Microfiltration Membranes

Two main types of membrane filters exist: screen Titers and depth Titers. Screen filters contain capillary-type pores; particles are retained on the membrane surface primarily by a sieving mechanism. Depth filters contain a random, tortuous porous structure; particles are retained through adsorption and mechanical entrapment within the bulk of the filter. Screen filters are absolute: particles larger than the pore size are retained, whereas particles smaller than the pore size can pass relatively easily through the membrane. Particle retention of depth filters is not that clearly defined: retention values increase slowly over a broad particle size range and only reach 100% for very large particles. Depth filters are often used for dead-end filtration, as they can retain a high particle load.

Membrane Materials and Membrane Preparation

Microfiltration membranes are available in a wide variety of materials and methods of manufacture. Many membranes are made of polymers, such as cellulose acetate, polysulfone, and polyvinylidene fluoride (PVDF). Most of these membranes are solvent cast, through a phase inversion process. Other preparation techniques are stretching (polytetrafluoro-ethylene, PTFE, membranes) and track-etching (polycarbonate membranes). The track-etching process results in cylindrical pores with a very narrow size distribution.

Other microfiltration membranes available are made from glass, from ceramics, such as alumina, titania, and zirconia, and from metals, such as silver and stainless steel. Advantages of these inorganic materials are their higher stability towards extreme process conditions, such as high temperature, extreme pH values, and solvents different than water. Most metal and some ceramic membranes are produced by a sintering process, whereas other ceramic membranes are produced by sol-gel processing or by anodic oxidation. Some novel membranes are prepared by lithographic techniques.

In Table 1, a number of different commercial membranes and some of their key properties are presented, and in Figure 3 SEM (scanning electron microscopy) and AFM (atomic force microscopy) images of some membranes are shown. Note that the membranes shown here are only a fraction of the total number of membrane materials and membrane manufacturers available.

Table 1 Various microfiltration membranes and their water fluxes

Manufacturer

Trade name

Materialb

Preparation method

Pore sizea (pm)

Water permeability at 20°C(Lm-2h-1 bar)

US Filter/SCT

Membralox®

a-Al2O3

Sintering

0.2

2000

Anotec

Anopore®

a-Al2O3

Anodic oxidation

0.2

3600

Carbon Lorraine

Carbon

Pyrolysis

0.2

1 500

Tech Sep

Carbosep®

ZrO2

Sintering

0.14

400

Millipore

Durapore®

PVDF

Phase inversion

0.22

5900

Fluoropore®

PTFE

Stretching

0.22

12 000

MF-Millipore®

Mixed cellulose

Phase inversion

0.22

14 400

esters

Osmonics

PCTE

Polycarbonate

Track-etching

0.2

14 600

PES

Polyethersulfone

Phase inversion

0.2

20500

MCS

Mixed cellulose

Phase inversion

0.22

15 400

esters

Whatman

Cyclopore®

Polycarbonate

Track-etching

0.2

16 000

Aquamarijn

Microsieve™

Silicon nitride

Photolithography

0.2

87000

aAll these membranes are available with pore sizes in large ranges. The pore sizes closest to 0.22 ^m are mentioned here to compare water fluxes of the different membranes. bPVDF, polyvinylidene fluoride; PTFE, polytetrafluoroethylene.

aAll these membranes are available with pore sizes in large ranges. The pore sizes closest to 0.22 ^m are mentioned here to compare water fluxes of the different membranes. bPVDF, polyvinylidene fluoride; PTFE, polytetrafluoroethylene.

Membrane Characterization

Originally the main goal in characterization of porous membranes was to determine the pore-size distribution. It has however been realized more recently that membrane surface properties, such as hy-drophobicity, zeta potential and surface roughness, play an important factor in fouling and retention properties of membrane processes. Characterization is therefore nowadays performed by various techniques, measuring different structural and physico-chemical parameters. The relatively novel technique of AFM microscopy has been shown to provide information on many membrane properties of interest: pore size distribution, surface roughness, and adhesion behaviour. In Table 2, various measurement techniques are summarized.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment