EP2074195A2 - Absorption recovery processing of fcc-produced light olefins - Google Patents

Abstract

Processing schemes and arrangements are provided for the processing a heavy hydrocarbon feedstock (212) via hydrocarbon cracking processing with selected hydrocarbon fractions being obtained via absorption-based product recovery.

Description

ABSORPTION RECOVERY PROCESSING OF FCC-PRODUCED LIGHT OLEFINS

BACKGROUND OF THE INVENTION

[0001] This invention relates generally to hydrocarbon processing and, more particularly, to the processing of hydrocarbon-containing materials having a high light olefin content, such as produced or formed in or by the cracking of a heavy hydrocarbon feedstock. [0002] Light olefins serve as feed materials for the production of numerous chemicals. Light olefins have traditionally been produced through the processes of steam or catalytic cracking of hydrocarbons such as derived from petroleum sources. Fluidized catalytic cracking (FCC) of heavy hydrocarbon streams is commonly carried out by contacting a starting material whether it be vacuum gas oil, reduced crude, or another source of relatively high boiling hydrocarbons with a catalyst such as composed of finely divided or particulate solid material. The catalyst is transported in a fluid-like manner by transmitting a gas or vapor through the catalyst at sufficient velocity to produce a desired regime of fluid transport. Contact of the oil with the fluidized material catalyzes the cracking reaction.

[0003] The cracking reaction typically deposits coke on the catalyst. Catalyst exiting the reaction zone is commonly referred to as being "spent", i.e., partially deactivated by the deposition of coke upon the catalyst. Coke is comprised of hydrogen and carbon and can include, in trace quantities, other materials such as sulfur and metals such that may enter the process with the starting material. The presence of coke interferes with the catalytic activity of the spent catalyst. It is believed that the coke blocks acid sites on the catalyst surface where the cracking reactions take place. Spent catalyst is traditionally transferred to a stripper that removes adsorbed hydrocarbons and gases from catalyst and then to a regenerator for the purpose of removing the coke by oxidation with an oxygen-containing gas. An inventory of catalyst having a reduced coke content, relative to the spent catalyst in the stripper, hereinafter referred to as regenerated catalyst, is collected for return to the reaction zone. Oxidizing the coke from the catalyst surface releases a large amount of heat, a portion of which escapes the regenerator with gaseous products of coke oxidation generally referred to as flue gas. The balance of the heat leaves the regenerator with the regenerated catalyst. The fluidized catalyst is continuously circulated between the reaction zone and the regeneration zone. The fluidized catalyst, as well as providing a catalytic function, serves as a vehicle for the transfer of heat from zone to zone. FCC processing is more fully described in US 5,360,533 to Tagamolila et al., US 5,584,985 to Lomas, US 5,858,206 to Castillo and US 6,843,906 to Eng, the contents of each of these patents are hereby incorporated herein by reference. Specific details of the various contact zones, regeneration zones, and stripping zones along with arrangements for conveying the catalyst between the various zones are well known to those skilled in the art.

[0004] The FCC reactor serves to crack gas oil or heavier feeds into a broad range of products. Cracked vapors from an FCC unit enter a separation zone, typically in the form of a main column, that provides a gas stream, a gasoline cut, light cycle oil (LCO) and clarified oil (CO) which includes heavy cycle oil (HCO) components. The gas stream may include dry gas, i.e., hydrogen and Ci and C₂ hydrocarbons, and liquefied petroleum gas ("LPG"), i.e., C₃ and C₄ hydrocarbons, also sometimes commonly referred to as wet gas. [0005] In view of an increasing need and demand for light olefins such as ethylene and propylene for various petrochemical uses such as for the production of polyethylene, polypropylene and the like as well as the desire to produce relatively less of heavier olefins such as butylenes and pentenes which are generally less desirable as gasoline blending components due to environmental considerations, it may be desired to practice the cracking reaction processing of heavy hydrocarbon feedstock to increase the relative amount of light olefins in the resulting product slate. [0006] Research efforts have led to the development of an FCC process that produces or results in greater relative yields of light olefins, i.e., ethylene and propylene. Such processing is more fully described in US 6,538,169 to Pittman et al., the contents of which are hereby fully incorporated herein by reference. As disclosed therein, a hydrocarbon feed stream can desirably be contacted with a blended catalyst comprising regenerated catalyst and coked catalyst. The catalyst has a composition including a first component and a second component. The second component comprises a zeolite with no greater than medium pore size wherein the zeolite comprises at least 1 wt-% of the catalyst composition. The contacting occurs in a riser to crack hydrocarbons in the feed stream and obtain a cracked stream containing hydrocarbon products including light olefins and coked catalyst. The cracked stream is passed out of an end of the riser such that the hydrocarbon feed stream is in contact with the blended catalyst in the riser for less than or equal to 2 seconds on average. [0007] In view of the increasing need and demand for light olefins such as ethylene and propylene, there is a need and a demand for improved processing and arrangements for the separation and recovery of light olefins from such FCC process effluent.

SUMMARY OF THE INVENTION

[0008] A general object of the invention is to provide an improved process and system for catalytically cracking a heavy hydrocarbon feedstock and obtaining selected hydrocarbon fractions.

[0009] The general object of the invention can be attained, at least in part, through a specified process such as involves contacting a heavy hydrocarbon feedstock with a hydrocarbon cracking catalyst in a fluidized reactor zone to produce a hydrocarbon effluent comprising a range of cracked hydrocarbon products including light olefins. In accordance with one preferred embodiment, the hydrocarbon cracking catalyst is desirably of a composition that includes a first component comprising a large pore molecular sieve and a second component comprising a zeolite with no greater than medium pore size, with the zeolite with no greater than medium pore size comprising at least 1.0 wt-% of the catalyst composition. The hydrocarbon effluent is separated in a separation section to form at least one separator liquid stream and a separator vapor stream. The at least one separator liquid stream includes C₃+ hydrocarbons. The separator vapor stream includes C₃- hydrocarbons. The separator vapor stream is contacted with a first absorption solvent in an absorption zone to remove C₃+ hydrocarbons therefrom and form a process stream that includes C₂- hydrocarbon materials. C₂- hydrocarbon materials can desirably be stripped from the at least one separator liquid stream to form a C₃+ hydrocarbon process stream substantially free of C₂- hydrocarbons. C₅+ hydrocarbon materials are separated from the C₃+ hydrocarbon process stream to form a first product process stream that includes C₅+ hydrocarbon materials and a second product process stream that includes C₃ and C₄ hydrocarbons. At least a first portion of the first product stream is desirably introduced into the absorption zone as at least a portion of the first absorption solvent.

[0010] The prior art generally fails to provide processing schemes and arrangements for obtaining light olefins via the catalytic cracking of a heavy hydrocarbon feedstock in an as effective and efficient a manner as may be desired. More particularly, the prior art generally fails to provide such processing schemes and arrangements that advantageously utilize absorption-based product recovery to produce or otherwise obtain process streams containing specifically desired ranges of hydrocarbons.

[0011] A process for catalytically cracking a heavy hydrocarbon feedstock and obtaining selected hydrocarbon fractions, in accordance with another embodiment involves contacting a heavy hydrocarbon feedstock with a blended cracking catalyst that includes regenerated catalyst and coked catalyst in a fluidized reactor zone at hydrocarbon cracking reaction conditions to produce a hydrocarbon effluent stream that includes a range of hydrocarbon products including light olefins. The catalyst desirably is of a composition that includes a first component comprising a large pore molecular sieve and a second component comprising a zeolite with no greater than medium pore size. The zeolite with no greater than medium pore size comprises at least 1.0 wt-% of the catalyst composition.

[0012] The process further involves separating the hydrocarbon effluent in a separation section to form at least one separator high pressure liquid stream and a separator high pressure vapor stream. The at least one separator high pressure liquid stream comprises C₃+ hydrocarbons. The separator high pressure vapor stream comprises C₃- hydrocarbons. The separator high pressure vapor stream is contacted with a first absorption solvent in a primary absorber to form a first primary absorber process stream that includes primarily C₂- hydrocarbons and residual amounts of C₃+ hydrocarbons. The first primary absorber process stream is contacted with a second absorption solvent to form a process stream including C₂- hydrocarbon materials and a process stream including residual C₃+ hydrocarbons and the second absorption solvent. C₂- hydrocarbon materials are stripped from the separator high pressure liquid stream to form a C₃+ hydrocarbon process stream substantially free of C₂- hydrocarbons. C₅+ hydrocarbon materials are separated from the C₃+ hydrocarbon process stream to form a first product process stream including C₅+ hydrocarbon materials and a second product process stream including C₃ and C₄ hydrocarbons. The process further involves introducing at least a first portion of the first product stream to the primary absorber as a majority of the first absorption solvent.

[0013] A system for catalytically cracking a heavy hydrocarbon feedstock and obtaining selected hydrocarbon fractions is also provided. In accordance with one preferred embodiment, such as system includes a fluidized reactor zone wherein the heavy hydrocarbon feedstock contacts a blended catalyst including regenerated catalyst and coked catalyst at hydrocarbon cracking reaction conditions to produce a cracked effluent stream containing hydrocarbon products including light olefins.

[0014] The system also includes a separation section for separating the cracked effluent stream to form at least one separator liquid stream and a separator vapor stream. The at least one separator liquid stream comprises C₃+ hydrocarbons. The separator vapor stream comprises C₃- hydrocarbons.

[0015] The system further includes an absorption zone to absorb C₃+ hydrocarbons from the separator high pressure vapor stream in a first absorption solvent and to form an absorption zone effluent stream that includes C₂- hydrocarbons, including ethylene. A stripper is provided for stripping C₂- hydrocarbon materials from the separator liquid stream to form a C₃+ process stream substantially free of C₂- hydrocarbons. A debutanizer is provided for separating C₅+ hydrocarbon materials from the C₃+ hydrocarbon process stream to form a first process stream including C₅+ hydrocarbon materials and a second process stream including C₃ and C₄ hydrocarbons. The system also includes a process line for introducing at least a first portion of the first product stream to the absorption zone as a majority of the first absorption solvent.

[0016] As used herein, references to "light olefins" are to be understood to generally refer to C₂ and C₃ olefins, i.e., ethylene and propylene, alone or in combination. [0017] References to light olefin materials or appropriate process streams as being "substantially free of carbon dioxide" are to be understood to generally refer to such light olefin materials or process streams as desirably generally containing less than 100 ppm of carbon dioxide, preferably containing less than 10 ppm of carbon dioxide and, more preferably, desirably containing less than 1 ppm of carbon dioxide. [0018] References to a process stream as "ethylene-rich" are to be understood to generally refer to such process streams as generally contain at least 20 percent ethylene and, in accordance with at least certain preferred embodiments alternatively contain at least 25 percent ethylene, at least 30 percent ethylene, at least 35 percent ethylene, at least 40 percent ethylene or 40 to 60 percent ethylene. [0019] References to "C_x hydrocarbon" are to be understood to refer to hydrocarbon molecules having the number of carbon atoms represented by the subscript "x". Similarly, the term "C_x-containing stream" refers to a stream that contains C_x hydrocarbon. The term "C_x+ hydrocarbons" refers to hydrocarbon molecules having the number of carbon atoms represented by the subscript "x" or greater. For example, "C₄+ hydrocarbons" include C₄, C₅ and higher carbon number hydrocarbons. The term "C_x- hydrocarbons" refers to hydrocarbon molecules having the number of carbon atoms represented by the subscript "x" or fewer. For example, "C₄- hydrocarbons" include C₄, C₃ and lower carbon number hydrocarbons. [0020] Other objects and advantages will be apparent to those skilled in the art from the following detailed description taken in conjunction with the appended claims and drawing.

BRIEF DESCRIPTION OF THE DRAWING

[0021] The Figure is a simplified schematic diagram of a system for catalytic cracking a heavy hydrocarbon feedstock and obtaining selected hydrocarbon fractions, including light olefins via an absorption-based product recovery, in accordance with one preferred embodiment.

DETAILED DESCRIPTION

[0022] Processing schemes and arrangements are provided for effectively and efficiently processing a heavy hydrocarbon feedstock via hydrocarbon cracking processing with selected hydrocarbon fractions being obtained via absorption-based product recovery.

[0023] The Figure schematically illustrates a system, generally designated by the reference numeral 210, for catalytic cracking a heavy hydrocarbon feedstock and obtaining light olefins via absorption-based product recovery, in accordance with one embodiment of the invention. Those skilled in the art and guided by the teachings herein provided will recognize and appreciate that the illustrated system has been simplified by the elimination of various usual or customary pieces of process equipment including some heat exchangers, process control systems, pumps, fractionation systems, and the like. It may also be discerned that the process flow depicted in the figures may be modified in many aspects without departing from the basic overall concept of the invention. [0024] In the system 210, a suitable heavy hydrocarbon feedstock stream is introduced via a line 212 into a fluidized reactor zone 214 wherein the heavy hydrocarbon feedstock contacts with a hydrocarbon cracking catalyst zone to produce a hydrocarbon effluent comprising a range of hydrocarbon products, including light olefins. [0025] Suitable fluidized catalytic cracking reactor zones for use in the practice of such an embodiment may, as is described in above-identified US 6,538,169 to Pittman et al., include a separator vessel, a regenerator, a blending vessel, and a vertical riser that provides a pneumatic conveyance zone in which conversion takes place. The arrangement circulates catalyst and contacts feed in a specifically described manner.

[0026] More specifically and as described therein, the catalyst typically comprises two components that may or may not be on the same matrix. The two components are circulated throughout the entire system. The first component may include any of the well-known catalysts that are used in the art of fluidized catalytic cracking, such as an active amorphous clay-type catalyst and/or a high activity, crystalline molecular sieve. Molecular sieve catalysts are preferred over amorphous catalysts because of their much-improved selectivity to desired products. Zeolites are the most commonly used molecular sieves in FCC processes. Preferably, the first catalyst component comprises a large pore zeolite, such as a Y-type zeolite, an active alumina material, a binder material, comprising either silica or alumina and an inert filler such as kaolin. [0027] The zeolitic molecular sieves appropriate for the first catalyst component should have a large average pore size. Typically, molecular sieves with a large pore size have pores with openings of greater than 0.7 nm in effective diameter defined by greater than 10 and typically 12 membered rings. Pore Size Indices of large pores are above 31. Suitable large pore zeolite components include synthetic zeolites such as X-type and Y-type zeolites, mordenite and faujasite. It has been found that Y zeolites with low rare earth content are preferred in the first catalyst component. Low rare earth content denotes less than or equal to 1.0 wt-% rare earth oxide on the zeolite portion of the catalyst. Octacat™ catalyst made by W. R. Grace & Co. is a suitable low rare earth Y-zeolite catalyst. [0028] The second catalyst component comprises a catalyst containing, medium or smaller pore zeolite catalyst exemplified by ZSM-5, ZSM-11, ZSM- 12, ZSM-23, ZSM-35, ZSM-38, ZSM-48, and other similar materials. US 3,702,886 describes ZSM-5. Other suitable medium or smaller pore zeolites include ferrierite, erionite, and ST-5, developed by Petroleos de Venezuela, S.A. The second catalyst component preferably disperses the medium or smaller pore zeolite on a matrix comprising a binder material such as silica or alumina and an inert filer material such as kaolin. The second component may also comprise some other active material such as Beta zeolite. These catalyst compositions have a crystalline zeolite content of 10 to 25 wt-% or more and a matrix material content of 75 to 90 wt-%. Catalysts containing 25 wt-% crystalline zeolite material are preferred. Catalysts with greater crystalline zeolite content may be used, provided they have satisfactory attrition resistance. Medium and smaller pore zeolites are characterized by having an effective pore opening diameter of less than or equal to 0.7 nm, rings of 10 or fewer members and a Pore Size Index of less than 31. [0029] The total catalyst composition should contain 1-10 wt-% of a medium to small pore zeolite with greater than or equal to 1.75 wt-% being preferred. When the second catalyst component contains 25 wt-% crystalline zeolite, the composition contains 4 to 40 wt-% of the second catalyst component with a preferred content of greater than or equal to 7 wt-%. ZSM-5 and ST-5 type zeolites are particularly preferred since their high coke resistivity will tend to preserve active cracking sites as the catalyst composition makes multiple passes through the riser, thereby maintaining overall activity. The first catalyst component will comprise the balance of the catalyst composition. The relative proportions of the first and second components in the catalyst composition will not substantially vary throughout the FCC unit. [0030] The high concentration of the medium or smaller pore zeolite in the second component of the catalyst composition improves selectivity to light olefins by further cracking the lighter naphtha range molecules. But at the same time, the resulting smaller concentration of the first catalyst component still exhibits sufficient activity to maintain conversion of the heavier feed molecules to a reasonably high level. [0031] The relatively heavier feeds suitable for processing in accordance herewith include conventional FCC feedstocks or higher boiling or residual feeds. A common conventional feedstock is vacuum gas oil which is typically a hydrocarbon material prepared by vacuum fractionation of atmospheric residue and which has a broad boiling range of from 315° to 622°C (600° to 1150⁰F) and, more typically, which has a narrower boiling point range of from 343° to 551°C (650° to 1025⁰F). Heavy or residual feeds, i.e., hydrocarbon fractions boiling above 499°C (930⁰F), are also suitable. The fluidized catalytic cracking processing the invention is typically best suited for feedstocks that are heavier than naptha range hydrocarbons boiling above 177°C (350⁰F). [0032] The effluent or at least a selected portion thereof is passed from the fluidized reactor zone 214 through a line 216 into a hydrocarbon separation system 220, such as includes a main column section 222 and a staged compression section 224. The main column section 222 may desirably include a main column separator with an associated main column overhead high pressure receiver wherein the fluidized reactor zone effluent can be separated into desired fractions including a main column vapor stream, such as passed through a line 226, and a main column liquid stream, such as passed through a line 230. [0033] To facilitate illustration and discussion, other fraction lines such as including a heavy gasoline stream, a light cycle oil ("LCO") stream, a heavy cycle oil ("HCO") stream and a clarified oil ("CO") stream, for example, may not here be shown nor hereinafter specifically described.

[0034] The main column vapor stream line 226 is introduced into the staged compression section 224, such as constituting a two-stage compression. The staged compression section 224 results in the formation of a high pressure separator liquid stream in a line 232 and a high pressure separator vapor stream in a line 234. While the pressure of such high pressure liquid and high pressure vapor can vary, in practice such streams are typically at a pressure in the range of 1375 to 2100 kPag (200 to 300 psig). The compression section 224 may also result in the formation of a stream of spill back materials largely composed of heavier hydrocarbon materials and such as can be returned to the main column section 222 via a line 235.

[0035] The high pressure separator liquid stream includes C₃+ hydrocarbons and is substantially free of carbon dioxide. The high pressure separator vapor stream includes C₃- hydrocarbons and typically includes a quantity of carbon dioxide. [0036] The separator vapor stream line 234 is introduced into an absorption zone, generally designated by the reference numeral 236, via a line 237. The absorption zone 236 includes a primary absorber 240 wherein the separator vapor stream contacts with a debutanized gasoline material provided by the line 242 and the main column liquid stream provided by the line 230 to absorb C₃+ and separate C₂ and lower boiling fractions from the gas to the primary absorber 240. In general, the absorption zone 236 includes a primary absorber that suitably includes a plurality of stages with at least one and preferably two or more intercoolers interspaced therebetween to assist in achieving desired absorption. In practice, such a primary absorber typically includes five absorber stages between each pair intercoolers. Thus, a primary absorber to achieve desired absorption in accordance with one preferred embodiment desirably includes at least 15 ideal stages with at least 2 intercoolers appropriately spaced therebetween. In another preferred embodiment, a suitable preferred primary absorber to achieve desired absorption desirably includes at least 20 ideal stages with at least 3 intercoolers appropriately spaced therebetween. In yet another preferred embodiment, a suitable preferred primary absorber to achieve desired absorption desirably includes at 20 to 25 ideal stages with 4 or more intercoolers appropriately spaced therebetween. While the broader practice of the invention is not necessarily so limited, in at least certain preferred embodiments, it has been found advantageous to employ propylene as a refrigerant in one or more of such primary absorber the intercoolers to assist in achieving the desired absorption.

[0037] C₃+ hydrocarbons absorbed in or by the debutanized gasoline and main column liquid can be passed via a line 243 for further processing in accordance with the invention as later described herein. [0038] The off gas from the primary absorber 240 passes via a line 244 to a secondary or sponge absorber 246. The secondary absorber 246 contacts the off gas with light cycle oil from a line 250. Light cycle oil absorbs most of the remaining C₄ and higher hydrocarbons and returns to the main fractionators via a line 252. A stream of C₂- hydrocarbons is withdrawn as off gas from the secondary or sponge absorber 246 in a line 254 for further treatment as later described herein.

[0039] The separator liquid stream in the line 232 and contents from the line 243 are passed through a line 260 into a stripper 262 which removes most of the C₂ and lighter gases in a line 264. In practice, such a stripper can desirably be operated at a pressure in the range of 1650 to 1800 kPag (240 to 260 psig) with a C₂/C₃ molar ratio in the stripper bottoms of less than 0.001 and preferably with a C₂/C₃ molar ratio in the stripper bottoms of less than 0.0002 to 0.0004.

[0040] As shown, C₂ and lighter gases in the line 264 can desirably be combined with high pressure separator vapor from the line 234 to form the line 237 that is fed into the primary absorber 240. The stripper 262 supplies a liquid C₃+ stream via a line 266 to a debutanizer 270. A suitable such debutanizer, in accordance with one preferred embodiment, includes a condenser (not specifically shown) that desirably operates at a pressure in the range of 965 to 1105 kPag (140 to 160 psig), with no more than 5 mol-% C₅ hydrocarbons in the overhead and no more than 5 mol-% C₄ hydrocarbons in the bottoms. More preferably, the relative amount of C₅ hydrocarbons in the overhead is less than 1 to 3 mol-% and the relative amount of C₄ hydrocarbons in the bottoms is less than 1 to 3 mol-%.

[0041] A stream of C₃ and C₄ hydrocarbons from the debutanizer 270 are taken overhead by a line 272 for further treatment, such as later described herein. A line 274 withdraws a stream of debutanized gasoline from the debutanizer 270. In accordance with one preferred embodiment, the stream of debutanized gasoline returned to the primary absorber 240 via the line 242 serves as the majority of the first absorption solvent therein.

[0042] Another portion of the stream of debutanized gasoline is passed in a line 276 to a naptha splitter 280.

[0043] In accordance with one preferred embodiment, the naptha splitter 280 is desirably in the form of a dividing wall separation column, such as having a dividing wall 281 positioned therewithin. Such a dividing wall separation column naptha splitter is desirably effective to separate the debutanized gasoline introduced therein into a light fraction stream comprising compounds containing five to six carbon atoms, an intermediate fraction stream comprising compounds containing seven to eight carbon atoms, and a heavy fraction stream comprising compounds containing more than eight carbon atoms. More specifically, such a dividing wall separation column may generally operate at a condenser pressure in the range of 34 to 104 kPag (5 to 15 psig) and, in accordance with one embodiment operated at a condenser pressure of 55 to 85 kPag (8 to 12 psig).

[0044] Such light, intermediate and heavy fraction streams, are appropriately passed via corresponding lines 282, 284, and 286, respectively, for further processing or product recovery, as may be desired. [0045] Returning to the processing of the stream of C₂- hydrocarbons withdrawn from the secondary or sponge absorber 246 in the line 254, such stream materials can be passed through a further compression section 290 to form a line 292 that is passed into a compression or discharge drum 294. The discharge drum 294 forms a knockout stream generally composed of heavy components (e.g., C₃+ hydrocarbons that liquefy in the discharge drum 294) and such as withdrawn in a line 296. The discharge drum 294 also forms an overhead stream that primarily comprising C₂- hydrocarbons, with typically no more than trace amounts (e.g., less than 1 wt-%) of C₃+ hydrocarbons, withdrawn in a line 300. [0046] The overhead stream in the line 300 is passed to an amine treatment section 302 such as may be desired to effect CO₂ removal therefrom. The utilization of amine treatment systems for carbon dioxide and/or hydrogen sulfide removal are well known in the art. Conventional such amine treatment systems typically employ an amine solvent such as methyl diethanol amine [MDEA] to absorb or otherwise separate CO₂ from hydrocarbon stream materials. A stripper or regenerator is typically subsequently used to strip the absorbed CO₂ from the amine solvent, permitting the reuse of the amine solvent. [0047] While such amine treatment has proven generally effective for removal of carbon dioxide from various hydrocarbon-containing streams, the application of such amine treatment to ethylene-rich hydrocarbon and carbon dioxide-containing streams, such as being processed at this point of the subject system, may experience some undesired complications as some of the olefin material may be co-absorbed with the CO₂ in or by the amine solvent. Such co-absorption of olefin material undesirably reduces the amounts of light olefins available for recovery from such processing. Moreover, during such subsequent stripper processing of the amine solvent, the presence of such olefin materials can lead to polymerization. Such polymerization can lead to degradation of the amine solvent and require expensive off-site reclamation processing.

[0048] In view thereof, it may be desirable to utilize an amine treatment system such as includes or incorporates a pre-stripper interposed between the amine system absorber and the amine system stripper/regenerator. Such an interposed pre-stripper, can desirably serve to separate hydrocarbon materials, including light olefins such as ethylene, from the carbon dioxide and amine solvent prior to subsequent processing through the regenerator/stripper. [0049] A stream containing C₂- hydrocarbons substantially free of carbon dioxide is passed through a line 304 to a drier section 306 with water withdrawn therefrom in a line 307. A stream containing stripped hydrocarbons and possibly minor amounts (e.g., typically less than 1 wt-%) of CO₂ is conveyed via a line 308 such as back to the compression section 224 such as for further processing such as consistent with the above description. A stream containing CO₂ is conveyed from the amine treatment section 362 via a line 309. [0050] A stream containing dried C₂- hydrocarbons substantially free of carbon dioxide is passed via a line 310 to an acetylene conversion section or unit 320. As is known in the art, acetylene conversion sections or units are effective to convert acetylene to form ethylene. Thus, an additionally ethylene-enriched process stream is withdrawn in a line 322 from the acetylene conversion section or unit 320. [0051] As the acetylene conversion may result in additional formation of water, the process stream in the line 322 can, if desired, be introduced into an optional drier unit 324 such as with water being withdrawn therefrom in a line 326 and with the resulting dried process stream passed via a line 330 to an optional further treatment section 332 such as in the form of a CO?, carbonyl sulfide ("COS"), Arsine and/or Phosphine treater as is known in the art to effect removal of CO?, COS, Arsine and/or Phosphine, withdrawn in a line 334, and a treated stream such as withdrawn in a line 336.

[0052] The treated stream in the line 336 may desirably be introduced into a demethanizer 340. A suitable such demethanizer, in accordance with one preferred embodiment, includes a condenser (not specifically shown) that desirably operates at a temperature of no greater than -90⁰C (-130⁰F), more preferably operates at a temperature in the range of -90° to -102⁰C, preferably -96°C (-130° to -150⁰F, preferably at -140⁰F). In addition, a preferred demethanizer for use in the practice of the invention desirably operates with a methane to ethylene molar ratio in the bottoms of no greater than 0.0005 and, more preferably at a methane to ethylene molar ratio in the bottoms of no greater than 0.0003 to 0.0002.

[0053] A stream of methane and hydrogen gas from the demethanizer 340 is taken overhead via a line 342 such as for use as a fuel or, if desired for further processing or treatment such as to a pressure swing absorption unit (not shown) for H₂ recovery.

[0054] A line 344 withdraws a stream of demethanized material from the demethanizer 340. The line demethanized material 344 is passed to an ethylene/ethane splitter 346. A suitable such ethylene/ethane splitter, in accordance with one preferred embodiment, includes a condenser (not specifically shown) that desirably operates at a pressure in the range of 1930 to 2105 kPag (280 to 305 psig), and desirably operates such that there is no more than 0.5 vol-% ethane in the ethylene product stream, preferably less than 0.1 vol-% ethane in the ethylene product stream and, more preferably, less than 0.05 vol-% ethane in the ethylene product stream. [0055] The ethylene/ethane splitter 346 forms a vapor stream of remaining light ends, a partial condensate stream of ethylene and a bottoms stream of ethane which are passed through lines 350, 352 and 354, respectively, such as either for product recovery or further desired processing, as is known in the art.

[0056] Returning to the processing of the stream containing C₃ and C₄ hydrocarbons taken overhead from the debutanizer 270 via the line 272, as such a process stream may contain some significant relative amounts of hydrogen sulfide, the line 272 can desirably be passed to a sulfide removal treatment unit 360 such as known in the art, such as in the form of an amine treatment section, such as to form a treated stream passed via a line 362. The hydrogen sulfide content of the treated stream is desirably reduced down to 20 ppm, with hydrogen sulfide being removed via a line 364.

[0057] If desired or required, the treated stream line 364 can be introduced into an optional caustic treatment or the like section 366 such as to effect further hydrogen sulfide removal such as down to a hydrogen sulfide content of 1 ppm or less. Hydrogen sulfide is shown as removed from the caustic treatment section 366 via a line 370. [0058] A treated stream with an appropriately reduced hydrogen sulfide content is passed via a line 372 to a mercaptan treatment section 374, such as to effect mercaptan removal from the stream materials such as via caustic wash as is known in the art. Mercaptans are shown as removed via a line 376.

[0059] The resulting stream is passed via a line 380 to C₃/C₄ splitter 382. A suitable such C₃ZC₄ splitter, in accordance with one preferred embodiment, includes a condenser (not specifically shown) that desirably operates at a pressure in the range of 1650 to 1800 kPag (240 to 260 psig), preferably at a pressure of 1724 kPa (250 psig) and desirably operates such that there is no more than 5 mol-% C₄'s in the overhead product stream, preferably less than 1 mol-% C₄' s in the overhead product stream and no more than 5 mol-% C₃' s in the bottoms stream, preferably less than 1 mol-% C₃ 's in the bottoms stream. [0060] The C₃/C₄ splitter 382 forms a stream of C₄+ hydrocarbons which is passed through a line 384 such as either for product recovery or further desired processing, as is known in the art.

[0061] The C₃/C₄ splitter 382 also forms a stream composed primarily of C₃ hydrocarbons which is passed through a line 386.

[0062] The stream in the line 386 can be passed to a propylene/propane splitter 390. A suitable such propane/propylene splitter, in accordance with one preferred embodiment, desirably operates such that at least 98 wt-% and, preferably, at least 99 wt-% of the propylene recovery is in the overhead stream and the propylene in the overhead stream is at least 99.5% pure.

[0063] The propylene/propane splitter 390 forms a stream of propylene and a stream of propane which are passed through lines 392 and 394, respectively, such as either for product recovery or further desired processing, as is known in the art.

[0064] Thus processing schemes and arrangements are desirably provided for obtaining light olefins via the catalytic cracking of a heavy hydrocarbon feedstock. More particularly, processing schemes and arrangements are provided that advantageously utilize absorption- based product recovery to produce or otherwise form process streams containing specifically desired ranges of hydrocarbons.

[0065] The invention illustratively disclosed herein suitably may be practiced in the absence of any element, part, step, component, or ingredient which is not specifically disclosed herein.

[0066] While in the foregoing detailed description this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention. For example, while the invention has been described above making specific reference to an embodiment wherein the amine treatment section 302 is placed downstream of the further compression section 290, those skilled in the art and guided by the teachings herein provided will appreciate that the broader practice of the invention is not necessarily so limited. This in certain embodiments it may be desired that the amine treatment section be disposed upstream of such further compression.

Claims

CLAIMS:

1. A process for catalytically cracking a heavy hydrocarbon feedstock and obtaining selected hydrocarbon fractions, the process comprising: contacting a heavy hydrocarbon feedstock (212) with a hydrocarbon cracking catalyst in a fluidized reactor zone (214) to produce a hydrocarbon effluent (216) comprising a range of cracked hydrocarbon products including light olefins; separating the hydrocarbon effluent (216) in a separation section (222) to form at least one separator liquid stream (232) and a separator vapor stream (234), the at least one separator liquid stream comprising C₃+ hydrocarbons, the separator vapor stream comprising C₃- hydrocarbons; contacting the separator vapor stream (234) with a first absorption solvent in an absorption zone (236) to remove C₃+ hydrocarbons therefrom and form a process stream (254) comprising C₂- hydrocarbon materials; stripping C₂- hydrocarbon materials from the at least one separator liquid stream (232) to form a C₃+ hydrocarbon process stream (266) substantially free of C₂- hydrocarbons; separating C₅+ hydrocarbon materials from the C₃+ hydrocarbon process stream to form a first product process stream (274) comprising C₅+ hydrocarbon materials and a second product process stream (272) comprising C₃ and C₄ hydrocarbons; and introducing at least a first portion (242) of the first product stream (274) to the absorption zone (236) as at least a portion of the first absorption solvent.

2. The process of claim 1 wherein said contacting of the heavy hydrocarbon feedstock with a hydrocarbon cracking catalyst comprises contacting the heavy hydrocarbon feedstock with a blended catalyst comprising regenerated catalyst and coked catalyst in a fluidized reactor zone at hydrocarbon cracking reaction conditions to produce a cracked stream containing hydrocarbon products including light olefins, said catalyst having a catalyst composition including a first component comprising a large pore molecular sieve and a second component comprising a zeolite with no greater than medium pore size, said zeolite with no greater than medium pore size comprising at least 1.0 wt-% of the catalyst composition.

3. The process of claim 1 wherein the first portion (242) of the first product stream introduced to the absorption zone comprises a majority of the first absorption solvent therewithin.

4. The process of claim 1 additionally comprising splitting at least a second portion (276) of the first product stream in a divided wall separation column (280) to form a light fraction (282) comprising compounds containing four to six carbon atoms, an intermediate fraction (284) comprising compounds containing seven to eight carbon atoms and a heavy fraction (286) comprising compounds containing more than eight carbon atoms.

5. The process of claim 1 additionally comprising splitting the second product process stream (272) in a C₃-C₄ splitter (382) to form a first C₃-C₄ splitter process stream

(386) comprising primarily C₃ hydrocarbons and a second C₃-C₄ splitter process stream (384) comprising primarily C₄ hydrocarbons.

6. The process of claim 5 additionally comprising separating propylene from the first C₃-C₄ splitter process stream (386) to form a propylene process stream (392) comprising primarily propylene and a propane process stream (394) comprising primarily propane.

7. The process of claim 1 wherein the second product process stream (272) comprising C₃ and C₄ hydrocarbons includes a quantity of mercaptans, the process additionally comprising selectively removing at least a portion of the mercaptans from at least a portion of the second process stream.

8. The process of claim 1 wherein the process stream (254) comprising C₂- hydrocarbon materials additionally comprises a quantity of carbon dioxide, the process additionally comprising treating at least a portion of the process stream comprising C₂- hydrocarbon materials to remove at least a portion of the quantity of carbon dioxide therefrom.

9. The process of claim 1 wherein the process stream (254) comprising C₂- hydrocarbon materials additionally comprises a quantity of acetylene, the process additionally comprising hydrogenating at least a portion of the quantity of acetylene to form additional ethylene.

10. A system (210) for catalytically cracking a heavy hydrocarbon feedstock and obtaining selected hydrocarbon fractions, the system comprising: a fluidized reactor zone (214) wherein the heavy hydrocarbon feedstock (212) contacts a blended catalyst comprising regenerated catalyst and coked catalyst at hydrocarbon cracking reaction conditions to produce a cracked effluent stream

(216) containing hydrocarbon products including light olefins; a separation section (222) for separating the a cracked effluent stream to form at least one separator liquid stream (232) and a separator vapor stream (234), the at least one separator liquid stream comprising C₃+ hydrocarbons, the separator vapor stream comprising C₃- hydrocarbons; an absorption zone (236) to absorb C₃+ hydrocarbons from the separator vapor stream

(234) in a first absorption solvent and to form an absorption zone effluent stream

(254) comprising C₂- hydrocarbons, including ethylene; a stripper (262) for stripping C₂- hydrocarbon materials from the separator liquid stream (232) to form a C₃+ process stream (266) substantially free of C₂- hydrocarbons; a debutanizer (270) for separating C₅+ hydrocarbon materials from the C₃+ hydrocarbon process stream to form a first product process stream (274) comprising C₅+ hydrocarbon materials and a second product process stream (272) comprising C₃ and C₄ hydrocarbons; and a process line (242) for introducing at least a first portion of the first product stream

(274) to the absorption zone (236) as a majority of the first absorption solvent.