8/14/2019 Configuracin de membranas

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Department Editor: Kate Torzewski

Membrane

Configurations

Membrane polymers are pack-aged into a configuration,commonly called a device

or an element. The most-commonelement configurations (figure) aretubular, capillary fiber, spiral wound,and plate and frame.

TUBULAR

Made from ceramic, carbon,stainless steel or a number ofthermoplastics, tubular elementshave inside diameters from 1/4in. up to about 1 in. Typically, themembrane is coated on the inside

of the tube, and the feed solutionflows through the interior (lumen)from one end to the other, withthe permeate passing through thewall and collected on the outsideof the tube.

CAPILLARY (HOLLOW FIBER)

These elements are similar to thetubular element in design. Theyare, however, smaller in diameterand usually consist of unsupport-

ed membrane polymers, which re-quire rigid support on each end.This support is provided by anepoxy potting of a bundle of thefibers inside a cylinder. The feedflow is either down the interior ofthe fiber or around its outside.

SPIRAL WOUND

This type of element is made froman envelope of sheet membrane,wound around a permeatetube that is perforated to al-low collection of the permeate.Feed water becomes purified bypassing through one layer of themembrane and flowing into thepermeate tube. This is by far themost common configuration inwater-purification applications.

PLATE AND FRAME

This kind of element employs

sheet membrane, stretched over aframe to separate the layers andfacilitate collection of the perme-ate, which goes to a center tube.

Feed

Feed

Feed

Feed

Concentrate

Concentrate

Concentrate

Concentrate

Membranes

Membranes

Feedspacer

Membrane

Permeate

Permeateflow

Permeateflow Permeate

flow

Poroussubstrate

Permeate

Cartridge

Hollow fibers

Plate and frame

Tubular

Spiral wound

Capillary

CHOOSING A CONFIGURATION

In selecting a membrane configu-ration, it is important to consider

how the packing density and con-centration polarization of eachconfiguration affects membranefouling resistance.

Packing density. From the perspec-tive of cost and convenience, it isbeneficial to pack as much mem-brane area into as small a volumeas possible. The higher the packingdensity, the greater the membranearea enclosed in a device of agiven volume, and, generally, the

lower the cost of the membraneelement. The disadvantage ofmembrane elements havinghigh packing density istheir greater propensityfor fouling, as outlined inthe table.

Concentration polarization.Concentration polarization is the ac-cumulation of rejected particles to anextent that transport to the membranesurface becomes limited. It reduces

the permeability of the solvent andcan lead to a limiting flux and ahigher fouling tendency.

The type of membrane module usedin a process affects the influence ofconcentration polarization; and it isdifficult to balance high fluxes andlow fouling with low investment andoperating costs. Tubular modules canaccommodate high cross flow andlarge particles, but their capital costsand ratio of relative price to mem-

brane area are considerably higherthan those for spiral-wound modules.Spiral-wound modules, on the otherhand, enjoy the advantages of lowerinstalled costs and easier changeout.Channel height can be varied by theuse of distance keepers, also knownas spacers.

Capillary membrane modules can bebackwashed inline during filtration toremove particles from the membraneor to add chemicals from the permeate

side. Like tubular modules, they havehigh investment costs, but their ability tobackwash at regular intervals reduces thepotential for fouling.

References

1. Cartwright, P., Membranes for Process WaterReuse, Chem. Eng.,June 2004, pp. 3842.

2. Baird, A., Making High-Purity Water, Chem.Eng.,May 2005, pp. 3643.

3. Buecker, B., Microfiltration for CPI Wastewater,Chem. Eng.,May 2007, pp. 6365.

4. Lipnizki, J., Strategies for Controlling MembraneFouling, Chem. Eng.,September 2007, pp.6264.

COMPARISON OF MEMBRANE ELEMENTCONFIGURATIONS

Elementconfiguration

Packingdensity*

Foulingresistance**

Capillary filter medium highPlate and frame low high

Spiral wound medium moderate

Tubular low high

* Membrane area per unit volume of element

** Tolerance to suspended solids