Gaseous mass transport in porous media through a stagnant gas
Post on 14-Feb-2017
Embed Size (px)
72 Ind. Eng. Chem. Res. 1987, 26, 72-77
xIw, xZw = as above, in water phase XAp, Xsp = adsorbed mass of AN and ST, respectively, per
unit mass of polymer, kg/kg of polymer XAW, XsW = dissolved mass of AN and ST, respectively, per
unit mass of water, kg/kg of water YA = p a / ( p A + p s ) , relative mole fraction of AN in the vapor
phase, mol of AN/mol of monomers 2 = global mass-fractional conversion ZAN = limiting conversion of AN in polymerization runs
(Figure 4) Zr, = limit mass-fractional conversion Greek Symbols u, /3,6 = liquid-liquid distribution coefficients of AN, ST, and
u, p = adsorption equilibrium constants of AN and ST, mass
ylo, y20, yH2O0 = activity coefficients of AN, ST, and H20,
yIw, yZw, T~~~~ = activity coefficients of AN, ST, and H20, 7, t , u, = in Kelen-Tudos method, defined in the text Superscripts o = organic phase w = aqueous phase sat = saturation P = polymer Subscripts 1, A = acrylonitrile 2, S = styrene
H,O, respectively, mass-fraction basis, eq 5-7
basis, eq 21 and 22
respectively, in organic phase
respectively, in water phase
L = limii min, max = minimum and maximum values in Kelen-Tudos
method Registry No. (AN)(ST)(copolymer), 9003-54-7; ST, 100-42-5;
Literature Cited Abbey, K. J. ACS Symp. Ser. 1981, 165, 345. Alonso, M.; Oliveres, M.; Puigjaner, L.; Recasens, F. Proceedings of
the 3rd Congress of International Information on Genie Chimique; Society of Chimie Industrielle: Paris, 1983; Vol. 1, p 47-1.
Ballard, M. J.; Napper, D. H.; Gilbert, R. G. J . Polym. Sci., Polym. Chem. Ed. 1981, 19, 934.
Comberbach, D. M.; Scharer, J. M.; Young, M.-Y. Chromatagr. Newsl. 1984, 12, 4.
Elias, H.-G. Macromolecules; Wiley: New York, 1977; Vol. 2. Fineman, M.; Ross, S. D. J . Polym. Sci. 1950, 5, 259. Gardon, J. L. Symp. Polym. React. Eng. 1972, 137. Guillot, J.; Rios, L. Makromol. Chem. 1982, 183, 1979. Guyot, A.; Guillot, J.; Pichot, C.; Rios, L. ACS Symp. Ser. 1981, 165,
Hanna, R. J. Ind. Eng. Chem. 1957, 49(2), 208. Haskell, V. C.; Settlage, P. H. Presented a t the AIChE Annual
Hatate, Y.; Nakashio, F.; Sakai, W. J . Chem. Eng. Jpn. 1971,4, 348. Hendy, B. N. Adv. Chem. Ser. 1975, 142, 115. Johnson, A. F.; Khaligh, B.; Ramsay, J. Presented at the National
Meeting of the American Chemical Society, New York, Aug 1981. Kelen, T.; Tudos, F. J . Macromol. Sci., Chem. 1975, A9, 1. Kikuta, T.; Omi, S.; Kubota, H. J . Chem. Eng. Jpn. 1976, 9, 64. Kiparissides, C.; McGregor, J. F.; Hamielec, A. E. Can. J . Chem.
Kolb, B. Applied Headspace Gas-Chromatography; Heyden: Lon-
Min, K. W.; Ray, W. H. J . Macromol. Sci., Chem. 1974, C I l , 177. Ray, W. H.; Gall, C. E. Macromolecules 1969, 2, 425. Redlich, 0.; Kister, A. T. Ind. Eng. Chem. 1948, 40, 345. Schork, F. J.; Ray, W. H. ACS Symp. Ser. 1981, 165, 505. Shaw, D. A.; Anderson, T. F. Ind. Eng. Chem. Fundam. 1983,22,79. Smith, W. V. J . Am. Chem. Sac. 1948, 70, 2177. Tirrell, M.; Gromley, K. Chem. Eng. Sci. 1981, 36, 367.
Meeting Chicago, 1970.
Eng. 1980, 58, 48.
Received for review April 22, 1985 Revised manuscript received January 21, 1986
Accepted March 13, 1986
Gaseous Mass Transport in Porous Media through a Stagnant Gas
Ali Una1 Department of Metallurgy and Materials Science, Imperial College of Science and Technology, London, SW7 2BP England
An analysis is presented for gas-phase mass transport in porous media through a stagnant gas. The natural flux ratio rule for isobaric conditions, which requires that the fluxes be inversely proportional to the square roots of the molecular weights, is not met in this system. Accordingly, a pressure difference develops and this superimposes viscous flow upon diffusion. Equations are derived for the flux and the pressure difference, and it is shown that even when the pressure difference is small, the contribution of viscous flow to the flux can be considerable. Flux and pressure difference measurements carried out on the transport of carbon dioxide through stagnant nitrogen in a septum of coarse pores (average pore size 2.5 pm) are in agreement, within experimental error, with those predicted by the equations and hence provide support for the theories available for describing combined transport in porous media.
Gas transport in porous media is an important factor in designing isotope separation equipment and nuclear reactors, in predicting reaction rates in catalysts and in gas and solid reactions, and in drying of porous solids. Accordingly, this field has received a considerable amount of attention over the past 2 decades, and the main prob- lems appear to have been sorted out. The dusty gas model developed by Mason and Malinauskas (1983) has been instrumental in the attainment of the present level of understanding.
In transport under isobaric conditions (is., diffusion only), the fluxes of the components in a mixture are not
independent; they are related by
CNi(Mi)1/2 = 0 (1)
Nl/N, = -t(M2/M1)12 (2)
which, for binary mixture, becomes
This relationship, first reported by Graham (1833), has been verified experimentally, and its applicability is not restricted to the Knudsen diffusion regime; it applies also in the transition and mutual diffusion regimes. Indeed, deviations from this ratio are frequently regarded as prime evidence of surface effects in mass transport.
0 1987 American Chemical Society
Ind. Eng. Chem. Res. Vol. 26, No. 1, 1987 73
The isobaric flux ratio of eq 1 and 2 is exact, provided that the mean free path is much larger than the diameter of the pores, for then Knudsen diffusion mechanism will be operating and the diffusivity is inversely proportional to the square root of the molecular weight. In the mutual diffusion regime, the ratio holds for a different reason: the total momentum transferred to the walls, which is pro- portional to Ni and the mean molecular momentum (re- lated to the square root of the molecular weight), by all the molecular collisions must be zero for isobaric condi- tions. The ratio is approximate in this case, however, but as discussed by Mason and Malinauskas (1983), this is a good approximation. Hence, the flux ratio equations are applicable to gaseous diffusion in porous media over all pressures and pore sizes. When the ratio of fluxes is fixed externally, as is the case in chemical reactions or in drying, the system itself creates pressure gradients to satisfy the requirement of zero net momentum transfer to the walls of the pores. Gas transport in such cases then involves both diffusion and viscous flow. Flux equations for de- scribing combined transport were derived by Mason and Malinauskas (1983) and Gunn and King (1969), and the total flux was shown to be simply the sum of viscous and diffusive fluxes.
The general flux equations have been used to account for the pressure difference observed in equimolar coun- tercurrent transport (Jackson, 1977) and have provided good agreement with measurements. McGreavy and Asaeda (1982) applied them to unsteady-state diffusion in monodisperse porous solids and showed that diffusion fluxes can induce total pressure gradients. These authors used a nonisobaric model to interpret their data and showed that the general flux equations applied well. Mason and Malinauskas (1983) have reviewed the various applications of the equations.
In the present work, the transport of one gas (1) through a stagnant second gas (2) was investigated. Equations were derived for the created pressure difference and for calcu- lating the flux. These equations were tested by experi- mental measurements of the flux and the pressure dif- ference for the transport of carbon dioxide through stag- nant nitrogen in a porous septum of monodisperse pores (average pore size 2.5 pm). Reasonable agreement with theory was obtained when the values of the Knudsen diffusivity and the viscous flow parameter, determined separately from permeability measurements, were used in the calculations. The present system is of relevance to the drying of porous solids and calcination reactions and has been used in some previous work for measuring diffusiv- ities (Turkdogan et al., 1971; Unal, 1986). It also provides an effective and simple steady-state test for the general flux equations.
Theory Pressure Gradient. The flux equation for combined
diffusion and flow of a binary gas mixture may be written as
By making use of the identities P1 = X l P P2 = x,P x1 + x2 = 1
and noting that N2 = 0 for the transport of species 1 through stagnant gas 2, we obtain the following equation between the gradients of composition and pressure when
1 ,.' / U L
Figure 1. Variation of the pressure difference created in diffusion through a stagnant gas with the mean pressure. T = 293 K, x,(O) = 0.2, xl(L) = 0, p = 15 X 10" Paes, Bol2 = 1.5 N d, and M = 28 kg kmol-I.
N, is eliminated between eq 3 and the corresponding equation for species 2,
where x1 x2 (Dm)-l = - + -
and p refers to the viscosity of the mixture. Assuming DM and p do not vary greatly across the porous septum and may be represented by suitable average values, eq 4 may be integrated to obtain
This equation predicts a negative pressure difference; that is, the pressure gradient (resulting from the composition gradient imposed) acts in such a way as to stop the transport of gas 2 that would be obtained under isobaric conditions.
Curves plotted from eq 6 for four porous media with pore sizes of 0.1, 0.5, 1, and 3 pm are shown in Figure 1. In these calculations, the composition dependencies of p and D K A were