JPH0323587B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to an olefin production method for producing ethylene, propylene, and other useful olefins by pyrolysis of naphtha or heavy gas oil, and particularly to improvements in the pyrolysis tube structure and residence time in the pyrolysis tube. Generally, a pyrolysis furnace with a built-in pyrolysis tube is used to produce olefins by thermally cracking naphtha or heavy gas oil, which are hydrocarbons.
The method used is to raise the temperature to 800 to 950 degrees for thermal decomposition, then rapidly cool it. In this case, various measures have been taken to increase the efficiency of heat transfer to the fluid within the pyrolysis tube, such as providing fins or ridges on the inner or outer surface of the pyrolysis tube, or making the tube elliptical. In addition, in order to improve the yield of olefin, especially the yield of useful ethylene, efforts have been made to shorten the residence time in the pyrolysis tube and cool it as quickly as possible. However, if you try to improve the conversion rate by tightening the reaction conditions, the coke produced in the pyrolysis tube will adhere to the inside of the pyrolysis tube, and it will be necessary to remove the adhering coke frequently. failed to improve the overall efficiency of the pyrolysis process. An object of the present invention is to provide a method for producing olefin, which has a high yield of olefin, reduces coke precipitation in a pyrolysis tube, and can improve the operating rate of the entire pyrolysis apparatus. The present invention is a pyrolysis tube with an average inner diameter of 15
~45mm, and the inner surface of the tube has a waveform with continuous semi-circular irregularities in the cross section.5~10
It has 7 to 7 grooves in the longitudinal direction of the tube.
Using a spiral-shaped tube with an inclination angle of 15 degrees, naphtha or heavy gas oil is passed through the pyrolysis tube and heated to 800 to 950℃ so that the residence time in the pyrolysis tube is 150 milliseconds or less. The purpose is to achieve the above object by increasing the temperature. Further, the pyrolysis furnace used in the present invention may be any type of general pyrolysis furnace, but a multi-tube pyrolysis furnace is preferable because it does not have a curved pipe section. As mentioned above, the pyrolysis tube needs to have an average inner diameter of 15 to 45 mm from the viewpoint of the residence time of the reaction fluid in the tube, the amount of heat transfer, and economic efficiency. In other words, if the inner diameter of the tube is larger than 45 mm, the heat transfer area per unit weight of the fluid in the tube becomes small, and therefore, the residence time of the fluid in the tube in the pyrolysis tube becomes shorter in order to raise the temperature of the fluid in the tube. This is because it becomes long. On the other hand, if the inner diameter of the tube is made smaller than 15 mm, the throughput per pyrolysis tube will be reduced, which will require an increase in the number of pyrolysis tubes, which will complicate the structure of the furnace and quencher, reducing economic efficiency and work efficiency. This is because it will lead to a decline in sexual quality. Regarding the inner surface shape of the pyrolysis tube, as mentioned above, the inner peripheral shape in the cross section has a groove with a corrugated shape consisting of unevenness, and this groove is formed in a spiral shape in the longitudinal direction of the tube. Use. The number of grooves provided on the inner surface of the tube is preferably about 5 to 10.
This is because when the number of grooves is less than 5 relative to the inner diameter of the tube, the difference in wall thickness of the pyrolysis tube becomes significant, and the resulting local differences in thermal stress may cause damage to the pyrolysis tube during long-term use. On the other hand, the number of grooves is 10.
This is because if it exceeds the width of each groove, the width of each groove will become narrower, making it easier for coke to adhere, which is undesirable. The angle of inclination of the spiral in the groove of the pyrolysis tube with respect to the longitudinal direction of the tube is preferably 7 to 15 degrees. This is because if the inclination angle is less than 7 degrees, the effect of providing the spiral groove will not be sufficient, while if it exceeds 15 degrees, the pressure loss within the pipe will increase, which is not preferable. Suitable pipes provided with such spiral grooves are molten pipes manufactured by twisting molten metal before it solidifies. Regarding the conditions of pyrolysis, the pyrolysis temperature is 800~
Preferably, the temperature is 950℃, the pressure is 2Kg/ ^cm2・G or less, and the weight ratio of steam to raw material (naphtha or heavy gas oil) is 0.3 to 1.0.Such conditions are suitable for the thermal decomposition of ordinary naphtha or heavy gas oil. It has been adopted. Also,
In the present invention, it is essential that the residence time of the reaction fluid be 150 milliseconds or less; if the residence time is longer than this, the effect of suppressing coke precipitation in the pyrolysis tube will be reduced, and the efficiency of the entire pyrolysis device will be reduced. This is because it reduces the Examples and comparative examples of the present invention will be described in more detail below with reference to the drawings. Example FIG. 1 shows a schematic configuration of a pyrolysis apparatus used in this example. In this figure, a pyrolysis furnace 10 has a burner 12 and a plurality of straight pyrolysis tubes 14, for example 12 straight pipes. The inlet sides of these pyrolysis tubes 14 are connected by an inlet header 16, and the outlet sides are connected by four outlet headers 18 each. A quencher 22 is connected outside the furnace. Naphtha to heavy gas oil and steam are supplied as raw materials to the raw material supply pipe 20, respectively, and these raw materials are passed through a preheater 24 provided in the supply pipe 20,
After being preheated by 26, the inlet header 1
6. As shown in FIG. 2, the cross section of each of the pyrolysis tubes 14 has a plurality of grooves 14A in which the inner surface of the tube has a continuous semi-circular arc-shaped unevenness to form a wave shape. 14A, as shown in FIG.
It is formed into a spiral strip with an inclination angle α and a pitch P in the longitudinal direction of the pipe. Pyrolysis tube 14 having such a shape
The specific structure is made of ASTM standard HP made of a nickel-chromium alloy, and the outer diameter D ₀ = 47.6 mm, the diameter between the groove bottoms d ₁ = 32.35 mm, and the diameter between the protrusions d ₂ = 23.1 mm. Therefore, average inner diameter D _A = 27.7 mm, groove depth h =
4.62 mm, minimum wall thickness t = 7.63 mm, number of grooves is 8, groove inclination angle α = 10 degrees, groove pitch P = 400 mm,
The length L of each straight pyrolysis tube (see FIG. 1) was 11 m, and 12 of these pyrolysis tubes 14 were used to carry out pyrolysis. Here, the average inner diameter D _A is the value obtained by subtracting twice the average value of the maximum (t + h) and minimum (t) wall thickness from the outer diameter D ₀ of the pyrolysis tube 14, or the diameter between the groove bottoms d ₁ and the inter-convex diameter _d2 .
In addition, the actual inner circumference = 124 mm and the average inner circumference C =
When the area expansion ratio S/C is calculated from π・D _A =86.978, S/C=1.426, which shows that the contact area of the raw materials has increased significantly. As raw materials, naphtha, vacuum gas oil (vacuum gas oil), and hydrodesulfurized gas oil having the properties shown in Table 1 were used.

【table】 The conditions for thermal decomposition are as shown in Table 2.

[Table] By continuing thermal decomposition under these conditions,
Coking occurred inside the pyrolysis tube 14, the pressure loss inside the tube increased, and the heat transfer effect of the pyrolysis tube 14 decreased, resulting in an increase in the outer surface temperature of the pyrolysis tube 14.
When this temperature reaches 1092℃, pyrolysis tube 1
Considering the strength and life of No. 4, thermal decomposition was interrupted and decoking was performed. The time required for this decoking was 5 hours. Since there is a correlation between the outer surface temperature limit of the pyrolysis tube 14 and the pressure loss inside the pyrolysis tube 14, a specific value of increase in pressure loss is used as a guideline for stopping pyrolysis, and this specific value is used. Table 3 shows the values for the period during which pyrolysis can be operated continuously when

[Table] Table 4 also shows the yield of thermal decomposition products in this case. Comparative Example Naphtha was pyrolyzed under the same conditions as in the previous example, except that a smooth circular tube with an outer diameter of 39.9 mm and an inner diameter of 26.9 mm, with concentric inner and outer diameters, was used as the pyrolysis tube, and the surface temperature of the pyrolysis tube was Decoking was performed when the temperature reached 1092℃. The time required for this decoking was approximately 10 hours. In this case, the period during which continuous operation was possible was 13 days. Furthermore, the yields of thermal decomposition products of the various raw materials in Table 1 are shown in Table 4.

[Table] From the above Examples and Comparative Examples, it is clear that according to the method for producing olefin of the present invention, coking can be significantly suppressed as a whole and the operation rate of the entire pyrolysis equipment can be improved. It can be seen that the yield can be improved. Although the numerical value of these improvements in yield is small, since the amount of oil passed is large, the economic effect is large. In addition, in the case where naphtha having the same properties as in the above example is used, the pyrolysis conditions are the same, and the operation is performed to obtain the same yield of pyrolysis products, the groove inclination angle of the pyrolysis tube is used as a parameter. Table 5 shows the pressure loss, continuous operation period, and decoking time. Here, the time required for decoking refers to the time required to flow steam at a temperature of 1000°C into the pyrolysis tube to perform a water gasification reaction and remove coke inside the tube.

[Table] As a result, the angle of inclination of the helix in the groove 14A of the pyrolysis tube 14 with respect to the longitudinal direction of the tube is preferably 7 to 15 degrees from the viewpoint of pressure loss, continuous operation period, time required for decoking, etc. be understood. If this angle of inclination is less than 7 degrees, the time required for decoking will be longer and the effect of providing the spiral groove will not be sufficient.On the other hand, if it exceeds 15 degrees, the pressure loss in the pipe will be large and the continuous operation time will be shortened. This is because it is not desirable.

[Brief explanation of the drawing]

Figure 1 is a schematic diagram of the pyrolysis furnace used in the examples and comparative examples of the present invention, Figure 2 is an enlarged cross-sectional view of the pyrolysis tube used in the examples of the present invention, and Figure 3 is FIG. 3 is a schematic developed view of the inner surface of the pyrolysis tube showing the shape of a single groove in the pyrolysis tube of FIG. 2; 10...Pyrolysis furnace, 14...Pyrolysis tube, 14A...
Groove, 20...raw material supply pipe.

Claims (1)

[Claims] 1. Naphtha or heavy gas oil is placed in a pyrolysis tube.
A method for producing olefin in which olefin is produced by pyrolysis while raising the temperature to 800 to 950°C, the pyrolysis tube having an average inner diameter of 15 to 45 mm, and a shape of the inner surface of the tube in a cross section. Using a tube that has 5 to 10 wave-shaped grooves made of continuous semicircular arc-shaped unevenness, and these grooves are formed in a spiral shape with an inclination angle of 7 to 15 degrees in the longitudinal direction of the tube, A method for producing olefin, which comprises flowing naphtha or heavy gas oil through a pyrolysis tube such that the residence time in the pyrolysis tube is 150 milliseconds or less.