Anal. Chem. 1900, 52, 1738-1742
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Zone Refiner with an Immersed Helical Heater A. R. McGhie Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pennsylvania
19 104
P. J. Rennolds and G. J. Sloan” E. I. du Pont de Nemours and Company, Central Research and Development Department, Experimental Station, Wilrnington, Delaware 19898
A novel zone refiner is described, in which a single helical heater rotates in an annular sample space. The device is easily built and operated in both batch and continuous modes. I t was tested on benzene doped with anthracene and was found to remove the anthracene effectively. It was also applied to batch and continuous purification of 1-nitronaphthalene.
We have been interested for some time in improved methods of laboratory-scale zone refining which might be useful in larger scale a n d continuous operation. This report describes a novel zone refiner, fulfilling both objectives, that was built a n d evaluated in the purification of benzene and 1-nitronaphthalene (1-NN). Many workers have tried to solve the problems that beset the purification of organic compounds by crystallization of their melts. One of the most troublesome of these problems is their high viscosity, which makes difficult the removal of impurity rejected from the advancing solid/liquid interface. Stirring of the liquid a t the interface has been used to create shear a t the interface, to achieve higher separation efficiency ( I ) . Another problem, especially in attempts to purify multikilogram quantities, is the low thermal conductivity of organic compounds, which limits the rate a t which the latent heat of fusion may be applied to the sample and removed from it. Immersion of a heating element in the ingot has been proposed as a means of improving heat transfer (2). Movable internal heat conductors have been used both to stir the melt and to speed the flow of heat through both solid and melt ( 3 ) . T h e large expansion that accompanies fusion of many organic solids leads to breakage of glass containers; although several remedies have been proposed ( 4 , 5 ) ,breakage of cylindrical zone refining tubes still occurs. Finally, i t must be pointed out that attempts to convert batch zone refiners to continuous operation have resulted in complex machinery, costly to fabricate and cumbersome to operate (6). Optimum heating is attained when the heater is immersed within the sample to be purified. The removal of heat from the sample is facilitated by reducing the distance through which heat must flow, that is, by working with a relatively thin charge. Moreover, breakage of a glass container will be minimized if only a minuscule fraction of the charge is melted by the zone. Other advantages derive from this stratagem, as will be seen later. These ideas have been used in a number of proposed and actual designs for zone refiners. For example, Pfann proposed 0003-2700/80/0352-1738501 OO/O
several zoning configurations involving annular charges with external heating (2). Anderson extended those proposals to devices in which a centrifugal field is additionally used to speed up the zone refining process (7). Saylor’s “freezing staircase method” makes use of a single molten zone of varying cross section (8). We describe here a purification apparatus that involves a slender helical heater, rotating within an annular sample space, as shown schematically in Figure 1,to form a film of melt on the surface of the heater. Heat may be transferred simply and effectively from the solid/liquid interface t o adjacent cooling jackets. Rotation of heater 1 is functionally equivalent to conventional zone transport along the axis of the annulus. This may be seen by considering only the section of the annulus designated ABCDEFGH, which is threaded by segments of the helical heater. A given point on the helix remains in a fixed horizontal plane during rotation, but the segment visible in “window” BCDE will appear a t progressively lower positions during clockwise rotation (indicated by an arrow). Hence, rotation of the helix produces effective axial transport. Further, we will show that a zone refiner of this configuration can easily be adapted to continuous operation.
EXPERIMENTAL SECTION Helizone I. Apparatus. A glass vessel was constructed to provide an annular sample space 30 cm deep as shown in Figure 2. Tube 1is of 10 cm o.d. and tube 2 is of 7 cm o.d. standard-wall Pyrex tubing, providing an annular gap about 1 cm wide. The inner tube was made so that coolant could enter through tubulation 3 and exit through tubulation 4. The top of the vessel was closed by cover 5 of 1.3-cmacrylic sheet. Heater 6 was constructed by inserting a bifilar length of Nichrome wire in a sheath of Teflon fluorocarbon resin into a soft copper tube of 0.48 cm o.d., which was then closed at one end; the heater provided 700 W at 120 V. The copper tube was wound on a mandrel to give five turns of a right-handed helix with a 4-cm pitch; its upper end made a snug f i t in a hole through cover 5. Cover 5 was rotated via a central shaft connected to the output shaft of synchronous motor 7, at a speed of 1 rph. The rotation rate was variably reduced by providing power to motor 7 through a Flex-0-Pulse on-off cyclic timer, The electrical leads to motor 7 were made sufficiently long so that they could wind up on a shaft as the heater rotated. Counterclockwise rotation of the heater resulted in downward zone movement. Operation. Purification of Benzene. The apparatus described above was placed in a low-temperature thermostat (Forma Model 2095W window bath) which contained a circulation pump. The bath liquid was pumped through the inner jacket of the container and back into the main body of the bath. In this way, W 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
I ,
CO = 675 ppm
k
sample space
Flgure 2. Cross section of a glass vessel containing an internal helical
heater both inner and outer surfaces of the annulus were maintained at the constant bath temperature. For purification of benzene (mp 5.5 “C), a bath temperature of -5 “C was found adequate. The heater was energized and set a t a temperature sufficient to maintain a liquid zone (30 V). A charge of 475 mL of benzene (thiophene free) containing 65 mg of anthracene (Eastman X480) was poured into the annular space and allowed to solidify. This gave an ingot 16 cm long. When the liquid zone was stable and narrow, the motor was switched on and adjusted to give a zoning speed of 1 cm h-’ (1.4 revolution h-’) and 12 revolutions were executed. On completion, the heater was switched off, and
/
l
001
Flgure 1. Schematic view of an internal helical heater in an annular
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1.1
00
04 06 o FRACTION OF CHARGE
02
s
I O
Figure 3. Distribution of anthracene in benzene after helical zone refining
fractions of the charge were removed from the top down, by partial melting. Five fractions were taken and analyzed for anthracene by measuring UV absorption a t 383.5 nm, using a Beckman DB spectrophotometer. Figure 3 shows the distribution of anthracene through the charge after zone melting. Approximately half of the charge contained about 5% of the starting concentration of anthracene (675 ppm). This indicates that the anthracene was effectively segregated from benzene. The minimum in the solute concentration near the start of the charge may be a result of supercooling at the head of the ingot, a phenomenon often observed in the zone melting of organic chemicals. Helizone 11. On the basis of the successful operation of ths simple device described above, a second unit was constructed which allowed better control of the operating variables and better sealing against environmental contamination. Container. The principal working element of Helizone I1 is the vessel shown in Figure 4; it was constructed of standard-wall Pyrex tubing. The outside diameter of the inner tube 1 of the sample space is 5.1 cm, and the inside diameter of the outer tube 2 of the sample space is 6.5 cm. The annular gap is thus about 0.7 cm. Tubes 1 and 2 are connected a t their lower ends by standard-taper joint 3 (360/50). The length of 1above the joint is 65 cm, giving a sample volume of about 0.8 L. Tubes 4 and 5, of outside diameter 8.0 and 3.8 cm, respectively, define outer and inner jackets through which coolant is circulated. A selfheated stainless-steel tube 6, used to introduce the starting charge, enters the sample space through tubulation 7. Thermocouples and a stainless-steel vent tube also enter through this tubulation; for clarity, these are not shown in the diagram. Heater 8 was provided to melt any solid in or near joint 3 and thus to facilitate disassembly. Heater 9 was provided to maintain a liquid annulus in which the lowermost turn of the helical heater is to rotate; a gap in the winding provides a viewing window. Heater 10 was provided for discharging the unit at the end of a run (see “Operation” below). Joint 3 rests upon rubber gasket 11 in end-plate 12, which is connected by four tie rods 13 (one of which is shown),to a matching upper end-plate (Figure 6, no. 9). The terminations of the tie rods are spring loaded to allow for lengthwise expansion of the glassware. Heater. The active heater element consists of a helically wound resistance wire sheathed with glass-fiber insulation (Glasohm); it is 0.23 cm 0.d. X 3.7 m long and provides 500 W a t 120 V. The heater was doubled, so that its free ends were together, and was inserted into a stainless-steel tube 0.64 cm 0.d. X 2.5 m long. The tube was then passed through rollers to flatten it to an elliptical cross section 0.48 cm X 0.74 cm. The tube containing the heater was wound on a grooved mandrel to provide a helix which was 5.32 cm i.d. X 6.33 cm o.d., with a pitch of 5.1 cm. The long axis of the elliptical cross section is vertical, as shown in Figure 5. The final helix had 13 turns, terminated by 3 / 4 turn and 11/4 turns
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9
11
Figure 6. Cross section of the closure and drive mechanism of a helical zone refiner
slip rings (not shown). Lead wires 6 connect the heater element to the slip rings. The top of the glass container 7 is sealed by rubber gasket 8 in end-plate 9, which is also sealed to flange 2 by carbon face seal 10. The glass vessel is clamped between end-plate 9 and a lower end-plate (see Figure 4, no. 12) by the tie rods 11,one of which is shown. A reversible, variable-speed motor provides rotational speeds of 1-10 cm h-'. A shear pin is
Figure 4. Cross section of an improved glass vessel for helical zone
refining
J
& c ...... ~
--_ - - _*-- -- ____ Figure 5. Helical heater, with "flat" t u r n s at top and bottom
at top and bottom, respectively, perpendicular to the helical axis. The upper flat turn was clamped to the rotation mechanism (see below) and the lower one served to maintain a molten ring, for ease of rotation. The lower end of the stainless-steel tube was cut off 2.5 cm from the end of the heater and welded closed, taking care not to overheat the end of the heater. The heater was energized through an autotransformer to provide a molten film surrounding the stainless-steel sheath; the height of the molten zone was adjusted to about 1 cm (see Figure 6). The ratio of ingot length to zone length is thus about 60. Drive Mechanism. In Figure 6, helical heater 1 is shown clamped between flange 2 and ring 3 by bolts 4. Flange 2 is threaded onto shaft 5 which in turn is connected to the output shaft of the drive mechanism (not shown) via a set of commutator
provided between the output shaft of the drive motor and the slip-ring assembly, to avoid application of excessive torque to the heater in case of inadvertent freezing of the molten zone. Batch Operation. A mixture of water and ethylene glycol was pumped from a constant-temperature bath through the inner and outer jackets of the vessel shown in Figure 4. Its temperature was set at 60 "C, about 4 "C above the melting point of 1nitronaphthalene (1-NN); a batch of 1-NN was melted in a flask and charged into the sample space of the refiner through a self-heated, stainless-steel transfer line (Figure 4, no. 6) of 0.32 cm diameter, which entered through a rubber closure on tubulation 7 of Figure 4. A total of 952 g was charged. The temperature of the glycol/water mixture was reduced to 52 "C and the helical heater was energized; during most of the run, the voltage was set to provide about 70 W. The heater was rotated to provide zoning at 1.3 cm h-'. After 21 revolutions (120 h), heating was discontinued, and the entire ingot was allowed to solidify. The coolant was drained until its surface was about 5 cm above the bottom of the annular sample space of the refiner. Heater 10 of Figure 4 warmed the liquid in the jacket until the lowermost 5 cm of the annular ingot melted. The melt was aspirated through self-heated transfer line 6 of Figure 4 into a receiver from which a sample was removed for analysis by gas chromatography. The liquid in the jacket was raised stepwise and heated to melt successive fractions whose total weight was 98.6% of the amount charged; see Table I. Modification of Container for Continuous Operation. The vessel shown in Figure 4 was modified in such a way as to allow continuous introduction of starting material and simultaneous removal of product and waste. Figure 7 shows how the outer jacket has been replaced by two separate jackets, with tubulation 1 providing entry into the sample space. This entry port is positioned so that two-thirds of the ingot is above it and one-third is below. This is approximately the place at which the steady-state solute profile has the same concentration as the starting material for the system under study here (see Table I). The tubulations for coolant and for process streams were offset circumferentially to facilitate connections. It must be noted that inlet 1 can communicate with the rotating, helical zone only once per revolution. To achieve constant communication, it is only necessary to provide a small molten zone, perpendicular to the axis of the cylindrical vessel; this is afforded by a narrow heater 4 wound around the sample tube, above and below tubulation 1. Continuous Operation. The feed was melted in a 5-L round-bottomed flask positioned above the inlet so that molten 1-NN could flow under a static liquid head, through the self-heated
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
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Table I. Analysis of Fractions of 1-Nitronaphthalene Recovered from Batch Operation of Helizone
weight, g
fraction 1
38.7
2 3 4 5 6 7
61.0
area % in gas chromatographic peaku at retention time 2.7 min 5.4 min 6.8 min 10.0 min 11.2 min (2-nitro(naph(chloro(l-nitro(2-nitronaphthoquinone) thalene) naphthalene) naphthalene) naphthalene) 0.002 0.003 0.007 0.019 0.023 0.065 0.104 0.254 0.332 0.120
98.8 131.6 128.6 146.6 132.5 105.4 95.9
8
9 s.m.
99.998 99.997 99.993 99.981 99.976 99.836 99.704 98.968 98.372 99.507
0.007 0.010
0.004
0.034 0.052 0.013
0.010 0.017 0.004
0.092 0.177 0.731 1.227 0.358
939.1 a Chromatographic conditions: column, 8 f t x '/, in., 7% SE-30 + 3% Silar on Gaschrom Q, 60-80 mesh; column temperature, 100 to 250 "C at 1 0 "C/min; injection temperature 240 'C; detector, flame ionization, 240 'C; carrier gas, helium, 40 cm3/min. b Starting material.
Table 11. Continuous Operation of Helizone Purification of 1-Nitronaphthalene operating condition no.
hours at steady state
zoning speed, cm h - '
heater wattage
1
71 120 97 65 66 64 73 73
1.2 1.3 1.3 0.4 3.1
61 61 61 61 74
1.3
74
1.2 1.2
74 74
2 3 4
5 6 7 8 a
feed stream, g day.'
product stream, g day-'
waste stream, g day-'
219 39 92 82 95 92 105 81
128 27 66 59 70 68 70 35
91 12 26 23 25 24 35 46
area 70 impurity product waste 0.31 0.04 0.05 0.08 0.000
0.00 0.00 0.00
0.76 1.46 1.59 1.67 1.58 2.03 2.31
1.73
Limit of detection approximately lo-' area %, by gas chromatography under conditions described in Table I.
transfer line and a heated sight glass, into tubulation 1, and ultimately into the transverse annular zone. The sight glass made it possible to measure the feed rate by drop counting. After stable conditions were established in the heater and coolant loop, rotation of the heater was started. Waste was removed from tubulation 3 a t a suitable drop rate. Product overflowed into a collection flask through tubulation 2 at an average rate given by the difference between the controlled feed and waste-removal rates. The conditions given in Table I1 were attained after a period of trial-and-error adjustment. In all, the unit was in continuous operation 45 days, without breakage, binding of the heater, or other mechanical difficulty. The instantaneous rate of product delivery varied considerably, since surface tension had to be overcome before a quantity of molten product could overflow. It was found desirable to include an in-line filter a t the intake point of the feed line to avoid blockage of the transfer line by insoluble particles. Infrared heaters were provided as needed to keep the 1-NN molten throughout the transfer lines.
RESULTS A N D D I S C U S S I O N To evaluate the results of the batch zone refining of 1-NN, we compared the impurity distribution along the charge a t the end of zoning in the Helizone in Figure 8 t o results previously obtained in an efficient conventional zone refiner. The profile generated by the Helizone is different in shape from that of the conventional zone melter. It is not known to what extent this is the result of the smaller number of zone passes carried out. In any case, diffusive and convective mixing of impurities along the length of the helical liquid zone appear t o be minimal and far from disabling, as was originally feared. The decision to zone downward led to an overall temperature gradient which opposes convective mixing. T h a t is to say, depositing the lower melting material a t the bottom of the apparatus gives a convectively stable situation. Moreover, this configuration provides local convection between the heater and the crystallizing interface, which results in efficient mixing.
Figure 7. Glass vessel for
helical zone refining, modified for continuous
operation
Our experience shows that Helizones may be built for batch charges ranging u p to a few kilograms, using the design given
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
3 c 00
.
HELIZONE 31 PASSES AT 1 3 c m hr co 0 4 3 I t %
02
04 06 08 FRACTION O F C H A R G E
'
i I O
Figure 8. Distribution of 2-nitronaphthalene in 1-nitronaphthalene after conventional and helical zone refining
here. Operation a t temperature below ambient is simplified and made more efficient since heat does not flow across air gaps between moving parts, as is the case in many conventional zone refiners. An additional benefit is that formation of ice in air gaps is eliminated. Because heat transfer is so efficient in the annular configuration, the energy required is small; in the 1-NN runs, the electrical energy dissipated amounted to about 5 W/helical turn. While fluctuations in heater power can cause diminished purification efficiency, they cannot cause container breakage. This can be understood easily if one considers that the single zone is "open" a t one end and can expand into the free space above the ingot. T h e results of our batch experiment with 1 - N N show that a single helical liquid zone can support a large concentration gradient along its length with no disabling diffusive or convective backmixing. This is understood to be a general attribute of the design rather t h a n peculiar to this particular design and set of operating conditions. Further, the mechanical requirements are modest. T h e drive motor is only required to move the heater through the melt; very small torque is involved. In Helizone I, a 3-W clock motor sufficed. Construction and operation are straightforward, and product recovery is very high. Finally, i t should be noted that a continuous mass of solid extends over the length of the annulus, and it is not necessary to renucleate the solid with each zone passage. This can be a major benefit in refining substances which tend to supercool. T h e Helizone described here resembles the column crystallizers described by Schildknecht (9) only in geometry. In function, the Helizone is very different: heat is introduced uniformly along the helical heater rather than at the column end only. The column crystallizer, with end heating, operates best Lnder adiabatic conditions. In the Helizone, the solid present between the turns of the heater is monolithic, in contrast to the mobile slurry which must be present in the column crystallizer. T h e monolithic solid is completely recrystallized with each rotation of the helical heater so as to
perform the essential function of a zone refiner, namely, multiple recrystallizations to achieve high purity. By contrast, in column crystallizers, the primary action is washing impure mother liquor from the suspended crystals. With monolithic solid growth, it is possible to avoid capture of impure liquid within the growing solid and thus attain an effective zone pass per rotation. T h a t this is in fact close to the case is shown by Figure 8, as discussed above. In evaluating the results of continuous zone refining of 1-NN, Table I1 shows that less purification took place under operating conditions 1-4 than under conditions 5-8, and under conditions 5-8, no impurity was detected in the product. The principal change responsible for the improvement was the increased heater wattage, which presumably caused more effective removal of rejected impurities from the freezing interface by enhanced convective mixing. Under conditions 5-8, the difference in impurity concentration between the top and bottom of the ingot was as great as in batch zone refining. However, the yield under operating conditions 5 and 6 was 74% vs. 21% in the batch operation. Furthermore, the capacity in continuous operation was 70 g day-l vs. 30 g day-' for the batch operation. In general, continuous operation offers the possibility of purifying relatively large amounts of material in a small device. A major advantage of continuous operation over batch operation is that steady-state distribution needs to be attained only once. Thereafter, product and waste are removed from the ingot ends at their minimum and maximum impurity concentrations (IO). In batch operation after attainment of steady state, product is obtained by removing a significant fraction of the charge, which will, of course, have a lower average purity than the material at the head of the ingot. Additional product can only be obtained by repeating the process with a fresh batch having uniform starting concentration. Another advantage of continuous operation which becomes important a t larger scale is the elimination of the handling of solid materials: the feed, product, and waste streams are liquid.
ACKNOWLEDGMENT I t is a pleasure t o recognize the skillful and enthusiastic technical assistance of Anthony J. Lewis. We also wish to thank W. G. Pfann for helpful comments on the manuscript.
LITERATURE CITED Bollen, N. J.; Van Essen, M. J.; Smit, W. M., Anal. Chirn. Acta 1967, 38, 279-284. Pfann, W. G. "Zone Melting", 2nd ed.; Wiley: New York, 1966; pp 75-77. Sloan, G. J. US. Patent 3844724, 1974. Maire, J. C.; Delmas. M. A. R e d . Trav. Chirn. Pays-Bas 1966, 85, 268-274. Wynne. E. A,; Zief, M. I n "Fractional Solidification"; Zief, M., Wilcox, W. R., Eds., Marcel Dekker: New York, 1967; VoI. 1 Kennedy, J. K.; Moates, G. H. I n "Fractional Solidification"; Zief, M., Wilcox. W. R.. Eds.: Marcel Dekker: New York, 1967; Vol. 1. Anderson, E. L. I n "Purification of Inorganic and Organic Materials"; Zief, M., Ed.; Marcel Dekker: New York. 1969. Saylor, C. P. I n "Purification of Inorganic and Organic Materials"; Zief, M., Ed.; Marcel Dekker: New York, 1969. Schildknecht, H. Chirnia 1963, 17, 145-157. Reference 2, p 171.
RECEIVED for review April 14, 1980. Accepted June 23, 1980. A. R. McGhie acknowledges support from the National Science Foundation Materials Research Laboratory Program, under Grant No. DMR-76-80994.
