ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Literature cited (1) Bjerrum, J., Kol. D a n k Videnskab. Selskab. Math.-fus. Medd., 11, No' 5 , 3-58 (Ig3'); No' 3-63 12, No' 15, (1934). ( 2 ) Brown, E. H., Cline, J. E., Felger, M. h'I., and Howard, R. R., Jr., I n d . E n g . Chem., Anal. Ed., 17, 280-2 (1945). ( 3 ) Egalon, R., Vanhille, R., and Willemyns, M., private communication, 1942-45; Egalon, R., presented a t XXIst Intern. Congr. Ind. Chem., Brussels, 1948; Egalon, R., and Vanhille. R.,
presented at XXIInd Intern. Congr. Ind. Chem., Barcelona, 1949; Egalon, R., Vanhille, R., and M'illemyns, M., presented before Lille section, Soci6tb Chimique de France, May 27, 1949; Egalon, R., and Vanhille, R., to be published.
(4) Langmuir, O., J. Am. Chem. Soc., 38, 2221 (1916); 39, 1885 (1917); 40, 1361 (1918). (5) I , ~ A. ~T., ~ and ~~ ~ ~ , iC. S., ~ lbid,,~ 44, 2878-85 ~ ~ (1922). ~ ~ (6) Patry, M., and Duguet, R., Compt. rend,, 225, 1158-9 (1947); 226, 255-6 (1948). (7) Pavlov, K. F., and Lopatin, K. I., J. A p p l . Chena. (U.S.S.R.), 20, 1223-34 (1947). (8) Zhavoronkov, N. M., and Reshchikov, P. hI., J. Chirn. Ind. (U.S.S.R.), 10, Yo. 8, 41-9 (1933) ; Zhavoronkov, N. hf., and Tikhmenev, S. N., Ibid., 16, No. 9, 35-6 (1939); Zhavoronkov, N. bl., and Chagunava, V. T., Ibid., 17, No. 2, 25-6 (1940). ACCEPTED May 24, 1954.
RECEIVED for r e n e w March 3, 1962.
High Temperature Thermal Conductivity of Gases Measurements on Nitrogen, Carbon Dioxide, Argon, and Nitrogen-Carbon Dioxide Mixtures at Temperatures up to 775' C. ALBERT J. ROTHMAN1 AND LEROY A. BROMLEY Radiation Laboratory and Division of Chemical Engineering, University of California, Berkeley, Calif.
T
HERRIAL conductivity values for gases are essential in
making calculations for conductive and convective heat transfer. I n addition t o their practical value, high temperature conductive measurements may ultimately prove to be of value in clarifying the mechanism of energy transfer in gaseous molecular collisions. The primary object of the present research n7as t o design and develop a cell and auxiliary equipment suitable for measuring conductivities of gases up t o moderately high temperatures (800" C.). I n order to check the operation of the equipment, data were t o be obtained both for purc gases and for a mixture of two gases. Heart of equipment i s a pair of concentric silver cylinders
Silver Cell. The basic design is similar t o t h a t used bv Keyes and Sandell (15). The heart of t h e equipment (Figure l") is a pair of concentric silver cylinders 7 inches long (Figure 2). This rnetal was chosen because of its low emissivity and high thermal conductivity. The annular space between the cylinders is about 0.025 inch (0.033 inch in later runs). This close spacing minimizes convection and is particularly valuable in reducing the percentage radiation correction b y increasing t h e gaseous conduction alone. The annular symmetry is kept by means of six Lava spacers sjrmmetrically placed and of small dimensions so as t o minimize conduction through them. (Lava is a natural stone which may he machined, product of American Lava Corp., Chattanooga, Tenn.) They maintain concentricity even a t 800' C. to within 0.002 inch, which is sufficient t o keep the conductivity error due to inch eccentricity below 0.3%. The spacers contact small in diameter) stainless steel inserts (not shown in Figure 1) in the emitter to prevent deformation of t h e soft silver a t high temperatures.
' Present address, Shell Oil C o . , Martinez, Calif. May 1955
T h e emitter, a solid silver piece about 1.45 inches in diameter, contains a Nichrome wire heater 0.016 inch in diameter in a shell of thin-gage (0.015-inch) stainless steel. The heater in turn has short leads of platinum 0.015 inch in diameter, which are fused to gold leads 0.015 inch in diameter and 24 inches long. The platinum wire reduces the temperature of the leads leaving the emitter and the gold is used to minimize the voltage drop along the long leads between the emitter and the external measuring circuit. The outer. cylinder (receiver) is 1.50 inches in inside diameter and 2.5 inches in outside diameter. Both inner and outer cylinders contain holes for thermocouple wells. These and all other silver parts v-ere stress-relieved a t 300' C. before machining. Fastened a t the bottom of the receiver is a solid silver disk which acts as part of a pair of parallel plates, along 13-ith thc lower
Figure 1.
Equipment
W h i t e potentiometer not shown
INDUSTRIAL AND ENGINEERING CHEMISTRY
899
h
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT @ CASING (STAINLESS S T E E L 1 @ RECEIVER CYLINDER [SILVER) @ EMITTER CYLINDER (SILVER) @ EMITTER HEATER @ OUARD (SILVER) @ GUARD HEATER @ THERMOCOUPLE WELLS @ EMITTER SUPPORT TIPStLAV.41 @ SUPPORT HOLDERS @ FLANGES @ CASKET (STAINLESS STEEL) @ OUTLET PIPE ISTAINLESS STEEL)
EMITTER SUPPORT DETAIL
Figure 2.
Thermal conductivity cell assembly
end of the emitter. Here, too, the emitter is supported by a Lava tip. inch above the emitter is a heat guard, so called beAbout cause its purpose is to prevent heat flow axially upward. I t has its own thel;mocouple well and Sichrome heater, so that it can be maintained a t the temperature of the emitter. This equipment is all contained uritliin a lower stainless steel casing which is flanged and bolted to a matching upper flange. The heater and thermocouplc tubes, as well as an outlet pipe, are welded to the upper flange. Vacuum tightness is ensured by the use of a stainless steel gasket compressed between the flanges. Furnace. The cell is placed in the center of a furnace (Figure 3) especially designed t o keep a constant temperature. The center of the furnace consists of a stainless steel can 13 inches in diameter, filled with molten tin (or water for room temperature runs). Tin was chosen because of its good thermal conductivity, low melting point (232" C.), and relative safety-noncombustibility as compared with heat transfer salts, and low toxicity (17) as compared with lead and other low-melting metals. I t was later found that corrosion by tin was excessive and the equipment is being redesigned to use a solid copper block. A Lightnin' mixer (Mixing Equipment Co., Rochester, N. Y.) was provided to agitate the liquid to aid in the maintenance of uniform temperature. The can is surrounded by a 2-inch thickness of Johns-Manville Superex, a form of diatomaceous inch in this layer at earth. Nichrome V coils are embedded the top, center, and bottom of the furnace, each section independently controlled. Successive turns are spaced about '/2 inch apart. The coils are backed by another 6-inch Supexex layer, which in turn is held in a sheet iron container. Copper cooling water coils are soldered to this casing and serve to keep it a t a constant and moderate temperature. The furnace interior is accessible by means of a lid opening a t the top of the furnace. Two basic input circuits are provided: one 8-kw. arrangement talcen directly from house current for rapidly heating up the furnace, and one 3-km, circuit fed from Sola voltage regulators. which keep voltage fluctuations to 1%. The latter circuit is used during steady-state operation. Temperature Controls. In addition to voltage control of the windings, and the isolation of these windings from the tin bath by 2 inches of low conductivity insulation, two stages of automatic temperature control were provided (dl ). One stage measures the temperature at tlie main windings by means of a
900
Chromel-Alumel thermocouple 0.025 inch in diameter and controls the current input to a 75-watt auxiliary heater which parallels the main windings. The other stage measures the temperatwe of the central bath by means of a platinum resistance thermometer encased in fused quartz and operates a 30-watt heater immersed in the bath to keep the measured temperature within f0.01" C. To broaden the range of bath temperature control, a second manually controlled bath heater of about 300 watts is provided. I n practice the first stage of automatic control \?-as found unnecessary, and was not used, the second stage taking over the burden of control. The desired degree of temperature control was achieved. For several hours the cell and bath temperatures remained constant to f0.02 " C. Measurements along the length of the silver cell indicated a temperature uniformity of f0.01 " C. Loading and Evacuation System. A aj4-inch I.P.S. stainless steel pipe (1 inch in inside diameter) welded to the flange of the conductivity cell connects it t o the vacuum piping. A Welch Duo-Seal No. 1405 vacuum pump is the forepump for a VMF20 Distillation Products oil diffusion pump, which provides the low pressure required. Pressure-measuring devices used are a McLeod gage (down to 0.03 micron) and thermocouple gage (down to about 1 to 5 microns). .4n alternate piping path is connected through a pair of magnesium perchlorate drying tubes to the required gns cylinders. An absolute mercury manometer for vacuum and open-end mercury mnnon~etersfor pressure and vacuum are in this branch of the circuit, as \?-ellas a pair of rotameters for metering gas mixtures. Stainless steel is used as the piping material in the gas loading circuit, excrpt for the vacuum shutoff valves alone (brass bellows type), as it is envisioned that corrosive gases will eventually be employed. For the relatively inert gases used in these experiments, clean copper connection tubes are used between the gas cylinders and the permanent piping. The vacuum piping and valves are iron and brass. Cell Heating Circuits. To provide stable heating current for both the guard and emitter, a series of from one to seven storage batteries and a source of rectified direct current are used in parallel for each heater. By means of variable transformers (Variacs), the current through the storage batteries is adjusted t o read a t or slightly above zero. I n this manner, the batteries are "floating" and act as a reservoir to keep the current through the heaters constant. The currents through the two heater circuits are
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY
Vol. 47, No. 5
ENGINEERING. DESIGN. AND PROCESS DEVELOPMENT varied roughly by varying the number of storage batteries in series, and more closely by a series of wire-wound rheostats ranging from 1 t o 800 ohms. T h e emitter and guard heater resistances are, respectively, about 90 and 35 ohms. I n operation, t h e current through the enlitter heater is obtained by measuring t h e voltage drop across a standard 0.1ohm resistor using a Leeds & Northrup No. 8662 portable potentiometer. T h e voltage across the heater and standard resistor together is obtained by placing a calibrated Leeds & Northrup high-resistance box in parallel with them and measuring the voltage drop across a small known part of the resistance. A correction is made for the drop along the gold arid platinum lead wires. Temperature Measurement. The emitter, receiver, and guard are provided with wells in which t h e hot junctions of thermocouples are inserted. At 50" C. and in some runs a t 350° C. Chromel-Alumel couples were used, while in most runs a t 350' C. and in all above t h a t temperature platinum-platinum-l0yo rhodium couples were used. The latter are less sensitive hut more stable in practice. Couples were annealed as rerornnlended by the National Bureau of Standards and calibrated by the bureau T h e cold junctions are placed in a thermos contalnlng distilled water and finely crushed, well-packed ice. Copper leads run from the ice bath t o the White double potentiometer Fractions of a microvolt are obtained by deflection readings of a Leeds & Northrup No. 2285-b high-sensitivity ga1v:tnometer. The sensitivity of the galvanometer and scale as used is about 0.05 t o 0.07 pv. per millimeter scale deflection. The entire memuring system is kept a t the same potential by intercorineeted slnelds. GASES USED. All gaseS used were available commerc~ially in cylinders. No attempt was made t o purify the gases, except t o pass them through tubes of magnesium pcrchlorate to dry them. In addition, the liquid carbon dioxide in the cylinder was purified somewhat by opening the valve and blowing off t h e gas near the top of the cylinder. This procedure lowcrs the air content of the gas. Manufarturer's specifications are:
Figure 3. 1.
2. 3.
4. 5. 6.
Constant-temperature furnace
Stainless can (tin bath) Conductivity cell Cell supports Furnuce cover Collar for mixer shaft Lifting eyes
May 1955
7. 8. 9. 10. 1 1. 12.
Furnace can (steel) Cooling coils (copper) Nichrorne wire heaters Insulation (Superex) Furnace support channels Electrical insulator terminals
Sitrogen (water-pumped), Linde. Minimum 99.7% nitrogen. Rare gases (krypton, argon, neon, helium) total
