JPS6259754B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a reaction tube device in a pyrolysis furnace for producing olefins by heating and pyrolyzing raw material hydrocarbons. A so-called reaction tube pyrolysis furnace for producing lower olefins such as ethylene and propylene by heating and pyrolyzing feedstock hydrocarbons such as naphtha and kerosene generally has the configuration shown in Figures 1 to 3. It's summery. That is, in each of these figures, this pyrolysis furnace 1 has a convection section 2 in the upper part of the furnace and a radiant section 3 at the base, and the raw material hydrocarbon is preheated through a preheating coil (not shown) disposed in the convection section 2. , and mixed with diluted steam, further heated to near the reaction temperature, and then introduced into the reaction tube device 4 disposed in the radiant section 3. In the case of this conventional example, the reaction tube device 4
, a reaction coil is formed by connecting a straight pipe section 5 in series with a bent pipe section 6, and the two systems 7A and 7B of this reaction coil are joined by a header 8 at the outlet section, and are connected by a hanger 9. The reaction coil is a heat-resistant alloy pipe with an inner diameter of 25 to 150 mm. In addition, burners 10 are arranged on the furnace wall and the furnace bottom forming the radiant section 3, and the feedstock hydrocarbons introduced into the reaction coil at a temperature of 550 to 650°C are usually heated to 800 to 800°C near the outlet.
Heated to a temperature of 950°C. Here, in the coil of the reaction tube device 4 , conventionally,
A pipe with a constant diameter is used from the inlet to the outlet, and the fluid temperature profile in the case of such a constant diameter coil is such that the temperature rises at the middle part, as shown by curve A in Figure 8. It has a tendency to slow down. In other words, the thermal decomposition of raw material hydrocarbons is an endothermic reaction, and decomposition occurs violently in this part, but a fixed diameter coil like this cannot provide enough heat, so the temperature rise is slow. It will become. On the other hand, in order to obtain olefins in high yield by thermally decomposing feedstock hydrocarbons, it is necessary to rapidly raise the reaction temperature and lower the reaction pressure within a short residence time in the reaction coil. It has been found that a small-diameter coil has a larger surface area ratio per unit coil volume than a large-diameter coil, and therefore a larger amount of absorbed heat per unit coil volume. In other words, if the mass transfer speed within the reaction coil is the same, a small diameter coil will absorb a larger amount of heat than a large diameter coil. From this point of view, the present invention consists of a series of reaction coils, in which the coil portion where the decomposition reaction is particularly intense is constituted by a plurality of small-diameter coils, and the amount of heat absorbed in the same portion is increased as much as possible. By rapidly raising the decomposition temperature, a high olefin yield can be obtained. Hereinafter, embodiments of the reaction tube apparatus according to the present invention will be described in detail with reference to FIGS. 4 to 10. 4 and 5 and FIGS. 6 and 7 are different embodiments in which the present invention is applied to a reaction coil consisting of one system, and FIGS. 4 and 5 show the intermediate section. shows the case where there are two channels, and FIGS. 6 and 7 similarly show the case where there are three channels. In each of these embodiments, the reaction coil 11 at the inlet is connected to the header 14a at a portion where the ratio to the total length per reaction coil flow path is 0.15 or more and less than 0.5, and in the case of FIGS. 4 and 5, the reaction coil 12a,
12b and the reaction coils 12c, 12d, or in the case of FIGS. 6 and 7, the reaction coils 12a, 12b and the reaction coils 12c, 1.
2d and the reaction coils 12e and 12f, each of these branched channels is again connected to the header 14b at a portion where the ratio to the total length per reaction coil channel is 0.6 or more. , the reaction coil 13 at the outlet is combined into one.
The reaction coils 12a to 12f are formed so as to reach the inner diameters of the reaction coils 11 and 13 at the inlet and outlet portions, and are branched into respective ones.
The inner diameter is made small. That is, in the entire length of a series of reaction coils, the middle part of the coil where the decomposition reaction is particularly intense is divided into small-diameter reaction coils 12a to 12a with multiple channels smaller than the inner diameter of each reaction coil 11, 13 at the inlet and outlet sections. 12f. Furthermore, Fig. 8 shows the fluid temperature decomposition distribution with respect to the coil length per flow path, and Fig. 9 shows the absorbed heat amount distribution per unit volume with respect to the coil length per flow path. In each of these figures, curve A represents a conventional example using a constant diameter coil.
Curve B represents an example in which the intermediate portion of the present invention is a small-diameter coil with multiple channels, but as is clear from the comparison of these curves A and B, in the case of a reaction coil to which this invention is applied. Since the raw material hydrocarbon is thermally decomposed at a higher reaction temperature level than in conventional examples, the conversion rate is high and the olefin yield can be increased, and at the same time, the reaction temperature can be rapidly increased. This makes it possible to shorten the residence time within the coil. On the other hand, in the thermal decomposition of hydrocarbons, tar substances, coke, etc. are produced as by-products in the reaction coil.
A so-called coking phenomenon occurs where the substance adheres to and accumulates on the inner surface of the coil, and as operation continues, the effective inner diameter of the reaction coil decreases, pressure loss within the reaction coil increases, and the reaction pressure increases. It is known. FIG. 10 shows the thickness distribution of tar substances and coke adhering to the inner surface of the coil with respect to the length of the coil per flow path, and it can be seen that this coking phenomenon is significant at the coil outlet. When the reaction pressure inside the coil increases in this way, the thermal decomposition reaction of hydrocarbons is naturally suppressed, and reactions such as polymerization of the produced olefin are also promoted, resulting in a decrease in olefin yield. It turns out. Since this pressure loss is approximately inversely proportional to the fifth power of the coil inner diameter, when tar substances, coke, etc. adhere to the inner surface of the coil, the pressure loss of a small diameter coil is significantly greater than that of a large diameter coil. This has a negative impact on the thermal decomposition of hydrocarbons. However, in order to deal with the increase in pressure loss due to the adhesion of tar substances, coke, etc. to the inner surface of the reaction coil, in this invention, the reaction coil at the outlet section connected to the multi-channel small-diameter coil at the middle section is made into a large-diameter coil. Therefore, the coking phenomenon can be effectively suppressed. In this case, since the above-mentioned large-diameter exit coil rejoins the small-diameter coils with multiple channels in the middle portion to form a single channel, it is possible to avoid an increase in the residence time due to a decrease in the flow velocity in the coil. In other words, it is possible to prevent deterioration of the thermal decomposition characteristics of the reaction coil due to coking, and maintain optimal decomposition reaction characteristics during long-term continuous operation. As detailed above, according to the present invention, in the reaction tube disposed in the radiating section, the reaction tube is branched into a plurality of channels at the middle part, and the reaction tube groups of these plurality of channels are merged again. As a result, three reaction sections, an inlet section, an intermediate section, and an outlet section, were constructed, and the inner diameter of each reaction coil in the plurality of channels in the intermediate section was configured to be smaller than the inner diameter of the inlet section reaction coil and the outlet section reaction coil. Therefore, it is possible to rapidly raise the reaction temperature of the raw material hydrocarbon in the reaction coil, lower the reaction pressure, and shorten the residence time in the coil, thereby improving the yield of olefin. be.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view showing the general structure of a conventional pyrolysis furnace, FIGS. 2 and 3 are cross-sectional views of the portions lined in FIG. 1, and FIG. 4 is an embodiment of the present invention. A front configuration diagram showing the reaction coil, FIG. 5 is a cross-sectional explanatory diagram of the line part shown in FIG. 4, FIG. 6 is a front configuration diagram of a reaction coil according to another embodiment of the same as above, and FIG. A cross-sectional diagram of
FIGS. 8, 9, and 10 are characteristic diagrams showing the fluid temperature distribution, unit absorbed heat amount distribution, and coking adhesion thickness distribution with respect to the length per channel of the reaction coil, respectively. 1 ...Pyrolysis furnace, 2...Convection section, 3...Radiation section, 4 ...
Reaction tube device, 10... Burner, 11... Inlet reaction coil, 12a to 12f... Intermediate reaction coil, 13... Outlet reaction coil, 14a, 14b...
Hetzder.

Claims (1)

[Claims] 1. In a pyrolysis furnace in which feedstock hydrocarbons are introduced into a reaction tube placed in a radiant section, heated and thermally decomposed to produce olefins, the reaction tube is branched into multiple channels at the middle section, and these channels are By merging the reaction tube groups of multiple channels again, three reaction sections, an inlet section, an intermediate section, and an outlet section, are constructed, and the inner diameter of each reaction coil of the multiple channels in the intermediate section is changed to the inlet section reaction section. A reaction tube device characterized in that the inner diameter is smaller than that of a coil and an outlet reaction coil.