Dead-end mode

Gas permeance through the membranes was measured in the pressure-controlled dead-end mode [18], The disc-shaped membranes were placed in the commercially permeance cells, K250 and K500 as mentioned before. Maximum operation temperatures were 300 and 600°C respectively. The membrane was fitted in the cell with the microporous top-layer at the gas feed side. The pressure difference over the membrane was adjusted by an electronic pressure controller. The gas flow through the membrane was measured by electronic mass flow meters. A schematic representation of the permeance set-up is given in Figure 5. 

Hollow fibers can be created with the dense side on the inside or lumen of the fiber or on the outside of the fiber, or they can be doubleskinned, where both the lumen and the outside of the fiber are dense. Location of the denser side of the membrane determines whether the service flow is outside-in or inside-out. Outside-in systems are typically used in a dead-end mode (or some variation thereof), while... 

Operation Pressurized configurations can be operated in either cross-flow or dead-end modes while submerged configuration are essentially only operated in dead-end mode. Submerged systems operate with outside-in flow, while pressurized can be either outside-in or inside out flow. 

Microfiltration is a unit operation for the separation of small particles. The separation limits are between 0.02 and 10 (jum particle dimensions. Microfiltration can be carried out in a dead-end mode and a cross-flow mode. In downstream processing, the cross-flow filtration is carried out continuously or discontinuously. The most important parameters that determine the productivity of cross-flow microfiltration are transmembrane pressure, velocity, particle size and surface, viscosity of the liquid and additives such as surfactants, and changing the surface and surface tension. 

Membrane operations are conducted either in a direct flow filtration (also called dead end) mode or in a tangential flow filtration (TFF) mode. Direct flow filtration is simple and easy to implement but has limited capacity for applications with high-solid mass. TFF is capable of processing large-solid masses but is more complex and capital intensive. 

Two process modes, namely, dead-end and cross-flow modes, are widely used for microfiltration (14). For the dead-end mode, the entire solution is forced through the membrane. The substances to be separated are deposited on the membrane, which increases the hydraulic resistance of the deposit. The membrane needs to be renewed as soon as the filtrate flux no longer reaches the required minimum values at the maximum operation pressure. This mode is mostly used for slightly contaminated solutions, e.g., production of ultra-pure water. For the cross-flow mode, the solution flows across the membrane surface at a rate between 0.5 and 5.0 m/s, which prevents the formation of a cover layer on the membrane surface. A circulation pump produces the cross-flow velocity or the shear force needed to control the thickness of the cover layer. The system is most widely used for periodic back flushing, where part of the filtrate is forced in the opposite direction at certain intervals, and breaks up the cover layer. The normal operating pressure for this mode is 1-2 bars. 

The simplest design is a dead-end operation, as shown in Fig. 16A. As the feed is forced through the membrane, the concentration of rejected components in the feed increases and accumulates at the membrane interface, hence the permeate quality decreases with time. Therefore, for industrial applications, a cross-flow operation, as shown in Fig. 16B, is preferred for its lower fouling tendency comparing to the dead-end mode. 

Fig. 4.2 A scheme of a generic pure hydrogen feeding sub-system in flow-through mode. Without ejectors the same scheme refers to dead-end mode...

Membrane Formation. In earlier work. 2.) it was found that fumed silica particles could be dispersed in aqueous suspension with the aid of ultrasonic sound. Observations under the electron microscope showed that the dispersion contained disc-like particles, approximately 150-200 1 in diameter and 70-80 1 in height. Filtration experiments carried out in the "dead-end" mode (i.e., zero crossflow velocity) on 0.2 urn membrane support showed typical Class II cake formation kinetics, i.e., the permeation rate decreased according to equation (12). However, as may be seen from Figure 7, the decrease in the permeation rate observed during formation in the crossflow module is only t 1, considerably slower than the t 5 dependence predicted and observed earlier. This difference may be expected due to the presence of lift forces created by turbulence in the crossflow device, and models for the hydrodynamics in such cases have been proposed. 

Conventional filtration processes are usually performed in a dead-end mode. By contrast, membrane separations are designed to be operated in a cross-flow mode (Figure 2). Such a turbulent cross-flow prevents an undesired concentration-polarization of the membrane, i.e., a concentration of solutes at the membrane... 

FIGURE 23.6 Schematic drawing of a module of membrane filtration used in laboratory scale in perpendicular to the dead-end mode. 

Dead-end Mode of operation where there is only one feedstream and one outlet stream (the filtrate of permeate). Refers to modules that do not have a means for cross-flow, e.g. stirred cells. Cross-flow modules could be operated in the dead-end mode by shutting off the reject outlet of the module. 

Mo90teguy P, Druart F, Bultel Y, Besse S, Rakotondrainibe A (2007) Monodimensional modeling and experimental study of the dynamic behavior of proton exchange membrane fuel ceil stack operating in dead-end mode. J Power Sources 167 349-357... 

These in sim fabricated membranes are able to overcome most of the drawbacks associated with the devices that use prefabricated membranes. In addition, depending on the material, designers have a high degree of flexibility with respect to the pore size distribution of the membrane. Moreover, the location of their pores is such that these membranes can be used in a cross-flow mode - a mode of operation that (as will be discussed later in the entry) is more effective than the other commonly used (dead-end) mode. The main drawback is that fabricating devices with such pores is difficult and expensive. Also, and perhaps more importantly, it has been possible to fabricate such devices in only a few limited materials. 

As shown in Fig. 5, there are two main modes in which a filter may be operated - the dead-end mode and the cross-flow mode. In the dead-end mode, the particle-laden fluid is directed normally against the Alter surface. While particles of size larger than that of the pores are prevented from passing through, all of the fluid passes through the filter. The particles being excluded remain on the upstream end of the Alter, and when a large number of them are accumulated, they impede further fluid flow. Hence, dead-end filtration is typically used for small sample volumes or suspensions with a small number of suspended solids. 

For most microfluidic applications involving beads and for removing contaminant particulates from environmental samples, filters are operated in the dead-end mode. Also to purify small volumes ( 100 pi) for one-shot diagnostic... 

Duveen (1998) has suggested the use of a venturi loop reactor for oxidations with pure oxygen in a manner exactly analogous to the Praxair Uquid oxidation reactor. The operation in a dead-end mode has been claimed to produce practically no vent gas. It must be noted here that burning of acetic acid and the consequent products cannot be avoided. CO and other products formed must be purged. To this extent, the operation is likely to be similar to the Praxair liquid oxidation reactor. 

To validate the proposed equivalent circuit other, additional EIS has been performed, for example flooding the cathode in ihe dead end mode of operation of the cathode. The fuel cell was operated over 8 h in the galvanostatically mode of operation at constant load, so that one can assume that the impedance change with time can be attributed to the change of and in the cathode, as shown by Schiller et al. [2001]. 

For industrial applications, a cross-flow operation is preferred because of the lower fouling tendency relative to the dead-end mode (figure VIII - 14b). In the cross-flow operation, the feed flows parallel to the membrane surface with the inlet feed stream entering the membrane module at a certain composition. The feed composition inside the module changes as a function of distance in the module, while the feed stream is separated into two a permeate stream and a retentate stream. The consequences of fouling in dead-end systems are shown schematically in figure VUI - 15. In dead-end filtration, the cake grows with time and consequently the flux decreases with time.Hux decline is relatively smaller with cross-flow and can be controlled and adjusted by proper module choice and cross-flow velocities. 

Figure S.l Schematic diagram of filler in dead-end mode...

Figure III.9. Modes of reactant supply a) dead-end mode, b) flow-through mode, c) recirculation mode.

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