if there is not already enough sulphur in the feed. In this case, perhaps 20 ppmw

to 100 ppmw of sulphur is added to minimize the coke and CO production.

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

Because a Fischer-Tropsch derived product contains almost no detectable levels of sulphur, sulphur will have to be added to this feedstock before it can be used as steam cracker feedstock. This may be done by adding a sulphur additive like dimethyl disulfide (DMDS) or by blending the Fischer-Tropsch feed with a high sulphur content material as described in more detail in ^λ Preliminary Survey on GTL Business Based on SMDS technology, June 2001, Japan External Trade Organization (JETRO), section 6.2.3.

Adding sulphur to a Fischer-Tropsch derived feed in a steam cracker process is also described in a more recent patent publication US-A-2003/0135077. This publication describes the production of a low sulphur Fischer-Tropsch derived naphtha at a remote location, transporting this product to a steam cracker location and adding a sulphur compound to the naphtha before using said naphtha as steam cracker feed.

WO-A-9937736 discloses a process wherein a field condensate as separated from raw natural gas is hydrotreated to reduce the sulphur level. According to

this publication sulphur needs to be removed in order to use it as fuels or for blending with other hydrocarbons and as raw materials for chemical processes.

JP-A-2005/163046 discloses a process to make a sulphur containing steam cracker feedstock from crude natural gas by performing the following steps:

(a) separating a sulphur containing liquid field condensate fraction from the natural gas,

(b) further separating a sulphur containing fraction comprising hydrocarbon compounds having 3 or more carbon atoms from the natural gas to obtain a plant condensate,

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

(d) preparing a paraffin product by performing a Fischer- Tropsch reaction using the carbon monoxide and hydrogen from step (c) , and

(e) combining the paraffin product or part of said product with the sulphur containing fraction comprising hydrocarbon compounds having 3 or more carbon atoms obtained in step (b) to obtain the sulphur containing steam cracker feedstock having a sulphur content of between 50 and 1000 ppm.

The resulting steam cracker feedstock has the disadvantage that it still contains toxic mercaptans, which makes the feedstock smelly, and requires suitable installations to prevent leakage during handling, while the feedstock itself can only be transported under the necessary safety precautions.

The object of the present invention is to provide a process to prepare a sulphur containing hydrocarbon product from crude natural gas in a more efficient manner as in the prior art processes.

This object is achieved by the following process. Process to make a sulphur containing hydrocarbon product from crude natural gas by performing the following steps:

(a) separating a sulphur containing liquid field condensate fraction from the natural gas,

(b) further separating a sulphur containing fraction comprising hydrocarbon compounds having 3 or more carbon atoms from the natural gas to obtain a plant condensate,

(c) oxidizing the mercaptans compounds as present in the plant condensate fraction to disulphide compounds in the presence of an oxidation catalyst in an oxidation zone,

(d) preparing a mixture of carbon monoxide and hydrogen from the gas obtained in step (b) ,

(e) preparing a paraffin product by performing a Fischer- Tropsch reaction using the carbon monoxide and hydrogen from step (d) , and

(f) combining the paraffin product or part of said product with the disulphide containing fraction comprising hydrocarbon compounds having 3 or more carbon atoms obtained in step (c) or a fraction thereof to obtain the sulphur containing hydrocarbon product.

Applicants have found that with the process according to the invention a disulphide compound containing fraction of C3+ compounds as separated from the natural gas can be advantageously be combined with the paraffin product as obtained in the Fischer-Tropsch process. Applicants found that this stream may contribute to the volume of paraffin product comprising sulphur. This product is found to be an excellent feed for a steam cracker to prepare ethylene and propylene among other products. A further advantage is that the level of mercaptans as present in the liquid plant condensate are

reduced resulting in a less smelly and safer product, which can also be handled easier.

Step (a) may be performed according to well-known processes. Typically the natural gas is won from subterrain wells. The gaseous hydrocarbons of 1 to 5 carbon atoms are typically separated from the C5+ hydrocarbons after the gas reaches the surfaces, for example in so-called slug catchers. The liquid product so obtained is also referred to as field condensate and may have very high sulphur levels, for example above

1500 ppmm and even above 1900 ppmm. The typical lower level is 1000 ppmm sulphur. The natural gas wherein field condensate is separated will be referred to as pre- cleaned natural gas . This liquid condensate as obtained in step (a) may have a very high final boiling point, even above 300 ⁰ C, while at the same time the majority of the products boils below 300 ⁰ C.

Prior to performing step (b) the majority of the sulphur is preferably separated from the pre-cleaned natural gas as obtained in step (a) by various possible techniques. Preferably sulphur is removed from the natural gas by so-called absorbent type processes. These well known processes include a step wherein the natural gas is contacted with a liquid mixture which contains a physical and a chemical absorbent. In such a process the gas mixture is treated in two steps consecutively with two different liquid mixtures which contain a physical absorbent and a chemical absorbent. Typical chemical absorbents are tertiary amines, particularly tertiary amines which contain at least one hydroxyalkyl group. Triethanol amine, diethyl ethanol amine (DEMEA) , and methyl diethanol amine (MDEA) are very suitable. Typical

physical absorbents are propylene carbonate N-methyl pyrrolidone, dimethyl formamide, dimethyl ether of polyethylene glycol, tetraethyleneglycoldimethylether and sulfolane are very suitable. Examples of such processes are described in detail in US-A-4372925 and in textbooks such as Gas Purification, 5th ed. /Arthur Kohl and Richard Nielsen, 1997, Gulf Publishing Company, ISBN 0-88415-220-0 Chapter 1.

In step (b) hydrocarbon compounds having 3 or more carbon atoms are separated from the natural gas prior to converting the gas to synthesis gas. Preferably separation is performed by cooling the gas to a temperature and pressure conditions at which these hydrocarbons liquefy. The liquid products may then the easily separated from the gas. Cooling may preferably be performed by indirect heat exchange against liquid nitrogen. Liquid nitrogen may preferably be the excess nitrogen as obtained when separating air to obtain purified oxygen for use in step (e) . A more preferred method is by lowering the pressure, separating the condensed C3+ hydrocarbons, and optionally increasing the pressure level before making use of the gas to prepare synthesis gas. Preferably the gas is lowered from a pressure exceeding 50 bars to a pressure below 40 bars, preferably below 30 bars. After separation of the liquid hydrocarbons, the pressure of the gas is increased to a level above 50 bars, preferably between 50 and 80 bars. All of the C3+ compounds thus obtained may be used as feed to step (c) . Optionally the C3 and C4 hydrocarbons are isolated and preferably blended with the LPG product as isolated from the product prepared in step (e) . The remaining C5+ compounds are preferably used as the plant condensate feed to step (c) .

In step (c) the mercaptans compounds as present in the liquid plant condensate fraction are converted to disulphide compounds in the presence of an oxidation catalyst and an alkaline solution in an oxidation zone. Such a process is sometimes referred to as a sweetening process because the content of corrosive and toxic SH functional group found in mercaptans is reduced when mercaptans are converted to disulphide compounds. The alkaline solution is preferably NaOH. The oxidation of the mercaptans to disulfides, can be done with a variety of oxidants, for example air, pure oxygen, enriched air, chemical oxidants such as hydrogen peroxide or mixtures thereof. Air is preferred because of its low cost and for safety reasons. The oxidation of mercaptans to disulfides is preferably performed in the presence of a suitable catalyst in order to improve the oxidation rate. Preferred catalysts are typically metals, and the most suitably lead, for example PbS, copper, for example as a copper chloride, or a phthalocyanine complex of copper, iron, nickel or cobalt, preferably cobalt. Such sweetening processes are well known for refinery streams. The oxidation zone is preferably a reactor vessel comprising a fixed bed of said catalyst to which the oxidant, the plant condensate feed and the alkaline solution is provided to. The alkaline solution can be suitably be separated from the plant condensate enriched in disulphide compounds by phase separation and recycled to the oxidation zone. Example of a commercially available and suitable process for the process according the present invention is UOP' s Merox^ ^m Process for Fixed- Bed Sweetening, as also described in US-A-5, 286, 372.

The synthesis gas to be used for the Fischer-Tropsch reaction is made in step (d) from the gas obtained in

step (b) by catalytic or non-catalytic partial oxidation, autothermal steam reforming, traditional steam reforming or convective steam reforming or combinations of said processes . To adjust the H2/CO ratio in the synthesis gas, carbon dioxide and/or steam may be introduced into the partial oxidation process. The H2/CO ratio of the synthesis gas is suitably between 1.3 and 2.3, preferably between 1.6 and 2.1. If desired, (small) additional amounts of hydrogen may be made by steam methane reforming, preferably in combination with the water gas shift reaction. The additional hydrogen may also be used in other processes, e.g. hydrocracking.

In another embodiment the H2/CO ratio of the synthesis gas obtained in the catalytic oxidation step may be decreased by removal of hydrogen from the synthesis gas. This can be done by conventional techniques as pressure swing adsorption or cryogenic processes. A preferred option is a separation based on membrane technology. Part of the hydrogen may be used in the hydrocracking step of especially the heaviest hydrocarbon fraction of the Fischer-Tropsch reaction.

The synthesis gas obtained in the way as described above, usually having a temperature between 900 and 1400 ⁰ C, is cooled to a temperature between 100 and 500 ⁰ C, suitably between 150 and 450 ⁰ C, preferably between 300 and 400 ⁰ C, preferably under the simultaneous generation of power, e.g. in the form of steam. Further cooling to temperatures between 40 and 130 ⁰ C, preferably between 50 and 100 ⁰ C, is done in a conventional heat exchanger, especially a tubular heat exchanger. To remove any impurities from the synthesis gas, a guard bed may be used. Especially to remove all traces of HCN and/or NH3

specific catalysts may be used. Trace amounts of sulphur which may still be present in the gas used in step (d) may be removed by an absorption process using iron and/or zinc oxide. The purified gaseous mixture, comprising predominantly hydrogen, carbon monoxide and optionally nitrogen, is contacted with a suitable catalyst in the catalytic conversion stage, in which the normally liquid hydrocarbons are formed. The catalysts used for the catalytic conversion of the mixture comprising hydrogen and carbon monoxide into hydrocarbons in step (e) are known in the art and are usually referred to as Fischer-Tropsch catalysts. Catalysts for use in this process frequently comprise, as the catalytically active component, a metal from

Group VIII of the Periodic Table of Elements. Particular catalytically active metals include ruthenium, iron, cobalt and nickel. Cobalt is a preferred catalytically active metal in view of the heavy Fischer-Tropsch hydrocarbon which can be made. The natural gas feedstock usually results in synthesis gas having H2/CO ratio's of about 2, cobalt is a very good Fischer-Tropsch catalyst as the user ratio for this type of catalysts is also about 2. The catalytically active metal is preferably supported on a porous carrier. The porous carrier may be selected from any of the suitable refractory metal oxides or silicates or combinations thereof known in the art. Particular examples of preferred porous carriers include silica, alumina, titania, zirconia, ceria, gallia and mixtures thereof, especially silica, alumina and titania.

The amount of catalytically active metal on the carrier is preferably in the range of from 3 to 300 pbw

per 100 pbw of carrier material, more preferably from 10 to 80 pbw, especially from 20 to 60 pbw.

If desired, the catalyst may also comprise one or more metals or metal oxides as promoters. Suitable metal oxide promoters may be selected from Groups HA, IHB,

IVB, VB and VIB of the Periodic Table of Elements, or the actinides and lanthanides. 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 for the catalyst used to prepare the waxes for use in the present invention are manganese and zirconium oxide. Suitable metal promoters may be selected from Groups VIIB or VIII of the Periodic Table. Rhenium and Group VIII noble metals are particularly suitable, with platinum and palladium being especially preferred. The amount of promoter present in the catalyst is suitably in the range of from 0.01 to 100 pbw, preferably 0.1 to 40, more preferably 1 to 20 pbw, per 100 pbw of carrier. The most preferred promoters are selected from vanadium, manganese, rhenium, zirconium and platinum.

The catalytically active metal and the promoter, if present, may be deposited on the carrier material by any suitable treatment, such as impregnation, kneading and extrusion. After deposition of the metal and, if appropriate, the promoter on the carrier material, the loaded carrier is typically subjected to calcination. The effect of the calcination treatment is to remove crystal water, to decompose volatile decomposition products and to convert organic and inorganic compounds to their respective oxides. After calcination, the resulting catalyst may be activated by contacting the catalyst with

hydrogen or a hydrogen-containing gas, typically at temperatures of about 200 to 350 ⁰ C. Other processes for the preparation of Fischer-Tropsch catalysts comprise kneading/mulling, often followed by extrusion, drying/calcination and activation.

The catalytic conversion process may be performed under conventional synthesis conditions known in the art. Typically, the catalytic conversion may be effected at a temperature in the range of from 150 to 300 ⁰ C, preferably from 180 to 260 ⁰ C. Typical total pressures for the catalytic conversion process are in the range of from 1 to 200 bar absolute, more preferably from 10 to 70 bar absolute. In the catalytic conversion process especially more than 75 wt% of C5+, preferably more than 85 wt% C5+ hydrocarbons are formed. Depending on the catalyst and the conversion conditions, the amount of heavy wax (C20+) ^ma Y be up to 60 wt%, sometimes up to 70 wt%, and sometimes even up till 85 wt%. Preferably a cobalt catalyst is used, a low H2/CO ratio is used (especially 1.7, or even lower) and a low temperature is used (190-240 ⁰ C), optionally in combination with a high pressure. To avoid any coke formation, it is preferred to use an H2/CO ratio of at least 0.3. It is especially preferred to carry out the Fischer-Tropsch reaction under such conditions that the ASF-alpha value (Anderson- Schulz-Flory chain growth factor) , for the obtained products having at least 20 carbon atoms, is at least 0.925, preferably at least 0.935, more preferably at least 0.945, even more preferably at least 0.955. Preferably the Fischer-Tropsch hydrocarbons stream comprises at least 40 wt% C30+, preferably 50 wt%, more

preferably 55 wt%, and the weight ratio C50+/C30+ is at least 0.35, preferably 0.45, more preferably 0.55.

Preferably, a Fischer-Tropsch catalyst is used, which yields substantial quantities of paraffins, more preferably substantially unbranched paraffins. A most suitable catalyst for this purpose is a cobalt-containing Fischer-Tropsch catalyst. Such catalysts are described in the literature, see e.g. WO-A-9934917.

The Fischer-Tropsch process may be a slurry Fischer- Tropsch process or a fixed bed Fischer-Tropsch process, especially a multitubular fixed bed.

The paraffin product may be isolated from the product as obtained in step (e) directly by means of distillation. Preferably such a fraction is first hydrogenated in order to remove any olefins and oxygenated compounds that could negatively influence the properties of the resulting blend for use as steam cracker feedstock. The paraffin product may also be isolated from the effluent of a hydrocracking/ hydroisomerisation step as performed on the entire or part of the product as obtained in step (e) . Such a paraffin product will comprise a larger content of iso- paraffins .

In step (f) a paraffin product as obtained in step (e) is used. A preferred paraffin product is a distillate fraction, preferably a subjected to the above- described hydrogenation and/or hydrocracking/ hydroisomerisation process, which comprises for more than 90 wt% of C5 to compounds boiling at 370 ⁰ C. Even more preferably the paraffin product boils in the naphtha range, preferably comprising for more than 90 wt% compounds starting from C5 and up to compounds boiling at 204 ⁰ C.

The paraffin product is combined in step (e) with the C3 plus fraction or the C5 plus fraction obtained in step (b) . The weight ratio of step (e) product to step (b) product is preferably between 30:1 and 5:1. The resulting blend preferably has the following properties:

Specific Gravity of between 0.65 to 0.74,

Colour Saybolt of more than +20,

Unsaturates < 1 vol%,

Paraffin content of between 65% and 100 vol.%. disulphide content as expressed as atomic sulphur of between 30 and 650 ppmw.

Final boiling point of between 150 and 204 ⁰ C, and a

Reid Vapour Pressure at 37.8 ⁰ C of not more than 13 psi. The invention is also directed to this new composition comprising the specifically made disulphide compounds .

The invention is also directed to the a process to prepare ethylene and propylene by subjecting the sulphur containing hydrocarbon product as obtained by the process according the present invention and a dilution gas to a thermal conversion step.

Examples of dilution gases are dilution steam (saturated steam at its dew point) , methane, ethane, nitrogen, hydrogen, natural gas, dry gas, refinery off gases, and a vaporized naphtha. Preferably, the dilution gas is dilution steam, carbon dioxide, hydrogen/carbon monoxide mixtures, also referred to as synthesis gas, a refinery off gas, a Fischer-Tropsch (gas-to-liquids) plant off-gas, more preferably a propane comprising off- gas, vaporized naphtha, or mixtures thereof.

The use of a gas-to-liquids plant off-gas, carbon dioxide or synthesis gas can be advantageously be used if the gas-to-liquids facility and the thermal conversion

step, e.g. the pyrolysis furnace (s) of said conversion step, are located close enough to benefit from a synergy. In such a situation even more preferably use is made of the excess synthesis gas, which is contaminated with carbon dioxide and/or methane as obtained after performing the Fischer-Tropsch synthesis reaction. In the cold box of the olefin process purified synthesis gas will then be advantageously obtained, which synthesis gas can be recycled to the Fischer-Tropsch synthesis step of such a combined process.

If carbon dioxide is used as dilution gas part of the carbon dioxide will be converted to carbon monoxide in the thermal conversion step. Carbon dioxide is preferably separated from the cracked effluent in the CO2 absorber located upstream of or integrated with the olefin process's cracked gas compressor. The carbon dioxide is preferably reused in said thermal conversion step. The carbon monoxide and hydrogen is preferably separated from the effluent of the thermal conversion step as a mixture of methane, carbon monoxide and hydrogen. This mixture may be advantageously recycled to the syngas manufacturing step (d) of the process to prepare the sulphur containing hydrocarbon product.

A thermal conversion step will comprise a step wherein the hydrocarbon feed is evaporated and a part wherein the evaporated hydrocarbon feed in the presence of a dilution gas is thermally converted at elevated temperature. The thermal conversion is preferably performed in a gas heated furnace provided with reactor tube in which the conversion, sometimes referred to as pyrolysis, takes place. Evaporation is suitably performed by indirect heat exchange against the flue gasses of the furnace. In the pyrolysis furnace the gaseous

hydrocarbons are thermally cracked to olefins and associated by products. Products of an olefins pyrolysis furnace include, but are not limited to, ethylene, propylene, butadiene, benzene, hydrogen, and methane, and other associated olefinic, paraffinic, and aromatic products. Ethylene is the predominant product, typically ranging from 15 to 40 wt%, based on the weight of the vaporized feedstock. The second important product is propylene. The pyrolysis furnace may be any type of conventional olefins pyrolysis furnace operated for production of lower molecular weight olefins, especially including a tubular gas cracking furnace. The tubes within the convection zone of the pyrolysis furnace may be arranged as a bank of tubes in parallel, or the tubes may be arranged for a single pass of the feedstock.

The temperature of the hydrocarbons in the thermal conversion step is preferably between 750 and 860 ⁰ C. This latter temperature is sometimes referred to as the coil outlet temperature. The temperature of this gas is quickly reduced to terminate any unwanted reactions to a temperature of below 300 ⁰ C. Examples of reducing the temperature are by means of well known transfer line exchangers and/or by means of a quench oil fitting. Preferably the temperature is reduced to below 440 ⁰ C by means of a transferline exchanger and further reduced to below 240 ⁰ C by means of a quench oil fitting. The product gas or cracked gas is further separated into the different products as listed above by well known and described processes known to the skilled person. Ethylene and propylene can be isolated from the effluent by means of distillation.

The present invention further relates to a steam cracker feed hydrocarbon product having the following

properties: (i) a specific gravity of between 0.65 to 0.74, (ii) a Colour Saybolt of more than +20, (iii) unsaturates less than 1 vol%, (iv) a paraffin content of between 65% and 100 vol.%, (v) a disulphide content as expressed as atomic sulphur of between 30 and 650 ppmw,

(vi) a final boiling point of between 150 and 204 ⁰ C, and (vii) a Reid Vapour Pressure at 37.8 ⁰ C of not more than 13 psi, wherein the hydrocarbon product is at least in part derived from a Fischer-Tropsch reaction. Preferably, in the steam cracker feedstock, the weight ratio of step (e) product to step (b) product is preferably between 30:1 and 5:1.