BARRER control

DRAWING NUMBER ENERGY AMOUMl TYPE BARRERS (CONTROLS POTENTIAL TARGETS RAC NO. UIALYSS OF BARRIER EFFECTIVENESS RECOMMENDED ACTK3NS COKTROLLED RAC APPLICABLE STANDARDS... 

From the condition 21a it immediately follows that if the clathrate is formed in the presence of a number of compounds which are potential solutes, i.e., sufficiently small to have 0 for some i, all these compounds contribute to its stability. As has already been pointed out by Barrer and Stuart4 this at once explains the stabilizing influence of "Hilfsgase" such as air, C02, or H2S on the formation of gas hydrates discussed by Villard49 and von Stackelberg and Meinhold.47 If there is only one solute, Eq. 21a with the = sign determines the minimum vapor pressure fiA necessary to make the clathrate stable relative to Qa. Since all cavities contribute to the stabilization, one cannot say that this minimum pressure is controlled by a specific type of cavity. 

T iffusion in porous pellets is often the rate-limiting process in industrial adsorption or catalytic processes. Much useful work in this field has been done by Smith and coworkers (3, 5), but for molecular sieve pellets the situation is complicated by diffusion in the zeolite crystal itself, as well as through the pores formed between the crystals. Few studies have been made of zeolite crystal diffusion, but Barrer and Brook (1) reported some results on diffusion of simple gases in various cation-substituted mordenites, and Wilson (7) gives some indirect results from the study of separation of CO2 from air using a fixed bed of type 4A zeolite pellets. In the present work, results have been obtained by studying self-diffusion of CO2 in a single pellet of type 5A zeolite under controlled conditions. The experimental results were fitted satisfactorily by a very simplified model of the pellet structure, which made it possible to deduce approximate values of the self-diffusion coefficients for both pore and crystal diffusion. 

In the experiments, both the feed and the sweep gases were mixed with controlled amounts of water before they entered the permeation cell. The effects of water content in gases on the membrane separation performance at 120 and 150 °C were investigated using a membrane with a thickness of -70pm on the BH A microporous Teflon support. The feed gas consisted of 20% C02, 40% H2, and 40% N2 (on dry basis). Figure 9.7 depicts the C02 permeability as a function of the water content on the sweep side at 120 and 150 °C. As the water content on the sweep side increased, C02 permeability increased almost linearly. When the water mole concentration increased from 58% to 93%, the C02 permeability increased from 3700 Barrer to as high as 8200 Barrer at 120 °C and from 920-2700 Barrer at 150 °C. 

Because of the rigid crystal structure and small window size, ionic diffusion in zeolites is slow and the activation energy is high (Barrer, 1980). Except on samples of very fine particle size, the exchange rate is controlled by intracrystalline rather than liquid-phase mass transfer. 

In this way, Barrer and Riley 46) and Barrer and Brook 47) obtained apparent diffusion coefficients that were decreasing with concentration for propane, butane, and other gases in chabazites. However, the initial slopes frequently cannot be measured accurately if only very small crystals are available. In one case, it was found that diffusion in the macropores of pellets controls the rate of sorption 48) in another case at least the rate of desorption seems to be influenced by the transfer through the phase boundary, as different diffusivities were reported for sorption and desorption 49). The determination of diffusivities from sorption rates is thus impeded by several difficulties. The results that have been obtained thus far can be generalized only with respect to a few points ... 

E. M. Flanigen 1) We can only attribute differences in the results of Barrer and Marshall and those reported here to variation in the method of preparing the gel network, and in some cases perhaps to different crystallization temperatures. We believe the critical point in achieving framework substitution is incorporation of all of the phosphate in the insoluble gel network, rather than in a soluble phosphate form. This is controlled by the exact method of coprecipitation and gelation. 

This volume is a complete progress report on the various aspects of zeolite systhesis on a molecular level. It provides many examples that illustrate how zeolites can be crystallized and what the important parameters are that control crystallization. Forty-two chapters cover such topics as crystallization techniques gel chemistry crystal size and morphology the role of organic compounds and novel synthesis procedures. It offers a complete review of zeolite synthesis as well as the lastest finding in this important field. Contains benchmark contributions from many notable pioneers in the field including R. M. Barrer. H. Robson, and Robert Milton. 

Some authors have been concerned with influence of flow or diffusion on measurements of and Anderson et al. discussed diffusion of spins between compartments, characterized by different states of longitudinal magnetization, leading to diffusion-driven longitudinal relaxation. The effects were explored experimentally and analyzed quantitatively. Herold and co-workers described an on-line NMR rheometer, able to measure NMR relaxation data. The corrections required for the analysis of relaxation data measured under flow conditions were discussed. The opposite problem how to avoid the detrimental effects of unequal relaxation rates on the diffusion measurements in complex mixtures - was discussed by Barrere et Relaxation can also cause problems in other kinds of NMR experiments. Skinner and co-workers described the optimal control design of band-selective excitation pulses that accommodates both relaxation and inhomogeneity of rf fields. 

Case 3. Barrer (i4a) treated the problem of diffusion through a slab of thickness Z, when in addition to pure diffusion two other rate-controlling surface processes are important ... 

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