RU2566244C2 - Chemical treatment method for inner surface of hydrocarbon pyrolysis reactor - Google Patents

Abstract

FIELD: oil and gas industry.

SUBSTANCE: before start-up of the reactor its surface is treated by flushing with vapour-liquid acid solution at temperature of 100-200°C and further neutralisation with aqueous base solution 0.0001-1 wt %. Neutralisation is ceased when flow medium reaches value of pH 8-9.5 at the reactor output.

EFFECT: reduced coke formation at inner surface of the reactor.

4 cl, 3 tbl, 12 ex

Description

The invention relates to surface treatment processes for reactors for conducting high-temperature processes, such as tube furnaces for thermal cracking or pyrolysis of hydrocarbons, in which coke formation is possible, to reduce coke formation and can be used in chemical and petrochemical industries.

Cracking and pyrolysis processes aimed at the decomposition of hydrocarbons are carried out in reactors, which are tube furnaces made of heat-resistant alloys containing elements such as nickel, iron, chromium, etc. In addition, during the operation of furnaces in reactors, various metals that may be contained as impurities in the feed or introduced with steam dilution. For example, solutions of alkali metal hydroxides, which can be carried into coils, are used to maintain the alkaline environment of the dilution vapor. The listed metals under the conditions of thermal processes are catalysts for the formation of coke on the inner surface of the coils. During high-temperature processes, coke grows on the surface (especially at the start-up times of the furnaces), which worsens the heat transfer through the wall of the coils and leads to the need for the furnaces to stop for oxidative coke burning. The firing is carried out by supplying air to the coils together with steam, which requires additional costs for the production of steam, the consumption of fuel gas to maintain the temperature in the furnace, the loss of olefins due to downtime of the furnace and generally contributes to the cost of the process. In addition, in the presence of catalytically active metals and water vapor, coke gasification occurs with the formation of oxide (CO) and carbon dioxide (CO ₂ ), which pollute the target products.

Reagents based on phosphorus-containing substances (US Pat. Nos. 6,368,494, 6454995), phosphorus amines or imidosalines (US Pat. No. 4,842,716), tripiperidine phosphine oxides (US Pat. No. 5,360,531) are effective coke inhibiting agents. The principle of action of these substances is to reduce the activity of catalytic centers on the walls of the coils. However, this direction has a rather large number of shortcomings, such as: the difficulty of uniform dosing, uniform distribution over the steam-feed stream, limitation of the use of coke formation inhibitors in the pyrolysis of raw materials with sulfur content (straight-run gasoline, atmospheric gas oil). In addition, phosphorus compounds lead to the formation of phosphine, which can deactivate hydrotreating catalysts from acetylene pyrolysis products.

A known method of producing olefin hydrocarbons according to the USSR AS No. 482490, IPC 10G9 / 16, publ. 08/30/75, according to which a coke formation inhibitor is added to the pyrolyzable hydrocarbon - bottom residue of the production of synthetic fatty acids with a carbon number of 19-26. However, this method has a serious main drawback - this is the dosage of the inhibitor in the raw material, which will necessarily lead to contamination of the pyrolysis products, namely ethylene and propylene, with impurities that are formed upon decomposition of this inhibitor.

The method described by the patent of the Russian Federation No. 2057584, MPK is described.⁶ C10G 9/16_; publ. 04/10/1996, in which metal salts are used as additives IIA groups, phosphorus-containing compounds and optional metal salts IA and sulfur compounds. As a polar solvent water or alcohol is used, and the dissolution of additives is carried out before introducing hydrocarbon feed into the stream. The use of these salts leads to the removal of coke from the coils as a result of its gasification with the formation of carbon monoxide and dioxide. However, this method has a drawback - it is the dosing of the inhibitor in the feedstock and, as a result, the contamination of the target products of CO and CO₂. In addition, Group IIA and IA metals accumulate in the coils, which leads to their alkaline corrosion.

A known method of reducing the formation of coke, CO and CO ₂ based on processing the walls of the reactors by passing sulfur-containing compounds with steam through the coils during the process. Of these, dimethyldisulfide and tert-butyl polysulfide are most commonly used. In the presence of sulfur-containing additives, the formation of metal sulfides occurs, which have less catalytic activity. As a result, coke deposition on the walls of the reactor is reduced by 4–20 times (US Patent No. 4116812 IPC C10G 9/16, publ. 09/26/1978; US Patent No. 8057707 IPC C10G 75/04, publ. 11/19/2009). The continuous introduction of these compounds into the coils strongly inhibits the formation of CO and CO ₂ and extends the mileage of the furnace. The disadvantage of this method is the need for continuous dosing of sulfur-containing compounds, contamination of decomposition products with sulfur-containing compounds and the occurrence of hydrogen sulfide corrosion of the coils.

These disadvantages are eliminated by pretreating the inner surfaces of the reactors with chemicals before carrying out a high-temperature process, such as pyrolysis.

Closest to the claimed technical solution is a method of passivation of the walls of the rectors by applying a protective coating of silicon. Passivation of the inner surface of the reactor is carried out by decomposition of the organometallic silicon compound in the vapor phase. For decomposition, an oxygen-free silicon compound is used, and the deposition is carried out in an inert or reducing atmosphere in the reactor tube until a thin layer of ceramic material forms on its surface. The result is a thin coating (RF patent No. 2079569, IPC ⁶ C23C 8/28, C10G 9/16, publ. 05/20/1997). The disadvantage of this technical solution is the insufficient strength of this coating, which leads to destruction of the layer, contamination of the decomposition products with silicon compounds and intensive coke formation on the exposed sections of the coil.

The problem to which the invention is directed is the development of an effective method for processing the inner surface of reactors for conducting high-temperature thermal processes.

The technical result of the claimed invention is to reduce the formation of coke by treating the inner surface of the reactor with a vapor-liquid acid solution, which leads to the formation of a catalytically inactive film and, as a result, an increase in the interregeneration path of the reactors, lower operating costs of the reactors, and a decrease in the formation of CO and CO ₂ at high temperature thermal processes.

The technical result of the claimed invention is achieved by the fact that:

1) in the idle mode of the reactor without heating by burners, a vapor-liquid acid solution is supplied to the coils at temperatures of 100-200 ° C;

2) washing the coils with a steam-liquid acid solution is carried out to ensure a constant concentration of metal ions in the spent acid solution at the outlet of the reactor;

3) after acid washing, an aqueous solution of the base is supplied to the coils to neutralize the medium in the coils;

4) both organic and inorganic acid with a concentration of 0.1-15 wt.% Can be used as an acid. %;

5) as the base can be used both organic and inorganic base with a concentration of 0.0001-1 wt. %;

6) to exclude corrosion of the coils, a nitrogen-containing acid corrosion inhibitor, for example, catapine, urotropine, gyroxylamine, etc., can be fed into the acid solution.

A method of reducing the formation of coke in reactors for carrying out high-temperature thermal processes according to the claimed invention is as follows.

Before acid washing, a reactor for conducting high-temperature thermal processes (pyrolysis furnace) is prepared, for which its coils are subjected to the standard procedure of oxidative coke burning, after which the burners are turned off and the reactor is cooled. Then steam is supplied to the coils with a temperature of 100-200 ° C and gradually the flow of a solution of inorganic (nitrous, sulfurous, etc.) or organic (citric, acetic, oxalic, etc.) acids with a concentration of 0.1-15 wt. % and in an amount providing a pH of the flow at the inlet of the coils 0-6 units. During the vapor-liquid acid washing, the content of metal ions is monitored at the outlet of the reactor. At the initial moment of acid washing, an increased concentration of metal ions in the spent acid solution is observed. This occurs as a result of the dissolution of salts, corrosion products and the surface layer of the coil alloy with an acid solution. To exclude corrosion of the coils, a nitrogen-containing corrosion inhibitor (catapine, urotropine, gyroxylamine, etc.) can be fed into the acid solution. As the procedure proceeds, the concentration of metal ions decreases and reaches a practically constant value. After stabilizing the concentration of metal ions, the acid solution is stopped and the base solution is supplied, which can be used as organic (monoethanolamine, diethanolamine, diethylamine, etc.) or inorganic alkalis (ammonia solution, sodium hydroxide, etc.) with concentrations of 0.0001-1 wt. % When the pH of the flow medium reaches 8-9.5 units, neutralization is stopped. As a result of the procedure, a protective catalytically inactive film is formed on the inner surface of the coils and salts and corrosion products are removed.

The start-up of the reactor and the further decomposition of hydrocarbons proceeds without significant formation of coke and CO and CO ₂ .

Example 1 (comparative without processing the coil).

The experiments were carried out on a laboratory pyrolysis unit described in RF patent No. 3290, IPC ⁷ C10G 1/00, publ. 09/20/2003, patent holder of OAO Nizhnekamskneftekhim. Instead of a quartz reactor, as described in the patent, a metal reactor made of steel grade 12X18H10T was used. Since sodium is the main metal polluting the coil of furnaces in ethylene production, pollution of the pyrolysis reactor was initially simulated with sodium. For this, an aqueous solution of sodium acetate with a concentration of 1000 mg / dm ^{3 was} supplied to the reactor heated to 800 ° C at a flow rate of 1 ml / min for 2 hours. After that, a series of 3 parallel experiments was conducted on the pyrolysis of n-butane without treatment of the inner surface of the reactor. Gaseous n-butane was supplied at a flow rate of 0.5 l / min under normal conditions; water was supplied to dilute hydrocarbons under normal conditions at a flow rate of 1.0 ml / min. The pyrolysis temperature was 850 ° C; the time spent on one experiment was 30 minutes. The amount of CO and CO _{2 formed was} determined 10 minutes after the start of the experiment by chromatographic method. At the end of the experiment, oxidative burning of coke was carried out by supplying air with a flow rate of 0.1 l / min, which led to the burning of coke with the formation of CO ₂ , which was sorbed in a tube filled with ascarite installed at the outlet of the reactor. Based on the weight gain of ascarite, taking into account the molecular weights of carbon dioxide and carbon, the yield of coke on the missed feed was calculated. The experiments showed that, with the untreated inner surface of the reactor, the content of CO and CO ₂ in the pyrolysis products averaged 1.10 wt. % and 0.09 wt. % respectively. The yield of coke on the missed feed was 0.23 wt. %

Example 2 (according to the invention with acid washing).

The experiment was carried out in a laboratory setup described in example 1. After the oxidative burning of coke, the surface of the reactor was processed. Organic acid - citric acid was used as the acid. For this purpose, demineralized water heated to a temperature of 110-150 ° С was supplied to a reactor cooled to room temperature with a pH of 7 equal to 1 unit and a flow rate of 1 ml / min. After 30 minutes, the water was replaced with an aqueous solution of organic citric acid with a concentration of 5 wt. % at similar temperatures and flow rates. 5 minutes after the acid solution was supplied, a sample of the spent solution was taken at the outlet of the reactor for analysis of the content of Na, Fe, Ni, Cr ions. The analysis was repeated every 30 minutes. When the concentration of metal ions was stabilized, as shown in Table 1, the aqueous acid solution was replaced after 215 minutes with demineralized water with pH = 7 to remove acid from the reactor and after 120 minutes when the pH of the reactor outlet reached 6.5, the water washing was completed .

After the surface treatment of the reactor was carried out, a series of 3 experiments was carried out on the pyrolysis of n-butane under the same conditions described in Example 1. The experiments showed that the content of CO and CO ₂ in the pyrolysis products of n-butane averaged 0.12 wt. % and 0.01 wt. % respectively. The coke yield on the skipped feedstock was 0.08 wt. % Thus, preliminary preparation of the reactor surface helps to reduce the formation of CO, CO ₂ and coke during the pyrolysis of hydrocarbons by an order of magnitude. In this case, there was no increase in the formation of these substances from experiment to experiment, and the discrepancy between the experiments did not exceed 10% rel., Which indicates that the effect of treating the surface of the reactor remains for a long time.

Example 3

The experiment was carried out under the conditions of example 2, only to reduce the corrosion of the reactor an acid corrosion inhibitor was used, and an ammonia solution was used to remove acid after treatment. Due to the fact that the processing effect remains for a long time, the reactor was replaced with a new one with the same dimensions and steel grades. After replacement, the reactor was contaminated with sodium by feeding an aqueous solution of sodium acetate at a concentration of 1000 mg / dm for 2 hours into a reactor heated to 800 ° C with a flow rate of 1 ml / min. Acid washing was carried out under similar conditions described in example 2, but the concentration of organic citric acid was 10 wt. % and a corrosion inhibitor was added - catapine in an amount of 0.1 wt. %

The use of a corrosion inhibitor allowed to reduce the amount of metals leached from the surface of the reactor, as shown in table 2.

After the acid wash, the aqueous acid solution was replaced with a 0.1% ammonia solution, as an inorganic base, to neutralize the medium and remove acid from the reactor. After 10 minutes, when the pH of the medium at the outlet of the rector reached 8–9.5 units, washing with ammonia solution was completed.

Example 4

Similar to example 3, but the concentration of organic citric acid was taken in an amount of 0.1 wt. %, and as a neutralizing reagent taken base - monoethanolamine with a concentration of 1 wt. %

Example 5

Similar to examples 2 or 3, but for acid washing was taken organic acid - acetic acid with a concentration of 15 wt. % As a neutralizing agent, an inorganic base is used - sodium hydroxide with a concentration of 0.0001 wt. %

Example 6

Similar to examples 2 or 3, but for acid washing was taken inorganic acid - nitrous with a concentration of 0.5 wt. % As a neutralizing agent, an inorganic base is used - sodium hydroxide with a concentration of 0.01 wt. %

Example 7

Similar to examples 2 or 3, but for acid washing was taken inorganic acid - nitrous with a concentration of 15 wt. % As a neutralizing agent, an organic base is used - diethanolamine with a concentration of 0.5 wt. %

Example 8

Similar to examples 2 or 3, but for acid washing was taken inorganic acid - sulfur dioxide with a concentration of 0.1 wt. % As a neutralizing agent, an organic base is used - diethanolamine with a concentration of 0.01 wt. %

Example 9

Similar to examples 2 or 3, but for acid washing was taken inorganic acid - sulfur dioxide with a concentration of 5 wt. % As a neutralizing agent, an organic base is used - diethylamine with a concentration of 0.0001 wt. %

Example 10

Similar to examples 2 or 3, but for acid washing was taken organic acid - oxalic with a concentration of 1 wt. %, As a neutralizing agent, an inorganic base is used - an ammonia solution with a concentration of 1 wt. %

Example 11 (comparative on an industrial pyrolysis furnace without processing a coil).

The tests were performed on a straight-run gasoline pyrolysis furnace type SRT-VI from Lummus. Comparative pyrolysis of straight-run gasoline without acid treatment of the coils was carried out. The process was carried out at a pyrolysis temperature of 835 ° C, a contact time of 0.20 seconds, the mass ratio of water vapor: raw materials = 0.5: 1.0. Coke formation was estimated by the rate of change of the temperature of the outer wall of the coils, measured using an optical pyrometer. This indicator characterizes the rate of coke formation during pyrolysis, because an increase in coke on the inner surface of the coils leads to a deterioration in heat transfer and an increase in the temperature of the outer wall of the coils. After starting the furnace, we measured the temperature of the outer wall of the coils of the pyrolysis furnace, which averaged 958 ° C. After 3 hours, the measurements were repeated and the wall temperature was 1015 ° C. The rate of increase in wall temperature was 19 deg / h, which indicates the rapid coking of the inner wall of the coils. The content of CO and CO ₂ in the pyrolysis products, determined by chromatographic method, 3 hours after starting, amounted to 1.21 wt. % and 0.12 wt. % respectively. Further measurements of the wall temperature of the coils and the contents of CO and CO ₂ were carried out once a day. The increase in the temperature of the outer wall averaged 0.16 deg / h; the content of CO and CO ₂ in the pyrolysis products averaged 0.25 and 0.04 wt. % respectively. The increased formation of CO and CO ₂ led to the need for additional purification of the obtained product, ethylene, and the mileage of the furnace was reduced to 853 hours.

Example 12 (according to the invention with acid washing).

After the standard oxidative burn procedure, the coils were acid-treated in Lummus type SRT-VI furnaces. To do this, the burners were turned off, the furnace was cooled, and steam was supplied to the coils with a temperature of 110-150 ° C and a flow rate of 12000 kg / h. The steaming of the coils was carried out for 30 minutes, after which a 5% citric acid solution containing 1 wt. % corrosion inhibitor (urotropin) with a flow rate of 2000 kg / h. Every 30 min, the spent acid solution was analyzed at the outlet of the furnace for the content of metal ions, as shown in table 3.

After stabilization of the concentration of metal ions in the spent solution, the acid washing was stopped and the medium in the coils was neutralized by supplying, in steam, an ammonia solution with a concentration of 0.01 wt. %, with a flow rate of 100 l / h for 10 minutes

After processing the inner wall of the furnace coils, pyrolysis of straight-run gasoline was carried out similarly to that described in Example 7. The rate of increase in the temperature of the outer walls of the coils in the first 3 hours was 6 deg / h, which indicates a significantly lower coke formation during the start-up of the furnace. The content of CO and CO ₂ in the pyrolysis products 3 hours after start-up was also reduced and amounted to 0.25 wt. % and 0.06 wt. % respectively. Further mileage of the furnace showed an increase in the temperature of the outer wall of the coils on average 0.09 deg / h, the content of CO and CO ₂ , on average, 0.11 and 0.01 wt. % respectively. This made it possible to exclude an additional stage of ethylene purification, and the furnace run was 1810 hours.

Thus, the use of the invention leads to a decrease in coke formation, which allows to increase the mileage between the stages of oxidative burning of reactors, and to reduce the formation of CO and CO ₂ , which pollute the decomposition products of hydrocarbon feedstocks.

The invention can be used to inhibit coke formation in any tubular reactors for carrying out high-temperature hydrocarbon decomposition processes, such as thermal cracking at a temperature of 500-700 ° C or pyrolysis at a temperature of 700-900 ° C, but also for any other tubular reactors operating at high temperatures.

Claims (4)

1. The method of chemical treatment of the inner surface of the reactor for the pyrolysis of hydrocarbons, characterized in that before starting the reactor, the surface is treated by washing with a steam-liquid acid solution at a temperature of 100-200 ° C and subsequent neutralization of 0.0001-1% by weight of an aqueous solution of the base, the neutralization is stopped when the medium reaches a stream at the outlet of the reactor pH 8-9.5. 2. The method according to p. 1, characterized in that the acid used is an organic acid with a concentration of 0.1-15 wt. % 3. The method according to p. 1, characterized in that the acid used is an inorganic acid with a concentration of 0.1-15 wt. % 4. The method according to p. 1, characterized in that the acid washing of the reactor is carried out with the addition of a nitrogen-containing acid corrosion inhibitor.