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results it has been concluded, however, that extrapolation with the Antoine equation gives more consistent results than extrapolation with the Frost-Kalkwarf equation. On this basis the values in the last column of Table I are recommended as the best acentric factors for these compounds. The Antoine coefficients used to obtain these values were generally those of API Research Project 44 (1977). The values reported by Passut and Danner for the acentric factors of 1-butyne and propadiene also appear t o be in error. However, the vapor pressure data for these compounds are either unavailable or appear to be in error in the reduced temperature range of 0.7. In addition there are problems with the critical properties for these compounds. Thus although these acentric factors are highly suspect and probably should not be used, no better values could be obtained.
Literature Cited American Petroleum Institute Research Project 44, “Selected Values of Physica! and Thermodynamic Properties of Hydrocarbons and Related Compounds, Thermodynamic Research Center, Texas A&M University, College Station, TX (loose-leaf sheets extant 1977). Passut, C. A . , Danner, R. R., Ind. Eng. Chem. Process Des. Dev., 12, 365 (1973). Pitzer, K . S.,J. Am. Chem. SOC.,77, 3427 (1955a). Pitzer, K. S., J. Am. Chem. SOC.,77, 3433 (1955b).
Department of Chemical Engineering T h e Pennsylvania State University University Park, Pennsylvania 16802
W. P. Henry R. P. Danner*
Receiued f o r reuieu, January 16, 1978 Accepted January 31, 1978
DISCUSSION
Electrolytic Reduction of Alumina with Activated Cryolite Sir: When Acton et al. (1976) initiated a small-scale test of the Diller patents, it was presumed that they were seeking to duplicate my 3000% enhancement of the overall conductivity of a Hall cell, operating a t an industrial voltage. In their experiment no. 6, they show data for a lower, but still substantive, activation, Le. 400% current a t 3.6 V, or 50% voltage reduction from a normal of 5.3 v. The greater activation would have enabled both the lower voltage and the higher current. However, in the same experiment, there is an incidental 2 V transient. They inexplicably selected this datum as their sole basis of evaluation. Thus, they derived an apparently sweeping and unconstructive position. Diller (1977) responded with an analysis of the constructive part of the experiment, and he related the data to the copious normal-state data of Schlain e t al. (1963), the progenitors of the particular cell. Printing back to back, the Acton group explained the rationale of selecting the 2 V datum as the basis for evaluation. The rationale pertains to small voltages and small power supplies. In this paper, I will continue to draw upon my experience with the more than 150 experiments I have personally conducted with respect to the Diller effect. I will validate the pertinent readings, establish the normals, and show my computations. I will discuss the new reactions, the back emf as a major variable in my activation, and the alleged rationale.
Data Interpretation The interpretation of the Acton data is dependent upon a clear, activation-free normal base. The data of experiment no. 1,Figure 3, which may have been intended for this purpose, had to be discarded. As Schlain had demonstrated ample tolerance for mixing data from different cells of this group, and since the tolerance is negligible in relation to the gains that have been achieved, the Schlain data are a reservoir from which a suitable comparison base could be drawn. The significant variables in the Schlain tables are (a) anode material and (b) voltage. On p 25, they otherwise summarize that “operation is not greatly affected by relatively large changes in temperature, electrode spacing, current density, 0019-7882/78/1117-0374$01.00/0
and alumina content”. The anode material of this inquiry is graphite and the voltage range of special concern is 2.85-3.6 V. The Schlain experiments that are compatible with these conditions are no. 8-15,17-21,24,26-28,34-35. The current densities of these experiments were averaged and multiplied by Schlain’s (and Acton’s) anode area, as determined by them. This current, 11.8 A, and the average corresponding voltage, 3.18 V, become the normal base for this inquiry. Figure 3 requires a moderately activated cell in which a straight line balances the opposing Tafel and Diller curvatures. At its maximum 2.62 V, the current density is about 300% of a normal base for that cell. The same cell was deactivated in experiment no. 5, where it shows current density comparable to the normal base derived from Schlain. No pulse was reported. The failure of Acton et al. to notice the enormous and stable currents a t their 2.85 and 3.6 V, respectively, was not understood. They now explain that they wanted to save energy by operating a t 2 V normal, notwithstanding that the normalstate industrial voltages range from 3.9 to 5.3 V, and that the pots have to sustain their temperature through the in situ production of heat. Had they attained Diller’s 3000% level of activation, they could obtain the normal 5 V current a t 1.7 V and twice that current a t 2.1 V (Diller, 1969). Moreover, they state that they wanted to be able to use a 50-A power supply. Figure 1of my (1969) paper required 2400 A. My supply was capable of 3000 A steady and 6000 A for timed short periods. Back emf is an important variable in the Diller effect. The altered emf is indicative of the new reactions that have been induced. In high level activation, the emf actually reverses and feeds the system and, in part, enables the enormous current densities. Its measurement is more history-dependent and responsive to current density than usual. I t cannot be determined by extrapolation to zero current density. Ordinarily, it does not vary much. In commercial cells, it centers about 1.6 V. It is determined by shutting off the electrolysis and reading rapidly so as to catch the preceding cell dynamics. The Acton group did not monitor the back emf. Fortunately, in this case, I was able to calculate it, as will be seen in the section on
0 1978 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 17,No. 3, 1978 375
computation of back emf. Short of measurement in about 10 ns, the computed value is as accurate as direct reading. The New Reactions The following model (Diller, 1977b) shows one possible set of reactions whereby the activation is induced and maintained and its consequences occur. Such reactions do not rule out the simultaneous occurrence of other reactions, including normal reactions and competing effects. In the initiation step, some of the NaF leaves the cryolite lattice, becoming vibrationally excited NaF*. The NaF* splits the oxide, of which the parts also become excited. Thermal energy is drawn from the melt. The ensuing chain of reactions is carried by the cryolite, notwithstanding any depletion of oxide. It is seen that the carbon is a direct participant in the reduction, and that three charges deal with 2 atoms of aluminum. Na3A1F6
+ A1203+ activation = NaF* + A10+* + A102-* t a)
+ NaF* + 3e- = A1 + NaO-* + F-* NaO-* + F-* + C + 2e+ = NaF* + CO AlOz-* + C + e+ = A1 + COz A1203+ NaF* = A10+* + A102-* + NaF*
A10+*
(b) (C)
(d) (e)
The Pertinent Data In all experiments, whether by Schlain or Acton, the anode was 15/8in. diameter graphite and it exposed a height of 318 in. to the melt. Acton’s experiment no. 6 opened with an activating pulse, went through a brief induction period, and reached a reasonably stable activated run that terminated 18 min later due to burn-out of the power supply. Several additional pulsings, single shots minutes apart, during the 18-min period, produced no further material change in the activated operating conductance, and this is normal for the specific condition. The six readings taken during the 18-min period, separated by from 2 to 8 min, were (1)20 A a t 2.28 V; (2) 20 A a t 2.25 V; (3) 20 A a t 2.25 V; (4) 40 A a t 2.85 V; (5) 40 A a t 2.85 V; and (6) 58 A at 3.6 V. Within the range covered by these data, they are reasonably linear. The readings most involved in the computations which follow are numbers (5) and (6), which cover the final 8-min interval. The average baseline voltage of 3.18 V was selected so as to be centered between the 2.85 and 3.6 V of readings 5 and 6, respectively. Computation of Back Emf Let b, = the normal 1.6 V back emf and b, = activated back emf. Then, since the relationship is essentially linear over the range of readings 5 and 6
-E6-ba E5- ba
_ -1 6
(1)
15
where the subscripts refer to the respective readings, and
Calculation of Enhancement T o measure the enhancement a t reading 6, we extrapolate the baseline current for this cell to 3.6 V and compare it with Acton’s observed current. From eq 1
I,’ =
Z,(E,’ - b,)
E, - b n
where the subscript p indicates preactivated baseline, and I,‘ is the preactivated (baseline) current a t 3.6 V (Ep’)with normal back emf. Since Schlain’s baseline for this cell is 11.8 A at 3.18 V, with b, = 1.6 V
I,‘
=
11.8(3.6 - 1.6) = 14.9 A 3.18 - 1.6 16 _
58 - 389% I,’ 14.9 Similarly, extrapolating the baseline to 2.85 V gives a preactivated current of 11.8t2.85 - 1.6) = 9.3 A 3.18 - 1.6 and the current enhancement is
I,’’
=
I5 40 430% I,” 9.3 Acton’s 58 A reading 6 is thus seen to be 389% of the preactivated extrapolated baseline current for 3.6 V, and the 40 A of reading 5 is 430% of the preactivated baseline current for 2.85 V. Voltage and Current Density Acton et al. state that their interest is limited to saving energy, so that only voltage differentials concern them. However, the Diller effect produces an increased operating (overall) conductance and this is observable and usable as voltage reduction, current enhancement, or as a compromise of the two. These factors can be observed directly, and, within the linear range of the data, they can be converted to each other by straightforward,simple computations. My patents start mostly with 5.0 V, which is the most used voltage in the U.S. aluminum industry. In addition, I run both constant current and constant voltage tests. Acton et al. prefer to use a small power supply and therefore wish to start with 2 V. This severely contracts the scale. For example, at this low level a 400% enhancement would be seen as approximately 0.2 V above back emf, which is about the level of the low-frequency electrochemical ripple. (Higher level of enhancement would result in an even smaller, hence less detectable, differential.) Conversion of Enhanced Current to Voltage Reduction The same approach can be used to convert Acton’s enhanced current into equivalent voltage reduction. Since the activated back emf is 1.18 V, from (1)we have
Ab = b, - b, = 1.6 V
- 1.18 V
= 0.42 V
where Ab is the loss in back emf due to Diller effect.
E g - 1.18
(7)
where E, is the post-activation voltage for I,’, and
E, = and
(6’)
-e-=
16
(3)
(6)
and
& = E, - 1.18 Substituting the following data from Acton’s readings 5 and 6 ( 1 5 = 40 A; 1 6 = 58 A; E5 = 2.85 v; E6 = 3.6 v) into reading 5 gives
(5)
I,’(Ep, - 1.18) 16
Substituting I,’ gives
(4)
E, =
+ 1.18
= 14.9 A, 1 6 = 58 A, and E6 = 3.6 V into (8)
14.9(3.6 - 1.18) 58
+ 1.18 = 1.80 V
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Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 3, 1978
To compute the equivalent voltage reduction
The same result would of course obtain if the data from reading 5 were used instead of data from reading 6. Acton's results are thus equivalent to a voltage reduction of 50%. In addition to the constant-current baseline from Schlain, a voltage baseline is also contained in Acton's nearly activation-free experiment no. 5 , where 5.3 V drives 40 A. By comparing reading 5 in experiment no. 6 (40 A a t 2.85 V) with the 40 A a t 5.3 V in experiment no. 5, the 400% current density of experiment no. 6 is seen directly as a 50 f 5% voltage reduction. Credibility Subject only to a test of the credibility, the Acton readings clearly confirm the principle of the Diller effect to the extent of prolonged and unprecedented enhancement of the operating conductance. I believe the readings to be credible for the following reasons. 1.The activation parameters that precede the formation of the pertinent readings of experiment no. 6 are within range of the patents. (Other factors remain to limit the level of result due to laches.) 2. The activation is preceded by induction transients. (These were misinterpreted by Acton to be the activation.) 3. Reading pairs 5 and 6 pertain to the same conductance while their ratios further show that the conductance is ionic by requiring a back emf factor in order for the reading pairs to equate. 4. The back emf a t the current density of reading 6 was computed at 1.18V, while the normal back emf for this current density is a t least 1.6 V. This differential is characteristic, in principle, of the Diller effect. Its comparative smallness (0.42 V) is due to a combination of a comparatively low enhancement (400%) and a comparatively inert anode. 5 . The readings per se appear to be carefully made. 6. The experimental voltage reduction (50%) is identical with that of eq 10. 7 . Once the activation developed, additional firings produced no change in the new conductance. This accords with my experience in the course of more than 150 experiments. 8. Other explanations are unsupported and fail to explain the observed results. Bubbles would reduce conductance. Pieces of graphite are dimensionally, mechanically, and electrochemically (back emf) insupportable. Conclusion I showed my computations, illustrated the new reactions, dealt with the explanations of Acton et al. and otherwise
sought t o provide additional insight and method pertaining to the Diller discovery, while focusing on only a few of its aspects. My position is based primarily on the more than 150 experiments that I have personally made. In their letter of 1977, Acton et al. give their reasons for deliberately selecting the 2 V transient as the basis for their negating conclusion. The same experiment no. 6 shows a stabilized 400% current or a 50% reduction, in relation to the normal state. The data of Acton et al. were rescued by replacing their Figure 3 with the data of Schlain for similar cells in the normal state. Apparently, test firing had started activation in each of the two Acton cells. Acton et al. did not monitor back emf. Due to the large current densities and the new reactions, in the activated state, back emf is an important variable. A fortunate array of data enabled its computation. The 400% level of current density reached by Acton et al. is short of the 3000% reached by Diller (1969). Where the former would enable 2X productivity 25% volt reduction, relative to the normal state at 5 V, the latter would enable 2X productivity 60% volt reduction, or 55X productivity a t the 5 V, etc. Acton's rebuttal fails to correct their position with respect to in situ heating as a function of the intense carbonic reactions and high current densities under this activation. My position is backed by the experimental production of a thermal runaway in a pot that is otherwise too small (7 kg of cryolite) merely to sustain the melt a t 5 V in the normal state. Moreover, as seen in the section on the new reactions, these are induced and primarily supported by thermal phonons. These reactions are extremely efficient in thermally reducing alumina to lower energy oxides and in reacting ionized oxides to produce significant forward voltage. Particularly a t the higher current densities, the normal-state second-law reaction, which is a synergetic part of the complex, becomes secondary with respect to the energy transfers. My processes may be viewed as means whereby chemical energy of the carbon is used in greater proportion, a t higher speed and with more efficiency, both internally and externally.
+
+
Literature Cited Acton, C. F., Nordine, P. C., Rosner, D. E., Ind. Eng. Chem. Process Des. Dev., 15, 285 (1976); 16, 261 (1977). Diller, I. M., Nature (London),224, 877 (1969). Diller, I. M., Ind. Eng. Chem. Process Des. Dev., 16, 259 (1977a). Diller, i. M., "Light Metals 1977," VoI. 1, p 266, TMS-AIME, New York, N.Y., 1977b. Schlain, D.,Kenahan, C. B.,Swift, J. H..U.S.Bur. Mines Rept. Invest.,No.6265 (1963).
50 Park Avenue New York, New York 10016
Isaac M. Diller
