ENRICHMENT OF OXYGEN-18 BY THE CHEMICAL EXCHANGE OF

Publication Date: August 1962. ACS Legacy Archive. Cite this:J. Phys. Chem. 66, 8, 1480-1487. Note: In lieu of an abstract, this is the article's firs...
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S. C. SAXENAAND T. I. TAYLOR

1480

Vol. 66

ENRICHMENT OF OXYGEN-18 BY THE CHEMICAL EXCHANGE OF NITRIC OXIDE WITH NITRIC ACID SOLUTIONS BY S. C. SAXENA AND T. I. TAYLOR Chemistry Department, Columbia University,New York,N. Y. Received March 8. 1866

The rapid exchange of oxygen-18 between nitric oxide and nitric acid solutions occurs with a single stage enrichment factor of 1.020 =k 0.002 in approximate agreement with calculations from spectroscopic data. Since oxygen-18 concentrates in the gas phase, the use of this system required the conversion of nitric oxide to water and nitric acid. This was accomplished by partial decomposition of the nitric oxide in an arc, and reaction of the remainder with hydrogen t o form water. The water extracted NO2 from the gas stream to form nitric acid of the desired concentration. Some of the characteristics of the exchange reaction were investigated during the operation of exchange columns for the concentration of oxygen-18. For 8 M nitric acid, the height equivalent to a theoretical plate (H.E.T.P.) increases approximately linearly with flow rates above 60 mg. atoms of O/min.-cm.z. At 25’ and a flow rate of 85 mg. atoms of O/min.-cm.2, the maximum over-all separation Although the rate of the exchange reaction increases with increasing acid concentration, the was obtained for G M “02. l)/H.E.T.P. is maximum for 6 M “0,. The over-all separation was not greatly value of ( C Y - I ) decreases so that ( OL affected when the temperature was increased from 25 to GO’, indicating that the product of the transfer coefficient and (a-1) is approximately constant. With a decrease in temperature, the decrease in the transfer coefficient was not compensated by an increase in ( a - l),so that the over-all separation decreased when the temperature was lowered t o 0’. Cohen’s theoretical equations for the increase in isotope concentration with time were used to analyze the oneration of the exchange column and to determine the ratio of hold-up a t the reflux end to the hold-up in the column. The optimum operating conditions for the system were selected and a comparison was made with other methods of concentrating oxygen-18. The exchange system described here appears t o be somewhat more favorable than the distillation of water, but not as promising &s the low temperature distillation of nitric oxide.

-

Introduction

satisfactory catalyst is found this exchange reaction will not be very useful for producing high concentraA variety of methods for separating the isotopes tions of oxygen-18. of oxygen have been investigated including absorpThe exchange reaction between nitric oxide and tion, chemical exchange, diffusion, electrolysis, nitric acid has been used to prepare 99.9% nitrofractional distillation, thermal diffusion, e k z P 3 The most successful of these have been the thermal gen-15,l6J7 and since this exchange must involve diffusion of oxygen415and the fractional distillation water, oxygen-18 exchange between nitric acid of w ~ t t e r ,carbon ~ ~ ~ monoxide,8 or nitric oxide.g solutions must also occur rather rapidly. PreAlthough chemical exchange methods have been liminary measurements of Taylor and Clarke1*on successful for the separation of the isotopes of a the enrichment of oxygen-18 appeared promising, number of the lighter element’s,no satisfactory ex- and in this paper we give the results of our further change reaction has been found for the isotopes of reasearch on the system. Enrichment Factors.-Although enrichment facoxygen. Among the exchange reactions investigated,3 the one between carbon dioxide and tors have been calculated for a number of exchange waterf0-15 has been of particular interest because reactions invoIving the isotopes of oxygen,10Jg of its relatively large enrichment factor, 1.038 a t values were not available for several exchange 25’. Unfortunately, the exchange reaction be- reactions involving the oxides and oxy-ions of tween carbon dioxide and water is slow, and until a nitrogen. Taylor and Clarke1*calculated a number of these a t 25’) and the calculations reported here (1) This research was supported in part by a grant from the U. 8. extend them and Urey’s tablelgto include NO, KO,, Atomic Energy Commission. NOa-, and KO2-- a t several temperatures. The ( 2 ) G. M. Begun, ”Isotope Separation and Isotope Exchange, methods of Urey19 and Bigeleisen and Mayer2” a Bibliography with Abst,raots” ORNL-2852, Office of Technical were used to calculate the partition function ratios Services, Deprtrhment of Commerce, Washington, D. C., 1R58. (3) D. Samuel and P. F, Xteekel, “Bibliog~aphg of the Stable and the enrichment factors from spectroscopic Isotopes of Oxygen,” Pergarnon P r e ~ Ino., , Naw York, N. Y., 1959. dat,a, The frequencies used for n T 1 4 0 1 a , W402 1 (4) K, Clusius, CT, Pickel, and E. W. Bsokar. Naturwiss., 81, 210 Ni40218--1 and N14t&l6- were those summarized by (18481, (5) K. CIuaius and C . Dioket, Z. physik. Chew., LB8, 274 (1944). Begun and and the frequencies for ( 6 ) I. Dostrovsky and A. Raviv, “Proceedings of the Interflation&l €12016 and H2018 were those given by Urey.lQ Symposium on Isotope Separation,” North-Holland Publishing Co., Amsterdam, Netherlands, 1958, p. 336. References to earlier Vibrational frequencies for the isotopic molecules work on the distillation of water are given in this paper. with oxygen-18 were calculated from the equations (7) M. Thurkauf, A. Narten, and W Kuhn, Nelu. Chim. Acta, 43, given by Herzberg,22 assuming the validity of the 989 (1960). (8) H. London, “Proceedings of the International Symposium on Isotope Separation,” North-Holland Publishing Co., Amsterdam, Netherlands, 1958, p. 331. (9) K. Clusius, K. Schleich, and M. Vecchi, Nelu. Chim. Acta, 44, 343 (1961). (10) H. C. Urey and L. J. Greiff, J . A m . Chem. Sac., 67, 321 (1935). (11) L. A. Webster, M. H. Wahl, and H. C. Urey, J. Chem. Phys., 3, 129 (1935). (12) M. Cohn and H. C. Urey, J . A m . Chem. Soc., 60, 679 (1938). (13) G. A. Mills and H. C. Urey, ibid., 68, 1019 (1940). (14) A. F. Reid and H. C. Urey, J . Chem. Phys., 11, 403 (1943). ( 1 5 ) W. T. Boyd and E. R. White, Ind. Eng. Chem., 44, 2202 (1 952).

(16) W. Spindel and T. I. Taylor, J . Chem. Phys., 83, 981 (1955): 24, 626 (1956).

(17) T. I. Taylor and W. Spindel, “Proceedings of the International Symposium on Isotope Separation,” North-Holland Publishing Co., Amsterdam, Netherlands, 1958, p. 158. (18) T. I. Taylor and J. C. Clarke, J . Chem. Phys., 31, 277 (1959). (19) H. C. Urey, J. Chem. Sac., 562 (1947). (20) J. Bigeleisen and RI. G. Mayer, J . Chsm. Phys., 15, 261 (1947). (21) G. hl. Begun and W. H. Fletcher, ibid., 38, 1083 (1960). (22) G. Herzberg, “Infrared and Raman Spectra of Polyatomio Molecules,” D. Van Nostrand Co., Ina., New York, N. Y.,1946.

August, 1962

ENRICHMENT OB OXYGEN-18

BY

EXCHANGE OF N o

WITH

"03

1481

SOLUTIONS

TABLB I ENRICHMENT FACTORS FOR OXYGEN EXCHANGE OF WATERAND OXLDES AND OXY-IONS OF NITROGEN

[SI [SI'^ [

3 l ' n

[g]

1.1103 1.0982 1.0880 1.0810 1.0000

[

F3&J'

[E],.

T,oC

1.0979 1.0847 1.0740 1.0667

1.0888 1.0780 1.0710 1.0642

1.0825 1.0719 1.0632 1.0573

1.0770 1,0694 1.0630 1.0584

0 25 50 70

1.0066 1.0072 1.0076 1.0077

1.011 1.012 1.013 1.013

1.020 1.019 1.017 1,016

1.026 1.025 1.023 1.022

1.031 1.027 1.024 1 021

0 25 50 70

1.0000

1.0046 1,0052 1.0054 1,0056

1.013 1.011 1.0091 1.0080

1.019 1.017 1.016 1.015

1.024 1.020 1.016 1.014

0 25 50 70

1.000

1.0084 1.0062 1.0036 1.0023

1.014 1.012 1.010 1.0089

1.019 1.014 1.010 1.0078

0 25 50 70

1.0000

1.0058 1.0057 1.0065 1.0065

1.011 1.0080 I.0067 1.0055

0 25 50 70

1.0000

1.0051 1.0023 1.0002 0.9990

25 50 70

valence force meth0d.~3 Calculated values of the partition ifunction ratios along with the enrichment factors a t temperatures 0, 25, 50, and 70' are recorded in Table I. The partition function ratios for liquid water were generated from those of gaseous water by multiplying the partition functions of the latter by the ratio of the vapor pressures of H2016 and H201*. The vapor pressure ratios were calculated a t each temperature according to the formula given by Urey. l 9 Because of the complexity of the system SOHN08-H20, the computed values of the enrichment factors mill not apply exactly. When nitric oxide and solutions of nitric acid are equilibrated, several ionic and molecular species result. The gas phase contains NO, KO2, x203,N204, HZO, HK03, and HXOz, while the liquid phase, in addition to H K 0 3 and H20, contains KO, NOz, K203, N204, HNOz, H30+, KO,-,and KO2-. The equilibrium amounts of these species depend on temperature, gas pressure, and concentration of the It is conventional, therefore, to determine experimentally an effective single stage enrichment factor, a, defined as (01s)/(016)gas liquid

NOz'8 [F-]'"

1.1030 1.0903 1.0798 1.0727

rNOa'8-7'"

(018)/(016)

[%]I.

(1)

where the symbols in parentheses refer to the atom fractions of the isotopic species in the two phases. This a is a weighted mean of the enrichment fac(23) S. C. Saxena, D. N. Bhatnagar, and S. Ramaswamy. Details of the calculations for the molecules of interest here along with others

will be published elsewhere. (24) E. Abel, H. Sohmid, and H. Stein-Wein, 2. Elektrochen., 86, 692 (1930).

0

tors for exchanges between various species. Since according to Table I, the enrichment factor which is most favorable for enriching oxygen-18 in the gas phase is the one between nitric oxide and water, and as the concentration of the other compounds increases with acid concentration, one would expect a to fall with increasing acid molarity at the same temperature and pressure. The enrichment factor, a, was determined by equilibrating nitric oxide with 25 ml. of nitric acid in a flask (1200 ml.) a t room temperature ( 2 5 O ) , and a t one atmosphere pressure. The nitric acid was added to the flask and, after the solution was frozen, the flask was evacuated. To out-gas the nitric acid, the flask was warmed to room temperature and evacuated again after freezing the solution. This procedure was repeated two to three times. Nitric oxide then was added with continuous shaking of the flask to saturate the solution at atmospheric pressure. The flask vas turned on its side and rotated with a motor for 24 hr. or more a t room temperature to ensure complete isotopic equilibration. Samples of the oxides of nitrogen from the gas phase were taken directly in a discharge tube for decomposition into K2 and O2 for mass spectrometric determination of the 32/34 ratio. A sample of the liquid phase was analyzed after decomposition in a similar discharge tube.25 Values of a calculated according to eq. 1 are summarized in Table 11, along with the composition of the gas phase for several concentrations of nitric acid. These values are in approximate agreement with the theoretical calculations from spectroscopic data and the preliminary value of 1.020 =k 0.002 (25) T. I. Taylor, to be published elsewhere.

8. c. SAXENA

1482

reported by Taylor and Clarkei8 for 6 25' and I atm. pressure.

AND

HXO3 a t

TABLE I1 ENRICHMENT FACTORS FOR THE EXCHANGE OB NITRIC OXIDEWITH NITRIC ACIDSOLVTIONSAT 25" A N I ) ONE Arbiosrwimu GASPHESKRI?: Nitric acid concii., A4 Volume of acid phase, ml. Volume of gas phase, ml. Composition of gas phase: Mole % NO Mole % NOz

NzOe

Mole

Mole % HzO h b l e % HNOz

Mole % HNOs Atom % 0 in N O Atom % 0 in NOz Fhrichment factor, a

25.0 1135

6.2 25.0 1138

8.0 28.0 1135

9.7 25.0 1135

95.4 0.63 0.02 2.6 1.3

95.7 1.7 0.21 2.2 0.2

94.2 4,s 1.4 1.9

80.7 10.3 7.4 1.6

4.1

+ NrO4

93.9 1.1 1.028 f0.004

..

..

..

..

93.1 4.4 1.020

84.9 13.4 1.018 3~0.003 10.003

..

0.03 62.5 36.3 1.015 10.003

Experimental The exchange column used in these investigations was made of glass, 145 cm. long, 13 mm. inside diameter, and packed with KO. 3012 stainless "Helipak" column packing. Previous experiments on the concentration of nitrogen-15 in packed columns, bubble cap columns, and bubble plate columns showed that "Helipak" gave the highest separations. Since the exchange systems for nitrogen-15 and oxygen-18 are similar in many respects, the same packing was used. A relatively small diameter was selected for the column so that a wide range of flow rates could be run with the same reflux equipment. The column had a concentric jacket through which water was circulated a t the desired temperature from a conventional thermostatically controlled bath. The feed to the column was a stream of waste gases, from another column enriching nitrogen-15, and is described in detail by Spindel and Taylor.leJ7 The feed consisted of a mixture of most,ly nitric oxide and other oxides of nitrogen in a proportion determined by the equilibrium composition (Table 11) of the gas phase in contact with the nitric acid in the nitrogen-15 column at that temperature, pressure, and concentration. The column can, of course, be fed with the oxides of nitrogen from other sources. These nitrogen oxides rise countercurrently against the nitric acid in the column and exchange mostly according to the reaction

T. I. TAYLOR

Vol. 66

cient t o maintain the reaction without a furnace. The small amount of ammonia formed was decomposed on a 6 in. section of stainless steel wire cloth at 800-900° in the back section of the same stainless steel tube. Most of the water was condensed from the gas stream by condensers cooled to about 0'. The remainder of the mater was recovered and I eturnrd to thr column by means of molecular sieve adsorbmts. Two glass tubes, each filled with 10 to 15 g. of type 5 8 Molecular Sieve adsorbents, were connected in parallel so that one could be used while the other was being regenerated by backflushing with dry nitrogen. A timer with microswitches connected to solenoid valves and heaters automatically controlled this operation. Since i t was found'sr20 that the molecular sieve adsorbents exchange nTith oxygen-18 water a t the temperature of regeneration (200-250"), the quantity used was reduced to a minimum. This exchange constitutes a holdup a t the top of the column and reduces the rate of build-up of oxygen-18 to the steady state. The effluent nitric acid was collected and titrated from time to time so that adjustments could be made to maintain the same acid concentration a t the top and bottom of the column. The volume of acid collected per minute was used to calculate the flow rate of oxygen in the column. Gus samples of enriched KO were taken from the top of the column and decomposed into oxygen and nitrogen in a discharge tube. A mass spectrometerz7 with a single collector was used to determine the 32/34 ratio in the oxygen of the samples. Samples of the gas fed to the bottom of the column were analyzed in the same way. Since the 32/34 ratio in the feed gas was practically the same as that in tank oxygen, the latter was used as a reference for calculating the over-all separations. The procedure adopted for making a run was essentially the same for all experiments. The temperature of the column was adjusted to the desired value and the column was flooded slowly with an auxiliary supply of nitric acid of the concentration to be maintained in the column during the run. After the column was flooded, the flow of auxiliary nitric acid was adjusted to the operating rate and maintained a t this rate while the column was drained and while the reflux system was started. To start the reflux system, the whole system was flushed with nitrogen, the furnaces were preheated, the molecular sieve system was turned on, and then the appropriate flow of hydrogen was established. The feed gases from the nitrogen-16 system were admitted and after nitric acid started to drip from the refluxer, the flow from the auxiliary supply of nitric acid was stopped. This procedure maintained the proper wetting of the packing in the column. For operational convenience, all of the experiments were carried out a t atmospheric pressure.

Results and Discussion Although there is some similarity between the NOle(g) HzOis(liq.) = N0l8(g) HzOi6(liq.) exchange reactions for the nitrogen-15 system16J7 The exchange here is rather complex because the gas and and the oxygen-18 system described here, the difliquid phases consist of several ionic and molecular species ferences between the two systems influence the as mentioned previously. A mobile equilibrium among the optimum operating conditions. This arises prinvarious species undoubtedly provides the mechanism for cipally because the oxygen of both the water and rapid isotope exchange between the major component's, nitric oxide and mater. For reflux, the nitrogen oxides a t the oxygen compounds of nitrogen must exchange the top of the column must be converted into water and rapidly with the gas phase. Also, as seen from nitric acid without any loss or addition of oxygen. Various Tables I and 11, high concentrations of nitric acid designs were tried for the reflux system and after considerable experimentation and experience, it was found that' the reduce the effective enrichment factor per stage, Consequently, it mias necessary to evaluate the following method worked satisfactorily. A part of the nitric oxide was decomposed into NZand 0 2 effect of the various parameters on the over-all by means of an electric discharge between platinum elec- separation, including flow rate, acid concentration, trodes using a neon-sign transformer. The 0 2 reacts with and temperature. The results of a number of some of the excess NO to form NOZ. This was absorbed to form nitric acid according to the equation 3N02 + HzO +. experiments for this purpose are summarized in 2HN0, + KO as the gases passed countercurrently to Table 111. water in a short section of a packed column. The concenEffect of Flow Rate.-The over-all separation, tration of the nitric acid so produced and hence the amount of nitric oxide t o be decomposed was adjusted by the arc X,, in a multistage fractionation column such as size and voltage. A photocell was used to monitor the con- the one used for these studies, operating under total centration of NO2 in the gas stream. reflux, is given by CohenZ8as

+

"0,

+

The remainder of the nitric oxide was reacted with hydrogen to form water according t o the reaction 2 N 0 2H2 +. 2Hz0 N2. This reaction occurred on a roll of stainless steel screen about 3 in. long in the front section of a stainless steel tube, l 3 / * in. diameter and 28 in. long. Bft'er the reaction was started a t 700-800", the heat evolved was suffi-

+

+

(26) S. C. Saxena and T. I. Taylor, details t o be published (27) 4. 0. C. Nier, Rev. Scz. Instr., 11, 212 (1940); 18, 398 (1947). (28) K Cohen, "The Theory of Isotope Separation," MoGraw-Hi11 Book Co., Inc., New York, N Y., 1951, Chapters 3 and 7; see also J . Chem. Phys., 8, 588 (1940).

ENRICHMEST OF OXYGEN-18 BY EXCHANGE OF NO

August, 1962

WITH

H S 0 3 SOLUTIOXS

1483

h plot of H.E.T.P. lis. flow rate for 8 M HKOs is TABLE I11 shown in Fig. 1. The flow rates in mg.-atoms of' SUMMARY OF OVER-ALLSEPARATIONS AS A FUNCTION OF O/min. were evaluated from measurements of the ACIDCONCEKTRATION, FLOW RATE,AND TEMPERATURE IN A volume and concentration of effluent nitric acid as COLUMN1.3 CM. I.D. AND 145 CM. LOW PACKED WITH No. 30112 STAINLESS STEELHELIPAKCOLUMN PACKINGwell as from measurements on the gas stream fed A Pprox.

I%I\Tos Run no.

11-0 111-0 VII-0 VII-0 VI-0 VI-0

VI-0 W-0 W-0 W-0 V-0

U-0 X-0 IV-0 IX-0

L,mg.-

ml.

rttoms

conon., nitric moles/ Temp., acid/ 1. OC. min.

4 4 6 6 6 6 6 8 8 8 8 8 8 10 13.7

Flow rate,

Plow rate,

min.-

Overall sep.

cm.2

m

O/

1.81 83.4 1.34 1.89 86.4 1.34 1.36 64.7 1.87 1.42 67.6 1.86 1.77 84.2 1 . 5 1 1 . 7 8 84.7 1.73 1.77 84.2 1.63 0.85 41.9 2.00 0.86 42.6 2.12 0.85 41.8 2.14 1.30 64.0 1.86 1.74 85.7 1.52 2.12 104.4 1.44 1.50 76.0 1..53 1.80 94.6 1.31.

25 60 25 25 0 25 60 25 45 70 25 25 25 25 22

In S,

kZ(a

- 1)

= -

e

1X.E.T.P., a

mi.

1.028

13.7

1.020

4.6 4.6 7.0 5.2 5.9 3.7 3.4 3.4 4.2 6.2 7.1 5.1

1.018

1.015

...

..

(2)

for systems in which a is near 1. Here Z is the length of column and L is the flow rate. The coefficient IC occurs in the equations for the rate of transfer of desired isotope across the interface per unit length of column and its form depends upon the rate determining process. This may be the rate of attainment of isotope equilibrium or the rate of diffusion through the fluid boundary layers. If the interfacial concentrations for the diffusion limited case do not differ appreciably from the concentrations in the body of the fluid, the rate of transfer T for both cases isz8

T = -k[n(l - N ) - aN(1 - n ) ] (3) where IC = R'cC for the reaction limited case and l / k = B/DCo b / d m for the diffusion limited case. In these equations N , C, B, and D are, respectively, the mole fraction of desired isotope, the concentrations of reacting species in the liquid phase, the thickness of the liquid film, and the diffusion coefficient in the liquid. The lower case letters refer to the same quantities for the gas phase, u is the area of interface per unit length of column, and IC' i s the rate constant for the exchange reaction. Since In X,/(a! - 1) is the number of plates, s, in the column, it follows from eq. 2 that kZ/Lmay be identified with s so that H.E.T.P. = L / k . Thus, for conditions to which eq. 2 and 3 apply, a plot of R.E.T.P. 11s. L should be a straight line if a and Z are constant and IC does not change significantly with flow rate. For the limited range of flow rates normally used for isotope separation, these conditions apply and the plot for both of the rate-limiting processes should be a straight line for a given acid concentration.

+

to the column. The total moles of nitrogen oxides fed to the column per minute was known from the flow of nitric acid fed to the nitrogen-15 column. A Cary spectrophotometer was used bo determine the concentra,tion of NOz so that the flow rate of the gas phase in mg.-atoms of O/min. could be calculated. The results shown in Fig. 1 indicate that the H.E.T.P. increases approximately linearly with flow rate above about 60 mg.-atoms of O/min.cm.2. At low flow rates, improper wetting of packing may cause an increase in H.E.T.P. and a t high flow rates, one would expect a departure from linearity because of turbulence and flooding. On the basis of the results of these experiments, flow rates from 60 to 85 mg.-atoms of O/cm.z-min. were selected for measurements on the influence of the other variables. Tliese flows represent a compromise between over-all separation and the transport or time to reach a steady state. Comparison of the results for 6 and 8 M "03 (Table 111)at a flow rate of about 85 mg.-atoms of O/cm.2-min., where the values of H.E.T.P. are, respectively, 5.2 and 6.2 em., indicates that the optimum flow may be higher for 6 M than for 8 M "03. Effect of Acid Concentration.-Little information is available on the kinetics and mechanism of oxygen-18 exchange between NO and water in solutions of nitric acid. Bunton, Halevi, and L l e ~ e l l y nhave ~ ~ studied the exchange of oxygen-18 between nitric acid and water. The rate was markedly dependent upon the concentration of nitrous acid. In the absence of nitrous acid, no appreciable exchange occurred below about 40 mole % HxO3. Above this concentration, the exchange was rapid. I n our system, in which nitric oxide is in equilibrium with nitric acid, other oxides of nitrogen and nitrous acid (Table 11) are continuously present to catalyze the exchange. As the concentration of nitric acid increases, the concentration of the higher oxides of nitrogen increases and presumably also the rate of exchange. But, as shown from the calculated values of the enrichment factors (Table I), a for NO and NO,- is appreciably smaller than that for KO and HzO. Furthermore, the increase in NO2 in the gas phase will further decrease 01. Thus, even though an increase in acid concentration may increase the rate of exchange and giTre a shorter H.E.T.P., the decrease in may be sufficient to decrease the over-all separation. Since the over-all separation, S, is given by S = e S ( a - 1 ) and since the number of stages, s, in a length of column 2 is s = Z/H.E.T.P., we desire to known the acid concentration for which ( a - l)/H.E.T.P. is a maximum. This can be obtained from measurements of the over-all separation as a function of acid concentration, keeping the other parameters (Y

(29) C . A. Bunton, E. A. Halevi, and D. R. Idewellyn, J . Chem. SOC., 4913 (1952).

S,

1484

C.SAXENA ASD T. I. TAYLOR

Vol. 66

over-all separation is slightly higher than a t 60°, but the separation dropped from 1.73 to 1.51 when the temperature was lowered to 0'. Apparently, any increase in a is more than compensated for by a decrease in k. In another experiment with 6 M HXO3a t a flow rate of 65 mg.atoms of O/cm.2-min. (Table 111), the over-all separation did not change significantly when the / temperature was increased from 25 to 45'. /' For 8 M "03, the over-all separations increased slightly as the temperature was increased from 25 to 45' and 75' (Table 111). No significant change in over-all separation was observed for 4 M I -1 0 / HNOs when the temperature was increased from 10 40 80 EO 100 IPO 25 to 60'. Any decrease in a! with increase in FLOW RATE, L . temperature apparently is balanced by an increase Fig. 1.-Effect of flow rate on the over-all separation for 6 in IC. The above results show that the system is and 8 M "Os. not very sensitive to temperature changes in the range from 25 to 75'. Thus it is not necessary to control the temperature closely and, for convenience, the system can be operated a t room temperature. I, 0 Approach to Steady State.-An important property of a system for separating isotopes is the rate of approach to the steady state. A comparison of the experimental behavior of the system with that predicted from theory enables one to check or to determine some of the important constants such as a, the hold-up, or the flows. Estimates of the hold-up in the reflux system a t the product end of the exchange column indicated that it probably was responsible for the relatively slow rate of approach 1.0 4 a 10 IP 14 to the steady state. Since the hold-up was difficult HNOa, MOLES/ LITER. to measure and since the extent of oxygen-18 exFig. 2.-Over-all separation as a function of acid concen- change with the molecular sieve adsorbents and the tration a t approximately 25' and a flow rate of approxi- catalyst for decomposition of NHz was uncertain, mately 85 mg.-atoms of O/cm.2-min. the effective hold-up in the reflux system was evaluated by comparison with theory. constant, such as flow rate, temperature, length, Cohen28 has derived theoretical equations that column packing, etc. give the increase mole fraction of desired isotope I n Fig. 2, the over-all separation a t steady state as a function of in the time of operation of a fracand total reflux, S,, is plotted against acid concen- tionating column such as ours. The feed end (bottration for approximately the same flow rate (85 tom) was maintained a t a constant mole fraction, mg.-atoms of O/cm.2-min.) and temperature (25'). No, of the desired isotope by supplying fresh mateFor the 10 and 13.7 M HN03, a small correction rial continuously, but there was an appreciable to the values in Table I11 was made assuming the hold-up, H,, gram-atoms of 0, a t the product end validity of eq. 2. A maximum in the over-all (top). The value of a is small and the extent of separation occurs a t about 6 M HNOa. The separation was not great, so that the mole fraction separation falls rather rapidly for more dilute solu- N of oxygen-18 in the feed and in the product was tions, probably because of a low exchange rate re- always small compared to I, i e , , N