Transmembrane, 235 velocity

Conductivity. Conductivity is an electrical property of excitable tissue which ensures that if one area of a membrane is excited to full activity, that area excites adjacent areas. Conduction of an impulse varies direcdy with the rate of development of phase 0 and the ampHtude of the action potential. Phase 0 is faster, and ampHtude of the action potential is greater, the more negative the transmembrane potential at the time of initiation of the impulse. Conduction velocity is faster when phase 0 is fast. 

Decoupled Driving Force and Depolarization Needs for improved fractionation motivate designers to reduce autofiltration. Using fluid velocity for depolarization means that hydrodynamic pressure drop will be additive to the transmembrane pressure driving force. Schemes to hmit this effeci confront a harsh economic reahty. Two novel schemes decouple the driving from the depolarizing force. 

The factors to consider in the selection of crossflow filtration include the flow configuration, tangential linear velocity, transmembrane pressure drop (driving force), separation characteristics of the membrane (permeability and pore size), size of particulates relative to the membrane pore dimensions, low protein-binding ability, and hydrodynamic conditions within the flow module. Again, since particle-particle and particle-membrane interactions are key, broth conditioning (ionic strength, pH, etc.) may be necessary to optimize performance. 

Cross-flow filtration systems utilize high liquid axial velocities to generate shear at the liquid-membrane interface. Shear is necessary to maintain acceptable permeate fluxes, especially with concentrated catalyst slurries. The degree of catalyst deposition on the filter membrane or membrane fouling is a function of the shear stress at the surface and particle convection with the permeate flow.16 Membrane surface fouling also depends on many application-specific variables, such as particle size in the retentate, viscosity of the permeate, axial velocity, and the transmembrane pressure. All of these variables can influence the degree of deposition of particles within the filter membrane, and thus decrease the effective pore size of the membrane. 

Maiorella et al. [93] observed that fouhng behavior was dependent on cell size. For smaller cells or suspensions containing significant levels of debris, high flux rates could not be maintained without inducing high transmembrane-pressures. This behavior is in qualitative agreement with the hydrodynamic hft theory, since lift velocity is predicted to increase with particle diameter to the second [100] or third power [101]. 

The change of flux velocity with transmembrane pressure can be explained by the concentration polarisation phenomenon. The physical processes at the membrane surface during the filtration procedure may be described by theo-... 

The performance of a hollow-fiber or sheet bioreactor is primarily determined by the momentum and mass -transport rate [15,16] ofthe key nutrients through the biocatalytic membrane layer. Thus, the operating conditions (transmembrane pressure, feed velocity), the physical properties of membrane (porosity, wall thickness, lumen radius, matrix structure, etc.) can considerably influence the performance of a bioreactor, the... 

Fluid-dynamic operating conditions, such as axial or angular velocity (i.e., shear stress that determines drag force value) and transmembrane pressure (that determines disperse-phase flux, for a given disperse-phase viscosity and membrane... 

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. 

In this situation, if the pressure filtration stays unchanged, the filtrate rate will decrease with time. When unacceptable values of the filtrate rate are reached, the process must be stopped and the membrane cleaned or replaced. This mode of operation is uneconomical. One solution to this problem is to increase the transmembrane pressure in order to maintain the flow rate but, in this case, the pumping flow rate has to be reduced because pumps generally present a pre-established and characteristic flow rate-pressure relation which is, a priori, unchangeable. Consequently, when the pressure is continuously increased, the clogging rate will increase faster than when a high tangential velocity is used in the unit. 

Successful performance of inorganic membranes depend on three types of variables and their interactions. The first type is related to the characteristics of the feed stream such as the molecular or particulate size and/or chemical nature of the species to be separated and concentration of the feed to be processed, etc. The second type is membrane dependent Those factors are the chemical nature and pore size of the membrane material and how the membrane and its accessory processing components are constructed and assembled. The third type is processing conditions such as pressure, transmembrane pressure difference, temperature, crossflow velocity and the way in which the membrane flux is maintained or restored as discussed earlier in this chapter. 

Meunier [1990] tested the feasibility of this new approach with 0.8 pm alumina membranes under a transmembrane pressure of 3 bars and a crossflow velocity of 3 m/s at 5 C. The initial permeate flux declines to a level of about 20 L/hr-m. ... 

Alumina and other ceramic membranes of various microfiluaiion pore sizes have been used for the separation of yeast (saccharomyces cerevisiae) from the broth and the clarification of thin stillage [Cheryan, 1994]. A typical flux of 110 L/hr-m can be obtained with a crossflow velocity of 4 m/s and a transmembrane pressure of 1.7 bars. The crossflow velocity is found to markedly affect the membrane flux. Concenuation factors (ratios of final to initial concentrations) of 6 to 10 for both the broth and the stillage can be achieved. Backflushing with a frequency of every 5 minutes and a duration of 5 seconds helps maintain the flux, particularly in the initial operating period. The permeate flux for both types of separation reaches steady state after 30 to 90 minutes. 

Applied Transmembrane Pressure and Velocity along the Membrane Surface. These process parameters may be varied extensively at the laboratory scale to achieve the ophmal combinahon of process parameters for a given beer or application. Variations in industrial installahons are normally possible, but limited due to given sizes of the connechng piping, pump capacity etc., which might not allow the process to be conducted under desired process condihons for each individual application. 

Thomassen JK, Faraday DBF, Underwood BO, and Cleaver JAS. The effect of varying transmembrane pressure and crossflow velocity on the microfiltration fouling of a model beer. Separat. Purif. TechnoL, 2005 41(1) 91-100. 

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