US20090022635A1

US20090022635A1 - High-performance cracker

Info

Publication number: US20090022635A1
Application number: US11/880,189
Authority: US
Inventors: Baozhong Zhao; Edward F. Stanley; John R. Garland; Samir J. Serhan
Original assignee: Selas Fluid Processing Corp
Current assignee: Linde Engineering North America Inc
Priority date: 2007-07-20
Filing date: 2007-07-20
Publication date: 2009-01-22
Also published as: WO2009014949A1

Abstract

A furnace for thermally cracking hydrocarbon process fluid is provided. The furnace comprises a convection section including a plurality of convection sub-sections, a radiant section comprising a plurality of radiant sub-sections, and an enclosure defining a plurality of discrete sectors. Each discrete sector of the enclosure includes a convection sub-section having at least one convection conduit for carrying hydrocarbon process fluid, at least one radiant sub-section having at least one radiant conduit connected to receive preheated hydrocarbon process fluid from the convection conduit, and a heat source configured to deliver heat into the discrete sector. Adjacent sectors of the furnace are segregated from each other, such that the heat delivered from the heat source is substantially localized in the discrete sector of the enclosure.

Description

FIELD OF THE INVENTION

The present invention relates to a pyrolytic furnace for cracking hydrocarbon process fluid. The pyrolytic furnace includes multiple convection and radiant sub-sections that are arranged in zones for cracking of hydrocarbon process fluid.

BACKGROUND OF THE INVENTION

The rapidly expanding global demand for ethylene has necessitated an industry need for efficient, expandable, large capacity pyrolytic furnaces. Generally, pyrolytic furnaces are employed in the petroleum industry to reduce the molecular weight of hydrocarbons by breaking their molecular bonds to yield olefins. Specifically, cracking of hydrocarbons is a petrochemical process that is widely used to produce olefins such as ethylene, propylene, butenes, butadiene, from a hydrocarbon feedstock such as naphtha, ethane, propane, gas oil or other fractions of whole crude oil.
The cracking process is commonly carried out in a pyrolytic furnace (also referred to herein as a cracker or steam cracker). The process may also be carried out in a heater, boiler, kiln, or kettle. Pyrolytic furnaces generally include a convection section and a radiant section. In practice, a hydrocarbon process fluid is first delivered into the convection section of the pyrolytic furnace, whereupon the hydrocarbon process fluid is preheated and diluted with steam. The preheated and diluted hydrocarbon process fluid is then delivered into the higher temperature radiant section, where cracking of the hydrocarbon process fluid occurs, yielding olefins. The resulting olefins are available for consumption, distribution or storage.
A conventional pyrolytic furnace is illustrated in U.S. Patent Application Publication No. 2003/0209469 to Kivlen, for example, which is incorporated by reference herein in its entirety. As disclosed in the Kivlen publication, the hydrocarbon cracking process begins by supplying a hydrocarbon feedstock to a furnace. The hydrocarbon feedstock is mixed with steam, which serves as a diluent to keep the hydrocarbon molecules separated. The hydrocarbon feedstock mixture is preheated in the convection section by flue gas from the burners positioned in the radiant section of the furnace. Once the hydrocarbon feedstock has been preheated, the feedstock is transferred to the radiant section of the furnace. The radiant section contains a plurality of radiant tubes through which the preheated hydrocarbon feedstock is distributed. The radiant section also contains a plurality of heat sources that supply the heat that is necessary for cracking the preheated hydrocarbon feedstock contained within the radiant tubes.
In the course of operation of such a pyrolytic furnace, carbon deposits (referred to herein as coke) typically form on the interior walls of the radiant coils. This phenomena is commonly referred to in the art as coking. Coke tends to reduce the heat exchange efficiency between the heat sources and the hydrocarbon feed within radiant coils, necessitating a greater quantity of thermal energy to crack the hydrocarbon feed within each radiant coil, consequently increasing the tube wall temperature and the process side pressure drop. The build-up of coke may vary from coil to coil, as the combustion heat may fluctuate from heat source to heat source, or the inner diameter, surface roughness or surface profile of each coil may also vary. After a sufficient amount of coke forms on the radiant coils, the radiant coils must be decoked in order to restore the efficiency of the furnace, and to maintain the tube metal temperature below its maximum permissible design temperature and also to maintain the process side pressure drop within design limits.
Decoking is the process of removing carbon deposits from the interior walls of the radiant coils, and is part of routine maintenance in an effort to maintain high-efficiency of a pyrolytic furnace and safe operation. Furnaces may be decoked by distributing air and steam through the radiant coils, for example.
Typically, the entire pyrolytic furnace is off-line during the decoking cycle. Unscheduled or frequent decoking cycles may lead to a reduction in olefin production, shortened radiant tube life, and higher furnace maintenance costs.
In the interest of limiting down-time of the pyrolytic furnace, ‘on-line’ decoking processes have been proposed, whereby hydrocarbon cracking and decoking are intended to be carried out in a furnace simultaneously. A method of online decoking is described in U.S. Pat. No. 6,027,635, which is incorporated by reference herein in its entirety. In such an on-line decoking process, a mixture comprising hot air and steam is distributed through select radiant coils of the furnace requiring decoking, while a hydrocarbon feed is simultaneously distributed through the remaining radiant coils.
On-line decoking is not a realistic option for many conventional furnaces, however. Specifically, all of the radiant coils and the convection coils of conventional furnaces, such as the one disclosed in the Kivlen publication, are maintained at substantially the same temperature, by virtue of the relative arrangement of their convection coils, radiant coils and burners. In contrast, in an on-line decoking cycle, the radiant coils being decoked require less heat input than the radiant coils carrying the hydrocarbon feed. Reducing the heat emitted by the heat sources to achieve decoking in some coils, consequently decreases the heat within the radiant section to a level insufficient for cracking the hydrocarbon feed within the other radiant coils at the design flow rate. Conversely, increasing the heat emitted by the heat sources to achieve cracking of the hydrocarbon feed within the respective radiant coils, may overheat and deform the radiant coils carrying hot air and steam. Thus, on-line decoking is not a realistic option for many conventional furnaces.
In an effort to achieve benefits such as on-line decoking, pyrolytic furnaces have been developed including plural radiant sections. Each radiant section receives hydrocarbon feed from a common convection section. Such a pyrolytic furnace may be referred to in the art as having a ‘twin radiant box’ or ‘twin radiant cell’ design. In an on-line decoking cycle, one radiant box undergoes decoking, while the other radiant box undergoes cracking. However, because one radiant box is decoking and therefore off-line for production, the furnace is only operating at 50% efficiency.
Accordingly, there remains a need to develop and improve upon the design of pyrolytic furnaces in the interest of facilitating on-line decoking while maintaining sufficient efficiency.

SUMMARY OF THE INVENTION

The invention relates to a pyrolytic furnace for thermally cracking hydrocarbon process fluid, and a method of operating the pyrolytic furnace. According to one aspect of the invention, the furnace comprises a convection section including a plurality of convection sub-sections, a radiant section comprising a plurality of radiant sub-sections, and an enclosure defining a plurality of discrete sectors. Each discrete sector of the enclosure includes a convection sub-section having at least one convection conduit for carrying hydrocarbon process fluid, at least one radiant sub-section having at least one radiant conduit connected to receive preheated hydrocarbon process fluid from the convection conduit, and a heat source configured to deliver heat into the discrete sector. Adjacent sectors of the furnace are segregated from each other, such that the heat delivered from the heat source is substantially localized in the discrete sector of the enclosure.
According to another aspect of the invention, each convection conduit is contained entirely within its respective convection sub-section.
According to still another aspect of the invention, a method of operating a pyrolytic furnace is provided. The method comprises the step of distributing hydrocarbon process fluid into a convection section of a discrete sector of the furnace. The pre-heated hydrocarbon process fluid is transferred from the convection section of the discrete sector into at least one corresponding radiant section of the discrete sector. Heat is delivered into the at least one radiant section of the discrete sector of the furnace to crack the pre-heated hydrocarbon process fluid, wherein the heat is substantially concentrated in the discrete sector of the furnace.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are shown schematically and may not be to scale. Included in the drawing are the following figures:

FIG. 1 is a front elevation schematic view of an exemplary pyrolytic furnace;

FIG. 2A is a front elevation schematic view of a pyrolytic furnace according to an exemplary embodiment of the invention;

FIG. 2B is a perspective view of a single radiant box arrangement of the pyrolytic furnace of FIG. 2A shown schematically;

FIG. 2C is a perspective view of another pyrolytic furnace having a single radiant box arrangement shown schematically;

FIG. 2D is a right side elevation schematic view illustrating a twin radiant box arrangement of the pyrolytic furnace of FIG. 2A;

FIGS. 3-5 are front elevation schematic views of various pyrolytic furnaces according to exemplary embodiments of the invention;

FIG. 6 is a perspective view of a convection section of a furnace shown schematically (radiant section omitted) according to an exemplary embodiment of the invention; and

FIG. 7 is a perspective view of another convection section of a furnace shown schematically (radiant section omitted) according to an exemplary embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The invention will next be illustrated with reference to the figures. Such figures are intended to be illustrative rather than limiting and are included herewith to facilitate explanation of the present invention. In the figures, like items numbers refer to like elements throughout.
Referring generally to the figures, according to one aspect of the invention, a furnace 200, 300, 400, 500 comprises a convection section 210, 310, 410, and 510 including a plurality of convection sub-sections 210(A-D), 310(A-D), 410(A-D), and 510(A-D) a radiant section 250, 350, 450, and 550 comprising a plurality of radiant sub-sections 250(A-D), 350(A-D), 450(A-D), and 550(A-D) and an enclosure 212, 312, 412 and 512 defining a plurality of discrete sectors. Each discrete sector of the enclosure includes a convection sub-section 210(A-D), 310(A-D), 410(A-D), and 510(A-D) having at least one convection conduit for carrying hydrocarbon process fluid, at least one radiant sub-section 250(A-D), 350(A-D), 450(A-D) and 550(A-D) having at least one radiant conduit connected to receive preheated hydrocarbon process fluid from the convection conduit, and a heat source 230, 330, 430, 530 configured to deliver heat into the discrete sector. Adjacent sectors of the furnace 200, 300, 400 and 500 are segregated from each other, such that the heat delivered from the heat source is substantially localized in the discrete sector of the enclosure 212, 312, 412 and 512.
Referring now to FIG. 1, an exemplary pyrolytic furnace 100 is shown schematically and for purposes of illustration. The pyrolytic furnace 100 includes a convection section 110 for preheating and diluting hydrocarbon feed 132, and a radiant section 150 for cracking the hydrocarbon feed to yield effluent 136 comprising olefins and byproducts. Unlike the conventional pyrolytic furnace shown in the Kivlen publication, which includes one set of coils within the convection section, the convection section 110 of the furnace 100 includes two sets of coils 114 and 122 positioned within an enclosure 112 of the furnace. The coils 114 and 122 extend along the entire length “L” of the convection section 110.
The hydrocarbon feed 132 goes through two sections of convection section 110. In the first section (feed preheat), the hydrocarbon feed 132 is distributed through feed preheat coils 114 that extend along the entire length “L” of the convection section 110. The preheated hydrocarbon feed 132 is then distributed through a set of conduits 140 (four shown) positioned exterior to the enclosure 112. Steam is injected into each conduit 140 for diluting the preheated hydrocarbon feed 132. Conduits interconnecting the preheat coils, such as conduit 140, may also be referred to in the art as ‘jumpovers.’
In the second section (mixed preheat) of the convection section 110, the diluted and preheated hydrocarbon feed is distributed through the mix-feed preheat coils 122 that extend along the entire length “L” of the convection section 110. The term ‘mix’ refers to the mixing that occurs between the hydrocarbon feed and the steam within the coils 122. The coils 114 and 122 are heated by thermal energy generated by a heat source 130 positioned within the radiant section 150. Thermal energy emitted by the heat source 130 rises through the radiant section 150 and into the convection section 110, consequently heating the hydrocarbon feed within the coils 114 and 122.
The individual diluted and preheated hydrocarbon feed streams are then transported through individual conduits 126 into the radiant section 150. Conduits interconnecting the convection section with the radiant section, such as conduits 126, may also be referred to in the art as ‘crossovers.’ The diluted and preheated hydrocarbon feed streams are then distributed through a series of radiant coils 128 extending along the height “H” of the radiant section 150. The diluted and preheated hydrocarbon feed streams are heated within the radiant coils 128 by the heat source 130. The hydrocarbon feed is heated to a temperature of above 1200 degrees Fahrenheit, for example, which is sufficient to crack the hydrocarbon mixture and yield effluent 136.
Referring now to FIGS. 2A and 2B, front and perspective views of another pyrolytic furnace 200 are illustrated schematically, according to an exemplary embodiment of the invention. The furnace 200 includes a convection section 210 for preheating a diluting hydrocarbon feed stock 232 and a radiant section 250 for cracking the hydrocarbon feed stock to yield effluent 236 comprising olefins and byproducts. The convection and radiant sections are optionally contained within a single enclosure 212.
The convection section 210 includes a plurality of convection sub-sections 210(A), 210(B), 210(C) and 210(D), collectively referred to as 210(A-D). Only convection sub-section 210(D) is illustrated in FIG. 2B for clarity. Similarly, the radiant section 250 optionally comprises four radiant sub-sections 250(A), 250(B), 250(C) and 250(D), collectively referred to as item 250(A-D). Only radiant sub-section 250(D) is illustrated in FIG. 2B for clarity. The sub-sections of the convection and radiant sections are preferably organized in sectors within the enclosure 212. The furnace 200 shown in FIG. 2A includes four sectors. Each sector includes a convection sub-section 210(A-D), a corresponding radiant sub-section 250(A-D), and a set of heat sources 230 (8 per sector). For example, the first sector comprises convection sub-section 210(A), radiant sub-section 250(A) and 8 heat sources 230. The second sector comprises convection sub-section 210(B), radiant sub-section 250(B) and 8 heat sources 230, and so forth.
Each convection sub-section 210(A-D) is configured to preheat and dilute the hydrocarbon feed 232 for distribution into a corresponding radiant sub-section 250(A-D). The convection sub-sections 210(A-D) optionally comprises a feed preheat section 214 and a mix-feed preheat section 222. Each coil is a thermally conductive hollow tube capable of carrying hydrocarbon feed. The individual coils are preferably positioned within the boundaries of their respective sector, the significance of which will be explained in greater detail below.
According to one exemplary use of the invention, hydrocarbon feed 232 is first distributed through the feed preheat section 214 of each sector. Each feed preheat section 214 is heated by a set of heat sources 230 positioned with its respective sector. The pre-heated hydrocarbon feed 232 is then delivered into a series of individual conduits 240 (three conduits per sub-section shown). As shown in FIG. 2B, the individual conduits 240 optionally extend outside of the enclosure 212. Steam 234 is then delivered into each conduit 240 through an inlet port (not shown). The steam 234 dilutes the preheated hydrocarbon feed within the conduits 240. The diluted and pre-heated hydrocarbon feed is thereafter distributed through the mix-feed preheat section 222 of each convection sub-section 210(A-D) for further heating. The diluted and twice pre-heated hydrocarbon feed is then distributed through a series of individual conduits 226 (three conduits shown) and into respective sub-sections of the radiant section 250. As shown in FIG. 2B, the individual conduits 226 optionally extend outside of the enclosure 212.
Each radiant sub-section 250(A-D) comprises a plurality of radiant coils 228, and a set of heat sources 230 (8 heat sources per sector). Each radiant coil 228 is a thermally conductive hollow tube through which the hydrocarbon feed is distributed. The radiant coils 228 are heated by heat sources 230 for cracking the hydrocarbon feed within each coil 228 to yield effluent 236.
The discrete sectors are oriented in a side-by-side arrangement along the length “L” of the furnace 200, whereby adjacent convection and radiant sub-sections are each separated by a gap “G.” According to one exemplary embodiment, the gap “G” may be about two times as large as the outer diameter of a conduit 228. Accordingly, no conduit or coil of a convection sub-section 210(A-D) or a radiant sub-section 250(A-D) overlaps or intrudes upon the space boundaries of an adjacent sector. By virtue of the gap “G” between adjacent sectors, heat generated by each set of heat sources 230 is substantially concentrated in that sector. For example, heat emitted by the heat sources 230 of the first sector provides heat to radiant sub-section 250(A) and convection sub-section 210(A), and so forth. Of course, it should be understood that heat may radiate into an adjacent sector regardless of the gap “G.”.
The heat generated by each set of heat sources 230 may be adjusted for controlling the temperature of each sector. As noted above, in an on-line decoking cycle, the radiant coils carrying hot steam and air (i.e., undergoing decoking) require less heat from the heat sources than the radiant coils carrying the hydrocarbon feed. Accordingly, because the temperature of each sector may be independently controlled, it is possible to achieve on-line decoking by (1) reducing the temperature of sectors operating in decoking mode, and (2) maintaining the temperature of the sectors operating in cracking mode.
Conventional furnaces typically undergo decoking once one of the coils needs to be decoked even though the remaining coils may not need to undergo decoking. In contrast, with the use of the invention, when one sector of the furnace 200 undergoes decoking, the remaining sectors may continue to crack hydrocarbon feed, consequently maximizing on-stream time and minimizing decoking cycles. This represents a considerable advantage over conventional single and twin radiant box furnaces.
In view of the increasing demand for cracking capacity, the furnace 200 may include provisions such that its length “L” may be increased to accommodate a greater number of sectors (each sector including at least one radiant sub-section and a corresponding convection sub-section).
In addition to achieving greater overall cracking efficiency, the furnace 200 is also uniquely adapted to maximize the olefin yields of each sector. By virtue of the segregated sectors of the furnace 200, the flow rate and/or the temperature of a single sector may be adjusted without adversely impacting the temperature of an adjacent sector to a significant degree. Thus, the olefin yields may be maximized for each sector of the furnace. Segregating the convection and radiant section of the furnace into sectors may be particularly advantageous if hydrocarbon feeds 232 of varying compositions (i.e., hydrocarbon feeds having varied cracking temperatures) are distributed through different sectors of the furnace 200. In that regard, the temperature of each sector may be tailored for the hydrocarbon feed distributed therethrough.
In contrast, olefin yields of conventional furnaces vary from radiant coil to radiant coil. Adjusting the flow rate and/or temperature of a single radiant coil to maximize the olefin yield of the hydrocarbon feed within one coil may adversely impact the temperature of an adjacent radiant coil. Thus, adjustment of either the flow rate and/or temperature of a single hydrocarbon stream is limited by the temperature constraints of the adjacent hydrocarbon streams. Once the temperature variation between the radiant coils becomes too great, the conventional furnace would be off-line for early non-routine maintenance, which is undesirable from the operational and cost perspectives.
In the exemplary embodiments shown in FIGS. 2A and 2B, each hydrocarbon stream 232 is consecutively distributed through a coil of feed preheat section 214, a conduit 240, a coil of mix-feed preheat section 222, a conduit 226 and a radiant coil 228. At no point are the individual hydrocarbon streams 232 of each sub-section combined. However, in another exemplary embodiment, the individual hydrocarbon streams may be combined together at one or more junctions to promote even heating and dilution of the hydrocarbon feed.
FIG. 2C depicts an alternative exemplary embodiment of the invention. Furnace 200′ is similar to furnace 200 of FIGS. 2A and 2B, with the exception of the manifold 226′ and the modified radiant coils 228′.
According to this exemplary embodiment illustrated in FIG. 2C, three individual hydrocarbon feed streams are combined in a manifold 226′ after the hydrocarbon feed is distributed through the mix-feed preheat section 222 of each convection sub-section 210(A-D). The manifold 226′ promotes even heating and dilution of the hydrocarbon feed streams 232. The manifold 226′ optionally extends outside of the enclosure 212. Although only one sector of the furnace 200′ is shown in FIG. 2C, it should be understood that each sector of the furnace 200′ may include a manifold 226′. Once combined, the hydrocarbon feed is then distributed through two radiant coils 228′.
The radiant coils 228′ differ from the radiant coils shown in FIGS. 2A and 2B, in that the radiant coils 228′ are “U”-shaped. Thus, instead of distributing the effluent 236 at the bottom of the enclosure 212, the radiant coils 228′ distribute the effluent 236 at an elevation above the radiant sub-sections 250(A-D).
FIGS. 2B and 2C indicate that furnaces 200 and 200′ have a single radiant box arrangement, i.e., one radiant sub-section corresponding to one convection sub-section. However, in the alternative embodiment of furnace 200″ shown in FIG. 2D, the furnace 200″ encompasses a twin radiant box arrangement, i.e., two radiant sub-sections corresponding to each convection sub-section.
FIG. 2D depicts a right side elevation schematic view of a twin radiant box furnace 200″ having two radiant sub-sections corresponding to each convection sub-section. Specifically, FIG. 2D depicts one sector of furnace 200″ that includes two radiant sub-sections 250(D″) and 250(E) corresponding to one convection sub-section 210(D″). Each radiant sub-section includes one or more heat sources 230 for heating the hydrocarbon feed. It should be understood that the furnace 200″ optionally includes four (4) convection sub-sections and eight (8) radiant sub-sections.
Each convection sub-section of furnace 200″ comprises a feed preheat section 214″ and a mix-feed preheat section 222″. The feed preheat section 214″ and the mix-feed preheat section 222″ may optionally include two times the number of coils as the convection section of FIG. 2A, because each feed and mix-feed preheat section 214″ and 222″ supplies hydrocarbon feed to twice the number of radiant sub-sections.
According to one exemplary use of the furnace 200″, hydrocarbon feed 232 is first distributed through the feed preheat section 214″ of each sector. The pre-heated hydrocarbon feed 232 is then delivered into a series of individual conduits 240. Steam 234 is then delivered into each conduit 240 through an inlet port (not shown). The diluted and pre-heated hydrocarbon feed is thereafter distributed through the mix-feed preheat section 222″ of each convection sub-section for further heating. The diluted and twice pre-heated hydrocarbon feed is then distributed through a series of individual conduits 226 and into the “U”-shaped radiant coils 228″ of respective radiant sub-sections 250(D″) and 250(E). The effluent is then distributed from the “U”-shaped radiant coils 228″ at an elevation above the radiant sub-sections 250(D″) and 250(E).
Referring now to FIG. 3, another exemplary embodiment of a pyrolytic furnace 300 is shown. The pyrolytic furnace 300 is similar to the furnace 200 shown in FIG. 2A, with the exception that each convection sub-section 310(A-D) includes a manifold 340 interconnecting the feed preheat section 314 with the mix-feed preheat section 322. The hydrocarbon feed streams 332 (five shown) combine together in the manifolds 340. Steam 334 is injected into each manifold 340 at an inlet port (not shown) for mixing with the combined hydrocarbon feed streams 332. Each convection sub-section 310(A-D) also includes a manifold 326 interconnecting the mix-feed preheat section 322 with the radiant coils 328.
The manifolds 326 and 340 promote even heating and dilution of the hydrocarbon feed streams 332. As mentioned above, the build-up of coke may vary from coil to coil, consequently affecting the flow rate and temperature of each hydrocarbon feed stream. Thus, combining the hydrocarbon feed streams 332 of each convection sub-section 310(A-D) at one or more locations promotes even heating and dilutions of the hydrocarbon feed streams.
In FIG. 4 another exemplary embodiment of a pyrolytic furnace 400 is shown. The pyrolytic furnace 400 is similar to the furnace 200 shown in FIG. 2A, with the exception that each convection sub-section 410(A-D) includes an additional mix-feed preheat section 423 and steam injection point 435. The additional mix-feed preheat section 423 and steam injection points 435 meet special process requirements of heavy feed stock. Although not shown, the individual conduits 440, 441 and 426 extending between the feed preheat sections 414, 423 and 422 may be replaced by manifolds, similar to the manifolds shown in FIG. 3, to promote even heating of the hydrocarbon feed streams 432.
Referring now to FIG. 5, the pyrolytic furnace 500 is similar to the furnace 200 shown in FIG. 2A, however, the furnace 500 includes insulating partitions 564, 566 and 560 positioned between adjacent sectors of the furnace. Specifically, partitions 564 are positioned between adjacent feed preheat sub-sections 514, partitions 566 are positioned between adjacent mix-feed preheat sub-sections 522, and partitions 560 are positioned between adjacent radiant sub-sections 550(A-D). By shielding or partially shielding the respective sectors, the temperature of each sector may be adjusted without substantially affecting the temperature of the adjacent sectors. As described above, it is desirable to shield the sectors to achieve broad flexibility in operating each sector under different operating conditions. It should be understood, however, that the partitions or partial partitions are optional features of the furnace, and are not required to achieve flexible operation of individual furnace sectors.
Each partition 560 extends upwards from the base 562 of the enclosure 512 for shielding adjacent radiant sub-sections 550(A-D). The partitions 560 may extend along a portion of the height of the radiant section (as shown), or, alternatively, the partitions 560 may extend along the entire height of the radiant section 550. Also, although not shown, the partitions 560 may optionally be suspended from the roof 570 of the radiant section 550 to achieve a similar effect.
In addition to the aforementioned partitions, each sector of the furnace optionally includes a series of flue gas shutters 572 positioned between each convection sub-section, e.g., 510(A), and its corresponding radiant section e.g., 550(A). The flue gas shutters 572 are moveable to control the amount of heat delivered to each convection sub-section 510(A-D).
As described previously with reference to the furnace shown in FIGS. 2A-5, it is desirable to control the temperature of each radiant and convection sub-section to achieve broad flexibility in operating each sector under different operating conditions. Nevertheless, depending upon user requirements, one or more feed preheat coils may be positioned across the entire length “L” of the convection section, as shown in FIG. 6.
Referring now to FIG. 6, a schematic perspective view of a convection section 610 is shown according to an exemplary embodiment of the invention. The radiant section of the furnace has been omitted. The convection section 610 is similar to the convection section 310 of the furnace 300 shown in FIG. 3, however the feed preheat sections 314 of FIG. 3 are replaced by four stream feed preheat coils 614(1), 614(2), 614(3), and 614(4), referred to as 614 collectively.
The feed preheat coils 614 are staggered along the width “W” of the interior of the enclosure 612. Each feed preheat coil 614 includes five 180 degree bends such that each coil 614 traverses the entire length “L” of the enclosure 612 six times. It should be understood that the convection section 610 may include any number of preheat coils, and each preheat coil may be configured to traverse the length “L” of the enclosure 612 any number of times.
The convection section 610 further comprises four mix-feed preheat sections 622(A-D) staggered along the length “L” of the interior of the enclosure 612. The mix-feed preheat sections 622(A-D) are substantially equivalent to mix-feed preheat sections 322 of FIG. 3. Accordingly, the individual coils 625 of each mix-feed preheat sections 622(A-D) traverse the width of the enclosure 612. The mix-feed preheat sections 622(A-D) are fluidly coupled to the feed preheat coils 614 by a manifold 640. Although not shown, manifold 640 may be replaced by individual conduits, if so desired. The outlet of the coils 625 of each mix-feed preheat section 622(A-D) is fluidly coupled to another manifold 626. Each manifold 626 is fluidly coupled to two (2) radiant conduits 628. The radiant conduits 628 extend into a corresponding radiant sub-section (not shown).
According to one exemplary use of the convection section 610, hydrocarbon feed 632 is delivered through a manifold 613 into feed preheat coils 614. The hydrocarbon feed 632 is then delivered into a manifold 640 where all four hydrocarbon feed streams are combined together and mixed with steam 634. Combining the hydrocarbon feed streams into a common manifold 640 promotes even dilution and heating of the hydrocarbon feed. The diluted hydrocarbon feed stream is then delivered into four mix-feed preheat sub-sections 622(A-D). The diluted hydrocarbon feed stream is pre-heated within each coil 625 of the mix-feed preheat sub-sections 622(A-D) by heat sources (not shown). Thereafter, the twice-preheated and diluted hydrocarbon feed streams 624 are delivered into a respective manifold 626, where the streams are combined together and optionally mixed with steam (not shown). The hydrocarbon feed within each manifold 626 is then distributed into two radiant conduits 626 and delivered into a corresponding radiant sub-section (not shown) for cracking.
Referring now to FIG. 7, another exemplary embodiment of a convection section 710 is shown. The convection section 710 is similar to the convection section 610 shown in FIG. 6, with the exception that convection section 710 includes two feed preheat sections 714 and 723 extending across the length “L” of the enclosure 712 instead of one feed preheat section. More particularly, the convection section 710 comprises four additional feed pre-heat coils 723(1), 723(2), 723(3), and 723(4), referred to herein as 723 collectively. Each feed preheat coil 723 includes three 180 degree bends such that each coil 723 traverses the entire length “L” of the enclosure 712 four times. The coil 723 can be used as a feed preheat coil or a mixed feed preheat coil.
Similar to the mix-feed preheat sections 622(A-D) illustrated in FIG. 6, the convection section 710 also includes four mix-feed preheat sections 722(A-D) staggered along the length “L” of the interior of the enclosure 712. The mix-feed preheat sections 722 are fluidly coupled to the feed preheat coils 714 by a manifold 741.
The outlet of the coils 725 of each mix-feed preheat section 722 is fluidly coupled to another manifold 726. Each manifold 726 is fluidly coupled to two (2) radiant conduits 728. The radiant conduits 728 extend into a corresponding radiant sub-section (not shown).
According to one exemplary use of the convection section 710, hydrocarbon feed 732 is delivered through a manifold 713 into four feed preheat coils 714 staggered along the length of the enclosure. After traveling through the coils 714, the hydrocarbon feed 732 is then delivered into a manifold 740 where all four hydrocarbon feed streams are combined together and mixed with steam 734. The diluted hydrocarbon feed is then distributed through four mix-feed preheat coils 723 and delivered into a manifold 741 where all four hydrocarbon feed streams are combined together and mixed with steam 735 again. The twice-diluted hydrocarbon feed is then delivered into four mix-feed preheat sub-sections 722(A-D). The mix-feed preheat sub-sections 722(A-D) are substantially equivalent to the mix-feed preheat sub-sections 622(A-D) of FIG. 6, and no further explanation is required. Although not shown, the manifolds 713, 740 and 735 may be replaced by individual conduits, if so desired.
In addition to facilitating on-line decoking, exemplary embodiments of this invention provide high capacity pyrolytic furnaces that can meet the rapidly expanding global demand for ethylene. To increase the capacity of a single cracking heater, the radiant coil length and the number of radiant coils may be increased, resulting in a larger radiant enclosure. Additionally, and according to exemplary embodiments of this invention, expansion of the radiant section is not constrained by limitations of the convection section. For example, the number and size of convection conduits or coils should preferably increase as the number and size of radiant conduits or coils increase.
According to exemplary embodiments of this invention, the number and size of convection conduits or coils can optionally be increased (or decreased) along with the number of radiant conduits or coils. This additional benefit conferred by exemplary embodiments of this invention can be incorporated to avoid several disadvantages. For example, independently increasing the length of convection conduits or coils (without changing their number) may result in excessive thermal expansion and/or a high pressure drop. Furthermore, independently increasing the width “W” (see FIG. 1) of the convection section enclosure to accommodate a greater number of convection conduits or coils typically would require additional mechanical supports for retaining all of the additional convection conduits or coils, at an added expense, which would reach economical design limits. Accordingly, these additional disadvantages can be overcome by exemplary embodiments of this invention, which allow for enlarging the capacity of the furnace.
While exemplary embodiments of the invention have been shown and described herein, it will be understood that such embodiments are provided by way of example only. Numerous variations, changes and substitutions will occur to those skilled in the art without departing from the spirit of the invention. For example, the convection and radiant sections may include any number of coils and conduits. Moreover, each sector may include any number of heat sources, convection sub-sections, or radiant sub-sections to meet any desired purpose.
Accordingly, it is intended that the appended claims cover all such variations as fall within the spirit and scope of the invention.

Claims

1. A furnace for thermally cracking hydrocarbon process fluid, said furnace comprising:

a convection section comprising a plurality of convection sub-sections, each convection sub-section configured for preheating hydrocarbon process fluid;

a radiant section comprising a plurality of radiant sub-sections, each radiant sub-section configured for cracking preheated hydrocarbon process fluid;

an enclosure defining a plurality of discrete sectors; and

each discrete sector of said enclosure including a convection sub-section having at least one convection conduit for carrying hydrocarbon process fluid, at least one radiant sub-section having at least one radiant conduit connected to receive preheated hydrocarbon process fluid from said convection conduit of said convection sub-section, and a heat source configured to deliver heat into said convection sub-section for preheating the hydrocarbon process fluid within said convection conduit of said convection sub-section and into said radiant sub-section for cracking hydrocarbon process fluid within said radiant conduit of said radiant sub-section,

wherein adjacent sectors of the furnace are segregated from each other, such that the heat delivered from said heat source is substantially localized in said discrete sector of said enclosure.

2. The furnace of claim 1 wherein the discrete sectors of the furnace are arranged in a side-by-side relation.

3. The furnace of claim 2 wherein the discrete sectors of the furnace are spaced apart from each other within said enclosure.

4. The furnace of claim 2 wherein the discrete sectors of the furnace are at least partially separated by partition walls positioned between adjacent sectors of the furnace.

5. The furnace of claim 1, wherein at least one of said discrete sectors is maintained within a single enclosure.

6. The furnace of claim 1, wherein at least one of said discrete sectors of said furnace further comprises at least one shutter positioned between said convection sub-section and said radiant sub-section of said discrete sector, said shutter being movable for adjusting the transfer of heat from said heat source into said convection sub-section.

7. The furnace of claim 1, wherein said heat source of each of said discrete sectors is independently operable for selectively delivering heat into a respective one of said discrete sectors.

8. The furnace of claim 1, wherein said convection conduit of said convection sub-section is contained entirely within a respective sector of said enclosure.

9. The furnace of claim 1, wherein at least one discrete sector of said furnace comprises a single convection sub-section and at least two radiant sub-sections, said at least two radiant sub-sections each having a radiant conduit being connected to receive hydrocarbon process fluid from a convection conduit in said single convection sub-section.

10. The furnace of claim 1, wherein the at least one convection conduit comprises a coil.

11. The furnace of claim 1, wherein the at least one radiant conduit comprises a coil.

12. The furnace of claim 1, wherein the heat source comprises one or more burners.

13. A pyrolytic furnace for hydrocarbon cracking, said furnace comprising:

an enclosure having a longitudinal axis;

a convection section divided into convection sub-sections in side-by-side relation along the longitudinal axis of said enclosure;

a radiant section divided into radiant sub-sections in side-by-side relation along the longitudinal axis of said enclosure, each of said radiant sub-sections corresponding to one of said convection sub-sections together forming a furnace sector;

at least one convection conduit positioned in each of said convection sub-sections for carrying hydrocarbon process fluid, and at least one radiant conduit positioned in each of said radiant sub-sections for carrying preheated hydrocarbon process fluid, wherein each convection conduit is contained entirely within its respective convection sub-section and is coupled for fluid flow communication with said at least one radiant conduit; and

plural heat sources positioned to introduce heat into said radiant section and said convection section of said furnace, wherein each heat source is positioned to deliver heat into a single sector of said furnace.

14. The pyrolytic furnace of claim 13, wherein deactivation of one of said heat sources associated with one of said furnace sectors reduces the heat introduced into that sector, while the heat delivered into an adjacent furnace sector remains substantially constant.

15. The pyrolytic furnace of claim 13, wherein at least two of the sectors of said furnace are adjacent and are at least partially separated by a partition wall positioned between the adjacent sectors of the furnace.

16. The pyrolytic furnace of claim 13, wherein at least one sector of said furnace further comprises a shutter positioned between said convection sub-section and said radiant sub-section, said shutter being movable for controlling the flow of heat from said heat source of said sector into said convection sub-section of said sector.

17. The pyrolytic furnace of claim 13, wherein said heat source of each sector is independently controllable for delivering heat into said sector.

18. The pyrolytic furnace of claim 13, wherein at least one sector of said furnace comprises a single convection sub-section and at least two radiant sub-sections configured to receive hydrocarbon process fluid from said single convection sub-section.

19. A method of operating a pyrolytic furnace comprising the steps of:

(a) distributing hydrocarbon process fluid into a convection section of a discrete sector of the furnace;

(b) transferring pre-heated hydrocarbon process fluid from the convection section of the discrete sector into at least one corresponding radiant section of the discrete sector; and

(c) delivering heat into the at least one radiant section of the discrete sector of the furnace to crack the pre-heated hydrocarbon process fluid within the radiant section of the discrete sector and into the convection section of the discrete sector to pre-heat the hydrocarbon process fluid within the convection section of the discrete sector, wherein the heat is substantially concentrated in the discrete sector of the furnace.

20. The method of claim 19, further comprising the step of performing steps (a) through (c) for plural discrete sectors of the pyrolytic furnace.

21. The method of claim 20, further comprising the step of reducing a quantity of heat delivered to a selected one of said discrete sectors of the furnace, while maintaining a quantity of heat delivered to an adjacent one of said discrete sectors of the furnace, such that the internal temperature of the selected discrete sector of the furnace is substantially reduced while the internal temperature of the adjacent sector of the furnace remains substantially constant.

22. The method of claim 19, further comprising the step of at least partially isolating the heat delivered within the radiant section and the convection section of the discrete sector of the pyrolytic furnace from an adjacent discrete sector.

23. The method of claim 22, wherein the isolating step comprises the sub-step of at least partially retaining the heat within the discrete sector using a partition wall positioned between the discrete sector and the adjacent discrete sector of the furnace.