CN106631659B - Steam cracking method - Google Patents
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- CN106631659B CN106631659B CN201510716141.8A CN201510716141A CN106631659B CN 106631659 B CN106631659 B CN 106631659B CN 201510716141 A CN201510716141 A CN 201510716141A CN 106631659 B CN106631659 B CN 106631659B
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Abstract
The invention relates to the field of chemical industry, and discloses a steam cracking method, which is implemented in a cracking furnace, wherein the cracking furnace comprises a convection section and a radiation section, a radiation furnace tube bank consisting of a plurality of groups of single-pass radiation furnace tubes is vertically arranged in the radiation section, and a bottom burner is arranged at the bottom of the radiation section, the method comprises the following steps: vaporizing and preheating cracking raw materials in a convection section, and then feeding the cracking raw materials into a radiation section for cracking reaction, wherein the one-way radiation furnace tube is a variable-diameter furnace tube with a twisted sheet tube, the bottom burner adopts oxygen-enriched air as combustion-supporting gas, and the heat supply of the bottom burner to the materials in the radiation furnace tube row at least accounts for 60% of the total heat supply; thereby obtaining a cracking method with ultrahigh selectivity, and simultaneously effectively improving the thermal efficiency of the cracking furnace, reducing the energy consumption and increasing the operation period of the cracking furnace.
Description
Technical Field
The invention relates to the field of chemical industry, in particular to a steam cracking method.
Background
Low carbon olefins such as ethylene, propylene and butadiene are important basic materials for petrochemical industry. At present, the method for producing low-carbon olefin is mainly a tubular furnace petroleum hydrocarbon steam cracking process. Statistically, about 99% of the world's ethylene, over 50% of the world's propylene, and over 90% of the world's butadiene are produced by this process.
The core equipment of the tubular furnace petroleum hydrocarbon steam cracking process is a tubular cracking furnace (hereinafter referred to as a "cracking furnace"), and when cracking raw materials such as ethane, propane, naphtha and hydrogenation tail oil are heated to high temperature in the cracking furnace, a carbon chain breaking chemical reaction can occur to generate low-carbon olefins such as ethylene, propylene, butadiene and the like. However, the thermal cracking reaction process is very complicated, and side reactions such as dehydrogenation, isomerization, cyclization, superposition and condensation can occur in addition to the target product of low-carbon olefin, thereby generating other by-products. Therefore, how to control the reaction conditions so that the target product, namely, the low-carbon olefin, is the most important subject of research in this field.
The results of long-term studies at home and abroad show that the raw material hydrocarbon is beneficial to the generation of olefin under the conditions of high temperature, short retention time and low hydrocarbon partial pressure. At the initial stage of the reaction, in terms of pressure drop, the conversion rate of the reaction is low, the volume of the fluid in the pipe is not increased much, the linear velocity of the fluid in the pipe is also not increased much, the pressure drop cannot be increased too much due to the small pipe diameter, and the increase of the average hydrocarbon partial pressure cannot be seriously influenced; from the aspect of heat intensity, the raw material is heated rapidly to absorb a large amount of heat, so that the heat intensity is required to be high, and the specific surface area can be increased by a small pipe diameter, so that the requirement is met; from the coking trend, the secondary reaction can not occur due to the lower conversion rate, the coking rate is lower, and the smaller pipe diameter is also allowable. In the later stage of the reaction, from the aspect of pressure drop, the conversion rate is higher at the moment, the volume of the fluid in the pipe is increased more, meanwhile, the linear velocity of the fluid is also increased sharply, and the larger pipe diameter is more suitable; from the aspect of heat intensity, the heat intensity begins to be reduced because the conversion rate is higher, and the heat transfer effect cannot be obviously influenced by the larger pipe diameter; from the aspect of coking trend, as the conversion rate is higher, the secondary reaction is more, the coking rate is increased, and the larger tube diameter of the furnace tube can ensure that the furnace tube is smooth and cannot cause too large pressure drop. In summary, in general, a cracking furnace tube is designed to have a smaller tube diameter at the inlet of the cracking furnace tube (i.e., at the initial stage of the reaction) and a larger tube diameter at the outlet of the cracking furnace tube.
In order to achieve the aims of high temperature, short residence time and low hydrocarbon partial pressure, almost all new furnace pipes with the structure adopt a method for shortening the pipe length, such as that the lummus company shortens the pipe length from eight-way 73m to two-way 25m or so; shiwei changes the tube length from 45m in the fourth pass to 21m in the second pass; KTI reduced the tube length from four passes 46m to two passes 23 m. Along with the shortening of the length of the tube, the retention time is reduced from more than 0.5s to 0.15-0.25 s. KBR even reduced the tube length of the furnace tube to about 12m and the residence time to less than 0.1 s.
The pressure drop of the materials in the furnace tube is reduced while the length of the tube is shortened, and the pressure drop is reduced to 0.04MPa or lower from about 0.15 MPa. The selectivity is improved due to the reduced hydrocarbon partial pressure, but the shorter tube length also brings the disadvantage of insufficient heat transfer area.
CN100338182C proposes a cracking furnace with single-pass reducing furnace tube, which comprises: the boiler comprises a boiler body, a high-pressure steam pocket, a convection section, a boiler tube, a combustor, a combustion chamber and a waste heat boiler, and is characterized in that the one-way reducing boiler tube is a vertical boiler tube, and the inner diameter of the outlet end of the one-way reducing boiler tube is larger than that of the inlet end of the one-way reducing boiler tube.
CN101062881B proposes a novel ethylene cracking furnace with one-way radiation furnace tubes, which also comprises a high-pressure steam drum, a convection section, radiation section furnace tubes, a burner, a radiation section and a quenching boiler, and is characterized in that the inner diameter of the outlet end of each one-way vertical reducing furnace tube is larger than that of the inlet end, a plurality of furnace tubes are connected to a collecting tube to form a furnace tube bundle, and the collecting tube in each furnace tube bundle is horizontally arranged. According to the furnace tube bending device, the bending connecting piece in front of the bend section and the furnace tube inlet section is arranged in the middle of the reducing furnace tube, so that the thermal stress condition of the heated furnace tube is improved, and the bending of the furnace tube is avoided.
CN10169012B proposes an ethylene cracking furnace with a once-through radiant furnace tube, which comprises a radiant section, a convection section, a quenching boiler, a collecting tube and a distribution tube, wherein one end of the distribution tube is connected to the collecting tube arranged at the lower part of the cracking furnace, and the other end is connected to the radiant furnace tube. The invention aims to utilize the space of the furnace tube to the maximum extent and reduce the geometric dimension and the occupied area of the cracking furnace; the furnace tubes of the adjacent groups are in a symmetrical structure, so that the furnace tubes are uniformly heated, and the operation period is prolonged; because the radiation furnace tubes are arranged in two rows in the radiation section, the radiation furnace tubes are connected by adopting the bent tubes so as to reduce the bending degree of the furnace tubes.
The above patent documents focus on how to design and arrange the single pass radiant tubes in the radiant section of the cracking furnace to ensure that the single pass radiant tubes do not distort at high furnace temperatures while being able to rapidly increase in temperature within a very short residence time. That is, these patent documents focus only on improvements in furnace tube configuration to accommodate the high temperatures and short residence times of the cracking furnace.
For the single-pass radiant furnace tube, because the retention time of the material in the furnace tube is short, the material is generally considered to be heated quickly after entering the single-pass radiant furnace tube, namely the heat intensity of the inlet part of the single-pass radiant furnace tube is high, so that the material can be heated quickly after entering the furnace tube, the heat supply quantity and the tube wall temperature of the rear section of the single-pass radiant furnace tube can be correspondingly reduced, the coking of the rear section is reduced, and the operation period of the cracking furnace is prolonged. Therefore, the design that the pipe diameter of the inlet end is small and the pipe diameter of the outlet end is large is often adopted in the design of the one-way radiation furnace pipe, so that the material temperature is low at the inlet end, the coking rate is slow, but the required heat flux is large, and the design of the small pipe diameter is favorable for the heat transfer process; at the outlet end, the material temperature is high, the coking rate is high, the pressure drop of the furnace tube is high, and the large pipe diameter is favorable for controlling the pressure drop. In the design of the single-pass radiant furnace tube, only one side of the furnace tube is considered, but one side of the hearth is not considered, namely, the combustion process of the fuel gas in the hearth is not well considered so as to adapt to the characteristics of the single-pass radiant furnace tube.
From the standpoint of the heat transfer of the furnace, fuel gas (primarily methane and hydrogen) is combusted to provide heat within the furnace chamber of the furnace, which is transferred into the furnace tubes by both radiant and convective heat transfer. Cracking furnaces typically employ mixed combustion of fuel gas and air to provide the heat required for the cracking reaction. Generally, the combustion reaction is caused by high energy collisions between combustible molecules in the fuel and oxygen molecules, so the supply of oxygen determines the combustion process.
Therefore, how to consider from two aspects of hearth combustion and furnace tube design, the characteristics of hearth combustion are matched with the characteristics of furnace tube design, and further the respective maximum advantages are exerted, and a novel cracking method with proper operation period, high selectivity, high thermal efficiency and low energy consumption is obtained, and further development and research are needed.
Disclosure of Invention
The invention aims to solve the problems of short operation period, low selectivity, low thermal efficiency and high energy consumption in the steam cracking process caused by insufficient heat supply at the bottom of a combustion system of a single-pass furnace tube cracking furnace, low flue gas blackness and mismatching with a single-pass radiant furnace tube.
In order to achieve the above object, the present invention provides a steam cracking method, which is implemented in a cracking furnace, the cracking furnace comprises a convection section and a radiation section, a radiation furnace tube row composed of a plurality of groups of single-pass radiation furnace tubes is vertically arranged in the radiation section, and a bottom burner is arranged at the bottom of the radiation section, the method comprises: the cracking raw material is vaporized and preheated in the convection section and then enters the radiation section for cracking reaction, wherein the one-way radiation furnace tube is a reducing furnace tube with a twisted sheet tube, the bottom burner adopts oxygen-enriched air as combustion-supporting gas, and the heat supply of the bottom burner to the material in the radiation furnace tube bank at least accounts for 60% of the total heat supply.
The traditional cracking furnace generally adopts air as combustion-supporting gas, because oxygen content in the air is only 21%, most is nitrogen gas, consequently in the combustion process, the burning velocity of fuel gas is slower, and burning flame is longer, and in cracking furnace's direction of height, furnace temperature is the curve and distributes, and is few in furnace bottom heat supply, and furnace middle part is then the heat supply is the most, and furnace upper portion heat supply begins to reduce. For a cracking furnace with a multi-pass furnace tube, because the residence time is long, the contradiction between the heat supply of a hearth and the heat absorption of the furnace tube is not outstanding, the contradiction is highlighted for a one-pass radiant furnace tube, the material is continuously and rapidly heated at the inlet end of the furnace tube, a large amount of heat is continuously provided, and the heat supply at the bottom of the traditional combustion system is less; at the outlet end of the furnace tube, the coking rate of the materials is increased sharply, secondary reaction needs to be controlled, and the heat supply at the middle upper part of the traditional combustion system is maximized. That is, there is a matching problem between the combustion system and the single pass radiant coils.
If the combustor of the cracking furnace adopts oxygen-enriched air with higher oxygen concentration than air for combustion, compared with air combustion, the method has the following advantages: firstly, because the radiation heat transfer is the main mode of pyrolysis furnace heat transfer, according to the characteristics of gas radiation, only the triatomic gas and polyatomic gas have the radiation ability, and the diatomic gas has almost no radiation ability, and under the combustion-supporting condition of conventional air, the proportion of nitrogen gas without radiation ability is very high, and the blackness of flue gas is very low, has influenced the radiation heat transfer process of flue gas to the boiler tube bank. Oxygen-enriched air is adopted for combustion supporting, and the nitrogen content is low, so that the air quantity and the flue gas quantity are both obviously reduced, the flame temperature and the blackness are obviously improved along with the increase of the oxygen proportion in the combustion air, the flame radiation intensity is further improved, and the radiation heat transfer is enhanced; secondly, oxygen-enriched air is adopted for supporting combustion, the flame of combustion is shortened, the combustion intensity is improved, the combustion speed is accelerated, the complete combustion reaction is facilitated, the use efficiency of fuel is improved, and the thermal efficiency of the cracking furnace is further improved; and thirdly, oxygen-enriched air is adopted for combustion supporting, so that the excess air coefficient can be properly reduced, the smoke exhaust volume is reduced, the smoke quantity after combustion is reduced, the smoke exhaust loss is further reduced, and the energy conservation of the cracking furnace is promoted.
The inventors of the present invention have found, through research, that by changing the combustion-supporting gas in the bottom burner disposed at the bottom of the radiant section of the tubular cracking furnace to oxygen-enriched air, and the heat supply of the bottom burner to the materials in the radiant furnace tube rows at least accounts for 60 percent of the total heat supply, and the one-way radiant furnace tube is a reducing furnace tube with twisted sheet tubes, can well solve the problems that the combustion system of the single-pass radiant furnace tube cracking furnace has insufficient heat supply to the bottom of the single-pass radiant furnace tube, has low flue gas blackness and is not matched with the single-pass radiant furnace tube, but also can obtain ultrahigh selectivity when the tubular cracking furnace is used for preparing low-carbon olefins such as ethylene, propylene, butadiene and the like, thereby obtaining a cracking method with ultrahigh selectivity, and simultaneously effectively improving the thermal efficiency of the cracking furnace, reducing the energy consumption and increasing the operation period of the cracking furnace.
Additional features and advantages of the invention will be set forth in the detailed description which follows.
Drawings
The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention and not to limit the invention.
In the drawings:
FIG. 1 is a schematic representation of steam cracking using the process of the present invention.
Description of the reference numerals
1. Blower 2, convection section
3. Tube row of radiant furnace tube
4. Combustion system
5. Radiation section 6, quenching boiler
Detailed Description
The following describes in detail specific embodiments of the present invention. It should be understood that the detailed description and specific examples, while indicating the present invention, are given by way of illustration and explanation only, not limitation.
The invention provides a steam cracking method, which can be implemented in a cracking furnace, wherein the cracking furnace can comprise a convection section and a radiation section, a radiation furnace tube row consisting of a plurality of groups of once-through radiation furnace tubes can be vertically arranged in the radiation section, and a bottom burner can be arranged at the bottom of the radiation section, and the method comprises the following steps: the cracking raw material is vaporized and preheated in the convection section and then enters the radiation section for cracking reaction, wherein the one-way radiation furnace tube is a reducing furnace tube with a twisted sheet tube, the bottom burner adopts oxygen-enriched air as combustion-supporting gas, and the heat supply of the bottom burner to the material in the radiation furnace tube bank at least accounts for 60% of the total heat supply.
According to the steam cracking method of the present invention, the cracking feedstock is not particularly limited, and preferably, the cracking feedstock may be at least one of ethane, propane, liquefied petroleum gas, naphtha, and hydrogenated tail oil.
In the present invention, the cracking raw material is preferably naphtha.
According to the steam cracking method of the invention, the cracking raw material enters the radiation section for cracking reaction after being vaporized and preheated in the convection section, wherein the temperature of preheating the cracking raw material in the convection section, namely the cross-over temperature (XOT) of the cracking furnace, is not particularly limited and can be selected conventionally by the person skilled in the art, preferably 550-630 ℃, and the temperature (COT) of the outlet of the radiation section of the cracking furnace is not particularly limited and can be selected conventionally by the person skilled in the art, preferably 820-860 ℃.
According to the steam cracking method, the heat supply of the bottom burner to the materials in the radiant furnace tube rows accounts for 60-90% of the total heat supply, and is further preferably 70-85%; in the present invention, the term "total heat supply amount" refers to the sum of the heat supply amount of the bottom burner to the material in the radiant furnace tube row and the heat supply amount of the sidewall burner to the material in the radiant furnace tube row.
According to the steam cracking method, the cracking furnace can also comprise a high-pressure steam drum, a combustion system and a quenching boiler, wherein the combustion system of the cracking furnace can adopt methane or a methane-hydrogen mixture as fuel and oxygen-enriched air as combustion-supporting gas, so that the nitrogen content is reduced, and the fuel is saved.
According to the steam cracking method of the present invention, the volume fraction of oxygen in the oxygen-enriched air may be 22% to 60%, preferably 25% to 40%, more preferably 27% to 33%; wherein, the oxygen-enriched air can be obtained by adopting a pressure swing adsorption method or a membrane permeation method.
According to the steam cracking method, the one-way radiation furnace tube can be a reducing furnace tube with twisted sheet tubes, in the invention, a boring machine can be adopted to bore a gradually-changing pipe diameter, the pipe inner diameter of the inlet end of the one-way radiation furnace tube is smaller than that of the outlet end, and the one-way radiation furnace tube simultaneously has a twisted sheet shape.
According to the steam cracking method of the present invention, preferably, the ratio of the inner diameter of the outlet end of the once-through radiant furnace tube to the inner diameter of the inlet end of the once-through radiant furnace tube may be greater than 1 and equal to or less than 1.4, and preferably 1.1 to 1.4. Wherein, the tube inner diameter refers to the diameter of the inner part of the tube orifice of the one-way radiation furnace tube.
According to the steam cracking method, the inner diameter of the outlet end of the once-through radiation furnace tube can be 35mm-65mm, and preferably 45mm-60 mm.
According to the steam cracking method of the present invention, the tube inner diameter of the inlet end of the once-through radiant furnace tube may be 25mm to 50mm, preferably 35mm to 45 mm.
According to the steam cracking method, the inlet end of the furnace tube is connected with the collecting tube, and materials are uniformly distributed through a Venturi or other distribution devices; the outlet end of the furnace tube is connected with a waste heat boiler.
According to the steam cracking method, preferably, the tube cavity of the once-through radiation furnace tube can be also provided with an enhanced heat transfer element to facilitate heat transfer. The enhanced heat transfer element is not particularly limited and may be a conventional choice for one skilled in the art, and in the present invention, the enhanced heat transfer element may be selected from one or more of a spiral sheet insert, a twisted ribbon insert, a crossed zigzag insert, a coil core insert, a filament wound porous body, and a spherical matrix insert; further preferably, the same or different enhanced heat transfer elements can be arranged in the tube cavity of the single-pass radiation furnace tube; still further preferably, non-identical enhanced heat transfer elements may be disposed within the lumens of the single pass radiant coils.
According to the steam cracking method of the present invention, the bottom burners may be disposed on both sides of the tube rows of the radiant furnace tubes; preferably, side wall burners can be arranged on the side wall of the radiant section of the tubular cracking furnace, and the side wall burners are arranged on two sides of the tube row of the radiant furnace tubes; thus, in the present invention, the combustion system of the cracking furnace may be composed of only bottom burners or bottom burners and side wall burners, which are distributed on both sides of the tube rows of the radiant furnace tubes in the furnace, wherein the side burners may also be symmetrically arranged in multiple rows in the height direction of both sides of the furnace tubes.
According to the steam cracking method of the present invention, the bottom burners may be symmetrically arranged on both sides of the tube row of the radiant furnace tubes, and the side wall burners may be symmetrically arranged on both sides of the tube row of the radiant furnace tubes.
Preferably, the bottom burners and the sidewall burners are each symmetrically arranged along the row of radiant furnace tubes.
According to the steam cracking process of the present invention, the number of bottom burners corresponding to each set of said once-through radiant coils is from 2 to 8, preferably from 3 to 6.
According to the steam cracking process of the present invention, when the tubular cracking furnace further has side wall burners, the number of the side wall burners corresponding to each set of the once-through radiant furnace tubes may be 2 to 16, preferably 4 to 10.
According to the steam cracking method of the present invention, the bottom burner and the sidewall burner can use, but are not limited to, methane or a mixture of methane and hydrogen as fuel.
The present invention will be described in detail below by way of specific examples.
Example 1
This example is intended to illustrate the steam cracking carried out by the process of the invention.
The steam cracking schematic diagram shown in fig. 1 is adopted to carry out cracking reaction, and the specific process comprises the following steps:
the method is implemented in a cracking furnace comprising a fan 1 and a quenching boiler 6, wherein the cracking furnace comprises a convection section 2 and a radiation section 5, naphtha of 60 ℃ is vaporized and preheated by the convection section 2 and then enters a radiation furnace tube bank 3 consisting of three groups of single-pass radiation furnace tubes for cracking reaction, wherein the preheating temperature of the naphtha in the convection section, namely the crossing temperature (XOT) of the cracking furnace is 590 ℃, and the outlet temperature (COT) of the radiation section of the cracking furnace is 841 ℃;
the radiant section is internally and vertically provided with a radiant furnace tube row 3 consisting of three groups of single-pass radiant furnace tubes, 12 bottom burners are arranged at the bottom of the radiant section, 36 side-wall burners are arranged on the side surface of the radiant section, a combustion system 4 of the radiant section 5 adopts a mode of combining the bottom burners and the side-wall burners, and the heat supply of the bottom burners to materials in the radiant furnace tube row accounts for 80% of the total heat supply; oxygen-enriched air is adopted as combustion-supporting gas, and the concentration of oxygen contained in the oxygen-enriched air is 30 volume percent (V/V);
wherein, the radiant section furnace tube 3 adopts a one-way furnace tube, the one-way radiant furnace tube is a reducing furnace tube with a twisted sheet tube, the diameter of the inlet of the furnace tube is 41mm, the diameter of the outlet of the furnace tube is 53mm, the length of the furnace tube is 12.8m, and the furnace tube adopts a downward feeding and an upward discharging; the cracked product is then selectively collected via quench boiler 6. And a spiral sheet insert is arranged in the tube cavity of the single-pass radiation furnace tube.
Other process parameters of the cracking furnace are shown in table 1;
the composition of the furnace fuel gas is shown in Table 2, as determined by the analysis of the furnace fuel gas.
As can be seen from Table 1, after the oxygen-enriched combustion was adopted, the amount of fuel gas used in the cracking furnace was reduced due to the reduction in the amount of nitrogen carried by the combustion-supporting oxygen, as compared with 7050Nm in comparative example 13Reduction of/h to 6933Nm3H, fuelGas saving is about 1.66%; and the bottom burner which adopts oxygen-enriched air as combustion-supporting gas is matched with the one-way radiation furnace tube which adopts a reducing furnace tube with a twisted sheet tube, so that ultrahigh selectivity can be obtained when the tubular cracking furnace is used for preparing low-carbon olefins such as ethylene, propylene, butadiene and the like.
Comparative example 1
Steam cracking was carried out in the same manner as in example 1, except that air was used as a combustion-supporting gas and the air contained oxygen at a concentration of 21 volume% (V/V);
TABLE 1
TABLE 2
| Components | mol％ |
| Hydrogen gas | 3.6 |
| Methane | 95.8 |
| Ethane (III) | 0.23 |
| Propane | 0.08 |
| Others | 0.29 |
| Total up to | 100.00 |
As can be seen from the results in Table 1, after combustion with air, the amount of fuel gas used in the cracking furnace increases due to the high amount of nitrogen carried in the combustion-supporting gas; and the mutual matching of a bottom burner which adopts oxygen-enriched air as combustion-supporting gas and a one-way radiation furnace tube which adopts a reducing furnace tube with a twisted sheet tube is not adopted, so that the selectivity obtained when the tubular cracking furnace is used for preparing low-carbon olefins such as ethylene, propylene, butadiene and the like is low.
Example 2
This example is intended to illustrate the steam cracking carried out by the process of the invention.
Steam cracking was carried out in the same manner as in example 1, except that the heat supplied by the bottom burner to the material in the radiant tube bank accounted for 70% of the total heat supplied; oxygen-enriched air is adopted as combustion-supporting gas, and the oxygen concentration in the oxygen-enriched air is 27 volume percent (V/V);
the radiant section furnace tube adopts a one-way furnace tube, the one-way radiant furnace tube is a reducing furnace tube with twisted sheet tubes, the diameter of an inlet of the furnace tube is 35mm, the diameter of an outlet of the furnace tube is 45mm, the length of the furnace tube is 12.8m, and the furnace tube adopts a downward feeding mode and an upward discharging mode; then selectively collecting the cracked product through a quenching boiler.
Comparative example 2
Performing steam cracking according to the same method as the embodiment 2, except that the one-way radiation furnace tube is not a reducing furnace tube with a twisted sheet tube, and the diameter of the inlet tube of the furnace tube and the diameter of the outlet tube of the furnace tube are both 53 mm;
as can be seen from Table 3, the selectivity obtained when using a tubular cracking furnace to produce lower olefins such as ethylene, propylene and butadiene is lower because the bottom burner using oxygen-enriched air as the combustion supporting gas is not matched to the single pass radiant furnace tube using a variable diameter furnace tube with twisted finned tubes.
TABLE 3
As can be seen from Table 3, after the oxygen-enriched combustion was adopted, the amount of fuel gas used in the cracking furnace was reduced due to the reduction in the amount of nitrogen carried by the combustion-supporting oxygen, as compared with 7050Nm in comparative example 23Reduction of/h to 6958Nm3Per hour, about 1.31% fuel gas savings; and the bottom burner which adopts oxygen-enriched air as combustion-supporting gas is matched with the one-way radiation furnace tube which adopts a reducing furnace tube with a twisted sheet tube, so that ultrahigh selectivity can be obtained when the tubular cracking furnace is used for preparing low-carbon olefins such as ethylene, propylene, butadiene and the like.
Example 3
This example is intended to illustrate the steam cracking carried out by the process of the invention.
Steam cracking was carried out in the same manner as in example 1, except that the bottom burner supplied heat to the material in the radiant tube bank accounted for 85% of the total heat supplied; oxygen-enriched air is adopted as combustion-supporting gas, and the oxygen concentration in the oxygen-enriched air is 33 volume percent (V/V);
the radiant section furnace tube adopts a one-way furnace tube, the one-way radiant furnace tube is a reducing furnace tube with twisted sheet tubes, the diameter of an inlet of the furnace tube is 45mm, the diameter of an outlet of the furnace tube is 60mm, the length of the furnace tube is 12.8m, and the furnace tube adopts a downward feeding mode and an upward discharging mode; then selectively collecting the cracked product through a quenching boiler.
Comparative example 3
Steam cracking was carried out in the same manner as in example 3, except that air was used as a combustion-supporting gas and the air contained an oxygen concentration of 21 volume% (V/V); the one-way radiation furnace tube is not a reducing furnace tube with a twisted sheet tube, the diameter of the inlet tube of the furnace tube is 45mm, and the diameter of the outlet tube of the furnace tube is 45 mm;
as can be seen from Table 4, the selectivity obtained when using a tubular cracking furnace to produce lower olefins such as ethylene, propylene and butadiene is lower due to the fact that the bottom burner, which does not use oxygen-enriched air as a combustion-supporting gas, is matched to the single-pass radiant furnace tube, which uses a variable-diameter furnace tube with twisted finned tubes.
TABLE 4
As can be seen from Table 4, after the oxygen-enriched combustion was adopted, the amount of fuel gas used in the cracking furnace was reduced due to the reduction in the amount of nitrogen carried by the combustion-supporting oxygen, as compared with 7050Nm in comparative example 33Reduction of/h to 6902Nm3Per hour, about 2.09% fuel gas savings; and the bottom burner which adopts oxygen-enriched air as combustion-supporting gas is matched with the one-way radiation furnace tube which adopts a reducing furnace tube with a twisted sheet tube, so that ultrahigh selectivity can be obtained when the tubular cracking furnace is used for preparing low-carbon olefins such as ethylene, propylene, butadiene and the like.
From the data in examples 1-3 and comparative examples 1-3 above and tables 1-4, it can be seen that: the inventor of the invention changes the combustion-supporting gas in the bottom burner arranged at the bottom of the radiation section of the tubular cracking furnace into oxygen-enriched air, and ensures that the heat supply of the bottom burner to the materials in the radiation furnace tube row at least accounts for 60 percent of the total heat supply, and the single-pass radiation furnace tube is the reducing furnace tube with the twisted sheet tube, thereby well solving the problems of insufficient heat supply to the bottom of the single-pass radiation furnace tube, low smoke blackness and unmatched with the single-pass radiation furnace tube of the combustion system of the single-pass radiation furnace tube cracking furnace, and obtaining ultrahigh selectivity when the tubular cracking furnace is used for preparing low-carbon olefins such as ethylene, propylene, butadiene and the like, thereby obtaining a cracking method with ultrahigh selectivity, and simultaneously effectively improving the heat efficiency of the cracking furnace, reducing the energy consumption and increasing the operation period of the cracking furnace.
The preferred embodiments of the present invention have been described in detail, however, the present invention is not limited to the specific details of the above embodiments, and various simple modifications may be made to the technical solution of the present invention within the technical idea of the present invention, and these simple modifications are within the protective scope of the present invention.
In addition, any combination of the various embodiments of the present invention is also possible, and the same should be considered as the disclosure of the present invention as long as it does not depart from the spirit of the present invention.
Claims (10)
1. A steam cracking process implemented in a cracking furnace comprising a convection section and a radiant section, wherein a radiant furnace tube bank consisting of a plurality of single pass radiant furnace tubes is vertically disposed in the radiant section, and a bottom burner is disposed at the bottom of the radiant section, the process comprising: the method is characterized in that a single-pass radiation furnace tube is a variable-diameter furnace tube with a twisted finned tube, the bottom burner adopts oxygen-enriched air as combustion-supporting gas, the heat supply of the bottom burner to materials in a radiation furnace tube row at least accounts for 70-85% of the total heat supply, and the concentration of oxygen contained in the oxygen-enriched air is 27-33% by volume; the ratio of the inner diameter of the outlet end of the one-way radiation furnace tube to the inner diameter of the inlet end of the one-way radiation furnace tube is 1.1-1.4, the side wall of the radiation section of the tubular cracking furnace is also provided with side wall burners, and the bottom burners and the side wall burners are respectively and symmetrically arranged along the tube row of the radiation furnace tube.
2. The method of claim 1, wherein the tube inner diameter of the outlet end of the single pass radiant furnace tube is from 35mm to 65 mm; the inner diameter of the tube at the inlet end of the one-way radiation furnace tube is 25mm-50 mm.
3. The method of claim 1, wherein the tube inner diameter of the outlet end of the single pass radiant furnace tube is from 45mm to 60 mm; the inner diameter of the tube at the inlet end of the one-way radiation furnace tube is 35mm-45 mm.
4. The method of claim 1, wherein an enhanced heat transfer element is disposed within a tube lumen of the single pass radiant furnace tube.
5. The method of claim 4, wherein the enhanced heat transfer element is selected from one or more of a spiral sheet insert, a twisted ribbon insert, a cross-zigzag insert, a coil core insert, a filament wound porous body, and a spherical matrix insert.
6. The method of claim 4, wherein the same or different enhanced heat transfer elements are disposed within the lumens of the single pass radiant coils.
7. The method of claim 1, wherein the number of bottom burners corresponding to each set of single pass radiant coils is 2-8.
8. The method of claim 1, wherein the number of bottom burners corresponding to each set of single pass radiant coils is 3-6.
9. The method of claim 1, wherein the number of sidewall burners corresponding to each set of single pass radiant furnace tubes is 2-16.
10. The method of claim 9, wherein the number of sidewall burners corresponding to each set of single pass radiant furnace tubes is 4-10.
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