Description:TRANSFER LINE HEAT EXCHANGER

FIELD OF THE INVENTION:

The present invention relates to the field of heat exchangers and, more particularly, the present invention relates to an improved vertical transfer line heat exchanger used in petrochemical plants for the production of olefins by the thermal cracking of liquid or gaseous hydrocarbons.

BACKGROUND OF THE INVENTION:

Pyrolysis or steam cracking is the primary process utilized to manufacture olefins from large hydrocarbon molecules. It is well known that thermal cracking occurs in pyrolysis or cracking furnaces in the presence of steam at high temperatures in the range of 800oC to 900oC.

The cracked gas is rich in olefins such as ethylene and propylene as well as higher olefins. The cracked gas needs to be cooled rapidly from the cracking temperature to a temperature below which secondary reactions do not occur. The secondary reactions decrease the yield of olefins and cause coke formation with a consequent decrease in the run time of the cracking furnace. The cooling of cracked gases is carried out in vertical shell and tube type of multi-tubular heat exchanger(s). These types of heat exchangers are known as Transfer Line Exchangers (TLEs) or Quench Coolers. The TLEs are closely coupled to the outlet of the radiant coils of the cracking furnace to minimize the residence time. The higher residence time promotes coke formation.

The TLEs comprise a tube bundle having a plurality of tubes whose opposite ends are welded to upper and lower tube sheets. The peripheral edges of the tube sheets are welded to a cylindrical shell, thus forming a closed vessel with a path for the passage of the shell side fluid. The cracked gas flows on the tube side of the TLEs. The heat is recovered by partial vaporization of high-pressure boiler feed water (BFW) flowing on the shell side of the heat exchanger. The steam-water mixture flow (F) exits from the TLEs through outlet nozzle(s) (hereinafter ‘the outlet (N)) located on the shell side at the top of the TLEs at a certain height below the top tube sheet (TS). A typical sketch of the location and arrangement of the outlet nozzle(s) in the prior art is shown in Figures 1 and 2.

The outlet (N) is connected to riser pipes (R) through which the steam-water mixture flows to an overhead steam drum (SD), where the mixture is separated to generate practically dry, saturated steam. The separated water is re-circulated to the TLEs through downcomer pipes (DC), which are connected to nozzle(s) on the shell side located at a certain distance above the bottom tube sheet. The TLEs have a shroud (S) located inside the shell near the shell outlet (N).

As is well known to those skilled in the art, this type of heat exchanger has a poor reliability record. Within a short span of operation, often in a couple of years from the date of commissioning of the plant, the heat exchanger is prone to tube failures below the top tube sheet. The primary cause of such tube failures is an accumulation of steam (steam blanketing) below the shell side face of the top tube sheet. The typical fluid dynamics at the outlet (N) of the shell side lead to the separation of steam from the two-phase mixture. In the prior art, venting arrangement (V) is provided below the top tube sheet to remove steam accumulated below the top tube sheet. The typical details of the venting arrangement (V) are shown in Figure 1. However, the venting arrangement (V) provided in the prior art is largely inadequate to significantly reduce steam blanketing. The steam blanketing can lead to flow-accelerated corrosion and erosion of the tubes at the liquid–steam boundary. This boundary, which forms at a certain height below the shell side face of the top tube sheet is highly unstable and keeps fluctuating dynamically. The alternate drying and wetting of the tubes in the zone of the fluctuating vapor-liquid boundary lead to the flow-accelerated corrosion of tubes.

Accordingly, there exists a need to provide an improved design of vertical type transfer line exchangers to eliminate the steam blanketing.

OBJECTS OF THE INVENTION:

An object of the present invention is to provide an improved vertical shell and tube-type transfer line, exchanger.

Another object of the present invention is to minimize the separation of a two-phase mixture.

Yet another object of the present invention is to reduce/eliminate steam accumulation below the top tube sheet of the transfer line exchanger.

Still, another object of the present invention is to prevent failures of tubes in transfer line exchangers.

SUMMARY OF THE INVENTION:

The present invention provides a vertical shell and tube-type transfer line exchanger (TLE) for cooling the cracked gases from the cracking furnaces. The transfer line heat exchanger (100) comprises a cylindrical shell (10), an upper tube sheet (20) and a lower tube sheet with peripheral edges welded with the cylindrical shell (10), a tube bundle (30) having a plurality of tubes with opposite ends welded to the upper tube sheet (20) and the lower tube sheet, a peripheral extension element (40) making an annular cavity (2), and at least one shell side outlet arrangement.

The shell side outlet arrangement is provided with elevation to the riser connection with reference to the bottom surface of the upper tube sheet using an annular cavity and a distributor or a venting arrangement, facilitating the effective flow of steam-water mixture from the tubed region towards the riser pipe with significant reduction/elimination of the separation of steam from the two-phase steam-water mixture and eliminating the steam blanketing below the upper tube sheet.

BRIEF DESCRIPTION OF THE DRAWINGS:

The objects and advantages of the present invention will become apparent when the disclosure is read in conjunction with the following figures, wherein

Figure 1 shows a schematic of a transfer line exchanger, in accordance with the prior art invention;

Figure 2 shows a typical arrangement of the transfer line exchanger with steam drum, in accordance with the present invention;

Figure 3 shows a schematic of the transfer line heat exchanger with a header distributor arrangement at the shell side outlet (N), in accordance with an embodiment of the present invention;

Figure 4 shows a schematic of the transfer line heat exchanger with a half-header distributor arrangement at the shell side outlet (N), in accordance with another embodiment of the present invention;

Figure 5 shows a schematic of the transfer line heat exchanger with a shroud distributor arrangement at the shell side outlet (N), in accordance with another embodiment of the present invention; and

Figure 6a and 6b show a schematic of the transfer line heat exchanger with venting arrangement and the shell side outlet (N) arrangement, in accordance with other embodiments of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS:

The foregoing objects of the present invention are accomplished and the problems and shortcomings associated with the prior art, techniques, and approaches are overcome by the present invention as described below in the preferred embodiments.

The present invention provides an improved design of the transfer line exchanger. The transfer line exchangers are used for cooling the cracked gases from the olefin producing cracking furnaces.

The present invention is illustrated with reference to the accompanying drawings, throughout which reference numbers indicate the corresponding parts in the various figures. These reference numbers are shown in a bracket in the following description.

Referring to figures from 2 – 6b, there is shown an improved transfer line exchanger (hereinafter ‘TLE (100)’). The TLE (100) is a vertical shell and tube type improved transfer line exchanger. The TLE (100) comprises a cylindrical shell (10), an upper tube sheet (20), a lower tube sheet (not shown), a tube bundle (30), a peripheral extension element (40), an inlet channel (not shown), an outlet channel, and at least one shell side outlet arrangement.

The cylindrical shell (10) includes at least one inlet and at least one outlet (N). In an embodiment, the cylindrical shell (10) may include a plurality of inlets and a plurality of outlets. Further, the inlet(s) is located just above the lower tube sheet for entry of the shell side fluid. Similarly, the outlet(s) (N) located just below the top tube sheet are configured for the exit of the shell side fluid.

The inlet and outlet channel of the TLE (100) are connected with the lower and upper tube sheet (20) respectively. In an embodiment, the inlet and outlet channel are flanged with the lower and upper tube sheet (20). Additionally, the inlet and outlet channel can be connected to the tube sheet with any of the specific configurations commonly used for such joints. Further, the inlet and outlet channels provide passage for the entry and exit of the tube side fluid respectively. The inlet channel is normally provided with refractory lining having a special profile. In an embodiment, the inlet channel of the TLE (100) is typically conical in shape and the outlet channel is selected from any of the conical or cylindrical shapes.

The cracked gas from the outlet of the cracking furnace flows on the tube side of the TLE (100), typically entering the TLE (100) through the inlet channel situated at the bottom. The cracked gas is cooled rapidly in the TLE (100) by high-pressure boiler feed water (BFW) flowing on the shell side. The boiler feedwater (BFW) gets partially vaporized (8-10%) by heat absorption from the cracked gas. The steam-water mixture exits from the shell side outlet arrangement and flows to a steam drum (not shown), where separation of the steam-water mixture occurs. The separated water is re-circulated to the cylindrical shell (10) along with make-up water.

The TLE (100) is connected to the steam drum (SD) through a set of downcomer (DC) pipes and riser pipes (R), as shown in fig. 2. The steam drum (SD) is normally placed at an elevation such as at the top of the convection section of the cracker furnace. The flow of the boiler feed water (BFW) from the steam drum (SD) to the TLE (100) occurs by natural circulation. This re-circulation is achieved by the difference between densities of water in the downcomers (DC) and the steam-water mixture in the risers (R).

The lower tube sheet and the upper tube sheet (20) are configured to separate the TLE (100) into two parts having a cylindrical shell side region and a tube side region. The peripheral edges of the lower and upper tube sheet are welded to the cylindrical shell (10), thus forming a closed vessel with a path for the passage of the shell side fluid. The tube bundle (30) includes a plurality of tubes with two ends. Further, the opposite ends of the plurality of tubes are welded to the upper tube sheet (20) and the lower tube sheet. The plurality of tubes of the tube bundle (30) can be connected to the tube sheet with any of the specific configurations commonly used for such joints. Additionally, the upper tube sheet is attached to the cylindrical shell (10) using a peripheral extension element (40).

The extension element (40) has a curved cross-section located between the upper tube sheet (20) and the point of attachment to the cylindrical shell (10). Further, the extension element (40) along with the upper tube sheet (20) and the cylindrical shell (10) forms an annular cavity (2). The annular cavity (2) is configured at a higher elevation than the bottom surface (3) of the upper tube sheet (20), thus making the water-steam mixture flow outwards from the tubed region on the bottom surface (3) of the upper tube sheet (20). The extension element (40) also provides the necessary flexibility to the joint between the upper tube sheet (20) and the cylindrical shell (10) by absorbing the differential axial movement between the tube bundle (30) and the cylindrical shell (10) caused by operating temperatures and pressures. In an embodiment, the dimensions of the extension element (40), and the tube sheet can be determined considering various operating and design parameters to take care of pressure on both sides of the tube sheet and the extension element (40), temperature distribution, and differential thermal expansion of various parts of the TLE (100).

For suitability of fabrication, the extension element (40) is divided into two parts. The first part is integral with the tube sheet which can be formed to the desired shape. The second part is integral to the cylindrical shell in the form of a forging including the part of the cylindrical shell (10), the extension element (40), and the junction thereof. The two parts are thus fabricated together as shown in Figure 3. However, the invented parts can also be fabricated in different ways as per any other suitable fabrication process.

The steam-water mixture from the annular cavity (2) exits the TLE (100) through the shell side outlet arrangement, which is configured at the outlet (N) of the cylindrical shell (10). The shell side outlet arrangement is configured at the equal or higher elevation of the annular cavity (2). The shell side outlet arrangement is selected from any one of a distributor arrangement (50a) and a venting arrangement (50b). The distributor arrangement (50a) is further selected from an outer distributor arrangement (60) and an inner distributor arrangement (70). Additionally, the outer distributor arrangement (60) is selected from any one of a header distributor arrangement (60a) and half-pipe header distributor arrangement (60b).

The header distributor arrangement (60a) includes a plurality of openings (1) spaced on the cylindrical shell (10) periphery near the annular cavity (2). The plurality of openings (1) may be arranged in a radial direction or may have offset concerning the equipment axis. The plurality of openings (1) has an inclination in the vertical plane or the horizontal plane passing through the axis of the equipment as shown in Figure 3. This inclination provides a higher elevation of the riser connections relative to the bottom surface (3) of the upper tube sheet (20). This facilitates the outflow of the steam-water mixture from the openings (1) of the cylindrical shell (10) towards the riser pipe (R) with significant reduction/elimination of the separation of steam from the two-phase steam-water mixture. This results in the reduction/elimination of the steam blanketing below the upper tube sheet and leads to a stable flow pattern below the upper tube sheet unlike the fluctuation flow behavior in the prior art. Thereby this avoids the resulting failure of the tubes observed in the prior arts near the upper tube sheet.

The number of the peripheral openings (1) and the spacing therebetween can be varied to suit the process conditions in each application. The steam-water mixture exiting from the peripheral openings (1) is carried to a header (5) through a plurality of connectors (6) as shown in Figure 3. This steam-water mixture is carried further to the steam drum or any other utility through at least one or more main openings (4) in the header (5). In this embodiment, the header (5) is a common ring header.

Reasonable variations and modifications will become apparent to those skilled in the art such as an exemplary embodiment of the present invention shown in Figure 4 with the provision of a half-pipe header distributor arrangement (60b) directly connected to the cylindrical shell (10) to collect the shell side outlet fluid and thus eliminating the ring header (5) as of shown in Figure 3. In this embodiment, the steam-water mixture flows from the annular cavity (2) to the half-pipe header (7) through a plurality of peripheral openings (1) in the cylindrical shell (10). The steam-water mixture further flows to at least one or more main openings (4) which con to the riser pipes (R). The half-pipe header (7) further comprises a cavity (7a) providing a fluid connection between the plurality of peripheral openings (1) in the cylindrical shell (10) for inflow of the steam-water mixture therein.

Another embodiment of the present invention is the inner distributor arrangement (70) as shown in Figure 5. The inner distributor arrangement (70) is an annular flow distribution shroud (hereinafter ‘the shroud (70)). The shroud (70) includes a horizontal annular plate (70a) and a vertical plate (70b). Further, the vertical plate (70b) has at least one or more openings (1). The opening (1) forms a flow passage from the shell side region near the tube bundle (30) to the annular cavity (2), thus playing the function of the ring header (5) as shown in Figure 3, and the half-pipe header (7) in Figure 4. But the variations and modifications are not limited to these and can be made from this invention without departing from the spirit and scope thereof.

Another embodiment of the present invention includes the venting arrangement (50b), as shown in Figures 6a and 6b. The venting arrangement (50b) includes a venting pipe (8). The venting pipe (8) is provided with a first end (8a) in the annular cavity (2) and a second end (8b) connecting to the riser connection (R) as shown in Figure 6a. The steam-water mixture accumulated in the annular cavity (2) flows through the venting pipe (8) to the riser pipe (R).

In another embodiment, in addition to the annular cavity (2), the venting arrangement (50b) is provided. The venting arrangement (50b) includes a plurality of venting pipes (8), where the venting pipes (8) are more in number than the riser connections (R). The venting pipes (8) are connected to a ring header (9). The ring header (9) is connected to the riser pipe (R) through a connecting piece (11) for steam-water mixture flow to the riser pipe (R). With this arrangement, the shell side outlet arrangement can be shifted to a lower elevation compared to the other embodiments, thus making it more convenient for fabrication. The venting arrangement (50b) eliminates the need for the ring header (5) as shown in Figure 3, and the half-pipe header (7) in Figure 4. The variations and modifications are not limited to these and can be made from this invention without departing from the spirit and scope thereof.

Advantages of the invention:
1. The shell side openings of the TLE (100) of the present invention provide elevation to the riser connection concerning the bottom surface of the upper tube sheet. This facilitates the effective flow of steam-water mixture from the tubed region towards the riser pipe with significant reduction/elimination of the separation of steam from the two-phase steam-water mixture. This results in the reduction/elimination of steam blanketing below the top tube sheet. The present invention thus avoids the resulting failure of tubes observed in prior arts near the top tube sheet.
2. The unique shell side outlet configuration of the present invention eliminates the need for special venting arrangements or makes the venting arrangements more effective than provided in the prior art.
3. The shell side openings of the TLE (100) of the present invention provide improved flow with a more uniform velocity distribution across the flow regime on the shell side part.
4. The extension element of the TLE (100) of the present invention acts as a flexible element at the tube sheet to shell joint. This reduces the stresses due to the pressure and thermal loading during operation and more specifically reduces the thermal stresses due to relative thermal expansion of the tubes and the shell, thus resulting in the improved structural integrity of the equipment.

The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the precise forms disclosed and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the present invention and its practical application, and to thereby enable others skilled in the art to best utilize the present invention and various embodiments with various modifications as are suited to the particular use contemplated. It is understood that various omissions and substitutions of equivalents are contemplated as circumstances may suggest or render expedient, but such omissions and substitutions are intended to cover the application or implementation without departing from the scope of the claims of the present invention.
, Claims: We claim:

1. A transfer line heat exchanger (100) for cooling the cracked gases from the cracking furnaces, the transfer line heat exchanger (100) comprising:
a cylindrical shell (10) having at least one inlet for feed water entrance, at least one outlet (N), a shell side region configured to produce a high-pressure water-steam mixture therein, and a tube side region configured to allow the flow of the cracked gas for cooling therefrom;
an upper tube sheet (20) and a lower tube sheet configured to separate the shell side region and the tube side region;
a tube bundle (30) having a plurality of tubes with two ends, wherein opposite ends of the tubes are welded to the upper tube sheet (20) and the lower tube sheet;
a peripheral extension element (40) configured between the upper tube sheet (20) and the cylindrical shell (10), the peripheral extension element (40) configured to make an annular cavity (2) at an elevation higher than the cylindrical shell side bottom surface (3) of the upper tube sheet (20);
an inlet channel of predefined form connected to the lower tube sheet at an inlet side thereof, the inlet channel having an opening nozzle configured for the entry of the cracked gas therefrom;
an outlet channel of predefined form connected to the upper tube sheet (20) at an outlet side thereof, the outlet channel having an opening nozzle for the exit of the cracked gas therefrom; and
a shell side outlet arrangement configured at the outlet (N) of the cylindrical shell (10) for the steam-water mixture exit from the annular cavity (2) towards a riser pipe (R), wherein the shell side outlet arrangement is selected from any one of a distributor arrangement (50a) and a venting arrangement (50b).

2. The transfer line heat exchanger (100) as claimed in claim 1, wherein the inlet of the cylindrical shell (10) is located above the lower tube sheet.

3. The transfer line heat exchanger (100) as claimed in claim 1, wherein the outlet (N) of the cylindrical shell (10) is located on the outer periphery thereof and near the annular cavity (2).

4. The transfer line heat exchanger (100) as claimed in claim 1, wherein the cylindrical shell (10) has a slightly larger diameter near the upper tube sheet (20) to accommodate the space required for the extension element (40).

5. The transfer line heat exchanger (100) as claimed in claim 1, wherein the annular cavity (2) is formed at the outlet (N) region at an elevation higher than the bottom surface of the upper tube sheet (20) to facilitate easy movement of the steam-water mixture from the tube bundle (30) towards the outlet (N).

6. The transfer line heat exchanger (100) as claimed in claim 1, wherein the maximum elevation of the shell side outlet arrangement is configured to be equal to or higher level compared to the annular cavity (2).

7. The transfer line heat exchanger (100) as claimed in claim 1, wherein the distributor arrangement (50a) is selected from an outer distributor arrangement (60) and an inner distributor arrangement (70).

8. The transfer line heat exchanger (100) as claimed in claim 7, wherein the outer distributor arrangement (60) is selected from any one of a header distributor arrangement (60a) and half-pipe header distributor arrangement (60b).

9. The transfer line heat exchanger (100) as claimed in claim 8, wherein the header distributor arrangement (60a) includes
a plurality of openings (1) spaced on the cylindrical shell (10) periphery near the annular cavity (2) facilitating the outflow of steam-water mixture therefrom;
a header (5) with a main opening (4) configured at a higher elevation than the outlet (N) of the cylindrical shell (10) for connecting to the riser pipe (R); and
a plurality of connectors (6) for adjoining the plurality of openings (1) with the header (5), wherein the plurality of connectors (6) configured to allow the outflow of the steam-water mixture from the plurality of openings (1) to the header (5) therethrough.

10. The transfer line heat exchanger (100) as claimed in claim 8, wherein the half-pipe header distributor arrangement (60b) includes
a plurality of openings (1) spaced on the cylindrical shell (10) periphery near the annular cavity (2) facilitating the outflow of steam-water mixture therefrom; and
a half-pipe header (7) with a main opening (4) connecting to the riser pipe (R), the half-pipe header (7) directly connects to the outlet (N) of the cylindrical shell (10) through the plurality of openings (1) to collect the shell side steam-water mixture.

11. The transfer line heat exchanger (100) claimed in claim 7, wherein the inner distributor arrangement (70) is an annular flow distribution shroud (70) located inside the cylindrical shell (10) near the outlet (N), guiding the fluid of the cylindrical shell (10) into the annular cavity (2), wherein the shroud (70) includes
a horizontal plate (70a), and
a vertical plate (70b) with at least one or more openings (1) forming a flow passage for the steam-water mixture from the shell side region near the tube bundle (30) to the annular cavity (2).

12. The transfer line heat exchanger (100) claimed in claim 11, wherein the shroud (70) starts slightly below the outlet (N) and extends up to the extension element (40) with a gap between the extension element (40) and the shroud (70) to accommodate the thermal expansion caused during various operating conditions.

13. The transfer line heat exchanger (100) claimed in claim 1, wherein the venting arrangement (50b) includes at least one or more venting pipes (8) with a first end (8a) in the annular cavity (2) and a second end (8b) connecting to the riser pipe (R) for allowing the outflow of the steam-water mixture therethrough.

14. The transfer line heat exchanger (100) claimed in claim 13, wherein when the venting pipes (8) are more in number than the riser pipe (R), the venting arrangement (50b) includes a ring header (9) for connecting the venting pipes (8) to the riser pipe (R) through a connecting piece (11) for allowing the outflow of the steam-water mixture therethrough.

Dated this on 23rd day of June 2022
Prafulla Wange
(Agent for the applicant)
(IN/PA-2058)