Separation of isotopes

Transcription

1 Separation of isotopes By Jan Ove Odden and Dag Øistein Eriksen Kristiansand, 2006

2 1 (6) Isotopes in general Atoms of the same element with different numbers of neutrons are called isotopes Most common isotope of hydrogen has no neutrons at all The second isotope of hydrogen has one neutron deuterium The third isotope has two neutrons - tritium Hydrogen Deuterium Tritium

3 Isotopic distribution of silicon: The distribution of the three different silicon isotopes in the nature is: 28 Si 92 % 29 Si 5 % 30 Si 3 % Si = 28 Si Si Si = 29 Si = 30 Si This isotopic distribution is also seen in end products like silicon-chips

4 Different isotope separation techniques: Diffusion based Membrane based Distillation Chemical exchange Electrolysis Electromagnetic Centrifugation Separation nozzle Selective excitation by laser Ion-mobility Chromatography Light elements and large scale Heavy elements and large scale

5 Separation Nomenclature Isotope separation, enrichment & depletion are concepts used when the concentration of a specific isotope is altered from its natural occurrence The enrichment factor (beta) is a measure of the separation of isotopes Beta = 1 No separation took place Beta > 1 Indicates enrichment Beta < 1 Indicates depletion The cut (theta) is a measure of the amount of feed that ends up in the product stream The beta and the cut are the determining factors defining the size and cost of a plant

6 Separating Unit, Stage and Cascade Separating Unit is the smallest element of a plant that effects separation single centrifuge, ASP single stationary wall pipe A Stage is a group of parallel-connected separating units that is fed the same composition and produces product streams with the same composition Stages are connected in series until the desired separation between product and waste is achieved. This is known as a Cascade

7 Cascade Enrichment Separating Element Feed Product Waste Simple Cascade Recycle Cascade

8 4-up 1-down Cascade j = 1, n j-1 j J

9 Separation Nomenclature, continue Separative Working Unit (SWU) is the amount of separation work done by a cascade to obtain one unit of product of the desired enrichment The specific energy consumption (E/δu) is the amount of energy needed to produce one SWU. For instance if the cost of electricity is $0.03 per kwh, then for a E/δu=1000 the electricity cost would be $30 per SWU.

10 Laser Isotope Separation When different isotopes have slightly different levels of excitation Radiation of the right frequency must be available The excited species must have the ability to be easily separated The selectivity for the desired isotope must be good Still to complex to be used industrially Laser-based isotope enrichment of Carbon 12/13:

11 Different separation techniques based on centrifugation: Rosegard Vortex Extraction October, 1976 Enrichment: (Argon) Cut: 6-8%

12 Wikdahl Vortex Separation March, 1976: Enrichment: Cut: 50%

13 UCOR Vortex Process Enrichment is achieved under pressurized conditions by centrifugal means in a stationary-wall centrifuge : Enrichment: 1.03 Cut: 5% NO PATENT

14 Separation based on chromatographic methods Theory on diffusion of gaseous species through the chromatographic column: Fick s 1 st law: J x D x c x, the flux (J) along the direction x is proportional to the concentration (c) gradient. D is the diffusion coefficient. Fick s 2 nd law: c t D x 2 x c 2, when D is constant. 1f()x2f()x3f()xx The compound containing 3 isotopes is released at time t=0. 1f()x2f()x3f()xx % 20 % % Distribution at t=1 Distribution at t=4

15 Results: Chemical separation Com ment: Silan puls File Na me: S ILAN2.D Analysis date: 14 Nov 2001 Analysis time: 9:58 am A bundanc e: T IC: 4.601e TIC Ar SiH m in Retention time (min)

16 Intencities Results: Chemical separation ,0 47,5 50,0 52,5 55,0 57,5 60,0 62,5 65, Capillary column L=20m D=100 m Temperature: 30 o C 0 45,0 47,5 50,0 52,5 55,0 57,5 60,0 62,5 65,0 40 Ar 0 45,0 47,5 50,0 52,5 55,0 57,5 60,0 62,5 65,0 Retention time (min) Mass 29 Mass 33 Conclusion: The retention of silane is not of kinetic nature since argon is heavier than silane and should therefore move slower. The retention must be due to molecular interactions between the porous material in the column and silane.

17 Results: Mass separation selectivity coefficient 1f()x2f()x3f()xx I i t 2 t 1 0 I I c c i i ( L, t) dt ( L, t) dt, is the selectivity coefficient is calculated from the ratio of the areas under the flanks of the mass distribution

18 Intencity Intencity Separation factor Yield Results: Mass separation selectivity Particles: "Hydro 3" Carrier gas: He at 50 o C Flow: 0,35mL/min Data: Mass 33 Model: Gauss Chi^2 = R^2 = y0 18 ±9 xc ±0.03 w ±0.07 A ± Retention time (min) Data: Mass 29 Model: Gauss Chi^2 = R^2 = y0 124 ±67 xc ±0.03 w ±0.07 A ± ,15 0,10 0,05 0,00 Particles: "Hydro 3" Carrier gas: He at 50 o C Flow: 0,35mL/min Mass 33/ mass Retention time (min) 1,8 1,7 1,6 1,5 1,4 1,3 1,2 1,1 1,0 0,9 1,0 0,9 Separation factor, 0,8 Yield 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0,0-0, Upper integration limit (min) Separation factors in the order of 1.10 is possible, but on the expense of the yield