Ethylene Cracking Furnace With Multi-Pass Radiant Coil

PATENT
Ethylene Cracking Furnace With Multi-Pass Radiant Coil
United States Patent Application 20120219466
Abstract:
The invention relates to an ethylene cracking furnace having a multi-pass radiant coil, comprising at least one radiant section. In the radiant section there are provided with bottom burners and/or sidewall burners, and at least one set of multi-pass radiant coil longitudinally arranged in the radiant section. The multi-pass radiant coil is a four- to ten-pass type radiant coil. At least one tube of the multi-pass radiant coil is arranged to be spatially adjacent to a tube which is not consecutive to said at least one tube. With this arrangement, the thermal radiation influence between tubes with high temperature can be reduced, so that the tubes with low temperature can absorb the radiation heat from the tubes with high temperature. Therefore, the surface temperature of the tubes with high temperature can be reduced, thus extending the lifetime of the radiant coil and the operational cycle of the cracking furnace.

Inventors:
He, Xiou (Beijing, CN)
li, Changli (Beijing, CN)
Zhang, Zhaobin (Beijing, CN)
Liu, Jingkun (Beijing, CN)
Yuan, Mujun (Beijing, CN)
Zhou, Cong (Beijing, CN)
Guo, Yuping (Beijing, CN)
Zhao, Yonghua (Beijing, CN)
Shen, Hainu (Beijing, CN)
Application Number:
13/504117
Publication Date:
08/30/2012
Assignee:
China Petroleum & Chemical Corporation (Beijing, CN)
China Petroleum & Chemical Corporation Beijing Research Institute of Chemical Industry (Beijing, CN)
SINOPEC Engineering Incorporation (Beijing, CN)

TECHNICAL FIELD
The present invention relates to petrochemical engineering field, and more specifically, to the radiant coil structure of an ethylene cracking furnace used in petrochemical engineering.

BACKGROUND
The ethylene cracking techniques used in petrochemical ethylene equipments mainly include those developed by LUMMUS Co. (USA), Stone & Webster Co. (USA), Kellog & Braun Root Co. (USA), Linde Co. (Germany), Technip KTI Co. (Netherlands), and the CBL cracking furnace developed by China Petrochemical Corporation.

FIG. 1A shows a typical ethylene cracking furnace 10, which comprises a radiant section 11, a convective section 13, and a flue section 12 located between the radiant section 11 and the convective section 13. Within the radiant section 11 a set of multi-pass radiant coil 14 is provided in the central plane P of the radiant section 11 along the longitudinal direction thereof In addition, the radiant section 11 is further provided with bottom burners 15 and/or side burners 16 for heating. Moreover, the ethylene cracking furnace 10 further comprises a transfer line exchanger 17, a high-pressure steam drum 18 and an induced draft fan 19, etc. Nowadays, in most companies a four- to six-pass (˜60 meter) medium-selectivity radiant coil with or without branches of variable diameters is used and the residence time thereof is controlled within the range of 0.4 to 1.0 second, so that the radiant coil suitable for cracking gas material can be also adapted for liquid material, with a proper operation cycle and a satisfied material adaptability. The first tube or the first two tubes of the radiant coil are of small diameter. Therefore, a quick temperature rise can be achieved since the specific surface area of the small-diameter tubes is relatively large. The tubes following the second tube are of large diameter, in order to reduce the influences on coking sensitivity. The four-pass medium-selectivity radiant coil used can be configured as 4-2-1-1 type, 2-2-1-1 type, 1-1-1-1 type and 2-1-1-1 type, etc.

For the radiant coil 14 of the prior arts, all tubes thereof are generally spatially arranged in sequence along the flow direction of fluid. In the meantime, the tubes are connected to each other with common elbows.

FIG. 1B shows a typical arrangement of multi-pass radiant coil of an ethylene cracking furnace in the prior art. As shown in FIG. 1B, the multi-pass radiant coil 30 is a four-pass radiant coil, comprising the first tube 1, the second tube 2, the third tube 3 and the fourth tube 4 along the fluid flow direction (i.e., from left to right in the drawing). The first tube 1 is a Y-shaped branched tube with varied diameters. All the four tubes are spatially arranged in sequence, i.e., the second tube 2 being arranged spatially between the first tube 1 and the third tube 2, and the third tube 3 being arranged spatially between the second tube 2 and the fourth tube 4, and so on. That is to say, each tube is arranged spatially adjacent to the one or two tubes that are consecutive to said tube. In addition, the tubes are connected to each other by means of common elbows 35. FIG. 1B further shows all tubes and the elbows are located in one single plane, i.e., the central plane P of the radiant section.

In this typical arrangement, the tubes are spatially arranged in sequence. Since the fluid flows along the multi-pass radiant coil from the first tube to the second tube, then to the third tube and finally to the fourth tube, the tube wall temperature of these tubes is gradually increased in this order. In other words, the temperature of the first, second, third and fourth tubes is gradually increased in this order. Therefore, an uneven temperature distribution is generated in the radiant section. Moreover, radiant heat exchange will also exist between the high-temperature tubes, i.e., the third and the fourth tubes in this case, which would negatively influence on reducing the tube wall temperature and extending the operation cycle of the furnace.

Furthermore, in the multi-pass radiant coil of the prior arts, the tubes are connected to each other by means of common elbows. This is undesirable for absorption of heat expansion generated in each of these tubes. In a long time operation, it will easily lead to bending of tubes, lowering the lifetime of the radiant coil and therefore shortening the operation cycle thereof.

SUMMARY OF THE INVENTION
Considering the defects in the prior arts, the present invention aims to provide an ethylene cracking furnace having a multi-pass radiant coil, which includes a novel tube arrangement that can improve the heating condition so as to enable the tubes being heated more evenly. In addition, the present invention further aims to provide an ethylene cracking furnace having a multi-pass radiant coil, which can enhance the mechanical properties of the radiant coil, increase the on-line percentage of the cracking furnace, reduce the operation cost, and extend the lifetime of the coil and the operational cycle of the cracking furnace.

According to the present invention, an ethylene cracking furnace having a multi-pass radiant coil is provided, comprising at least one radiant section. In the radiant section there are provided with bottom burners and/or sidewall burners, and at least one set of multi-pass radiant coil longitudinally arranged in the radiant section; wherein the multi-pass radiant coil is a four- to ten-pass type radiant coil. At least one tube of the multi-pass radiant coil is arranged to be spatially adjacent to a tube which is not consecutive to said at least one tube.

It should be noted that with regard to the first tube, the tube consecutive thereto is the second tube; with regard to the second tube, the tubes consecutive thereto are the first tube and the third tube, and so on. With at least one tube of the multi-pass radiant coil being arranged to be spatially adjacent to a tube which is not consecutive to the at least one tube, said at least one tube can be spatially adjacent to a tube having a temperature not close to that of said at least one tube. Therefore, the even temperature distribution within the radiant section can be effectively improved. Consequently, the surface temperature of the radiant coil can be lowered, and thus the lifetime of the radiant coil and the operational cycle of the cracking furnace can both be extended.

According to an embodiment of the present invention, the last two tubes of the multi-pass radiant coil are arranged to be spatially non-adjacent to each other. Since the last two tubes have the highest temperatures among all tubes, the thermal radiation influence from the high-temperature tubes can be effectively reduced. Therefore, the surface temperature of the radiant coil can be further lowered, and thus both the lifetime of the radiant coil and the operational cycle of the cracking furnace can be further extended.

According to another embodiment of the present invention, the first tube and the last tube are arranged at the opposite outer sides of the whole multi-pass radiant coil respectively. Alternatively, in one example, at least one of the first tube and the last tube of the multi-pass radiant coil is not arranged at the opposite outer sides of the whole multi-pass radiant coil, but in the middle thereof.

According to one embodiment of the present invention, the tubes of the multi-pass radiant coil are connected with each other by means of connectors. Since the temperatures on the walls of the tubes in the multi-pass radiant coil are different from each other, the thermal expansion between two adjacent tubes will be different, thus resulting in thermal stress. In one example, at least one connector located in the lower part of the radiant section is a combined connector consisting of a U-shaped elbow and two S-shaped elbows located at the opposite sides of the U-shaped elbow respectively. With the above combined connector consisting of S-shaped elbows and U-shaped elbow, the thermal stress generated from heating and uneven burning at the opposite outer sides of the coil can be absorbed. Accordingly, tube bending due to the thermal stress resulted from any possible reasons can be prevented. In this way, the mechanical properties of radiant coil are effectively improved. Consequently, partial overheating due to tube bending can be avoided, and thus both of the lifetime of the radiant coil and the operational cycle of the cracking furnace can be extended.

In one example, the tubes in the central plane of the radiant section are arranged as not being spatially adjacent to each other, and at both sides of the central plane there are provided with equal quantity of S-shaped elbows connected by a U-shaped elbow. In such an arrangement, the temperature in the radiant section is more evenly distributed, and the distortion caused by thermal stress can be more evenly absorbed. These S-shaped elbows have downward projections parallel to each other, or at least one of them has a downward projection is not parallel to downward projections of the others due to structural requirements. Likewise, all U-shaped elbows can also have downward projections parallel to each other. These arrangements are advantageous for obtaining a very compact structure.

In all of the above embodiments, the combined connectors, consisting of S-shaped elbows and U-shaped elbow, for connecting the tubes in the lower part of the radiant section, form a profile of a closed, symmetrical and continuous curve in the side view, which will be described below in details with reference to FIGS. 32-35. The U-shaped elbow may be formed in various forms.

Furthermore, some conventional elbows which are not in form of the above-mentioned combined connectors can be placed in the central plane of the radiant section. These arrangements will be advantageous for obtaining a compact structure. As required in particular processes, the first tube can be a branched tube with varied diameters, or both of the first and the second tubes are branched tubes with varied diameters.

The present invention presents the following advantages over the prior arts: (i) since the high-temperature tubes and the low-temperature tubes are arranged alternately or in a form of partial interleaving arrangement, the heat radiation influence from the high-temperature tubes can be reduced, and the tubes with low temperature can absorb the heat from those with high temperature; as a result, the temperature of surface of the high-temperature tubes can be lowered, and both of the lifetime of the radiant coil and the operational cycle of the cracking furnace can be extended; and (ii) by using combined connectors other than common elbows to connect the bottoms of the tubes, the mechanical properties of the radiant coil can be significantly improved, which is advantageous for absorbing thermal stress generated by temperature differences between two adjacent tubes, thus preventing the tubes from bending due to the thermal stress, avoiding partial overheating due to tube bending, and eventually extending both the operation cycle of the cracking furnace and the lifetime of the radiant coil.
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