Plate and frame systems

For ultrafiltration appHcations, hollow-fine fibers have never been seriously considered because of their susceptibiUty to fouling. If the feed solution is extremely fouling, tubular or plate-and-frame systems ate still used. Recentiy, however, spiral-wound modules with improved resistance to fouling have been developed, and these modules are increasingly displacing the more expensive plate-and-frame and tubular systems. Capillary systems are also used in some ultrafiltration appHcations. 

Pervaporation operates under constraints similar to low pressure gas-separation. Pressure drops on the permeate side of the membrane must be small, and many prevaporation membrane materials are mbbery. For this reason, spiral-wound modules and plate-and-frame systems ate both in use. 

Plate-and-frame systems are competitive in this appHcation despite their high cost, primarily because they can be operated at high temperatures with relatively aggressive feed solutions, for which spiral-wound modules might fad. 

Plate and Frame. Plate-and-frame systems consist of plates (Fig. 15) each with a membrane on both sides. The plates have a frame around their perimeter which forms flow channels ca 1 mm wide between the plates when they are stacked. The stack is clamped between two end plates, sealing the frames together. 

For many years the plate-and-frame systems have used flush plates with separate frames to contain the cake. These filter presses had many sealing surfaces which were the main cause for leakages. As a result recessed plates were introduced in order to cut the number of surfaces in half and reduced the problem of drippings. The development of recessed plates has gone hand in hand with advances in cloth technology which enabled three-dimensional stretching as opposed to simple plate-and-frame where the cloth remains in one plain. Present recess depths are 16, 20 and 25 mm so the corresponding cake thicknesses are 32, 40 and 50 mm at... 

Pervaporation Most pervaporation systems are small so plate-and-frame systems were used in the first systems. Spiral-wound and capillary modules are being introduced. 

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). 

The labor intensive replacement of membranes in plate and frame systems has been facilitated in the "leaf-module" design of Dorr Oliver (Figure 16). Here a number of plates are assembled in a disposable cartridge where the process stream flows over the plates and the permeate is ducted to a common header. 

In current practice, turbulence promoters most often take the form of a net or screen material which also serves as a feed channel spacer between two membranes. For example, the familiar spiral wound modules (Figures 29) used extensively in reverse osmosis and to a lesser extent in ultrafiltration use a plastic screen material as the feed channel spacer. This is also used in some plate and frame systems (Figure 30). 

Plate and Frame. Plate-and-frame systems consist of plates each with a membrane on bodi sides. 

Plate and frame systems Here the feed solution flows through flat, rectangular channels (Figure 42.5c) [34]. Packing densities of about 100-400 m /m are achievable. For NF, plate and frame design has been excluded due to their expensive nature. They are primarily used to treat fouling-prone feed streams and to produce potable water in small-scale applications. 

The spiral-wound module is in fact a plate-and-frame system wrapped around a central collection pipe, similar to a sandwich roll. The basic structure of this module is illustrated in Fig. 10. Membrane and permeate-side spacer material are then glued along three edges to build a membrane envelope. The feed-side spacer separating the top layer of the two flat membranes also acts as a turbulence promoter. The feed flows axial through the cylindrical module parallel along the central pipe and the permeate flows radially toward the central pipe. In order to make the membrane length shorter, several membrane envelopes are wound simultaneously. The spiral-wound module is featured by... 

During these 20 years, a large number of module systems has been made. The first idea was to use the conventional filter press, the plate-and-frame system, as done by Aerojet General Corp. However, this system never proved to be very successful in the U.S.A., and consequently new configurations, such as tubes, hollow fibers, and spiral-wound systems were invented. 

In Europe, and especially by our company. The Danish Sugar Corporation, DDS, in Denmark, we attacked the problem in a different way. Primarily, we saw a great number of advantages in the plate-and-frame system, and by solving some of the problems, such as sealings, manufacturing of plates and frames by injection moulding, etc., in 1970 we came up with a system which at that time we considered to be a competitive product. 

I think, these ten factors explain some of the obvious advantages obtained by a plate-and-frame system. However, I would like to add that without the great work carried out by Dr. R.F. Madsen in our company who started a research programme within this field some 15 years ago, there would be no DDS plate-and-frame system today P). 

Figure 1. The principal configuration of DDS plate-and-frame systems...

Plate-and-Frame Systems. The membrane cast on support paper is exchanged and replaced by membranes cut out in the proper size. 

In respect to manufacturing costs, transport costs, and handling, the plate-and-frame system should have the most economical exchange procedure. The drawback is the necessary work in making the exchange which is usually made by the customer, but for desalination plants this is less important since they are often situated in areas where labour costs are low. 

Security in delivery is most likely best obtained by dealing with companies of a reasonable size and well introduced in the field of membrane filtration. But also here, the plate-and-frame system has an additional advantage, because membranes from other manufacturers who can cast membranes in sheet form can be used direct in our modules, though this is not something DDS is very much in favour of. 

Madsen, R.F., Ed. "Hyperfiltration and Ultrafiltration in Plate-and-Frame Systems", Elsevier, Amsterdam, 1977. 

In plate and frame systems (see Figure 2.36), the filtrate itself is often used as the backwash fluid. Sometimes, larger membrane housings are used to serve as an accumulation reservoir for the filtrate. A backwash fluid outlet is provided up-stream of the membrane to purge "backwash debris" from the system. Of course, the membrane must be supported on both sides. 

Figure 2.36 Schematic of plate and frame system with back-wash capability. 

Figure 2.61 Harvested cell viability during CFF in open-channel plate and frame system.

Millipore Corp. and New Brunswick Scientific Co. sell a "cassette" plate and frame system. The "cassette" is a membrane packet (schematically shown in Figure 3.77) comprised of two membranes enclosing a filtrate collection screen (sealed around the edges). Alternating holes on the edge of the cassette carry re-tentate or filtrate to the appropriate manifold. The cassettes are separated by screens or channel plates (see Figures 2.47 and 2.48 in Chapter 2). When filtering cellular or colloidal suspensions, the channel plates are preferred over the... 

The Asahi Glass system is designed to produce oxygen enriched air of up to 40% for medical applications. The driving force is maintained by a vacuum on the permeate side of the plate and frame system. Air at atmospheric pressure is the feed. 

Membrane surface area to vohime of ace occupied varies from around 8000 m m for hollow fibres, down to a few hundred for tubular systems with plate and frame systems providing an intermediate membrane packing denaty. 

Because plate-and-frame and tabular configurations are used widi the most contaminated fluids, mechanical cleaning techniques are used. In the case of plate-and-frame systems, die equipment may be disassembled and scrubbed, while in the tube configurations oversized soft foam plugs are driven through the tubes by pressure. 

Plate and frame Flat porous plates covered with polymeric membrane material are assembled together with alternate hollow spacers to produce a cross-flow system where feed moves through the annular spaces between adjacent membrane surfaces. Although now largely superseded by other designs, the plate and frame system is still available with membrane areas up to 80 m. ... 

In plate-and-frame systems, such as in electrodialysis, turbulence promoters are introduced to increase the mass transfer. Figure VII - 8 is a schematic drawing of a flow channel in which corrugations have been introduced and where A1 is the distance between successive corrugations. 

Developments continue on citrus fruit processing. The membrane configurations most often used in juice clarification are tubular membranes or hollow fibre modules, as well as plate and frame systems using flat sheet membranes. The traditional method of concentrating juices and purees has been evaporation, but nowadays reverse osmosis is proving successful. Reverse osmosis plant performance depends on juice viscosity, the osmotic pressure of the solution, and the constraints imposed by the need for a particular product quality. 

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