Average transmembrane pressure

Figure 4. Concentration of Cells (AG Techn. 500K MWCO Hollow Fibers) Flux and Average Transmembrane Pressure Profiles for Constant Permeate Flow-Controlled Operation...

Figure 9. Enzyme Concentrations Step Effect of Recirculation Rate on Flux Performance for the Amicon Diaflo S10Y40 Spiral Ultrafilter (10,000 MWCO, 40 ft2) at Constant Average Transmembrane Pressure (30 psi)...

Figure 9. Influence of average transmembrane pressure on membrane flux (0.45 pirn microporous). Recirculation rate (40 psi) is kept constant as TMP is varied. Operation is in the total recycle mode.

The relationship of wall shear rate to flux for two membrane devices is shown in Figure 5. Wall shear rate is proportional to velocity divided by channel height. These experiments were run with E. coli grown in a defined salts medium. The average transmembrane pressure was not held constant but allowed to rise as a natural consequence of the increasing flow. The net result was a dependence of flux to shear rate to the 1/2 power. 

Energy consumption for each of the two membrane systems included suction pumps assuming an average transmembrane pressure of 35 kPa and scouring aeration as described above. 

The sensitivity of productivity or flux to transmembrane pressure (TMP) is referred to as the permeability L = flux/transmembrane pressure. TMP refers to a module average. Pure-component permeability (e.g., water permeability) refers to membrane properties while the more industrially relevant process permeability includes fouling and polarization effects. 

Normalized permeate flow (NPF) is a function of the average applied transmembrane pressure, the osmotic pressures of the feed and permeate, and temperature, as shown in Equation 11.1. Factors that cause an increase or decrease in the NPF are discussed below. 

Hgure 15 13 Comparison between experimentally obtained droplet sizes and deseriptions by the model (data from Schrrider et al. 1998, 1999). The membrane used was a eeramic membrane with average pore size of 0.4 pm, and with transmembrane pressure of 3 bar sunflower oil was emulsified in water with different surfaetants. One only obtains a good quantitative fit with the data when one assumes that the interfaeial tensions are 18 mN/m with 0.05% Tween-20 11 mN/m with 0.5% Tween-20 and 1.7 mN.m with 2% SDS. These values are not physically realistic therefore, the model does not quantitatively deseribe the data. 

Recycling some portion of the permeate or retentate stream and introducing feed at intermediate locations are effective methods for improving the reaction conversion. The relative permeabilities of the reactant(s) and product(s) and whether the permeate or retentate is recycled all affect the effectiveness of these measures for conversion enhancement. To compensate for the variations in the transmembrane pressure difference and consequently in the permeation rate, the concept of a location-dependent membrane ]x rmeability has been proposed. The effects of this approach and the average permeation rate arc discussed. 

Spacers—A mesh-like material used to separate successive layers of membrane and/or support backing. Transmembrane Pressure (AP)—Average pressure difference between the upstream and downstream sides of the membrane, which is the driving force. 

Membrane Flux Performance for the Hollow Fiber Units. Figures 3a and 3b show the results of a cell separation pilot scale run in which transmembrane pressure was maintained constant at 4.0 psi and linear velocity through the fibers was maintained at 1.0 m/sec (100 1pm). Transmembrane pressure was controlled with a backpressure valve on the combined permeates from the hollow fibers. Note that the flux vs. concentration curve for this run does not conform to theoretical predictions for concentration polarization (see Figure 3b). Flux on this semi-log plot does not decline linearly, but, rather, shows two phases of fouling. The initial phase (approximately 20 minutes in duration) is characterized by a rapid loss in flux, while the second stage (approximately 90 minutes in duration) is more gradual. The initially rapid decline caused an unacceptably low average flux of 10 l/m -hr. 

Transmembrane pressure It is the average driving force for permeation across the membrane. Neglecting osmotic pressure effects for most MFAJF applications, it is defined as the difference between the average pressure on the feed (or retentate) side and that on the permeate (or shell side). 

Transmembrane pressure (TMP) The driving force for flux. In cross-flow systems, it is measured as the average of the feed inlet and concentrate outlet pressures minus the permeate back-pressure. 

The transmembrane pressure is the average pressure difference between the inside of the crossflow filter and the permeate pressure. It is often assumed to be equal to the mean average of the feed and retentate pressures minus the permeate line pressure. It is usual, however, to employ pressure measurem t upstream and downstream of the filter and in different diameter tubes, hence the transmembrane pressure calculated in this way is only very approximate. One irrportant consequence of the effect illustrated in Figure 10.5 is that increasing the filtration pressure and, therefore, the operating costs may lead... 

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