| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 30.05.2000 |
| PATENT TITLE |
Process and compositions for Mn containing catalyst for carbo-metallic hydrocarbons |
| PATENT ABSTRACT |
An improved manganese and/or chromium-promoted catalytic process, catalyst and method of manufacture for heavy hydrocarbon conversion, optionally in the presence of nickel and vanadium on the catalyst and in the feed stock, to produce lighter molecular weight fractions, including lower olefins and higher isobutane than normally produced. This process is based on the discovery that two "magnetic hook" elements, namely manganese and chromium, previously employed as magnetic enhancement agents to facilitate removal of old catalyst, or to selectively retain expensive catalysts, can also themselves function as selective cracking catalysts, particularly when operating on feeds containing significant amounts of nickel and vanadium, and especially where economics require operating with high nickel- and vanadium-contaminated and containing catalysts. Under such conditions, these promoted catalysts are more hydrogen and coke selective, have greater activity, and maintain that activity and superior selectivity in the presence of large amounts of contaminant metal, while also making more gasoline at a given conversion. |
| PATENT INVENTORS | This data is not available for free |
| PATENT FILE DATE | October 21, 1996 |
| PATENT REFERENCES CITED | Wm. P. Hettinger Jr. Magnetic and Chemical Properties of Mag. Sep'd . . . Catalysts laden with Iron etc. Catalysis Today, 13 (1992) pp. 157-189 Elsevier Science Publishers (Amsterdam). |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A composition of matter for selectively converting hydrocarbons to gasoline comprising microspheres in the range of 5 to 200 microns comprising at least about 5200 ppm Mn and about 5% or more zeolite selected from the group consisting of: X, Y, USY and ZSM-5 and additionally comprising a matrix. 2. A composition according to claim 1 additionally comprising Ni+V and having a weight ratio of at least 0.5 Mn: (Ni+V) on catalyst. 3. A composition according to claim 1 in which Mn is deposited substantially uniformly throughout the microspheres. 4. A composition according to claim 1 in which cracking activity exists in both the zeolite and the matrix. 5. A composition of matter according to claim 1 comprising at least 5200 ppm Mn and additionally comprising about 0.1-20 wt. % Cr on the catalyst. 6. A composition of matter according to claim 1 comprising 1 to 15 wt. % Cr. 7. A composition of matter according to claim 1 comprising about 5 to 50 wt. % ZSM-5. 8. A composition of matter according to claim 1 consisting essentially of at least 5200 ppm Mn and 5% or more zeolite X, Y, USY, or ZSM-5. 9. A composition of matter for selectively converting hydrocarbons to gasoline according to claim 1 comprising microspheres in the range of 5 to 200 microns and further comprising at least 0.5 wt. % Na. 10. A composition according to claim 1 wherein the catalyst comprises at least 0.05-10% rare earth. 11. A composition according to claim 1 having a magnetic susceptibility of at least 2.times.10.sup.-6 emu per gram when measured on a Johnson Mathey balance. 12. A process for preparing a selective catalytic cracking catalyst comprising about 5% or more of a zeolite selected from the group consisting of X, Y, USY and ZSM-5 and at least 5200 ppm Mn in a microspherical cracking catalyst comprising mixing a manganese compound with a cracking catalyst to form said microspherical catalyst. 13. A process according to claim 12 wherein the manganese compound comprises at least one of the following precursors: manganese (II) carbonate, manganese (II) octoate, manganese (II) acetate tetrahydrate, manganese (II) acetate, manganese (III) acetate, manganese (II) fluoride, manganese (III) fluoride, manganese (II) iodide, manganese (II) methoxide, manganese (II) naphthenate, manganese (II) nitrate, manganese (III) nitrate, manganese (III) orthophosphate, manganese (III) oxalate, manganese (II) 2,4-pentanedionate, manganese (III) 2,4-pentanedionate, manganese (II) perchlorate, manganese (II) phosphate, manganese (II) phthalocyanine, manganese (II) sulfate, MMT, and optionally adding chromium to the solution as chromium (II) acetate, chromium (III) acetate hydroxide, chromium (III) acetylacetonate, chromium (III) bromide, chromium (II) chloride, anhydrous chromium (III) fluoride, chromium (III) hexafluoro 2,4-pentanedionate, chromium (III) iodide, chromium (II) iodide, anhydrous chromium (III) nitrate, chromium (III) 2,4-pentadionate, chromium (III) perchloate, chromium (III) phosphate, chromium (III) sulfate, and/or chromocene. 14. A process according to claim 12 wherein the catalyst comprises at least 0.84 wt % Mn. 15. A process according to claim 12 wherein the manganese compound is added to a hydrocarbon feedstock to be cracked and deposits on the catalyst at a level above 5200 ppm Mn. 16. A process according to claim 12 wherein the catalyst comprises at least 0.05-10% rare earth. 17. A process according to claim 12 in which the Mn comprises salt solutions of nitrate, sulfate, chloride, carbonate or acetate of Mn. 18. A process according to claim 12 comprising depositing Mn substantially on the outer shell of the microsphere or uniformly throughout the microsphere or a combination of both. 19. A process according to claim 12 for preparing a selective cracking catalyst, said process comprising adding at least a portion of said Mn as Mn acetate. 20. A process according to claim 12, said process further comprising exchanging or impregnating spray-dried catalyst with Mn solution and drying. 21. A process according to claim 12 for preparing a selective cracking catalyst by incorporating zeolite and 5200 ppm to 20 weight % Mn in a microspherical cracking catalyst, said process further comprising mixing a Mn solution with gelled catalyst and spray drying. 22. A process according to claim 12 for preparing a selective cracking catalyst by incorporating zeolite and 5200 ppm to 20 weight % Mn in a microspherical cracking catalyst, said process further comprising mixing a solution of Mn salts and spray-drying the mixture. 23. A process according to claim 12 further comprising preloading the zeolite with a solution of Mn, and spray drying. 24. A process according to claim 12 further comprising impregnating calcined catalyst with Mn. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
BACKGROUND OF THE INVENTION I. Field of the Invention This invention relates to the field of adding manganese and/or chromium to hydrocarbon cracking catalysts, generally classified in Class 208, subclass 253 of the United States and in International Class C10G-29/D4. II. Description of the Prior Art U.S. Pat. No. 4,412,914 to Hettinger et al. is understood to remove coke deposits on sorbents by decarbonizing and demetalizing with additives including manganese (claim 4, column 26). U.S. Pat. No. 4,414,098 to Zandona et al. uses additives for vanadium management on catalysts (column 15, line 6). U.S. Pat. No. 4,432,890 to Beck et al. mobilizes vanadia by addition of manganese, inter alia, Table A; column 9, line 35-48; column 10, line 40; and column 27, line 3; Table Y; etc. U.S. Pat. No. 4,440,868 to Hettinger et al. refers to selected metal additives in column 11, line 20, but does not apparently expressly mention Mn. U.S. Pat. No. 4,450,241 to Hettinger et al. uses metal additives for endothermic removal of coke deposited on catalytic materials and includes manganese as an example of the additive (colunnn 11, Table C). U.S. Pat. No. 4,469,588 to Hettinger et al. teaches immobilization of vanadia during visbreaking and adds manganese to sorbent materials (column 11, lines 1-13, line 53 and line 65; column 23, line 59 and line 20; claim 1 and claim 17. U.S. Pat. No. 4,485,184 to Hettinger et al. is understood to teach that trapping of metals deposited on catalytic materials includes manganese as an additive (column 8, line 32; column 10, line 50, Table A; column 11, line 34; column 29, line 55, Table Z; column 31; column 32; claims 5-9. U.S. Pat. No. 4,508,839 to Zandona et al. mentions metal additives including manganese at column 17, line 44 for the conversion of carbo-metallic oils. U.S. Pat. No. 4,513,093 to Beck et al. immobilizes vanadia deposited on sorbent materials by additives, including manganese; column 9, line 35, Table A; column 10, lines 8-9; column 10, line 21. U.S. Pat. No. 4,515,900 to Hettinger et al. is understood to teach that additives, including Mn, are useful in visbreaking of carbo-metallic oils (column 10, line 64 and column 23, line 52, Table E; column 25, line 13, Table 5. U.S. Pat. No. 4,549,958 to Beck et al. teaches immobilization of vanadia on sorbent material during treatment of carbo-metallic oils. Additives include manganese mentioned at column 9, line 37, Table A; column 10, line 10; column 10, line 21; column 21, line 27, Table Y; column 21, line 56, Table Z; claim 37-38. U.S. Pat. No. 4,561,968 to Beck et al. is understood to teach carbo-metallic oil conversion catalyst with zeolite Y-containing catalyst includes immobilization by manganese; column 14, line 43. U.S. Pat. No. 4,612,298 to Hettinger et al. teaches manganese vanadium getter mentioned at column 14, line 31-32. U.S. Pat. No. 4,624,773 to Hettinger et al. is understood to teach large pore catalysts for heavy hydrocarbon conversion and mentions manganese at column 18, line 27. U.S. Pat. No. 4,750,987 to Beck et al. teaches immobilization of vanadia deposited on catalysts with metal additives including manganese; column 9, line 10; column 11, line 6, Table A; column 11, lines 47-49; column 11, lines 67; column 24, lines 14-25; column 28, line 52, Table Y. U.S. Pat. No. 4,877,514 to Hettinger et al. teaches the incorporation of selected metal additives, including manganese, which complex with vanadia to for in higher melting mixtures; column 10, lines 43-49; column 14, lines 34-35; column 29, line 37; claims 2, 10 and 13. U.S. Pat. No. 5,106,486 to Hettinger teaches the addition of magnetically active moieties, including manganese, for magnetic beneficiation of particulates in fluid bed hydrocarbon processing; column 4, line 64; Claims 1, 2, 11, 32, and 44-48. U.S. Pat. No. 5,198,098 to Hettinger uses magnetic separation of old from new equilibrium particles by means of manganese addition (see claims 1-30). U.S. Pat. No. 5,230,869 to Hettinger et al. is understood to teach the addition of magnetically active moieties for magnetic beneficiation of particulates in fluid bed hydrocarbon processing; column 5, line 4 and claim 1. U.S. Pat. No. 5,364,827 to Hettinger et al. teaches the composition comprising magnetically active moieties for magnetic beneficiation of particulates in fluid bed hydrocarbon processing; column 5, line 4 and claim 5. U.S. Pat. No. 4,836,914 to Inoue et al. mentions magnetic separation of iron content in petroleum mineral oil but is not understood to mention manganese. U.S. Pat. No. 4,956,075 to Angevine et al. adds manganese during the manufacture of large pore crystalline molecular sieve catalysts and particularly uses a manganese ultra stable Y in catalytic cracking of hydrocarbons. U.S. Pat. No. 5,358,630 to Bertus et al. mentions manganese in claims 28 and 40, but not in the specification. The patent relates primarily to methods for " . . . contacting . . . catalyst with a reducing gas under conditions suitable countering effects of contaminating metals thereon and employing at least a portion of said reduced catalysts in cracking said hydrocarbon feed" (column 7, lines 10-12). U.S. Pat. No. 2,575,258 to Corneil et al. mentions manganese as accumulating in the catalysts as a result of erosion of equipment (column 3, line 34). U.S. Pat. No. 3,977,963 to Readal et al. mentions manganese nitrate and manganese benzoate and other manganese compounds, e.g., in the second paragraph under "Descriptions of Preferred Embodiments" and in the Tables under "Detailed Description" and in claim 4. It is directed to the contacting of catalysts with a bismuth or manganese compound to negate the effects of metals poisoning. U.S. Pat. No. 4,036,740 to Readal et al. teaches use of antimony, bismuth, manganese, and their compounds convertible to the oxide form to maintain a volume ratio of carbon dioxide to carbon monoxide in the regeneration zone of a fluid catalytic cracker of at least 2.2. Cimbalo et al., May 15, 1972, teaches the effects of nickel and vanadium on deleterious coke production and deleterious hydrogen production in an FCC unit using zeolite-containing catalyst. SUMMARY OF THE INVENTION I. GENERAL STATEMENT OF THE INVENTION According to the invention, an improved "magnetic hook"-promoted catalytic process, catalyst and method of manufacture for heavy hydrocarbon conversionl, stock to produce lighter molecular weight fractions, including lower olefins and higher isobutane than normally produced has been discovered. This process is based on the discovery that two elements, namely manganese and chromium, previously employed as magnetic enhancement agents to facilitate removal of old catalyst, or to selectively retain expensive catalysts, can also themselves function as selective cracking catalysts, particularly when operating on feeds containing significant ainotmts of nickel and vanadium, and especially where economics require operating with high nickel- and vanadium-contaminated and containing catalysts. Under such conditions, these promoted catalysts are more hydrogen and coke selective, have greater activity, and maintain that activity and superior selectivity in the presence of large amounts of containment metal, while also making more gasoline at a given conversion. II. UTILITY OF THE INVENTION Table A summarizes approximate preferred, more preferred, and most preferred levels of the more important parameters of the invention. Briefly stated, the invention comprises improving gasoline selectivity in a process for the conversion of hydrocarbons, e.g., gas oils, preferably those containing more than 1 ppm of nickel and/or more than 1 ppm of vanadium to lower molecular weights comprising gasoline by contacting said hydrocarbons with a circulating zeolite-containing cracking catalyst, which is thereafter regenerated and recycled to contact additional hydrocarbons, the improvement comprising in combination the steps of: a) maintaining a catalyst:oil weight ratio of at least about 2, more preferably at least 3; and b) adding to at least a portion of said cracking catalyst from about 0.1 to 20 wt. % of manganese and/or chromium, in the form of a compound, based on the weight of the catalyst; whereby gasoline selectivity is increased by at least 0.2 wt. % points as compared to said process without said manganese or chromium. More preferably the portion of cracking catalyst to which manganese is added comprises from 5-100 wt. % of the total weight of the circulating catalyst. This portion can optionally contain more than 0.5% by weight of sodium. This process and catalyst are especially effective when used in conjunction with a circulating catalyst containing nickel and vanadium and/or when operating at higher steam and/or temperature severity. When cracking reside, the weight of manganese is preferably maintained at about 0.3 or above times the total nickel-plus-vanadium or total metals or total vanadium on the circulating catalyst. The carbon remaining after regeneration is preferably no more than 0.1% of the weight of the carbon deposited on the catalyst during hydrocarbon conversion. Particularly preferred is a process wherein the fresh catalyst is added over time to the circulating catalyst, particularly where the fresh catalyst comprises 0.1-20 wt. % manganese and/or a similar concentration of chromium. The manganese-containing cracking catalyst added can be the same or different from that circulating and can preferably comprise a paraffin-selective cracking catalyst such as Mobile's ZSM-5. One important advantage of the invention is that the cracking catalyst can be rendered more gasoline selective, coke selective, and/or hydrogen selective when it contains 0.1-20 wt. % manganese and/or chromium and is even more selective when the catalyst is contaminated with nickel and/or vanadium, as compared to the selectivity of an equivalent catalyst without manganese. The manganese and/or chromium is preferably deposited onto the outer periphery of each microsphere but can be deposited uniformly throughout the microsphere, where the most preferred microspherical catalysts particles are used. Cracking activity can exist in both the zeolite and the matrix. Manganese preferably can also serve as an oxidation catalyst to accelerate the conversion of carbon to CO and CO.sub.2 and any sulfur in the coke to SO.sub.2, SO.sub.3 or sulfate and can act as a reductant in the conversion reactor to convert greater than 10% of the retained sulfate in the reactor to SO.sub.2, sulfur and H.sub.2 S. Cracking catalyst can be prepared by incorporating manganese into a microspherical cracking catalyst by mixing with a solution of a manganese salt with a gelled cracking catalyst and spray drying the gel to form a finished catalyst or a solution of manganese salt can be combined with the normal catalyst preparation procedure and the resulting mixture spray-dried, washed and dried for shipment. Manganese can be added to the microspherical catalyst by impregnating the catalyst with a manganese-containing solution and flash drying. Preferred salts of manganese for catalyst preparation include nitrate, sulfate, chloride, and acetate of manganese. The selective cracking catalyst can be prepared by impregnating spray-dried catalyst with MMT (methylcyclopentadienyl manganese tricarbonyl) and drying. The MMT can be dissolved in alcohol or other solvent which can be removed by heating. Alternatively, spray-dried or extruded or other catalyst can be impregnated with a colloidal water suspension of manganese oxide or other insoluble manganese compound and dried. The continuous or periodic addition of a water or organic solution of manganese salts with or without methyl cyclopentadienyl manganese tricarbonyl in a solvent can also be employed with the invention. Manganese compounds, preferably MMT or manganese octoate in mineral spirits or a water solution of a manganese salt, can also be added directly to the catalytic cracker feed and subsequently deposited on the circulating catalyst. The virgin catalysts will preferably possess a magnetic susceptibility of greater than about 1.times.10.sup.-6 emu/g and this can be promoted to a magnetic hook in the range of about 1-40.times.10.sup.-6 emu/g or even greater. (Magnetic hooks are discussed in detail in U.S. Pat. Nos. 5,106,486; 5,230,869 and 5,364,827 to Hettinger et al.) The coke produced in the conversion is burned off by contact with oxygen-containing gas in a conventional regenerator and the manganese can serve as an oxidation catalyst in the regenerator to accelerate the conversion of carbon to carbon monoxide and/or carbon dioxide, enhancing the regeneration process. As an additional advantage of the invention, the sulfur in some gasolines can be reduced by 10% or even more as compared to gasoline produced without manganese in the catalyst. A portion of the circulating catalyst can be removed from the process of the invention and treated with nitrogen, steam and greater than 1% oxygen (preferably in the form of air) for 10 minutes to 1 hour or even more at 1200.degree. F. or greater, then returned to the process, to effect a partial or complete regeneration of the catalyst. TABLE A ______________________________________ PROCESS More Most Parameters Units Preferred Preferred Preferred ______________________________________ V in Gas wt. 0.1 or more -- -- Oil Feed ppm Ni in Gas Wt. 0.1 or more -- -- Oil Feed ppm V in Resid Wt. 1 or more 10 or more 50 or more Feed ppm Ni in Resid Wt. 1 or more 10 or more 50 or more Feed ppm Ni + V on Wt. above .increment. 500 above 1000 above 5000 Catalyst ppm (resid) V on Wt. 100-100,000 above 500 above 1000 Catalyst ppm (resid) Mn on Wt. % 0.05-20 0.1-15 0.2-10 Catalyst Catalyst compos. Zeo- USY- ZSM-5 containing containing containing Cat:Oil Wt. 2 or more 2.5-12 3-20 ratio Mn/Cr on Wt. % 0.1-20 0.5-15 1-10 Catalyst Mn/Cr any exchange or impregnation Addition impregnation Meth. Gasoline Wt. % +0.2 or more +0.4 or more +1 or more Selectivity .increment. "Portion" Wt. % 5-100 10-50 15-25 with Mn/Cr Na in Wt. % more than more than more than "Portion" 0.5 0.6 0.7 Mn:(Ni + V) Wt. above 0.3 above 0.5 above 1 on Catalyst ratio (resid) Mn:V Wt. above 0.3 above 0.5 above 1 on Catalyst ratio (resid) Cr:(Ni + V) Wt. above 0.3 above 0.5 above 1 on Catalyst ratio (resid) Concarbon Wt. % above 0.1 above 0.3 1-7 in feed % of Carbon % of 0.5 or less 0.1 or less 0.05-0.1 on Cat. orig. remaining after regen. Zeolite-in- Wt. % 1 or more 5 or more 10 or more Catalyst Hydrocarbon Wt. % above 0.1 above 0.5 above 4 Concarb. S in Wt. % above 0.2 above 0.5 above 2 Hydrocarbon feed S retention Wt. % 10 or more 12 or more 15 or more by Mn of S % Sulfate in Wt. % above 10 above 12 above 15 Reactor of S Converted Catalyst Form Form any microspheres spray-dried microspheres Cat. Mag. 10.sup.-6 above 1 2-40 3-40 Suscept. emu/g Cat. Mag. 10.sup.-6 1-50 2-40 3-40 Hook Mag. emu/g Susceptibility Increase Reduction of % of 10 or more 12 or more 15 or more SOx in Flue S in ______________________________________ The present invention is useful in the conversion of hydrocarbon feeds, and preferably metal-contaminated residual feeds, to lower molecular, weight products, e.g., transportation fuels. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plot of relative activity (by Ashland Inc. test, see e.g., U.S. Pat. No. 4,425,259 to Hettinger et al.) versus cat:oil weight ratio for AKC #1 catalyst (the same catalyst except for FIG. 7 and including catalyst AKC #2 in FIGS. 19 and 20, used in FIGS. 1-18) catalyst with and without manganese. (See Example 3 and Table 3b.) FIG. 2 is a plot of wt. % gasoline selectivity versus wt. % conversion in a typical cracking process and compares catalysts with and without manganese. (See Example 3 and Table 3b.) FIG. 3 is a plot of gasoline yield versus conversion rate constant and compares catalysts with and without manganese. (See Example 3 and Table 3b.) FIG. 4 is a plot of gasoline wt. % selectivity versus conversion comparing catalysts with and without manganese and contaminated with 3000 ppm nickel plus vanadium. (See Example 4 and Table 4.) FIG. 5 is a plot of relative activity versus cat:oil ratio comparing catalysts with and without manganese. (See Example 4 and Table 4.) FIG. 6 is a plot of wt. % gasoline in product versus conversion rate constant for the catalysts with and without manganese showing the improved gasoline percentage with manganese. (See Example 4 and Table 4.) FIG. 7 is a plot of gasoline selectivity versus weight ratio of (X) manganese:vanadium, and (O) manganese:nickel+vanadium. (See Example 8 and Table 8.) FIG. 8 is a plot of relative activity versus cat:oil ratio comparing no manganese with 9200 ppm manganese added by an impregnation technique and with 4000 ppm manganese added by an ion exchange technique. (See Example 10 and Tables 10a, 10b and 10c.) FIG. 9 is a plot of gasoline selectivity versus conversion comparing no manganese versus 9200 ppm impregnated manganese and 4000 ppm ion exchanged manganese. (See Example 10 and Tables 10a, 10b and 10c.) FIG. 10 is a plot of Ashland relative activity versus cat:oil ratio comparing catalysts with and without manganese at different levels of rare earth. (See Example 11 and Table 10.) FIG. 11 is a plot of gasoline selectivity versus gasoline conversion comparing no manganese with impregnated rare earth elements and ion-exchanged manganese, showing manganese, surprisingly, is more effective than rare earths. (See Example 11 and Table 10.) FIG. 12 is a plot of wt. % isobutane (in mixture with 1-butene/isobutene) versus wt. % conversion for catalysts with no manganese and with 9200 and 4000 ppm manganese. (See Example 12 and Table 10.) FIG. 13 is a plot of the ratio of C.sub.4 saturates to C.sub.4 olefins versus wt. % conversion comparing manganese at levels of 4000 ppm, 9200 pm with no manganese and no manganese plus 11,000 ppm rare earth. (See Example 12 and Table 10.) FIG. 14 is a plot of the CO.sub.2 :CO ratio versus percent carbon oxidized off during generation (See Example 14) with and without manganese. FIG. 15 is a plot of wt. % gasoline versus wt. % conversion for catalysts with and without manganese and 3200 ppm Ni+V showing improved gasoline yield with manganese. (See Example 15 and Table 12.) FIG. 16 is a plot of hydrogen-make versus conversion showing the improved (reduced) hydrogen make with manganese being deposited as an additive during cracking. (See Example 15 and Table 12.) FIG. 17 is a plot of coke-make versus conversion showing the improved (reduced) coke make with manganese being deposited as an additive during cracking. (See Example 15 and Table 12.) FIG. 18 is a plot of conversion versus cat:oil ratio showing the improved conversion with manganese at cat:oil ratios above about 3. (See Example 15 and Table 12.) FIG. 19 is a plot of AOI relative activity versus manganese content. (Example 2, Table 3a.) FIG. 20 is a plot of selectivity versus manganese content. (See Example 2 and Table 3a.) FIG. 21 is a plot of the relationship between magnetic properties and manganese on catalyst. (Examples 1 and 16, and Tables 1 and 13.) FIG. 22 is a plot of relative activity versus weight percent manganese. (See Example 1 and Table 1.) FIG. 23 is a plot of gasoline selectivity versus weight percent conversion using catalyst containing manganese and catalyst with no manganese addition. (see Example 1 and Table 1.) FIG. 24 is a plot of gasoline selectivity versus weight percent manganese on catalyst at 75% conversion. (See Example 1 and Table 1.) DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples are presented to illustrate preferred embodiments of the invention, but the invention is not to be considered as limited by the specific embodiments presented herein. |
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