JP2001262159A - Cracking coil - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a new cracking coil capable of preventing the yield of ethylene and the like from being lowered by heightening the heat transfer performance to a fluid in a pipe without increasing pressure loss, and enabling the compactification of an apparatus, the facilitation of operational control, and the like. SOLUTION: This cracking coil has a protrusion 20 formed only on the inner surface of a straight pipe 14 at the outlet side. A fin 22 is exemplified as the protrusion 20 formed on the inner surface of the outlet pipe 14.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cracking coil capable of simultaneously improving ethylene production capacity and reducing the size of production equipment.

[0002]

2. Description of the Related Art Olefins such as ethylene and propylene are produced by pyrolyzing a hydrocarbon raw material gas such as naphtha, natural gas and ethane. In the pyrolysis operation, a hydrocarbon raw material gas is introduced together with steam (steam) into a cracking coil provided in a heating furnace, and while passing through the cracking coil at high speed, the raw material gas is supplied by heat supply from outside the pipe. It is carried out by heating to the reaction temperature range. Normally, the cracking coil is formed in a U-shape or a W-shape by connecting a plurality of straight pipes sequentially with a bent bend pipe. The upstream side of the cracking coil is connected to a supply source of the hydrocarbon raw material gas, and the preheated raw material gas is introduced into the cracking coil through a supply pipe. On the downstream side of the cracking coil, a heat exchanger for gas cooling, a separation / purification device for generated gas, and the like are provided, and purify olefins such as ethylene from the produced gas.

[0003] In the above-mentioned thermal decomposition operation, free carbon is deposited on the inner wall of the cracking coil, that is, so-called coking occurs with the thermal decomposition reaction. This caulking is noticeable in the exit tube of the cracking coil. This is because coking is promoted by the contact of the decomposition gas with the inner surface of the outlet pipe of the cracking coil, which becomes high in temperature. As the pressure loss of the cracking coil increases due to coking, and the heat transfer efficiency of the cracking coil decreases,
Since the surface temperature of the cracking coil rises and reaches the upper limit temperature for use, it is necessary to perform a decoking operation for removing free carbon. Also, in the above pyrolysis operation,
It is necessary to avoid an excessive thermal decomposition reaction of the hydrocarbon feed gas. Specifically, the longer the residence time in the high-temperature region, the excessive lightening of hydrocarbons (generation of methane and free carbon), the polycondensation reaction of decomposition products, etc. occur, and the recovery of the target products The rate is reduced, further promoting coking.

[0004] By increasing the heat transfer performance of the cracking coil, the surface temperature of the cracking coil can be lowered,
There is a cracking coil in which the diameter of the cracking coil is reduced to increase the heat transfer area with respect to the internal volume in order to quickly heat and raise the temperature of the fluid in the pipe. However, when the diameter is reduced, there is a problem that the influence of coking is increased, such as an increase in pressure loss. In addition, in order to maintain the same ethylene production capacity, it is necessary to increase the number of cracking coils to be provided for the reduced diameter, which is problematic in terms of increasing the size of the equipment and complicating operation management.

[0005]

In order to increase the heat transfer area of the cracking coil, some cracking coils have fins formed on the inner surface thereof over the entire length. However, there is a problem that the formed fins increase the pressure loss of the gas flowing in the cracking coil and increase the residence time in a high-temperature region, thereby lowering the yield.

[0006] In view of the above, the present invention improves the heat transfer performance to the fluid in the pipe without increasing the pressure loss,
It is an object of the present invention to provide a new cracking coil that can prevent a decrease in the yield of ethylene and the like, and that can reduce the size of the device and facilitate operation management.

[0007]

In order to solve the above-mentioned problems, a cracking coil (10) according to the present invention comprises a straight pipe on an outlet side.
The protrusion (20) is formed only on the inner surface of (14). Fins (2) are formed as protrusions (20) formed on the inner surface of the outlet pipe (14).
2) can be exemplified.

[0008]

By forming the projection (20) only on the outlet pipe (14), the heat transfer of the outlet pipe is enhanced, and the rise in the temperature of the outlet pipe can be suppressed. Therefore, occurrence of over-decomposition and caulking in the outlet pipe is prevented. Also, except for the outlet side portion, since no protrusion is provided, the pressure loss in the portion other than the outlet side does not increase, suppressing an increase in residence time in a high temperature region,
A decrease in yield can be suppressed.

Further, according to the cracking coil of the present invention, even if the supply amount of the raw material gas is increased, it is possible to avoid over-decomposition, an increase in pressure loss and a remarkable rise in surface temperature. Can be increased.

[0010] Further, the cracking coil of the present invention comprises:
By forming the protruding part (20) only on the outlet pipe, even if the length of each straight pipe is shorter than the conventional cracking coil, sufficient thermal decomposition can be performed while avoiding a significant rise in surface temperature Thus, it is possible to reduce the cost of the construction equipment and the installation space.

[0011]

FIG. 1 is a sectional view showing an embodiment in which the present invention is applied to a W-shaped cracking coil.
In case of W-shaped cracking coil, straight pipe (12) (12) (12)
(14) are arranged in parallel, and flow from the straight pipe (12) on the inlet side (left side in the figure) where the hydrocarbon raw material gas is introduced to the straight pipe (14) on the outlet side (right side in the figure) So that a path is formed,
Adjacent straight pipes are bent bend pipes (16, 16)
Connected at (16). As shown in FIG. 2 (b), no protrusion is formed on the inner surface of the straight pipe (12) other than the outlet pipe (14).

As shown in FIG. 2 (a), the outlet pipe (14)
A projection is formed on the inner surface over substantially the entire length. Projection (2
The fin (22) can be exemplified as (0). The fin (22) may be in the form of a spiral ridge that spirals the inner surface of the tube wall in the tube axis direction. Further, it may be formed as an annular ridge that orbits around the pipe axis. Height (h) and pitch (p) of the protrusion (20)
Is appropriately set in accordance with the diameter D of the outlet pipe (14). When the diameter D is about 150 mm or less, the height h is about 1 to 15 mm,
The pitch p is desirably about 20 to 350 mm. By forming the projection (20) only in the outlet pipe (14), pressure loss does not increase in the straight pipes (12), (12), (12) other than the outlet pipe (14). In the outlet pipe (14), the heat transfer area is increased by the protrusions (20), so that the heat transfer performance is improved, and the heat transfer performance is also improved by the strong stirring action that the fluid receives from the protrusions (20). In the outlet pipe (14), the pressure loss increases due to the formation of the protrusion (20), but the increase in pressure loss that occurs is smaller than when the protrusion is formed over the entire length of the cracking coil (10). At a level that does not hinder the performance of the pyrolysis operation. By increasing the heat transfer performance of the outlet pipe (14), an excessive rise in temperature of the outlet pipe (14) is prevented, and the occurrence of over-decomposition or the like is prevented.

The outlet pipe (14) can be obtained by producing a pipe by centrifugal casting and forming a projection (20) on the inner surface of the produced pipe by overlay welding or the like. The outlet pipe (1
4) can also be prepared by hot extrusion. Outlet pipe (1
4) is desirably composed of a material containing 0.4% by weight or more of C. By setting the content of C to 0.4% by weight or more, the high-temperature strength and heat resistance of the outlet pipe (14) can be increased. As described above, in the case of a material having a C content of more than 0.4% by weight, it is difficult to produce the material by hot extrusion, so that the material is produced by centrifugal casting.

The straight pipes (12), (12) and (12) other than the outlet pipe (14) can also be manufactured by centrifugal casting or hot extrusion. As described above, in order to increase high-temperature strength and heat resistance, C
When a material having a content of 0.4% by weight or more is used, it is manufactured by centrifugal casting instead of hot extrusion.

Outlet pipe (14) and straight pipes other than the outlet pipe (12) (12)
(12) The length L is appropriately set according to the number of installations, the diameter, the amount of the hydrocarbon raw material gas introduced, and the like. The cracking coil (10) of the present invention can achieve improved heat transfer performance while suppressing pressure loss by improving the outlet pipe (14), thereby improving thermal decomposition efficiency. Therefore, the total length of the cracking coil is about 10 times that of the conventional cracking coil.
３０30%, which can reduce the cost of construction equipment and the installation space.

The straight pipes (12), (12), (12) and the outlet pipe (14) having the above configuration.
Are arranged almost in parallel, and the ends of the adjacent straight pipes are bent (16) (1
6) By connecting with (16), cracking coil (10)
Is formed. Placing the formed cracking coil (10) in a heating furnace (not shown), heating and raising the temperature of the cracking coil (10) by radiant heat, etc., while the straight pipe (12) on the inlet side
, A pyrolysis operation is performed.

In the above description, the W-shaped cracking coil (1
Although 0) was described, the number of straight pipes is not limited to four, and a U-shaped cracking coil having two straight pipes and a cracking coil having three straight pipes or five or more straight pipes are also described. , By forming a protrusion on the inner surface of the outlet pipe,
Needless to say, the same effect can be obtained.

[0018]

EXAMPLES Example 1 Naphtha was introduced as a hydrocarbon raw material gas into a W-shaped cracking coil for pyrolysis, and the gas pressure in the cracking coil and the surface temperature of the cracking coil were measured. In the measurement, as shown in Table 1, Comparative Examples 1 and 2 in which no fin was formed on the inner surface of the cracking coil, and Comparative Example in which the fin was formed on the inner surface over the entire length of the cracking coil. 3
The cracking coil of Inventive Example 4 in which fins (22) were formed only on the inner surface of the outlet pipe (14) of the cracking coil (10) was used. The cracking coils used had a straight pipe length of 9.5 m, a diameter of 70 mm, and a wall thickness of 8 mm. The protrusions of Inventive Example 4 and Comparative Example 3 were formed in a spiral shape with a height h of 3 mm and a pitch p of 120 mm. In Tables 1 to 3, "straight pipe number" indicates a number where the straight pipe on the inlet side is 1 and the straight pipe on the outlet side is 4. The straight pipe provided with the protrusion is marked with a circle. I have.

[0019]

[Table 1]

In each of the cracking coils of the inventive example and the comparative example, naphtha having a flow rate shown in Table 1 was introduced together with steam.
Gas pressure at the outlet of cracking coil is 0.2MPa
abs., the temperature of the gas at the inlet of the cracking coil was set to 600 ° C., and the temperature of the gas at the outlet was set to 820 ° C. The ratio of steam to naphtha is 0.1.
The ratio is set to 4: 1. For each cracking coil, the measurement results of the pressure are shown in Tables 2 and 3, and the measurement results of the temperature are shown in Tables 3 and 4, respectively.

[0021]

[Table 2]

[0022]

[Table 3]

With respect to the internal pressure, inventive example 4 has a higher internal pressure on the inlet side than comparative examples 1 and 2 in which no protrusion is formed. Is provided. However, it can be seen that the internal pressure on the inlet side is kept low as compared with Comparative Example 3 in which the protrusion is formed over the entire length.

On the other hand, with respect to the surface temperature of the cracking coil, the invention example 4 is different from the comparative example 1 in which the flow rate of naphtha is smaller by about 10% and the comparative example 2 in which the flow rate of naphtha is the same as the invention example 4 in the outlet pipe. It can be seen that the surface temperature of is low. This is because the heat transfer performance at the outlet portion was enhanced by forming the protrusion only on the outlet tube, and the rise in the temperature of the outlet tube could be suppressed. Also, comparing Inventive Example 4 and Comparative Example 3, it can be seen that forming the protrusion only on the outlet pipe has the effect of suppressing the temperature rise as much as forming the protrusion over the entire length.

Comprehensively judging the internal pressure and the temperature, the invention example 4 can suppress the internal pressure to a level that does not hinder the thermal decomposition operation by forming the projection only on the outlet pipe, and The rise in the temperature of the outlet pipe can be suppressed. In Comparative Example 1 and Comparative Example 2, the rise in the internal pressure could be suppressed because no protrusion was formed, but the surface temperature of the outlet pipe was high because the heat transfer performance in the outlet pipe was low, and the flowing gas was over-decomposed. And caulking may occur. In Comparative Example 3, as a result of forming the protrusions over the entire length, the heat transfer performance was good over the entire cracking coil.
Although the rise in the temperature of the cracking coil can be suppressed, the internal pressure has risen due to the increase in the pressure loss, which may cause a decrease in the yield in the pyrolysis operation.

Example 2 Pyrolysis was carried out in the same manner as in Example 1 except that the length of the straight pipe constituting the cracking coil was changed, and the gas pressure in the cracking coil and the temperature of the cracking coil were measured. For the measurement, as shown in Table 4,
Comparative Examples 5 and 6 in which no protrusion is provided on the inner surface over the entire length of the cracking coil, Comparative Example 7 in which fins are formed on the inner surface over the entire length of the cracking coil, and an outlet pipe of the cracking coil (10) Fins (2
The cracking coil of Invention Example 8 formed in 2) was used. The length of the straight pipe of the cracking coil was 9.5 m in Comparative Example 5, and 8.5 m in Comparative Example 6, Comparative Example 7, and Invention Example 8 which were 1 m shorter than Comparative Example 5. The diameter and thickness of the cracking coil and the protrusions of Inventive Example 8 and Comparative Example 7 are the same as in Example 1. In Tables 4 to 6, "straight pipe number" indicates the number where the straight pipe on the inlet side is 1 and the straight pipe on the outlet side is 4, and the straight pipe provided with the protrusion is marked with a circle. I have.

[0027]

[Table 4]

In each of the cracking coils of the invention example and the comparative example, naphtha having a flow rate shown in Table 1 was introduced together with steam.
Gas pressure at the outlet of cracking coil is 0.2MPa
abs., the temperature of the gas at the inlet of the cracking coil was set to 600 ° C., and the temperature of the gas at the outlet was set to 820 ° C. The flow rate of naphtha is 1200kg / h
The ratio of steam to naphtha is set to be 0.4: 1. For each cracking coil, the measurement results of pressure are shown in Tables 5 and 5, and the measurement results of temperature are shown in Tables 6 and 6.
Are shown below.

[0029]

[Table 5]

[0030]

[Table 6]

Regarding the internal pressure, the internal pressure at the inlet side of the invention example 8 is higher than that of the comparative example 5 in which each straight pipe is 1 m longer and the comparative example 6 in which the length of the straight pipe is the same as that of the invention example 8. I have. This is due to the fact that a projection was formed on the outlet pipe of Invention Example 8. However, the internal pressure on the inlet side is kept low as compared with Comparative Example 7 in which the protrusion is formed over the entire length.

When the comparative example 5 and the comparative example 6 are compared with respect to the temperature of the cracking coil, the comparative example 6 in which the length of the straight pipe is shorter by 1 m than the comparative example 5 has a very high outlet pipe temperature. You can see that there is. In other words, it can be seen that simply shortening the length of the cracking coil causes an increase in the temperature of the cracking coil and causes over-decomposition and the like. On the other hand, in Comparative Example 7 and Invention Example 8, the surface temperature was lower than that of Comparative Example 5 despite the shorter length of the cracking coil because the protrusions were formed inside. Comparing Invention Example 8 with Comparative Example 7,
The surface temperature of the outlet pipe is almost the same, and it can be seen that the formation of the protrusion only on the outlet pipe has almost the same effect as the formation of the protrusion over the entire length.

Comprehensively judging the internal pressure and the temperature, the invention example 8 shows that even if the total length of the cracking coil is shortened, the protrusion is formed only in the outlet pipe, so that the cracking operation is not easily performed.
The internal pressure can be suppressed to a level that does not hinder, and
The rise in the temperature of the outlet pipe can be suppressed. Comparative Example 5
In Comparative Example 6, since no protrusion was formed, an increase in internal pressure could be suppressed. However, since the heat transfer performance in the outlet pipe was low, the surface temperature of the outlet pipe was high, and excessive decomposition and coking of the flowing gas were prevented. May occur. In particular, in Comparative Example 6 in which the total length of the cracking coil was shortened, the surface temperature increased significantly. In Comparative Example 7, as a result of forming the protrusions over the entire length, the heat transfer performance was good over the entire cracking coil, and the rise in the temperature of the cracking coil could be suppressed. Therefore, the internal pressure may increase, which may cause a decrease in yield in the thermal decomposition operation.

The description of the above embodiments is for the purpose of illustrating the present invention, and should not be construed as limiting the invention described in the appended claims or reducing the scope thereof. Further, the configuration of each part of the present invention is not limited to the above embodiment, and various modifications can be made within the technical scope described in the claims.

[Brief description of the drawings]

FIG. 1 is a sectional view of a W-shaped cracking coil to which the present invention is applied.

2A is an enlarged sectional view of an outlet pipe, and FIG. 2B is an enlarged sectional view of a straight pipe other than the outlet pipe.

FIG. 3 is a graph showing results of Example 1 in which gas pressures of cracking coils were compared.

FIG. 4 is a graph showing the results of Example 1 in which the surface temperatures of cracking coils were compared.

FIG. 5 is a graph showing results of Example 2 in which gas pressures of cracking coils were compared.

FIG. 6 is a graph showing the results of Example 3 in which the surface temperatures of cracking coils were compared.

[Explanation of symbols]

 (10) Cracking coil (14) Outlet pipe (20) Projection (22) Fin

Claims (4)

[Claims] 1. A cracking coil provided in a heating furnace for introducing a hydrocarbon raw material gas into the inside thereof for thermal cracking, wherein a plurality of straight pipes (12) and (14) are sequentially bent by a bend pipe (16). A cracking coil, characterized in that a projection (20) is formed on the inner surface of only the most downstream outlet pipe (14) in the connection. 2. The cracking coil according to claim 1, wherein the protrusion (20) is a fin (22) protruding from an inner surface of the outlet pipe (14). 3. The cracking coil according to claim 1, wherein the outlet pipe is a cast pipe containing 0.4% by weight or more of C. 4. The cracking coil according to claim 1, wherein the outlet pipe is manufactured by centrifugal casting.