Hydrocarbon Pyrolysis

Abstract

The invention relates to hydrocarbon pyrolysis, to equipment and materials useful for hydrocarbon pyrolysis, to processes for carrying out hydrocarbon pyrolysis, and to the use of hydrocarbon pyrolysis for. e.g., natural gas upgrading. The pyrolysis can be carried out in a reverse-flow reactor.

Claims

1. A hydrocarbon pyrolysis process, the process comprising: (a) providing a feed comprising 1 wt. % of C.sub.2+ hydrocarbon; (b) providing an elongated flow-through reactor having (i) an internal volume which includes first and second regions and (ii) opposed first and second openings in fluidic communication with the internal volume, wherein the first and second openings are separated by a reactor length (L.sub.R); (c) providing at least one heated thermal mass located in the first region, wherein (i) the thermal mass includes first and second apertures and at least one internal channel, (ii) the first and second apertures are in fluidic communication with the channel, and are separated by a flow-path of length L.sub.M through the channel, (iii) L.sub.M is 0.1*L.sub.R, and (iv) the first opening is proximate to the first aperture; (d) establishing a flow of the feed in the channel toward the second aperture by introducing the feed through the first opening and through the first aperture; (e) pyrolysing the feed flow's C.sub.2+ hydrocarbon in the channel under pyrolysis conditions to produce a flow of a pyrolysis product comprising molecular hydrogen, ethylene, propylene, and butadiene, the pyrolysis conditions including: (i) a first bulk gas temperature profile at the start of the pyrolysis which increases from a first temperature (T.sub.1) proximate to the first aperture to a second temperature (T.sub.2) proximate to the second aperture, T.sub.2 being in the range of from 800 C. to 1400 C.; and (ii) a peak gas temperature (T.sub.p) within the internal volume, T.sub.p being located within the second region, wherein during the pyrolysis (A) the peak gas temperature decreases from a temperature that is >T.sub.2 at the start of the pyrolysis and (B) the location of T.sub.p remains substantially constant; and (f) conducting the flow of the pyrolysis product into the second region of the internal volume via the second aperture, and away from the reactor via the second opening.

2. A hydrocarbon pyrolysis process, the process comprising: (a) providing a feed comprising 1 wt. % of C.sub.2+ hydrocarbon; (b) providing an elongated flow-through reactor having (i) an internal volume which includes first and second regions and (ii) opposed first and second openings in fluidic communication with the internal volume the first and second openings being separated by a reactor length (L.sub.R); (c) providing at least one heated thermal mass located in the first region, wherein (i) the thermal mass includes first and second apertures and at least one internal channel, (ii) the first and second apertures are in fluidic communication with the channel, and are separated by a flow-path of length L.sub.M through the channel, (iii) L.sub.M is 0.1*L.sub.R, and (iv) the first opening is proximate to the first aperture; (d) establishing a flow of the feed in the channel toward the second aperture by introducing the feed through the first opening and through the first aperture; (e) pyrolysing the feed flow's C.sub.2+ hydrocarbon in the channel under pyrolysis conditions to produce a flow of a pyrolysis product comprising molecular hydrogen, ethylene, propylene, and butadiene, the pyrolysis conditions including: (i) a first bulk gas temperature profile at the start of the pyrolysis which increases from a first temperature (T.sub.1) proximate to the first aperture to a second temperature (T.sub.2) proximate to the second aperture, T.sub.2 being in the range of from 800 C. to 1400 C.; and (ii) a conversion profile for the feed's C.sub.2+ hydrocarbon which at the start of the pyrolysis increases from a first conversion (X.sub.1) at a reference location R.sub.1 positioned between the first and second apertures to a second conversion (X.sub.2) proximate to the second aperture, wherein X.sub.1 is in the range of from 25% to 85%, and X.sub.2 is in the range of from 65% to 98%; and (f) conducting the flow of the pyrolysis product into the second region of the internal volume via the second aperture, and away from the reactor via the second opening.

3. The process of claim 1, wherein (i) the reactor is a reverse-flow thermal pyrolysis reactor, the reactor further comprising a second thermal mass located in the second region of the internal volume, the second thermal mass having at least one internal channel in fluidic communication with the internal channel of the first thermal mass, and (ii) the process further comprises conducting the pyrolysis product through the internal channel of the second thermal mass before the pyrolysis product is conducted away from the reverse-flow reactor, and cooling the pyrolysis product by transferring heat from the pyrolysis product to the second thermal mass.

4. The process of claim 1, wherein the pyrolysis conditions at the start of the pyrolysis further include T.sub.1750 C., T.sub.2 in the range of from 975 C. to 1100 C., a partial pressure of 7 psia (48 kPa), a total pressure of 5 psig (34 kPag), and a total gas residence time in the range of from 0.01 second to 0.8 second.

5. The process of claim 1, wherein the feed comprises one or more of ethane, propane, butanes, saturated and unsaturated C.sub.6 hydrocarbon, including those derived from one or more of Fischer-Tropsch synthesis products, shale gas, biogas, associated gas, natural gas and mixtures or components thereof, steam cracked gas oil and residues, gas oils, heating oil, jet fuel, diesel, kerosene, gasoline, naphtha (including coker naphtha, steam cracked naphtha, and catalytically cracked naphtha), hydrocrackate, reformate, raffinate reformate, Fischer-Tropsch liquids, natural gasoline, distillate, virgin naphtha, crude oil, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, wide boiling range naphtha to gas oil condensates, heavy non-virgin hydrocarbon streams from refineries, vacuum gas oils, heavy gas oil, naphtha contaminated with crude, synthetic crudes, shale oils, coal liquefaction products, coal tars, tars, atmospheric resid, heavy residuum, C.sub.4residue admixture, naphtharesidue admixture, cracked feed, coker distillate streams, and hydrocarbon streams derived from plant or animal matter.

6. The process of claim 2, wherein the feed comprises 90 wt. % of (i) ethane and/or (ii) propane.

7. The process of claim 6, wherein (i) X.sub.1 is in the range of from 25% to 60%, (ii) T.sub.2 is in the range of from 1025 C. to 1075 C., (iii) X.sub.2 is in the range of from 85% to 98%, (iv) the bulk gas temperature profile includes a bulk gas temperature at the reference location in the range of from 925 C. to 975 C., and (v) the reference location R.sub.1 is positioned within 0.2*L.sub.M and 0.4*L.sub.M of the second aperture.

8. The process of claim 6, wherein the pyrolysis conditions at the start of the pyrolysis further include: (i) an acetylene selectivity in a range of from 0% to 1% at the reference location, which acetylene selectivity increases to a range of 5% to 10% at the second aperture, (ii) an ethylene selectivity in a range of from 85% to 95% at the reference location, which ethylene selectivity decreases to a range of 70% to 85% at the second aperture, (iii) a propylene selectivity in a range of from 0.7% to 0.9% at the reference location, which propylene selectivity varies non-monotonically to a range of 0.4% to 0.6% at the second aperture, and (iv) a butadiene selectivity in a range of from 0.5% to 1.5% at the reference location, which butadiene selectivity increases to a range of 4% to 5% at the second aperture.

9. The process of claim 6, wherein the pyrolysis conditions further include a second bulk gas temperature profile at the end of the pyrolysis, the second bulk gas temperature profile being substantially congruent with the first bulk gas temperature profile.

10. The process of claim 6, further comprising carrying out the pyrolysis for a time duration in the range of about 110.sup.3 seconds to 10 seconds and then terminating the pyrolysis, wherein the pyrolysis conditions further include (i) a second conversion profile at the end of the pyrolysis, (ii) the first and second conversion profiles are substantially congruent, and (iii) the conversion X.sub.1 at the end of the pyrolysis is achieved at a second reference location R.sub.2, with R.sub.2 being positioned between the R.sub.1 and second aperture.

11. A hydrocarbon pyrolysis process, the process comprising: (a) providing a feed comprising 1 wt. % of C.sub.2+ hydrocarbon; (b) providing an elongated flow-through reactor having (i) an internal volume which includes first and second regions and (ii) opposed first and second openings in fluidic communication with the internal volume the first and second openings being separated by a reactor length (L.sub.R); (c) providing at least one heated thermal mass located in the first region, wherein (i) the thermal mass includes first and second apertures and at least one internal channel, (ii) the first and second apertures are in fluidic communication with the channel, and are separated by a flow-path of length L.sub.M through the channel, (iii) L.sub.M is 0.1*L.sub.R, and (iv) the first opening is proximate to the first aperture; (d) initiating at a time t.sub.1 a flow of the feed in the channel toward the second aperture by introducing the feed through the first opening and through the first aperture, and terminating the flow of feed at a later time t.sub.2, wherein the time duration t.sub.P of feed flow is substantially equal to t.sub.2t.sub.1, (e) pyrolysing the feed flow's C.sub.2+ hydrocarbon in the channel under pyrolysis conditions during t.sub.P to cool the thermal mass and produce a flow of a pyrolysis product comprising molecular hydrogen, ethylene, propylene, and butadiene, the pyrolysis conditions including: (i) a peak gas temperature T.sub.p located in the internal volume, the peak gas temperature being located along L.sub.M, and (ii) a first bulk gas temperature profile at t.sub.1 which varies continuously along L.sub.M from a first temperature (T.sub.1) proximate to the first aperture to a second temperature (T.sub.2) proximate to the second aperture, wherein T.sub.1<T.sub.2, T.sub.2<T.sub.p, and T.sub.2 is in the range of from 800 C. to 1400 C., and (iii) during t.sub.P, T.sub.p continuously decreases and the position of T.sub.p along L.sub.M remains substantially constant; and (f) during t.sub.P, conducting the flow of the pyrolysis product into the second region of the internal volume via the second aperture, and away from the reactor via the second opening.

12. A hydrocarbon pyrolysis process, the process comprising: (a) providing a feed comprising 1 wt. % of C.sub.2+ hydrocarbon; (b) providing an elongated flow-through reactor having (i) an internal volume which includes first and second regions and (ii) opposed first and second openings in fluidic communication with the internal volume the first and second openings being separated by a reactor length (L.sub.R); (c) providing at least one heated thermal mass located in the first region, wherein (i) the thermal mass includes first and second apertures and at least one internal channel, (ii) the first and second apertures are in fluidic communication with the channel, and are separated by a flow-path of length L.sub.M through the channel, (iii) L.sub.M is 0.1*L.sub.R, and (iv) the first opening is proximate to the first aperture; (d) initiating at a time t.sub.1 a flow of the feed in the channel toward the second aperture by introducing the feed through the first opening and through the first aperture, and terminating the flow of feed at a later time t.sub.2, wherein the time duration t.sub.P of feed flow is substantially equal to t.sub.2t.sub.1, (e) pyrolysing the feed flow's C.sub.2+ hydrocarbon in the channel under pyrolysis conditions during t.sub.P to cool the thermal mass and produce a flow of a pyrolysis product comprising molecular hydrogen, ethylene, propylene, and butadiene, the pyrolysis conditions including: (i) a first conversion profile for the feed's C.sub.2+ hydrocarbon at t.sub.1 which continuously varies from a first conversion (X.sub.1) at a reference location between the first and second apertures to a second conversion (X.sub.2) at the second aperture and exhibits at least one peak of conversion X.sub.p located between the reference location and the second aperture, wherein X.sub.p>X.sub.2, X.sub.2>X.sub.1, X.sub.2 is in a range of from 55% to 95%, X.sub.p continuously decreases during t.sub.P, and the location of X.sub.p remains substantially constant during t.sub.P, and (ii) a first bulk gas temperature profile at t.sub.1 which continuously varies from a first temperature (T.sub.1) proximate to the first aperture to a second temperature (T.sub.2) proximate to the second aperture, wherein (A) the first bulk gas temperature profile at t.sub.1 includes a peak gas temperature T.sub.p at substantially the same location as X.sub.p, (B) T.sub.1<T.sub.2, (C) T.sub.2<T.sub.p and (D) T.sub.2 is in the range of from 800 C. to 1400 C.; and (f) during t.sub.P, conducting the flow of the pyrolysis product into the second region of the internal volume via the second aperture, and away from the reactor via the second opening.

13. The process of claim 11, wherein (i) the pyrolysis conditions further include a partial pressure of 7 psia (48 kPa), a total pressure of 5 psig (34 kPag), and a total gas residence time in the range of from 0.01 second to 0.8 second, and (ii) the time duration t.sub.P is in a range of from 110.sup.3 seconds to 10 seconds.

14. The process of claim 11, wherein (i) the pyrolysis product further comprises coke and one or more of acetylene, benzene, methane, and at least a portion of any unconverted feed, and (ii) at least a portion of the coke remains in the internal channel as a deposit.

15. The process of claim 11, wherein the feed comprises one or more of ethane, propane, butanes, saturated and unsaturated C.sub.6 hydrocarbon, including those derived from one or more of Fischer-Tropsch synthesis products, shale gas, biogas, associated gas, natural gas and mixtures or components thereof, steam cracked gas oil and residues, gas oils, heating oil, jet fuel, diesel, kerosene, gasoline, naphtha (including coker naphtha, steam cracked naphtha, and catalytically cracked naphtha), hydrocrackate, reformate, raffinate reformate, Fischer-Tropsch liquids, natural gasoline, distillate, virgin naphtha, crude oil, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, wide boiling range naphtha to gas oil condensates, heavy non-virgin hydrocarbon streams from refineries, vacuum gas oils, heavy gas oil, naphtha contaminated with crude, synthetic crudes, shale oils, coal liquefaction products, coal tars, tars, atmospheric resid, heavy residuum, C.sub.4residue admixture, naphtharesidue admixture, cracked feed, coker distillate streams, and hydrocarbon streams derived from plant or animal matter.

16. The process of claim 12, wherein the feed comprises 90 wt. % of (i) ethane and (ii) propane.

17. The process of claim 16, wherein at the time t.sub.1 (i) T.sub.2 is in the range of from 925 C. to 1075 C. and (ii) X.sub.2 is in the range of from 85% to 98%.

18. The process of claim 16, wherein the reference location is within 0.2*L.sub.M and 0.4*L.sub.M of the second aperture.

19. The process of claim 16, wherein the pyrolysis conditions further include: (A) an acetylene selectivity in a range of from 5% to 10% at the second aperture at t.sub.1, which acetylene selectivity decreases into a range of about 0% to 1% at t.sub.2, (B) an ethylene selectivity in a range of from 75% to 80% at the second aperture at t.sub.1, which ethylene selectivity increases into a range of about 90% to 95% at t.sub.2, (C) a propylene selectivity in a range of from 0.7% to 0.9% at the second aperture at t.sub.1, which propylene selectivity varies by no more than about +/20% during t.sub.P, and (D) a butadiene selectivity in a range of from 4% to 5% at the second aperture at t.sub.1, which butadiene selectivity decreases into a range of about 0% to 1.5% at t.sub.2.

20. The process of claim 16, wherein the pyrolysis conditions further include (i) a second gas temperature profile at t.sub.2, the second bulk gas temperature profile exhibiting a continuously decreasing gas temperature at the second aperture during t.sub.P and being substantially congruent with the first bulk gas temperature profile, and (ii) a second conversion profile for the feed's C.sub.2+ hydrocarbon at t.sub.2 which is substantially congruent with the first conversion profile.

21. A hydrocarbon pyrolysis process, the process comprising: (a) providing a feed comprising gaseous C.sub.2+ hydrocarbon; (b) providing an oxidant and a fuel; (c) providing an elongated flow-through reactor having (i) an internal volume which includes first and second regions and (ii) opposed first and second openings in fluidic communication with the internal volume the first and second openings being separated by a reactor length (L.sub.R), and (iii) a first temperature profile; (d) providing at least one heated thermal mass located in the first region, wherein (i) the thermal mass includes first and second apertures and at least one internal channel, (ii) the first and second apertures are in fluidic communication with the channel, and are separated by a flow-path of length L.sub.M through the channel, (iii) L.sub.M is 0.1*L.sub.R, and (iv) the first opening is proximate to the first aperture; (e) during a first time interval having a duration t.sub.P, (i) establishing a forward flow of the feed through the first opening, through the first aperture, into the into the channel, and toward the second aperture, (ii) pyrolysing the feed flow's C.sub.2+ hydrocarbon in the channel under low-severity pyrolysis conditions which cools the thermal mass to achieve a second thermal profile, deposits coke in the channel, and establishes a forward flow of a pyrolysis product comprising molecular hydrogen and olefin, (iii) conducting the forward flow of the pyrolysis product into the second region of the internal volume via the second aperture, and away from the reactor via the second opening; and (iv) decreasing or halting the feed flow; wherein the low-severity pyrolysis conditions include: (A) a first bulk gas temperature profile at the start of the first time interval which increases substantially monotonically from a first temperature (T.sub.1) proximate to the first aperture to a second temperature (T.sub.2) proximate to the second aperture, T.sub.2 being in the range of from 800 C. to 1400 C.; and (B) a peak gas temperature (T.sub.p) within the second region, wherein during t.sub.P the peak gas temperature decreases from a temperature that is >T.sub.2 and the location of T.sub.p remains substantially constant; and (e) during a second time interval having a duration t.sub.H, (i) establishing a reverse flow of the fuel and a reverse flow of the oxidant through the second opening and into the second region of the internal volume, the oxidant flow comprising first and second portions of the oxidant, (ii) combusting the first portion of the oxidant flow under combustion conditions with at least a portion of the fuel flow in the second region to produce a reverse flow of a first combustion product toward the thermal mass, (iii) establishing a reverse flow of the first combustion product and the second portion of the oxidant into the channel at the second aperture toward the first aperture, (iv) combusting in the internal channel the second portion of the oxidant flow with at least a portion of the deposited coke to produce a reverse flow of a second combustion product, (v) conducting the reverse flows of the first and second combustion products away from the first aperture and out of the reverse-flow reactor via the first opening, wherein heat is transferred to the thermal mass from the combustion of the first and second oxidant portions to re-heat the thermal mass to substantially achieve the first temperature profile; and (vi) decreasing the reverse flow of fuel and the reverse flow of oxidant.

22. The pyrolysis product of claim 1.

23. A reverse-flow reactor, comprising: (a) a reactor vessel having an internal volume which includes opposed first and second heat-transfer zones, a pyrolysis zone located between the first and second heat-transfer zones, and a combustion zone, wherein the pyrolysis zone, the combustion zone, the first heat transfer zone, and the second heat transfer zone are in fluidic communication; (b) at least one feed conduit in fluidic communication with the first heat-transfer zone to convey a forward flow of a gaseous feed comprising C.sub.2+ hydrocarbon through the first heat transfer zone and into the pyrolysis zone, the pyrolysis zone being adapted to: (i) pyrolyse at least a portion of the feed and produce a pyrolysis product comprising coke, molecular hydrogen, and olefin, and (ii) establish a forward flow of the pyrolysis product out of the pyrolysis zone and through the second heat-transfer zone and deposit at least a portion of the coke in the pyrolysis zone, wherein (A) the reactor has a peak gas temperature (T.sub.p) at a location within the pyrolysis zone, and (B) the location of T.sub.p remains substantially constant during the pyrolysis; (c) at least one pyrolysis product conduit in fluidic communication with the second heat-transfer zone to convey a forward flow of the pyrolysis product away from the heat-transfer zone and out of the reverse-flow reactor; (d) at least one fuel conduit in fluidic communication with the combustion zone to convey a reverse flow of a fuel to the combustion zone; (e) at least one oxidant conduit in fluidic communication with the combustion zone to convey a reverse flow of an oxidant to the combustion zone, wherein (i) the combustion zone is adapted to combust at least a portion of the fuel with a first portion of the oxidant and convey away from the combustion zone at least (A) a reverse-flow of a first combustion product and (B) a reverse flow of un-combusted oxidant and (ii) the pyrolysis zone is adapted to oxidize the coke deposits with the un-combusted oxidant flow to produce a second combustion product; (f) at least one combustion product conduit in fluidic communication with the combustion zone to convey a reverse-flow of the first and second combustion products away from the pyrolysis zone and out of reverse-flow reactor; and (g) at least one flow controller to (i) establish during a first time interval the forward flows of the gaseous feed and the pyrolysis product and (ii) establish during a second time interval the reverse flows of flow of the fuel, the oxidant, and the combustion product.

24. The reverse flow reactor of claim 23, wherein (i) the reactor vessel has the form of an elongated tube of circular, elliptical, or polygonal cross section, (ii) the elongated tube includes opposed first and second openings, the first opening being in fluidic communication with the feed conduit and the first heat-transfer zone, and the second opening being in fluidic communication with the pyrolysis product conduit and the second heat-transfer zone

25. The reverse flow reactor claim 23, wherein (i) the pyrolysis zone and first heat transfer zone include a first thermal mass for (A) transferring heat to the feed during the first interval and (B) for transferring heat from the combustion product during the second interval, and (ii) the pyrolysis zone and second heat transfer zone include a second thermal mass for (A) transferring heat to the reverse flow of the fuel and/or to the reverse flow the oxidant during the second time interval and (B) for transferring heat away from the pyrolysis product during the first time interval.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0014] FIG. 1 schematically shows one form of a reverse flow reactor that is suitable for carrying out certain aspects of the invention.

[0015] FIGS. 2-5 schematically show forms of a reverse flow reactor and representative bulk gas temperature profiles at the start (solid lines) and end (dashed lines) of pyrolysis.

[0016] FIG. 6 shows the variation of conversion as a function of T.sub.av for the reverse flow reactor of FIG. 2 during pyrolysis mode.

[0017] FIGS. 7-9 show the selectivities of the desired hydrocarbon compounds for the reverse flow reactor of FIG. 2 during pyrolysis mode.

DETAILED DESCRIPTION

Definitions

[0018] For the purpose of this description and appended claims, the following terms are defined.

[0019] The term C.sub.n hydrocarbon means hydrocarbon having n carbon atom(s) per molecule, wherein n is a positive integer. The term C.sub.n+ hydrocarbon means hydrocarbon having at least n carbon atom(s) per molecule. The term C.sub.n hydrocarbon means hydrocarbon having no more than n carbon atom(s) per molecule. The term hydrocarbon means a class of compounds containing hydrogen bound to carbon, and encompasses (i) saturated hydrocarbon. (ii) unsaturated hydrocarbon, and (iii) mixtures of hydrocarbons, and including mixtures of hydrocarbon compounds (saturated and/or unsaturated), such as mixtures of hydrocarbon compounds having different values of n.

[0020] The terms alkane and paraffinic hydrocarbon mean substantially-saturated compounds containing hydrogen and carbon only, e.g., those containing 1% (molar basis) of unsaturated carbon atoms. The term unsaturate and unsaturated hydrocarbon refer to one or more C.sub.2+ hydrocarbon compounds which contain at least one carbon atom directly bound to another carbon atom by a double or triple bond. The term olefin refers to one or more unsaturated hydrocarbon compound containing at least one carbon atom directly bound to another carbon atom by a double bond. In other words, an olefin is a compound which contains at least one pair of carbon atoms, where the first and second carbon atoms of the pair are directly linked by a double bond. The term aromatics and aromatic hydrocarbon mean hydrocarbon compounds containing at least one aromatic ring.

[0021] The term selectivity refers to the production (weight basis) of a specified compound in a reaction. As an example, the phrase a hydrocarbon pyrolysis reaction has 100% selectivity for methane means that 100% of the hydrocarbon (weight basis) that is convened in the pyrolysis reaction is converted to methane. When used in connection with a specified reactant, the term conversion means the amount of the reactant (weight basis) consumed in the reaction. For example, when the specified reactant is ethane, 100% conversion means 100% of ethane is consumed in the reaction. With respect to hydrocarbon pyrolysis the term conversion encompasses any molecular decomposition by at least pyrolysis heat, including cracking, breaking apart, and reformation. Average conversion in a reaction zone, e.g., a pyrolysis zone, is the conversion achieved at T.sub.av. Yield (weight basis) is conversion times selectivity.

[0022] The term pyrolysis means an endothermic reaction for converting molecules into (i) atoms and/or (ii) molecules of lesser molecular weight, and optionally (iii) molecules of greater molecular weight, e.g., processes for converting ethane and/or propane to molecular hydrogen and unsaturates such as ethylene, propylene and acetylene. Certain aspects of the invention feature a pyrolysis zone exhibiting selectivities (e.g., of desired products) which vary as a function of position along the length of the pyrolysis zone but which do not vary appreciably as a function of time during pyrolysis mode, e.g., within about +/25%, such as +/10%, or +/5% from selectivity at the start of t.sub.P. More particularly, for certain aspects in which T.sub.av and/or T.sub.p decrease by 100 C. during pyrolysis mode, the yield of many desired products, e.g., light olefin yield, such as ethylene and/or propylene yield, do not vary appreciably as a function of time during pyrolysis mode even though the product selectivities vary as a function of position along the length of the pyrolysis zone. For example, yield is typically within about +/25%, such as +/10%, or +/5% of yield at the start of t.sub.P. In these aspects, average conversion might not vary appreciably as a function of time during pyrolysis mode, and is typically within about +/25%, such as +/10%, or +/5% of average conversion at the start of t.sub.P.

[0023] A hydrocarbon feed is subjected to thermal pyrolysis when <50.0% of the heat utilized by the pyrolysis is provided by exothermically reacting the hydrocarbon feed, e.g., with an oxidant, e.g., 25%, such as 10%, or 1%. The severity threshold temperature for pyrolysis is the lowest temperature at which acetylene selectivity is at least 10% for a residence time 0.1 second. High-severity pyrolysis conditions are those carried out at a peak gas temperature that is greater than or equal to the severity threshold temperature. Low-severity pyrolysis conditions are those carried out at a peak gas temperature that is less than the severity threshold temperature, i.e. conditions under which substantially no hydrocarbon pyrolysis is carried out at a pyrolysis gas temperature that exceeds the severity threshold temperature. High-severity conditions include those which exhibit (i) a methane selectivity 5 wt. % and/or (ii) a propylene selectivity at a temperature 1000 C. of 0.6 wt. %. With respect to pyrolysis reactors, the term residence time means the average time duration for non-reacting (non-converting by pyrolysis) molecules (such as He, N.sub.2, Ar) having a molecular weight in the range of 4 to 40 to traverse a pyrolysis region of a pyrolysis reactor.

[0024] The terms reactor, reactor system, regenerator, recuperator, regenerative bed, monolith, honeycomb, reactant, fuel, and oxidant have the meanings disclosed in U.S. Pat. No. 7,943,808, which is incorporated by reference herein in its entirety. A pyrolysis reactor is a reactor, or combination of reactors or a system for hydrocarbon pyrolysis. The term pyrolysis stage means at least one pyrolysis reactor, and optionally including means for conducting one or more feeds thereto and/or one or more products away therefrom. A region or zone is a location, e.g., a specific volume, within a reactor, a location between two reactors and/or the combination of different disjointed locations in one or more reactors. A pyrolysis region is a location where pyrolysis is carried out, e.g., in a location which contains or is proximate to components, such as at least one thermal mass, which provides heat for the pyrolysis. A reactor or reaction stage can encompass one or more reaction regions. More than one reaction can be carried out in a reactor, stage, or region.

[0025] A pyrolysis region can include components having conduits, channels, and passages. The term conduit refers to means for conducting a composition from one location to another. The term encompasses (i) elementary conducting means, such as a pipe or tube, and (ii) complex means such as tortuous pathways through conducting means, e.g., pipes, tubes, valves, and reactors, that are filled with random packing. The term passage means a geometrically contiguous volume element that can be utilized for conveying a fluid within a reactor, regenerator, recuperator, regenerative bed, monolith, honeycomb, etc. The term channel means a plurality of passages that can be utilized together for conveying a fluid within the reactor, regenerator, recuperator, regenerative bed, monolith, honeycomb, etc. For example, a honeycomb monolith can comprise a single channel, with the channel having a plurality of passages or sets of passages.

[0026] The term bulk gas temperature means the temperature of a bulk gas steam as measured by a device (such as a thermocouple) that is in contact with the bulk gas but not in contact with a solid thermal mass. For example, if the gas is traveling through an internal channel of length L.sub.c of a thermal mass in the pyrolysis zone of a thermal pyrolysis reactor, the bulk gas temperature at a location along L.sub.c is the average temperature (arithmetic mean) over the channel's cross sectional area at that location. The peak gas temperature is the greatest cross-sectional-averaged bulk gas temperature achieved along a flowpath. One skilled in the art will appreciate that a gas temperature immediately proximate to a solid thermal mass, such as a partition between passages within a thermal mass at any particular location may exceed the bulk gas temperature, and may, in some infinitesimal layer, actually approach the solid's temperature. The average bulk gas temperature T.sub.av over a region of the reactor, e.g., the pyrolysis zone or, a flow path, a channel, a passage, etc. T.sub.av at a particular time (e.g., at the start of pyrolysis) is obtained using the formula:

[00001]$\mathrm{Tav}=\left[\frac{1}{b-a}.Math.{}_a^b.Math.T\left(x\right).Math.\mathrm{dx}\right]$

Parameters a and b are the boundaries of an interval (distance) along the long axis of the reactor. For example, referring to FIG. 1, parameter a can be the position of aperture 51 and parameter b can be the position of aperture 9. T(x) is a function representing the variation of bulk gas temperature over the interval of from a to b. When T(x) is a bulk gas temperature profile of a pyrolysis zone, e.g., the pyrolysis zones indicated (at the start of t.sub.P) by the shaded regions in FIGS. 2 and 3, parameters a and b are the locations where the bulk gas temperature profile intersects the line T.sub.MIN, which corresponds to the minimum temperature at which feed conversion is 10% under the selected pyrolysis conditions and feed. Since the bulk gas temperature profile typically changes during the pyrolysis time interval t.sub.P, T.sub.av will typically decrease during t.sub.P. The portion of the profile having a temperature T.sub.MIN can be continuous, but this is not required. For example, when a profile that intersects T.sub.MIN at more than two locations in the pyrolysis zone (e.g., a, b) and touches T.sub.MIN at a location c (not shown, but between a and b), additional integrations are carried out, e.g.:

[00002]$\mathrm{Tav}=\frac{1}{b-a}.Math.{}_a^b.Math.T\left(x\right).Math.\mathrm{dx}+\frac{1}{c-b}.Math.{}_b^c.Math.T\left(x\right).Math.\mathrm{dx}.$

When the portion of the profile that is T.sub.MIN is in the form of discrete segments, the integrations are performed over each of the segments.

[0027] The term Periodic Table means the Periodic Chart of the Elements, as it appears on the inside cover of The Merck Index, Twelfth Edition. Merck & Co., Inc., 1996.

[0028] Certain aspects of the invention relate to carrying out pyrolysis mode and heating mode under the specified conditions in one or more reverse flow reactors. Representative reverse flow reactors will now be described in more detail with respect to FIG. 1. The invention is not limited to these aspects, and this description is not meant to foreclose the use of other reactors within the broader scope of the invention.

Representative Reverse Flow Reactors

[0029] Reverse-flow reactor 50 can be in the form of an elongated tubular vessel having an internal volume which includes a pyrolysis zone for carrying out the pyrolysis. Typically, the internal volume includes three zones: a first heat-transfer zone, a second heat transfer zone, with the pyrolysis zone being located between the first and second heat transfer zones. The zones are in fluidic communication with one another. The reactor vessel's cross sectional shape and/or cross sectional area can be substantially uniform over the length of the reactor, but this is not required. For example, one or more segments of the reactor vessel's length can have a circular, elliptical, or polygonal cross section. Reactor 50 has opposed first and second openings 51 and 52 which are in fluidic communication with the internal volume and are located at terminal ends of the reactor vessel.

[0030] The reactor 50 includes first and second thermal masses 1 and 7 for transferring heat to/from reactants and products during the pyrolysis and heating modes. Typically, a first segment of the first thermal mass 1 is located in the first heat transfer zone, with a second segment being located in the pyrolysis zone. Likewise, a first segment of the second thermal mass can be located in the second heat transfer zone, with a second segment being located in the pyrolysis zone. Typically, the thermal mass is in the form of a channeled tubular member comprising bedding or packing material that is effective in storing and transferring heat, such as glass or ceramic beads or spheres, metal beads or spheres, ceramic (e.g., ceramics, which may include alumina, yttria, and zirconia) or honeycomb materials comprising ceramic and/or metal, other forms of tubes comprising ceramic and/or metal, extruded monoliths and the like. Typically, the thermal mass has an open frontal area 55%. Those skilled in the art will appreciate that the compositions of the first and second thermal masses should be selected from among those that substantially maintain integrity (structural and compositions) and functionality during long term exposure to pyrolysis feeds, products, and reaction conditions, e.g., temperatures 750 C., e.g., 1200 C., or for increased operating margin 1500 C. Regenerative beds, such as those described in U.S. Pat. No. 8,754,276 (incorporated by reference herein in its entirety) are suitable for use as thermal mass, but the invention is not limited thereto. Typically, thermal masses 1 and 7 have the form of an elongated refractory tube having (i) at least one internal channel for conveying the feeds and products through the pyrolysis zone and (ii) opposed apertures in fluidic communication with the internal channel(s). Thermal mass 1 has a length L.sub.3 (typically L.sub.3 is the length of the internal channel, L.sub.M) and thermal mass 7 has a length L.sub.1. L.sub.3 is 0.1*L.sub.R, e.g., in the range of from 0.1*L.sub.R to 0.9*L.sub.R such as 0.1*L.sub.R to 0.4*L.sub.R. Optionally, L.sub.3 is of substantially the same length as L.sub.1, and is of substantially the same cross-sectional shape and of substantially the same cross sectional area. As shown in FIG. 1, thermal mass 1 includes first and second apertures 3 and 5, and thermal mass 7 includes first and second apertures 9 and 11. Typically, aperture 3 is adjacent to opening 51. Optionally, particularly in aspects (not shown) in which thermal mass 7 is omitted, aperture 5 can be adjacent to opening 52. Turning again to the aspects illustrated in FIG. 1, thermal masses 1 and 7 each have the form of an elongated honeycomb comprising at least one channel, the channel having a plurality of passages. When a thermal mass is a segmented thermal mass, the honeycombs can be arranged adjacent to one another (e.g., end-to-end, in series). As may be appreciated, it is desirable, e.g., to lessen reactor pressure drop, to a align passages of a honeycomb's internal channel or channels with those of neighboring honeycombs to facilitate fluidic communication through all of the segments constituting the thermal mass. Optionally, the segments are of substantially the same composition, e.g., refractory, such as refractory comprising metal oxide-containing ceramic and/or ceramics which do not contain metal oxide.

[0031] The internal volume of reactor 50 also includes a combustion zone, typically located between terminal segments of the first and second thermal masses. Although the combustion zone can occupy less than all of the region between apertures 5 and 11, it is within the scope of the invention for the combustion zone to include all of the reactor's internal volume between apertures 5 and 11, e.g., the entire length L shown in FIG. 1. Typically, however, the combustion zone is centered in the region between apertures 11 and 5, e.g., with L.sub.2 being substantially equal to L.sub.4. As may be appreciated, the combustion zone occupies a region of reactor 50's internal volume that is within the pyrolysis zone. However, since in the aspects illustrated in FIG. 1, a heating mode is not carried out at the same time as pyrolysis mode, appreciable combustion does not occur in the combustion zone during pyrolysis and appreciable pyrolysis does not occur in the pyrolysis zone during heating.

[0032] The combustion zone is configured for (i) mixing the fuel and a portion of the oxidant during heating mode for efficient combustion, (ii) increasing distribution uniformity over third zone's internal cross sectional area of the combustion products, unreacted oxidant, and optionally unreacted fuel, and (iii) lessening undesirable pressure-drop effects during pyrolysis mode. The combustion zone can have the form of an open volume within the internal volume of reactor 50, e.g., an open volume having a length L and substantially constant circular cross section of diameter D and cross sectional area A (not shown). As may be appreciated, an open volume having an appropriate L: A ratio will provide at least some mixing and distribution during heating mode without creating too great a pressure drop during pyrolysis mode. More typically, since it provides improved mixing and distribution and allows a lesser overall length for the combustion zone, the combustion zone includes at least one mixer-distributor apparatus 10. The mixer-distributor, which can have the form of a relatively thin member (e.g., a plate) having one or more orifices effective for carrying out the mixing and distribution during heating mode. Generally, the orifices have sufficient cross sectional area to prevent an undesirably large pressure drop across the third zone during pyrolysis mode. Conventional mixer-distributors can be used, such as those described in U.S. Patent Application Publication No. 13-0157205 A1 and U.S. Pat. No. 7,815,873 (incorporated by reference herein in their entireties), but the invention is not limited thereto. Optionally, the combustion zone contains at least one selective combustion catalyst. Suitable selective combustion catalysts are described in U.S. Pat. No. 8,754,276, but the invention is not limited thereto. When used, a fixed bed of the selective combustion catalyst can be included as a component of mixer-distributor 10, e.g., with one or more of the mixer-distributor's plate members serving as a catalyst support. When used, mixer-distributer 10 can be located at any location within the combustion zone. Typically, however, it located approximately mid-way between apertures 11 and 5. In certain aspects, however, such as those where the amount of coke deposits in thermal mass 1 exceed that of thermal mass 7, the combustion zone is shifted downstream (with respect to fuel-oxidant flow) toward thermal mass 1. The amount of shift is typically 25% of L, e.g., 20%, such as 10%.

[0033] The sum of lengths L.sub.1, L, and L.sub.3 is typically 90% of the total length of reactor 50 (L.sub.R), e.g., as measured between openings 51 and 52. Since it is desirable to direct fuel and oxidant flows into appropriate passages of thermal mass 7 during heating mode and to direct pyrolysis feed flow into appropriate passages of thermal mass 1 during pyrolysis mode, it is typically desired to limit the internal volume between aperture 9 and opening 52 and between aperture 3 and opening 51, to that needed for convenient reactor assemble and to prevent component interference as might otherwise occur from thermal expansion during use. The pyrolysis zone, which generally encompasses all of region L, a segment of L.sub.1, and a segment of L.sub.3, is typically 10% of the total length of reactor 50, e.g., 15%, such as 20%. It is also typical for the pyrolysis zone to encompass 80% of the total length of reactor 50, e.g., to leave sufficient internal volume of thermal mass 1 for pre-heating the pyrolysis feed and sufficient internal volume of thermal mass 7 for quenching the pyrolysis product, e.g., 60%, such as 40%. In certain aspects, the pyrolysis zone has a length in the range of from 10% to 60% of the total length of the reactor, e.g., in the range of from 20% to 40%. The combustion zone's length L is typically 50% of that of the length of the pyrolysis zone, e.g., 40%, such as 30%, or 20%.

[0034] Values for L, L.sub.1, L.sub.2, L.sub.3, L.sub.4, and D generally depend on the pyrolysis feed used and the rate at which it is conducted into the reactor, the fuel and oxidant compositions, and the rate at which these are conducted into the reactor, etc. Although larger and small reactors are within the scope of the invention, (i) D is typically 1 cm, e.g., in the range of from about 1 cm to 10 m, such as 0.1 m to 7.5 m. (ii) L.sub.R is typically 1 cm, e.g., in the range of from about 1 cm to 20 m, such as 0.1 m to 7.5 m, (iii) L is typically 25% of L.sub.R, e.g., 10%, (iv) L.sub.1 is typically 35% of L.sub.R e.g., 45%, (iv) L.sub.3 is typically 35% of L.sub.R, e.g., 45%, L.sub.3 being optionally of substantially the same size and shape as L.sub.1, and (v) L.sub.2 is typically within about +/25% of L.sub.4, e.g., +/10%, such as +/5%. Aperture 3 is typically adjacent to opening 51, and aperture 9 is typically adjacent to opening 52. For example, the spacing between aperture 3 and opening 5 can be 0.1*L.sub.R, such as 0.01 L.sub.R, or 0.01*L.sub.R. These spacing ranges also typically apply to the spacing between aperture 9 and opening 52.

[0035] In certain aspects (not shown) at least a portion of the fuel-oxidant combustion is carried out in a location other than within the internal volume of reactor 50. For example, fuel combustion can be carried out at a location external to reactor 50, with the combustion products, unreacted oxidant, and optionally unreacted fuel being conveyed to the vicinity of the pyrolysis zone for (i) heating the pyrolysis zone to provide a desired temperature profile for efficiently carrying out the pyrolysis and (ii) combusting catalyst coke deposits with at least a portion of the unreacted oxidant.

[0036] In aspects illustrated schematically in FIG. 1, reactor 50 is heated during heating mode by conveying a heating mixture 19 comprising fuel and oxidant through opening 52, through aperture 9 of thermal mass 7, and out of aperture 11 toward mixer-distributor 10. Typically, the fuel and oxidant are conveyed separately through different channels of thermal mass 7, and are combined to form the heating mixture downstream of thermal mass 7. Typically fuel and oxidant are heated by a transfer of heat from thermal mass 7 as the fuel and oxidant are conveyed through the channels of thermal mass 7. Combustion of the fuel and oxidant produces a combustion product. Combustion product, any un-combusted oxidant, and any un-combusted fuel enter aperture 5. When there is un-combusted oxidant in thermal mass 1, this can react with coke deposits and any un-combusted fuel to produce additional combustion product. An aggregated combustion product 45 is conducted out of aperture 3 and away from the reactor via opening 51. The aggregate combustion product typically comprises the combustion product produced in combustion zone 10, additional combustion product, typically from combustion of coke in passages of thermal mass 1, and any unreacted fuel and/or any unreacted oxidant. Reactor 50 is switched from heating mode to pyrolysis mode after achieving the desired reactor temperature profile.

[0037] Continuing with reference to FIG. 1, a pyrolysis feed 15 is conducted into reactor 50 during pyrolysis mode via opening 51. The pyrolysis feed is preheated in an upstream segment of thermal mass 1 and is typically pyrolysed in (i) a downstream segment of thermal mass 1, and optionally also in (ii) the region between thermal mass 1 and thermal mass 7, and (iii) in an upstream segment of thermal mass 7. A volatile portion 49 (typically gaseous) of the pyrolysis product is cooled in a downstream segment of thermal mass 7, and is conducted away from thermal mass 7 via aperture 9, and is conducted away from reactor 50 via opening 52. A non-volatile portion of the pyrolysis product remains in the reactor, typically as coke deposits.

[0038] Heating mode is carried out for a time duration t.sub.H to achieve a desired temperature profile in the internal volume of reactor 50 for the start of pyrolysis mode, primarily by fuel-oxidant combustion in the combustion zone, coke-oxidant combustion in passages of thermal masses 1 and 7, and optionally additional fuel-oxidant combustion in internal passages of thermal mass 1 and (less typically) thermal mass 7. Pyrolysis mode is carried out for a time interval t.sub.P. Pyrolysis is endothermic, and, consequently, the bulk gas temperature profile of reactor 50 transformed over the course of time interval t.sub.P to one that is not appropriate for efficient pyrolysis. Reactor 50 is then switched from pyrolysis mode to heating mode to reheat the reactor, so that the desired bulk gas temperature profile is exhibited at the start of a following pyrolysis mode. Typically, valve means, e.g., a plurality of valves, are provided to (i) establish forward flows of the pyrolysis feed and the pyrolysis product during pyrolysis mode for a time duration t.sub.P and (ii) establish reverse flows of the fuel, the oxidant, and the combustion product during heating mode for a time duration t.sub.H. The valve means can be operated by one or more flow controllers.

[0039] Pyrolysis mode and heating mode are typically repeated in sequence, for semi-continuous or continuous operation. Intervening steps between successive pyrolysis and heating modes, e.g., one or more steps for admitting a forward or reverse flow of sweep gas to the reverse-flow reactor, can be carried out between pyrolysis mode and heating mode operation, and vice versa. Continuous or semi-continuous operation can be characterized by a cycle time, which constitutes the time duration from the start of a pyrolysis mode to the start of the next pyrolysis mode in the sequence, and includes the time duration of heating mode(s) and any intervening steps (when used). Cycle time can be substantially constant over a plurality of repeated cycles, but this is not required. The invention is typically practiced with relatively short cycle times compared to that of conventional processes for pyrolysing similar feed hydrocarbon at a peak pyrolysis temperature 1200 C. For example, cycle can be 60 seconds, e.g., 30 seconds, such as 15 seconds, or 5 seconds, or 1 second, or 0.1 second. Typically, cycle time is in the range of from 0.1 second to 60 seconds. e.g., 0.1 second to 15 seconds, such as 0.1 second to 10 seconds, or 0.1 second to 5 seconds, or 0.1 second to 1 second. When (i) the pyrolysis feed is introduced into the reactor in an average flow direction that is substantially opposite to the average flow direction of fuel and oxidant flow and/or (ii) when the flow of pyrolysis product away from the reactor is substantially opposite to the average flow direction of combustion product flow, the reactor is called a reverse-flow reactor.

[0040] Certain aspects of heating mode operation, during which reactor 50 is preheated for initial pyrolysis mode operation, or reheated for continued pyrolysis mode operation, will now be described in more detail. The invention is not limited to these aspects, and this description is not meant to foreclose other ways to operate a heating mode within the broader scope of the invention.

Representative Heating Mode Conditions

[0041] Operating conditions during heating mode are selected to accomplish (i) reheating the pyrolysis zone to establish a temperature profile in the reactor corresponding to the desired bulk gas temperature profile at the start of a following pyrolysis mode and (ii) removing sufficient coke deposits from within the reactor's internal volume, which would otherwise lead to an increase in reactor pressure drop. When it is desired to quench the pyrolysis product within the reactor, heating mode optionally includes cooling thermal mass within the reactor at a location that is both upstream (with respect to fuel-oxidant flow) of the combustion zone and downstream (with respect to the flow of pyrolysis product) of the pyrolysis zone.

[0042] Combustion is carried out during heating mode by reacting fuel and oxidant, e.g., fuel and oxidant contained in a heating mixture. The fuel and oxidant can be the same as those disclosed in U.S. Pat. No. 7,943,808. Optionally, the fuel is derived from, comprises, consists essentially of, or consists of one or more of hydrogen, CO, methane, methane containing streams, such as coal bed methane, biogas, associated gas, natural gas and mixtures or components thereof, etc. The fuel typically comprises one or more of molecular hydrogen, synthesis gas (mixtures of CO and H.sub.2), and hydrocarbon, such as 10.0 wt. % hydrocarbon, or 50.0 wt. % hydrocarbon, or 90.0 wt. % hydrocarbon. The oxidant is typically one or more of molecular oxygen, ozone, and air, including molecular oxygen in air. Those skilled in the art will appreciate that feed flow rate will depend on factors such as feed composition, reactor volume, pyrolysis conditions, etc. Accordingly, the invention can be carried out over a very wide range of heating mixture flow rates. e.g., at a flow rate 0.001 kg/s, such as 0.1 kg/s, or 10 kg/s, or 100 kg/s, or more.

[0043] Once a fuel of the desired caloric content (heating value) has been selected, the amounts of fuel and oxidant to the reactor during heating mode can be specified in terms of the amount of oxidant needed for combusting the accumulated coke deposits (OC.sub.a) and the amount of oxidant (OC.sub.b) needed for the substantially stoichiometric combustion of the fuel. Typically, the amount of oxidant supplied during heating mode is Z.Math.(OC.sub.a+OC.sub.b), wherein Z is generally 0.5, e.g., 0.8, such as in the range of 0.5 to 5.0, or 0.5 to 3.0, or 0.8 to 3.0. The amounts OC.sub.a and OC.sub.b are on a molar basis. When Z>1.0, the excess oxidant can be utilized, e.g., for one or more of removing at least a portion of any accumulated coke deposits, moderating the reaction temperature during heating mode (as disclosed in U.S. Pat. No. 7,943,808), and conveying heat within the reactor from one zone to another. Generally, a first portion of the oxidant is combusted with the fuel in the combustion zone, and a second portion is combusted with accumulated coke deposits. Typically, the first portion comprises 50 wt. % of the total amount of oxidant supplied during heating mode, e.g., 75 wt. %, or 90 wt. %, with the second portion comprising at least 75 wt. % of the remainder of the total oxidant, e.g., 90 wt. %. It is also typical for oxidant flow rate and fuel flow rate to remain substantially constant for the duration of heating mode. These flow rates are selected to achieve the desired amount of combustion heating and the desired amount of coke removal during t.sub.H. The invention is compatible with conventional methods for lessening coke accumulation in thermal masses during heating mode, e.g., those described in U.S. Pat. No. 9,187,382, which is incorporated by reference in its entirety.

[0044] Other streams can be provided to the reactor during heating mode, e.g., one or more diluent streams can be provided, such as by addition to the heating mixture. When used, diluent can be provided with the oxidant and/or fuel. Suitable diluents (which can be a diluent mixture) include one or more of, e.g., oxygenate (water, carbon dioxide, etc.), non-combustible species, such as molecular nitrogen (N.sub.2), and fuel impurities, such as hydrogen sulfide. For example, the oxidant can comprise 60.0 mole % to 95.0 mole % diluent and 5.0 mole % to 30.0 mole % molecular oxygen per mole of the oxidant, such as when the oxidant is air. Optionally, the oxidant has a mass ratio of diluent to molecular oxygen in the range of 0.5 to 20.0, e.g., in the range of 4.0 to 12.0.

[0045] In order to lessen or prevent the occurrence of a sharp temperature gradient in the bulk gas temperature profile at the start of pyrolysis mode and during the course of pyrolysis mode, it was expected that a relatively long-duration t.sub.H would be needed, e.g., a t.sub.H of 30 seconds, or 50 seconds. Surprisingly, this is not the case: a t.sub.H27 seconds is typically sufficient for reheating the reactor to achieve the desired bulk gas temperature profile at the start of pyrolysis mode, e.g., 25 second, such as 10 seconds, or 1 second, or 0.1 second. For example, t.sub.H can be in the range of from 0.01 second to 25 seconds, or 0.05 second to 10 seconds, or 0.05 second to 5 seconds, or 0.05 second to 1 second.

[0046] It was also expected that fuel-oxidant combustion should be distributed through the reactor's pyrolysis zone to achieve the desired non-constant bulk gas temperature profile in the pyrolysis zone during t.sub.P, and to lessen or prevent the occurrence of a sharp temperature gradient in the bulk gas temperature profile during t.sub.P. Surprisingly, it has been found that this is not the case. The desired bulk gas temperature profile for pyrolysis mode is established during heating mode by carrying out fuel-oxidant combustion primarily in the central region of the reactor (e.g., a region of length L as shown in FIG. 1). While not wishing to be bound by any theory model, it is believed that concentrating combustion in the central region of the reactor leads to an improved reactor temperature compared to that which is achieved by distributed combustion for mainly two reasons. First, the greater fuel and oxidant flow rates needed to achieve the desired amount of combustion during t.sub.H, and the resulting increased flow rate of combustion product, leads to more favorable distribution of combustion heat within the reactor. Second, during heating mode the combination of radiative heat transfer to a thermal mass proximate to the combustion zone and heat conduction within the thermal mass sufficiently moderates the reactor temperature profile so as to broaden temperature gradients in the pyrolysis zone that would otherwise be undesirably sharp.

[0047] Referring again to FIG. 1, an appropriate combustion zone of length L can be achieved by conventional methods, e.g., by use of one or more mixer-distributors 10, use of a selective combustion catalyst, etc. For example, it has been found that even when mixer-distributors and selective combustion catalysts are not used, limiting Z to a value 5.0, e.g., 3.0, and especially <2.0, results in a combustion zone length L that is 50% of that of the length of the pyrolysis zone, e.g., 40%, such as 30%, or 20%.

[0048] After the reactor is sufficiently reheated to establish the reactor temperature profile needed for the start of pyrolysis, heating mode operation is lessened or terminated, typically by decreasing or terminating fuel and oxidant flow into the reactor. Pyrolysis mode operation in then increased or commenced by introducing a pyrolysis feed into the reactor. Representative pyrolysis feeds will now be described in more detail. The invention is not limited to these pyrolysis feeds, and this description is not meant to foreclose the use of other pyrolysis feeds within the broader scope of the invention.

Representative Pyrolysis Feeds

[0049] The pyrolysis feed comprises C.sub.2+ hydrocarbon. e.g., 1 wt. % of C.sub.2+ hydrocarbon, such as 10 wt. %, or 25 wt. %, or 50 wt. %, or 75 wt. %, or 90 wt. %. Typically 90 wt. % of the remainder of the pyrolysis feed comprises diluent, e.g., one or more of methane, CO.sub.2, water, etc. In certain aspects, the pyrolysis feed consists essentially of or even consists of C.sub.2+ hydrocarbon. The pyrolysis feed's hydrocarbon (the feed hydrocarbon) generally includes any hydrocarbon compounds or mixture of hydrocarbon compounds that when subjected to the specified pyrolysis conditions produce the desired pyrolysis product. Suitable pyrolysis feeds include those disclosed in U.S. Patent Application Publication No. 2016-176781, which is incorporated by reference herein in its entirety. In certain aspects, particularly those aspects where the feed comprises 50 wt. % ethane (or propane, or a mixture of ethane and propane). e.g., 75 wt. %, such as 90 wt. %, conversion during pyrolysis is based on the amount of C.sub.2+ hydrocarbon that is converted. In other aspects, e.g., those where the feed includes components such as (i) saturated C.sub.4+ hydrocarbon and/or (ii) aromatic and/or non-aromatic cores having one or more substantially-saturated C.sub.2+ side chains, the conversion is based on the aggregate amount of C.sub.2+ hydrocarbon components converted, including such substantially saturated side chains as may be converted.

[0050] Although the feed hydrocarbon typically includes C.sub.2+ compounds which contain hydrogen and carbon only, feed hydrocarbon compounds having covalently-bound and/or non-covalently-bound heteroatoms are within the scope of the invention. When present in the feed hydrocarbon, the amount of such heteroatom-containing hydrocarbon compounds is typically 10 wt. % based on the weight of the feed's hydrocarbon. Feed hydrocarbon that is substantially-free of non-aliphatic hydrocarbon is within the scope of the invention, as is feed hydrocarbon that is substantially free of aromatic hydrocarbon and/or substantially free of olefinic hydrocarbon, particularly C.sub.2-C.sub.5 olefinic hydrocarbon. Substantially-free in this context means <1 wt. % based on the weight of the feed hydrocarbon, such as 0.1 wt. %, or 0.01 wt. %, or 0.001 wt. %.

[0051] The feed hydrocarbon can be obtained from one or more sources of hydrocarbon, e.g., from natural hydrocarbon sources including those associated with producing petroleum, or from one or more synthetic hydrocarbons sources such as catalytic and/or non-catalytic reactions. Examples of such reactions include catalytic cracking, catalytic reforming, coking, steam cracking, etc. Synthetic hydrocarbon sources include those in which hydrocarbon within a geological formation has been purposefully subjected to one or more chemical transformations. The feed can include recycle components, e.g., a portion of the pyrolysis product. Such recycle, when used, can include, e.g., methane, molecular hydrogen, and C.sub.2+ hydrocarbon, typically C.sub.2 to C.sub.5 hydrocarbon.

[0052] The feed hydrocarbon can include one or more of ethane, propane, butanes, saturated and unsaturated C.sub.6 hydrocarbon, including those derived from one or more of Fischer-Tropsch synthesis products, shale gas, biogas, associated gas, natural gas and mixtures or components thereof, steam cracked gas oil and residues, gas oils, heating oil, jet fuel, diesel, kerosene, gasoline, naphtha (including coker naphtha, steam cracked naphtha, and catalytically cracked naphtha), hydrocrackate, reformate, raffinate reformate, Fischer-Tropsch liquids, natural gasoline, distillate, virgin naphtha, crude oil, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, wide boiling range naphtha to gas oil condensates, heavy non-virgin hydrocarbon streams from refineries, vacuum gas oils, heavy gas oil, naphtha contaminated with crude, synthetic crudes, shale oils, coal liquefaction products, coal tars, tars, atmospheric resid, heavy residuum, C.sub.4residue admixture, naphtharesidue admixture, cracked feed, coker distillate streams, and hydrocarbon streams derived from plant or animal matter. The feed hydrocarbon can comprise volatile and non-volatile hydrocarbon, as described in U.S. Patent Application Publication No. 2016/176781. Those skilled in the art will appreciate that feed flow rate will depend on factors such as feed composition, reactor volume, pyrolysis conditions, etc. Accordingly, the invention can be carried out over a very wide range of feed flow rates. e.g., at a flow rate 0.001 kg's, such as 0.1 kg/s, or 10 kg/s, or 100 kg/s, or more.

[0053] Although the invention is not limited thereto, the specified process can be used for upgrading relatively refractory hydrocarbon, such as ethane. Accordingly, the feed hydrocarbon can comprise ethane in an amount 1 wt. %, e.g., 5 wt. %, such as 10 wt. %. Suitable feeds include those comprising >50 wt. % ethane, such as 75 wt. %, or 90 wt. %, or 95 wt. %. For example, the feed can comprise an amount of ethane in the range of from 1 wt. % to 99 wt. %, such as 5 wt. % to 95 wt. %, or 10 wt. % to 90 wt. %. One representative feed hydrocarbon comprises (i) 10 wt. % ethane, or 50 wt. %, or 90 wt. %, such as in the range of from 10 wt. % to 99.5 wt. % ethane, with 95 wt. % of the balance of the feed hydrocarbon comprising one or more of methane, propane, and butanes. In other aspects, the feed comprises 90 wt. % of (i) ethane and/or (ii) propane. The light paraffinic hydrocarbon can be provided from any convenient source, e.g., from synthetic and/or natural sources. Light paraffinic hydrocarbon can be provided from petroleum or petrochemical processes and/or sources of geological origin, e.g., natural gas. In particular aspects, the pyrolysis feed comprises 90 wt. % of (i) ethane and/or (ii) propane.

[0054] The pyrolysis feed optionally includes diluent, typically comprising compositions that are essentially non-reactive under the specified pyrolysis conditions. Examples of useful diluents include, but are not limited to, one or more of methane, water (e.g., steam), hydrogen, nitrogen and the noble gases, such as helium, neon and argon. Diluent present in the pyrolysis feed's source (e.g., methane and/or CO.sub.2 present in natural gas) and diluent added to the pyrolysis feed are within the scope of the invention. Diluent, when present, is typically included in the pyrolysis feed in an amount 60 wt. % based on the weight of the feed, e.g., 50 wt. %, such as 40 wt. %, or 30 wt. %, or 20 wt. %, or 10 wt. %, or in the range of from 1 wt. % to 50 wt. %. Diluent is also suitable for use as a sweep gas, e.g., for (i) removing at least a portion of any deposits in the reactor after the pyrolysis mode and/or after heating mode and/or (ii) adjusting the reactor's temperature profile-heat carried by the sweep gas from warmer regions of the reactor for transfer to cooler regions will increase the temperature of the cooler regions and further lessen or prevent sharp gradients in the reactor temperature profile.

[0055] A flow of the pyrolysis feed is conducted to the pyrolysis reactor during pyrolysis mode, typically in a reverse-flow direction, e.g., one that is opposed to that of oxidant flow. During pyrolysis mode, at least a portion of the feed hydrocarbon is pyrolysed to produce a desired pyrolysis product. Certain pyrolysis conditions that are useful for pyrolysing the specified pyrolysis feeds will now be described in more detail. The invention is not limited to these pyrolysis conditions, and this description is not meant to foreclose the use of other pyrolysis conditions within the broader scope of the invention.

Representative Pyrolysis Mode Conditions

[0056] Pyrolysis mode is operated for a time duration t.sub.P. At the start of pyrolysis mode and during t.sub.P, the reactor's pyrolysis zone exhibits a bulk gas temperature profile that results in a range of light olefin yields, typically without excessive coke yield. When heating mode is carried out under the specified conditions, the bulk gas temperature profile at the start of pyrolysis mode continuously varies over the length of the pyrolysis zone. Although the peak gas temperature in the pyrolysis zone decreases during the course of the pyrolysis, its position along the length of the pyrolysis zone and the general shape of the bulk gas temperature profile typically remain substantially the same during t.sub.P. Regions of substantially-constant temperature along the length of the pyrolysis zone are therefore avoided. Sharp gradients in the bulk gas temperature profile within the pyrolysis zone are also substantially avoided. Although the pyrolysis can be carried out at high severity, typically it is carried out at low severity.

[0057] In order to substantially maintain the shape of the bulk gas temperature profile as the peak gas temperature decreases during pyrolysis mode, the duration of pyrolysis mode t.sub.P is kept relatively short as compared to conventional reverse-flow pyrolysis reactors pyrolysing substantially similar pyrolysis feeds, particularly at a T.sub.av1400 C. e.g., 1200 C. In certain aspects, the duration of pyrolysis mode t.sub.P is 20 seconds, e.g., 15 second, such as 10 seconds, or 1 second, or 0.1 second, or 0.01 second. For example, t.sub.P can be in the range of from 110.sup.3 second to 20 seconds, e.g., 0.01 second to 15 seconds, such as 0.05 second to 5 seconds, or 0.05 second to 1 second, or 0.05 second to 0.5 second. Conventional methods can be used to achieve these ranges of t.sub.P, e.g., using one or more poppet valves and/or hydrodynamic valving, but the invention is not limited thereto. Using the specified ranges of t.sub.P avoids the appearance of pyrolysis zone segments having a substantially-constant bulk gas temperature profile. Using these t.sub.P values also substantially prevents relatively sharp temperature gradients in the pyrolysis zone. For example, at any time during the pyrolysis variations in the bulk gas temperature are typically 140 C. within any pyrolysis zone segment having a length 10% of I.sub.C, e.g., 100 C. such as 50 C. More particularly, using these t.sub.P values typically limits temperature variations to 75 C. within any segment of thermal mass 1 that has a length 10% of I.sub.B, e.g., 50 C., such as 25 C.

[0058] FIG. 2 schematically shows a representative regenerative reverse-flow reactor, similar to that shown in FIG. 1, and a representative gas temperature profile. The solid line represents the bulk gas temperature profile at the start of pyrolysis mode, and the dashed line represents the bulk gas temperature profile at the end of t.sub.P. At the start of pyrolysis mode, the flow of combustion mixture 19 is curtailed a flow of pyrolysis feed 15 is established. The reactor's pyrolysis zone encompasses the combustion zone, the shaded region 16 of thermal mass 7, and the shaded region 14 of thermal mass 1. Particularly at relatively large flow rates of fuel and/or oxidant during heating mode, and/or when t.sub.H is of relatively long duration, the peak gas temperature T.sub.p during the pyrolysis can be displaced away from the reactor's center. In such aspects, the length of the pyrolysis zone's downstream segment I.sub.A is less than that of the upstream segment I.sub.B, e.g., at least 10% less, such as at least 25% less, or at least 50% less. The total length of the pyrolysis zone I.sub.C is the sum of I.sub.A and I.sub.B. Typically, I.sub.C is in the range of from 10% to 50% of the total length of reactor 50. e.g., in the range of 20% to 40%. For example, I.sub.C can be in the range of from 20% to 40% of L.sub.1+L.sub.2+L.sub.3+L.sub.4. The location of the terminal ends of I.sub.A and I.sub.B is determined by the minimum temperature T.sub.MIN needed for appreciable pyrolysis of the selected pyrolysis feed under the specified pyrolysis conditions, e.g., in the range of about 500 C. to about 950 C.

[0059] FIG. 2 shows aspects having a slight displacement of T.sub.p away from the reactor center. FIG. 3 shows an even greater displacement, with T.sub.p being located within shaded region 14 of thermal mass 1. Other aspects of the invention are schematically illustrated in FIG. 4. In these aspects, a mixer-distributor 10 is located within the combustion zone. In such aspects, the bulk gas temperature profile at the start of pyrolysis mode (profile PR.sub.3) exhibits at least two local maxima, as does profile PR.sub.4 which represents the bulk gas temperature profile at the end t.sub.P. While not wishing to be bound by any theory or model, it is believed that the bi-modal bulk gas temperature profile results from heat radiated from the mixer distributor during heating mode toward thermal masses 1 and 7. Since the downstream end of the mixer-distributer achieves a greater temperature than its upstream end during heating mode, and since radiative heating is a relatively short-range phenomena (the inverse-square law applies), thermal mass 1 is heated more than thermal mass 7. The resultant bulk gas temperature profile PR.sub.3 in the pyrolysis zone at the start of pyrolysis mode is therefore believed to be a substantially linear combination of bulk gas temperature profile PR.sub.1, which is related to the heating of thermal mass 7, and bulk gas temperature profile PR.sub.2, which is related to the heating of thermal mass 1. As in the case of aspects illustrated by FIG. 3, greater fuel-oxidant flow rates during heating mode lead to additional heating of thermal mass 1, e.g., by convective heat transfer from the combustion product, which displaces the peak temperature of profile PR.sub.2 toward (or even into) shaded region 14. Such aspects are schematically illustrated in FIG. 5. The maximum gas temperature of profile PR.sub.1 is typically 20% to 70% of the maximum gas temperature of profile PR.sub.2, such as 30% to 70%. In FIGS. 1-5, components and streams performing similar functions have the same index number.

[0060] In aspects such as those illustrated in FIGS. 2 and 4, the pyrolysis conditions include a bulk gas temperature profile which at the start of the pyrolysis typically increases from a first temperature (T.sub.1) proximate to the first aperture 3 of thermal mass 1 to a second temperature (T.sub.2) proximate to the second aperture 5. The peak gas temperature T.sub.p, located at a position that is at or downstream of face 5, is greater than T.sub.2. T.sub.pT.sub.2 at the start of pyrolysis is typically in the range of from 10 C. to 400 C., or 25 C. to 300 C. or 50 C. to 200 C. Typically, the location of T.sub.p within the pyrolysis zone remains substantially constant during the pyrolysis. Substantially constant in this context means that the location of T.sub.p changes during pyrolysis mode from its initial position by +/20% of I.sub.c, e.g., +1-15%, such as +/10%, or typically +/5%. T.sub.1 is less than T.sub.2, with T.sub.2 at the start of pyrolysis typically being 1400 C. e.g., 1300 C., such as 1200 C. or 1100 C., or 1000 C. T.sub.2T.sub.1 at the start of pyrolysis is typically in the range of from 50 C. to 500 C., such as from 100 C. to 400 C., or 100 C. to 300 C. In particular aspects utilizing a feed comprising ethane and/or propane, the pyrolysis conditions at the start of pyrolysis include T.sub.1900 C., e.g., 750 C., such as 500 C.; T.sub.2 in the range of from 975 C. to 1100 C., T.sub.p1150 C., and T.sub.MINT.sub.1 in the range of from 10 C. to 400 C., or 25 C. to 300 C., or 50 C. to 200 C. At the start of the pyrolysis, the conversion of the feed's C.sub.2+ hydrocarbon exhibits a profile (not shown in FIG. 2) which increases from a first conversion (X.sub.1) at a reference location R.sub.1 positioned between the first and second apertures to a second conversion (X.sub.2) proximate to the second aperture, wherein X.sub.1 is in the range of from 25% to 85%, and X.sub.2 is in the range of from 65% to 98%. Reference position R.sub.1 is typically proximate to the end of the pyrolysis zone, as shown. The peak gas temperature decreases during t.sub.P, but the bulk gas temperature profile typically maintains substantially the same shape as shown. Although the bulk gas temperature profile at the start of t.sub.P is substantially congruent with that at the end of t.sub.P, the location in the pyrolysis zone at which conversion X.sub.1 is achieved translates during t.sub.P from R.sub.1 toward aperture 5 to reference position R.sub.2 at the end of t.sub.P. In particular aspects where the feed comprises ethane and/or propane, the process can include one or more of (i) X.sub.1 in the range of from 25% to 60%, (ii) T.sub.2 in the range of from 1025 C. to 1075 C., (iii) X.sub.2 in the range of from 85% to 98%, (iv) the bulk gas temperature profile includes a bulk gas temperature at the reference location in the range of from 925 C. to 975 C., and (v) the reference location R.sub.1 is positioned within 0.2*L and 0.4*L of the second aperture. More particularly, conditions at the start of the pyrolysis can include (i) an acetylene selectivity in a range of from 0% to 1% at the reference location, which acetylene selectivity increases, e.g., monotonically, to a range of 5% to 10% at the second aperture, (ii) an ethylene selectivity in a range of from 85% to 95% at the reference location, which ethylene selectivity decreases, e.g., monotonically, to a range of 70% to 85% at the second aperture, (iii) a propylene selectivity in a range of from 0.7% to 0.9% at the reference location, which propylene selectivity varies monotonically or non-monotonically to a range of 0.4% to 0.6% at the second aperture, and (iv) a butadiene selectivity in a range of from 0.5% to 1.5% at the reference location, which butadiene selectivity increases, e.g., monotonically, to a range of 4% to 5% at the second aperture.

[0061] In other aspects, shown schematically in FIGS. 3 and 5, the pyrolysis zone at time t.sub.1 (the start of t.sub.P) has a peak gas temperature T.sub.p at a position located between apertures 3 and 5. T.sub.p is >T.sub.2, and T.sub.2 is >T.sub.1. T.sub.p is typically 1400 C., e.g., in the range of from 1000 C. to 1400 C., such as from 1000 C. to 1300 C., or from 1025 C. to 1175 C. T.sub.pT.sub.2 is typically in the range of from 10 C. to 400 C., or 25 C. to 300 C., or 50 C. to 200 C. T.sub.2-T.sub.1 is typically in the range of from 50 C. to 500 C., such as from 100 C. to 400 C. T.sub.MIN is located between apertures 3 and 5. In particular aspects utilizing a feed comprising ethane and/or propane, T.sub.1 can be 900 C., e.g., 750 C., such as 500 C.; with T.sub.2 being in the range of from 975 C. to 1100 C., T.sub.p being 1150 C., and T.sub.MINT.sub.1 being in the range of from 10 C. to 400 C., or 25 C. to 300 C., or 50 C. to 200 C. In aspects where the feed comprises ethane and/or propane, T.sub.1 is typically 750 C. and T.sub.2 is in the range of from 900 C. to 1100 C. As in the aspects illustrated in FIGS. 2 and 4, the aspects illustrated in FIGS. 3 and 5 exhibit a favorable conversion profile for the feed's saturated C.sub.2+ hydrocarbon. At t.sub.1, the conversion profile continuously varies from a minimum conversion X.sub.min at a reference location between the first and second apertures to a second conversion (X.sub.2) at the second aperture. Proximate to the location of T.sub.p, the conversion profile exhibits a peak (X.sub.p), wherein X.sub.p is >X.sub.2. At t.sub.1, X.sub.2 is in a range of from 55% to 95% and X.sub.p is 98%. During t.sub.P, the conversion profile (not shown) exhibits a continuously decreasing conversion at the second aperture and a continuously decreasing X.sub.p. In particular aspects where the feed comprises ethane and/or propane, at the time t.sub.1 (i) T.sub.2 is in the range of from 925 C. to 1075 C. and (ii) X.sub.2 is in the range of from 85% to 98%. Typical pyrolysis conditions include one or more of (A) an acetylene selectivity in a range of from 5% to 10% at the second aperture at t.sub.1, which acetylene selectivity decreases into a range of about 0% to 1% at t.sub.2, (B) an ethylene selectivity in a range of from 75% to 80% at the second aperture at t.sub.1, which ethylene selectivity increases into a range of about 90% to 95% at t.sub.2, (C) a propylene selectivity in a range of from 0.7% to 0.9% at the second aperture at t.sub.1, which propylene selectivity varies by no more than about +/20% during t.sub.P, and (D) a butadiene selectivity in a range of from 4% to 5% at the second aperture at t.sub.1, which butadiene selectivity decreases into a range of about 0.% to 1.5% at t.sub.2.

[0062] In any of the foregoing aspects, the pyrolysis conditions include a feed hydrocarbon partial pressure 7 psia (48 kPa), or 10 psia (69 kPa), or 20 psia (138 kPa), or 30 psia (207 kPa). The total pressure is typically 5 psig (34 kPag), or 15 psig (103 kPag), or 40 psig (276 kPag), or 80 psig (552 kPag), or 120 psig (827 kPag). Particularly when the pyrolysis feed includes diluent, the total pressure can be 500 psig (3447 kPag), or 300 psig (2068 kPag), or 150 psig (1034 kPag). Total gas residence time in the pyrolysis zone is typically of 1.0 second, or 0.8 second; preferably 0.75 second or 0.6 second; or in the range of 0.001 second to 1.0 second, or in the range of 0.01 second to 0.8 second, or in the range of 0.01 second to 0.75 second. For example, the pyrolysis feed can be passed through thermal mass 1 at a total gas residence time at a bulk gas temperature 800 C. that is 0.100 second, such as 0.060 second, such as 0.040 second, or in the range of 0.001 second to 0.100 second, or in the range of 0.002 second to 0.060 second, or in the range of 0.002 second to 0.040 second. The pyrolysis conditions in the pyrolysis zone during t.sub.P generally include T.sub.p1400 C., T.sub.av1200 C. Typically, the pyrolysis conditions include T.sub.p1200 C., e.g., 1100 C., such as 1000 C. or in the range of from 1000 C. to 1400 C.; and T.sub.av1100 C., e.g., 1000 C., such as 900 C., or in the range of from 900 C. to 1100 C., or 925 C. to 1075 C. During pyrolysis mode (e.g., during t.sub.P), T.sub.p and/or T.sub.av (e.g., during t.sub.P) typically decrease by 100 C. or less, e.g., 75 C. such as 50 C. or 25 C., or 10 C., or 5 C. This is particularly the case when a channeled member in the pyrolysis zone has an OFA55%, e.g., 45%, such as 40%, or 30%, or in the range of from 10% to 55%, or 15% to 50%, or 20% to 45%, or 25% to 35%. Since the temperature profile during pyrolysis varies over the length of the pyrolysis zone, and typically over the entire length of the reactor, T.sub.p is >T.sub.av. Generally, when T.sub.av in the channel exceeds 800 C., it is beneficial for T.sub.av to be 60 C., when T.sub.av in the channel exceeds 900 C., it is beneficial for T.sub.av to be 50 C., when T.sub.av in the channel exceeds 1000 C., it is beneficial for T.sub.av to be 40 C., and when T.sub.av in the channel exceeds 1100 C., it is beneficial for T.sub.av to be 20 C.

[0063] By modulating bulk gas temperature over the length of the pyrolysis zone during pyrolysis mode, the pyrolysis product conducted away from the reactor comprises a range of desired hydrocarbon products, including a desirable range of C.sub.2-C.sub.5 olefin. Typically, one or more of the desired hydrocarbon compounds is separated from the pyrolysis product, e.g., for storage and/or further processing. For example, one or more of ethylene, propylene, butadiene butenes, etc. can be separated from the pyrolysis product, e.g., for recovery and use in producing products such as fuels and fuel additives, oxygenates, polymer, etc. Molecular hydrogen and methane can be separated and recovered from the pyrolysis product, e.g., as a tail gas. Light paraffinic hydrocarbon can be separated recovered, e.g., for use as a fuel, such as a fuel for heating mode. Aromatic hydrocarbon, such as one or more of benzene, toluene, and xylenes, can be separated and recovered, e.g., for producing chemical and petrochemical products including fuel, solvents, polymer, etc. Conventional separations and recovery methods can be used, e.g., those described in U.S. Patent Application Publication No. 2016/176781, but the invention is not limited thereto.

[0064] Certain representative pyrolysis products will now be described in more detail. The invention is not limited to these products, and this description is not meant to foreclose the production of other pyrolysis products within the broader scope of the invention.

Representative Pyrolysis Products

[0065] In certain aspects, the pyrolysis product conducted away from the reactor is primarily gaseous and comprises molecular hydrogen; methane; ethane; ethylene; propane; propylene; butanes; butenes; butadiene; C.sub.5 hydrocarbon, including normal, iso, and cyclo C.sub.5 olefin and paraffin, and C.sub.6+ hydrocarbon, including aromatics and normal, iso, and cyclo C.sub.6+ olefin and paraffin. For example, when utilizing one representative pyrolysis feed comprising light paraffinic hydrocarbon and representative heating mode and pyrolysis mode conditions, the pyrolysis product can comprise 2 wt. % to 10 wt. % methane, 50 wt. % to 95 wt. % ethylene, 0.2 wt. % to 1 wt. % propylene, 0.1 wt. % to 5 wt. % butadiene, and up to about 3 wt. % benzene, based on the weight of the pyrolysis product. As may be appreciated, these ver desirable compositional ranges for the identified hydrocarbon compounds are achieved not only at the start of pyrolysis mode, but during the duration of t.sub.P. This stands in sharp contrast to conventional processes operating at a gas temperature 1200 C., such as steam cracking. Since the conventional processes operate with little temperature variation in the pyrolysis zone, they produce a pyrolysis product having very narrow compositional ranges for the desired hydrocarbon compounds.

Example

[0066] In this prophetic example, a pyrolysis feed consisting essentially of ethane is exposed to the specified pyrolysis conditions in a representative reverse-flow reactor configured to be similar to the one illustrated in FIG. 2. The peak gas temperature of 1200 C. at the start of pyrolysis mode is achieved at about midway along the reactor's length (proximate to the midpoint of I.sub.C). About 40% of the reactors length is in the pyrolysis zone. The bulk gas temperature profile in the pyrolysis zone, which is similar to that illustrated in FIG. 2, exhibits a bulk gas temperature of about 900 C. at position R.sub.1 at the start of pyrolysis mode (t.sub.1), and at R.sub.2 at the end of pyrolysis mode (t.sub.2). R.sub.2 is located about midway along the length of thermal mass 1. The duration of pyrolysis mode (t=t.sub.2t.sub.1) is within the specified range. Even though T.sub.p and T.sub.av decrease during the pyrolysis, the location of T.sub.p within the reactor remains substantially constant during the pyrolysis.

[0067] The pyrolysis product is quenched inside the reactor as it flows through the unshaded segment of thermal mass 1. FIG. 6 shows the ethane conversion achieved over a segment of the reactor from location R.sub.1 to the midpoint of I.sub.C, over which distance the temperature varies from about 925 C. to 1200 C. FIGS. 7-9 show the selectivities of the desired hydrocarbon compounds over the same segment. As shown in the figures, the conversion and desired product selectivities exhibit substantially continuous variation over the segment.

[0068] All patents, test procedures, and other documents cited herein, including priority documents, are fully incorporated by reference to the extent such disclosure is not inconsistent. It is not intended that the scope of the claims appended hereto be limited to the descriptions set forth herein but rather that the claims be construed as encompassing all the features of patentable novelty which reside herein, including all features which would be treated as equivalents thereof by those skilled in the art to which this disclosure pertains. When lower and upper limits are specified, ranges from any lower limit to any upper limit are expressly within the scope of the invention. The term comprising is synonymous with the term including. When a composition, an element or a group of components is preceded with the transitional phrase comprising, the same composition or group of components is within transitional phrases consisting essentially of, consisting of, selected from the group of consisting of, or is preceding the recitation of the composition, component, or components, and vice versa.