Nornographs for Distillation of Low-Boiling Hydrocarbons Conversion of Distillation Temperatures and Pressures to Boiling Point at One Atmosphere FRED R9. NELSEN, FRANCIS R. BROOKS, AND VICTOR Z r l H S Shell Development Company, Emeryville, Calij. c
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7-50
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-40
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a 100 7
-20
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Figure 1. Conversion of Distillation Temperatures and Pressures to Boiling Point at 1 .4tmosphere from -85' to 20' C.
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N T H E fractionation of a liquefied gaseous sample by means of 'Ion-temperature distillation, it is usually necessary to carry out a t least part of the distillation a t reduced pressures. Inasmuch as the distillate is conveniently identified by its boiling point, by means of a thermocouple located in the condenser section of the column, it is desirable to have a rapid and simple means for converting distillation temperatures and pressures t o the more familiar boiling points a t 760-mm. pressure. This conversion can be closely approximated for the aliphatic hydrocarbons by means of nomographs. Such nomographs have been used in these laboratories for some time. To cover the full temperature range encountered in distillation of low-boiling hydrocarbons, it was found most convenient to construct two nomographs, one covering a distilling range +21" to -85" C. and the other from -80" to -175' C. (Figures 1 and 2). These nomographs have been constructed empirically from vapor pressure data taken from the tables of selected values of properties of hydrocarbons issued by the American Petroleum Institute (1). In constructing the nomographs, the distillation pressure was plotted in millimeters of mercury, from 30 to 1000 mm., on a vertical two-cycle logarithmic scale. Then a parallel line of nearly the same height was drawn for the distillation temperature scale. A hydrocarbon for which vapor pressure data were available throughout most of the desired temperature range was selected. The highest temperature for which the vapor pressure of the reference hydrocarbon was known was then arbitrarily estab-
Figure 2. Conversion of Distillation Temperatures and Pressures to Boiling Point at 1 Atmosphere from -175' to -80" C.
lished near the bottom of the distillation temperature scale and the lowest temperature for which the vapor pressure was available was established near the top of the scale. Intersecting lines were then drawn between the two values on the distillation temperature scale and the corresponding vapor pressures on the distillation pressure scale for the reference hydrocarbon. A straightedge placed on the intersection point of these two lines and any given value on the distillation pressure scale ww used to determine the proper location of the corresponding temperature on the distillation temperature scale. In this manner a number of values were located on the distillation temperature scale and the scale was further subdivided by interpolation and by similarly using vapor pressure data of other hydrocarbons. %-Butane was used as the principal reference hydrocarbon for Figure 1 and ethane for Figure 2. From the determined distillation temperature and pressure scales, intersection points were established for various hydrocarbons using available vapor pressure us. temperature data. I t was found that these intersection ioints lay very nearly on a straight line, which was then drawn in for the 760-mm. boiling point scale. The values for this scale were filled in as much as possible by laying a straightedge from the 760-mm. point on the distillation pressure scale to various temperature values on the distillation temperature scale (at 760-mm. mercury pressure the readings on the distillation temperature scale and those on the 760-mm. boiling point scale should be identical). The scale was then further extended by interpolation.
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OCTOBER 1947
815
It has been found simple, with the aid of these nomographs, to follow the course of a distillation performed a t reduced pressure in terms of the boiling point a t atmospheric pressure. The nomographs have been used for the distillation of a large number of low-boiling paraffins, olefins, and diolefins and have proved RCcurate to within =t1 C. throughout their working range with all hydrocarbons through Cr thus far encountered except acetylene. The anomalous behavior of acetylene is not surprising, however, because it5 vapor pressure curve rises more sharply than do those of the other hydrocarbons.
ACKNOW€EDGMENT
The present nomographs are an outgrowth of one developed by Ritchie R . Ward of these laboratories. LITERATURE CITED (1) American Petroleum Institute, Research Project 44, National Bureau of Standards. “Tables of Vapor Pressures and Boiling Points, at 10 to 1500 Mm. Hg”: S o . l k , “Paraffins, CI to Ci” (June 30, 1944); No. 2k (Part 1 ) . “Paraffins, CS” (March 31. 1944); No. 2k (Part 2 ) . “Paraffins, Ci” (Max 31, 1944); No. 8k (Part l ) , “Monoolefins, Cz to C I (March 31, 1945). R ~ C E I V ESeptember D 27, 1946.
Simplified Still Head with Automatic Control of Reflux Ratio RICHARD KIESELBACH, Bakelite Corpora tion, Bound Brook, jV. J . ’HE literature contains numerous designs of still heads with provision for automatic control of reflux ratlo, independent of boil-up rate. One principle much used is the intermittent take-off, in which the entire condensate is directed alternately to the receiver and to the column as reflux (1-4). The valve controlling IRON WIRE SEALED IN this flow is operated by a commercial I 24/40 P X 2 4 / 4 0 cdectric cyclc timer. The still head described hereisof this typc, but has some
A
EXPANSION BELLOWS. NO LIQUID TRAP AT’ LOWEST POINT OF DRIP TIP MUST BE OPPOSITE SIDE TUBE.
7 I.D. MINIMUM 3 12,2 MM. BORE
2 I.D. MINIMUM
PLUG MUST FIT CENTERING TIPS.
advantages over each of the previousll- publlshed designs. I t is compact, rugged, and relatively easily constructed. The take-off valve is readily removed, facilitatlng cleaning. No lubricant is required on the valves exposed to the condensate. S o stopcocks are required on the head when used at atmospheric pressure, n.hile lvith vacuum, only one stopcock, exposed only to cold vapor. need he manipulated to TI ithdraw condensate from the receiver. The still head is shown in Figure 1. A 110-volt alternating current relay coil, such as Struthers-Dum type D,.sLipped over the 12-mm. tube, A , projecting from the top of the head, operates the take-off valve. Vapor temperature may be measured with a standard-taper thermometer fitted in the 10/30 joint, E, a t the right. The spherical joint,, F , connects to the column. Yo provision has been made for cooling the condensate coming from the take-off valve, D. This is unnecessary with the highboiling materials for which the head was designed. For use with loner-boiling compounds, t,he addition of a water jacket below the take-off valve would be desirable. The still head is constructed in two par?.;. to facilitate cleaning, and to permit its convenient use for either vacuum or atmospheric pressure. The spherical joint, B , connects to the pressure regulator and vacuum pump, The trap, C, to Thich it connects may he immersed in a dry-ice bath, if desirable. In normal oDeration. the 3-viav stODcock. G . is turned to connect t,he uppir and lower seedons of the receiver. Atmospheric pressure then closes the lower check valve, H , and condensate collects in the lower section. To remove the condensate, the stopcock is turned to admit air into the loner section, thereby closing the upper check valve, H , and opening the IoLver one. Condensate then collects in the upper section until the stopcock is returned to its original position. Lubrication of stopcock G is no problem, since it is not exposed to the condensate. When slightly viscous liquids art) distilled, the check valves tend to st,ick closed. Although t’hey may be loosened by tapping, a more convenient arrangement is to keep them warm with a few turns of B&S 32 Nichrome wire wrapped over asbestos paper, operated from a bell-ringing transformer.
A slight amount of air leakage inevitably ocscurs at the check valves of this still head, since unlubricated ground joints cannot he perfectly airtight. However, if the valves are carefully lapped in, and assembled so as to prevent imperfect seating, the amount of leakage a t 10-mm. pressure is insignificant. Dirt lodgin’g in the valve seat,s will, of course, cause leakage. Such dirt usually can be dislodged by one or two operations of t’he valve. I n practice, this occasional inconvenience has been found far less troublesome than the many ills afflicting lubricated stopcocks, particularly when distilling high-boiling organic mat,erials.
2 I.D. MINIMUM
Figure 1. All dimensions in millimeters.
LITERATURE CITED
Still Head
(1) Bartleson, J. D., Conrad, A . L., and Fay, P. S.,IND.ENG.CHEM., A X ~ LED., . 18, 724 (1946). (2) Collins, F. C., and Lante, V.,Ibid., 18, 673 (1946). (3) Ferguson, B., Jr., Ibid., 14,493 (1942). (4) Muller, R . H., Ibid., 12, 605 (1940).
Tubing sizes outside diameter
RECEIVED July 16, 1946.
