ZONE PRECIPITATION Separation Technique Based on Diferences in Solubilities I
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A separation technique with potential use in the chemical, pharmaceutical, and petroleum industries has resulted from efforts to fractionate
petroleum waxes.
The new tech-
nique, called zone precipitation, separates crystalline o r noncrystalline materials through differences in solubility. While zone precipitation has much in common with zone melting, it operotes on a different principle.
In zone pre-
cipitotion, the mixture to be separated is mixed with a solvent, resulting in a gel-like mixture; the solvent itself does not have to solidify.
A zone in a column of the solid is heated;
the portion in immediate vicinity o f the hot zone liquefies. As the zone moves, the liquid behind it solidifies, rejecting the most soluble components.
After the heater has passed
over the entire slab, the solids of greatest solubility will be depleted behind it and concentrated in direction of movement.
Fractionation i s improved by use o f solvents which
reduce liquid viscosity in the molten zone.
the two most important physical characteristics of petroleum waxes are boiling points and melting points. However, since waxes are solids, they are generally marketed by melting points. I n most refineries, wax processing is dictated by the quality requirements of the lubricating oils being produced. Waxes are removed from the lube fractions, down to the melting points necessary to meet cloud and pour specifications on the finished lubricating oils. To produce pour points in the region of 0 to 30" F., it is necessary to remove waxes which melt at temperatures as low as 100 O F. Generally, only the final processing of the crude waxes for oil removal and melting point separation can be directed to produce the desired wax quality. Oil may be removed by sweating (7) or by the use of solvents. Once the oil is removed. additional processing of the wax takes the form of separations because of both molecular weight and type. Ideally, if the wax can be fractionated into portions of different melting points and then recombined, sufficient quality advantages can be obtained. This is particularly important, since waxes have changed from general purpose materials to specialty products designed for particular needs. This kind of fractionation may be carried out by direct melting point separation, as in the case of sweating, or indirectly by separation according to related properties. Several processing schemes were considered : ROM A PROCESSING POINT O F VIEW,
Separation Technique Distillation Crystallization Urea adduction Thermal diffusion 2
Mechanism Volatility Solubility Molecular dimensions Molecular configuration
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B, Process Research Division, Esso Research €5 Enpineerzng
Co., Linden, N . J .
Each of these techniques has real disadvantages.
For example:
Sweating becomes inoperable for the higher melting microcrystalline fractions, because of poor crystal structure and high viscosity of any associated oil. Distillation shows very poor melting point separations on the high boiling waxes. Fractional crystallization, even though ideally suited, is uneconomical because of the excessive cost of interseparations and recombinations. This is especially true when a mixture needs to be separated into more than two fractions. Urea adduction cannot separate nonnormal paraffins by differences in melting point. Thermal diffusion throughputs are extremely small to be commercially feasible. Studies were undertaken to explore new techniques such as zone melting ( 8 ) . However, the results showed that a fractionation of petroleum microcrystalline waxes which depends on differences in melting point cannot be achieved. Zone melting experiments were conducted on a plant microcrystalline wax [175' F. petrolatum ( 7 ) melting point], and no segregation of the wax into components of different melting points was obtained. The failure of zone melting to fractionate the petroleum waxes is attributed to poor crystallinity of the wax. According to Ball, Helm, and Ferrin ( Z ) , an essential requirement for a separation or purification by zone melting is that the compound exists in a crystalline form or that it can be crystallized by cooling. These investigators also point out that solids, such as glass or plastics, that do not have a definite crystalline form cannot be purified by zone melting. Photomicrographs showed that the petroleum microcrystalline wax used in this study has a very poor crystalline structure compared to normal paraffin wax. The former looks amorphous while the latter has a needle-type appearance. Petroleum microcrystalline waxes consist of a mixture of normal and isoparaffins, as well as naphthenes with long side chains. T h e wide variety of hydrocarbons from which the microwax is made makes for a poor crystalline structure. Crystallinity is important for a separation by zone melting because in poorly defined crystalline or amorphous substances, the mother liquor is occluded in the solidifying interface thus preventing the diffusion of the impurity into the molten liquid zone. Since zone melting did not fractionate the petroleum microwax, a different technique was needed. To be useful, this technique should not require a good crystalline system and should separate on the basis of melting point. As mentioned earlier, separations by differences in melting point can be achieved indirectly through separations by differences in solubility. Tiedje (9) found a definite correlation between melting point and solubility. As a result, zone precipitation was developed to fractionate the waxes. I n zone precipitation, a hot liquid zone is moved down a column of solid made from the solvent and the material to be fractionated. After the zone has passed over the entire slab,
the components of greatest solubility will be depleted behind the zone and concentrated in the direction of its movement. If the components to be separated have properties which correlate with solubility, such as melting point, molecular weight, or others, then zone precipitation can separate these components. Considerations such as the scale-up factors and the efficiency of zone precipitation passes deserve additional study to put the design oflarge scale equipment on a sound basis. Theoretical
Zone precipitation, while similar to zone melting, operates on a different principle. Zone precipitation is based on the fact that, in fractionating a mixture of solids dissolved in an inert solvent by cooling, the first solids precipitated are those which are least soluble. The solid material to be fractionated is always admixed with a solvent, the mixture still being a solid or gel-like material. A heater is moved down a column of solid made from the solvent and the material to he fractionated. The solid, which is in the immediate vicinity of the heater, will then liquefy. As the beater moves on, the liquid behind the heater will solidify, rejecting the most soluble components. After the heater has passed over the entire slab, the components of greatest solubility will be depleted behind the moving heater and concentrated in the direction of its movement (Figure 1). By repeated passes down the column, greater separations can be obtained. Briefly, in zone melting, solidification is done from the melted solid, while in zone precipitation it is done from the solvent. Separations by zone Precipitation depend on differences in solubility, not melting point. The use of solvents eliminates the necessity for good crystallinity, and the variation of solvents used could make the separation more selective to certain constituents. Experimental
Equipment. All preliminary zone precipitation experiments were conducted in a bench-scale Fisher zone refiner (Model 5-712-100) operating with one vertical glass tube container of about 75-cc. capacity. Two resistance wire heaters (nickel-chrome) approximately 4 inches apart provided the necessary heat to melt the charge and give a molten zone.
LEAST SOLUBLE COMPONENTS
Both heaters were connected to a drive mechanism within the unit which operated in either a vertically upward or downward direction a t controlled travel rates u p to 2.4 inches per hour. I n the work described here, the heaters always moved downward. Cooling behind the molten zone was accomplished by means of air streams from jets mounted behind each heater on the heater drive mechanism. The distance between heater and the following cooler was adjusted to give a molten zone length approximately one tenth that of the total ingot treated. The container tubes for the zone precipitator were constructed from 18-mm. (inside diameter) borosilicate glass tubing. Each tube was sealed a t the bottom and contained a smaller concentric tube (approximately 8 mm. outside diameter) inside, which was introduced through a one-hole stopper a t the top (Figure 2). The smaller internal tube. also sealed at the bottom, was equipped with a stopcock located above the one-hole stopper. The annular space between the two concentric tubes could be evacuated through the stopcock and a pinhole in the wall of the internal tube located just below the stopper. The inner tube was used because in some cases the material at the center of the tube did not solidify after being melted. A three-tube zone precipitator with 2-liter capacity was specially constructed for fractionating larger microwax samples. This piece of equipment was similar to the bench model unit but larger (approximately 6 us. 3 feet) to accommodate the longer and bigger diameter tubes. The external and internal tubes of the container used with this unit were constructed from 32- and 12-mm. (outside diameter) borosilicate glass tubing, respectively. Cooling with air at 75O F. was too slow. The air used was therefore passed over a bed of solid C O X and entered the cooling zone at about 20" F. A small portion of a zone precipitation container is shown in Figure 3. Procedure. Far fractionation by zone precipitation, the wax was first dissolved in the necessary amount of hot solvent. The solution of wax plus solvent in a liquid form was poured inside the tube. After charging, the solution was allowed to
TOP
t
&STOPCOCK
0
9
8 MM O.D. GLASS CENTER-11
HEATER
MOST SOLUBLE COMPONENTS BOTTOM
Figure 1 . Zone Precipitation and zone melting are mechanically similar
Figure 2. Container tube for zone precipitotion experiments
Figure 3. Heater followed b v cooler. passing over a tube Solvent plus w a x is still lure, despite 3 / 1 ratio
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1962
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rn
rTHERMOMETER
M ERCURY
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THERMOCOUPLE
EATING MANTLE MAGNETIC STIRRER Figure 4. solvents
Apparatus
for measuring w a x diffusion in
cool in the tube to form a gel-like ingot. Solidification was achieved by atmospheric air cooling. In the larger diameter tubes it was slow, but since the charge was left overnight before processing, complete solidification was always obtained. Laboratories in rchich these studies were made were maintained at room temperature. The unfilled annular space above the ingot was evacuated to about 25 to 30 inches of Hg through the pinhole and stopcock arrangement described above to prevent possible pressure build-up upon heating. After each run, the ingot was extruded from the container and separated into several fractions of approximately equal weight. Extrusion was accomplished by cutting the sealed end of the glass container and applying gas pressure behind the ingot through the stopcock and pinhole in the center tube. Each fraction from the extruded ingot was melted, and the solvent was completely evaporated with vacuum and slight heating. The solventfree wax cuts were then submitted for petrolatum melting point determinations. Materials. A plant microcrystalline wax (175 O F. melting point) obtained from a North Louisiana crude was used. This wax has already been deoiled and hydrofined. The microwax was zone precipitated in sec-butyl acetate (SBA) with a solvent wax ratio of 1 to 1, 3 to 1, and 6 to 1 (weight basis). I t was also zone precipitated in a variety of other solvents including methyl ethyl ketone (MEK), ethylene dichloride, chloroform, toluene, and carbon tetrachloride. Analysis. Since fractionation by zone precipitation depends on transport phenomena within the liquid molten zone, properties of the liquid such as viscosity and of the solute such as diffusivity and solubility were investigated. Viscosity measurements were made on wax-solvent solutions over the temperature range at which the fractionation was assumed to take place. T o define this range, samples of wax plus solvent in the same proportions as those used in the charge to the zone precipitator were prepared. The mixture was heated in a n 18-mm. (inside diameter) glass tube, similar to that used in the zone precipitation experiments, until it became a clear liquid; it was then'cooled while being mildly stirred with a thermometer. The volume of each sample was equal to that occupied by the liquid in the molten zone of a n 18-mm. inside diameter tube, Temperatures were recorded a t the points where the wax started to come out of solution and where the solution became opaque. These temperatures are reported here as the cloud and opaque points, respectively. During zone precipitation the temperature of the liquid in the molten zone was slightly higher than the cloud point, and this incre4
I&EC PROCESS DESIGN A N D DEVELOPMENT
ment was assumed to be 5" F. 1-iscosity measurements uere then carried out over a range of temperatures ranging from 5' F. above the cloud point to the opaque point. After the opaque point was determined the solutions were cooled d o v n further until they became solid. and this temperature \vas recorded as the solid point. The significance of this point is brought out in subsequent discussions. Diffusivity coefficients of microwax in a number of solvents used in this study were measured by the capillary technique. This technique, as presented by TVilcox (70). depends on diffusion of the wax molecules from a capi1lar)- sealed at one end and containing a wax-solvent solution into a less concentrated surrounding bulk solution. From a knowledge of the bulk and initial capillary solution concentration as uell as the average capillary concentration after diffusion. the diffusivity coefficient may be calculated from the following equation developed by Carslaw and Jaeger (5) for heat transfer and now adapted for mass transfer:
where
X. affects the rate of heat transfer ( 4 ) . Small values of r2 ‘71 cause faster transfer of heat. I n scaling up, r2irl \vas 2.25 for the bench-scale tube and 2.66 for the large-scale tube; again these ratios were not too different. In general. the large-scale tube effected a slightly better separation than the bench scale tube (Figure 13). Seither D,/L nor 12 ‘ r l \vas significantly different. HoLvever, the
Table VIII.
Geometry of Bench-Scale Unit Was Close to LargaScale Unit Bench Scale Large Scale 1 cm. 2 cm. 9 . 1 5 cm. 3.81 cm. 0.26 0.22 DJL U, = equivalent diameler = 4 Ru R H = Hydraulic radius - Diameter of outer tube - diameter of inner tube
--‘.
d
L
= length of molten zone
higher heat input needed for the large tube. compared with the small one. is believed to cause stronger convection currents \vhich effect better separations. Better separations were also found in larger tubes by Ball. Helm. and Ferrin (2) when they purified 9-meth) lcarbazole by zone melting. These authors did not. however, report on the scale-up factors considered above. Applications. T h e technique of zone precipitation as described for fractionating petroleum microwaxes can be extended to the fractionation of: Synthetic polymers having a wide molecular weight distribution into fractions of narrow molecular weight ranges. Heat-sensitive protein materials. Mixtures of hydrocarbons such as p - and *xylene, or polynuclear aromatics such as naphthalene, anthracene, and phenanthrene. Synthetic and natural dyes. Mixtures of noncrystalline natural materials such as rubber, animal fats (butter or lard). gum. asphaltenes. vitamins or carbohydrates. glass, plastics. Zone precipitation could also be used in refining spent nuclear fuels with a uranium-zinc mixture as solvent and in the separation of isotopes. Acknowledgment
T h e author appreciates the contributions made to this work by George E. Charles and George R. Chludzinski, for portions of the data, D. L. Baeder and P. J. Lucchesi, for their valuable suggestions, and Robert J. French, who assisted in building the equipment and processing the samples.
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190
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E” LARGE SCALE L A R G E TUBE SOLV.,‘WAX = 3.1 10 ZOIVE P A S S E S I
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130
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40 60 ‘/. OF CHARGE MOVING ZONE-
80
100
Figure 13. Fractionation b y zone precipitation was slightly better in the larger scale equipment; AT was greater for the largescale tube
(1) Am. Soc. Testing Materials, Philadelphia, Pa., “Melting Point of Petrolatum and Microcrystalline Wax,” ASTM Standards on Petroleum Products and Lubricants, D-127-49, p. 65, October 1959. (2) Ball: J. S., Helm, R. V., Ferrin, C. R., Rejining Engr. 30, C36C39 (December 1958). (3) “Beilstein Handbuch der Oreanischen Chemie.” 4th ed.. ’3rd Suppl., Springer-Verlag, Be&, 1961. (4) Brown, G. G., “Unit Operations,” 3rd ed., p. 431, Wiley, New York, 1951. (5) Carslaw, H. S., Jaeger, J . C., “Conduction of Heat in Solids,” 2nd ed., p. 96, Clarendon Press, Oxford, 1959. (6) “Handbook of Chemistry and Physics,” 42nd ed., Chemical Rubber Publishing Co., Cleveland, Ohio, 1960-61. (7) Nelson. W. L., “Petroleum Refinery Engineering,” 3rd ed., pp. 323-6, 334, 337, McGraw-Hill, New York, 1949. (8) Pfann, W. G., “Zone Melting,” Wiley, New York, 1958. (9) Tiedje, J. L., Can. 021 Gas J., 79-88 (November 1955). (10) \27ilcox, W. R., “Fractional Crystallization from Melts,” Ph.D. thesis, Univ. California, June 1960. (11) \Vilke, C. R., Chem. Eng. Progr. 45, 218-24 (1949). RECEIVED for review January 3, 1961 ACCEPTED October 3, 1961 Symposium on Less Common Separation Methods in the Petroleum Industry, 139th Meeting, ACS, St. Louis, Mo., March 1961. VOL.
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