Membrane filtration velocity

Hydrodynamic. For a pressure driven process such as ultrafiltration the flow of solvent towards the membrane results in a drag which carries the solute in the same direction. This drag is a function of the distance of the solute from the pore entrance. At large distances it is equal to the isolated solute value (Stokes limit), but as the solute approaches and begins to enter the pore, the drag, for a constant filtration velocity, increases due to the restriction of solvent flow. This increase depends on the ratio of solute diameter to pore diameter. 

The impact of membrane filtration is not limited only to the quality of red wines. It is also ol rved with white wines. Given in Table 6.8. are the microfiltration results of a white and a red wine using 0.2 pun alumina membranes [Castelas and Serrano, 1989]. The TMP and crossflow velocity are 2 bars and 4.5 m/s, respectively. The changes on the wine quality are very obvious in almost every property. 

Recovery of catalyst from converted oil. Another way to process the residues is to add hydrogen to effect hydroconversion which avoids the formation of a large quantity of asphalt Solid catalyst is formed afterward by reaction. Membrane filtration is used to separate the converted oil from the catalyst This makes it possible to partially recycle the catalyst to the reactor. Alumina and zirconia membranes with pore diameters ranging from 30 to 600 nm have been tested for this application. The membrane with a pore diameter of 30 nm yields a stable flux and a catalyst retention better than 98% [Deschamps et al., 1989). Concentration polarization is significant and requires a high crossflow velocity and temperature to overcome it. 

Wai and Fumeaux [1990] applied CFD to crossflow membrane filtration to provide an array of data such as local pressures and fluid velocities on both sides of a membrane, shear stresses on the membrane surface and local concentrations of retentate species. This type of information is useful for designing the membrane unit as a separator or a reactor. With a commercial CFD code, the authors simulated the fluid flow, on both the feed and permeate sides, along the membrane channel and through the membrane. Frictional effects between the fluid and membrane surfaces depend on the nature of the fluid flow. For flow parallel to the essentially flat membrane surface, standard wall friction expressions based on logarithmic velocity profiles adjacent to the wall arc used. 

An assessment of the literature over a wide range of operating conditions and feeds indicates that flux enhancement for tubular membrane filtration can be affected by module configuration and operation conditions such as feed concentration, liquid velocity, bubble size, and TMP. The trends have been summarized by Cui et al. [77], as follows ... 

Recent research efforts brought about new and exciting developments in membrane technology, some with direct implications for the membrane filtration of beer. For example, Stopka et al. [21] reported flux enhancement in the microfiltration of a beer yeast suspension when using a ceramic membrane with a helically stamped surface. A relatively simple modification of the ceramic membrane surface resulted in modified hydrodynamic conditions and disturbance of the fouling layer. As compared with a regular, smooth ceramic membrane of the same nominal pore size, the stamped membrane leads to higher flux and lower power consumption per unit volume of permeate at the same velocity of the feed. 

In cross-flow filtration (Fig. IB) or delayed cake filtration, the slurry flows parallel to the cake surface with sufficient velocity to prevent partially or entirely the deposition of cake. It is used successfully to increase flow rate in membrane filtration. It is also employed for concentrating and recovering very fine particles in dilute suspensions when deep-bed or cake filtration would not applicable. 

A concentration boundary layer theory clearly is needed to relate C to C, so that membrane properties such as L, a, and P can be correlated with R, at various operating conditions. Slso, since ir in Equations 1 and 5 is an independently determined function of C, a boundary ayer theory could correlate the observed filtrate velocity, J (averaged along the fiber length), with average applied pressure AP. For sufficiently high axial flow velocities, C == C, and a major theoretical barrier to data analysis is removeS. Some early work in reverse osmosis ( ) was done with flat-sheet membranes and large feed stream velocities. 

Problem 5-12. Flow Through a Porous Tube. Let us consider flow through a cylindrical porous tube, which occurs in many membrane filtration processes. The tube is very long with radius R. At the inlet of the tube, the pressure is/ /. Fluid permeates or leaks out through the wall of the tube with a velocity k(P — Ps) ///, where P is the local pressure in the fluid, Ps is the pressure on the other side of the membrane, A is a permeation coefficient, and // is the viscosity of the fluid. We wish to determine how much fluid is filtered as a function of the length of the tube. 

Secondary flow is a flow region where the flow velocity and direction are significantly different from the primary flow region. The secondary flow could be generated by curved channel, coiled tubes, and rotating movement. The secondary flow generated close to the membrane surface can reduce the concentration polarization and enhance membrane filtration. This section discnsses the secondary flows, including Dean vortices, Taylor flow, and helical membrane modules for the membrane performance enhancement. 

Membrane filtration has many similarities to conventional filtration, and the mathematical description of the process uses many ccmcepts already introduced in Chapter 2. However, there are rignificant differences in the terminology enqiloyed the filtrate is referred to as the permeate , the residual slurry or suspension from the filtration is called the retentate and the permeate filtration rate is the flux rate , which in microfiltration is conventionally reported in the emits of litres per square metre of membrane area per hour (1 m h ). This rate is equivalent to the superficial liquid velocity through the menibrane. In nearly aU the instances of constant-pressure... 

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