138
INDUSTRIAL AND ENGINEERING CHEMISTRY
the Fh values of fractionating molecular stills, but they should remain in the neighborhood of Fh 0.0. hlultiple redistillation may be done on a single rotating cone or an many centrifugal plates grouped as a single still or as a collection of separate stills. The thermal exposure generally decreases slightly with each step of separation, so that a 10-plate operation, other things being equal, should cause a little less than ten times the unit‘ hazard, The performance area for a cone molecular fractionator (8) is shown at I and the mvltiplate fractionator a t J ( 7 ) ,both allowing I second per step. If area K represents the present limit of unit distillation a t 0.02 second, evidently the lowest hazard for fractional redistillation which can now be foreseen lies along area L, using for high separations a fantastic number of small high speed plates. The hazard ratings (but not the plate hazards) of various stills not mentioned in the text are given on Figure 2. The data and the scheme presented are, of course, approximations which require considerable refinement and laboratory investigation before they become quantitatively reliable. A first requisite is the compilation of the decomposition constants, of a number of phlegmatic substances and mixtures in contact with different structural materials. A second requisite is a knowledge of the geometric average pressure in the still, which will be the summation of the pressures in different parts of the column and boiler. These pressures can be computed from the compositions of the distilland and reflux in various sections or they may be determined directly by automanometers (9) attached a t various points as shown in Figure 3. If readings are taken in the external limbs with the vacuum still in operation and also immediately after admission of enough air t o stop distillation, the difference between the two, multiplied by the specific gravity of the liquid in each Iipb, gives the net pressures. To these must he added the pressure of residual gas a t the head of the still.
Vol. 40, No. 1
ACKNOWLEDGMENT
The authors wish t o thank various critics (13) and reviewers (1) for material hclp with the final draft of this paper. LITERATURE CITED
Bowman, J. R., “A Concentric Tube Thermal Rectifying Column,” presented at Cambridge High Vacuum Symposium, Cambridge, Mass., Oct. 30, 1947. Bragg, L., IND. ENG.CHEM.,33, 376 (1941). Division of Industrial and Engineering Chemistry, A.C.S., Ibid.. 39, 686-804 (1947).
Geniesse, J., and Reuter, R., Ibid., 24, 219 (19321. Hickman, K., Am. Scientist, 33, 213 (1945). Hickman, K., Chem. Rev., 34, 51 (1944). Hickman, K., IND.ENG.CHEM.,39, 686 (1947). Hickman, K., E. 9. Patent 2,234,166 (March 11, 1941). Hickman, K., and Weyerts, W., J . Am. Chem. SOC.,52, 4714 (1930).
Layton, L., Distillation Products laboratory report (unpublished). New York Paint and Varnish Production Club, Report of Technical Subcommittee 19, Group 4, Joseph Mattiello, chairman, p. 55 (1936).
Privett, 0. S., McFarlane, W. D., and Gass, J. E., J. A n . Oil Chem. SOC.,24,2@ (1947).
Shields, J. R., private communication, Nov. 5,1947. (14) Taylor, H. S., “Treatise on Physical Chemistry,” 2n’d ed., Vol 2, p. 981, New York, D. Van Nostrand Co., 1931.
(13)
(15) Ibid., p. 1040. (16) Von Mikusch, J., IND. ENG.CHEM.,32, 1061 (1940). (17) Willingham, 0. B., Sedlak, V. A., Westhaver, J. W . , and
Rossini.
F. D., Xatl. Bur. of Standards, Am. Petroleum Inst. Research Project 6 (1946).
RECEIVED October 29, 1947. Communication 126 from the Laborstorien of Distillation Products, Inc.
Degradation of at Elevated
ratures
R. C. WALLER. I
