A Tilting Arc Flow Divider Suitable for Reflux Ratio Control’ SAMUEL PALKIN AND S. A. HALL Bureau of Agricultural Chemistry and Engineering, U. S. Department of Agriculture, Washington, D . C.
T
HE simple stopcock is perhaps one of the most widely used devices for control of reflux ratio of laboratory col-
TABLEI.
umns. Under conditions of reduced pressure, difficulty is ordinarily experienced when using the stopcock type of divider through contamination of distillate by soluble lubricants and the development of leaks at the stopcock barrel, or through obstruction by insoluble lubricant and consequent interference with “product” flow. Even very small obstructions and leaks may seriously disturb the reflux ratio. Controls of the stopcock type, and also improved controls (3, 6) of this nature that require fixed settings by way of partial valve closure, provide a measure of constancy only with respect to product rate; they depend for ratio control on the maintenance of constancy of distillation rate, which of itself constitutes a difficult problem ( 4 , 5 , 7 ) . Moreover, in the case of stopcocks, additional complications are introduced because of the lubricant. Devices in which reflux ratio settings are independent of distillation rate have obvious advantages (1,2). The reflux divider described in this paper is of the continuously dividing type (in contrast with intermittent take-off, 2 ) ; it has the advantage that ratio control is reasonably independent of rate; it is, moreover, not subject to difficulties arising through contact of distillate with lubricant; has a negligible holdup; and is relatively simple in construction. At low reflux ratios when continuous division of reflux may become an important factor the reflux divider meets this demand and its operation, as contrasted with the intermittent type (Z),is thus not complicated by discontinuity of reflux to the column. The simple flow-dividing mechanism presents a somewhat radical departure from the flow dividers commonly used. Its design is based on the observation that when a stream of liquid is delivered to some point of an “arc” (curvature circular or nearly so, the plane of curvature substantially vertical,
WATER INLET
1 2 3 4 5 6 7
0.69 0.68 0.68 0.66 0.63 0.60 0.61 0.60 0.60 0.60 0.58 0.58
S 9 10 11 12
13 14
15 16
17 18 19 20 21
1.77 1.72 1.70 1.73 1.35 1.40 1.31 1.18 1.17
REFLUX RATIOCONTROL Time Elapsed Min. 38.0 26.5 26.2 24.0 20.8 13.8 13.7 13.4 12.9 10.3 9.2 3.9
Average Throughput Ml./hr. 79 113 115 125 144 218 219 224 233 293 326 766
New Setting (Lower Ratio) 81 37.0 29.9 100 107 27.9 27.6 109 189 15.9 13.9 216 13.5 223 333 9.0 356 8.4
Reflux Ratio 71.5 72.6 72.6 74.8 78.3 82.4 81.0 82.4 82.4 82.4 85.2 85.2 27.3 28.1 28.4 27.9 36.0 34.7 37.2 41.4 41.7
and the surface of the arc made “even-wetting”) there results a division of the incident stream into two streams which fall from the extreme ends of the arc. The relative volumes of the two streams depend upon the point of the arc where the liquid is dropped, and the angle or tilt of the arc. Two modes of effecting the flow division are thus possible: (1) when the point on the arc a t which the liquid is delivered is restricted to the center, the flow division (ratio of the two streams) is determined by the angle or tilt; and (2) when the point of contact (with the incident stream or drops) is not restricted to the center, then tilting of the arc will alter the relative length of arc extending on either side of the point of contact and correspondingly alter the reflux ratio. The f i s t method (restricting liquid to the center and tilting the arc to obtain the desired ratio) has been found to give better control than the second. A small glass bead a t the center of the arc facilitates the transfer of the dropping liquid to the even-wetting surface. A wetting or spreading mechanism that has been found satisfactory consists of a close winding of glass fiber around the arc.
,CONDENSER
--. d
Fraction Collected
M1.
1 Device (in several forms and modifications) constitutes subject matter of application for Service Patent.
0
Test NO.
The divider proper, which with its winding of fiber glass constitutes the flow-dividingmechanism, is simply a curved segment (curvature a proximately circular) of glass rod. Wire or metafiic rod has also been found satisfactory. Dimensicns d e pend on the maximum reflux rate for which it is t o be used. Single segments, using rod of small diameter (about 3 mm., such as the one used here) are serviceable for liquid rates up to about 700 ml. per hour. For higher rates rod of somewhat larger diameter and with additional layers of winding may be used, or for much higher rates multiple segments, made of two or three arallel curved contactin segments with an over-aE winding, have been foun8 serviceable. The remainder of the device consists of a sup orting rod sealed to a female ground-glass joint anian arm for setting the angle of the segment for the desired reflux ratio. The end of this arm is drawn out to a tip and may serve as a pointer on an arbitrary scale (not shown in the drawing). This
DROPPER,
FIGURE1
901
INDUSTRIAL AND ENGINEERING CHEMISTRY
902
by a heat-compensated enclosure (an extended part of the box housing of the column proper) provided with a window for visibility. For the experiments in Table I, 50 ml. of turpentine were allowed to drop from a buret to the flow divider. Readings were made a t comparatively short intervals, long enough to collect approximately 0.5 to 1.5 ml. of product. The rate of delivery was determined by timing with a stop watch. It can be seen from the data thus obtained that, a t rates of 200 ml. per hour and above, there is very little variation in the reflux ratio a t a given setting or tilt of the arc. However, a 100 per cent decrease in rate (to about 100 ml. per hour) with the same setting, gives a lower reflux ratio. The reflux ratios following such a change in rate are approximately 10 and 20 per cent lower t,han the original ratios of around 80 and 35, respectively. When the rates are low (100 ml. per hour and below), con-
TABLE 11. REFLUX RATIO CONTROL
Fraction 1
Time Elapsed between Fraationi Hours 1.5
Distillate Collecteda Ml. 8.2
Average Reflux Return b (Throyghput minus Take-off) Ml./hr. 155
Take-off (Product) MI. / hr 5.5
Reflux Ratio
Remarks
28
Set for approx. 30 to 1 reflux ratio
.
2 3
1.0 3.0
5.5 7.2
152 127
5.5 2.4
28 53
4 5
16.0 1.5 3.5
39.5 3.5 7.6
127 118 108
2.5 2.3 2.2
51 51 49
6
7
2.0
3.9
93
2.0
47
8
17.0
25.0
67
1.5
45
9
8.0
9.5
84
1.2
70
10 11 12 13
16.0 8.0 72.0 44.5
19.8 9.7 85.2 32.8
85 91 87 87
1.2 1.2 1.2 0.74
71 76 73 118
14 15
8.0 16.0
4.9 6.3
74 45
0.61 0.39
121 115
a
b
o-Pinene. Measured by calibrated dropper.
Vol. 14, No. I1
Throughput decreased. Set for aDDI‘OX. 50 t o 1 reflux r a t i i Throughput decreased Throughput further decreased Throughput further decreased Throughput further decreased Set for approx. 70 t o 1 reflux ratio Set for approx. 120 t o 1 reflux ratio Throughput decreased Throughput further decreased
denser, -a reflux return cup, calibrated dropper, and take-off or product cup with its capillary delivery tube as shown in Figure 1.
Literature Cited
Some indication of the dependabibty of and constancy of ratio made possible by this device may be seen from the data in Tables I and 11. Data in Table I were obtained on the flow divider independent of column application (divider set up on a laboratory table). Data in Table 11were obtained on a cO1umn equipped with this divider, the Operating at 20-mm. pressure. Here the still head was thermally insulated
(1) Brum, J. H.9 IND. ENQ.CHEM., ANAL.ED., 7,359 (1935). (2) Carter, A. S., and Johnson, F. W., U. S. Patent 2,251,185. (3) Ferguson, B., IND. ENQ.CHEM., ANAL.ED., 14,493 (1942). (4) Hall, S. A., and Palkin, S., Ibid., 14, 652 (1942). (5) Othmer, D. F.,IND.ENQ.CHEM.,22,322 (1930). (6) Rossini, F. D., and Glasgow, A. R., J. R ~ K WNatl. C ~ BUT.Standards, 23,509 (1939). (7) Selker, M. L., Burk, R. E., and Lankelma, H. P., IND. ENQ.CHQM., AXAL.ED.,12,352 (1940).
Leakproof Stopcock for Regulation of Take-off during Distillation MELVIN S. NEWiMAN The Ohio State University, Columbus, Ohio
T
HE stopcock illustrated represents a modification of previous designs for leakproof stopcocks, useful in controlling the rate of flow in a variety of apparatus. The author has found the new stopcock particularly valuable in the regulation of take-off in distillation heads (I), as it embodies a number of features which make it preferable to the ordinary straight-bore stopcock. When adjusting the take-off, the rate a t which liquid is being taken is observed immediately at the drip point of the stopcock plug. Since the rate of flow through the stopcock is governed at only one place where the hole in the barrel and the hole in the plug intersect, regulation is easier to accomplish. Furthermore, the small vertical fall in the bore of the plug acts as a siphon and thereby aids in regulation. Mercury may be put in the well at the top. This prevents leakage during vacuum distillation and loss of volatile substances during prolonged fractionations a t atmospheric pressure.
-
Lubrication may be accomtdished effectively with the liquid being distilled. The stopcock as described has been made according to the author’s specif i c a t i o n s b y Ace Glass, Inc., Vineland, N. J.
Literature Cited (1) Morton, A. A., “Laboratory Technique in Organic Chemistry”, 1st ed., P P . 8 3 , 84, New York, McGraw-Hill BookCo.. 1938
