PROCESS FOR STEAM CRACKING OF HYDROCARBON FEEDSTOCKS

CONTAINING ASPHALTENES

FIELD OF THE INVENTION [0001] The present invention relates to a process for the cracking of hydrocarbons present in hydrocarbon feedstocks containing asphaltenes, wherein the asphaltenes are removed from the feedstocks before the hydrocarbons undergo steam cracking.

DESCRIPTION OF THE PRIOR ART [0002] Steam cracking has long been used to crack various hydrocarbon feedstocks. Conventional steam cracking utilizes a pyro lysis furnace which has two main sections: a convection section and a radiant section. The hydrocarbon feedstock typically enters the convection section of the furnace as a liquid (except for light feedstocks which enter as a vapor) wherein it is typically heated and then vaporized by indirect contact with hot flue gas from the radiant section and by direct contact with steam. The vaporized feedstock and steam mixture is then introduced into the radiant section where the cracking takes place. The cracked product can include olefins such as ethylene, propylene, butenes, and butadiene and aromatics such as benzene, toluene, and xylenes. [0003] Conventional steam cracking systems have been effective for cracking high- quality feedstock, which contain a large fraction of volatile hydrocarbons, such as gas oil and naphtha. However, steam cracking economics sometimes favor cracking lower cost heavy feedstocks such as, by way of non-limiting examples, crude oil and atmospheric resid. Some of the heavy feedstocks, e.g., crude oil and atmospheric resid, can contain asphaltenes in an amount greater than 2 ppm by weight. Also, contaminates and full range vacuum gas oil (FRVGO) can contain up to 10 percent by weight of asphaltenes. Since asphaltenes do not vaporize, but decompose to form coke when heated above 600 ⁰ F (315°C), the asphaltenes present in these feedstocks lay down as a foulant in the convection section of conventional pyrolysis furnaces, which increases pressure drop across the convection section. In addition, deposited coke can cause erosion of the metal tubing and associated metal elements during decoking operations. Only very low levels of asphaltenes can be tolerated in the convection section downstream of the point where the lighter components have vaporized. Additionally, during transport some naphthas are contaminated with heavy crude oil containing asphaltenes. Conventional pyrolysis furnaces do not have the flexibility to process crudes, or

many resid or crude contaminated gas oils or naphthas, which are contaminated with asphaltenes.

[0004] Asphaltenes represent a wide variety of hydrocarbon molecules that are typically poly-nuclear-aromatics in nature with some degree of alkylation present and which may or may not contain heteroatoms such as oxygen, nitrogen, and sulfur and metal atoms in their structures. Asphaltenes are dark brown to black high molecular weight hydrocarbons that have no definite boiling point, and when heated, usually leave a carbonaceous coke residue. Asphaltenes are generally determined in accordance with ASTM D6560. [0005] Various techniques have been employed for treating petroleum hydrocarbon feeds for the removal of coke precursors, such as asphaltenes, contained therein in order to render the feed suitable for steam pyro lysis. One such technique, as exemplified in U.S. Patent 3,617,493, involves vaporizing materials contained in crude oil that boil below 450 ⁰ F (232°C), i.e., naphtha fraction, by passing the crude oil through the convection section of a pyrolysis furnace to form a vapor fraction, separating the vapor fraction from the liquid fraction, and feeding the vapor fraction into a pyrolysis furnace to crack the hydrocarbons to olefins. Another effort, as exemplified in U.S. Patent 4,715,946, involves a solvent extraction pretreatment of the hydrocarbon feed to remove asphaltenes. This technique involves breaking the existing equilibrium between asphaltenes and the maltene surrounding medium by addition of a solvent which decreases the viscosity and, overall, the interfacial tension of the oil medium. A further attempt as exemplified in U.S. Patent 4,065,379, involves the thermal pre -treatment of resids to yield a heavy hydrocarbon, then catalytically hydrotreating a portion of the heavy hydrocarbon feedstock before the steam cracking step. These processes all improve the cracking of hydrocarbon feeds containing asphaltenes, however, in most instances, the processes suffer from high capital and operating costs due to the processing steps used, high capitol expense of the separation equipment, and/or unsatisfactory reduction in the amount of asphaltenes in the feeds.

[0006] The present invention provides a process for the steam cracking of a hydrocarbon feedstock containing asphaltenes that uses a cost effective technique for removing asphaltenes from the feedstock before the feedstock undergoes steam cracking. SUMMARY OF THE INVENTION

[0007] The present invention relates to the cracking of hydrocarbons and is based on the inventors' recognition that when asphaltene-containing hydrocarbon feedstocks are heated to sufficient temperature to form a vapor phase and a liquid phase, asphaltenes, which are non-

volatile and remain in the liquid phase, can be separated from the hydrocarbon feedstocks using low cost equipment.

[0008] The present invention is a process for the cracking a hydrocarbon feedstock containing asphaltenes, wherein the asphaltenes are removed from the feedstock before the hydrocarbons undergo cracking. The process comprises: (a) heating an asphaltene-containing hydrocarbon feedstock upstream with respect to a plurality of pyrolysis furnaces comprising at least two pyrolysis furnaces to a temperature sufficient to form a vapor phase that is essentially free of asphaltenes and a liquid phase containing asphaltenes; (b) separating the vapor phase from the liquid phase; (c) transferring at least a portion of the separated vapor phase in parallel flow to at least two of the plurality of pyrolysis furnaces; and (d) cracking at least a portion of the hydrocarbons present in the separated vapor phase in the at least two of the plurality of pyrolysis furnaces to form a cracked product.

[0009] In another embodiment, the present invention comprises: (a) heating an asphaltene-containing hydrocarbon feedstock in a heating unit to a temperature sufficient to form a vapor phase that is essentially free of asphaltenes and a liquid phase containing asphaltenes, the heating unit being located upstream with respect to a plurality of pyrolysis furnaces comprising at least two pyrolysis furnaces; (b) separating the vapor phase from the liquid phase in a separation unit; (c) transferring at least a portion of the separated vapor phase in parallel flow to at least two of the plurality of pyrolysis furnaces; (d) heating the at least a portion of the separated vapor phase in the convection section of the at least two of the plurality of pyrolysis furnaces; and (e) cracking at least a portion of the hydrocarbons contained in the separated vapor phase in the radiant section of the at least two of the plurality of pyrolysis furnaces to form a cracked product. [0010] In a further embodiment, the present invention further comprises condensing the separated vapor phase that is essentially free of asphaltenes, storing the condensed hydrocarbons, and subsequently using the condensed hydrocarbons in steam cracking. BRIEF DESCRIPTION OF THE DRAWING

[0011] The FIGURE is a simplified process flow diagram illustrating an embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] As used herein, the expression "essentially free of asphaltenes" means that concentration of asphaltenes in the vapor phase is reduced to an extremely low level. Those skilled in the art know that it is difficult to obtain a complete separation of asphaltenes from

hydrocarbon feedstock such as crude oil. As a result, the vapor phase may contain a trace amount of asphaltenes. Therefore, in the context of the present invention, while it is the objective that the vapor phase contain no asphaltenes, it is recognized that the vapor phase may contain a trace amount of asphaltenes, e.g., an amount of 2 ppm (by weight) or less, but still be essentially free of asphaltenes. The vapor phase preferably contains less than 1 ppm by weight of asphaltenes. More preferably, the vapor phase contains less than 0.5 ppm by weight of asphaltenes.

[0013] The FIGURE is a simplified schematic flow diagram of a non-limiting embodiment of the invention. The addition of steam at various points in steam cracking is known in the art and, for simplicity, is not shown in the FIGURE or described in detail herein.

[0014] Referring to the FIGURE, a hydrocarbon feedstock containing asphaltenes is sent via line 1 to heat unit zone 3. Any hydrocarbon feedstock containing asphaltenes can advantageously be utilized in the process. Examples of such feedstock include one or more of steam cracked gas oil and residues, gas oils, heating oil, jet fuel, diesel, kerosene, gasoline, coker naphtha, steam cracked naphtha, catalytically cracked naphtha, hydrocrackate, reformate, raffinate reformate, Fischer-Tropsch liquids, Fischer-Tropsch gases, natural gasoline, distillate, virgin naphtha, crude oil, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, wide boiling range naphthas, heavy non-virgin hydrocarbon streams from refineries, FRVGO, heavy gas oil, naphtha contaminated with crude, atmospheric resid, heavy residium, C4's/residue admixture, contaminated condensate, and naphtha residue admixture.

[0015] The hydrocarbon feedstock will usually have a nominal end boiling point of at least 500 ⁰ F (260 ⁰ C). Preferred hydrocarbon feedstocks include crude oil, atmospheric resids, contaminated condensate, and FRVGO.

[0016] The amount of asphaltenes present in the hydrocarbon feedstock will vary depending upon the feedstock. For example, contaminates, FRVGOs, and petroleum crude oils often contain relatively high levels of asphaltene molecule, i.e., up to 10 percent by weight of asphaltenes. The hydrocarbon feedstock used in the process of the present invention will usually contain asphaltenes in an amount of from about 5 to about 400 ppm by weight.

[0017] In heat unit 3, the hydrocarbon feedstock is heated to a temperature that is sufficient to form a vapor phase and a liquid phase. The heating of the hydrocarbon

feedstock is not limited to any particular technique. For example, the heating can be conducted by means of a heat exchanger, steam injection, or a fired heater. Although the temperature to which the hydrocarbon feedstock is heated will vary depending upon composition of the hydrocarbon feedstock, usually the feedstock is heated to a temperature at which at least significantly greater than 50 percent of the feedstock vaporizes, e.g., from about 400 to about 1000 ⁰ F (204 to 538°C). Preferably, the hydrocarbon feedstock is heated to a temperature from about 500 to 860 ⁰ F (260 to 460 ⁰ C). Since the asphaltenes contained in the hydrocarbon feedstock are essentially non-volatile, they remain in the liquid phase. The amount of vapor/liquid phase is a function of both the hydrocarbon partial pressure and the temperature to which the hydrocarbon feedstock is heated. Usually about 50 to about 98 percent by weight of the heated feedstock will be in the vapor phase and, preferably, about 90 to 98 by weight percent of the heated feedstock will be in the vapor phase. [0018] The heated feedstock is transferred via line 5 to separation unit 7, where the vapor phase is separated from the liquid phase. Examples of equipment suitable for separating the vapor phase from the liquid phase include knock-out drums and flash drums. It is important to effect the separation so that the vapor phase is essentially free of asphaltenes. Otherwise, the asphaltenes entrained in the vapor phase will be carried into the pyrolysis furnace and cause coking problems. [0019] Heat unit 3 and separation unit 7 are located upstream with respect to the pyrolysis furnaces. Although the heat unit and separation unit are shown in the FIGURE as separate units, they can be combined into a single unit ("heat/separation unit"). Examples of suitable heat/separation units include distillation towers as well as knock-out drums and flash drums having a means within the drum for heating the hydrocarbon feedstock. Examples of suitable techniques for heating of the hydrocarbon feedstock contained within the heat/separation unit include injecting steam into the hydrocarbon feedstock present in the heat/separation unit and heaters immersed into the hydrocarbon feedstock present in the heat/separation unit. Additionally and preferably, fired heaters are used to heat the hydrocarbon feedstock. [0020] Although the heat unit, separation unit, and heat/separation unit are each shown in the FIGURE as a single unit, each of these units can comprise a plurality of units, e.g., separation unit can include more than one knock-out drum or flash drum.

[0021] It is preferred to maintain a predetermined constant ratio of vapor to liquid in the separation unit or, as the case may be, the heat/separation unit, but such ratio is difficult to measure and control. However, the temperature of the heated feedstock before separation can

be used as an indirect parameter to measure, control, and maintain an approximately constant vapor to liquid ratio in the unit. Ideally, when the feedstock temperature is higher, more volatile hydrocarbons will be vaporized and become available, as part of the vapor phase, for cracking. However, when the feedstock temperature is too high, asphaltenes could be present in the vapor phase and carried over to the convection furnace tubes, eventually coking the tubes. If the temperature of the heated feedstock is too low, this can result in a low ratio of vapor to liquid with more volatile hydrocarbons remaining in the liquid phase and not be available for cracking. [0022] The temperature of the heated feedstock is also dependent upon the composition of the hydrocarbon feedstock. If the feedstock contains higher amounts of lighter hydrocarbons, the temperature of the feedstock will be lower. If the feedstock contains a higher amount of less-volatile or non- volatile hydrocarbons, the temperature of the feedstock will be higher. For example, with respect to FRVGO, the temperature of the heated feedstream will usually be maintained in the range of from about 500 ⁰ F (260 ⁰ C) to about 950 ⁰ F (510 ⁰ C).

[0023] In addition to temperature, it is usually also desirable to maintain a substantially constant hydrocarbon partial pressure in order to maintain a constant ratio of vapor to liquid in the separation vessel. Typically, the hydrocarbon partial pressures for the heated feedstream are dependent upon the amount of steam present in the feed. [0024] The amount of vapor phase produced can vary widely which allows a wide flow rate range. For example, the vapor phase flow rate can vary from a vapor flow rate that has a partial furnace load to a flow rate that provides sufficient load to a plurality of furnaces. Still further, vapor phase flow rate can exceed the furnace load with excess material being condensed and stored for subsequent use in steam cracking. For example, the condensed material can be stored for at least a day, week or even longer. The determination of total furnace load is known to persons skilled in the art. For example, it can be calculated from the heat requirements for pyrolysis in the radiant section of the furnaces as well as heat requirements in the convection section. Additional fuel combustion provides the heat needed in the convection section. Pyrolysis capacity is sometimes limited by the heat output capabilities of the furnace and efficiency with which that heat is utilized. Improved heat transfer in both the radiant and convection sections will allow total pyrolysis throughput to be increased.

[0025] The liquid phase is withdrawn from separation zone 7 as a bottoms stream via line 9. This material can be sold as fuel oil or processed, e.g., subjected to fluidized catalytic cracking (FCC), to produce higher value products. The liquid phase may also contain resins in addition to asphaltenes. Resins differ from the asphaltenes primarily in having lower molecular weight, less polynuclear aromatics, more solubility in aliphatic hydrocarbons, and lower in metal content.

[0026] The vapor phase is withdrawn from separation unit 7 as an overhead stream via line 11 and passed in parallel flow via lines 13 and 15 to a plurality of pyro lysis furnaces, which are shown in the FIGURE as pyro lysis furnaces 17 and 19. Although two pyro lysis furnaces are shown in the FIGURE, three or more pyrolysis furnaces, e.g., four to ten pyrolysis furnaces, can be used. Alternatively, the vapor phase essentially free of asphaltenes can be removed via line 21, cooled or condensed to a liquid in cooling unit 23, and then transferred via line 25 to storage unit 27. Although the cooling unit and storage unit are each shown in the FIGURE as a single unit, they can comprise a plurality of units, e.g., storage unit can comprise a plurality of tanks. The condensed liquid (or a portion thereof) can be transferred from storage unit 27 via line 29 to line 11 and passed in parallel flow via lines 13 and 15 to pyrolysis furnaces 17 and 19.

[0027] The feed is passed via line 13 to convection section 31 of pyrolysis furnace 17 and via line 15 to convection section 33 of pyrolysis furnace 19. The amount of feed transferred to pyrolysis furnace 17 and pyrolysis furnace 19 will usually be an amount that is in excess to the furnace load of either pyrolysis furnace 17 or pyrolysis furnace 19. The feed is heated in convection sections 31 and 33 to a temperature sufficient for limited cracking to occur. The preferred temperature of the feed entering radiant section 35 of pyrolysis furnace 17 and radiant section 37 of pyrolysis furnace 19 is usually in the range of from about 425 to about 700 ⁰ C (797 to 1293°F). After the feed is heated in convection section 31 of pyrolysis furnace 17 and convection section 33 of pyrolysis furnace 19, the feed heated in convection section 31 is transferred via line 39 to unit 41 and the feed heated in convection section 31 via line 43 to unit 45. In units 41 and 45, adjustments can be made to the heated feed. [0028] Next, the heated feed is transferred from unit 41 via line 46 to radiant section 35 and from unit 45 via line 47 to radiant section 37 where the hydrocarbons are cracked into different products. Typical conditions include a residence time from 0.1 to 1.0 seconds and a pressure from 7 to 40 psig (48 to 276 kPa). The cracked product leaves radiant section 35 via

line 49 and radiant section 37 via line 51. The effluent is thereafter quenched and product molecules are recovered.

[0029] The effluent contains gaseous hydrocarbons of great variety, e.g., from methane to tar. These gaseous hydrocarbons can be saturated, monounsaturated, polyunsaturated, and aromatics. The cracked gas also contains significant amounts of hydrogen.

[0030] While the present invention has been described and illustrated with respect to certain embodiments, it is to be understood that the invention is not limited to the particulars disclosed and extends to all equivalents within the scope of the claims.