Composite catalytic membrane reactor analysis for the water gas shift reaction in the tritium fusion fuel cycle

ELSEVIER

Fusion Engineering and Design 30 (1995) 217-223

Fusion Engineeri.ng and Design

Composite catalytic membrane reactor analysis for the water gas shift reaction in the tritium fusion fuel cycle V. Violante, A. Basile t, E. Drioli 2 Associazione EURATOM-ENEA sulla Fusione, ('.R.E. Frascati, C.P. 65-00044 Frascati, Rome, Italy

Received 17 February 1994 Handling Editor: G Casini

Abstract A mathematical model has been developed to analyze the behavior of a composite or integrated membrane-fixed bed tubular reactor by considering the catalytic reaction of water with carbon monoxide. Such a reaction is a typical example of an equilibrium controlled reaction, where the conversion is enhanced when one of the products (i.e. the hydrogen) is removed selectively from the reactor. The application considered for such a reactor concept is that of tritium recovery from the solid blanket purge gas and/or from the plasma exhaust gas of a fusion reactor, where a small number of process components, modularity and continuous operations are required. The analysis has been performed taking into account that parameters such as the hydrogen gas diffusion coefficient, film mass transfer resistance and gas linear velocity are not constant along the reactor axis. In an adiabatic reactor, the gas specific heat, temperature and the kinetic parameters are not constant. For this reason, the reactor has been assumed as a series of tubular catalytic membrane reactors. The results show that, with proper optimization, it is possible to achieve very high conversion.

I. Introduction M u c h effort has gone into the d e v e l o p m e n t o f t e c h n o l o g y to i m p r o v e fuel cycle p l a n t safety a n d reliability. Recently, in several l a b o r a t o r i e s , men] b r a n e process units have been p r o p o s e d as c a n d i d a t e c o m p o n e n t s in fuel clean-up technology. In

t Present address: Research Institute on Membranes and Chemical Reactors, 87036 Arcavacata di Rende (CS), Italy. 2 Present address: Department of Chemistry, Chemical Engineering Section, University of Calabria, 87036 Arcavacata di Rende (CS), Italy.

this work, a catalytic m e m b r a n e r e a c t o r ( C M R ) , acting also as a s e p a r a t o r , is p r o p o s e d for tritium recovery f r o m tritiated water. The interest in C M R s resides in their c a p a b i l i t y for the c o n t r o l o f the r e a c t i o n zone c o n f i n e m e n t a n d o f the selectivity. F u r t h e r m o r e , when the feed in a C M R is passing t h r o u g h the m e m b r a n e , which selectively c o n t r o l s the mass transfer rate o f the r e a c t a n t s (this is n o t the case in the c o n s i d e r e d application), r e d u c e d catalyst d e a c t i v a t i o n can be obtained. In Refs. [1-3], I t o h describes e x p e r i m e n t a l a n d theoretical w o r k on a p a l l a d i u m m e m b r a n e reac-

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V. Violante et al. : Fusion Engineering and Design 30 (1995) 217 223

218

a)

ceramic support CO2+ unreacted H20 + CO excess

feed H20 -4- CO catalyst ~ J J J l J J i ~ J N U M l J H J ~ I I J l ~ J W ~ I ~ N I ~ U I ~

1 =

I I

l

~'~.Pd/Ag membrane

b)

Z

. . . . . . . . . , . , . . . .

========================================

::.

: : •:

: : :- : :.:::

:.:: : : : :

: : :::: :.:.:.::.: ::: : :::.

J i

+:<

>:.::

• : .:+

1

5

: :2~::::~:::::::

.,.,. He, . . . . . . . . . . .

:::.::::::.::

D g D g g g

, ,~ .,.,.

2

i

\

n reactor shell

Fig, 1. Schematic model of catalytic membrane shift reactor.

tor and proposes such a reactor as supporting both dehydrogenation and oxidation. In Ref. [4], Wu and Liu present a mathematical analysis of the catalytic dehydrogenation of ethylbenzene, by using a ceramic membrane reactor. Sloot and Zaspalis have presented two Ph.D. theses on membrane reactors for catalytic gas phase reactions [5,6]. Recently, many groups in Europe, the USA and Japan have studied alternative solutions for a fusion reactor fuel cycle, based on membrane process units [7 13]. Uemiya and coworkers recently published two interesting papers [14,15] concerning the development of ceramic membranes coated with 20 25 lam of low-defect Pd Ag. The aim of this work is to obtain a preliminary theoretical evaluation of the performance of a CMR for the water-gas shift reaction, which

integrates into one step both the shift reaction and the separation of the molecular hydrogen isotopes produced. The effect of the integration should be not only a reduction of the process components but also an improvement in the performance, as a result of the continuous hydrogen isotope extraction involved in the reaction equilibrium. The analysis was developed mainly for isothermal conditions; however, some evaluations also have been performed for the adiabatic case. The reactor consists of a porous tubular ceramic membrane as support (1 mm thick), and contains a catalyst [16,17] for the water gas shift reaction in the lumen. The external surface of the ceramic support is coated with a P d - A g layer 25 tam thick which is assumed to be defect-free [14,15] (see Fig. l(a)). An advantage of this con-

V. Violante et al. / Fusion Engineering and Design 30 (1995) 217 223

cept is lower costs, as a result of two different considerations: the first point is the need to reduce the amount of expensive materials, such as P d Ag alloy (which, in this reactor configuration, is just a layer, since the mechanical support is provided by the ceramic material); the second point is the possibility to have a material with a higher global permeability than that of a traditional metallic membrane. The technology proposed in the literature [14,15] (electroless and/or electrochemical) for the thin film production, in principle, is not expensive. In our model, we have considered only hydrogen as the permeating species.

negligible surface defects in the metallic coating; use of large-sized catalyst pebbles or pellets to keep the pressure losses in the bed negligible. By considering the water gas shift reaction C O + H 2 0 - - C O 2 + H2 the typical Langmuir-Hinselwood kinetics [18] are assumed. The axial differential mass balance, in terms of partial pressures, for each chemical species gives the following system of differential equations for the generic stage i:

d=p~i

v, dp,i R'Ti _ Rki D,, dz ~-~ J, + ~ R'Ti

dz 2 2. The model

d2 Pbi

Vi dpbi

dE 2 -

To develop a model that describes the system under adiabatic or isothermal conditions, the CMR has been divided into a number n of elements or stages (several hundred), each of which resembles a small tubular reactor (see Fig. 1). Such an approximation has been made because, along the reactor axis, parameters such as the diffusivity, film resistance, gas velocity and, in the adiabatic case, temperature, gas specific heat and the kinetic parameters are not constant. In each small element, however, all these parameters can be assumed to be reasonably constant. Thus, the differential mass balance equations along the axis, and the global stage energy balance equations are solved by assuming as inlet boundary conditions for each stage the outlet value of the preceding stage. The large number of stages also gives a reasonable thermal balance approximation. The assumptions are as follows: negligible effect of the competitive reactions; plug flow fluid dynamic regime in each stage; negligible effect of the catalyst pebble (or pellets) bed on the evaluation of the resistance of the interface film between the ceramic membrane and the gas, which means that the pebbles or pellets are large enough, compared with the film thickness, to produce a significant effect on the film resistance, which is evaluated by considering a fluid flow through a tube; ideal gas behavior inside the reactor;

219

Rk,

Dbi dz ~- Dbi R ' T i

d2 Pci vi dpci dz 2 - Dci dz d2

Pdi

dz 2

=c, R'Ti

Vi dpdi

Rki

- - Ddi dz

Dd,

R'T,

The boundary conditions are

~P~'

=0

z=O

Pal=0,

2=]

100 f

ix.

80

/

°

60

Ip

~

40

T= 600 K

L

Initial: Pco = PH~O= 50 kPa, Pe =9.4

PH2 -- ~

r'l,

20

t

~

PCO 2

. . . . . . . PCO and PH20

'~ k ....

0

..i . . . . . . . . . .

50

J, . . . . . . . . . .

100

1

150 Z (cm)

Fig. 2. Partial pressure axial profiles for H 2, CO 2, CO and H=O at 600 K, Pe = 9.4 and an upstream total pressure of 1 atm.

V. Violante et al. / Fusion Engineering and Design 30 (1995) 217. 22.3

220

250

f

/

2oo

-~ 150 L_

Q.

/

T:

600

K

Initial: Pco= PHzO = 125 kPa, Pe=9.4

-~ 100 PH~ - - w w P c o ~

50

,~

. . . . . . . PCO and PH20

0

50

100

150

Z (cm) Fig. 3. Partial pressure axial profiles for H2, CO2, CO and H 2 0 at 600 K. Pe = 9.4 and an upstream total pressure of

In each stage i, let P~i be the hydrogen partial pressure inside the membrane, let P2i be the hydrogen partial pressure at the gas-membrane interface, let P2, be the hydrogen partial pressure at the ceramic (Pd-Ag) interface, and let P3i be the hydrogen partial pressure at the external side of the coated membrane; P3i can be kept constant along the reactor axis under a technological vacuum (e.g. 0.1-0.01 atm), since hydrogen is the only permeating component on the strip side (the metallic coating without defects is practically nonpermeable to the other gases). Thus, the fluxes through the film, porous ceramic membrane and metallic layer are respectively given by Pli-

Jl

1)'1i--P2i

J2

R2i

2.5 atm.

dpb

dz z = / : 0,

dpc ~:/=0 dz

dpd dz z=/ = 0 ,

P'l,

Rli

p2i ]/2 -- p3i ]/2

Phi

=

0,

_ o Pd-Pcl, Pall=P°1,

-~ 0

J3-

R3i

=

z=0

Z=0

The subscripts a, b, c and d indicate the chemical species H2, CO2, CO and HzO respectively; i indicates the stage, i.e. i = 1, 2 . . . . . n; p is the partial pressure of each component in the gas within stage i; R~, is the reaction rate ( m o l c m 3s J); R' is the gas constant; T is the temperature (K); z is the axial coordinate (cm); r is the membrane internal radius (cm); v is the gas linear velocity (cm s ~); D is the diffusion coefficient (cm 2 s-l); and J is the hydrogen molar permeation flux. All the parameters containing the stage index are evaluated, on the basis of the local conditions, stage by stage. The hydrogen flux is controlled by three mass transfer resistances: Rl~, resistance through the film at the interface between the inside ceramic membrane wall and the gas; R2i, resistance through the ceramic membrane: R3,, resistance through the Pd Ag layer.

(the flux through the ceramic membrane can be considered as a Knudsen permeation) where Rli is evaluated by means of the Colburn analogy

R'T, R~i= k, 100 f

ca. -~v

/ 80

/ I T=600 K

"~

I 60

c~ -~ ~

40

I I ,I

Initial" PCO =PH2o = 50kPa, Pe = 94

ro

\

~

PH2

Pco z

2O

0

50

100

150 Z (cm) H2, CO,, CO and

Fig. 4. Partial pressure axial profiles for H 2 0 at 600 K, P e = 9 4 and an upstream total pressure of 1 atm.

221

V. Violante et al. / Fusion Engineering and Design 30 (1995) 217 223

N s hDai

k,

250

2r /

a_

where Nsh is the Sherwood n u m b e r in stage i. Also, we have

/

200

/ T=600 K

h._ :D

g~ R 2 - p~

150

I I J

with 62 and P2 being the ceramic thickness and the permeability respectively. The permeability t h r o u g h the ceramic (P~) was assumed to be 1.0x 1 0 - S m o l m ~s ' [10]. Furthermore, we have R3 =

e

Initial: Pco=PH~o = 125 kPa, Pe = 94

100

a_

50

g3

,._~k

PH2 - - _ _

.......

Pco and PH20

-~

0

where 63 and P3 are the thickness and the permeability o f the P d - A g coating respectively. With some algebraic operations and the condition that, at the steady state, all the fluxes are equal, the following expression for the hydrogen flux t h r o u g h the m e m b r a n e is obtained:

.

.

.

.

.

.

50

a---=~r

. . . .

100

l

150 Z (cm)

Fig. 5. Partial pressure axial profiles for H2, C 0 2 , CO and H20 at 600 K, Pe = 94 and an upstream total pressure of 2.5 atm.

-- (R,,. + R2, -- 2R3iP3i I/2) + {(R1~ + R2i-- 2R3ip3, I/2:2 ) Ji -

Pco 2

~

4R3i-(P3~ -- p,~)} 1/2

2R3/

It should be observed that, for the above-mentioned reasons, P3i is the same for all the elements. The heat balance for element i gives

Cp(i

I)Q,

,T~

It can be observed that the exchanged heat Qi can be evaluated for the isothermal working condition by setting Ti = Ti ~. In contrast, by assure-

,-Cp~Q~T~-J~SiCp~,T~-q~

100

= A H i R k i V,

where Cp~ is the specific heat in the element considered, Qi is the flow rate t h r o u g h the element, S~ is the m e m b r a n e element exchange area, V~ is the element reaction volume, AH~ is the reaction enthalpy evaluated by considering the element conditions, and q~ is the heat exchanged in the element considered (see Fig. l(b)). The index i - 1 refers to the inlet values. The different terms in this last equation have the following meanings. The first term is the a m o u n t o f heat entering the element with the gas. The second term is the heat leaving the element with the gas. The third term is the heat leaving the element with the permeating hydrogen. The last term on the right-hand side is the reaction heat produced.

._. 80

" .... T= 600

K

initial: PEao__~PHzo= 50 kPa,

v ¢-

o 60

initial: C

o

u

40

. . . . . . . PCeOg;H20= 125 kPa,

20

0

I

i

I

50

100

150 Z (cm)

Fig. 6. Hydrogen conversion profiles for Pe = 9.4 and Pe = 94 and for upstream total pressure of 1 atm and 2.5 atm at 600 K.

V. Violante et all / Fusion Engineering and Design 30 (1995) 217-223

222

100

._. 8O

~

v

--t

cO

60

initial: PCO=_PH20= 50 kPa,

//

//

F e ----~ 4

/ 40

L)

20

0

.--

/

l

(u

g

2; . . . . . . . . .

/

525 K

- - - - -

......

550K

~

600 K

_.

660 K

m

1

I

50

1O0

I

150

Z (cm) Fig. 7. Hydrogen conversion profiles along the reactor axis at different temperatures for Pe = 94 and an upstream total pressure l atm.

ing that q = 0, the reactor is adiabatic, except for the heat leaving the element with the permeating hydrogen.

3. R e s u l t s and d i s c u s s i o n

A numerical simulation has been performed to analyze the reactor behavior. Two fluid dynamic regimes have been investigated for the isothermal conditions: low Peclet number values (Pe = 9.4, i.e. low axial diffusion effect) and high Peclet number values (Pe = 94.0, i.e. axial diffusion negligible). The assumed working temperatures were 525, 550, 600 and 660 K. The maximum conversion (greater than 99.8%) was obtained at 600 K. This temperature is the same as that utilized in some experimental work (the reason for such a choice is because of the P d - A g CO poisoning at a lower temperature [8,9]). The membrane length is 150 cm, with a membrane radius of 0.5 cm. The hydrogen pressure in the permeate side is assumed to be 0.01 atm. Figs. 2 and 3 show the profiles of the partial pressures along the axis of the reactor for H 2 , C O 2 , CO and H 2 0 at Pe =9.4, with upstream total pressures of 1 atm and 2.5 atm respectively. Figs. 4 and 5 show the partial pressure axial profiles for H2, CO2, CO and H 2 0 at Pe = 94.0, with up-

stream total pressures of 1 atm and 2.5 atm respectively. Fig. 6 gives the hydrogen conversion profile along the reactor axis for the conditions presented in Figs. 2 and 5 at 600 K. It can be seen that the conversion profiles do not change significantly, either by varying the Pe number in the fluid dynamic regime investigated or by varying the upstream total pressure. In Fig. 7, the hydrogen conversion profile along the reactor axis is plotted for different temperatures, with Pe = 94.0 and an upstream total pressure of 1 atm. At low values of the axial coordinate (z), the temperature effect is significant; for z = 100 cm, the optimum temperature is 600 K, giving more than 99.8% conversion. Fig. 8 shows the axial profile of the thermal load resulting from the reaction, under different conditions. As can be easily observed, there is a strong heat production at the reactor inlet, which needs to be removed to maintain the isothermal conditions necessary to produce the calculated conversion profile. A quasi-isothermal working condition could be achieved by using an appropriate catalyst concentration profile along the reactor axis (increasing from the inlet to the outlet section) and working in the real stage mode with an intercooling system, and/or introducing an appropriate inert

/."

100 A H (W) 8O

T = 600 K initial:

60 .......

initial: Pco =~PH~O= 125 kPa, F e - - ~ 4

40

20 L I

0

5

10

100

Z (cm) Fig. 8 Axial profiles of the thermal load vs. axial coordinate for Pe = 9.4 and Pe = 94 and for upstream total pressures of 1 atm and 2.5 atm at 600 K.

v. Violante et al. / Fusion Engineering and Design 30 (1995) 217 223

gas fraction into the inlet stream to give a thermal flywheel effect. W h e n the temperature increases above a threshold value along the reactor axis, the reaction rate a n d the heat p r o d u c t i o n rate are reduced; the temperature then drops until the reaction rate again increases, a n d so on, giving spatial oscillations.

4. Conclusions If operated u n d e r appropriate conditions, the C M R enables good levels of conversion a n d hydrogen separation. The ceramic material allows operation at high temperature levels (below its calcination temperature), within a reasonable range of pressure and, generally, without significant problems of chemical compatibility. Recently, the authors experimented for some m o n t h s (up to 550 K) with a C M R consisting of a thin p a l l a d i u m film on rnicroporous a l u m i n a [19]. The permeability of the multilayer metallic/ceramic m e m b r a n e is higher t h a n that o f a traditional metallic m e m b r a n e . The exchange area is thus reduced, m i n i m i z i n g the a m o u n t of expansive material ( P d - A g ) , since b o t h the exchange surface and the metallic film thickness are reduced. F r o m the given flux expression, it arises that, w o r k i n g with the same pressure difference across the m e m b r a n e a n d with the same exchange area, the h y d r o g e n flux t h r o u g h the multilayer m e m b r a n e (having the considered characteristics) is five times larger t h a n the flux t h r o u g h a P d - A g m e m b r a n e (0.15 m m thick). Moreover, the calculations show that such a C M R should be operated in the a p p r o p r i a t e c o n d i t i o n s of t e m p e r a t u r e a n d pressure to reach good conversion a n d to avoid stability problems.

Acknowledgement This work was partially supported by C N R , Progetto Finalizzato Chimica Fine e Secondaria II.

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and hydrogen-permeable activities of palladium, Proc. 5th World Filtration Congress, Nizza, June 1990, pp. 178 183. [3] N. ltoh, Simulation of bifunctional palladium membrane reactor, J. Chem. Eng. Jpn., 23 (1990) 81 87. [4] C.S.J. Wu and P.K.T. Liu, Mathematical analysis on catalytic dehydrogenation of ethylbenzene using ceramic membranes, Ind. Eng. Chem. Res., 31 (1992) 322 327. [5] N.J. Sloot, A non-permselective membrane reactor for catalytic gas phase reactions, Ph.D. Thesis, Twente University, May 199l. [6] V.T. Zaspalis, Catalytically active ceramic membranes, Ph,D. Thesis, Twente University, November 1990. [7] C. Hsu and R.E. Buxbaum, Palladium catalysed oxidative diffusion for tritium extraction from breeder blanket fluids at low concentration, J. Nucl. Mater., 141-143 (1986) 238 243. [8] H. Yoshida, S. Konishi and Y. Naruse, Preliminary design of a fusion reactor fuel clean-up systemby the palladium-alloy membrane method, Nucl. Technol./Fusion, 3 (1983) 471-484. [9] H. Yoshida, H. Takeshita, S. Konishi, H. Ohno, T. Kurasawa, H. Watanabe and Y. Naruse, A feasibilitystudy of the catalytic reduction method for tritium recovery from tritiated water, Nucl. Technol./Fusion, 5 (1984) 178-188. [10] V. Violante, A. Basileand E. Drioli, Membrane separation technologies: their application to the fusion reactors fuel cycle, Fusion Eng. Des., 22 (1993) 257 263. [11] C. Latger and S. Seller, Fuel clean-up system, experimental study and modelling of a palladium-silver permeator, Proc. Syrup. on Fusion Technology, London, 1990, pp. 699 703. [12] M. Glugla, R.D. Penzhorn, R. Rodriguez, D. Herbrechter, P. Dinner and D. Murdoch, Evaluation of concepts for a NET plasma exhaust clean-up system, KfK Rep. 4752, July 1990. [13] R.D. Penzhorn, R. Rodriguez, M. Glugla, K. Gtinther, H. Yoshida and S. Konishs, A catalytic plasma exhaust purification system, Fusion Technol., 14 (1988) 450 455. [14] S. Uemiya, N. Sato, H. Ando, Y. Kude, T. Matsuda and E. Kikuchi, Separation of hydrogen through palladium thin film supported on a porous glass tube, J. Membr. Sci., 56 (1991) 303 313. [15] S. Uemiya, T. Matsuda and E. Kikuchi, Hydrogen permeable palladium silver alloy membrane supported on porous ceramics, J. Membr. Sci,, 56 (1991) 315-325. [16] M.E. Agnelli, M.C. Demicheli and E.N. Ponzi, Catalytic deactivation of methane steam reforming catalysts, 1. Activation, Ind. Eng. Chem. Res., 26 (1987) 1704-1707. [17] M.E. Agnelli, E.N. Ponzi and A.A. Yeramian, Catalytic deactivation of methane steam reforming catalysts. 2. Kinetic study, Ind. Eng. Chem. Res., 26 (1987) 1707 1713. [18] W.F. Podoski and Y.G. Kim, Modelling the water-gas shift reaction, Ind. Eng. Chem. Process Des. Devel., 13 (1974) 414-421. [19] V. Violante, E. Drioli and A. Basile, Catalytic ceramic membranes reactor design for hydrogen separation from inert gas via oxidation, J. Membr. Sci., in press.