Coker operation without recycle

Abstract

A process for coking a heavy oil feedstock with elimination of recycle is disclosed. In a preferred embodiment, heavy hydrocarbon feed is directly passed to the coking vessels, coker overhead vapors are combined and passed directly to a fractionator and fractionator bottoms are recovered as product for further processing in other refining systems. Distillate coker product is not used to reduce the heavy hydrocarbon feed viscosity or to manage coke fouling in the coker furnace.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to the field of heavy hydrocarbon refining by the delayed coking process.

BACKGROUND OF THE INVENTION

[0002] This invention relates to an improved method for delayed coking and involves elimination of all recycle, in particular, heavy coker gas oil and fractionator bottoms recycle to improve heavy hydrocarbon feed throughput. Delayed coking is a well-known oil refining process that is used to convert very heavy hydrocarbon feed stocks into useful liquid fuel products. In this process the heavy hydrocarbon feed is heated rapidly to cracking temperatures at elevated pressure and fed into a coke drum. The heated feed drops in pressure as it enters the coke drum causing lower boiling range components to vaporize. The larger molecules in the heated feed rapidly crack into lower boiling volatile components, leaving behind a solid carbonaceous material known in the art as petroleum coke or, simply, coke.

[0003] The volatile component, a vaporous hydrocarbon mixture, is fed overhead to a fractionator vessel which serves the dual purpose of separating (fractioning) the volatile component into coker products and serving as a coker feed surge drum. Coker system fractionator products typically include process gas, light and heavy naphtha, light and heavy gas oil, and fractionator bottoms. Heavier components of the vapor mixture that condense before reaching the fractionator trays fall to the bottom of the fractionator where they are mixed with incoming fresh feed. These heavier components that condense into the fresh feed in the fractionator are known as “natural recycle”. Lighter components condense on the fractionator trays, are collected and fed to other refining systems for further processing. As this process continues, coke accumulates in the active drum until it is filled to a safe level, at which time the heated feed is diverted to a “sister” coke drum to continue the above process. The full drum is isolated from the coking system and the accumulated coke is removed to prepare the drum for repetition of the above described cycle.

[0004] A conventional coking process is illustrated in FIG. 1. This conventional process utilizes two coke drums for coking a heavy feedstock, and a fractionator for separating a vapor product from the coke drums and for recovering one or more liquid distillate products. Fresh coker feedstock from line 10 passes through heat exchangers 12 and 14, where it is preheated. The fresh coker feedstock may be unstable to heating, and, as such, may cause deposits to form on the heat exchange surfaces during heating. To minimize this coke deposition, a portion of coker distillate product 86 is combined with the fresh feed through line 88 prior to preheating in heat exchanger 12. Amounts of distillate are generally from about 2 to about 50 parts by volume of distillate per 100 parts of fresh feed, and preferably about 5 to about 30 parts for most cases. The preheated feed is then introduced through line 16 to the bottom of coker fractionator 22, where it is combined with the bottoms from the fractionator. Alternatively, a portion of the heavy coker gas oil 18 may be combined with the fresh feed 10 through line 80.

[0005] Fractionator bottoms 26 are heated in furnace 28 and passed from line 30 via valve 90 and into either coke drum 36 through conduit 32 or into coke drum 38 through conduit 34. Feed to the coke drum 36 is a mixture of fresh feed 10, heavy gas oil recycle product 20, distillate product recycle 88, and bottoms product from fractionator 22. The heavy coker gas oil recycle 20 may be combined with the fractionator bottoms product either internal to the fractionator 22 (as shown), or externally into line 26. The heavy gas oil comes from several sources. As shown in FIG. 1, heavy coker gas oil is withdrawn from fractionator 22 via line 18, and a portion of the heavy gas oil is returned to the fractionator via line 20 where it is utilized to knock down entrained material and condense the heavier components of the vapor in fractionator 22. Heavy gas oil 18 withdrawn from the fractionator 22 is also used to quench the vapor overhead product 50 through line 24 and to condense the heavier boiling material in the overhead product 50.

[0006] The feed for coking is thus passed from the fractionator through furnace 28 for heating the feedstock to coking temperatures and from there, alternatively, to the coke drums. The mixture 26 is heated in furnace 28 to temperatures normally in the range of about 850° F. to 1100° F., and preferably in the range of 900° F. to 975° F. A furnace that heats the mixture rapidly to such temperatures, such as a pipe still, is normally used. The mixture exits the furnace through line 30 at substantially the above-indicated temperatures and is introduced into the bottom of coke drum 36. The mixture is charged to the coke drum at pressures usually ranging between about 20 to 200 psig, though higher pressures may be used if desired. The coke drum is insulated and may also be heated, such as by introduction of hot gas and vapor from a sister vessel into the drum, so as to maintain the drum's contents at a temperature in the range of about 800° F. to about 1200° F., more usually 750° F. to 950° F. Inside the drum the heavy hydrocarbon in the mixture thermally cracks to form cracked vapors and coke.

[0007] The vapors are continuously removed overhead from the drum through line 40. Coke accumulates in the drum until it reaches a predetermined safe level at which time the feed to the drum is shut off and switched to the alternate coke drum 38. The operation of drum 38 is identical to that of drum 36. This switching permits drum 36 to be taken out of service, opened, and the accumulated coke removed therefrom using conventional techniques. The hydrocarbon vapors that are taken overhead from the coke drum(s) are carried by line 50 to a fractionator 22. Even though the coker vessels 36 and 38 are operated alternately the overhead hydrocarbon vapor products flow continuously via line 50 to the fractionator 22.

[0008] After the Coke drum 36 is filled with coke, the feed 30 is redirected to the alternate coke drum 38, steam 92 is immediately introduced to drum 36 to strip out any remaining hydrocarbon liquid. The drum is stripped with steam and the resulting vapor is fed to the fractionator through line 50 for a period of time, then the vapors are redirected to the blowdown system 48 via line 44. Heavier oils stripped out of the drum to the blowdown system are condensed. This condensed material is then pumped into the feed stream 16 through line 58 on the way to the fractionator 22 and, thus, represents a source of recycle.

[0009] Coke in drum 36 is removed by a drilling operation. This process is well known, and does not require detailed explanation here. After drum 36 is drilled and is empty of coke, the drum is preheated, using product vapor from drum 38. This may be accomplished by diverting a portion of the vapor from drum 38 through drum 36 in a reverse direction through line 96. The resulting flow of condensed liquid and uncondensed vapor exiting through the bottom of the drum through line 52 is routed to a coke condensate drum 54. Vapor leaves the coke condensate drum 54 through a balance line 56 with the fractionator 22. Liquid 98 is routed from the coke condensate drum 54 into the blowdown system 48, and from there via line 58 into the feed stream 16. This represents another source of recycle.

[0010] Likewise, after the Coke drum 38 is filled with coke, the feed 30 is redirected to the first coke drum 36. Steam 94 is immediately introduced to drum 38 in order to strip out any remaining hydrocarbon liquid. The drum is stripped with steam to the fractionator through line 50 via line 42 for a period of time and then the drum vapors are redirected to the blowdown system 48 via line 46. Preheating drum 38 using product vapor from drum 36, with condensed liquid and uncondensed vapor out of the drum bottom of vessel 38 being routed to the coke condensate drum 54 is accomplished in the way described above for drum 36. However, the product vapor stream (corresponding to line 96) and the condensed liquid and uncondensed vapor stream (corresponding to line 52) are not illustrated in FIG. 1.

[0011] Products recovered from fractionator 22 include heavy Naphtha 86, light naphtha 76 and a process gas overhead product 70. In FIG. 1, a distillate stream in line 78 is recovered from fractionation, and stripped using stripper 82. The bottoms from the stripper are cooled in exchanger 12 and recovered as distillate product 86. A portion of the distillate product is combined with fresh feed via line 88, using the method previously described. The overhead from stripper 82 is returned as reflux to fractionator 22 via line 84. A fractionator overhead product 62 is cooled in exchanger 64 and passed, via line 66 to separation zone 68. A portion of separation zone liquid 72 is returned as reflux to the fractionator 22 via line 74, and at least a portion of the remainder recovered as light naphtha through line 76. A process gas overhead product 70 is also recovered from separation zone 68.

[0012] As is evident from the above description, conventional coking systems use substantial amounts of “recycle” in the coker feed, principally to reduce viscosity of the feed and minimize fouling of the coker furnace. However, recycle in the feed effectively reduces coking capacity and the production of valued light hydrocarbons; thus, there is a need in the industry to improve the coking process to enhance recovery of valued, light hydrocarbons. In this regard, the process taught in U.S. Pat. No. 4,394,250 includes adding a catalyst and hydrogen to the coker feed to facilitate production of high amounts of useful light products. However, '250 also includes recycle of the bottoms from the fractionator to the coker feed.

[0013] Similarly, U.S. Pat. No. 4,455,219 describes a conventional coking process in which feed to the coker includes fresh feed, heavy fractionator bottoms recycle and coker gas oil added as a diluent to minimize recycle of heavier fractions. As noted above, the heavy coker gas oil is a cracked product from the coking reactions in the coke drum. This gas oil can be processed elsewhere in the refinery for the production of useful liquid products; it does not require additional reaction in the coker. Thus, the addition of the heavy coker gas oil in the coker feed consumes coke drum capacity which is more economically utilized for the raw, heavy feed which must be coked.

[0014] Whereas, the '219 patent does not resolve the problem of heavy coker gas oil in the coker feed U.S. Pat. No. 4,518,487 directly addresses the problem and purports to eliminate fractionator bottom and heavy coker gas oil recycle. In '487, feed to the coker furnace is not combined with fractionator bottoms; instead a higher boiling range distillate is combined with the fresh feed in amounts necessary to prevent fouling of the furnace tubes. This higher boiling range distillate is drawn off the coker product stream and, thus represents recycle, although not of heavy fractionator bottoms.

[0015] The present invention further improves upon '487 and provides for complete elimination of all coker product recycle and thereby increases throughput of feed to the delayed coker process resulting in greater and mores economic recovery of valued light hydrocarbon products. Coker furnace tube fouling is minimized and managed by non-recycle methods, such as by on-line steam spalling.

SUMMARY OF THE INVENTION

[0016] The present invention is directed to a process for coking a heavy oil feedstock with elimination of all of recycle. Conventional coking processes require recycle of lighter distillate product streams to reduce the viscosity of feed streams, to reduce carbon deposition in heating zones, and to facilitate recovery of liquids from high temperature streams exiting the coking vessel or within fractionation zones. However, adding recycle streams to the coking vessel effectively reduces the amount of heavy oil feed stocks that may be processed for coke production.

[0017] The process of the present invention provides for a greater throughput of heavy oil feedstock in a coking process. Among other factors, the present invention is based on the discovery of methods for increasing recovery of products from a coking process rather than recycling the products back to the process, as in the prior art. In one preferred embodiment, the entire bottoms product from the coking fractionator is recovered as a product stream rather than being recycled to the coking process for additional coking. In another preferred embodiment, fresh feed is first passed through a surge drum then directly to the coking drums without passing through the coker fractionator. In another preferred embodiment, liquid products from the coking process are passed directly to the fractionator rather than being blended with the fresh coker feed. Thus, the most important and unique aspects of the present invention include: (1) directly passing heavy hydrocarbon feed to coking vessels; (2) combining coker overhead vapors; (3) passing the overhead vapor combination directly to a fractionator and (4) recovering fractionator bottoms as product for further processing in other refining systems.

[0018] Accordingly, the present invention provides a method for coking a heavy hydrocarbon feed comprising:

[0019] (a) directly feeding the heavy hydrocarbon feed through at least one pre-heater to a coking furnace;

[0020] (b) heating the feed in the coking furnace to coking temperatures;

[0021] (c) alternately feeding the heated feed directly to a first coke vessel and a second coke vessel to form a first hydrocarbon vapor product and a first coke accumulation in the first vessel and a second coke accumulation in the second vessel.

[0022] Another aspect of the invention comprises combining coker overhead vapor and liquid products from a blowdown system and condenser in a quench step and feeding the combination to the fractionator for separation into coker products. In a preferred embodiment, the product stream recovered from the coker fractionator includes coker fractionator bottoms, heavy coker gas oil, light coker gas oil, a jet fuel cut, light naphtha, heavy naphtha, and process gas.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 illustrates the prior art coking process and depicts various points in the prior art process where product recycle takes place.

[0024] FIG. 2 illustrates a specific embodiment of the present invention and depicts the elimination of coker product recycle.

DETAILED DESCRIPTION OF THE INVENTION

[0025] Suitable hydrocarbon feed stocks for delayed coking are described in the art. The feedstock may be derived from petroleum, shale, coal, tar and/or other hydrocarbon sources. It is typically heavy, low-grade oil such as heavy virgin crude, reduced crude, topped crude, residua from refining processes such as thermal or catalytic cracking processes or blends of such stocks. These feed stocks may be hydrotreated, if desired, before being fed to the coking process to remove sulfur, metals, and other contaminants.

[0026] One embodiment of the present invention is illustrated in FIG. 2. In the present invention, a surge drum or tank 104 is for temporary in-line storage of the fresh coker feed 110 and serves the purpose of absorbing or minimizing sudden changes in the pressure or flow rate of feed into the coking system. Surge drum 104 has a capacity of between 0.1 and 100 minutes of feed throughput, based on the design feedrate of the coking process. The fresh feed 110 is fed to at least one heat exchanger 114. Then, in contrast to the conventional process, the preheated feed is passed through the surge drum 104 and line 126 to furnace 128 and heated therein to coking temperatures. From the furnace 128, the heated feed is passed via line 130, through valve 190 and into either coke drum 136 through conduit 132 or into coke drum 138 through conduit 134. In the preferred process of the present invention, the fresh feed is not passed to the fractionation column 122, nor does it contain bottoms product from the fractionator 122. More preferably, the heated coker feedstock 130 does not contain any fractionator bottoms product, heavy coker gas oil or any coker product recycle.

[0027] Furnace 128, typically a pipe still type furnace, heats the feedstock mixture to coking temperatures, normally in the range of about 850° F. to 1100° F., and preferably in the range of 900° F. to 975° F. The mixture exits the furnace 128 through line 130 and is alternately introduced into the bottom of either coke drum 136 or 138, at substantially the above-indicated temperatures and at pressures usually ranging between about 20 to 200 psig, though higher pressures may be used if desired. The coke drum is insulated and may also be pre-heated, such as by introduction of hot gas and vapor from the sister vessel into the drum, so as to maintain the drum's contents at a temperature in the range of about 800° F. to about 1200° F., more usually 750° F. to 950° F. Inside the drum, the heavy hydrocarbon in the mixture thermally cracks to form cracked hydrocarbon vapors and coke.

[0028] The vapors are continuously removed overhead from the active drum through either line 140 or 142. The vapors that are taken overhead from the coke drum(s) are carried by line 150 to a fractionator 122. Coke accumulates in the active drum until it reaches a predetermined level at which time the feed to the drum is shut off and switched to the second, sister coke drum 138. The operation of drum 138 is identical to that of drum 136. This switching permits drum 136 to be taken out of service, opened, and the accumulated coke removed therefrom using conventional techniques.

[0029] After the coke drum 136 is filled with coke, and the feed 130 is redirected to the second coke drum 138, steam is immediately introduced through 192 to remove any remaining hydrocarbon liquid. The steam-stripped liquid is passed to fractionator 122 through line 150 for a period of time and then the drum vapors are redirected to the blowdown system 148 via line 144. Heavier oils stripped out of the drum to the blowdown system are condensed. While the prior art processes use coker gas oil recovered from the fractionator for quenching the vapors, the present process provides that the oil accumulation in the blowdown system 148 is injected into the coke drum overhead vapor 150 as quench, thus delivering this oil to the fractionator 122 for further distillation and product recovery. This arrangement provides the refiner the capability of increasing the quench 158 of the coke drum overhead vapor 150 and reducing the flash zone temperature in the fractionator 122 without increasing recycle to the coking zones. Increased quench of the overhead vapor will further reduce coking of the products streams in the fractionator, thus extending fractionator run time between turnarounds.

[0030] After drum 136 is drilled and is empty of coke, the drum is preheated, using product vapor from drum 138. This may be accomplished by diverting a portion of the vapor from drum 138 through line 196 into drum 136. The resulting flow of condensed liquid and uncondensed vapor out the drum bottom through line 152 is routed to coke condensate drum 154. Vapor leaves the coke condensate drum 154 through a balance line 156 and into fractionator 122. Liquid 198 is routed from the coke condensate drum 154 into the blowdown system 148, from where it is combined with other liquid products, e.g. stream 144, for quenching the vapors in stream 150 through stream 158.

[0031] Likewise, after the Coke drum 138 is filled with coke, the feed 130 is redirected to the first coke drum 136. Steam 194 is immediately introduced to drum 138. The liquid produced during the steam stripping operation is passed to fractionator 122 through lines 142 and 150 for a period of time and then the drum vapors are redirected to the blowdown system 148 through line 146. Preheating drum 138 using product vapor from drum 136, with condensed liquid and uncondensed vapor out of the drum bottom of vessel 138 being routed to the coke condensate drum 154 is accomplished in the way described above for drum 136.

[0032] Products recovered from fractionator 122 may include a bottoms product 108, a heavy coker gas oil product 118, a light gas oil product 119, a jet fuel distillate 120 heavy naphtha 121, a light naphtha 176 and a process gas overhead product 170. The heavy coker gas oil is typically returned to the fractionator 122 via line 116 to cool the fractionator bottoms product and reduce coke formation in the fractionator 122. Boiling ranges of the various product fractions are broadly defined in Table I, below.

Product                   Boiling range
Bottoms Product 108       >650° F.
Heavy Coker Gas Oil 118   650° F.-1150° F.
Light Coker Gas Oil 119   350° F.-750° F.
Jet Fuel Distillate 120   250° F.-570° F.
Heavy Naphtha Product 121 180° F.-400° F.
Light Naphtha Product 176 50° F.-250° F.
Process Gas 170           <100° F.

[0033] The above description of preferred embodiments of the invention is intended to be descriptive and not limiting as to the scope of the invention, which is defined by the following claims.

Claims

1. A method for coking a heavy hydrocarbon feed comprising: (a) directly feeding the heavy hydrocarbon feed through at least one pre-heater to a coking furnace; (b) heating the feed in the coking furnace to coking temperatures; (c) alternately feeding the heated feed directly to a first coke vessel and a second coke vessel to form a first hydrocarbon vapor product and a first coke accumulation in the first vessel and a second coke accumulation in the second vessel.

2. The method of claim 1 further comprising, alternately steam stripping the first and the second coke accumulations to form a second hydrocarbon vapor product and feeding the second hydrocarbon vapor product to a blowdown system to form a first condensate.

3. The method of claim 2 further comprising, alternately emptying the first coke accumulation from the first coke vessel and the second coke accumulation from the second vessel and alternately feeding the second hydrocarbon vapor through the empty vessel to a condensate drum to form a second condensate.

4. The method of claim 3 further comprising, combining the first condensate, the second condensate and the first hydrocarbon vapor product to form a quenched combination and feeding the quenched combination to the fractionator to form one or more hydrocarbon products.

5. The method of claim 1 wherein the heavy hydrocarbon feed is heated to temperatures in the range of about 850° F. to about 1100° F.

6. The method of claim 5 wherein the heavy hydrocarbon feed is heated to temperatures in the range of about 900° F. to about 975° F.

7. The method of claim 4 wherein the hydrocarbon products are selected from the group consisting of coker fractionator bottoms, heavy coker gas oil, light coker gas oil, a jet fuel cut, light naphtha, heavy naphtha and process gas.

8. The method of claim 7 wherein the heavy coker gas oil product has a boiling range between about 650° F. and 1150° F.;

9. The method of claim 7 wherein the light coker gas oil product has a boiling range between about 350° F. and 750° F.;

10. The method of claim 7 wherein the jet fuel product has a boiling range between about 250° F. and 570° F.;

11. The method of claim 7 wherein the heavy naphtha product has a boiling range between about 180° F. and 400° F.;

12. The method of claim 7 wherein the light naphtha product has a boiling range between about 50° F. and 250° F.;

13. The method of claim 7 wherein the process gas product has a boiling range less than about 100° F.;

14. The method of claim 7 wherein the coker fractionator bottoms product has a boiling range above about 650° F.

15. The method of claim 1 wherein step (a) further comprises feeding the heavy hydrocarbon feed through a surge drum before feeding to the coker furnace.

16. The method of claim 7 further comprising feeding the coker fractionator bottoms product to other refining processes.

17. The method of claim 7 further comprising feeding at least a portion of the heavy coker gas oil to the fractionator.

18. The method of claim 18 further comprising feeding the entire stream of heavy coker gas oil to the fractionator.