INDUSTRIAL AND ENGINEERING CHEMISTRY
592
There is no loss of liquid if the column floods when unattended. There is less f i e hazard than with the usual type of column. In case of breakage there is little liquid in the vaporizer and column proper and the distillation can be stopped immediately. rhe jacket bath can be enclosed and insulated, so that i d a m mable liquids will not be ignited. Thermal decomposition of heat-sensitive materials can be decreased by keeping the feed liquid cold. The removal of the heavy ends first should also decrease thermal decomposition. There is no tendency for liquids to bump or froth with the type of vaporization employed. Where there is lack of head space, several columns can be oper-
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Vol. 17, No. 9
ated in series, using a vaporizer as an automatic pump for taking liquid from the bottom of one column to the top of the next. DISADVANTAGES. Large amounts of solids or nonvolatile viacous liquids will plug up the vaporizer. The vaporizer has more liquid holdup than the condenser used in the reverse type of operation. LITERATURE CmD (1) Bosschart, R. A. J., IND.ENO.CHBY.,ANAL.ED.,6,29-33 (1934).
(2) Fenske, M. R.,Tongberg, C. O., Quiggle, D., and Cryder, D. S., IND.ENO.CHBM.,28, 644-5 (1936). (3) Langdon, W. M.,and O’Brien, G. M., Jr., IND. ENG.CEEY., ANAL.ED., 16, 83940 (1944). (4) Oldershaw, C . F., Ibid., 13, 267 (Figure 6) (1941).
Constant-Volume Pump for Circulating Gases 1. E. PUDDINGTON Division of Chemistry, National Research Council, Ottawa, Canada
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N APPARATUS that has been devised to circulate gases
in a system whereit is necessary to keep the volume constant consists of two Toepler pumps of about 100-cc. volume, connected as shown in the figure, so that the exhaust from one becomes the intake of the other. The assembly moves freely in a vertical plane about e, the amount and timing of this movement being controlled b a cam, Q. Free movement about e is accomplished, with all-gLs construction, by means of two helices, attached as shown. The operation of the pump ie simple. As bulb b is raised by the cam, the mercury contained in it flows by gravity into a, and completely fills it. This forces any gas in 5 t h o u h c, and a t the same time allows b to fill with gas through d. As t i s lowered, the mercury in a flows into b and forces the gas in b through tube f into a. The cam is shaped to maintain b in the raised and lowered osition for an ual period of time, and ita speed is adjustei’ to give just cient time for the mercury to flow from one bulb to the other when b is in the raised or lowered position. With each rotation of the cam 100 cc. of gas are forced through the pump. In practice, f was made of 2-mm. capilla to cut down the de8d space and to give added strength where x e pump is supported by the cam.
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Since the time required for the mercury to flow from one bulb to the other is less a t low than a t high pressures, the rate of rota-
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tion of the cam and consequently the pumping speed may be considerably increased a t low pressures. The absence of mechanical valves allows this pump to operate a t pressures as low as 0.001 mm. and no adjustment is necessary as the pressure within the system changes. This device operates satisfactorily against the backing pressures normally found in systems where circulation is required. By increasing the length of the arms on bulb b and increasing the amplitude of ita motion, the backing pressure can be substantially increased without stopping circulation. The lass helices were made by winding about 1.5 meters of 6-mm. f’yrex tubing, a t one turn per 2 cm., about a piece of 5cm.(2-inch) iron pipe, covered with a layer of wet asbestos paper. This may be done conveniently by sup orting the pipe in two ring clamps and turning i t siowly, whilee!t glsss tubing is heated with a Bunsen burner, equipped with a wing top. For this model, each helix was about 15 cm. long, a!though it seems probable that half this len th would give sui3cient flexibility for sign of failure was found after one the motion of the pump. month’s continuous operation. In assembling the apparatus it is necessary to have the bottoms of the helices in line with or slightly below the center of rotation, e. This procedure eliminates much of the strain on the helices when the pump is in operation. The cam used in this case was made of laminated Bakelite. A strip of wood placed under tube f spread the load sufficiently to keep the fae: from cracking. A 600 to 1 reucing gear, coupled directly to the shaft of a 1725-r.p.m. motor, gave .f about the correct speed for the cam. This gave a pumping speed of about 300 cc. per minute.
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The high density of mercury probably limits the size of the bulbs that can be used in this pump. However, there seems no reason why, with a suitable supporting mechanism, they could not be of the order of several hundred cubic centimeters, should increased capacity be desired. ACKNOWLEDGMENT
It is. a pleasure to acknowledge the assistance of George Richard, who aided materially in the construction of this apparatus. PUBLIBH~ED sa N.R.C. No. 1310
