CN111533636A - Industrial cracking furnace with shielding-type distributed radiation section furnace tubes - Google Patents

Abstract

The invention discloses a novel industrial cracking furnace with shielding-type distributed radiation section furnace tubes, which comprises a convection section, a radiation section connected with the convection section, a plurality of groups of radiation furnace tubes vertically arranged in the radiation section, a burner and a quenching boiler. Wherein the outlet tube of the radiant section furnace tube is physically shielded by the inlet tube, having a smaller direct radiating area than the inlet tube. The invention can effectively reduce the heat load intensity of the outlet pipe. The invention has the advantages of improving the productivity by about 15 percent, or prolonging the operation period by about 25 percent and improving the selectivity of olefin by about 1 percent under the condition of the same cracking depth or conversion rate.

Description

An industrial cracking furnace with shielded arrangement of radiant section furnace tubes

technical field

The invention relates to an industrial cracking furnace with a shielded arrangement of radiant section furnace tubes.

Background technique

The ethylene industry is an industry that produces basic raw materials for the petrochemical industry such as trienes (ethylene, propylene, 1,3-butadiene) and triphenyls (benzene, toluene, xylene), which are widely used in the three major synthetic materials (plastics). , synthetic fibers, synthetic rubber) and other organic chemical products. The rapid growth of the ethylene industry is conducive to promoting the development of the national economy. The production capacity of ethylene plants is one of the important indicators to measure the development level of a country's petrochemical industry.

As the core of the ethylene plant, the cracking furnace accounts for about 65% of the energy consumption of the entire process. The intensive and large-scale production of the cracking plant can significantly reduce its initial construction cost and later operating cost. During the development process, the production capacity of a single cracker gradually increased from the initial 100,000 tons/year to 1 million tons/year, while the standard energy consumption per ton of ethylene gradually decreased to one third of the original. At present, the theoretical maximum production capacity of a single ethylene plant is 1.4 million tons per year. Due to the load limitation of the downstream compression and separation system, the upper limit cannot be exceeded in a short period of time. Therefore, the means to further reduce the energy consumption of the ethylene plant still have to return to the ethylene cracking furnace. Efficiency improvement.

The cracking furnace is located at the most upstream of the entire ethylene plant. The mixture of cracking raw materials (usually hydrocarbons) and dilution steam enters the radiation chamber through one or more rows of cracking furnace tubes arranged in the radiation section, where a cracking reaction occurs and is converted into small molecules of alkenes. The residence time of the cracked gas in the tube is generally 0.1s-0.5s. During this time, the cracked raw material undergoes a cracking reaction to generate a large amount of olefin products and rapidly heats up to about 780-900°C at the outlet of the furnace tube. The heat required for the cracking reaction is provided by the bottom burner and side wall burners in the furnace.

One of the main problems in the process of hydrocarbon thermal cracking is the deposition of hydrocarbon compounds on the surface of the inner wall of the cracking furnace tube, also known as coking. On the one hand, the coking of the cracking furnace tube hinders the heat transfer between the furnace and the furnace tube and increases the consumption of fuel; on the other hand, it causes the temperature of the outer wall of the furnace tube to gradually increase, causing problems such as high temperature creep of the furnace tube; The furnace needs to be periodically shut down to clean the coke to eliminate the negative effects caused by the coking, and the cleaning will lead to the loss of production capacity of the cracking furnace, and it also requires additional maintenance costs. The heating condition of the furnace tubes in the radiant section is very important to the operation cycle and economic benefits of the cracking furnace.

Theoretically, the ideal cracking process should allow the cracked raw materials to be rapidly heated in a short time to reach the corresponding cracking temperature and crack into the desired small molecular olefins, and then rapidly cool the cracked products to prevent secondary reactions of the cracked products, thereby maximizing the Olefin yield. This mode of operation requires that the inlet tube of the radiant furnace tube of the cracking furnace absorbs a large amount of heat, while the heat absorbed by the outlet tube should be relatively low, which can not only improve the yield of the target product, but also prevent the outlet tube from being accelerated due to excessive heat load. Coking, shortening the operation period of the cracking furnace. At present, most of the cracking furnaces adopt the arrangement of the inlet pipe and the outlet pipe on the center line of the radiant section, as shown in Figure 1, in which the inlet pipe row and the outlet pipe row are grouped corresponding to different burners. The problem is that the outlet pipe directly faces the burner with the same radiation power as the inlet pipe, and the heat flux is large, which will cause the yield to drop and accelerate the coking of the outlet pipe. Therefore, it is an important content to improve the operation efficiency of cracking furnace to adjust the structure of the tube row in the radiant section of the cracking furnace to achieve a reasonable heat distribution between the inlet and outlet tubes.

SUMMARY OF THE INVENTION

In order to solve the problems existing in the above-mentioned prior art, the present invention discloses an industrial ethylene cracking furnace, which includes two passes of radiant furnace tubes, namely an inlet tube and an outlet tube, wherein the inlet tube is the first pass of the radiant furnace tube and is also the raw material for cracking. The channel entering the radiant section, the outlet tube is the second pass of the radiant furnace tube, and is also the channel for the pyrolysis products to leave the radiant section. In the present invention, the outlet tube of the radiant furnace tube is physically shielded by the inlet tube, which has more Small direct radiation area.

The core of the ethylene plant is the cracking furnace, in which the hydrocarbon mixture feed is converted into cracked products by thermal cracking in the presence of steam, which acts as a diluent. The cracked gas is usually rich in ethylene and propylene, and the hydrocarbon feedstock can include ethane, propane, liquefied petroleum gas, naphtha, hydrogenated tail oil, etc. according to the degree of severity.

The cracking furnace includes at least one radiant section, which includes a burner for supplying heat to the cracking process, and the cracked raw materials enter the radiant section through a plurality of sets of radiant furnace tubes. The feed in the tube is heated to a temperature at which rapid cracking of the feed occurs, producing the desired olefins such as ethylene and propylene. It is generally believed that the non-ideal cracking conditions are caused by the mismatch between the heat flux distribution and the requirements of the cracking process. According to the above analysis, in order to improve the yield and selectivity of the cracking process, the heat flux of the inlet pipe should be increased to a certain extent. flux, and reduce the heat flux in the outlet pipe. The present invention realizes the regulation of heat flux by designing the arrangement of the inlet pipe and the outlet pipe of the radiant furnace tube.

Among them, the physical shielding of the outlet tube of the radiant furnace tube by the inlet tube is defined as: only part of the heat radiation emitted from the high temperature flame above the burner or the refractory brick can directly reach the surface of the outlet tube due to the physical barrier of the inlet tube. That is, observing the radiant furnace tube from the position of the burner, it can be observed that part of the outlet tube is blocked by the inlet tube. Due to this shielding effect, the outlet pipe receives less heat flux than the inlet pipe, and the degree of physical shielding indicates the extent to which the heat generated by the burner cannot reach the outlet directly due to this obstruction during the operation of the cracking furnace Tube.

In principle, the physical shielding described above can also be achieved by placing other objects between the outlet pipe and the burner, such as the use of thermally insulating coatings or shielding layers. The shielding method mainly adopted in the present invention is realized by using the inlet section of the radiant furnace tube as a shielding layer.

The invention is beneficial to reduce the cracking temperature difference in the outlet pipe, so that the outlet pipe can be operated within a relatively uniform temperature range. From the heating curve of the cracked gas in the radiation furnace tube, it can be seen that the temperature rise of the cracked gas in the outlet tube of the conventional cracking furnace is relatively fast, about 130 °C, while the temperature rise of the cracked gas in the outlet tube of the cracking furnace of the present invention is relatively small, about 115 °C, Therefore, the present invention allows the temperature rise of cracking in the outlet pipe to be reduced by about 15°C, which can effectively reduce the occurrence of secondary reactions from the perspective of cracking reaction.

Compared with the furnace tube arrangement in the radiant section of the conventional cracking furnace, the invention can effectively increase the average temperature of the cracked gas in the radiant tube, thereby allowing a relatively shorter residence time to improve the conversion rate of small molecular olefin products. The residence time of the cracked gas in the conventional two-pass radiant furnace tube is usually 0.20-0.25 seconds, while the residence time in the radiant furnace tube of the present invention can be reduced to about 0.15-0.22 seconds. Thus, the present invention allows for a reduction in residence time of about 15% to achieve higher olefin selectivity.

The present invention can also effectively reduce the heat load intensity of the outlet pipe. Fig. 3 shows the distribution of the heat flux along the length direction of the radiant furnace pipe simulated by the cracking furnace simulation software. Compared with the traditional cracking furnace, it can be seen that the heat flux of the inlet pipe of the present invention is significantly increased, while the heat flux of the outlet pipe is significantly reduced. This heat flux distribution has two significant benefits: on the one hand, it reduces the cracking temperature rise in the outlet pipe, which helps to reduce the occurrence of secondary reactions and improves the selectivity of cracked products; on the other hand, compared with the existing cracking furnaces , the coking rate of the cracking furnace outlet pipe designed by the present invention is significantly reduced. Cracking products (including ethylene, propylene and other small molecular olefins) are potential coking precursors, which are easy to catalyze reactions with metals on the surface of furnace tubes at high temperatures, and deposit to form coke, thereby hindering the heat transfer process and reducing the cracking gas flow area , increasing the pressure drop in the tube.

Due to the higher olefin concentration and temperature, coking mainly occurs in the outlet pipe, and the present invention reduces the heat flux of the outlet pipe and the temperature of the metal surface of the pipe wall, thereby effectively reducing the coking rate of the outlet pipe, as shown in Figure 4 shown. The invention can significantly improve the service life of the radiant furnace tube and the operation period of the cracking furnace, reduce the frequency that the cracking furnace needs to be shut down for coking, thereby increasing the production capacity of the cracking furnace. Table 1 compares the comparison results of the present invention and the traditional cracking furnace in terms of production capacity, operation period and olefin selectivity. Under the conditions of the same cracking depth or conversion rate, the present invention can increase the production capacity by about 15%, or prolong the operation period by about 25%. %, increasing the olefin selectivity by about 1%.

Table 1: Comparison of traditional furnace tube arrangement and the present invention

The invention is suitable for different kinds of hydrocarbon thermal cracking processes, and the feed can cover gas phase hydrocarbons such as ethane, propane and liquefied petroleum gas, to liquid phase hydrocarbons such as carbon five, light naphtha, hydrogenated tail oil, etc.

The present invention is applicable to the radiant section of the cracking furnace comprising two rows of burners (including bottom burners or side wall burners and their combinations), usually the two rows of burners are located on both sides of the radiant section, and the radiant furnace tubes enter and exit from the top, and are mainly arranged Near the centerline of the radiant section, it includes at least 2 rows of inlet pipes and 1 row of outlet pipes. The outlet pipe row is located between the inlet pipe rows, and the inlet pipe row is located between the burners on both sides. - the arrangement of the outlet pipe row - the inlet pipe row - the burner).

In the arrangement method of the radiant furnace tubes of the cracking furnace of the present invention, the inlet tubes and the outlet tubes can be arranged in sequence or in a staggered arrangement, the ratio of the tube center distance to the outer diameter of the tubes is between 2 and 6, and the outlet tubes can be arranged in a single row on the radiant tube. The centerline of the segment, or multiple rows are arranged at positions deviating from the centerline of the radiating segment.

The diameter of the inlet pipe and the outlet pipe of the present invention can be the same or different. Usually, the diameter of the outlet pipe is slightly larger than that of the inlet pipe. On the one hand, the heat absorption per unit volume of the cracked gas in the outlet pipe is reduced, which is beneficial to prevent the occurrence of secondary reactions. On the other hand, due to the increase in the number of molecules in the cracking process, the increased outlet pipe diameter can reduce the pressure drop in the pipe, and prolong the operation period of the cracking furnace while improving the olefin selectivity.

The invention can adjust the parameters such as the arrangement of the inlet and outlet pipes, the distance between the tube centers to adapt to different lengths of the radiant furnace tubes. The starting and ending positions of the radiant furnace and the bending radius of the U-shaped return bend are suitable for different lengths of the radiant furnace tubes. The U-shaped return bend is located at the bottom of the radiant section, with a diameter between 100mm-1000mm, and the suspension height is between 300mm-1000mm from the bottom surface of the radiant section. The S-bend can be set on one or both sides of the U-shaped return bend. In the case of different diameters of inlet and outlet pipes, a section of reducing pipe can be added to the front or rear of the U-shaped return bend.

The radiant furnace tube used in the present invention can add internal components to enhance heat transfer, so as to further improve the olefin selectivity and prolong the operation period.

Description of drawings

The above summary of the present invention and the following detailed description will be better understood when read in conjunction with the accompanying drawings. It should be noted that the accompanying drawings are merely illustrative of the claimed invention. In the drawings, the same reference numbers represent the same or similar elements.

FIG. 1 shows a front view, a top view and a left side view of a set of two-pass radiant furnace tube arrangement of a conventional cracking furnace.

FIG. 2 is a temperature distribution of cracked gas along the length of a radiant furnace tube according to an embodiment of the present invention.

3 is a heat flux distribution along the tube length of a radiant furnace tube according to an embodiment of the present invention.

4 is a distribution of coking rates along the length of a radiant furnace tube according to an embodiment of the present invention.

FIG. 5 is a schematic diagram of an ethylene cracking furnace according to an embodiment of the present invention.

6 is a front view, a top view and a left side view (Example 1) of a set of two-pass radiant furnace tube arrangement according to an embodiment of the present invention.

7 is a front view, a top view, and a left side view of a set of two-pass radiant furnace tube arrangement according to an embodiment of the present invention (Example 2).

Detailed ways

The detailed features and advantages of the present invention are described in detail below in the specific embodiment, and the content is sufficient to enable any person skilled in the art to understand the technical content of the present invention and implement it accordingly, and according to the description, claims and drawings disclosed in this specification. , those skilled in the art can easily understand the related objects and advantages of the present invention.

Fig. 5 shows the schematic diagram of the ethylene cracking furnace involved in the present invention, wherein (1) is the convection section, which is mainly used for the preheating of cracked raw materials and the waste heat recovery of flue gas; (2) is the radiation section, which is connected with the convection section (1) ) connection is the main place where the cracking process occurs. The raw material is heated to a specified temperature by the fuel gas in the furnace in the radiant section, and small molecular olefins are cracked to produce small molecular olefins; there are multiple rows of radiant furnace tubes (4) arranged in the radiant section, the furnace tubes from the top Entering the radiant section, it is mainly arranged near the centerline of the radiant section, and returns to the top through a U-shaped turn to leave the cracking furnace, enter the quenching boiler indicated by (3), rapidly cool and send it to the subsequent separation device; the radiant section also It includes a bottom burner (6) and a side wall burner (5), which are respectively located on both sides of the radiant section close to the refractory bricks and away from the radiant furnace tube, so as to avoid the generated high temperature flame from damaging the radiant furnace tube.

Figure 6 shows an example of the arrangement of a group of two-pass radiant furnace tubes of the present invention, including a front view, a top view and a left view, wherein (1) is the outlet tube, (2) is the inlet tube, and the lengths are about 8 About meters, the inlet and outlet pipes are connected by two S-shaped bends (3) and a U-shaped return bend (4).

The length of the S-shaped bend connected to the inlet pipe is about 2 meters, and the bending radius of the U-shaped return bend is 350 mm. The pipe diameters of these two parts are the same as the inlet pipe.

The length of the S-shaped elbow connected to the outlet pipe is about 2 meters, and the radius is the same as that of the outlet pipe.

The outlet pipes are all arranged on the A plane in the top view, while the inlet pipes are divided into two groups, which are arranged on the B plane and the C plane respectively, and the outlet pipes are physically shielded from both sides. In this example, the distance between the inlet tube row and the outlet tube row and the tube center distance are relatively large, and the furnace tube arrangement is relatively scattered, which is suitable for the radiant section of the cracking furnace with moderate radiation intensity.

Figure 7 shows an example of the arrangement of a group of two-pass radiant furnace tubes of the present invention, including a front view, a top view and a left view, wherein (1) is the outlet tube, (2) is the inlet tube, and the lengths are about 8.5 About meters, the inlet and outlet pipes are connected by an S-shaped bend (3) and a U-shaped return bend (4).

The length of the S-shaped bend connected to the inlet pipe is about 1.7 meters, and the bending radius of the U-shaped return bend is 400 mm. The pipe diameters of these two parts are the same as the inlet pipe.

The length of the S-shaped elbow connected to the outlet pipe is about 1.7 meters, and the radius is the same as that of the outlet pipe.

The outlet pipes are all arranged on the A plane in the top view, while the inlet pipes are divided into two groups, which are arranged on the B plane and the C plane respectively, and the outlet pipes are physically shielded from both sides. In this example, the distance between the inlet tube row and the outlet tube row and the tube center distance are relatively small, and the furnace tube arrangement is relatively concentrated, which is suitable for the radiation section of the cracking furnace with high radiation intensity.

It should be understood that within the scope of the present invention, the above-mentioned technical features of the present invention (such as the pipe diameter, arrangement and pipe length of the inlet and outlet pipes, the pipe length and pipe diameter of the S-shaped bend, and the bending radius of the U-shaped return bend) , the placement direction of the U-shaped return bend, the position and length of the reducing pipe, the plane spacing of the pipe row, etc.) can be combined with each other to form a preferred technical solution.

By adopting the optimal combination of the above technical features, the present invention can effectively reduce the thermal load intensity of the outlet pipe, reduce the cracking temperature rise of the outlet pipe, allow a shorter residence time of the cracked gas, and under the conditions of the same cracking depth or conversion rate, It can effectively increase the production capacity of cracking furnace, prolong the operation period and improve the olefin selectivity.

The terms and expressions used herein are for description only, and the present invention should not be limited to these terms and expressions. The use of these terms and expressions is not intended to exclude any equivalents of those shown and described (or portions thereof), and it should be recognized that various modifications that may exist should also be included within the scope of the claims. Other modifications, changes and substitutions may also exist. Accordingly, the claims should be deemed to cover all such equivalents.

Also, it should be pointed out that although the present invention has been described with reference to the current specific embodiments, those skilled in the art should realize that the above embodiments are only used to illustrate the present invention, without departing from the present invention. Various equivalent changes or substitutions can also be made under the spirit of the present invention. Therefore, as long as the changes and modifications to the above-mentioned embodiments are within the scope of the essential spirit of the present invention, they will fall within the scope of the claims of the present application.

Claims (8)

1. An ethylene cracking furnace with a two-pass radiant furnace tube, comprising: a convection section, a radiant section connected to the convection section, a plurality of sets of radiant furnace tubes, burners, and quenching boilers vertically arranged in the radiant section; Wherein, for each of the radiant furnace tubes, it has a first pass and a second pass; the first pass is the channel through which cracked raw materials enter the radiant section, also called an inlet pipe; the second pass is the product The channel leaving the radiant section is also called outlet pipe; a plurality of the inlet pipes constitute at least two rows of inlet pipes, and a plurality of the outlet pipes constitute at least one row of outlet pipes; observed from the position of the burner , the outlet tube of the radiant furnace tube is physically shielded by the inlet tube, and has a smaller direct radiation area than the inlet tube. 2. The ethylene cracking furnace according to claim 1, wherein the outlet pipe row is located between the inlet pipe rows, the inlet pipe row is located between the burners on both sides, and the overall composition is as follows: Arrangement of burner - inlet pipe row - outlet pipe row - inlet pipe row - burner. 3. The ethylene cracking furnace according to any one of claims 1-2, characterized in that, the arrangement between the inlet pipe row and the outlet pipe row is from the distance of the radiant section of the cracking furnace. From the top view, they are arranged in sequence or staggered, and the ratio of the tube center distance to the tube outer diameter is between 2 and 6; wherein, the outlet tubes are arranged in a single row on the centerline of the radiating section, or in multiple rows. at a position deviating from the centerline of the radiating segment. 4. The ethylene cracking furnace according to any one of claims 1-3, wherein the inlet pipe passes through an S-shaped bend, a U-shaped return bend, and an S-shaped bend, and is connected to the The outlet pipes are connected; wherein, according to the actual arrangement and relative position of the pipe rows, the S-shaped elbows connected with the inlet pipes or the outlet pipes can be omitted. 5. The ethylene cracking furnace according to any one of claims 1 to 4, wherein the diameter of the inlet pipe is less than or equal to the diameter of the outlet pipe, if the inlet pipe and the outlet If the diameter of the pipe is different, it is necessary to add a section of reducing pipe to the front or rear of the U-bend. 6. The ethylene cracking furnace according to any one of claims 1-5, wherein the U-shaped return bend is located at the bottom of the radiant section, the diameter is between 100mm-1000mm, and the suspension height is 300mm-1000mm from the bottom of the radiant section. between 1000mm. 7 . The ethylene cracking furnace according to claim 1 , wherein the inside of the radiant furnace tube is a smooth tube or an enhanced heat transfer member is arranged. 8 . 8. The ethylene cracking furnace according to any one of claims 1 to 7, characterized in that, for the radiant section of the cracking furnace having the tube row, all the burners are located at the bottom of the furnace, or some of the burners are located at the bottom of the furnace. The burners are located at the bottom of the radiation section, and part of the burners are located at the side walls of the radiation section.

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