Plate and frame units

Plate and frame systems offer a great deal of flexibility in obtaining smaller channel dimensions. Equations 4 and 5 show that the Increased hydrodynamic shear associated with relatively thin channels Improves the mass-transfer coefficient. Membrane replacement costs are low but the labor involved is high. For the most-part, plate and frame systems have been troublesome in high-pressure reverse osmosis applications due to the propensity to leak. The most successful plate and frame unit from a commercial standpoint is that manufactured by The Danish Sugar Corporation Ltd. (DDS) (Figure 15). 

Plate-and-frame units have been developed for some small-scale applications, but these units are expensive compared to the alternatives, and leaks through the gaskets required for each plate are a serious problem. Plate-and-frame modules are now only used in electrodialysis and pervaporation systems and in a limited number of reverse osmosis and ultrafiltration applications with highly fouling feeds. An example of one of these reverse osmosis units is shown in Figure 3.39 [111],... 

Residual moistures will be reduced in comparison to standard plate and frame units and RVF s by up to one-third, due to the high driving force created by the hydraulic membrane of up to 375 psi. Particles can be retained to one micron, which can eliminate the need for a precoat and save on waste disposal. As the cake developed in the pressure chamber is relatively even, and the wash delivered is also consistent, washing efficiency is high. (See Fig. 12.)... 

Plate and frame units are also used for cross-flow filtration. Some units take sheet stock MF membranes while others work best with a preassembled membrane cassette-a sandwich of two outer layers of membrane sealed to an inner filtrate collection screen (see Figure 2.47). A cross-flow spacer is placed between the filter packets and stacked in a plate and frame arrangement (see Figure 2.48). In some systems, the cross-flow spacer is a screen, but the "flow-channel" spacer shown in Figure 2.47 is less prone to fouling. 

Dorr-Oliver began to search for other polymers suitable for casting asymmetric UF membranes. By 1965, the first laboratory-scale UF membranes and cells appeared on the market. The ten-year period between 1965 and 1975 was a period of intense development where chemically and thermally resistant membranes were made from polymers like polysulfone (PS) and even polyvinylidene difluoride (PVDF) in molecular weight cut-offs (MWCO) from 500 to 1,000,000. Hollow fibers were also developed during this decade and a whole host of module configurations. Tubes, plate and frame units, and spiral-wound modules became available. 

Fortunately, hollow fibers may be cleaned by back-washing which tends to compensate for their propensity to foul. Manufacturers of tubes, plate and frame units, and spiral wound modules do not recommend back-washing due to problems with membrane delamination and glue line seal rupture. Because hollow fibers are self-supporting and hold up well under the compression force of a reverse transmembrane pressure drop, they can easily withstand back-wash pressures of 15 to 20 psi. However, the back-wash fluid should be filtered to remove any particles which would tend to lodge in the porous wall of the fiber. 

HBndbook of Industrie Membrane Technology Plate and Frame Units... 

Spiral-wound modules cannot be unwrapped for cleaning lest the glue line seal rupture and most cannot be autoclaved. They are more prone to fouling than tubes and some plate and frame units (depending on the type of feed channel spacer), but they are more resistant to fouling than hollow fibers. 

Finally, some process equ ment, e.g. large-scale plate and frame units used in dge separations, may operate in a mixed mode. Filling of the press and early build rq> of cake deposit may ensue at a controlled constant flow final deposits, with dewatering, may be effected at a hi er-kvel, constant pressure differential [Svarovsky, 1981]. 

Plate-and-frame coolers have made some inroads into this application. These will be much smaller than the typical shell-and-tube coolers, at the expense of a greater pressure drop. Titanium plates are necessary, and welded construction of the plates is highly recommended [16]. Plate-and-frame units will probably become more common as the increasing use of the newest membrane cells allows operating pressures to increase. Few units are found today in sizes above 100 tpd chlorine. Section 9.3.2.4 on caustic cooling briefly discusses the general characteristics of plate exchangers. 

Flat sheet membranes in a plate-and-frame unit offer the greatest versatility but at the highest capital cost (P6). Membranes can easily be cleaned or replaced by disassembly of the unit. Spiral-wound modules provide relatively low costs per unit membrane area. These units are more prone to foul than tubular units but are more resistant to fouling than hollow-fiber units. Hollow-fiber modules are the least resistant to fouling when compared to the three other types. However, the hollow-fiber configuration has the highest ratio of membrane area per unit volume. 

A slurry of a solid (3.0 specific gravity, concentration 1(X) kg/m water) is filtered in a plate and frame unit which takes 900 sec to dismantle, clean, and reassemble. If the cake is incompressible (porosity of 0.4, viscosity of slurry is 1 x 10 N sec/m ), find the optimum cake thickness for a pressure of 1000 kN/m. ... 

A 20-frame (0.3-m square, 50-mm frame thickness) plate and frame filter press is operated first at constant rate for 300 sec to a pressure of 350 kN/m (one-fourth total filtrate obtained). Filtration then takes place at 350 kN/m for 1800 sec until the frames are full. Total filtrate volume per cycle is 0.7 m. Breaking down and reassembly requires 500 sec. A replacement rotary drum filter (1.5 m long, 2.2-m diameter) is to give the same overall filtration rate as the plate and frame unit. Assume that resistance of the filter cloth is the same and that the filter cakes are incompressible. What is the speed of rotation if pressure difference is 70 k N/m and drum submergence is 25 percent ... 

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