Patent Application
20080093261
The terms “hydrocarbon,” “hydrocarbons,” and “hydrocarbonaceous” as used herein do not mean materials strictly or only containing hydrogen atoms and carbon atoms. Such terms include materials that are hydrocarbonaceous in nature in that they primarily or essentially are composed of hydrogen and carbon atoms, but can contain other elements such as oxygen, sulfur, nitrogen, metals, inorganic salts, and the like, even in significant amounts.
The term “gaseous” as used in this invention means one or more gases in an essentially vaporous state, for example, steam alone, a mixture of steam and hydrocarbon vapor, and the like.
The term “coke” as used in this invention means any high molecular weight carbonaceous solid, and includes compounds formed from the condensation of polynuclear aromatics.
An olefin producing plant useful with this invention would include a pyrolysis (thermal cracking) furnace for initially receiving and cracking the feed. Pyrolysis furnaces for steam cracking of hydrocarbons heat by means of convection and radiation, and comprise a series of preheating, circulation, and cracking tubes, usually bundles of such tubes, for preheating, transporting, and cracking the hydrocarbon feed. The high cracking heat is supplied by burners disposed in the radiant section (sometimes called “radiation section”) of the furnace. The waste gas from these burners is circulated through the convection section of the furnace to provide the heat necessary for preheating the incoming hydrocarbon feed. The convection and radiant sections of the furnace are joined at the “cross-over,” and the tubes referred to hereinabove carry the hydrocarbon feed from the interior of one section to the interior of the next.
Cracking furnaces are designed for rapid heating in the radiant section starting at the radiant tube (coil) inlet where reaction velocity constants are low because of low temperature. Most of the heat transferred simply raises the hydrocarbons from the inlet temperature to the reaction temperature. In the middle of the coil, the rate of temperature rise is lower but the cracking rates are appreciable. At the coil outlet, the rate of temperature rise increases somewhat but not as rapidly as at the inlet. The rate of disappearance of the reactant is the product of its reaction velocity constant times its localized concentration. At the end of the coil, reactant concentration is low and additional cracking can be obtained by increasing the process gas temperature.
Steam dilution of the feed hydrocarbon lowers the hydrocarbon partial pressure, enhances olefin formation, and reduces any tendency toward coke formation in the radiant tubes.
Cracking furnaces typically have rectangular fireboxes with upright tubes centrally located between radiant refractory walls. The tubes are supported from their top.
Firing of the radiant section is accomplished with wall or floor mounted burners or a combination of both using gaseous or combined gaseous/liquid fuels. Fireboxes are typically under slight negative pressure, most often with upward flow of flue gas. Flue gas flow into the convection section is established by at least one of natural draft or induced draft fans.
Radiant coils are usually hung in a single plane down the center of the fire box. They can be nested in a single plane or placed parallel in a staggered, double-row tube arrangement. Heat transfer from the burners to the radiant tubes occurs largely by radiation, hence the thermo “radiant section,” where the hydrocarbons are heated to from about 1,450° F. to about 1,550° F. and thereby subjected to severe cracking.
The initially empty radiant coil is, therefore, a fired tubular chemical reactor. Hydrocarbon feed to the furnace is preheated to from about 900° F. to about 1,000° F. in the convection section by convectional heating from the flue gas from the radiant section, steam dilution of the feed in the convection section, or the like. After preheating, in a conventional commercial furnace, the feed is ready for entry into the radiant section.
In a typical furnace, the convection section can contain multiple zones. For example, the feed can be initially preheated in a first upper zone, boiler feed water heated in a second zone, mixed feed and steam heated in a third zone, steam superheated in a fourth zone, and the final feed/steam mixture preheated to completion in the bottom, fifth zone. The number of zones and their functions can vary considerably. Thus, pyrolysis furnaces can be complex and variable structures.
The cracked gaseous hydrocarbons leaving the radiant section are rapidly reduced in temperature to prevent destruction of the cracking pattern. Cooling of the cracked gases before further processing of same downstream in the olefin production plant recovers a large amount of energy as high pressure steam for re-use in the furnace and/or olefin plant. This is often accomplished with the use of transfer-line exchangers that are well known in the art.
Radiant coil designers strive for short residence time, high temperature and low hydrocarbon partial pressure. Coil lengths and diameters are determined by the feed rate per coil, coil metallurgy in respect of temperature capability, and the rate of coke deposition in the coil. Coils range from a single, small diameter tube with low feed rate and many tube coils per furnace to long, large-diameter tubes with high feed rate and fewer coils per furnace. Longer coils can consist of lengths of tubing connected with u-turn bends. Various combinations of tubes can be employed. For example, four narrow tubes in parallel can feed two larger diameter tubes, also in parallel, which then feed a still larger tube connected in series. Accordingly, coil lengths, diameters, and arrangements in series and/or parallel flow can vary widely from furnace to furnace. Furnaces, because of proprietary features in their design, are often referred to by way of their manufacturer. This invention is applicable to any pyrolysis furnace, including, but not limited to, those manufactured by Lummus, M. W. Kellog & Co., Mitsubishi, Stone & Webster Engineering Corp., KTI Corp., Linde-Selas, and the like.
Downstream processing of the cracked hydrocarbons issuing from the furnace varies considerably, and particularly based on whether the initial hydrocarbon feed was a gas or a liquid. Since this invention uses whole crude oil and/or liquid natural gas condensate as a feed, downstream processing herein will be described for a liquid fed olefin plant. Downstream processing of cracked gaseous hydrocarbons from liquid feedstock, naphtha through gas oil for the prior art, and crude oil and/or condensate for this invention, is more complex than for gaseous feedstock because of the heavier hydrocarbon components present in the liquid feedstocks.
With a liquid hydrocarbon feedstock downstream processing, although it can vary from plant to plant, typically employs an oil quench of the furnace effluent after heat exchange of same in, for example, the transfer-line exchanger aforesaid. Thereafter, the cracked hydrocarbon stream is subjected to primary fractionation to remove heavy liquids, followed by compression of uncondensed hydrocarbons, and acid gas and water removal there from. Various desired products are then individually separated, e.g., ethylene, propylene, a mixture of hydrocarbons having four carbon atoms per molecule, fuel oil, pyrolysis gasoline, and a high purity hydrogen stream.
In accordance with this invention, a process is provided which utilizes crude oil and/or condensate liquid that has not been subjected to fractionation, distillation, and the like, as the primary (initial) feedstock for the olefin plant pyrolysis furnace in whole or in substantial part. By so doing, this invention eliminates the need for costly distillation of the condensate into various fractions, e.g., from naphtha, kerosene, gas oil, and the like, to serve as the primary feedstock for a furnace as is done by the prior art as first described hereinabove.
By this invention, the foregoing advantages (energy efficiency and capital cost reduction) while using crude oil and/or condensate as a primary feed are accomplished. In so doing, complete vaporization of the hydrocarbon stream that is passed into the radiant section of the furnace is achieved while preserving distillate fractions initially present in the liquid condensate feed essentially in the liquid state for easy separation of same from the lighter, vaporous hydrocarbons that are to be cracked.
This invention can be carried out using, for example, the apparatus disclosed in U.S. Pat. No. '961. Thus, this invention is carried out using a self-contained vaporization facility that operates separately from and independently of the convection and radiant sections, and can be employed as (1) an integral section of the furnace, e.g., inside the furnace in or near the convection section but upstream of the radiant section and/or (2) outside the furnace itself but in fluid communication with the furnace. When employed outside the furnace, crude oil and/or condensate primary feed is preheated in the convection section of the furnace, passed out of the convection section and the furnace to a standalone vaporization facility. The vaporous hydrocarbon product of this standalone facility is then passed back into the furnace to enter the radiant section thereof. Preheating can be carried out other than in the convection section of the furnace if desired or in any combination inside and/or outside the furnace and still be within the scope of this invention.
The vaporization unit of this invention (for example section 3 of U.S. Pat. No. '961) receives the condensate feed that may or may not have been preheated, for example, from about ambient to about 350F, preferably from about 200 to about 350F. This is a lower temperature range than what is required for complete vaporization of the feed. Any preheating preferably, though not necessarily, takes place in the convection section of the same furnace for which such condensate is the primary feed.
Thus, the first zone in the vaporization operation step of this invention (zone 4 in U.S. Pat. No. '961) employs vapor/liquid separation wherein vaporous hydrocarbons and other gases, if any, in the preheated feed stream are separated from those distillate components that remain liquid after preheating. The aforesaid gases are removed from the vapor/liquid separation section and passed on to the radiant section of the furnace.
Vapor/liquid separation in this first, e.g., upper, zone knocks out distillate liquid in any conventional manner, numerous ways and means of which are well known and obvious in the art. Suitable devices for liquid vapor/liquid separation include liquid knock out vessels with tangential vapor entry, centrifugal separators, conventional cyclone separators, schoepentoeters, vane droplet separators, and the like.
Liquid thus separated from the aforesaid vapors moves into a second, e.g., lower, zone (zone 9 in U.S. Pat. No. '961). This can be accomplished by external piping. Alternatively this can be accomplished internally of the vaporization unit. The liquid entering and traveling along the length of this second zone meets oncoming, e.g., rising, steam. This liquid, absent the removed gases, receives the full impact of the oncoming steam's thermal energy and diluting effect.
This second zone can carry at least one liquid distribution device such as a perforated plate(s), trough distributor, dual flow tray(s), chimney tray(s), spray nozzle(s), and the like.
This second zone can also carry in a portion thereof one or more conventional tower packing materials and/or trays for promoting intimate mixing of liquid and vapor in the second zone.
As the remaining liquid hydrocarbon travels (falls) through this second zone, lighter materials such as gasoline or naphtha that may be present can be vaporized in substantial part by the high energy steam with which it comes into contact. This enables the hydrocarbon components that are more difficult to vaporize to continue to fall and be subjected to higher and higher steam to liquid hydrocarbon ratios and temperatures to enable them to be vaporized by both the energy of the steam and the decreased liquid hydrocarbon partial pressure with increased steam partial pressure.
The pre-heated crude oil cracking feed is then passed by way of pipe (line) 10 to the aforesaid vaporization unit 11, which unit is separated into an upper vaporization zone 12 and a lower zone 13. This unit 11 achieves primarily (predominately) vaporization of at least a significant portion of the naphtha and gasoline boiling range and lighter materials that remain in the liquid state after the pre-heating step. Gaseous materials that are associated with the preheated feed as received by unit 11, and additional gaseous materials formed in zone 12, are removed from zone 12 by way of line 14. Thus, line 14 carries away essentially all the lighter hydrocarbon vapors, e.g., naphtha and gasoline boiling range and lighter, that are present in zone 12. Liquid distillate present in zone 12, with or without some liquid gasoline and/or naphtha, is removed there from via line 15 and passed into the upper interior of lower zone 13. Zones 12 and 13, in this embodiment, are separated from fluid communication with one another by an impermeable wall 16, which can be a solid tray. Line 15 represents external fluid down flow communication between zones 12 and 13. In lieu thereof, or in addition thereto, zones 12 and 13 can have internal fluid communication there between by modifying wall 16 to be at least in part liquid permeable by use of one or more trays designed to allow liquid to pass down into the interior of zone 13 and vapor up into the interior of zone 12. For example, instead of an impermeable wall 16, a chimney tray could be used in which case vapor carried by line 17 would pass internally within unit 11 down into section 13 instead of externally of unit 11 via line 15. In this internal down flow case, distributor 18 becomes optional.
By whatever way liquid is removed from zone 12 to zone 13, that liquid moves downwardly into zone 13, and thus can encounter at least one liquid distribution device 18. Device 18 evenly distributes liquid across the transverse cross section of unit 11 so that the liquid will flow uniformly across the width of the tower into contact with packing 19.
Dilution steam 6 passes through superheat zone 20, and then, via line 21 into a lower portion 22 of zone 13 below packing 19. In packing 19 liquid and steam from line 21 intimately mix with one another thus vaporizing some of liquid 15. This newly formed vapor, along with dilution steam 21, is removed from zone 13 via line 17 and added to the vapor in line 14 to form a combined hydrocarbon vapor product in line 25. Stream 25 can contain essentially hydrocarbon vapor from feed 2, e.g., gasoline and naphtha, and steam.
Stream 17 thus represents a part of feed stream 2 plus dilution steam 21 less liquid distillate(s) and heavier from feed 2 that are present in bottoms stream 26. Stream 25 is passed through a mixed feed preheat zone 27 in a hotter (lower) section of the convection zone of furnace 1 to further increase the temperature of all materials present, and then via cross over line 28 into the radiant coils (tubes) 29 in the radiant firebox of furnace 1. Line 28 can be internal or external of furnace conduit 30.
Stream 6 can be employed entirely in zone 13, or a part thereof can be employed in either line 14 and/or line 25 to aid in the prevention of the formation of liquid in lines 14 or 25.
In the radiant firebox section of furnace 1, feed from line 28 which contains numerous varying hydrocarbon components is subjected to severe thermal cracking conditions as aforesaid.
The cracked product leaves the radiant fire box section of furnace 1 by way of line 31 for further processing in the remainder of the olefin plant downstream of furnace 1 as shown in USP '961.
Section 13 of unit 11 provides surface area for contacting liquid 15 with hot gas or gasses, e.g., steam 21. The counter current flow of liquid and gas within section 13 enables the heaviest (highest boiling point) liquids to be contacted at the highest hot gas to hydrocarbon ration and with the highest temperature gas at the same time.
Pursuant to the refinery integration aspect of this invention bottoms stream 26 of unit 11, which contains a substantial amount, if not most or all, of the distillate(s) in feed 2, is passed by way of line 26 to atmospheric distillation zone (column) 32 in a crude oil refinery which, in conventional fashion, separates feed 26 into various fractions thereof such as one or more kerosene fractions 33 and 34, atmospheric gas oil 35, and an atmospheric residue 36. Bottoms 36 can be sold as a product of the process or used as a feedstock for a catalytic cracking unit or employed in the production of heavy fuel oil or any combination thereof.
In a conventional olefin production plant, the preheated feed 10 would be mixed with dilution steam 21, and this mixture would then be passed directly from preheat zone 3 into the radiant section 29 of furnace 1, and subjected to severe thermal cracking conditions. In contrast, this invention instead passes the preheated feed at, for example, a temperature of from about 200 to about 350F, into standalone unit 11 as shown in the embodiment of
In the embodiment of
The embodiment of
The addition of stream 26 to conventional crude oil feed 37 has a very distinct advantage in that the quantity of distillates 33 through 35 recovered from unit 32 is very substantially increased over what would otherwise have been recovered from the processing in unit 32 of solely crude oil feed 37. Other advantages for the integration of section 13 with the normal operation of a crude oil refinery will be apparent to one skilled in the art, and are within the scope of this invention.
In the illustrative embodiments of
Thus, if feedstock 2 boils in the range of from about 100 to about 1,350F, and contains naphtha (boiling in the range of from about 100 to about 350F) plus at least one distillate fraction (boiling, for example, mostly in the range of from about 350 to about 650F) that feed can, pursuant to this invention, be preheated in unit 3 and further heated in unit 11 to vaporize essentially all the naphtha present for removal by way of lines 14 and 17. This could thereby leave essentially only liquid distillate to be recovered by way of line 26. The temperature of operation of units 3 and 11 to achieve this result can vary widely depending on the composition of feed 2, but will generally be in the range of from about 150 to about 500F.
In the alternative, should it be desired to leave some naphtha in the liquid state with the distillate, as recovered by way of line 26, the temperature of operation of units 3, if used, and 11 can be altered to accomplish this result. When it is desired not to have essentially only distillate in stream 26, the amount of naphtha left in the liquid state for stream 26 can, with this invention, vary widely, but will generally be up to about 30 wt. % based on the total weight of naphtha, and distillates in stream 26. The temperature of operation of unit 3, if used, and unit 11 to achieve this result can vary widely depending on the composition of feed 2 and the amount of steam and pressure used, but will generally be in the range of from about 150 to about 450 F.
Stream 15 falls downwardly from zone 12 into lower, second zone 13, and can be vaporized as to any amounts of undesired liquid naphtha fractions initially present in zone 13. These gaseous hydrocarbons make their way out of unit 11 by way of line 17 due to the influence of hot gas 21, e.g., steam, rising through zone 13 after being introduced into a lower portion, e.g., bottom half or one-quarter, of zone 13 (section 22) by way of line 21.
Of course, units 3 and 11 can also be operated so as to leave some distillate in vaporous streams 14 and/or 17, if desired.
Feed 2 can enter furnace 1 at a temperature of from about ambient up to about 300F at a pressure from slightly above atmospheric up to about 100 psig (hereafter “atmospheric to 100 psig”). Feed 2 can enter zone 12 via line 10 at a temperature of from about ambient to about 500F at a pressure of from atmospheric to 100 psig.
Stream 14 can be essentially all hydrocarbon vapor formed from feed 2 and is at a temperature of from about ambient to about 400F at a pressure of from atmospheric to 100 psig.
Stream 15 can be essentially all the remaining liquid from feed 2 less that which was vaporized in pre-heater 3 and is at a temperature of from about ambient to about 500F at a pressure of from slightly above atmospheric up to about 100 psig (hereafter “atmospheric to 100 psig”).
The combination of streams 14 and 17, as represented by stream 25, can be at a temperature of from about 170 to about 400 F at a pressure of from atmospheric to 100 psig, and contain, for example, an overall steam/hydrocarbon ratio of from about 0.1 to about 2, preferably from about 0.1 to about 1, pounds of steam per pound of hydrocarbon.
Stream 28 can be at a temperature of from about 900 to about 1,100F. at a pressure of from atmospheric to 100 psig.
Liquid distillate 26 can contain essentially only middle distillate boiling range and heavier components, or can be a mixture of such components and lighter components found in streams 14 and/or 17. Distillate stream 26 can be at a temperature of less than about 550F at a pressure of from atmospheric to 100 psig.
In zone 13, dilution ratios (hot gas/liquid droplets) will vary widely because the composition of condensate varies widely. Generally, the hot gas 21, e.g., steam, to hydrocarbon ratio at the top of zone 13 can be from about 0.1/1 to about 5/1, preferably from about 0.1/1 to about 1.2/1, more preferably from about 0.1/1 to about 1/1.
Steam is an example of a suitable hot gas introduced by way of line 21. Other materials can be present in the steam employed. Stream 6 can be that type of steam normally used in a conventional cracking plant. Such gases are preferably at a temperature sufficient to volatilize a substantial fraction of the liquid hydrocarbon 15 that enters zone 13. Generally, the gas entering zone 13 from conduit 21 will be at least about 350F, preferably from about 650 to about 1,000F at from atmospheric to 100 psig. Such gases will, for sake of simplicity, hereafter be referred to in terms of steam alone.
Stream 17 can be a mixture of steam and hydrocarbon vapor that has a boiling point lower than about 350F. It should be noted that there may be situations where the operator desires to allow some distillate to enter stream 17, and such situations are within the scope of this invention. Stream 17 can be at a temperature of from about 170 to about 450F at a pressure of from atmospheric to 100 psig.
Packing and/or trays 19 provide surface area for the steam entering from line 21. Section 19 thus provides surface area for contacting down flowing liquid with up flowing steam entering from line 21. The counter current flow within section 13 enables the heaviest (highest boiling point) liquids to be contacted at the highest steam to oil ratio and, at the same time, with the highest temperature steam.
It can be seen that steam from line 21 does not serve just as a diluent for partial pressure purposes as does diluent steam that may be introduced, for example, into conduit 2 (not shown). Rather, steam from line 21 provides not only a diluting function, but also additional vaporizing energy for the hydrocarbons that remain in the liquid state. This is accomplished with just sufficient energy to achieve vaporization of heavier hydrocarbon components and by controlling the energy input. For example, by using steam in line 21, substantial vaporization of feed 2 liquid is achieved. The very high steam dilution ratio and the highest temperature steam are thereby provided where they are needed most as liquid hydrocarbon droplets move progressively lower in zone 13.
Unit 11, instead of being a standalone unit outside furnace 1, can be physically contained within the interior of convection zone of that furnace so that zone 13 is wholly within the interior of furnace 1. Although total containment of unit 11 within a furnace may be desirable for various furnace design considerations, it is not required in order to achieve the benefits of this invention. Unit 11 could also be employed wholly or partially outside of the furnace and still be within the spirit of this invention. Combinations of wholly interior and wholly exterior placement of unit 11 with respect to furnace 1 will be obvious to those skilled in the art and also are within the scope of this invention.
A natural gas condensate stream 5 characterized as Oso condensate from Nigeria is removed from a storage tank and fed directly into the convection section of a pyrolysis furnace 1 at ambient conditions of temperature and pressure. In this convection section, this condensate initial feed is preheated to about 350F at about 60 psig, and then passed into a vaporization unit 11 wherein a mixture of gasoline and naphtha gases at about 350F and 60 psig are separated from distillate liquids in zone 12 of that unit. The separated gases are removed from zone 12 for transfer to the radiant section of the same furnace for severe cracking in a temperature range of 1,450° F. to 1,550° F. at the outlet of radiant coil 29.
The hydrocarbon liquid remaining from feed 2, after separation from accompanying hydrocarbon gases aforesaid, is transferred to lower section 13 and allowed to fall downwardly in that section toward the bottom thereof. Preheated steam 21 at about 1,000F is introduced near the bottom of zone 13 to give a steam to hydrocarbon ratio in section 22 of about 0.5. The falling liquid droplets are in counter current flow with the steam that is rising from the bottom of zone 13 toward the top thereof. With respect to the liquid falling downwardly in zone 13, the steam to liquid hydrocarbon ratio increases from the top to bottom of section 19.
A mixture of steam and naphtha vapor 17 at about 340F is withdrawn from near the top of zone 13 and mixed with the gases earlier removed from zone 12 via line 14 to form a composite steam/hydrocarbon vapor stream 25 containing about 0.5 pounds of steam per pound of hydrocarbon present. This composite stream is preheated in zone 27 to about 1,000F at less than about 50 psig, and introduced into the radiant firebox section of furnace 1.
Bottoms product 26 of unit 11 is removed at a temperature of about 460 F, and pressure of about 60 psig, and passed to atmospheric distillation unit 32 which is operated at an overhead temperature of about 250 F at about 3 psig to allow the removal from unit 32 of separate streams containing light kerosene boiling in the range of from about 330 to about 450 F, heavy kerosene boiling in the range of from about 450 to about 540 F, and atmospheric gas oil boiling in the range of from about 540 to about 650 F. The bottoms stream 36 is removed from unit 32 is removed at a temperature of about 650 F and pressure of about 5 psig.
It can be seen from the foregoing that this invention provides for the efficient separation of straight run naphtha boiling range and lighter material from whole crude oil, natural gas condensate, and mixtures thereof, while the separation of naphtha and lighter materials is integrated directly into the thermal cracking process to produce olefins in an energy and capital cost efficient manner, and while preserving the heavier materials for integration directly into the crude oil refining process to produce middle distillate boiling range components. One result of the refinery integration feature of this invention is the production from a refinery atmospheric distillation unit of light and heavy kerosene fractions that are best used directly in jet fuel and diesel fuel production. A further result of the refinery integration feature of this invention is the use of the atmospheric distillation unit bottoms as feed for a vacuum distillation unit for maximum upgrading. Vacuum gas oil from the vacuum distillation unit can be sent to a fluid catalytic cracking unit for gasoline production. This maximizes, for example, the efficient utilization of the crude oil feed by cracking the low octane straight run naphtha in a pyrolysis cracking furnace, separating the less abundant straight run middle distillate components, and maximizing high octane gasoline production through the use of vacuum gas oils as feed to a catalytic cracking unit.
