US2525812A - Combination hf-treating-catalytic cracking process - Google Patents

Description

Oct. 17, 1950 A. P. LIEN ErAL 2,525,812

COMBINATION HFTREATINGCATALYT1C cRAcKING PRocEss Filed Dec. 23, 194s Patented Oct. '17,

COMBINATION HF-TREATING-CATALYTIC CRACKING PROCESS Arthur P. Lien, Hammond, Ind., and Bernard L. Evering, Chicago, Ill., assignors to Standard Oil Company, Chicago, Ill., a corporation of Indiana Application December 23, 1946, Serial No. 718,038

3 Claims.

This invention relates to a process of producing high quality motor fuel from heavier-thangasoline hydrocarbons by a new and improved combination of HF treating and catalytic cracklng and it pertains more particularly to processes wherein the catalytic cracking is effected by silica-alumina type catalysts which are activatable by small amounts of HF.

An object of the invention is to provide a method and means for obtaining maximum yields of high quality motor fuel from catalytic cracking charging stocks containing large amounts of sulfur compounds, objectionable polycyclic hydrocarbons, and/or other components -which are not efficiently handled by catalytic cracking processes and/or which may be detrimental in catalytic cracking systems. A further object is to improve the effectiveness and efficiency of catalytic cracking systems employing silica alumina type catalysts, i. e. to obtain increased motor fuel production, decreased coke formation and superior product quality. A still further -object is to provide a process wherein the catalyst used in the second step is promoted in situ by traces of treating agent employed in the iirst creased. 'Thel treating step itself eliminates sulfur and other undesirable components and it may produce a utilizable icy-product tar from material which would otherwise be deposited as coke on the catalyst in the cracking step. Furthermore, the treating step may remove organic and/or inorganic material from the charge that would otherwise contaminate or deactivate the catalyst and lead to lower conversion, undesirableA product distribution, excessive gas and coke formation, etc.

The initial step of the process is a light treatment with HF, i. e. a treatment under such mild conditions as to avoid any substantial amount of cracking but which is sufficient to eliminate the bulk of the sulfur and other undesirable components in the gas oil. In this treating step a step so that the expense of removing such traces of treating agent is largely avoided so that the combination of steps produces results heretofore unobtainable in the art. Other objects will be apparent as the detailed description of the invention proceeds.

Briey, the invention contemplates the production of high quality motor fuel from heavierthan-gasoline charging stocks which have heretofore presented serious problems to the refining industry. For example, a West Texas gas oil may contain 2 to 3% of sulfur and when such gas oil is charged to a, conventional catalytic cracking process the finished gasoline will contain about .3% sulfur and the catalytic gas oil about 11A, to 2% of sulfur. By giving gas oil a preliminary treatment with HF and then subjecting it to substantially the same conditions of catalytic cracking, we obtain a marked and substantial increase in gasoline production per pass anda marked improvement in gasoline quality, the gasoline 'containing only about onetenth as much sulfur and being characterized by a higher A. P. I. gravity, a lower bromine number, and agreatly increased responsiveness to lead tetraethyl. In spite of treating losses in the initial step the overall yield of gasoline may be as high as the yield that would be obtained from untreated charging stock., The effective catalytic cracking capacity is markedly inrelatively large amount of HF, e. g. about 20 to 200% by volume based on charge may be employed at a relatively low temperature of about 50 to 150 F. but preferably about 80v to 100 F. and with a time of contact of the order of about 5 minutes to an hour or more. The treated charging stock may be separated from gases and from' the bulk of HF in a settling zone wherein the undesirable polycyclic hydrocarbons and sulfur compounds are withdrawn in the bottom HF layer and gases flashed overhead. The dissolved HF in the treated charge may be partially removed by a simple stripping operation. The treated charge which contains traces of HF is then directly introduced into a catalytic cracking reactor containing a solid catalyst of the silica alumina type and operated under conditions for converting the charge to motor fuel. The traces of HF remaining in this charging stock apparently effect an in situ activation of the cracking catalyst so that the yields per pass of gasoline are substantially increased. The net result of the combined process is the production of remarkably high quality gasoline of low sulfur content, an increase in the. capacity of the cracking unity the substantial elimination of the expense of defluorinating and a net increase in production of valuable by-product materials, the increase in gasoline production made possible 'by the treating step substantially balancing losses of by-product production in the treating step.l

The invention will be more clearly understood from the following description read in conjunction with the following drawing which is a schematic flow diagram illustrating a practical application Yof our invention.

While the invention is applicable to any heavier-than-gasolne hydrocarbon charging messia A. which are deleterious to or are inefectively handled in a catalytic cracking system, it will rst be described in connection with the hanmore-butane, pentane and gasoline than the untreated gas oli charge. The treated gas oil gave a lower yield of ethane and a higher yield oi propylene. Gasoline from the HF treated gas oil contained only about one-tenth as much sulfur Ei dling of a heavy West Texas gas oil. This gas as that fromthe treated gas oil. The lower denoil was treated with HF at room temperature in sity, lower bromine number and lower refractive four successive stages with fresh l-H", using in index of gasoline from the HF treated gas oil all each stage about 1 volume of l-F to 5 `volumes of indicate a lower olefin content and perhaps a Charging stock. In each treatment the HF chargn 1c lav/er aromatic content than that 'obtained from ins stock mixture was vigorously agitated with the untreated feed. An' unexpected result is the a 1725 R. P. M. mechanical stirrer for a period fact that the cycle stock from the HF' treated of 30 minutes and the HF was then separated by gas oil actually shows an inc ease in sulfur consimple settling from the treated charge. At the tent over the charging stock the cracking step. end of the first Stage the percent of sulfur had It may therefore be desirable to subject the cycle been reduced from 2.13 to 1.16% with a treating stock to the HF treating step before it is returnedy loss of about 18.5 weight percent. By the end of to the cracking step. The lower density and the fourth stage the sulfur content had been relower refractive index of the cycle stock from duced to .65% and the total cumulative treating the HF terated gas oil charge indicate lower aroloss was 22.5 weight percent. Inspections of maticity, indicating that the cycle stock is, less the West Texas gas oil before and after the HF refractory than the cycle stock from the cracktreatlng steps are as follows: ing of untreated gas oil. From a series of studies tio from which the abov data Were obtained it was Inspec ns shown that with a given amount of catalyst the HF treated gas oil gives higher conversion with lsarg FG'geiti'l lower coke formation. It is also indicated that at the same conversion level the HF treated gas oil b 2.13 o' 65 requires markedly less catalyst and produces only Per Centsunurom 0. 08 0.05 about half as much coke, and that at the same D w j (11%? Mgg; 30 coke level the HF treated gas oil gives remarkseih''ispersicn- 143 123 ably higher conversion. In spite of the 22.3% As'llgnllistmatiom 42B 450v treating losses in the HF treating step, the total 155g gg converted product will be about the same as from 602 609 a given amount of driginal feed, a considerable gg gg advantage accruing tto the fact that the cycle 679 csa stock from the treated feed is less refractory 32(1) gg? than that from the untreated feed. "143 739 As a second example, allower boiling West o Texas gas oil was treated on a pilot plant scale Representative samples of the eas oil charge with HF under conditions oflcounter-current-exand the HF extracted gas oil respectively were traction with a continuous acid phase at a temcatalytically cracked in a fixed-bed unit with pemture of 100 F" and with zovolume pep Super Fmrol under-substantm11y the Same son' cent HF, based on hydrocarbon. The following anions employing a temperature of 910 F' 45 tabulation shows the inspections of the original Super Filtrol is the trade name for acid-treated gas ou as compared with the treated gas oir bentonite clay, the principal constituent of which is the clay mineral, Montmorillonite. A Inspections comparison of the products obtained is set forth in the following table:

tg Extracted Products from:

Weight Per Cent Sulfur l. 50 0. 56 oss 011 HF Treated .411111115 Pom 154 112 charge Gas ou ND 1.4836 1.4141 D. 0.8745 0.8534 Specific Dispersioxn; 130. 9 113. 4 Total Analysis (Weight Per Cent on Bromine No 12.9 5.6

ee ASTM Distllation: Per Cent Dry Gas 9. 8 9.5 Initial 357 353 c. 5.2 8.7 422 421 4.2 as 480 466 24.6 29.7 517 501 47.6 35.7 550 543 6.0 4.4 585 511 62o 612 as 2.5 654 641 20.5 19.4 69s ses 5.1 5.6 747 126 1a ai: 78 2 276 250 65 64 19 These two stock were then separately cracked 53? gbg vin a pilot plant fluid catalytic cracking unit. 0.178 0. 76e Cracking was carried out with a Super Filtrol C D M429 1-4383 catalyst of high iron content under substantially ycle Stock 01g 0%32 the same conditlons for unextracted and exgjjjjjjjj 1 5344 1 '5231 tracted charge stocks using a temperature of about 900 F. and a space velocity (Wt. of oil per From the above data if, Wm be Seen that the hour per wt. of catalyst in the reactor) of about HF treated gas oil gave less dry gas and coke but Although the coke formation was unduly high in both cases due to the particular catalyst employed, the markedly lower coke formation in the case of the HF treated charge is noteworthy along with the other indicated improvements in over-al1 results.

A commercial system for practicing our invention is illustrated in the drawing and our invention will now be described in connection therewith. Charging stock which may be West Texas gas oil, reduced crude or any other heavierthan-gasoline hydrocarbon containing objectionable components,v is introduced through line I by pump II to line I2 where it meets HF from storage tank I3, line I4 and pump I5. VKnothole mixers or other mixing means may be employed for attaining intimate Acontact of the two streams in line I2 and/or said intimate contact may be obtained in treating chamber I8 by mechanical stirrer II driven by motor I8. It should be understood that instead of a stirred reactor we may employ a packed or unpacked tower with mixing orifices, a circulating system of the type commonly employed for effecting sulfuric acid alkylation or any other eiectlve contacting means. Instead of the illustrated concurrent flow through the reactor the HF may be introduced at the top thereof, while the charge is introduced at the base, such countercurrent treatment being particularly desirable in towers where the treating is to be effected in a countercurrent manner and the separation is effected in the tower itself,

the treated material Ibeing in that kcase taken overhead while the HF and impurities are withdrawn from the base of the tower. In the system illustrated in the drawing a mixture of HF and treated charging stock is introduced by line I9 to settler 20, a cooler 2| being employed if the treating is at very high temperature/ The treating should be effected under relatively mild conditions so that sulfur and/or other undesirable components may be removed from the charging stock without materially changing its boiling lrange, i. e. without effecting substantial amounts of cracking. The treating may be effected at room temperature with about 10 to 200% by volume oi HF and any number of stages may be employed in the treating step (although only one stage is shown in the drawing). When it is desired to remove the sulfur compounds as materials soluble in HF the treating should be in the range of 50 to 150 F. The pressure in all cases should be sufllcient to maintain the HF in liquid phase.

In settler 20, which may operate at about cooling water temperature, the treated hydrocarbons may be withdrawn throughcooler 24 and cold settler 25 for throwing cut additional amounts of HF which may thus be recycled directly by line 26, thereby diminishing the load on the stripper tower 23.

The HF layer from settler 20' is withdrawn through line 21 and recycled to treater I6 via line I2. At least a part of 'the HF tar in line 21 is -directed through line 21--a to tar stripper 28 which is provided at its base with conventional heater or reboiler 29.` The tar and sulfur adducts 1 with HF are decomposed by heating to Z50-500 F.

The overhead from this stripper is passed through condenser30 to settler 3I in which the HF separates out as a lower liquid and is withdrawn through line 32 to storage tank I3. The upper layer in settler 3I consists of light hydrocarbons and it may be recycled through line 33 by pump 34 to serve as a stripping oil in tower 28 by virtue of azeotrope formation with HF, any deficiency or excess of light hydrocarbonsbeing supplied to or removed from this circuliting system through line 35. Tar and sulfur compounds are withdrawn -from the base of the stripper through line 28a and such tar may be utilized per se as a by-product or it may be subjected to coking or cracking (for exa-mple with HF) to lcracking temperature of about 250 to 500 F. and

allowing sufficient time of contact whichmay be about 10 minutes to an hour or more, much ofA `the tar can be converted into gas oil and/or lighter hydrocarbons before its introduction into stripper 28 and in this case the gasoline components may be recovered in a suitable fractionation system( not shown) or introduced by line 35 to supplement the treated gas oil stream.

If -the HF treating is effected at sufliciently high temperature to convert sulfur compounds into H2S,fgases may be removed from the top of the settler 2D through line 36 to knock-back tower 31 which is provided with cooling, scrubbing or reux means 38 so that methane, H2S, HC1 (from desalting) and other fixed Igases may be vented through line 39 while HF and'condensable hydrocarbons are returned by line 40 to stripper 23.

Stripper 23 is provided with suitable heater or reboiler 4I at its base and overhead from this stripper passes through condenser 42 to settler 43. Liquid HF accumulates in the bottom of this settler and is withdrawn through-line 44 to storage tank I3. A light hydrocarbon is withdrawn from the upper part of the settler through line 45 and recycled by pump 46 to serve as a stripping gas in tower 23, any excess or` deflciency of such light hydrocarbons Ibeing removed from or supplied to the system through line 41.

The stripped gas oil leaving the vbase of stripper 23 through line 48 may pass through a preheater 4B, then pick up hot regenerated catalyst ,from the base of standpipe 50 and carry this The fluid type catalytic cracking system thus separate as an upper layer and they may be withdrawn through line 22 to gas oil stripper 23. Settler 2li may, however, operate at higher temperature and in such cases the treated gas oil briey and diagrammatically represented is now well known to those skilled in the art and. requires no detailed description. The catalyst-to-oil ratios employed may be in the range of 2:1 to 20:1

on a weight basis, the cracking temperature may i i 12 to separator B8. Gases from this separator are be in the range of about 800 to 1000 F. e. g. about 900 F. and the weight space velocity may be in the range of about 0.2 to 20 pounds of oil charged per hour per pound of catalyst in the reactor. In

other words, the cracking conditions may be approximately the same as those heretofore employed for conventional gas oil stocks although somewhat smaller amounts of catalyst or higher throughputs may be obtained by virtue of the promoting eiect of the residual amounts of I-IF V vention is equally applicable to fixed bed systems and to moving bed systems. The catalyst itself is of the silica alumina type and may be either conventional Super'Filtrol or a synthetic silica alumina catalyst of any type known to the art.

Apparently Super Filtrol and other silica alumina type cracking catalysts generally are markedly improved in their activity by treatment with HF which perhaps results in the formation of aluminum iluosilicate. In our process this activation of the catalyst is particularly effective because it is produced in situ. .The traces of HF remaining in the treated gas oil are thus effectively removed therefrom in the cracking opera` tion and at the same time the effectiveness of the catalyst is increased so that higher conversions per pass are obtained with smaller amounts of carbon deposition on the catalyst. Usually the amount of HF introduced is so small that the make-up catalyst requirements in the cracking step are suiiicient to take care of it. In some cases, however, it may be necessary or desirable to pass a portion of the treated gas oil through bauxite chambers or other HF removal means 56 (note U. S. Patent 2,391,149). Our-,invention minimizes the required HF removal and may entirely eliminate it.

The products from catalytic cracking reactor 5I pass by line 51 to tower 58 which may be provided with a settling section 5,9 at its base so that catalyst slurry may be returned through line 60 to the stream entering the reactor and decanted oil may be Withdrawn through line 6i and either removed from the system or recycled lines 62 and I2 to the HF treating step. Heavy gas oil may be withdrawn as a side stream through line G3, recycled bylines 6d and l2 to the HF treating step, and/or recycled by lines 64 and 64a to the catalytic cracking step. Cycle gas oil from this process is usually in most respects superior to the untreated original gas oil charge but if all of it is recycled through line 64a a buildup in sulfur compounds or aromatics might result; at least a part of the gas oil stream shouldfusually therefore either be withdrawn through line 63 or recycled through line I2 to the treating step in order to prevent any buildup of sulfur compounds or aromatics in the catalytic cracking step. A light gas oil stream may be withdrawn through line 65 and it likewise may be recycled either to the HF treating step or to the catalytic cracking step. Gasoline and lighter components are taken overhead through line 66 and condenser 61 to vsettler 68 from which any water from diluent steam in the cracking step "or oth-erwise accumulated in the system may be withdrawn through 69. Any gasoline produced in the HF treating or tar cracking and taken overhead from strippers 23 or 28 may be passed via lines 35 or 41 through line 10, HF removal chambers 1 I and line compressed by compressor 13, liquid hydrocarbons are pumped by pump 14, and the combined stream is passed through line 15 to superatmospheric pressure .fractionation system diagrammatically yillustrated by tower 16 from which heavy naphtha isl withdrawn through line 11, light naphtha through line 18 and one or more lighter naphtha streams through lines 18 and A19. A C4 or Ca-C4 stream may be withdrawn through line B0 and such` butane or propane-butane mixtures may be introduced by lline 8i and line 10 te supply any deficiency of stripping gas for towers 23 and 28 respectively. Dry gas is removed from the system through line 82. In actual practice of course a conventional absorber and. stripper system may be employed instead of a simple tower but since this fractionation step per se forms no part of the claimed invention it will not be described in furtherv detail.

From the above description it will be seen that we have accomplished the objects of our invention. The mild HF treating step removes sulfur compounds and/or polycyclic aromatics which are not readily amenable to cracking and makes possible the recovery of tar and sulfur compounds as utilizable by-products. This particular method of pretreating the catalytic cracking charging stock (coupled perhaps'with traces of HF left in the treated charging stock) results in greatly increased cracking yields, substantially less carbon formation on the catalyst and an improved cracked gasoline which is less olenic, more amenable to improvement by lead tetraethyl and `which contains only about one-tenth as much sulfur as would be obtained by the cracking of the same stock without the initial treating step. The catalytic cracking apparently concentrates the sulfur compounds in the cycle gas oil so that when this-gas oil is recycled to the HF treating step still further amounts of sulfur can be eliminated from the system and kept out of the i'lnal gasoline products. The butane or proane-butan-e hydrocarbons produced in the catalytic cracking step` serve as stripping gases and form .HF azeotropes which facilitate HF removal from the treated gas oil and the tar respectively. Thus this new and improved unitary combination accomplishes results which have heretofore been unattainable in the cracking of low grade charging stocks and particularly those containing large amounts of sulfur.

The step per se of removing certain polycyclic aromatics by chemical condensation is claimed in our copending application Ser. No. 135,166, led December 27, 1949, which in turn was a continuation-in-part of copending application Ser. No. 760,061, filed July 10, 1947. The combination process of effecting chemical condensation by use of HF at temperatures in the range of 150 to 400 F. for removing certain polycyclic aromatics prior to catalytic cracking with solid catalysts such' as silica alumina type catalystsis claimed in our Y copending application Ser. No. 168,112, led June those skilled in the art. When salt bearing reduced crudes are employed the process will oier the additional advantage of salt removal. No specic details as to structural materials and gention in effecting alkyla-tion.

. 9 eral safety precautions have been recited herein. since those skilled in the art are familiar with the handling of HF in view oi.' its commercial utiliza-'- and cracking conversion of the resultant purified gas oil with a silica-alumina/type catalyst, the

- steps consisting essentially of contacting said gas oil charging stock with between about 0.1 and about 2 volumes of liquid hydrogen fluoride per volume of said charging stock at a temperature between about 50'o F. and about 150 F., stratifying the mixture resulting from said contacting into a hydrogen fluoride solution of organic sulfur compounds and polycyclic aromatic hydrocarbons and a hydrogen iluoride-immiscible layer of puried ygas oil ofsubstantially reduced sulfur and polycyclic aromatic hydrocarbon content, said purified gas oil having substantially the same 10 percentl to 80 percent distillation'temperatures as said charging stock, a substantially reduced carbon residue and containing sulfur compounds which are not readily adsorbed on silica-alumina catalyst during said cracking conversion.

2. The process of converting a gasoil charging stock containing sulfur compounds and polycyclic aromatic hydrocarbons to low `sulfur gasoline while minimizing coke formation, which process consists essentially oi' contacting said charging stock with between about 0.1 and about 2.vo1 umes of liquid hydrogen iluoride per volume of said charging stock at a temperature between about 50 F. and about 150 F., stratifying the 4o mixture resulting from said` contacting into ahydrogen uoride solution of organic sulfur compounds and polycyclic aromatic hydrocarbons and a hydrogen fluoride-immiscible layer of puri-V fied gas oil of substantially reduced sulfur and polycyclic aromatic hydrocarbon content, saidv` puriiled gas oil having substantially the same 10 percent to 80 percent distillation temperatures as said charging stock, converting said purified gas 011 with a Simca-nummer type cracking catalyst at a temperature between about 800 F. and about 1000 F., fractionating the conversion products` into a low sulfur gasoline fraction and a high sulfur cracked gas oilfraction, said cracked gas oil fraction having a higher sulfur content than said puried gas oil, recycling at least a part of said cracked gas oil fraction to a hydrogen uol0 ride liquid-liquid contacting step, and reducing the sulfur contento! said cracked gas oil fraction in the last-named contacting step to a value not greater than that of said purified gasoil.

3. The process of converting a gas oil charging i stockjcontaining sulfur compounds and p'olycyclic 4aromatic hydrocarbons to low suliur gasoline while minimizing coke' formation, which process consists essentially of contacting said charging stock with between 'about 0.1 and about 2 volumes of liquid hydrogen fluoride per volume of said charging stock at a temperature between about 50 F. and about 150 F.,stratifying the mixture resulting from said contacting into a hydrogen liuoride 4solution of .organic sulfur. compounds and polycyclic aromatic hydrocarbons and a hydrogen fluoridc-immiscible'layer of purified gas oil of substantially reduced sulfur and polycyclfc aromatic hydrocarbon content, said purified gas oil having substantially the same 10 percent to percent distillation temperatures as said charging stock, converting said purified gas oil with a silica-alumina type cracking catalystv at a temperature between about 800` FL and about 1000" F., fractionating the conversion ,products into a low sulfur gasoline fraction and a. hig'h sulfur cracked gas oil fraction, saidV cracked gas oil fraction having a higher sulfur content than said puried gas oil, and contacting said cracked gas oil with between about 0.1 and about 2 volumes of liquid hydrogen fluoride per-volume of said cracked g'as oil at a temperature between about 50F. and about 150 F. to produce a gas oil having a sulfur content not greater than that of said purified gas oil.

` ARTHUR P. LIEN.

. BERNARD L; EVERING.

REFEnENoEs tn'rm)Y The followingreferences are of record in the file of this patent:

UNITED STATES PATENTS Number Narne Date 1,932,174 Gaus et al Oct. 24, 1933 2,203,470 Pier et al June'4, 1940 2,330,685 Connolly Sept. 28, 1943 2,351,154 Schulze L June 13, 1944 2,352,236 Thomas June 27, 1944 2,375,675 Matuszak May 8, 1945 2,378,762 Frey .June 19, 1945 2,379,966 Johnson July 10, 1945 2,428,741 Plank Oct.7, 1947 2,430,724 Meadow Nov. 11, 1947 2,434,623 Meadow et ai. Jan. 13, 1948 2,444,316 Vautrain June 29, 1948 2,450,588 Evering et al. Oct. 5, 1948 Y e Certificate of Correction v Patent No. -2,525,812- October 17, 1950 ARTHUR P. LIEN ET AL.

It is hereby certified that error appears in the printed speciiction of the above numbered patent requiring correction as follows:

Column 3, in the table headed Products from, column 1`thereof, under Dry Gas, fifth chemical formula, for C21-I6 read 08H6; column 4, line 10, for lawer read lower; line 19, for terated read treated; column 7, line 49, after recycled insert through; column 9, line 30, after silica-alumina insert 'f1/P6; y and. that the said Letters Patent should be read as corrected above, so that the' same may conform to the record of the case in the Patent O fce.

Signed and sealed this 26th day of December, A. D; 1950.

time@ OMAS 1F. MURPHY,

Assistant Uommzssoner of Patents.

Claims (1)

1. IN A PROCESS FOR CONVERTING A GAS OIL CHARGING STOCK CONTAINING SULFUR COMPOUNDS AND POLYCYCLIC AROMATIC HYDROCARBONS TO LOW SULFUR GASOLINE WHILE MINIMIZING COKE FORMATION BY A COMBINATION OF STEPS COMPRISING SELECTIVE EXTRACTION OF SAID SULFUR COMPOUNDS AND POLYCYCLIC AROMATIC HYDROCARBONS FROM SAID GAS OIL CHARGING STOCK AND CRACKING CONVERSION OF THE RESULTANT PURIFIED GAS OIL WITH A SILICA-ALUMINA TYPE CATALYST, THE STEPS CONSISTING ESSENTIALLY OF CONTACTING SAID GAS OIL CHARGING STOCK WITH BETWEEN ABOUT 0.1 AND ABOUT 2 VOLUMES OF LIQUID HYDROGEN FLUORIDE PER VOLUME OF SAID CHARGING STOCK AT A TEMPERATURE BETWEEN ABOUT 50*F. AND ABOUT 150*F., STRATIFYING

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