Quench exchange with extended surface on process side

Abstract

A quench exchanger and quench exchanger tube with increased heat transfer area on the process side of the tube are provided. The exchanger provides increased heat transfer efficiency relative to a fixed tube length and at the same time eliminates stagnant and low velocity areas as well as recirculation eddies. The tubes incorporate a fin profile on the process side of the tube with alternating concave and convex surfaces. Additionally, the fins are preferably aligned with the tube center line as opposed to being twisted or spiraled.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further explained in the description that follows with reference to the drawings illustrating, by way of non-limiting examples, various embodiments of the invention wherein:

FIG. 1 is a cross-sectional view of a TLE process tube best suited for use with a short-residence time cracking furnace according to the present invention in a preferred embodiment thereof;

FIG. 2 is a cross-sectional view of a TLE process tube best suited for use in close coupling an exchanger to a serpentine cracking coil furnace according to the present invention in a preferred embodiment thereof;

FIG. 3A is a cross-sectional view of a double pipe quench exchanger incorporating a TLE process tube of the present invention in one embodiment thereof;

FIG. 3B is a sectional view taken along the line 3b-3b of the quench exchanger illustrated in FIG. 3A; and

FIG. 4A is a cross-sectional view of a double pipe quench exchanger incorporating a TLE process tube of the present invention in another embodiment thereof;

FIG. 4B is a sectional view taken along the line 4b-4b of the quench exchanger illustrated in FIG. 4A.

DETAILED DESCRIPTION OF THE INVENTION

The present invention for a novel heat exchanger and related TLE tube is now described in specific terms sufficient to teach one of skill in the practice the invention herein. In the description that follows, numerous specific details are set forth by way of example for the purposes of explanation and in furtherance of teaching one of skill in the art to practice the invention. It will, however, be understood that the invention is not limited to the specific embodiments disclosed and discussed herein and that the invention can be practiced without such specific details and/or substitutes therefor. The present invention is limited only by the appended claims and may include various other embodiments which are not particularly described herein but which remain within the scope and spirit of the present invention.

One of the key aspects of the present invention is the use of a fin profile having alternating concave and convex surfaces on the process side of the TLE tube. Further, it is preferred that the fins are aligned with the center line (longitudinal axis) of the tube as opposed to twisting or spiraling the fins within the interior of the TLE tube. Through the use of the finned process tube of the present invention, various advantages may be obtained. For example, stagnant and low flow zones are reduced and/or eliminated as are recirculation eddies. As such, fouling problems are mitigated. Further, increased heat transfer function is obtained with the desirable result that tubes can be shortened while still meeting the required heat transfer characteristics for the furnace and process.

In one embodiment of the present invention, a heat exchanger tube is provided. The heat exchanger tube has a longitudinal axis, an interior surface defining the flow area of the tube, and an interior circumference in a plane perpendicular to the longitudinal axis; wherein the interior surface comprises a plurality of axially extending grooves aligned with the longitudinal axis; the grooves formed along the length of the tube and formed as a series of alternating concave and convex surfaces along at least a portion of the interior circumference; and wherein the length of the perimeter of the interior surface in the plane is at least about twenty percent longer than the interior perimeter of a circular tube having substantially the same flow area as the heat exchanger tube.

FIG. 1 illustrates a cross-sectional view of a TLE process tube suited for use with a short-residence time hydrocarbon cracking furnace according to the present invention in a preferred embodiment thereof is presented. TLE tube 100 incorporates, on its process (internal) surface a finned surface 110 which is present, in this embodiment, around the complete circumference of the process side of the tube 100. Finned surface 110 is comprised of alternating concave 120 and convex 130 surfaces. In one embodiment, the fins, both concave 120 and convex 130, are aligned with tube center line 140 which is an imaginary line running along the longitudinal center (longitudinal axis) of tube 100.

In a preferred embodiment, each of concave 120 and convex 130 surfaces are of similar size and shape to one another such that each of the concave 120 surfaces has a concave nadir 160 and each of said convex 130 surfaces has a convex pinnacle 150, and wherein each of said convex pinnacles 150 are located at substantially the same distance from said longitudinal axis 140 and each of said convex nadirs 160 are located at the same distance from said longitudinal axis 140. In a preferred embodiment, all concave surfaces 120 and convex surfaces 130 have the same radius.

The thickness of the tube wall 170 is determined by the steam pressure in the quench exchanger, which is in turn, determined by the particular application. For example, in one embodiment a typical wall thickness for a tube on the order of about 2 to about 3 inches (about 50 to 76 mm) inside diameter (measured from valley to opposing valley of the fins) may have a wall thickness of about 0.3 to about 0.5 inches (about 7.6 to 12.7 mm). In another embodiment, a typical wall thickness for a tube on the order of about 2 to about 6 inches (about 50 to about 152 mm) inside diameter may have a wall thickness of about 0.2 to about 0.6 inches (about 5.1 to 15.2 mm). FIG. 1 shows a finned surface on the interior of TLE tube 100 having 14 fins within a nominal inner tube diameter of about 2.35 inches (about 60 mm).

As will be readily skilled in the art, the teachings of the present invention may alternatively be applied to tubes of larger or small diameters and the number of fins per unit of diameter may also be increased or decreased as desired for the particular application. That being said, typical dimensions for the inside diameter of the exchanger tube may be in the range of about 2 inches to about 3 inches (about 50 to about 76 mm) for single pass and U-tube type radiant coil units.

For units which are close coupled to serpentine radiant coils, the corresponding exchanger tube inside diameter (measured from valley to opposing valley of the fins) may lie in the range of about 3 inches to 6 about inches (about 76 to about 152 mm) depending upon the diameter of the radiant coil. When used in connection with shell and tube type exchangers, or double pipe units in a bundle arrangement, the corresponding exchanger tube inner diameter may be in the range of about 1.5 inches to about 2.5 inches (about 38 to about 64 mm). Of course, larger and smaller sizes may also be used without departing from the scope or spirit of the present invention.

With specific reference to FIG. 1 and an exemplary embodiment with a nominal inner tube diameter of 2.35 inches (60 mm), an exemplary fin height may be on the order of about 0.1 inches to about 0.3 inches (about 2.5 to 7.6 mm) with about 0.18 inches (4.6 mm) being a preferred embodiment. The number of fins located around the interior circumference of tube 100 will typically vary with the diameter of the tube. However, in a preferred embodiment, the number of fins is in the range of about 5 to about 7 times the inside diameter, measured in inches (0.2 to 0.3 times the inside diameter, measured in mm), of tube 100. For example, in one preferred embodiment, a tube with an interior diameter (from base of fin valley to base of fin valley) of about 2 inches (50 mm) may have on the order of 12 fins. In a preferred embodiment, the length of exchanger tube 100 may be as short as 15 feet (4.5 meters) or as long as 60 feet (18 meters). For use with a hydrocarbon cracking furnace, anything shorter will generally not offer the desired heat transfer characteristics and anything longer may suffer from fabrication and other size-related issues.

The tube illustrated in FIG. 1 may be fed by two single-pass radiant tubes or by a single U-tube from the radiant section. Other embodiments are also possible using the tube design of the present invention.

Turning now to FIG. 2, another possible embodiment of the present invention is now described in connection therewith. FIG. 2 is a cross-sectional view of a TLE process tube best suited for use in close coupling an exchanger to a serpentine cracking coil furnace. In a preferred embodiment, tube 200 shown in FIG. 2 is used to close couple a quench exchanger with a serpentine cracking coil furnace where the radiant outlet tube inside diameter is in the range of about 5.00 to about 5.75 inches (about 127 to about 146 mm). Of course, the tube and finning arrangement illustrated in FIG. 2 can also be applied to a wide variety of other applications.

As can be seen from FIG. 2, similar elements to the embodiment shown in FIG. 1 are present in this embodiment. However, in this case, a typical inside diameter from valley to valley is on the order of 5.75 inches (146 mm). Tube 200 preferably includes finned surface 210 having an alternating concave 220 and convex 230 surface. Again, the surface is preferably centered on center longitudinal axis 240. Convex pinnacles 250 and concave nadirs 260 are also present. It is also preferred that all concave surfaces 220 and convex surfaces 230 have the same radius. The thickness of tube wall 270 is determined by the steam pressure in the quench exchanger, which is in turn, determined by the particular application. For example, a typical wall thickness for a tube on the order of about 5 to about 5.75 inches (127 to about 146 mm) in diameter may have a wall thickness of about 0.5 to about 0.75 inches (about 12.7 mm to about 19 mm).

Turning now to FIGS. 3A and 3B, a cross-sectional view of a double pipe quench exchanger incorporating a TLE process tube and a sectional view taken along the line 3b-3b of the quench exchanger illustrated in FIG. 3A are presented. In this case, the finned TLE process tube 300 of the present invention is incorporated into double pipe quench exchanger 380 of the present invention. Process tube 300 includes wall 370 and finned interior surface 310 as described above with reference to FIGS. 1 and 2. In FIG. 3A, BFW inlet flow 390 can be seen. BFW with steam flows along the outside wall of tube 300 as is known in the art.

In the embodiment illustrated by FIGS. 3A and 3B a TLE tube with a so-called “oval-header” design is employed. An alternative embodiment is also possible and is shown in FIGS. 4A and 4B. In this embodiment, a TLE tube without such an oval-header design is used. As shown in FIG. 4A, a cross-sectional view of a double pipe quench exchanger incorporating a TLE process tube is shown. In FIG. 4B, a sectional view taken along the line 4b b of the quench exchanger illustrated in FIG. 4A is presented. In this case, the finned TLE process tube 400 of the present invention is incorporated into double pipe quench exchanger 480 of the present invention. Process tube 400 includes wall 470 and finned interior surface 410 as described above. In FIG. 4A, BFW inlet flow 490 is also depicted. BFW, with steam, flows along the outside wall of tube 400 as is known in the art. As may be appreciated by those skilled in the art, this design may also be readily implemented in commercial double-pipe TLE applications as described above.

An example of an application of the teachings of the present invention along with the achieved benefits thereof is now discussed. In this case, an older steam cracking furnace employs a four-pass serpentine radiant coil with each of the four radiant outlet tubes having an inside diameter of about 5.25 inches (133 mm). Prior to employing the teachings of the present invention, the four outlet passes are manifolded together and fed to a circular quench exchanger of the type shown in FIGS. 4 and 5 of the Herrmann paper, discussed above. In this state, the furnace experiences an undesirably long “unfired residence time” due to the time required for manifolding and the time required for the effluent to traverse the inlet chamber of the quench exchanger.

It is desirable to use a close-coupled double-pipe quench exchanger to quench the effluent from each of the four radiant passes. The geometrical constraints imposed by the existing furnace limit the length of the double-pipe quench exchanger to a maximum of thirty feet. Using a conventional, circular quench exchanger tube profile, the predicted outlet temperature from the double pipe exchanger is about 1190° F. (645° C.), which provides insufficient operating margin from the practical upper limit of about 1200 to about 1250° F. (about 650° C. to about 675° C.).

By using an internally finned quench exchanger tube such as the one described in connection with FIG. 2 herein, a quench exchanger outlet temperature of about 1114° F. (600° C.) is predicted according to a software simulation using heat transfer correlations well known to those skilled in the art. This temperature is sufficiently low to allow the effluent from the four double pipe close-coupled quench exchangers to be manifolded together before passing to the existing circular TLE for further heat recovery.

The tubes and the extended surface feature of the present invention as described above may be incorporated into a variety of quench exchanger types and designs. For example and without limitation, the teachings herein may be applied to double pipe exchangers and shell and tube type exchangers. In the case of double pipe units, the design may be linear arrangements or arrangements with multiple units positioned in a bundle with a common inlet changer and a common outlet chamber. If arranged as a linear unit, the unit may be close coupled to the radiant coil to minimize adiabatic time between leaving the furnace fired zone and entering the quench exchanger.

When used as a linear, close coupled unit, it is preferable that one or more radiant tubes be included with one or more radiant tubes feeding each quench exchanger tube. It is preferable in this case that if the radiant coil is a single pass coil, 2 or 4 radiant tubes feed each quench exchanger tube. Alternatively, if the radiant coil is a two-pass or U-tube coil, it is preferable that one or two radiant coil feeds each quench exchanger tube. Further, if the radiant coil is a serpentine coil, it is preferred that one radiant coil feeds each quench exchanger tube.

In the event that the TLE tube of the present invention is incorporated into a shell and tube type exchanger or a double pipe exchanger with multiple double pipe units mounted together in a bundle with a common inlet chamber, multiple radiant coils can be fed to multiple quench exchanger tubes regardless of the radiant coil type.

The teachings of the present invention have particular application to processes with light feeds such as gas and naphtha cracking applications. Additionally, the TLE tubes and the heat exchanger incorporating said tubes may have application in other processes such as gas-oil and other heavy feed based processes. This includes, by way of example and not limitation, gas-oil cracking applications, other heavy feed applications including atmospheric and vacuum gas-oils as well as virgin and hydro-treated gas-oils (to include both mildly hydro-treated gas-oils and severely hydro-cracked gas-oils). Other feeds may include, for example, crude oil and crude oil fractions from which non-volatile components have been removed. Further, the invention may also have application to feeds comprising field condensates with high final boiling points (e.g. above about 600° F. (315° C.).

The foregoing disclosure of the preferred embodiments of the present invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many variations and modifications of the embodiments described herein will be apparent to one of ordinary skill in the art in light of the above disclosure. The scope of the invention is to be defined only by the claims, and by their equivalents.

Claims

1. A heat exchanger tube having a longitudinal axis, an interior surface defining the flow area of said tube, and an interior circumference in a plane perpendicular to said longitudinal axis; wherein said interior surface comprises a plurality of axially extending grooves aligned with said longitudinal axis; said grooves formed along the length of said tube and formed as a series of alternating concave and convex surfaces along at least a portion of said interior circumference; and wherein the length of the perimeter of said interior surface in said plane is at least about twenty percent longer than the interior perimeter of a circular tube having substantially the same flow area as said heat exchanger tube.

2. The heat exchanger tube of claim 1, wherein said heat exchanger tube is used to cool effluent from a hydrocarbon cracking furnace and is fed by at least one radiant tube associated with said furnace.

3. The heat exchanger tube of claim 1 wherein said concave and convex surfaces form a plurality of convex fins, and wherein the number of convex fins is equal to about 5 to about 7 times the inside diameter of said heat exchanger tube when measured in inches.

4. The heat exchanger tube of claim 1 wherein each of said concave surfaces has a concave nadir and each of said convex surfaces has a convex pinnacle; and wherein each of said convex pinnacles are located at substantially the same distance from said longitudinal axis and each of said convex nadirs are located at the same distance from said longitudinal axis.

5. The heat exchanger tube of claim 4 wherein the inside diameter of said heat exchanger tube is about 2 to about 6 inches (about 50 to about 152 mm) and the wall thickness of said tube is about 0.2 to about 0.6 inches (about 5.1 to 15.2 mm).

6. The heat exchanger tube of claim 5 wherein each of said convex pinnacles are located from about 0.75 inches (19.0 mm) to about 2.75 inches (69.8 mm) from said longitudinal axis; and each of said concave nadirs are located from about 1.0 inches (25.4 mm) to about 3.0 inches (76.2 mm) from said longitudinal axis.

7. The heat exchanger tube of claim 4 wherein the inside diameter of said heat exchanger tube is about 2 to about 3 inches (about 50 to about 76 mm) and the wall thickness of said heat exchanger tube is about 0.3 to about 0.5 inches (about 7.6 to 12.7 mm).

8. The heat exchanger tube of claim 7 wherein each of said convex pinnacles are located from about 0.75 inches (19.0 mm) to about 2.75 inches (69.8 mm) from said longitudinal axis; and each of said concave nadirs are located from about 1.0 inches (25.4 mm) to about 3.0 inches (76.2 mm) from said longitudinal axis.

9. The heat exchanger tube of claim 1 wherein the length of said heat exchanger tube is from about 15 feet to about 60 feet (about 4.5 meters to about 18 meters).

10. The heat exchanger tube of claim 1 wherein the height of each groove is from about 0.1 inches (2.54 mm) to about 0.3 inches (7.62 mm).

11. The heat exchanger tube of claim 1 wherein: the inside diameter of said heat exchanger tube is about 2 to about 6 inches (about 50 to about 152 mm); the wall thickness of said tube is about 0.2 to about 0.6 inches (about 5.1 to 15.2 mm); the height of each groove is from about 0.1 inches (2.54 mm) to about 0.3 inches (7.62 mm); and the length of said heat exchanger tube is from about 15 feet to about 60 feet (about 4.5 meters to about 18 meters).

12. A transfer line heat exchanger unit connected to a steam cracking furnace, in which heated furnace effluent flows from a furnace outlet into at least one heat exchanger tube for cooling said furnace effluent; wherein said at least one heat exchanger tube has a longitudinal axis, an interior surface defining the furnace effluent flow area of said tube, and an interior circumference in a plane perpendicular to said longitudinal axis; and wherein said interior surface comprises a plurality of axially extending grooves aligned with said longitudinal axis; said grooves formed along the length of said tube and formed as a series of alternating concave and convex surfaces along at least a portion of said interior circumference; and wherein the length of the perimeter of said interior surface in said plane is at least about twenty percent longer than the interior perimeter of a circular tube having substantially the same flow area as said heat exchanger tube.

13. The heat exchanger tube of claim 12, wherein said heat exchanger tube is used to cool effluent from a hydrocarbon cracking furnace and is fed by at least one radiant tube associated with said furnace.

14. The heat exchanger tube of claim 12 wherein said concave and convex surfaces form a plurality of convex fins, and wherein the number of convex fins is equal to about 5 to about 7 times the inside diameter of said heat exchanger tube when measured in inches.

15. The heat exchanger tube of claim 12 wherein each of said concave surfaces has a concave nadir and each of said convex surfaces has a convex pinnacle; and wherein each of said convex pinnacles are located at substantially the same distance from said longitudinal axis and each of said convex nadirs are located at the same distance from said longitudinal axis.

16. The heat exchanger tube of claim 15 wherein the inside diameter of said heat exchanger tube is about 2 to about 6 inches (about 50 to about 152 mm) and the wall thickness of said tube is about 0.2 to about 0.6 inches (about 5.1 to 15.2 mm).

17. The heat exchanger tube of claim 16 wherein each of said convex pinnacles are located from about 0.75 inches (19.0 mm) to about 2.75 inches (69.8 mm) from said longitudinal axis; and each of said concave nadirs are located from about 1.0 inches (25.4 mm) to about 3.0 inches (76.2 mm) from said longitudinal axis.

18. The heat exchanger tube of claim 12 wherein the inside diameter of said heat exchanger tube is about 2 to about 3 inches (about 50 to about 76 mm) and the wall thickness of said heat exchanger tube is about 0.3 to about 0.5 inches (about 7.6 to 12.7 mm).

19. The heat exchanger tube of claim 18 wherein each of said convex pinnacles are located from about 0.75 inches (19.0 mm) to about 1.4 inches (35.6 mm) from said longitudinal axis; and each of said concave nadirs are located from about 1.0 inches (25.4 mm) to about 1.5 inches (38.1 mm) from said longitudinal axis.

20. The heat exchanger tube of claim 12 wherein the length of said heat exchanger tube is from about 15 feet to about 60 feet (about 4.5 meters to about 18 meters).

21. The heat exchanger tube of claim 12 wherein the height of each groove is from about 0.1 inches (2.54 mm) to about 0.3 inches (7.62 mm).

22. The heat exchanger tube of claim 12 wherein: the inside diameter of said heat exchanger tube is about 2 to about 6 inches (about 50 to about 152 mm); the wall thickness of said tube is about 0.2 to about 0.6 inches (about 5.1 to 15.2 mm); the height of each groove is from about 0.1 inches (2.54 mm) to about 0.3 inches (7.62 mm); and the length of said heat exchanger tube is from about 15 feet to about 60 feet (about 4.5 meters to about 18 meters).