Pressure transmembrane

Feed High pressure feed side AP = Transmembrane pressure difference An = Osmotic pressure difference Membrane Concentrate... 

Solution—Diffusion Model. In the solution—diffusion model, it is assumed that (/) the RO membrane has a homogeneous, nonporous surface layer (2) both the solute and solvent dissolve in this layer and then each diffuses across it (J) solute and solvent diffusion is uncoupled and each is the result of the particular material s chemical potential gradient across the membrane and (4) the gradients are the result of concentration and pressure differences across the membrane (26,30). The driving force for water transport is primarily a result of the net transmembrane pressure difference and can be represented by equation 5 ... 

Fig. 6. Solute transport in hemodialysis. Clearance vs solute mol wt for dialy2ers prepared from the two different membranes illustrated in Figure 5. Numbers next to points represent in min /cm calculated from equations 10 and 5. Data is in vitro at 37°C with saline as the perfusion fluid. Lumen flow, dialysate flow, and transmembrane pressure were 200 ml,/min, 500 mL/min, and 13.3 kPa (100 mm Hg) area = 1.6. Inulin clearance of the SPAN...

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 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. 

Bubble Point Large areas of microfiltration membrane can be tested and verified by a bubble test. Pores of the membrane are filled with liquid, then a gas is forced against the face of the membrane. The Young-Laplace equation, AF = (4y cos Q)/d, relates the pressure required to force a bubble through a pore to its radius, and the interfacial surface tension between the penetrating gas and the liquid in the membrane pore, y is the surface tension (N/m), d is the pore diameter (m), and P is transmembrane pressure (Pa). 0 is the liquid-solid contact angle. For a fluid wetting the membrane perfectly, cos 0 = 1. 

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. 

The objective of the present study is to develop a cross-flow filtration module operated under low transmembrane pressure drop that can result in high permeate flux, and also to demonstrate the efficient use of such a module to continuously separate wax from ultrafine iron catalyst particles from simulated FTS catalyst/ wax slurry products from an SBCR pilot plant unit. An important goal of this research was to monitor and record cross-flow flux measurements over a longterm time-on-stream (TOS) period (500+ h). Two types (active and passive) of permeate flux maintenance procedures were developed and tested during this study. Depending on the efficiency of different flux maintenance or filter media cleaning procedures employed over the long-term test to stabilize the flux over time, the most efficient procedure can be selected for further development and cost optimization. The effect of mono-olefins and aliphatic alcohols on permeate flux and on the efficiency of the filter membrane for catalyst/wax separation was also studied. 

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. 

As presented in Figure 15.10, after 100 h TOS the permeate valve was closed, and thus the transmembrane pressure fell to zero. The pilot plant remained in a standby mode and unmanned for approximately 75 h. During this period, the catalyst slurry was circulated through the cross-flow filter, without permeate flow radially through the filter membrane (i.e., test conditions were constant with the... 

To help understand the performance of membranes, brief explanations of a few terminologies are in order. Permeability of a membrane is determined by dividing permeate flux by the transmembrane pressure. It indicates the membrane s throughput per unit area (flux) per unit pressure difierence. An important factor afiecting flux and retention ability of the membrane is the direction of the feed flow relative to the membrane surface. In through-flow configuration, the feed flow is perpendicular to the membrane surface. In cross-flow configuration, the feed stream flows parallel to the membrane... 

Microfiltration with uniform transmembrane pressure Commercial and developing liquid phase applications Gas separations using inorganic membranes... 

The removal of PhCs by NF membranes occurs via a combination of three mechanisms adsorption, sieving and electrostatic repulsion. Removal efficiency can vary widely from compound to compound, as it is strictly correlated to (a) the physicochemical properties of the micro-pollutant in question, (b) the properties of the membrane itself (permeability, pore size, hydrophobicity and surface charge) and (c) the operating conditions, such as flux, transmembrane pressure, rejections/recovery and water feed quality. 

Figure 4. Dependence of equilibrium foulant film thickness on effective transmembrane pressure...

Osmotic pressures of chain polymers in the semi-dilute region (2-20% v) are appreciable and no longer molecular-weight dependent. At 50% v, osmotic pressures of chain polymers are of the order of 100 atm, far greater than any transmembrane pressure used in ultrafiltration T). 

Note that pressures of more than 60 psi can be developed, which are fairly typical of transmembrane pressures in ultrafiltration. Needless to say, there are also cases where the expected asjnmp-totic pressure is not reached because the energy barrier is overcome on the way there and coagulation occurs (14, 15). However, for the case of a stable latex, we can approximate the curves of Figure 3 by... 

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