INDUSTRIAL AND ENGINEERING CHEMISTRY
6.52
ml. per hour for 48 hours, since the rate is not controlled by narrow openings which may become blocked with grease or, as may happen in gasoline fractionation, by traces of moisture. The auxiliary systems of both designs 14 and 15 may of course be used satisfactorily with simple main condensers as in Figure 1, 2.
The flexibility of these systems of vapor take-off may be denionstrated by two other modifications which have proved useful for special applications.
A closed tube may be inserted axially through the condenser in design 15 and the upper end of this tube brought out through the thermometer support tube above the va or line and sealed into the permanent gas inlet line. The tip ofthis closed tube is located close to the thermometer bulb in the product vapor so that an increase in “top” temperature will cause the air in the closed compensating tube to expand and, as a consequence, drive down the blanket in chamber B. If the volume of air in the compensator is adjusted (by the addition of a few drops of oil) it is possible to oppose the increase in At due to a rise in top temperature by a reduction in available condensing surface, so that the product rate remains substantially constant. It is sometimes an advantage in precise fractionation to increase the reflux ratio as the cut point is approached. Design 9 may be modified to do this automatically, as shown in 16 in diagrammatic form. The vapor line communicates directly to a vertical condenser which is emptied intermittently by a siphon. As product condenses in the lower part of this condenser-receiver,
Vol. 14, No. 8
the product level rises and cuts off part of the available condenser surface. As a result there is a progressive decrease in product rate as the distillation proceeds until the siphon is primed and the cycle repeated. The volume of the cut depends on the volume of the receiver up to the bend in the siphon, the final product rate on the residual condenser surface above the top of the siphon, and the initial product rate on the total condenser surface. It is possible therefore to design such a head to meet any specified conditions of distillation. If the to temperature is increasing throughout the run there will be a increase in product rate unless the water temperature is raised or the area of condenser remaining above the siphon progressively reduced by an air blanket in the water jacket.
SEW
Acknowledgment Thanks are due to Trinidad Leaseholds Limited for permission t o publish this laboratory application of still-head design.
Literature Cited (1) Arthur, P., and Nickolls, C. L., IND. ENG.CHEX, A N A L . ED., 13, 356 (1941). (2) Bruun, J. H., Bur. Standards J . Research, 7, 851 (1931). (3) Bruun, J. H., IND.ENG.CHEM.,SNAL. ED., 7, 359 (1935). (4) Bush, M.T., and Schwarts, A. M., Zbid., 4, 138 (1932). (5) Whitmore, F. C., and Lux, A. R., J . Am. Chem. Soc., 54, 3448 (1932).
An Oil Manometer-Manostat to Control Column Throughput S. A. HALL AND SARIUEL PALKIN, Bureau of Agricultural Chemistry a n d Engineering, Washington, D. C.
T
HE importance of control of distillation rate (through-
put) for optimum column performance is well recognized. Control of the heat input to the still pot by rheostat or Variac alone, except over short periods of time, has been found undependable because of the marked effect, especially a t low throughput, of even small variations in line voltage on the total heat supply t o the assembly. This is particularly true where tall columns are used and where the column insulation is dependent upon heat compensation. [For short columns (not over 90 cm., 3 feet) in which column and still pot are vacuum-jacketed, such as Podbielniak type (4),Variac control is probably much more dependable.] Any serious departure from t h e optimum throughput of a given column (especially a packed column) by increase or decrease of rate may seriously impair the column efficiency (HETP); if t h e rate is too fast, it may cause flooding; if too slow, it may fail to supply adequate reflux. The latter condition is especially likely t o result where “product” or “take-off” is maintained a t a constant value b y a stopcock setting, or some similar means is used for reflux ratio control.
Manostatic control of heat input to the still pot, by taking advantage of the relationship of column pressure drop t o distillation rate, has been used b y Othmer (2), Rossini and Willingham (5), and Selker, Burk, and Lankelma (6). The manostatic liquid (mercury or other conductor) provides the switch mechanism for opening and closing the control circuit to the still-pot heater. Mercury, as the manostatic liquid, can be used advantageously only with columns having appreciable pressure drop. With recent developments in columns having very low pressure drop, mercury is too heavy and lighter liquids, such as are used in the control described by Selker et al. ( B ) , are necessary. When a two-fluid manostatic system is used and the conducting manostatic liquid is an aqueous salt solution, use of the control device is restricted to column operation at atmospheric pressure because of the excessive volatility a t reduced pressure of the water in the manostat. The manostatic control described in this paper utilizes oil alone (mineral oil of very low vapor pressure) as the manostatic liquid. The movement of the meniscus, as a result of
TABLE I. THROUGHPUT
Test No.
r -
Min.
ThroughputMax.
Cc./hour
AV.
Min.
A p N0.a-
Max.
M m . of oil
Difference between No. of Minimum and Total Time Throughput Maximum Elapsed Determinattonsb Throughput
Hours
28 30 16 35 301,’~ 1 26 291/2 32 2 2 32 42 37 40 43 72 100 96 92 3 49 52 17 4 132 146 139 a Pressure drop as measured by distance between upper and lower levels of manometric fluid b Obtained at still head by counting drops a t varying intervals during test.
29 34 46 22
Deviation from Average Throughput
Cc./hou
Cc./hour
%
9 10 8 14
-4.5 -4
*14.8 *13.5 i4 . 2
*7
f
*El
5.0
August 15, 1942
653
ANALYTICAL EDITION
pressure variations corresponding to variations in column throughput, actuates a photoelectric relay which controls the heat input to the still pot. The results of some tests on throughput constancy when this device was used are shown in Table I. The manometer-manostat assembly is shown in Figure 1. The mineral oil used (Nujol), previously degassed under a high vacuum, fills the apparatus to level C. It has a density, a t room temperature, of 0.862; therefore, when vertical distance AC is 15.7 mm. of oil it represents a pressure drop ( A p ) of only 1mm. of mercury. The oil as a manometric fluid is thus almost sixteen times more sensitive than mercury.
Since the volume of oil in bulb B is relatively large, the level of oil in it is practically stationary for all probable values of A p , or distances between the upper and lower levels of oil in the two sides of the manometer. The photocell and the light source are contained in a brass housing, F , which can be moved by means of clamps G and GI to any desired setting along the manometer by-pass tube, K L , which acts as the manostat. The micrometer screw, M,, permits a slow raising or lowering of the mechanism for a Drecise setting of the reflux rate. T and ?=‘I are trap Tubes. Directly below the photocell mechanism is a small metal box, iV, containing a two-stage alternating current-operated photoamplifier relay circuit connected to the phototube in housing F. The amplifier circuit operating a small sensitive relay mounted inside the box was .L described by Shepard and Schrader (7). .D The two thermionic tubes and electrolytic condenser are mounted on top of the box. The other circuit parts, together with the sensitive relay, are mounted inside. Figure 2 is a detailed drawing of the metal housing for the phototube and 6-watt, 115-volt lamp constituting the movable phototube mechanism. Figure 3 shows the electrical hooku between the amplifier circuit and the stilf-pot heater.
Operation M.
I
I
CM.
l
m o b 0 0 0 0 0 0 0 0 0 0 0 0 00
I
The heat input to the still pot is set roughly by means of a Variac (Figure 3) until a reflux a t the top of the column, slightly above the rate desired, is attainedas measured by a drop counter or other suitable means. The phototube mechanism is then moved down along arm K L (Figure 1) until the sensitive relay is actuated to close the circuit. This signifies that the transparent oil, acting as a cylindrical lens to give a line image of the light source on the phototube cathode, W , has been interposed between the light source, s, and the aperture, 0 (Figure 2). [Thus its operating principle is different from the “opaque meniscus” principle described by Josten ( I ) . ] The mechanism is then moved slowly upward by means of the micrometer screw, M , until the sensitive relay breaks contact. A pilot light (not shown) across the power relay is used to facilitate the setting. When the relay breaks contact it signifies that the light from S is no longer focused through aperture 0 t o the phototube compartment but, instead, is diffused by the empty glass tube. This change in light intensity is sufficient to operate the phototube, which is sensitive to a rise or fall of about 0.33 mm. of the oil column when a t the level of the aperture. The sensitive relay operates a power relay (Figure 3) which intermittently interposes and cuts out a resistance, R, in series with the still-pot heater, H , as the oil level in the control arm of the manostat falls or rises indicatin an increase or decrease in throughput. l$hen using a 50-watt Chromalox bayonet-type heater immersed in an oil-filled center heater (a device to prevent bumping and promote boiling, 3 ) in a 500-cc. round-bottomed insulated flask, an intermittent resistance of 75 ohms was found adequate to compensate for line voltage variations. A heater of larger wattage would call for a proportionately lower intermittent resistance.
I
J
At a n extremely low throughput (30 cc. per hour) the percentage of fluctuation is naturally greatest ( * 14.8 per cent), as shonn in Table I. TT’ithout automatic control, however, it had been found virtually impossible to maintain such low
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 14, No. 8
7 LG. ROD3
FIGURE 2. METALHOUSING FOR PHOTOTUBE AND LAMP SLOTS
- x 32”
RELAY
Sangamo A1310 condenser (100 micro micro farads), one Mallory TP438 condenser (0.1 microfarad), one Mallory FPSl2O condenser (20 microfarads), one I. R. C. BT-1 resistor (1 megohm), one Ohmite “Brown Devil” resistor (30 ohms 10-watt), one Ohmite “Brown Devil” resistor (50 ohms 10-watt), and one Ohmite 0568 variable resistor (250 ohms 50-watt) with 3 connect-
I
inp dins. ---o ---r - STILL POT
H HEATER
(50 WATTS)
1
RELAYS. One sensitive relay (12 milliamperes direct current), Type PC5 (Allied Control Co., New York, N. Y.), and one alternating current mercury lunger relay Type MP1-M (H. B. Electric Co., Philadelphia, genna.) The total cost of these parts mas less tha; $17.
1 d
I l O V . A.C.
FIGURE 3. HOOKUP
throughput with any degree of constancy. The fluctuation in A p for a given throughput amounts to about 3 mm. of oil. This is due to the time lag between heat input change and throughput change.
Parts Used in Assembly PHOTOELECTRIC MECHANISM.One R. C. A. 922 phototube with socket and cathode clip, one G. E. 6-watt, 115-volt Mazda lamp (candelabra screw base), and one Dialco pilot light mounting for lamp. PHOTOTUBE AMPLIFYIXG CIRCUIT. One G. E. 6 J 7 radio tube with socket, one G. E. 25 A 6 radio tube with socket, one
Acknowledgment
The writers wish to acknowledge the valuable assistance rendered b y R. U. Bonnar of the Food and Drug Administration in the design of the phototube mechanism; also the helpful suggestions of J. J. Hopfield, physicist of the Naval Stores Research Division.
Literature Cited Josten, G. W., ISD.ENG.CHEM.,ANAL.ED., 10, 163 (1938). Othmer, D. F., IND.ENG.CHEM.,22, 322-5 (1930); IND.ENG. CHEY.,AXAL.ED., 2, 97 (1929). Palkin, S., and Chadwick, T. C., Ibid., 11, 509-10 (1939). Podbielniak, W.J., Ibid., 13, 6 3 9 4 5 (1941). Rossini, F. D., and Willingham, C. B. (publication pending). Selker, M. L., Burk, R. E., and Lankelma, H. P., IXD.ENG. CHEM., A N A L . ED.,12, 352-5 (1940). Shepard. F. H.. and Schrader, H. J.,Electronics, 9,36 (1936).
