JPS6311394B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to steam cracking (pyrolysis) of hydrocarbons, and more particularly to a new and improved method for carrying out steam cracking of hydrocarbons. In accordance with the present invention, an improved method for steam cracking hydrocarbons is provided by providing a cylindrical working coil with an internal insert for providing a radiation absorbing surface within the working coil. According to the invention, the use of such an insert increases the heat flux in the tube wall, reduces the required residence time and lowers the internal tube wall temperature, while keeping the external tube wall temperature unchanged. . The insert is designed and placed within the cylindrical coil to provide a radiation absorbing surface to increase heat flux without increasing pressure drop. Usually and preferably the insert is located at the exit of the processing coil so that the most intense pyrolysis takes place at the exit of the coil. The tube insert preferably has arms or vanes extending outwardly from a central portion spaced from the inner tube wall and configured to contact or nearly contact the inner tube wall; has been found to provide a heat absorbing surface that provides the desired increase in heat flux without undesirably increasing pressure drop. This insert must divide the free internal cross-section of the tube into equal area sections to provide uniform gas flow. The insert should also preferably be centered (most preferably the center should be axially aligned with the tube axis) to avoid misdistribution of gas flow through the free cross section. The insert should also be arranged so that it cannot move or vibrate within the tube, resulting in mechanical damage. The heat absorption of the insert can be increased, for example, by darkening the surface or by providing an absorbent coating. The inserts are placed in the heating device in only a small portion of the entire cylindrical coil (the entire coil may consist of one or several tubes), most preferably at the coil outlet (at the outlet of the last tube of the coil). ) to be located. The insert usually has a length of at least 5 feet and accounts for 15-100% of the total length of the final tube. As already mentioned, the cross-section of the insert is dimensioned to provide the desired increase in heat flux without undesirably increasing the pressure drop across the tube containing the insert, and the increase in pressure drop for the insert is normally Less than 5 psi, preferably less than 3 psi.
Typically the pressure drop increase will be at least 0.3 psi, most commonly at least 0.7 psi. Increased pressure drop
It is desirable that the pressure be less than 5 psi so that the equilibrium of the reaction is not affected rapidly, so the pressure should be as small as possible. According to the present invention, there is a significant increase in heat flux (BTU/hr-ft ² ) in the tube section with the insert. Usually this heat flux is at least
The increase is on the order of 10%, preferably at least 20%, but usually not more than about 50%. The increased heat flux is due to improved direct convection from the tube to the cracked stream as well as radiant heat transfer from the tube wall to the insert, which transfers heat to the cracked stream by convection. An increase in heat flux of at least 10% is important from the point of view of energy savings; the higher the increase, the better. Also, since metal creep of the tube may occur as the temperature increases,
It is also advantageous to be able to save energy in terms of the effect on the metal material of the tube. According to the invention, the temperature of the inner wall of the portion of the tube containing the insert decreases due to the increased heat flux. Therefore, caulking of the tube is reduced. It has been recognized that because the insert is much cooler and has virtually no tendency to coke, increased heat flux can be achieved without increasing the risk that the entire surface area will be exposed to coking. The hydrocarbon feedstock can be any of a wide variety of cracking feedstocks. INDUSTRIAL APPLICATION This invention is utilized for the production of olefins, especially ethylene. Typical raw materials include ethane, propane, butane and mixtures thereof, naphtha, light oil, etc. The product stream contains a wide variety of components, and the product composition is due in part to the feedstock selection. Steam pyrolysis of hydrocarbons is carried out at gas outlet temperatures above 1400°C, usually between 1500° and 1800°C. Tube outlet pressure is typically 0 to 50 psig. Residence time is 0.05 seconds to 2 seconds. This pyrolysis is usually carried out at a steam to hydrocarbon ratio of 0.2:1 to 1.5:1 (by weight). The overall conditions described above are similar to those commonly used in conventional processes, but at a constant tube diameter, other conditions (exit pressure, feed velocity, steam to hydrocarbon ratio, combustion rate, etc.) The total residence time can be reduced if the method of the invention is carried out with fixation. In other words, the use of inserts reduces the total residence time. The present invention will be explained below with reference to the accompanying drawings. FIG. 1 shows a vertical cylindrical heating device supported on a structural steel frame 10 mounted on piers and consisting of outer walls 11 and 12, inner walls 13 and 14, end walls 15 and floors 16 and 17. ing. The outer walls 11 and 12 are substantially parallel to the inner walls 13 and 14, and the outer walls 11 and 12 extend higher than the inner walls 13 and 14. Outer wall 1
1 and 12 and inner walls 13 and 14 are provided with a number of vertical rows of high-intensity radiant burners, indicated by 18. Floors 16 and 17 extend between outer walls 11, 12 and inner walls 13, 14, respectively. floor 1
6 and 17 are provided with floor burners 19, preferably flame-type burners. The outer wall 11, the inner wall 13, the floor 16 and the end wall 15 constitute a radiant heating zone 20, and the outer wall 12, the inner wall 1
4, another radiant heating zone 2 at the floor 17 and end wall 15
1. The end wall 15 has an inverted U-shape and defines an open area allowing access to the burner groups 18 provided on the inner walls 13 and 14. An internal roof 25 is provided horizontally on the internal walls 13 and 14. An upper roof 26 extending horizontally inwardly from the outer wall 11 is provided on the outer wall 11 and the end wall 15, and similarly an upper roof 27 is provided on the outer wall 12 and the end wall, from which upper walls 28 and 29 extend and extend horizontally over the end wall 15. A convection region 30 is created by the upwardly extending portion of the convection region 30. All walls, floors and roofs are made of suitable fire-resistant materials. The radiant heating zones 20 and 21 are provided with a support structure 10.
There are a number of vertical tube groups 31 held in place by hangers 33. These many vertical tubes 31 are connected to the outer wall 11 and the inner wall 1.
3, and between the outer wall 12 and the inner wall 14, respectively. The tubes are arranged to provide processing coils in multiple passages and are provided with suitable return bends and outlet means. A horizontally placed conduit 35 is provided within the convection zone 30 . This conduit 35 has a straddle joint 36
This allows fluid communication with the tube 31. A further horizontal conduit 38 is provided within the convection zone 30. Inlet and outlet manifolds 38a and 38b are in fluid communication with this conduit 38. Burner 18 is supplied with fuel through line 40 from a number of manifolds 39 . Fuel is manifold 4
1 and into manifold 39 under the control of valve 42. The flow of fuel to the burners 18 is varied from column to column depending on the desired burnup of the tube group 31. Each burner can also be regulated by a manual valve 44 in line 40, with overall fuel flow to the heating device controlled by valve 45. It will be appreciated that the burners in the outer walls 11 and 12 and the inner walls 13 and 14 have similar manifold means. The number of fluid passages through the vertical tubes within the radiant heating zone will vary depending on feedstock, product specifications, etc. That is, these tubes represent a number of vertical coils placed vertically within the radiant heating zone. A basic heating device is described in US Pat. No. 3,274,987. A portion of the pyrolysis tube 31, particularly the outlet tube of each passage, includes internal inserts to provide a radiation absorbing surface within the exit tube of the coil. Second
As shown in Figure A, this insert 100 has the shape of a three-bladed spider, and has a central body portion 111 and wings 112, and the tips of these wings, that is, the portions that contact the tube wall, are curved. There is. The spider is pressed and secured within the tube to provide the desired positioning constraints and divide the interior of the tube into three passages. The central portion 111 of the spider is positioned in line with the tube axis. Figure 2b shows an insert 120 having a central body portion 121 and three outwardly extending bendable vanes 122, which divide the interior of the tube into three separate passages. Figure 2c shows an insert 130 in the form of a four-bladed spider, the ends of which are curved outwardly to divide the interior of the tube into four passages. FIG. 2d shows an insert 140 having a central body portion 141 and four outwardly extending vanes 142.
, which divides the interior of the tube into four separate passages. The present invention will be explained below with reference to Examples. EXAMPLE A three-bladed spider tube insert as shown in FIG. 2a was used. These vanes were 1/8" thick, which reduced the free flow cross section somewhat and increased the pressure drop somewhat. Basic tube size 6" ID x 1"/4w. t Processing amount: Hydrocarbon 8800 1b/hr Water Steam 4400 1b/hr Total 13200 1b/hr Outlet pressure 22psia Outlet tube wall temperature 1080℃ (1976〓) Gas temperature From inlet to last tube 825℃ (1517〓) Outlet 850℃ (1562〓) Conversion rate or decomposition rate (% of coil conversion rate):
75% from inlet to last tube 100% at outlet Decomposition heat 28000BTU/1b hydrocarbon product 1 mole calculation duty, last tube (sensible heat + decomposition)
1980000BTU/hr Results

[Table] Using the insert increased the heat flux on the tube wall by approximately 33.3%. In addition, the insert temperature does not become extremely high (the insert temperature is 1610
〓) The temperature of the inner wall of the tube decreased. Also, the increase in pressure drop was very small. The present invention is highly advantageous because the heat flux can be increased, the residence time can be decreased, and the selectivity can be improved. Also, such results are achieved without increasing tube wall temperature (in fact, tube wall temperature is lower), reducing the tendency to coking. It should also be understood that for the same residence time, higher gas outlet temperatures can be obtained according to the invention.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a pyrolysis furnace in which the heating device of the present invention can be incorporated, and FIG. 2a,
Figures 2b, 2c and 2d are each a cross-sectional view of a portion of a pyrolysis tube incorporating an insert that may be used in the present invention.

Claims (1)

[Claims] 1. A steam pyrolysis process for hydrocarbons in which steam and a hydrocarbon feed are passed through a cylindrical processed coil in a decomposition heating device to carry out the pyrolysis; The coil is passed in a separate continuous longitudinal channel between the insert and the inner wall of the coil at the outlet of the coil, the insert providing a radiation absorbing surface for increasing the heat flux, and the insert providing a radiation absorbing surface for increasing the heat flux. A process for steam pyrolysis of hydrocarbons, characterized in that the outlet section is positioned to increase the pressure drop by 10% and limit the pressure drop increase to less than 5 psi. 2. The method of claim 1, wherein the hydrocarbon feed and steam are passed through the longitudinal channels in a uniform gas flow.