Membranes thickness

consider that the membrane thickness is reduced to 10 pm, thereby reducing the mass of palladium per unit area of membrane. Assuming that the initial 25 pm thick membrane of unit area contained US 2000 worth of palladium, the new 10 pm thick membrane will contain (US 2000)(10 pm/25 pm) or US 800 worth of palladium. The hydrogen flux will increase by the ratio 25 pm/10 pm or 2.5. Because the flux increases by a factor of 2.5, the total membrane area will be reduced by the same factor in order to keep the permeate hydrogen flow rate con- 

The data presented in Fig. 5.6 were derived from membrane modules using Pd- Cu foil (planar) membranes with the membrane area sized to deliver 0.78 Nm h i of product hydrogen. Even though palladium alloy membranes are often criticized as being too expensive for commercial applications due to the cost of palladium, it is clear that as membrane thickness is reduced to 10 pm and less, the cost associated with the value of palladium in the membrane can be very reasonable. 

if thin membranes are a good idea, what about ultra-thin membranes that are only 1 or 2 (tm thick. Unfortunately, fabrication and handling costs do not decrease linearly with reduction in thickness of the permselective membrane. Experience has shown that qualitatively the relationship between fabricated cost and membrane thickness is approximately as shown in Fig. 5.7. Initially, as the thickness of the membrane is reduced, there is a corresponding reduction in the cost, largely driven by a reduction in the mass of expensive palladium used in the membrane. However, as the membrane thickness is reduced to approximately 5 i-im, the fabrication cost increases faster than can be offset by the reduction in the amount of palladium. A major reason that fabrication costs increase so dramatically is the reduction in the yield of satisfactory membrane product (i.e., membrane product without pinholes, tears, and similar defects). The membrane must be free of defects which would compromise the permselectivity of the membrane. This requirement becomes increasingly difficult to satisfy as membranes become exceptionally thin. 

as membranes, whether tubular or in the form of a flat membrane foil, become exceptionally thin ( 5 pm), the handling of the membranes becomes increasingly difficult, leading to additional expense in the fabrication of the membrane modules. Also, the probability of damaging membranes during assembly of the modules increases, resulting in yield declines and corresponding cost increases. 

If the permselective membrane is a thin metal layer deposited onto a porous support, the challenge of making defect-free metal coating increases as the thickness of the layer is reduced. This challenge is compounded by the fact that the underlying support is porous and rough. The net effect is reduced yield of acceptable membranes and increased cost of fabricating membrane modules. 

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Although microporous membranes are a topic of research interest, all current commercial gas separations are based on the fourth type of mechanism shown in Figure 36, namely diffusion through dense polymer films. Gas transport through dense polymer membranes is governed by equation 8 where is the flux of component /,andare the partial pressure of the component i on either side of the membrane, /is the membrane thickness, and is a constant called the membrane permeability, which is a measure of the membrane s ability to permeate gas. The ability of a membrane to separate two gases, i and is the ratio of their permeabilities,a, called the membrane selectivity (eq. 9). 

The primary site of action is postulated to be the Hpid matrix of cell membranes. The Hpid properties which are said to be altered vary from theory to theory and include enhancing membrane fluidity volume expansion melting of gel phases increasing membrane thickness, surface tension, and lateral surface pressure and encouraging the formation of polar dislocations (10,11). Most theories postulate that changes in the Hpids influence the activities of cmcial membrane proteins such as ion channels. The Hpid theories suffer from an important drawback at clinically used concentrations, the effects of inhalational anesthetics on Hpid bilayers are very small and essentially undetectable (6,12,13). 

PVC. Poly(vinyl chloride) (PVC), a very versatile polymer, is manufactured by the polymerisation of vinyl chloride monomer, a gaseous substance obtained from the reaction of ethylene with oxygen and hydrochloric acid. In its most basic form, the resin is a relatively hard material that requites the addition of other compounds, commonly plasticisers and stabilisers as well as certain other ingredients, to produce the desired physical properties for roofing use. The membranes come in both reinforced and nonreinforced constmctions, but since the 1980s the direction has been toward offering only reinforced membranes. The membrane thickness typically mns from 0.8—1.5 mm and widths typically in the range of 1.5—4.6 m. 

The simplest osmotic dosage form, ALZA Corporation s OROS elementary osmotic pump (Fig. 7), combines the dmg and sometimes an osmotic agent in a monolithic core and deflvers the dmg in solution (102). The mass dehvery rate with time dm df) of the dmg solution is described by equation 4, where is the hydrauHc permeabiUty of the membrane, a is the membrane reflection coefficient, Atz is the osmotic pressure gradient, APis the hydrostatic back pressure, A is the area of the membrane, C is the dissolved concentration of the dmg, and b is the membrane thickness. 

An important consideration is that the goodness of the separation is almost independent of the membrane thickness, z, and the rate of the process is inversely proportional to z, giving rise to a major emphasis on maldng the separating layer of a membrane very thin. It is rare that z is known for a commercial membrane, and Jj is stated without regard to z. 

Flux is the term used to describe how fast a product passes through a membrane. A velocity, almost always reported as volume/area-time, it does not take membrane thickness into account. For most users. 

Membrane thickness is a factor in microbial retention, Tortiioiis-pore membranes rated at 0,22 jlrn tvpicallv have surface openings as large as 1 jlrn (Fig, 22-71), Narrovv er restrictions are found beneath the surface. In challenge tests, P. diminnta organisms are found vv ell beneath the surface of an 0,2 jlrn membrane, but not in the permeate. 

The liquid membrane (thickness 0.2 cm) was separated from the aqueous solutions by two vertical cellophane films.The electrode compartments were filled with 0.05 M sulfuric acid solutions and were separated by the solid anion-exchange membranes MA-40. Binary mixtures contained, as a mle, 0.04 M Cu(II) and 0.018 M Pt(IV) in 0.01 M HCl. 0.1 M HCl was used usually as the strip solution. 

T is a time lag term, easily read from the test h is membrane thickness... 

FIGURE 14 In vitro rate of release of testosterone from a PCL capsule (reservoir device), illustrating rate control by drug dissolution when the polymer membrane thickness is small. (From Ref. 68.)... 

Methanol crossover was also investigated as a function of the ionomeric membrane thickness " and it was found to decrease with... 

The membranes also have a certain, though small ionic conductivity. The electrical resistance of membranes when referred to unit surface area is 10 to 10 Q-cm, which when allowing for the small membrane thickness (about 10 nm) corresponds to the rather high value of specific (volume) resistance of 10 to 10 Q-cm. 

In accordance with observed data, this model shows that water flux increases linearly with applied pressure AP, decreases with higher salt concentration through its impact on osmotic pressure Jt, increases with a smaller membrane thickness I, and increases with temperature through the temperature dependence of the water permeability P . The model also demonstrates that the solute or salt flux J, increases linearly with applied pressure AP, increases with higher salt concentration c , increases with a smaller membrane thickness I, and increases with temperature through the temperature dependence of the solute permeability Pj. Polarization, as described early in this section, causes the wall concentration c to exceed the bulk concentration ci,. 

Such bimetallic alloys display higher tolerance to the presence of methanol, as shown in Fig. 11.12, where Pt-Cr/C is compared with Pt/C. However, an increase in alcohol concentration leads to a decrease in the tolerance of the catalyst [Koffi et al., 2005 Coutanceau et ah, 2006]. Low power densities are currently obtained in DMFCs working at low temperature [Hogarth and Ralph, 2002] because it is difficult to activate the oxidation reaction of the alcohol and the reduction reaction of molecular oxygen at room temperature. To counterbalance the loss of performance of the cell due to low reaction rates, the membrane thickness can be reduced in order to increase its conductance [Shen et al., 2004]. As a result, methanol crossover is strongly increased. This could be detrimental to the fuel cell s electrical performance, as methanol acts as a poison for conventional Pt-based catalysts present in fuel cell cathodes, especially in the case of mini or micro fuel cell applications, where high methanol concentrations are required (5-10 M). 

Membrane thickness fluctuations were initially discussed in the local approach by Hladky and Gruen (HG) [102] in conjunction with their possible effect on membrane capacitance. They are directly related to the spectrum of 5-modes ... 

Further progress in understanding membrane instability and nonlocality requires development of microscopic theory and modeling. Analysis of membrane thickness fluctuations derived from molecular dynamics simulations can serve such a purpose. A possible difficulty with such analysis must be mentioned. In a natural environment isolated membranes assume a stressless state. However, MD modeling requires imposition of special boundary conditions corresponding to a stressed state of the membrane (see Refs. 84,87,112). This stress can interfere with the fluctuations of membrane shape and thickness, an effect that must be accounted for in analyzing data extracted from computer experiments. 

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