Decomposition Hazard of Vacuum Stills K. C. D. HICKMAN AND N. D. EMBREE Distillation Products, Inc., Rochester, N .
A system is presented for caiculating the thermal hazard
P. All stills at present known operate in the range between Dh 12 and -1 for total thermal hazard, and in the range Fh 9 to -1 for hazard per plate. While this system
which labile substances are likely to encounter in stills of various design. Three terms and a unit of measurement are introduced: D = decomposition hazard = time (seconds) X pressure (microns) during residence in the still; Dh * index of.hazard = log D ; F = hazard per plate of fractionation = DIP where P = number of theoretical plates; and F h = plate hazard index = log F = D h log
of rating should have some practical usefulness, particularly after it has been tested and refined, for appropriate purposes the ease of operation of a vacuum column still may far outweigh the advantage of a low hazard and a position towards the lower, right-hand corner of Figure 2.
-
I
and 2(a); the latter becomes more significant as the still becomes "emptier" in type or the contents are more thoroughly agitated. This particular coefficient is low (16)-between 1.1 and 1.4 per IO" rise- and can be controlled by choice of construction materials and chepical additives. The coefficients of the homogeneous reaction are higher (14) and, therefore, dominate the situation. Monomolecular changes; chain reactions resulting in fission of the molecule, elimination of water, carbon dioxide, etc.; and bimolecular changes, such as polymerization of like molecules or interaction between the two components of a binary mixture, accelerate with temperature over the very wide temperature range under consideration according to:
N TWO previous publications (6) the author has had occasion t o compare the thermal hazards which different types of stills ake likely t o impose on labile distillands. A rule of thumb system (6) was used in which the hazard was considered to be proportional to time of exposure and doubling with every 10" rise of temperature. This led to the conclusion that a pot still distillation lasting one hour would be about 30 billion times as destructive as a rapid molecular distillation, completed within a second. The need for a formalized method of comparison which could also he represented in graphical form has now become pressing, not only to further design in stills, but to place in proper pefspective the performance of existing stills and to serve as a guide for the distillation of particularly labile substances. At the distillation symposium a t Pittsburgh (3)in December 1946, stills were described with separatory powers ranging from 0.5 t o 1.8 plates a t 1 M , through 10 to 15 plates a t 3 mm., t o stills furnishing 60 t o 100 plates a t atmospheric pressure. The operator wishing t o separate a thermally labile mixture must evidently choose between high separation and gross thermal hazard on one hand and poor separation under safer conditions on the other. If no compromise satisfactory for the purpose can he found, other methods than distillation must be adopted. Thermal hazard is contributed by two factors, temperature and time. But whereas the time factor has the mme meaning in most calculations of hazard, temperature has not, the variation of reactivity with rise of temperature differing widely according to circumstances. It is not possible to construct a general scale applicable t o all kinds of labile mixtures in which functions of time and temperature are interchangeable. It is, however, just such a general scale that is needed, so that, for instance, a rapid flash still may be compared with a pot still of the same kind or with a multistage evaporator of totally different design-all three constructions being considered for distillands ranging from a polymerizable oil to, say, an unstable amino acid hydrochloride; the problem is t o seek the best compromise. At least five variants must be considered in computing the thermal coefficients: 1.
2.
y.
log k = C
- E/2.3RT
where k is the rate of reaction ( decomposition), C is a constant, E is the activation energy, R is the gas constant, and T is the absolute temperature. To make this discussion treat general conditions, the equation may be put in a form to compare rates a t different temperatures:
The value for R is 1.985 calories per degree C. per mole and for k any unit may be used. Values of E for distillands of general interest are scarce, but the suthors have figures for the thermal alteration of vitamin A esters (10) where E equals 28,400 between 180" and 270' C.; and E equals 30,000 for the bodying of linseed oil (11) between 280' and 315" C. Similar factors have been obtained for soybean and castor oils (19, 16). Bragg (z), studying the cracking of paraffin wax, and Geniesse and Reuter ( d ) , studying the decomposition of gas oils, found E values of 64,000 and 53,000, but the measurements were made a t much higher temperatures and pressures than those treated in this paper. It is highly convenient for the present purpose that vapor pressures are related t o temperature in a manner similar t o reaction constants:
Homogeneous reactions in distilland, reflux, and vapor ( a ) Monomolecular ( b ) Bimolecular (c) Higher molecular orders Heterogeneous reactlons between (a) Distilland, reflux: and container (b) Distilland, reflux:and foreign gas
where AH is the heat of evaporation in calories per mole, and t,he units of pressure, p , are immaterial. Values of AN from the vapor pressure curves of high boiling fluids measured in the laboratory range from 15,000 for mercury t o 30,000for synthetic pump fluids
A sixth reaction, between vapor and container, need not be considered because the walls are usually wetted with distilland or reflux. Exceptions would be a molecular still or column still improperly operated. The three important reactions are l(a), l ( b ) ,
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Vol. 40, No. 1
to be investigated are large enougb to permit certain simplifying assumptions without distorting the final conclusions. First, we shall assume that most of the high boiling fluids handled in organic and petroleum laboratories have values of AH around 30,000. Second, and with less foundation, we assume that the value of E is the same as the value of A H . This is roughly borne out by the decomposition data mentioned above, and it does not seem incongruous that the rate of decomposition should vary in the same manner as the vapor pressure. The relationship is apparent from Table I and Figuie 1, whence it follows that Equation 3 can be extended: D = t X klkp
= t
X pjpp = t X p
(4)
when p is measured in microns. Our excursion into reaction types and vapor pressures thus brings us to the ultrasimple approximation that the decomposition hazard is measured by the average pressure in the still multiplied by the time of residence or passage of the filling. Because of the inconstancy of both E and A H , the relation will be inaccurate near absolute zero and near the critical point, but for the high vacuum still and the kind of substances treated therein, the relationship happens to hold sufficiently well. If the reasonable pressure limits are taken as 0 . 1 ~to 100 mm., or 108 times, and the time limits as 0.1 second to 24 hours, or lo8 times, the two ranges are almost identical. Seconds and microns become units of equal magnitude in their effect on decomposition, and time and temperature become interchangeable as elemrnts influencing design. The total range of decomposition hazard from atmospheric pressure downwards, so far experienced experimentally, is from TEMPERATURE I N DEGREES
D
CENTIGRADE
Figure 1. Relation of Temperature and Pressure
A group oi heavy petroleum oils have values of AH in the region 25,000 t o 30,000. These data are shown in Table I. To compare the different hazards of decomposition in various stills, the decomposition hazard, D , is defined to have a value of 1 at an exposure of 1 second at the temperature, T,, corresponding to distillation at a saturation pressure of 1 micron and to have values under other conditions defined by the following relation: D
t X k/k,
(3)
where t equals the number of seconds’ exposure, k equals the decomposition rate, and k, equals the decomposition rate a t T,. The value of k can be calculated through Equations 1 and 2 when the proper values of E and AH are given. Fortunately, the effects
TABLE I. E
AH VALUES
ASU
E Heavy petroleum distillates 2-Ethyl hexyl phthalate Tripalmitin Tristearin Polymerization, linseed oil Polymerization, castor oil Polymerization, spybean oil Decomposition, vitamin A
AH
(Decomposition)
(Pressure)
. . . . ....
24,000-25,100 26,600 37,800 42,500
.. .. .. .. .. .... .. ........
24,000-30,000 27,000 35,000 28,400
........ ........ ....,... .. ......
=
0.02 to D = 100,000,000,000
This wide range is most conveniently expressed in logarithms and the authors suggest that the decomposition hazard, D , be referred to in terms of hazard index, Dh, where Dh equals log D. A still with a hazard, D , of 53,000 would have a hazard index of Dh 4.72. According to this nomenclature, the centrifugal rim still with the smallest known hazard would have an index of Dh-1.7 for a single pass; and a pot still a t atmospheric pressure would have Dh 9.5 to Dh 11. There is a minor difficulty in comparing continuous distillations with pot orbatch distillations because we do not know the number of fractions that will be removed, and hence the number of repasses throigh the still, in any particular case. A laboratory “analytical” distillation will require 15 to 20 repasses; a commercial distillation may require only two or three. Whereas the pot distillation exposes the last fraction and residue t o the total or maximum hazard, the continuous still exposes only the contents of the column, and the hazard incurred by the last pass ill be the unit hazard multiplied by the number of passes. (The calculation must always be made for the last fraction or pass because this fraction is important to the operator; othcrlnise he would not distill it,) In what follol\s the continuous stills have been rated for ten fractions or ten times the individual pass hazard. Continuing with this concept, the chart of decomposition hazard constructed in Figure 2 has the hazard, D , as ordinate plotted on the left against the number of plates of separation, as abscissa. On the right the lower half of the scale is related to
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137
formed in a tubular flash boiler, average time of exposure for 10 passes of 10 seconds, would be situated in a position (C). 36 times or 1.56 Dh units lower.
9 LL
0
IO
TYPES OF FRACTIONATING S T I L L S
9
So much for individual stills. When we consider types of rectification stills or multiple fractionating assemblies, the matter becomes slightly more complicated. I n a continuous column fractionator which holds considerable reflux or feed but has substantially no pot, the feed being admitted laterally and “instantaneously” brought to distilling temperature, the number of plates will be nearly proportional t o the length ofthe column and, hence, to the holdup and time of exposure. A 50-plate still will impose a time hazard ten times greater than a 5-plate still as’well as an increased thermal hazard due to the greater pressure drop in the column; and the type will be represented as a long area sloping upwards towards the right-hand side of the chart. If the same type of fractionator were used with a pot instead of lateral feed, the ratio of pot contents to reflux would be so great that the length of the still would have less effect on the total hazard. The time factor, perhaps 10 hours, would raise the whole rating t o B nearly horizontal area much higher up on the chart. An example of this condition, shown a t D on the chart, is the rotary column still recently described by Rossini ( l 7 ) ,which operates a t atmospheric pressure, the pot filling taking 12 to .18 hours to distill. A laboratory Fenske packed column, operating a t 10 to 30 mm., with a pot filling taking 8 hours to distill, would occupy area E. The same still, operated continuously, drops down toward, but nat beyond, the points of the arrows. At the extreme end of the hazard scale comes centrifugal molecular distillation with Dh 0.5 + Dh -1.7, albeit with a fractionating power of one plate, or less. Evidently one more index is required t o evaluate the decomposition hazard per plate given by a fractionating still. The hazard per plate of fractionation is F = D / P , whence the plate hazard. index is
8 VI 0
&g X
H
7
6
5
Y 4
P
3 2 I 0 -I -2 -3 b4 1/2 I 2 4 8 16 32 64 128 NO. OF THEORETICAL PLATES Figure 2.
Decomposition Hazards of Stills
Molecular plate operating a t lfi for 1 second B. Pot still operating a t atmospheric pressure for 1 hour C. Flash distillation a t 1 atmosphere D . Rossini rotary fraationating oolumn and pot, 1 atmosphere E . Fenske packed columns, 10 mm.. pot and Bash stills F Bowman hot-cold still ot and flash models FF. Bowman cold stirrer &, pot and flash models C. Molecular fractionator, Brewer and Madovski H Molecular fractionator, Wollner I. Centrifugal cone molecular fractionator Multiple plate centrifugal molecular fractionetor J. K. Limit of single plate molecular distillation Probable limit of multiplate molecular fractionation L. A.
)I
pressure in microns, the upper to time in seconds. This is for convenience of the user in arranging the position of a still. All three scales are geometric but with widely different exponents. If they were plotted linearly with the horizontal scale occupying 128 mm., the vertical scales would reach to the moon-such are the differences between stills and the factors underlying the safe distillation of any particular substance. The thermal hazard scale applies to compounds and mixtures just as it does to stills, A crude glyceride oil will survive 95% intact to horizontal hazard line 3 (left-hand scale), a refined oil t o line 4, while natural vitamin D scarcely survives line 1. Petroleums, some lubricating oils, terpenes, and many essential oils extend far up the scale and are properly handled by stills rated a t such levels, Hydrocarbons in the range Cq t o Cs would be assigned ratings above Dh 12 to 14, expressive of their ability t o withstand boiling indefinitely at, atmospheric piessure. On this scale a single-pass centrifugal molecular distillation at l p is represented by point A on the chart, while the complete 10-pass distillation is represented by the area surrounding point A denoting the probable variation in time (0.2 to 20.0 seconds) of treatment and the probable limits of separation (0.5 to 1.8 plates). An ordinary pot distillation lasting an hour or two at atmospheric pressure is likewise shown as an area located at B about Dh 9.44, the position being determined by the thermal hazard of 7.6 X lo6 multiplied by 3.6 x 1 0 8 seconds. The same distillation per-
Fh = log
Figure 3. Diagram
DIP
= Dh
- log P
The plate hazards range from indexes of Fh 9.2 for Rossini’s still (17),through Fh 7.3 for a convent,ional laboratory column to Fh 5.2 for the Bowman cold stirrer still (1) which latter improves the rating over a hundred times. When this still is operated continuously without a pot, its index drops correspondingly lower, to Fh 3 to 4. No measurements are yet available t o show
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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
