V O L U M E 19, NO. 6
432
Table I.
Flash Vaporization of Hydrocarbon Mixture (50 atmospheres, 200’ F.) T=,
Cornponent
CHI CaHa CaHa
i-C4Hl@ n-CdHlo i-CsH1z n-CoHn
COH14
=F
0,125 0.150 0,200 0,100 0,150 0,100 0,100 0,075
6.30 2.35 0,850 0,670 0,610 0,400 0,345 0,215
p =
0.5Q Y 0,216 0.210 0.184
0.40 Y
0.252 0.229 0.181 0,077 0.080 0,114 0,109 0.057 0.053 0.051 0,045 0.026 0,023 0.938 0,969
p =
0.30 Y 0,304 0.251 0,178 0.074 0,104 0,049 0.043 0.021, 1,024
r =
0.34 Y 0.281 0,241 0.181 0,075 0,106 0,050 0,044 0.022
0,045 0.103 0,213 0,112 0,173 0,124 0,129 0.102
1,000
1,000
2
values of x are determined by means of Equation 6,.after a check has been obtained. What the nomograph accomplishes, then, is the solution of Equation 5 by graphical means, instead of a series of two sliderule settings and one addition and one subtraction, which would be required normally for each component, for each trial. That this actually represents a considerable saving in time can be shown by comparing the two methods of calculation for a typical problem. Such B problem is represented by the case of a hydrocarbon mixture containing 12.5 mole yo methane, 15% ethane, 20% propane, 10% isobutane, 15’% n-butane, 10% isopentane, 10% n-pentane, and 7.5% hexane. Assume that such a mixture is Bash-vaporized a t 50 atmospheres and 200’ F. A normal set of calculations would probably go somewhat as indicated in Table I. Values of K were taken from Robinson and Gilliland (9). Comparison of the normal method of calculations with the nomographic method indicates the saving in time and effort effected by the latter method.
The use of the nomograph rather than the analytical method of calculation introduces no inaccuracies other than those associated with graphical methods in general. The larger the nomograph is made, the more accurate it can be, but the more unwieldly it becomes and hence the less useful. With the size shown, a relative error of about 270 can generally be obtained. If greater precision is desired, the nomographical solution may be regarded as a hrst approximation, and the calculations may then be completed analytically. However, since the values of K are not absolutely accurate (depending, for example, upon the assumption that the solutions are ideal), it would seem that the nomographical result, though less precise, is about as accurate ae the analytical method. For most practical cases, therefore, the use of the nomograph is justified. NOMENCLATURE
F Kr L r V
= number of moles of feed = equilibrium constant of component i
= number of moles of liquid residue = fraction vaporized = number of moles of vapor
Z F ~=
mole fraction of component i in feed
z i = mole fraction of component i in liquid residue yj = mole fraction of component i in vapor LITERATURE CITED
(1) Dodge, B. F., “Chemical Engineering Thermodynamics,” 1st ed., p. 599,New York, McGraw-Hill Book Co., 1944. (2) Gilbert, M.,C h m . M e t . Eng., 47,No. 4, 234 (1940). (3) Robinson, C. S., and Gilliland, E. R., “Elements of Fractional Distillation,” 3rd ed., p. 45,New York, McGraw-HillBook Co., 1939.
Melting Point Bath liquids Useful up to 440” C. LAWRENCE RI. WHITE, Western Regional Research Laboratory, U.S . Department of Agriculture, Albany, Calif.
ACK of a clear, mobile, heat-stable fluid suitable for use in L high-temperature melting point baths has caused many workers to devise more or less complicated melting point blocks, hot stages, and air baths for use in determining the melting points of high-melting organic compounds. Melting point blocks and hot stages require extensive calibration with compounds of known melting point, and Markley (5)has shown that the use of air baths may lead to erroneous results even a t relatively low temperatures. Materials available as melting point baths above 300” C. have been limibed to sulfuric acid containing potassium bisulfate (8) or potassium sulfate (6), mixtures of ortho- and metaphosphoric acids ( I ) , and “hard” hydrogenated vegetable oil ( 7 ) . Temperatures in excess of approximately 350” C. cannot be reached with these baths and there are numerous disadvantages in their use. Two groups of organosilicon compounds having remarkable heat stability have been developed in recent years. The tetraaryl ofthosilicates (4)boil a t approximately 400’ C. and are said to be stable at their boiling points, particularly in an atmosphere of nitrogen. ?;one of these materials was available for study at the time of this investigation. Silicone fluids are members of a group of orgariosilicon oxide polymers. Some of these fluids have properties that render them suitable for use in melting point baths up to 425‘ t o 440” C. This paper reports the behavior of three silicone fluids xhen heated and cooled repeatedly in two types of melting point equipment. APPARATUS AND MATERIALS
The melting point apparatus described by Conte ( 2 ) was made by sealing a jet and delivery tube into a Pyres No. 0640 Thiele tube and winding a 20-ohm heating coil on the curved portion. The coil was insulated with asbestos. Figure 1 shows how the melting point apparatus described by
Hershberg (3) has been modified to allow the bath liquid to be kept under an inert atmosphere. The ball bearings used to guide the stirrer shaft in the original equipment have been replaced by short glass tubes sealed to each end of aT29/42 inner joint which replaces the loose cap used by Hershberg. The glass tubes that serve as bearings and the stirrer rod were selected to give a good fit, so that the gas entering through the side tube is forced through the by-pass tube and out around the thermometer cap. Since the bath fluid undergoes an expansion of approximately 35y0 on being heated to 425“ to 440’ C. the arms of the apparatus must be sufficiently long to accommodate the increased volume. A 20-ohm heating coil was wound on the curved portion of the tube and insulated with asbestos. A second Hershberg apparatus, without provision for using an inert gas, was used for heating the fluids in contact with air. Silicone fluids type 550-84 centistoke grade, type 550-142 centistoke grade, and type 703-64 centistoke grade were used in this study. (Type numbers are the descriptions used by the manufacturer.) Compressed air, water-pumped nitrogen, hydrogen, and carbon dioxide were bubbled through concentrated sulfuric acid before being introduced into the melting point apparatus. PROCEDURE
The fluid was introduced into the clean, dry apparatus and heated to the desired temperature in about 25 minutes. The temperature was maintained about, 5 minutes, then the liquid was permitted to cool to room temperature. The gas used to stir the bath or to maintain an inert atmosphere flowed during the entire test. after the heating and cooling cycle had been repeated the desired numbcr of times, a portion of the liquid was placed in a 25-mm. cuvette and the per cent light transmittance over the range 400 to 700 millimicrons n-as recorded by a reoording spectrophotometer. .kfter the per cent light trasmittance had been recorded the fluid \vas returned t o the apparatus for additional heating at the same or a higher temperature. The per cent transmittance at 550 millimicrons was selected w a measure of the darkening of the fluid, since each of the unheated silicone fluids transmitted virtually all of the incident light of
433
JUNE 1947 ~~
Table I. Light Transmittance by Silicone Fluid T y p e 550-84 Centistoke Grade Heated under Various Conditions
Table 11. Light Transmittance by Silicone Fluids Heated
Type of Apparatus and Atmosphere Conte Nitrogen Conte Air TransTrammitthnee. No. of mittance. No. of mittance, pen- No. of 550 rnp 550 r n p heatings S b u rnp heatings ire he#,ti.mgs C. % % % Hemhberi Air
Trans-
226 275 275
100
310 350
g?;. .
d50
E
50° 50°
95
85
.. ..
59
50 50*
..
53'
18 0
25'
..
.. 98
95 88 50
.. ..
50 50'
19'
..
..
81
15 0
..
Hea..nae id addition to those in line above
Conte
Nitrogen Air
50 50' 10'
350
50 25'
300
300
99
425
94 24
94 92 87 45
97 0
350
301,
Heating8 in addition to those indioated in line above. Discontinued because of high visoosity sfter 11th heatine at this temi perature. a b
Tahl? 111. Influence of Atmosphere on Darkening of Heated Silicone Fluid Type 703 (25 heatinpa at 425' to 440' C. in Conte tube)
Atmosphere Nit rogen
Transmittance at 550 ma. 9 c 99
frozen 2; bon dioxide
95 89 20
Air
..,. m . air. . mowever, uariiening , > under similar conumons IS no5 senous with this grade 550 silicone. When heated in air in a Conte tube the viscosity increased much more rapidly than when heated under the other conditions studied and heatine had t o he discontinued before the temperature was raised above 350" C. After 10 heatinge at 425" t o 440' C. in addition t o previous heetings at 300" and 350' C., the viscosity of the fluids in the Hershberg apparatus and in the nitrogen stirred Conte apparatus had become so high that i t was considered advisable t o discontinue the tests. With care the fluids in the Hershherg apparatus could have withstood severd more heatings, but those in the Conte tube circulated bv nitrogen had nearly reached their limit of utility. Since silicone fluid 703 darkened appreciably faster when heated in air than in nitrogen, the influence of other relatively iuert gases on the darkening of this fluid wm studied. The study was conducted in a Conte tube, since the data in Table I1 show that the influence of the atmosphere is magnified in thw apparatus. Table I11 shows that, when heated to 425' to 440' C., this fluid darkens much less rapidly in nitrogen, hydrogen, or carbon dioxide than in air. The sample heated in air could not be used after the 25th heating because of its high viscosity, but the other s m ples, particularly the one that bad been heated in nitrogen, could have been used a few more times. The relatively low viscosity coefficients (compared to v e g e table and mineral oil), and low freezing points of the silicone fluids tested, suggest their use for melting point bath fluids throughout the range of -30" to 440" C. I
ready use without B special lighting system transmitted none of the inciaent light of this wave length. RESULTS AND DISCUSSION
Table I shows that silicone 550-84 centistoke nade is mu01h more stable when heated in a Conte tube under anatmosphere o'f nitroKen than under the same conditions in air. This is especiall:Y masked a t temperatures above 300" C. The fluid darkens less rapidly in air in a Hershberg apparatus than when heated in ai in a Conte apparatus. Since the silicone fluids are nonconductors of electricity ani are noncorrosive t o metal, the heating coil may be immerse< directly in the fluid, According to Hershberg (S), thi is highly desirable, since it permits more accurate temperaturi control and requires less energy. A sample of silicone type 550 84 centistoke grade was heated to 310' C . or higher 320 timeS under nitrogen in a Conte tube having an internal heating coi 1 before the viscosity mas too high for further use. At this point the fluid transmitted 20% of the incident light of 550 milli microns. Table I1 shows that neither of the fluids, type 703 nor 550-14 centistoke grade, darkened as rapidly when heated in nitrogen a
ACKNOWLEDGMENT
The author wishes t o thank Iva V. Streeter for making the recorded spectrograms. LITERATURE CITED Christensen, B. E., and Kim, King, A. E., IND.ENC. CHEM.,ANAL. Christensen.
Eo., 8, 194 (1936). Conte, Ernest, Ibid., 2,200 (1930). Hershberg, E. B.,Ibid..8,312 (1936). Johnston. L.H.(to Arthur D. Little. Ino.). U. 8. S. Patent 2,335.012 (Nov. 23, 1943). Markley, X.S., IND. ENC.CHEM.,ANAL.ED..6,475 (1934). Morton, A. A,, "Laboratory Teohnique in Organic Chemistry," p. 31, New York, MeGraw-Hill Book Co., 1938. Robertson, G . R., I d . Eng. Chem.. 15,701 (1923). Sando,C. E., IND.END.CHEM.,ANAL.Eo., 3.65 (1931).
