Partial Oxidation Using Membrane Reactors - American Chemical

Chapter 33

Partial Oxidation Using Membrane Reactors Catalytic Selective Oxidation Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 01/12/17. For personal use only.

Lewis A. Bernstein, Sunil Agarwalla, and Carl R. F. Lund Department of Chemical Engineering, State University of New York, Buffalo, NY 14260

The viability of one particular use of a membrane reactor for partial oxidation reactions has been studied through mathematical modeling. The partial oxidation of methane has been used as a model selective oxidation reaction, where the intermediate product is much more reactive than the reactant. Kinetic data for V O /SiO catalysts for methane partial oxidation are available in the literature and have been used in the modeling. Values have been selected for the other key parameters which appear in the dimensionless form of the reactor design equations based upon the physical properties of commercially available membrane materials. This parametric study has identified which parameters are most important, and what the values of these parameters must be to realize a performance enhancement over a plug-flow reactor. 2

5

2

Membrane reactors can offer an improvement in performance over conventional reactor configurations for many types of reactions. Heterogeneous catalytic reactions in membrane reactors [1] and the membranes used in them [2,3] have been reviewed recently. One well studied application in this area is to remove a product from the reaction zone of an equilibrium limited reaction to obtain an increase in conversion [4-10]. The present study involves heterogeneous

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428

CATALYTIC SELECTIVE OXIDATION

catalytic partial oxidations of hydrocarbons, where conversion generally is not limited by thermodynamics. Instead, the key to successful partial oxidation is stopping the reaction short of complete conversion (to C 0 and FÔ). In one approach to doing this, a membrane can be used to supply one of the reactants, e.g. oxygen, at a controlled rate or level throughout the reactor [1, 11-14]. It is also possible to use a solid oxygen anion electrolyte with an applied potential in this approach [1]. The present study investigates a different approach. The membrane is used to allow the desired intermediate product to escape from the reaction zone before it is consumed by further reaction. This use of a membrane reactor was first suggested by Michaels [15]. The partial oxidation of methane, which is a challenging reaction of the type proposed for this application of membrane reactors, has been analyzed herein. There is no thermodynamic limitation for the production of carbon dioxide and water, actually these products are favored. It is desired to remove any partial oxidation product, for example formaldehyde, before it has a chance to be further oxidized. A schematic diagram of a concentric-tube membrane reactor configuration for doing this is given in Figure 1. In this configuration, reactants are fed to the tube side, which is a packed bed reactor whose walls are constructed of a membrane material. Inert gas is fed to the shell side to sweep away the products that have diffused to that side. Due to the temperatures required for partial oxidations, choices of membrane materials are limited to ceramic or porous glass membranes such as Vycor. For such membranes, reactants, products, and intermediates normally diffuse through the membrane according to the Knudsen mechanism. Relative diffusion rates are then fixed by the molecular weight (or size) of the diffusing species.

Catalytic Selective Oxidation Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 01/12/17. For personal use only.

2

Impermeable Tube Wall Inert GasPermeable Tube Wall Reactant Catalyst

Figure 1. A schematic representation of a membrane reactor for improving yields of intermediate products in a partial oxidation reaction.

33. BERNSTEIN ET AL.

Partial Oxidation Using Membrane Reactors 429

Agarwalla and Lund [16] analyzed the use of a membrane reactor for a generalized catalytic series reaction of the form, A-> Β


Β -> C where Β is the desired intermediate product. They concluded that in order for a membrane reactor to show improvement over a conventional plug flow reactor the membrane must be permselective for the intermediate product, B. That is, Β must pass through the membrane faster than A. However, the methane partial oxidation reaction is not purely a series reaction, but a series-parallel reaction. One of the reactants is reacted in series, the other in parallel as below:

A +B -> R + T A + R-» S + Τ This is seen in the kinetic sequence proposed by Spencer and Periera [17]. C H + Q2-> H C H O + H 0 4

2

HCHO +i-Q2 -> CO + H 0 2

C O + l Q 2 - > CQ2 + H2O

It is expected that the conclusions reached in the analysis of the series reaction will also be valid for methane partial oxidation. The first objective of this study was to verify this expectation. The second objective of the study was to determine how much faster than methane formaldehyde must permeate for the membrane reactor to begin to outperform a plug-flow reactor. Methods The reactor was evaluated through mathematical modeling. Mole balances for all species on both the shell and tube sides were solved. The assumptions used in writing and solving the equations are as follows: The reactor operates at steady state, plug flow conditions exist on both sides of the membrane, boundary layer effects between the bulk fluids and membrane surface are negligible (that is the bulk and surface concentrations are equal), and radial diffusion that obeys Fick's Law is the only mechanism of transport within the membrane. All of these assumptions are made routinely in modeling inorganic membrane reactors of ceramic or Vycor glass construction [19-23]. Kinetic expressions for the three step pathway given above for the partial oxidation of methane to formaldehyde over a vanadium oxide-silica catalyst were determined by Spencer and Periera [17]. The kinetic parameters


430


were measured in the temperature range from 773 to 873 K, at one atmosphere pressure, and with a nine to one ratio of methane to oxygen in the feed stream. Since these conditions were used in determining the kinetic data, the modeling studies were constrained to these values of temperature, pressure and feed composition. Mole balance expressions were developed for a general series reaction by Agarwalla and Lund [16], and the same procedures were used here to develop the species balance equations shown in Table I. Boundary conditions and parameter definitions are presented in Tables II and III. Note that the boundary conditions are given only for co-current flow of reactants and inert, which is the only configuration studied. Previous work [16], has shown that counter-current operation is less effective than co-current operation. From Table III it is evident that many parameters exist. The Damkohler number, Da, is a ratio of the time scale for reactor residency to the time scale for chemical reaction. The Peclet number, Pe, as defined for this problem is a ratio of the time scale for reactor residency to the rate of permeation through the membrane. The product, DaPe, is a ratio of the rate of reaction to the rate of permeation. Choosing a temperature fixes κ , κ , and all of the Knudsen diffusivities, Sj. Four main parameters remain once these values are determined. These are the Damkohler number, the Damkohler-Peclet product, DaPe, the ratio of the shell side to tube side pressure, φ, and the inert gas sweep rate ratio, Y,, all of which can be varied independently. In all simulations the Damkohler number was allowed to vary over whatever range was required to generate conversions from zero to ca. ninety-nine percent of the limiting reagent, oxygen. The Damkôhler-Peclet number varies with the thickness or the pore diameter of the membrane. Realistic values of membrane thickness and pore diameter (based on commercially available materials) were used when determining the range in which to vary the Damkôhler-Peclet number. The ratio of inert gas to reactant gas flow rate, Y and the ratio of pressure on the shell side to pressure on the tube side, φ, were studied over reasonable ranges. The relative permeability of the intermediate product, S , was varied in some cases as well. Once all of the conditions were determined and parameters chosen, the equations were solved by an implicit Euler method. The program was written with a self adjusting step size and analytic Jacobian to reduce error and run time. 2

3

v

HCHO

Results In all cases studied, the membrane reactor offered a lower yield of formaldehyde than a plug flow reactor if all species were constrained to Knudsen diffusivities. Thus the conclusion reached by Agarwalla and Lund for a series reaction network appears to be true for series-parallel networks, too. That is, the membrane reactor will outperform a plug flow reactor only when the membrane offers enhanced permeability of the desired intermediate product. Therefore, the relative permeability of HCHO was varied to determine how much enhancement of permeability is needed. From Figure 2 it is evident that a large permselectivity is not needed, usually on the order of two to four times as permeable as the methane. An asymptotically approached upper limit of

33.

Partial Oxidation Using Membrane Reactors

BERNSTEIN ET AL.

Table I. Mole balances on all species in the reactor F^CHt CH* — Ç ^ = Da dZ F , DaPe t

F

CHt

φ

1

r

F

9L


dZ

F

= Da

CHi

CHO_= Da dZ

CQ _

dZ

= Da

CH,

,

K

dF}

K

A

P

E

D A P E S

c o > l F : ,total

dZ

dP

L

co. r

dZ dZ dlfnert

dZ

co -s total/

r

cg

-s total/

total

total r

-Φ: CHt

FÔ2

O2

1

DaPeSozlptotal

F

s

r

total

r

Da | HCHO φ ^ Η Ο ] DaPeS cHo\F'total F;total r

dZ

r

r

H

V>.

total /

p?t es Inert _ φ Mnert

Da ( CH>

Fi H Q Da Ho ΦF DaPeSjfeolF'total F

2

2

s

r

total

Fçp Da DaPeSco \ρ[total

φ Fç

Da DaPe S e a

Φ:-s

Da

r

Q

total 7 Fi COi

\ F |t total F r

H Q 2

C o l F Ï total ,

F^CQ2

F

=

r

CO

\F

total

H 2 0 \ F Î total ,

D a P e S

DaPeSinen

CH> dZ

FE,total

2

Da

dFH CHO^

H C H 0 \

F iH o

D a P e S

Fioui

n

dtf dZ

S

COi

dF

I

D

F^

K3

F

H C H O a HCHC>1

FrCO

3

total /

-Φ-

PU,

dZ

D

F

FLd

Fi,total

dtf ^ = Da dZ

O T A L

F'HCHQ

2

FkHO.

2

FÎ

Fioul

F|,total

K

2

FiHCHO

2

Fi,total F

3

F}.total K

F

1 ( cO2 DaPeSa^ttotal

K F^Q

2

CH*

ô=Da dZ

\ total

K FHCHO

F 'total

total

r

total

1

Inert

DaPeSinen \ F Î total

total 8

F a Mnert s

F total 1

431


432


Table II. Boundary conditions for the membrane reactor

Variable

Boundary

Value

CH*

z =o

1

*k

z =o

1/9

HCHO

z =o

0

H 0

z =o

0

CO

z =o

0

CQz

z =o

0

Inert

z =o

0

CH*

z =o

0

z =o

0

HCHO

z =o

0

H 0 2

z =o

0

CO

z =o

0

CQj

z =o

0

Inert

z =o

Y.

F

F

F

2

F

F

r

F

r

F

F

F

r

33.


BERNSTEIN ET AL.

433


Table III. Critical dimensionless parameters for series reactions in a membrane reactor Definition

Ratio Characteristic of

2

Da =

Reaction vs. Flow Rate

nd Lk l* x

RT

2

DaPe= -

Reaction vs. Permeation Rate

=

d h i \d n i+- r2Vtl

Shell Side Flow vs. Tube Side Flow

Shell Side Pressure vs. Tube Side Pressure Membrane Permeability of i vs. C H

p'

5, =

4

^

Dciu

Relative Reaction Rate of Reaction 2

κ = ^2.

Relative Reaction Rate of Reaction 3

K =|l

2

3

Moles C H of Reacted/Mole CH fed 4

4

Sel =

Methane Reacted to form HCHO

flHCHO+nHCHO {ncH$ -ή -ncth - nctu

i = CH , 0 , HCHO, H 0, CO, C 0 , or Inert 4

2

2

2

434


enhanced performance from the effect of the permselectivity of formaldehyde is seen as well. The Damkôhler-Peclet product also had an impact on performance; the optimal value ranged from is 1.0 χ 10* at 773K, to 1.0 χ 10" at 873K. Little or no improvement was observed when the pressure in the tube was larger than the pressure in the shell, and no improvement was seen when the shell pressure exceeded the tube pressure. When the inert gas sweep rate was increased, the membrane reactor improved until the amount of sweep gas to reactant gas was approximately one hundred as seen in Figure 3. Once again there was an asymptotic limit to the amount of enhancement seen. There was no improvement when the permeabilities of any other component were increased over the permeability of methane.


1

Figure 2. Effect of permselectivity of HCHO, S reactor performance at 873K.

HCHO

, upon membrane

Discussion In order for a membrane reactor to produce yields of HCHO greater than in a plug flow reactor, the membrane must be permselective for this species. The more permselective the membrane is to formaldehyde the better the membrane reactor performs until the formaldehyde is approximately one thousand times more permeable than methane. At this limit, the concentration of HCHO is essentially equal on both sides of the membrane at all times. No further improvement is possible by increasing the diffusivity of the formaldehyde further because there is


33.


BERNSTEIN ET A L

435

no longer any concentration gradient. Existing membranes separate only on the basis of size and hence to not offer the requisite permselectivity. New materials are under development which use pore surface affinity for the permeants to alter permselectivities. The present study shows that if such materials can be developed, membrane reactors may represent a viable reactor alternative. At each temperature an optimal range of DaPe exists. These represent a reactor system where the time scale for reaction and permeation are similar. At higher values of DaPe, corresponding to thick membranes and/or narrow pores, there is virtually no permeation. The performance of the membrane reactor then approaches that of a plug flow reactor consisting of just the inner tube. For low values of DaPe, corresponding to thin membranes and/or large diameter pores, there is virtually no resistance to permeation for any species. Here the performance of the reactor approaches that of a plug flow reactor with some bypass. The bypass is due to a certain amount of methane never coming into contact with catalyst because it has diffused to the shell side where no catalyst is present and therefore never reacted. That is, there is near perfect mixing across the membrane.

0

1

2 3 4 5 6 Conversion (% of methane reacted)

Figure 3. Effect of sweep gas flow rate on reactor performance at 873K. As sweep gas flow rate is increased, the performance of the reactor improves until the flow rate is about one thousand times the reactant flow rate. The concentration of all species, but most importantly formaldehyde decreases in the shell side of the reactor as this happens. This increases the driving force for permeation of all species. After increasing this flow rate to a certain point further increases in inert gas flow rate do not change the concentration gradient of any species along the reactor because the shell concentrations of all species is

436


essentially zero. However, this decrease in concentration is also a negative from a separations viewpoint. The products must then be recovered from both the tube and shell sides of the reactor, where they are present at very low concentrations.


Conclusions Partial oxidation of methane in the membrane reactor configuration shown in Figure 1 will not lead to higher yields of desired products than a plug flow reactor unless the diffusivity of the intermediate product, formaldehyde, is approximately four times that of methane. Presently available membranes that can withstand partial oxidation temperatures do not satisfy this criterion. This suggests areas for further study: developing high temperature molecular affinity based membranes or developing a low temperature methane partial oxidation catalysts to take advantage of existing permselective polymer materials. For example, Kapton polyimide films with a glass transition temperature of about 625K possess the necessary permselectivity [23]. Acknowledgments This work is based upon work supported by the National Science Foundation under Grant No. CBT-8857100. The Government has certain rights in this material. The authors would also like to acknowledge an equipment grant from Sun Microsystems, Inc. for the 3/260 workstation used in the calculations. Literature Cited [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

Eng, D., and Stoukides, M., Proc. 9th Int. Congr. Catal., Calgary, Vol 2, p. 974. The Chem. Soc. of Canada, Ottawa, 1988. Hsieh, H. P., AIChE Symp. Ser. 85 (268), 53 (1989). Hsieh, H. P., Catal. Rev.-Sci. Eng. 33, 1 (1991). Mohan, K., and Govind, R., AIChE J., 32, 2083 (1986). Oertel, M., Schmitz, J., Weirich, W., Jendryssek-Nuemann, D,. and Schuleten, R., Chem. Eng. Tech. 10, 248 (1987). Raymont, M. E. D., Hydrocarbon Proc. 54, 139 (1975). Uemiya, S., Sato, N., Ando, H., and Kikuchi, E., Ind. Eng. Chem. Res. 30, 585 (1991). Uemiya, S., Sato, N., Ando, H., and Kikuchi, E.,Appl.Catal.67, 223 (1991). Champagnie, A. M., Tsotsis, T. T., Minet, R.G., and Webster, I. Α., Chem. Eng. Sci. 45, 2423 (1990). Kameyama, T., Dokiya, M., Fujishige, M., Yokokawa, H., and Fukuda, K., Ind. Eng. Chem. Fundam. 20, 97 (1981). Gryaznov, V. M. Platinum Metals Rev. 30, 68 (1986). Gryaznov, V. M., Smirnov, V. S., and Slinko, M. G., Proc. 5th Int. Congr. Catal., p. 80 - 1139. J. W. Hightower, ed. Elsevier, New York, 1973. Nagamoto, H., and Inoue, H., Chem. Eng. Comm. 34, 315 (1985). Dicosimo, R., Burrington, J. D., and Grasselli, R. K., U. S. Patent 4,571,443 (1986).

33. BERNSTEIN ET AL. [15] [16] [17] [18]


[19] [20] [21] [22] [23]


Michaels, Chem. Eng. Prog. 64, 31 (1968). Agarwalla, S., and Lund, C. R. F., J. Mem. Sci., 70 (1992) 129-141 Spencer, N. D., and Pereira, C. J., J. Catal. 116, 399 (1989). N. Itoh, A Membrane Reactor Using Palladium, AIChE J., 33 (1987) 1576. Y.-M. Sun and S.-J. Khang, Ind. Eng. Chem. Res., 27 (1988) 1136. N. Itoh, Y. Shindo, K. Haraya and T. Hakuta,J. Chem. Eng. Jpn., (1988) 399. Y.-M. Sun and S.-J. Khang, Ind. Eng. Chem. Res., 29 (1990) 232. S. Uemiya, N. Sato, H. Ando, and E. Kikuchi, Ind. Eng. Chem. Res., 30 (1991) 585. M . C. Hausladen, M.S. Thesis, SUNY-Buffalo, 1992.

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