DD282927A5 - METHOD FOR REDUCING COZING DEPOSITS IN THERMAL PROCESSES - Google Patents

Abstract

The invention relates to a method for lowering coke deposits in thermal processes in the chemical industry, in which organic compounds in the temperature range of 450-1 100C converted, cooled and / or heated, the surface of the pipe systems required for the Prozeszfuehrung with one or more silicon-containing Compounds covered, then at atmospheric temperatures of 500-1 100C with air and after successful gas exchange with oxygen-free and / or nitrogen-containing nitrogen-reactive gases is contacted. {Koksablagerung; thermal process; Surface; Pipe system; silicon-containing compound; Air; Gas exchange; nitrogenous gases}

Description

Field of application of the invention

The invention relates to a method for lowering Koksablagorungen in thermal processes of the chemical industry, in which organic compounds in the temperature range of 450-1100 ⁰ C gewandnlt. be cooled or heated.

Characteristic of the known state of the art

In the enothermal processes of olefin and aromatics production, undesirable side reactions in the reaction and cooling systems, which are favored by certain alloying constituents of the high-alloy steels used, lead to the formation and deposition of coke-like products.

These coke deposits in tube reactors and downstream cooling systems resulted in a drastic deterioration of the heat transfer as well as an increase in the pressure difference in the system, resulting in a deterioration of the operating regime and the overall economy of the process.

When certain limits are reached. the removal of the coke deposits by burning out with air and / or steam or other suitable methods required. This considerably reduces the technical availability of the pyrolysis plants.

In addition, the service lives of the aggregates, particularly the tubes of the reaction system, are greatly shortened by the frequent alternation of oxidizing and reducing conditions in the production cycle (L.F. Albright, Industrial and Laboratory Pyrolysis, ACS, Symp. Serr. 32, Washington 1976).

Methods are known which aim to suppress the formation and deposition of coke-like products.

Besides processes which work with additives (substances which are added to the pyrolysis starting product) (SU 427979, SU 518959), methods are also described in which pipes with passivated surfaces are used.

According to NL 8401804, tubes of the reactor and fission gas cooler are provided with a protective layer by taurhening or brushing, which consists of a mixture of 50 to 70% silicone resin and 30 to 50% carbides and / or nitrides.

In GB 1483144 a process is described in which the pipe surface is oxidized with air or steam at temperatures of about 800 ° C. Subsequently, at 200 to 1 200 ° C, preferably at "100 to 900 ° C, an occupancy of the surface with alkoxysilanes.

Both of these methods reduce coke formation only over a certain period of time, and rapid activation of the passive layer with respect to coke growth occurs.

In addition, methods for surface treatment in mechanical engineering are known, the wear-resistant coatings and hard coatings on heavily used machine parts, vie ζ. B. on cutting tools, the goal.

For the production of hard coatings on heavily used machine parts made of unalloyed and low-alloyed steels, according to DD 200 573, the surface is first treated with an oxidic and / or hydroxidic primary layer, the elements such. B. Al, Si, Cr, Ti, Mo, W, V, Nb, provided and subsequently subjected to a thermo-chemical treatment in aufstickender and / or carburizing atmosphere.

The primary layer is applied to the parts from an aqueous phase (usually from aqueous electrolytes), and then these are then subjected to a thermal treatment in an oven in the presence of NH ₃ , CH ₄ , CO, CO ₂ . As a result, hard nitride and / or carbide-containing layers are obtained on the components or tools, so that they no longer have to be made of high-alloy steels because of their improved utility properties (skill behavior, increased wear and corrosion resistance).

Object of the invention

The aim of the invention is a simple and effective method for reducing coke deposits in thermal processes, in which a passivation of the tube surface over a long period, so that a high availability of plants is achieved, in which such thermal processes are integrated.

Explanation of the essence of the invention

The object of the invention is to develop a method for lowering coke deposits in thermal processes, after a layer application on the inner surface of the pipe system used by special treatment for a long time to an effective passivation of the surfaces of the heating, reaction and / or Cooling system leads.

According to the invention, the object is achieved by a method for lowering coke deposits in thermal processes in the temperature range of 450 to 1100 "C, in a C- and H-containing organic compounds, preferably hydrocarbons, passed through pipe systems, converted, heated and / or cooled, dissolved by the surface of the pipe systems required for the process control with one or more silicon-containing compounds covered, then 1 to 60 hours at wall temperatures of 500 to 1100 ⁰ C with air and after gas exchange 1 to 50 hours with oxygen-free and / or nitrogen-containing Gases at 500 to 1100 ⁰ C and flow rates of 5 to 80 m / s is contacted by reactive.

Suitable nitrogen-containing gases are, in particular, ammonia and nitrogen.

In the first step of the process according to the invention, a silicon-containing compound or a mixture of silicon-containing compounds is applied to the pipe surface by technologies known per se, for example by introducing the silicon-containing compounds into the pipe system by means of steam as diluent (IESundgren et al., J. Vac ,

Be. Technol. A4 (5) 1986, 259).

Suitable silicon-containing compounds are gaseous and liquid substances which consist of silicon, carbon, hydrogen and oxygen or only of silicon, carbon and hydrogen or nitrogen-containing silicon compounds.

After application of the silicon-containing compound (s), in the second step treatment of the tubes of the reaction system, preferably 1 to 60 hours, takes place at wall temperatures of 800 to 1100 ° C. with air and, depending on the spatial configuration of the reaction system, 1 to 50 hours. preferably 4 to 30 hours, with ammonia at flow rates of 10 to 25m / s and 800 to 1100 ⁰ C.

The tubes of the cooling systems, in particular the quench cooler, 1 to 60 hours, preferably 5 to 30 hours at wall temperatures of 500 to 800 ° C with air and then 4 to 30 hours with ammonia at flow rates of 5 to 25m / s and 500 to 800 ⁰ C treated.

The passive layers on the alloyed steels produced by the process according to the invention cause in the pyrolysis process up to 90% reduction of the coke-like deposits on the pipe surfaces and thus a substantial extension of the running time of the plant up to a necessary decoking. The time between two kilns is increased up to ten times that of untreated pipes, which in addition to a higher availability of the plant and an improvement in the overall economy of the process, also leads to reduced material damage to the pipe materials. This significantly extends the service life of the pipe systems. The passivating layer does not cause a negative change in the product composition in the thermal conversion.

The inventive method can be used for the passivation of pipe systems both before their initial start-up and after appropriate cleaning.

If the passivating effect of the surfaces leaves, it can be restored in the manner described.

The invention will be explained in more detail with reference to the following examples.

embodiments

example 1

In a laboratory tube reactor of quality X8 stainless steel CrNiTi 18.10 with the dimensions 200 χ 15 χ 1.5 mm was at a reactor temperature of 75O ⁰ C and a residence time of 0.25s using water vapor as a diluent (weight ratio steam / hydrocarbon = 0, 6) Pyrolysis of gasoline of the quality indicated in Table 1. Water vapor and gasoline were dosed separately and evaporated, combined at the reactor inlet and cracked in the electrically heated reactor tube. At the reactor outlet, the reaction product was quenched by indirect cooling, the liquid product condensed and the gas product collected in a storage vessel. The analysis of the gaseous and liquid reaction products was carried out by gas chromatography.

To determine the amount of coke formed in the reaction tube, the tube was burned out with air at 800 ° C for about 1 hour. The carbon oxides formed were introduced after conversion of CO to CO ₂ via cupric oxide at 700 ° C in sodium hydroxide solution and the amount of carbon determined by back titration of the non-consumed sodium hydroxide with hydrochloric acid. The results of the coke formation characterization for three consecutive pyrolysis experiments (run time 1 h each) are given in Table 2. To characterize the proportion of steam reforming reactions under pyrolysis conditions, the amount of CO + CO ₂ , based on the ethylene yield, is given. The amount of coke in the pipe is reported in mass fractions in%, based on feedstock.

Belsplol2

The reactor tube of the pyrolysis apparatus according to Example 1 was coated with hexamethyldisiloxane in the CVD method at 750 ° C and then kept for 60 min at the same temperature in the air stream. In the thus treated pipe straight-run gasoline was then pyrolyzed according to the specified method and at comparable parameters. As Table 2 shows, gaps 2, steam reforming and coke deposition on the freshly passivated pipe are significantly reduced compared to the untreated reactor; However, after three pyrolysis tests, the passivation effect is canceled again.

Example 3

The reactor tube of Pyrolyseappantur according to Example 1 was coated with silicone oil in the CVD process at 750X, but then after-treated in the air stream for 1 h at 900 ° C and another 15h at 800 ⁰ C. Steam reforming and coke formation are initially as low as in Example 1; However, after only six pyrolysis decoking cycles, the steam reforming activity is again very high, while coke formation only increases slowly (see Table 2, column 3).

Example 4

The reactor tube is wetted with silicone oil at room temperature, the liquid film remaining on the tube inner surface is then baked with a ring burner at 1000 ⁰ C. As a result, a very effective and long-term stable passivation is achieved, as the consistently low accumulation of CO and CO ₂ and achieved until the 35th pyrolysis decoking cycle low coke formation. Only then does coke formation increase again (see Table 2, column 4).

Example 5

The reactor tube of the pyrolysis apparatus according to Example 1 was added at room temperature with silicone oil Benetz Jer on the tube inner surface remaining liquid film is then baked with a ring burner h at about 1000 ⁰ C1 in the air stream and subsequently a 5-hour post-treatment with gaseous ammonia at the same temperature and at a flow rate of 20l / h subjected. Thereafter, a total of 60 pyrolysis tests were carried out with straight-run gasoline under the conditions given in Example 1, followed by decoking in each case. As shown in Table 3, the amount of CO and CO ₂ in the pyrolysis gas as well as the amount of coke formed are extremely low and both side reactions, steam reforming and coke formation, maintain their low level even after 60 pyrolysis decoking cycles. The structure of the gaseous and liquid pyrolysis products is not adversely affected by the coating of the tubes (see Table 4).

Example 6

The reactor tube of a laboratory pyrolysis reactor is wetted with Siüconöl at room temperature. The remaining on the tube inner surface a liquid film is then reacted with an annular burner at 1000 ⁰ C for 1 h baked in the air stream and subsequently a five-hour post-treatment with nitrogen at 1000 ° C at a flow rate of 20 l / h subjected. The Koksbildu.ig reaches similar low levels as in Example 5 (0.02-0.05 parts by mass in%). After 40 test cycles, no increase in coke formation or steam reforming could be detected.

Example 7

A laboratory pyrolysis reactor with a reactor tube coated according to Example 5 exhibited a reduction in the passivation effect after half a year of operation (200 pyrolysis decoking cycles), which is reflected in a slight but steady increase in coke deposits in successive pyrolysis tests. Aftertreatment of the reactor with hexamethyldisiloxane, which was passed through the reactor in vapor form with steam at reaction temperature and after-treated with ammonia (according to Example 5), restores the original protective action. In subsequent pyrolysis tests, low coke values were again achieved (see Table 5, line 3).

Examples

A laboratory pyrolysis reactor with a coated reactor tube according to Example 5, the passivation after a year Laufzoit has subsided, is again subjected to a coating with liquid silicone oil, heat treatment at 1000 ⁰ C and subsequent treatment with gaseous ammonia at 1000 ° C. Very low coke values and low steam reforming activity were found again in subsequent pyrolysis tests (see Table 5, line 4). Investigations on the effect of the protective layer thus produced over an extended period of time were terminated after 200 cycles of pyrolysis decoking without an increase in the (CO + CO ₂ ) amount and coke formation.

Table 1: Characterization of the straight-run gasoline used for the pyrolysis tests

Siedabereich 42-177 "C d? 0.725 g / cm ³ nj ° 1.4100 Fabric Type Composition paraffins 71.4 mass shares in% naphthenes 17.1 mass shares in% aromatics 10.6 parts by mass in% BMCI value 7.0

Table 2: Pyrolysis of straight-run gasoline in differently treated reactors; Accumulation of CO + CO ₂ , based on the ethylene yield (A), coke attack (B)

, CO (% by mass) + CO ₂ (% by mass) "". .... ·, · ",>

^A - B = coke (mass%)

C ₂ H ₄ (mass fractions in%)

experimental untreated treated reactors Example 3 Example 4 Number / reactor Reactor Example 1 Example 2 A B A B A B A B

10 15 20 25 30 35 40

0.89 0.63 1.45 0.48 0.94 0.84

0.02 0.03 0.92 0.09 0.31 0.69

0.04 0.04 0.09 0.06 0.14 0.03 0.39 0.05 0.44 0.10 0.59 0.14

0.02 0.04 0.01 0.04 0.01 0.05 0.01 0.05 0.01 0.04 0.03 0.07 0.04 0.14 0.02 0.14 0.03 0.17 0.05 0.17 0.05 0.33 0.06 0.38 0.03 0.34

Table 3: Coke formation tendency and steam reforming activity in pyrolysis tests in a reactor tube pretreated with silicone oil and after-treatment with ammonia at 1000 ° C. (data in% by weight)

CO + CO ₂ 0.02 coke C ₂ H ₄ 0.02 Trial 1 0.03 0.08 2 0.01 0.09 3 0.01 0.07 4 0.04 0.06 5 0.05 0.04 10 0.08 0.02 20 0.08 0.05 30 0.05 0.07 40 0.06 0.10 50 0.06 60 0.05

Table 4: Comparison of the gaseous and liquid pyrolysis products obtained in the untreated and pre-treated in Example 5 reactor.

(T = 730 ⁰ C; τ = 0.25 s; m _KW / m _Mj o = 0.6)

reactor untreated treated Products ( % by mass ) according to Example 1 according to example 5 Mei tan 12.1 11.8 ethylene 28.3 28.8 propylene 17.4 17.5 butenes 5.7 5.9 butadiene 4.8 4.9 pyrolysis gasoline 24.6 25.0 ΒΊΧ aromatics 4.8 5.0 pyrolysis 2.2 2.0

Table 5: Coke formation tendency and steam reforming activity in a pretreated according to Example 5 reactor which has been post-treated after a long period. (Figures in% by weight)

1. Experiment after fresh coating according to Example 5

2nd trial after six months (about 200 attempts)

3rd trial after CVD treatment with

Silicone oil and water vapor according to Example 7 0.11

4. Experiment after re-coating according to Example ξ

CO + CO ₂ coke C ₂ H ₄ 0.02 0.04 0.06 0.38 0.11 0.16 0.04 0.04

Claims (4)

1. A method for lowering coke deposits in thermal processes in the temperature range of 450 to 1100 ⁰ C, where by piping C- and H-containing organic compounds, preferably hydrocarbons, passed, converted, heated and / or cooled, wherein the protected Surface of the tubes is coated permanently with silicon-containing compounds, characterized in that the surface of the pipe systems covered with one or more silicon-containing compounds, then 1 to 60 hours at wall temperatures of 500 to 1100 ° C with flowing air and after gas exchange 1 to 50 hours with oxygen-free and / or poor nitrogen-containing gases at 500 to 1100 ⁰ C and flow rates of 5 to 80 m / s is contacted by reactive. 2. The method according to claim 1, characterized in that at wall temperatures of 800 to 1100 ⁰ C mitströmender air and then 4 to 30 hours with nitrogen-containing gases at flow rates of 10 to 25m / s and 800 to 1100 ⁰ C with silicon-containing compounds covered pipe wall surface the reaction system is treated. 3. The method according to claim 1, characterized in that at wall temperatures of 500 bit-800 ° C with flowing air and then 4 to 30 hours with nitrogen-containing gases at flow rates of 5 to 25m / s and 500 to 800 ⁰ C with silicon-containing compounds covered pipe wall surface of the cooling system is treated. 4. The method according to claim 1 to 3, characterized in that ammonia is used as the nitrogen-containing gas.