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A new type separation column for the water–hydrogen isotope catalytic exchange process
Fusion Engineering and Design 58 – 59 (2001) 433– 438 www.elsevier.com/locate/fusengdes
A new type separation column for the water–hydrogen isotope catalytic exchange process O.A. Fedorchenko *, I.A. Alekseev, V.D. Trenin Petersburg Nuclear Physics Institute, Gatchina, Leningrad district 188350, Russia
Abstract The catalytic water/hydrogen isotope exchange process is by right considered the most attractive for the solution a number of urgent problems of hydrogen isotope separation. A new type exchange reaction column is described and studied in details by computer simulation and with the help of McCabe– Thiele diagrams. It is shown that the new column in comparison with a traditional one needs less catalyst quantity and a smaller diameter for the solving of the same separation tasks. Generalized calculation data are presented in graphical form. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Water–hydrogen isotope; Catalytic exchange process; McCabe– Thiele diagrams
1. Introduction In the fusion reactors tritium should be recovered from a large amount of contaminated water: cooling water from plasma facing components and water waste from the tritium system. The combined electrolysis and catalytic exchange (CECE) process is ideally suited for the solution of this problem and a number of problems of hydrogen isotope separation owing to its high separation factors and near-ambient operating condition. Overall rate of hydrogen isotope exchange in an exchange column is limited by the hydrogen gas/ * Corresponding author. Tel.: + 7-812-714-6975; fax: +7812-713-1985. E-mail address: [email protected] (O.A. Fedorchenko).
water vapour exchange or the water vapour/liquid water exchange depending upon which exchange is proceeding at the slower rate. Let assume that equilibrium composition in hydrogen –vapour mixture is established after going through a catalytic bed and water vapour composition leaving a scrubbing bed is equilibrium to entering liquid water composition. Even so, one can see that traditional CECE flow sheet could not make full use of the high separation factor of catalytic exchange. This paper deals only with the CECE process as applied to upgrading of heavy water because of descriptive-geometric presentation with the help of McCabe –Thiele diagrams. However, the principle can be readily extended to tritium recovery from light and heavy water.
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 4 8 5 - 9
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O.A. Fedorchenko et al. / Fusion Engineering and Design 58–59 (2001) 433–438
2. Traditional CECE process computer study At first the traditional CECE process flow sheet has been investigated. In this scheme water flowing from top to bottom in the opposite direction to hydrogen stream is successively fed into all scrubbing beds, which alternate with catalytic beds. Simulation code ‘EVIO-3’ is a Windows 95 application and one of the descendants of the code ‘KIO’ [1]. The Murphree-type factor eff is introduced in the code to consider a catalyst bed efficiency and factor, named kpd —to consider scrubbing bed efficiency in term of ratio to one theoretical plate. The significance of the liquid– vapour exchange step as well as great influence of a vapour stream value on a separation degree (SD) is demonstrated in Fig. 1. A vapour stream value (that is quantity of water moles taken away by hydrogen stream) depends on conditions (temperature, pressure and concentration). One can see translation of maxi-
Fig. 2. The dependence of optimal conditions, expressed in terms of water vapour stream value to hydrogen stream value ratio (V/G), upon scrubbing bed efficiency to catalytic bed efficiency ratio (kpd/eff ) for traditional type column at Bot = Top=0.1 G.
mum of separation degree in the line of temperature increase with increase of ratio of kpd to eff. The dependence of optimal conditions, expressed in terms of water vapour stream value to hydrogen stream value ratio (V/G), on kpd/eff is shown in Fig. 2.
3. New type exchange column
Fig. 1. Temperature dependence of traditional type column separation degree at following conditions: P= 130 kPa, Nb = 30, Nsc = 1, Xw = 0.5, Bot = Top= 0.1 G.
A new type column flow sheet of principle is represented in Fig. 3. The scheme contains n scrubbing beds (2) and, accordingly, n catalytic beds (3), where n is any integer number more than 2. The scheme in Fig. 3 corresponds to a column operating at the total reflux. Two different water streams: from a catalytic oxidiser (5) (the first one) and the water stream from condenser (4) (the second one) do not merge to one another and are separately fed into different scrubbing beds flowing from the top down in the opposite direction to hydrogen–water vapour stream. In doing so the first water stream is fed into the first from the top scrubbing bed and then successively into all odd
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(numbering from the top) scrubbing beds. And the second water stream is fed accordingly into the second from the top scrubbing bed and then successively into all even (numbering from the top) scrubbing beds. Such arrangement of the water streams leads to the fact that the ratio of liquid water stream value to water vapour stream value changes on a scrubbing stage. Besides relative moving away of starting concentrations in vapour and liquid water streams, which enter a scrubbing stage, takes place. All this made it possible to do better a phase isotopic exchange process and as a result of this to conduct a catalytic process more effectively as well as the overall process.
Fig. 4. Two type column flow sheets comparison: (a) a new type column and (b) traditional one.
4. Two schemes comparison
Fig. 3. A new type column flow sheet of principle: 1 —electrolytic cell, 2i — the ith scrubbing bed, 3i — the ith catalytic bed, 15i 5n, 4— condenser, 5 — catalytic oxidiser.
However to come to the point is easier comparing two schemes (traditional and new one) with a concrete number of scrubbing and catalytic beds. Thus, for instance, two schemes with n= 5 are shown in Fig. 4 (a— a new column and b— traditional one). Under the same conditions (P=105 kPa, DP = 1 kPa, t=83 °C, tc = 15 °C, Nb =5, eff= 1, Nsc = 8, Bot= Top = 0.1 G, Xw =0.5) SD= 25.684 in a new column and SD= 10.005 in a traditional one. The example is illustrated with the help of McCabe –Thiele diagram in Fig. 5 (a new type column) and in Fig. 6 (a traditional one), which should be treated together with corresponding schemes in Fig. 4. The vapour composition Z is designated on axis of ordinates and deuterium atomic fractions in water X and in hydrogen Y— on axis of abscissas. Two arch-lines above the diagonal line are catalytic equilibrium lines corresponding to maximum and minimum separation factors. Equilibrium line of a phase
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isotopic exchange is placed under the diagonal line (almost coincides with it). The bottom part of a column corresponds to the upper part of a diagram. Five scrubbing operating lines are well seen in Fig. 6. For example, ((X1, Z %1), (X2, Z2)) and ((X4, Z %4), (X5, Z5)) — the first and the fourth operating lines. Every one of these lines almost touches equilibrium line of a phase isotopic exchange by one of its extremity. As a result right triangles corresponding to theoretical plates from the 5th to the 8th are practically not seen. Operating lines of a co-current catalytic isotope exchange process for the example on a traditional column start approximately on the diagonal line (see Fig. 6). For example, point (Y3, Z4) is the beginning of the fourth catalytic operating line. McCabe – Thiele diagram of the process realized in a new type column looks in another way (see
Fig. 5). Though scrubbing operating lines overlap each other, because they are rotated (in comparison with a traditional scheme) in such a way that now they are almost parallel to the diagonal line (for example, ((X2, Z %1), (X4, Z2)) and ((X3, Z %2), (X5, Z3))—the second and the third operating lines), the main effect of the innovation is well seen. The catalytic operating lines start below the diagonal line (for example, (Y2, Z3) is the beginning of the third line), that is a hydrogen–vapour mixture composition is moved away farther from an equilibrium composition, it is due to more effective carrying out a scrubbing process. All mentioned above lead to increasing overall separation degree of a column. Calculated temperature dependence on SD of a new type column (1) and traditional one (2) are represented in Fig. 7. Maximum separation degree
Fig. 5. McCabe –Thiele diagram of the following steady-state process: P = 105 kPa, DP= 1 kPa, t= 83 °C, tc =15 °C, Nb = 5, eff= 1, Nsc = 8, Bot= Top=0.1 G, Xw = 0.5, Nw =3, that run in a new type column.
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Fig. 6. McCabe –Thiele diagram of the following steady-state process: P= 105 kPa, DP= 1 kPa, t =83 °C, tc =15 °C, Nb = 5, eff= 1, Nsc = 8, Bot= Top=0.1 G, Xw = 0.5, Nw =3, that run in a traditional type column.
of new type column relates to 91 °C. This separation degree is achieved by traditional type column at temperature equal to 100 °C only. So relatively small increasing of temperature nevertheless lead to the increase of vapour+ gas load two times that is: to retain efficiency of a catalytic bed the catalyst quantity must be increase at least twice and to retain linear gas mixture velocity the diameter of column must be increase accordingly more then 1.4 times. There are some concrete conditions for each separation task, which provide maximum degree of separation of a new type column. However these conditions for any concrete task do not differ heavily from the conditions providing maximum degree of separation for a column operating at the total reflux: temperature and pressure must be so that vapour stream value is equal to hydrogen stream value [2].
5. Conclusion Thus the realisation of a catalytic isotope exchange process between water and hydrogen in a new type column makes it possible to increase a separation degree in comparison with a traditional type column (with the same quantity of catalyst) or achieve the same separation degrees using less catalyst quantity and a smaller diameter. This is a great advantage especially when the processing of a big stream is required.
Appendix A. Nomenclature P DP t tc
pressure at the top of a column (kPa) pressure drop across a column (kPa) temperature in a column (°C) temperature in a condenser (°C)
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deuterium atomic fraction in liquid water stream Y deuterium atomic fraction in hydrogen stream Z deuterium atomic fraction in vapour stream Bot withdrawal of a bottom product (mol/h) Top withdrawal of a top product (mol/h) Nw feed scrubbing bed number (feed stream is W=Bot+Top) Xw feed composition (deuterium atomic fraction) eff Murphree-type efficiency of a catalytic bed kpd scrubbing bed efficiency in term of ratio to one theoretical plate (if Nsc\1 then kpd = Nsc) SD separation degree of a column SD= (XBot/(1−XBot))/(XTop/(1−XTop)) X
Fig. 7. Calculated temperature dependence of separation degree of a new type column (1) and traditional one (2) for the following conditions: P= 130 kPa, Nb = 8, Nsc = 8, Bot= Top= 0.1 G, Xw = 0.5.
References Nb Nsc G V L
the number of catalytic beds the number of theoretical plates in a scrubbing bed hydrogen stream value (mol/h) water vapour stream value (mol/h) liquid water stream value (mol/h)
[1] O.A. Fedorchenko, I.A. Alekseev, V.D. Trenin, V.V. Uborski, Computer simulation of the water and hydrogen distillation and CECE process and its experimental verification, Fusion Technol. 28 (1995) 1485 – 1490. [2] O.A. Fedorchenko et al., The method of hydrogen isotope separation, RUS Patent 2148426 (1998).
A new type separation column for the water–hydrogen isotope catalytic exchange process O.A. Fedorchenko *, I.A. Alekseev, V.D. Trenin Petersburg Nuclear Physics Institute, Gatchina, Leningrad district 188350, Russia
Abstract The catalytic water/hydrogen isotope exchange process is by right considered the most attractive for the solution a number of urgent problems of hydrogen isotope separation. A new type exchange reaction column is described and studied in details by computer simulation and with the help of McCabe– Thiele diagrams. It is shown that the new column in comparison with a traditional one needs less catalyst quantity and a smaller diameter for the solving of the same separation tasks. Generalized calculation data are presented in graphical form. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Water–hydrogen isotope; Catalytic exchange process; McCabe– Thiele diagrams
1. Introduction In the fusion reactors tritium should be recovered from a large amount of contaminated water: cooling water from plasma facing components and water waste from the tritium system. The combined electrolysis and catalytic exchange (CECE) process is ideally suited for the solution of this problem and a number of problems of hydrogen isotope separation owing to its high separation factors and near-ambient operating condition. Overall rate of hydrogen isotope exchange in an exchange column is limited by the hydrogen gas/ * Corresponding author. Tel.: + 7-812-714-6975; fax: +7812-713-1985. E-mail address: [email protected] (O.A. Fedorchenko).
water vapour exchange or the water vapour/liquid water exchange depending upon which exchange is proceeding at the slower rate. Let assume that equilibrium composition in hydrogen –vapour mixture is established after going through a catalytic bed and water vapour composition leaving a scrubbing bed is equilibrium to entering liquid water composition. Even so, one can see that traditional CECE flow sheet could not make full use of the high separation factor of catalytic exchange. This paper deals only with the CECE process as applied to upgrading of heavy water because of descriptive-geometric presentation with the help of McCabe –Thiele diagrams. However, the principle can be readily extended to tritium recovery from light and heavy water.
0920-3796/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 0 - 3 7 9 6 ( 0 1 ) 0 0 4 8 5 - 9
434
O.A. Fedorchenko et al. / Fusion Engineering and Design 58–59 (2001) 433–438
2. Traditional CECE process computer study At first the traditional CECE process flow sheet has been investigated. In this scheme water flowing from top to bottom in the opposite direction to hydrogen stream is successively fed into all scrubbing beds, which alternate with catalytic beds. Simulation code ‘EVIO-3’ is a Windows 95 application and one of the descendants of the code ‘KIO’ [1]. The Murphree-type factor eff is introduced in the code to consider a catalyst bed efficiency and factor, named kpd —to consider scrubbing bed efficiency in term of ratio to one theoretical plate. The significance of the liquid– vapour exchange step as well as great influence of a vapour stream value on a separation degree (SD) is demonstrated in Fig. 1. A vapour stream value (that is quantity of water moles taken away by hydrogen stream) depends on conditions (temperature, pressure and concentration). One can see translation of maxi-
Fig. 2. The dependence of optimal conditions, expressed in terms of water vapour stream value to hydrogen stream value ratio (V/G), upon scrubbing bed efficiency to catalytic bed efficiency ratio (kpd/eff ) for traditional type column at Bot = Top=0.1 G.
mum of separation degree in the line of temperature increase with increase of ratio of kpd to eff. The dependence of optimal conditions, expressed in terms of water vapour stream value to hydrogen stream value ratio (V/G), on kpd/eff is shown in Fig. 2.
3. New type exchange column
Fig. 1. Temperature dependence of traditional type column separation degree at following conditions: P= 130 kPa, Nb = 30, Nsc = 1, Xw = 0.5, Bot = Top= 0.1 G.
A new type column flow sheet of principle is represented in Fig. 3. The scheme contains n scrubbing beds (2) and, accordingly, n catalytic beds (3), where n is any integer number more than 2. The scheme in Fig. 3 corresponds to a column operating at the total reflux. Two different water streams: from a catalytic oxidiser (5) (the first one) and the water stream from condenser (4) (the second one) do not merge to one another and are separately fed into different scrubbing beds flowing from the top down in the opposite direction to hydrogen–water vapour stream. In doing so the first water stream is fed into the first from the top scrubbing bed and then successively into all odd
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(numbering from the top) scrubbing beds. And the second water stream is fed accordingly into the second from the top scrubbing bed and then successively into all even (numbering from the top) scrubbing beds. Such arrangement of the water streams leads to the fact that the ratio of liquid water stream value to water vapour stream value changes on a scrubbing stage. Besides relative moving away of starting concentrations in vapour and liquid water streams, which enter a scrubbing stage, takes place. All this made it possible to do better a phase isotopic exchange process and as a result of this to conduct a catalytic process more effectively as well as the overall process.
Fig. 4. Two type column flow sheets comparison: (a) a new type column and (b) traditional one.
4. Two schemes comparison
Fig. 3. A new type column flow sheet of principle: 1 —electrolytic cell, 2i — the ith scrubbing bed, 3i — the ith catalytic bed, 15i 5n, 4— condenser, 5 — catalytic oxidiser.
However to come to the point is easier comparing two schemes (traditional and new one) with a concrete number of scrubbing and catalytic beds. Thus, for instance, two schemes with n= 5 are shown in Fig. 4 (a— a new column and b— traditional one). Under the same conditions (P=105 kPa, DP = 1 kPa, t=83 °C, tc = 15 °C, Nb =5, eff= 1, Nsc = 8, Bot= Top = 0.1 G, Xw =0.5) SD= 25.684 in a new column and SD= 10.005 in a traditional one. The example is illustrated with the help of McCabe –Thiele diagram in Fig. 5 (a new type column) and in Fig. 6 (a traditional one), which should be treated together with corresponding schemes in Fig. 4. The vapour composition Z is designated on axis of ordinates and deuterium atomic fractions in water X and in hydrogen Y— on axis of abscissas. Two arch-lines above the diagonal line are catalytic equilibrium lines corresponding to maximum and minimum separation factors. Equilibrium line of a phase
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isotopic exchange is placed under the diagonal line (almost coincides with it). The bottom part of a column corresponds to the upper part of a diagram. Five scrubbing operating lines are well seen in Fig. 6. For example, ((X1, Z %1), (X2, Z2)) and ((X4, Z %4), (X5, Z5)) — the first and the fourth operating lines. Every one of these lines almost touches equilibrium line of a phase isotopic exchange by one of its extremity. As a result right triangles corresponding to theoretical plates from the 5th to the 8th are practically not seen. Operating lines of a co-current catalytic isotope exchange process for the example on a traditional column start approximately on the diagonal line (see Fig. 6). For example, point (Y3, Z4) is the beginning of the fourth catalytic operating line. McCabe – Thiele diagram of the process realized in a new type column looks in another way (see
Fig. 5). Though scrubbing operating lines overlap each other, because they are rotated (in comparison with a traditional scheme) in such a way that now they are almost parallel to the diagonal line (for example, ((X2, Z %1), (X4, Z2)) and ((X3, Z %2), (X5, Z3))—the second and the third operating lines), the main effect of the innovation is well seen. The catalytic operating lines start below the diagonal line (for example, (Y2, Z3) is the beginning of the third line), that is a hydrogen–vapour mixture composition is moved away farther from an equilibrium composition, it is due to more effective carrying out a scrubbing process. All mentioned above lead to increasing overall separation degree of a column. Calculated temperature dependence on SD of a new type column (1) and traditional one (2) are represented in Fig. 7. Maximum separation degree
Fig. 5. McCabe –Thiele diagram of the following steady-state process: P = 105 kPa, DP= 1 kPa, t= 83 °C, tc =15 °C, Nb = 5, eff= 1, Nsc = 8, Bot= Top=0.1 G, Xw = 0.5, Nw =3, that run in a new type column.
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Fig. 6. McCabe –Thiele diagram of the following steady-state process: P= 105 kPa, DP= 1 kPa, t =83 °C, tc =15 °C, Nb = 5, eff= 1, Nsc = 8, Bot= Top=0.1 G, Xw = 0.5, Nw =3, that run in a traditional type column.
of new type column relates to 91 °C. This separation degree is achieved by traditional type column at temperature equal to 100 °C only. So relatively small increasing of temperature nevertheless lead to the increase of vapour+ gas load two times that is: to retain efficiency of a catalytic bed the catalyst quantity must be increase at least twice and to retain linear gas mixture velocity the diameter of column must be increase accordingly more then 1.4 times. There are some concrete conditions for each separation task, which provide maximum degree of separation of a new type column. However these conditions for any concrete task do not differ heavily from the conditions providing maximum degree of separation for a column operating at the total reflux: temperature and pressure must be so that vapour stream value is equal to hydrogen stream value [2].
5. Conclusion Thus the realisation of a catalytic isotope exchange process between water and hydrogen in a new type column makes it possible to increase a separation degree in comparison with a traditional type column (with the same quantity of catalyst) or achieve the same separation degrees using less catalyst quantity and a smaller diameter. This is a great advantage especially when the processing of a big stream is required.
Appendix A. Nomenclature P DP t tc
pressure at the top of a column (kPa) pressure drop across a column (kPa) temperature in a column (°C) temperature in a condenser (°C)
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deuterium atomic fraction in liquid water stream Y deuterium atomic fraction in hydrogen stream Z deuterium atomic fraction in vapour stream Bot withdrawal of a bottom product (mol/h) Top withdrawal of a top product (mol/h) Nw feed scrubbing bed number (feed stream is W=Bot+Top) Xw feed composition (deuterium atomic fraction) eff Murphree-type efficiency of a catalytic bed kpd scrubbing bed efficiency in term of ratio to one theoretical plate (if Nsc\1 then kpd = Nsc) SD separation degree of a column SD= (XBot/(1−XBot))/(XTop/(1−XTop)) X
Fig. 7. Calculated temperature dependence of separation degree of a new type column (1) and traditional one (2) for the following conditions: P= 130 kPa, Nb = 8, Nsc = 8, Bot= Top= 0.1 G, Xw = 0.5.
References Nb Nsc G V L
the number of catalytic beds the number of theoretical plates in a scrubbing bed hydrogen stream value (mol/h) water vapour stream value (mol/h) liquid water stream value (mol/h)
[1] O.A. Fedorchenko, I.A. Alekseev, V.D. Trenin, V.V. Uborski, Computer simulation of the water and hydrogen distillation and CECE process and its experimental verification, Fusion Technol. 28 (1995) 1485 – 1490. [2] O.A. Fedorchenko et al., The method of hydrogen isotope separation, RUS Patent 2148426 (1998).