Membrane modules technique

Within a few years of the invention of the L-S membrane, development of the RO membrane modules technique was reaHsed. Haven and Guy developed tubular RO membranes in the mid-1960s. In the late 1960s, Westmoreland and later Bray invented the spiral-wound module, which was more efficient than the tube-in-sheU module. The spiral-wound membrane can be viewed as a plate-and-frame (PAF) arrangement that has been rolled up. The original module had a single leaf of membrane whereas modem spiral-wound modules contain multi-leaf membranes. 

Because membranes appHcable to diverse separation problems are often made by the same general techniques, classification by end use appHcation or preparation method is difficult. The first part of this section is, therefore, organized by membrane stmcture preparation methods are described for symmetrical membranes, asymmetric membranes, ceramic and metal membranes, and Hquid membranes. The production of hollow-fine fiber membranes and membrane modules is then covered. Symmetrical membranes have a uniform stmcture throughout such membranes can be either dense films or microporous. 

Owiag to the variety of situations encountered ia RO appHcatioas, there is ao single analytical technique to predict membrane module performance. The module and the feed stream, along with the operatiag parameters, determine system performance. To predict module performance, a model that... 

The technology to fabricate ultrathin high-performance membranes into high-surface-area membrane modules has steadily improved during the modem membrane era. As a result the inflation-adjusted cost of membrane separation processes has decreased dramatically over the years. The first anisotropic membranes made by Loeb-Sourirajan processes had an effective thickness of 0.2-0.4 xm. Currently, various techniques are used to produce commercial membranes with a thickness of 0.1 i m or less. The permeability and selectivity of membrane materials have also increased two to three fold during the same period. As a result, today s membranes have 5 to 10 times the flux and better selectivity than membranes available 30 years ago. These trends are continuing. Membranes with an effective thickness of less than 0.05 xm have been made in the laboratory using advanced composite membrane preparation techniques or surface treatment methods. 

Figure 6.14 Backflushing of membrane modules by closing the permeate port. This technique is particularly apphcable to capillary fiber modules...

Another new trend is called membrane distillation. This is based on open hydrophobic membranes that enable the passage of water vapor only. The product quality is expected to be better than RO since only water vapor may pass through the membrane. Vapor condensation is allowed on colder surfaces adjacent to the membranes or outside the membrane module, where vapors are pumped out. Another way is to condense the vapor in direct contact with a cold-water stream. The main problem using this technique is the need to evaporate the water. The energy demand for this is around 650kWh/m3. This enormous amount of energy may be reduced when energy reuse is possible, in a similar way to the multieffect distillation... 

The distinguishing feature of membrane emulsification technique is that droplet size is controlled primarily by the choice of the membrane, its microchannel structure and few process parameters, which can be used to tune droplets and emulsion properties. Comparing to the conventional emulsification processes, the membrane emulsification permits a better control of droplet-size distribution to be obtained, low energy, and materials consumption, modular and easy scale-up. Nevertheless, productivity (m3/day) is much lower, and therefore the challenge in the future is the development of new membranes and modules to keep the known advantages and maximize productivity. 

If the normalized salt rejection is low or the normalized permeate flow is high, the integrity of the membrane may be in question. The vacuum decay test is a direct test for the integrity of a spiral wound RO membrane module. The test is best used to identify leaks within the membrane modules rather than leaks due to chemical attack. The test requires the isolation of an individual membrane module or the entire pressure vessel. A vacuum is then pulled on the membrane(s) and the rate of decay in pressure is observed. A decay of greater than 100 millibar per minute is indicative of a leaky membrane. Refer to ASTM Standards D39235 and D69086 for a more detailed review of the technique. 

There are also techniques involving the use of nonporous, solid or liquid membranes that separate the donor phase from the receiving phase by an evident phase boundary. Most often used are three-phase systems (donor phase, membrane, and acceptor phase) or two-phase systems, in which one of the surrounding phases is the same as the membrane. Solid membranes are made of chemically resistant, hydrophobic polymers (PTFE, PVDF, PS, PP, silicates), metals (Pd alloys), or ceramic materials. Channels of membrane modules have a volume ranging from 10 to 1000 pL and, according to their geometry, can be classified as planar or fibrous. For setting up a membrane system, two modes can be used the membrane can be immersed in a sample (membrane in sample, MIS) or the sample can be introduced into a membrane (sample in membrane, SIM). In both systems, only a small amount of sample is in direct contact with membrane, because ratio of the membrane surface area to the sample volume is small. 

Vibratory mechanism was one more interesting technique for reducing fouling negative effect. Vibrations can be apphed to flat sheet membrane modules, but many researchers tested the technique on performance of hollow fibers for different separation processes [3, 136]. One example of construction is shown in Fig. 9.10 [136] to assess the effect of axial membrane vibrations on mass transfer in a hollow fiber oxygenator. 

Another advantage of PMo-PSF-DMF is that porosity of the film catalyst can be controlled by the membrane preparation technique. The homogeneous PMo(4.76 wt %)-PSF(23.81 wt %)-DMF(71.43 wt%) solution was used for the preparation of microporous film catalyst by the phase separation method. Water vapor was used as a non-solvent for PSF. Phase separation rate was controlled by modulating water vapor concentration (RH). RH might affect DMF evaporation rate and phase separation rate. 

---