REDUCING CARBON DIOXIDE EMISSIONS IN STEAM CRACKING OPERATIONS

Abstract

A method for reducing CO.sub.2 emissions from steam cracking operations can include: introducing an oxygen-rich stream comprising oxygen and from 0 wt % to 15 wt % nitrogen to a vessel; introducing hydrocarbon combustion fuel to the vessel; combusting oxygen and hydrocarbon combustion fuel in the vessel to (1) produce a flue gas comprising carbon dioxide and water and (2) heat a cracking coil passing through the vessel; and performing a steam cracking reaction in the cracking coil passing through the vessel.

Claims

1. A method comprising: introducing an oxygen-rich stream comprising oxygen and from 0 wt % to 15 wt % nitrogen to a vessel; introducing hydrocarbon combustion fuel to the vessel; combusting oxygen and hydrocarbon combustion fuel in the vessel to (1) produce a flue gas comprising carbon dioxide and water and (2) heat a cracking coil passing through the vessel; and performing a steam cracking reaction in the cracking coil passing through the vessel.

2. The method of claim 1 further comprising: heating a steam coil and a hydrocarbon coil both passing through the vessel with the flue gas; and supplying steam from the steam coil and hydrocarbon cracking feed from the hydrocarbon coil to the cracking coil.

3. The method of claiml, wherein a temperature in the vessel at an outlet of the cracking coil from the vessel is about 600 C. to about 1000 C.

4. The method of claim 1, wherein a pressure in the vessel at an outlet of the cracking coil from the vessel is about ambient pressure to about 600 psig.

5. The method of claim 4, wherein a pressure in the vessel at an outlet of the cracking coil from the vessel is about 300 psig to about 600 psig.

6. The method of claim 1, wherein a temperature in the cracking coil at an outlet of the cracking coil from the vessel is about 400 C. to about 900 C.

7. The method of claim 1, wherein the oxygen is diluted with steam and/or carbon dioxide.

8. The method of claim 1, wherein an excess of oxygen is used during combustion to produce the flue gas consisting essentially of carbon dioxide and water.

9. The method of claim 1 further comprising: condensing the water out of the flue gas to produce a water stream and a carbon dioxide stream, wherein the carbon dioxide stream comprises at least 75 wt % carbon dioxide.

10. The method of claim 10 further comprising: condensing the water out of the flue gas to produce a water stream and a carbon dioxide stream, wherein the carbon dioxide stream comprises at least 90 wt % carbon dioxide.

11. The method of claim 1, wherein the hydrocarbon combustion fuel comprises a sulfur- containing contaminant, and the method further comprises: passing the flue gas through a selective amine scrubber.

12. The method of claim 1, wherein a fluidized bed is present in the vessel.

13. The method of claim 1 further comprising: heating steam in a bottom coil passing through the vessel and located between the burners and the cracking coil.

14. A system comprising: a vessel that contains burners and a cracking coil passing through the vessel; an oxygen line that supplies an oxygen-rich stream comprising 0 wt % to 15 wt % nitrogen to the burners; a hydrocarbon combustion fuel line that supplies hydrocarbon combustion fuel to the burners; and a flue gas line that transports a flue gas comprising combustion product of the oxygen and hydrocarbon combustion fuel from the vessel.

15. The system of claim 14, wherein the vessel is capable of being operated at a temperature in the vessel at an outlet of the cracking coil from the vessel is about 600 C. to about 1000 C.

16. The system of 14, wherein the vessel is capable of being operated at a pressure in the vessel at an outlet of the cracking coil from the vessel is about ambient pressure to about 600 psig.

17. The system of claim 16, wherein the vessel is capable of being operated at a pressure in the vessel at an outlet of the cracking coil from the vessel is about 300 psig to about 600 psig.

18. The system of claim 14 further comprising: a line that supplies carbon dioxide and/or steam to the oxygen line to dilute the oxygen before being supplied to the burners.

19. The system of claim 14 further comprising: a condenser that receives the flue gas from the flue gas line and produces a water stream and a carbon dioxide stream.

20. The system of claim 14, wherein the hydrocarbon combustion fuel comprises a sulfur-containing contaminant, and the system further comprises: a selective amine scrubber that receives the flue gas from the flue gas line.

21. The system of claim 14 further comprising: a steam coil that passes through the vessel to supply steam to the cracking coil; and a hydrocarbon coil that passes through the vessel to supply hydrocarbon cracking feed to the cracking coil.

22. The system of claim 14 further comprising: a fluidized bed in the vessel.

23. The system of claim 14 further comprising: a bottom coil containing steam, passing through the vessel, and located between the burners and the cracking coil.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The following figures are included to illustrate certain aspects of the embodiments, and should not be viewed as exclusive embodiments. The subject matter disclosed is capable of considerable modifications, alterations, combinations, and equivalents in form and function, as will occur to those skilled in the art and having the benefit of this disclosure.

[0010] FIG. 1 schematically illustrates an example steam cracking unit of the present invention.

[0011] FIG. 2 schematically illustrates an example system that includes a steam cracking unit of the present invention.

DETAILED DESCRIPTION

[0012] The present application relates to systems and methods that reduce CO.sub.2 emissions from steam cracking operations. More specifically, a steam cracking unit of the present invention is operated with a hydrocarbon combustion fuel and O.sub.2, which produces a flue gas that is essentially CO.sub.2 and water. Then, the CO.sub.2 and steam can be readily separated by condensing the steam to water. As a result, essentially pure CO.sub.2 and pure H.sub.2O are produced. The CO.sub.2 can be used as a chemical (e.g., in the beverage industry) or as an enhanced oil recovery agent. The H.sub.2O can be used in other processes at the plant. Preferably, the O.sub.2 contains an optimum amount of N.sub.2 to meet maximum allowed N.sub.2 levels per CO.sub.2 specifications.

[0013] Examples of hydrocarbon combustion fuels include, but are not limited to, natural gas, methane, ethane, propane, steam cracker tar, and combinations thereof. Oxygenates, biomass, or waste fuels are also acceptable fuels for this process. An advantage of this invention is being able to fire low value heavy fuels instead of expensive fuels like natural gas. If the fuel contains sulfur, appropriate sulfur treatment step(s) should be added to the process to remove SO.sub.x as required from captured CO.sub.2.

[0014] FIG. 1 schematically illustrates an example steam cracking unit 100 of the present invention. The steam cracking unit 100 includes a vessel 102 that contains burners 104 and has several coils 106, 108, 110, 116 passing therethrough. The burners 104 are illustrated at the bottom of the vessel but could additionally or alternatively be placed in other locations along the walls of the vessel including up the sides of the vessel. The burners 104 introduce the oxygen-rich feed and hydrocarbon combustion fuel supplied to the vessel 102 by oxygen line 112 and hydrocarbon combustion fuel line 114. As used herein, oxygen-rich feed contains 50 wt % to 100 wt % O.sub.2, preferably at least 75 wt % O.sub.2. Optionally, the oxygen-rich feed can be oxygen diluted with CO.sub.2 and/or steam to moderate the combustion temperature rise and/or improve heat transfer coefficient for the coils 106, 108, 110, 116. The oxygen and hydrocarbon combustion fuel combust in a substantial absence of nitrogen in the vessel 102 and heat the coils 106, 108, 110, 116. Preferably, the oxygen-rich feed contains an optimum amount of N.sub.2 to meet maximum allowed N.sub.2 levels per CO.sub.2 specifications. In some embodiments, up to 15 wt % N.sub.2 in the oxygen-rich feed is acceptable, preferably less than 10 wt % N.sub.2 in the oxygen-rich feed, and more preferably less than 5 wt % N.sub.2 in the oxygen-rich feed.

[0015] As with a typical steam cracking furnace, a hydrocarbon coil 106 contains and heats hydrocarbon cracking feed, and a steam coil 108 contains and heats steam. The hydrocarbon cracking feed and steam are mixed and passed through a cracking coil 108, which is closest to the closest coil to the burners 104, where steam cracking occurs.

[0016] Optionally but preferably, a bottom coil 116 is included above the burners 104 and below the cracking coil 110 to generate very high pressure steam to make electricity for the plant, for example, to power refrigeration units used in the product recovery. The bottom coil 116 tempers the temperature to which the cracking coil 110 is exposed. The steam in the bottom coil may be at about 1000 psig to about 2500 psig, preferably about 1500 psig to about 2500 psig

[0017] The sizes, locations, and arrangements of the steam coils 108, hydrocarbons coils 106, and/or the cracking coils 110 can be optimized based on the feed, fuel, O.sub.2 content of the oxygen-rich stream, CO.sub.2/H.sub.2O recycle to the burners, and desired product slate. The combustion product exits the vessel 102 via a flue gas line 118. Examples of hydrocarbon cracking feeds include, but are not limited to, ethane, propane, butanes, pentanes, light naphtha and heavier naphtha up to vacuum gas oil (VGO), light crude, condensate, and combinations thereof.

[0018] An example method of the present invention that could be performed with the steam cracking unit 100 includes: combusting an oxygen-rich feed and hydrocarbon combustion fuel in the vessel 102 in a substantial absence of nitrogen to produce flue gas and heat coils 106, 108, 110, 116 passing through the vessel 102; and performing a steam cracking reaction in a coil (illustrated as the cracking coil 110) passing through the vessel 102.

[0019] Because the combustion reactants are oxygen and hydrocarbons, the products are carbon dioxide, carbon monoxide, and water. The amounts of oxygen and hydrocarbon combustion fuel will vary based on the composition of the hydrocarbon combustion fuel. Preferably, the combustion reaction is performed with a stoichiometric excess of oxygen to maximize carbon dioxide and minimize carbon monoxide in the flue gas. The excess of oxygen is preferably low so as to minimize the amount of unreacted oxygen in the flue gas. The amount of excess oxygen should be in an amount to produce a flue gas with 0.1 wt % to 15 wt % O.sub.2, preferably 1 wt % to 10 wt % O.sub.2, and more preferably 2 wt % to 5 wt % O.sub.2.

[0020] In some instances, the hydrocarbon combustion fuel may have sulfur-containing contaminants, which when fully combusted form SO.sub.x. In such instances, a known scrubber (e.g., a selective amine scrubber) can be used to remove the produced SO.sub.x from the flue gas.

[0021] The steam cracking unit 100 can be operated at ambient pressures or elevated pressures. Preferably, the steam cracking unit 100 is operated at elevated pressures (e.g., above about 100 psig) to maximize heat transfer from the gas in the vessel and the coils. Further, lower pressure operation can lead to uneven mixing of the gases in the vessel, which can cause coking. The steam cracking unit 100 can be operated at ambient pressure to about 600 psig; preferably about 100 psig to about 500 psig; and more preferably about 300 psig to about 500 psig. When operating at pressures above atmospheric, the steam cracking unit 100 will be a pressure vessel and must be made of a material suitable for safely operating the steam cracking unit 100 at the operating temperature and pressure. One skilled in the art would recognize the proper material and design requirements for such operating conditions.

[0022] Oxygen and natural gas are typically transported and stored at higher pressures (e.g., above 500 psig). Accordingly, additional compressors may not be necessary when operating the steam cracking unit 100 at preferred higher pressures. The steam cracking unit 100 can be operated so that the temperature at the cracking reaction (i.e., inside the cracking coil 110 measured where the cracking coil 110 leave the vessel 102) of greater than about 400 C. (e.g., about 400 C. to about 900 C.); preferably about 500 C. to about 800 C.; and more preferably about 700 C. to about 800 C. The temperature necessary for the steam cracking reaction will vary based on the hydrocarbon cracking feed. Because the cracking is an endothermic process, the combustion in the vessel 102 provide heat to the coils. Therefore, the steam cracking unit 100 can be operated at a temperature inside the vessel 102 measured around steam cracker coil outlet surface about 20 C. to about 300 C. greater than the steam cracker coil outlet temperature. This temperature varies depending on many factors including the presence of bottom coils 116 upstream of the cracking coils 100.

[0023] Optionally, the steam cracking unit 100 can be operated as a fluidized bed reactor and contain particulates that assist with heat transfer into the coils 106, 108, 110, 116. The particulates can have a weight average diameter of less than about 100 microns (e.g., about 10 microns to about 100 microns); preferably about 20 microns to about 80 microns; preferably about 70 microns to about 80 microns; preferably about 40 microns to about 60 microns; and more preferably about 30 microns to about 40 microns. The particle size is optimized based on the operating conditions and heat transfer coefficient desired.

[0024] FIG. 2 schematically illustrates an example system 220 that includes a steam cracking unit 222 of the present invention. Similar to as illustrated in FIG. 1, oxygen line 224 and hydrocarbon combustion fuel line 226 supply oxygen and hydrocarbon combustion fuel to the vessel 228 of the a steam cracking unit 222. The oxygen can be supplied as compressed oxygen or, optionally, be supplied from an air separator 230 that removes the nitrogen from the air provided by air supply line 232.

[0025] A flue gas line 234 supplies the combustion product (comprising carbon dioxide and water) exiting the vessel 228 to a condenser 236. The condenser 236 condenses the water from the combustion product. The water is transported via water line 238 to another part of the plant where it can be stored or used in another process or other beneficial purposes.

[0026] The remainder of the combustion product is preferably essentially all carbon dioxide (preferably at least 75 wt % carbon dioxide; more preferably at least 90 wt % carbon dioxide; preferably at least 95 wt % carbon dioxide; and most preferably at least 99 wt % carbon dioxide). The remainder of the combustion product is transported via carbon dioxide line 240 to another part of the plant where it can be stored or used in another process. Optionally, a portion of the remainder of the combustion product can be transported via recycle line 242 to dilute the oxygen in the oxygen line 224.

[0027] Like the description of FIG. 1, the system 220 can be operated at ambient pressures or elevated pressures. Preferably, the steam cracking unit 222 and lines 224, 226, 234, 238, 240 are operated at elevated pressures (e.g., above about 100 psig). The system 220 can be operated at ambient pressure to about 600 psig; preferably about 100 psig to about 500 psig; and more preferably about 300 psig to about 500 psig. When operating at pressures above atmospheric, the system 220 must be made of a material suitable for safely operating the system 220 at the operating temperature and pressure. One skilled in the art would recognize the proper material and design requirements for such operating conditions.

[0028] When operating the system 220 at elevated pressures, the oxygen and hydrocarbon combustion fuel can be provided at the operating pressure of the system 220. If needed, the system 220 can include compressors and/or pressure reducing valve to provide the oxygen and hydrocarbon combustion fuel to the vessel 228 at the proper operating pressure. For example, if the hydrocarbon combustion fuel is provided at a pressure of 50 psig but the operating pressure of the steam cracking unit 222 is 150 psig, a compressor can be included along hydrocarbon combustion fuel line 226 to increase the pressure of the hydrocarbon combustion fuel. Alternatively, in another example, if hydrocarbon combustion fuel is provided at a pressure of 50 psig but the operating pressure of the steam cracking unit 222 is closer to the ambient pressure, a pressure reducing valve can be included along hydrocarbon combustion fuel line 226 to decrease the pressure of the hydrocarbon combustion fuel.

[0029] The operating and reaction temperatures and additional optional components described relative to the steam cracking unit 100 in FIG. 1 apply to the steam cracking unit 222 of FIG. 2. For example, the steam cracking unit 222 can be operated with a fluidized bed of particulates.

[0030] One skilled in the art will recognize that the steam cracking unit 100 and system 220 of FIGS. 1 and 2 can further include hardware (e.g., distributors, cyclones, filters, valves, pressure meters, sensors, and the like) to ensure the proper and safe operation thereof.

[0031] A nonlimiting example method of the present invention comprises: introducing an oxygen-rich stream comprising oxygen and from 0 wt % to 15 wt % nitrogen to a vessel; introducing hydrocarbon combustion fuel to the vessel; combusting oxygen and hydrocarbon combustion fuel in the vessel to (1) produce a flue gas comprising carbon dioxide and water and (2) heat a cracking coil passing through the vessel; and performing a steam cracking reaction in the cracking coil passing through the vessel.

[0032] Optionally, the nonlimiting example method can include one or more of the following: Element 1: the method further comprising: heating a steam coil and a hydrocarbon coil both passing through the vessel with the flue gas; and supplying steam from the steam coil and hydrocarbon cracking feed from the hydrocarbon coil to the cracking coil; Element 2: wherein a temperature in the vessel at an outlet of the cracking coil from the vessel is about 600 C. to about 1000 C.; Element 3: wherein a pressure in the vessel at an outlet of the cracking coil from the vessel is about ambient pressure to about 600 psig; Element 4: wherein a pressure in the vessel at an outlet of the cracking coil from the vessel is about 300 psig to about 600 psig; Element 5: wherein a temperature in the cracking coil at an outlet of the cracking coil from the vessel is about 400 C. to about 900 C.; Element 6: wherein the oxygen is diluted with steam and/or carbon dioxide; Element 7: wherein an excess of oxygen is used during combustion to produce the flue gas consisting essentially of carbon dioxide and water; Element 8: the method further comprising: condensing the water out of the flue gas to produce a water stream and a carbon dioxide stream, wherein the carbon dioxide stream comprises at least 75 wt % carbon dioxide; Element 9: method further comprising: condensing the water out of the flue gas to produce a water stream and a carbon dioxide stream, wherein the carbon dioxide stream comprises at least 90 wt % carbon dioxide; Element 10: wherein the hydrocarbon combustion fuel comprises a sulfur-containing contaminant, and the method further comprises: passing the flue gas through a selective amine scrubber; Element 11: wherein a fluidized bed is present in the vessel; and Element 12: the method further comprising: heating steam in a bottom coil passing through the vessel and located between the burners and the cracking coil. Examples of combinations include, but are not limited to, Elements 11 and 12 in combination; Elements 1, 11, and 12 in combination; Elements 1, 2, 3 (or 4), 11, and 12 in combination; Elements 1, 2, 3 (or 4), 5, 11, and 12 in combination; Elements 1, 2, 3 (or 4), 5, 6, 7, 11, and 12 in combination; Elements 1, 2, 3 (or 4), 5, 6, 7, 8 (or 9), 11, and 12 in combination; Elements 1, 2, 3 (or 4), 5, 6, 7, 8 (or 9), 10, 11, and 12 in combination; Elements 1, 2, 3 (or 4), 5, 6, 7, 8 (or 9), and 10 in combination; Elements 1, 2, 3 (or 4), 5, 6, 7, and 8 (or 9) in combination; Elements 1, 2, 3 (or 4), 5, 6, and 7 in combination; Elements 1, 2, 3 (or 4), 6, and 8 (or 9) in combination; and Elements 2, 3 (or 4), 6, 11, and optionally 12 in combination.

[0033] A nonlimiting example system of the present invention comprises: a vessel that contains burners and a cracking coil passing through the vessel; an oxygen line that supplies an oxygen-rich stream comprising 0 wt % to 15 wt % nitrogen to the burners; a hydrocarbon combustion fuel line that supplies hydrocarbon combustion fuel to the burners; and a flue gas line that transports a flue gas comprising combustion product of the oxygen and hydrocarbon combustion fuel from the vessel.

[0034] Optionally, the nonlimiting example system can include one or more of the following: Element 13: wherein the vessel is capable of being operated at a temperature in the vessel at an outlet of the cracking coil from the vessel is about 600 C. to about 1000 C.; Element 14: wherein the vessel is capable of being operated at a pressure in the vessel at an outlet of the cracking coil from the vessel is about ambient pressure to about 600 psig; Element 15: wherein the vessel is capable of being operated at a pressure in the vessel at an outlet of the cracking coil from the vessel is about 300 psig to about 600 psig; Element 16: the system further comprising: a line that supplies carbon dioxide and/or steam to the oxygen line to dilute the oxygen before being supplied to the burners; Element 17: the system further comprising: a condenser that receives the flue gas from the flue gas line and produces a water stream and a carbon dioxide stream; Element 18: wherein the hydrocarbon combustion fuel comprises a sulfur-containing contaminant, and the system further comprises: a selective amine scrubber that receives the flue gas from the flue gas line; Element 19: the system further comprising: a steam coil that passes through the vessel to supply steam to the cracking coil; and a hydrocarbon coil that passes through the vessel to supply hydrocarbon cracking feed to the cracking coil; Element 20: the system further comprising: a fluidized bed in the vessel; and Element 21: the system further comprising: a bottom coil containing steam, passing through the vessel, and located between the burners and the cracking coil. Examples of combinations include, but are not limited to, Elements 20 and 21 in combination; Elements 13 and 14 (or 15) in combination; Elements 16 and 19 in combination; Elements 13, 14 (or 15), and 21 in combination; Elements 13, 14 (or 15), and 20 in combination; Elements 16, 17, 20, and optionally 21 in combination; Elements 13, 14 (or 15), 16, 17, 20, and optionally 21 in combination; and Element 18 in combination with any of the foregoing combinations.

[0035] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth used in the present specification and associated claims are to be understood as being modified in all instances by the term about. Accordingly, unless indicated to the contrary, the numerical parameters set forth in the following specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the embodiments of the present invention. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claim, each numerical parameter should at least be construed in light of the number of reported significant digits and by applying ordinary rounding techniques.

[0036] One or more illustrative embodiments incorporating the invention embodiments disclosed herein are presented herein. Not all features of a physical implementation are described or shown in this application for the sake of clarity. It is understood that in the development of a physical embodiment incorporating the embodiments of the present invention, numerous implementation-specific decisions must be made to achieve the developer's goals, such as compliance with system-related, business-related, government-related and other constraints, which vary by implementation and from time to time. While a developer's efforts might be time-consuming, such efforts would be, nevertheless, a routine undertaking for those of ordinary skill in the art and having benefit of this disclosure.

[0037] While compositions and methods are described herein in terms of comprising various components or steps, the compositions and methods can also consist essentially of or consist of the various components and steps.

[0038] To facilitate a better understanding of the embodiments of the present invention, the following examples of preferred or representative embodiments are given. In no way should the following examples be read to limit, or to define, the scope of the invention.

EXAMPLES

[0039] Simulations of five systems for steam cracking were run with the same temperatures within the furnace/cracking vessel. In the simulations, different combustion gases, types of furnace/cracking units, and optional fluidized beds according to Table 1 were used, and the resultant process conditions including the heat transfer to the coils for cracking were calculated. More specifically, Simulation 1 is a steam cracking furnace that has a radiant section and a convection section using air (i.e., including nitrogen) in the combustion fuel. Simulation 2 is a steam cracking furnace that has a radiant section and a convection section using oxygen (i.e., no nitrogen) diluted with carbon dioxide from the flue gas in the combustion fuel. Simulation 3 is a steam cracking unit of the present invention using air (i.e., including nitrogen) in the combustion fuel. Simulations 4 and 5 are steam cracking units of the present invention using oxygen (i.e., no nitrogen) diluted with carbon dioxide from the flue gas in the combustion fuel. Simulation 5 as compared to all other simulations has a higher radiant area.

TABLE-US-00001 TABLE 1 Simulation Inputs and Results Sim. 1 Sim. 2 Sim. 3 Sim. 4 Sim. 5 Flame Conditions Radiant sect. yes yes Fluid bed (75 micron wt. avg. diam. yes yes yes particles) Air combustion yes yes O.sub.2/CO.sub.2 recycle combustion yes yes yes Hydrocarbon combustion fuel flow 8259 8344 7049 7142 8344 rate (steam cracker tar) (lb/hr) Flue gas rate (lb/hr) 156050 146456 110008 130147 146456 Flame temp. ( F.) 3494 3486 Crossover* temp. ( F.) 1900 1900 2500 2135 1900 Convection sect, out temp. ( F.) 1361 1352 2150 1509 1352 Process Conditions** Hydrocarbon cracking feed flow 173240 173232 173240 173232 173232 rate (VGO) (lb/hr) Conv. sect, in temp. ( F.) 974 974 974 974 974 Conv. sect, out temp. ( F.) 1174 1174 1174 1174 1174 Radiant sect, out temp ( F.) 1600 1600 1600 1600 1600 Heat transfer (10.sup.6 BTU/hr) 77.48 77.48 77.48 77.48 77.48 Radiant/fluid duty (10.sup.6 BTU/hr) 77.48 77.48 67.76 77.47 64.77 Radiant area (ft.sup.2) 1256.4 1256.4 1256.4 1256.4 1679.8 Pressure (psia) 150.0 150.0 150.0 150.0 150.0 Fluid bed velocity (feet per 2.16 2.04 2.04 second fps) Conv. sect. U.sub.max (fps) 4.18 4.15 1.52 4.15 4.15 Conv. sect, duty required 25.01 25.01 25.01 25.01 25.01 (10.sup.6 BTU/hr) U.sub.conv (BTU/hr/ft.sup.2/ F.) 43.08 51.96 39.43 50.62 50.32 U.sub.fluid (BTU/hr/ft.sup.2/ F.) 65.33 76.42 84.83 Fluid flux (BTU/hr/ft.sup.2) 53934 61663 38558 Radiant flux (BTU/hr/ft.sup.2) 61668 61668 *The crossover portion is above the fluid bed, when present, or above the cracking coils 110 when the fluid bed is not present. **U.sub.conv and U.sub.fluid the heat transfer coefficients for the convection section and the fluid bed, respectively. U.sub.max is the maximum fluid velocity.

[0040] With air combustion vs O.sub.2 combustion, the CO.sub.2 is easily recovered from the flue gas of O.sub.2 combustion while steam cracking duties can be maintained.

[0041] Therefore, the present invention is well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. The particular embodiments disclosed above are illustrative only, as the present invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein.

[0042] Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular illustrative embodiments disclosed above may be altered, combined, or modified and all such variations are considered within the scope and spirit of the present invention. The invention illustratively disclosed herein suitably may be practiced in the absence of any element that is not specifically disclosed herein and/or any optional element disclosed herein. While compositions and methods are described in terms of comprising, containing, or including various components or steps, the compositions and methods can also consist essentially of or consist of the various components and steps. All numbers and ranges disclosed above may vary by some amount. Whenever a numerical range with a lower limit and an upper limit is disclosed, any number and any included range falling within the range is specifically disclosed. In particular, every range of values (of the form, from about a to about b, or, equivalently, from approximately a to b, or, equivalently, from approximately a-b) disclosed herein is to be understood to set forth every number and range encompassed within the broader range of values. Also, the terms in the claims have their plain, ordinary meaning unless otherwise explicitly and clearly defined by the patentee. Moreover, the indefinite articles a or an, as used in the claims, are defined herein to mean one or more than one of the element that it introduces.