Hydrocarbon cracking

Abstract

A method and apparatus for extending the operating life of a tube-type heat exchanger that has an upstream tube sheet face that carries a plurality of hollow tubes wherein the upstream side of the tube sheet face is coated with at least one refractory.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the thermal cracking of a hydrocarbonaceous material in a pyrolysis furnace. More particularly, this invention relates to the transfer line exchanger (TLE) of a pyrolysis furnace.

2. Description of the Prior Art

Thermal cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, and aromatics such as benzene, toluene, and xylenes. In an olefin production plant, a hydrocarbonaceous feedstock such as ethane, naphtha, gas oil, or other fractions of whole crude oil is mixed with steam which serves as a diluent to keep the hydrocarbon molecules separated.

This mixture, after preheating, is subjected to hydrocarbon thermal cracking using elevated temperatures (1,450 to 1,550 degrees Fahrenheit or F.) in a pyrolysis furnace (steam cracker or cracker). This thermal cracking is carried out without the aid of any catalyst.

The cracked product effluent of the pyrolysis furnace (furnace) contains hot, gaseous hydrocarbons of great variety (from 1 to 35 carbon atoms per molecule, or C1 to C35 inclusive, both saturated and unsaturated). This product contains aliphatics (alkanes and alkenes), alicyclics (cyclanes, cyclenes, and cyclodienes), aromatics, and molecular hydrogen (hydrogen).

This furnace product is then subjected to further processing to produce, as products of the olefin plant, various, separate and individual product streams such as hydrogen, ethylene, propylene, fuel oil, and pyrolysis gasoline. After the separation of these individual streams, the remaining cracked product contains essentially C4 hydrocarbons and heavier. This remainder is fed to a debutanizer wherein a crude C4 stream is separated as overhead while a C5 and heavier stream is removed as a bottoms product.

Such a C4 stream can contain varying amounts of n-butane, isobutane, 1-butene, 2-butenes (both cis and trans isomers), isobutylene, acetylenes, and diolefins such as butadiene (both cis and trans isomers).

The hot, cracked furnace product, upon leaving the furnace, is first introduced into a tube-type heat exchanger wherein, for example, boiler feed water is indirectly heat exchanged with the hot furnace product stream to cool that product stream to a more manageable level, and to generate high pressure steam for use elsewhere in the cracking plant. The tube-type heat exchanger (exchanger) employed is a unit that contains a plurality of closely spaced heat exchange tubes, e.g., typically from about 25 to about 100 tubes. The number of tubes varies widely depending on a number of variables such as exchanger and tube internal diameters. The tube ends are carried and spaced apart by a metal member that is termed a tube sheet face.

The transfer of product from the furnace to the exchanger is accomplished through a transfer line and a truncated cone adapter which expands from the smaller diameter transfer line to the larger diameter exchanger. The truncated adapter, unlike the tube sheet face and exchanger tubes, is typically refractory lined, and incorporates various conical or trumpet style designs intended to distribute the flow evenly across the larger diameter exchanger. The mass flow rate (pounds/second/square foot) of furnace product through the transfer line and cone, and into and through the exchanger tubes is relatively constant under normal conditions.

The exchanger is an elongated unit, since the tubes in its interior are long in order to achieve as much heat transfer from the hot product to the boiler feed water as reasonably possible. The exchanger, including its upstream tube sheet face and tube interiors, are formed of uncoated metal, and are exposed to the hot furnace product.

In the cracking process coke is unavoidably formed in the furnace, and just as unavoidably, coke fines find their way into the furnace product that passes into the transfer line exchanger. Thus, the exposed metal upstream side of the upstream tube sheet face of the exchanger and the interior of the exchanger tubes are both constantly impacted with hot furnace product containing coke particulates (fines). The furnace product also carries steam which can cause scale deposition on the upstream tube sheet face and the interior of the exchanger tubes. Thus, the upstream side of the tube sheet face and the interior surface of the exchanger tubes carried by that tube sheet face are subjected to the peak heat flux (Btu/hour/square foot) of the exchanger under severe particulate erosion and scaling deposition conditions.

Accordingly, there are no less than three major problems encountered by an upstream tube sheet face and its accompanying tubes in a TLE. These are 1) physical erosion due to normal operation of the transfer line exchanger in conjunction with a pyrolysis furnace which includes de-coking operations that employ combustion, in the presence of added oxygen, of coke deposits on the upstream surface of the upstream tube face sheet and tube interior surfaces; 2) high temperature degradation of such surfaces during normal operation, including de-coking; and 3) scale deposition from the water contacting such surfaces with accompanying corrosion of such surfaces under the scale deposits.

Therefore, it is desirable to have the most robust tube sheet face as is economically possible to provide as long an operating life as possible for a transfer line exchanger. This invention does just that, and addresses all three problem areas as aforesaid at the same time.

SUMMARY OF THE INVENTION

In accordance with this invention, the foregoing transfer line exchanger problems are minimized by coating the exposed metal upstream side of the upstream tube sheet face with at least one refractory.

Pursuant to this invention, it has been found that, contrary to what was thought, it is not necessary to protect the exposed inner metal surface of the exchanger tubes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a conventional pyrolysis furnace with a cracked product stream being removed from the furnace to the first TLE heat exchanger to be encountered downstream of the furnace.

FIG. 2 shows one embodiment of a typical transfer line and adapter cone operating between a furnace and a vertical exchanger located on the top of the radiant section of that furnace.

FIG. 3 shows a more detailed configuration for a transfer line, adapter cone, and exchanger.

FIG. 4 shows the upstream side of a tube sheet face for an exchanger.

FIG. 5 shows a side view of the tube sheet face of FIG. 4.

FIG. 6 shows a side view of a tube sheet face within this invention.

FIG. 7 shows a ferrule system for supporting refractory on a tube sheet.

FIG. 8 shows a modification of the anchoring system of FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows an olefin production plant pyrolysis furnace 1 having hydrocarbonaceous feed 2 and steam 3 fed thereto for thermal cracking of feed 2 as described here in above. The cracked product is recovered from the interior of the radiant box of furnace 1 and passed in a first flow direction by way of transfer line 4 to tube-type heat exchanger 5 to cool such product and generate steam for use elsewhere in the plant. Transfer line 4 can be straight as shown in the Figure or can contain one or more curved sections, all of which are within the scope of this invention. The thus cooled product is removed from exchanger 5 and passed in a second flow direction by way of line 6 to other downstream processing units of the plant (not shown) as described here in above.

FIG. 2 shows in greater detail a configuration for bridging the distance between the outlet of furnace 1 and the upstream inlet of exchanger 5. In this Figure, transfer line 4 is fixed to, and in fluid communication with, adapter 7. Adapter 7 is conical in nature, and can be a straight sided classic cone shape or a curved sided trumpet bell shape, both of which are known in the art and within the scope of this invention. Cracked product 8, which also contains suspended therein substantial amounts of coke fines and steam, passes in a straight first flow direction through transfer line 4, expands inside cone 7, and then passes into the interior of heat exchange tubes (FIG. 3) carried in the interior of exchanger 5.

FIG. 3 shows transfer line 4 carrying in its hollow interior cracked product 8 from furnace 1 to the hollow interior 19 of a classical straight sided cone 7. Product 8 expands into the larger, open interior of cone 7. From interior 19, product 8 is split at the exposed metal, upstream side of tube sheet face 11 and passes into and through the exposed metal, hollow interiors 26 of multiple, longitudinally extending, spaced apart, heat exchange tubes 12 whose long axes are essentially in parallel alignment with the first flow direction 8, and the long axis of exchanger 5.

Tubes 12 are carried by the upstream tube sheet face 11, and extend along their long axes to downstream tube sheet face 13. Tubes 12 terminate at sheet face 13, and are in fluid communication with downstream outlet chamber 14. Chamber 14 is typically cylindrical or conical in shape. Sheet faces 11 and 13 enclose opposing ends of interior 20 of exchanger 5. The cooled furnace product then passes out of exchanger 5 in a conduit (not shown) in a second flow direction as shown by arrow 6.

As shown in FIG. 3, transfer line 4 has a substantially smaller cross-sectional diameter than exchanger 5. Cone 7 adapts from its small end 24 which is adjacent to and contiguous with the outlet end of transfer line 4 to its large end 25 which is adjacent to and contiguous with upstream tube sheet inlet face 11 of exchanger 5.

FIG. 4 shows the upstream side 28 of tube sheet face 11, and shows that face 11 carries a plurality of essentially circular transverse cross-section tubes 26 across its entire upstream surface area.

FIG. 5 shows a side view of tube sheet face 11, and shows that the inlet ends 27 of tubes 12 are carried by face 11 so that their interiors 26 start at upstream side 28 of face 11.

Thus, the mass flow rate (mass flow) of product 8 in line 4 passes into interior 19 and impinges on upstream side 28 of tube sheet inlet face 11 at high velocity under peak temperature flux conditions in the presence of coke fines that are also traveling at a high velocity, thus leading to the three major problems enumerated here in above.

This invention alleviates, if not eliminates, all three problems.

Pursuant to this invention, as shown in FIG. 6, upstream side 28 of upstream tube sheet face 11 is coated with at least one refractory 30 in the interstices 29 (FIG. 4) between the inlet ends 27 of tubes 12. Pursuant to this invention, it has been found that the exposed metal interior surface 34 (FIG. 7) of tubes 12 need not be coated with refractory.

By this invention, the combination of tube sheet face 11 and refractory member 30 shown in FIG. 6 prevents erosion due to impingement of coke fine particles on the sheet face and tube interior surface. This tube sheet face/refractory combination has been found also to provide sufficient thermal insulation to reduce both high temperature degradation and oxidation of the metal from which the tube sheet face and tubes 12 are composed during normal operation and de-coking operations. Thus, the refractory combination of this invention not only serves as a physical barrier, but at the same time, provides sufficient thermal insulation to reduce oxidation and scaling, as well as erosion, of upstream tube sheet face 11 and tubes 12 sufficiently to make a significant improvement in the operating life of this equipment, thereby extending the operating cycle of the pyrolysis furnace.

The refractory employed in this invention can be any refractory which contains a substantial amount of alumina. The alumina content can be at least about 20 weight percent (wt. %) alumina, preferably at least about 80 wt. % alumina, based on the total weight of the refractory. The refractory can contain other known refractory materials such as magnesium oxide, phosphoric acid, and/or silica. High alumina refractory is well known in the art, commercially available, and further detail is not necessary to inform the art. The refractory coatings of this invention can vary widely in thickness depending on the size of the equipment, particularly the interior diameter of tubes 12, but will generally vary from about 0.25 to about 6 inches.

Refractory can be affixed to tube sheet face surface 28 in any known manner. Generally, such refractory has a putty-like consistency when first molded to the desired carrying surface. Thereafter the molded refractory is fired at a high temperature to convert the soft refractory to the desired rigid refractory coating. Known anchoring systems can be used to help fix and hold the refractory coating on the tube sheet face during operation of the plant.

An anchoring system pursuant to this invention is shown in FIG. 7 wherein a ferrule 31 is rolled into inlet end 27 of tube 12. Refractory 30, while in the moldable state, is formed around this ferrule member 31 filling adjacent interstices 29 so that after firing of refractory 30 to a hard, rigid state it is firmly supported by one or more ferrule members 31.

Member(s) 31 is of a curvilinear, e.g., essentially circular, transverse cross-section similar to that of tube(s) 12. Member 31 extends upstream longitudinally outside (beyond) wall 28 for all or essentially all of the thickness 33 (FIG. 8) of refractory 30, and extends longitudinally for a finite distance 31, e.g., at least about two inches, along the interior wall 34 of tube 12. Ferrule 31 is fixed to wall 34 in any desirable manner obvious to one skilled in the art.

Pursuant to this invention, a method of fixing member 31 inside tube 12 involves inserting an undersized essentially curvilinear cross-section ferrule into tube interior 26, member 31 essentially matching the curvilinear cross-section of that tube, and then mechanically rolling or otherwise expanding the outer surface of the ferrule into tight physical contact with all or any part of the 360 degree interior of tube surface 34. In this manner, the rolled ferrule is held in place by physical, frictional gripping between the outer wall of the ferrule and inner wall of the tube. The foregoing construction can be employed in lieu of welding, riveting, or other mechanical means for fixing the ferrule to the tube; however mechanical fixing means are also within the scope of this invention.

Ferrule 31 can carry one or more projection members that are fixed to the ferrule and extend into the interior of the body of the refractory coating 30 to better support refractory 30 when in place on tube sheet face 11, and to better fix refractory 30 to face 28. FIG. 8 shows a projection member system within this invention. In FIG. 8, member 40 is a rectangular cross-section projection that is fixed to and extends outwardly from and away from the outer surface 41 of ferrule 31. Member 40 can extend completely or partly around outer surface 41. Thus, member 40 can be one or a series of individual projections, or a single annular projection that surrounds the entire 360 degree outer surface 41 of member 31. Member 31 can be rectangular in cross-section as shown in FIG. 8, or can be square, rounded, or the like in cross-section, or can be shaped like a “Y,” “T,” or otherwise, or any combination thereof. Ferrule 31 can be employed in all or any number of tubes 12 in a particular tube sheet 11.

The upstream side 28 of tube sheet 11 is, for example, routinely exposed to a gas stream 8 which contains coke particulates and which is at a temperature of about 1,560 F, while its downstream side 42 is routinely exposed to water at 610 F. This creates a substantial temperature gradient across tube sheet 11. This temperature gradient can cause cracks to form in the tube sheet itself and/or in tubes 12. It can also cause scale deposits to form on the upstream side 42 of tube sheet 11, particularly in the corners where the tube sheet and tubes are contiguous with one another.

By this invention, the upstream side 28 of tube sheet 11 is reduced to a temperature of from about 700 to about 900 F, thereby significantly reducing the temperature gradient across tube sheet 11, and substantially reducing, if not eliminating, one or both of cracking in tube sheet 11 and/or tubes 12, and scale deposition on upstream side 42 and tube 12.

EXAMPLE

A tube-type heat exchanger as shown in FIG. 3 containing 56 carbon steel tubes 12 each having an inside diameter of 1.75 inches and a length of 26 feet is employed downstream of a conventional cracking furnace. The feed to the furnace is ethane to be cracked to ethylene. The mass flow rate of cracked ethane furnace product in transfer line 4 is about 35,000 pounds/hour/square foot of cross section of line 4. The temperature at upstream side 28 of tube sheet face 11 is about 1,550 F. at a pressure of about 12 psig. The mass flow rate of cracked furnace product in interior 26 of tubes 12 is about 6 pounds/second/square foot of tube cross section. Upstream side 28 of tube sheet face 11 is coated with a refractory containing about 80 wt. % alumina, the remainder being a mixture of magnesium oxide, phosphoric acid, and silica, based on the total weight of the refractory. The exposed metal interior surfaces of the carbon steel tubes 12 are left uncoated with refractory.

Claims

1. In a method for thermally cracking a hydrocarbonaceous material to form a cracked product wherein said material is passed through at least one pyrolysis furnace to cause said cracking and form a furnace product, said product being transferred from said furnace to at least one tube-type heat exchanger which has a first upstream tube sheet face and a second downstream tube sheet face and which contains a plurality of longitudinally extending spaced apart hollow interior heat exchange tubes having an interior surface and upstream and downstream ends, said tubes extending in overall length from said upstream face sheet to said downstream face sheet for transporting said product through said heat exchanger, the longitudinal axes of said tubes extending from said upstream face sheet to said downstream tube sheet face, said upstream tube sheet face having an upstream side, said upstream ends of said tubes being product inlet ends that terminate with said upstream side of said upstream tube sheet face, the improvement comprising coating said upstream side of said upstream tube sheet face between said tube inlets with at least one refractory.

2. The method of claim 1 wherein said refractory contains at least about 40 wt. % alumina based on the total weight of said refractory.

3. The method of claim 1 wherein said interior surfaces of said exchange tubes are not coated with refractory.

4. In a thermal cracking transfer line tube-type heat exchanger which has a first upstream tube sheet face and a downstream tube sheet face and which contains a plurality of longitudinally extending spaced apart hollow interior heat exchange tubes having an interior surface extending in overall length from said upstream tube sheet face to said downstream tube sheet face and upstream inlet and downstream outlet ends, the longitudinal axes of said tubes extending from said upstream tube sheet face to said downstream tube sheet face, said upstream tube sheet face having an upstream side, said upstream inlet ends of said tubes terminating with said upstream side of said upstream tube sheet face, the improvement comprising said upstream side of said upstream tube sheet face having a coating of at least one refractory.

5. The apparatus of claim 4 wherein said refractory coating is from about 0.25 to about 6 inches.

6. The apparatus of claim 4 wherein said refractory coating contains at least about 40 wt. % alumina based on the total weight of said refractory.

7. The apparatus of claim 4 wherein said refractory coating is supported by at least one ferrule that is carried at the inlet end of at least one of said exchange tubes.

8. The apparatus of claim 7 wherein said at least one ferrule carries at least one projection that extends away from said ferrule and into the interior of said refractory coating.

9. The apparatus of claim 4 wherein said interior surfaces of said exchange tubes are not coated with refractory.