Patent Application
20090177022
The present invention relates to cracking tubes for use in thermal cracking reactors for producing ethylene or the like, and more particularly to a cracking tube which is provided on the inner surface thereof with fins for stirring the fluid therein and which is adapted to suppress pressure losses to the greatest possible extent while effectively promoting heat transfer to the fluid therein.
Olefins such as ethylene, propylene or the like are produced by thermally cracking material gases of hydrocarbons (naphtha, natural gas, ethane, etc.). The thermal cracking reaction is conducted by introducing the hydrocarbon material gas and steam into a cracking coil disposed within a heating furnace supplied with heat from outside, and heating the mixture to a reaction temperature range while the mixture flows through the coil at a high velocity.
Typically, the cracking coil comprises a plurality of (straight) tubes which are connected into a zigzag assembly by bends.
To conduct the thermal cracking reaction efficiently, it is important to heat the fluid flowing inside the coil at a high velocity to the reaction temperature range radially inward to the central portion of the tube channel within a short period of time and to avoid heating at a high temperature to the greatest possible extent. If the gas is heated at a high temperature over a prolonged period of time, lighter fractions of hydrocarbons (methane, free carbons, etc.) will be produced in excessive amounts or the product of cracking will undergo, for example, a polycondensation reaction to reduce the yield of the desired product. Promoted coking (deposition of free carbon on the tube inner wall) will also result to lower the coefficient of heat transfer, giving rise to a need to perform decoking frequently.
Accordingly it is practice to provide fins on the tube inner surface of the cracking coil as elements for stirring the fluid within the tubes. The fluid flowing at a high velocity produces turbulence by being stirred by the fins, and can be heated to a higher temperature rapidly. As a result, the reaction is completed within a shortened period of time, while production of lighter fractions due to excessive cracking is avoided. Furthermore, an improvement in the coefficient of heat transfer of the tubes makes it possible to lower the temperature of the tubes, producing an effect to improve the serviceable life of the tubes.
These examples have a great effect to stir the fluid within the tubes and are highly efficient in heat transfer to the fluid within the tubes, whereas the internal pressure of the fluid inside the tubes builds up owing to a great pressure loss of the fluid, entailing the drawback that the cracking operation produces ethylene, propylene or the like in a lower yield.
In view of the above problems, an object of the present invention is to suppress pressure losses to the greatest possible extent while maintaining an effect to promote heat transfer to the fluid within the tube.
To fulfill the above object, the present invention provides a cracking tube which has fins formed on an inner surface thereof and inclined with respect to an axis of the tube for stirring a fluid inside the tube, the fins being discretely arranged on one or a plurality of helical loci, the tube inner surface having regions wherein no fins are present over the entire axial length of the tube from one axial end of the tube to the other axial end thereof.
The tube of this construction is adapted to minimize the pressure loss of the fluid inside the tube while permitting the helical fins to transfer heat to the inside fluid with a high efficiency.
The cracking tube of the present invention will be described below in detail with reference to the illustrated embodiments.
Fins 1 are formed discretely along a helical locus which is positioned at a predetermined angle of inclination θ with respect to the axial direction x of the tube. The helical locus is indicated by slanting dotted lines, and connections of the helix are indicated by vertical dotted lines. Horizontal chain lines show regions ZA wherein fins are arranged in the axial direction, and regions ZB of nonfin portions 2 wherein no fins are present.
In the embodiment of
Thus, according to all the embodiments of
In the case of a tube having an inside diameter D of about 30 to 150 mm, for example, the angle of inclination θ can be about 15 to about 85 degrees, and the pitch p, about 20 to 400 mm. The pitch p is increased or decreased for adjustment depending on the angle of inclination θ of the helix and the number N of helixes (p=E/N wherein E is helix lead).
The height H (the height of projection from the tube inner surface) of the fins is, for example, about one-thirtieth to one-tenth of the inside diameter of the tube. The length L of the fins is, for example, about 5 to 100 mm, and is determined, for example, according to the inside diameter D of the tube and the number of divided fins along every turn of helical locus.
Incidentally, the proportion of the total circular arc length TW of the fins to the circumferential length C (C=n D) of the tube inner surface, namely, R (R=TW/C), is preferably about 0.3 to 0.8 in order to ensure a minimized pressure loss while permitting the helical fins to promote heat transfer to the fluid inside the tube. If this value is too small, the effect to promote heat transfer will be lower, whereas if the value is excessively great, an excessive pressure loss will result.
The helical fins can be efficiently formed as beads by an overlaying method such as plasma powder welding (PTA welding).
A tube 50 is horizontally supported by a rotary drive apparatus (not shown) and rotatable about its axis x. A welding torch 51 is fixed to a support arm 52, which is held parallel to the tube axis and is movable forward or rearward axially of the tube.
A powder (material for overlaying) is supplied by a pipe 53 to the welding torch 51, which forms beads on the tube inner surface. Plasma welding is performed intermittently by the rotation of the tube 50 and the horizontal movement (in the direction of the tube axis) of the welding torch 51 to form helical fins comprising beads formed by overlaying.
In the case where two welding torches 51 are installed as illustrated, fins are formed along two helical loci.
The number of helixes of fins to be formed, the angle of inclination θ, pitch p, the number and width (circular arc length of projected image in
Helical fins are arranged over the entire length of tube channel from the inlet end of the tube to the outlet end thereof, or at suitable portion or portions of the channel, for example, in at least one of a region in the vicinity of inlet end of the channel, intermediate region thereof and a region in the vicinity of the outlet end.
The material for forming the helical fins is the same kind of heat-resistant alloy as the tube, such as 25Cr—Ni(SCH22), 25Cr-35Ni(SCH24) or Incoloy(Trademark). Also suitably usable are other heat-resistant alloys which are serviceable in the environment wherein the tube is to be used.
The present invention will be further described with reference to specific examples.
Test Tubes T1 to T5 were prepared and checked for film heat transfer coefficient h (W/m2/K) and pressure loss dP (Pa).
T1 is according to the invention, and T2 to T5 are comparative examples. Table 1 shows the particulars about these test tubes.
Experimental conditions are as follows.
The results of measurement are shown in
However, Test Tubes T2 and T3 are greater than Test Tube 1 in pressure loss and result in a lower yield as will be described later.
On the other hand, Test Tube T4 is inferior to Test Tube T1 in heat transfer characteristics and therefore has the problem of permitting coking in addition to a lower yield.
Test Tube T5 is a smooth-surfaced tube having no fins and accordingly superior to Test Tube T1 of the invention with respect to pressure loss, but is inferior in heat transfer characteristics and involves the problem of yield and coking like Test Tube T4.
In contrast, Test Tube T1 of the invention is adapted to ensure a minimized pressure loss while maintaining the desired heat transfer characteristics.
Next, a thermal fluid analysis was conducted using a W-shaped coil shown in
The coil shown in
Test Tube T6 is according to the invention, and Test Tubes to T9 are comparative examples. As to the arrangement of fins on the tube (straight tubular portion), Test Tube T6 is the same as is shown in
The analysis conditions are fluid pressure at the coil outlet of 1.98 kg/cm2 (absolute pressure), coil inlet temperature of 600° C. and coil outlet temperature of 830° C. Naphtha was caused to flow through one coil at a flow rate of kg/h, and steam at a flow rate of 420 kg/h.
Table 3 shows the temperature of the first to fourth passes of the coil.
Table 4 shows the results of analysis, i.e. the pressure and temperature at the coil inlet and outlet, pressure loss and ethylene and propylene yields.
Table 3 reveals that T6 is comparable to T7 and T8 in tube temperature and about 2° C. lower than T9. This means that T6 to T8 are comparable in heat transfer efficiency and can be operated at a lower temperature.
Table 4 shows that T6 is smaller than T7 and T8 in pressure loss and excellent in ethylene and propylene yields. Although small in pressure loss, T9 is inferior in heat transfer efficiency and therefore lower in ethylene and propylene yields.
The arrangement of helical fins formed on the inner surface of the cracking tube of the invention enables the tube to minimize the pressure loss of the fluid inside the tube while permitting the tube to maintain high heat transfer characteristics due to the stirring action of fins. Accordingly the tube achieves improved ethylene and propylene yields, diminishes decoking work for the tube, has a prolonged life and is therefore useful as a cracking tube for thermal cracking furnaces for producing ethylene or the like.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2002-332257 | Nov 2002 | JP | national |
This application is a Divisional Application of prior application Ser. No. 10/534,016 filed on May 5, 2005.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10534016 | May 2005 | US |
| Child | 12379845 | US |
