Patent Application
20080081937
A suitable heavy hydrocarbon feedstock can be cracked and the effluent resulting therefrom processed using a dividing wall separation column, in accordance with a preferred embodiment, to produce or form hydrocarbon product streams having desirably sharper splits of the hydrocarbon products than have heretofore been commonly obtainable and, more particularly, to do so in a manner that may desirably also be more energy efficient.
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.
Suitable fluidized catalytic cracking reactor zones for use in the practice of such an embodiment may, as is described in above-identified U.S. Pat. No. 6,538,169 B1 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.
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.
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 about 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 about 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.
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. U.S. Pat. No. 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 10-25 wt. % or more and a matrix material content of 75-90 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.
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 4-40 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.
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.
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 315-622° C. (600-1150° F.) and, more typically, which has a narrower boiling point range of from 343-551° C. (650-1025° F.). Heavy or residual feeds, i.e., hydrocarbon fractions boiling above 499° C. (930° F.), are also suitable. The fluidized catalytic cracking processing the invention is typically best suited for feedstocks that are heavier than naphtha range hydrocarbons boiling above about 177° C. (350° F.).
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.
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.
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 about 1375 kPag to about 2100 kPag (about 200 psig to about 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.
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 includes a quantity of carbon dioxide.
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+ hydrocarbons 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 may desirably include about 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 about 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 about 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 about 20 to about 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.
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.
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. The light cycle oil absorbs most of the remaining C4 and higher hydrocarbons and returns to the main fractionator 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 or processing such as known in the art. In accordance with a preferred embodiment, the stream withdrawn from the secondary or sponge absorber 246 in the line 254 is desirably an ethylene-rich hydrocarbon-containing stream, as herein defined.
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 about 1650 kPag to about 1800 kPag (about 240 psig to about 260 psig) with a C2/C3 molar ratio in the stripper bottoms of less than 0.001 and preferably no more than about 0.0002 to about 0.0004.
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 feeds 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 about 965 kPag to about 1105 kPag (about 140 psig to about 160 psig), with no more than about 5 mol % C5 hydrocarbons in the overhead and no more than about 5 mol % C4 hydrocarbons in the bottoms. More preferably, the relative amount of C5 hydrocarbons in the overhead is less than about 1-3 mol % and the relative amount of C4 hydrocarbons in the bottoms is less than about 1-3 mol %.
A stream of C3 and C4 hydrocarbons from the debutanizer 270 are taken overhead by a line 272 for further treatment or processing such as known in the art.
A line 274 withdraws a stream of debutanized gasoline from the debutanizer 270. A portion of the stream of debutanized gasoline is returned to the primary absorber 240 via the line 242 to serve as the first absorbent solvent. Another portion of the stream of debutanized gasoline is passed in a line 276 to a naphtha splitter 280.
In accordance with one preferred embodiment, the naphtha 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 naphtha 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 about 34 kPag to about 104 kPag (about 5 psig to about 15 psig) and, in accordance with one embodiment operated at a condenser pressure of about 55 kPag to about 85 kPag (about 8 psig to about 12 psig).
Such a dividing wall separation column typically operates in a more energy efficient manner than a simple sidedraw column and also desirably produces a sharper product split than normally obtainable with conventional sidedraw columns.
Further, in accordance with a preferred embodiment, the products produced or formed by or from the dividing wall column may desirably include a distillate having a Total Boiling Point (TBP) at the 95% cut point in the range of about 72° to about 78° C. (about 162° to about 172° F.) and, more specifically, about 75° C. (167° F.) and a side product having a TBP at the 5% cut point the range of about 72° to about 78° C. (about 162° to about 172° F.) and, more specifically, about 75° C. (167° F.) and a TBP at the 95% cut point the range of about 167° to about 173° C. (about 333° to about 343° F.) and, more specifically, about 170° C. (338° F.).
As will be appreciated by those skilled in the art and guided by the teachings herein provided, such light, intermediate and heavy fraction streams may desirably be appropriately passed such as via corresponding lines 282, 284, and 286, respectively, either for further processing or product recovery, as may be desired.
For example, in the illustrated embodiment, the C5-C6 containing stream in line 282 is passed to a light fraction compound cracking zone 283 wherein at least a portion of the light fraction compounds containing five to six carbon atoms, e.g., C5-C6 olefins, are cracked such as in a manner known in the art to form a cracked olefin effluent comprising C2 and C3 olefins, shown as passing in a line 288 as well as possibly a paraffin purge stream such as in a line 289.
The C7-C8 containing stream in the line 284 can, if desired, be passed for further desired processing such to an aromatics recovery zone 285, for example, wherein aromatic hydrocarbons desirably can be recovered from such the intermediate fraction compounds as a stream in a line 291, in a manner such as is known in the art.
The heavy fraction stream comprising compounds containing more than eight carbon atoms in the line 286 can be passed to a blending zone 287 wherein the heavy fraction compounds containing more than eight carbon atoms are selectively blended into a gasoline hydrocarbon-containing stream, shown as a line 293.
Turning to
In accordance with a preferred embodiment and as shown, a naphtha feed can desirably be introduced via a line 320 into the central dividing wall section 312. A light fraction, in accordance with a preferred embodiment, can be withdrawn via a line 322 from the upper or top section 314. An intermediate fraction, in accordance with a preferred embodiment, can be withdrawn as a side product via a line 324 from the central or middle section 312. A heavy fraction, in accordance with a preferred embodiment, can be withdrawn via a line 326 from the lower or bottom section 316.
Thus, through the incorporation and use of dividing wall separation column as herein described, the invention provides sharper desired product splits for FCC effluent processing and does so in a generally more energy efficient manner than heretofore has been realized in the processing of such effluent streams.
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.
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.
