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N. F. CHAMBERLIS, J. A. DIYU'WIDDIE, AND J. L. FRANKLIN Humble Oil & Rejining Company, Buytown, Tex.
A ftrndnriientai approach to the problem of propane dewaxing has been made through microscopic studies of wax crystals from a mid-continent paraffin distillate, Wax crystals were observed i n their normal environment a t various stages of the chilling period by means of a pressure cold btage. When no dewaxing aid was present, the wax crystallized in plates which were arranged in either a flat or a honeycomb struclure. K7hen a small amount of asphalt w-as added as dewaxing aid, the way crlstallized rapidly immediately below the crg stal point into spherical particles of unknown structure, arbitrarily termed ('nuclei." As chilling was continued, each nucleus was quickly surrounded by a shell of radiating needles. When large amounts of asphalt were used, a large number of small niirlei formed at the crystal point and grew slowly until the filtering temperature was reached; these formations never developed the surrounding shell of radiating needles. Filtering characteristics were critically dependent upon the type of w-ax structure present. URING the course of a seriesof pilot unit studies designed to establish optimum process conditions and to determine product yields and quality obtainable upon propane deaaxing a mid-continent paraffin distlllate, it became apparent that there was need for supplementary information of a more fundamental nature. It was desired to develop (a)a real understanding of the action of dewaxing aids, ( b ) a dependable correlation of way crystal structure with filter rate and behavior, and (c) improved methods of controlling crystal structure. Such knowledge could be used in interpreting previous propane den axing Operations of both plant-scale and pilot unit&,in improvmg piesent and future operations, and perhaps in developing a dewaxing aid that m~ould not adversely affect the color of the deu axed oil. This knowledge could be gained only through systematic microscopic examination of n ax crystals. Extensive studies of thii type have been carried out on residual stocks and the results recorded in the literature (I), but little information on parafin and lubricating oil distillatcs is available. Accordingly, there was initiated a comprehensive microscopic stud> of wax crystals as they are precipitated under various conditions from propane solutions of a mid-continent parafin distillatc stock. Although the study is not complete, certain regularities have been observed and it is believed that a, report of the work performed is justified.
inch low-temperature steel pipe; ihe annular space thus formed served as a propane cooling jacket. The jacket had a cutout portion to provide room for 3/1e-inch thick Pyrex observation window A in t,he wall of the 1-inch pipe. The slurry to be examined was introduced through a/,-inch copper tube J which directed the stream toward the window. Light was introduced through Pyrex tube E , sealed and flattened a t both ends. A small disk of Polaroid J film, I , held by friction insidc the top of the tube, was the polarizer. Rubber packing around the tube's lower end afforded sufficient freedom of movement to prevent breakage during adjustment. A double packing gland arrangement was used to obtain vertcial atljustmont without applying torquc to the glass tube. Dryer F , which contained Ascarite in a perforated trough around its side wall, prevented iroxt,ing of the microscope objective and window. The dryer was sealed a t the top by a thin rubber diaphragm, G, which allowed the objective to move freely. Metal shield H was at,tached to tho microscope as a safety precaution to protect the operator if the glass window should break. No such breakage was experienced, however, during the course of this work. The microscope objective, of course, had to have a sufficiently long working distance to allow objects to be focused through the thick window. A standard Bauach & Lomb 16-mm. ( X 10) achromatic objective was used in this apparatus. The microscope was equipped with x ivicol prism a,s an analyzer. hlthough this unit proved adequate for our purpose, a number of desirable improvements mi ht be suggested. Window A and tube B were made of standarcf Pyrex, and no attempt was made t o polish their surfaces optically flat. This probably resulted in some distortion and did result in some undesirable scattering of t,he polarized light. Also, the depth of field obtainable with the arrangement described was insufficient t o permit photographing of both large and small wax particles in sharp focus a t the same time. This situation might be improved by inverting the cold stage so that the visible sides of all the wax particles would lie in t,he same plane against the window. I t is also desirable t o be able to rotate the plane of polarized light. with respect to the sample or vice versa. The light source employed, not shown in Figure 1, was a vaporproof water-cooled projector, using a 300-watt7 120-volt Mazda project,ion lamp. Heat was removed from the light beam by P wa,ter-cooled plastic filter. A mirror and lens system was em-
DESCRIPTION OF APPARATUS YALE
Figure 1 shows the pressure cold stage used in this work. It consisted of a 1-inch low-temperature alloy stre! pipe concentric with a standard 2-
e _---I" Figure P.
566
Pressure Cold Stage
--A*'
March 1949
567
INDUSTRIAL AND ENGINEERING CHEMISTRY
It.
Crystal point (11' C . )
A.
Fiillv rhillrd (-40'
C.)
Vo Dewaxing Aid I're~enl
C.
D.
UndiRturbed (8" C . )
Crushed (-40'
C.)
0.2% Asphalt Present
Figure 2.
Wax Cyystals from a Mid-continent Paraffin Distillate-Propane Solri tioii ( X 156)
ployed to form a sharply focused image of the light filament at point B on the glass tube. KOcolor filters were used. The pressure cold stage was connected to a suitably insulated pressure vessel which was made of low-temperature alloy steel and equipped with oil and propane charge lines, vacuum line, and compressor for chilling by autorefrigeration, etc. The connection was made in such a way that solution could be withdrawn from the pressure vessel, pumped through the pressure cold stage, and returned to the pressure vessel through insulated lines. OmRArioN
Briefly, the procedure used in making observations was as follows: Twenty to sixty liters of warm oil containing the desired amount of wax modifier were added to the chiller. To this were added two and one half volumes of liquid propane per volume of oil. The liquids were thoroughly mixed with a mechanical stirrer and were heated, if necessary, until the final lemperature was 52" C. (126" Fa). Autorefrigeration was then initiated, and the total amount of propane in the chiller was held essentially
constant by adding liquid propane at room temperature. The usual chilling rate was 1 C. per minute down to about -30' C. (-22" F.). Below this temperature the chilling rate decreased somewhat. The pressure cold stage was not considered safe for pressures in excess of 150 pounds per square inch gage; therefore it was not possible to obscrve the mixture until the temperature reached about 24" C. (75" F.). When the mixture reached this temuerature. the uumo was started to circulate the mixture through the cold stage and thence back to the chiller. Samples for microscopic observation were trapped from the moving stream by moving the vertical glass tube from a lowered to an elevated position. Field thickness could be varied by moving the tube up and down. A fresh sample could be obtained by lowering and raising the tube through a distance of approximately '/8 inch. Using this method, mixtures could be examined a t all temperatures below 24' C. to the desired minimum of about -43" C.
-~
(-45OF.).
INDUSTRIAL AND ENGINEERING CHEMISTRY
568 TABLE
I. TESTD A T A
ON XID-CONTINENT STOCKS
Parafftn Distillate Gravity, A.P.I. 32.3 Viscosity, Saybolt Universal soc. 13OC F. 66 210° F. 40 Pour point, F. 95 Wax" weight 70 22.5 M.P. bf dry waxa, F. 124 (I wax, free of oil and solvent, obtained by dewaxing t h e of Oo F. with see-butyl acetate as solvent.
Medium Motor Oil Distillate 27.9
...
63 120
19.6
140 to a
point
Most photographs were taken with Nicols parallel, showing the wax as dark particles on a light background; but additional information was gained by observing the samples between crossed Nicols, showing the wax as bright particles and dark shadows against a dark background. It is difficult to obtain a field suitable for photographing that shows both- representative distribution and structure of the wax formations present, especially when there is a wide range in particle size. Hence, the usual procedure was to attempt to find a field which illustrated the types and structure of the wax present with only secondary importance being given to showing a representative distribution. Table I gives test data on the charge stocks employed in thi.; work. WAX FROM PARAFFIN DISTILLATE
WITHOUT DEWAXING AID. -4t temperatures above 11" C . (52"F.) no wax or other solid particles were observed. At about 11 C., however, the was formations shown in Figure 2 4 , appeared; this temperature was arbitrarily termed the "crystal point." An outstanding characteristic feature of these formations wm their looseness. They were probably white, although they appeared t o have the light yellow color of the solution. Except for oolor they appeared w r y much like a sheet of crushed clear cellophane suspendcd in water, t,hc creases and edges appearing &r, needles. However, it^ is not to be infcrrrd that they could be flattened out into a single plane. Except that the geometrical symmetry was lacking, the st,ructure was similar to that of a honeycomb in which the plane intersections appear as needles. It was never possible to identify needle crystals in the fragments resulting from tearing up these honeycomb structures, but plate crystals were invariably identified. A small number of plate
A.
Undisturbed formations (cover glass put gently in place)
Figure 3.
Vol. 41, No. 3
groups always accompanied the large loose formations, but it is not known whet,her they were separately formed or were torn from the large formations. The honeycomb structures were formed almost exclusively from t'he crystal point to about -7" C. (20' F.). As chilling was continued below t,his temperature, another type of cryfital formation appeared-waferlike micaceous clusters. Figure 2,B, showe some of these clusters. They are composed apparently of plate crystals lying with their flat sides together. When viewed from, their flat side, thcy appeared roughly elliptical and almost transDarent. When viewed from the edEe. thev" aoueared as dark .. lines. S l o rates ~ of chilling (about 0.25 C. per minute) favored. the formation of the micaceous plate clusters whereas fast, rates (1 to 3' C. per minute) favored the honeycomb structure. 111 addition to thc micaceous cluster just considered, a mushy, fragmentary form of wax also appeared. Its struct'ure could not be definitcly charact,crized, but it seemed to bn composed of snialt needles and plates. Hence, the \Tax appearing in a fully chilled batch of paraffin distillate consisted of honeycomb plate formations, micaceous clusters, and small amounts of mushy, fragmentary material, apparently composed of small needles and plates. Pilot-unit filter operation on t,hie material was charact,erized by a very high filter rate of about 30 gallons of dewaxed oil per squarr foot per hour. This rate was obtained with a 15-second pickup time (t,hat time during which the slurry is being filtered, prior t o subsequent washing and dixharging of the wax cake). The rather short pickup time was necessary t o obtain a wax cake sufficiently thin so that it could be propane-wvashed. With this condition rather large amounts of propane as wash were required to obtain a satisfactory yield of dewaxed oil. Furt,hermore, the wax cake was quite fluffy, arid it eroded sevei:ely upon being washed with propane. All of these filtering cllara.ct~eristicsseem to correlate well with the type of wax cryst,als observed to be present,. WITH~ P I I A L TDEWAXIXG SID.It has long been known that. an asphalt produced by propane precipitation from a mid-continent residuum is highly effective as a deavaxing aid when added to distillate stocks prior to propane dewasing. This material is inexpensive and easily handled; the only serious objection to its use in processing oils of light color is the fact t,hat it affects this color adversely. Such a material was used in this work. A t temperatures above about 11" C. only dark brown, trans'
B . Clulrters crushed under cover glasa
Wax Crystals with 0 . 3 q ~Asphalt, Examined w i t h Crossed N i d s at R o o m Temperature in Absence of Propane ( X 156)
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1949
Parent (when thin), resinous, lacylike, fragmentary particles were visible in the propane-oil solution when asphalt was present as a dewaxing aid in *he amount of 0.15 to 0.7% by weight of the oil charged. The amount of this material appeared to be directly proportional to the asphalt concentration, and as far as could be determined, it exerted no influence on the crystals that formed a t a lower temperature. At a temperature between 10" and 11 C., dense spherical wax particles appeared, and apparently varied in number with asphalt concentration. They grew in size very rapidly, and appeared to have the same light yellow color as the solution but were much more brilliant. The structure of these particles could not be discerned, but they seemed to be composed of a number of smaller units. Their diameter appeared to be inversely proportional to the asphalt concentration. These spherical wax particles which appeared first upon chilling paraffin distillate containing asphalt were arbitrarily termed "nuclei" with the full realization that they are not nuclei in the ordinary sense. (The I&tterare usually considered to be submicroscopic in size.) At about 10' C. a second growth of crystals appeared suddenly as needles radiating from these nuclei and formed a structure greatly resembling that of a cocklebur. The length of these needles apparently varied inversely with the asphalt concentration. Figure 2,C, shows this structure. It may be visualized more clearly by considering Figure 2,D,which shows fan-shaped segments formed by crushing the cocklebur formations between the observation window and glass tube. The formations just described were also examined a t atmospheric pressure and temperature. The method consisted of drawing a sample of the chilled mixture (-43' C. or below) into a Dewar flask. A drop of this mixture was then placed on a slide and was observed by means of standard microscope equipment, during and after the evaporation of the propane. Figure 3 presents photomicrographs obtained by this method. A shows the undisturbed cocklebur formations, and B the fan-shaped segments formed by crushing the formation. A black radial extinction line passed through these segments when they were rotated on a stage between crossed Nicols. This is considered to be good evidence that the segments are composed of a large number of individual oriented needles. The ratio of the diameter of the wax nucleus to the outside diameter of the shell varied from about 1:2 to 1: 1 as the amount of asphalt was increased from 0.15 to 0.7%. Thus, the shell of
A.
0.3% asphalt, -43" C.
Figure 4.
569
radiating needles is practically absent when the asphalt concentration is high, the major portion of the wax being present as nuclei. In many instances this outside shell has been separated from its nucleus while under microscopic observation. Occasionally, still a third growth in the form of portions of a second shell composed of radiating needles was observed. -4thin secondary shell can be seen around the center sphere (and others) in Figure 2,C. This second shell apparently was seldom formed completely and was readily separated from the primary shell. In addition to the crystal forms just described, a mushy, fragmentary material began to form a t about -7" C. and was present in considerable amount in the fully chilled batch (Figure 4,A) The structure of this material was not clear, but seemed to be composed of needles and plates. Since the second shell of radiating needles referred to above was apparently very unstable, it is conceivable that a t least a portion of this fragmentary material resulted from tearing off this second shell by agitation of the solution. Figure 4,B, shows typical wax formations with a relatively high concentration of asphalt (0.7%). The shell of radiating needles is absent, essentially only spherical nuclei being present. It*has long been known from pilot and commercial scale plant operation that one important effect of a dewaxing aid is t o increase the compactness of thc wax filter cake, with a resultant decrease in its oil content (giving an increased yield of dewaxed oil). Another effect noted in pilot unit and filter leaf tests is that filter rate decreases with increasing concentrations of aciphalt as a dewaxing aid. These effects are shown in Table I1 for the paraffin distillate stock.
TABLE11. EFFECT OF ASPHALT CONTENT ON OIL YIELDAND FILTERRATE^
Asphalt, Wt. % 0.0 0.3 0.7
Pickup Time, Sec. b 15 35 45
Filter Rate, Yield of DePropane Wash Gal. Dewaxed Oil with Required for 72 waxed Oil/Sq. No Propane Wt. % Dewaxed Oil, Wash. Wt. Yo Vol. Ca/Vol. Chargee Ft./Hr. 30 60 2.4 10 64 1.4 9 64 1.4
a Based on filter leaf studies, obtained from submerged filtering area and pickup time. inch in each caee b Varied in order to obtain a cake thickness of l/g to Somewhat better yield-propane wash relationship was obtained in pilot unit studies.
B . 0.7% asphalt, -42'
C.
Typical Crystals from a Mid-continent Paraffin Distillate ( X 156)
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
The compact cake formed as the result of using a duwasing aid
h more rugged and more easily washed with propane than the wax cake obtained without an aid. The filtering characteristics described above can be explained on the basis of the characteristics of the wax crystals present which, in turn, are determined by the amount of dewaxing aid present. The decrease in size and increase in density of the wax crystal clusters account for the changes in filter rate and character of the cake. It is well known that filter rate is dependent on uniformity of particle size as well tts on the actual average size. Maximum rates are obtained when the wax crystallizes into particles of maximum size mith a minimum of size variation. WITH OTHER DEWAXIXG AIDS. Several different maaterials were tried as dewaxing aids in the paraffin distillates. Those which posseszed any wax-modifying power produced the same general changes in %-axstructure as did the asphalt, so t>hatthe photomicrographs shonTn illustrate the structures obtained n-ith all the active wax modifiers invest,igated. However, the concentration of dewaxing aid required to produce a given degree of modification varied widely among the different materials. In an attempt to isolate an oil-free wax modifier, a sample of asphalt was separated into a butane-solublc and a butanc-insoluble portion (at 30" C., 86" F . ) j and the two portions were tested as dewaxing aids. Both were effective dewaxing aids, but the insoluble portion appeared to be the more effective. The butane-insoluble portion was sufficiently soluble in the propaneoil solution, ho-ivever, t o affect adversely the color of the demaxed oil: hence it had no material advantage over the original sample. h sample of cracking-coil tar (obt,aincd from a coil and tank unit) was also examined as a dewaxing aid. I t possessed crystalmodifying properties, but was inferior to asphalt because of excessive quantities (above 4.0T0) required to produce a marked degree of m-ax modification. A sample of asphalt that had been b l o m with air a t an elevated temperature was found to have little or no wax-modifying properties. Paraflow (a high-molecular-Feighb polymer used commercially as a pour depressant for lubricating oils) was also examined as a crystal modifier and it was found t o produce results similar to those produced by asphalt. Ilowever, only about one fifth as much Paraflow as asphalt was required for a given amount of crystal modification. Paraflow has two distinct advantages over asphalt: (I) It affords a higher filter rate for a given amount of crystal modification. This may result, a t least partially, from thc fact that no resinous particles, similar to those always present in asphalt-aided batches, are
VOL
41, No. 3
formed at any stage of thi. chiliirig perivd. ',2) It has a relatively light color and therefore does not adversely affect the color of the deffaxed oil. It has the disadvantage, however, of being relatively expensive. It was thought that small solid particles might serve as nuclei (similar to the wax nuclei previously described) about which wax would precipitate. h fincly divided solid substance, insoluble in propane and possessing niodifying properties, would be desirable as a dexwxing aid in that it would be filtered with the petrolatum arid hence would h a w no adverse effect on the color of the dewxed oil. Crushed cracking-coil coke and commercial lampblack were investigated for this purpose. but were found to possess no crystal-modifying properties. The cracking-coil coke particle size varied approximately from 3 to 30 microns, the average being about 10 microns. The lanipblack particle size was somewhat, smaller. W A X FROM A MEDIUM MOTOR OIL DISTILLATE
A study similar to that just described was also carried out on L rnid-cont,inent medium motor oil distillate. T e s t data on this stock are shown in Table I. The general effect of dem-axing aids on motor oil was observed to be the 8ame as t,hut on paraffin distillate, but, presumably because of the presence of entrained residuum, smaller additional quantities of aid were required t o obtain the same degree of wax modification. The wax precipitated from medium motor oil was very sticky (as indicated by its tendency to stick to the observation window in the cold stage), especially during the early part of the chilling period, whereas the wax from paraffin distillate did not pass through a sticky stage. The crystals precipitated from motor oil iwre about, one third to one half the size of those precipitated under similar conditions from paraffin distillate. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance and cooperation of J. B. Beaugh during the course of this investigation. Appreciation is also expressed to the Humble Oil & Refining Company for permission to publish this x-ork. LITERATIIRE CITED
(I) Anderson, A. I?., and Talley, S. K., IND.E N GGEEX., ~ 29, 484 (1937). RECEIVEDDecember 6 , lY-47. Presented hefore t h e Third Southwest Regional l l c c t i n p of the Avertrcnh' (:NERIICAL SOCIGTP,Houston, Tex., Decem. her 12 and 18, 1947.
H. L. JIITCHELL, R., G. SCNREXIQ, AND H. H. KIN(; Kansas Agricultural Experiment Station, Manhattan, Kan. Experiments were performed on several finely ground solids to determine their effectiveness as carriers for preparation of solid carotene concentrates. Results of experiments show that use of less highly refined carriers resulted in a more stable concentrate. Of the carriers tested, cottonseed meal had the greatest stabilizing effect.
D
URING World War I1 much consideration was given to the preparation of carotene concentrates from plant materials a3 a means of augmenting the short supply of vitamin A obtained
from fish oils. In general, the concentrates were prepared by dissolving the extracted plant lipides in edible oils such as soybean and cottonseed oils. However, the carotene of such concentrates was destroyed during storage. By storing a t lorn ternperatures (IO)or by adding certain antioxidants ( d ) the stability of the carotene in such concentrates was increased. Carotene concentrates are still of much interest because of the continued shortage and increased cost of fish oils. To facilitate the incorporation of such concentrates into the rations of farm animals, it perhaps would be more desirable to prepare solid,
