EP1226223A1

EP1226223A1 - Coking reduction in cracking reactors

Info

Publication number: EP1226223A1
Application number: EP00964312A
Authority: EP
Inventors: Francis Humblot; Claude Brun; Harry M. Woerde; Paul F. Van Den Oosterkamp
Original assignee: Technip SA; Atofina SA
Current assignee: Arkema France SA; Technip Energies France SAS
Priority date: 1999-09-24
Filing date: 2000-09-18
Publication date: 2002-07-31
Also published as: PL192646B1; PL354579A1; NO20021425L; FR2798939B1; TWI286569B; CZ294442B6; AU7527600A; CA2385372C; BR0014221A; AR025643A1; US7604730B1; RU2002110818A; WO2001021731A1; CN1399670A; KR20020068327A; KR100729188B1; MXPA02003075A; NO20021425D0; ZA200202939B; CN1263828C

Abstract

The invention concerns a method for reducing coking on the metal walls of a reactor cracking hydrocarbons or other organic compounds and on the metal walls of a heat exchanger placed downstream of the cracking reactor, whereby the metal surfaces coming into contact with the organic substance to be cracked are pre-treated with a water vapour stream containing at least silicon and at least a sulphur compound.

Description

REDUCING COKAGE IN CRACKING REACTORS

The present invention relates to the field of cracking of hydrocarbons or other organic compounds and more particularly relates to a process for reducing coking on the walls of cracking reactors and heat exchangers used to cool the compounds resulting from the reaction of cracking.

In order to produce ethylene and other short olefins, certain petroleum fractions of hydrocarbons are thermally cracked in metal tube reactors. The resulting cracking gases are suddenly quenched in heat exchangers operating by adding water and steam under pressure.

The tubular reactors used are preferably made of steels rich in chromium and nickel while the heat exchangers, subjected to less severe constraints, are made of carbon steels. This same type of apparatus also meets to produce other organic compounds such as vinyl chloride by pyrolysis of 1, 2-dichloroethane.

The efficiency of these steel reactors and heat exchangers depends on their resistance to the formation of a coke deposit on their internal walls in contact with the hydrocarbon to be cracked. Not only is this deposit detrimental to heat transfer, but it reduces the cross-section of the tube. The thickness of this coke deposit becomes such that the unit must be shut down and undergo costly cleaning operations. In most cases, the deposit of coke is removed by gasification at high temperature by a mixture of water vapor and air which converts the coke to carbon oxides and restores the initial characteristics of the cracking tube. When the deposit occurs in the heat exchangers, it is not structurally possible to carry out an online decoking by gasification since the maximum admissible temperatures are too low to allow this reaction. Manual disassembly and decoking is necessary, a long and difficult operation.

Despite optimized procedures that completely eliminate coke, hydrocarbon cracking units such as steam crackers are frequently shut down to undergo further decoking cycles (after 20 to 60 days of operation). In addition, the decoking oxidizing treatment leads to an increase in the catalytic activity of the cracked metal surface, which increases the rate of coke formation. Thus, with the increase in the number of decoking operations undergone by the unit, the operating time decreases and the annual number of decoking operations increases. This long term effect is technically and economically damaging since maintenance costs become increasingly heavy with the age of the unit for a lower annual operating rate. This is the reason why many efforts have been made for years to find solutions which avoid the rapid coking of the internal metal walls of such units (cracking tubes and heat exchangers). Among the many solutions described in the literature, the following may be mentioned more particularly:

1) A first method, described in US Pat. No. 4,099,990 and a subsequent publication by D.E. Brown et al. in ACS Symp. Ser. 202 (1982) 23, consists of forming, from alkyloxysilane, a coating of silica by thermal degradation in steam. Some improvement in the quality of the deposit can be obtained by using a silicone oil under specific conditions (Chem. Techn. (Leipzig) 42 (1990) 146). However, the process is quite expensive and the silica layer is not very stable above 750 ° C., the usual temperature for cracking tubes in industrial installations.

2) US Pat. No. 4,410,418 describes a method for depositing a film of silica from halogenosilane. The silylated compound is deposited liquid, in film, on the metal surface to be treated then, by exposure to humidity, a layer of silica is formed by hydrolysis. This technique is difficult to apply to industrial installations because of its delicate implementation; it is also accompanied by the release of acids which can corrode the metal walls. 3) In patents EP 540 084, EP 654 544 and EP 671 483, a protective layer of ceramic type is obtained from silylated compounds which do not contain alkoxy groups and which are cracked in the presence of vapor or inert gas.

4) US Patents 4,692,243, US 5,565,087, US 5,616,236, US 5,656,150, EP 698,652 and EP 770,665 all deal with a method of reducing the formation of coke in a cracking tube. hydrocarbon. This method uses a silicon compound mixed with a tin compound. Certain improvements have been made to it, such as the use of a reducing gas as a carrier fluid for pretreating the cracking tube (US Pat. No. 5,616,236) or the cracking of a desulphurized filler (Patent EP 770,665). This type of treatment remains expensive and the long-term effects of tin on the metallurgy of the cracking tube and in the downstream sections are not known.

5) US Pat. No. 5,849,176 describes a process in which an additive composed of sulfur and silicon is added to the charge of the cracking unit. Coke formation is reduced more significantly than with a silylated compound alone or a sulfur compound alone. This patent claims the use of compounds based on sulfur and silicon to reduce coking in the cracking tubes and also in the heat exchangers placed in line following the cracking reactor. The quantities of silicon thus introduced end up being non-negligible and blockages are to be feared either in the cracking tube or in the section for treating cracked gases.

6) Patent application WO 95/22588 claims a process in which the cracking tube is pretreated in an inert gas (nitrogen, methane, hydrogen) with an additive based on sulfur and silicon. A significant reduction in the amount of coke formed during the cracking of the hydrocarbon feed is obtained. A real synergy exists between sulfur and silicon since no additive based on sulfur or silicon alone leads to such results. The use of an inert carrier gas however seems essential to these performances. Example 6 and Figure 7 of this patent application show that the use of steam as a carrier gas with an additive consisting of trimethylsilylmethylmercaptan does not lead to any inhibition of the formation of coke.

Surprisingly, it has now been found that an additive consisting of a mixture of sulfur compound and silylated compound can be used to pretreat a hydrocarbon cracking tube in vapor and thus significantly reduce the formation of coke which accompanies the hydrocarbon cracking reaction.

Compared to the process described in patent application WO 95/22588, this new process is easier to set up in steam cracking units since, as carrier gas, it uses steam, a fluid already available usually in said units .

The first object of the invention is therefore a process for reducing coking on the metal walls of a reactor for cracking hydrocarbons or other organic compounds and on the metal walls of a heat exchanger placed after the cracking reactor, characterized in that the metal surfaces coming into contact with the organic substance to be cracked are pretreated with a stream of water vapor containing at least one silicon compound and at least one sulfur compound, at a temperature between 300 and 1100 ° C, preferably between 400 and 700 ° C for the heat exchanger and preferably between 750 and 1050 ° C for the cracking tube, for a period of between 0.5 and 12 hours, preferably between 1 and 6 hours.

The silicon compounds which can be used in the process according to the invention may contain one or more silicon atoms and be of inorganic or organic nature.

As inorganic silicon compounds, mention may be made more particularly of halides, hydroxides and oxides of silicon, silicon acids, ques, and the alkaline salts of these acids. Among the inorganic silicon compounds, those which do not contain halogens are preferred.

In the context of the present invention, it is preferred to use organic silicon compounds and, among these, those which contain only silicon, carbon, hydrogen and, optionally, oxygen. The hydrocarbon or oxycarbon groups linked to silicon can contain from 1 to 20 carbon atoms and are, for example, alkyl, alkenyl, phenyl, alkoxy, phenoxy, carboxylate, ketocarboxylate or diketone groups. As non-limiting examples of such compounds, mention may be made of tetramethylsiiane, tetraethylsilane, phenyltri-methylsilane, tetraphenylsilane, phenyltriethoxysilane, diphenyldimethoxysilane, tetraethoxysilane, tetramethoxysilane, triethyl , poly (dimethylsiloxanes) and especially hexamethyldisiloxane.

May also be used organic silicon compounds containing heteroatoms such as halogen, nitrogen or phosphorus atoms. As examples of such compounds, mention may be made of chlorotriethylsilane, (3-aminopropyl) triethoxysilane and hexamethyldisilazane.

As sulfur compounds which can be used in the context of the present invention, mention may be made of carbon disulfide and the compounds corresponding to the following general formula:

R ¹ - Sx - R ²

in which R ¹ and R ² , identical or different, each represent a hydrogen atom or a hydrocarbon group, and x is a number greater than or equal to 1. As examples of hydrocarbon groups, mention may be made of alkyl, alkenyl groups, cycloalkyl, aryl and combinations thereof such as, for example, alkylaryl groups. As nonlimiting examples of organic sulfur compounds, mention may be made more particularly of alkyl mercaptans, dialkyl sulfides, -disulfides and -polysulfides, as well as the sulfur compounds present in certain petroleum fractions (naphtha) such as thiophenic and benzothiophenic compounds. . Preferably, dimethyl sulphide, diethyl sulphide, hydrogen sulphide and especially dimethyldisulphide are used.

The atomic ratio (Si: S) defining the proportions between the sulfur compound (s) and the silylated compound (s) is preferably between 5: 1 and 1: 5. Advantageously, an Si: S ratio of between 2: 1 and 1: 2 is used. The concentration of the additive constituted by the mixture of the sulfur compound (s) and the silylated compound (s) can range from 50 to 5000 ppm by mass in the carrier fluid consisting of steam alone or mixed with an inert gas (nitrogen, hydrogen, methane or ethane). Preferably, this concentration is between 100 and 3000 ppm.

The pressure of the carrier fluid is generally equal to that usually used in cracking furnaces (between 1 and 20 bars absolute, advantageously between 1 and 5 bars absolute).

The pretreatment according to the invention can be implemented in any new cracking unit or in any existing unit after each decoking operation.

The invention also relates to a cracking process in which a sulfur compound and, optionally, a silylated compound is added during cracking to the charge of organic compounds. The temperature at which this addition takes place depends directly on the cracking conditions; it generally varies between 400 and 1000 ° C. and is preferably between 700 and 950 ° C.

The sulfur compounds and, optionally, those of silicon to be used in the context of this embodiment are the same as those mentioned above. The sulfur-containing compound can be used alone or as a mixture with a silylated compound in an Si: S atomic ratio less than or equal to 2: 1, preferably less than or equal to 1: 2.

When the organic compound to be cracked already contains sulfur in organic form, only the silylated compound can optionally be added. In this case, an Si: S atomic proportion of less than or equal to 2: 1, preferably less than or equal to 1: 2, must be respected, the concentration of silicon in the compound to be cracked should not exceed 500 ppm.

The concentration of sulfur additive, with or without silylated compound, is chosen so that the sulfur concentration in the organic compound to be cracked is between 10 to 1000 ppm by mass, preferably between 20 and 300 ppm by mass.

The following examples illustrate the invention without limiting it.

EXAMPLE 1

This example shows the effectiveness of a pretreatment based on sulfur and silicon diluted in steam to inhibit the formation of coke during the cracking of an petroleum cut rich in n-hexane (composition given in the following table 1). Table 1: composition of the load to be cracked

The cracking tube with an internal diameter of 9 mm and a length of 4.6 m, was made of Incoloy 800 HT steel and included an additional length of 1.45 m from the same tube for preheating fluids.

During the pretreatment of the cracking tube, 1.92 kg / h of water vapor was introduced while maintaining a temperature at the outlet of the tube of 850 ° C. The additive is a mixture of dimethyldisulfide and hexamethyldisiloxane having an atomic ratio Si: S = 2: 1. This mixture, diluted in a nitrogen flow of 30 g / h, was injected into the steam after the preheating section, at the rate of 5.7 g of additive per hour for 60 minutes. The additive concentration in the water vapor was 2970 ppm by mass.

The cracking conditions were as follows:

- outlet gas temperature 850 ° C

- pressure 1.7 bar

- contact time 260 ms

- flow rate of the load to be cracked 4.8 kg / h

- water vapor flow rate 1.92 kg / h

- dilution 0.4 kg of steam / kg of hydrocarbons

- cracking time 6 hours

The decoking of the reactor was carried out by means of a mixture of air (1.2 kg / h) and water vapor (4.5 kg / h) brought to 800 and then 900 ° C. in order to completely oxidize coke to carbon oxides. Carbon oxide concentrations were continuously measured by an infrared detector. Part of the coke that came off was entrained by the gas flow and then trapped by a cyclone. The mass of coke initially formed in the cracking tube is given by the sum of the coke which has been entrained and the coke which has been oxidized.

A reference test was carried out under the same conditions (pretreatment, coking and decoking) but without the addition of the dimethyldisulfide-hexamethyldisiloxane mixture.

By comparison with this reference test, the mass of coke was reduced by 66% when the tube was pretreated with the dimethyldisulfide-hexamethyldisiloxane mixture.

EXAMPLE 2

This example shows the effectiveness of a pretreatment based on sulfur and silicon diluted in steam to inhibit the formation of coke during cracking of propane.

The cracking tube was made of Incoloy 800 HT steel with an inside diameter of 7.7 mm and a length of 9 meters. The gases were preheated to 200 ° C before their introduction into the cracking tube.

The pretreatment used a mixed flow of steam (0.7 kg / h) and nitrogen (3.5 kg / h) for 4 hours. The temperature of the gases leaving the cracking tube was 1010 ° C. The additive was a mixture of dimethyldisulfide and hexamethyldisiloxane having an atomic ratio Si: S = 1: 2. This additive was injected at the inlet of the pyrolysis tube at the rate of 5.63 g / h, ie a concentration of 1340 ppm mass in the gas flow.

The cracking conditions were as follows:

- gas outlet temperature 910 ° C - pressure 1, 4 bar

- contact time 150 ms

- flow rate of the load to be cracked 2.33 kg / h

- water vapor flow 0.7 kg / h

- dilution 0.3 kg of steam / kg of propane - conversion of propane 88-92%

- ethylene selectivity 73-77%

- selectivity in propylene 23-27%

- cracking time 20 hours

The decoking was carried out using air (240 g / h) diluted in nitrogen (1, 2 kg / h) at a temperature between 900 and 1000 ° C. Carbon oxide concentrations were continuously measured by an infrared detector.

The coke entrainment phenomena were negligible which allowed directly calculate the mass of coke formed from the total quantities of carbon oxides.

A reference test was carried out under strictly identical conditions but without the addition of the additive based on dimethyldisulfide and hexamethyldisiloxane.

Compared to this reference test, the coke mass was reduced by 27% when the tube was pretreated with the dimethyldisulfide-hexamethyldisiloxane mixture.

EXAMPLE 3

This example shows the coke-inhibiting properties of a pretreatment based on sulfur and silicon diluted in steam, to which is added a continuous addition of dimethyldisulfide to the feed.

The general experimental conditions as well as those of the pretreatment were identical to those of Example 2. Dimethyldisulfide was injected at the inlet of the cracking tube at a rate of 1.8 g / h during the 20 hours that lasted propane cracking.

A reference test was carried out under identical conditions but without adding the pretreatment additive based on dimethyldisulfide and hexamethyldisiloxane.

Compared to this reference test, the mass of coke was reduced by 18% when the tube was pretreated with a dimethyldisulfide-hexamethyldisiloxane mixture.

EXAMPLE 4

This example shows the coke-inhibiting properties of a pretreatment based on sulfur and silicon diluted in steam, to which is added a continuous addition to the feed of a dimethyldisulfide-hexamethyldisiloxane mixture.

The general experimental conditions as well as those of pretreatment were identical to those of Example 2. An additive composed of dimethyldisulfide and hexamethyldisiloxane having an atomic ratio Si: S equal to 1:20 was injected at the inlet of the tube. cracking at a rate of 1.88 g / h during the 20 hours that the cracking of propane lasted.

A reference test was carried out under identical conditions but without adding the pretreatment additive and the silylated compound during the cracking.

Compared to this reference test, the mass of coke was reduced by 17%. Comparative example 5

The coke-inhibiting properties of a pretreatment based on an organic silicon compound alone (hexamethyldisiloxane) were compared with those of a pretreatment without the addition of hexamethyldisiloxane.

The general experimental conditions were identical to those of Example 2 but using as an additive the hexamethyldisiloxane injected at the inlet of the cracking tube at the rate of 2.3 g / h during the 4 hours of pretreatment.

Compared to a reference test, carried out under strictly identical conditions but without the addition of hexamethyldisiloxane, the coke mass increased by 5% in the tube pretreated with hexamethyldisiloxane.

EXAMPLE 6

This example shows the effectiveness of a pretreatment using an additive based on sulfur and silicon diluted in water vapor to inhibit the formation of coke in a heat exchanger.

Apparatus and operating conditions

The micropilot was divided into two parts, a cracking reactor followed by a heat exchanger. A small metal coupon (P-22 type carbon steel containing 2.25% chromium and 1.0% molybdenum) was placed in the gas flow passing through this heat exchanger. The coking reactions took place on the surface of this coupon, causing an increase in its mass which could be translated into coking speed per unit of area.

The pretreatment conditions were as follows:

- cracking reactor temperature 600 ° C

- contact time of the cracking reactor 2 seconds

- steam flow 21 l / h - nitrogen flow 7 l / h

- additive concentration 1000 ppm by mass

- heat exchanger temperature 600 ° C

- duration 2 hours

The sulfur-silicon additive was a mixture of dimethyldisulfide and hexamethyldisiloxane with an atomic ratio Si: S = 2: 1. This additive was injected into the vapor stream at the inlet of the cracking reactor. The cracking conditions (coking phase) were as follows: temperature of the cracking reactor 850 ° C. contact time of the cracking reactor 0.5 second hydrocarbon to be cracked isobutane isobutane flow rate 10 l / h nitrogen flow rate 10 l / h cracking severity (propylene / ethylene) 0.6 heat exchanger temperature 500 ° C duration 1 hour

The coke formed in the cracking reactor and the heat exchanger was removed (decoking) by a high temperature air treatment to transform the carbon into carbon oxides gas.

Results

After the pretreatment using the sulfur and silicon additive, a coking / decoking cycle was applied in order to obtain a coupon having a used metal surface, representative of the heat exchangers used on industrial units. After this preliminary treatment, the anti-coke properties generated by the sulfur-silicon pretreatment as well as their stability were tested during 6 coking / decoking cycles.

The following table 2 indicates the coking speeds observed on the metal coupon placed in the heat exchanger, under standard cracking conditions, during each coking phase. The coking speeds of the coupon pretreated with the sulfur and silicon-based additive are compared to the coking speeds obtained on a coupon of the same kind, under the same conditions, but having not undergone any pretreatment.

The anti-coke properties of the sulfur-silicon pretreatment are expressed by the term "inhibition of coke" defined as follows:

Coking speed Coking speed on coupon not pretreated on coupon pretreated by S-Si

"Coke inhibition" (%) = 100

Coking speed on untreated coupon Table 2

Coking speeds of metal coupons placed in the heat exchanger

Claims

1. Method for reducing coking on the metal walls of a cracking reactor for hydrocarbons or other organic compounds and on the metal walls of a heat exchanger placed after the cracking reactor, characterized in that the metal surfaces coming into contact with the organic substance to be cracked are pretreated with a stream of water vapor containing at least one silicon compound and at least one sulfur compound, at a temperature between 300 and 1100 ° C for a period of 0.5 to 12 hours.

2. Method according to claim 1, wherein the pretreatment of the cracking reactor is carried out at a temperature between 750 and 1050 ° C.

3. Method according to claim 1 or 2, wherein the pretreatment of the heat exchanger placed following the cracking reactor is carried out at a temperature between 400 and 700 ° C.

4. Method according to one of claims 1 to 3, wherein the pretreatment is carried out for a period of 1 to 6 hours.

5. Method according to one of claims 1 to 4, wherein the water vapor, used as a carrier fluid, further contains an inert gas.

6. Method according to one of claims 1 to 5, in which a compound containing only silicon, carbon, hydrogen and, optionally, oxygen is used as silylated compound.

7. The method of claim 6, wherein the silylated compound is hexamethyldisiloxane.

8. Method according to one of claims 1 to 7, in which carbon disulfide or a compound of general formula R ¹ -Sx-R ² , in which R ¹ and R ² , which are identical or different, are used as sulfur compound , each represent a hydrogen atom or a hydrocarbon group, and x a number equal to or greater than 1.

9. The method of claim 8, wherein the sulfur compound is dimethyldisulfide.

10. Method according to one of claims 1 to 9, wherein the atomic ratio Si: S is between 5: 1 and 1: 5, preferably between 2: 1 and 1: 2.

11. Method according to one of claims 1 to 10, in which the mass concentration of sulfur and silylated additives in the carrier fluid is between 50 and 5000 ppm, preferably between 100 and 3000 ppm.

12. Method according to one of claims 1 to 11, wherein the pressure varies between 1 and 20 bar absolute, preferably between 1 and 5 bar absolute.

13. Method according to one of claims 1 to 12, wherein, after the pretreatment, a sulfur compound and or a silylated compound are added to the charge of the organic compound to be cracked.

14. The method of claim 13, wherein the sulfur compound is dimethyldisulfide.

15. The method of claim 13 or 14, wherein the silylated compound is hexamethyldisiloxane.

16. Method according to one of claims 13 to 15, wherein the atomic ratio Si: S does not exceed 2: 1 and is preferably less than or equal to 1: 2.

17. Method according to one of claims 13 to 15, in which, to a charge of organic cracking compound containing sulfur, a silylated compound is added in an amount such that the atomic ratio Si: S does not exceed 2: 1, preferably less than or equal to 1: 2, and that the silicon concentration does not exceed 500 ppm.

18. Method according to one of claims 13 to 17 wherein the mass concentration of sulfur in the organic compound to be cracked is between 10 and 1000 ppm, preferably between 20 and 300 ppm.