Isotope Separation Processes

5 Isotope Separation Processes WILLIAM SPINDEL

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Department of Chemistry, Yeshiva University, New York, Ν. Y. 10033

Stable isotopes can be separated by a v a r i e t y of methods. A few of the methods which have been used or proposed f o r concentra t i n g isotopes are listed i n Table 1, which is n e i t h e r complete nor exhaustive.

An a p p r e c i a t i o n of the v a r i e t y of conceivable s e p a r a t i o n pro cesses f o r a p a r t i c u l a r task can be gleaned from the f a c t that a survey (9) c a r r i e d out i n 1953 by a group at the Esso Research and Engineering Company f o r the U.S. Atomic Energy Commission, of pos s i b l e methods f o r the production of heavy water (D O), examined 98 p o t e n t i a l processes. In a s i m i l a r v e i n , an advisory committee of the AEC i n 1971 examining the p o t e n t i a l merits of known pro cesses f o r the s e p a r a t i o n of uranium i s o t o p e s , evaluated at l e a s t 25 processes other than gaseous d i f f u s i o n and gas c e n t r i f u g e meth ods (10). Almost all of the processes l i s t e d i n the t a b l e are probably u s e f u l in varying degrees f o r separating isotopes of any of the elements i n the p e r i o d i c t a b l e . None of the processes listed i s c l e a r l y s u p e r i o r to all the others f o r every isotope separating purpose. 2

Although

the s i n g l e - s t a g e s e p a r a t i o n f a c t o r is the best s i n g l e

77

In Isotopes and Chemical Principles; Rock, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1975.

78

ISOTOPES

AND

CHEMICAL

PRINCIPLES

measure of the r e l a t i v e ease of s e p a r a t i o n , i t alone does not determine the optimum process f o r a s p e c i f i c separation task. The best method of separation depends on the p r o p e r t i e s of the element, the degree of s e p a r a t i o n d e s i r e d , and the s c a l e of the operation. Even f o r a given i s o t o p e , there i s not one best method.


Some general conclusions regarding isotope separation methods drawn by Manson Benedict (11) almost twenty years ago, have not been i n v a l i d a t e d to date by any d i r e c t experimental demonstration. 1. The most v e r s a t i l e means f o r the production of research q u a n t i t i e s of isotopes i s the electromagnetic method. 2. The simplest and most inexpensive means f o r s m a l l - s c a l e separation of many isotopes i s the C l u s i u s T h e r m a l - d i f f u s i o n column. 3. D i s t i l l a t i o n and chemical exchange are the most economic c a l methods f o r the l a r g e - s c a l e separation of the l i g h t e r elements. 4. Gaseous d i f f u s i o n and the gas c e n t r i f u g e are most economi c a l f o r the l a r g e - s c a l e separation of the h e a v i e s t elements. In t h i s paper a number of isotope separating processes w i l l be examined, p a r t i c u l a r l y those u t i l i z e d on a l a r g e i n d u s t r i a l s c a l e , and the bases f o r the above conclusions w i l l be presented. F i n a l l y , the fundamental p r i n c i p l e s of a photochemical method of isotope separation based upon e x c i t a t i o n by l a s e r l i g h t , which has e x c i t e d a great deal of current i n t e r e s t , w i l l be o u t l i n e d . Electromagnetic

Separators

Electromagnetic separators, r e a l l y l a r g e - s c a l e mass spectrometers , c a l l e d Calutrons because of t h e i r e a r l y development at the U n i v e r s i t y of C a l i f o r n i a C y c l o t r o n Laboratory, were o r i g i n a l l y constructed f o r the Manhattan D i s t r i c t during the second world war i n order to separate 235y ^ kilogram q u a n t i t i e s . At the height of t h i s e f f o r t some 1100 u n i t s were i n o p e r a t i o n , i n two s i z e s , a 48-inch radius u n i t c a l l e d an alpha Calutron and a 24-inch radius u n i t , the beta Calutron. Figure 1 shows a schematic view of a beta separator. In 1945 production of U by t h i s method was discontinued because the gaseous d i f f u s i o n process could be operated at much lower cost and most of the Calutrons were d i s mantled. Only two of the alpha u n i t s and 72 of the beta u n i t s remain i n operation today at the Oak Ridge N a t i o n a l Laboratory. These have been devoted f o r the past 25-30 years to the separation of a most amazing v a r i e t y of i s o t o p e s , both s t a b l e and r a d i o a c t i f . Some 200 kilograms of enriched isotopes i n c l u d i n g over 250 n u c l i d i c species have been produced by the f a c i l i t y . A recent r e t r o s p e c t i v e paper by W. 0. Love (12) o u t l i n e d the accomplishments of the Oak Ridge Electromagnetic Separations Department over n


5.

SPINDEL

Isotope Separation Processes

79

the past three decades. Figure 2 and Table 2 summarize the production of separated isotopes by this f a c i l i t y , and indicate the levels of isotopic purities obtainable by the electromagnetic method. The highly purified samples listed i n Table 2. required two or at most three passes through the separator.


These results elegantly demonstrate the extreme v e r s a t i l i t y of the electromagnetic method. Table 2. Uranium and plutonium isotope separations. (12)

Mass

Weight Collected (g)

Assay range (%)

233 234 235 236

Uranium 220 16 530 280

99.99-99.9986 30.86-94.49 98.64-99.9988 85.07-99.996

238 239 240 241 242 244

Plutonium 5 232 238 120 360 3

99.48-99.9988 95.4-99.999 97.03-99.993 83.31-99.997 81.45-99.987 0.61-99.06 Science

Thermal Diffusion The thermal diffusion effect, namely that i n a gaseous mixture subjected to a temperature gradient, as for example i n a vessel with walls at different temperatures, a concentration gradient is established, was f i r s t predicted theoretically by Enskog (13) and by Chapman (14) and confirmed experimentally by Chapman and Dootson (15). It remained for Clusius and Dickel (16) i n 1938 to transform the thermal diffusion effect from a laboratory curiosity into a useful and simple method for separating gaseous, l i q u i d , and isotopic mixtures by devising the thermal-diffusion (or thermo-gravitational) column whose operational principle i s illustrated i n Figure 3. Heating the inner wire (or tube) and cooling the outer wall of a column produces a convective flow pattern, as shown, descending along the cold wall and rising along the heated wire. This convective flow i s super-imposed upon the radial concentration gradient produced by the thermal diffusion effect. The



ISOTOPES

Figure 1.

Η· 2989 35.9

Να

K

Be

M f l

8200 45 A

O.O03 ,.3 2ββ9 47.7

Co

Β

«3.6 659 45.2

Ag 12.4 Ct

C

d

Bo

Au

Hg "

Fr

Ro

8985 78Λ 2,63 30.5 «963 40.9 648 422

6

0

C

N

ÎE°3 3757 ,46

8 1

Se

18.2 Sr

63 9.5

Al

Cu Rb

PRINCIPLES

Diagram of a beta calutron separator ( 12 )

H U

AND CHEMICAL

ΤΙ

7,3 ,4.6

6

,663 22.,

β

Y

Zr

In

Sn

L

°

ΤΙ

33, 7.» 294 8.6 2539 ,7.0

256, 4,.5

Mf Pb

23,4 59.7 4932 ,09 602 24.5 5280 ,05

4, 2.5

P

0

3447 ,27

S

,86 5.8

V

A»

C

0.022

f

s..

4088 35.3 7 8 4

3,0 No

S

b

To

Β

'

^9603 ,50 384 5.4 ,427 28.7 5, 0.6

Τ

·

W Po

227, 27.9 9576 92.9

F

C l

Ν·

43, 26.,

Ar

Μη

* Te

IPar

r «ir

ir

•

I

"*

Ι*»HT 572 22.8

370 6.7

At

Ac

r

v.

0

I· all" nil" al

39.5 Pu

,506 24.3

_ ^ T O T A L ESTIMATED WEIGHT (grwna)

""1,11 "». """^ THOUSANDS e

OF TANK-HOURS

Science Figure 2. Summary of isotope separations by Oak Ridge Electromagnetic Separations Department through 1972 (12)


5.

SPINDEL

81



counter-current flow multiplies the single-stage separation effect and concentrates the component which diffuses preferentially toward the hot wall at the top of the column, while the component which diffuses preferentially toward the cold wall concentrates at the bottom. This multi-stage separation i s essential because the concentration difference between hot and cold walls i s quite small (^10""2) for isotopic mixtures. Usually rather than separating isotopes i n a single long column, a series or series-parallel arrangement of columns (a cascade) i s constructed with individual units perhaps 3-5 meters in length, and appropriate interconnections to permit flow from the top of one unit to the bottom of a succeeding unit. A typical cascade of eleven thermal diffusion columns used at the Mound Laboratory of the U.S.A.E.G. for separating the isotopes of argon (17) i s illustrated i n Figure 4. The separations obtained and the rate of production of enriched material i s indicated. It i s worth noting that 38ΑΓ concentrates i n the middle of the cascade. This i s typical behavior for an isotope intermediate in mass between two others. The v e r s a t i l i t y of the thermal diffusion method and the separations achieved with simple laboratory cascades by Clusius laboratory for various isotopes i s elegantly demonstrated by the data i n Table 3. 1

Table 3.

Isotopes Separated by K. Clusius by Thermal Diffusion (8)

Year

Isotopes

1939 1939 1942 1942 1950 1950 1953 1955 1956 1959 1959 1960 1962

35C1

371 C

Kr Kr Ne »N 13C

8 4 8 e 2

2

*Ne 0» Ar Ne Ar

1 8 3 8 22 3 6

Natural Abundance

Separation Factor

Final Purity

75.7 24.3 57.1 17.5 90.5 0.37 1.09 8.9 0.275 0.204 0.064 9.21 0.37

53 775 45 940 210 135,000 45,000 810 96,500 200,000 9,750,000 12,500 3,300,000

99.4 99.6 98.3 99.5 99.95 99.8 99.8 99.0 99.6 99.75 99.984 99.92 99.991

Adv. Chem,



ISOTOPES

A N DCHEMICAL

Countercurrent Separation Processes Figure 3.

Thermal diffusion column ( 7 )

.0.5ml/hr (Impurities) -2.0ml/hr (Product)

Concentra tion.% 36 38 40 99.8 0.2 0

92.8 7.0 0.2

8.5 30.3 61.2

2.0

1.3 97.7

L^S LnS ί-S LnS L ^ u Î

Feed

1

1

3

1

3

3

3

1

2

0.27 0.06 99.66 500 ml/hr

Gaseous Isotope Separation at Mound Lab. Figure 4.

Hot wire thermal diffusion cascade for separating argon isotopes (17)


PRINCIPLES

5.

SPINDEL


83

The foregoing d i s c u s s i o n f a i r l y w e l l demonstrates that on a small s c a l e any d e s i r e d isotope can be separated e i t h e r by the electromagnetic or the thermal d i f f u s i o n method. In contrast to these l a b o r a t o r y - s c a l e processes, the separations o f the heavier isotope D, of the l i g h t e s t element, hydrogen, and o f the l i g h t e r isotope 235u, o f the h e a v i e s t n a t u r a l element uranium, are c a r r i e d out on a l i t e r a l l y enormous i n d u s t r i a l s c a l e .


Uranium Enrichment by Gaseous D i f f u s i o n The United States has three gaseous d i f f u s i o n p l a n t s (18) f o r separating 235u l o c a t e d a t Oak Ridge, Tenn., Paducah, Kentucky; and Portsmouth, Ohio. The t o t a l cost o f these p l a n t s i s about 2.4 b i l l i o n d o l l a r s and the annual operating costs (at f u l l capacity) i s about 460 m i l l i o n d o l l a r s . E l e c t r i c a l power consumption i s about 6000 megawatts, which correspond to an annual e l e c t r i c a l energy cost o f over 300 m i l l i o n d o l l a r s . The production capacity o f these p l a n t s i s the equivalent o f 75000 kg. (75 m e t r i c tons) of 90% 5 u per year. Further, the AEC has estimated that even i f the e x i s t i n g p l a n t s are upgraded with newer technology and operated at a higher power l e v e l , to produce an a d d i t i o n a l 60% of enriched uranium, by 1980 a d d i t i o n a l e n r i c h ing capacity w i l l be required i f the U.S. i s t o supply most o f the free-world needs f o r uranium enrichment. A new p l a n t i s p r o j e c t e d a t a cost of 1-1.2 b i l l i o n d o l l a r s with an annual pro duction capacity equivalent to 38.5 m e t r i c tons o f 90% 235TJ. 2 3

Figure 5 shows an a e r i a l view of the Portsmouth, Ohio p l a n t (the newest one), t y p i c a l o f the three i n s i z e . The b u i l d i n g s cover 93 acres of ground but have a f l o o r area about three times as great. The l a r g e r two b u i l d i n g s (χ-330 and x-333) are each about a h a l f mile long by 550 f e e t wide! The operating p r i n c i p l e s o f multi-stage isotope separating processes, such as the gaseous d i f f u s i o n process, were f i r s t developed by K a r l Cohen (19), and f u r t h e r exposed i n a form more d i r e c t l y u s e f u l t o chemical engineers by Benedict and P i g f o r d (1). B i g e l e i s e n has presented a concise summary o f the theory i n h i s review (8). At the heart of an isotope separation cascade i s the elemen tary separating u n i t , or stage which separates a feed stream c a r r y i n g F moles/time of an i s o t o p i c mixture c o n t a i n i n g a mole f r a c t i o n x f o f the d e s i r e d i s o t o p e , i n t o a product (enriched) stream, Ρ at mole f r a c t i o n Xp, and a waste (depleted) stream, W at mole f r a c t i o n x^. For that matter, the same separation process i s c a r r i e d out by the e n t i r e cascade. Two m a t e r i a l balance equations govern the operation o f a separating u n i t o r an e n t i r e cascade.


ISOTOPES

84

Total: Isotopic:

AND

CHEMICAL

PRINCIPLES

F = Ρ + W

(l)

Fx- = Px + Wx f ρ w

(2)

The elementary separation factor for a single-separating unit on a two component mixture i s defined as


-ι it Elementary factor:

_ (x/l-x)enriched α = ; , —r—:—, ^ , (x/l-x) depleted

13;

Λ

For the separation of uranium isotopes by gaseous diffusion of UF^, the theoretical limiting value of a i s given by Graham's law

m

/ U S

1.0043

The stages of a cascade are usually combined as shown i n Figure 6. The feed to each stage i s made-up by combining the de pleted stream from the stage above with the enriched stream from the stage below. A material balance anywhere inside the cascade shows that the depleted flow from the (n + l ) t h stage i s just equal to the enriched flow from the η th stage minus the product withdrawn at the top of the cascade. From the material balance equations 1 and 2, and material balances within the cascade, the operating parameters for an isotope separating cascade can be determined (1,8,19). The minimum number of separating stages, N, required to achieve a given overall separation at total reflux (no enriched product withdrawn) i s (x /1-x ) Separation, S = , / ^ ) = w w Ρ

Ρ

1

N

min

=

l n

S / l n

χ

α

N

α

,

(4)

( 5 )

The minimum reflux ratio required, at the feed point, to produce a given product rate Ρ of material at isotopic composition Xp, in systems where α i s close to 1, is
mm

(S"i)? (i-x ) f

( 6 )

£

This minimum feed/product rate would require an i n f i n i t e number of separating stages in the cascade. Neither of these limiting cascade parameters are suitable for an actual isotope production task; the minimum stage cascade



5.

SPINDEL

85


AEC Gaseous Diffusion Plant Operations Figure 5.

Aerial view of Portsmouth, Ohio gaseous diffusion plant ( 18 )

Product stream Ρ mole/sec molt fraction y

p

Top stage

Stage η+ 2

*«+2 Stage η + 1

L -P n

Stage η

Enriching section

Feed stage

Feed stream F mole/sec mole fraction x

f

Bottom stage Waste stream W mole/sec mole fraction *

Stripping section

w

Encyclopedia of Chemical Technology Figure 6.

Separation stages arranged to form a simple cascade (6)


86

ISOTOPES

AND

CHEMICAL

PRINCIPLES

produces no material at the desired concentration, the minimum flow cascade requires an i n f i n i t e number of stages. The ideal cascade i s one in which the separation achieved per stage i s just sufficient to make the composition of the depleted stream from the n+1 stage exactly equal to that of the enriched stream from the n-1 stage. Thus no entropy of remixing i s produced when these streams are combined to form the feed of the η th stage. This cascade i s often called the no-remixing cascade. In the ideal cascade, Downloaded by UNIV OF MICHIGAN ANN ARBOR on December 22, 2014 | http://pubs.acs.org Publication Date: June 1, 1975 | doi: 10.1021/bk-1975-0011.ch005

ff

χ

lf

n+1 n-1 η = χ = x w ρ f £

N

ideal

= 2 N

min

ideal

(7) mm

Since the reflux ratio (flow/product) required at each stage in a cascade (eqs. 6 and 7) i s related to the isotopic concentra tion at that point, the size of the separating units are tapered from the feed point towards the ends of the cascade in order to minimize the size and cost of the plant, the pumping energy con sumed, the hold-up of enriched material, and the time required to reach steady-state enrichment. The characteristic shape of an ideal cascade i s indicated i n Figure 7 which diagvomatioa1Vy depicts the parameters for an ideal gaseous diffusion cascade to produce 1 kg of 90% 235TJ from natural U F 5 containing 0.711% 235TJ and discarding waste UF6 at 0.200% 235TJ. The v e r t i c a l height at any point in the figure i s proportional to the stage number measured from the waste end of the cascade, the width i s propor tional to the total interstage flow at that stage. The quantities of uranium feed required and waste produced, the numbers of stages in the enriching and stripping sections (above and below the feed point), and the inter-stage flow at any stage are a l l calculated by eqs. 1-7 and the elementary factor 1.0043. Thus, from eq. 7, and the product and feed isotope concentrations, the interstage flow at the feed point i s calculated α/Ρ) . 0Q711) ^'^ideal · (.0043) (.00711) (1-.00711) 2

m

5 8

Ζ

D O

8 2 9

>

(8)

o z y

Almost 60,000 moles of natural abundance uranium flow through the cascade, at the feed stage for each mole of 90% 235TJ withdrawn. The total interstage flow,J, in the cascade, i.e. the total area enclosed in Figure 7, i s (20) J

=

"(odl

2

p

t -V(*p) + W-V(x ) - F-V(x )] w

f


(9)

5.

SPINDEL


87


where V(x) i s the function (2x-l) In x$.-x), often called the value function or separation potential. The separation potential is a function of composition only and i s dimensionless. It has a value of zero at χ = 0.5 and i s positive for a l l other values of x, symmetrically about the minimum at χ = 0.5. Eq. 9 i s of major importance for estimating the size and cost of an isotope separation plant. It indicates that the total flow i s a product of two factors; the f i r s t of these, proportion al to 1/(a-1)2 i s a function only of the elementary separation factor which i s determined by the separation process used. The second factor [in square brackets], which i s usually called the separative duty or separative work units (S.W.U.) i s a function only of quantities and concentrations of feed, product, and waste. It has the same dimensions as those used for the quan t i t i e s of material, and i t s value i s independent of the process used to accomplish the separation task. The significance of the magnitude of the elementary factor i s immediately apparent; a two-fold reduction i n (a-1) requires an increase i n the total flow by a factor of 4. Since for a gaseous diffusion process, the total flow rate i s closely related to the total area of porous barriers, the total pumping capacity and the total power consumption required, a l l the associated costs vary proportion ately. The S.W.U. provides a quantitative measure of the isotope separation task for any conceivable process. For the task des cribed i n Figure 7, 227.3 kg S.W.U. are required per kg of 90% 235TJ. For gaseous diffusion, with (a-1) = .0043, 98.2 million kg U must be pumped through the cascade i n order to produce 1 kg of product. It i s most interesting to note from the shape of the ideal cascade i n Figure 7, and from eq. 9, that most of the area in the figure, and therefore most of the volume of the cascade, and the energy which must be supplied, i s associated with enriching the isotope from natural abundance by the f i r s t factor of ten, say from 0.711% to 7% 235u. To prepare 1 kg of 235TJ t 7% con centration requires 77% of the S.W.U. required to produce a kg of 235TJ at 90% enrichment. The cost of enrichment by any combina tion of processes i s essentially determined by the process used at the base of the cascade. a

To give some feeling for the magnitude of cost of enriching uranium, the A.E.C. estimates (18) that in the most efficient projected gaseous diffusion plants, 0.266 W of continuous elec t r i c a l energy w i l l be consumed per S.W.U.»yr. This corresponds to a consumption of 530,000 kW-hr per kg of 90% 35TJ, At 5.5 mills per kw*hr the cost of e l e c t r i c a l energy i s $2900 per kg of 9Œ 235u. 2


88

ISOTOPES

AND CHEMICAL

PRINCIPLES


The essential elements of a gaseous diffusion cascade for 235TJ enrichment, their physical arrangement and inter-connections are indicated i n Figure 8. The inter-relationships between the motors, compressors, heat exchangers, diffusion membranes, and stage control valves are a l l shown. The largest size diffusion stages used in one of the AEC plants are seen i n the photograph in Figure 9, which shows the equipment size i n comparison to a man. Deuterium Enrichment by Exchange and D i s t i l l a t i o n Now l e t us examine deuterium enrichment—first the scale of the current effort, and a projection of the deuterium requirements in the future. The united States has constructed two plants, each capable of producing 500 tons of D2O per year. One of these plants i s dismantled, and the second plant i s currently operating at about one third of i t s capacity, producing 180 tons of D2O per year. In Canada, two plants with a combined output of 1600 tons D2O per year have recently been completed (21). D2O i s used as a neutron moderator and heat transfer medium for power reactors. The D 0 requirement i s about one ton D2O per megawatt of e l e c t r i c a l capacity. In the United States the development of power reactors has moved toward reactors fuelled with enriched 235u, using light water as moderator and either gas or liquid metals as coolants. In Canada the direction has been toward the use of natural abundance uranium (0.7% 235TJ) as fuel, with D2O as moderator. 2

It i s of interest to project the deuterium requirements for use as a fusion fuel i n controlled thermonuclear reactors (CTR). Robert Hirsch, Director of the U.S.A.E.C. Division of Controlled Thermonuclear Research has predicted the production of s i g n i f i cant amounts of fusion energy by 1980, and fusion power commer cialization before the turn of the century (22). A report pre pared by a group at Brookhaven National Laboratory (23) can be used for estimating energy and deuterium requirements i n a future fusion-based energy regime, i n which a l l f o s s i l fuels (except for ship and petrochemical feed requirements) are replaced with synthetic fuels produced by D-D fusion reactors, and a l l electrici ty production i s by CTR. They estimate that by the year 2020, 500 χ 1015 BTU of fusion energy would be required to supply a l l U.S. energy needs (with exceptions noted above). The total amount of deuterium needed to produce this energy (assuming 100% e f f i ciency for the D-D fusion reaction) would be only 1000 tons D2 per year (5000 tons D 0). If the technical problems can be over come, the fusion process promises to be astonishingly efficient from the standpoint of resource consumption! 2

Four, of the many processes proposed and used for deuterium



5.

SPINDEL


174.7 kg WASTE x =0.0020 RATIO OF HEADS TO PRODUCT w

HEADS FLOW RATE vs STAGE NUMBER IN IDEAL CASCADE α = 1.0043

Figure 7. Parameters of an ideal separation cascade f uranium isotope separation or

AEC Gaseous Diffusion Plant Operations Figure 8.

Arrangement of gaseous diffusion stages ( 18 )


89

90

ISOTOPES

AND CHEMICAL

PRINCIPLES

enrichment, w i l l now be examined. Two are d i s t i l l a t i o n process es and the remaining two are chemical exchange processes. The basic operational parameters of the d i s t i l l a t i o n processes, water d i s t i l l a t i o n and hydrogen d i s t i l l a t i o n , are compared in Table 4. The water d i s t i l l a t i o n system was actually used i n early plants


Table 4. D i s t i l l a t i o n process requirements to produce deuterium at 99.8% D from natural feed at 0.0149% D. (4,24)

effective α temperature pressure min.no.stages min.reflux ratio percent recovery moles feed/moles product Operating Cost ($/lb D 0) 2

Water Distillation

Hydrogen Distillation

1.05 50°C ^100 mm 308 141,000 5 133,940 176

1.52 23 Κ ^1.6 atm 41 19,600 90 7442 16

built and operated in the U.S. between 1943 and 1945. These plants were shut down in 1945 because of the high operating costs. The hydrogen d i s t i l l a t i o n process appears extremely favorable be cause of the large effective fractionation factor, and the total energy cost for producing deuterium by this process has been estimated (24) to be about 1700 kw · hr per pound of D 0. This corresponds to an energy cost of $9.30 per pound of D£0 (at 5.5. mills/kW · hr) ; the energy consumption i s probably the lowest of any of the deuterium separating processes. Several plant designs for producing deuterium by d i s t i l l a t i o n of liquid hydrogen were actually carried out during the decade 1941-1951 (25-27). Be cause of the technical problems of handling large amounts of liquid hydrogen at cryogenic temperatures of ^20 Κ , and because of the unavailability of sufficiently large quantities of hydro gen gas to serve as feed for a large-scale deuterium enriching plant, no hydrogen d i s t i l l a t i o n plant has ever been constructed in the U.S. Several relatively small hydrogen d i s t i l l a t i o n plants have been constructed and operated in the past 15 years in France, Germany, India and the Soviet Union. The reported magnitude of D2O production i n these plants i s in the range 3-14 tons D2O per year. 2

It seems of interest to re-examine the hydrogen d i s t i l l a tion system, now, in light of current liquid hydrogen technology, and of projected hydrogen production as a synthetic fuel. At present, the U.S. production capacity for liquid hydrogen, mainly for use in the space program, i s about 150 tons of liquid hydro gen per day. This, by i t s e l f , would permit the extraction of 15



5.

SPINDEL

91


tons deuterium (75 tons D2O) per year. More to the point i s a developing interest i n the use of liquid hydrogen as an aircraft fuel, particularly for hypersonic a i r c r a f t . Design studies have shown that the flying range of a large transport could be i n creased 30-40% using liquid hydrogen as fuel (28). Projections regarding liquid hydrogen consumption by aircraft (29) indicate that i f only ^3.5% of a i r transport i n the U.S. in the year 2000 were hydrogen fuelled, liquid hydrogen production would be 4.25 million tons per year, and the extractable deuterium, by d i s t i l l ation, would be 1100 tons per year. It should be noted that the energy consumption for the extraction of deuterium from liquid hydrogen, by a d i s t i l l a t i o n process operating parasitioally on a liquid hydrogen plant, would probably be lower by one or two orders of magnitude than the energy consumption previously e s t i mated for hydrogen d i s t i l l a t i o n systems. Previous estimates were based on using gaseous hydrogen as feed, where a l l refrigeration energy would be supplied for the purpose of isotope extraction; in parasitic operation on a liquid hydrogen f a c i l i t y only the heat losses from the d i s t i l l a t i o n columns would correspond to energy required for the deuterium extraction. Two significant chemical exchange reactions useful for the concentration of deuterium are HDS(gas) + H 0(liq) = H S(gas) + HDO(liq) o

2

0

2

HD(gas) + NH (liq) = H^gas) + NH^Diliq) 3

(10) .

A simple system for u t i l i z i n g these exchange reactions for deu terium concentration i s illustrated i n Figure 10. In both the exchange reactions l i s t e d above, the desired isotope, deuterium concentrates i n the liquid phase, so the phase conversion unit i n Figure 10 consists of a chemical reactor which converts the en riched compound (HDO or NH2D respectively for the reactions listed) into the isotopically depleted compound (hydrogen gas or hydrogen sulfide gas). The exchange tower must contain sufficient separating stages to multiply the single stage enriching factor, α to produce the desired degree of overall separation. As indicated earlier, interstage flows are very large i n isotope separation processes, and the quantities of processing chemicals required for reflux reactions (phase conversion) repre sent a major expense in any chemical exchange separation system, unless the process i s parasitic on a chemical manufacturing process and/or the by-products of the reflux reactions upgrade the value of the reflux chemicals. An elegant way to avoid the need for reflux chemicals i s provided by using a dual-temperature exchange system of the type f i r s t proposed, independently by Spevack (30) and by Geib (31). Such a system uses the variation of the exchange equilibrium constant with temperature, to substi-



92

ISOTOPES

A N DC H E M I C A L

PRINCIPLES

AEC Gaseous Diffusion Plant Operations Figure 9. Close-up view of actual equipment (diffusers and compressor) in a gaseous diffusion plant (18)

LIQUID FEED "

WASTE GAS"*"

EXCHANGE TOWER

-PRODUCT

PHASE CONVERSION Figure 10. Simple chemical exchange system for isotope separation for an isotope concentrating in the liquid phase


5.


SPINDEL

93


tute thermal r e f l u x i n s t e a d of chemicals f o r r e t u r n i n g the des i r e d isotope from the enriched compound to the i s o t o p i c a l l y dep l e t e d compound. The magnitude of the exchange constants f o r deuterium exchange, and t h e i r v a r i a t i o n with temperature, make a dual-temperature exchange process p a r t i c u l a r l y s u i t a b l e f o r conc e n t r a t i n g isotopes of hydrogen. Figure 11 i l l u s t r a t e s a s i m p l i f i e d arrangement of components and the magnitude of operating parameters f o r the concentration of deuterium by dual-temperature exchange between hydrogen s u l f i d e gas and l i q u i d water. i s c i r c u l a t e d i n a closed loop countercurrent to a descending stream of water. In the c o l d e r column deuterium concentrates i n the l i q u i d phase; the e q u i l i b r i u m constant f o r the exchange at the c o l d temperature, 30°C, i s 2.20. In the h o t t e r column, at 130°C, the exchange e q u i l i b r i u m constant i s 1.69, and deuterium i s returned from the aqueous phase to the gaseous phase. Thus, dual-temperature operation avoids the need f o r a chemical r e a c t i o n to return the d e s i r e d isotope from the phase i n which i t enriches to the phase i n which i t i s depleted. In operation, feed water at the n a t u r a l deuterium abundance of 145 ppm i s fed to the top of the c o l d column, depleted water at 120 ppm deuterium i s discarded from the bottom of the hot column, and enriched D2O i s withdrawn between the c o l d and hot columns. The e f f e c t i v e s i n g l e - s t a g e enrichment f a c t o r i s simply the r a t i o °f c o l d / h o t 1·26, f o r t h i s process at the i n d i c a t e d operating temperatures. This e f f e c t i v e s i n g l e - s t a g e enrichment f a c t o r determines the number of separating stages required i n the hot and c o l d columns r e s p e c t i v e l y , to achieve the d e s i r e d degree of enrichment. The maximum f r a c t i o n of the isotope which i s e x t r a c t able from the feed i n t h i s operating mode, i s simply the d i f f e r ence i n the a s at the two operating temperatures d i v i d e d by the l a r g e r of the two values. These r e l a t i o n s h i p s e f f e c t i v e l y l i m i t the use of a dual temperature system to the separation of hydrogen isotopes. The operating temperatures shown i n Figure 11 f o r the H2S-water exchange system are determined by the p r a c t i c a l considerations that I^S-hydrate p r e c i p i t a t e s i n the low temperature column below 30°C, and at the operating pressure of 300 p s i , the mole f r a c t i o n of water i n the gaseous phase becomes excessive above 130°C. a

a

=

T

A dual-temperature system requires twice the number of separating stages (hot and c o l d columns) needed f o r a s i n g l e temperature system with the same e f f e c t i v e a, but the need f o r r e f l u x i n g chemicals i s eliminated, and the heat energy required to maintain the temperature difference can be g r e a t l y reduced by the use of heat exchangers at appropriate points to pre-heat the gas and l i q u i d streams entering the hot column, and to p r e - c o o l the gas entering the c o l d column. A l l of the major plants c u r r e n t l y producing

deuterium i n



94

ISOTOPES

AND

CHEMICAL

PRINCIPLES

quantities >20 tons per year u t i l i z e the I^S-water dual-tem perature exchange system. Figure 12 shows schematically the cas cade arrangement used at the Savannah River plant for the produc tion of 500 tons of D2O per year (32). Concentration of deuterium from the 15% level produced by exchange, to the f i n a l product concentration of 99.8% D i s accomplished by vacuum d i s t i l l a t i o n of water. It should be noted however, that practically a l l of the separative work i s expended i n getting from the feed concentration of 145 ppm to 15% D. This task requires 7383 SWU; about 0.7% additional SWU are needed to produce the f i n a l product. The economics of the process i s determined at the base of the cascade! Fundamental parameters for the ammonia-hydrogen exchange reaction which i s listed in eq. 10, are much more favorable than the equivalent factors for the I^S-water system. The discovery by Claeys, Dayton and Wilmarth (33) that amide ion serves as an efficient homogeneous catalyst for the exchange immediately stimulated a group at Brookhaven National Laboratory, led by Bigeleisen to carry out extensive experimental and theoretical studies of this system (34,35). They determined that a dualtemperature system operating with hot column at 70°C (a = 2.9) and a cold column at -40°C (a = 5.9) would have an effective α = 2.0, and would permit extraction of 50% of the deuterium from the ammonia feed. They also showed that the exchange, when catalyzed by NaNH2 dissolved i n the liquid, was sufficiently rapid even at the lower temperature to reach equilibrium i n reasonably sized exchange columns. To date, this system i s being used i n a single-temperature plant in France producing about 20 tons D2O per year, about which only few details have been published. The major limitation on the use of this process has been the a v a i l a b i l i t y of sufficient quantities of ammonia for plant feed. Even a plant producing 1000 tons ammonia per day would only provide sufficient feed to permit production of 60-70 tons D2O per year. Again, one sees the interplay between science and technology, and one i s faced with the fact that technological rather than s c i e n t i f i c considerations are often the over-riding determinants for large seale isotope separation. Photochemical Isotope Separation

Processes

For many years attempts have been made to use photochemical processes for the separation of isotopes. The basic idea i s to u t i l i z e the difference in the absorption spectra of different isotopic species, and by use of sufficiently monochromatic light of an appropriate wavelength to excite only one of the species to an upper energy level. The excited species may then be separated by chemical or physical means from i t s isotopic partners; the separating process need not have any inherent isotopic selectivity. A particularly successful application of the method was to the separation of Hg isotopes by Gunning et a l . (36,37). For example,


5.

SPINDEL


Fraction of isotope extracted

95

a cold — ahot

WATER 145 ppm

I COLD » 2.20 H S


2

300 pel PRODUCT 15 % D 0 2

DEPLETED WATER

| f J_

Figure 11. Simplified flow sheet for a dual-temperature exchange system for concentrating deuterium

Feed 0.0145 % D

Product 15 % D

Waste 0.012 %D' Stage I

Stage Π Chemical Engineering Progress

Figure 12. Simplified schematic of Savannah River exchange unit showing principal towers and liquid flow paths. There are 24 such units (500 tons D 0/year). Stage 1: hot towers, 12' dia. X 70 trays; cold towers, 1Γ dia. χ 70 trays. Stage 2: hot towers, 2 X 6.5' dia. X 70 trays ea.; cold towers, 2 X 6.5' dia. X 85 trays ea. (32). 2


96

ISOTOPES AND CHEMICAL PRINCIPLES

a monoisotopic 202Hg resonance lamp was used to i l l u m i n a t e mercu ry vapor containing the n a t u r a l abundance mixture of Hg isotopes (196, 198-202, 204). E s s e n t i a l l y only H g isotopes were e x c i t e d to an upper e l e c t r o n i c s t a t e , i n which they reacted with an added gas such as H2O, to form HgO which could then be sepa rated e a s i l y from unreacted Hg. Values o f α as high as 3 were obtained f o r 202Hg.


2 0 2

The e s s e n t i a l requirements f o r any photochemical isotope separation scheme f o l l o w : 1. Absorption spectrum with a w e l l resolved isotope s h i f t ( v i b r a t i o n a l - r o t a t i o n a l or v i b r o n i c f o r molecules, e l e c t r o n i c f o r atoms) 2. A l i g h t source s u f f i c i e n t l y monochromatic and tunable to e x c i t e absorption by one isotope and not the other, s u f f i c i e n t l y intense to e x c i t e a s u b s t a n t i a l f r a c t i o n of the i s o t o p i c species i n the gas mixture. 3. D e a c t i v a t i o n o f e x c i t e d species by c o l l i s i o n s with other species must be minimized; energy t r a n s f e r from one i s o t o p i c species to another must not occur before step 4. 4. A chemical or p h y s i c a l process which separates e x c i t e d species from the others. The f i r s t and l a s t of these requirements are dependent on the s p e c i f i c i s o t o p e s , molecular species and chemical or p h y s i c a l processes. They cannot be changed, but the appropriate s e l e c t i o n depends on the ingenuity o f the i n v e s t i g a t o r . Requirements two and three, on the other hand, have been v i r t u a l l y unobtainable u n t i l the development of l a s e r s , which produce very l a r g e outputs of r a d i a n t energy with an extremely narrow band width. A l s o , the a b i l i t y o f a l a s e r t o emit t h i s l a r g e amount o f monochromatic radiant energy w i t h i n a time i n t e r v a l as short as s e v e r a l nano seconds i s important i n meeting the energy t r a n s f e r requirements. Two recent papers by Letokhov (38) and by Moore (39) contain e x c e l l e n t and d e t a i l e d d i s c u s s i o n s o f the a p p l i c a t i o n o f l a s e r s to i s o t o p e s e p a r a t i o n . The approaches f a l l i n t o two broad cate gories which may be c h a r a c t e r i z e d as one-step and two-step pro cesses. The one-step process i s p a r t i c u l a r l y simple conceptually but not as g e n e r a l l y a p p l i c a b l e . I t i n v o l v e s s e l e c t i v e e x c i t a t i o n o f a s u i t a b l e molecule to an upper p r e - d i s s o c i a t i v e s t a t e . This upper s t a t e i s a n o n - d i s s o c i a t i v e one whose p o t e n t i a l energy surface i n t e r s e c t s another surface corresponding to a d i s s o c i a t i v e s t a t e . Such a system i s i l l u s t r a t e d i n Figure 13. I f the d i s s o c i a t i v e l i f e t i m e i s s h o r t e r than the r a d i a t i v e l i f e t i m e , then s e l e c t i v e p h o t o - e x c i t a t i o n can produce i s o t o p i c a l l y enriched


5.

SPINDEL


97

dissociation products. Yeung and Moore (40) irradiated an equimolar mixture of D2CO and H2CO using a frequency-doubled Ruby laser (3472 Â ) . The hydrogen gas formed by photo-dissociation of the formaldehyde showed a 6:1 D to H ratio! To maximize isotopic separation, the compound should be irradiated with a narrow line in a spectral region where the undesired isotopic species i s r e l atively transparent. Moore (41) has observed the absorption spectra of the isotopic (^C and C) formaldehydes and concluded that irradiation with light of an appropriate wavelength should yield l^CO preferentially as a dissociation product.


12

Selective two-step processes are more generally applicable to isotope separation. Several approaches are indicated i n Figure 14. The diagram on the l e f t (a) shows approaches to selective photo-ionization. A photon V-^ selectively excites isotopic atoms to the upper state, 2, and a second photon λ^, with s u f f i cient energy to ionize atoms in state 2, but insufficient to ion ize those i n the ground state, then ionizes the excited atoms. It should be noted that the second photon, V2 need not be highly monochromatic. Alternatively, the frequencies V-^ and V2 may be identical, as indicated by V3. The V3 frequency must be reso nant with a transition of one of the isotopic species, and of sufficiently high frequency to ionize the excited atom. The dia gram on the right of Figure 14, illustrates a two-step, photo-dis sociation process. The selective photon, excites a vibration a l transition. The level excited should be sufficiently high so that i t s thermal population i s negligible, and i t s absorption co efficient for i>2 is appreciable, while the corresponding c o e f f i cient from the thermally populated state i s negligible. If the A and Β fragments are chemically stable, one need only separate isotopically enriched A or Β species from the remaining AB mole cules by a simple chemical separation. If the fragments are re active, a chemical trapping scheme which does not induce isotopic scrambling must be devised. Notice that the two-step processes are really limited to laser light sources, because the two light pulses must be closely synchronized to avoid energy transfer. One can only estimate very crudely at this time the energy requirements for laser isotope separation. Moore has pointed out (39) that a process yielding one separated atom for each 3300 Â photon absorbed, requires 0.1 kw. hr. of light energy per mole of separated isotope. Assuming a laser efficiency of 10"* this corresponds to an energy consumption of 1000 kw. hr. per mole. Naturally, these calculations essentially represent a theoretical minimum goal to be approached. To date, the experimentally demonstrated f e a s i b i l i t y of uranium isotope separation has been l i m i t ed to the preparation of about 10^ 235jj atoms per second (42), but the magnitude of the industrial enterprise involved i n isotope separation certainly appears to j u s t i f y an expanded level of fundamental research i n this direction.


A N DC H E M I C A L


ISOTOPES

Figure 14. Schematic diagram of selective turnstep processes: (a) photo-ionization, (b) photodissociation (39)


PRINCIPLES

5.

SPINDEL


99

Acknowledgement The author i s most pleased to acknowledge the assistance of his colleagues, George W. Flynn and Ralph E. Weston through numer ous discussions of isotopic separation "by laser excitation. Literature Cited


1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Benedict, Μ., and Pigford, Τ. Η., "Nuclear Chemical Engineer ing", McGraw-Hill, New York, 1957. Kistemaker, J . , Bigeleisen, J . , Nier, A.O.C., (Eds.), "Pro ceedings of the International Symposium on Isotope Separa tion", North-Holland Publ. Co., Amsterdam, 1958. Koch, J., (Ed.), "Electromagnetic Isotope Separators and Ap plications of Electromagnetically Enriched Isotopes", NorthHolland Publ. Co., Amsterdam, 1958. London, H., (ed.), "Separation of Isotopes", George Newnes, Ltd., London, 1961. J . Chim. Phys., (1963), 6 0 , (1 and 2), Symposium on "Physical Chemistry of Isotope Separation". Schacter, J . , Von Halle, Ε., Hoglund, R. L., "Encyclopedia of Chemical Technology", Standen, Α., (Ed.), 7, 9 1 , Wiley and Sons, New York, 1965. Pratt, H. R. C., "Countercurrent Separation Processes", Elsevier Publ. Co., Amsterdam, 1967. Bigeleisen, J . , i n "Isotope Effects i n Chemical Processes", Advances i n Chemistry Series, 89, Am. Chem. Soc., Washington, D. C., 1969, Chapter 1. Barr, F. T., Drews, W. P., Chemical Engineering Progress, (1960), 56, 49. Benedict, M., Berman, A. S., Bigeleisen, J . , Powell, J . Ε., Shacter, J . , Vanstrum, P. R., "Report of Uranium Isotope Separation Review Ad Hoc Committee", ORO-694, Oak Ridge, Tenn., June 1972. Ref. 1, p. 516. Love, L. O., Science, (1973), 182, 343. Enskog, D., Z. Physik, (1911), 12, 56, 533. Chapman, S., P h i l . Trans, Roy. Soc. London Ser. A, (1916), 216, 279. Chapman, S., Dootson, F. W., P h i l . Mag., (1917), 33, 248. Clusius, K., Dickel, G., Naturwiss., (1938), 2 6 , 546. Haubach, W. J., Eck. C. F., Rutherford, W. M., Taylor, W. L., "Gaseous Isotope Separation at Mound Laboratory-1963", MLM1239, Miamisburg, Ohio, June 1965. "AEC Gaseous Diffusion Plant Operations", ORO-684, Washington, D. C., January 1972. Cohen, K., "The Theory of Isotope Separation", McGraw-Hill, New York, 1951. Ref. 1, p. 396.


ISOTOPES

100

21.


22. 23.

24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

AND CHEMICAL

PRINCIPLES

Bancroft, A. R., "The Canadian Approach to Cheaper Heavy Water", AECL-3044, Chalk River, Ontario, February, 1968. New York Times, February 2 8 , 1974. Powell, J . , Salzano, F., Sevian, W., Hoffman, Κ., Bezler, P., "An Evaluation of the Technical, Economic and Environmental Features of a Synthetic Fuels Economy Based on Fusion Reactors", ENL-18430, Upton, Ν. Y., November 1973. (Figure 3.5). Benedict, Μ., "Survey of Heavy Water Production Processes", Proc. Int'l Conf. on Peaceful Uses of Atomic Energy, (1956), 8, 377, United Nations, Ν. Y. Clusius, Κ., Starke, Κ., Z. Naturforschung, (1949), 4A, 549. Hydrocarbon Research, Inc., "Low Temperature Heavy Water Plant", ΝΥO-889, Ν. Υ., Ν. Υ., March 1951. Murphy, G. M., (Ed.), "Production of Heavy Water", N.N.E.S. III, 4F, McGraw-Hill, New York, 1955. TID-26136, "Hydrogen and Other Synthetic Fuels--A Summary of the Work of the Synthetic Fuels Panel", September, 1972, Superintendent of Documents, Washington, D. C. AET-8, "Reference Energy Systems and Resource Data for use in The Assessment of Energy Technologies", A p r i l , 1972, Assoc. Univ. Inc. Upton, New York. Spevack, J . , Report MDDC-891(1947). Clusius, Κ., et al., FIAT Review of German Science (19391946), Physcial Chemistry, p. 19ff. Bebbington, W. P., Thayer, V. R., Chem. Eng. Progress, (1959) 55, 70. Claeys, Y., Dayton, J . C., Wilmarth, W. K., J . Chem. Phys., (1950), 18, 759. Perlman, M., Bigeleisen, J . , Elliot, N., J . Chem. Phys., (1953), 2 1 , 70. Bigeleisen, J . , Ref. 2., p. 121. Pertel, R., Gunning, Η. Ε., Can. J . Chem., (1959), 37 35. Gunning, Η. Ε., Strausz, O. P., Adv. i n Photochem., (1963), 1, 2 0 9 . Letokhov, V. S., Science, (1973), 180, 451. Moore, C. B., Accounts of Chem. Res., (1973), 6 , 323. Yeung, E. S., Moore, C. B., Appl. Phys. Lett., (1972), 2 1 , 109.

4 1 . Moore, C. B., private communication. 42. Chem. and Eng. News, (1974), 52, No. 27, p. 24 (July 8). *Research supported by U.S.A.E.C. under Contract AT(11-1)-3581. †Present address:

Division of Chemistry and Chemical Technology National Academy of Sciences-National Research Council, 2101 Constitution Avenue, Washington, D. C. 20418


Isotope Separation Processes

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