Membrane modules pressure losses

A number of factors can lead to high pressure drop, including membrane scaling, colloidal fouling, and microbial fouling. These three factors all involve deposition of material onto the surface of the membrane as well as onto components of the membrane module, such as the feed channel spacer. This causes a disruption in the flow pattern through the membrane module, which, in turn, leads to frictional pressure losses or an increase in pressure drop. 

Pressure drop must also be assessed. Most projection programs assume a piping loss of about 5 psi (35 kPa) per stage plus the pressure drop through the membrane modules in the projection program. Should the actual pressure drop exceed the predicted pressure drop, there could be two explanations ... 

The hollow-fiber module is often used when the feed stream is relatively clean, such as in gas separation and pervaporation. It has also been used in the case of seawater desalination, but pretreatment is needed. The module construction given in Fig. 15 A is a typical RO module, where a central pipe is used to uniformly distribute the feed solution throughout the module. This is to avoid the problem of channelling in outside-in model, which means the feed has a tendency to flow along a fixed path, thus reducing the effective membrane surface area. In gas separation, as shown in Fig. 15B, the outside-in model is used to avoid high pressure losses inside the fiber and to attain a high membrane area (13). 

As a result of tlie construction of the membrane module, the interface area for mass transfer per volume is very high compared to extraction towers and is not influenced by flow volumes. The flow volumes are only restricted by the phase breakthrough into the other phase caused by the pressure loss along the contactor. HTU values are lower compared to other extraction units therefore the membrane contactor has higher extraction efficiency. HTU values increase with higher loading of the membrane module. To reach a higher theoretical plate munber, comparable to extraction towers, more module units have to be set in series. 

In general, the optimization of a membrane column must consider molar flow rate Na, reflux ratio and the pressure losses. The mutual dependencies, which have been only shortly discussed here, are the reason why hollow fiber modules for the membrane column are relatively long (L/di 20,000) compared to hollow fiber modules for the separation of liquid mixtures. 

Industrial-size plate-and-frame modules, for example, consist of a stack of tightly packed membranes over which the feed solution is recirculated (Mulder, 1997). The membranes are separated by spacers and the permeate is withdrawn by a central permeate pipe (Stiirken, 1994). Pressure losses occur on both the feed and the permeate side of the packed membranes and need to be accounted for in the module design. On the feed side, the fluid dynamic conditions over the membrane may be less uniform than on the laboratory scale, resulting in more pronounced concentration polarization. On the permeate side, the packed configuration of the membranes may lead to considerable pressure losses, rendering the instantaneous removal of solutes from the membrane downstream surface more difficult. Both aspects may cause solute fluxes lower than expected (Chapter 3.2) and a possible... 

Raghuraman and Wiencek [11] developed a hybrid technique where an emulsion is fed into a hollow fiber contactor on the tube side. Since the solid membrane support is hydrophobic, the continuous phase of the water-in-oil emulsion easily wets the pores of the tube wall and permeates to the shell side. On the shell side of the hollow fiber, the aqueous feed phase is exposed and held at an elevated pressure that prevents the permeating liquid membrane phase from exiting the pores. Thus, extraction occurs on the shell side, and stripping on the tube side of the hollow fiber membrane module. This methodology is closely related to SLMs, but the key difference is the presence of the emulsion on the tube side, which allows for long-term stability because the membrane liquid is continuously replenished to make up for any loss by solubility. 

Membrane module geometry - Can the membrane formed be incorporated into a module geometry that accommodates conduits for feed and product gases, optimum driving force for the separation, efficient membrane area density, and with minimal pressure head loss (energy) ... 

Additional pressure losses caused by hydrodynamic resistances in the permeate pass from the permeate side of the membrane to the condenser or the vacuum pump will be even more detrimental to the performance of the pervaporation process. When an alcohol-water mixture has to be dehydrated to a final water content of 1000 ppm even at 100 °C the partial water vapor pressure at the feed side will be of the order of 10 mbar. Using a high-selective membrane the partial water vapor pressure at the permeate side of the membrane will have to be kept at a few millibar. As this pressure is determined by the temperature of the condensing liquid permeate there has to be an unobstructed flow of the permeate vapor from the membrane to the condenser. It is obvious from Eq. (24) that even a pressure drop of one or two millibar in the permeate channel of a module will have a severe effect on the ratio of the partial vapor pressure and thus on the performance of the system. 

Usually all membranes in a module are arranged for parallel flow of the feed. The feed channel, between membrane and supporting plate, has a height between 0.5 to 1 mm, linear flow velocities of the liquid feed are of the order of a few centimeters per minute. Serial flow would be desirable in order to allow for higher linear flow velocities and higher Reynolds numbers, but then feed-side pressure losses will become too high. When used with a vaporous feed the feed channels need to be widened and linear velocities over the membrane should be of the order of 1 m per minute. 

The following calculation for the vacuum operation is based on the assumptions as (i) negligible pressure drop in fiber lumen (ii) no concentration and temperature gradients in the membrane module (iii) pure oxygen product and (iv) 10% heat loss for the membrane system. Figure 18.9 shows the flow patterns of the membrane system combined with heat exchangers for heat recovery where the parameters for energy calculation are also presented. 

Provided that the pressure difference between the two sides of the membrane is set to a value lower than the difference in their osmotic pressure, a water flux establishes across the membrane from the low- to the high-pressure side (i.e. from the low to the high salt concentration side). Therefore, the water flow rate on the high-pressure side of the membrane increases. The seawater stream diluted by pure water in the membrane module (brackish stream B1) is then sent to a hydrauhc turbine connected to the same shaft of the main pump. Since the turbine head equals the pump head (apart from the pressure losses along the circuit and the small head provided by the EPl pump) and the turbine flow rate is higher than the pump flow rate because of the water permeated across the membrane, a net mechanical power output is available at the shaft. A generator connected to the shaft finally converts mechanical to electric power. 

Reducing the energy requirements of RO Updates to module construction and modifications to membrane materials to reduce pressure losses are required. Note that the operating pressure of current RO membranes is near the thermodynamic limit such that any membrane improvements would have minor impacts on performance. However, changes in module design could improve pressure losses and reduce energy requirements of the system. 

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."... 

If the fiber diameter varies either effect must taken into account. In case of a fiber diameter distribution within a module, small fibers could consume the product. If the whole feed stream entering a fiber permeates through the membrane, the pressure within the fibers will drop under the pressure of the retentate pressure, leaving larger fibers so that a backflow into the small fiber is induced. This results in a loss of product and will decrease the module recovery. The reduced pressure on the feed side also decreases the mass transfer through the membrane, which is indicated by eqn (5.19) ... 

The membrane material and all other parts of the membrane module which come in contact with the gases should be stable and chemically compatible not only with the gas mixture to be separated but also with the trace impurities present therein. All materials used in the membrane module should also have sufficient mechanical properties to resist any creep at operating temperature and pressure. The membrane should offer high gas ffux and high selectivity for compact design, higher gas purity and lower gas losses. 

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