membrane configurations

- Membrane can be classified in several ways, butalways there are arbitrary classifications.

• Structure: symmetric, asymmetric

• Configuration: flat, tubular, hollow fiber

• Material: organic, inorganic

• Surface charge: positive, negative, neutral

• ...and even other divisions and subdivisions

Membrane Separations

- Structure:

• Symmetric. Also called homogeneous. A crosssection shows a uniform porous structure.

• Asymmetric. In a cross section, one can see twodifferent structures, a thin dense layer and below aporous support layer.

- Integral: the layers are continuous.- Composites: the active layer (thickness0.1-0.5 �m) is supported over a highlyporous layer (50-150 �m), sometimesboth layers are of different materials.


Symmetric UF membrane of 0.45 µm made ofcellulose acetate (Millipore).


Symmetric ceramic membrane of 0.2 µm made ofalumina (Al2O3) (AnoporeTM).

Surface Cross section


Asymmetric ceramic membrane madeof γ-Al2O3 (Membralox).


UF integral asymmetric membrane made ofpolypropylene.


RO composite membranes.

Cellulose acetate

Polyamide


- Configuration and modules

• Configuration: geometric form given to thesynthetic membranes.

• Module: name of the devices supporting oneor several membranes (housing).

The module seals and isolates the different streams. Thegeometry and specific fluid movement through the confinedspace characterises each module. The type of f lux, thetransport mechanism and the membrane surface phenomenadepend on the module design.


- Configuration:

• Flat.

- The active layer is a flat.

- Synthesised as a continuous layer.

- Later, one can select a desired geometry(rectangle, circle,...) to be placed in the module.

- Used in two kind of modules: plate-and-frameand spiral wound.

- High surface area/volume ratio.


Plate-and-Frame Membrane System.


Consists of layers of membranes separated by corrugatedstructural sheets, alternating layers with feed materialflowing in and retentate flowing out in one direction,while permeate flows out in the other direction.

Spiral-wound module.



Spiral-wound module.

- Configuration:

• Tubular.

- It is like a tube.

- Usually the active layer is inside.

- The permeate crosses the membrane layer tothe outside (this is, the feed flows inside).

- Low surface are/volume ratio.

- Several lengths and diameter (>10 mm).

- Modules grouping one or various membranes.


Different types of tubular modules.


- Configuration:

• Capill ary (0.5-10 mm) and hollow fiber (<0.5 mm).- Active layer inside.

- Hollow fiber: usually ∅in from 40 to 120µm and ∅out from 85 to 250 µm.


• Capill ary, hollow fiber.

- Usually, the permeate goes from outside toinside, but the flux also can be reversed.

- Very high surface area/volume ratio.

Out-in

In-out


Capillary membranemodules.


Hollow fiber module.



Hank of polyamide hollow fiber for RO (DuPont).

Cross section of hollow fiber (Monsanto).Comparison with a clip.


Hollow fiber cross section of polyamidefor RO (DuPont).


Hollow fiber made of polysulfone(∅ ≈ 1 mm) for UF (detail ).


Hollow fiber cross section of ∅ ≈ 1 mm(Monsanto).


Hollow fiber surface of polypropylene (Celgard).


Hollow fiber ceramic membranes (CEPAration).


- Comparison between modular configurations.

Module

Parameter Tubular Spiral-wound Hollow fiber

Specific surface area (m2/m3) 300 1000 15000Inside diameter or spread (mm) 20-50 4-20 0.5-2Flux (L/m2 day) 300-1000 300-1000 30-100Production (m3/m3 per module & day) 100-1000 300-1000 450-1500Space velocity (cm/s) 100-500 25-50 0.5Pressure loss (bar) 2-3 1-2 0.3Pretreatment Simple Medium HighPlugging Small Medium ElevatedReplacement Easy Difficult ImpossibleCleaning:

MechanicalChemical

PossiblePossible

Not possiblePossible

Not possiblePossible


- Comparison between modular configurations.

Modular configurations and processes.

Module

Operation Tubular Spiral-wound Hollow fiber

Reverse Osmosis A VA VA

Ultrafiltration VA A NA

Microfil tration VA NA NA

Pervaporation A VA VA

Gas Permeation NA VA VA

VA = Very appropriate; A = Appropriate; NA = Not appropriate


membrane configurations

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