High Permeability

The main problem with electrodes coated with membranes is the higher resistance to the diffusion of the chemical species and the slower electron transfer mechanism. Chemical species that are blocked by the membrane cannot be detected. Species that decrease their diffusion can be oxidized or reduced but they show lower electrochemical currents and, for irreversible processes, a shifted peak potential, as expected from classical theory of electrochemical mechanism. 

This problem can be overcome by using membranes prepared by electrospinning. The fibrous structure of membranes reduces the mass-transport resistance encountered by the chemical species passing through the channels of the membrane. As a result, electrodes coated with nanofibrous membranes offer, in principle, a negligible barrier to the analyte diffusion and high solvent permeability. 

There is not a general agreement if this morphology can be referred as porous. lUPAC defines pores as cavities, channels, or interstices which are deeper than they are wide [61, 62], Within this generous definition, the high permeability of the nanofibrous membranes can be attributed to their inherent high porosity. 

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Carbonate rocks are more frequently fractured than sandstones. In many cases open fractures in carbonate reservoirs provide high porosity / high permeability path ways for hydrocarbon production. The fractures will be continuously re-charged from the tight (low permeable) rock matrix. During field development, wells need to be planned to intersect as many natural fractures as possible, e.g. by drilling horizontal wells. 

The macroscopic sweep efficiency s the fraction of the total reservoir which is swept by water (or by gas in the case of gas cap drive). This will depend upon the reservoir quality and continuity, and the rate at which the displacement takes place. At higher rates, displacement will take place even more preferentially in the high permeability layers, and the macroscopic displacement efficiency will be reduced. 

If water or gas breakthrough occurs (in an oil well) from a high permeability layer it can dominate production from other intervals. Problems such as this can sometimes be prevented by initially installing a selective completion string, but in single string... 

In high permeability reservoirs, wells may produce dry oil for a limited time following a shut-in period, during which gravity forces have segregated oil and water near the wellbore. In fields with more production potential than production capacity, wells can be alternately produced and shut in (intermittentproduction) to reduce the field water cut. This may still be an attractive option at reduced rates very late in field life, if redundant facilities can be decommissioned to reduce operating costs. 

At the opposite limit of bulk diffusion control and high permeability, all flux models are required to he consistent with the Stefan-Maxwell relations (8.3). Since only (n-1) of these are independent, they are insufficient to determine all the flux vectors, and they permit the problem to be formulated in closed form only when they can be supplemented by the stoichiometric relations (11.3). At this limit, therefore, attention must be restricted from the beginning to those simple pellet shapes for ich equations (11.3) have been justified. Furthermore, since the permeability tends to infininty, pressure gradients within the pellet tend to zero and... 

It is seen that the pressure variation tends to zero when - , so In coarsely porous pellets with high permeability the pressure change Induced by reaction may be very small compared t/ith the absolute pressure. In this sense, then, the pellet approaches an isobaric system at high values of the permeability. 

The approximations of Kehoe and Aria and of Hite and Jackson (denoted "high permeability" in the diagrams) are indistinguishable from the exact results, while the method of Apecetche et al. introduces substantial error. 

Hite s treatment is based on equations (5.18) and (5.19) which describe the dusty gas model at the limit of bulk diffusion control and high permeability. Since temperature Is assumed constant, partial pressures are proportional to concentrations, and it is convenient to replace p by cRT, when the flux equations become... 

The same device was used in section 11.7 when discussing steady states at the limit of bulk diffusion control and high permeability. 

In section 11.4 Che steady state material balance equations were cast in dimensionless form, therary itancifying a set of independent dimensionless groups which determine ice steady state behavior of the pellet. The same procedure can be applied to the dynamical equations and we will illustrate it by considering the case t f the reaction A - nB at the limit of bulk diffusion control and high permeability, as described by equations (12.29)-(12.31). 

Addition of Inert Filter Aids. FUtet aids ate rigid, porous, and highly permeable powders added to feed suspensions to extend the appheabUity of surface filtration. Very dilute or very fine and slimy suspensions ate too difficult to filter by cake filtration due to fast pressure build-up and medium blinding addition of filter aids can alleviate such problems. Filter aids can be used in either or both of two modes of operation, ie, to form a precoat which then acts as a filter medium on a coarse support material called a septum, or to be mixed with the feed suspension as body feed to increase the permeabihty of the resulting cake. 

Fig. 38. Permeability as a function of molar volume for a mbbery and glassy polymer, illustrating the different balance between sorption and diffusion in these polymer types. The mbbery membrane is highly permeable the permeability increases rapidly with increasing permeant size because sorption dominates. The glassy membrane is much less permeable the permeability decreases with increasing permeant size because diffusion dominates (84).

Gas Permeability. Crystalline PMP is relatively highly permeable to various organic and inorganic gases. Permeabilities to oxygen, nitrogen. 

Because of the highly permeable nature of the ced wad of gram-positive organisms, they produce P-lactamases which are not only found throughout the ced wad, but also in the extracedular environment. Hence the extracedular P-lactamases can act on the antibiotic before the ced is entered. 

Toxicity. The toxicity of barium compounds depends on solubility (47—49). The free ion is readily absorbed from the lung and gastrointestinal tract. The mammalian intestinal mucosa is highly permeable to Ba " ions and is involved in the rapid flow of soluble barium salts into the blood. Barium is also deposited in the muscles where it remains for the first 30 h and then is slowly removed from the site (50). Very Httle is retained by the fiver, kidneys, or spleen and practically none by the brain, heart, and hair. 

Use of filter aids is a technique frequently applied for filtrations in which problems of slow filtration rate, rapid medium blinding, or un-satisfactoiy filtrate clarity arise. Filter aids are granular or fibrous solids capable of forming a highly permeable filter cake in which veiy fine solids or slimy, deformable floes may be trapped. Application of filter aids may allow the use of a much more permeable filter medium than the clarification would require to produce filtrate of the same quahty by depth filtration. 

State of the Art A desirable gas membrane has high separating power (ot) and high permeability to the fast gas, in addition to critical requirements discussed below. The search for an ideal membrane produced copious data on many polymers, neatly summarized by Robeson [J. Membrane ScL, 62, 165 (1991)]. Plotting log permeability versus log selectivity (ot), an upper bound is found (see Fig. 22-73) which all the many hundreds of data points fit. The data were taken between 20-50°C, generally at 25 or 35°C. 

A ferrite toroid or E core can be used for a drive transformer. No gap is needed since the input coupling capacitor guarantees that the core will operate in a bipolar fashion. A high permeability core is also suitable for this purpose. The wire that is going to be used will be in the range of 32 to 36 AWG. The core size will be approximately 0.4 to 0.6 inches (10 to 15mm). 

A high permeability ferrite is used such as the W material from Magnetics, Inc. which has a permeability of 10,000. 

Corrosion-resistant coatings and high-permeability separation membranes. 

The polyols used are of three types polyether, polyester, and polybutadiene. The polyether diols range from 400 to about 10,000 g/mol. The most common polyethers are based on ethylene oxide, propylene oxide, and tetrahydrofuran or their copolymers. The ether link provides low temperature flexibility and low viscosity. Ethylene oxide is the most hydrophilic and thus can increase the rate of ingress of water and consequently the cure rate. However, it will crystallize slowly above about 600 g/mol. Propylene oxide is hydrophobic due to hindered access to the ether link, but still provides high permeability to small molecules like water. Tetrahydrofuran is between these two in hydrophobicity, but somewhat more expensive. Propylene oxide based diols are the most common. 

The warming up of the mat is performed by the so-called steam shock effect [173,219]. A prerequisite for this is the high permeability of the particle or fiber mat. Higher moisture contents of the face layers and spraying of water sustain this effect. 

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