Membrane module filtration mode

In a general way, most of ceramic membrane modules operate in a cross-flow filtration mode [28] as shown in Figure 6.18. However, as discussed hereafter, a dead-end filtration mode may be used in some specific applications. Membrane modules constitute basic units from which all sorts of filtration plants can be designed not only for current liquid applications but also for gas and vapor separation, membrane reactors, and contactors, which represent the future applications of ceramic membranes. In liquid filtration, hydrodynamics in each module can be described as one incoming flow on the feed side gf, which results in two... 

The concept of the helical membrane module has been tested in a submerged membrane filtration mode with bubbling used for the membrane fouling control. Liu et al. [30] showed that the helical membrane with a twisted angle of 180° could achieve a 1.46-1.69 flux enhancement, compared to the membrane modules with 0° twisted angle, in the filtration of 500 mg/L kaolin suspension under a constant TMP of 2.8 and 3.2 kPa. The particle image velocimetry (PIV) analysis [31] showed that the tortured membrane surface of the helical membrane could generate rotational flow near the membrane surface and increase the wall shear rate. 

The filtration unit operated in a batch mode for six hours each day, for six days, and processed approximately KXX) gallons of feed per day. Over the six day test period, permeate flux was a relatively constant 0.0085 gallons/min/ft (coefficient of variation < 10%). Based on a total membrane area of 300 for the system, the permeate flow rate for the four-module filtration unit averaged 2.6 gpm. 

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. 

Inside-out, Outside-in Filtration in Hollow-Fiber Membranes Hollow-fiber membrane modules can be operated in two different flow modes— inside-out and outside-in —based on the direction of filtration flow. In the inside-out configuration, pressurized feed water flows through the bore of a hollow fiber, and permeate is collected on the outside of the membrane fibres. In the outside-in configuration, the pressurized feed stream flows from the outside of a hollow fiber, and permeate is collected inside the bore of the hollow fiber. 

The application of MF and UF technology in water treatment is still relatively new, as many full-scale plants are less than 10 years old. Consequently, the design of MF and UF systems varies from plant to plant as no single design has proved itself to be the best (see Table 6.2). For example, MF/UF systems may be positioned horizontally or vertically, operate in deadend or cross-filtration mode, the membranes may be immersed/submerged in a feed tank where permeate is sucked (via vacuum) into the inside of the hollow fiber (outside-in filtration) or the membranes may be housed in modules where pressurized feed water is forced through the fiber and permeate is collect on the outside (inside-out filtration). 

A typical UF pilot plant has been used in this study. Examples of application for these membranes can be found in the literature [40, 58]. The UF unit woks in deadend mode (2.5 m h ) and it can be operated in filtration, backwash and chemically enhanced backwash (CEB) modes as described in the literature for similar UF systems [40]. The specifications of the hollow fiber UF modules and the operating conditions are summarized in Table 5. 

Boundary layer formulation. Many membrane processes are operated in cross-flow mode, in which the pressurised process feed is circulated at high velocity parallel to the surface of the membrane, thus limiting the accumulation of solutes (or particles) on the membrane surface to a layer which is thin compared to the height of the filtration module [2]. The decline in permeate flux due to the hydraulic resistance of this concentrated layer can thus be limited. A boundary layer formulation of the convective diffusion equation can give predictions for concentration polarisation in cross-flow filtration and, therefore, predict the flux for different operating conditions. Interparticle force calculations are used in two ways in this approach. Firstly, they allow the direct calculation of the osmotic pressure at the membrane. This removes the need for difficult and extensive experi-... 

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. 

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

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