US3691058A - Production of single-ring aromatic hydrocarbons from gas oils containing condensed ring aromatics and integrating this with the visbreaking of residua - Google Patents

Abstract

SINGLE-RING AROMATIC HYDROCARBONS ARE MAXIMIZED BY HYDROCRACKING A HEAVY HYDROCARBON FEED BOILING BELOW 1050*F. CONTAINING POLYNUCLEAR AROMATICS OR THEIR PRECURSORS OVER ZEOLIT BASE HYDROCRACKING CATALYST AT LOW CONVERSIONS OF 5 TO 40% AND RECYCLING THE 90-160*F. AND 350*F.+ FRACTIONS TO EXTINCTION. INTEGRATING THIS STEP WITH THE VISBREAKING OF RESIDUA IN PRESENCE OF A HYDROCARBON MODIFIER OR FREE-RADICAL ACCEPTOR IN WHICH THE 90160*F. FRACTION FROM THE HYDROCARACKING IS USED AT LEAST IN PART AS THE MODIFIER IS INCLUDED.

Description

Sept. 12, 1972 I G. P. HAMNER ETAL 3,691,058

PRODUCTION OF SINGLE-RING AROMATIO HYDROOARBONS FROM GAS OILS OONTAINING CONDENSED RING AROMATICs AND INTDGRATING THIS WITH THE VISBREAKING OF RESIDUA Filed April 15, 1970 2 Sheets-Sheet 1 sept. l2, 1972 G. P. HAMNER ETAL 3,691,058 PRODUCTION 0F SINGLE-RING AROMATIC HYnRoCARBoNs FROM GAS OILS CONTAINING CONDENSED RING AROMATICS AND INTEGRATING THIS WITH THE VISBREAKING OF RESIDUA Flled April 15, 1970 2 Sheets-'Sheet 2 United States Patent O U.S. Cl. 208-73 8 Claims ABSTRACT F THE DISCLOSURE Single-ring aromatic hydrocarbons are maximized by hydrocracking a heavy hydrocarbon feed boiling below 1050 F. containing polynuclear aromatics or their precursors over zeolite base hydrocracking catalysts at low conversions of to 40% and recycling the 90-160 F. and 350 R+ fractions to extinction. Integrating this step with the visbreaking of residua in presence of a hydrocarbon modifier or free-radical acceptor in which the 90- 160 F.. fraction from the hydrocracking is used at least in part as the modifier is included.

RELATED APPLICATIONS This is a continuation-in-part of Ser. No. 840,986, filed July 1l, 1969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to the hydrocracking of gas oils containing condensed ring aromatic hydrocarbons or their precursors under conditions to maximize the formation of single-ring aromatics.

Heavy crude oils and various hydrocarbon residua such as those made by vacuum distillation, tars obtained by steam-cracking of hydrocarbon fractions and the like are of low economic value in'terms of utility in petroleum rening to produce conventional refinery products. Attempts at upgrading such fraction by conventional means result in the production of substantial proportions of Bunker C fuel oil which, because of the high sulfur content of the residual feeds, also contain large amounts of sulfur which must be reduced to very low limits as a result of recent stringent air pollution restrictions, thus further decreasing the economic value of oils made from these residual feeds.

SUMMARY `OF THE INVENTION It has now been discovered that residues can be economically upgraded to concentrate aromatic hydrocarbons or their precursors in the products by hydrocracking the portion of these residues boiling below 1050 F.. and recycling, if desired, to extinction the 90 to 160 F. fraction as well as the bottoms fraction boiling above 350 F. and controlling the conversion in the reactor to 5 to 40%. A fraction boiling below 90 is removed overhead from the product fractionator and further treated to recover C3 and C4s and an isopentane fraction. A fraction boiling from 160 F.| to any point from l60-430 F. is recovered which contains all of the single ring aromatics and can be used as blending stock for motor fuel or may be further treated to separate the individual aromatic hydrocarbons.

It is also a part of this invention to integrate this process with the process described in Ser. No. 29,629, filed Apr. 17, 1970, by the instant applicants in which hydrocarbon residues, such as steam cracked tar, are visbroken in the presence or absence of hydrogen with or without a freeradical acceptor and in which at least a part of the freeradical acceptor used is the -160 F. fraction obtained from the hydrocracking.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic representation of one embodiment of this invention in which a gas oil containing condensed ring aromatic hydrocarbons is hydrocracked in accordance with this invention.

FIG. 2 is a diagrammatic representation of another embodiment of this invention in which a hydrocarbon residuum is visbroken in the presence of hydrogen and a free radical acceptor to prepare a feed stock particularly suited for hydrocracking in accordance with this invention and in which the hydrocracking step is used to prepare a recycle stream suitable as free radical acceptor in the visbreaking step.

DESCRIPTION OF THE PREFERRED EMBODIMENTS .Referring now to FIG. 1, a hydrocarbon oil boiling below 1050 F. and containing polynuclear aromatic hy drocarbons is fed by line 1 and mixed with total recycle from line 2. The total hydrocarbon feed is carried by line 3 and mixed with fresh hydrogen from line 4 and recycle hydrogen from line 5 and passed into hydrocracker 6 by line 7.

The hydrocarbon feed may be any suitable fraction containing polynuclear aromatic hydrocarbons such as reformate polymer, catalytic clarified oil, gas oil, distillate gas oils from steam-cracked tars, and the like.

'I'he hydrocarbon feed ows downwardly through reactor 6 which is filled with a suitable contact material such as silica-alumina, alumina or a steam-treated crystalline zeolite, such as faujasite, having a suitable catalyst dispersed thereon. Broadly, the zeolites used are those having silica-to-alumina mole ratios above about 3, preferably 4 to 5.5 Suitable catalysts include noble or nonnoble elements. One such suitable catalyst is platinum or palladium on faujasite. The nonnoble catalysts used are the suldes of metals Group IAB, AII-B, and VIII of the Periodic Table (Handbook of Chemistry and Physics, 38th edition, Chemical Rubber Publishing Company) mixed with the sulfdes of nonnoble metals from Groups IV, VAB and VI-B. The preferred metals from the Irst-named groups are copper, zinc, iron, cobalt, and nickel. Those from the last-named groups are molybdenum and tungsten. The details on the preparation of these catalysts may be found in U.S. Pat. No. 3,549,518.

The conversion in reaction zone 6 is maintained at 5- 40% by controlling the conditions therein. Thus the ternperature may range between 500 and 950 F., preferably 700:-800 F. The pressure will range between 400 and 3000 p.s.1.g., preferably 500 and 1500 p.s.i.g. The space velocity should range from 0.5 to 5 v./v./hr., preferably 3 to 4.5 v./v./hr. and the exit hydrogen rate from 500 to 5000 s.c.f./bbl., preferably 750-2000 s.c.f./bbl. Cracked products are removed from the bottom of the reactor by line 8 and are taken to hot separator 9 from which liquid products are recycled by lines 10 and 2. Vapors are removed by line 11 and condensed in condenser 12 and passed by line 13 to gas separator 14 from which uncondensed gas, mostly hydrogen, is recycled by line 5. Liquid products are removed by line 15 and passed to fractionator 16 from which an overhead fraction boiling below 90 containing C3-C4 a-nd isopentane is removed by line 17.

A fraction boiling at 90-l60 F. is removed by line 18 and, if desired, recycled to the hydrocracking reactor 6 by line 18 and 2 are removed from the system by line 21.

A side stream boiling 1GO-350 F. or higher, e.g., up to 430 F., containing single ring aromatics such as benzene, toluene and the like are removed by line 19. This fraction may be used as blending stock for motor fuel or further fractionated, extracted or sievated to separate the individual aromatic hydrocarbons.

A bottoms fraction having an initial boiling point between 350 F. and 430 F. is withdrawn through line 20 and mixed, if desired, with the 90-l60 F. fraction flowing in line 18 and recycled to the hydrocracker by lines 2, 3, and 7 or removed from the system by line 22.

By operating as described above, the ratio of single ring aromatics to total cracked products boiling up to 430 F. is increased by operating at the low conversions specified and by recycling the unreacted portions. The desired low conversion per pass are achieved at high feed rates which with recycle provide approximately the same throughput of fresh feed as obtained at the usual lower rates which give nearly complete conversion.

Referring now to FIG. 2, a process is disclosed in which the hydrocracking process described in FIG. l is integrated with the visbreaking of steam-cracked tar or other residue in the presence of free-radical modifiers as described in S.N. 29,629, filed Apr. 17, 1970 by the instant applicants and which is incorporated herein by reference. The C-C6 fraction boiling at 90-l60 F. produced by the hydrocracking step of FIG. l is used at least in part as the free-radical acceptor in the visbreaking step.

A hydrocarbon residue having a Conradson carbon number between 5 and 40 and a substantial amount boiling at 1000 F. and above, such as thermal tar from steam cracking, reduced crude, shale oil residue, liquefied coal fractions, and the like is fed by line 101 and mixed with lower boiling material, such as gas oil, preferably recycled from a later stage of the process which enters through line 102, mixed with 50-1000 s.c.f. of hydrogen or a nonoxidizing gas per barrel of feed introduced by line 103. Gas oil or the like acts as a solvent for the tar and permits easy pumping at moderate temperatures and prevents coking at hot spots in the system. The mixture is passed by line 104 into the bottom of depolymerizer or visbreaker 105 where the mixture is maintained at a temperature of 700-900 F. (preferably 750-770 F.) and under suicient pressure to maintain it in the liquid phase, e.g., 50-1000 p.s.i.g. A free-radical acceptor or modier, preferably an acrylic hydrocarbon, which may be a paraffin or iso-parafiin of 4 to 20 carbon atoms per molecule, or an olefin or iso-olefin of 2 to 20 carbon atoms per molecule or mixtures thereof is added by lines 106 and 107. Suitable hydrocarbons include the paraflins n-heptane and n-pentane, the olefins 2,2,4 trimethyl pentane-l and 2,4,4,trimethyl pentene-2, as well as other olefins of similar skeletal configuration low octane unsaturated naphtha fractions, catalytic heavy naphtha, heavy alkylates, and the like. It may also be a naphthene hydrocarbon, such as described hereinafter. If desired, a mild alkali may be slurried with the feed as described in S.N. 29,629, filed Apr. 17, 1970. Similarly, a catalyst such as Harshaw nickel may be used as described in U.S. Pat. No. 3,622,502, both cases being in the name of the instant applicants. The hydrocarbons are added in amounts of about 1 to 25% based on tar feed and are sprayed, jetted or otherwise passed through the liquid tar phase in depolymerizer 105, into the vapor phase and removed overhead through line 108. The alkali, if used, is added in amounts of 0.01 to 0.1 part per part of hydrocarbon. The residence time of the modifier added through line 107 should range from about S minutes to one hour. The presence of the hydrocarbon modifiers at such short residence times results in reduced coke and gas loss. However, some of the modifier is consumed in the process. When n-heptane is the modifier, the degradation products are predominantly normal hydrocarbons, namely, nbutane, n-pentane, n-hexane, etc., whereas when isooctane is used the degradation products are predominantly branched, i.e., isobutane, isopentane and branched C6 and C7 parafiins. The use of 2,2,4-trimethyl pentane and olefins of a similar skeletal configuration results in the production of the important blending agent, triptane, Without intending to limit the invention to any theory of what occurs, it is believed that the mechanism is one in which the modifier is being consumed with accompanying hydrogen exchange, demethanation, alkylation, isomerization, aromatic disproportionation and probably every known hydrocarbon reaction. The most plausible explanation is a free-radical mechanism in which the condensed ring aromatic components of the tar depolymerize with the formation of free radicals which attach themselves to the modifier as a sink In doing so, the modifier in turn forms free radicals involving stepwise degradation and rearrangement reactions leading to gaseous products, coke, etc.

From the above it appears that conditions of short residence times for the modifier (less than one hour) coupled with fairly long residence times for the tar feed (one to six hours) is important for best results.

The modifier leaving depolymerizer through line 108 is passed to separator 109 from which hydrogen and uncondensed gas is recycled by line 110. Condensate from separator 109 is passed by line 111 to fractionator 112 from which low boiling products are removed by line 113 and unreacted modifier and entrained higher boiling components by line 114. This unreacted modifier is recycled to depolymerizer 105 and by lines 115 and 107.

Liquid products are withdrawn from depolymerizer 105 by line 116 and passed through filter 122 where coke and/or other solid contaminants are removed and then passed to vacuum flash chamber 117 where they are separated into high-boiling products and low-boiling products. The low-boiling products have a boiling range of 350 to 1000 F. The high-boiling products, therefore, may have an initial boiling point anywhere from 650 to 1000 F. The low-boiling products are either drawn off as make products through lines 118 and 121 or are partially recycled to depolymerizer by lines 118, 102, and 104. The required high-boiling products for equilibrium conditions are recycled by lines 120, and 107.

The amount of low-boiling products recycled by lines 118, 102 and 104 is critical, and must be between 20 and 50% of the total composition regardless of the distribution of the other materials. This control is made possible by withdrawing an amount of 350 F.-1000 F. material from the system by line 121 necessary to maintain the proper recycle ratio. The product drawn off through line 121 is processed further in accordance with this invention as described below.

The recycle of the high-boiling material on the other hand is controlled so that the amount of this material fed to the depolymerized based on total feed will be substantially the same as the amount of this high boiling material found in the product, based on tar blend. Generally, this is between 45 and 50%, preferably 46-47%. While it is not intended to be bound by any theory as to the mechanism involved, it is believed that the beneficial results obtained are due to an equilibrium phenomenon in which an equilibrium exists between the condensed ring aromatic-containing high-boiling fraction and the lower boiling depolymerized fraction. Excessive amounts of the low-boiling fraction will retard the depolymerization, limit throughput of the depolymerization feed and incur excessive handling costs.

Conversion products having boiling range of 350- l000 F. initial boiling fraction are drawn off through line 121 and mixed with high-boiling product having an initial of 430 F.'+ flowing in line 123 and passed to hydrocracker 126 by line 127. Fresh hydrogen is introduced by line 124 and mixed with hydrogen recycle fiowing in line 125. Catalyst and conditions in hydrocracker 126 are maintained substantially the same as those set forth in connection with FIG. l so as to maintain conversion levels of to 40%. Bottoms from the -fiash chamber 117 are recycled to the visbreaker through lines 120, 115, and 107.

Cracked products are withdrawn through line 128 and passed to hot separator 129. Hot separator 129 is operated in a manner to knock out a maior portion of the 650 F.| unconverted material from stream flowing in line 128 which is recycled to the hydrocracker by lines 130 and 123. Vapors pass by line (131 to condenser 132 and thence by line 133 to gas separator 134 from which hydrogen and other gaseous products are recycled to the hydrocracker by lines 125. Condensate is passed by line 135 to fractionator 136. These products are separated into an overhead fraction composed of C3-C4, and isopentane by line 137. A light side stream boiling at 90- 160 F. is removed by line 138 and may be sent back to the visbreaker 105 where it serves as part of the hydrocarbon modiier used in the process, or withdrawn from the system by line 143.

A side stream boiling from 160 F. up to any point above 350 =F., e.g., up to 430 F., is withdrawn as product through line 139. This stream contains the single ring aromatics and may be further fractionated, extracted or sievated to yield the individual compounds or used as blending stock for gasoline.

A bottoms fraction 430 |F.| is withdrawn through line 140 and is recycled to the hydrocracker by line 123 or Withdrawn from the process by line 142. -If desired, this bottoms fraction may also be recycled, either in toto or in part, to the visbreaker by lines 141, 138 and 107 The feeds used in the above processes may be any fraction obtained from heavy crudes or their residues, which fraction boils below 1050 F. If the fraction contains major proportions of polynuclear aromatic hydrocarbons, the steps outlined in FIG. l are sufficient, but if the polynuclear aromatic hydrocarbon content of the feed is low, then the process described only concentrates the aromatic hydrocarbon precursors and must be submitted to a conventional reforming to convert the precursor to single-ring aromatic hydrocarbons.

The following examples are included to illustrate the eiectiveness of the instant process without, however, limiting the same.

Example 1 Feed rates, v./v./hr 1. 0 2. 1 4.0 4. 5 Exit hydrogen rate, cf./b 6, 860 4, 200 930 1, 760 Wt. percent 430 F., based on feed.. 95. 2 00 25 20 Wt. ratio single ring aromatic yield- 0. 13 0. 19 0. 39 0. 41 Potentia single ring aromatic yield:

Wt. percent 13 19 39 41 Vol. percent 16 23 47 50 The above data show that maximum single ring aromatic yields are obtained at low conversion per pass and that these low conversions are obtained at high feed rates.

Example 2 A 430-650 F. gas oil, produced by the depolymerizaton of steam cracked tar simulating the process o-f passing the tar to visbreaker 105 of FIG. 2 and subsequent recovery was passed over a commercial palladium faujasite catalyst under conditions and with results as follows:

Process conditions: Temperature, F.: 710; pressure, p.s.i.g.: 800; ieed rate, v./v./hr.: 3.5; hydrogen rate, s.c.f./bbl.: 4,000 selectivity per unit, 430 F.+ converted: 80.5%

C3 and lighter C5 Production distribution and inspections (wt. percent) C4 430 F. 430 F.+

Yield, vol. percent. 3. 4 3. 1 24. 7 75. 2 Gravity, API 3l. 0 9. 9 Research Octane Number 99.8

The 24.7 vol. percent C5- 430 IF. and the single ring aromatics contained therein are distributed as follows:

Total C5- 430 F. naphtha product:

According to the preferred embodiments of segregating the single ring aromatics in a l60-350 F. fraction with recycle of the indanes, naphthenes and naphthalenes and with about 9% yield of single ring aromatics per pass, a yield of 60-70% is estimated upon recycle of extinction. Thus, in the combined modiiied visbreaking and controlled hydrocracking, single ring aromatic yield of 50-60% based on steam cracked tar is available.

The foregoing description does not by any means cover the possible uses of this invention or the forms it may assume, but serves to illustrate its fundamental principles and a process in which the novel features have been incorporated. yIt is obvious that changes in the details may be made without departing from either its novel characteristics or the spirit and scope of the invention as defined in the claims. For example, more than one hydrocracking unit may be provided in both FIGS. 1 and 2. In such a case, the rst reactor would contain the nonnoble catalyst as described so as to reduce the sulfur and nitrogen level to a point which could be tolerated by a sulfur and nitrogen sensitive catalyst. Thus, the second reactor could use a noble metal catalyst such as platinum or palladium or nonnoble metal hydrogenation components on molecular sieves such as faujasite, alumina, silica-alumina or nickel suliide on silica-alumina.

Example 3 A 430l000 F. gas oil produced by the depolymerization of steam cracked tar is processed as in Example 2, except that the catalyst is nickel-tungsten on faujasite under conditions and with results as follows:

Process conditions:

Temperature, F. 700 Pressure, p.s.i.g 1500 Feed rate, v./v./hr. 0.8 Hydrogen rate, s.c.f./b. 6000 Conversion to 430 E+, wt. percent 28.2 Selectivities per unit, 430 F.| converted:

C5 430 F. 86.5 Single ring aromatics plus naphthenes 42.2 C-Cs aromatics 17.6

When the entire steam cracked tar is hydrocracked over a nickel-molybdenum on alumina catalyst under the same conditions, the conversion to 430 F.- is 9.2 wt. percent but the selectivity to C5- 430 F. is 91.5 wt. percent, thus indicating that neither the catalyst nor the particular fraction processed is critical. Note tha-t in Example 2, a selec- 7 tivity to C- 430 F. of 80.5% was obtained with the 430-650 F. fraction as feed t0 the hydrocracking step.

Example 4 Example 1 was repeated to compare the results obtained 5 by using palladium on hydrogen faujasite and nickel sulfide on silica-alumina. The following results were 0btained:

Pd-H- Ni-si/Al Catalyst faujasite (sulfided) Process conditions:

Temperature, F- 700 820 Pressure 1, 000 1,000 v./v.lhr 4. 2 1.0 H2, gas rate, s.c.f./b 3, 000 3, 000 Conversion to 430 F.+ L., wt. percent 33. 7 23. 2 15 Seleotivitles per unit 430 F.+ Converted:

C5- 430" F 57. 3 s0. 2 Single ring aromatics plus naphthenes.. 34. 1 41. 4 Ce-Ca aromaties 16. G 17. 5

The above data show that excellent single ring aromatics yield can be obtained using other catalysts.

Example 5 Example 1 is repeated using a catalyst consisting of 10% zinc and 5% tin on faujasite to show the effect of total recycle of the 430 F.| fraction in the hydrocrack- 25 ing operation. In these runs the total amount of 430 F.+ product formed in Runs A and B is blended and used as feed in Run C. The total amount of 430 R+ material formed in Run C is used as feed to Run D. The following data were obtained:

Run number A B C D Catalyst Zn-Sn-faujnsite Feediraetion 430-1,000 F. 430 F.+ 1 430 F.+2

Process conditions:

Temperature, F 774 800 823 838 Pressure 1, 000 v/v./l1r 1.06 H2gasratc,s.e.f./b 7,000 1,400 Conversion to 430 F. and L.,

wt. percent 11. 1 19. 1 17. 5 15. 0 Selectivities per unit 430 F.4-

converted:

C5- 430 F 70.0 75.0 74.3 65.4 Single ring aromatics plus naphthenes 42. 4 44. 1 47. 3 C-Cgaromatlcs 26.0 27.3 29.0 36.1

1 Recycle from A and B. 2 Recycle from C.

The above data show that the highest yields of single ring aromatics and naphthenes are Obtained by the use of a single pass but that the CG-Cs aromatics increase with the number of passes indicating the maximum yields are Obtained by recycling the 430 F.{- fraction to extinctlOIl.

The nature and advantages of the present invention having thus been fully described and illustrated and specific examples of the same given, what is claimed as new, useful and unobvious and desired to be secured by Letters Patent is:

1. A process for preparing single-ring aromatic hydrocarbons from hydrocarbon residues having Conradson carbon numbers between 5 and 40 which comprises visbreaking said hydrocarbon residues in a visbreaker in the presence of hydrogen under pressure suticient to maintain the residue in the liquid phase and at a temperature between about 700 F. and 900 F., separating the cracked liquid products from said visbreaking into a high boiling fraction having an initial boiling point between about 650 F. to 1000 F. and a low boiling fraction boiling in the range between about 350 F. to 1000 F., recycling a portion of the said low boiling fraction to said visbreaker such that the feed to the visbreaker contains 20 to 50% of said low boiling recycle fraction, recycling a portion of the Said high boiling fraction to the visbreaker at such a rate that the amount of said high boiling fraction in the total feed to the visbreaker is the same ratio as the amount of said high boiling fraction in the product, contacting another portion of said 10W boiling fraction from said cracked liquid product in a hydrocracking reaction zone in the presence of hydrogen and a hydrocracking catalyst at temperatures between about 500 F. and 950 F., pressures between about 400 and 3000 p.s.i.g., feed rate space velocity between about 0.5 and 5 v./v./hr. and an exit hydrogen rate between about 500 and 5000 s.c.f./bbl. whereby conversion levels of 5 to 40% per pass are maintained, recovering from the products of the hydrocracking reaction zone a fraction containing C3, C4 and isopentane, a fraction boiling in the range between about F-l60" F., a fraction boiling from about 160 F. to a point between about 350 F. and 430 F. containing the single ring aromatic hydrocarbons and a bottoms fraction having an initial boiling point between 350 F. and 430 F. and a final boiling point above about 650 F., at least a portion of said bottoms fraction being recycled to the hydrocracking reaction zone.

2. The process of claim 1 in which the visbreaking takes place in the additional presence of 1 to 25% of a hydrocarbon modiiier.

3. The process of claim 2 in which the said bottom fraction recycled to the hydrocracking zone -boils above 430 F.

4. The process of claim 2 in which the said low-boiling fraction recycled to the visbreaker boils above 650 F.

5. The process of claim 2 in which the said high-boiling fraction recycled to the visbreaker boils above 1000 F.

6. The process of claim 2 in which 1 to 28 wt. percent of the 90-160 F. fraction is recycled to the visbreaking zone as hydrocarbon modifier therein.

7. The process according to claim 1 in which the catalyst used in the hydrocracking step is a mixture of tungsten and nickel suldes on faujasite.

8. The process of claim 1 in which the feed to the visbreaker is steam cracked tar and the catalyst used in the hydrocracking step is palladium on faujasite.

References Cited UNITED STATES PATENTS 3,147,206 9/ 1964 Tulleners 208-56 3,392,108 7/1968 Mason et al. 208-111 2,900,327 8/1959 Beuthe 208-106 3,352,776 11/1967 Masol et al 208-125 3,551,323 12/ 1970 Hambiin 208-58 DELBERT E. GANTZ, Primary Examiner G. E. SCHMITKONS, Assistant Examiner U.S. Cl. X.R.

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