RU2232791C1 - Method of pyrolysis of liquid hydrocarbons in tubular furnaces - Google Patents

Abstract

FIELD: chemical technologies; methods of conducting of pyrolysis of hydrocarbon in tubular furnaces.

SUBSTANCE: the invention presents a method of conducting of pyrolysis of fluid hydrocarbons in tubular furnaces and is dealt with the field of chemical technologies, in particular, with the methods of conducting of pyrolysis of hydrocarbons in tubular furnaces. The method provides for injection of a preheated in the preliminary heating zone hydrocarbon raw material into a torrent of a hot products of a fuel combustion in a torch with supply of water steam into a zone of pyrolysis. At that they additionally create a zone of the high-temperature pyrolysis initiated by a flame and using as an initiator an open hydrogen - oxygen or hydrocarbon-oxygen flame torch of a stoichiometric composition having temperature of 1900-2500°С and power of 30-60 % wit a time of their presence in a zone of high-temperature pyrolysis of 0.06-0.12 seconds, which is gained with the help of a torch of diffusive type located at the exit from the zone of pyrolysis. At that the temperature in the zone of preliminary heating is kept within the range of 400-600°С. The presence of the liquid hydrocarbon raw material in the zone of pyrolysis is kept for 0.3-0.5 seconds under the temperature of 800-900°С and the ratio of water steam to the raw materials taken equal to 1 : 0.5-0.8. The invention allows to increase efficiency of the method at the expense of an increase of output of main products and speed of the process.

EFFECT: increased efficiency of the method at the expense of an increase of output of main products and speed of the process.

3 ex, 1 tbl

Description

The invention relates to chemical technology, and in particular to methods for pyrolysis of hydrocarbons in tube furnaces.

Pyrolysis of oil fractions or hydrocarbon gases with water vapor in tube furnaces is the main, mastered and widespread technological process for the production of lower olefins, dienes, aromatic hydrocarbons - intermediates for the main organic synthesis.

Despite the fact that over the past 25-30 years the technology of tubular pyrolysis has been significantly improved, the possibilities of this process are limited. Difficulties are associated with the need to use heat-resistant high alloy steels for the manufacture of pipes, the limited possibility of supplying large amounts of thermal energy to the reaction zone through the surface of the coil, the limited pyrolysis of raw materials due to the deposition of coke and resins in tubular reactors, and the insufficiently high yields of target products.

Known pyrolysis of hydrocarbons in the presence of homogeneous initiators, for example, halogen-containing substances or hydrogen peroxide [Adelson SV, Vorontsova TA, Melnikova SA et al. // Petrochemistry. 1979.V.19. Number 4. p. 577-582.]. However, when pyrolysis of hydrogen halides is used as an initiator of the reaction due to the aggressiveness of the latter, significant technical difficulties are created associated with corrosion of the main equipment, and pyrolysis with the addition of peroxides is accompanied by a significant consumption of expensive substances, for example, for the effective initiating action of Н ₂ O _2, its addition to raw materials should be in the amount of an order of magnitude greater than Hcl.

Closest to the proposed method for pyrolysis of liquid hydrocarbons is high-temperature pyrolysis with gaseous heat carrier [Bowen C.R., Wooderd W.P. // Chem. Progr. 1983. V.79. No. 1. p. 68-75.]. The fuel is a methane-hydrogen fraction burned in oxygen in a multi-nozzle burner with steam supply to control the temperature of the combustion products at 2000 ° C. The raw material heated to 300-400 ° C is injected into the flow of hot combustion products, where the raw materials are mixed, evaporated and pyrolyzed. The contact time is 0.015-0.03 s, the flow temperature decreases from 1200 ° C at the beginning of the reactor and to 900 ° C at the outlet of it. The main disadvantage of this method is the significant cost of obtaining a large mass of coolant, which indicates insufficient process efficiency.

The task of the invention is to increase the efficiency of the method by increasing the yield of target products and the speed of the process.

The problem is achieved in that in a method for carrying out the pyrolysis of liquid hydrocarbons in tube furnaces, comprising injecting heated in the preheating zone of hydrocarbon feed into the stream of hot fuel combustion products in the burner with the supply of water vapor in the pyrolysis zone, an additional zone of high-temperature pyrolysis initiated by the flame is additionally created using as an initiator, an open hydrogen-oxygen or hydrocarbon-oxygen flame torch of a stoichiometric composition having a temperature of 1900-2500 C and a power of 30-60% when the residence time in the zone of high-temperature pyrolysis is 0.06-0.12 s, which is obtained using a diffusion type burner located at the outlet of the pyrolysis zone, while the temperature of 400-600 ° is set in the preheating zone C, in the pyrolysis zone, the residence of liquid hydrocarbon feedstock is carried out for 0.3-0.5 s at a temperature of 800-900 ° C, and the ratio of water vapor: feedstock is taken equal to 1: 0.5-0.8.

In the pyrolysis zone at temperatures from 800 to 900 ° C, the feedstock is partially decomposed, and then undergoes conversion at a certain residence time in the high temperature zone (the area of the flame). At shallow conversion depths, this process is reduced mainly to carbon chain cleavage or dehydrogenation. Alkanes and alicyclic hydrocarbons included in the composition of liquid raw materials are sources of light hydrocarbons during pyrolysis. With increasing depth of conversion of the feedstock, the role of secondary reactions increases. The resulting olefins can undergo decomposition, polymerization, and condensation. Compared to alkanes and alicyclic hydrocarbons, aromatic hydrocarbons are less reactive and undergo mainly condensation and densification reactions, being the main intermediate products in heterogeneous coke formation processes on the walls of the reactor. The increase in the pyrolysis selectivity is facilitated by the suppression of the secondary decomposition reactions of the target products — olefins with a sufficient degree of primary reactions, the reduction of the partial pressure of the hydrocarbon part in the reactor by adding water vapor, which is an inert diluent.

Placing a diffusion-type burner inside the reactor at the outlet of the pyrolysis zone allows creating a zone of high-temperature pyrolysis initiated by flame in a hydrogen medium.

Flame at torch temperatures of 1900-2500 ° C (depending on the type of fuel and oxidizer) is a powerful source of heat supplied to the flow of raw materials and water vapor from the surface, tens of times smaller than the area of the pyrolysis zone. This heat is very effective, without losses, due to direct contact of the combustion products with the pyrolyzable mixture is transferred to the flow of raw materials and water vapor. According to calculations, the proportion of heat introduced by the torch was 15–20% in the total heat balance of pyrolysis. To transfer aromatic hydrocarbons, especially condensed aromatic hydrocarbons and coke, to gaseous products (ethylene, propylene, acetylene, divinyl, isoprene), a reducing medium is needed - hydrogen. The formation of hydrogen in the initiation zone is possible during the gasification of part of the feedstock, pyrolysis and coke products during high-temperature interaction with water vapor in a nonequilibrium state in the flare region:

- (CH ₂ ) _n- + nН ₂ О → nCO + (n + 1) Н ₂

C + H ₂ O → CO + H ₂

CO + H ₂ O → CO ₂ + H ₂

The flame is also not only a powerful source of heat, but also a supplier of a large number of hydrogen atoms up to 2.2-10 ¹⁷ cm ^-3 and other reactive radicals HO ₂ $\genfrac{}{}{0ex}{}{\text{.}}{\text{,}}$ HE $\genfrac{}{}{0ex}{}{\text{.}}{\text{,}}$ CH ₃ $\genfrac{}{}{0ex}{}{\text{.}}{\text{,}}$ C ₂ H ₅ $\genfrac{}{}{0ex}{}{\text{.}}{\text{,}}$ SSS $\genfrac{}{}{0ex}{}{\text{.}}{\text{,}}$ About ^.. and others. The combination of these two contributions - a high concentration of hydrogen atoms and temperature allows to increase the reaction rates and the completeness of conversion of the starting materials into final ones. High reaction rates provide high process performance. Since it is practically not necessary to introduce energy from the outside to create a high-temperature zone enriched with hydrogen in the reactor, this in turn determines the simplicity and reliability of the process equipment, as well as favorable economic indicators of the process.

An example of the implementation of the proposed method for the pyrolysis of liquid hydrocarbon raw materials is considered on the example of diesel fuel with the following main characteristics: boiling point 180-360 ° C, molecular weight 250, density at 20 ° C 0.83 kg / m ³ , C content - 85.65 wt. .%, H - 13.19%, S - 0.092%.

Example 1. The fuel is a mixture of propane-butane, burned in oxygen in a diffusion type burner with a temperature of combustion products at the level of 1900 ° C. The raw material heated to 600 ° C is injected with water vapor at a ratio of water vapor: raw material = 1: 0.8 into the pyrolysis zone (T = 900 ° C), where evaporation and pyrolysis take place. The residence time in the pyrolysis zone is 0.5 s. At the exit from the pyrolysis zone, the combustion products are mixed with the pyrolyzable mixture. The residence time is 0.12 s. The flow temperature decreases from 1400 ° C at the beginning of the high-temperature zone to 900 ° C at the exit from it. The power of the torch is 60%. The output of the main products under these conditions: 33.2% ethylene, 10.1% propylene, 7.5% butenes; the degree of gas generation is 75.4%, while pyrolysis with a gaseous coolant (prototype) allows to obtain: 30.9% ethylene, 10.7% propylene, 7.5% butenes; the degree of gas formation is 61.4%.

Example 2. When using hydrogen as fuel, also burned in oxygen with a temperature of combustion products at 2500 ° C with the following process parameters: torch power 30%, the ratio of water vapor: raw materials = 1: 0.5, preheating temperature 400 ° C, pyrolysis temperature 800 ° C, residence time in the pyrolysis zone 0.3 s, in the torch zone 0.06 s, the flow temperature decreases at the beginning of the high-temperature zone from 1600 to 910 ° C at the exit from it. The yield of ethylene in this case reaches 32.1%, propylene - 8.3%, butenes - 9.8%, and the degree of gas formation - 77.2%.

Example 3. The use as fuel of methane burned in a mixture with pure oxygen, allows to obtain combustion products with a temperature of 2200 ° C. When the combustion products are displaced with water vapor and partially pyrolyzed raw materials, the flow temperature in the high-temperature zone decreases from 1500 to 900 ° C. In the preheating zone, a temperature of 500 ° C is set, in the pyrolysis zone, the pyrolyzable mixture stays for 0.4 s at a temperature of 850 ° C, the torch power is 40% with a residence time in the torch zone of 0.09 s, and the ratio of water vapor: raw materials are taken equal to 1: 0.6. The data obtained are close in terms of the yields of the main products to the previous case using hydrogen as fuel (see the table of examples).

The desired initiation mode is obtained by supplying fuel and an oxidizing agent to the burner in a stoichiometric ratio. The power of the torch is determined by the oxygen content to the pyrolyzable feedstock in wt.%. The effectiveness of the effect of flame on the pyrolysis process is evaluated by the yields of the main products during initiated pyrolysis and by pyrolysis without initiator. The yields of the target products characterize the efficiency of the pyrolysis and depend on the temperature, contact time, dilution of the feedstock with water vapor, as well as the presence and power of the initiating flare, the residence time in the flare zone.

The increase in yields of the target pyrolysis products in the presence of the proposed torch is calculated by the difference in the yields of the corresponding products under the same conditions in the presence of the torch and without it.

Examples of pyrolysis of liquid hydrocarbons by the proposed method (examples 1-26), as well as pyrolysis with gaseous heat carrier (prototype) are shown in the comparative table.

The table shows that the creation of a high-temperature zone enriched in hydrogen in the reactor allows increasing the yield of gaseous products from 61-62 to 84-86% and reducing the yield of liquid products from 38-39 to 14-16% (t> 160 ° С). The yield of the target products increases for ethylene by 6.2%, butenes by 1.4%, acetylene by 5.1%, for propylene the yield decreases by 0.2%.

In the initiation zone:

1) the aromatic rings are saturated, ethylene is retained and the reactions of the formation of dienes (divinyl) are partially suppressed:

2) alkyl radicals react faster with H ₂ than with RH, for example:

Hydrogen atoms are more active and less selective in reactions with hydrocarbons (upon addition and cleavage).

The proposed method for the pyrolysis of liquid hydrocarbon raw materials with a degree of gas formation of 83-86% succeeded in obtaining up to 35-37% of ethylene, 10-11% of propylene, 4.5-5.5% of butenes, 5-6% of acetylene. With the selective hydrogenation of acetylene on a palladium catalyst, the ethylene yield will be even greater. The use of technology to initiate the process of pyrolysis of liquid hydrocarbon feedstock with a flame made it possible to obtain an ethylene yield equal to the maximum ethylene yield during pyrolysis with a gaseous coolant, reaching 28-30% of the processed feed, at a temperature in the pyrolysis zone of less than 800 ° C. In this case, the temperature of external heating is reduced by 25-100 ° C, which naturally simplifies the process. In addition, the possibility of secondary processes is sharply reduced, which allows to increase the selectivity.

The comparison table shows that due to the almost instantaneous transfer of heat, hydrogen atoms, and other active particles from the flame to the raw material subjected to pyrolysis, the pyrolysis process proceeds more efficiently with an increase in the yields of unsaturated hydrocarbons with minimal formation of tar and coke than the pyrolysis process with gaseous heat carrier ( prototype).

The advantage of the proposed method is that the necessary heating of the raw material in comparison with the prototype is carried out both through the wall and inside the reactor with direct contact of the raw material with the products of combustion of the flame.

Claims (1)

The method of pyrolysis of liquid hydrocarbons in tube furnaces, comprising injecting heated in the preheating zone of hydrocarbon feed into the stream of hot fuel combustion products in the burner with the supply of water vapor in the pyrolysis zone, characterized in that they additionally create a zone of flame-initiated high-temperature pyrolysis, using as an initiator open hydrogen-oxygen or hydrocarbon-oxygen flame torch of stoichiometric composition having a temperature of 1900-2500 ° C and a power of 30-60% at the residence time in the zone of high-temperature pyrolysis is 0.06-0.12 s, which is obtained using a diffusion type burner located at the outlet of the pyrolysis zone, while the temperature of 400-600 ° C is set in the preheating zone, the liquid hydrocarbon is in the pyrolysis zone raw materials are carried out for 0.3-0.5 s at a temperature of 800-900 ° C, and the ratio of water vapor: raw materials is taken equal to 1: 0.5-0.8.

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