A comparative structural study of cracking catalyst, porous glass, and carbon plugs by surface and volume flow of gases

1959 ◽  
Vol 251 (1266) ◽  
pp. 353-368 ◽  
Keyword(s):  
Steady Flow ◽  
Gas Phase ◽  
Surface Diffusion ◽  
Porous Glass ◽  
Diffusion Coefficients ◽  
Transient Flow ◽  
Time Lag ◽  
Surface Flow ◽  
Microporous Media ◽  
Pore Properties

Transient and steady states of volume and surface flow of gases and vapours in microporous media require for their description four different diffusion coefficients, namely, a surface and a gas-phase coefficient for transient and a surface and gas-phase coefficient for steady flow. The distinction between steady and transient flow arises in part because of the role of blind pores. Similarly, two different porosities ∊ and ∊ 8 and two different surface areas A and A 8 govern transient and steady-state flow respectively. By comparing the two gas-phase and the two surface diffusion coefficients with gas-phase and surface diffusion coefficients in a smooth cylindrical capillary having ∊/ A equal to that for the porous medium, four structure factors can be defined, in terms of which the pore properties of microporous media can be discussed. Data are presented for sorption, transient flow (time lag) and steady flow of He, Ne, A and N 2 in compressed alumina-silica catalyst plugs. These, and previously obtained data for Vycor porous glass (Barrer & Barrie 1952) and Carbolac carbon plugs (Barrer & Strachan 1955) have been analyzed according to the above procedure, and pore properties compared. In this way it was possible to obtain information regarding pore structure which could not have been derived by other methods.

Sorption and diffusion of simple paraffins in silica-alumina cracking catalyst

1960 ◽  
Vol 256 (1285) ◽  
pp. 267-290 ◽  
Keyword(s):  
Surface Diffusion ◽  
Diffusion Coefficients ◽  
Time Lag ◽  
Transient State ◽  
Surface Flow ◽  
Sorption Data ◽  
Silica Alumina ◽  
Pore Properties ◽  
Permanent Gases ◽  
And Diffusion

Sorption and diffusion measurements have been made of methane, ethane and propane in the silica-alumina cracking catalyst previously employed in similar measurements for permanent gases (Barrer & Gabor 1959). The sorption data show no evidence of molecular sieve behaviour in the micropore structure of the catalyst, for the species studied. The ratio of surface to volume flow increases with increasing molecular weight of hydrocarbons and with decreasing porosity. On the other hand, in or near the Henry’s law region of sorption, this ratio decreases as the temperature rises. In a micropore system there are many points where surface flow may be interrupted by roughnesses, blind pores or crevices. A treatment has been given which permits the influence of such pore properties upon surface diffusion to be considered. The procedure developed has been applied to the data obtained in this research, and estimates have been made of some properties of the surface diffusion coefficients which could be expected upon uninterrupted surfaces of the catalyst. Owing to differences in the roles of pores in steady and transient states of flow, transient state (time-lag) diffusion coefficients could depend on time. Some consequences of this have also been examined.

An experimental study of combined gas phase and surface diffusion in porous glass

1998 ◽  
Vol 140 (2) ◽  
pp. 165-184 ◽  
Author(s):  
Axel Tuchlenski ◽  
Petr Uchytil ◽  
Andreas Seidel-Morgenstern
Keyword(s):  
Experimental Study ◽  
Gas Phase ◽  
Surface Diffusion ◽  
Porous Glass

Diffusion and flow of gases and vapours through micropores II. Surface flow

1950 ◽  
Vol 203 (1073) ◽  
pp. 165-178 ◽  
Keyword(s):  
Surface Diffusion ◽  
Gaseous Phase ◽  
Diffusion Coefficients ◽  
Activation Energies ◽  
Surface Flow ◽  
Adsorbent Surface ◽  
Flow Through

Flow through porous plugs has been measured under conditions where surface flow of adsorbed gases makes an important contribution. An analysis of variables affecting surface flow enables surface diffusion coefficients to be calculated and has indicated conditions likely to reduce the correction for flow in the gaseous phase to small proportions. This has been confirmed by studying the effect of variations of porosity. Surface diffusion coefficients tend to increase with ‘coverage' of the adsorbent surface. This requires further study. Activation energies measured for one system were found to be rather small. Owing to the variation of surface diffusion coefficients with coverage, it proved difficult to obtain accurate values. Activation energies indicated a tendency to decrease as coverage increases. Further experiments on these points are also being carried out.

Sorption and surface diffusion in microporous carbon cylinders

1955 ◽  
Vol 231 (1184) ◽  
pp. 52-74 ◽  
Keyword(s):  
Surface Diffusion ◽  
Diffusion Coefficients ◽  
Surface Concentration ◽  
Surface Flow ◽  
Carbon Powder ◽  
Type I ◽  
Equilibrium Studies ◽  
Type Iv ◽  
Internal Surfaces ◽  
Helium Neon

An investigation has been made of adsorption and diffusion of helium, neon, hydrogen, argon, nitrogen and krypton in microporous cylinders of carbon compressed to different porosities e, and having very high internal surfaces, A . On the uncompressed carbon powder sorption of argon occurred at 90 and 78° K without hysteresis; in a compressed cylinder with e = 0.64 cm 3 per cm 3 and 2e A = 18.8 Å the isotherms changed from type II to type IV in Brunauer’s classification and a pronounced hysteresis loop appeared. In a highly compressed cylinder with e = 0.37 cm 3 per cm 3 and 2e/ A = 6.5 Å hysteresis again disappeared and the isotherms changed from type IV nearly to type I. The areas A and heats of sorption were measured, and special attention was paid to sorption equilibria involving dilute films of nitrogen, argon and krypton between 273 and 323° K. Surface and volume diffusion were then investigated over the same temperature interval, 273 to 323° K, in the compressed cylinders already carefully characterized by the equilibrium studies. Surface diffusion coefficients, D s , were evaluated for all the gases including helium. The properties of D s indicate that dilute films of helium, neon and hydrogen are best regarded as one- or two-dimensional gases but that argon, nitrogen and krypton still give localized adsorbed films in which surface flow is by jumps, each unit diffusion requiring an energy of activation which was 0.53 to 0.62 of the heat of adsorption. It was further shown that adsorption isotherms in the Henry’s law region can be measured by flow methods, and are in agreement with those obtained by direct measurement. The surface diffusion coefficients increase and the activation energies for diffusion decrease for argon, nitrogen and krypton as the surface concentration of adsorbate increases. In some cases, especially for krypton, surface flow was much the most important mode of transport within the micropore system.

Sorption and surface flow in graphitized carbon membranes - I. The steady state

1967 ◽  
Vol 299 (1459) ◽  
pp. 434-454 ◽  
Keyword(s):  
Gas Phase ◽  
Diffusion Coefficients ◽  
Surface Flow ◽  
Inert Gases ◽  
Sulphur Hexafluoride ◽  
Carbon Membranes ◽  
Carbon Particles ◽  
Influence Of Temperature On ◽  
Pass Through ◽  
The Individual

A study has been made of the flow of inert gases and of sulphur hexafluoride through, and of the adsorption of argon and sulphur hexafluoride on, two carbons chosen for the energetic homogeneity of the sorbing surfaces and non-porosity of the individual carbon particles. A wide temperature range has been covered, ‘surface’ and ‘gas phase’ flows of the sorbed gases have been measured using helium as non-sorbed calibrating gas, and the corresponding permeability and diffusion coefficients derived. The smooth, non-porous sorbents chosen were able to produce extra flows considerably greater than those on much larger but porous and broken surfaces of some other sorbents (e. g. active carbons and alumina-silica cracking catalyst). The concentration dependence of the surface diffusion coefficients, D ss , agrees with a treatment of this dependence according to which, in the regime when gas phase diffusion coefficients are independent of pressure, D ss varies inversely as the slope of the isotherm. When fluxes at each of a series of pressures are plotted against temperature, the fluxes pass through maxima as the temperature is lowered. This behaviour can be explained in terms of the simultaneous influence of temperature on D ss and on the concentration gradients.

Gas phase transport, adsorption and surface diffusion in a porous glass membrane

2005 ◽  
Vol 104 (2-4) ◽  
pp. 344-351 ◽  
Author(s):  
Jianhua Yang ◽  
Jiřina Čermáková ◽  
Petr Uchytil ◽  
Christof Hamel ◽  
Andreas Seidel-Morgenstern
Keyword(s):  
Gas Phase ◽  
Surface Diffusion ◽  
Porous Glass ◽  
Glass Membrane ◽  
Gas Phase Transport ◽  
Phase Transport

Sorption and surface diffusion in porous glass

1952 ◽  
Vol 213 (1113) ◽  
pp. 250-265 ◽  
Keyword(s):  
Surface Diffusion ◽  
Porous Glass ◽  
Diffusion Coefficients ◽  
Hysteresis Loops ◽  
Transient State ◽  
Substantial Contribution ◽  
Henry's Law ◽  
Henry’S Law ◽  
Type Iv ◽  
Adsorbed Films

Sorption, gas-phase diffusion in the region of molecular streaming, and surface diffusion in very dilute adsorbed films have been studied quantitatively for a number of gases in membranes of porous glass. The sorption of oxygen, nitrogen, argon, sulphur dioxide and ammonia near their liquefying temperatures resulted in type IV isotherms with very characteristic hysteresis loops. From the isotherms porosity, surface area and mean pore radius were evaluated. Heats of sorption, Δ H , have been obtained in very dilute adsorbed films and in films where v/v m approaches or exceeds one. These heats showed the porous glass to be an energetically non-uniform sorbing surface. The Henry’s law region of sorption was studied and Henry’s law sorption constants evaluated at 273, 290, 323 and 343° K for the most dilute adsorbed films of oxygen, nitrogen, argon, krypton, methane and ethane. Nearly all this information is essential for measurements of surface diffusion by the transient state method of Barrer & Grove (1951). The micropore structure was so fine that non-sorbed gases diffused within the porous glass only by molecular streaming at pressures up to half an atmosphere or more. Effects of sorption upon this diffusion were inappreciable for helium, neon and possibly hydrogen; for oxygen, nitrogen, argon, krypton, methane and ethane the influence of sorption upon timelags and diffusion coefficients became progressively more marked. In the steady state of flow there was little evidence of surface diffusion when, in the most dilute adsorbed films, Henry’s law is obeyed. On the other hand, in the transient state of flow, surface diffusion makes a substantial contribution to flow. Reasons are given for the difference in the extent to which surface mobility can be detected in the two states of flow. Consistent values of the surface diffusion coefficients, D 8 , were obtained for oxygen, nitrogen, argon, krypton, methane and ethane. These values of D 8 were compatible with the Arrheniusequation D 8 = D 0 exp ( – E / RT ) over the temperature range of 273 to 343° K investigated. The ratio of E to Δ H for very dilute adsorbed films lay between 0·5 and 0·6, and should in order of magnitude be characteristic of a heterogeneous sorbing surface. The numerical values of D 8 have been compared with D for liquids, and analyzed in terms of entropy and energy of activation.

Sorption and surface flow in graphitized carbon membranes - II. Time-lag and blind pore character

1968 ◽  
Vol 304 (1479) ◽  
pp. 407-425 ◽  
Keyword(s):  
Diffusion Coefficient ◽  
Time Dependence ◽  
General Expression ◽  
Diffusion Coefficients ◽  
General Equation ◽  
Time Lag ◽  
Surface Flow ◽  
Time Lags ◽  
Carbon Membranes ◽  
Graphitized Carbon

A general expression has been obtained for the time-lag in transport through membranes, based only on the conservation of mass condition and independent of any assumed equation of flow. When considered in relation to Fick’s equation the time-lag expression covers all situations in which diffusion coefficients, D , are functions of concentration, distance or time, or combinations of these variables. When D is a function only of concentration, C , two new ways have been given for exact treatments of the time-lag. These avoid difficulties which arise in using the formula originally given by Frisch (1957) and previously used in time-lag studies. Differences can arise between time-lags where D is a function only of C and the time lag L given by the general equation. These differences serve for the study of non-Fickian diffusions in which D is a function also of distance or time. This treatment, applied to a number of microporous membranes, leads to the conclusion that in these membranes non-Fickian components sometimes arise which are mostly dominated by time-dependence in the overall diffusion coefficient. It is shown how this behaviour can result from the partial blind-pore character of the channels in the membranes. In this and other ways the time-lag has been shown to give information about micropore systems which is not readily found from other measurements.

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