Pipe flow with permeation

Membrane gas-separation systems have found their first applications in the recovery of organics from process vents and effluent air [5]. More than a hundred systems have been installed in the past few years. The technique itself therefore has a solid commercial background. Membranes are assembled typically in spiral-wound modules, as shown in Fig. 7.3. Sheets of membrane interlayered with spacers are wound around a perforated central pipe. The gas mixture to be processed is fed into the annulus between the module housing and the pipe, which becomes a collector for the permeate. The spacers serve to create channels for the gas flow. The membranes separate the feed side from the permeate side. 

Commercial cross-flow filtration units are often supplied as packaged units where the membrane modules, feed pumps, feed and permeate tanks, heat exchangers, pipes, and controls are all arranged on a frame, or skid, which is preassembled and tested in the fabricator s workshop. It is possible to provide these units with a high level of automation and control, depending on the application. 

The testing system (Fig. 1) was a 1.2 volume pressure apparatus made of metaplex (1). The har support covered with the membrane (2) of an effective surface area of 49.2 cm was fixed in the lower part of the apparatus. To maintain the dye concentration on the level required, continuous circulation of the permeate between the feeding tank (5) and the apparatus was applied. The solution was mixed with a magnetic stirrer (3) which prevents excess concentration of dye on the membrane surface. Pressure was generated by feeding the apparatus with an inert gas (nitrogen) from a cylinder (8). Samples for flow rate measurements and determinations of dye concentration in the permeate were taken through a stub pipe (4). 

If cross-flow filtration in porous tubes is carried out at constant pressure, the permeate flux drops, initially approaching steady state, which ideally would result in a constant rate at which cake deposition and stripping are equal and the resistance of the precoat layer is unchanged. In practice, however, the rate decreases slowly with time and back-flushing or even chemical treatment must be used for clearing the pipes. 

In many applications, elastomers are used as sealing materials in contact with typical gases present in oil and gas recovery like NBR, FKM (Viton ), EPDM, or recently also TFEP (AFLAS ), acopolymeroftetrafluoroethylene (TFE) and propylene (P) that can be used up to 200 °C. Like in the case of the flexible pipes described above, the gas diffuses into the sealing material to some extent leading to loss of flows into the atmosphere. Especially H2S and CO2 dissolve to a considerable extent residting in high permeation coefficients [44]. 

The probe comprises a small electrochemical cell with one electrode, a thin inert metal foU, usually palladium, in direct contact with the pipe wall. As the hydrogen diffuses through the vessel or pipe wall and the metal foil, it is electrochemically oxidized on the inner face of the foil that is in contact with the cell electrolyte. The current flowing in the cell is directly proportional to the rate of hydrogen permeation through the wall of the equipment and provides a direct measure of hydrogen activity. 

A father development of the plate-and-frame module is the envelope module. In the GKSS module (Figure 5.5) flat sheet membranes are designed in the envelope type that enables membranes exchange easily. These modules are applicable in nanofiltration, vapor permeation and gas permeation processes and are suited for high operation pressure up 120 bars. The flat sheet membrane module can be used with sweep gas or vacuum operation on the permeate side. The feed gas flows around the membrane envelopes and the permeate is collected over a central collecting pipe, which lead to low pressure losses. The flexible distance between the membrane envelopes makes it possible to create relative constant retentate flows."... 

Permeate water is recovered from both ends of the BOE reverse osmosis module and is collected with each supporting plate, respectively. At the opposite side of the feed water inlet (brine side), permeate water is collected through the supporting plate and passes through the permeate pipe in the inside of the concentric pipe. This permeate combines with the feed-side permeate water and flows to the permeate port of the pressure... 

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