ANALYTICAL CHEMISTRY
638 In this design 8. liquid-sealed reflux return line ( I , Figure 1 ) by-passes the reflux valve. This prevents the valve from t r a p ping liquid in the reflux condenser and so smooths out the operation of the column and tends to minimize flooding. Figure 1 gives elevations, tap view, and details of the head. Operation. The head has been operated -with a variety of cypes of columns and under pressures varying from atmospheric to a few millimeters. It is equally satisfactory for either pressure condition, although its mechanical advantages are pitrticularly noticeable under vacuum. 'Under atmospheric pressure its maximum capacity without flooding is about 3600 ml. per hour. When i t does flood, it is a t the base of the reflux valve. The throughput can be increased at the expense of a slightly larger holdup by increasing the diameter of the liquid seal (I,Figure 1) in the reflux line. The valves are operated by either alternating or direct current adenoids, which, in turn, are actuated by t~ timer. The timer
must be arranged so that it simultaneously cuts one solenoid into the circuit as the other is out out. Both mechanical and electronic timers have been used and units have been fabricated which pennit the adjustment of the reflux ratio with a single dial. Ratios varying from 1:l to 1OO:l are easily obtainable and the actual reflux ratios are very close t o the "off-on" ratios of the timer. The head and its accessories are readily adapted to multiple setups and as many as six stills have been operated in a battery by one man. Figure 2 shows the finished vapor dividing head unsilvered. LITERATURE CITED
(1) Collins, F. C., and Lantu, V.,IND. ENO.CHEM.. AN\,.. fin. 18
673 (1946). R n c r r r m J u n ~24. 1048
Small laboratory Centrifugal Molecular Still R. M. BIEHLER, K. C. D. HICKMAN, AND E. S. PERRY Distillation Products, Inc., Rochester, N . Y . efficient, quantitative molecular disAREQUIREMENT , . . that thefordistilland tillation shall be exposed in a thin film
CYCLIC BATCH CENTRIFUGAL . ~ O L E C U L A KSTILL
IS
with uniformity of area, thickness, and rate of feed. This has been done in various ways, the most promising of which at present is the use of s heated centrifugal cone for spreading a distilland supplied a t constant rate.
with modiccd contour.
From the disprammstio elevation in
east-aluminum rotary evaporator, which contain? an embeddd
4
DISTILLATE
t
FEED
F i g u r e 1. Ce n t r i f u g a l Molecular Still of C:yolio B a t c h T y p e The simplest, complete distillation unit embodying this arrangement is a cyclic batch still with double reservoir (Z). Figure 1 shows this to comprise a still head, two reservoirs, a circulating pump, and a manifold for attaching a vaouum system. There are necessarily many campoqent parts and constructional pieces in making a versatile unit of this kind. The problem in designing a small centrifugal still suitable for laboratory bench use IS to diminish the number of parts without sacrificing the functions. The progress that has been made to date in meeting these requirements is described in this paper. In Figure 2 is shown a :nodel of the complete still unit.
F i g u r e 2.
Five-Inch Centrifugal Molecular Still
V O L U M E 21, NO. 5, M A Y 1 9 4 9
639 of readings obtained TTith this arrangement and others devised iri the laboratory indicates that they reflect the true temperature oE the evaporating surface within =+=5" C. I n special cases where t h r residues a t the end of the distillation are extremely viscous, the knife with thermocouple may be damaged if i t is allowed to freeze in the residue. T h i s c a n be prevented by flushing the viscous residue out while the evaporator is still hot or by lifting the c o u p l ~oiit of the groove xhile the rotor i. still hot.
Figure
3.
Elevation of Centrifugal 3Iolecular Still
electrical resistance heating element. The cast-in element supplies heat rapidly and uniformly t.o the distilland. No hot glowing elements are exposed to the vapors in the still and the hot,test part of the still is the rotor surface itself. Leads from the heater pass through the shaft to slip rings which are contained, but separately housed, in the bell jar. This design feature obviates the necessity of placing the vacuum seal on the hot rotor shaft. Electrical contact to the slip rings is made through two spring-loaded carbon brushes. The bearing housing is an aluminum casting, suspended from the top plate. Drive is by a coiled spring wire belt which connects with a countershaft hcld in a projection of the housing on the upper side of the top plate. Small commercial seals serve to maintain a vacuum. The shaft is carried by shielded ball bearings which can be lubricated by heavy grease or oil, but they are inevitably lubricated by small quantities of distillate. The mechanism for feeding the distilland consists of a long tube and internal shaft extending from the top plate to the bottom of the glass bell. At the far end there is a tiny gear pump which is designed with a wide entrance port, for receiving viscous fluids under loiv head and for minimizing vapor lock. The speed of the gear pump :ai? tw varied from 25 to 180 cc. per minute by a simple transmiwon, even while the still is in operation under vacuum. The distilland is led onto the rotor by a slanting tube and spreads out in a film yhich,is 0.01 to 0.1 mm. thick a t the periphery, according to the viscosity. From the edge, the dist,illand passes into a rotating gutter fastened to the rot,or. At the back of the gutter thew is a depression where the spent distilland collects momentarily and in this depression there rides a thin metal knife to which is attached a thermocouple (Figure 4). Liquid spills over the rotating edgt, of the gutter into a stationary gutter, whence it, falls by gravi . onto a small loop of tubing which is cooled by air or water. T h same tube embraces the bearing housing to keep the latter at.a lovw temperature than the rotor, so that the bearings will rimam lubricated by traces of condensate. The arrangement for ascertaining the temperature of the dis . tillnnd is one of a number that have been tried, including movablz thermocouples riding on the liquid film in front of the stili, radiation thermocouples, and embedded thermocouples with slip ring connections. The dragging couple described has proved to be the most reliable and trouble-proof under ordinary conditions. The shielding which the two gutters provide and their proximity to the hot rotor keep the temperature readings reliable. Comparison
Thc distilland, considerably chilled by contact with the watw. cooled loop x i t h consequent reduction of thermal hazard ( 3 ) passes to the upper reservoir. When the bell jar which forms the lower reservoir is empty, the contents of the upper reservoir can be dumped by means of an externally operated ball valve. The condenser is formed by a bulge blown into the side of the glass bell jar. It is intended that this portion shall be cooled by a small electric fan or blast of compressed air. Cooling may be interrupted when it is necessary to melt down solid distillates. K i t h adequate air cooling, condensation is virtually complete. Keverthelcss, small quantities of "light ends" condense on the main bod>-of the bell jar and gradually fall down into the residue. There is thus an inherent tendency to contaminate later fraction> with traces of earlier ones, particularly if the material distills over a wide temperature range. This could be remedied in variow ways, such as splitting the bell jar and placing an intermediate collar to collect the drainings, or providing a second internal metal reservoir, thus leaving the bottom of tht. bell jar to collect the drainings. These could then be drained off separately. The advantages to be gained by including this feature appeared small in comparison to the inconvenience in operation which would result from their inclusion. The present desigii waR, therefore. adopted in favor of the more complex arrangement. The pumping system which produces vacuum consists of a booster-type, double jet, glass, oil diffusion pump (designed by G. Kuiprrs of this laboratory), conveniently employing butyl phthalat:. The maximum micron per liter per second thioughpur capacity of the vapor pump has been arranged t o give its preferred service in the range 1 to l o p in which the still is intended to operate. One or more freezing traps may be placed in series with the stili and the booster and the mechaiiiral fortipump. OPERATION
Thermocouple issembly
IJipire 1..
It is usually prvferahle to charge a cyclic still with distilland while the still is uridrr rough vacuum. Tht. material then untlorgoes a preliminar? (16.gassing, the e.ctc.nt i i t xhich is governed 113 the rate of charging
I n the present still, the distilland is admitted from the external glass funnel through the second small tube onto the rotor while the latter is running. Bubbles are broken up by the shear forces 01, th: rptor and the liquid is substantially degassed by the act of admission. h second cycle over the hot rotor with dry ice and acetone placed in the trap near the fore-pump generally rendrra the distilland sufficiently degassed for distillation to be coninienced. If desired, distilland may be charged into the lower re+ ervoir through the tubulation provided for withdraQ?hg thr residue. \Then the vacuum has fallen to less than 1 mm., the boo5tc.r pump is energized and as soon as this is seen to be in operatioil refrigerant is placed in the trap next t o thp ftill. The pressure
ANALYTICAL CHEMISTRY
640 should now fall to below 10p and the cycle of distillation can be commenced. In the molecular still time and temperature take the place of temperature and pressure for the management of distillation; hence it is convenient to adjust the still to the exact conditions required and allow the preliminary distillates t o pass back into the distilland in the lower reservoir. This is accomplished by the three-way stopcock which connects the cdndenser to the lower portion of the reservoir. Distillation is usually performed in cycles at regular temperature intervals, taking off a fraction a t each temperature. At the end of each cycle, the contents of the upper reservoir are dumped into the lower reservoir, to become the feed material for the next cycle. At the end of the distillation, the oil pump and motor are switched off. The pump fluid and residue are allowed to cool before air is admitted into the still. The rotor is best coo!ed by constant recirculation of the residue. After the vacuum is released, the residue is drained from the lower reservoir. Cleaning of the still after distillation is a relatively simple operation and is, indeed, one of the points that received special attention during design. S o dismantling is usually required, although for occasional thorough cleanin the bell jar can be removed. After most distillations it is sufacient merely to circulate a nonflammable solvent through the system by means of the feed pump. Aspiration of air through the apparatus after draining the solvents leaves it clean and dry. Some heavy organic mixtures will reach the consistency of a stiff asphalt before the end of distillation. If the still is shut down and the viscous residues are allowed to cool in place, it becomes difficult t o clean. It is imperative, therefore, to rinse out the still with a suitable solvent while the rotor is still hot, which can be done under vacuum or atmospheric pressure according to choice of solvent. The rapid evaporation of the solvent floods the whole interior with distillate and washes the viscous residues into the lower reservoir, from which they may be drained and replaced by successive charges of clean solvent. The still is not intended to be used a t any substantial pressure or Tl-ith flammable materials, and a vacuum-operated safety sv, itch prevents current reaching the rotor a t preswres corresponding with flammable materials, such as acetone.
tion pressures from a fraction of a micron to lop, a t which upper limit between 5 and 7 grams of distillate Jyill collect each second from each square meter of evaporating surface. The little 5-inch rotor has performed distillations a t 30 or more microns saturation pressure and has evaporated soybean oil, for instance, in a continuous stream of distillate with a rotor temperature ranging between 350" and 400" C.; both rotor and residual oil have remained bright. The still has also been used on Fischer-Tropsch residues and asphalts which failed to yield distillates in other apparatus.
PERFORMANCE
SCOPE AND USE OF STILL
The authors have as yet no absolutely accurate data on the conditions required to give optimum separation in a molecular still; it is not knoivn when one full molecular plate has been achieved. The most certain appraisal is, therefore, a quantitative comparison of performance at different rates of feed and xyith the performance of other stills.
The still described shows features of interest and marks certain advantages. There is, however, no finality in the design of surface evaporators and there are many additions to be made in the present still before it can meet all needs. Constructed as it is from aluminum and steel, it offers in the rotor excellent heat transfer and low catalytic activity to the distilland even a t the high temperatures employed. Plain steel is used only where the parts could not be obtained in noncorrosive metals, and these parts a t no time contact the hot distillands. While definitely not intended for use with corrosive materials such as acid chlorides or free fatty acids, if such substances must be handled, corrosion is reduced to a minimum. Again, the reservoirs and feed lines are not serviced by heating coils or cooling means for manipulating materials of abnormal viscosity and thermal lability. Such modifications as are needed under these circumstances can, however, be applied without much difficulty. The material in the upper reservoir can he kept molten by an infrared heat lamp and the contents of the bottom reservoir by a Glass-Col jacket.
3 FALLING FILM WITH CHANNELLING
0
a
1 I
m
I, 0
20
Figure 5.
40
60
1
I
80 I00 I20 140 DISTILLATION RATE IN CCIHR.
160
180
U
90 100 110 I20 130 140 150 160 170 180 190 200 TEMPERATURE .C.
Figure 6.
Performance Curves for Centrifugal and Falling Film Stills
Operating Features R.p.m. of rotor Diameter of rotor. cm.
Using the binary plate mixture described elsewhere (Q), the authors have compared the performance of the 12.5-cm. (5-inch) centrifugal with the falling film, cyclic batch still for different throughputs of Octoil and Octoil-S. The degrees of separation, compared with a unit plate measured a t loop, are s h o w in Figure 5. I t is seen that the separatory pox-er of the centrifugal rotor remains unimpaired for relatively high rates of feed. Another means of asqessment is the elimination curve (1)techniquc. Results of comparing the separation of celanthene red and dimethylanthraquinone on both falling f l m and centrifugal stills are presented in Figure 6 . The centrifugal still gives sharper and narrower peaks, representing a better separation of constituents. The usual rate of molecular distillation is in a range of satura-
Elimination Curves
cm.
, mm. sec.
g./min. Size of charge, liters
1750 12.7 100 0-500
0.01-0.1 0.03-1.O D = 0.03-1.0 35-100 0.1-1.5
LITERATURE CITED
(1) (2) (3) (4)
Embree, N. D., I n d . Eng. Chem., 29, 975 (1937). Hickman, K., Ibid., 29, 968 (1937). Hickman, K., and Embree, N. D., Ibid., 40, 135 (1948). Perry, E. S., and Fuguitt, R. E., Ibid., 39, 782 (1947).
RECEIVED July 16, 1948. Communication 138 from the laboratories of Distillation Products, Inc. Demonstrated before High Vacuum Symposium, Cambridge, Mass., October 30, 1947.
