Poisoning of the acid sites decreases hydrocracking activity. HoLvever, no indication has been given of the magnitude of simultaneous poisoning of the platinum surface. While a catalyst containing 1% platinum experienced a 5070 decrease in hydrocracking activity, the hydrogenation activity decreased only about 20% in the same interval of 20 hours. A catalyst containing 570 platinum decreased in hydrocracking activity by 6070 and in hydrogenation activity by 25y0. Thus, it is suggested that the drop in activity for hydrocracking is more strongly related to the poisoning of acidic than of hydrogenation sites.
sintering of the platinum crystals begins to be significant in this range. The chemisorption method of measuring platinum surface is an excellent tool for predicting both hydrogenation and hydrocracking activity. Finally, the use of ammonia for differentiating the strength of acid sites on hydrocracking catalysts appears interesting. However, the utility of the method is questionable, since sites which may not be active in hydrocracking are measured. Acknowledgment
The authors express appreciation to R. T. Barth for his ~ v o r k on the ammonia chemisorption measurements.
Conclusions
Platinum is selectively adsorbed on the strong acid sites. perhaps on an atomic scale. Since these sites are active in hydrocracking, an optimum quantity of platinum exists for a given support. For a silica-alumina containing 25y0alumina. this ranges from 1.0 to 2.570 platinum by weight \{Then a simple impregnation procedure is used. Such a range exists because hydrocracking requires both hydrogenation and acidic sites. Stocks that do not contain poisonous nitrogen compounds can best be used in combination with the higher platinum levels, while relatively high concentrations of nitrogen \vi11 Jvork best with low platinum levels. The degree of dispersion can be varied by the chemical form of the impregnant and the thermal history of the catal>st. Temperatures higher than 650” C. are to be avoided. since
Literature Cited (1) Archibald, R. C., Greensfelder? B. S., Holzman, G.. Rolvr. D. H., IND.ENG.CHEM.52, 745-50 (1960). (2) Barth, R. T.: Ballou, E. V.: Anal. Chem. 33, 1080 (1961). (3) Ciapetta, F. G. (to Socony Mobil Oil Co.), L.S. Patent 2,945,806 (July 19, 1960). (4) Coonradt, H. L.. Ciapetta. F. G.: Earwood. \V. E., Leaman. lv. K.. Miale. J. h’.:IND. E S G . CHESI. 53, 727 (1961). (5) Flinn, R. .4..Larson! 0. h.: Beuther, H.; Ibid., 5 2 , 153-6 (1960). (6) Maatman. R. W.. Prater. C. D.. Ibid., 49, 253 (1957). ( 7 ) Scott, J. \V., Jr. (to California Research Corp.), U. S . Patent 2,944,005 (July 5>1960). (8) Spenadel, L., Boudart. M.. J . Phys. Chem. 64, 204 (1960). (9) \.\:ebb. A. N..IND.ENG.CHEM. 49, 261 (19.57). RECEIVED for review December 7. 1961 ACCEPTEDMay 9 : 1962
Division of Petroleum Chemistry, 140th Meeting, ACS, Chicago. 111.: September 1961.
LIQUID-LIQUID EXTRACTOR Application to Separation of Strontium and Barium H A R L E Y A. W I L H E L M A N D M A U R I C E L. A N D R E W S Iowa State University of Science and Technoloqy, Ames, Iowa
A laboratory-size, 20-stage, multiple-contact, “unlimited-feed,” countercurrent-flow, liquid-liquid extractor i s described. The apparatus consists mainly of glass parts, joined b y polyethylene tubing and mounted in a steel cradle that can rotate on its horizontal axis. The reservoirs for feed liquids are an integral part of the assembly. Proper rotation of the assembly causes the flow of liquid to, through, and from the extractor. Use of this extractor i s exemplified b y a study on the liquid-liquid separation of strontium and barium. Starting with an aqueous feed 0.5M in SrC12 and 0.5M in BaC12, a barium product containing less than 100 p.p.m. of strontium was delivered in the aqueous phase, while moderately pure strontium was obtained in the organic product phase.
s
and purification of metals through liquidliquid extraction of their compounds have received considerable attention and study in recent years. Derivation of such a liquid-liquid process generally requires that single-stage distribution data be obtained in the early development work. Ho\vever. information on the behaviors of the components of an extraction system under actual or simulated operational ccnditions is often essential to the subsequent derivation of a workable process. This article presents a small-scale, 20-stage, multiple-contact. countercurrent-flow extractor in which the operating variables can be readily tested. Some types of data that may be obtained on a liquid-liquid system through EP.\R.ATIO~
use of this extractor are exemplified by results of experiments on rhe separation of strontium and barium. Design and Operation of Extractor
The extractor consists essentially of an orderly assembly of feeder systems, mixer-settler chambers, interstage transfer bulbs, and connecting tubes. These parts are all made of glass and are joined by polyethylene or other practically inert flexible tubing. This assembly (Figure 1) is mounted in an open structure steel cradle constructed of two steel end plates connected by four lengths of angle iron. The cradle is supported in a horizontal position by two end shafts that rest in bearings on an over-all steel framework. The feeding of VOL.
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and Foas (7, 8). The apparatus developed by Craig and by Lathe and Ruthven were, however, designed to move only one of the liquid phases in a stagewise manner. Their extractors may be referred to as the “limited-feed” type, because they operate on essentially single batches rather than on continued additions of feed. All of the extractors considered here have a common feature, in that they depend for their operation on repetition of a sequence of mixing, scttling, and separate flaw of the phases far each stage in an assembly
or
Figure 1.
Figure 2.
Extractor assembly
Arrangement of ports for a single stage
Figure 3.
Feeder system
liquids to the extractor, the multistage extraction, and the delivery of product solutions from the extractor are performed through rotation of the cradle and its extractor assembly an the horizontal axis. Since these operations take place repeatedly as rotations of the cradle are continued, the extractor may he referred to as the “unlimited-feed” type. Extractors that have some physical resemblances to the one presented here have been described by Craig ( Z ) , Lathe and Ruthven ( 3 ) ,von Metzsch (4,Alderweireldt ( I ) , and Wilhelm 306
l&EC PROCESS DESIGN A N D DEVELOPMENT
The two phases are separated in a stage in most of the above extractors after settling by drawing off the less dense liquid through a tube opening positioned a t or near the liquid-liquid interface. Since the volume of the more dense liquid essentially determines the height (or position) of this interface after the settling, it consequently determines the optimum position of the opening through which the less dense phase is removed. I n extraction systems set up to operate under conditions designed for production, significant concentration and volume changes often occur in the portions of the immiscible phases as they move stagewise through the extractor and as steady-state operation develops. Should the volume change for the more dense phase in any stage in an extractor of the type being considered become such that the position of the interface is moved significantly, the tube opening for carrying away the less dense phase may need repositioning. The extractor described here has a design that permits adjustment of the positions of these openings. Consequently, this extractor has broader adaptability in liquid-liquid extraction studies and operations than those described by Craig, von Metzsch, and Alderweireldt which do not provide far such adjustments. The extractor of Lathe and Ruthven, although different in design, also lacks this broader adaptability. The extractor presented here is, in the main, an orderly side-by-side arrangement of 20 units, each unit corresponding to a single stage such as that sketched in Figure 2. I n operation, p?rtions of the two ‘immiscible liquid phases, differing in density, are in the mixer-settler chamber, M . The two liquids are mixed by oscillating the chamber about its axis of rotation, between about + Z O O and -20‘ from the horizontal position represented in Figure 2. After adequate mixing, chamber M is held at a nearly horizontal position to permit settling of the liquid phases. The unit is then turned slowly clockwise through about 90’ and as the mixer-settler chamber goes to this vertical position, the light liquid flows out through tube L toward the interstage transfer bulb, T . The position of the extension of L is adjustable within chamber M , so that the liquids can be properly separated at a liquid-liquid interface for different volumes of the heavier phase. Further rotation of the unit in the clodtwise direction to the 180‘ position permits the heavier liquid phase to flow from M through tube H to the interstage transfer bulb, T’. Tubes H’ and L’ in Figure 2 are actually connected to inlet spouts on the mixer-settler chambers on either side of the chamber shown. Further clockwise rotation of the unit, then, allows the separated liquids to flow from the transfer hulhs to mixer-settler chambers M 1 and M - 1 that are juxtaposed to chamber M in thr total assembly. This completes one cycle of operation far a single stage. For the entire assembly, the mixing-settling-flow sequences far all 20 stages take place concurrently. The net result is countercurrent flow, with the two liquid phases being delivered at opposite ends of the extractor.
+
An explanation of the essentially automatic feeder system may be facilitated by reference to Figure 3. Here the relative positions of a feeder and the mixer-settler chambers are indicated with respect to their common axis of rotation. Liquid from the cylindrical reservoir tank of Figure 3 flows
into and floods the graduated feeder, F, at a point near or shortly beyond the 180' of clockwise rotation in the operation cycle described above. continuation of this clockwise rotation causes the excess liquid in the tubes connected to the feeder to flow back to the reservoir. When this rotation is such that the liquid level is within the upper enlarged tube section of F, a short quick rotary movement is imparted to the assembly to bring the graduated section of the feeder to a vertical position. This position is held until the excess liquid has flowed from the graduated section of the feeder through the adjustable tube. By this manipulation, a measured volume of liquid is retained for delivery to the extractor on completion of the cycle. For an extraction three such feeder systems are generally adequate. One feeder may introduce organic solvent at one end of the extractor, one may introduce aqueous scrub a t the other end, and another may introduce feed solution a t some intermediate stage. So on each cycle of operation of this extractor, predetermined volumes of liquids are taken from three reservoirs and delivered simultaneously to the extractor a t the selected stages. The breathers indicated in the figures allow free movement of air in or out as required by proper flaw of the liquid phases during the cycling. Four reservoir breather tubes, each having a constriction as indicated by that sketched in Figure 3, are connected a t YO" intervals around the reservoir tank. Figure 4 is a more detailed sketch of the reservoir shown in perspective. The mixer-settler chambers are made from 25-mm. glass tubing and each will conveniently accommodate 60 ml. of combined liquid phases per cycle. The reservoir tank in the feeder system of Figure 3 is a glass cylinder about 4.5 inches in diameter and 9.5 inches in length. I t has a maximum operatThis maximum ing capacity of almost 2 liters of liquid. operating capacity is about 80% of the tank's total volume; a tank should not contain liquid at a level greater than three fourths of the vertical diameter of the tank cylinder. The main bodies of the graduated feeders and transfer bulbs were made from 25- and 30-mm. tubing, respectively. Systematic procedures for starting, adjusting, and bringing to steady-state operation an extractor of this design can be derived from considerations based on the above description. The adjustable tubes in the mixer-settler chambers should be positioned to ensure that none af the heavy phase will flaw through them. Further suggestions on procedures in operating, as well as miscellaneous considcrations in con3tructing an extractor of this design, ate given in some detail by Wilhelm (6).
Use of Extractor The course of the development work employing multistage extraction experiments in the derivation of a practical liquidliquid process is based largely on observations made on the behavior of the system and on analytical data on the product solutions. The extractor described here is useful in such development work, since thorough visual inspection of the system in all stages at all times is possible and interruption of an extraction experiment far analytical data, for changes in the conditions of operation, or for the convenience of the operator, is possible without interfering with the proper functioning of the extractor. This extractor also permits extending operations to essentially steady state far an extraction system. I n addition to obtaining satisfactory products by an extraction and obtaining information on which to base operation of a large scale continuous extraction column, interest may also bear to some degree on an interpretation of the mech anism of the extraction process. Samples of both phases may be readily obtained Irom all stages of the extractor after mixing the phases in the stages following a cycle of operation. Phase volume measurements and complete analytical data on such samples can supply information pertinent to an interpretation of the behavior of the constitucnts of the liquidliquid system during an extraction. This small extractor is also convenient to use where the amounts of materials tu he processed are limited or where only small amounts of products are desired. The amount of product can be extended, however, by continued operation, and extractors of this desian hut with larger capacity parts can be used to product3 significant amou nts :tractor has been empk,yed in the limited area of liquid-liquid extraction dealin g wi th metal separations in l,II.....l, :r.j ,,pears tho+ .nnl:rst:-ns +aqueous-organic I./ltsmC other problems in the broader field of liquid-liquid extraction are entirely within its possibilities (5). Some results of the type that can be obtained with this extractor in the development of an extraction system and on studies of the behavior of the components of the system are included in the following treatment on a separation of two
.
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LlluI
ayy.Lb~b2v..1
A method was developed for the separation at Strontium and barium by liquid-liquid extraction. In the preliminary work the chlorides of these elements in aqueous solution did not show adequate tendency tc distribute to any of a large number of organic liquids that were tested. The presence of 3 moles of ammonium thiocyanate to 1 mole of strontium plus barium in the aqueous solution, it was found, gave promising transfer to some organic liquids. Tri-n-butyl phosphate was selected for further development work, but preliminary data indicated that n-amyl alcohol, cyclohexanol, isophorone, and tert-amyl alcohol might also he considered. I n experiments with 2, 3, and 4 moles of thiocyanate to 1 mole ot" strantium plus barium (0.5 mole of each), the amount of distribution to the tributyl phosphate phase increased with increasin!;ratio of thiocyanate to metal. However, 2 moles of thiocy.anate to 1 of the metals gave adequate transfer with aration. Strontium favored the organic phase more the barium. Incidentally, tests a n calcium indicated a pouibi lity of separating it from strontium or barium in such a sy:stem containing thiocyanate. Tests TMere made to determine the effects on distribution
.
Figure 4.
Details of feeder syst em reservoir In perrpedive
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Figure 5. Strontium (and barium) content in a number of stages
'Oo'
t
+-ORGANIC
f-
\
I
I ,
'I II
Figure 6. Amounts of strontium and barium in phases in each of 20 stages
behavior caused by additions of hydrochloric acid and of ammonium hydroxide to the aqueous feed solution. Small additions of the acid reduced the strontium and barium transfer markedly and proportionately, while the base, even in concentrations nearing precipitation conditions, had little effect on the distribution. The substitution of equal moles of sodium thiocyanate for ammonium thiocyanate in the aqueous solution gave a marked increase in transfer of the strontium plus barium to the organic phase with a good separation factor. Furthermore, addition of sodium chloride to the feed containing the sodium thiocyanate seemed to improve the separation factor with little effect on total transfer. However, excessive sodium chloride concentration caused formation of a precipitate during a trial countercurrent extraction test. Treatment of the organic phase with thiocyanate before charging the organic into the countercurrent operation was found to be unnecessary. O n the basis of the preliminary development work, operating conditions were set up to include the following compositions of the liquids to be fed to the extractor at the stages indicated. Aqueous scrub. 3M NaCl added at stage 1. Aqueous feed. 0.5M BaC12, 0.5M SrC12, and 2.2M NaSCN added at stage 11. Organic solvent. Tri-n-butyl phosphate, washed with water, added at stage 20. Employing these conditions and flow rates of 15, 15, and 30 ml. of scrub, feed, and organic, respectively, per cycle (feeding ratio 1:1:2) gave high purity strontium in the organic product solution and moderately pure barium in the aqueous product solution. The scrub. feed, and organic flow rates were then readjusted to 12, 12 and 36 ml., respectively, per cycle (feeding ratio 1 :1 :3) for the next experiment. These flow rates gave barium that was 99.99+ weight % pure with respect to the barium and strontium in the aqueous product and moderately pure strontium in the organic product. At the end of this experiment, which continued for 160 cycles, quantitative data were obtained on the constituents in the portions of the liquid phases in each stage. The qualities of the barium and strontium found in the portions of aqueous and organic liquids in stages throughout the extractor are in308
I & E C PROCESS D E S I G N A N D DEVELOPMENT
dicated by the points plotted in Figure 5. The barium in the portion of the aqueous phase that is ready for discharge from the extractor at stage 20 has less than 0.01 weight % strontium. The strontium in the portion of the organic phase that is ready for discharge from the extractor at stage 1 is of moderate purity. The barium and strontium contents of the phases were determined by first precipitating them as mixed sulfates and then analyzing the solid mixtures by x-ray fluorescence techniques. Precipitation of the sulfate was done on the aqueous phase directly; the organic phase was treated with an aqueous solution 2M in ammonium sulfate, which extracted and then precipitated the sulfates of barium and strontium. Rough quantitative data on sodium in the organic phase in a number of stages were obtained by igniting and weighing the sulfate residues left by evaporating the aqueous filtrates and washes following the strontium and barium precipitations. Thiocyanate contents of the stages were determined by a colorimetric method. The chloride was then determined by difference, employing another colorimetric technique that gives the combined thiocyanate and chloride contents. Analytical data indicated that the chloride and the sodium entering the extractor through the scrub and feed solutions both remained largely in the aqueous phase and passed through the extractor without build-up. The thiocyanate ion, which entered only at the feed stage with the barium and strontium, followed both barium and strontium in both phases in the scrub section of the extractor with a thiocyanate ion to metal ion ratio of slightly more than 2 to 1, respectively. From the feed stage to the aqueous discharge end, however, this ion ratio was somewhat higher, indicating that the excess thiocyanate tended to follow the aqueous phase. There was a build-up of barium in the extract section and of strontium in the scrub section (Figure 6). From the data for this figure and stagewise phase volume measurements, dis-
tribution quotients for strontium. Dsr, and barium, DBa, were calculated for stages throughout the extractor. I n the scrub section, Ds, had values in the range of 0.7 to 1.1. while DBs had values, except for stage 3, in the range of 0.2 to 0.3. The separation factors, p values, for the scrub section were in the range of 3.0 to 4.0. I n the extract section, Dsr ranged from about 2.0 to 5.0, \vhile DBs ranged from 0.4 to 0.55, giving /3 values from about 5.0 up to about 10.0. The higher distribution quoticnts of strontium and barium in the extract section are in part due to the higher thiocyanate concentration in that section. Discussion .A multistage extractor of the design described here could be set up to operate with more than 20 stages. The number 20 was chosen for convenience in separations work where fairly good separation factors may be developed. A larger number of stages would require more cycling before steadystate operation could be adequately satisfied. However, in the experiments on strontium and barium separation, a few more stages probably would have made it possible to prepare high purity strontium and high purity barium simultaneously from a mixture using the same liquids. Any extraction test requiring less than 20 stages could be operated on this extractor by merely changing interstage connections to bypass some of the stages. Since the analytical data indicated that only relatively small amounts of chloride and sodium were distributed to the organic phase, but that approximately 2 gram ions of thiocyanate per gram ion of combined strontium and barium were in the
organic, it appears that the species distributed to that phase contained (Sr, Ba) (SCN)z. Possible extension of the extraction system containing thiocyanate to separations involving calcium has been pointed out. Speculation based on known behaviors of the tested alkaline earth elements in a thiocyanate system leads one to predict that a system similar to that employed here for separating strontium and barium could be effective for separating barium and radium. Acknowledgment
The authors are indebted to Robert Heidel for supplying the x-ray fluorescence data used in the quantitative determination of strontium and barium, and to Bruce Raby for the colorimetric determinations of chloride and thiocyanate. literature Cited
(1) .4lderweireldt, F., Anal. Chem. 33, 1920 (1961). (2) Craig, L. C., Zbid., 22, 1346 (1950). (3) Lathe, G. H., Ruthven, C. R. J., Biochem. J . 49, 540 (1951). (4) Metzsch, F.-A. von, Chem. Zng. Tech. 31, 262 (1959). (5) Scheibel, E. G., “Technique of Organic Chemistry.” 2nd ed.,
Vol. 111, p. 332, Interscience, New York, 1956. (6) Wilhelm, H. A., U. S. At. Energy Comm.. Rept. IS-309 (June 1961). (7) Wilhelm, H. A., Foos, R. A,, IND.ENG.CHEkf. 51, 633 (1959). (8) IVilhelm, H. A., Foos, R. A., U. S. .4t. Energy Comm., Rept. ISC-458 (September 1954). RECEIVED for review October 13, 1961 . ~ C C E P T E D March 12, 1962 Contribution 1072. Work performed in the .4mes Laboratory of the U. S. .4tomic Energy Commission.
HYDROGEN DONOR DILUENT VISBREAKING OF RESIDUA A. W . L A N G E R , J O S E P H S T E W A R T , C. E. T H O M P S O N , H . T. W H I T E , AND R. M. H I L L Esso Research and Engineering Co., Linden, II’. J .
Thermal cracking of crude residuum mixed with a selected hydrogen donor diluent was investigated as a means for obtaining higher conversions and improved product quality. In visbreaking operations, an effective hydrogen donor can reduce residual fuel yield more than 20% when residuum conversion is limited b y asphaltene formation. The minimum residual fuel yields are obtained b y operating at the lowest severity which produces the desired viscosity.
to lower boiling products mild cracking in the presence of a hydrogen donor diluent has been described (2-4). I n this work, tetrahydronaphthalene and partially hydrogenated refinery streams containing a high proportion of condensed-ring aromatic compounds were used as the thermal hydrogen transfer agents. High conversion of residua to more valuable lower boiling products, with very low yields of coke and dry gas, was demonstrated. HE CONVERSION OF CRUDE RESIDUA
The present work is concerned with another application for this hydrogen transfer technique which is called hydrogen donor diluent visbreaking (HDDV). The use of hydrogen donor diluents in visbreaking reduces residual fuel yields by improving visbreaker tar quality. The need for such a process is evident wherever visbreaker operation is limited by fuel oil qualities other than viscosity. One of the most important of these limitations is the content of high molecular weight condensed-ring compounds commonly called asphaltenes. VOL.
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